Investigation on the Simultaneous Removal of COD and Phosphorus from Saponification Effluent in Rare-Earth Separating Process by Fenton Oxidation and Chemical Precipitation

In general, the saponification effluent result from rare earth metallurgical process is often characterized by high chemical oxygen demand (COD) and high phosphorus and salt concentrations. This paper presents a method for simultaneously removing COD and phosphorus from saponification effluent via Fenton oxidation and chemical precipitation. A series of experiments were conducted to enhance the removal of COD and phosphorus from the effluent which contained 924 mg/L of COD and 15.5 mg/L of phosphorus. Experimental results showed that pH of the solution, reaction time, and H2O2 and Fe2+ concentrations exert significant effects on the removal of COD and phosphorus by Fenton oxidation and chemical precipitation, while a change of temperature makes no difference. Under optimal conditions with [H2O2] = 11.9 g/L, [FeSO4 center dot 7H(2)O] = 4.8 g/L, pH of the solution = 3.0, and reaction time = 360 min, 82.3% of COD, and 72.3% of phosphorus were removed and the effluent was decolorized completely. The X-ray diffraction pattern of the precipitation indicated that the organic phosphorus in the saponification effluent had been changed into inorganic phosphorus and precipitated by calcium chloride. An economic evaluation indicated that it is more economic to adjust the pH with calcium oxide prior to the chemical oxidation process.