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3. Literature survey

Salt lake brine contains various minerals such as sodium chloride, sodium sulfate,

sodium carbonate, potassium chloride, magnesium chloride, calcium chloride,

hydrogen sulfide. Salt produced must not contain these minerals more than that

specified by the buyer. This requires sodium carbonate, sodium sulfate and other

impurities to be separated.

Production of common salt from salt lake thus involves operations such as elimination

of algae from lake brine, separation of sodium sulfate and other impurities from lake

brine and then the separation of sodium chloride from brine and then drying of

sodium chloride (Buch et. al., 1958). There are many processes available for step 2

and 3. Most processes exploit differences in their solubility in brine to separate them

out. Sodium chloride is crystallized out in all the processes and hence, it is possible to

get the crystal size distribution as required by the buyer. These processes of separating

sodium chloride from brine can be distinguished on the basis of means of energy

supply for evaporation of brine. Some use solar energy where as some processes use

artificial heat by means of steam for evaporation of brine. The skill lies in separation

of sodium sulfate from sodium chloride.

3.1. Elimination of algae from brine

There are various algae present in lake brine. Due to presence of algae, brine develops

foul smell at the time of crystallization. Dead algae form a jelly which adheres to

crystals, thus colouring the salt crystals and imparting foul smell to it. This

deteriorates the quality of salt. So they must be eliminated before any processing is

done on brine. One method for removing algae is adding sodium hypochlorite

solution in such quantity that available chlorine will be 0.5 g per liter of brine and

kept under constant stirring. The brine can be filtered to get clear solution. Passing

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chlorine gas has also been reported to be convenient method for elimination of algae

present in the brine (Seshadri et. al., 1957). An average dosage of 0.5 g of chlorine per

liter of chlorine has been found to be effective in destroying the algae and preventing

their further growth even for a year after treatment. Chorine gas can be bubbled

slowly into lake brine with occasional stirring. The chlorination is carried out till the

solution is slightly alkaline. The solution is then passed through filters to get clear

brine.

3.2 Separation of impurities from brine

3.2.1. Hydrogen sulfide separation

Hydrogen sulfide present is lowered to an acceptable operating level by aeration. An

open brine trough 25 ft. long, with perforated bottom for introducing air can be used.

Aeration will reduce 20 p.p.m H2S to about 2 p.p.m.(Kauffman D, 1960).

3.2.2. Calcium removal

Calcium is mostly present in the form of calcium sulfate and calcium chloride. For

removing calcium various strategies are there which are shown below.

Reagent used Details Reference

Na2CO3 Temperature-250C; 1.8 g per liter;

90% conversion.

(Kauffman D, 1960).

Na2CO3 CA 120:138613e

Red mud Added to solar salt evaporation pool WO 206158None Membrane separation CA 105:81661c

Na2CO3 92% yield CA 105:P175257

NaF Temp 350C, batch time 2 h, yield 87.5% CA 97:94860n


Na2CO3 CA 105:P193855

NaHCO3 Yield 93.33% CA 99:197420k

3.2.3 Removal of Magnesium

Magnesium exists in the form of magnesium chloride.

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Ca(OH)2,

NaOH

(Kauffman D, 1960).

Ca(OH)2 CA 120:138613e

Red mud Added to solar salt evaporation pool WO 206158None Membrane separation CA 105:81661c

None Crystallization at 0 0C CA 103:195506v

NaOH Yield 97.5% CA 105:175257c

CaF2 Temperature 350C, batch time 2 h, yield

89%

CA 97:94860

NaOH CA 105:P193855

Ca(OH)2 CA 101:113263k

3.2.3. Sodium sulfate and sodium carbonate recovery

Separation of sodium sulfate and sodium chloride is very difficult. Various strategies

to do that are given below.

Strategy Details Na2CO3

recovery

Reference

Adsorption Silica gel, at temp above

mp below 2000C

Yes CA 112:142281x

Floatation SDS as floatation agent,

pH 8.5, purity 97.5%

No CA 117: 10901t

Solar evaporation Na2SO4 crystallizes during

diurnal temp variations.

No CA 120: 138625k

Adsorption Zr(OH)4.xH2O ( x=4-200)

resin, 20-900C,pH 3,

followed by desorption

No EP 647474

Crystallization

followed by

Allowing simultaneous

crystallization, then

No CA 101:P133343

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dissolution leaching with cold water

takes away NaCl

Reaction with

BaCl2

Temperature 80- 850C, pH

3-5, yield 88%

No CA 97:25861x

Crystallization at 00C,Na2SO4 separates

at100C, Na2CO3 separates

Yes Seshadri et al, (1958)

Crystallization At1000C,Na2SO4,Na2CO3

separate as burkeite

Na2CO3.2Na2SO4

Yes Sapre et al (1959)

3.3. Separation of sodium chloride

Sodium chloride is separated from purified brine by evaporation of brine. It can be

done by using solar energy. Another means is to use artificial heat sources such as

steam to evaporate brine (Rossiter A.P.,1986). Another method suggested is Solvent

precipitation method (Ireland D.T., 1983). It uses a organic solvent which has specific

heat 0.7 cal/g 0C, heat of vaporization less than 200 cal/g and more than 20%

solubility in water. This solvent reduces solubility of salt in brine precipitates more

than 0.15 g of NaCl for each gram of solvent added. The organic solvent is distilled

from unsaturated brine is pumped back to brine well. Total energy consumed is 1300

cal/g NaCl.

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