Title of Invention

A PROCESS FOR PURIFICATION OF SODIUM CHLORIDE CONTAINING SODIUM SULPHATE IMPURITY

Abstract

A process for purification of sodium chloride containing sodium impurity which has definite composition. The invention consists of a composition of Didwana salts found in the Didwana region of Rajasthan State containing less percentage of impurities which are within the permissible limit for industrial grade salt.The principle of the process is based on the phase equilibrium of NaCl-Na2SO4-H2O.The salt obtained is the upgraded salt. It provides Glauber"s salt and is a cyclic process with cost effectiveness. The resultant salt is named as salt efficient brine (SDB) . Moreover, induced crystallization in the invention makes the process energy efficient which with high industrial applicability.

Full Text

This invention relates to a process for purification of the sodium chloride containing sodium sulphate impurity. The present invention particularly relates to separating or removing impurities of sodium sulphate from common salt of Didwana region of Rajasthan State by simple cyclic process having the induced crystallisation step, for the purification of Didwana salt as described hereunder. Common salt (NaCl), a basic raw material of vital importance for human consumption and industry, is generally obtained from inland and sea brine by solar evaporation. India produces about thirteen million tonnes of salt per year. Nearly ten per cent of this salt production goes to the credit of Rajasthan state. Nagore district of Rajasthan produces about 75 per cent of total salt production of this state. Didwana salt works provides nearly 2 lakh tonnes of common salt per year. The principal anionic impurity in this salt is sulphate. The utilisation of salt in industry namely caustic soda, soda ash, dairy butter-cheese etc. depends on the quality of salt and the extent of impurities in it. According to I.S.I. Specification of industrial grade-II salt (I.S. 797-1982), for chemical industry the minimum sodium chloride content should be 98.5 per cent and maximum permissible impurities are calcium 0.2, magnesium 0.1, sulphate 0.6, carbonate (as Na2CO3) 0.5 and insolubles 0.2 per cent respectively on dry basis. Didwana salt contains 8 to 16 per cent Na2SO4 as impurity, other impurities namely calcium, magnesium etc. are within permissible limit for industrial grade-II salt. On account of very high contents of Na2SO4, this salt is not suitable for human consumption and industrial use. In a chlor-alkali industry, salt is accepted which contains less than one per cent of sodium sulphate, so that it can be purified with minimum chemical treatment. We have developed a process for purification of common salt containing impurity of Na2SO4 to meet the requirement of grade- II salt and simultaneous recovery of sodium sulphate. V.H. Shah et al. [Res. & Ind. (India) 31, 162 (1986)] have developed a process for upgradation of Didwana salt. The process was based on the principle of phase equilibrium of NaCl-Na2SO4-H2O system. In this work salt containing 13 per cent of Na2SO4 and 84 per cent NaCl was upgraded for the salt having 97-98 per cent of NaCl. But this process needs a lot of energy, especially for getting 0°C for chilling purpose. Similarly, N. Sutherland [Seventh Symposium on Salt, Vol.1, 599 (1993) edited by H. Kakihana, H.R. Hardy, Jr., T. Hoshi, K. Toyokura, Elsevier Science Publishers, B.V. Amsterdam] has studied the inland brine containing NaCl and Na2SO4 and developed a method which is weather dependent. Many of salt mines in China contain 270-300 gram/litre of Na2SO4, therefore, H. Juemin [Seventh Symposium on Salt, Vol.11, 165(1993) edited by H. Kakihana, H.R. Hardy, Jr., T. Hoshi, K. Toyokura, Elsevier Science Publishers, B.V. Amsterdam] developed a process for combined production of NaCl and Na2SO4 from these sources. Heating the salt solution to nearly 100°C in this method, rendering the process energy intensive, creates corrosion and scaling problems alongwith handling of the system. The main object of the present invention is to develop a simple cyclic process for the purification of salt containing Na2SO4 impurity by avoiding all above drawbacks. Another objective is that the purified salt should be acceptable to chemical industry especially chlor-alkali industry. Besides purified salt the process offers Glauber's