In this study, magnesite was used as a low-cost magnesium source to remove ammonium as struvite from the wastewater generated in the rare-earth elements separation process. Since the solubility of magnesite is low, before it was used it was decomposed to magnesia which has a higher reaction rate than magnesite. To optimize its usage, the optimum temperature of decomposition of magnesite and the time required for the process were determined by batch experiments to be 700 °C and 1.5 h, respectively. Besides, batch experiments using the residues of magnesite decomposed under optimum conditions were undertaken to investigate the effects of solid (magnesite)/liquid (wastewater) ratio and reaction time on ammonium removal as struvite. Results indicated that for the solid/liquid ratios tested and for a reaction time of 6 h, phosphorus concentrations fell steeply from the initial 9105 mg/L to a range of 198.8–29.8 mg/L, and ammonium concentrations from the initial 5287 mg/L to 540–520 mg/L. An economic analysis conducted indicated that the operation cost of the struvite process could be reduced by about 34% using decomposed magnesite instead of pure MgCl2.

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