Field of the invention :
The present invention relates to a process for the recovery of sodium sulphate from the brine resulting from Nanofiltration system in chloralkali process. The sodium soleplate recovered from the process of the present invention is stable, crystalline and anhydrous which is useful for paper, detergent and pharma industries. The process of the present invention not only produces commercially valuable, stable, crystalline and anhydrous sodium sulphate but also but also helps in avoiding of sludge and waste generation thereby avoiding creating environmental pollution.
Background of the invention
In the chloral kali process, purified brine is fed to the electrolysers for the manufacture of Sodium hydroxide and chlorine. Pure brine is prepared from raw industrial grade salt (sodium chloride). Raw salt consists of impurities like calcium, Magnesium and soleplate. The concentration of the impurities varies and has to be precipitated and separated using sodium salts; barium salts etc to make the brine free of impurities to be fed to the membrane electrolysers.
In the membrane-cell chloralkali process, industrial grade salt is dissolved in a saturator and the brine with 280 - 300 gpl of sodium chloride is treated with soda ash, barium salts and sodium hydroxide for the removal of calcium, magnesium and suphate impurities. The clear brine free of impurities is fed to the electrolysers.

The electrolysers consist of anode, and cathode separated by an ion-exchange membrane. The ion-exchange membranes are sensitive to impurities such as calcium, magnesium and sulphates.
The sodium chloride in the brine fed to the electrolyser is electrolysed and sodium and water pass through the membrane electrochemically thus forming sodium hydroxide and hydrogen at the cathode and depleted brine and chlorine at the anode.
The brine coming out after the electrolysis is called the depleted brine. The sodium chloride concentrations in the depleted brine vary between 200 - 220 gpl. This brine is dechlorinated and recycled.
Membrane electrolysers particularly, membranes in chloralkali process, demand the best of brine treatment process. Stringent specifications on the presence of divalent and metallic ions in the brine fed to the electrolysers and the presence of anions such as sulphate above certain concentrations have made the brine treatment process challenging and complicated.
The divalent calcium, magnesium and sulphate impurities present in the brine come from the industrial grade salt and have to be virtually removed to render the brine suitable for the membrane electrolyser in chloralkali industry.
The principle anionic contaminant in the Industrial grade salt is the sulphate. Sulphate forms part of the calcium and magnesium salt present in the raw salt. In the chlor-alkali plant with brine recycle, the sodium sulphate which

is a soluble entity has to be periodically removed to maintain the concentration in the brine that is fed to the electrolysers. Various technologies are being employed currently in chloralkali industry for the removal of the sulphate in brine.
The methods employed are :
1) Addition of barium salts to the raw brine to precipitate the sulphate as barium sulphate.
2) Addition of calcium salts to the raw brine to precipitate out as calcium sulphate.
3) Purging of the depleted brine or brine with high concentration of sulphate to maintain the concentration of sulphate in the brine feed to the membrane electrolysers. Purging of brine is not allowed as per the norms of Pollution Control Board.
4) Removal of sodium sulphate from brine by nanofiltration, and purging out the reject brine with sodium sulphate.
The first two processes require addition of barium chloride or Barium carbonate or Calcium salts to the raw brine for the precipitation of the sulphates in the brine. The use of barium salts has the following disadvantages:

1) Availability of the barium salts on a long run as these salts are naturally occurring minerals and are likely to be exhausted in the near future.
2) Barium is toxic and is not environmentally friendly chemical.
3) Addition of Barium salts will also add other divalent and metallic ions in
the brine which will affect the performance of the ion-exchange
membrane in the electrolyses.
4) These chemicals generate sludge, which has to be disposed off as per the
pollution control board norms.
5) Use of calcium salts will lead to sludge formation and increased calcium
levels in the brine, which has to be ultimately precipitated using sodium
salts. The generation of sludge will pose disposal problems and increased
use of sodium salts for the treatment of the brine will add on to the
chemical treatment costs.
The amount of sludge generated varies with the quality of the raw material i.e., the salt (Industrial grade sodium chloride). The sludge generation will be to the tune of 52 -80 kgs/MT of salt treated.
Prior art
US patent 4556463 : This patent is about the removal of sulphate impurities from alkali metal chloride brine with a weakly basic anion exchange medium. The anion exchange resin regeneration solution has sulphate content to the extent of 15 - 18 gpl is recovered by cooling as decahydrate.

