The anion effect on sodium aluminosilicates formed under Bayer process digestion conditions

The removal of inorganic impurities Cl⁻, CO₃ ^2– and SO₄ ²⁻ from Bayer liquor largely relies on their incorporation into sodium aluminosilicates (ideal formulae Na₈(AlSiO₄)₆ X ₂•yH₂O; X = OH⁻, Al(OH)₄ ⁻, Cl⁻, ½ CO₃ ²⁻, ½ SO₄ ²⁻, y = 0–4), a waste by-product formed from the precipitation of solution silicates. This paper aims to examine the effect of anion incorporation on the structural properties of sodium aluminosilicates precipitated from synthetic Bayer liquors containing 5.0 M NaOH, 2.0 M NaAl(OH)₄, with 0–0.4 M NaCl or Na₂CO₃ or Na₂SO₄ under typical Bayer digestion conditions (150 °C/1 h, 250 °C/30 min). Added anions promoted desilication by up to 80% (as solution SiO₂) compared to conditions without added impurities. Al(OH)₄ ⁻ and CO₃ ^2– were found to be ubiquitous in Bayer sodium aluminosilicate regardless of synthesis conditions, and may occupy up to 75% and 85% of cages respectively; the maximum incorporations of Cl⁻ and SO₄ ²⁻ saw a similar extent. The incorporation of each inorganic anion increased with higher reaction temperature. Divalent anions CO₃ ^2– and SO₄ ²⁻ promoted the phase transformation from sodalite to cancrinite to a greater extent than the monovalent anions Cl⁻, OH⁻, Al(OH)₄ ⁻, which did not exhibit a phase change. The amorphous content in precipitates decreased significantly in the presence of inorganic anions. The modelling of a unique Bayer hydroxy-cancrinite phase was necessary to match the aluminosilicate XRD patterns, and atomic positions and occupancies were determined through Rietveld refinement.