Evaluation of different microwave-assisted dilute acid extracting reagents on simultaneous coal desulphurization and demineralization

Abstract

Coal combustion is one of the largest anthropogenic contributors to SOx and some other volatile elemental emissions to the atmosphere. Therefore, the current study investigated the effect of different dilute acidic reagents (HCl, HNO3, HCl–HNO3, HCl–H2O2, HNO3–H2O2 and HCl–HNO3–H2O2) on simultaneous coal demineralization and desulphurization under microwave irradiation. The applied conditions for microwave-assisted dilute acid extraction (MW-ADAE) process were as follows: 180°C, 5min, 3molL−1, 3molL−1, 0.1g and 2:1 for microwave temperature, extraction time, acid concentration, [H2O2], coal amount and acid-hydrogen peroxide volume ratios, respectively. The leachate solutions obtained from the MW-ADAE process were then analysed by using ICP–OES for multi-element determination. The ICP–OES analysis was performed in order to investigate the acidic reagent that resulted in highest extraction recoveries of the examined multi-element. All extracting reagents that contained HNO3 showed accepted extraction efficiencies (% R⩾80%) as indicated by the ICP–OES results. The proposed MW-ADAE procedure with HNO3–H2O2 reagent showed excellent desulphurization efficiency parallel to the other literature reported methods. The raw and acid treated coals were then characterized by Elemental analyser (CHNS), FT-IR, SEM, P-XRD and TGA to examine the influence of different acids on the coal structure. The FT-IR, SEM and P-XRD results showed insignificant changes throughout all the studied acidic reagents. However, the CHNS results of HNO3–H2O2 treated coal revealed sufficient coal desulphurization as compared to the other investigated acids. Alternatively, thermal studies of raw coal (A) showed high quantity of unburned mineral matter. In addition, most of the acid treated coals such as B (HCl), C (HNO3), E (HCl–H2O2 and F (HNO3–H2O2) indicated moderate removal of mineral matter while aqua-regia based extracting reagents (G (HCl–HNO3–H2O2) and D (HCl–HNO3)) caused drastic extraction of both carbon and mineral content. Therefore, the mixture of HNO3–H2O2 was selected as the potential extracting reagent for the proposed simultaneous coal desulphurization and demineralization process as it showed moderate attack on the carbon content of the coal and sufficient desulphurization (102%) and demineralization (70–115%).