Salt Methods
Salt Methods
Salt Methods
Limited
United Nations Industrial Development Organization
(UNIDO) UNIDO/IS.330
July 9, 1982
ENGLISH
FROM
by
* The views expressed in this paper are those of the author and do not necessarily reflect the
views of the secretariat of UNIDO. This document has been reproduced without formal editing.
** UNIDO consultant
V.82-28735
FOREWORD
The Industrial and Technological Information Bank (INTIB) came into existence in 1977 as a
UNIDO pilot operation in four industrial sectors. Since its successful inception, INITB has
become a permanent activity of UNIDO covering, at the moment, 20 industrial sectors. Its
main objective is to facilitate the choice of technology for decision makers in developing
countries.
In view of the importance of salt for human and animal consumption and of the availability of
this natural resource in most developing countries, this document has been prepared under the
INTIB programme of UNIDO.
It is hoped that this document will be of assistance to planners and promoters in developing
countries in identifying the most suitable technology for solar salt production according to their
local conditions.
-1-
CHAPTER 1
IMPORTANCE OF SALT
The story of salt is the story of mankind. Salt has played a predominant part in the
development of man’s activities, trade, politics and culture from prehistoric times. One reason for
its overwhelming influence is that it is a source for Sodium and Chlorine, two of the twelve
dominant elements in the human body. These two elements have important functions in the
metabolism of the body. Lack of these elements leads to decay and death. The other reason is
that the science of chemistry has used this inexpensive and abundantly available commodity as an
important raw material in today’s industry. Directly or in the form of derivatives, salt finds
application in more than 14, 000 ways. The word salt has become synonymous with Sodium
Chloride and will be considered equivalent in this publication.
Salt is principal constituent of extracellular body fluid i.e., fluids outside the cells as in the
tissues, blood serum and saliva. Its concentration varies with the type of fluid being almost the
same in the blood serum, cerebral and spinal fluids, but less in tissue fluids, sweat and gastric
juices. It becomes part of the human body even in the embryo stage since the foetus floats in a
saline solution. The amount of salt increases during the period of growth and reaches 230 grams
in the body of an adult.
In the physiological system, salt functions as sodium ion. Sodium controls muscular movement
including that of the heart muscles, the peristallic movement of the digestive tract and the
transmission of messages by nerve cells. The chloride ion produces hydrochloric acid required for
digestion. A principal function of salt is to regulate pressure and the exchange of fluids between
the intra cellular fluid and the extra cellular fluid.
For normal health, the salt concentration in the body can vary only within narrow limits. Salt that
goes out of the system has to be replaced. Salt is lost mainly through sweating. It is also out
through urine but the amount is so regulated by the kidneys that the salt remaining in the system
is maintained at the necessary level. Salt in the gastric juices and the digested food is mostly
reabsorbed in the intestines, except in cases of frequent loose motions which result in salt
depletion. Salt loss is high in the tropical summer under conditions of heavy manual work when
excessive sweating takes place. Such loss of salt through sweating and other processes has to be
made good by intake of fresh salt with food. The early man who subsisted on a meat diet had no
need to add salt to his food as meat contains an adequate amount. Even today, certain Eskimo
tribes living on seal meat or the Masai tribesmen of Kenya who drink the blood of cattle, do not
need salt. When man took to agriculture, cereals became his staple food. These contain more
potassium salts than sodium salts and hence addition of salt to the diet became important. The
rice eating population of the world requires more salt than others because rice is very deficient in
salt. In the tropics, where most of the population is rice eating, the condition is further aggravated
by sweating. In temperate climate the annual human requirement of salt is about
5 kg/year. In the tropics this is higher.
Chronic inadequacy of salt produces loss of weight, loss of appetite, inertia, nausea and muscular
cramps. Acute salt depletion as in gastroenterites, results in dehydration and reducing blood
volume, interfering with the supply of oxygen and other elements of the tissues. When this
happens, the body, in an attempt to maintain the normal balance of the fluids, releases vital
substances from within the cells, causing damage to health and hazard to life. Excessive heat,
like summer in deserts, results in salt depletion and causes heat strokes especially among children.
On the other hand, excess of sodium in salt and other foods, can contribute to hypertension, heart,
liver and kidney diseases. In such conditions, water (required to maintain sodium at the proper
concentration in blood) accumulates in the tissues leading to oedema. Patients are then advised to
be on a low sodium diet.
Salt is as important for the health of animals as for that of human beings. It is a part of an
animal’s body fluid in almost the same concentration as in humans. Experiments indicate that
insufficient salt stunts the growth of young animals and in the case of fully grown ones produces
lassitude, lowered production of milk, loss of weight and nervous diseases leading at times to
death. Since fodder and plant life have little salt, domestic animals have to be given salt with
their feed. Herbivorous animals in the wild get their salt from salt licks. The carnivorous animals
get it from the flesh and blood of herbivorous ones. In today’s modern farms, salt is also used as
a vehicle for mineral supplements that are essential for good health of livestock.
The primary use of salt for man, ever since he took to agriculture, is as an essential item of the diet
both for him and his cattle. Salt has also been used from prehistoric times for flavouring, pickling,
preserving, curing meat and fish, and in tanning. In view of such widespread importance, it has
become part of our culture and civilization. As one writer points out, “From cells in our brains
and bones to customs that spice our languages, salt penetrates every aspect of our existence”.
‘Worth his salt’, ‘above salt’, ‘old salt’, ‘loyal to one’s salt’, ‘the salt of life’, ‘salary’, are all
expressions and words used every day which originate from salt. Salt has been held as a
symbol
of divinity, of purity, of welcome and hospitality, of wit and wisdom in different cultures. In
Sanskrit the word ‘lavanya’ expressing grace, beauty and charm, is derived from the word for salt
‘lavana’.
Salt has been equally important in trade and politics. It was used as currency in earlier cultures.
Some primitive tribes gave gold, weight for weight, to purchase salt. The Hanseatic league
developed initially on the salt trade. Salt trade was a monopoly of the state in many countries.
The salt tax, among other things, provoked the French into revolution. The same salt tax was a
principal issue in Mahatma Gandhi’s civil disobedience movement against British control of India.
Salt for industrial consumption:
With the advent of industrial civilization, the uses and importance of salt have multiplied. Today
only 6 % of the world’s annual salt production is directly used for human consumption, the
balance being consumed mainly by chemical industries. It is one of the Big Five among
chemicals which form the base of the chemical industry, the other four being sulphur, coal,
limestone and petroleum.
The chlor-alkalil industry is the largest industrial consumer of salt. Chlor-alkalis consist of
chlorine, caustic soda (sodium hydroxide) and soda ash (sodium carbonate). Chlorine is used in
the production of vinyl chloride resins which form the base for a variety of plastic products, in the
paper industry, for water and sewage treatment, in laundry and textile bleaching, in the synthesis
of numerous organic and inorganic chemicals including hydrochloric acid and in the manufacture
of insecticides. Caustic soda and hydrogen gas are the co-products formed during manufacture of
chlorine. Caustic soda is used in the manufacture of chemicals, paper and pulp, soaps and
detergents and in vegetable oil refining, rubber reclaiming and petroleum refining. It is also used
to digest bauxite in the manufacture of aluminum metal. Caustic soda is so important that its
consumption is taken as an index of the industrial activity of a country. Hydrogen is used
mainly in the manufacture of ammonia. Soda ash (sodium carbonate) is used in glass making, in
the manufacture of chemicals, soaps and detergents. In recent years the discovery of very large
resources of trona (naturally occurring soda ash) has led to a decreased demand of salt
manufacture of soda ash.
A small quantity of salt is used in the production of other chemicals such as metallic
sodium, sodium sulphate, sodium nitrate, sodium chlorate, sodium cyanide and bisulphate.
Salt finds application in food industries such as canning, baking, processing of flour and other
foods, meat packing, fish curing, dairying and food flavouring. Salt is used in animal nutrition as
a vehicle for supplementary minerals which are added in controlled doses.
The leather industry consumes salt for tanning. Salt is used in de-icing of roads and highways.
Salt is used as a flux in the production of high purity aluminum alloys, as a floatation agent in
ore enrichment and in soil stabilizers and pond sealants. Salt is also used directly in the
manufacture of pulp and paper. Salt, alum and acetic acid are used to separate emulsified latex
in the production of synthetic underground salt formations are anticipated. In the textile and
dyeing industry it is used to salt out the dyestuff. Salt is used to regenerate cation exchange
water softeners for domestic and municipal purposes. Figure 1.1 indicates the various uses of
salt.
As the frontiers of the chemical industry grow, new applications for salt and its derivatives are
constantly being discovered. Salt will be playing an even important role in the future.
\
SODIUM'
CHLORIDE
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SOAP MANUFACTURE
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The production of common salt is one of the most ancient and widely distributed industries in the
world. Salt is produced by mining solid rock deposits and by the evaporation of sea water, lake,
playa and underground brines. The latter method accounts for over 50% of world salt production
today. The distribution of world salt production in 1980 was as follows:
80
70
60
50
40
30
20
10
0
Europe North America
Asia South America
Oceania Africa
Figure 1
Country-wise production during the years 1978-1980 is given in Table 2.1. Some countries
depend entirely on rock salt, some on solar salt. There are few countries where both forms are
produced. Figure 2.1 gives the location of solar salt operations in the world. While the process for
solar evaporation of brines is the same the world over, manufacturing techniques and product
quality vary considerably.
The demand for salt increases with growth of populations well as the development of industries.
Apart from consumption for human use, heavy chemical industries chiefly use chlorine, caustic
soda and soda ash require salt as raw material. In the developed countries, industrial
requirements of salt are several times that of the edible consumption. In the USA for instance,
over 95% of the total production of approximately forty one million tonnes is used for non-edible
purposes. In the
-7-
developing countries the trend towards increased industrial demand for salt has become
apparent only during the past decade.
