Effect of coal on mine tailings’ water permeability and water retention

Abstract

For safe and efficient mining operations to occur the management of waste materials is required, which often takes the form of geotechnical structures constructed from this waste. The safe use of these structures requires a number of resources, one of these being sufficient information about the waste material properties. For example, the drying process of a tailings dam is predicted with the water retention and permeability of the tailings. When considering coal tailings, which are comprised of coal and mineral soil particles (typically), the presence of coal may be problematic. The localised hydrophobicity of coal molecules may have a unique effect on water permeability and retention; this is relevant to geotechnical analysis where hydrophilic behaviour is assumed. To explore the possible effect of localised hydrophobicity, mine tailings were obtained from a coal mine of the Hunter Valley, NSW, Australia, and the coal fraction was separated via density separation. After this, three materials were available: unchanged mine tailings and a coal and mineral fraction of tailings. The goal was to characterise the three materials and allow deeper insight on what effect the addition of coal has on retention and hydraulic properties. Characterization involved measuring particle size distribution, pore size distribution, soil water retention curve, and saturated water permeability. The results show that there are distinct differences in the water retention and permeability properties of each material, and a number of these differences could be explained by the differing particle/pore sizes observed in each material. However, the coal containing materials desaturated at low suctions (< 10 kPa) compared to the mineral fraction, which could not be explained by particle/pore size differences and points towards localised hydrophobicity as a possible cause.