Magnesium and calcium extraction from phosphate mine waste rock using phosphoric acid: Thermodynamics, parameter optimization, kinetics, and reaction mechanism

Abstract

With rising global demand for phosphate-based fertilizers, phosphate mining has led to significant waste rock accumulation, primarily consisting of dolomite [CaMg(CO₃)₂]. This dolomite-rich waste rock (DRWR) presents both environmental risks, such as groundwater contamination, and opportunities for producing valuable calcium and magnesium phosphate compounds. This study explores the potential of phosphoric acid (PA), a phosphate industry product, for selectively leaching dolomite from DRWR to recover calcium and magnesium phosphates. Using a design of experiments (DOE) methodology, the research examines factors affecting leaching efficiency, including temperature, PA concentration, particle size, acid introduction method, solid–liquid (S/L) ratio, reaction time, and stirring speed. Thermodynamic analysis revealed that PA selectively reacted with dolomite in a temperature-sensitive manner, with optimal conditions identified as 44.97 °C, 3.07 mol/L PA, 47 min reaction time, and a S/L ratio of 5.69/50 (g/mL) using a Box-Behnken design. Kinetic studies, employing the shrinking core model, measured the dissolution rate across various conditions—leaching temperature (20–80 °C), stirrer speed (200–750 rpm), particle size (−90 µm to 600 µm), and PA concentration (0.5–4.5 M). An activation energy of 20.96 kJ/mol suggests a product layer diffusion-controlled mechanism. These results provide valuable insights for developing sustainable processes, and promoting circular economy practices in the phosphate industry.