SODIUM HYDROXIDE PRODUCTION WITH A CALCIUM

CARBONATE SEED CRYSTAL

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making sodium
hydroxide. In another aspect, the present invention relates to a process
for the production of sodium hydroxide from trona ore. In still another
aspect, this invention relates to the control of calcium carbonate
crystal size to facilitate the separation of the solid cal cium carbonate
from the desired liquid sodium hydroxide product, by utilization of
sodium carbonate during slaking.
2. Description of the Related Art
Sodium hydroxide is a white, somewhat translucent crystalline
solid, which is also known widely in the industry as caustic soda. A
major use of sodium hydroxide is to form sodium salts, thus
neutralizing strong acids and solubilizing water-insoluble chemicals
through the formation of the sodium salt. Sodium hy droxide is also
useful in the precipitation of heavy met als as their hydroxides, and in
the control of acidity of aqueous solutions.
Because of its varied chemical activity, sodium hydroxide ?nds
utility in a wide variety of processes. For example, sodium hydroxide is
utilized in processes of making tribasic sodium phosphate, sodium
chlorite,sodium chloroacetate, sodium cyanide and sodium formate.
Sodium hydroxide is also utilized in the refining of kraft process pulp to
higher content alpha cellulose,petroleum re?ning, manufacture of
detergents, manufacture of soaps, textile processing, and metal
processing. In addition, sodium hydroxide also ?nds utility inre?ning
vegetable oils, water and acid waste stream treatment, pH control,
alkaline bottle washing formula tions and in groundwood pulp
bleaching.
Sodium hydroxide has been produced from soda ash since shortly
after the development of the Leblanc synthetic soda ash process in the
early 18th century. In this lime-soda process,dry, pure soda ash
(sodium carbonate) was dissolved in water and mixed with milk of lime
(hydrated lime in water) to form sodium hydroxide in solution and
calcium carbonate solids. Separation of the solids from the liquor
resulted in a 10 to 12 percent sodium hydroxide solution that could be
used directly or concentrated to a desired concentration. However, this
process utilizes very pure sodium carbonate, i.e., greater than 99.5%
sodium carbonate, and is known for sodium hydroxide product that is
low in metals, organic carbons and insoluble impurities. The principle
disad vantage of this process is that it requires an extremely pure soda
ash feed, which is economically prohibitive.
When the electrolytic process for caustic and chlorine was
developed, this process could compete only with great difficulty and its
use has gradually dwindled. The lime-soda process is no longer used to
produce caustic- soda for sale, but is still used by industry in
processes such as kraft recovery and to a limited extent in the
production of alumina, in which lime and soda ash are charged to
barrite digesters. Under the digester operating conditions, the reaction
is not complete, resulting in a loss of efficiency and other difficulties.
Consequently, most alumina plants prefer to charge caustic soda
directly to the digesters.
Since the development of the electrolytic process over SOyears
ago, the lime-soda process has slowly been displaced. In the
electrolytic process, saturated sodium chloride brine is fed to an
electrolytic cell where 10 to 12 percent sodium hydroxide, along with
hydrogen gas are produced at the cathode, and chlorine is produced at
the anode. The reaction may be expressed as:
The sodium hydroxide thus produced may be concentrated to the
desired concentration, generally from 50 to 73 weight percent, by
evaporation. During this evaporation process, most of the unreacted
sodium chloride crystallizes, is separated from the caustic and is
recycled back into the electrolytic cell feed system.
This process is economically viable if the chlorine by-product can be
sold. However, in recent times the demand for chlorine has not
maintained a balance with sodium hydroxide demand. Environmental
concerns related to chlorine containing compounds impact the demand
for chlorine as the use of chlorine containing compounds is curtailed or
prohibited. Examples of such chlorine containing compounds include
vinyl chloride, a number of highly chlorinated high volume pesticides,
ethylene dichloride as lead scavengers in gasoline, ?Uo rocarbon
aerosol propellants, polychlorinated biphenyls, and chlorine and
chlorine-containing chemicals used water and sewage treatment.
Accordingly, the need exists in the industry for an economical
process for the production of sodium hy droxide, without chlorine
coproduction.

