Solubility of chromate hydrocalumite (3CaO center dot Al2O3 center dot CaCrO4 center dot nH(2)O) 5-75 degrees C

A secondary precipitate was consistently observed in a series of dissolutio n and precipitation experiments conducted on synthesized Ca-6[Al(OH)(6)](2) (CrO4)(3). 26H(2)O. X-ray diffraction (XRD) patterns, scanning electron mic roscopy, thermogravimetry, and solid digest analyses demonstrate that d-spa cings, crystal morphology, water content, and Ca/Cr/Al ratios for the secon dary solid are consistent with chromate hydrocalumite (3CaO . Al2O3. CaCrO4 . 15H(2)O). Steady-state aqueous ion concentrations indicate that the exper imental 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 3CaO . Al2O3. CaCrO4. 15H(2)O=4Ca(2+) + 2Al(OH)(4)(-) + CrO42- + 4OH(-) + 9 H(2)O at 25 degreesC is -30.38 0.28. The temperature dependence of the log K-SP o btained from six additional temperatures from 5 degreesC to 75 degreesC is logK(SP) = (-2042.1/T) -23.480. Delta Gr(r)(o) and DeltaH(r)(o) for the reaction are 173.1 +/- 3.7 and 39.1 +/- 3.2 0 mol(-1) and DeltaS(r)(o) is -450 +/- 10 J mol(-1) K-1. Using the se values and published partial molal quantities for constituent ions, we c alculate DeltaG(f,298)degrees= -9905 +/- 15.7 kJ mol(-1), DeltaH(f)(o) = -1 1303 +/- 8.3 kJ mol(-1), and DeltaS degrees = 1439 +/- 89 J mol(-1) K-1 for 3CaO . Al2O3. CaCrO4. 15H(2)O. (C) 2001 Elsevier Science Ltd. All rights r eserved.