Treatment of an industrial wastewater for phosphorus and fluoride recovery by a process coupling block freeze concentration and precipitation

Abstract

The work aims at developing a process for treating industrial wastewater from phosphoric acid and fertilizer production units by a coupled block freeze concentration-precipitation process. First, the block freeze concentration study, carried out on synthetic phosphoric acid solutions of concentrations ranging between 1.31 wt% 29.93 wt%, has shown that the purity of the ice block is strongly dependent on the initial concentration, the freeze duration, the thawing rate and the ice block temperature. Second, the block freeze concentration study, performed on a solution containing an orthophosphate-fluoride mixture, showed that the process increased the purity of the ice by the migration of occluded fluoride and orthophosphate ions until reaching a removal efficiency of F− and PO43− upper than 94 %. Further treatment by precipitation was studied on the first thawed fraction resulting from the freezing step. The results of the recovered liquid confirmed that, in the pH range between 2.4 and 12.73, the removal of fluoride and orthophosphate increased from 9.8 % to 96 % and from 6.3 % to 99 %, respectively. Precipitates characterization using methods such as X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy combined with energy dispersive X-ray analysis, indicated that CaF2, Ca5(PO4)3(OH)F and Ca5(PO4)3(OH) were successfully produced.