Solubility of chromate hydrocalumite (3CaO·Al 2O 3·CaCrO 4· nH 2O) 5–75°C

Abstract

A secondary precipitate was consistently observed in a series of dissolution and precipitation experiments conducted on synthesized Ca 6[Al(OH) 6] 2(CrO 4) 3·26H 2O. X-ray diffraction (XRD) patterns, scanning electron microscopy, thermogravimetry, and solid digest analyses demonstrate that d-spacings, crystal morphology, water content, and Ca/Cr/Al ratios for the secondary solid are consistent with chromate hydrocalumite (3CaO·Al 2O 3·CaCrO 4·15H 2O). Steady-state aqueous ion concentrations indicate that the experimental aqueous solutions were in apparent equilibrium with both the Cr(VI) analog of ettringite and chromate hydrocalumite. Over the pH range 10.9 to 12.2, the log of the solubility product (log K SP) for the reaction 3 CaO· Al 2 O 3· CaCrO 4·15 H 2 O⇄4 Ca 2++2 Al( OH) 4 −+ CrO 4 2−+4 OH −+9 H 2 O at 25°C is −30.38±0.28. The temperature dependence of the log K SP obtained from six additional temperatures from 5°C to 75°C is logK SP = −2042.1 T −23.480. Δ G r° and Δ H r° for the reaction are 173.1±3.7 and 39.1±3.2 kJ mol −1 and Δ S r° is −450±10 J mol −1 K −1. Using these values and published partial molal quantities for constituent ions, we calculate Δ G f,298°=−9905±15.7 kJ mol −1, Δ H f°=−11303±8.3 kJ mol −1, and ΔS°=1439±89 J mol −1 K −1 for 3CaO·Al 2O 3·CaCrO 4·15H 2O.