Unveiling the role of carbonate in dolomitic limestone for efficient leaching of magnesium from phosphate mine wastes

The recovery of magnesium as a critical metal from industrial wastes rather than traditional mining has attracted significant attention due to environmental concerns and mining limitations. In this research, we investigated the role of carbonate in acid leaching utilizing diluted sulfuric acid to efficiently extract Mg from dolomitic limestone, a phosphate mine waste rock. As the main objective, we elucidated how carbonate phase transformation influences impurities' co-leaching from thermodynamic and kinetic aspects. In this regard, the leaching behavior of dolomitic limestone was compared with calcined ore at 700 & ring;C and 900 & ring;C, where the CO2 was liberated from MgCO3 and CaCO3, respectively. The CO2 volatilization raised the hydrophilicity of the ore and increased the average surface area from 2 m2/g to 6 m2/g. During the leaching process, phase transformation impacted the lixiviant redox potential and caused a 35 mV decrease for dolomitic limestone leachate compared to calcined ore at 900 & ring;C. The alteration of physicochemical properties of feed and lixiviant redox potential arising from the carbonate phase could inhibit the access of H+ to impurities, reducing the co-leaching of impurities from 87 % to 36 %. Adopting a solid-to-liquid ratio of 10 g/l in 0.5 M H2SO4 at 80 & ring;C was the optimum condition for 100 % magnesium extraction and simultaneous crystallization of 75 % calcium as needle-shaped CaSO4 & sdot;2H2O. Based on the kinetic modeling, magnesium leaching obeyed the shrinking core model with shrinking particle size. The leaching rate was controlled by diffusion through the ash product layer, with an activation energy of 16 kJ/mol.