Metal Mobilization in Tailings Covered with Alkaline Residue Products: Results from a Leaching Test Using Fly Ash, Green Liquor Dregs, and Lime Mud

The in situ dynamics of repeated percolation of rainwater through unoxidized mine tailings was simulated using a modified column test to predict long-term weathering characteristics. Fly ash, green liquor dregs, and lime mud waste materials from the paper mill industry were used in the column tests to assess the effect of alkaline paper mill residue on pH buffering and controlling the mobility of metals and semi-metals from the tailings. The experiment was continued for 15 cycles, each comprising reaction, leaching, and idle steps (1 week per step). The cumulative percolated water through the tailings represented a liquid:solid ratio of 1.6. The leached content of inorganic elements decreased with time in both the covered and the uncovered tailings. The presence of the alkaline materials consistently reduced the mobility of most contaminants (e.g. Cd, Co, Cr, Mn, Ni and Zn), but not As. The leachate was modeled using PHREEQC. XRD analysis of the remaining tailings samples after termination of the test showed detectable variations in mineral composition. The alkaline residue products all fulfilled the requirements as a cover material from a geochemical prospective, with the exception of As. The green liquor dregs and lime cover increased the pH of the tailings more rapidly than did the fly ash.