Weathering and Oxidation Rates in Black Shales – A Comparison of Laboratory Methods

Kinetic testing has been conducted on six samples of sulfidic Mount McRae Shale from the Pilbara, West Australia. The total sulfur content of the samples ranged from 0.2 to 8.9%. Whilst the sulfide mineralization was predominantly pyrite, the bulk mineralogy of the samples was dominated by silicates and iron oxides. Acid neutralizing capacity was low and sample classification ranged from Uncertain (UC) for the low sulfide samples, to potentially acid forming (PAF) for the higher sulfide samples.

Various small scale test methods have been developed within the industry to assess the rate of oxidation and solute release from mine waste materials. Whilst humidity cell tests (HCTs) were developed in North America as part of the MEND program, an alternate but similar free draining method was developed in Australia, referred to as the AMIRA free-draining method. These methods are conducted at a similar scale, but differ primarily in the frequency of irrigation, and the temperature at which testing is undertaken. The methods were compared to assess the implications on leach rat es and water quality predictions.

Under acidic conditions, both test methods generated similar trends, and produced approximately equivalent results. One sample contained neutralizing capacity and the two methods yielded different pH profiles; the HCT r emained circum neutral, whilst the AMIRA test developed acidic conditions. A possible cause of the discrepancy is comparatively high consumption of neutralization capacity in the saline AMIRA leachate environment. Further work is required to develop a mo re robust understanding of neutralization processes, and their effectiveness, as a function of leachate salinity.

The test results for acidic conditions would yield similar outcomes when used as the basis for water quality predictions at a larger scale, however, this was not the case for the sample containing reactive ANC.