Kinetic testing typically consists of free draining column tests (e.g. humidity cells). Leachate concentrations are used to calculate solute release rates, which provide a measure of the material reactivity and allow assessment of the potential for acid and metalliferous drainage. These tests are not suited to materials that do not drain freely (e.g. fine grained tailings).
Coal processing wastes (i.e. tailings, mixed rejects and tailings, and coarse rejects) collected from coal mines in the Bowen Basin included a fine-grained fraction, clay minerals and coal (organic) macerals. Due to the abundance of clays and macerals, the samples for testing were not free draining, resulting in saturated solids with low to no leachate recovery.
An alternative kinetic test method was developed that involved increased duration between flushing events (8 weeks) and extended contact between reacted solids and deionised water (several days) to maximise dissolution/elution of solutes for leachate analysis. Leachate was recovered via Rhizon® pore water samplers operated under vacuum over a period of 1 – 2 weeks – leaving dewatered, reacted solids with a gravimetric water content (GWC) of around 15-18 %. The compact mass of dewatered solids was broken up to create aggregate like clods (<20 mm) for the next unsaturated weathering cycle.
The bed of clods in the weathering columns is loosely packed corresponding to a dry bulk density between 0.5-1.0 g/cm3. Weathering is undertaken using an incubator (30 oC) whilst maintaining the GWC within the above range. Physicochemical conditions during the weathering cycle are near-optimal for abiotic and biotic reactions, including oxidation of sulfides (predominantly pyrite) and complex coal polymeric organic compounds.
As well as leaching, gaseous measurements of oxygen and carbon dioxide were undertaken (30 oC) for the estimation of oxygen consumption and carbon dioxide release rates.
This paper describes the methodology in greater detail and presents selected outcomes from testwork. The method may be applied to other commodities with material of similar properties, overcoming limitations of free draining column tests and generating data more representative of the unsaturated conditions expected within waste stockpiles post closure.
