The occurrence of potentially hazardous trace elements in five Highveld coals, South Africa

• Published in: International journal of coal geology 2005-08, Vol.63 (3), p.228-246
• Main Authors: Wagner, Nicola J., Hlatshwayo, Bafana
• Format: Article
• Language: English
• Subjects: Coal; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Highveld coal field; Sample preparation; Sedimentary rocks; Soil and rock geochemistry; South Africa; Trace elements
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2005.02.014

Permian coals of the southern hemisphere are generally considered to contain lower concentrations of sulfides, halogens, and trace elements when compared to northern hemisphere Carboniferous coals. Few studies have considered the trace element content in South African coals, and little or no work has been published for Highveld coals. Of the nineteen coal fields in South Africa, the Highveld coal field is one of the nine currently producing, and is second largest in terms of production. Five run of mine samples and a high ash middlings product from the Number 4 Lower seam were analyzed, totaling six sample sets. Fourteen trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, V, and Zn) were selected for this study based on the global perception that these elements may be hazardous to human health and/or the environment when they are released during coal utilization. Several sample preparation techniques were tested using certified reference materials (SARMs 18, 19 and 20) to determine the most repeatable technique for these coals. The samples were analyzed by ICP-AES and CVAA (Hg only). Microwave digestion proved to be generally unreliable despite the utilization of several different methods. A slurry direct injection method into the ICP-AES provided good correlations with the reference material, but requires further development to enhance the confidence level in this relatively unexplored technique. Samples prepared based on three ASTM standards for the determination of trace elements in coal provided repeatable results in most instances, and were the preparation methods utilized for the Highveld coals. The trace element values determined for the Highveld coals are generally in good agreement with values available in literature for South African coals, with the exception of Hg, Mn and Cr. Hg values reported here are lower, Cr and Mn higher. Results generally agree well with analyses on the same samples conducted by the United States Geological Survey. When considering the global ranges for trace elements, the Highveld range values are within Swaine's range boundaries with the exception of Cr. Compared to the cited global average values for the fourteen trace elements determined, the values obtained for the Highveld coals generally fall below or well below these average values, with the exception of Cr and Mn. Concentrations of Cd and Cu are lower compared to global average values, and As, Mo, Pb, Se, Sb, and Zn can be considered low to very low.