Characterisation of coal slurries for introduction into ICP OES for multi-element determinations† M. Mujuru, * R. I. McCrindle and N. Panichev Received 7th November 2008, Accepted 22nd January 2009 First published as an Advance Article on the web 19th February 2009 DOI: 10.1039/b819963a The characterisation and stability of coal slurries for introduction into an inductively coupled plasma optical emission spectrometer (ICP OES) and the application of slurries in the analysis of coal was investigated. The importance of rheology and the stability of the coal slurries on the analytical results produced by ICP OES was demonstrated. The homogeneity and stability of various coal slurries were characterised by sedimentation tests, scanning electron microscopy (SEM), zeta potential and viscosity measurements. The porosities and surface areas of different South African coals and a mechanism for the adsorption of the dispersants onto the coals were determined using low-temperature nitrogen physisorption studies. The possibility of partially solubulising the coal in N,N-dimethylformamide (DMF) was investigated. SEM indicated that DMF solubulised coal fines and that 0.1% (m/v) Triton X-100 with 10% (v/v) DMF resulted in well dispersed slurries relative to other dispersants (glycerol, PEI and water). Slurry analysis of coarse and ground coal in 0.1% (m/v) Triton X-100, showed that higher emission intensities and better precision was obtained when using ground coal. This was confirmed by lower RSDs (less than 5% for most of the elements analysed) relative to the unground, coarse coal. Glycerol slurries gave higher RSDs (higher than 5% for most of the elements) most of the time, thus negatively influencing the precision of measurement. Physisorption studies indicated that ground coal had up to ten times higher surface areas than coarse coal. The higher surface areas for fine ground coals suggested why they were better dispersed and thus better transported into the plasma, resulting in higher emission signals. Zeta potentials indicated that for all dispersants, the coal particles were well repelled from each other, although SEM pictures indicated better dispersion for 0.1% (m/v) Triton X-100 with 10% (v/v) DMF. It was demonstrated that higher intensities and higher precision could be obtained with well dispersed and stable coal slurries. 1. Introduction Coal is a primary energy source and is used in the form of coke by metallurgical industries, where it is applied as a reducing agent during the manufacture of iron and steel, ferro-alloys and many non-ferrous metals. Worldwide coal production and interna- tional coal trade is projected to increase in the next several decades, leading to coal playing an increasingly visible role in global environmental, economic and energy issues. 1–4 Accurate information on the quantity and quality of coal is required by policy makers in order to make informed decisions regarding foreign policy objectives, technology transfer policies, environmental and health assessments, byproduct use and disposal of coal wastes. 5–7 Coal is a complex material with more than 20 variables that must be determined for complete characterisation. Real time information on coal quality leads to better and efficient manage- ment of coal stock piles and improved plant performance. 7–9 South Africa is the sixth largest producer of coal. These coals are mainly Permian and are considered to contain lower concentrations of trace elements than most world coals. Few studies have been published regarding the trace element concentration of South African coals. 4 Although trace elements, by definition, are present at low concentrations, the large tonnages used could produce significant quantities of toxic elements which may accumulate and be of concern to the envi- ronment and human health. 10–12 Due to the increased demand and usage of coal, the develop- ment of cost effective, faster and user friendly methods for analysis is urgent. Existing methods are costly, involve long sample preparation times that may result in dilution of the analyte, or are prone to contamination and loss of volatile elements. Slurry nebulisation with inductively coupled plasma optical emission spectroscopy (ICP OES) has been proposed as an alternative to traditional dissolution methods and the technique has received increasing attention over the last few years. 13–15 ICP OES is now the main method of trace analysis for inorganic elements. A number of inorganic, ceramic and geological materials have also been analysed by slurry nebu- lisation ICP OES. 16–20 Trace element analysis of coal by ICP OES using slurry sample introduction has been reported. These studies have shown that the coal must be of small particle size, even nanometre size. 21–23 Tshwane University of Technology, Department of Chemistry, Arcadia Campus, P. O. Box 56208, Arcadia, 0007, Pretoria, Republic of South Africa. E-mail: mccrindleri@tut.ac.za; Fax: +012 382 6286; Tel: +012 382 6284 † This article is part of a themed issue dedicated to Professor Jean-Michel Mermet, in recognition of his contributions to the field of atomic spectrometry. 494 | J. Anal. At. Spectrom., 2009, 24, 494–501 This journal is ª The Royal Society of Chemistry 2009 PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry Published on 19 February 2009. Downloaded by Open University on 13/01/2014 11:58:47. 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