Recovery of magnesium from Uyuni salar brine as hydrated magnesium carbonate

Abstract

Uyuni salar (Bolivia), the largest resource of lithium (Li) in the world not only contains a high level of Li but also ~1.3% w/w magnesium (Mg) in strong NaCl brines. The recovery of Mg is therefore essential as the resources represent high value of Mg by-products if recovered. High purity hydrated magnesium carbonate (HMC as dypingite or hydromagnesite) could be recovered from the Uyuni salar brines as an easy-saleable product. The precipitation process used for this recovery relies on the selective removal of Ca by adding excess oxalic acid at an oxalate/Ca molar ratio of 6.6:1. The excess oxalate has to be subsequently removed as Mg oxalate by adjusting the pH to pH 4–7. HMC products of 97.3–99.6% purity could be produced as a result (equivalent to 25.3–25.9% Mg in the product compared to a theoretical value of 25.99% Mg for hydromagnesite). Without removing Mg oxalate, products could only reach 23.3–24.3% Mg. The light HMC (hydromagnesite) product could be precipitated at ambient conditions as long as the drying was set at >65°C. Precipitation at >55°C also yields more crystalline hydromagnesite compared to those obtained at lower temperatures. The light HMC product has a heat adsorbing capacity (endothermic) of 1123–1196J/g dry product. The product has a sheet-like structure and is easily filtered, compared to magnesium hydroxide.