The behaviour of phosphate during jarosite precipitation

The behaviour of phosphate during the precipitation of sodium jarosite and potassium jarosite was investigated. At 97, 98 or 150 °C, single-phase PO4-bearing sodium jarosite or potassium jarosite precipitated from Fe(SO4)1.5 solutions containing less than about 4 g/L PO4. Higher PO4 concentrations resulted in the co-precipitation of other phosphate species. Phosphate is preferentially precipitated relative to sulphate under all conditions. The products formed from solutions having a constant PO4 concentration and containing > 0.15 M Fe(SO4)1.5 consisted of PO4-bearing sodium jarosite. The precipitates from more dilute Fe(SO4)1.5 solutions consisted of an amorphous phase plus sodium jarosite. Different behaviours were observed when the Fe(SO4)1.5/PO4 ratio of the solutions was kept constant; these precipitates were only PO4-bearing sodium jarosite. At temperatures < 170 °C, in the presence of 1.8 g/L PO4, only PO4-bearing sodium jarosite was detected. At 98 °C, PO4-bearing sodium jarosite was produced over the pH range from 1.0 to 2.0. At 150 °C, PO4-containing potassium jarosite was produced, but all the precipitates were contaminated with an unknown phosphate species. The precipitates made in predominantly chloride media were similar to those formed under comparable conditions in the all-sulphate systems. Single phase PO4-containing sodium jarosite or potassium jarosite precipitated from chloride solutions containing < 3 g/L PO4, but the precipitates formed from more concentrated PO4 solutions were contaminated with other phosphate species. Detailed mineralogical studies indicated a limited degree of PO4 solid solution in both sodium jarosite and potassium jarosite, with PO4 contents of 5–6% being achieved. It seems that the PO43− anion substitutes for SO42− in the jarosite structure, and that the limited extent of substitution is mostly limited by the different charges on the PO43− and SO42− anions.