salt also which is the additional objective of this invention. The process conditions are established from phase equilibrium study containing ternary system i.e. NaCl-Na2SO4-H20. To deal with such system is also one of the objectives. Further, induced crystallisation step is the inventive step which makes the process energy efficient, is our yet another object of this proposed invention. There are two kinds of impurity of sodium sulphate attached with Didwana salt containing sodium sulphate impurity in the range of 8 to 16 per cent, viz. trapped and superficial. The superficial impurity of Na2SO4 is easier to get rid of, than trapped impurity. Grinding of salt to very fine sizes helps in removing superficial impurity and hence, Didwana salt was crushed to the required size of 1 mm to 0.5 mm and used in this invention. Suitable seed material (Na2Cr04.10H2O/Na2SO4.10H2O/Na2SO4) were selected on the basis of "epitaxis" phenomenon considering the fact that nucleation may occur when the crystallographic parameters of the nucleation catalyst and the salt to be crys- tallised agree within 15 per cent. Accordingly the present invention provides a process for purification of sodium chloride containing sodium sulphate which comprises : i) contacting crushed sodium sulphate contaminated sodium chloride with sulphate deficient brine (SDB) for a period in the range of 15-45 minutes at a temperature in the range of 16-22°C to obtain purified sodium chloride and sulphate efficient brine (SEB); ii) cooling the SEB to a temperature in the range of 5 to 10°C, adding 300-1000 ppm of seeding material to the cooled SEB to induce crystallisation of Glauber's salt, separating the Glauber's salt and the resultant sulphate deficient brine (SDB) by known methods; iii) repeating step (i) if desired using SDB obtained in step (ii) . In an embodiment the contaminated sodium chloride may be crushed to a size in the range of 1 mm to 0.5 mm. In another embodiment of the present invention the seeding material used may be such as Na2SO4, Na2SO4.10H2O of particle size in the range of 0.5 mm to 0.2 mm. In yet another embodiments the seeding materials may be as such or mixed with clay particles of size 0.3 mm to 0.1 mm. The process of the present invention consists of following steps : 1) Didwana salt containing sodium sulphate impurity in the range of 8 to 16 per cent was contacted with sulphate deficient brine (SDB) for suitable time (15 to 45 minutes) under appropri ate temperature (16 to 22°C) conditions to get purified salt. 2) Alongwith purified salt, sulphate efficient brine (SEB) was also obtained simultaneously. 3) This SEB was cooled to temperature (5 to 10°C) and properly ground seeds (0.5 mm to 0.2 mm) of sodium sulphate/sodium sul phate Decahydrate (as such or mixed with clay particles) was added (300-1000 ppm) to it. 4) Glauber's salt - Na2SO4, 10H2O - was removed and washed with cold (5 to 10°C) 3 to 5 per cent solution of Na2SO4. After removing Glauber's salt (Na2SO4.10H2O) from the solution, the resultant solution is saturated with NaCl but unsaturated with respect to Na2SO4. 5) This sulphate deficient brine (SDB) was made up with water or low sulphate brine. The volume diminution taking place on cool ing the sulphate efficient brine, is due to removal of water as decahydrate with Na2SO4 and removal of good quantity of dissolved Na2SO4 as solid. 6) Now the sulphate deficient brine was contacted with further quantity of impure common salt. The proposed process is cyclic in nature and the end liquor can be used again and again, thus further saving in cost input. The process is further illustrated with the help of following examples and should not be construed to limit the scope of the invention. Example-1 a) Didwana salt containing 16 per cent sodium sulphate impurity was added to Didwana brine containing 19 per cent NaCl and 6 per cent Na2SO4 or water at 20°C and solution was stirred for 30 minutes to get solution containing 28 per cent NaCl and 7 per cent Na2SO4. b) 204 litres of above solution, as per capacity of cooling container, was cooled to 9°C, and Na2SO4 (500 ppm) was added to induce the crystallisation of Glauber's salt (Na2SO4.10H2O). Glauber's salt and sulphate deficient brine (28 per cent NaCl containing 4 per cent of Na2SO4) was separated by known methods. c) 15 litres of water or brine containing 19 per cent of NaCl