1. Department of Mining Engineering and Mine Surveying, University of Johannesburg, Doornfontein 2028, South Africa
2. Department of Mining Engineering, Federal University of Technology, Akure 340110, Nigeria
3. DSI/NRF Clean Coal Technology Research Group, School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, WITS, Johannesburg 2050, South Africa
4. Department of Civil and Mining Engineering, University of Namibia, Windhoek 13301, Namibia