Assessment of waste rock reactivity involving potential acid generation and/or metal
release generally includes kinetic testing methods such as humidity cells and column
tests. These tests are generally conducted over periods of tens to hundreds of weeks
for the assessment of a single waste rock sample. Each test can provide a single rate
of acid generation or rate of metal release. There is clearly a need for additional test
methods that can provide more cost effective data to assess waste rock reactivity.

The oxygen consumption method, that was initially developed as a tool to assess
reactive tailings in the laboratory and field was adapted to measure reaction rates on
waste rock samples in short term experiments. The technique provides reaction rate
data that can complement data from humidity cells. Many samples can be processed
simultaneously and, therefore, a larger body of kinetic data can be developed for
cost-effective statistical assessment and interpretation. This study represents the
development of the technique, demonstration of the interpretation and a comparison
among several waste rock types from selected mines and exploration projects.

An experimental method was developed to assess the rate of oxygen consumption in
sulphide waste rock. The technique involves placing a reactive rock sample in a sealed
chamber and measuring the gas-phase oxygen concentration over a period of 2 to 3
days. A research programme was undertaken to investigate the influence of particle
size, sulphur content, temperature and inoculation with Thiobacillus ferrooxidans on
the rate of oxygen consumption of waste rock samples. Waste rock and drill core
samples were collected from a number of locations and the oxygen consumption rates
of different rock types were compared and contrasted.

The oxygen consumption rate was expected to increase with decreasing particle size,
increasing temperature, higher sulphur content and inoculation with T. ferrooxidans.
The experimental results generally agreed with the expected trends. Particle size
effects were studied over a range of sizes from 0.07 to 100 mm. The oxygen
consumption rate was a function of 1/dn, where n was generally between 0.8 and 1.6
for uninoculated samples. Values of n were slightly lower for inoculated samples,
typically between 0.3 and 0.7. The oxygen consumption rate of samples inoculated
with T. ferrooxidans was linearly related to the total sulphur content of the waste rock.
Oxygen consumption rates of uninoculated rock samples did not show a strong
dependence on sulphur content. The rate of oxygen consumption in rock containing
pyrrhotite as the dominant sulphide mineral increased by a factor of 2 to 10 after
inoculation with T. ferrooxidans at temperatures of 20 and 30°C. The rates of oxygen
consumption in rocks with pyrite as the dominant sulphide increased by as much as a
factor of 60 after inoculation. The more pronounced increase for pyrite rates appears to
be related to the relatively high rates of pyrrhotite oxidation in the absence of bacteria
compared to those for pyrite under similar conditions. At temperatures below 20°C,
inoculation did not significantly enhance the rates of oxygen consumption.

Sulphate release rates from selected rock samples were measured on the samples
being tested for oxygen consumption and compared to oxygen consumption rates
based on the stoichiometry of pyrite and pyrrhotite oxidation. The oxygen consumption
rate consistently over-estimated the rate of sulphate release for samples containing
pyrrhotite. Partial oxidation of sulphide in the pyrrhotite leading to enrichment of
sulphur on the surface may account for 60 to 90% of the total oxygen consumed by
pyrrhotite. This is consistent with results observed in more fundamental studies of
pyrrhotite oxidation kinetics for early stages of oxidation (weeks to months). Samples
containing pyrite as the dominant sulphide mineral exhibit oxygen consumption rates
that correlate well with the amount of sulphate released.

Oxygen consumption measurements provide rates of reaction in waste rock samples
that can be used to interpret potential acid generation and metal leaching rates. The
data can be used to assess depletion rates for sulphide minerals and neutralization
potential. Although the method does not give water chemistry, the oxygen consumption
values can be used to infer drainage chemistry and can also provide complementary
data for other kinetic testing such as humidity cells. In addition, the results of the
oxygen consumption tests can be used as input values for sophisticated waste rock
models if such methods are required for further evaluation.