Pilot test and cost-based feasibility study of solar-assisted evaporation for direct preparation of high-purity magnesium sulfate hydrates from metastable Na+,Mg2+//Cl−,SO42−-H2O salt-water system

Abstract

To solve the longstanding problem of low purity that hampered the extraction of magnesium sulfate from the sea-type Na+,Mg2+//Cl−,SO42−–H2O system, solar collection was used to conduct pilot-scale evaporation and crystallisation at different temperatures, and brine was maintained in a metastable state to directly precipitate high-purity crystals of magnesium sulfate hexahydrate (MgSO4·6H2O). The precondition for preparing high-purity MgSO4·6H2O crystals was found to be an optimal ratio of 0.30–0.48 between the Jänecke indexes of SO42− and Mg2+. The purity of the directly precipitated MgSO4·6H2O crystals was >99.5%. Decreasing Cl− content in the feed brine was found to enhance the recovery ratio of Mg2+ and reduce the solar collector consumption. Solar-assisted evaporation technology can double the Mg2+ recovery and halve the water evaporation compared to high-purity salt production using the conventional natural evaporation and recrystallization technique. According to techno-economic analysis, the operating cost at a production scale higher than 20 t·d−1, brine evaporation temperature of 50 °C, and production from April to September can be adjusted to be as low as 41 USD·t−1. Compared to the market price of high-purity magnesium sulfate hydrates, the proposed solar-assisted evaporation/extraction method is economically feasible.