Treatment of an iron-rich ARD using waste carbonate rock: bench-scale reactor test results

The treatment of acid rock drainage (ARD) places extraordinary financial burdens on governments and companies worldwide, and an improved efficiency in treatment by as little as 1% can save many millions of dollars in rehabilitation. We investigated a system for treating Fe-rich ARD using a three-stage reactor design. In the first reaction cell, Fe-rich ARD was partially neutralised using rapid periodic carbonate resuspension with a rotating axial mixer. This was followed by an air-sparged oxidation chamber and then a second reaction cell, with more carbonate periodically resuspended until a pH of 6.3 was reached, which was followed by a settlement chamber. This reactor design has a high capacity for neutralisation, with an efficiency of ≈70% of acidity neutralised by the acid neutralising capacity (g of CaCO3 equivalent) added to the reactor. Axial mixers were tested because of their low-energy requirements and their high reliability. The intermediate chamber effectively removes Fe by oxidising Fe(II) to Fe(III). Given the amount of acidity neutralised, the sludge volume produced was low compared to other technologies, providing further potential savings in sludge handling. Waste carbonate rock proved to be an effective neutralising agent, even though it was about 60% dolomite and 40% magnesite, with minor calcite, and despite the fact that magnesite has substantially slower dissolution kinetics compared to the more dominant dolomite. The mixed waste carbonates were capable of raising the pH sufficiently to reduce the heavy metal loadings in Fe-rich ARD by more than two orders of magnitude. The final settlement stage of the process was shown to be essential for metal precipitation, for the carry-over of fine carbonates, and CO2 loss. This was associated with a rise in pH, from 6.3 to 7.5. In addition, residual slow-reacting magnesite from the mixed carbonate remains in the sludge from the first reactor and provides acid buffering capacity within the sludge, which is commonly lacking in the ARD neutralisation sludge of other systems.