Development of microwave based sample preparation methods for extraction of multi-element for coal clean-up and spectrometric determination

Abstract: During coal processing such as pyrolysis, liquefaction, gasification and combustion, significant quantities of toxic trace-metals and sulphur can be released into the atmosphere causing severe environmental problems (water, air & soil pollution). Some of these contaminants are corrosive in nature and can cause damages to the combustion reactors which might lead to the reduction of the reactor life span. Additionally, other elements in coal might be trapped inside the combustion boilers and cause slagging, abrasion and fouling. Coal impurities such as trace-metals and sulphur cannot be controlled or prevented as they occur naturally during coal genesis. Therefore, development of analytical procedures is urgently required for determination of coal impurities for future clean-up strategies. Concentration levels of the elemental contaminants in coal must be monitored and controlled in order to determine the coal quality and value. The aim of the current study was to develop cost-effective, novel and environmentally friendly sample preparation procedures based on the use of microwave radiation for quantitative extraction of trace-metals and sulphur in coal samples followed by ICP-MS/OES & IC analysis and coal clean-up approaches. Determination of trace-metals and total sulphur was conducted by using two microwave based methods followed by ICP-MS/OES analysis. The first developed sample preparation procedure was microwave-assisted acid extraction (MW-ADAE) using different dilute inorganic acid and their combination with hydrogen peroxide. Optimisation for trace-metals and total sulphur extraction was conducted by using multivariate and univariate approaches, respectively. In both cases (trace-metals and total sulphur optimisation), results indicated that dilute HNO3-H2O2 was the suitable mixture for simultaneous extraction of 22 elements and total sulphur in coal samples. The use of dilute nitric acid eliminates the carcinogenic nitrous oxides that are produced by concentrated nitric acid (normally used in classic acid digestion methods) and also reduces laboratory waste generated. The MW-ADAE method was then applied in three South African coals coded as coal sample A, B & C (CSA, CSB & CSC) for determination of trace-metals and total sulphur...Ph.D. (Chemistry