The precipitation of hematite from ferric chloride media

Abstract

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 ∼ 125°C; akaganeite, β-FeO · OH, is precipitated at lower temperatures for the < 6 h reaction times used. In the presence of hematite seed, Fe 2O 3 is precipitated at temperatures as low as 100°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 FeCl 3. Further increases in the FeCl 3 concentration have a negligible effect on the quantity of Fe 2O 3 precipitated or on the concentration of HCl generated. Increasing initial HClconcentrations, 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 FeCl 2, NaCl, CaCl 2, ZnCl 2 and PbCl 2 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.