SUMMARY OF THE INVENTION

According to one embodiment of this invention there is provided a

method of producing sodium hydroxide from trona ore, which utilizes
appropriately sized seed crystals in causticization to produce larger
sized crys tals of calcium carbonate to facilitate an easier separa tion
of the liquid sodium hydroxide and the calcium carbonate crystals. The
method comprises: (a) contacting trona ore with sodium hydroxide to
form sodium carbonate and water; (b) removal of solids greater than
32 mesh from the product of step (a); (c) reacting the sodium
carbonate of step (b) with calcium hydroxide in the presence a suitable
amount of calcium carbonate seed crystals with crystal size between
about 1 to about 10 microns to form calcium carbonate crystals with
particle size between about 30 to about 150 microns; and (d)
separation of the sodium hydroxide and calcium carbonate in step (c).
The seed crystals utilized in step (c) may be produced by any method.
In a preferred embodiment of the present invention, the seed crystals
are produced insi'tu in the lime slaker by reacting calcium oxide and
water in the presence of sodium carbonate. As the lime is slaked to
form calcium hydroxide, some of calcium hydroxide and sodium
carbonate react to form the calcium carbonate seed crystals which are
introduced into the causticization with the calcium hydroxide.
BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic process diagram of one embodi ment of the

present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, sodium hydroxide is produced from trona

