Water Science & Technology—WST Vol 59 No 6 pp 1093–1099 © IWA Publishing 2009 doi:10.2166/wst.2009.086

Recycle use of magnesium ammonium phosphate to remove ammonium nitrogen from rare-earth wastewater

H. M. Huang, X. M. Xiao and B. Yan

This paper presents a recycle MAP process (magnesium ammonium phosphate) to remove NH₄-N from a typical rare-earth wastewater. The optimum conditions for the MAP precipitation and recycle use of the MAP with a newly-designed process were investigated in laboratory. The results showed that the pH value and dosages of P (phosphate) and Mg reagents have a significant influence on NH₄-N removal, with a maximum removal efficiency of 99.4% at the conditions of pH = 9 and Mg:N:P molar ratio =1.2:1:1.2. In the process of recycle use of the MAP, adding some HCl to dissolve MAP decomposition residues could effectively enhance NH₄-N removal. The NH₄-N removal efficiency reached 99.6% by adding an HCl amount of H⁺:OH^- molar ratio = 0.8 into the reused MAP decomposition residues, whereas the NH₄-N removal efficiency without addition of HCl was only 96.4%. Moreover, the residual PO₄-P from the end of reaction was recovered and the optimum recovery efficiency was achieved at a Mg:P molar ratio = 6 and pH = 10. Under these optimum conditions, the residual NH₄-N and PO₄-P concentrations in the treated wastewater, through 6 times of the recycling, were less than 15 mg/L and 1 mg/L, respectively. On the basis of this, an economic evaluation of the recycling MAP was made, and this recycle process could save 48.6% cost used in the chemicals for treating per cubic meter of the rare-earth wastewater, compared to the conventional MAP process.

Keywords: MAP precipitation; phosphate recovery; rare-earth wastewater; recycling

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