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.

Abstract

Marble industry worldwide produces large amount of non-degradable marble dust powder (MDP) waste during mining and processing stages. MDP mainly comprises of CaCO3 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 Ca2+ and Mg2+ impurities but contains higher levels of SO42-. Therefore, the common salt (NaCl) produced from such brines is contaminated with Na2SO4 (8-30 wt%) depending upon SO42- 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 (CaCl2 and MgCl2 slurry) is used in stoichiometric ratio of Ca2+ to SO42- in brines to produce high-purity NaCl and gypsum (CaSO4·2H2O) via fractional crystallization. Remaining magnesium-containing solution was reacted with Na2CO3 to prepare high purity magnesium carbonate hydrate. Purity of crystallized NaCl, CaSO4·2H2O and MgCO3·6H2O 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 CaSO4·2H2O and MgCO3·6H2O production from sulphate-rich brines is simple and economic, and allows management of MDP generated in huge amounts, which poses problems of disposal.