This patent is about recovering sulphate from the ion exchange regenerant solution having a sulphate content of 15-18 gpl. Moreover in this process the brine have to be diluted for the sulphate recovery because of the limitation of using ion exchange resins. Such dilution necessitates evaporation for concentration which makes the process energy intensive. Hence such a process is not economical.
.US Patent 4,702,805 relates to the control of sulphate by precipitating sodium sulphate from the electrolyte stream in a crystalline chlorate plant.
The process disclosed in this patent relates to separation of sodium sulphate in the brine of a chlorate plant. This method is not concerned with chlor alkali brine Further if such a process is applied to the precipitation of sodium sulphate in the brine of chlor alkali process , only sodium deca hydrate is produced which is unstable at the ambient temperatures. Hence by employing the process disclosed in the above said US patent is applied to chlor alkali brine stable sodium sulphate cannot be produced.
US Patent 4,636,376 discloses removal of sulphate in aqueous chromate-containing sodium chlorate liquor using calcium salts, which forms a precipitate and is separated.
US Patent 5,071,563 : Describes the selective removal of sulphate using zirconium hydroxide slurry under acidic conditions. This process is more labour & energy intensive process and hence not economical

The recovery of sodium sulphate from the depleted brine coming out of the membrane electrolyser of chlor alkali plant producing caustic soda has become very essential to prevent its discharge on to the ground or sea which creates environmental pollution. Further a complicated treatment involving addition of toxic chemicals and generation of sludges one has to think of cleaner technologies for the future of the Chloralkali industry to eliminate or reduce the generation of sludges.
Hence, a process for the treatment of raw brine and recovery of sulphate from the brine as sodium sulphate overcoming the drawbacks of the hitherto known processes will help in eliminating the use of toxic chemicals like barium and calcium salts, avoid the problem of waste disposal, such a process will also be environmental friendly.
Objectives of the invention
The main objective of the present invention is, therefore, to provide a process for the recovery of stable, crystalline & anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process
Yet another objective of the present invention is to provide a process for the recovery of stable, crystalline and anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process ,as anhydrous sodium sulphate and Sodium sulphate Decahydrate by concentration and separation of sodium sulphate.

Still another objective of the present invention is to provide a process for the recovery of stable crystalline & anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process,
without the formation of any sludge thereby making the process more easy and viable.
Yet another objective of the present invention is to provide a process for the recovery of stable, crystalline and anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process which is clean and environment friendly.
Still another objective of the present invention is to provide a process for the recovery of stable, crystalline and anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process which reduces the liquid effluent thereby making the process eco-friendly.
The process of the present invention has been developed based on our findings after sustained research & development that sodium sulphate present in the depleted brine coming out of the nanofiltration membrane system which can be recovered without the addition of any chemicals. The process has been developed based on the fact that the solubility of sodium sulphate in water containing sodium chloride decreases at low temperatures. By maintaining the specified temperature of the brine the sodium sulphate in the brine separates out as decahydrate. The separation of sodium sulphate is

also assisted by the presence of sodium chloride in sufficient concentration
in the brine.
Such a process is not hitherto known and it is for the first time such a
process being reported and hence the process is novel
According to the present invention stable, crystalline & anhydrous sodium
sulphate is recovered as anhydrous sodium sulphate in two stages which is
more stable than sodium sulphate decahydrate.
The process of the present invention is explained with the help of the flow chart given below


Summary of the invention
Accordingly the present invention provides a process for the recovery of stable , crystalline and anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process, which comprises
(i) cooling the brine obtained from the nanofiltration membrane system , in a reactor (I) with constant stirring to a temperature in the range of -20 to + 2 Deg C and at a pH in the range of 3 to 12 to form sodium sulphate deca hydrate.
(ii) Centrifuging the cooled mass of sodium sulphate decahydrate obtained in step (i) at a RPM in the range of 600 -1200 and separating the solid mass
iii) Recycling the liquid phase obtained in step (ii) to the reactor (1).
(iv) dissolving the sodium sulphate decahydrate in saturated sodium sulphate solution and heated to a temperature in the range of 100-150 deg C in a Reactor (II) To get a slurry containing anhydrous sodium sulphate
(v) Centrifuging the resulting slurry to separate out stable ^crystalline and anhydrous sodium sulphate and leaving the mother liquor
(vi) Recycling the mother liquor obtained in step (v) to reactor (II) and
(vii) drying the stable , crystalline and anhydrous sodium sulphate obtained in step (v) at a temperature between 60-120 deg C

