NEW APPROACH TO LEACH TESTING OF BAUXITE RESIDUE TO SUPPORT MANAGEMENT DECISIONS IN VIEW OF BENEFICIAL APPLICATION AND STORAGE
Bauxite residue storage, treatment and re-use practices are receiving increasing attention as refineries strive to reduce the potential environmental impact of their operations. In the past, leaching tests such as the Toxicity Characteristic Leaching Protocol (TCLP) and the Australian Standard Leaching Protocol (ASLP) test have been used to evaluate wastes for their potential to release trace elements. These tests are limited in their usefulness, as they provide leaching data at a limited number of pH values, and do not provide information about the long-term behaviour of the residue. A new approach to characterisation of leaching behaviour, previously developed by the Energy Research Centre of the Netherlands (ECN) and Vanderbilt University (Nashville, US), has been applied to various bauxite residue samples. This approach involves characterising the leaching behaviour of bauxite residue using a standardised pH dependence leaching test over a pH range from 4–12, and a standardised percolation leaching test. The leaching data was modelled using a geochemical speciation program to determine which chemical processes are controlling the leaching behaviour of bauxite residue. This approach, when applied to bauxite residue, shows that the nine residue samples examined behave very similarly for most elements, even when generated by different refineries that utilise bauxite from different sources. This implies that the solubility controlling minerals and other release controlling factors are most likely the same for this wide range of samples. Residue treatments such as carbonation and seawater neutralisation do affect the leaching of some elements, resulting in a change in the leaching behaviour of the treated material as compared with the untreated sample. Using geochemical speciation modelling the changes are largely explained, which provides a basis for prediction of long term behaviour.