Characterization of South African Coal for Metals, Inorganic and Organic Sulfur Compounds

Abstract

This work characterized South African coal for metals, organic and inorganic sulfur compounds. Microwave assisted extraction (MAE) followed by Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) were used for extraction, identification and quantification of inorganic, organic, total sulfur forms as well as selected metals related to the presence of sulfur in coal. The total sulfur content was also determined in coal by direct elemental analysis for carbon, hydrogen, nitrogen and sulfur. Thiophene organic sulfur compounds in coal were extracted with ultrasonication system followed by identification and quantification using gas chromatography and flame ionization detector. The ultrasonication extraction system was optimized for the extraction organic solvent, the extraction time and influence of sample concentration. Microwave assisted extraction followed by ICP-OES gave an average of 8758, 4500, 3600 and 600 mg kg^–1 for total sulfur, pyrite sulfur, organic sulfur and sulfate sulfur, respectively. Pyrite and organic sulfur forms were found to compete for the largest amounts in coal samples. The total sulfur content in South African coal was found to be less than 20 000 mg kg^–1 which suggests that the coal may be of low sulfur content compared to others in the world. The mean percentages of carbon, hydrogen, nitrogen and sulfur in coal samples was 66.20, 2.98, 1.15 and 1.11 confirming that it is of bituminous rank. The concentration levels of selected metals were found in this decreasing order; Na>Ca>Fe>Mg>K>Ba>Mn>Cr>Pb> Zn>Cu>As>Co>Sb>Hg. Thiophene organic sulfur compounds were best extracted with dichloromethane with five sequential extractions of 15 min each. Recovery of extraction from spiked concentration ranged from 40 % to 70 % depending on the individual compound. More volatile organic sulfur compounds showed lowest recovery especially at low spiked concentrations. 2-methyl thiophene was the only compound identified in all coal samples. The concentration of thiophene sulfur compounds ranged from about 4–16 mg kg^–1. Dibenzothiophene was found with the highest concentration (15.5 mg kg^–1) while 2-ethyl thiophene was found with the lowest concentration.

Keywords: Coal, sulfur, metals, microwave assisted extraction, and ultrasound assisted extraction, inductively coupled plasma optical emission spectrometry, gas chromatography