Table - 2.1
Country Production of Salt:
North America
3
Bahamas (e) 1, 636 441 685
3
Canada 6,465 6,895 7,044
Costa Rica 35 46 40
Dominican Republic 38 38 38
El Salvador (e) 27 27 27
3
Guatemala 11 15 10
Honduras (e) 32 (e) 32 (e) 32
Leeward and 50 50 50
Windward Islands (e)
Mexico 5, 647 (e) 5, 636 6,000
Netherlands - - -
Antilles 400 400 400
Nicaragua (e) 18 (e) 18 20
Panama 15 15 19
USA including Puerto
Rico (e):
rock salt 13, 352 13, 357 10, 734
other salt:
U.S.A 25, 619 28,092 25, 949
Puerto Rico 25 25 25
-15-
COUNTRY (2) 1978 1979 (p) 1980 (e)
South America
rock salt 1 1 3
1
3
other salt 701 563 627
Europe
Albania (e) 50 64 68
Austria: rock
salt evaporated 1 1 3
1
salt salt in 322 381 3
411
brine 156 208 200
3
Bulgaria 87 86 122
Czechoslovakia 258 272 273
Denmark 325 381 382
France:
3
rock salt 459 574 301
brine salt: 1, 105 1, 191 3
1, 115
- marine salt 865 1, 805 3
1,277
- salt in solution 3, 867 4, 504 3
4, 424
-16-
Country 2 1978 1979 (p) 1980 (e)
German Democratic
Republic:
rock salt: 2, 694 3, 004 3, 091
marine salt: 53 55 56
Germany, Federal
Republic of:
Marketable:
rock salt 6, 860 8, 978 7, 909
Malta 1 (e) 1 1
Netherlands 2, 945 3, 959 (3) 3, 471
Poland:
rock salt 1, 438 1, 461 1, 091
other salt 2, 964 2, 977 2, 273
Portugal:
rock salt 327 408 409
marine salt 149 (e) 150 127
Romania:
rock salt 1, 418 1,420 1, 455
marine salt and other
evaporated salt (5) 1, 280 1,263 1, 355
-17-
Country 2 1978 1979 (p) 1980 (e)
-18-
Country 2 1978 1979 (p) 1980 (e)
-19-
India: rock
salt marine 5 5 5
salt 6, 710 7, 046 7, 273
-20-
Country 2 1978 1979 (p) 1980 (e)
Yemen, People`s
Democratic Republic
of (e): 75 75 80
Oceania:
Australia (marine salt 5, 578 5, 812 (3) 5, 326
and brine salt)
New Zealand 65 70 73
TOTAL (r) 167, 279 172, 214 165, 098
(e) Estimated
(p) Preliminary
(r) Revised
(2) Salt is produced in many other countries, but quantities are relatively insignificant and
reliable production data are not available.
(5) Includes production by Canary Islands (Sain’s provinces of Las Palmes and Santa Cruz de
Tenerife) totalling: 17, 434 short tons in 1977; 15, 766 short tons in 1978; and, 8, 685 short tons in
1979. (1976 and 1980 not available)
-21-
(6) Data captioned ‘brine salt’ for the United Kingdom are the quantities of salt obtained from the
evaporation of brines; that captioned ‘other salt’ are the salt content of brines used for the
purposes other than production of salt by evaporation.
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A brief review of the status of the salt industry in the world is presented below:
Europe:
Poland, East and West Germany, Czechoslovakia, Hungary and Holland mine rock salt. France
and Italy have facilities for both rock and solar salt production. As is to be expected in these
developed countries, the solar salt operations are large and are run on a completely mechanized
basis incorporating scientific techniques of layout design, brine density and biological control.
Countries like the USSR, Bulgaria and Romania have a relatively small proportion of solar salt
operations. Solar salt is produced in the USSR from the waters of the Black Sea and the Sea of
Azov. Greece and Spain depend almost entirely on solar salt making, although they have rock salt
deposits also. The size of salt works in these countries ranges from large mechanized works to
small units. In the hinterland of Spain, there are salt springs where salt is produced by boiling of
brine. Saline lagoons are also a source of brine in Spain. Portugal produces rock salt and solar
salt in almost equal amounts.
North America:
Canada does not produce any solar salt. Even in the USA, only 5% of the production is
accounted for by solar salt. Indeed, most of the salt is obtained by evaporation of brine from
the Great Salt Lake (to recover potassium and magnesium salts) is flushed back into the lake.
Solar salt is manufactured mostly along the California Coast by about a dozen operations and
around
the Great Salt Lake of Utah. Production in the USA has been almost static during the last decade
because of a reduction in demand owing to the closure of several synthetic soda ash plants
following the discovery of natural deposits. American salt works are noteworthy for the large
number of varieties produced for different applications.
In Mexico, solar salt operations play an important role although there are rock salt deposits. The
six million tonnes per year operation of the Exportadora de Sal in the Baja California desert is
considered the largest solar salt operation in the world today. There also exist in Mexico several
small and medium sized solar power operations managed by individuals or co-operatives. Small
quantities of solar salt are produced in the Central American countries like Costa Rica,
Nicaragua and Panama bordering the sea. In South America, Brazil is the largest producer of
salt, the major portion of which is solar. Venezuela is building a very large solar salt works
-9-
which is expected to become the world’s second largest salt producer. In Uruguay, solar salt
production has been
-10-
attempted but abandoned. Chile has no solar salt industry. In Argentina and Peru, all the solar
salt is produced from brine wells. In Cuba, salt is produced almost entirely by solar evaporation
of sea water. In this country there is potential for increasing production because of favourable
climatic conditions and availability of land. Bahamas and Netherlands Antilles have lately
emerged as large scale producers of solar salt from sea water. However, in recent years the
Bahamian operations have suffered production set backs due to heavy rainfall.
Asia:
In Asia, the Middle Eastern countries like Syria, Iraq and Iran depend mostly upon rock and plya
salt. Turkey is an exception and concentrates on solar salt making. Production methods are
traditional. However, a few modern solar salt operations are now being developed in Turkey, Iraq.
Israel produces a small quantity of solar salt form the Dead Sea. In the Indian subcontinent,
Pakistan has extensive deposits of rock salt but also produces some solar salt. In India, almost all
the salt is produced by solar evaporation of sea water, underground springs and inland lake brines.
Here salt works range from very small to large sized operations. Salt works are operated by
private and government owned companies, co-operatives and individuals. Solar salt is the only
form of salt produced in Sri Lanka, Bangladesh and Thailand where the size of salt works is
mostly small. In Indonesia, the government has a monopoly on salt production and the state
owned salt company produces salt in half a dozen locations. The production methods adopted in
Kampuchea and other south east Asian countries where small holdings predominate, are primitive
and the salt is not very pure. Salt making has had a very important role in China from ancient
times. In spite of rock salt resources, about seventy five percent of the production is solar salt
from sea water. Salt farms are worked in every coastal province of China. In the Philippines and
Taiwan salt is produced entirely by solar evaporation of sea water in small and medium sized
operations. In Japan, the pressure on availability of land has encouraged the development of
sophisticated ion exchange technique for recovery of salt directly from sea water. Direct
evaporation methods using a combination of solar and artificial evaporation are also employed.
However, the production by these methods is small and Japan today is the largest importer of
salt.
Oceania:
Australia has enormous reserves of rock salt as well as facilities for solar salt manufacture. The
main locations where solar salt is manufactured are the South Australian and West Australian
coasts. Five companies operating along the West Australian coast together produce nearly eight
million tonnes in highly mechanized operations. Most of this production is exported to Japan. In
terms of design and layout the Australian salt works can be considered to be the modern,
producing and exceedingly pure product. However, their profitability has been low owing to
low price realization. New Zealand produces a small quantity of solar salt to meet domestic
requirements despite unfavourable weather conditions. Recently rock salt deposits have been
discovered in the Antarctic region.
Africa:
In contrast to Australia the techniques in the African countries are conventional and in some areas,
ancient. Solar salt is produced only in certain countries while others depend on rock salt and other
salt sources. In the former category are Egypt, Libya and Ethiopia. In Egypt, there exist small
and medium salt plants. In Libya and Ethiopia there are several small operations. Ethiopia
produces salt along the Red Sea coast and exports it. There is very good potential for
improvement of production in this country if modern techniques are adopted and transport
facilities are improved. In general, the Red Sea area is well suited for solar salt production from
the point of view of salinity, climate and topography and deserves detailed investigation.
Mozambique is a country with a long coastline but very little salt is produced as climatic
conditions are not favourable. In Tanzania, conditions of humidity and rainfall are not conducive
for solar salt manufacture. Nevertheless, a small quantity of salt is manufactured from sea water
and the major portion by solar evaporation of playa and lake brines. In Somalia, small quantities
of solar salt are being produced for local consumption. In Mauritius, small shallow paved ponds
are used for solar salt production. Niger produces playa salt by solar evaporation almost as
cottage industry. This forms the mainstay of its economy. In Zambia, plants growing in brine
marshes are cut and squeezed in water to get a weak brine solution which is heated in pots to
recover salt. In Mauritania, salt is made by solar evaporation of brine from inland lakes. Although
South Africa has a long coastline and has a solar evaporation plant, the major portion is from solar
evaporation of playa salt brine after the rains. In Namibia, there is natural solar evaporation of sea
water which gets trapped in tidal basins. In most of West Africa conditions for solar salt
production are unsatisfactory because of the humid climate and low salt concentration of sea
water owing to dilution by large rivers. However, in countries such as Ghana, Guinea, Senegal
and Togo solar salt production is feasible. The North African countries have by far the most
favourable conditions in Africa for solar salt manufacture. These include Algeria, Morocco,
Tunisia and Egypt. In Algeria most of the salt is produced from salt lakes, although there is
good potential for sea salt production also.
There is tremendous potential for increasing output and improving the quality of the salt
produced in the African, Asian and South American countries. In several countries, simple
machines for the harvesting and handling of salt could be introduced. Improved techniques can
be adopted for designing and concentrating and crystalizing ponds and quality control procedures
introduced for the production of high purity salt. The recovery of chemicals like Bromine,
Magnesium and Potassium salts from bittern could be the next step.
A major portion of the world output of salt (>80 %) is consumed in the country in which it is
produced. Since bulk salt for industrial use has a relatively low value, transportation costs usually
form a large part of the delivered cost. This is a disadvantage in international trade. Japan is a
major industrial country that depends largely on salt imports of nearly ten million tonnes per
year from Australia, Bahamas, Canada, Mainland China, Mexico, Netherlands and Spain. There
is, however, fairly widespread international trade in refined grades of salt for table use.
