The crystallization of desilication products in the Bayer process and its impact on hematite solid/liquid separation properties

摘要

In the Bayer process; reactive silica in the bauxite ore reacts with the sodium hydroxide lixiviant used to dissolve alumina, precipitating as aluminosilicates called desilication products (DSPs). In the modern refinery, pre-desilication is purposely used to lower post-digestion silica in liquor levels in order to minimise the risk of downstream silicate scaling and product quality issues. In this work, information about how the DSP crystallizes from solution silicate and from kaolin is presented, then the impact this has on hematite settling properties, the main bauxite residue constituent, is discussed. The presence of DSP results in the poor flocculation of bauxite residue during its solid/liquid separation process, as evidenced by reduced settling rates, poor overflow clarity and low underflow density during gravity thickening.