Guidebook On Design, Construction and Operation of Pilot Plants For Uranium Ore Processing
Guidebook On Design, Construction and Operation of Pilot Plants For Uranium Ore Processing
Guidebook On Design, Construction and Operation of Pilot Plants For Uranium Ore Processing
Guidebook on Design,
Construction and Operation
of Pilot Plants
for Uranium Ore Processing
vsz
itWi ' NTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1990
GUIDEBOOK ON DESIGN,
CONSTRUCTION AND OPERATION
OF PILOT PLANTS
FOR URANIUM ORE PROCESSING
The following States are Members of the International Atomic Energy Agency:
The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the
IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Head-
quarters of the Agency are situated in Vienna. Its principal objective is "to accelerate and enlarge the
contribution of atomic energy to peace, health and prosperity throughout the world".
© IAEA, 1990
GUIDEBOOK ON DESIGN,
CONSTRUCTION AND OPERATION
OF PILOT PLANTS
FOR URANIUM ORE PROCESSING
The design, construction and operation of a pilot plant are often important
stages in the development of a project for the production of uranium concentrates.
Since building and operating a pilot plant is very cosdy and may not always be
required, it is important that such a plant be built only after several prerequisites have
been met.
The main purpose of this guidebook is to discuss the objectives of a pilot plant
and its proper role in the overall project. Given the wide range of conditions under
which a pilot plant may be designed and operated, it is not possible to provide
specific details. Instead, this book discusses the rationale for a pilot plant and
provides guidelines with suggested solutions for a variety of problems that may be
encountered.
This guidebook is part of a series of Technical Reports on uranium ore
processing being prepared by the IAEA's Division of Nuclear Fuel Cycle and Waste
Management. A report on the Significance of Mineralogy in the Development of
Flowsheets for Processing Uranium Ores (Technical Reports Series No. 196, 1980),
Methods for the Estimation of Uranium Ore Reserves: An Instruction Manual
(Technical Reports Series No. 255, 1985) and a Manual on Laboratory Testing for
Uranium Ore Processing (Technical Reports Series No. 313, 1990) have already
been published.
The IAEA wishes to thank the consultants who took part in the preparation of
this report for their valuable contributions: A. Abrao (Instituto de Pesquisas
Energ&icas e Nucleares/Comissao Nacional de Energia Nuclear, IPEN/CNEN,
Brazil), P. Gasds (Centro de Investigaciones Energ&icas Medioambientales y
Tecnologicas, CIEMAT, Spain) and D.C. Seidel (Bureau of Mines, United States
of America). Thanks are also due to the consultants' Member States and organi-
zations for their generous support in providing experts to assist in this work. The
IAEA officer with overall responsibility for this work was S. Ajuria of the Division
of Nuclear Fuel Cycle and Waste Management.
EDITORIAL NOTE
Although great care has been taken to maintain the accuracy of information contained
in this publication, neither the IAEA nor its Member States assume any responsibility for
consequences which may arise from its use.
The mention of names of specific companies or products (whether or not indicated as
registered) does not imply any intention to infringe proprietary rights, nor should it be con-
strued as an endorsement or recommendation on the part of the IAEA.
CONTENTS
1. INTRODUCTION 1
1.1. The pilot plant concept 1
1.2. Pilot plant objectives 2
1.3. Prerequisites for pilot plants 3
1.3.1. Ore body data 4
1.3.2. Mining methods 5
1.3.3. Bench scale metallurgical data 5
1.4. Planning of experiments 5
1.4.1. Experimental planning 5
1.4.2. Sampling 6
1.4.3. Reagent preparation 6
1.5. General considerations 6
1.5.1. Partial or total pilot plant 7
1.5.2. Pilot plant flexibility 7
1.5.3. Size of the pilot plant 8
1.5.4. Pilot plant location 9
1.5.5. Other considerations 10
1.6. Limitations of pilot plants 10
3. BASIC ENGINEERING 22
3.1. Introduction 22
3.1.1. Technological manual 22
3.1.2. Conceptual project document 22
3.1.3. Basic engineering project document 23
3.1.4. Detailed engineering project document 24
3.2. Siting 25
3.3. Plant layouts 27
3.4. Selection of equipment 28
3.4.1. Crushing 29
3.4.2. Grinding 29
3.4.3. Leaching 30
3.4.4. Solid-liquid separation 31
3.4.5. Equipment for uranium recovery 32
3.4.6. Equipment for precipitation and drying 33
3.5. Matching equipment sizes 34
3.6. Instrumentation and control 34
3.7. Piping and instrumentation diagrams 35
3.8. Utilities 37
3.8.1. Electrical services 37
3.8.2. Process water requirements 39
3.8.3. Drinking water 39
3.8.4. Demineralized water 39
3.8.5. Steam generation 40
3.8.6. Compressed air 40
3.8.7. Laundry 40
3.9. Effluents treatment and tailings disposal 41
3.9.1. Liquid effluents 42
3.9.2. Solid effluents 42
3.9.3. Waste treatment flow sheet 43
4. PROJECT ADMINISTRATION 47
4.1. Introduction 47
4.2. The pilot plant manager 48
4.3. Planning 49
4.3.1. Licensing 49
4.3.2. Action planning 50
4.3.3. Safety 53
4.3.4. Support services 55
4.4. Staffing 59
4.5. Operational controls 61
4.5.1. Staff meetings 61
4.5.2. Operating manuals 62
4.5.3. Data recording 62
4.5.4. Reports 63
APPENDIX I: EXAMPLE OF A URANIUM PILOT PLANT 65
1.1. Introduction 65
1.2. The Grand Junction Pilot Plant 65
1.2.1. Acid leach-CCD-solvent extraction pilot plant 66
APPENDIX II: GENERAL AND RADIOLOGICAL SAFETY 71
II. 1. General safety precautions 71
11.2. Radiological safety 71
11.3. Safety rules 72
REFERENCES 77
BIBLIOGRAPHY 79
The necessity of pilot plants for process development has been argued exten-
sively, with the dispute centred primarily on the need for piloting processes that
produce organic chemicals. The circumstances differ, however, for hydrometallurgi-
cal processes such as the recovery of uranium from its ores, because the raw
material, uranium ore, has a decisive influence on the process. Since the uranium
minerals and the composition of the gangue are both variable, it is risky to scale up
the laboratory results using standard chemical engineering principles and
projections.
Unless one has extensive experience with similar ores, it is advisable to verify
the laboratory results in a pilot plant: either in a complete flow sheet or, at least,
in the most complex parts of the process where recirculating streams could accumu-
late impurities over long periods of time.
The successful recovery of uranium from an ore body requires a sequence of
activities, beginning with the study of the ore body, followed by laboratory scale
exploratory studies of the first phases of the process, mainly the leaching operations.
As the knowledge of the ore body improves and representative samples become
available, a detailed experimental laboratory project is carried out. The aim of this
research is to define the most advantageous flow sheet and to develop the design and
operating information. This information will lead to the prefeasibility studies which,
if positive, will be used to develop engineering criteria for the industrial plant.
The design of a commercial plant can be unsuitable if the available experience
is not extensive enough to scale up the information from bench scale laboratory tests,
or if the uranium ore has some peculiarities in its uranium or host rock minerals.
Baekeland's [1] famous comment, "Commit your blunders on a small scale and
make your profit on a large scale", can be applied to this situation where the pilot
plant represents an intermediate stage between the laboratory studies and the indus-
trial plant. The necessity of the pilot plant will depend on the complexity of the ore
and the experience available.
The pilot plant must be understood not as a scale-up of laboratory equip-
ment [2], but as a small scale simulation of the future industrial operations. Results
of the laboratory studies will be used to choose the most suitable process for the ore
deposit and will lead to the selection of the equipment for each stage of the flow sheet
(for instance, pneumatic or mechanical stirred tanks for leaching, thickeners or
filters for counter-current washing, etc.). If the prefeasibility studies are positive and
if pilot plant studies are judged to be necessary, the pilot plant will be designed to
1
simulate the industrial operations. It is not always necessary that the pilot plant
include all of the flow sheet stages, but it must include at least those which (a) differ
fairly markedly from conventional practice; (b) require equipment not frequently
used; (c) have caused some problems at the laboratory scale; and (d) have some ele-
ments, even trace elements, that might build up in some streams, for example in sol-
vent extraction.
In pilot plants for new chemical processes it is usually desirable to use the same
materials of construction that will be used in the industrial plant. In the case of ura-
nium plants, however, there is extensive experience and information about materials
of construction. Therefore it is not generally necessary to use the same materials in
the pilot plant as in the full scale plant, unless some special ores must be tested.
The main objective of the pilot plant — to check, on a reduced scale, the
process developed in laboratory studies — has already been described. As a result
of experience with a pilot plant, the decision to proceed with the full scale plant
project will be based on a proven process, and on a more reliable economic estimate.
This, however, is not the only purpose of a pilot plant. There are other objectives
which can be fulfilled simultaneously and in some cases these other objectives may
be the definitive considerations for the decision to build the pilot plant.
For example, in the case of uranium recovery, because the raw material (ura-
nium ore) often varies throughout the life of the ore body, it may be necessary to
change the operating conditions in order to achieve the highest efficiency. In this
case, a pilot plant can be very useful. Studies of the process can be carried out in
the pilot plant while the industrial plant is working. In this way, it is possible to deter-
mine the most suitable economic operating conditions, which depend on the charac-
teristics of the ore. In a case such as this where both plants work simultaneously,
the objective of the pilot plant is to find the best conditions of operation. On the other
hand, when the pilot plant is built as an intermediate stage between the laboratory
studies and the industrial plant, its main objective is to confirm the design charac-
teristics of the full scale plant.
In some ores the uranium is associated with other elements which can be
profitable by-products. In this case, a pilot plant facilitates the study of alternative
flow sheets and the choice of the most suitable one. At the same time it is possible
to obtain samples of the by-products which can then be examined for potential com-
mercial value. Furthermore, in a pilot plant enough uranium concentrate can be
obtained to determine if commercial specifications are met. The yellow cake product
also can be useful for the study of subsequent stages of the fuel cycle.
Another important objective of a pilot plant is personnel training, especially
in countries where similar hydrometallurgical processes do not exist. In this case,
2
personnel will be trained not only for operation of the different types of equipment,
but also for control of the process. This training will help get the full scale plant in
operation, reducing the time needed to reach its design capacity, and avoiding
damage to some of the equipment as a result of improper operation. Training is a
very important objective, then, since a delayed start would probably imply lower out-
puts than those which were forecast in the project, and consequently would hinder
the economic success of the exploitation.
The objectives of a pilot plant, therefore, can differ depending on the specific
circumstances of each project, and the decision for its construction can include one
or several of the following objectives:
— To optimize the operating parameters of the process,
— To study the effects of recirculating process streams and of accumulation of
impurities over long periods,
— To obtain process information necessary to specify and design the full scale
plant,
— To test process control systems and procedures,
•r— To test materials of construction,
— To optimize the design of the equipment,
— To obtain sufficient information to prepare detailed and reliable estimates of
capital and operating costs and to prepare a reliable economic evaluation of the
project,
— To gain operating experience and to train the personnel that will operate the
full scale plant,
— To identify hazards in the process and ensure safety in design and operation,
including the disposal of radioactive wastes,
— To produce a reasonable amount of uranium concentrate for characterization
and for use in subsequent stages of the nuclear fuel cycle.
A pilot plant requires a substantial investment and often long term operation
is needed to achieve the desired objectives. Thus a thorough analysis must be carried
out before a decision is made to build a pilot plant. Drawbacks such as expenses and
length of the project must be weighed against the expected advantages to be gained
in fulfilling the objectives of a pilot plant.
A pilot plant is usually not considered until the project is sufficiently advanced
and well defined to the extent that there is reasonable assurance that the overall
project is feasible. This presupposes that an ore body has been identified and devel-
oped, that a mining method has been selected at least tentatively, that a suitable
hydrometallurgical process has been defined, that the capital and operating costs
3
have been estimated and that these estimates indicate that the ore can be processed
economically. With this information the pilot plant project can be undertaken.
Given the above prerequisites, it is also assumed that a process development
laboratory and an analytical laboratory are available. Both will be used during the
operation of the pilot plant.
The size and morphology of the ore body (in three dimensions) should be
known. The information must include plans and sections of the ore body as well as
borehole data and data from other development work.
Ore reserve estimates prepared by any of the standard procedures and esti-
mates of the mean ore grade should also be available.
The degree of homogeneity of the ore body or ore bodies should be known.
While no ore body is entirely homogeneous, there are many cases in which the
degree of homogeneity is sufficient to allow the use of a single metallurgical process.
In other cases the inhomogeneity is so great as to require process modifications
during the life of the plant.
4
1.3.2. Mining methods
From the ore body information outlined in the previous section it will be possi-
ble to define the expected production rate and production life of the mine.
A suitable mining method or methods can be selected or proposed on the basis
of the geological information and the production rate.
A mining method having been selected, the grade and tonnage of minable ore
and the ratio of ore to waste rock can be calculated.
One of the main objectives of the pilot plant is to verify and optimize the
process flow sheet and the process parameters.
5
comminution be? Overgrinding is expensive and in turn makes solid-liquid separa-
tion more difficult and expensive. How much acid will be consumed? How should
it be dosed? The use of excess acid means that an excess of lime will be needed for
neutralization. The appropriate retention time for leaching must be determined. Are
there any problems with the solid-liquid separation? The physical characteristics of
pulps are likely to be different in the pilot plant and in the laboratory. This may affect
solid-liquid separation operations such as filtration.
During laboratory testing one may produce only small amounts of yellow cake,
of the order of grams or tens of grams. These amounts may be sufficient for some
analytical determinations and thermogravimetric tests which require only a few
hundred milligrams of yellow cake, but it may not be possible to do much more with
such small quantities. The pilot plant, however, will provide larger amounts of yel-
low cake. The choice of the type of yellow cake may be confirmed (sodium
diuranate, ammonium diuranate, etc.) and filtration tests may be done, probably for
the first time. It may now be possible to determine the size of the filters for the indus-
trial plant. It is probably a good idea to stop the tests here. Drying and calcining of
yellow cake are very well known operations. It will be difficult to make meaningful
tests on the small amounts of concentrate available and too much ore would have to
be processed to provide enough yellow cake to make any meaningful tests.
