MAGNETITE SOLUBILITY AND PHASE-STABILITY IN ALKALINE MEDIA AT ELEVATED-TEMPERATURES

A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of magnetite (Fe3O4) in alkaline sodium phosphate and ammonium hydroxide solutions between 21 and 288 degrees 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)(-). Ar 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. Fe(OH)(3)(-) + H2O reversible arrow Fe(OH)(4)(-) + (1/2)H-2(g) Fe(OH)(2)(HPO4)(2-) + H2O reversible arrow Fe(OH)(3)(HPO4)(2-) + (1/2)H-2(g) For temperatures below 83 degrees C and for a dissolved hydrogen concentration of 234 mu mol-kg(-1), the activity of ferrous iron in aqueous solution is controlled by a hydrous Fe(II) oxide solid phase rather than magnetite.