1. Coal Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom 2520, South Africa
2. CSIR Materials Science and Manufacturing,
PO Box, 395, Pretoria 0001, South Africa
3. Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia QLD 4072, Australia
4. Eskom Research and Innovation Centre, Lower Germiston Rd., Rosherville, Johannesburg 2095, South Africa