Bayer process plant scale: Transformation of sodalite to cancrinite

An investigation of the deposition and in situ transformation of scale found in a Bayer process plant has been carried out using X-ray powder diffraction and FTIR studies. Scale samples were analysed as a function of their position in the Bayer process circuit. Scale precipitated during bauxite digestion at approximately 255 degrees C was found to be mostly cafetite but also contained haematite. At 120 degrees C boehmite has been identified as the main scale phase formed from ''spent'' liquor (i.e. liquors from which Al(OH)(3) crystallisation has previously occurred). Three sodium aluminosilicate phases were found to form between 150 and 255 degrees C, sodalite(1), sodalite(2) and cancrinite although thermonatrite (Na2CO3 . H2O) and calcite (CaCO3) were also observed periodically. The ratios of cancrinite to sodalite(1) and sodalite(2) to sodalite(1) were observed to increase with the temperature of formation, The scale phases found in a cross section of plant scale formed at 150 degrees C show a similar trend on increasing the in situ age of the scale. Comparison with precipitation from synthetic solutions has indicated that the aging mechanism of the sodium aluminosilicate deposits is the same in both cases: sodalite(1) (cubic, a approximate to 8.98 Angstrom) --> sodalite(2) (cubic, a approximate to 8.89 Angstrom) --> cancrinite (hexagonal, a approximate to 12.70 Angstrom, c approximate to 5.18 Angstrom). The transformation from sodalite(2) to cancrinite has been shown to be the rate determining step in cancrinite formation.