The effect of particle size distribution on redox zonation in gold mine tailings

Abstract

Sulphidic mine waste poses a threat to environmental health. This is due to its potential to generate acidic fluids that may contaminate ground- and surface water bodies. To be able to mitigate the release of these hazardous fluids from sulphidic mine tailings, accurate predictions of the capacity of the tailings to be able to generate AMD needs to be made. Current strategies use standardised tests and methodologies such as Acid-Base Accounting and applying them uniformly to all types of tailings for predicting the potential generation of AMD. These methods use assumptions about the development of oxidation zones and the depth of the phreatic head based on studies from other sites. This leads to uncertainty concerning the depth and evolution of oxidation zones in tailings and proves to be a shortcoming in geochemical assessments, predictive modelling and the establishment of accurate oxidation profiles and the determination of the phreatic head. To solve this problem, the physical characteristics of the tailings facility needs to be taken into consideration as well. Tailings are artificial and unlike natural soils, they are more angular due to the crushing processes used to extract desired minerals from rocks. The mechanics principles of soil can, however, be applied to tailings. Tailings dams are constructed with the materials it contains over time. Layers of volumes of sediments are deposited depending on the rate of excavation and extraction of minerals. These layers of sediments contain coarser and finer materials, the gradation of these materials have an important effect on geotechnical properties which determines parameters such as permeability and density of the tailings materials. The physical characteristics, such as the particle size and the type and distribution of void space of the waste, has an effect on the moisture content and rate of oxygen ingress. Moisture and oxygen are the primary components needed for the oxidation process to occur. Understanding the development of oxidation zones (i.e. the processes and depth) is crucial in predicting the quality of pore water in the tailings impoundment, as oxidation precedes acidification. The volume and quality of the AMD that could potentially be generated is directly related to the volume of the oxidized tailings. Thus, by correlating the physical characteristics with oxygen and moisture abundances, the results can be used to predict potential oxidation zones that could develop within the material. The study conducted was carried out on a decommissioned gold mine tailings facility just outside of Randfontein. Four sampling sites were chosen based on accessibility on the tailings facility. A manual auger hole was drilled at each sampling site. A sample was taken at a 30 cm interval throughout the auger hole profile.

The particle size distribution was determined by using a hydrometer method. The results indicated that the particle size distribution throughout the auger hole profile varied in layers that can be attributed to the deposition method or the milling method used at the time of deposition. The moisture content was determined by using a Neutron Moisture probe in the field, the moisture content was correlated with the particle size distribution and indicated that the larger the particles in the layer, the less the moisture content. The oxygen content was determined by planting probes in the auger holes, the oxygen content generally depleted at approximately 1,2 m below the surface. This can be due to the consumption of oxygen through chemical processes or the slow diffusion of oxygen through the material. The oxygen content was correlated with the moisture content and indicated that where the moisture content increased, oxygen content rapidly decreased. This is due to the slow rate of diffusion of oxygen in the water. Although there was an indication that the particle size distribution influenced the formation of redox zones in RF 1, but it could not be said for RF 2 to RF 4. The particle shape was not taken into consideration. The particle shape has an effect on void spaces in the tailings body which in turn has an effect on moisture and oxygen content within the profile and should be considered in future work.