Boron extraction using selective ion exchange resins enables effective magnesium recovery from lithium rich brines with minimal lithium loss

Abstract

Magnesium hydroxide is a commodity chemical, produced in almost pure form from seawater through precipitation. Even though Li brines contain high concentrations of Mg2+ cations, the precipitation of Mg(OH)2 from them is not common. Rather it is typically coprecipitated with other salts and becomes a waste. The crystallization of chemical-grade Mg(OH)2 from a Li+ rich brine that contained 3.1 g/L Mg2+ and 1.3 g/L Li+, was investigated by means of NaOH and CaO addition. Direct precipitation of Mg(OH)2 leads to considerable Li loss and impure crystals due to the brine uptake into the crystal cake. In order to increase the sedimentation rate and decrease the brine loss, a boron extraction step was introduced, which consisted of initial polishing using Amberlite IRA743. It was possible to extract B to below the ICP detection limit from native salt lake brines selectively and regenerate the ion exchangers for repetitive cycles. Upon the extraction of boron, the Mg(OH)2 sedimentation rate, increased while the loss of Li+ from the brine decreased from 13.7 ± 1.2 to 1.5 ± 0.4%. Precipitation with CaO as an alkalizing agent generated a mixture of crystals and a Li+ loss between 6.7 ± 0.8 and 3.8 ± 0.7%. When the boron-free brine was alkalized by NaOH, an increase in the degree of crystallinity of Mg(OH)2 crystal structure was verified by XRD and a purity of 95 ± 2% for the solid was obtained. The high water intake in the presence of B was associated with the presence of hydroboracite in intercalation within the brucite. It was expected that, if the hydroboracite was present as an intercalation, this structure would be disrupted more easily by the ultrasound, potentially resulting in a loss of the hydroboracite d-spacings. Concluding, subsequent B and Mg removal steps from Li+ rich brines result in the production of 95% pure Mg(OH)2 leaving a brine for high purity lithium salt precipitation, thereby bringing a second attractive product from brine processing.