Magnetite solubility and phase stability in alkaline media at elevated temperatures

AbstractA platinum-lined flowing autocláve facility was used to investigate the solubility behavior of magnetite (Fe3O4) in alkaline sodium phosphate and ammonium hydroxide solutions between 21 and 288°C. Measured iron solubilities were interpreted via a Fe(II)/Fe(III) ion hydroxo-, phosphato-, and ammino-complexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. A total of 14 iron ion species were fitted. Complexing equilibria are reported for 8 new species: Fe(OH)(HPO4)−, Fe(OH)2(HPO4)2−, Fe(OH)3(HPO4)2−, Fe(OH)(NH3)+, Fe(OH)2(PO4)3−, Fe(OH)4(HPO4)3−, Fe(OH)2(H2PO4)−, and Fe(OH)3(H2PO4)3−. At elevated temperatures, hydrolysis and phosphato complexing tended to stabilize Fe(III) relative to Fe(II), as evidenced by free energy changes fitted to the oxidation reactions. $$\begin{gathered} Fe(OH)_3^ - + H_2 O_ \leftarrow ^ \to Fe(OH)_4^ - + (1/2)H_2 (g) \hfill \\ Fe(OH)_2^{} (HPO_4 )^{2 - } + H_2 O_ \leftarrow ^ \to Fe(OH)_3 (HPO_4 )^{2 - } + (1/2)H_2 (g) \hfill \\ \end{gathered}$$ For temperatures below 83°C and for a dissolved hydrogen concentration of 234 μmol-kg−1, the activity of ferrous iron in aqueous solution is controlled by a hydrous Fe(II) oxide solid phase rather than magnetite.