Extraction of high-purity magnesium and calcium from phosphate mine waste rocks by a carbon-neutral and circular hydrometallurgy

Dolomitic limestone waste rock (DLWR), generated during phosphate ore extraction, represents a high-potential source of magnesium and calcium. However, its valorisation is hindered by CO2 emissions from carbonate ore leaching, the co-dissolution of impurities, and the difficulty of separating leached Mg and Ca due to their similar aqueous chemistries. In this study, a simplified and carbon-neutral hydrometallurgical process was developed to recover high-purity Mg(OH)2 and CaCO3 microspheres from DLWR. Thermodynamic simulations, combined with experimental validation, were used to identify insolubility domains for Mg, Ca, and impurities, as well as optimal recovery conditions. Using 0.5M H2SO4 enabled the separation of over 50% of Ca from Mg during leaching, forming needle-shaped CaSO4·2H2O particles. Adding 1M NaOH to the acid-leaching liquor and adjusting the pH to 7 separated all impurities, including Ti, Al, Fe, and Mn. Subsequent pH adjustment to 9 caused Mg(OH)2 precipitation, with 98% recovery and 99.5% purity. The residual CaSO4·2H2O was solubilised by water leaching and mixed with the remaining precipitation liquor containing Ca ions. When exposed to CO2 released during acid leaching, this mixture promoted CaCO3 precipitation, achieving a recovery of 98.6% and a purity of 99.7%. This developed process demonstrates a carbon-neutral and circular approach to valorising DLWR.