Separation of calcium and magnesium using dual precipitation/chelation scheme from saline solutions

Abstract

A process scheme has been developed for the selective separation of calcium (Ca) and magnesium (Mg) from the Red Sea water and reverse osmosis (RO) brine using combined dual precipitation/chelation units. Two precipitation stages have been adopted for sequential Ca and Mg removal using sodium carbonate and sodium hydroxide, respectively. Calcium removal using sodium carbonate approached 96.6 and 93.2% for Red Sea (pH 9) and RO brine (pH 9.2), respectively. Maximum Mg removal efficiencies achieved using sodium hydroxide at pH 12 for both Red Sea and RO brine were 99.6 and 74%, respectively. The decalcified filtrate of the second precipitation stage was further processed for Mg removal using ethylene glycol bis(2-aminoethyl ether)-N,N,N′,N′-tetra acetic acid (EGTA), diethylenetriaminepentaacetic acid (DTPA), and N-(2-Hydroxyethyl) ethylene diamine-N,N,N′-triacetic acid (HEDTA) as selected chelating agents. Almost complete removal of Mg was achieved using the three tested chelating agents with maximum Mg adsorption capacity of 490 mg/g. The final rich monovalent effluents could be directed to thermal or membrane desalination plants or selective monovalent ion separation/salt recovery units. The purified streams after Ca and Mg removal could be utilized immediately for lithium, potassium, and sodium chloride separation.