A pilot-scale investigation on the recovery of zinc and phosphate from phosphating wastewater by step precipitation and crystallization

A sustainable process to recover zinc and phosphate separately from phosphating wastewater was proposed, which involved a preliminary recovery of zinc by the adjustment of pH, followed by phosphate recovery through the formation of magnesium ammonium phosphate (MAP). Lab-scale batch experiments were first performed to investigate the effects of suspended solid (SS), linear alkylbenzene sulfonate (LAS), and Zn2+ on the crystallization of MAP. The results indicated that the presence of LAS and Zn2+, except SS, had a markedly inhibitory effect on the MAP crystallization. The inhibition ratio of Zn2+ on MAP crystallization markedly increased with an increase in the Zn2+ concentration; whereas, it progressively decreased at a pH range of 8.5-9.5, reached the minimum value at pH 9.5, and then increased gradually at a pH range of 9.5-10.5. Nevertheless, the presence of Zn2+ was conducive to the removal of phosphate. The results of the experiments for overdosing magnesium and ammonium salts demonstrated that excessive addition of magnesium was more conducive to the enhancement of phosphate recovery efficiency from phosphating wastewater compared to that of excess of ammonium. The investigation on separate recovery of zinc from phosphating wastewater showed that the zinc recovery efficiency reached 99% by adjusting pH to 7, accompanied by the removal of a small quantity of phosphate. The pilot-scale study revealed that 99% of the Zn2+ and 98% of the PO4-P could be recovered from phosphating wastewater by the proposed stepwise precipitation process. An economic analysis revealed that the treatment cost and the benefit of the recovered product by the proposed process could be basically offset. (C) 2017 Elsevier B.V. All rights reserved.