Digestion efficiency is a process performance metric in alumina refining that quantifies the proportion of total aluminium present in the bauxite feed that is successfully dissolved into the caustic soda solution during the Bayer Process digestion step. It is typically expressed as a percentage and is calculated by comparing the aluminium extracted in the sodium aluminate liquor against the total aluminium content of the ore feed, as determined by chemical assay. High digestion efficiency is directly linked to overall alumina recovery and plant profitability, as unextracted aluminium is lost with the bauxite residue (red mud). Digestion efficiency is influenced by several factors including the mineralogical composition of the bauxite (particularly the ratio of gibbsite, boehmite, and diaspore), particle size distribution after grinding, caustic concentration, digestion temperature and pressure, residence time in the digester, and the degree of pre-desilication. Diaspore-bearing bauxites inherently require more severe conditions to achieve equivalent digestion efficiency compared to gibbsite-dominant ores. Process engineers continuously monitor and optimise digestion efficiency through real-time sampling, liquor analysis, and mass balance calculations to maximise alumina output and minimise costly alumina losses to residue streams.