1.4.2. Sampling
Chemical and instrumental analyses are essential for monitoring and control-
ling the metallurgical process. A sampling programme must be carefully drawn up.
It is necessary to determine what samples should be taken, how often they should
be taken and which analytical methods should be used. The samples should be
properly homogenized by quartering or by other techniques commonly used. Pulp
samples should be fresh because aged samples have different characteristics.
Once the decision has been made to construct the pilot plant as an intermediate
stage between laboratory results and the industrial plant project, and once the main
6
objectives have been fixed, it is time to analyse a sequence of very important items
in order to achieve the highest efficiency in the final project.
First, a decision must be made as to whether or not the pilot plant has to com-
prise the whole process or just some of the different unit operations within the
process. Such a decision will depend, in part, on the complexity of the flow sheet
resulting from the laboratory studies. In a complex process, with recirculating
streams between stages that could result in buildup phenomena, it would be advisable
to check the whole flow sheet. Otherwise, if the laboratory studies reveal that the
uranium ore does not present any special problem and if it is possible to use a simple
flow sheet where there are doubts about only a few operations, it would be sufficient
to include only these operations in the pilot plant.
Apart from the laboratory results, if one of the pilot plant objectives is person-
nel training, or if the pilot plant is built to determine the best operating conditions,
working simultaneously with the full scale plant, then the pilot plant would have to
include the whole flow sheet.
Therefore, before deciding which activities are to be included and what the
appropriate size of the pilot plant should be, all factors, objectives and circumstances
relating to this decision have to be carefully analysed.
7
to increase the residence time of the leaching section or in a case where it has been
decided to incorporate a new stage in the counter-current washing system.
Laboratory test results provide design and operating parameters which will be
used to prepare the pilot plant project. While the values of these parameters (size
distributions, reaction time, reagent doses, temperatures, etc.) have to be considered
as central values, it may be necessary to make adjustments to obtain the most
favourable values for the process. Therefore, the pilot plant has to be designed with
adequate flexibility, keeping in mind the possibility of modification of any of these
parameters. It will also be important to analyse how these changes can affect the next
stages of the process. For example, if the leaching residence time has to be changed,
then a modification of the feed pulp rate may be necessary. Such a change could
affect the following stages in the flow sheet. Another way to adjust the residence time
without changing the pulp flow rate could be modification of the volume of the tanks.
Thus, as the project is being developed with the necessary flexibility for a plant
of this type, every stage has to be analysed, different ways to accomplish the possible
modifications have to be considered and the effects on the rest of the flow sheet must
be studied. On the basis of these considerations, the design which is easiest to imple-
ment can then be chosen.
A pilot plant is a reduced scale model of a commercial plant. Given this defini-
tion, the next problem is to select the scale-up factor, i.e. the ratio of the size of the
pilot plant to the size of the commercial plant.
A larger pilot plant will provide more reliable information and will reduce
risks in the overall project. It will, however, be more expensive to build and operate
and it will also be more difficult to provide feed material and to dispose of the mill
tailings.
On the other hand, controlling the flow rates and handling the pulps can be
difficult in a small pilot plant, which may be impossible to operate in a continuous
and uniform manner. The information obtained from such a plant may not be reliable
enough.
A compromise has to be found. In practice it has been found that capacities
below 100 kg of ore per hour should not be considered. This is the lower limit that
allows a uniform operation that will produce reliable results.
In a pilot plant of this capacity, not all of the equipment can be just a small
scale model of that in the full scale plant, because the smallest sizes of some of the
equipment available on the market have still higher capacities than required. This
happens, for example, with slurry pumping. Most uranium mills use different types
of centrifugal or diaphragm pumps for this operation, with minimum capacities
which are much higher than the required ones, especially in the case of centrifugal
pumps. There is extensive experience in slurry pumping, however, and for the usual
8
distances and heights in a uranium ore plant, the knowledge of the characteristics of
these pulps (size distribution of solids, density, viscosity, etc.) usually will be ade-
quate for the industrial plant project.
A similar situation can occur in the case of grinding, when the use of large
diameter autogenous or semiautogenous mills has been envisaged. In these and other
similar situations where larger scale pilot plant experimentation has been judged
necessary, a loop can be set up for testing a particular circuit or a sample can be sent
to an equipment supplier who will carry out the pertinent tests.
The upper capacity limit of a pilot plant will be determined by several factors:
economic considerations, the availability of uranium ore supply and the disposal of
tailings generated in the process.
In general, it is reasonable to say that an acid leach pilot plant with a capacity
of about 100-200 kg of ore per hour will be large enough to obtain all the necessary
data with an acceptable level of reliability. For production plants with capacities
between 1000 and 5000 tonnes of ore per day, this feed rate range will give scale-up
factors that are usually considered acceptable for most of the equipment.
If, however, ore reserves and prefeasibility studies justify a production plant
with a feed capacity greater than 5000 tonnes per day or if the ore has specific
processing problems, a larger pilot plant would be justified.
Initially, two possibilities may be considered for the location of a pilot plant:
the vicinity of the ore body or the research centre where laboratory studies have been
carried out.
The research centre has the advantage of having the necessary infrastructure
for plant operation (maintenance workshops, analytical laboratories, etc., are avail-
able). These centres, however, are usually located rather far from the ore bodies,
a factor of particular significance when the pilot plant throughput is large. Since it
would be necessary to transport large amounts of ore and dispose of the generated
tailings, such a location would increase the complexity and expense of the operation.
In the case of uranium ores, there is another problem with building a pilot plant
at the research centres, which are usually located in urban areas. The radioactivity
associated with uranium ores would involve the establishment of a security guarantee
in order to avoid the appearance of an undesirable risk in such areas. This complica-
tion would reduce the likelihood of such a location being chosen for the pilot plant.
Therefore the most suitable place for a pilot plant appears to be near the ore
body. On the one hand, the cost of transport of both ore and tailings is reduced, and
on the other hand, suitable zones for tailings are available and radiation protection
can be dealt with under existing ore body protection requirements. Nevertheless, it
will still be necessary to provide the infrastructure and services needed for plant
operation, if they are not already available. With regard to analytical services, a
9
laboratory can be prepared close to the plant for the more urgent analyses, while the
more complex can be done at a central laboratory.
If, however, the pilot plant is small or if only some unit operations will be
included in the pilot plant, it may still be advisable to install the plant near an existing
research centre where support facilities are available.
10
perature. Short periods of operation can result in overly optimistic information, far
removed from the industrial plant reality. If the full scale plant project is then
designed according to the results obtained during a short lived pilot plant operation,
serious problems can arise.
Likewise, it is necessary to warrant that the pilot plant has reached steady state.
This may require long periods of operation.
Some operations may be difficult to study at the pilot scale, especially if the
size of the plant is relatively small, because it is difficult to find reduced models
whose operations can be scaled up. This can be the case for some types of mills,
thickeners, clarifiers, etc.
In some circumstances, it is not possible to operate the whole plant continu-
ously. As uranium ores are generally of relatively low grade, it is necessary to work
with large amounts of ore in order to obtain a small amount of concentrate. There-
fore, studying the last phases of the process may require the storing of the product
from an earlier stage, in order to ensure a large enough flow for the later stages.
The ageing of the stored products, however, may affect the results.
11
In some cases, the throughput of the pilot plant may differ from one section
to another. Thus, the capacity of the first sections of the flow sheet is determined
by the amount of ore to be processed, while the capacity of the last sections (precipi-
tation, drying, etc.) is defined by the amount of uranium in the ore. In the case of
low grade uranium ores the flow rates in the last sections are generally very small
and these sections are often designed for intermittent operation, storing the stripped
product liquor and any intermediate stream that is to be recycled to other parts of
the process. In this way, the equipment in different sections can be so designed that
the system operates continuously.
The process that will be studied at pilot plant scale is based on the results of
laboratory tests. The flow sheet should be defined as precisely as possible, and the
most favourable alternatives chosen. Any alternatives not applicable to the ore being
studied should be rejected.
At this stage of the project the type of leaching reagent (acid or alkaline) has
been chosen and the various unit operations of the process have been defined. If
enough information is available, the most suitable kind of equipment will have been
selected (e.g. thickeners or filters in solid-liquid separation). In some instances,
however, the pilot plant may include the different alternatives. If corrosion problems
are expected, the pilot plant programme will include testing of suitable construction
materials.
In general, all sections of the pilot plant will use continuous flow operation,
as happens in a full scale plant. In some instances, however, it may be necessary,
or at least convenient, to use batch equipment for operations such as the final product
preparation (yellow cake).
By-product recovery should be considered if the uranium ore contains other
elements that can be economically recovered. In this case, the appropriate stages will
be included in the pilot plant.
At this point in the planning process it is advisable to seek advice from experts
who can analyse the available information and focus on those points that must be
studied in more detail. This procedure will help ensure that the pilot plant achieves
the desired objectives.
The design strategy for the pilot plant should be planned with all of these points
in mind. It is usually not necessary, and may not even be desirable, to prepare
detailed engineering such as that required for a full scale plant. Excessive documen-
tation is also unnecessary. Strict standards on drawings and other documents will
increase both engineering costs and the total time required to complete the project.
The pilot plant must be flexible; there will probably be changes not only in the flow
sheet but also in the equipment [3].
12
The first step in pilot plant design is to prepare a block diagram. This consists
of rectangular blocks which are connected by arrows according to the flow sequence;
each block represents a unit operation in the pilot plant.
These block diagrams are very useful in the early stage of a pilot plant project
because they give a general view of the overall process that has been envisaged from
laboratory information. In addition to the unit operations, the block diagram can
include the reagents and utilities that will be used in the pilot plant.
A block diagram of a uranium ore processing pilot plant is shown in Fig. 1.
This diagram includes:
— Ore comminution in two stages, crushing and grinding.
— Ore leaching with sulphuric acid, an oxidant and steam to heat the pulp up to
the temperature which has been defined by laboratory tests. This heating
method can be advantageously replaced by electric heating at pilot plant scale.
— Solid-liquid separation by counter-current decantation in thickeners using
water as washing liquid and flocculant for improving settling and thickening
characteristics of the pulps. Pregnant solution is clarified before sending it to
the next stage.
— Solvent extraction to purify and concentrate the uranium solution. The
raffinate is sent to effluent treatment. The organic phase is stripped with
ammonia and a solution of ammonium sulphate coming from the concentrate
thickening stage, which is made up with water to the appropriate concentra-
tion. The organic phase is recycled from stripping to extraction, new organic
phase being added to compensate for losses in the process.
— Yellow cake precipitation from the stripped product liquor with ammonia.
— Thickening, washing and filtration of yellow cake pulp. A part of the overflow
from the first thickener is recycled to the stripping stage.
— Drying of the yellow cake to obtain the uranium concentrate.
— Effluents neutralization with lime, and addition of barium chloride to the
effluents to decrease the concentration of radium, which is eliminated with the
barium sulphate.
— Tailings disposal.
The block diagram gives a general view of the whole plant and the main
streams, but it does not take into account the equipment and internal flows of the
different sections. These are developed in the following phases of the project.
The next step in the project is to calculate the different streams that enter or
leave the sections of the block diagram, and also reagent consumption in the process.
13
ORE
•
i
CRUSHING AND
GRINDING _ WATER
-2
+ SULPHURIC ACID
LEACHING OXIDANT
-3 11 HEAT
*
SOLID-LIQUID FLOCCULANT
SEPARATION
STERILE i -4
— 10
PULP i
EXTRACTION ORGANIC PHASE
q
STRIPPED
' " J- - 5 8 <' RAFFINATE
ORGANIC PHASE T
_ /
STRIPPING AMMONIA
"I
7 -6
*
1'
PRECIPITATION
+ ''
THICKENING
•
MOTHER LIQUOR
• *
FILTRATION
.+
DRYING
FILTRATE
*
URANIUM
CONCENTRATE
''
'—*- '
1
'
EFFLUENT i LIME
TREATMENT
»
TAILINGS
i BARIUM CHLORIDE
DISPOSAL
To carry out these calculations some basic data of the process are needed:
throughput, reagent dose requirements, solid-liquid ratios in the pulps, retention
times, temperatures in the different unit operations, etc. The basic data are presented
in the Annex. The figures in the Annex are for the calculation of mass balances relat-
14
ing to the block diagram of Fig. 1. Units are chosen in such a way that figures from
the calculations are easy to handle.
In this way main streams are computed, including all of the parameters (flow
rate, specific gravity, solids concentration in the pulps, uranium concentration, etc.)
that define the flow. Characteristic data of some streams of the block diagram in
Fig. 1 are given in Table I. These figures have been computed from the data in the
Annex. With this information it is possible to calculate reagent consumption and util-
ities that will be needed for pilot plant operation.
Energy balances have to be considered only as a rough estimation, because
equipment in a pilot plant is so small and its surface/volume ratio so large that heat
losses are very large as compared with heat needed for the process. Energy balance
is important only in the early sections of the flow sheet where all ore is processed
and, in general, it affects only the leaching stage in most of the processes. Drying
of yellow cake represents such small figures at this scale (0.28 kg U 3 0 8 /h in the
example) that it is not worth computing because batch equipment with electric heat-
ing will be used and it will not be possible to scale up the results.
When energy balances are calculated, not only must the process parameters be
considered, but also other conditions such as the climate and whether the equipment
will be indoors or outdoors.
In this kind of pilot plant, if it is not too large, electric heating has some advan-
tages over steam heating, particularly if a steam generator is not available and has
to be installed.
A block diagram provides a general view of the process but it does not give
any information about the different sections of the process. Consequently, it is neces-
sary to further develop these sections through process flow diagrams which include
all of the equipment in a given section, represented by standard symbols. Streams
between equipment are represented by lines with arrows to indicate the direction of
flow. Major streams are generally represented by heavy lines.
