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Rapid total sulphur reduction in coal samples using various dilute alkaline leaching reagents under microwave heating: preventing sulphur emissions during coal processing

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Abstract

Currently in South Africa, online flue gas desulphurisation (FGD) is being utilized as one of the most effective methods for total sulphur reduction in coal samples during the combustion process. However, the main disadvantage associated with FGD is the formation of its by-products (FGD gypsum). The latter is mostly formed in low grade quality, thereby bringing secondary pollution problems and extra disposal costs. Therefore, the current study describes the development of total sulphur extraction in coal under microwave heating using different dilute alkaline solutions such as NaOH, NaOH-H2O2, NH4OH, and NH4OH-H2O2. The experimental conditions were as follows: 150 °C, 5 min and 10% (m/v or v/v) for temperature, extraction time and reagent concentration, respectively. The most effective alkaline reagent for coal desulphurisation was observed to be NaOH-H2O2 with total sulphur reduction of 55% (from the inductively coupled plasma-optical emission spectrometry (ICP-OES) results). The NaOH-H2O2 reagent also showed significant morphological changes in coal as observed from the SEM images and effective demineralisation as revealed by the powder X-ray diffractometer (P-XRD) results. Additionally, desulphurisation results obtained from the developed microwave-assisted dilute alkaline extraction (MW-ADAE) method were quite comparable with the published work. The proposed total sulphur reduction method is advantageous as compared to some of the literature reported coal desulphurisation methods as it requires a short period (5 min) of time to reach its completion. Additionally, the proposed method shows excellent reproducibility (% RSD from 0.5 to 1); therefore, it can be utilized for routine analysis.

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Acknowledgements

The authors would like to acknowledge and thank the following parties: ESKOM-TESP program, National Research Foundation (NRF) Scarce Skills Program, L’Oréal-UNESCO for Women in Science for Sub-Saharan Africa (Doctoral Awards) for their financial support, and the University of Johannesburg (analytical facility) for providing ICP-OES, P-XRD, SEM, and some other laboratory facilities.

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Authors and Affiliations

  1. Analytical Facility, University of Johannesburg, PO Box 524, Auckland Park, Johannesburg, 2006, South Africa

    Nomvano Mketo

  2. Department of Applied Chemistry, University of Johannesburg, PO Box 17011, Doornfontein, Johannesburg, 2028, South Africa

    Nomvano Mketo, Philiswa Nosizo Nomngongo & Jane Catherine Ngila

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  1. Nomvano Mketo
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  2. Philiswa Nosizo Nomngongo
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  3. Jane Catherine Ngila
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Correspondence to Nomvano Mketo.

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Responsible editor: Philippe Garrigues

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Mketo, N., Nomngongo, P.N. & Ngila, J.C. Rapid total sulphur reduction in coal samples using various dilute alkaline leaching reagents under microwave heating: preventing sulphur emissions during coal processing. Environ Sci Pollut Res 24, 19852–19858 (2017). https://doi.org/10.1007/s11356-017-9632-y

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  • DOI: https://doi.org/10.1007/s11356-017-9632-y

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