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In this study, we developed a technology to recover Mg from seawater desalination brine in the form of high-purity MgSO 4 that did not contain Ca impurities. The MgSO 4 recovery consisted of pre-precipitation of Mg(OH) 2 using alkali, concentration of Mg using sulfuric acid, and precipitation of MgSO 4 using ethanol. The removal of Ca impurities by adding ethanol twice into the Mg concentrate in the precipitation process was subtle but a highly beneficial novelty. The first dosage of ethanol was added to the Mg concentrate by adjusting the quantity such that only Ca (without Mg) was precipitated in the form of CaSO 4. Then, the ethanol was further added to the Ca-free Mg concentrate to precipitate high-purity MgSO 4. This two-step ethanol addition ensures effective removal of Ca impurities without Mg loss using the difference in the solubility of CaSO 4 and MgSO 4 in ethanol. As the purity of the recovered MgSO 4 was up to 99.8%, it could be used to re-mineralize fresh water after the seawater desalination process. According to the cost assessment, high-purity MgSO 4 produced from seawater desalination brine is expected to be preferred over other products in pharmaceutical and food markets that require high purity and economic feasibility. • We develop a technology to produce pure MgSO 4 from seawater desalination brine. • Ca impurities are effectively removed without Mg loss by adding ethanol twice. • Purity of Ca-free MgSO 4 produced is very high up to 99.8%. • The technology uses the solubility difference between CaSO 4 and MgSO 4 in ethanol. • Mg recovery efficiency is 67%, producing 15.8 kg of MgSO 4 ·7H 2 O from 1 ton of brine. [ABSTRACT FROM AUTHOR]