In general, a process flow diagram is prepared for each block in the block dia-
gram, unless there is a particular relation between two blocks. In such a case, it is
advisable to include both sections in a single diagram. Thus the process flow dia-
grams, which include all of the equipment, provide a view of each one of the sec-
tions, with the process streams and operations quite apparent. It should be noted that
in this type of process flow diagram, pipes, electrical details and utilities distribution
are generally omitted.
A process flow diagram of the solid-liquid separation section in the block dia-
gram of Fig. 1 is shown in Fig. 2. Washing is carried out with five thickeners in
series, with repulping of streams entering the thickeners and pumping of overflows
15
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and underflows. In this diagram, flocculant preparation and distribution system and
sampling points are included. Clarifying equipment is not included.
It is very important that these diagrams give clear and accurate information at
a glance. A simple and precise reference has to be assigned to each piece of equip-
ment. References allotted to the equipment in the process flow diagram of Fig. 2
consist of one capital letter and three numbers. The letter gives the code associated
wim a type of equipment, the first number refers to the section in the block diagram
(for example, the number 3 has been assigned to the solid-liquid separation section)
and the last two numbers are a sequential listing for equipment in that particular
section.
Basic data and mass balances from block diagram calculations provide enough
information to compute the internal streams of each one of the sections. These calcu-
lations will be used for the sizing of pipes and equipment. The most important
characteristics of different flows in the solid-liquid separation section are given in
Table II. It is advisable to include these parameters in the process flow diagram; they
are used to size the equipment for the plant.
From the process flow diagrams, equipment lists with their references are pre-
pared, followed by calculation sheets for each piece of equipment (mills, pumps,
thickeners, etc.) including information on flow rates and basic data (Annex). After
sizing of the equipment, specification sheets listing characteristics, dimensions, con-
struction materials, fittings, standards, etc., are completed.
With this information, tenders for commercial equipment are prepared. If spe-
cial equipment is needed, the specification sheets will be used for its design and con-
struction. It is advisable, however, to use standard commercial equipment if
available.
The next step in the project is die preparation of piping and instruments (P&I)
diagrams. These drawings include equipment, instruments, pipes for streams
between equipment, valves, etc.
Each piece of equipment, with the same reference as in the process flow dia-
gram, is represented wim all its fittings for connection with process and utilities
pipes. Drawings of these fittings show their relative positions in the equipment.
Instruments are represented with symbols and Instrument Society of America
(ISA) standards are commonly used. A reference is assigned to every instrument,
using the same method as for equipment.
In these P&I diagrams, all pipes — utilities as well as process pipes —- are
included. A reference including diameter, type of fluid, section and ordinal number,
and specification is assigned to every pipe. An example is shown in Fig. 3.
18
PW 3.04 — A.01
Pipe specification
Section 3, pipe 4
Fluid identification
(process water)
Nominal diameter, I5 in
A list of fluids, with the symbols assigned to each one, has to be prepared. Pip-
ing specifications are also prepared for the pipes to be installed in the pilot plant.
These specifications will include sizes, schedule, fittings, type of valves for
different sizes, etc. It is advisable to use a standard for piping design specifications.
A list of lines is prepared for each section including all pipesin the section. This
list will show at least the following information (Fig. 3): pipe size^/l-j in), process
fluid (PF), series number (3.04), specification (A.01), connection pointy (references
of equipment connected by this pipe), working temperature and mermaHnsulation
(if any). \
Data sheets showing the flow rate, temperature, precision level, control
valves, etc., will be prepared for each of the different instruments. Specifications^
including general characteristics, standards, etc., are also prepared for every type
of instrument. At this point, enough information is available to prepare tenders for
instruments, valves, pipes, etc.
After the P&I diagrams are completed, a preliminary plan including all of the
equipment in the pilot plant must be made ready. As previously stated, since the pilot
plant will be subject to flow sheet and equipment changes during its operation, a
more detailed development of the project is not justified at this point. Finally, to
achieve a successful project, it is important to utilize the advice of experts whenever
that seems appropriate.
19
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3. BASIC ENGINEERING
3.1. INTRODUCTION
Once the preliminary process book (flow sheets, material balances and other
information developed from laboratory data) has been prepared, it is possible to start
development of the basic engineering project. For this purpose the first documents
required are: a technological manual, the conceptual project document, the basic
engineering project document and the detailed engineering project document.
The conceptual project document describes the studies and definition of the
process systems, including the main project criteria. The preliminary documents that
must be prepared for this purpose are listed below.
22
3.1.2.3. Instrumentation description
The basic engineering project consists of the establishment of the layout of the
unit operations of the process and the issuing of documents for the pilot plant. In the
basic project the criteria for the project must be described and the following main
topics considered: the chemical process, mechanical engineering, instrumentation,
piping, electrical engineering, services, and civil and architectural engineering.
The basic project document must include process information such as the fol-
lowing items: an engineering flow sheet, a list of equipment, equipment sizing calcu-
lations, data sheets for the equipment (dimensions, wall thickness, corrosion
problems, operating temperature, volume and mass of reagents), preliminary draw-
ings for each unit operation, data sheets for the instruments and a preliminary plant
layout.
The basic project document must also include mechanical engineering informa-
tion such as general and technical specifications for the equipment, detailed drawings
of the pieces of equipment and their sizing calculations, general arrangements,
mechanical assembly drawings and mechanical detail drawings.
For the instrumentation, the following documents must be provided: a list of
the instruments, their general and technical specifications and the instrumentation
diagram (dimensions, flow rate, volume and operating temperature).
For the piping, the project must define the design criteria, the specifications
for the piping material, detailed piping and plumbing layouts, and working and fabri-
cation details.
For the electrical engineering the following should be prepared: general
specifications and power requirements, a single line diagram, the equipment sizing
calculations for the electrical equipment and its technical specification, the electrical
layout and the lighting drawings and the circuit schedules and details. The electrical
drawings must include the single line diagrams, schematic diagrams, interconnection
diagrams, lighting layouts, electrical equipment layouts, instrumentation loop dia-
grams and panel layout.
With respect to services, a simplified description of the main utilities, espe-
cially the water and electricity requirements, should be provided.
23
The civil-architectural design must include the following documents: design of
earthwork, design of the structures, design of the concrete foundations, design and
detailing of all non-process buildings and structures, civil and structural drawings,
drainage plans, concrete drawings, structural steel drawings and architectural
arrangements.
The detailed engineering project is the most advanced stage of the plant project
and describes the details for all equipment, lines, instrumentation, structures for the
unit operations and systems, a revision of the documents issued for the preliminary
stage and final versions of all documents for this advanced stage. There must be
included here the following documents: isometric drawings of the pipelines, dimen-
sions for the pumps, equipment fabrication specifications, civil engineering design
and purchase and inspection procedures. Generally speaking, the manufacturer is
responsible for the construction drawings of the equipment.
The general manager for the engineering project must organize the project
according to the categories described below.
The chemical process description includes the process diagram; utilities dia-
gram; list of equipment; data sheets for the process; schematic drawings for tanks,
reactors, columns, vessels, heat exchangers, pumps, compressors and special equip-
ment; selection of piping materials; instrumentation data sheet; equipment layout;
engineering flow sheet; instrumentation layout; detailed layout and designs for
materials handling; the operating manual; and the analytical manual.
The mechanical project description shall include the specifications for the pur-
chase of vessels and tanks; the layout and procurement procedures for the solids
handling equipment; and design and procurement details for stairways, platforms,
skid-mounted unit operations and the ventilation system.
24
3.1.4.4. Instrumentation description
For the instrumentation, the engineering project description includes the pro-
posed criteria, the sizing calculations for each instrument, the data sheet and specifi-
cations for the purchase of instrumentation panels and materials, the instrumentation
plan and details for installation, the diagrams for the instrumentation, a list of alarms
and set points, the network diagram, the logic circuit, specifications for the installa-
tion, and checking procedures and tests for the instrumentation.
The following must be considered: details for the single line diagram and inter-
connections; plans and details for the power distribution and grounding; lighting and
communication plans; the list of materials and specifications for the mounting, instal-
lation and tests; and data sheets for the purchase of the equipment.
The engineering project description includes the criteria for the civil engineer-
ing structures and architecture, the details for the foundations and concrete struc-
tures, the project for the drainage and waste disposal system, and the industrial
engineering.
3.1.4.7. General
Finally, the overall engineering project description includes other items such
as a technical analysis of the suppliers' drawings, an estimate of the capital and oper-
ating costs and the schedule for the pilot plant construction.
3.2. SITING
The selection of a site for the installation of a pilot plant for uranium ore
processing is affected by various factors, including the water supply, the electrical
power supply, the ore supply, roads between the pilot plant site and the supply
centres, and a location for the waste disposal system.
Before a first pilot plant can be built in a country that has no such installation,
a decision must be made to install the pilot plant (a) where the ore is located, or (b)
near a city or village with sufficient infrastructure to support the enterprise, in which
case the ore will have to be transported to the pilot plant. Depending on circum-
stances such as distance and the occurrence of different ore bodies, the latter may
25
be the preferable choice. Such a pilot plant may be capable of treating several differ-
ent ores and can be supported by material infrastructure and manpower existing at
any centre, such as a university, technological institute or private industrial centre.
This type of arrangement would be quite desirable for a developing country.
In deciding on a suitable site, factors such as climate, meteorology, hydrology
and flooding, geology and mineralogy should be considered. In addition, it is impor-
tant to keep in mind that the impact of the facility (especially the industrial plant)
on the environment and human health must be acceptable to the competent authority.
The effect of the facility on land use, flora and fauna must be carefully considered.
For example, the annual precipitation and evaporation at a site virtually deter-
mine whether all of the liquid wastes can be retained in the waste retention system
or whether they will be discharged to the environment through controlled releases.
The magnitude and frequency of floods must also be considered in the siting and
design of a pilot plant and its waste management system.
A knowledge of the frequency, strength and duration of atmospheric inversions
is needed for the calculation of airborne contaminants arising from mining and mill-
ing operations. In order to assess the wind erosion of wastes and dust transport, wind
direction and speed should be considered, especially in arid locations. Temperature
inversions also have to be considered in assessing the effect of radon emanation from
the ore body and from wastes. AH of these factors must be taken into account for
the design and engineering of both the pilot plant and the waste management facility.
The processes used in both mining and milling operations will have a direct influence
not only on the performance, size and cost of the pilot plant but also on the waste
management requirements.
In planning the pilot project, the mineralogy of soils and the subsurfaces of
areas being considered for the mill site and the waste retention system must also be
studied. The choice of the uranium extraction process is largely determined by the
effects of gangue minerals which, as a consequence, have a major impact on waste
management technology.
The hydrology of the site is another important factor since the wastes could
contaminate surface water or groundwater.
The nature of aquatic and terrestrial habitats influences the degree of environ-
mental impact that would result from waste treatment practices. For example, dis-
charge acidity affects plant nutrition. Fish are very sensitive to heavy metal ions and
the softer the water the more sensitive they are. Discharge of long chain amines
reduces phytoplankton productivity, an effect which is more pronounced if kerosene
is also present.
The climate and operating conditions should be defined in detail so that the
instruments, piping and fittings that are used will be suitable for the environment and
not subject to damage such as corrosion due to humidity.
In order to gain public acceptance, concerns over the siting and its environ-
mental effects must be carefully considered. The site should be large enough to sup-
26
port all of the operations necessary for the facilities; however, the size of the
installations should be kept conveniently modest. The installations should be as sim-
ple as possible and problems related to site availability, adequate water and electric-
ity supply, and transportation must be dealt with responsibly.
Waste management issues are also significant and must be considered before
the installations are committed, constructed and placed in operation. Again, the two
major constraints for die pilot plant need to be stressed: (1) the supply of water and
electricity, and (2) the impact of the radwaste. From the point of view of the environ-
ment, virtual elimination of the heavy elements, especially manganese, from the mill
circuits and effluents is of great importance. This means that the decision to use pyro-
lusite (Mn0 2 ) or Caro's acid (H2SO5) as the oxidant for uranium must be carefully
considered, since the presence of manganese when pyrolusite is used is significant
and requires special treatment.
Although it is desirable to place the mill as near as possible to the ore body
or the mine, in the case of small pilot plants the ore can be transported to the site
where the pilot plant is being erected. If the pilot plant is near a city or village, the
potential for making use of the infrastructure of that location should be explored.
When the pilot plant is based on the heap leach process, the necessary site
preparation may include conditioning of the ground for heaps, ponds, roads, founda-
tions for buildings, and in some cases footways.
Another important aspect that must be taken into account is the presence of
natural radioactive isotopes — U, Th and their daughters — which involves authori-
zation, regulation and local laws. In planning for the plant location and ore body
exploitation, permission must be obtained from the local authorities and local legal
regulations covering such isotopes must be followed.
The project must also provide some orientation in matters pertaining to
licences and environmental concerns.
The basic philosophy for the design of the pilot plant is to keep it compact and
simple. The equipment should be distributed and assembled within a relatively small
area, but allowing sufficient space to facilitate equipment operation. The flow sheet
process must be optimized at the pilot plant, after which the engineering for the com-
mercial plant, if a decision is made to have one, shall be carried out.
A general layout for the arrangement of the construction and equipment
depends on the characteristics of the particular pilot plant and on the available land.
A rational and well planned layout contributes to the efficiency of the pilot operation,
to the ease of operation and maintenance, and to the safety and economy of the
installation.
27
The first step to take in planning the layout is to list all of the primary pilot
plant components: process unit operations; areas for stockpiles of raw materials,
reagents, by-products andfinalproducts; utilities such as power and water supplies;
the waste disposal system; the machine shop, warehouse, laboratories and office
building. After the list has been completed, the size of the area needed for each com-
ponent is estimated, with some space reserved for future expansion of the pilot plant.
The distribution of the land area for a pilot or commercial plant depends mainly
on the general operation of the installation. The total length of the piping (an eco-
nomic factor) and the movement of materials and people are primary considerations.
Generally speaking, the areal distribution may be according to the following criteria:
administration buildings (offices, restaurant, medical service) should be near the site
boundary or the main entrance; unit operations should be in the middle of the site;
areas reserved for oil, kerosene and other flammable products should be away from
me internal streets and roads; the power supply station and electricity installations
should be away from the areas and installations subject to dangerous atmosphere and
leakage of flammable products; and the storehouse, machine shop and warehouse
should be near the entrance.
