Full recycling of MgCl2 wastewater generated in rare earth extraction and separation process by spray pyrolysis

Wastewater recycling is the desired goal of the rare earth separation plants for current ecological requirements and economic benefit. The MgCl2 wastewater generated during rare earth extraction and separation process was treated by spray pyrolysis for the first time to achieve full recycling. The kinetic of spray pyrolysis process and the mechanism affecting the activity of MgO powder were investigated. The results showed that the rate determining step of MgCl2 solution spray pyrolysis was transformed from a chemical reaction control to a diffusion process control after the temperature exceeded 1073 K. The MgO powder prepared by spray pyrolysis was rough and porous, the embedding of Cl made MgO grains severely distorted, producing more defects. Meanwhile, polycrystalline basic magnesium chloride likewise had more defects. These two points resulted a higher density of defects of MgO powders, and causing higher activity. Moreover, at a temperature of 1173 K, MgCl2 concentration of 300 g/L and N2 flow rate of 40 L/min, the thermal decomposition rate of MgCl2 reached 99.54%, the alkalinity of the powder reached 0.045 mol/g, and the iodine absorption value reached 102.72 mg I2/g MgO. Meanwhile, the high purity hydrochloric acid with a concentration of 6.70 mol/L was obtained.