Management of solid waste marble powder: improving quality of sodium chloride obtained from sulphate-rich lake/subsoil brines with simultaneous recovery of high-purity gypsum and magnesium carbonate hydrate

Marble industry worldwide produces large amount of non-degradable marble dust powder (MDP) waste during mining and processing stages. MDP mainly comprises of CaCO 3 with small amounts of Mg, Fe or Si in various forms. In India, mainly in Rajasthan state, marble is quarried in huge amounts and MDP thus produced is collected improperly and dumped at any abandoned land or identified disposal sites. On the other hand, the composition of sub soil/lake brines of Rajasthan is typical in nature as it does not have much Ca 2+ and Mg 2+ impurities but contains higher levels of SO 4 2− . Therefore, the common salt (NaCl) produced from such brines is contaminated with Na 2 SO 4 (8–30 wt%) depending upon SO 4 2− concentration in the brine. Such a salt produced is suitable neither for edible purpose nor for industrial usage. Herein, we have reacted MDP with HCl, and the resulting solution (CaCl 2 and MgCl 2 slurry) is used in stoichiometric ratio of Ca 2+ to SO 4 2− in brines to produce high-purity NaCl and gypsum (CaSO 4 ·2H 2 O) via fractional crystallization. Remaining magnesium-containing solution was reacted with Na 2 CO 3 to prepare high purity magnesium carbonate hydrate. Purity of crystallized NaCl, CaSO 4 ·2H 2 O and MgCO 3 ·6H 2 O has been ascertained through analytical and spectral methods (TGA, FTIR, P-XRD). Field emission scanning electron microscopy was used to elucidate morphology of crystals. The method reported for improving purity of NaCl along with CaSO 4 ·2H 2 O and MgCO 3 ·6H 2 O production from sulphate-rich brines is simple and economic, and allows management of MDP generated in huge amounts, which poses problems of disposal.