The U.S. Department of Mines had made the following forecasts for growth in world demand
for salt:
In the developing countries the demand growth rate is expected to be faster (4.2%) than in
countries like the USA (2%). This is because many countries in various stages of economic
development are expanding industries that consume salt as raw material. Most countries produce
salt only at 70-80% of the full rated capacity since the production of salt by solar evaporation is
subject to weather fluctuations. To meet anticipated increases in demand, salt producers in many
countries will have to increase productivity of existing operations, increase the capacity of existing
plants or construct new facilities. Today, production economics and quality considerations
indicate the size of solar salt operations has to be progressively increased from a cottage level of
500 tonnes/year to at least a small scale level of 2000 to 3000 tonnes/year. The fact that solar salt
production utilizes the abundant and inexhaustible resources of the sun and the sea should
encourage such a growth.
CHAPTER III
The Chemistry of Solar Salt Manufacture
The oceans are the most prolific source of sodium chloride accounting for over fifty percent of
world production today. The reserves in the seas are estimated at fifty million billion tonnes.
Apart from Sodium Chloride, the seas are an important source for Potassium, Magnesium and
Bromine. 65% of Magnesium Metal and 68% of Bromine produced in the world are from
seawater. Almost every element, including gold and uranium, is found in traces in seawater.
Table 3.1 lists sixty elements found in seawater, giving their concentration and the amounts
present. The more important of these ones are:
Chlorine Sodium
Magnesium Sulphur
Calcium Potassium
Bromine
-23-
The major chemicals found in one litre of seawater are:
Sodium Chloride, also called Halite, dominates, constituting 80% by weight of the total salts in
seawater. One litre of seawater contains 35gms of dissolved salts, giving it a specific gravity of
1.034. This value is variable within a small range.
The salinity of seawater depends upon a number of factors including, location, season,
temperature and dilution by river discharges. It is lower at seashores and estuaries than in the
mid-sea. Geographical features like the partly stratified surface layers in bays with constricted
outlets as in the south Australian coast result in higher salinity. A remarkable fact about seawater
is that though salinity may vary, the relative proportion of the dissolved elements is consistently
the same everywhere in the world, in all seasons and, geologists believe, through all times.
For the manufacture of salt and its byproducts by the solar evaporation of seawater, a knowledge
of both the composition of seawater and its phase chemistry is necessary. Considerable research
has been done in this connection by the Italian scientist, Usiglio. His experiments on the
separation of salts by evaporation of sea brine have been performed at 25oC which is normal for a
temperate climate. The ruling temperatures over salt pans in the tropics are generally between
30oC and 35oC. Some work has been done over at these higher temperatures by Borschart (1940).
The conclusions are broadly the same because the pattern of phase relationship remains the same.
The actual values depend upon local conditions and results will not be absolutely identical at any
two locations.
In studying the phase chemistry, brine concentration is given the Beaume scale which is
defined as:
o
Be = 145 - 145
(Degree Baume) Specific gravity of the brine at 15.6oC
Table 3.2 gives the conversion figures from oBe to density (gmsÉml). Table 3.3 gives the
temperature corrections for solutions whose density is measured at different temperatures.
The order of separation of the dissolved salts depends on their relative solubility which is
given below. Calcium Carbonate being the least insoluble, separates out first. The highly
soluble magnesium salts are separated last.
Table 3.4 gives the amounts of precipitation of the various salts at different concentrations. Figure
3.1 represents graphically the deposition of different salts during concentration of sea water.
Here the extent of deposition of different salts is plotted against the brine concentration expressed
in oBe.
As indicated by this graph, the evaporation process is conveniently divided into four distinct
phases. This first phase is from 3o Be to 13o Be when most of the carbonates precipitate as salts of
iron, magnesium and calcium. While iron carbonate and magnesium carbonate crystallize
completely by 13o Be, calcium carbonate crystallizes up to 90%, the remaining 10% precipitating
by 15o Be in the next phase. These carbonates have little practical value.
The second phase, extending from 13o Be to 25.4o Be, centres round gypsum. This crystallizes as
needle shaped crystals as CaSo4 . 2H2 O from 13o Be to 16.4o Be and thereafter as anydrite CaSo4.
85% of the calcium sulphate present is precipitated in this phase. The precipitation of the
remaining 15% is spread over the third and fourth phases until evaporation is complete. Gypsum
is used in the production of cement and plaster of paris. It is also of immediate value in a solar salt
works since its crystals are used to pave pond floors thus preventing leakage of brine into soil.
The third phase extends between 24.5o Be and 30o Be. Common salt (NaCl) precipitates out in
this phase. Crystallization starts at 25.4o Be and its rate rapidly increases in the initial stages. 72%
of the total amount is precipitated by 29o Be and 79% by 30o Be. At higher levels of salinity the
crystallization slows down considerably and is complete only with the completion of evaporation.
The concentration at which sodium chloride starts to crystallize is known as the salting point and
the mother liquor at this point is called the pickle. At the end of the phase the, when the
concentration is 30o Be, the liquor is called bittern because of its characteristic bitter taste.
Sodium chloride is formed as cubic crystals. It is colourless, odourless and has a characteristic
taste. It has a specific gravity of 2.165 and a molecular weight of 54.45. Its solubility in water
varies only slightly with temperature.
Though salt is a predominant precipitate in the third phase, it is not pure because gypsum is also
precipitated, especially in the earlier stages. At the higher concentrations near 30 o Be, some
bromides, potassium chloride and magnesium sulphate from the fourth phase appear. The
technique of salt manufacture involves fractional crystallization of the salts to obtain sodium
chloride in the purest form possible.
According to typical standards adopted in developing countries, the purity required is 99% for
grade I industrial salt, 98.5% for grade II industrial salt, 96.0% for edible common salt, 99.6% for
dairy salt and 97.0% for table salt. In advanced countries the specifications are more stringent,
the minimum purity prescribed for table salt being 99.5%.
Figure 3.2 shows the considerable shrinkage in the volume of brine that takes place during salt
manufacture. The volume is 19% of the original at the beginning of phase I when gypsum starts
crystallizing, 9% at the salting point when salt crystallizes and 3% when phase III ends and bittern
is formed.
If the bittern is concentrated by evaporation without precipitating potassium salts, a mixture of the
remanant sodium chloride and magnesium sulphate separate out. This fraction is known as mixed
salt I. The mother liquor is further evaporated when a mixture of sodium chloride, potassium
chloride and carnallite called mixed salt II is obtained. Other potassium salts precipitating between
36o Be and 38o Be are kainite, schoenite, glasserite, sulphate and langbeinite. Beyond 38o Be, the
mother liquor consists predominantly of magnesium chloride with a small proportion of bromides.
This is crystallized as bischofite.
Underground brines:
The salinity of underground brines is much higher than that of seawater sometimes as much as
eight times. Underground brine is considered to be seawater cut off from oceans by early
geological changes and concentrated by periods of sunlight before being covered by further
geological deposits. There are also brines formed by the flow of underground water amongst
weak salt deposits.
The composition of underground brines varies widely. The composition of brines close to sea
coasts is similar to seawater evaporated to the same concentration with minor variations.
Sometimes the calcium sulphate content is higher. In other cases the Kcl : NaCl ratio is higher
than that of seawater. In certain inland underground brines, potassium and magnesium salts are
totally absent and sodium sulphate is the only constituent other than sodium chloride.
Underground brines occur at varying depths from very shallow levels of 3 metres to depths of
more than 200 metres. Underground sources are tapped by sinking borewells and pumping
the brine to the surface and subjecting it to an evaporation process similar to sea brine.
As in the case of underground brines, the composition of salt lakes depends upon the history
of formation of the lake. Salt lakes are found primarily in semi arid areas of the earth with low
rainfall and high evaporation. The salts of such lakes may have been derived by a portion of sea
being cut off from the ocean by geological upheavals in the past and their being subject to
concentration by solar heat. In such cases, the waters are characterised by the presence of
calcium and magnesium. An alternative method of formation is said to be from water collected in
a depression over rock deposits of an earlier era. Salt lakes could also derive their salinity from
sedimentary or igneous rock of the surrounding drainage areas. In such cases, neither the
composition nor the phase chemistry will be similar to sea water.
1) Sulphate type
2) Carbonate type
3) Bittern type
4) Sodium chloride type
In all types sodium chloride is present. Sulphate type lakes have derived their salts from
surrounding rocks that contain sodium sulphate. Sambhar lake, in India, is an example of this
type. Carbonate type lakes are alkaline lakes which contain sodium carbonate in addition to
sodium chloride and sodium sulphate. Searle`s lake, in the USA, is a carbonate lake. Bittern type
lakes contain more magnesium than sodium. They are often sulphate free but contain calcium.
The Dead Sea is one such lake. The Great Salt Lake of the USA is a sodium chloride lake, similar
in composition to sea brine of an identical density. Table 3.5 gives the composition of different
lakes.
Lake brines may also form over playas which are the sandy, salty or mud caked floors of desert
basins. After a rain, these plains are covered temporarily by a few centimetres of water. These
brines are sometimes drawn and used for salt production.
