Characterization of the iron arsenate–sulphate compounds precipitated at elevated temperatures

To help clarify the nature of the iron arsenate–sulphate compounds produced during the autoclave treatment of refractory gold ores and concentrates, systematic synthesis studies were undertaken; in addition to scorodite and Fe(SO4)(OH), two other compounds, designated as Phase 3 and Phase 4, were identified. Whereas Fe(SO4)(OH) is predominantly an orthorhombic compound, Phase 3 can have the same composition but is predominantly the monoclinic polytype, the formation of which is promoted by the solid-solution uptake of As; substitution of As results in a corresponding decrease in the OH required to maintain the charge balance; e.g., Fe[(SO4)0.60(AsO4)0.40]∑1.00[(OH)0.6(H2O)0.4]∑1.00. Phase 4 corresponds to Fe(AsO4)·¾H2O. In 0.4 M Fe(SO4)1.5 (22.3 g/L Fe), 0.41 M (40 g/L) H2SO4, 0.09 M (7 g/L) As(V) solutions, sulphate-containing scorodite was formed at 150–175 °C. Phase 3 precipitated at 175–210 °C, but mixtures of Phase 3 and Fe(SO4)(OH) formed above 210–220 °C. The Fe content of Phase 3 is about 30 mass %, whereas the AsO4 and SO4 contents vary widely and in an inversely proportional manner, reflecting the extensive mutual structural substitution of these anions. At 205 or 215 °C, Fe(SO4)(OH) was precipitated from 0.4 M Fe(SO4)1.5 (22.3 g/L Fe), 0.41 M (40 g/L) H2SO4 solutions containing < 0.03 M (2 g/L) As(V). Increasing As(V) concentrations enhance the precipitation of Phase 3, but only Phase 4 was precipitated from solutions containing > 0.33 M (25 g/L) As(V). The composition of Phase 4 is nearly constant and it contains < 1 mass % SO4. Acid concentrations > 0.2 M H2SO4 had little effect on the composition of the precipitates. At 205 °C in 0.41 M (40 g/L) H2SO4, 0.09 M (7 g/L) As(V) media, mixtures of scorodite and Phase 4 precipitated from 0.0–0.1 M Fe(SO4)1.5 (0.0–5.6 g/L Fe) solutions; for Fe(SO4)1.5 concentrations > 0.1 M, only Phase 3 formed. To provide a preliminary indication of the solubility of Phase 3 and Phase 4 in tailings impoundments, the various precipitates were leached at room temperature for 40 h in water. The As concentrations dissolved from Phase 3 were consistently < 0.1 mg/L, which suggests that Phase 3 might be an acceptable medium for arsenic disposal. In contrast, the soluble As concentrations from Phase 4 were 1–3 mg/L.