The following items must be considered for the layout as well: land topogra-
phy, location of the sources of water and electricity, and the predominant wind
direction.
The unit operation should be viewed as a whole and the equipment located in
accordance with the operational flow sheet and the requirements for the interconnec-
tion of equipment. All of the equipment that must be located at a higher position must
be placed close together so that the same support structure can be used. In general,
equipment is placed at the lowest possible height for ease of operation and main-
tenance as well as economy.
As a rule, the distance between the wall and equipment is kept at a minimum
although enough space must be provided for mounting, maintenance, repair and
operation of the equipment. For example, pumps must be kept one metre from the
wall, whereas the minimum distance for vessels can be one to two metres from the
wall. The rotating machines, for example centrifuge pumps, must be installed at
floor level and valves, instruments and equipment that require maintenance should
be located on a platform or other supporting structure to facilitate access.
Finally, the preparation of the plant layout must take into consideration the dis-
tribution of the exhaust and ventilation systems.
The selection of equipment is based upon the process flow sheet. The project
must organize a list and description of all equipment necessary for the pilot plant,
including the following:
28
— Equipment for crushing,
— Equipment for grinding,
— Equipment for leaching,
— Equipment for solid-liquid separation,
— Equipment for uranium recovery,
— Equipment for precipitation and drying.
Equipment for grinding, for example, can be ordered after a choice has been
made between having the ore wet ground or dry ground. The advantages of having
the ore wet ground include elimination of dust problems, lower energy consumption
and simpler classifiers. Dry ore dusts are normally generated during crushing and
dry screening operations and must be removed by dust collection, which requires
hoods, ducts, fans, cyclones and wet scrubbers in order to capture particles and
return them to the process stream.
3.4.1. Crushing
3.4.2. Grinding
Depending on the type of ore and the process selected, the mineral should be
ground from 32 mesh (0.5 mm) to about 325 mesh (0.044 mm). The dimensions of
the ground ore must be defined by the bench scale tests at the process development
laboratory before the pilot plant is developed and the mill should be equipped to
grind the ore to the required dimensions. It is strongly recommended that grinding
be a wet operation using either a conventional grinding circuit or a semi-
autogenous mill.
29
The characteristics of the ore determine how fine the grinding is to be and
whether an acid or alkaline treatment should be used.
3.4.3. Leaching
The leaching process requires more attention and technical innovations and
must be studied very carefully. It should be mentioned that quite frequently the pulp
(slurry) exhibits unexpected characteristics. Today a commercial autoclave usually
presents no special problems, but may require special attention to certain details.
This type of equipment, used for alkaline leaching, involves the technique of pres-
sure leaching usually applied to carbonate leaching. The technique requires high tem-
perature and pressure, and an autoclave constructed in low alloy steel. The leaching
of the uranium ore is improved by the addition of oxidants, air, oxygen and hydrogen
peroxide being the common reagents.
Pressure leaching, a technique used today for both alkaline (carbonate) and
acidic (sulphuric) solutions, is applied in the case of ores which are difficult to
leach [4-6]. The equipment consists of an autoclave operating at 50-150°C and a
pressure of 10-12 arm (1-1.2 x 106 Pa), usually under oxygen pressure. It is clear
that the decision to use an autoclave depends on knowledge of the ore characteris-
tics [6, 7], Although this type of equipment results in over 90% extraction of ura-
nium, there are problems with the lining of the autoclave (lead, acid proof brick,
titanium) and with the impellers. Some laboratory autoclaves of about 100 g ore
capacity are commercially available for treatment of highly refractory ore and ore
with high sulphide content. The scale-up of this technique to pilot or industrial plants
is a particularly difficult task.
The engineering project must recommend the type of autoclave to be selected.
Data sheets for different types of equipment must be compared in order to gain infor-
mation on the capability and performance of vertical or horizontal autoclaves and to
determine which type has the simplest layout and whether it is readily available and
offers flexibility. Before deciding on the purchase of an autoclave, the following fac-
tors should be considered: the actual volume of the equipment, the construction
material, whether it is a batchwise or continuous operation, the oxidant and the type
of impeller to be used.
Conventional acid leaching uses Pachuca type reactors or stirred reactors; acid
pressure leaching can be carried out in vertical or horizontal autoclaves.
In any case, the selection of the equipment for leaching the uranium ore has
to take into account chemical attack parameters such as granulometry (grinding),
pulp density, temperature and pressure, time of digestion, concentration of reagents
and oxidants, and whether acid or alkaline leaching is used [6].
30
3.4.4. Solid-liquid separation
31
of techniques produces a large volume of low grade solution which must be treated
to reclaim the uranium.
For economic reasons, the direct precipitation of uranium to obtain the yellow
cake is rarely used today since the concentration in the leached solution is usually
very low and would require preconcentration. The current methods of choice for ura-
nium recovery are the ion exchange (IX) and solvent extraction (SX) techniques [9].
One of these methods must be selected.
Currently the great majority of commercial plants recover the uranium by SX,
others apply solid IX and some use a combination of the two processes (IX-SX) (e.g.
the ELUEX process).
For SX, the sulphuric acid is the chemical system which requires the extraction
of uranium with a long chain amine diluent. For the common mixer-settler process,
the enormous volumes and low uranium concentration must be considered before the
equipment is chosen. The type and selection of the equipment depend on several fac-
tors such as technical feasibility, local suppliers or manufacturers, and economic and
even environmental considerations which will be important for the commercial plant
as well.
The designed mixer-settler unit must be tested at the process development
laboratory before the construction of the equipment for the pilot plant is ordered. The
mixer-settler equipment is used primarily for the extraction of uranium from com-
pletely clear solutions or partial solutions.
The type of equipment used for a fixed bed IX process is usually a conven-
tional column to accommodate the IX and requires a filtered, well cleaned pregnant
solution. The fixed bed columnar operation can be applied to the fixation of uranium
from acid or alkaline solution. While uranium can be reclaimed advantageously from
acid solutions (sulphuric or phosphoric medium), by either SX or IX, the only
suitable technique for obtaining the uranium from carbonate leach solutions is
anionic IX.
The primary requirement for loading uranium onto an ion exchanger inside a
fixed bed column is that the pregnant solution be well clarified. Alternatively, equip-
ment specially designed for operation as a fluid bed ion exchanger [10] or the well
known technique of resin in pulp (RIP) may be used. The fluid bed IX technique
allows the use of liquors of low suspended solids (a preclarified solution with a maxi-
mum of 1 % solids) while the RIP process can tolerate a high solids content (slurries).
Several models of the equipment mentioned for this technique are commercially
available [11]. The fluid bed IX or continuous IX technology is gaining more recog-
nition today and can be considered as a strong contender technology for uranium
recovery [11, 12].
32
One claimed advantage for the RIP process is that it avoids solid-liquid separa-
tion and for this reason reduces the plant capital costs. The selection of an RIP con-
tactor must be done after die preliminary experiments at the process development
laboratory; for example, the chemical composition of the pregnant uranium solution
must be known before experiments are performed using IX.
Before deciding to apply the RIP process, die following information on the
physical and chemical properties of the resin load should be obtained from experi-
ments performed at the laboratory: (a) me density and type of the resin, and (b) the
mesh size required for design parameters such as the screen selection, number and
size of stages, residence time, resin inventory and kinetics, and loading parameters.
It is important to know about the leaching rates of the ore and the treatment
of the pulp in order to determine the setding rate and filtration characteristics, factors
which must be considered in making me decision to filter die pulp or to go dirough
the RIP process. The RIP technique must be considered by the project since it is eco-
nomically attractive. One possible disadvantage of diis technique, however, is the
degradation of die resin due to me abrasiveness of die pulp. If die process develop-
ment laboratory determines diat the type of pulp is extremely abrasive, then die RIP
process should not be considered. It should be emphasized that each ore is different
and for each one the corresponding data available from die process development
laboratory must be studied.
The recovery of uranium from pregnant solutions usually takes into account
the large stream volumes involved, and for mis reason mixer-settler extraction
equipment is generally used. Until recently, pulsed columns had not been commonly
used. The Soci&e industrielle des minerais d'ouest (SIMO), in France, recently com-
missioned the first industrial pulsed column [13].
Equipment for the precipitation of yellow cake consists of one or two precipita-
tion reactors, pumps and flow meters, and a diickener with a pump and a storage
tank for the thickened uranium yellow cake product are also recommended.
Precipitation can be accomplished simply by die addition of base. Gaseous
ammonia (NH3), ammonium hydroxide or sodium hydroxide may be chosen for this
purpose. In a few cases magnesium oxide has been the chosen base. Alternatively,
hydrogen peroxide may be used for the precipitation. In each case the reactor may
simply be a conventional agitated tank. In the case of a small pilot plant a batch oper-
ation is recommended.
After the uranium is precipitated from the pregnant strip solution, the yellow
cake is dewatered by filtration or centrifugation in order to obtain a product with
about 50% moisture. At this point, in the case of a small pilot plant, the yellow cake
can be dried using a simple drying oven.
33
The dry diuranate (yellow cake) should be removed from the oven and handled
under adequate exhaust ventilation. Air containing yellow cake particles may be
scrubbed with dilute acid or water and filtered. Product drying and packing opera-
tions require similar dust removal and recovery equipment.
Pilot plant equipment must be selected carefully for each operation. Some
small pilot plants have been non-integrated operations, because the equipment sizes
required for full integration were not available. In such cases, either surge capacity
must be provided for intermittent operation or only a fraction of the output stream
is processed in the following section; the remaining portion is discarded. Other possi-
bilities can be considered, according to the specific requirements of each pilot plant.
The uranium ores most frequently treated have grades between 0.1 and 0.5% U 3 0 8 ,
although ores graded ten or more times higher are currently being processed. There-
fore a large amount of ore must normally be treated to obtain small quantities of yel-
low cake. Flows in the sections that handle the ore (for example grinding, leaching,
solid-liquid separation) are much greater than those of the last sections of the flow
sheet (yellow cake precipitation, drying, etc.).
If a pilot plant with a throughput of 200 kg/h of uranium ore with a grade of
0.2% U3Og is considered, the flow rate of pregnant solution coming from a CCD
section would be approximately 500 L/h, but the flow rate of the solvent extraction
product solution to the precipitation operation would be only 10-15 L/h. The very
small equipment sizes required for the 10-15 L/h flow may not produce reliable
scale-up information. If special design information is needed for this part of the
process, the solvent extraction product solution can be accumulated in a storage tank,
and the yellow cake precipitated periodically using larger equipment. The effect of
storage should be considered, however, as it can affect this unit operation. If the bar-
ren liquor from the uranium precipitation operation is to be recycled to the solvent
stripping section, an appropriate storage tank must be included in the flow sheet.
Storage of pulps should be avoided if at all possible because extended agitation
can affect the particle size distribution, and the information from the subsequent sec-
tions may not be reliable.
In contrast, the storage of solids does not pose any problems and it is generally
used between the crushing and grinding sections.
34
same size as those used in an industrial plant, it is recommended that sophisticated
instrumentation be avoided in the simple pilot plant.
Initially the pilot plant requires only simple instruments such as a pH meter
to control the leaching and precipitation stages, equipment to measure the oxidation
potential during leaching, manometers to control autoclave pressure, flow meters for
monitoring fluid flow and thermometers for temperature measurement. If uranium
is to be oxidized to the hexavalent state using Caro's acid (H 2 S0 5 ), the demand for
oxidant must be controlled so as to maintain the oxidation potential that was estab-
lished by laboratory testing [14]. This can be done using a simple valve with manual
control and a rotameter. A simple conductometric bridge may be used to control the
purity of the demineralized water.
35
Buried lines should have a minimum size of 2 in (5 cm). Solution lines should
be sized for economy, especially in the case of lines containing alloy materials.
Slurry lines should be sized according to velocity, with 1.5-2 m/s maintained
regardless of the materials used.
Compressed air lines should be sized so that the drop in pressure along the line
will not exceed 10% of the original line pressure, with full capacity being delivered
at the end of the line.
Steam lines should be installed to provide for the following conditions: conser-
vation of heat for piping over 120°C and adequate personnel protection in cases
where personnel may come in contact with the piping at temperatures over 60°C.
All process, service and utility piping should be hydraulically or pneumatically
tested in accordance with the governing codes or standards.
All piping materials should conform to the selected codes and standards where
applicable — ANSI, DIN (Deutsches Institut fur Normung) or ASTM (American
Society for Testing and Materials) — or even local regulations may be consulted.
Based on the process flow sheet, the piping and instrumentation diagrams are
designed in a document that gives in detail all of the unit operations for the chemical
process and designates the required equipment, piping and accessories, process
instrumentation and controls. In brief, the following items must be indicated: process
equipment, electric motors, equipment accessories such as reducing regulators,
auxiliary cold systems, lubricating and heating elements, safety valves, gauges,
steam separator, expansion joint, thermowell and insulators. The piping for
processes and utilities with indication number, diameter, material codes, flow codes
and insulation should also be included. A list of piping accessories is prepared
including insulation, vapour and electrical traces, orifice plate, relief valves,
purgers, filters, retention valves, check valves and plugs. In addition, a list of all
required instruments is prepared with their respective numbers, signal lines, connec-
tions, identification of signal type and loops. Instrument Society of America (ISA)
standards are recommended.
A process is any operation or sequence of operations where a variation of at
least one physical or chemical characteristic for any material is observed. Modifica-
tions of such characteristics are followed by the indirect control of the main process
variables such as pressure, temperature, flow, level, density, humidity and weight.
In some instances the measurements are made directly, using automatic controls.
These can be grouped as follows:
(a) Primary elements (sensors): flow, level, pressure, temperature, weight, vis-
cosity, velocity, density, electrical conductivity, pH.
(b) Secondary elements (intermediary): transmitters, controllers, converters,
switches, transducers.
(c) Final elements (regulators): alarms, control valves, recorders, indicators,
regulators. Examples are automatic regulators, hand control valves, level regu-
lators, pressure reducing regulators and back-pressure regulator.