TABLE 3.1
CONCENTRATION AND AMOUNTS OF THE ELEMENTS IN SEA WATER
-31-
Xenon 0.0001 0.5 150 x 109
Germanium 0.00007 0.3 110 x 109
Chromium 0.00005 0.2 78 x 109
Thorium 0.00005 0.2 78 x 109
Scandium 0.00004 0.2 62 x 109
Lead 0.00003 0.1 46 x 109
Mercury 0.00003 0.1 46 x 109
Gallium 0.00003 0.1 46 x 109
Bismuth 0.00002 0.1 31 x 109
Niobium 0.00001 0.05 15 x 109
Thallium 0.00001 0.05 15 x 109
Helium 0.000005 0.03 8 x 109
Gold 0.000004 0.02 6 x 109
Protactinium 2 x 10-9 1 x 10-5 3,000
Radium 1 x 10-10 5 x 10-7 150
Radon 0.6 x 10-15 3 x 10-12 1 x 10-3
-32-
-45-
TABLE - 3.2
\
RELATION BETWEEN BRINE DENSITY IN GMS/ML
AND DEGREES BAUME
Density Density
gm/litre gm/litre
. ------------~---------------------------------------------
1. 020 2.8 1.061 8.4
1.021 3.0 1.062 8.5
1.022 3.1 1.063 8.7
1.023 3.3 1.064 8.8
1.024 3.4
3.6 1.065 8.9
1.025 1.066 9.0
1.026 3.7 1.067
3.8 9.2
1.027 1.068 9.3
1.028 4.0
4.1 1.069 9.4
1.029 1.070 9.5
1.030 4.2
4.4 1.071 9.6
1.031 1.0~(2 9.7
1.032 4.5
4.7 1.073 9.9
1.033 1.074 10.0
1. 034 4.8
4.9 1.075 10.1
1.035 1.076 10.2
1.036 5.0
1.037 5.1 1..077 10.3
5.3 1.078' 10.5
1.038 1.079 10.6
1.039 5.4
1.040 5.5 1.080 10.7
1.041 5.7 1.081 10.8
1.042 5.8 1.082 11.0
1.043 6.0 1.083 11.1
1.04.4 6.1 1.084 11.2
6.2 1.085 11.3
1.045 1.086 11.5
1.046 6.4
6.5 1.087 11.6
1.047 1.088 11.7
1.048 6.7
6.8 1.089 11.8
1.049 1.090 11.9
1.050 6.9
1.051 7.0 1.091 12.0
7.2 1.092 12.1
1.052 1.093 12.3
1.053 7.3
7.5 1.094 12.4
1.054 1.095 12.5
1.055 7.6
1.056 7.7 1.096 12.6
7.9 1.097 12.7
1.057 1.098 12.8
1.058 8.0 1.099 13.0
1.059 8.1 1.100' 13.1
1.060 8.2 1.101 13.2
----------------------------------------------------------
-46-
Density Density
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-50-
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TABLE - 3.4 \
OF SEA WATER
--------------------------------------------------------------------------
Magnesium Sodium Potassium Total Cumulative Total
Chloride
MgC12
Bromide
NaB2
Chloride
KCL ;i.f:d s;;;:-----i~-----
r~ted solution
Total
separated 0.1532 0.22·24 29.9762
Remainder
in soln.
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0.0162
1itres 3.1640 0.3300 0.5339 8.4709
Total
content in
one litre 3.3172 0.8524 0.5339 38.4471
------------------------------------------------------ -----------~-------
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SEA WA T E ROE N SI T V (. B e )
Although a country may have a long coastline or inland lake shore line, not all locations along the
coast or lake shore become automatically suitable for manufacture of solar salt. There are a
number of factors to be considered before setting up a solar salt plant. The effects of these factors
may have to be studied for periods ranging from three months to a few years. Sometimes any one
of the factors may turn out to be critical. However, for a solar salt works to be technically and
economically viable, all the important factors must be at least reasonably favourable. These
factors are:
Ideally, the location should be flat or gently sloping block of impervious clay land unfit for
agriculture, with access to abundant quantities of undiluted seawater/subsoil or lake brine, with
provision for tidal intake during high tides in the case of seawater, with little or no rainfall, with a
hot dry breeze blowing all year round, with a rail head or deep sea port nearby. However, such
ideal locations are rare. The coast of West Australia and of the Baja California desert in Mexico,
where salt is manufactured all the year round and exported by ship to Japan, come closest to
these ideal conditions. Usually some compromises have to be made in the above factors and a
decision taken based on the overall techno-economic feasibility of the specific site.
Flat land closest to the sea, lake or underground sources of brine, unfit for agricultural or other
purposes is preferred for locating a salt works. Land that can be used for agricultural or other
purposes is generally too expensive to make a salt project economically viable. The land chosen
should be flat or very gently sloping ( not greater than 30 to 400 centimetre per kilometre) in one
direction so that it is possible to hold the brine in ponds at different stages and obtain an even
cover with shallow depths and at the same time allow the brine to flow from one pond to
another.
Brine in a salt works is held in large ponds with earth bottoms and allowed to concentrate by solar
evaporation. The characteristics of the top one metre layer of the soil determine the capacity of
the land to hold and evaporate brine at different stages of concentration. A prime indication of the
nature of the soil is its composition expressed in terms of percentages of gravel, sand, silt, and
clay. This composition is determined by mechanical analysis. Obviously, a gravelly or sandy
soil through which is impervious is suitable. However, too clayey a soil will be weak under wet
conditions and will impede proper harvesting of the salt. Also clay soil tends to crack under dry
conditions. In such a soil the presence of fine sand imparts a desirable characteristic viz.,
increased bearing strength. Ideally te soil should content not exceeding about 40% (as fine sand)
and the balance being accounted for by silt and clay, clay being more than silt. In order of
preference, clayey soils, clay loam and silty clay are suitable for establishment of a salt works.
Such a soil would fall under the groups ML, CL, MH, CH of the unified soil classification.
In addition to the mechanical analysis, certain index properties at the soil are to be measured:
The bearing capacity of the soil is to be determined from the point of view of assessing its strength
and suitability for construction of roads and embankments. A bearing capacity of at least 1
kilogram per square centimetre is required. If lower, it should be improved by addition of sand
during the preparation of crystalliser floors.
The consistency limits of the soil should also be determined as these data are useful in the
compaction of crystalliser floor and in the construction of embankments to achieve the desired
strength. These consist of the liquid limit, plastic, limit, plasticity index and shrinkage limits.
These limits are easily determined by simple laboratory tests. The procedures are described in any
standard book o Soil Mechanics. The ideal ranges for these consistency limits are 40% to 60%
for liquid limit, 15 to 30% for plastic limit and 15 to 40% for plasticity index. In such soils the
limit which shows that the soils are mostly fine grained plastic soils.
The critical factor that affects the production of salt is the evaporation rate in shallow ponds. It is
important that the net evaporation rate for the dry season should be positive , and at least 500
millimetres for the site to merit consideration. In a salt works several meteorological parameters
like temperature, incident radiation, relative humidity and wind velocity influence the net
evaporation of brine at different concentrations. The meteorological data for as long a period as is
possible at a station close to the project site should be collected and analysed.
The first and most important meteorological factor to consider is rainfall - the antitheses of
evaporation. The rainfall pattern over at least ten years will reveal the duration of the dry season
when salt production operations are feasible. The extent of salt production is determined by the
evaporation rate during the dry season. To operate a solar salt works successfully, the annual
rainfall should be as low as possible and its distribution restricted to a few months leaving a long
clear weather period for salt manufacture. An area receiving rainfall not in excess of 600
millimetres within any span of 100 days during a calendar year is considered suitable. However
solar salt plants can also be successfully operated in areas where the total annual rainfall is as
high as 1600 millimetres but is restricted to a very short and definite duration of 80 to 90 days
during the year. Even during the dry season, showers not exceeding 15 to 20 millimetres in a
single spell within 24 hours do not affect salt manufacturing operations.
The evaporation rate is influenced most by a high sunshine rate and air temperature which are
usually interrelated. It will be beneficial if the site is subject to a hot inland or desert air. If the
hot air is also dry it will further improve evaporation. The lower the relative humidity the greater
the capacity of the evaporating body to take up more water vapour. However, the benefit
conferred
by the heat is more important than that conferred by dryness. Wind, which is considered the least
significant factor, occasionally assumes importance. It helps in the removal of air saturated with
water vapour from the surface of the evaporating body and bringing in contact with it fresh
unsaturated layers of the atmosphere thus increasing evaporation. However, the wind velocity
should not be so high as to blow sand and dust into the works affecting the quality of salt. In
certain desert regions despite other favourable conditions, so much sand is blown by high velocity
winds that solar evaporation becomes infructuous and vacuum pan evaporation is preferred. A
desirable range of wind speed which aids evaporation is 3 km to 15 km É hour. The direction of
the wind is an equally important consideration. The wind blowing from over the des is normally
saturated with water vapour thus reducing evaporation. Wind blowing from over the land is dry
and can take up water vapour until it attains saturation and thus aids evaporation.
Areas prone to cyclonic storms and tidal waves are not suitable.
Once the site is fixed there is not much scope for altering the meteorological
parameters. However, maximum advantage of prevailing factors can be obtained by
suitable design.
For instance, the ponds could be aligned normal to the prevailing wind rather than parallel to it.
Also the ponds containing highly concentrated brines could be placed on the windward side. In
the crystalliser area, absorption of incident radiation can be maximised by addition of a suitable
dye.
The salinity of the intake brine and its availability in adequate quantities throughout the
manufacturing season is an important factor. In the case of sea salt works, this involves a study of
the salinity variation and a chemical analysis of seawater along the coast at different times during
the season. Sometimes seawater is diluted at locations close to estuaries due to influx of fresh
water. Generally the concentration of undiluted seawater is 3o to 3.5o Be (NaCL content 2.7% to
3%). If there is appreciable dilution below these values, it calls for serious investigation since the
output of the salt works is very much dependant upon the intake seawater salinity. If, instead of
3o Be, only 2o Be water is available, it could reduce the output of the salt works by as much as
50%. In the case of underground brines the concentration of brine available and its flow rate at
different depths should be studied. For lake brines the variation in lake levels and salinity at
different times during the season should be studied. In the case of seawater, at some places,
phosphate impurities have been noticed, possible brought in by river water or due to the influence
of rocks below.
Brine with such phosphates should be avoided, as phosphates are conducive to the growth of
algae which retard crystallization. Impurities in the form of algae, plankton, shell fish and
sea
weed all have adverse effects on production. In certain locations, as along the east coast of India,
there is sand drift during high tide and if the sand is not fully flushed back into the sea during the
low tide, a sand bar is gradually built up, choking the entry of seawater during the dry season.
Usually such bars are fully cleared by the floods during the rainy season when the rivers and
storm water drains are in full spate. But if the rains are poor for two or three successive years, the
bars are known to close completely in the subsequent dry season much ahead of time, affecting
brine supply. Before choosing the location for the entry of brine, its past history has to be studied
to determine the behaviour of the sea mouth and whether dredging operations will become
necessary and at what cost. This has a bearing on the economic viability of the project.
