Column leaching of fine coal tailings: impact on environment and thickener operation

Abstract

The coal mining sector is synonymous with generating large quantities of waste. A comprehensive fraction of the waste is classified as fines (-500 µm) and ultrafine (-100 µm) particles, which accounts eleven per cent of the run-of-mine coal mined in South Africa reported by England et al. These fines are transported to tailings dumps in slurry form as discard. Consumption of water is thus significant in the fines handling. Due to environmental restrictions, most of the water used is recovered – aided by thickeners and filters – and recirculated during beneficiation. The contact time between fine coal and water influences the quality of the recycled water. It is hypothesized that leaching occurs during fine coal tailings slurry handling. Effectively changing the water chemistry to some degree, leading to operational and environmental difficulties. The study focuses on predicting the impact of leaching on the mobilization and speciation of heavy metals and light metals which would possibly contaminate the environment. In addition, investigate the financial and operational impact the change in water properties may pose in the thickening process. Witbank seam 4 tailings were samples were obtained and a twelve-week column leaching experiment conducted. Chemical and mineral composition of the solid tailings were determined by means of X-ray fluorescence (XRF) and X-ray diffraction (XRD) respectively. The XRD specified quartz as the main mineral present. Other minerals were kaolinite, anatase, troilite, and magnetite. Metal analysis was conducted leachate using an inductively coupled plasma optical emission spectroscopy (ICP-OES). PHREEQC was used to model the metal speciation results. Furthermore, batch-settling experiments were conducted using the altered water quality to determine sedimentation properties. The results showed that the leaching phenomenon has a negative impact on the environment and the thickener operation