Low-temperature ash

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

Low temperature ashing with radio frequency induced oxygen plasma ... 

Fourier transform infrared (FTIR) spectroscopy of coal low-temperature ashes was applied to the determination of coal mineralogy and the prediction of ash properties during coal combustion. Analytical methods commonly applied to the mineralogy of coal are critically surveyed. Conventional least-squares analysis of spectra was used to determine coal mineralogy on the basis of forty-two reference mineral spectra. The method described showed several limitations. However, partial least-squares and principal component regression calibrations with the FTIR data permitted prediction of all eight ASTM ash fusion temperatures to within 50 to 78 F and four major elemental oxide concentrations to within 0.74 to 1.79 wt % of the ASTM ash (standard errors of prediction). Factor analysis based methods offer considerable potential in mineral-ogical and ash property applications. 

Experience in this laboratory has shown that even with careful attention to detail, determination of coal mineralogy by classical least-squares analysis of FTIR data may have several limitations. Factor analysis and related techniques have the potential to remove or lessen some of these limitations. Calibration models based on partial least-squares or principal component regression may allow prediction of useful properties or empirical behavior directly from FTIR spectra of low-temperature ashes. Wider application of these techniques to coal mineralogical studies is recommended. 

The author gratefully acknowledges the contributions of others to various aspects of this project. F. P. Burke and R. A. Winschel provided research guidance and helpful advice. M. S. Lancet provided low temperature ash samples and conventional analysis data. Experimental work was performed by J. M. Sariscak, L. L. Anthony, and L. K. Dahm. L. L. Schlutz and D. J. Simmons prepared the manuscript. D. M. Haaland of Sandia National Laboratory provided the PLS and PCR software and gave advice on its use. G. Ritter of Nicolet provided the MCOMP program source code. 

Table 8. Elemental concent rations determined in raw (i.e., feedstock). retorted. and after low-temperature ash (LTA) of oil shale from Green River Formation. Mahogany Zone, (GRFMZ). USA...

Fruchier rial. (1980), determined by X-ray fluorescence IXRF), except Aland Naby ncuiran activation analysis (NA At. Mg by flame atomic absorption tlidnum borate fusion (FAA), and B by plasma emission spectroscopy (sodium carbonate fusion) (PE5) Saether (1980), determined by XRF after low-temperature ashing (LTA) of raw oil shale samples In = 10). 

LDH LEU LIBD LAW LET LILW LIP LLNL LLW LMA LMFBR LOI LREE L/S LTA LWR Layered double hydroxide Low enriched uranium Laser-induced breakdown detection Low-activity waste Linear energy transfer Low- and intermediate-level nuclear waste Lead-iron phosphate Lawrence Livermore National Laboratory Low-level nuclear waste Law of mass action Liquid-metal-cooled fast-breeder reactor Loss on ignition Light rare earth elements (La-Sm) Liquid-to-solid ratio (leachates) Low-temperature ashing Light water reactor... 

Figure 1. Scanning electron photomicrographs of minerals from coals. The minerals were studied and photographed by a Cambridge Stereoscan microscope with an accessory energy-dispersive x-ray spectrometer at the Center for Electron Microscopy, University of Illinois. A. Pyrite framboids from the low-temperature ash of a sample from the DeKoven Coal Member. B. Pyrite cast of plant cells from the low-temperature ash of a sample from the Colchester (No. 2) Coal Member. C. Kaolinite (left) and sphalerite (right) in minerals from a cleat (vertical fracture), Herrin (No. 6) Coal Member. D. Calcite from a cleat in the Herrin (No. 6) Coal Member. E. Kaolinite books from a cleat in the Herrin (No. 6) Coal Member. F. Galena small crystals in the low-temperature ash of a sample from the DeKoven Coal Member.

Within the past decade the technique of electronic (radiofrequency) low-temperature ashing has been used to investigate mineral matter in coal. In a low-temperature asher, oxygen is passed through a radiofrequency field, and a discharge takes place. Activated oxygen thus formed passes over the coal sample, and the organic matter is oxidized at relatively low temperatures—usually less than 150°C (14). 

Estep et al. (16) used infrared absorption bands in the region 650-200 cm"1 to analyze quantitatively as well as qualitatively for minerals in low-temperature ash. O Gorman and Walker (2) also applied this technique in their investigations. 

Gluskoter, H. J., Electronic Low-temperature Ashing of Bituminous Coal, ... 

Two types of coal ash samples have been prepared routinely for analysis at the Illinois Geological Survey. Low-temperature ash samples (12), in which the bulk of the mineral matter remains unchanged, are prepared by reaction of the coal with activated oxygen in a radiofrequency field. The effective temperature produced by this device is approximately 150 °C. Such samples were unsatisfactory for emission spectroscopic analysis. It is postulated that the presence of largely unaltered mineral matter, such as carbonates, sulfides, and hemihydrated sulfates (12), caused the observed nonreproducibility of results. High-temperature ash samples, prepared in a muffle furnace, consisted mainly... 

Because of these encouraging results and previous work on brown coals by Sweatman et al. (4) and Kiss (5), which indicated that major and minor elements could be determined in whole coal, a series of 25 coals was prepared for x-ray fluorescence analysis. For each coal, a low-temperature ash, a high-temperature ash, and the whole coal itself... 

Gallium. Since investigation showed that gallium is not lost when coal is ashed in a low-temperature plasma asher not exceeding 150 °C, Santoliquido and Ruch (4, 23) determined gallium in the low-temperature ash of coal. 

Radiochemical yields are within 46-74%. The average relative standard deviation of the method is 8%. The accuracy of the method was checked by analysis of a U. S. Geological Survey rock standard. Gallium concentrations in 101 coals, calculated from the concentration in the coal ash and the percentage of low-temperature ash in the coal, range from 1.1 to 7.5 ppm, the median value being between 2.9 and 3.0 ppm (16). 

Arsenic. At the Illinois Geological Survey, arsenic has also been determined in low-temperature coal ash, and the concentration is calculated to a whole coal basis because it was found that negligible amounts of arsenic are lost in the low-temperature ashing of coal (4). 

Another method of direct determination of organic sulfur is to subject a small (20- to 30-mg) sample of <200-mesh coal to low-temperature ashing 1 to 3 hours and to collect the sulfur oxides evolved in a cold trap. They are then absorbed in hydrogen peroxide (H202) and determined chromatographically. 

Determination of a good value for the percent of mineral matter content (% MM) is a very important component of coal analysis. If this quantity cannot be determined directly by the acid demineralization or low-temperature ashing procedure discussed previously, or by other suitable methods, it is possible to calculate a reasonable value for the mineral matter in coal, provided that the necessary data are available. 

Factors that affect the rate of low-temperature ashing other than radiofrequency power and oxygen flow rate are the coal particle size and depth of sample bed. Typical conditions for ashing are a particle size of less than 80 mesh, a sample layer density of 30 mg/cm2, oxygen flow rate of 100 cm3/min, chamber pressure of about 2 torr, and a 50-W net radio-frequency power. The total time required is 36 to 72 hours, and specified conditions must be met during the procedure to obtain reproducible results. 

Figure 8. Fitting of two Gaussian curves with the curve difference between the sulfur dioxide evolution from PTO of the Provence coal and of the Low Temperature Ash derived from it.

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