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
2022
Sanghavi, Rahul J. | Upadhyay, Sumesh C. | Kumar, Arvind
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.
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