Development of a geochemical model to predict leachate water quality associated with coal mining practices

Abstract

South Africa mines coal to supply in the growing energy demands of the country. A majority of these mines are opencast resulting in back filled pits and above ground disposal facilities. Leachate emanating from these disposal sites are saline and in most cases highly acidic. Currently the standard testing procedure to quantify expected leachate qualities include Acid Base Accounting (ABA), Net-acid Generating test (NAG), static-and kinetic leaching. The aim of this study is to model standard humidity cell leach tests performed using the PHREEQC code. This model can then be scaled up to field conditions to model 1D reactive transport. It is commonly accepted that the rate of pyrite oxidation in backfilled pits and waste storage facilities is governed by the rate of oxygen ingress and that no pyrite oxidation take place in the saturated zone. This is not the case for humidity cells, as sufficient oxygen is available for reaction. Pyrite reactions rates in humidity cells is expected to be governed by a combination of available reaction surface and ash layer resistance. This is modelled in PHREEQC (Parkhurst & Appelo, 2003) using the KINETIC block. Leachate composition is then modelled in the column by making use of the TRANSPORT block. The experimental data is fitted by using the reactive surface and ash layer diffusion coefficient as a fitting parameter. PHREEQC does not have a gas transport module to model oxygen diffusion through the column. Due to this shortfall of PHREEQC, the influence of oxygen ingress in the system can not be directly modelled under kinetic conditions. Davis and Ritchie (1986a) proposed an anlylitical solution in which the integrated sulphate production rate can be calculated for a waste heap dump. This rate takes into account the influence of oxygen ingress and the development of an ash layer resistance to the pyrite oxydation rate. This intergrated rate can then be defined in a RATES block in PHREEQC. The Aproximate Analytical Solution (AAS) model proposed by Davis and Ritchie (1986a) is used to scale up the model used for the humidity leach cell experiment. It was found from the modelling results and comparison with PYROX that the model under predicts the integrated sulphate production rate in the initial stages of the reacting waste heap dump. It does however show results that are in close agreement with the results obtained from PYROX in later stages of the lifespan of the waste heap dump. This highlight limitations to the AAS model’s applicability on geochemical problems. The model can only be applied to describe waste heap dumps where the particles at the top of the heap are fully oxidized.