ore. The process of the present inven- 5 tion may best be described by
reference to the process ?ow diagram of FIG.1.
In the dissolution step of the present process, stream 1 comprising
trona ore and stream 2 comprising sodium hydroxide are contacted
together in a dissolution sys tem under suitable conditions to produce
sodium carbonate and water.
The dissolution temperature must at least be high enough to allow
the trona to be easily solubilized into solution, and therefore allow for a
reasonable and eco nomic retention time in the dissolution step. To
avoid having to provide costly pressure vessels and equip ment, the
upper range for the dissolution temperature is generally not much
higher than the boiling point of the dissolution solution. Suitable
dissolution temperatures generally include temperatures in the range
of about 100 F._ to about 250 F. Preferably, the dissolution
temperatures are in the range of about 120 F. to about 220 F.
Trona dissolution may be conducted under a broad range of
pressures, ranging from a vacuum to high pressures. However, to avoid
costly pressure equipment and process difficulties, trona dissolution is
generally con ducted at or near atmospheric pressure. Suitable trona
dissolution pressures are generally in the-range of about 0 psig to
about 50 psig, and preferably at about atmospheric pressure.
The dissolution residence time will be dependent upon the solubility
of the trona ore in the dissolution solution. Generally dissolution
residence times will range up to about 2 hours. Preferably, dissolution
resi dence times are in the range of about 10 minutes to about 60
minutes, most preferably in the range of about 15 minutes to about 50
minutes.
Trona ore is generally obtained from naturally occur ring
subterranean deposits, most of which are in Wyo ming, and consists
mainly of sodium sesquicarbonate, Naz-CO3 NaHCO3-2l-I2O. The trona
ore contains com paratively high quantities of insoluble matter, organic
carbon, and other impurities, for example such as iron and silica.
In an alternative embodiment not shown, trona ore may be
converted into sodium carbonate by heat calcining instead of
chemically with sodium hydroxide.
Stream 2 is an aqueous stream that generally com prises in the
range of about 5 to about 50 weight percent sodium hydroxide, and
preferably in the range of about 10 to about 12 weight percent sodium
hydroxide. The sodium hydroxide of stream 2 may be supplied from an
external source and/or supplemented by sodium hydroxide recycle
from within the process. In the embodiment shown, stream 21 is
sodium hydroxide supplied from an external source, and steam 22 is
dewatered sodium hydroxide recycled from the causticization step.
The dissolution of trona ore via its reaction with sodium hydroxide
results in the release of water of hydration from the trona ore and is
represented by:
The dissolution of the trona ore is accomplished in one or more
agitated tanks that may be arranged in parallel or series con?guration.
The composition of the ?nal aqueous dissolution product (stream 3)
will be limited by its solubility at the operating conditions. Generally,
the ?nal aqueous dis solution product comprises in the range of about
10 to about 30 weight percent sodium carbonate.
The dissolution step product (stream 3) is then pro cessed to
remove solids and other insolubles. These solids and insolubles are
generally introduced into the system via the trona ore. It is important
to remove these solids before substantial solubilization of the solids
has occurred by caustic and sodium carbonate solution present during
dissolution.
Generally, particles larger than about 32 mesh should be removed
in the solids separation step following the dissolution. Preferably,
particles larger than about 325 mesh should be removed in the solids
separation.
These solids may be removed by any suitable tech nique. Methods
of removing solids from liquids are well known to those of skill in the
art, and include methods utilizing ?lters, rake or screw classi?ers,
decanters, thickeners and centrifuges.
The removed particles are disposed of in a suitable manner via
stream 5. In the embodiment shown, the aqueous solution of sodium
carbonate, stream 6 is then further processed, with part of the stream
fed to the slaker, and most fed to the causticizer. This aqueous solution
generally comprises in the range of about 10 to about 30 weight
percent sodium carbonate.
In addition to sodium carbonate of stream 12, sodium carbonate
may optimally be recycled into the causti cizer from the causticizer
dewaterer (steam 16) and from the solids removal step (stream 30).
Generally an excess of up to about 40 mole percent of sodium
carbonate is present in the causticizer to drive the reaction toward
sodium hydroxide.
In the causticization step, sodium carbonate and calcium hydroxide
(milk of lime) are contacted together under conditions suitable to
form sodium hydroxide solution and calcium carbonate crystals. The
reaction may be represented as follows:
Causticization of sodium carbonate with calcium hydroxide is well
known. Suitable causticization tem peratures are generally at
temperatures near the boiling point of the causticization liquid.
Generally such tem peratures are in the range of about 50' F. below the
boiling point to about the boiling point of the liquid, and preferably in
the range of about 10' F. below the boiling point to about the boiling
point of the causticizer liquid. Typical causticization temperatures for
near ambient pressures are in the range of about 100 F. to about
220oF.
The causticization may be conducted at a wide range of pressures
ranging into the hundreds of pounds per square inch. However, for
ease of operation, and pro cess economics low pressures are generally
utilized. Typical causticization pressures range up to about 100 psig,
and are preferably at about ambient atmospheric pressure.
Without the proper crystal habit, the calcium carbonate crystals
formed in the causticizer will be ?nes on the order of 10 microns.
Such ?nes result in high settling times, and make it very dif?cult to
economically sepa rate the calcium carbonate crystals from the desired
sodium hydroxide.
According to one embodiment of the present inven tion an
economical way of separating the sodium hy droxide and calcium
