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.

A Simple Water Retention Model Based on Grain Size Distribution

“…Indeed, coal macerals are known to be more hydrophobic than minerals [37][38][39][40][41][42] and the contact angle of a given fraction depends on the relative proportion of macerals and minerals. In addition, it was found that those proportions vary with particle size [46].…”

“…Thus, consideration of the contact angle of a material is relatively simple to implement, at least for materials that have the same contact angle for every particle. However, coal tailings contain a mixture of coal and mineral particles, and the coal particles can be more prevalent for particular size ranges, as indicated by the correlation between specific gravity (G s ) and particle size for one studied coal tailings [46]. No models have been found that can consider contact angle variations with particle size; however, for GSD-WRC models in particular, it would be simple to implement such the correction in [37], as a suction is associated with each particle size range.…”

Coal mining wastes valorization as raw geomaterials in construction: A review with new perspectives

“…Moreover, except those wastes buried permanently below the water table, most CMWGs are unsaturated, yet there are few experimental researches in the literature characterizing the unsaturated properties of CMWGs. The most recent studies (Fityus and Li 2006;Vidler et al 2020) highlighted the importance of mineralogical composition, grain size distribution (GSD), pore size distribution (PSD), weathering and fabric, water retention and permeability.…”

“…Ding and Liu (2021) immersed a soft slate in water for different durations and found that it disintegrates into particles with GSDs obeying different fractal distributions. Latest studies show that the GSDs of many mine tailings (Qiu and Sego 2001;Vo et al 2020) and coal tailings (Islam 2021;Salam et al 2019;Vidler et al 2020) also exhibit fractal characteristics. Russell (2010) described a GSD exhibiting a single fractal scaling as:…”

“…They concluded that on drying from a saturated state, processed coal would have negligible suction until Sr < 0.5-0.6, hence much of the moisture in stockpiled coal would not contribute to forming films and adhesion to suppress the release of dust. Vidler et al (2020) obtained the soil water retention curves of a coal tailing, the mineral fraction and the coal fraction using a tensiometer and a dewpoint potentiometer. They found that the presence of coal in the tailing has a significant impact on the water retention behaviour of the coal tailing.…”