Dissolution Kinetics of Magnesium from Calcined Serpentine in NH4Cl Solution

Abstract

A process of extracting magnesium from serpentine to produce Mg compounds by using recyclable NH3 was proposed. This process includes calcination of serpentine; dissolution of magnesium by NH4Cl leaching to obtain NH3 and MgCl2-rich solution, and precipitation of Mg(OH)2 from refined MgCl2-rich solution by feeding NH3. The effects of various factors, such as roasting temperature and time, particle size, concentration of NH4Cl solution, solid-to-liquid ratio, and leaching temperature, on the dissolution kinetics of magnesium of the calcined serpentine were comprehensively investigated. It is found that the extraction rate of magnesium was enhanced by the removal of NH3 generated from the reaction system. Thus, the obtained optimum conditions for magnesium extraction from serpentine are a roasting temperature of 700 °C, a roasting time of 1 h, a particle size fraction of −0.20 + 0.10 mm, an initial ammonium chloride concentration of 4.0 M, a solid-to-liquid ratio of 20/500 g/mL, and a reaction temperature of 104 °C. With the removal of NH3 in batch leaching study, the maximum recovery of magnesium reached 83.87%, and the concentration of magnesium was ∼0.63 mol·L–1, satisfying the requirement of Mg(OH)2 precipitation. Finally, the reaction kinetics of the system was successfully modeled by an empirical diffusion-like equation. The apparent activity energy was obtained to be 23.56 ± 1.06 kJ·mol–1. The results from this study provide the fundamentals for magnesium extraction from serpentine by NH4Cl leaching.