The effect of calcium and temperature on the transformation of sodalite to cancrinite in Bayer digestion

The transformation of sodalite (SOD) to cancrinite (CAN) in Bayer digestion has been studied for a range of temperatures and time intervals relevant to high temperature bauxite refining. The transformation extent was determined by quantifying phases in digestion residues using an XRD method. It has been found that the transformation is slow in the absence of lime but dramatically promoted by three different Ca-containing sources (calcium oxide, calcite and hydrocalumite) commonly found in the Bayer process. The amount of CAN produced, however, is not proportional to the quantity of Ca charged. The transformation process can be modelled by a form of the Avrami equation. The high activation energy (~ 218 kJ/mol) suggests that the transformation is a chemical process controlled by the nucleation and growth of CAN. The advantages of CAN formation include a reduction of soda in digestion residues and better control of silica and carbonate levels in Bayer liquor. The role of Ca, the efficiency of lime charge and the mechanism of the transformation process are discussed, especially within the context of high temperature and high silica bauxite processing.