Effect of exit shape of compaction mold on coal log quality

The quality of coal logs formed by compacting a coal mixture in a cylindrical mold or die was found to depend to a large extent on the exit shape of the mold (die). An exit with a sharp edge causes cracks or microcracks in the logs due to stress concentration at the edge generated by elastic expansion of the part of the coal log exiting the mold (die), Such stress concentration and damage to coal logs can be greatly reduced by using either a tapered exit (i.e. an exit of gradual expansion) or a round-edged exit of proper dimensions. Tests conducted in this study suggest that a round-edged exit of proper dimensions is the most effective in reducing such damage and in preserving the quality and integrity of the coal logs produced. The log quality was assessed by visual observation, by measuring the density and tensile strength of the logs, by immersing the coal logs in water under high pressure, and by circulating the logs in a pipe flow to determine their abrasion resistance. The study is expected to have implications for the compaction and high-pressure extrusion of other brittle elastic materials, not just coal.