The area may get flooded due to accumulation of storm water discharging into it from the usually
large catchment area surrounding it, especially during rainy months. The extent of flooding will
determine the type of boundary embankment protection required. Innundation of the area due to
storm water from adjoining areas should be avoided since this completely removes the salinity in
the salt works year after year. For this purpose, The extent of catchment area must be
approximately determined and co-related to the corresponding rise in level of water in the area.
With this data, a suitable boundary channel can be designed to divert the storm water directly into
the sea or lake or, an adjoining without passing through the area.
There may be several other factors and ecological considerations that may influence the location
of a solar salt works. In certain cases, marshy lands identified as ideal for production of solar salt
and chemicals based on bitterns, have been found to be winter homes for rare birds. Therefore,
the development of salt works in these areas is being objected to by ecologists. Availability of
skilled labour in sufficient numbers, power for pumping, fresh water and basic township facilities
for the workers are other factors to be considered in locating the salt works.
From these observations, it will be appreciated that the factors affecting the location of a solar
salt plant are quite demanding. However, by careful analysis their restrictive effects can be
considerably reduced and salt works set up in locations previously considered unfavourable.
CHAPTER V
SMALL AND MEDIUM SCALE PRODUCTION OF SALT:
Today salt is manufactured by solar evaporation of sea water and other types of brines by adopting
a range of techniques from the most primitive to the most advanced. At one end of the scale, salt
is manufactured in cottage units, using labour intensive methods with no quality control. At the
other end, there are very large solar salt plants employing modern methods of production and
quality control and fully mechanized, so that the material is produced and handled completely by
machines. The aim of this guide is to outline design and manufacturing practices for small to
medium sized operations that will produce a high quality product at minimum cost, adopt
scientific methods of quality control and a selective mechanization without displacing labour.
The degree of mechanization must be carefully considered, not only from a financial but also
from an employment view point. In many developing countries, solar salt manufacture is a
traditional agriculture-type operation and mechanization is likely to raise social and economic
problems. Often in these countries employment in solar salt works is complementary with
agriculture. Nevertheless, with growing demand for larger quantities and a higher purity of salt for
industrial
and table applications in all developing countries, there has to be a gradual shift from cottage scale
to small scale. Also, existing solar salt works can frequently be made more productive by the
application of new techniques of evaporation control and harvesting.
The size of a salt works could be considered in terms of its output. Production yields vary
considerably, depending upon the several factors outlined in Chapter IV, and could be anywhere
between 10 tonnes/hectare of total evaporation area and 1000 tonnes/hectare. In certain
exceptional cases, where the feed brine is almost saturated with salt, the yield could be even
higher. In this book the following sizes are considered:
Area Production/year
C Small size 5 to 50 hectares 500 - 5000 tonnes
C Medium scale 50 to 250 hectares 5000 - 50 000 tonnes
The design and layout is outlined for this range of operation. For salt works of larger area and
production, capacity and same design principles outlined in this chapter hold good, with a higher
degree of mechanization of salt harvesting and handling operations.
A good solar salt works consists of a series of ponds with eastern embankments, into the first of
which saline water from the sea, or any other source, is drawn. The function of these
concentrating ponds is to increase the salinity of the brine that is slowly flowing through them, to
the point where salt will crystallize. At this point, the saturated brine is transferred to
crystallization ponds where the salt deposits as a uniform layer and is ready for harvest. The salt is
harvested manually or by the mechanical means, washed and stored. Figure 5.1 is a process flow
sheet for a solar salt works.
As elaborated the previous chapter, the production potential of site is evaluated based upon the
following inputs.
(i) Area - It is necessary to ensure the availability of sufficient are and to verify the contours
within this area that will determine the levels at which brine is held at different stages and
transferred
from one stage to another . In order to determine the area available, a traverse survey is conducted
starting from one point on the boundary and proceeding along the boundary measuring distances
and angles until the starting point is reached again. The area within the traverse is estimated.
Simultaneously, the levels within the area are determined within a specified grid of say 60 m x 60
m and plotted in the traverse survey plan. Points of equal levels are joined to form contour lines.
The contour map will then determine the brine holding capacity of the land at different levels
and will yield the data required to design the brine circuit.
(ii) Meteorological studies - The meteorological parameters recorded as close to the are as possible
will indicate the duration of the of the dry season for salt production and the net evaporation rate
during the dry season during the whole year. If the data is not readily available, the following
meteorological instruments are set up for recording the parameters over at least two years:
Parameter Instrument
Wet & dry bulb temperature and humidity Wet & dry bulb thermometer
Maximum & minimum temperature Maximum & minimum thermometer
Wind direction Wind Vane
Wind speed Anemometer
Rainfall Rain gauge
Evaporation Open pan evaporimeter
Radiation Solarimeter
In addition, tests can be set up in trays to study the effect of salinity on evaporation rate to allocate
areas required for brines at different concentrations.
(iii) Soil characteristics - The mechanical composition, proctor compaction, void ratio,
permeability, compressibility and consistency limits of the soil are determined at different
levels
up to a depth of 1 metre. In addition, bearing capacity tests are conducted in the are proposed to
be developed for the crystallisers.
(iv) Brine availability & salinity - In the case of seawater and lake brines, the salinity at different
points along the coast is measured to locate the best intake point of brine into the area and the
pumping capacity required. In the case of underground brine, a hydrographic study will
determine the location of the brine strata. Trial bores are then drilled to determine the dalinity of
the brine and the output.
(v) Flooding - The accumulation of rainwater n the area due to inflow from the adjoining
catchment area is studied to design suitable boundary drainage channels to divert
stormwater without passing through the area.
Once the site has been selected, the next step is to carefully prepare a layout which can take
maximum advantage of the climatic and other parameters prevailing in the locality which affect
the yield of salt per unit area, its collection, storage and transport to the market. The basic
principle to be followed in designing a salt works is to keep the investment and manufacturing cost
as low as possible by simplifying the brine circuit, minimising the pumping required and the
stages for the collection, handling and storage of salt.
The yield potential of the area depends upon the net evaporation rate and intake brine salinity.
The following example illustrates how this is determined.
Consider the intake brine as seawater with a salinity of 3o Be. This has a sodium chloride content
of 2.7% and about 0.3% of other salts. To recover sodium chloride, 97% of the seawater has to be
evaporated. Therefore 97 grams of water are evaporated to yield 2.7 grams of sodium chloride, on
a theoretical basis. Taking into account seepage losses, 48 grams of water are evaporated per from
of sodium chloride produced, exclusive of recovery losses. Assuming 70% recovery,
approximately 70 tonnes of water is evaporated per tonne of salt recovered. For an annual
evaporation rate of 500 millimetres through te system, this gives a yield 75 tonnes per hectare per
year. This indicates the production capacity of the area. If the evaporation area available is 100
hectares, the production capacity can be assessed at 7500 tonnes per year under normal weather
conditions.
Area Ratio:
The ratio of crystalliser ponds to concentrating ponds for the above example is determined
based on the following data:
If A1 is the concentrating area and A2 the crystalliser area, then the area ratio is given by:
A1 x 600 = 9.0
A2 x 400 6.7
A1 90 x 400 60 9
A2 6.7 x 600 6.7 1
Providing for a buffer in the concentrating area, in order to obtain saturated brine without
interruption into the crystallising ponds, a slightly higher ratio like 10:1 is advisable. The layout
of the salt works is designed to suit the above requirements and the allocation of land for the
different components of the salt works made on this basis.
Area allocation:
Based on the area ratio determined above, a rough subdivision of areas is done.
1. Concentrating ponds
a) Embankments (boundary bunds, drainage, channels, embankments for
roads, cross bunds for the concentrating ponds and bunds along
creeks) 2%
2. Crystalliser ponds
The area of brines at different stages is calculated based upon the volumetric reduction in that
stage and the evaporation rate during that stage. The volumetric reduction for brine is set out in
Figure 3.2. The reservoir area is to be provided only in the case of sea salt works handling large
volumes of dilute brine.
The layout of ponds and brine flow circuit are based largely on the elevation contours and
topography of the area. The concentrating ponds are made as large as is consistent with
maintaining the desired pond depth and preventing the formation of dead areas or short
circuiting. These are normally worked in series. To build up a graded density, weirs and gates
must be provided to enable control of flow between ponds. In the case of seawater, the division
of the
reservoir area may be restricted to only 2 or 3 parts to take maximum advantage of tidal
inflow. The compartments in the concentrating area are smaller than those in the reservoir area.
The brine entering the concentrating area (including the gypsum ponds) is at about 6oBe and the
concentration is to be raised to 24oBe. This is achieved by spreading the brine in a number of
compartments which prevents mixing brine of different densities, thus helping the progressive
increase in density. The flow path of the brine in the concentrating ponds is arranged so that it is
made to travel over a longer distance, as brine in motion evaporates more quickly than in a
stagnant condition. In deciding on the size of the reservoirs and concentrating ponds, a balance
has to be struck between the expenditure to be incurred for constructing the partition bunds and
the average accruing in zig zagging the brine. Concentrating ponds containing brines of widely
different densities are not normally adjacent to each other because of possible infiltration of the
weaker brine into the pond containing the concentrated brine. However, when the topography of
the land does not permit this, they are separated from each other by strong, well compacted
embankments. Figure 5.2 gives a typical layout for a solar salt works.
In the case of small and medium sized salt works, it is sufficient to restrict depths to about 60cms
in the reservoirs, about 30cms in the concentrating ponds and to 10-15cms in the crystallisers.
However, reservoirs should be deep since their main function is to store and maintain a steady
brine supply to the concentrating ponds. Large depths slow down the build up of salinity and
shallow depths may result in drying up. These are only thumb rules based on working
experience.
Sluice gates:
Where tidal conditions permit intake of brine by gravity, sluice gates are to be provided.
Normally, automatic sluice gates that allow tidal water to enter during tide but prevent water from
the reservoirs from returning during low tide are preferred. The gates should be located on the
main creek feeding brine to the salt works, so that maximum intake of tidal water is possible. The
size and number of gates is fixed, based on the height of the normal tide, bed level of the
reservoirs and pressure and period of high tides. The objective is to obtain the maximum level in
the reservoirs as close to the high tide as possible. Provision can be made to operate the gates on
both sides of their supports so that during the rains, these gates may operate as flood gates
discharging rainwater accumulated in the reservoirs and preventing tidal water from entering the
reservoirs.
