Polymer Flocculant Under The Bayer Process Red Mud Settling Behavior Study

Red mud generated from Chinese diaspore bauxite has a poor settling performance, so that red mud settlement requires excellent polymeric flocculants. However, most of these excellent polymeric flocculants are imported products. In the view of a lack of theoretical basis for the design and selection of polymeric flocculants used in red mud separation, the settling behaviour of red mud treated by different flocculants was studied in this paper to investigate the interaction mechanism between red mud and flocculants.Red mud surface charge properties were investigated using acid/basic potentiometric titration. The results show that the adsorption of sodium polyacrylate (SPA) causes the increase of red mud surface hydroxyl groups and the decrease of the red mud point of zero charge (PZC). However, the adsorption of polyacrylamide (PAM) causes the reverse result. The changed amount of red mud surface hydroxyl groups increases with the dosage increase of flocculants. The more the silica-containing compounds in red mud, especially in the case of sodalite, the more the red mud surface hydroxyl groups and the lower the PZC.The factors to affect red mud settlement were studied by red mud settling tests. The results show that excessive negative or positive surface charges result in a failed red mud settlement. Neither excessive nor insufficient flocculants are helpful for red mud settlement. Red mud treated by SPA has a lower settling rate than by PAM, but has a higher supernatant clarity degree. Red mud settling rate decreases obviously with the increase of the PAM hydrolysis degree. Red mud settling results from polyacrylamidomethyltrimethyl ammonium chloride (PATAC) are much worse than those from SPA.The hydrolysis degree and intrinsic viscosity of PAM dissolved in NaOH were measured by conductivity/pH titration and Ubbelodhe viscosimetric measurement, respectively. The results indicate that the hydrolysis degree of PAM dissolved in NaOH solution increases with the increase of NaOH concentration and the intrinsic viscosity increases at first and then decreases due to the formation of intramolecular hydrogen bonds. The results from thermogravimetric analysis and infrared spectral analysis show that PATAC is unstable in caustic solution at a higher temperature and its decomposition product HPATAH has two functional groups of -CH₂-OH and -CONH₂ to form a -CH₂-OH～-CONH₂ intramolecular hydrogen bond.Some interaction mechanisms between polymeric flocculants and red mud were discussed. SPA and PAM interact with red mud surface by hydrogen bond or Ca²⁺ bridge, which cannot effectively reduce the supernatant turbidity of red mud slurry. And the salicylate group forms a stable six-membered ring complex with Fe³⁺ from red mud surface to greatly improve the supernatant clarity degree. The formation of intramolecular hydrogen bonds in polymer chains results in the flocculants having a poor flocculation performance for red mud.