Dispersion and Sedimentation Characteristics of Red Mud

Red mud is the bauxite residue generated from aluminum industries, and due to its low utilization rate, a large volume of red mud in slurry form is stored. The high alkalinity ($\mathrm{pH}>11$) favors leachate and hinders the sedimentation of red mud. Occasional collapse of a red mud reservoir dam like Ajka in Hungary leaves an adverse impact on the greater vicinity due to spreading of red mud particles over a large area. In this research, an attempt has been made to control the dispersive nature of red mud and accelerating sedimentation by adding different additives. Based on pinhole and cylindrical dispersive test, biopolymers (xanthan gum and guar gum) were found to be effective in controlling the dispersiveness of red mud due to the formation of ionic and hydrogen bonds. The high magnitude of the zeta potential ($>30\mathrm{mV}$) of red mud slurry was found to be the hindrance in the sedimentation. Phosphogypsum and salt (NaCl) solutions were found effective in reducing the zeta potential and pH, thereby accelerating the rate of sedimentation of the red mud slurry. The column sedimentation test showed that phosphogypsum is more effective in accelerating the rate of sedimentation due to the formation of agglomerated particles as observed in scanning electron micrography.