In several sea salt plants, the design must permit maximum intake of brine by tide. But even
in such areas it is rare that the brine can travel through the system and reach the crystallisers
by gravity alone. Pumps are therefore required to supplement or take the place of tidal gates.
Suppose the expected output of a sea salt works is 10,000 tonnes per year. The volume of 3oBe
brine is required to produce one tonne of salt is 60 cubic metres. Providing for seepage and
recovery losses, the actual requirement varies from 100-150 cubic metres. If this quantity has
to be pumped in 100 days, the daily pumping requirement is 15, 000 cubic metres. Assuming
that the pumps operate for 15 hours a day, the pumping capacity is 20 cubic metres/minute.
Two pumps of 10, 000 litres/min. are required.
Even though provision is made for pumping of brine during a comparatively short season, the
reservoirs should be designed to provide a steady supply of seawater to the salt works in the event
of failure of pumps or fluctuation in seawater inflow due to tides.
In the concentrating area, advantage should be taken of the contours of the land available to
minimize additional re-lift pumping within the brine circuit.
While permanent type pumping stations should be provided for brine intake and transfer ponds, a
few portable pumps with diesel engines on trolleys are recommended to be available for
temporary transfer operations between ponds or for drainage ponds.
Low head, high volume pumps are recommended. Axial flow or centrifugal mixed flow pumps of
cast iron construction are commonly used. A typical elevation of a pumping station is shown in
Figure 5.3. Axial flow pumps do not need priming. Mixed flow and other centrifugal pumps need
priming. Priming can be done by fitting foot-valves on the pump suctions or by using vacuum
priming pumps. Figure 5.4 shows an intake brine pump installation. For permanent stations
where power supply is available, pumps can be directly coupled with totally enclosed, fan
cooled squirrel cage motors. The impellers should be of open or semi-open construction to
prevent clogging caused by salting. Pumps are usually made of cast iron with a 2% nickel
casting and suction flare.
While lifting concentrated brines above 20oBe, salting is frequently a problem. Pumps for such
applications must have provision for periodical flushing of the casing and impeller to clean out
the salt. For these pumps, the impeller should be preferably be made of bronze with stainless
steel shaft.
The crystallisers where the deposition of salt takes place are the productive and, therefore, the
most important part of the salt works. They constitute not less than 8% of the salt works area for
sea salt works and a higher proportion in the case of salt works based on richer underground or
lake brines. An approximate estimate of crystalliser area required for different feed brine
salinities is given below:
Salt
Salt content
content of feed
feed brine
brine (%NaCl) Crystalliser
Crystalliser area
area as percentage
percentage of total
total area
3 8
6 16
10 24
15 40
20 70
25 90
Since the crystallisers are fed with the most concentrated brine in the salt works, soil in this area
has to be impervious. The crystallisers are rectangular in shape, uniform size and even in level.
The beds are well consolidated and neatly maintained. The shorter dimension of the crystalliser is
aligned in the direction of the wind to maximize evaporation. The size of the crystallisers
depends upon the degree of mechanisation in harvesting and handling operations and the method
of
feeding followed, which in turn depends on the incidence of rain during the manufacturing
season. If the harvesting is done manually, the crystallisers cannot be very large. A minimum size
would be 30 metres x 15 metres and the upper limit would be about 100 metres x 30 metres. If
the harvesting is done semi-mechanically or mechanically, the crystallisers are larger running,
sometimes to several hectares. Manual harvesting involves removal of the salt with picks and
shovels and conveying it to the nearest transport roads in baskets or sacks. The work is arduous
and needs skill to collect the salt without adhering mud to it. However, in many countries it is a
source of employment and should not be disrupted.
Semi-mechanised harvesting involves manual removal of the salt and loading belt conveyors
to transfer it to the nearest transport roads.
In a solar salt works, it is advantageous to conserve brines of high concentration during the off
season for use in the succeeding manufacturing season. If the brine is stored in shallow depths, it
will get diluted by rain. To minimise dilution, storage ponds up to 3 M deep should be
constructed. At the end of the manufacturing season, all concentrated brines from the
crystallisers and final concentrating ponds are drained into these ponds and stored. The area
covered by these ponds is 3 - 5% of the crystalliser area. The ponds are provided with rainfall
overflow arrangements such that even when exposed to heavy rainfall, dilution is minimal.
Saturated brine has a density of about 1.23gm/ml. When rainwater strikes the surface it initially
remains on top as it is lighter. By topping off the rainwater quickly before it mixes with the brine
below, dilution is minimised. This principle is also adopted in the crystallisers where overflow
weirs are provided to drain rainwater during mid-season showers. A typical overflow arrangement
is shown in Figure
5.6.
After the rains, the brine from deep storage ponds can be used to accelerate the start of
crystallisation and as buffer ponds for settling gypsum before the brine enters the crystallisers.
Embankments:
The salt works is bounded by an embankment and a ring channel to protect it from storm water
flowing in from the adjoining catchment areas. The embankment must have a free board of 1
metre above the highest flood level recorded in the area. The top width and side slopes of the
embankment must be sufficient to provide the required strength to the bund. If the embankment
is to act as an inspection road a minimum top width of 5 metres is required.
Along the boundary embankment perimeter runs a ring channel to divert catchment storm water
into a nearby creek, lake or river. The ring channel must be designed with sufficient bed width
and slope to drain off water accumulated by a peak rainfall recorded in the area for a 24 hour
period.
The internal partition bunds separating different reservoirs and concentrating ponds need not be of
as great a height as the boundary bunds. Their height can be fixed, based on the height of water to
be stored, providing for of a free board of about 60cms. Their top width can be restricted to about
3 metres except where they also serve as inspection roads, in which case a minimum top width of
5 metres is required. Where wave action is excessive, embankments are lined with loose boulders.
Embankments are constructed from earth, from adjoining borrow areas. Typical embankment and
borrow pit cross sections are shown in Figure 5.7.
Where labour is cheap, embankments may be constructed manually in layers of 15-20cms and
compacted by hand drawn rollers to provide the requisite strength to the embankment. Where
equipment is available, bulldozers can be used to scrape off adjacent earth on both sides to form
the embankment and then run over it to compact it. The compaction required is detrmined by
the soil bearing capacity.
A cheap and effective way of protecting embankment slopes is to promote shrub growth along the
sides. Certain thorny bushes, like proscopis, grow quickly in saline soils. Sometimes dressing the
bund with paddy grass mixed with mud is also helpful.
Areas where top sand occurs over an impervious layer below, resulting in heavy percolation, can
be productively used by forming a key trench up to the clay layer and an impervious wall to seal
off cross percolation from the pond (Figure 5.8).
In areas where concentrated brines are available close to the surface up to a depth of 8 metres,
these are tapped by digging shallow open wells of about 10-15 metres diameter (Figure 5.9).
However, when the brines are available lower down and when the brine table tends to fall in
summer, it is advisable to sink borewells. This consists of drilling a hole vertically either by
using a hand auger or by using a pneumatic drill. The diameter of the bore is normally in the
range 15-
25cms. Where the earth is firm, no casting is provided. But, lower down, where the soil is weak
and tends to collapse, it is necessary to sink casing pipes to support the side walls. The drilling
should continue until the water strata is reached. Solid casing pipe can be used for the upper
portions of the borewell. However, in the lower portions where cross seepage is required to feed
the borewell, slotted casing pipes are used. Casing pipes are of cast iron, Polyvinyl Chloride or
High Density Polyethylene. A borewell cross section is shown in Figure 5.10.
Borewells vary in depth from about 20 metres to over 300 metres. The water table should not
drop below 8 metres, even under continuous pumping, if ordinary centrifugal pumps are to
operate. Where water tables are 10 metres below ground level, it is necessary to lower the pumps
to a level where suction does not exceed 8 metres or to use special deep well submersible pumps,
which are lowered into the water strata. Figure 5.11 shows the top end of a borewell fitted wiuth a
submersible pump.
Prospecting for good locations for borewells is still largely based on hydrographic studies and
systematic trial bore testing to identify the water strata. Electrical resistivity methods, commonly
used for fresh water prospecting, very often yield erroneous figures for saline brines and are not
useful. The proper location of a borewell is very crucial and borewells located as close as 100
metres apart may have widely varying outputs. The spacing between borewells should normally
be less than 100 metres. Except, in cases where the output of a single well is insufficient, a series
of wells closely spaced are combined and coupled to a single pump.
Transport of salt:
The salt harvested in the crystallisers has to be transported to the central stack yard/washery. This
is done by small locamotives with tipping trolleys on a trolley track that passes through the
crystalliser area. A more convenient method adopted nowadays is to use tractor drawn tipping
trailers of 4-6 tonne capacity.
The salt from the crystallisers may be subjected to washing and stacked. Salt washing is important
in that it improves the colour of the salt and removes some impurities both soluble and insoluble.
For a salt works to be able to afford a washery, a production capacity of at least 10, 000 tonnes per
year is required. There are several possible methods of ensuring a proper washing of
salt with saturated brine. One such method is described here. Figure 5.12 shows a layout for a
salt washery. The crude salt is fed into an agitated tank where it is mixed with saturated brine
from the crystallisers to form a slurry which is pumped into a screw classifier. The wash liquor
overflows. The salt then passes through a de-watering screen for further removal of
impurities. The washed salt is stored in large heaps using inclined belt conveyors (called
stackers).
The washery is located near the stack yard which forms the main storage area for the product. The
stack yard is normally situated close to the crystalliser area and should be about 1.5 metres above
the prevailing ground level. The top of the stack yard should be well consolidated, properly
levelled and given a gentle gradient to effectively drain out the rain water.
Salt in heaps exceeding 8 metres in height can be safely stored in the open area without any
covering to withstand a rainfall of even up to 100cms. After the first shower, the top crust of the
heap hardens and, thereafter, the rainwater slides off the surface of the heap. If the heaps are
smaller and rain is excessive it is necessary to protect the salt with palmmryah leaf covering
(where available) or with low density polyethylene covers.
