High temperature ashing

One of the more important considerations in determining the end use of synthetic graphite is its contamination with metallic components Metals such as iron, vanadium, and especially in nuclear applications, boron are deleterious to the performance of graphite Table 3 presented the extraction yields of NMP-soluble material for three bituminous coals. For these coals, mineral matter and insoluble coal residue were separated from the extract by simple filtration through 1-2 pm filter paper fable 13 lists the high-temperature ash content in the dry coal, and in their corresponding NMP-insoluble and NMP-soluble products. The reduced ash content of the extract is typically between 0.1 to 0.3 wt% using traditional filtration techniques for the small-scaled extraction experiments... 

Simultaneously with the efforts to determine the origin of mineral matter in coal, systematic efforts were underway to estimate the quantitative distribution of trace and minor elements in American coals. The early analyses were performed on high-temperature ashes, and as a consequence, the investigators had to be content with determining the nonvolatile metallic oxides. However, with the advent of the low temperature asher and improvisations and advances in wet chemical, radiochemical, and instrumental analytical techniques, we not only can analyze nondecomposed mineral matter but also can study the composition of whole coal. 

Clay Minerals. The clay minerals in coal all contain water bound within their lattices. Kaolinite contains 13.96%, illite 4.5%, and mont-morillonite 5% bound water. In addition, the montmorillonite in the mixed-layer clays also contains interlayer or adsorbed water. All of the water is lost during the high-temperature ashing. 

Calcite. The calcium carbonate is calcined to lime (CaO) during high-temperature ashing, with a loss of carbon dioxide. This results in a 44% weight loss. 

Quartz. The stable mineral quartz (Si02) is the only major mineral found in coal which is inert during high-temperature ashing. 

Until recently, chemical analyses of coals were done on ash produced from the coal at relatively high temperatures. This was the standard approach for many years, and analyses of trace elements in coals do have a long history. An early article on an element as rare as cadmium in coal was published 125 yrs ago (28). One limitation of high-temperature ash sample is that volatile elements may be lost during combustion and will not be detected. Another problem which applies especially to analyses for trace and minor elements is that there have not been any coal standards available until very recently. 

Dry ashing is still the simplest prior treatment and should be used where high temperature ashing is feasible. A comparison of wet ashing and HTA was made for nine elements. The results indicate no appreciable loss from volatilization (Table III). 

Quantitative spectrographic analyses for trace elements in high-temperature ash samples of coal have been reported by Abemethy et ah (1), Zubovic et ah (2,3,4,5), Rao (6), and Hunter and Headlee (7). We felt that it was desirable to develop additional analysis methods, especially for the direct-reading spectrometric technique, in which small changes of some matrix constituents might cause relatively large variations in results because of the increased sensitivity of the detection system. A... 

The samples (high-temperature ash) and standards, both diluted with an equal weight of SP-2X graphite powder, were ignited spectro-graphically under conditions shown in Table II. 

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... 

Two reliable optical emission spectroscopic methods have been developed for trace element analyses of high-temperature ash from coal samples. However, care must be taken in the ashing procedure to guard against contamination or loss of certain elements. 

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... 

Other metal carbonates behave similarly (i.e., the oxides are formed during the ashing procedure). The stable mineral quartz (silicon dioxide, silica, Si02) is the only major mineral found in coal that is inert during high-temperature ashing. 

Analyses of high-temperature ash (Table II) for different Texas lignites show near-surface Wilcox lignite to have the lowest Na 0 content and the highest CaO, MgO, and contents. 

Following each test, samples of the solidified slag and high-temperature ash are analyzed by X-ray fluorescence and X-ray diffraction techniques. A Kevex Model 0700 energy dispersive X-ray spectrometer is employed for X-ray fluorescence analysis. X-ray diffraction measurements are conducted using a Philips 3600 automated X-ray diffractometer. 

A procedure for the determination of molybdenum in serum, red blood cells, and urine is described. The low concentration of molybdenum in most unexposed individuals requires the sensitivity obtained using atomic absorption spectrophotometry and electrothermal atomization. Spike recovery tests indicate that low temperature ashing is required for accuracy. Severe matrix interferences preclude wet ashing or high-temperature ashing as sample pretreatments. Using the method described, it is possible to distinguish between industrially exposed and unexposed individuals. 

Extensive analytical efforts to fully characterize the oil shales are underway at Exxon Research and Engineering Company s Baytown Laboratories. No significant losses of any metals of concern are observed during high temperature ashing. An alternate means of rapid ash determination uses a Parr combustion bomb. The ash can be dissolved by alkaline fusion in a Claisse fluxer or by acid dissolution in a Parr bomb. The solutions thus prepared are analyzed by atomic absorption or by inductively coupled plasma emission spectrometry for major (Al, Ca, Fe, K, Mg, Na, Si, Ti) and trace elements (As, B, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Sr, U, V, Zn). Kerogen enriched shales need to be ashed before the dissolution, otherwise low recoveries are obtained. Overall accuracy and precision of metals determination is within... 

Minerals. Few analyses of the mineral contents are available despite the abundance of elemental analyses expressed as oxides of high temperature ashes (71,72). Sprunk and O Donnell (30) described and illustrated the microscopic occurrences of minerals in many U.S. coals, especially kaolinite, pyrite, calcite, siderite, and quartz. O Gorman and Walker (2 ) quantitatively identified 14 different mineral phases in dull durain and clarain layers in 12 samples from mines in Kentucky, Pennsylvania, West Virginia, North Dakota, and Wyoming. 

Samples were also ashed at 750 C in accordance with ASTM procedure D3174-73 ) and will be referred to as ASTM samples. Samples were then ashed at 1000 C following the same procedure for 750°C coal ashing and will be referred to as HTA (high temperature ash) samples. 

Table III. Spectrochemical Analysis of ASTM High-Temperature Ash (HTA) Residues From...

Thus, minerals in coal have been, and continue to be, a subject of much interest. Early studies approached the subject somewhat indirectly by means of chemical analysis of high-temperature ash and back calculation to obtain estimates of the mineral matter. Others supplemented chemical studies by handpicking the coarser minerals or performing density separations for chemical tests and optical microscopic studies. With the introduction of radio frequency ashing at low temperature (<150°C), it became possible to directly investigate all of the mineral constituents (Gluskoter, 1965 Miller, 1984). 

However, when coal ash is prepared for complete analysis, it has been considered necessary to ensure that sulfur is not retained in the ash. If sulfur is retained in the ash, the analysis will most likely be inaccurate unless corrections to a sulfur dioxide-free basis are made. In addition, attempts must be made to ensure that important elements are not lost during the ashing procedure by virtue of the higher temperatures (ca. SSO C, ISbO F) that are used. In fact, a low-temperature ash sample can be of the order of 150% w/w of the high-temperature ash sample. 

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