Coal desulfurization and demineralization by chemical/physical treatments

摘要

Several chemical and physical cleaning methods for removing sulfur and ash-forming mineral matter from various kinds of bituminous coal were investigated. These methods involved grinding coal to a fine size to expose or liberate the mineral matter and either physical separation or chemical leaching of the resulting particles. By combining these methods, a multistep process was developed and demonstrated for producing super clean coal. In this process, the coal is first leached with a dilute sodium carbonate solution under oxygen pressure at 150°C to oxidize pyritic sulfur and some of the organic sulfur to produce water-soluble sulfur species. Next, the temperature of the alkaline suspension is raised above 250°C but under non-oxidizing conditions to convert clays and other minerals to acid-soluble materials. The solids are subsequently separated by filtration and then extracted with a dilute mineral acid. As a result of this treatment, most of the mineral matter is removed from the coal and the sulfur content greatly reduced;To better understand the behavior of mineral matter in this process, individual minerals including various clays, quartz, carbonates, and pyrite were reacted with hot alkaline solutions and the solid reaction products characterized by X-ray diffraction analysis. The solubility of the reaction products in dilute mineral acids was also investigated in detail. Kinetic data for the reactions of quartz and pyrite with alkaline solutions were collected and analyzed. A shrinking unreacted core model was used to analyze the pyrite data and determine the rate-controlling step;The basic preoxidation, alkaline leaching, and acid washing steps of the chemical leaching process were demonstrated with several types of coal to establish optimum treatment conditions. Various process variables including leaching temperature and time, alkali type and concentration, particle size, acid type and concentration, washing temperature and technique, and pulp density were investigated. Reductions in ash and sulfur contents and coal recovery were determined for different treatment conditions. The phase transformations of ash-forming mineral matter in coal during the leaching treatments were monitored and identified by X-ray diffraction analysis. The results with coal were compared to those obtained when individual minerals were leached.