A new thermal-reduction-based approach for producing Mg from seawater

A new process is presented for producing metallic magnesium from seawater. Ion exchange is used to selectively extract Mg^{2 +} from seawater, without introducing Na⁺ and K⁺ impurities into the product Mg. The magnesium extracted from the resin is reacted with CaO and precipitates on the surface of fine ferrosilicon suspension to produce a Mg(OH)₂-Ca(OH)₂-FeSi solids cake, which, after further wash and decantation, yields raw material at a quality similar to that used in the classical dolomite-based Pidgeon silicothermic reduction technique. The filtrate of the raw cake decantation step, comprising predominantly of CaCl₂, is used as the ion-exchange regeneration solution. Ca^{2 +} ions are adsorbed on the resin and thereafter replenished through CaO dosage, applied for the Mg(OH)₂ precipitation. The close contact between the Mg(OH)₂ and the FeSi in the dry cake enables direct usage of the cake in ~ 1150 °C retort which is applied for single-step dehydration and thermal reduction of the Mg^{2 +}, thereby minimizing the problem of heat and mass transfer through the briquettes and eliminating the need for separate calcination, briquetting and reduction. Feasibility dehydration and reduction tests yielded 99.0–99.5% pure Mg. Materials and energy cost assessment revealed that the proposed process is ~$0.75 more expensive per kg of magnesium than the dolomite-based process, assuming equal thermal reduction yields. The cost estimation stimulates further optimization and parametric study of the proposed process and suggests that production of Mg in places where dolomite is not available, requiring seawater as the sole raw resource, is feasible.