Iron control in nitrate hydrometallurgy by autoclave hydrolysis of iron(III) nitrate

In a previous paper, we investigated the (auto) decomposition of iron(II) nitrate in solution. A filterable precipitate can only be produced by first oxidizing the iron(II) ion to iron(III) in the presence of nitric acid, followed by hydrolysis of the iron(III) nitrate formed at elevated temperature. In this paper, the hydrolysis of iron(III) nitrate in solution at temperatures from 110°Cto 180°C is reported. The precipitates obtained at temperatures lower than 140°C are a mixture of α-Fe2O3 and α-FeOOH, whereas at higher temperature, the goethite transforms to hematite. The rate and amount of iron(III) hydrolyzed was found to be more dependent upon temperature than on the iron(III) nitrate starting concentration. An increase in either increased the degree of hydrolysis. The particle size of the precipitates was found to increase significantly with increasing temperature, above 140°C. The morphology of the precipitate was also found to depend on the hydrolysis temperature, with a low temperature (140°C) favouring a loose structure, as opposed to high temperature (180°C) hydrolysis, which produced a compact structure.