Mixing of Acid Rock Drainage with Alkaline Ash Leachates: Formation of Solid Precipitates and pH-Buffering

Three metal-rich, acidic mine waters (from Bersbo and Ljusnarsberg, Sweden) were mixed with alkaline fly ash leachates in various proportions, representing a pH titration. Changes in pH and the loss of metals in solution due to precipitation of solid phases were tracked. Mineral equilibria and changes in pH and alkalinity were simulated using the geochemical code PHREEQC and the MINTEQv4 database, and the measured and simulated pH responses were compared. The formation of solid precipitates corresponded to fairly well-defined pH-buffering regions, reflecting the mine water compositions (notably the levels of Fe, Al, and Mn). Zn precipitation had a distinct buffering effect at near-neutral pH for the mine waters not dominated by iron. The formation of solid Mg phases (carbonate, as well as hydroxide) was indicated at high pH (above 9), but not formation of solid Ca phases, despite high sulfate levels. The phases that precipitated were various amorphous mixtures, mostly of the metals Fe, Al, Mn, Zn, and Mg. For the Fe-rich mine water, pH was poorly simulated with a simple MIX model, while alkalinity predictions agreed reasonably well with measured data. For the Al-rich mine waters, the simulated pH responses agreed well with the measurements. In an additional step, geochemical simulations were performed where selected proxy phases for major elements were forced to precipitate; this significantly improved the pH and alkalinity predictions. This approach may be more efficient than performing mixing experiments and titrations.