Optimization of magnesium recovery from reject brine for reuse in desalination post-treatment

In this work, the recovery of magnesium from desalination reject brine through reaction with ammonia has been evaluated and statistically optimized using response surface methodology. The process is based on precipitation of magnesium hydroxide by the reaction of MgCO ₃ in the brine with ammonium hydroxide. A software, which is designed for studying chemical reaction and equilibrium, was employed to perform the thermodynamic analysis of the reaction of magnesium carbonate with ammonia, which was found to be exothermic and spontaneous in the temperature range of 0–22 °C. Central composite design (5-level, 3-factor) was used to optimize the process to obtain the maximum response of magnesium recovery as a function of reaction temperature, brine salinity, and ammonia to magnesium molar ratio. Maximum recovery of 99% was obtained at a temperature of 15 °C, brine salinity of 85 g/L, and a molar ratio of 4.4NH ₃ : 1 Mg. The predicted response was in excellent agreement with the experimental results. The collected solid product at optimum conditions was characterized using X-ray Diffraction, Energy Dispersive Spectra analysis and thermogravimetric analysis. The analysis indicated the high purity of the recovered product in the form of Brucite (Mg(OH) ₂ ).