9. Bayer Process—Water and Energy Balance

This chapter aims to illustrate and discuss the inputs and outputs for both water and energy in the context of the Bayer process, highlighting throughout the sections their interconnectedness. It breaks down these inputs and outputs in light of the unit operations of the Bayer process and discusses the reasons as to the variations in consumption figures, based on refinery design and other raw material considerations. The section on water shows how different technological developments, in conjunction with the bauxite to be processed, can drive much of the Bayer loop water balance. Explanations are given as to why boehmitic bauxites have the potential to drive somewhat different water balances compared to gibbsitic bauxites, owing to higher digestion temperatures and the removal of water within their precipitation circuits. Going through the history of the Bayer process and the changes brought out by modern equipment, examples show how the alumina refining process has undergone significant change over time. From a design standpoint, examples discussing caustic washing systems for precipitators highlight the significant impact this critical system can have on both equipment availability as well as the plant water balance. Beyond the Bayer loop, the greater water balance of the refinery is considered, showing how much of this balance is a slave to the climatic conditions of the refinery, but also highlighting how residue disposal technology has significantly changed the net balance through adjusting the areas for water catchment versus evaporation. Within energy, a significant focus is placed on the importance of precipitation yield of alumina in the Bayer liquor, in determining a plant energy efficiency, and how this can perhaps help to explain the variability in different plant energy numbers. The quality of the bauxite is also discussed, including the analysis of energy requirements in high temperature digestion refineries, or boehmitic/diasporic bauxite operators. The subsequent section on clarification reinforces the importance of yield further, emphasizing how residue handling solutions in different parts of clarification have both hindered and helped yield and recovery. The sections on heat interchange and precipitation show how modern design improvements such as plate heat exchange between green and spent liquor as well as inter stage cooling have unlocked even more potential in energy efficiency.