Coupling oxidation to transient drying during multilayer deposition of thickened gold tailings

Abstract

Dewatering tailings prior to deposition is an attractive alternative to conventional practice, as it minimizes the use of embankments and therefore reduces the risk of catastrophic failure associated with dammed slurry impoundments. One of the potential detriments is the absence of a water cover and the consequent increase in oxygen ingress and generation of acid drainage. This paper investigates the amount of oxidation that occurs in shallow layers of desiccating gold tailings during deposition. Oxidation is modelled by coupling transient predictions of unsaturated flow to predictions of oxygen diffusive transport and consumption. A first-order rate law is adopted to model oxidation. Coupling is accomplished through the dependency of oxygen diffusivity on degree of saturation. Experiments and numerical predictions agree within the order of the measurement error, despite the relative simplicity of the oxygen consumption model. Though these tailings crack, the cracks do not influence the drying behaviour, nor do they significantly influence the horizontal distribution of oxidation. The modelling method is applied to the case of a fresh layer of tailings deposited over a deep stack. This analysis shows that some oxidation will occur during deposition of a fresh layer of thickened tailings, and the rate of drying and oxidation is strongly influenced by the underlying previously-deposited tailings. It is also illustrated how drying time to achieve a given density, and consequently oxidation, can be limited by hydraulically decoupling a fresh layer from the underlying stack using a capillary break.