Chemical Cleaning of Miike Coal for Exhaustive Demineralization and Desulfurization.

Chemical cleaning is an effective method to remove the fine mineral matter and organic sulfur from coal which are difficult to separate with physical cleaning method. The alkali-acid leaching processes, such as aqueous caustic process and molten caustic process, are well known as the effective processes which enable to remove both of mineral matter and sulfur. But it has not been reported on the comparison of the both treatment results, in spite of the similarity of these processes.In this paper, using Miike coal as a sample, the limit of demineralization and desulfurization and the optimum conditions of both processes are compared. In addition, the change of the coal substance during both processes is also investigated.In the aqueous caustic process, the majority of mineral matter is removed with high recovery of coal substance and low calorific loss, but most of organic sulfur is remained unremoved. Molten caustic process reduces the ash and sulfur contents of the product to less then 0.1% for both, but with low recovery of coal substance and high calorific loss.The relationships between ash or sulfur rejection and the other cleaning results (recovery of coal substance, calorific loss and the fuel ratio of clean coal) of both processes could be drawn as smooth continuous curves which might characterize the reactions between coal and alkali reagent in the processes. These curves are considered new measures indicating the facility of demineralization and desulfurization of coal.