First order characterisation of seepage quality from Coal Discard — Fly Ash mixtures

摘要

The purpose of the assessment is to compare likely seepage quality from alternativebackfilling configurations being considered for an open cast coal mine. The backfill materialwould include coal discard and ash from a nearby coal-fired power station. Alternativeconfigurations included a base case (coal discard only), mixed (ash and discard) and layered(ash and discard) options. Samples of these materials were combined in ash:discardproportions of 15%:85% and 25%:75% to assess the proposed mixed backfillingconfigurations. The conceptual model of seepage quality under unsaturated conditions considers thatinterstitial water is driven from the material by infiltration. Seepage is therefore determinedby the volume of leachate developed over time and the interstitial pore water quality. Unsaturated flow modelling of one-dimensional columns representing the proposedplacement configurations indicated moisture conditions varying from approximately 5% to20%. Simulation of seepage qualities was conducted using equilibrium modelling software, theresults of humidity cell testing and modelled moisture content. Modelling indicated lowersulphate loads associated with mixed ash-discard materials, however, acidification wouldpotentially result in significantly higher metal concentrations due to the high metalavailability in ash. Layering of ash over discard above the water table therefore appears to beassociated with the lowest long term seepage quality risk as it provides leachableneutralisation potential to mitigate acid production in underlying discards while maintaining anon-acidic environment to limit metal mobilisation from the ash.