36
The project must indicate the piping and instrumentation lines to be distributed
according to the following categories:
(1) Piping for the process — serving the main activities which constitute the
process itself, the stockpile of materials and the distribution of fluids.
(2) Piping for utilities — including the piping of auxiliary fluids for the cooling
and heating systems and piping used for maintenance and cleaning systems,
fire hydrants, service water, condensed steam and compressed air.
(3) Piping for hydraulics transmission — piping for liquids under pressure that
serve the command and hydraulics servomechanism.
(4) Piping for instrumentation — for transmission of signals by compressed air to
the control valves and automatic instruments.
(5) Piping for drainage — especially with the function of collecting the several
liquid effluents and sending them to the appropriate site.
To select and specify the materials needed for each service, it is necessary to
analyse the main parameters that influence their selection: (a) work service condi-
tions — pressure and temperature, and (b) fluids — concentration, pH, toxicity and
flow. Safety concerns, the cost of materials and their local availability, and the time
required for procurement and supply must also be considered. The most common
materials used for some conventional services are the following: carbon steel for
treated water, cooling water, compressed air and low pressure steam; PVC, poly-
propylene or stainless steel 304 for demineralized water; stainless steel 304 for
ammonia piping and condensed steam; stainless steel 304 L piping for nitric acid and
uranium solutions; and galvanized stainless steel for nitrogen piping.
3.8. UTILITIES
Services for the pilot plant start with electrical power and water supply. In a
small pilot plant, the water consumption is relatively low, probably in the range of
2000-4000 L/t ore. The lower value applies in cases where recycling of the extrac-
tion effluents is carried out.
Electrical energy is taken from the public grid, if available, or from a diesel
generator whose power must be calculated by the project. In cases where no grinding
is used (the heap leach process), the electrical power is relatively low.
After considering the source of electrical energy, it is advised that all master
controls for plant equipment be mounted on control and instrumentation panels from
which the entire pilot plant can be controlled and monitored.
37
Because of the corrosive conditions of the processes, all electrical wiring
should be run in PVC conduit and all conduit within the building should be exposed.
The only underground duct will be encased in concrete between the substation and
electrical equipment rooms.
In accordance with the layout, the electrical substation (or the power supply
generator) should be located as nearby as possible and the site should be free of cor-
rosive vapours and sources of waste, to avoid their accumulation on the electrical
insulators. If possible, the substation (or power supply) should be oriented in relation
to the wind in such a way as to avoid potential problems.
The voltage systems in common use include 110/220, 480, 2400, 4160, 6900
and 13 800 volts. The most commonly used voltages are:
— 110/220 V, three phases, two wires,
— 110 V, one phase, two wires,
— 220 V, one phase, two or three wires,
— 220 V, three phases, three wires.
The project must indicate the selected voltage for the pilot plant. The choice
of voltage will depend on the local electricity company supply, the size and type of
load and the special equipment requirements. The project should consult with the
company about the supply and other information concerning the voltage, number of
phases, power factor, frequency, measurement instrument and its location. The
project must provide information on the required maximum installed power, maxi-
mum power demand and estimated monthly demand. The project must also calculate
the power requirement for each pilot plant section.
The electricity supply system must have an efficient grounding to maximize
personnel protection and avoid damage to the equipment. Not only the electrical sys-
tem but also any equipment or part of the installation (metallic structures and sup-
ports, connectors, conduits, metallic fences, cabinets and equipment housing) must
be grounded as a safety precaution for personnel. The lighting system must also be
included in the grounding project.
It is desirable to have a provision for the installation of an emergency power
supply, operated with gasoline or diesel oil. The project should reserve space and
indicate a place for this equipment.
The safety aspect of the electrical project is of maximum concern. Accidents
and malfunction are a consequence of poorly designed and constructed installations.
The potential risk must be eliminated during the planning of the electrical system and
the preventive maintenance must be rigorous. Exposed parts, equipment or wiring
that can be subjected to mechanical or water damage are potential areas of risk. The
electrical insulation material can be damaged by heat.
The electrical system must be safe and reliable. Its reliability depends on such
factors as supply source, distribution grid, load capacity, voltage variation, fre-
quency variation, power factor and waveform.
38
3.8.2. Process water requirements
The project shall estimate the total amount of water necessary for the pilot
plant, indicating the annual consumption. The water demand must be distributed
among the following areas: crushing and grinding, leaching, extraction, precipita-
tion, effluents, fire, utilities, drinking water and steam generation.
Where there is no possibility of using municipal treated water, it is necessary
to provide a capture system for the water stream by means of a pump and a storage
tank. The capacity of this tank is a function of the size of the pilot plant. A reasonable
volume could be in the 10-20 m 3 range. Part of this volume (4 m3) can be reserved
for fire protection and the rest pumped to the treatment station.
The fire protection system must have hydrants located in the critical areas,
previously defined by the layout. The use of a foam system can be planned as well.
Each hydrant must be fitted with a 2.5 in ( — 6 cm) hose with a minimum length of
30 m and a jet to reach 10 m distance, with a flow rate of 40-60 m 3 /h. The project
must recommend a pump with the capacity to accommodate a demand of two or three
hose connections operating simultaneously.
For the chemical treatment, water is pumped from the storage tank to the sta-
tion. The conventional addition of aluminium sulphate and flocculant and calcium
hydroxide (lime) for pH control is followed by chlorination for oxidation of iron and
organic matter. The treated water is directed to a decantation tank and then filtered.
The filtered water is treated with lime for pH adjustment and then finally stored in
a tank with a capacity of about 10 m 3 . The slurry from the decantation tank is
pumped to the effluent treatment section.
39
conductivity meter on-line to follow the quality of the demineralized water and to
indicate when the resins are exhausted and require regeneration. The purified water
must be stocked in a lined tank with a capacity (1-5 m3) as outlined by the project.
The effluent from the regeneration and washing of the resins is sent to a deposit
and then neutralized before being disposed of in the waste pond.
3.8.7. Laundry
40
3.9. EFFLUENTS TREATMENT AND TAILINGS DISPOSAL
The waste treatment system is designed to meet industrial safety and regulatory
requirements. To achieve this goal, the project must organize the following items:
the control systems, process flow sheet, equipment data sheet, layout, specifications
for the equipment and instruments, piping plan, and procurement.
For the operation of waste management facilities the more specific information
which may be required includes the following: a description of the design, construc-
tion and operation of the facility; and a detailed description with drawings of the
facility, including all structures and equipment designed to retain and control the tail-
ings, the quality and quantities of all effluents and emissions from the facility, the
expected volume and flow rates of all liquids handled by the pilot plant, and the
points of discharge. An operational procedure manual, including the operating
procedures for all components and a description of the sampling techniques and
analysis methods, is also helpful.
The main pilot plant effluents are solid waste and acid washing solutions from
pulp treatment, and acid raffinate from uranium solvent extraction or ion exchange
effluent. Solid ion exchange has some advantage over solvent extraction because it
introduces less organic contaminants. These effluents must be pumped to their
respective storage units and then neutralized before disposal into the pond.
Acidic effluents must be neutralized with limestone or lime and then disposed
of in the tailings pond. All tailings should converge and be fed to the neutralization
operation as a slurry. Safety control at this point is mandatory to prevent the runoff
of liquids which may have leached 226Ra, a nuclide which could be transported to
the surface water or groundwater. The acid liquid barren effluents are also neutral-
ized and then specially treated to co-precipitate radium into barium sulphate, before
the effluents are sent to the pond. The radium co-precipitation must be done carefully
to guarantee that the depleted solution contains less than 0.37 Bq/mL.
The pond for the waste must be designed by the project in accordance with the
local legislation for accepting effluents depleted in radium. All of the discharged
slurries and solutions are contained in the tailings pond, the dimensions of which
must be determined by the engineering project. The pond, usually located at the site
where the ore body is exploited, must be completely constructed prior to startup of
the mill and must be ready to accept the solids from the leach wastes and 226Ra
sludge. These solids quickly settle in layers and incorporate the radioactive material,
contributing to diminished radiation. Normally, for economic reasons, the waste
facility is sited close to the mine and mill facility, but other factors can contribute
to the decision on the final location.
Wastes generated at the process laboratory (e.g. solid and liquid samples,
waste solutions, reagents and organic materials) should be contained in suitable con-
tainers and disposed of in appropriate places or at special waste sites while awaiting
disposal in the pond prepared for the pilot plant [15]. The same local legislation must
41
establish the limits of tolerance for other elements, especially heavy elements. Such
limits take into account the climate, seasonal evaporation rates and geology.
The principal characteristics of the liquid effluents are the acidity of the
raffinate after the extraction of uranium by solvent extraction or retention by ion
exchange and the alkalinity of the filtrate of the ammonium or sodium diuranate,
whose volume is quite small. The acid filtrate of the yellow cake must also be consid-
ered if peroxide is chosen for the precipitation.
The main chemical reactions in the liquid effluent treatment are shown below.
(a) Neutralization:
H 2 S0 4 + CaC03(s) -CaS0 4 (s) + C0 2 + H 2 0
H 2 S0 4 + Ca(OH)2 -CaS0 4 (s) + 2H 2 0
(b) Causticization:
temp.
(NH 4 ) 2 C0 3 + Ca(OH)2 — - * CaC03(s) + 2NH3 + 2H 2 0
The main solid effluent (leached ore slurry) must be treated in a mechanically
agitated reactor, and the final pH adjusted to 6.5-7.0 by the addition of lime. A mini-
42
mum residence time of 30-45 min is recommended. The used lime should have a
minimum of 90-95% available Ca(OH)2. The neutral pulp is pumped to the waste
dam. The main process for the treatment of acidic mill effluents is neutralization with
lime or a combination of limestone and lime [15]. With this treatment sulphuric acid
is neutralized, some sulphate is precipitated, most heavy metals are precipitated or
adsorbed, and to a large extent 226Ra is removed. Amines used in the solvent
extraction are removed by adsorption on precipitated solids. To complete the
removal of radium after the neutralization, barium chloride is added to the decanted
liquor.
After ore leaching and recovery of uranium the 226Ra content in the solution
must be drastically reduced by co-precipitation with barium sulphate. This technique
provides for the disposal of liquid effluents with radioactivity of less than 10 pCi/L
(0.37 Bq/L). The co-precipitation of radium is achieved with a solution of
50 g/L BaCl2 with an estimated consumption of 15 g BaCl2 , 2H 2 0/m 3 effluent. The
reaction time is relatively short, about five minutes, and the mixture in the reaction
vessel must be well agitated. Before disposal a flocculant should be added to the mix-
ture, followed by adequate clarification.
Preliminary studies for the identification and definition of the flow sheet for
the waste treatment are done at the process laboratory. These studies start with the
characterization of the effluents and include, for example, the chemical composition
of the solid waste, with special attention directed to (1) the residual uranium content
and the composition (free acidity and uranium), and (2) the specific activity (radium)
of the raffinate from solvent extraction and of the effluent from the anionic ion
exchanger. The radium content must be analysed throughout the leaching circuits and
in the neutralization circuits.
After the treatment of effluents and before their disposal into the pond, it is
necessary to establish an analytical control to determine the level of impurities such
as arsenic, cadmium, copper, manganese, mercury and selenium, and to ensure that
these levels are in accordance with the recommended legal values and the established
local environmental standards [16]. The streams to be disposed of must also be ana-
lysed for soluble species, especially for sodium, sulphate, fluoride and phosphate.
It is useful to compare these values with the limits established for drinking water.
Usually the solid tailings range from 0.004 to 0.05% U3Og depending on
several parameters, including ore mineralogy, grinding (size of grain), leaching con-
ditions (i.e. the amount of sulphuric acid used per tonne of ore), temperature, oxidant
and residence time. Generally an increase in the amount of acid per tonne of ore
43
decreases the tailings, but there is a practical and economic limit for the acid con-
sumption. Both parameters, i.e. ore mineralogy and acid requirements, have impor-
tant effects on the uranium leaching and, as a result, on the tailings.
Installations set up to operate with a simplified flow sheet based only on crush-
ing and heap leaching usually recover only 70-80% of the uranium. For a pilot plant
based on the heap leach process, the main solid wastes remain on the leaching pad
and can be covered with soil and stabilized, but regulatory requirements should be
checked. The liquid effluent must be neutralized and disposed of together with all
of the other wastes, including the barium sulphate which was used to retain the
radium.
For a pilot plant based on the heap leach process, the site for the piles must
be planned so that drainage from them can be collected and treated for the recovery
of uranium. This is done in an already existing mill when possible. The leaching
solution may be recirculated or, after the extraction of uranium, released with other
liquid effluents. Long after the recovery of uranium, however, rain water will seep
through the pile and may continue to extract small quantities of uranium, radium and
heavy metals. Care must be taken to analyse and properly dispose of all waste
solutions.
The more conventional plants, which include crushing, grinding, leaching and
solid-liquid separation, in many cases can recover over 90% of the uranium. The
tailings generated from this type of pilot plant still contain some uranium, and treat-
ment of these tailings must also be considered. Today the most common grade of
ore contains 0.1-0.2% U3Og. On the basis of data and assuming a 90% recovery of
uranium, a plan for the tailings disposal is drawn up.
The volume of liquid effluent should be kept as low as possible by recycling.
The section in charge of effluent treatment will consider the temperature and pH
recommendations, the biological and chemical oxygen demand and the amount of
total soluble salts, uranium and radium. In the case of some by-products recovered
from the process (in commercial plants), for example ammonium sulphate, ammo-
nium nitrate and sodium sulphate, special attention must be paid to any residual ura-
nium and radium. Reuse of mine water requires special treatment to eliminate solid
materials and radium.
For environmental protection, returning all solid residues from the leaching
operation to the exploited ore body site should be considered, especially when it is
an open pit mine.
Particularly in areas like the yellow cake section where solid products are han-
dled, the engineering project must guarantee an extremely low level of solid particu-
lates, about or less than 0.25 mg/m3 of uranium in the yellow cake handling area
and less than 5 /*g/m3 in the filtered air returned to the surrounding building. Treat-
ment of solid residue must be in accordance with the norms of each country.
