Removal of phosphorus, fluoride and metals from a gypsum mining leachate using steel slag filters

The objective of this work was to evaluate the capacity of steel slag filters to treat a gypsum mining leachate containing 11–107 mg P/L ortho-phosphates, 9–37 mg/L fluoride, 0.24–0.83 mg/L manganese, 0.20–3.3 zinc and 1.7–8.2 mg/L aluminum. Column tests fed with reconstituted leachates were conducted for 145–222 days and sampled twice a week. Two types of electric arc furnace (EAF) slags and three filter sequences were tested. The voids hydraulic retention time (HRTv) of columns ranged between 4.3 and 19.2 h. Precipitates of contaminants present in columns were sampled and analyzed with X-ray diffraction at the end of tests. The best removal efficiencies over a period of 179 days were obtained with sequential filters that were composed of Fort Smith EAF slag operated at a total HRTv of 34 h which removed 99.9% of phosphorus, 85.3% of fluoride, 98.0% of manganese and 99.3% of zinc. Mean concentration at this system's effluent was 0.04 mg P/L ortho-phosphates, 4 mg/L fluoride, 0.02 mg/L manganese, 0.02 zinc and 0.5 mg/L aluminum. Thus, slag filters are promising passive and economical systems for the remediation of mining effluents. Phosphorus was removed by the formation of apatite (hydroxyapatite, Ca5(PO4)3OH or fluoroapatite, Ca5(PO4)3F) as confirmed by visual and X-ray diffraction analyses. The growth rate of apatite was favored by a high phosphorus concentration. Calcite crystals were present in columns and appeared to be competing for calcium and volume needed for apatite formation. The calcite crystal growth rate was higher than that of apatite crystals. Fluoride was removed by precipitation of fluoroapatite and its removal was favored by a high ratio of phosphorus to fluoride in the wastewater.

► Use of slag filters for efficient removal of P, F and metals from mining leachates.
► Demonstration of the effect of P/F ratio on fluoride removal efficiency.
► Demonstration of the effect of slag type on fluoride removal efficiency.
► Description of hydroxyapatite and calcite crystal growth phenomenon in slag filters.