An Overview of the Chemistry of the Molten-Caustic-Leaching Process

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Abstract
The Molten-Caustic-Leaching (MCL) Process is quite effective in its ability to remove both organic and pyritic sulfur, mineral matter, and trace elements from coal. In recent years, the chemistry of various reactions taking place in the process has been examined in detail by several researchers. For example, Ames Laboratory of Iowa State University is actively studying the chemistry of MCL desulfurization and reagent regeneration reactions, and researchers at the U.S. Department of Energy's Pittsburgh Energy Technology Center are examining the reactions of molten caustic with the organosulfur compounds present in coal. As a result of these research efforts, reactions of mineral pyrite, benzothiophene, and minerals commonly found in coal (quartz, illite, etc.) with molten caustic are very well characterized. However, reactions of the organic components of coal with molten caustic are still difficult to understand because of the complexity of interaction between the various compounds present in coal and variations in the chemical composition of different coals. Also, considerable insight into the overall development of the process has been obtained as a result of these research efforts; and alternate, and probably more energy-effective, process development steps have been suggested. Various analytical techniques and methodologies that can remove some of the uncertainties associated with the characterization of coals have been developed.