The precipitation of hematite from ferric chloride media

The precipitation of hematite from ferric chloride media was systematically investigated in a series of autoclave experiments. The minimum temperature to form hematite in the absence of hematite seed is similar to 125 degrees C; akaganeite, beta-FeO . OH, is precipitated at lower temperatures for the < 6 h reaction times used. In the presence of hematite seed, Fe2O3 is precipitated at temperatures as low as 100 degrees C. The amount of iron precipitated and the concentration of co-generated HCl increase systematically as the initial concentration of ferric chloride increases from 0.025 to 0.3 M FeCl3. Further increases in the FeCl3 concentration have a negligible effect on the quantity of Fe2O3 precipitated or on the concentration of HCl generated. Increasing initial HCl concentrations, however, sharply reduce the amount of hematite precipitated. In contrast, pre-neutralization of the solution significantly increases the extent of hematite precipitation. The presence of soluble species such as FeCl2, NaCl, CaCl2, ZnCl2 and PbCl2 sometimes decreases the extent of iron precipitation but has only a minor effect on the purity of the hematite product; the precipitates typically contain > 65% Fe, < 0.5% Cl and less than 0.1% of Na, Ca, Zn or Pb. The precipitated hematite is very fine, but its filterability is greatly improved by the presence of hematite seed.