Iron control in nitrate hydrometallurgy by (auto) decomposition of iron (II) nitrate

Abstract

Iron removal from hydrometallurgical sulphate or chloride systems has not, to date, provided a lowcost route to a saleable iron product. Regeneration of the Fe 3+ ion as an oxidant still relies on slow gas/liquid transfer. The present work evaluates the possibilities in the nitrate system. Iron(III) nitrate leaching of metallic iron or reactive iron sulphides would yield iron(II) nitrate, which in this work was found to decompose spontaneously into iron(III) nitrate and iron(III) oxides in the temperature range 63–103°C, depending on the excess of nitric acid (HNO 3) present. An increase in nitric acid lowers the decomposition temperature. Stirring may have the opposite effect. (Auto) decomposition of Fe(NO 3) 2 involves reduction of nitrate and the gaseous end product was found to contain at least 95% NO in the absence of nitric acid. The iron(III) oxides formed were extremely fine (approximately 1 μm). Hydrolysis of the iron(III) nitrate formed above 120°C gave good filterable iron oxides. Hydrolysis at 160°C or 180°C gave a remarkable increase in particle size. In the more concentrated solutions, hematite spheroids of approximately 25 μm were produced. When sufficient nitric acid is present, only iron(III) nitrate is formed in the (auto)decomposition. A process possibility is proposed where iron(II) nitrate is first (auto)decomposed to iron(III) nitrate, from which a bleed stream is hydrolyzed to Fe 2O 3 and HNO 3. The bulk of the iron(III) nitrate is directly returned to the leaching operation. Air is used for oxidation of the NO evolved during (auto)decomposition of iron(II) nitrate.