Wastes from the mining and milling of uranium create potential health
problems as a result of radioactivity (U, Ra and daughters) and heavy metal contami-
44
nation. A technical report [17] describing the practices and options for confinement
of uranium mill tailings was published by the IAEA in 1981.
Professionals (managers) involved in this type of work must identify the wastes
and provide for their management in such a way as to minimize public health risks
and both radiological and non-radiological environmental impacts. Specialized litera-
ture on waste management [18-21] is available to the personnel responsible for plan-
ning, designing, constructing and operating the waste facilities. Requirements for die
management and control of radioactive solid, liquid and airborne (radon) wastes
must be fulfilled. Information about non-radiological contaminants in uranium mill
tailings must be considered as well [16]. It is quite common that wastes from the min-
ing and milling of uranium also contain potentially hazardous non-radioactive com-
ponents, such as arsenic, usually encountered in the barren solution. To avoid any
risk, the competent authority must consider this possibility.
Wastes from the mining and milling of uranium ores (and thorium as well)
include solids, liquids and airborne effluents containing radioactive gases (radon)
and particulates (uranium). Prior to operation of the pilot plant, each step of the
process (e.g. exploration, mining and milling) must be examined with assistance
from the process development laboratory. Liquid and airborne effluents may contain
a significant proportion of the daughter products of the uranium and thorium in the
ore. Radon and its daughters may be released from the tailings and dispersed into
the atmosphere. Radium and other daughter radionuclides may reach the surface
water or even the groundwater. For proper operation of a waste facility a manual
on operation, maintenance and monitoring as well as adequately trained personnel
are needed.
Usually the effluents from a pilot plant are very low in uranium. Liquid
effluents correspond to an estimated volume of about 1.5-4 m 3 /t ore.
The wastes generated by the mining, milling and leaching of the uranium ore
include radionuclides which give rise to radiological hazards. The radionuclides
involved are those descending from die first members of the natural decay series,
i.e. 238U and 235 U. Whenever the ore body contains 232Th its daughters must also
be included. Uranium-238 must be viewed as the most important parent nuclide since
it is the most abundant. Radium-226 is the most critical descendant to be considered.
It has a long half-life (1600 a) and is the source of the radioactive gas radon. Follow-
ing radium in importance and impact are 230Th (half-life 75 380 a) and at the end
of the 238U series 210Pb (half-life 22.3 a), which decays to 210Bi and 210 Po. The lat-
ter is a very powerful poison and a high energy alpha emitter.
Because of the risks associated with its possible inhalation by persons near the
mine, mill or waste sites, the emission of radon and its decay products (from the tail-
ings) must be considered. Special attention must be paid to ventilation, particularly
in the underground mines [22, 23]. The natural 232Th series includes 220Rn, a gase-
ous decay product, a descendant of radium. This family contributes two radium iso-
topes; the first,228Ra (mesothorium, half-life 5.75 a), deserves special attention.
45
Additional information on the natural uranium and thorium series is available in
Refs [17, 22].
Radon is released from open pits, in mine ventilation exhaust, ore dust, and
at all stages of the milling process, but especially during the crushing, grinding and
leaching stages. There is, at present, no practicable method for removing this gas
from ventilation exhausts. Thus the manner and rate of release have to be monitored
and adjusted to comply with authorized limits.
After the uranium is extracted from the ore, most of the remaining ore material
becomes a mill waste or tailings, commonly a slurry of finely ground solids. As the
major portion of the 226Ra remains undissolved throughout the leaching process, the
concentration of radium in the tailings is only slightly less than the concentration in
the ore [24, 25]. If unprotected, the tailings release radioactive material (1) to the
air as radon gas and airborne particulates, and (2) to waterways as soluble or particu-
late radionuclides.
Backfilling of all parts of the tailings into worked out portions of the mine
would appear to offer considerable promise as a means of isolating the material from
the external environment.
During the early development, operation and decommissioning of any uranium
or thorium mine, management of the mine waste rock is essential. This material
originates from the rock that is removed from either an open or an underground mine
in order to provide access to the ore body. Before rejection as waste or barren rock,
the material should be analysed for its uranium content. Any potentially economic
grade material, for example low grade rock, should be segregated. However, before
a decision is made to use the waste rock for any purpose (e.g. as a source of uranium
which can be recovered by heap leaching or to refill the trenches, especially in open
mines), the mineralogy, radioactivity and chemical reactivity of the waste rock
should be assessed.
If the rock is subjected to heap leaching to recover the uranium, then the
extracted heap becomes a waste. Care should be taken when siting and constructing
such heaps in order to minimize their eventual impact on the environment.
If the hydrogeological, radiological, engineering and economic aspects are
favourable, it may be preferable to dispose of the waste rock in the mine since in
this way the waste would be isolated from the external environment.
Mine drainage water consists mainly of surface water or groundwater which
has entered the workings through subterranean channels or fissures, or rain water
which has fallen into the open pit operations. Mine water may contact the ore body
for substantial periods of time and thus may contain dissolved uranium, thorium,
radium, radon, thoron or other metals. If sulphides are present, the mine water may
be quite acidic. Treatment and disposal of mine water depend on local conditions
such as the mineralogy of the ore body, climate, topography and the existence of an
operating mill. Treatment of the water may include separation of the uranium,
46
radium and other heavy metals followed by processes such as sedimentation, precipi-
tation, lime neutralization, ion exchange and precipitation with barium salts [17].
Finally, it is important to point out some differences in leaching processes car-
ried out under alkaline as compared with acid conditions.
In the alkaline leaching process, the barren liquor is recovered and reused, and
for all practical purposes only the washed tailings become a waste stream. The
leached, washed tailings are transported to the tailings impoundment system in a
water slurry which is slightly alkaline and may be contaminated with radioactive
nuclides and chemicals. These tailings may need treatment before disposal.
In the acid leaching process the solution that is normally used to transport the
tailings may contain a greater concentration of contaminants. These can include sul-
phuric acid, heavy metals, nitrates, sulphates, organic solvents, long chain amines,
chlorides and natural radionuclides [26, 27]. The acid effluents require neutraliza-
tion and confinement of the radium as previously described.
If because of climatic conditions the pilot plant must depend on the release of
treated waste streams into rivers, for example, information on the safe disposal of
radioactive waste into rivers, lakes and estuaries is available [28-30].
It has been recommended that the pilot plant have a laundry installed. It should
be noted that the laundry water will contain particulates and soluble materials washed
out of clothing and will probably be lightly contaminated. Such water can be directed
to the waste impoundment facility.
For other solid wastes from uranium milling operations — for example clogged
filters and filter cloths — it is a normal practice to consign such material to the tail-
ings impoundment or to a special disposal site.
4. PROJECT ADMINISTRATION
4.1. INTRODUCTION
Like all industrial programmes, a successful pilot plant requires high quality
administration and management. Depending upon the size and objectives of the pilot
plant programme, the management structure may vary from an almost informal to
a relatively complex organization. The management functions that must be carried
out, however, are the same regardless of the size or complexity of the management
structure. These functions include planning, staffing and controlling the operation.
The pilot plant manager is usually the key to a successful pilot plant operation.
47
4.2. THE PILOT PLANT MANAGER
The primary goals of the pilot plant manager are to ensure the technical quality
of the work, to control the cost of the project, and to complete the project safely and
on schedule. To a large extent the success of a pilot plant operation will depend upon
the ability of the project manager and the authority that he/she is given. The most
successful pilot plant operations have been directed by managers who are proficient
in a range of disciplines covering managerial as well as engineering functions. The
major disciplines include (1) engineering, (2) cost management, (3) human skills,
and (4) negotiations. The authority given to the project manager should be broad in
scope. Some of the more common areas of decision making authority include the
following:
Technical decisions
— Directing the design approach,
— Identifying and selecting the type and scope of tests,
— Selecting the equipment to be used.
Commercial decisions
— Whether to make or buy equipment,
— Selecting or recommending vendors or subcontractors.
Administrative decisions
— Selecting and assigning personnel,
— Scheduling personnel, equipment and other resources of the project.
Monetary decisions
— Determining the expenditure of budget funds.
48
In many instances the pilot plant manager must act as chief engineer and be
willing to trust his/her own judgement in making decisions of a highly technical
nature. The manager, however, should not become embroiled in the details of all
technical matters. The most successful pilot plant manager will have a balanced
combination of technical, managerial and leadership skills.
4.3. PLANNING
The decision to build a pilot plant and the definition of the mission of the pilot
plant usually involve an iterative process. The various options and needs are
evaluated, and the decision process then goes through a series of cycles that
culminate in a definition of the overall mission of the pilot plant operation. Once the
decision to build and operate a pilot plant has been made, and the pilot plant manager
has been selected, specific planning for the construction and operation of the pilot
plant can be initiated. This planning involves setting goals and objectives for the
organization and developing 'work maps' which show how these goals and objectives
are to be accomplished. After these plans have been formulated, organization, which
involves an integration of resources, becomes the primary concern. This means
bringing together people, capital and equipment in such a way that the pilot plant will
effectively accomplish its mission and goals.
Along with the planning and organizing, the motivation and spirit of the pilot
plant team play a large role in determining how effectively the pilot plant goals will
be met. Since motivation strongly influences performance levels, the planning
process should carefully consider and recognize the importance of motivation for
successful pilot plant operation.
Critical components of the planning function are listed below:
— Initiating and following through on licensing requirements,
— Developing action plans for budgeting, programming, and scheduling,
— Developing safety plans for the construction, operation, and shutdown of the
pilot plant.
4.3.1. Licensing
Licensing requirements for a pilot plant operation may vary considerably for
different countries and also for different locations within a given country. Since
licensing requirements will affect nearly every aspect of the pilot plant construction,
operation and closure, these requirements must be among the very first consider-
ations of the planning process. Lead times to complete the licensing can be long
because it may be necessary to deal with many different regulatory agencies.
Assigning and designating specific responsibility for the licensing function are
49
necessary. Licensing costs can be significant and should be considered carefully in
the budgeting process. The use of consultants or organizations experienced in the
licensing procedures can be cost effective, particularly when the licensing process
is complex.
The term 'action planning' has been used to designate the process of devel-
oping budgets, programmes and schedules for the pilot plant. Action planning is an
iterative process, and several planning cycles may be necessary. This planning
process continues throughout me pilot plant programme since modifications of the
original programme are often necessary. Each planning cycle should consider
potential options and provide for contingencies.
4.3.2.1. Budgeting
Preparing a budget estimate for a pilot plant involves essentially all of the steps
and components of a capital and operating cost estimate for industrial scale
operation. Cost estimating is primarily an art based upon a combination of factual
and empirical relationships. The process consists of developing capital and operating
cost estimates based on the flow sheet, material balance and energy balance that have
been prepared for the pilot plant. A wide variety of books and articles on cost
estimation, which can serve as references and guidelines for preparing the pilot plant
budget estimate, are available [31-34].
Capital investment — Capital investment is the amount of money that must
be available for construction of the pilot plant facilities and their ultimate operation.
There are two types of capital: fixed capital and working capital. The investment in
buildings, equipment and auxiliary facilities is called fixed capital. Working capital
refers to the funds required for operation of the pilot plant. Typical components of
a fixed capital estimate are listed below.
Purchased equipment
Equipment installation
Piping
Instrumentation
Insulation
Electrical system
Buildings
Land and yard improvements
Utilities
Physical plant cost (subtotal)
50
Engineering and construction
Direct plant cost (subtotal)
Contractor's fee
Contingency
Fees and contingency (subtotal)
Working capital — The working capital requirement for a pilot plant is the
amount of money necessary for normal conduct of the operation. Since the pilot plant
does not produce a saleable product, the working capital requirement includes
all of the funds needed for completing the pilot plant programme, i.e. the total
operating cost.
Operating cost — The operating cost is the sum of all the direct, indirect
and fixed expenses incurred in the actual operation of the pilot plant. The principal
components of an operating cost estimate are listed below.
Direct costs
Raw materials and reagents
Supervision
Operating labour
Maintenance labour
Parts and supplies
Utilities
Safety/environmental supplies
Miscellaneous
Direct operating cost
Indirect costs
Payroll overhead
Laboratory services
Engineering services
Administrative services
Safety services
Security services
Transportation
Communication
Warehousing
Miscellaneous
Indirect operating cost
51
Fixed costs
Depreciation
Taxes
Insurance
Fixed operating cost
One of the most frequent objections to pilot plant operations is that they take
too much time. Thorough programming and scheduling will minimize the time
requirement and reduce costs. The number and type of experimental runs needed to
meet the research objectives strongly influence all of the economic aspects of the
programme.
Planning and scheduling the programme of pilot plant investigations start after
the bench scale research and preliminary economic evaluations have defined the
potential of the processing operations. A primary objective of the pilot plant
programme is to provide the data and information needed to design and construct a
plant that the operating department can put into maximum production with a
minimum of problems. Careful planning of the experiments may also reveal needed
alterations and may even demonstrate that a pilot plant is not the best method to
obtain the desired design data. Additional, less costly laboratory work may be
needed to support the pilot plant operation.
Statistical designs for the experiments usually uncover synergistic factors
affecting performance or product quality and will provide maximum information at
minimum cost. Statistically planned experiments reduce the element of human bias,
and eliminate less productive avenues of experimentation by taking advantage of
earlier data. In addition, the number of runs needed to define the effect of variables
is reduced, and confidence in the experimental results is increased.
Scheduling for the design and construction of a uranium processing pilot plant
is critical particularly if long lead times are required for the purchase and delivery
of equipment. The time elapsed between project inception and data generation is
often between 12 and 24 months. Table III shows typical requirements.
As mentioned above, identification of long lead time equipment deliveries can
be one of the most critical scheduling items. Construction, however, can be started
52
TABLE III. TIME REQUIREMENTS
Task Months
even before the final design is complete. For example, utilities can be brought in,
and support structures can be started. It also may be desirable to build and operate
subsections of the pilot plant. This procedure may prolong the overall construction
time but can significantly reduce the startup time. Many short cuts are possible, but
each carries a risk that should be evaluated before implementation.
