Surface properties of red mud particles from potentiometric titration

Acid/basic potentiometric titration has been used to obtain data on the surface charge and the amount of surface hydroxyl groups on red mud particles generated from different bauxite sources. It has been demonstrated that this can be used to quantify the surface properties of red mud particles. Red mud particles carry a significant negative charge under the basic conditions that exist in the Bayer process for alumina production due to ionized hydroxyl groups on their surfaces. Hydroxyl groups and corresponding surface charge are primarily due to the silica-containing compounds formed in the Bayer process. Addition of a synthetic, organic flocculant to red mud slurries causes the negative charge on the surface of mud particles to rise, which is reflected in the shift of the point of zero charge (PZC) to lower pH values. Under the conditions studied, red mud slurries have the same PZC after flocculation with synthetic flocculant, independent of the bauxite origin and variation in digestion conditions. In general the PZC was found to be sensitive to the presence of flocculant in the slurry and the mineralogical composition. When compared to sodium ions, potassium ions have a higher affinity to the surface of red mud particles and exhibit preferential adsorption. (C) 2001 Elsevier Science B.V. All rights reserved.