and 6 per cent of Na2SO4 was added to sulphate deficient brine (SDB). d) This diluted SDB was contacted with 20 Kg of Didwana salt containing 16 per cent of Na2SO4 for 30 minutes at 20°C. Solid to liquor ratio was kept 1:4 (W/V). Resultant purified salt was analysed for sulphate contents with standard gravimetric method. Sodium sulphate impurity was reduced to only 0.6 per cent from 16 percent. This purified salt with a little chemical treatment can be used in chlor-alkali industry. Apart from purified salt, sulphate efficient brine (SEB) was also obtained by this step, which was processed as per step (b). e) Obtained Glauber's salt (Na2SO4.10H2O) containing 10 per cent impurity of NaCl was washed with 5 per cent solution of Na2SO4. The temperature of the wash solution was 10°C. The Glauber's salt was dried at 110°C for 4 hrs. to get 98 % pure anhydrous sodium sulphate. Alternatively, Glauber's salt was mixed with calculated quantity of crushed Didwana salt (0.4 mm) containing 16 per cent Na2SO4 at 60°C, the resultant anhydrous Na2SO4 was 99 per cent pure. This alternative method is less energy intensive compared to drying of Glauber's salt. Rxample-2 10 Kg. of common salt containing 12 per cent impurity of Na2SO4 was contacted with sulphate deficient brine, obtained in example 1, Step (b), for selected time (30 minutes) and temp. 20°C. Salt to sulphate deficient brine ratio was kept 1:3 (W/V). Resultant purified salt contained only 0.6 per cent impurity of Na2SO4. Glauber's salt (Na2SO4.10H2O) 1000 ppm added as seed material to sulphate efficient brine (SEB) and system was cooled to desired temperature (9°C). Obtained sulphate deficient brine and Glauber's salt (Na2SO4.10H2O) were processed as per Example-1. Kxanple-3 10 Kg of Didwana salt containing 8 per cent impurity of Na2SO4 was contacted 30 minutes with sulphate deficient brine (obtained in example-1, step b) at 20°C temperature. Solid to liquid ratio was selected as 1:2 (W/V). Purified sodium chloride was analysed for sulphate contents with standard gravimetric method. Result suggests that 8 per cent impurity of Na2SO4 in Didwana salt was reduced to 0.6 per cent level. Seed material -500 ppm of Na2SO4 - was added to sulphate efficient brine and system was cooled to 9°C. Obtained sulphate deficient brine and Glauber's salt (Na2SO4.10H2O) were processed as per example-1. The main advantages of the present investigation are to provide simple, cyclic but energy efficient process for the removal of Na2SO4 impurity from common salt and simultaneous recovery of Na2SO4. The purified salt can be used in chemical industry. The loss of salt by this process is hardly 2-5 per cent which is also beneficial step in this process. Major finding of the process is that the purified salt has got the purity of 98.5 per cent NaCl, and the purity of Na2SO4 recovered simultaneously is 98 per cent. We claim : 1. A process for purification of sodium chloride containing sodium sulphate which comprises : i) contacting crushed sodium sulphate contaminated sodium chloride with sulphate deficient brine (SDB) for a period in the range of 15-45 minutes at a temperature in the range of 16-22°C to obtain purified sodium chloride and sulphate efficient brine (SEB); ii) cooling the SEB to a temperature in the range of 5 to 10°C, adding 300-1000 ppm of seeding material to the cooled SEB to induce crystallisation of Glauber's salt, Separating the Glauber's salt and the resultant sulphate deficient brine (SDB) by-known methods; iii) repeating step (i) if desired using SDB obtained in step (ii) • 2. A process as claimed in claim-I wherein crushed sodium chloride used in the size in the range of 1 mm to 0.5 mm. 3. A process as claimed in claims 1-2 wherein the seeding material used is such as Na2SO4, Na2SO4.10H2O of particle size in the range of 0.5 mm to 0.2 mm. 4. A process as claimed in claims 1-3 wherein seeding materials used is as such or mixed with clay particles of size 0.3 mm to 0.1 mm. 5. A process for purification of sodium chloride containing sodium sulphate substantially as herein described with reference to the examples.

Documents:

1676-del-1998-abstract.pdf

1676-del-1998-claims.pdf

1676-del-1998-correspondence-others.pdf

1676-del-1998-correspondence-po.pdf

1676-del-1998-description(complete).pdf

1676-del-1998-form-1.pdf

1676-del-1998-form-19.pdf

1676-del-1998-form-2.pdf