4.3.3. Safety
The safety responsibilities of the pilot plant management encompass all aspects
of the pilot plant operation. Safety considerations are a critical component of the
initial planning phase and should continue throughout the design, construction,
operation and close-out phases of the pilot plant programme. Safety must be built
into the pilot plant from the beginning. It is particularly desirable to have one
engineer responsible for safety considerations; this engineer should report directly
to the project manager. Establishing a sound safety policy and developing the
necessary rules and regulations are of utmost importance.
Potential hazards that may have been overlooked or missed in the laboratory
can become critical during pilot plant operations. This is particularly true if the pilot
plant is testing new technology or chemistry. Hazard assessment during both the
planning and operational phases of the pilot plant operation should include the
considerations listed below.
PRELIMINARY ASSESSMENT
(a) Reagent hazards
— Toxicity
— Flammability
— Explosibility
(b) Reaction hazards
— Runaway or explosive reactions
— Contaminant effects
53
(c) Corrosion hazards
— Effect on equipment
— Effect on reactions
(d) Radiation hazards
— Direct radiation
— Indirect radiation (dust, etc.)
DESIGN PHASE
(a) Space requirements
(b) Pilot plant layout
(c) Buildings
(d) Equipment
(e) Piping
(f) Electrical system
(g) Instrumentation
(h) Emergency systems
CONSTRUCTION PHASE
(a) Inspections
— Buildings and equipment
— Utilities
— Safety equipment
(b) Design changes and modifications
OPERATIONS
(a) Preparation of safety instructions
(b) Participation in preparation of operating instructions
— Startup
— Normal operation
— Shutdown
— Emergency procedures
(c) Participation in operator training
(d) Waste disposal
(e) Housekeeping
(f) Provisions for first aid and medical services
(g) Personal safety equipment (glasses, shoes, etc.)
54
CLOSE-OUT
(a) Final shutdown procedures
(b) Waste disposal
(c) Disposal or mothballing of equipment and facilities
Safety planning is particularly critical in pilot plants because for the first time
operations will be conducted by and entrusted to non-technical personnel. The
laboratory studies will have been handled by technically trained personnel, but the
pilot plant is a new phase of the overall programme. Also, the pilot plant is the origin
or 'birthplace' for the safety practices and operating procedures that will be used in
the full scale plant.
The pressure of pilot plant schedules and problems can easily result in the mind
of the engineer being compartmentalized. Technical criteria and specifications
are carefully followed, but when it comes to even the most elementary safety
precautions, these sometimes seem to be excluded from consideration. Both the pilot
plant manager and the staff must recognize that any accident may retard the pilot
plant programme to a much greater extent than minor errors in operation.
Additional information on pilot plant safety is presented in Appendix II.
Planning for successful pilot plant operations must also include provisions for
adequate support services such as:
— Sample preparation,
— Analytical laboratory and mineralogy support,
— A metallurgical laboratory,
— Shop and warehouse facilities.
Although the need for these support services is crucial, the scope and extent
of the on-site facilities may vary considerably. For example, an isolated pilot plant
location may require all of these services to be located at the site, but a pilot plant
located near an operating plant or laboratory may be able to use existing facilities.
Factors that should be considered are discussed in the following sections.
Pilot plants produce essentially all of the control samples that would be found
in a commercial operation. These include dry or moist solids, slurry samples, and
a variety of liquid samples. The equipment and procedures required for handling and
preparing samples are described in Ref. [35]. The necessary equipment includes
crushers, splitters, drying ovens, pulverizers and filtration equipment. Unless
existing sample preparation facilities are adjacent to the pilot plant, it is almost
55
always desirable to have equipment for filtering and handling slurry samples in the
pilot plant. Sample preparation equipment should be located in a separate room with
adequate dust collection facilities. It is particularly important that facilities for drying
and preparing yellow cake samples be isolated from the areas where tailings samples
are handled.
Bins or shelves for systematic storage of bulk samples, sample pulps and
solution samples should be provided. The need for repeat analyses or additional
metallurgical laboratory tests commonly arises in pilot plant operations.
Sample handling and preparation are particularly important in pilot plant
operations. Cross-contamination or other mishandling can produce misleading or
inconsistent data that could require expensive additional pilot plant tests.
Timely and accurate analytical results are a crucial requirement for successful
pilot plant operations. Providing adequate analytical support must be an intrinsic
component of the pilot plant planning process. Prompt analysis of all pilot plant
samples is desirable, but results for leach circuit tailings and solvent extraction
raffinates are particularly critical.
Provisions should also be made for mineralogical support services. Contract-
ing out mineralogical services normally provides adequate support for a pilot plant
operation, but occasions can arise when a fast response is needed. For example, a
sudden drop in uranium extraction may require prompt recharacterization of the ore.
Planning for support services should include provisions for this type of contingency.
The extent and scope of any new analytical facility that is required depend
largely on the location of the pilot plant. If the pilot plant is located adjacent to an
existing uranium mill or metallurgical laboratory, no new facilities may be required.
Additional personnel and a two shift operation, however, will probably be needed
while the pilot plant is operating. Pilot plants in isolated locations will require at least
some analytical support facilities. In all instances the options of transporting samples
to an existing company or contracting the analytical work should be considered and
carefully evaluated.
56
Again, the location of the pilot plant affects the scope of the required testing
facility. If a metallurgical laboratory is close by, most of the test work probably can
best be carried out at this laboratory. If the laboratory is more than 50 km from the
pilot plant, at least some metallurgical testing facilities will be required. For
example, equipment should be available for moisture determinations, screen tests,
filtrations, acid consumption determinations, leach tests and solvent extraction
shake-out tests.
The IAEA Manual on Laboratory Testing for Uranium Ore Processing [35] is
a useful reference for planning the required metallurgical testing facilities. The
manual also presents details of sampling and experimental laboratory procedures for
uranium ore processing.
57
If relatively complete on-site maintenance facilities are not available, the pilot
plant schedule should recognize and plan for the downtime that almost invariably will
occur.
Warehouse space should be provided not only for equipment and instrument
spare parts but also for the pilot plant reagents and general operating supplies. Safety
considerations are particularly important when providing storage space for chemicals
such as oxidants and acids. Separate storage areas are usually needed. Equipment for
safe handling of bulk chemicals should also be provided.
The availability of spare parts for the pilot plant equipment is particularly
important. Adequate supplies of items that take a long time to be delivered must be
available. Replacement parts such as spare pH electrodes, pump parts, and replace-
ment motors are almost always critical items.
The importance of pilot plant maintenance skills should not be underestimated.
The availability of one or two experienced and versatile maintenance personnel can
be a particularly valuable asset for any pilot plant operation.
The pilot plant planning should also include provisions for a number of other
support services such as the following:
Office space
Functional office space for both technical and administrative personnel will be
required. Overcrowding can be a detriment to effective pilot plant operation. If
rented office trailers are available, they can provide cost effective temporary space.
Changehouse
Safety regulations will require shower and changing facilities for pilot plant
operating personnel. Special clothing for the pilot plant operators and arrangements
for laundering the clothing are likely requirements.
Communications
If the pilot plant is isolated, provisions for outside communications are
essential. Telephone or radio telephone service is particularly important for all
emergency situations.
Transportation
Pilot plant planning should carefully consider transportation needs. For
example, if samples must be taken to another location for analysis, a specifically
assigned vehicle and driver may be necessary. Special ore handling equipment could
be needed for larger pilot plants.
58
PILOT PLANT
MANAGER
SAFETY
CONSULTANTS ENGINEER
••
ANALYTICAL
SERVICES
INSTRUMENT MAINTENANCE
REPAIR SERVICES
PILOT PLANT
OPERATORS
FIG. 4. Typical uranium pilot plant organization. Solid lines: direct project assignment;
broken lines: as needed.
4.4. STAFFING
59
require only one process engineer, whereas six or more engineers may be needed
for round the clock operations. It has often been desirable to use pilot plant
operations as a training ground for the engineers or chemists who will staff the future
uranium mill. Experience has shown that the best pilot plant results are most often
achieved when the process engineers are directly involved in the hands-on operation
of the pilot plant equipment. If the pilot plant is to be operated under a union shop
arrangement, an agreement whereby process engineers can perform hands-on
operation of the equipment is particularly desirable.
The number of operators or technicians required for effective pilot plant
operation may vary throughout the course of the programme. If personnel from other
parts of the organization are available, overstaffing during the startup phases can
promote smooth operations. Pilot plants have also been used to train future mill
operators, particularly in locations where experienced operating personnel may not
be available.
Pilot plant safety responsibilities should be a specific assignment. As discussed
in Section 4.3.3, the safety engineer should report directly to the pilot plant
manager.
If possible, at least one maintenance specialist should be assigned to the
project, and more could be required if extended round the clock operations are to
be conducted. This would be true particularly if company or union rules prevent
engineers or operators from doing maintenance work. Provision for instrument
maintenance and repair should also be evaluated carefully.
The analytical requirements and particularly the turnround time for pilot plant
operations are critical. Even if separate analytical services are available, pilot plant
staffing for routine analytical determinations should be considered. Delays while
waiting for analyses needed for the evaluation and planning of the experimental
programme can be extremely expensive.
Almost all pilot plant operations require parallel support work from bench
scale tests and experiments. Often this work can be performed by the pilot plant
personnel, but for large pilot plant operations separate staffing for this function
should be considered.
Consultants can also be an effective component of the pilot plant staff if the
experience level of available personnel is relatively low. Consultants are probably
most valuable during the planning and design stages of the programme.
Personnel expenses are almost always the major cost component of the overall
pilot plant programme. Careful planning of the staffing requirements can be a major
contribution to the success of the programme. Skimping on labour can seriously
diminish pilot plant effectiveness, but overstaffing can also cause problems. As the
work force exceeds the optimal number required, the rate of progress slows down.
Communications fall off and co-ordination of the operation becomes more difficult.
The ability of each engineer to participate in the setting of the goals towards which
he/she is working becomes diminished and as a result tension can arise. It is advanta-
60
geous to keep the organization as small as possible in order to stimulate rapid feed-
back between all stages of the pilot plant programme. Optimal staffing is a major
challenge for the pilot plant management.
Control of the pilot plant operation requires co-ordination of the various efforts
of the pilot plant staff. This co-ordination is one of the primary responsibilities of
the pilot plant management; effective communication is a must. Tools used for this
co-ordination include staff meetings, operating manuals, data recording and reports.
61
Experience indicates that when the accountability meeting approach is used,
surprises between the supervisor and the subordinate are minimized, and communi-
cation remains open, focused and consistent.
The pilot plant operating manual is a key document for training personnel in
the operation and maintenance of the plant. Even a preliminary manual can help
prevent costly startup and operating errors. The pilot plant manuals also become the
basis for the preparation of subsequent commercial plant operating manuals. The
scope of the pilot plant operating manual can vary, but in general the manual should
cover such functions as startup, routine operations, trouble shooting, normal and
emergency shutdowns and equipment maintenance. Since the pilot plant operating
manual may require significant changes as new insights are gained, it is particularly
important that the manual be planned so that subsequent revisions can be made
easily.
A modular approach has proven desirable for pilot plant operating manuals.
Each module of the manual is devoted to a specific unit operation. A module
typically contains the following information:
— Introduction: a discussion of the process concept for the unit operation;
— Description: a detailed description of the equipment;
— Routine operation: a detailed discussion of how the equipment is to be operated
and controlled;
— Trouble shooting: a discussion of procedures for diagnosing equipment
malfunctions;
— Maintenance: procedures for disassembling equipment and replacing parts,
and schedules for testing and lubrication;
— Emergency operation: detailed procedures for emergencies such as leaks, fire
and shutting down the process operations.
It is also desirable that the operating manual have an appendix containing the
manufacturers' literature on the equipment provided by them together with lists and
drawings of the spare parts.
The full benefit of the pilot plant can be obtained only if the data are taken and
recorded properly. Inadequate planning of the data recording procedures has often
proven costly in pilot plant operations. Project personnel must decide in advance
what information is required for the necessary calculations and projections.
Hypothetical calculations can help to ensure that all of the required data are
collected. Experimental data can be recorded in many ways. Filling in log sheets and
62
filling in blanks on an apparatus sketch are two of the most common methods used.
Computer data acquisition systems may also be desirable for some parts of the pilot
plant operations. The application of these systems may depend at least partially upon
the availability of suitable sensing and interfacing devices. The costs of data
acquisition systems for a pilot plant are often equivalent to those for a commercial
scale operation.
All data recording or acquisition systems should be supplemented with a log
book for recording all operational difficulties, suggestions for changes and methods
for overcoming difficulties. Complete and detailed records of all flow sheets should
also be kept along with flow sheet changes, drawings, curves and descriptions of the
progress of runs. A running photographic record of all equipment changes and
modifications can be particularly useful when reports are being prepared.
4.5.4. Reports
Formal reporting of the pilot plant operations not only provides the basis for
the design of the commercial operation, but also preserves the lessons of the pilot
plant so that they can be used for trouble shooting future operational problems. The
reporting process includes not only the final reports but also periodic progress
reports. Relatively short monthly progress reports supplemented by summary reports
on each completed phase of the pilot plant operation together with a final overall
report have proven effective. The reporting procedure should also include final
updating of the operating manuals. The pilot plant manager should allow sufficient
time and funds to complete the final reports before the pilot plant team is dispersed.
63
Appendix I
1.1. INTRODUCTION
The equipment selected for a uranium pilot plant is unique for each operation.
A generic approach has significant limitations because each pilot plant has different
requirements. As mentioned in previous sections of this guidebook, pilot plant sys-
tems can vary from fully integrated flow sheets to isolated unit operations. Many of
the smaller pilot plants have been non-integrated operations because the equipment
sizes needed for full integration were not available. These systems required surge
capacity between the various unit operations. In some instances, only a portion of
the output stream from a given unit operation was processed in subsequent sections
of the pilot plant; the remaining portion was discarded. Many variations on this
approach are possible, and equipment selection must be tailored to fit the specific
requirements of each pilot plant. The services of a consultant experienced in the
selection of pilot plant equipment can be cost effective and should be carefully con-
sidered. The following sections describe various equipment that has been used suc-
cessfully in several different types and sizes of uranium ore processing pilot plants.