In an embodiment of the present invention the liquid phase obtained in step (i) may be recycled to the saturator of the chlor alkali brine treatment system
The characteristics of the brine obtained from the nanofiltarion membrane of a chlor alkali process may usually have on an average a composition given in the table 1

Detailed description about the process of the present invention
The brine obtained from the nanofiotration membrane rejects was collected in the reactor (i) and stirred and cooled to a temperature between -20 to +2 deg C. Due to the property of low solubility of sodium sulphate at lower temperatures and also due to the presence of sodium chloride, sodium sulphate decahydrate separates out due to common ion effect.
The slurry mass is put into a centrifuge and the solid and liquid separated. The mother liquor which comes out of the centrifuge was collected and recycled to the reactor (I) . In an embodiment of the invention the mother liquor may also be put back into the chloralkali brine system via the saturator.

The composition of the mother liquor obtained on an average will be as follows:
a) Sodium chloride: 190 - 220 gpl
b) Sodium sulphate: 10 - 35 gpl c)PH:5-8
The composition of the solid sodium sulphate decahydrate separated will be on an average:
a) Assay as sodium sulphate: 35 - 38 %
b) Sodium chloride : 4 - 5 %
c) Moisture including hydrated : 58 - 60 %
The solid mass of sodium sulphate decahydrate obtained from step (iii) was made to dissolve in a saturated solution of sodium sulphate in reactor (II). Initially the saturated sodium sulphate solution was prepared using the anhydrous sodium sulphate as a start-up media. The saturated solution of sodium sulphate used may have the concentration in the range of 250 -400 gpl
The reaction mixture is heated to 100 - 150 deg C where the decahydrate will lose the water of crystallization and convert to anhydrous sodium sulphate and settle at the bottom of the reactor(ii).

The solid mass will be put in a centrifuge and separated as anhydrous sodium sulphate. The liquid phase is again recycled to the reactor(II). The liquid phase on an average may have the following composition:
a) Sodium sulphate : 390 - 410 gpl
b) Sodium chloride : 15-30 gpl c)pH:6-8
The solid mass of sodium sulphate: resulting from step (v) may have the following composition
a) Anhydrous sodium sulphate: 93 - 95%
b) Sodium chloride : 0.2 - 0.5 %
c) Moisture : 4.5-6.8%
The stable crystalline sodium sulphate obtained in step (v) was dried preferably in a hot-air oven between 60-120 deg C to get stable, crystalline and anhydrous sodium sulphate,
The overall recovery of the stable, crystalline & anhydrous sodium soleplate according to the process of the present invention may be between 70 - 85 %.
The details of the invention are given in the Examples given below which are provided to illustrate the invention only and therefore should not be construed to limit the scope of the invention

Example 1
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 10 by addition caustic soda. The liquid was cooled to zero deg C with constant stirring. A solid mass of sodium sulphate decahydrate was separated. The solid material was centrifuged. At an RPM of 600.to separate the solid phase from the liquid slurry which contains sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate. The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 105 deg C.
Example 2
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 4 by addition of hydrochloric acid. The liquid was cooled to zero deg C with constant strirring. A solid mass of sodium sulphate decahydrate was separated. The solid material was centrifuged. at an RPM of 600.to separate the solid phase from the liquid slurry which contains sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The

Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate. The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 105 deg C.
Example 3
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and maintained at pH to 7. The liquid was cooled to zero deg C with constant strirring. A solid mass of sodium sulphate decahydrate was separated. The solid material was centrifuged. at an RPM of 600.to separate the solid phase from the liquid slurry which contains sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium soleplate. The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 105 deg C.
Example 4
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 8.5 by addition caustic soda. The liquid was cooled to