carbonate is by making larger calcium carbonate crystals which are
easier to separate from the liquid sodium hydroxide.
Larger calcium carbonate crystals can be obtained by introducing a
sufficient amount of seed crystals of cal cium carbonate into the
causticizer, so that larger parti cles of calcium carbonate will be formed
in the causti cizer. Generally, the introduction of calcium carbonate
seed crystals on the order of about 1 to about 10 microns will allow the
production of calcium carbonate crystals in the causticizer on the order
of about 30 to about 150 microns. Thus the calcium carbonate crystals
are of suf?cient particle size and density so that an economic settling
time may be achieved in the causticizer, and separation of the calcium
carbonate crystals and the desired sodium hydroxide is more easily
facilitated.
The calcium carbonate seed crystals may be added to the
causticizer from an outside source. In the embodi ment shown, the
calcium carbonate crystals are produced insitu in the lime slaker and
then fed to the causticizer in the milk of lime.
 The calcium carbonate seed crystal concentration may vary from
about 0.2 to about 20 weight percent based on the weight of the
reactants. Preferably the concentration is in the range of about 2 to
about 6 weight percent. '
Generally the calcium hydroxide utilized in the caus ticizer is
produced as milk of lime in a slaking process in which lime (calcium
oxide) is reacted with water. The presence of a small amount of sodium
carbonate in the lime slaker produces a small amount of about 1 to
about 10 micron calcium carbonate crystals through reaction with the
calcium hydroxide. It is this calcium carbonate that helps to control the
ultimate particle size of the calcium carbonate crystals in the
causticizer.
Generally sodium carbonate is present in the lime slaker in an
amount suitable to form suf?cient calcium arbonate nuclei to properly
control the crystal habit in the causticizer. Generally sodium carbonate
is present in the lime slaker in an amount in the range of about 0.2 to
about 20 weight percent, based on the total weight of the slaker
reactants. Preferably, sodium carbonate is present in the slaker in an
amount in the range of about 4 to about 16 weight percent, and
preferably in the range of about 4 to about 6 weight percent.
In the embodiment shown, sodium carbonate may be fed to the
lime slaker via stream 11 from the solids separation step, or recycled
from the solids removal step (stream 30) or from the causticization
dewatering step (stream 24). Optionally, sodium carbonate may be
supplied to the lime slaker from an external source.
The lime or calcium oxide utilized in the slaker (stream 4) may be
from an external source (stream 7) and/or recycled from the
causticizer, once it has been dewatered and calcined (stream 8).
Once the calcium oxide and water have been reacted in the slaker
to form calcium hydroxide, the resulting product must be processed to
remove particles such as ?nes, grit and other insolubles. The major
impurity to be removed is insoluble silica which comprises in the range
of about 2 to about 3 weight percent of the lime being fed to the slaker.
The amount of silica removed in the grit removal step will depend
on the caustic product desired. Generally the ?nal caustic product will
comprise less than about 800 ppm silica, and preferably less than
about 500 ppm silica.
The grit removal step is generally accomplished by any suitable
technique. Methods of removing solids from liquids are well known to
those of skill in the art, and include methods include ?lters, rake or
screw classi?ers, decanters, thickeners and centrifuges. .
In the embodiment shown, stream 9 from the slaker comprising the
particulate material is fed to the lime slaker. The removed particle
material is removed via stream 14 in any suitable manner. The calcium
hydroxide product is then fed to the causticizer via stream 10.
After sodium carbonate and calcium hydroxide have been reacted
in the causticizer to form aqueous sodium hydroxide and calcium
carbonate crystals, this resultant product mixture is then processed in
a dewatering step in which any appropriate means may be utilized to
separatethe solid calcium carbonate crystals from the aqueous sodium
hydroxide solution. Such methods in clude countercurrent decantation,
or countercurrent ?ltration. In such multistage processes, aqueous
sodium hydroxide is recovered from the ?rst stage as product with
optionally part of the sodium hydroxide recycled to assist in trona ore
dissolution. Water is generally fed to such multistage processes from
the bottom or last stage and flow countercurrent from the solids and
exists in the second to last stage with alkali values. The calcium
carbonate crystals are recovered in the bottom stage and recovered
via centrifuging and/or ?ltering.
In the embodiment shown, causticizer product stream 13 is fed to
the dewatering step. Sodium carbonate is recycled via stream 15 to
the causticizer (stream 16) and/or to the slaker (step 24). Water is
supplied to the dewatering step via stream 28. Calcium carbonate is
recovered via stream 17 where it may be utilized exter nally via stream
18, or recycled via steam 19 to the calciner and ultimately recycled
back into the slaker via stream 8.
Calcium carbonate from the last wash stage can be sent for
reburning to be converted into calcium oxide (lime) by any of the well
known methods, or recovered
as product to be used externally. Alternatively, the calcium carbonate
could be recycled into the causti cizer to function as sud crystal.
Aqueous sodium hydroxide is recovered via stream 16 and may be
recycled via stream 22 to the trona disso~ lution step, or further
processed via stream 23. The
sodium hydroxide product of stream 23 is ?rst concen trated by water
evaporation. Evaporation product stream 31 is fed to a solids removal
step where sodium carbonate crystals are recovered via stream 26
which may be recovered as external product via stream 29 and/or
recycled. Sodium carbonate recycle stream 30 may be diluted with
water stream 27 both of which comprise causticizer feed steam 32.
Sodium hydroxide product is recovered from the solids removal
step as 50% aqueous sodium hydroxide (stream 25).