An assessment of requirements of neutralising materials of partially oxidised pyritic mine waste rock

The objective of this study was to assess the ameliorating properties of selected neutralising materials added to partially oxidised pyritic mine waste rock from Martha mine pitwall, Waihi, New Zealand. Chemical characterisation showed that the partially oxidised pyritic pitwall rock material had very low pH (<2.1), high electrical conductivity and elevated levels of Al, SO₄/^2-, Fe, Mn, and a total nonoxidised sulfide-S content of 2.6%. Acid-base accounting analysis of the material indicated that it had nil acid-neutralising capacity and a net acid-producing potential of 82 kg CaCO₃/t. The standard buffer method indicated that a lime requirement of 29 kg CaCO₃/t was required to neutralise the total acidity generated from oxidation of liberated pyrite grains. Incubation for 90 days of the pyritic pitwall rock with varying rates of amendments of neutralising materials showed that limestone, dolomite, and fluidised bed boiler ash were all equally effective in raising the pH above 6.0 at a CaCO₃ content equivalent rate (C(ER)) of 30 kg CaCO₃/t. The reactive phosphate rock, however, did not raise the pH above 6.0 even at the highest CER of 50 kg CaCO₃/t. Amendment with neutralising materials also significantly reduced EC, Al, SO₄/^2-, Fe, and Mn in the incubated pitwall rock. Physical examination of the incubated pitwall rock material showed that a large fraction of the coarse-grained neutralising material remained unreactive due to oxyhydroxide coatings.