Recovery of high-purity magnesium solutions from RO brines by adsorption of Mg(OH)_2(s) on Fe₃O₄ micro-particles and magnetic solids separation

A new approach is presented for cost effective recovery of Mg(II) from 1st stage seawater reverse-osmosis brines (salinity: twice seawater concentration). The process is based on precipitation of Mg(OH)_2(s) on the surface area of self-synthesized magnetite (Fe₃O₄) micro-particles and magnet-assisted separation of the solids-slurry from the Mg(II)-depleted brine. Once separated from solution, the solids slurry is subjected to acidic conditions (pH~4-6) under which Mg(OH)_2(s) is recovered as Mg(II) with the counter anion being either SO42-, Cl^- or HCO3-, depending on the choice of strong acid used in the dissolution step. The magnetite solids are then used in the following adsorption cycle. This paper focuses on proof-of-concept of the suggested process and on defining ranges for the major process operational conditions (Fe₃O₄ particle concentration; pH range maintained during Mg(OH)_2(s) dissolution step; determination of the favorable solid-aqueous separation technique, etc.). Once defined, the chosen operational conditions were applied and shown to result in three high purity (>97%) Mg(II) solution products at costs which are comparable with equivalent commercial products.