| In this work, the kinetic and equilibrium profiles of each individual reaction step involved in the production of hematite at 200°C via oxydrolysis of ferrous sulphate in concentrated zinc sulphate media were established. Crystallization of ferrous sulphate was found to play a crucial role in the overall process due to its relatively low solubility and fast crystallization kinetics at elevated temperatures. In fact, the overall kinetics of the oxydrolysis process were found to be limited by the re-dissolution of ferrous sulphate. Pre-crystallization of ferrous sulphate prior to oxidation was found to result in enhanced overall kinetics, cutting down the required retention time from 3 to 2 hours. Enhanced kinetics were also achieved by performing oxydrolysis in two-temperature stages: a low temperature (T = 180°C) first stage with retention time ≥20 min and an elevated temperature (T = 200°C) second stage with retention time ≥100 min.;The typical composition of the hematite material produced in this work was 64.3% Fe, 1.3% S (as SO4), 0.6% Zn, and 4.6% H2O. The sulphur content was found to be predominantly (0.6--0.8%) due to formation of sodium jarosite with the remaining (0.3--0.5%) attributed to SO4 chemisorption, and (to less extent) basic ferric sulphate formation. In the absence of zinc sulphate, the majority of sulphate contamination was due to basic ferric sulphate formation. Hematite was found to form via a predominantly homogeneous nucleation mechanism with sub-micron crystallites clustered together as aggregates of 5--10 mum size and around 7 m 2/g specific surface area. In contrast, hematite produced by direct hydrolysis of ferric sulphate possessed one order of magnitude higher specific surface area. As for the industrial hematite product, its composition was found to be 52.6% Fe, 4.6% S, 1.0% Zn, and 8.8% H2O. The sulphur contamination in the industrial product was mainly due to co-precipitation of jarosite and basic ferric sulphate compounds. Hydrothermal trans-formation of the industrial hematite product at elevated temperatures (≥200°C) and retention time of ≥60 min with solids loading as high as 16 wt.% proved to be effective in reducing the sulphur content to less than 1%. |
