Desulfurization of coal by HNO3 leaching: Optimization of influential factors using Box-Behnken design

Sulfur is one of the most significant impurities in coal, which reduces the quality of coal and also results in environmental pollution. This work was aimed to investigate the removal of sulfur from coal by the leaching method employing parameters expected to affect the removal rate such as acid concentration (10-30%), temperature (40-80 °C), and reaction time (40-100 min). A response surface methodology using Box-Behnken design was employed to maximize, model, and evaluate the factors affecting the desulfurization process. The results obtained indicated that the desulfurization value increased with increase in the acid concentration, temperature, and reaction time. A quadratic model with a high correlation coefficient (R2=0.98) is proposed and developed for the relationship between the removal value and the influential factors. The modeling results demonstrated that the significance degree of factors was in the order of acid concentration>temperature>reaction time. It was also found that the maximum desulfurization (about 87%) could be obtained under the optimal conditions of acid concentration=25%, temperature=80 °C, and leaching time=84 minutes.