Hematite formation from jarosite type compounds by hydrothermal conversion

The hydrothermal conversion of K jarosite, Pb jarosite, Na jarosite, Na-Ag jarosite, AsO(4) containing Na jarosite and in situ formed K jarosite and Na jarosite to hematite was investigated. Potassium jarosite is the most stable jarosite species. Its conversion to hematite in the absence of Fe(2)O(3) seed occurred only partially after 5 h reaction at >240 degrees C. In the presence of Fe(2)O(3) seed, the conversion to hematite was nearly complete within 2 h at 225 degrees C and was complete at 240 degrees C. The rate of K jarosite precipitation, in situ at 225 degrees C in the presence of 50 g L(-1) Fe(2)O(3) seed, is faster than its rate of hydrothermal conversion to hematite. In contrast, complete conversion of either Pb jarosite or Na-Pb jarosite to hematite and insoluble PbSO(4) occurs within 0.75 h at 225 degrees C in the presence of 20 g L(-1) Fe(2)O(3) seed. Dissolved Fe(SO(4))(1.5) either inhibits the conversion of Pb jarosite or forms Pb jarosite from any PbSO(4) generated. The hydrothermal conversion of Na-Ag jarosite to hematite was complete within 0.75 h at 225 degrees C in the presence of 20 g L(-1) Fe(2)O(3) seed. The Ag dissolved during hydrothermal conversion and reported to the final solution. However, the presence of sulphur or sulphide minerals caused the reprecipitation of the dissolved Ag. The conversion of AsO(4) containing Na jarosite at 225 degrees C in the presence of 20 g L(-1) Fe(2)O(3) seed was complete within 2 h, for H(2)SO(4) concentrations,<0.4M. Increasing AsO(4) contents in the Na jarosite resulted in a linear increase in the AsO(4) content of the hematite, and similar to 95% of the AsO(4) remained in the conversion product. Increasing temperatures and Fe(2)O(3) seed additions significantly promote the hydrothermal conversion of in situ formed Na jarosite at 200-240 degrees C. However, the conversion of previously synthesised Na jarosite seems to proceed to a greater degree than that of in situ formed Na jarosite.