Advanced high internal phase water-in-oil technique for the beneficiation of weathered ultra-fine coal discards

Mashaba, Poitah Manche
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Continuous use of mechanised mining and the convectional beneficiation of South Africa’s run of mine coal has led to over one billion five hundred million tons of fine discard coal (-1 mm) stored/dumped in South Africa (Department of Energy, 2001). This study was carried out using HIP W/O technique, known as advanced high internal phase water-in-oil emulsion binder. The as-received weathered discards coal comprised of fixed carbon content ranging from 17.79% to 43.29%, with calorific values ranging from 8.48 MJ/kg to 20.94 MJ/kg. The ash content of the coal feeds ranged from 28.63% to 48.71%. Out of the four prepared HIP W/O emulsion binders, emulsion binder 1 was stable and used to beneficiate all the coal discards. The beneficiation of the coals was conducted at different concentrations (5 kg/ton, 7 kg/ton and 9 kg/ton) for the emulsion binder and kerosene at a constant solid’s concentration (25%) and conditioning time of 30 seconds. A clean coal product with an ash content of 11.76%, a mass yield of 15.61%, and calorific value of 28.61 MJ/kg was produced using 5 kg/ton of HIP W/O emulsion from coal 1. In addition, clean coal floats with an ash content of 17.47%, calorific value of 26.42 MJ/kg, and a recovery of 21.59% was also obtained from 5 kg/ton of kerosene using coal 1. The clean coal products were upgraded from coal 1, with a feed ash of 29.34% and calorific value of 20.94 MJ/kg under the second stage of froth flotation. Four highly weathered discard coal feeds (discard coal 2, 3, 4 and 5) were also beneficiated using 5 kg/ton of the emulsion binder. The calorific values (8.48 MJ/kg and 16.29 MJ/kg) for the feed discard coal 2 and 3, respectively, were upgraded to 18.16 and 25.77 MJ/kg, respectively. Moreover, discard coal 4 and 5 produced clean coal products with a heat content of 27.62 MJ/kg and 26.28 MJ/kg. from the feed calorific value of 20.61 MJ/kg and 18.55 MJ/kg, respectively. The response surface method was applied to the optimisation of low-grade coal 2 and 3. Clean coal products were achieved under the emulsion concentrations (5 kg/ton, 7 kg and 9 kg/ton) and flotation agitation speeds of 1200 revolutions per minute (rpm), 1500 rpm and 1800 rpm. The beneficiation results showed that the calorific value increased with increased agitation speed and decreased emulsion concentration. The feed calorific value of discard coal 2 and 3 was increased from 8.48 MJ/kg and 16.29 MJ/kg to 19.45 MJ/kg and 28.12 MJ/kg, respectively, with 5 kg/ton of emulsion binder and an agitation speed of 1800 rpm. With kerosene and at 5 kg/ton, a clean coal product with 19.05 MJ/kg and 27.20 MJ/kg was achieved. In this research, the HIP W/O emulsion approach was established as an alternative approach to upgrade weathered discard coal dumped for over three decades and achieved better quality product compared to kerosene. The produced clean coal products can be used directly for combustion and co-combustion solid fuels in power generation. However, the HIP W/O emulsion binder requires only 0.25 kg/ton of organic liquids (kerosene and Span 80). In contrast, 5 kg/ton is required under kerosene to produce clean coal products. The study showed that the HIP W/O process could recover high-grade coal from oxidised fine coal discards with calorific values ranging from 8.48 MJ/kg to 20.94 MJ/kg.
A research report submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and Built Environment, University of Witwatersrand, Johannesburg, 2022