- 4 deg C with constant stirring. A solid mass of sodium sulphate
decahydrate was separated. The solid material was centrifuged. at an RPM
of 600.to separate the solid phase from the liquid slurry which contains
sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate same the solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 105 deg C.
Example 5
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 8.5 by addition caustic soda. The liquid was cooled to
- 20 deg C with constant stirring. A solid mass of sodium sulphate
decahydrate was separated. The solid material was centrifuged. at an RPM
of 600.to separate the solid phase from the liquid slurry which contains
sodium sulphate deca hydrate 00
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulpate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate asame The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the

bottom of the reactor, which was removed centrifuged and dried. In a hot air oven at 105degC.
Example 6
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 8.5 by addition caustic soda. The liquid was cooled to + 2 deg C with constant stirring. A solid mass of sodium sulphate decahydrate was separated. The solid material was centrifuged. at an RPM of 600.to separate the solid phase from the liquid slurry which contains sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate. The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 105 deg C.
Example 7
Taken 1 lit of the reject sample containing 120 gpl of the sodium chloride and adjusted the pH to 8.5 by addition caustic soda. The liquid was cooled to + 2 deg C with constant stirring. A solid mass of sodium sulphate decahydrate was separated. The solid material was centrifuged. at an RPM

of 600.to separate the solid phase from the liquid slurry which contains sodium sulphate deca hydrate.
The solid sodium sulphate decahydrate separated as described above was dissolved in saturated sodium sulphate solution in another reactor. The Saturated sodium sulphate solution (having concentration 400 gm / liter) was prepared using the AR sodium sulphate. The solution was boiled to 120 deg C for about 1 hour. The anhydrous sodium sulphate settled at the bottom of the reactor, which was removed centrifuged and dried, in a hot air oven at 70 degC
Advantages of the process:
1) The process is clean and environment friendly.
2) There is no sludge formation
3) Reduction in the liquid effluent.
5) Production of valuable stable, crystalline & anhydrous sodium
sulphate which is useful for paper, detergent and pharma industries.
6) The process results in the recovery of sodium sulphate in the purest
form (99.8 %).
7) No discharge of brine to land or sea.

We Claim
1. A process for the recovery of stable, crystalline and anhydrous sodium sulphate from the depleted brine coming out of the nanofiltration membrane system of a chlor alkali process, which comprises
(i) cooling the brine obtained from the nanofiltration membrane system, in a reactor (I) with constant stirring to a temperature in the range of-20 to + 2 Deg C and at a pH in the range of 3 to 12 to form sodium sulphate deca hydrate.
(ii) Centrifuging the cooled mass of sodium sulphate decahydrate obtained in step (i) at a RPM in the range of 600 -1200 and separating the solid mass
iii) Recycling the liquid phase obtained in step (ii) to the reactor (1).
(v) dissolving the sodium sulphate decahydrate in saturated sodium sulphate solution and heated to a temperature in the range of 100-150 deg C in a Reactor (II) To get a slurry containing anhydrous sodium sulphate
(viii) Centrifuging the resulting slurry to separate out stable ,crystalline and anhydrous sodium sulphate and leaving the mother liquor
(ix) Recycling the mother liquor obtained in step (v) to reactor (II) and

(x) drying the stable , crystalline and anhydrous sodium sulphate obtained in step (v) at a temperature between 60-120 deg C
2. A process as claimed in claiml wherein the liquid phase obtained in step
(i) is recycled to the saturator of the chlor alkali brine treatment system
3. A process as claimed in claims 1 or 2 wherein the characteristics of the
brine obtained from the nanofiltarion membrane of a chlor alkali process is
on an average a composition given in the Table 1

4. A process as claimed in claims 1 to 3 wherein Te composition of the
mother liquor obtained on an average is follows:
a) Sodium chloride : 190 - 220 gpl
b) Sodium sulphate : 10 - 35 gpl
c)pH:5-8

5. A process as claimed in claims lto 4 wherein the composition of the solid sodium sulphate decahydrate separated is on an average:
a) Assay as sodium sulphate : 35 - 38 %
b) Sodium chloride : 4 - 5 %
c) Moisture including hydrated : 58 - 60 %
7. Stable, crystalline and anhydrous sodium sulphate wherever prepared
by the process as claimed in claims 1.
8. A process for the recovery of stable , crystalline and anhydrous
sodium sulphate from the depleted brine coming out of the nanofiltration
membrane system of a chlor alkali process substantially as herein described
with reference to the Examples 1-7