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

摘要

The removal of inorganic impurities Cl-, CO32- and SO42- from Bayer liquor largely relies on their incorporation into sodium aluminosilicates (ideal formulae Na-8(AlSiO4)(6) X (2)center dot yH(2)O; X = OH-, Al(OH)(4)(-), Cl-, 1/2 CO32-, 1/2 SO42-, 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 degrees C/1 h, 250 degrees C/30 min). Added anions promoted desilication by up to 80% (as solution SiO2) compared to conditions without added impurities. Al(OH)(4)(-) and CO32- 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 SO42- saw a similar extent. The incorporation of each inorganic anion increased with higher reaction temperature. Divalent anions CO32- and SO42- 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.