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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]

The economic challenges arising from resource depletion and uneven distribution of Mg in land reserves have spurred interest in technologies for Mg extraction from seawater. Thus, this study introduces a novel method to recover high-purity magnesium carbonate, specifically hydromagnesite, from seawater through carbonation using Ca(OH)₂ as an alkali source. A significant achievement of this method is the production of high-purity hydromagnesite despite the presence of large amounts of Ca, realized only through carbonation without the need for expensive additives such as NaOH. Experimental results reveal that the Ca(OH)₂ content predominantly influences the Mg dissolution efficiency during carbonation and the overall Mg recovery rate. Maximizing the efficiency of this technology relies on using an appropriate amount of Ca(OH)₂. Notably, the appropriate amount of OH− supply enhances CO₂ dissolution, facilitating Mg dissolution. Conversely, an excess of Ca(OH)₂ impedes Mg dissolution. The overall Mg recovery rate reaches a peak of 67 %, enabling the recovery of 3.4 kg of hydromagnesite from one ton of seawater. The findings highlight the effectiveness of the proposed technology as a promising and cost-effective method for extracting high-purity Mg from seawater, utilizing Ca(OH) 2 , previously considered an impurity, as a cost-effective alkali source. • Recovered high-purity hydromagnesite from seawater via carbonation using Ca(OH) 2 • Used Ca(OH)₂, previously deemed an impurity, as a cost-effective alkali • Optimal amounts of Ca(OH)₂ enhanced Mg dissolution but excess content inhibited it • Mg recovery peaked at 67 %, yielding 3.4 kg hydromagnesite per ton of seawater [ABSTRACT FROM AUTHOR]

Previous studies on the utilization of seawater and brine have focused on either recovering Mg from seawater/brine using alkalis or extracting Ca from industrial byproducts using solvents. Therefore, the present study was aimed at recovering Mg and Ca simultaneously from simulated seawater/brine and slaked lime [Ca(OH) 2 ] using a simple process, with seawater/brine employed both as a Mg source and as a solvent for extracting Ca from the Ca source. Manipulating the concentration of the sucrose additive and the ionic strength of seawater/brine enabled Mg and Ca recovery with high efficiency and purity. The highest Mg recovery from brine was 75 %, and ~ 100 % pure MgSO 4 was produced; specifically, ~28 kg of MgSO 4 •7H 2 O was produced from 1 ton of brine. The impacts of sucrose concentration and seawater ionic strength on the Ca elution, carbonation efficiency, and size and shape of the resulting vaterite particles were analyzed. The purity of the vaterite produced was 94 %–100 %, and the maximum CaCO 3 yield per ton of the Ca(OH) 2 was estimated as 770 kg, with a carbonation efficiency of up to 92 %. Therefore, our cost-effective method—featuring the use of seawater/brine and Ca(OH) 2 —can concurrently produce economically demanding minerals such as MgSO 4 and vaterite, thereby facilitating resource conservation. • MgSO 4 and CaCO 3 are produced simultaneously from seawater and Ca(OH) 2. • The sucrose content and seawater ionic strength affect the mineral yield and form. • The Mg recovery efficiency is 75 %, and the MgSO 4 •7H 2 O yield is 28 kg/ton of brine. • The carbonation efficiency is 92 %, and the CaCO 3 yield is 770 kg/ton of Ca(OH) 2. • The MgSO 4 and vaterite powders obtained through the devised process are ~100 % pure. [ABSTRACT FROM AUTHOR]