Chemical-Physical Pretreatment of Phosphogypsum Leachate

In landfills for phosphogypsum disposal, a large volume of leachate wastewater generated by the residual process water and by the rainfall percolation through the solid material must be managed. The leachate shall be properly treated before its discharge to surface water or its reuse; it has a very low pH and high concentrations of hazardous contaminants such as fluoride, nitrate, phosphate, ammonia, and heavy metals. On the basis of a technical-economical evaluation, a membrane filtration process for the treatment of the leachate was proposed. The membrane process produces a very good quality permeate and a concentrate that shall be reinjected in the stack. From the results of a calculation model that defines the thermodynamic saturation conditions of the salts, it can be expected that the high salt content of the wastewater will reduce the performance of the membranes in terms of low recovery, higher operative pressure, and fouling problems. Moreover, as the concentrate obtained in the membrane process results in metastable oversaturated conditions, it will cause a rapid occlusion of the injection wells. In this study, a pretreatment of the leachate wastewater with lime was defined through laboratory batch tests, to obtain, under controlled conditions, the crystallization of low solubility salts and their separation from the effluent before entering the membrane plant. When operating at a pH of 6.7 with an addition of 10 g/L of lime, very high fluoride and phosphate removals were obtained (96% and 80%, respectively), similar to those predicted by the mathematical model. This approach was proposed as a pretreatment at demonstrative scale to evaluate its feasibility in a membrane process application.