Investigating the pyritic sulfur reduction and the coal ash content using nitric acid leaching

Coal is one of the most important sources of energy which is used in the production of coke, but presence of impurities including sulfur and ash in coal affect its properties and lead to environmental pollution. Hence, in this study, the reduction of the pyritic sulfur and ash content from coal using leaching method were investigated. The investigations indicated that among different leaching reagents, nitric acid had the highest effect in the reduction of sulfur. The influence of important factors showed that the removal value of pyritic sulfur increased with increasing the temperature, acid concentration and leaching time and the most removal percentage (91%) was obtained at leaching time of 75 min, acid concentration of 30% and temperature of 80 °C. The pyritic sulfur dissolution kinetics was also examined according to shrinking core models and the results revealed that the dissolution rate was controlled by surface chemical reaction model with the activation energy of 32.319 kJ/mol. In addition, the design of experiments method was used to determine the desirable conditions of factors affecting the reduction rate of ash content. The findings indicated that the highest removal percentage of ash was achieved under optimal conditions: nitric acid concentration of 30%, temperature of 80 °C and leaching time of 40 min.