Abstract
Potentially acid generating (PAG) waste rock, that is unmitigated, typically produces acidic drainage requiring treatment prior to discharge to the environment. The intensity of acid production drives the operating cost for water treatment that is highly dependent on the quantity of lime used per unit volume of water treated. The assessment of long-term treatment requirements has not been as well advanced as other aspects of predicting water quality associated with PAG rock. A new dynamic acidity production model was therefore developed to quantify the acid loadings from PAG rock, by considering the intensity and duration of production. The model is based on a stochastic Monte Carlo approach to address the inherent uncertainties in geochemical properties of the rock and the spatial representation of those properties in the waste rock pile.
Application of the model to PAG rock with low to moderate sulphide and carbonate contents at an operating mine provided several interesting insights into the potential outcomes for intensity and duration of acid production and lime requirements – both during operations and after mine closure. A key condition in the mixed PAG/non-PAG rock pile was the availability of reactive sulphide minerals that produce acidity and carbonate minerals that provide alkalinity prior to acidification. With median bulk sulphide contents on the order of 0.5% S, depletion of the available sulphur inventory in much of the PAG rock can occur within several decades, implying the cessation of acid production after that period. Additionally, the variable distribution of sulphide contents in many rock deposits can limit the impacts from the comparatively small portions of ‘high-sulphide’ PAG waste rock if much of the waste rock balance is net-alkaline. The environmentally available sulphide and carbonate inventories, along with the distribution of waste rock geochemical characteristics can substantially reduce the required water-treatment periods for drainage from a PAG waste rock pile.
Even with a large portion of the pile occupied by PAG rock, a combination of low sulphide content and available carbonate minerals may limit acid generation to decades and suggest that water treatment may not be needed in perpetuity.
The modelling approach has identified some key aspects of acid generation and potential requirements for water treatment at mine closure that have not, before now, been considered in mine waste management strategies.