Challenges related to solute analysis of bauxite residue filter cakes

Solute analysis of bauxite residue filter cakes is an important but not very well understood operation carried out at refineries and in scientific research. As more and more refineries rely currently on dry cake disposal after filtration, washing and deliquoring of the cakes, the importance of correctly performed cake analysis will increase. Chemical analysis of bauxite residue filter cakes regarding their soluble compounds, such as the total caustic, soda and aluminate, is performed after forming a slurry from the cake and pure water by using a selected liquid/solid (L/S) ratio. This paper shows that there is currently no cake analysis procedure which could be used without unexpected variation in the results. This is because several factors affect the analysis result at the filtration and sample preparation stages. The factors discussed in this paper are divided into two groups, including the filtration and washing conditions and the sample preparation procedures. In the filtration stage, the filtration temperature and pressure were found to have a considerable effect on the solute recovery. On the other hand, the sample preparation procedure was observed to be affected by factors like the L/S ratio, the composition of the diluting liquid (with respect to its Na2CO3 concentration), the temperature, and the cake leaching time. In addition to the cake solute analysis, particle size measurements and calculation of alkali recovery for the solid-liquid separation stage are discussed as well. To summarize the main findings, it can be stated that significant differences in the results can be seen when any detail in the sample preparation procedure is changed.