5.2 Operation of the salt works:
At the commencement of the manufacturing season, sea water/brine is drawn into reservoirs either
by tidal action or by pumping. The average depth of charge in the reservoirs is maintained around
60cms. The brine remains in the reservoirs until it reaches a concentration of about 6 oBe. During
this period the suspended impurities settle down at the bottom and the volume of the brine gets
reduced to almost 60% of the original quantity drawn into the reservoirs. The brine, at a
temperature of about 6oBe, is transferred to the first series of ponds in the concentrating area. In
the concentrating ponds the depth of charge is maintained between 40cms initially and, 20cms in
the final stages. Between 6oBe and 10oBe calcium carbonate settles down. The brine is further
concentrated to 16oBe and transferred to gypsum ponds. Here the bulk of the calcium sulphate in
the brine separates out and settles down as long needle shaped crystals. The brine is retained in
these ponds until it reaches 24oBe, which is almost the saturation point with respect to sodium
chloride. In the reservoirs and concentrating ponds, all the suspended matter gets removed.
Density at different stages in the salt pond system is measured by using a simple glass hydrometer
calibrated in oBe (0-40).
The brine close to saturation is passed through deep storage buffer ponds before filling the
crystallisers. This is because the gypsum has a tendency to supersaturate. If brine enters the
crystalliser under-saturated or has rapidly reached salting point, it can deposit several times more
gypsum than otherwise. The buffer pond will aid deposition of as much gypsum As possible
before the brine enters the crystalliser.
Before being fed with saturated brine, the crystallisers are cleared of all slush and well compacted
with the addition of sand and rolled. Cast iron drum rollers with a dead weight of about one ton
are suitable for this rolling operation. These rollers can be filled with water so as to increase the
weight up to 3 tons depending upon the type of soil. After the crystallisers are rolled they are
fed with a small quantity of saturated brine and a thin layer of salt is allowed to form. This layer
is uniformly dragged over the bed of the pond. The crystallisers are then charged to a depth of 8
to
10 cms through the brine supply channels. As described earlier, the crystallisers are charged in
series, the brine entering the first pond at 25o Be and leaving the last pond of the series at 30o Be.
In the series system, the salt formed in the first pond has a low magnesium content and a little
more calcium whereas the salt formed in the last pond will have a high magnesium content and a
low calcium content. Both in quality and quantity such a method is more advantageous than
direct charging of brine separately into each crystalliser since the average evaporation rate in a
series system is higher than the average rate in independent crystallisers. The brine is 30 o Be, the
volume of brine reduces to about 3% of the original. A little over 70% of the sodium chloride in
solution crystallisers on the well compacted clay beds of the crystallisers. When the salt crust
reaches a thickness of 1-3 cms, the mother liquor is discharged and fresh saturated brine is
admitted. Figure 5.13 shows a crystalliser ready for harvest. The salt is rinsed in this brine and
then extracted manually. The extracted salt is collected in small heaps within the pans (figure
5.14) where it is allowed to drain before it is transferred manually to the roads. From the roads
it is transported by tract9r-trailers (Figure 5.15) or trucks to the washery and stacked.
The bitterns rejected at 30o Be is transferred to another crystalliser where it can remain till it
reaches 32o Be. There is a further reduction in volume to nearly 2% of the original. An additional
quantity of sodium chloride, contaminated with magnesium sulphate can be taken out of this
pond, which is generally called the crude salt crystalliser. By careful washing of the crude salt
with under-saturated brine in the concentration range of 20-22o Be, it is possible to separate the
magnesium impurities which cling to the salt crystals as mother liquor. The washed salt will
then be fit for edible use. At the end of the season all high density brines in the crystallisers and
gypsum ponds are drained into deep storage ponds adjoining the crystalliser area specially
provided for this purpose.
Before stacking (Figure 5.16) about 5 ppm sodium ferro cyanide can be added as an anti-caking
agent. Drainage continues in the heap before the material is offered for sale.
The quality of the final product depends upon the application and the standard specifications
prescribed by the country. For a medium sized salt works it may be worthwhile to set up a small
quality control laboratory.
After washing, solar salt crystals range in size from 2 mm to 10 mm. In the case of industrial salt,
the crystals can be despatched without crushing. For edible purposes, the salt has to be ground to
a size of 0.5 mm to 1 mm. This is done using roll crushers or pin mills as shown in Figure 5.17.
Prior to crushing wet salt may have to be dried. This can be done by spreading the salt in thin
layers in the open sun for a few hours or with the help of a rotary drier. If required, additives are
added for speciality salt production after grinding. Caking caused by moisture absorption in table
salt is reduced by adding up to 1.5% free flowing agents, such as magnesium carbonate and
tricalcium phosphate. Iodised salt contains 0.01% potassium iodide, 0.01% calcium hydroxide
and 0.1% sodium thisulphate as stablisers when free flowing agents are used.
The material is packed in 50/75/100 kg bags for despatch as shown in Figure 5.18. In the case
of bulk transport by road the material is directly loaded into trucks and despatched to the
destinations. For table salt, machines are available for packaging in 1/3 kg/’1 kg polythene bags.
5.3 Estimation of project costs, production cost, and profitability:
Project cost:
The construction of the salt works is spread over a period of two to three years depending upon
the length of the dry season when development work is possible. During this period, the project
area is progressively developed. Simultaneously, buildings and civil works are erected,
machinery and equipment are procured and installed. Miscellaneous accessories are also
purchased. The capital investment in the project is classified under five heads:
i) Earthwork
ii) Buildings and civil works
iii) Plant and equipment
iv) Miscellaneous accessories
v) Preliminary and pre-operative expenses
i) Earthwork
This item cover all land development operations classified under three sub heads:
a) Concentrating area earthwork
b) Crystalliser area earthwork and,
c) Miscellaneous earthwork and roads
ii) Buildings
Space estimates for the buildings are based upon the requirements of the salt
works when it reaches full production. Construction costs for standard height
buildings with reinforced cement concrete or asbestos cement sheet roofs depend
on local cost of materials and wage rates.
a) Vehicles
b) Furniture and fittings
c) Laboratory equipment and accessories
Laboratory tables, wooden racks and cupboards, glassware and chemicals, ovens,
muffle furnace and hot plates, electric balance, vacuum pump, sieving machine and
sieves, sink and water supply, drainage fittings, flame photometer, reference books
and charts, distilled water plant, soil testing equipment, miscellaneous laboratory
equipment and fittings.
d) Workshop equipment - Lathe, flexible shaft grinder and bench grinder, welding set,
tools set and accessories, electrical testing equipment, drilling machine, workshop
racks, chain pulley blocks, miscellaneous equipment not elsewhere specified.
e) Garage equipment - Washing ramp, diesel oil storage tank and metering pump,
compressor and air pressure gauge, pump connections and fittings, oil storage
tanks, miscellaneous and sundry items.
g) Survey and meteorological equipment - Dumpy level, compass, levelling staffs and
theodolite, double stevenson’s screen, thermohygrograph, rainfall recorder and rain
gauge, evaporimeter, hydrometers, miscellaneous fittings.
h) Staff amenities - Fresh water trailers, canteen and accessories, toilet facilities for
staff and workers.
Production Cost
The items of expenditure that constitute the cost of production of salt can be broadly
grouped under the following heads:
4) Fixed Expenses:
Staff Salaries
General administrative expenses
Rent, rates and taxes
Maintenance and service expenses
Earthwork repairs
2) Variable Expenses:
Staff salaries - This item covers salaries and prequisites for a small batch of supervisory
and technical staff and shift operators. An organization chart for a small to medium sized
salt works is presented in Figure 5.19.
General administration expenses - This head includes all expenditure of a general and
fixed nature incurred in connection with the operation of the salt works like postage,
telephone, stationery and printing, insurance, office maintenance, staff travelling and
conveyance expenses.
Rents, rates and taxes - The lands leased to most solar salt projects generally belong to
the government of the country. The policy of rental varies from country to country and
within countries themselves, varies from province to province. Normally a ground rent
per hectare of area leased and a royalty per ton of salt produced is levied. The leases are
granted for varying periods which could be as long as 99 years. Usually a two or three
years moratorium is given for payment of ground rent and royalty when construction is in
progress and there is no production.
Maintenance and service expenses - This item provides for routine maintenance of all
plant and equipment, civil structures and foundations. It includes cost of parts
replacement, lubrication, painting and other general maintenance required for upkeep of
the assets. Since the environment in which the plant and equipment operate is extremely
corrosive, periodic maintenance and painting is essential to ensure the longevity of the
equipment. Buildings need to be frequently white washed since even cement plaster is
subject to saline corrosive. In addition, painting has to be done with a good primer
followed by an anti-corrosive epoxy paint. Plant and equipment maintenance cost per
year could be as high as 8-10% of the value of the equipment.
Earthwork repairs - Salt fields comprising of embankments, channels and roads will
require routine maintenance every year after the rains, especially in areas where the silt
and sand content of the soil is high. All earth embankments are subject to erosion and
formation of ruts along the sides. Gravel topped roads have to be frequently checked to
prevent the formation of pot holes. The repair work is usually carried out year round.
Provision for this item of expenditure is made based on the prevailing site conditions and
could vary from 5% to 10% of the capital cost of the earthwork.
Brine pumping and distribution - The consumption of electric power/diesel oil and
lubricants for operating motors and engines to drive the brine intake and relift pumps is
covered under this head of account.
Washing and storage - The cost of operating the washery, washing losses, storage and
heap covering expenses fall under this head.
Packing and forwarding - Under this head the cost of crushing and additive mixing
(where applicable), packing and loading trucks/railwagons/ships is provided.
In addition to the above expenses, depreciation is normally charged on the assets. The following
depreciation rates are typical:
Earthwork 25%
Buildings and civil works 10%
Plant and equipment 20 - 25%
Miscellaneous 10-20%
Profitability:
Based on the prevailing market factors, the value of the product that can be realized ex-factory
is estimated. This is done by determining the “economic marketing zone” that lies within the
reach of the salt works, i.e., the region within which the salt produced is competitive in terms of
the landed cost in the market.
Based on the ex-factory price and production schedule, a schedule of revenues can be worked out.
These are normally made at constant price without providing for escalation so that the estimation
of the profitability is in real terms i.e., after adjusting for the effect of inflation. The absence of
any differential changes in price indices is a conservative assumption, since the profitability
worked
out would be lower than the estimate based on a progressive rise in revenues and costs.
