Neutralization of Bauxite Residue with Acidic Fly Ash

The objectives of this work were to assess the potential of fly ash to neutralize red mud slurry, an alkaline residue from bauxite ore processing for alumina production, and to evaluate the kinetics of the process. Two samples of fly ash were used to neutralize red mud, both of which were acidic (fly ash 1 [FA1], pH = 3.95 and fly ash 2 [FA2], pH = 4.2) according to the standard evaluation method of pH measurement 5 min after suspending the fly ash in water. Time variant base neutralization capacity (BNC) values were measured for both fly ash types to obtain an estimate of the dose of fly ash required to neutralize a given quantity of red mud. Amorphous silica dissolution was found to be the major contributor of the fly ash BNC. Experimental results demonstrated that both the varieties of fly ash at fly ash: red mud w/w ratio of 0.6-1 could neutralize red mud with an initial pH of 12.5 to pH 11 in approximately 150 days. A chemical equilibrium model developed for the fly ash/red mud system was used to investigate the reactions governing the acid-base chemistry of the system and to obtain estimates of the pH that can be achieved for various doses of fly ash (modeled as silica) added to red mud. Overall, the results of this study indicate that large doses of acidic fly ash are required to neutralize red mud, and the rate of neutralization is slow.