Integrated flow sheets are usually possible only with relatively large pilot
plants. During the period from 1954 through 1959, the United States Atomic Energy
Commission operated several integrated pilot plants at Grand Junction, Colorado.
One of the primary objectives of this pilot plant operation was to confirm the
projected flow sheet and metallurgical data development during laboratory studies
on a variety of commercial uranium deposits. The overall pilot plant operation
included the following processing circuits:
Feed preparation (crushing and sampling)
Carbonate leach-filtration
Carbonate leach-resin in pulp
Acid leach-resin in pulp
Acid leach-CCD-column ion exchange
Acid leach-CCD-solvent extraction.
The individual pilot plant capacities ranged from 5 to 15 tons (4.5 to 13.5 t)
of ore per day. All pilot plants were operated on a 24 hour day, 7 day week schedule.
The pilot plant results and related laboratory investigations are reported in Ref. [36].
65
The following section discusses the flow sheet and equipment used in the acid
leach-CCD-solvent extraction pilot plant.
Approximately 560 tons (~ 5101) of Holly Blend sandstone uranium ore were
treated in the acid leach-CCD-solvent extraction pilot plant. The ore was from
Section 14, R10W, T14N, of the Ambrosia Lake uranium district in New Mexico.
The average feed assays of the ore were 0.313% U 3 0 8 , 0.151% V 2 0 5 ,
4.07% CaC0 3 and 0.021% Mo. Overall test results indicated that the ore was
amenable to the acid leach-CCD-solvent extraction process.
A leach extraction of 96.8% was obtained with a reagent consumption of
158 lb H2S04/ton (79 kg H 2 S0 4 /t), 1.5 lb Mn0 2 /ton (0.75 kg Mn0 2 /t) and a leach
retention time of approximately 24 h.
When five thickeners were used in the CCD circuit, and the wash ratio was
3:1, the soluble loss was approximately 0.5 % of the uranium in the mill feed. Under
these conditions, flocculant consumption was 0.25 lb/ton (0.125 kg/t) of ore.
Essentially complete uranium recovery was obtained in the solvent extraction
circuit. Three amine extractants were tested: a secondary unsaturated amine, a
trilauryl amine, and a trifatty amine. The secondary unsaturated amine performed
satisfactorily with a minimum amount of operational problems. Poor phase disen-
gagement occurred when using the trilauryl amine, and the formation of a
molybdenum-amine precipitate caused operating problems when the trifatty amine
was used.
When the loaded secondary unsaturated amine was stripped with a sodium
chloride solution and MgO was added to precipitate the uranium, the yellow cake
product was found to contain 79.3% U3Og.
A material balance over the entire 87 day run, which included conditions not
necessarily optimal, resulted in a theoretical uranium recovery of 94.2% compared
with an actual recovery of 94.0%. Projections indicate that optimal conditions would
result in an approximately 96% recovery.
66
8 ft X 6 ft VIBRATING SCREEN
OVERSIZE
ROTARY IMPACT BREAKER
SAMPLER No. 1
CRUSHING ROLLS
SAMPLER No. 2
HjO
H S0 IRON No. 4
p-—ET-; f— 2 « No. 3
•H20
.(totalis)
GRINDING lEAC „ , N G *-> fetrSj]
COUNTER-CURRENT DECANTATION
PREGNANT^
1 LIQUOR [— Mnnnnnnj—'
CLARIFICATION PRESS
fitp.
STRIP SOLUTION
•"PREGNANT r— n?so 4
EXTRACTION
STRIPPED ORGANIC ISSi*!"? STRIP SOLUTION skr Hj°2PlPRECIPITATION
STRIPPING
AIR b=fcl ° FILTRATE
(totalis)
STEAM''
YELLOW CAKE
The flow sheet for the feed preparation section is shown in Fig. 5. The flow
sheet for the grinding, acid leach, CCD, solvent extraction, and precipitation opera-
tions is shown in Fig. 6.
67
Feed preparation and sampling
The 560 tons of sandstone ore were hauled to the pilot plant location in trucks.
The 'as received' ore, which was approximately —6 in (approximately - 1 5 cm),
was dumped into the 30 ton (27 t) receiving bins, and then fed to the 24 in (61 cm)
rotary breaker at a rate of about 10 tons/h (9 t/h). The product was approximately
-3/16 in (approximately - 5 mm). The roll crusher in the sampling section had a
diameter of 10 in (25 cm) and 6 in (15 cm) wide rolls. The —3/16 in material was
stored in five 30 ton bins and then hauled by truck to the grinding circuit feed bin.
Grinding
The -3/16 in ore was fed to a 3 ft X 5 ft (91.5 cm X 152.5 cm) ball mill
that was operated in closed circuit with a 24 in (61 cm) spiral classifier. The feed
rate averaged 6.5 dry tons (5.9 t) per day. Water was added to the ball mill to main-
tain a discharge density of approximately 1.6 and additional water was added to the
classifier to maintain an overflow density of 1.4 (48% solids). The grinding circuit
product was 99% - 3 5 mesh (-0.417 mm) and 35% -325 mesh (-0.044 mm).
Leaching
Counter-current decantation
The pulp from the fifth leach tank was pumped to the first of five 12 ft x 8 ft
(366 cm x 244 cm) rubber lined thickeners in die CCD circuit. Wash water was
introduced to the fifth thickener at a rate of approximately 3.0 tons of water per ton
of ore fed to the grinding circuit. The thickener underflows were pumped with air
operated diaphragm pumps. The overflow from the first thickener was polished in
a 1 ft x 1 ft x 3 ft (30.5 cm x 30.5 cm x 91.5 cm) stainless steel filter press.
The clarified solution was fed to the solvent extraction circuit.
68
VENT
INTERFACE
CONTROL
BOX
The loading circuit was afivestage mixer-settler system, and the feed rate was
approximately 4.3 gal/min (~ 16.3 L/min). This pilot plant used an internal mixer-
settler system shown in Fig. 7. This system was adopted for at least one commercial
uranium operation in the USA, but most plants chose to use solvent extraction sys-
tems with external mixer-settler arrangements. Each of the internal mixer-settler
units consisted of an 11 in x 13 in (27.5 cm X 32.5 cm) mixer immersed in the
organic phase of a 2 ft X 3 ft (61 cm X 91.5 cm) settler. Four 1.5 in (3.75 cm)
holes in the side of the mixer near the bottom permit the aqueous-organic mixture
to flow into the annular settling chamber. The aqueous to organic ratio in the mixer
can be changed by varying the size of the holes in the mixer. The organic phase in
the settler overflowed into a 1.5 in (3.75 cm) diameter standpipe. The aqueous phase
underflowed through an adjustable jack leg into a 2 in (5 cm) airlift. The basic unit
69
was constructed from 316 SS and plastic piping was used for all aqueous streams.
Mixing was done by a 4 in (10 cm) diameter turbine type impeller powered by a
1/4 hp (188 W) variable speed electric motor.
Since molybdenum was present in the leach liquors, a solvent scrubbing system
was used to prevent significant amine losses as a result of the limited solubility of
molybdenum-amine complexes. A one stage scrubbing circuit was provided. This
mixer-settler unit had essentially the same dimensions as each of the four units in
the stripping section.
The pilot plant used batch type yellow cake precipitation. Two 6 ft x 6 ft
(183 cm X 183 cm) rubber lined tanks were provided for the precipitation and preg-
nant strip solution storage. Each tank was equipped with a slow sweep agitator.
When NaCl strip solutions were used, the pregnant liquor was precipitated by neu-
tralizing to a pH of 7.0 to 7.2 by adding MgO. After precipitation the uranium was
filtered on a 1 ft x l f t x 3 f t (30.5 cm x 30.5 cm x 91.5 cm) plate and frame
filter press. The precipitate was washed with water, dried in a gas fired oven and
packaged in small barrels. The 87 day run produced about 4000 lb (—1800 kg) of
79.5% U 3 0 8 yellow cake from the 560 tons ( - 5 1 0 t) of 0.313% U3Og ore fed to
the pilot plant.
70
Appendix II
Pilot testing of uranium ore involves risks that must be understood and guarded
against. Two broad types of risk may be considered: general risks, common to most
chemical plants, and radiological risks.
Most of the safety precautions that apply to a chemical processing plant also
apply to a uranium pilot plant. Standard precautions against the following should be
observed:
(a) Corrosive substances, such as sulphuric and other acids and caustic solutions.
(b) Explosive and flammable chemicals, including diluents and extractants used
for solvent extraction.
(c) Dusts, especially ore dusts and uranium concentrate dusts.
(d) Chemical spills, including acids, leach solutions and other process pulps and
liquids.
(e) Mechanical hazards, especially during the crushing, grinding and other han-
dling of ores.
(f) Electric shock.
(g) Toxic or irritating chemicals. Virtually all chemicals used in the plant are toxic
or irritating in some degree. Special care must be taken with liquids used in
heavy media separations, extraction solvents and ammonia used for the
precipitation of concentrates.
(h) Pressurized vessels, such as autoclaves. It is extremely important to study care-
fully and follow the manufacturer's instructions when using this type of
equipment.
(i) Burns.
Overalls should be worn at all times. Appropriate safety equipment such as
hard hats, face shields, safety goggles, gloves, respirators and hard tipped boots
should be used as required.
The radiological risks in a uranium pilot plant are relatively small and can be
easily controlled [23, 38, 39]. The main risks concern the inhalation of uranium ore
71
dust and uranium concentrate dust. External irradiation and inhalation of radon and
radon daughters are also possible although minor risks.
Substantial amounts of dust can be generated during sample preparation: crush-
ing, grinding, splitting, screening, sieving and blending. Sample preparation rooms
should be isolated from other areas and should be equipped with dust control systems
such as hoods and filters. Access to these rooms should be controlled. The personnel
preparing the samples should be properly attired with overalls, boots, gloves, dust
masks and caps. Workers should also shower and change into their normal clothing
before leaving the controlled area.
Inhalation of uranium concentrate dust is less likely because the amounts han-
dled in the pilot plant are usually small, seldom more than a few hundred grams.
Nonetheless, dry concentrates should be handled with care, especially during screen-
ing, blending or any other operation that can generate dust. These operations can be
carried out under a hood or, better still, in a glove box. Appropriate attire should
also be worn.
Radon and radon daughters normally are not a problem, provided that the plant
is well ventilated. The risk of external irradiation is usually negligible, except
perhaps when working with very high grade ores. It is advisable to check the level
of radioactivity in the plant, especially in the sample storage room and in the area
where uranium concentrates are stored. If the levels of radioactivity are significant,
the risk can be controlled by limiting the residence time in these areas, although this
time is likely to be small in any case.
Care should be taken to keep the plant clean at all times. Special care must be
taken to clean up all spills of uranium bearing pulps or liquids. Surfaces such as
bench tops and floors should be monitored regularly for possible contamination.
Eating, drinking and smoking must must not be allowed in any area of the
plant.
Safety rules, covering both general and radiological safety, should be drafted
and put into effect. It is strongly recommended that specialized books on these sub-
jects [40-42] be consulted.
72
Annex
BASIC DATA
The data listed below were compiled from laboratory tests and general infor-
mation. These figures were used to calculate the mass balances in Tables I and II.
Basic data are classified by sections.
A - l . GENERAL
A-2. COMMINUTION
Bond index 14
Grinding size, Tyler mesh -20
Solids in leaching pulp, wt% 55
A-3. LEACHING
73
A-4. SOLID-LIQUID SEPARATION
Washing ratio, m 3 /t 3
Solids in thickeners underflow, wt% 50
Total dose of flocculant, g/t 80
Flocculant concentration, g/L 1
Number of thickeners 5
Flocculant distribution, %
— First thickener 40
— Second thickener 30
— Third-fifth thickeners 10
Unit area, m 2 - r ' - d 0.4
A-5. EXTRACTION
Organic phase
— Amine, vol. % 3
— Alcohol, vol. % 3
— Kerosene, vol. % 94
— Specific gravity 0.8
— Saturation capacity, g U 3 0 8 /L 3.5
— Loading, % of saturation capacity 85
Raffinate, g U3Og/L 0.001
Phase ratio in mixers, A/O a 1.5
Phase ratio in settlers, A/O 1.0
Number of stages 4
Retention time in mixers, min 0.5
— Settler unit area, as referred to the organic phase,
L-min~'-m 2 25
— Retention time of the organic phase in settlers, min 20
A-6. STRIPPING
74
Phase ratio in settlers, A/O 1
Retention time in mixers, min 5
Retention time of the organic phase in settlers, min 20
Settling unit area, as referred to the organic phase,
L-min" 1 -m 2 25
A-7. PRECIPITATION
Temperature, °C 35
Ammonia dose, kg NH3/kg U 3 0 8 0.2
Tanks in series 3
Total retention time, h 1.5
pH distribution in tanks
— Tank 1 5.0
— Tank 2 6.5
— Tank 3 7.5
Ammonia distribution in tanks, %
- Tank 1 75
— Tank 2 20
— Tank 3 5
Grade of concentrate, % U3Os 85
Specific gravity of concentrate 4.5
Uranium concentration in mother liquors, g U3Og/L 0.004
A-8. THICKENING
75
A-9.1. Pulps
A-9.2. Liquids
76
REFERENCES
77
[18] INTERNATIONAL ATOMIC ENERGY AGENCY, Basic Safety Standards for
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78
[33] GUTHRIE, K.M., Process Plant Estimating, Evaluation, and Control, Craftsman Book
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BIBLIOGRAPHY
FRANKS, R.G., Modelling and Simulation in Chemical Engineering, Wiley, New York
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79
IAEA PUBLICATIONS ON
URANIUM ORE PROCESSING
1980 Production of Yellow Cake and Uranium Fluorides (Panel Proceedings Series
- STI/PUB/553)
1985 Inorganic Ion Exchangers and Adsorbents for Chemical Processing in the
Nuclear Fuel Cycle (Technical Documents Series — IAEA-TECDOC-337)
1986 Ion Exchange Technology in the Nuclear Fuel Cycle (Technical Documents
Series — IAEA-TECDOC-365)
81
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