Based on the year-wise schedule of revenue and expenditure the gross profit is worked out. After
deducting depreciation, the profit before tax, tax payable to government, profit after tax and
operating cash flow can be worked out. In the cash flow calculation any special taxes/incentives
applicable may be incorporated.
The discounted cas flow (DCF) method is used to assess the profitability of the project. This
involves the calculation of a DCF rate of return at which the present values of net annual inflows
and outflows are equated. Due to the consideration of the time value of money DCF returns are
generally lower than the conventional return on investment of projects with a long gestation
period. DCF return expected may vary from country to country depending on the opportunity
cost of investment.
The following financial data are then worked out to establish the viability of a project.
1) Capital outlay
2) Share capital
3) Maximum long term debt
4) Maximum debt equity ratio
5) Repayment period for long term debt
6) DCF rate of return
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CHAPTER - VI
Modernization & Mechanization of the Solar Salt Industry
The traditional method of making salt has been to bale out brine from creeks or backwaters into
a pond and evaporate it using solar heat in shallow pans. The slat that is formed is scooped out
using shovels or scraping planks and sold without further processing. This has been the method
practiced for centuries. It is an inefficient method of production and yields a low quality product
since the salts, other than NaCl, are not separated out. This crude salt is suitable only for
consumption as edible salt or as preservative in villages. It cannot achieve the specifications
prescribed for table salt and is not suitable for use in any chemical industry on account of its
impurities. The size of the farms that can be controlled by this method is a few hectares and such
small sized farms can support neither a modern chemical industry or an export trade. Today a
number of countries follow the traditional method to some extent purely to meet limited local
requirements for edible purposes.
With the advancement of science and technology, several refinements have been developed in
order to produce a high purity salt at minimum cost. Modernization can go hand in hand with
manual labour being employed up to a particular level of production. Above that level,
mechanisation operations becomes more economical. This level is different for different
countries depending upon the prevailing wage and price levels. For a developing country where
labour is cheap, this level is in the region of 40, 000 tonnes per annum.
Modernisation is applicable in every phase of operation of a salt works viz., layout, intake of brine,
pumping systems, concentrating ponds, crystallisers, harvesting, storage and transport. The
salient developments in each of these aspects is now recounted.
Layout:
Scientific design methods of layout are now available to maximize production. Mathematical
models and computer programs have been developed for solar pond systems to specify optimum
pond depths to be maintained at different stages of concentration and prescribe procedures for
brine transfer and concentration control at different times of the year based on prevailing weather
conditions and other inputs.
-79-
system ensures continuous draw of undiluted sea water. Large turbine type pumps that require no
priming and consume very little power are being increasingly used for intake brine and re-lift
pumping (Figure 6.2). An added advantage in using these axial flow pumps is that they can be
erected in the open, as shown in Figure 6.3. Among diesel engines used as prime movers, marine
type engines are preferred since they are cooled by the intake brine itself. New plastic materials
are replacing older metallic piping. These are poly vinyl chloride (PVC), high density
polyethylene
(HDPE) and fibre glass-reinforced plastics (FRP). These are non-corrosive and often
less expensive than cast iron or steel pipes.
Concentrating ponds:
It is now the practice to maintain the depth concentrating ponds fairly high so that a permanent
buffer of brine is created within the salt works. Several salt works in advance countries maintain
reservoir and concentrating pond depths of 2-3 metres. The concentrating pond embankments are
well protected with boulders to prevent breakage as shown in Figure 6.4. Thus, a salt works is
designed to receive only direct rainfall which can be allowed to overflow. The salinity within the
salt works is thus preserved and increased year after year. In several salt works brines that are two
to three years old are still in storage.
Biological control:
Several salt works are implementing conservation and biological control by encouraging birds to
feed in the salt works. Salt works with leaking ponds have improved the situation by use of
fertilizers to encourage the growth of native pond organisms to multiply and produce bottom mats
which reduce percolation. The presence of shrimp and fish which feed on algae in the brine
attracts birds whose droppings are rich in nitrogen and phosphorous, the presence of which
stimulates algae growth and the entire cycle of life is repeated. It is now believed that a solar salt
works with a properly performing biological system can produce quantities of salt close to the
theoretical maximum.
Crystalliser area:
Crystallisers are now made of large size of more than two or three hectares each (as shown in
Figure 6.5). The soil is properly compacted and treated chemically when required to ensure
sufficient bearing strength and to make the crystalliser beds as impervious as possible. It is also
common practice to charge the crystallisers deep (say 15-20cms) so as to facilitate uninterrupted
crystallisation of salt. When the concentration reaches 29oBe the crystalliser is replenished with
fresh brine. The process is repeated two or three times before the brine is discharged fully.
Colouring the brine with chemicals like naphthol green B dye helps absorption of radiation and
increases production by 15-20%. Also, the growth of certain algae bacteria (Duna liella saline) in
the brine imparts a pink colour to it. This improves evaporation rate. Thus a thick layer of salt,
10-
15cms is formed for harvesting mechanically. Harvesting equipment are available in different
shapes and sizes. They can be independently driven, pushed or pulled, mounted on tracks, rollers,
caterpillars, wheels or floats. Most of them operate on a common principle of scooping the salt
off the crystalliser pond by inserting a blade underneath the salt layer and transporting it through a
drag conveyor/truck moving alongside the harvesting machine inside the crystalliser. Figures 6.6
and 6.7 show different types of harvesters. Where the crystalliser beds do not permit plying of
vehicles inside the pond, a series of portable belt conveyors are used to transfer the harvested salt
to the nearest road, as shown in Figure 6.8. The harvesting operations are restricted to about four
to six weeks at the end of the season during which the machines work for nearly 14-16 hours per
day. Where the climate and ground conditions permit, a permanent salt floor of a thickness of one
or more years harvest is maintained. This is the most preferred form of harvesting since the salt
floor provides the necessary ground bearing capacity to support heavy harvesting equipment and
there is little chance of contamination of the salt with clay or sand. However, in areas where
rainfall is high, maintenance of a salt floor is not possible. In ponds with high seepage rats,
percolation is being arrested by lining floors with low density polyethylene (LDPE) sheets.
Salt washing:
The harvested salt is transported by trailers or trucks to the washery where it is dumped into a pit
and subjected to a thorough agitation in agitators and screw classifiers (Figure 6.9).
Hydrocyclones are also used in some salt works (Figure 6.10). The thorough scrubbing with
brine removes most of the insoluble impurities, a good portion of the magnesium salts and about
50-
70% of the calcium salts. Today, solar salt works are able to supply salt having a purity of nearly
99.5% NaCl with calcium and magnesium contents below 0.05% and sulphate content below
0.2%.
Storage of salt:
The traditional practice of covering salt heaps is now being replaced by mechanised stacking of
salt in large heaps. The larger the size of the heap, the smaller is the area exposed per unit quantity
to rainfall. As a result, the loss is reduced to less than 8% when salt is stored in heaps with height
in excess of 8 metres in quantities of more than 5000 tonnes, even when the heap is subjected to a
total rainfall of 1200 mm. Both linear and radial stackyards are used (Figure 6.11 and 6.12). The
stacking belt conveyors move on rails.
Destacking:
Stored salt over a period of time hardens. Bucket wheel excavator type destackers have now been
developed to mechanically remove the salt from the heaps.
However, in view of rising fuel costs several alternative salt refining methods that do not involve
re-crystallisation have been developed. These involve a multi-stage washing and scrubbing
process followed by centrifuging, fluid bed drying and screening.
Varieties of salt:
In the advanced countries, a high degree of sophistication has been achieved in producing several
varieties of salt for different applications. While the chemical composition of the salt is the same
in all cases the physical properties and the additives used vary to suit different applications. Some
of the varieties are iodised salt, trace mineralized salt for cattle, salt blocks, compressed water
softener salt, enriched salt dough, etc.. A process has been recently developed to fortify salt with
iron to reduce iron deficiency anaemia.
Transport:
A major feature of all recent solar salt developments is their access to direct ship loading facilities
in bulk (Figure 6.13). The high rate of loading achieved by these salt works running up to 30, 000
tonnes per day has helped them capture the large Japanese and south Korean markets for
industrial salt. In addition to the normal methods, transport, transmission of salt as a
slurry through pipelines is being adopted in some areas.
Artificial methods:
High cost of land, low temperature, high humidity, sandy soil are inhibiting factors in the
manufacture of solar salt and the Japanese have developed special techniques to overcome these
handicaps. One method uses multiple effect vacuum evaporates to concentrate brine. In
another method, initial concentration is effected by making the sea water flow down a series of
slopes resulting in partial evaporation. The resultant brine is collected, pumped up to a height
from where it drips down elongated bamboo poles which causes further evaporation. When the
required concentration is reached, the brine is fed into thermo- compression evaporators. Salt is
obtained in the form of a slurry from which impurities are then removed and the salt is dried. A
recently developed technique is the electrodialysis of sea water in chambers partitioned by ion
exchange membranes alternately arranged. By this process a high degree of initial concentration is
achieved and the concentrated brine is fed to evaporating crystallisers provided with external
heating. Salt comes out in the form of a slurry and is centrifuged and then dried. This process has
eliminated salt fields and replaced them by factories and converted salt making from an
agricultural type of operation to a purely chemical industry. Research is also being conducted to
improving desalinisation processes and recover salt as a byproduct.
Bittern salts:
Where salt fields are of medium size and the production is of the order of 50, 000 tonnes or less,
the amount of bittern recovered is inadequate for any further use. Above this level the bitterns
form the basis for recovery of bromine and magnesium and sometimes of potash on a
commercial scale. On a medium scale, production of the following salts from bitterns is now
commercially feasible:
C Magnesium Sulphate
C Magnesium Chloride, magnesium hydroxide and magnesium tri silicate
C Bromine and Bromo compounds
C Plaster of paris (from gypsum)
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APPENDIX I - BIBLIOGRAPHY
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•
Proceedings of the Second World Salt
Symposium, Vol.2, Section 4, Cleveland, Ohio,
USA, 1964.
- p~ge five -
\
APPENDIX· -
II
SOURCES OF INFORMATION ON THE SOLAR
SALT
INDUSTRY
TRAINING PROGRAMMES
TRAINING PROGRAMMES