Desilication is a chemical process used in the refining of bauxite ore to reduce the concentration of dissolved or reactive silica (SiO₂) in aluminate liquors, thereby preventing the formation of scaling deposits (desilication product or DSP) that would otherwise precipitate throughout the Bayer Process equipment and reduce process efficiency. It is a critical unit operation in alumina refineries that process bauxite ores with high reactive silica content, particularly monohydrate bauxite ores such as diaspore and boehmite. In the Bayer Process, bauxite is digested in hot caustic soda (sodium hydroxide) solution to dissolve the aluminium-bearing minerals while leaving inert minerals — including silica — as undissolved residue. However, reactive silica dissolves into the caustic liquor as sodium silicate and subsequently reacts with sodium aluminate to form sodalite-type minerals (collectively called desilication product), which deposit as scale on heat exchanger surfaces and pipe walls, impeding heat transfer and flow. The desilication process involves holding the caustic liquor — after initial digestion — at elevated temperatures and pressures (typically 150–175°C) in agitated vessels (desilication tanks) for an extended residence time. This promotes the controlled precipitation of dissolved silica as sodalite or cancrinite crystals before the liquor enters downstream heat exchangers and clarifiers. The precipitated DSP is removed from the liquor by filtration or settling along with the red mud residue. Effective desilication reduces silica concentrations in the pregnant liquor, improves the quality of the resulting alumina trihydrate product, and extends the service life of process equipment. Desilication is particularly important for bauxite deposits with reactive silica contents above approximately 3% by weight.
