Some aspects of rapid analysis of coal slurries using direct current plasma emission spectrometry

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Virginia Polytechnic Institute and State University


The direct current plasma is an excitation cell that should be well suited to rapid analysis of coal slurries by virtue of its tolerance for various sample matrices. Problems which are encountered in coal analysis by emission spectrometry include incomplete atomization of analyte by formation of metal oxides, lack of adequate methodology for sulfur analysis, and ineffective sample transport for coarse coal slurries.

Atomization of metal oxides can be improved by addition of small amounts of propane (Ca 45 mL/min) to the nebulizing argon of the direct current plasma. However, the improved atomization is manifested above the normal viewing zone, and the enhancement effect of propane on analytical signals is offset by severe depression of emission signals caused by temperature reduction in the lower regions of the plasma.

Sulfur in coal can be determined by direct current plasma emission spectrometry. Emission lines accessible to the echelle grating of the DCP are not suitable for such analysis, so that the deep-UV lines from 180-183 nm must be utilized for such work. A relatively simple purge system with low argon consumption (5 L/min) is adequate for sulfur analysis, and the beforementioned analytical lines provide detection limits that are adequate for sulfur determination in 1% slurries of coal. However, transport of the coal sample to the plasma is incomplete when compared to that of aqueous solutions, precluding the use of such solutions as calibration standards.

Transport of the coal can be improved by increasing the viscosity of sample and standards, which increases the droplet size from the nebulizer and hence the particle size transportable• The increased droplet size causes a decrease in sensitivity due to reduced desolvation/vaporization, but does permit the use of aqueous solutions and as calibration standards for determination of sulfur, iron, aluminum, silicon in coal.