Solubility and phase behavior of Cr(III) oxides in alkaline media at elevated temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr2O3 and FeCr2O4 in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288 degrees C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51 degrees C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)(2) . 3H(2)O solid phase rather than Cr2O3; above 51 degrees C the saturating solid phase is gamma-CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)(3)(H2PO4)(-), Cr(OH)(3)(HPO4)(2-), Cr(OH)(3)(PO4)(3-), and Cr(OH)(4)(HPO4)-(H2PO4)(4-). The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.