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

The objective of this study was to assess the ameliorating properties of se lected neutralising materials added to partially oxidised pyritic mine wast e rock from Martha mine pitwall, Waihi, New Zealand. Chemical characterisation showed that the partially oxidised pyritic pitwal l rock material had very low pH (<2.1), high electrical conductivity and el evated levels of Al, SO42-, Fe, Mn, and a total non-oxidised sulfide-S cont ent of 2.6%. Acid-base accounting analysis of the material indicated that i t 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 roc k with varying rates of amendments of neutralising materials showed that li mestone, 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 abo ve 6.0 even at the highest CER of 50 kg CaCO3/t. Amendment with neutralisin g materials also significantly reduced EC, Al, SO42-, Fe, and Mn in the inc ubated pitwall rock. Physical examination of the incubated pitwall rock mat erial showed that a large fraction of the coarse-grained neutralising mater ial remained unreactive due to oxyhydroxide coatings.