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

摘要

This paper presents a recycle MAP process (magnesium ammonium phosphate) to removenNH4-N from a typical rare-earth wastewater. The optimum conditions for the MAP precipitationnand recycle use of the MAP with a newly-designed process were investigated in laboratory.nThe results showed that the pH value and dosages of P (phosphate) and Mg reagents have ansignificant influence on NH4-N removal, with a maximum removal efficiency of 99.4% at thenconditions of pH ¼ 9 and Mg:N:P molar ratio ¼ 1.2:1:1.2. In the process of recycle use of thenMAP, adding some HCl to dissolve MAP decomposition residues could effectively enhance NH4-Nnremoval. The NH4-N removal efficiency reached 99.6% by adding an HCl amount of H+n:OH2 molarnratio ¼ 0.8 into the reused MAP decomposition residues, whereas the NH4-N removal efficiencynwithout addition of HCl was only 96.4%. Moreover, the residual PO4-P from the end of reactionnwas recovered and the optimum recovery efficiency was achieved at a Mg:P molar ratio ¼ 6 andnpH ¼ 10. Under these optimum conditions, the residual NH4-N and PO4-P concentrations in thentreated wastewater, through 6 times of the recycling, were less than 15mg/L and 1mg/L,nrespectively. On the basis of this, an economic evaluation of the recycling MAP was made, andnthis recycle process could save 48.6% cost used in the chemicals for treating per cubic meter ofnthe rare-earth wastewater, compared to the conventional MAP process.