Removal of calcium from industrial-grade magnesium sulfate solution

Abstract

The aim of this work was to find a method to remove calcium impurities from a concentrated magnesium sulfate solution. Different methods were evaluated, including (i) selective precipitation using ethanol, sodium carbonate, oxalic acid, and sodium hydroxide, (ii) partial crystallization, and dissolution of magnesium sulfate solutions, (iii) ion-exchange using cationic exchange resin, and (iv) membrane separation using a calcium ion-selective membrane. The changes of Ca2+ and Mg2+ ions in each method were evaluated using ICP-OES analysis. Among the additives used in the selective precipitation experiments, ethanol and sodium hydroxide were the most efficient in removing calcium up to 65% and concentrating magnesium ions by six times (Mg/Ca molar ratio) as compared to the original solution, respectively. Sodium carbonate and oxalic acid did not cause any precipitation of calcium due to the high ionic strength as well as the inhibitory effect of Mg2+ ions present in the solution. Furthermore, partial crystallization and Ca2+-selective membrane separation were found to be ineffective in removing calcium. Ion-exchange, using Amberlite® IR 120 resin, reduced the concentration of calcium ions in the solution by 30%, but also removed 20% of the magnesium ions in the solution, which signifies inadequate selectivity of the ion-exchanger. Lastly, partial dissolution of magnesium heptahydrate salts showed that calcium solubility decreases at low temperatures.