Coal Preparation » Dewatering
Dewatering of ultrafine coal and tailings is a big challenge to the coal industry. In this project, the dewatering of ultrafine coal and tailings in the presence of chemicals (anionic and cationic surfactants, and anionic flocculants) were investigated using surface characterisation techniques, lab scale dewatering facilities, and pilot scale solid bowl centrifugation.
Specifically, the pilot scale experiments were conducted at a CPP in the Bowen Basin using a pilot scale solid bowl centrifuge. The degree of difficulty associated with fine coal tailings dewatering differs from mine to mine as the process water and surface properties of tailings can change significantly. These dewatering and handling problems are caused by the complex surface properties and gelation due to the swelling characteristics of smectite-type clays, which can result in high yield stress, high dosages of dewatering chemical aids, low settling rates, and poor supernatant clarity. Therefore, further studies on samples received from different coal preparation plants in the Bowen and Surat Basins using a pilot scale solid bowl centrifuge were conducted. In this regard, pilot scale trials on tailings thickener underflow were completed at different pool depths and with different differential rates to maximise capture of ultra-fine clay mineral particles without using chemicals.
Characterisation and lab scale dewatering studies showed that applied anionic surfactant in the filtration process could decrease the final moisture significantly through decreasing surface tension of water while in the settling process acted as a dispersant and decreased the settling rate in both coal and tailings dewatering practice. Cationic surfactant adsorption was observed on both coal and tailings and its neutralisation effect and increasing hydrophobicity of the particles was the main driving force in improving dewatering performance. Flocculants increased solid recovery considerably, but increasing final moisture by capturing water between flocculated particles was the adverse effect of the flocculants.
The effect of mixed anionic surfactant/anionic flocculant and cationic surfactant/anionic flocculant on dewatering coal and tailings were also studied. In both coal and tailings trials, mixed anionic surfactant/anionic flocculant yielded a product with lower final moisture by rupturing flocculated particles structure and releasing entrapped water. However, this effect was stronger for coal dewatering rather than tailings dewatering. Mixed cationic surfactant/anionic flocculant increased dewatering rate and solid recovery with lower moisture in comparison with single flocculant condition. There was not a significant difference when the sequence of adding the addition was changed. Pilot scale trials on flotation concentrate, tailings, and diluted tailings thickener underflow were conducted at different residence times and centrifugal forces.
The effect of chemicals to improve the performance of the centrifugal dewatering was also investigated. The results showed a minimum filter cake moisture of 23% at ~ 1500 g-force for coal and 29% at ~1000 g-force for tailings. However, the effluent water quality obtained under this condition was higher than expected. The laboratory results for total dissolved solids (TDS) were around 0.46% for coal and 0.45% for tailings. The anionic flocculant and the cationic surfactant used in the test work showed an improvement in capturing ultrafine coal and tailings particles. Increasing anionic flocculant dosage up to 0.2 kg/t captured more ultrafine coal and tailings particles than the cationic surfactant. It also produced a better quality of the effluent water (with around 0.34% TDS for coal and 0.38% TDS for tailings), but there was a 3% increase in the cake moisture. The cationic surfactant at 2 kg/t dosage also resulted in an improved quality of effluent water with 0.40% TDS, and there was an adverse effect on the final moisture of the cake. The different feed type studies results showed that in the presence of clay tailings containing more negative surface charge and swelling types, pool depth in the clarification zone of the centrifuge should be increased to recover an acceptable clear centrate with some sacrifice in cake moisture content. It was also discovered that process water chemistry played a critical role in affecting the dewatering performance through neutralisation of the particles surface charge. Losing ultrafine tailings particles during centrifugal dewatering (in the presence and absence of the flocculant) with increased pool depth in spite of improved recovery is still an unsolved problem and detailed studies are required for designing novel chemicals for dewatering fine coal tailings efficiently.