The effect of pH adjustment on the properties and pressure filtration characteristics of bauxite residue slurries

Abstract

The influence of the slurry pH on the characteristics of a topical unit operation, pressure filtration of bauxite residue slurries, was investigated in this experimental study. The primary aim of pH adjustment is to precipitate aluminate off from the liquid phase and to facilitate safe disposal of the residue. In the investigated cases, pH adjustment was performed with hydrochloric acid. The experiments were carried out by using two types of pressure filters, a Nutsche filter unit used for the acquisition of average filtration data, and a piston press used for measuring local cake characteristics. The Nutsche filter was used to separate the bauxite residue slurry with a pH of 13.3 and 11.0 at three different filtration pressures (300, 450 and 600 kPa), while the piston press was operated under a constant filtration pressure of 1200 kPa and with a wider pH range (13.3, 11.0 and 7.0). The average particle/agglomerate size increased as the pH was reduced, however, the width of particle size distribution increased somewhat as well when the pH was adjusted to 7.0. Based on the average cake properties, it could be concluded that the specific cake resistance and cake compressibility index increased, while cake solidosity decreased as the slurry pH was reduced from 13.3 to 11.0. The final average solidosity (at 300 kPa) decreased from 0.43 to 0.31 when the pH was adjusted from 13.3 to 11.0. The local filtration properties also showed that the cake became more compressible and the structure of the cake more porous as a consequence of pH reduction. The local hydrostatic pressure measurements indicated that the expression rate of the cakes was decreased for filter cakes formed from a slurry with a lower pH level (pH 11 and 7 compared to a pH of 13.3).