Mineral matter removal, caustics

Chemical treatment of coal has also been used to further reduce the mineral matter content. Usually the physical techniques mentioned earlier are used to reduce the inorganic sulfur species (mostly pyrite) to a very low level. Since there is a great concern about the sulfur contents of coal, and the organic sulfur content exceeds the permissible standards for many high-sulfur coals, non-physical or chemical techniques have been tried to further reduce the sulfur content. Strong caustic reagents are used most often to remove the more refractory sulfur forms. 

Previous work has shown that ash-forming mineral matter, including iron pyrites, can be removed from coal by leaching the finely divided material with a hot caustic solution under pressure followed by washing with a dilute mineral acid (1-6). Recently, similar results have been achieved in leaching fine-size coal with hot sodium carbonate solutions (J7). In both cases, quartz appears to dissolve in the hot alkaline solution, while clay minerals and iron pyrite are converted into acid-soluble compounds which are removed in the acid washing step. Acid also removes any carbonate minerals which are present in the coal. 

The aqueous caustic recovered from the first coim-tercurrent washing circuit is limed to remove the coal-derived mineral matter, sulfur compounds, and carbonates. The mixture is provided with sufficient residence time to permit precipitation of the impurities before being centrifuged. The purified hquid is preheated and sent to a caustic evaporator where the water is recovered for recycle to the first wash train, while producing anhydrous caustic (as either a molten liquid or as flakes) for reuse in the initial leaching of the coal. 

---