Microwave beneficiation of coal to improve grindability and handleability
Experimental results and analyses have shown that significant improvements in coal grindability (reductions in Relative Work Index) can be achieved by exposing coals to microwave radiation. Experimental data have indicated that low rank coals are highly responsive to microwave radiation, possibly due to their higher inherent moisture content. There is evidence to suggest that gaseous evolution (water vapour and volatile matter) and localised zones of differential expansion (arising for example from occluded mineral matter) in coal during heating give rise to crack formation and hence are the probable causes for the measured increase in coal grindability. The composition of the various coals treated by microwave radiation remained relatively unaltered and there was no significant change in coal calorific value or the proximate and ultimate analyses (dry, mineral matter free basis). Initial (laboratory-scale) microwave trials and pilot-scale testwork demonstrated an improvement in the grindability of various coals. However, the gross energy input for these tests were excessively high (220k WhIt) in comparison to that used mechanically for pulverised coal production (15-20kWhlt). Improvements in microwave cavity design and increased electric field strengths may increase the energy efficiency of the process; however, further work would be required. Additional studies were carried out to evaluate the potential use of microwave technology for coal desulphurisation. The results were encouraging and show that substantial improvements in pyrite separation can be achieved with some coals. Fundamental studies have shown that there is significant change in coal flowability following microwave exposure.