Lime properties and dose effects on causticisation of synthetic Bayer liquor

Abstract

The production of alumina (Al2O3) from bauxite via the Bayer process is underpinned by the use of lime in a variety of process reactions. One of the most important is the causticisation of process liquors, in which lime is used to convert soluble sodium carbonate into caustic, i.e. sodium hydroxide. Causticisation is complicated by unproductive side reactions with sodium aluminate that limit both the extent of causticisation and the efficiency of lime use. Substantial industrial effort and literature has been devoted to optimising causticisation process parameters (e.g. temperature, reaction time, liquor concentrations, and reactor mixing regimes) but little attention has been given to determining the impacts of lime properties and dose. In this study a test procedure was developed and used to evaluate the impact of lime dose, in both slaked and dry quicklime formats, on causticisation performance. This was used to determine the effects of various lime properties, including (1) chemical composition (purity), (2) particle size and (3) reactivity on the causticisation of synthetic Bayer liquors. The results show that, of these parameters, the differences in lime chemical composition had the most significant impact. The methodology can be used to facilitate optimal lime reagent choice and dosage in industrial alumina Bayer refineries. The potential impact of lime utilisation efficiency is also illustrated in the context of overall CO2 emission intensity.