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A Waste to Wealth Initiative
V P Mohandas, A U Hamidani, M R Gandhi, A M Bhatt, A Kumar, U P Saraiya, R J Sanghvi and P K Ghosh. Highly purifi ed common salt is in much demand nowadays, for both industrial and edible purposes. This case study elaborates on Solvayĺs approach of recycling distiller waste into brines, applied by the scientists of Central Salt and Marine Chemicals Research Institute for obtaining highly purified common salt for the soda ash industry.

There has been a steady rise in the demand for common salt [Sodium Chloride (NaCl)] of increasingly higher purity be it for industrial or edible purposes. The two most important industrial applications are in the manufacture of Caustic Soda (NaOH) and Chlorine (Cl) by membrane cell electrolysis and soda ash by the Solvay process. On the other hand, the need to maintain high stability of iodine in iodised salt and to impart greater aesthetic appeal to salt has led to the demand for high purity and whiteness of edible salt.

Almost all of the common salt in the country is produced from seawater/sub-soil brine through solar evaporation. The principal impurities in the salt are divalent ions, mainly  Calcium (Ca) and Magnesium (Mg) cations, and Sulphate (SO4) anion. As expected, these are present, for the most part, in the form of Calcium Sulphate (CaSO4) and Magnesium Sulphate (MgSO4), although there can be entrapped impurities of the chloride salts also. Although washing the salt can remove much of the adhering soluble impurities, calcium sulphate (gypsum) is especially difficult to remove in view of its sparing solubility. Figure 1 shows the crystallisation behavior of gypsum and common salt (NaCl) as a function of brine density [oBe╩=145-(145/specific gravity)]. It can be seen from the figure that there is a region of overlap which leads to inevitable contamination of the NaCl with gypsum, the latter continuing to co-crystallise even beyond 25 oBe╩, this being the normal density at which the salt crystallisers are charged.1

The compositions of seawater and typically available sub-soil brine in the Bhavnagar area are shown in Table 1. The [SO42-]/ [Ca2+] molar ratio works out to 3.0 and 1.5 for the two cases, respectively. It can thus be seen that under normal circumstances, formation of gypsum is limited by the availability of Ca2+. It occurred to us that if forced desulphatation of the seawater/ sub-soil brine can be carried out in practical manner, it would be feasible to eliminate almost completely the region of overlap indicated in Figure 12. This would, therefore, lead to a purer salt besides yielding additional amounts of common salt due to the common ion effect and nearly 3x and 1.5x more gypsum in the cases of seawater and sub-soil brine, respectively.

Moreover, most of the major soda ash manufacturers in India are backward integrated to salt production and should therefore be able to mitigate the problem of waste discharge while producing superior quality salt which would reduce the cost of downstream purification operations Our initial studies were conducted in the institute╩s experimental salt farm utilising sub-soil brine and distiller waste brought from M/s Nirma Limited. Under a cluster development project supported by the Industries Commissionerate, Government of Gujarat, the know-how was implemented in the Little Rann of Kutch in partnership with Self-Employed Women╩s Association of India (SEWA). Sub-soil brine was used and the distiller waste was obtained from DCW, Dhrangadhra. A study was subsequently undertaken with M/s Hindustan Salts Limited in the same location. Both sets of data are compiled in Table 2 on next page. A demonstration was also made in the salt works of M/s TATA Chemicals Ltd, Mithapur The remarkable improvement in the quality of salt would be evident from these tables.

As per the stoichiometry of the Solvay process, 1 mole of Calcium chloride (CaCl2) is obtained from 2 moles of NaCl reacting to produce one mole of Sodium carbonate (Na2CO3). Based on the conventional assumption that 50 L of seawater is required to obtain 1 kg of solar salt, and further given that the sulphate concentration in seawater is 2.74 g/L, the usage of distiller waste would be around 20 per cent if salt is produced only for soda ash production. On the other hand, if a company produces salt for other purposes also, then the consumption of distiller waste can increase proportionately and, in principle, it is feasible to use up all of the distiller waste thereby eliminating the problem of its discharge (Figure 3). There is an additional gain that accrues from our invention. The bittern devoid of sulphate becomes suitable for production of crude carnallite (KCl.MgCl2.3H2O + NaCl). Upon decomposition with water, it yields low sodium salt (NaCl + KCl) directly through another patented invention developed and licensed by the institute4. Alternatively, this salt can be further processed to obtained pure Potassium chloride (KCl).

It is reported herein that the distiller waste of Solvay process can be gainfully utilised when industries engaged in manufacture are backward integrated to common salt production. CaCl2 in this waste is used for the purpose of forced separation of sulphate from the brine, thereby avoiding the zone of co-crystallisation of gypsum and NaCl.

Although in recent years, the institute has developed other approaches to pure solar salt,5,6 the methodology of distiller waste addition is of special importance given that there are other benefits also: in particular, (i) additional production of gypsum, (ii) ca. 5 per cent higher yield of common salt, and (iii) cost-effective production of KCl/low sodium salt via carnallite. Most importantly, the adverse consequence of discharge of the distiller waste into the sea, as evident from Figure 3, can be prevented altogether if a plan is devised whereby soda ash manufacturers are compelled to produce as much salt as necessary to use up the entire distiller waste generated. With India╩s aspiration to export pure salt in much larger amounts, the present inventions would enable the country to accomplish this goal with responsible care.