Ashing techniques

There are many minerals in coal which will produce as ash residue (Table 7.6) and, although the ash composition varies around wide limits, the ash may appear to be composed of only a few of the more common elements (Table 7.4). But it is often the trace elements (Table 7.5) that can give ash some of its more obnoxious properties. 

The determination of the mineral matter content of coal (determined as mineral ash after combustion) has been an essential part of coal evaluation for many years (Shipley, 1962 Rees, 1966 Given and Yarzab, 1978 Huggins et al., 1982 Nadkarni, 1982) (Chapter 8). For example, when coal is cleaned by various processes to reduce the sulfur and mineral content, it is an advantage to be aware of the mineral (ash) content to determine the best cleaning method insofar as the various cleaning methods have different tolerance levels for the mineral constituents of coal. Fnrthermore, the ash content is also a means of assessing the adequacy of the various sampling procednres (Chapter 8), and it is one of the criteria normally specified in contracts between a pnrchaser and a supplier of coal. 

The amount of mineral matter in coking coal (Chapter 16) is an indication of the amonnt of ash that will eventually be part of the coke made from the coal. Thus, the higher the ash content of the coal, the lower the proportion of usable carbon in the coke and the more the fluxing limestone that must be added to the furnace to assist in ash (mineral) removal. However, the fouling tendency of a coal is dependent upon several factors, not the least of which is the nature of the mineral matter and the resulting ash (Beer et al., 1992). 

On the other hand, the potential benefits that could arise from the presence of this same mineral matter should not be ignored catalytic effects in processes designed for the liquefaction (Chapters 18 and 19) and the gasification (Chapters 19 and 20) of coal may be cited as examples. 

Mineral matter in coal is often determined indirectly with the ash analysis (determined by direct combustion of the sample) (Chapter 8) forming the basis of the calculation. However, determination and chemical analysis of the ash content of coal gives the average content of the inorganic elements in a particular coal but is not an indication of the nature or distribution of the mineral matter in coal. Nevertheless, ash analysis can provide valuable data which, when used with data from other sources, may give a representation of the mineral content of coal. 

---

The combination of oxidi2ing effect, acidic strength, and high solubiHty of salts makes perchloric acid a valuable analytical reagent. It is often employed in studies where the absence of complex ions must be ensured. The value of wet ashing techniques, in which perchloric acid is used to destroy organics prior to elemental analysis for the determination of trace metals in organics, has been well estabHshed (see Trace and residue analysis). 

A wet-ashing technique used for dissolution of graphite in perchloric acid involved boiling a mixture of 70% perchloric acid and 1% of chromium trioxide as an aqueous solution. This method was later applied to 6-14 mesh charcoal, and after boiling for 30 min the reaction rate increased (foaming) and accelerated to explosion. The charcoal contained traces of extractable tar. 

Several formulas have been proposed for calculating the amount of mineral matter originally in the coal using data from ashing techniques as the basis of the calculations. Of these formulas, two have survived and have been used regularly to assess the proportion of mineral matter in coal the Parr formula and the... 

The evaluation of coal mineral matter by the ashing technique can be taken further insofar as attempts can then be made to determine the individual metal constituents of the ash. On the occasion when the mineral matter has been separated from the coal successfully, it is then possible to apply any one of several techniques (such as x-ray diffraction, x-ray fluorescence, scanning electron microscopy and electron probe microanalysis) not only to investigate the major metallic elements in coal but also to investigate directly the nature (and amount) of the trace elements in the coal (Jenkins and Walker, 1978 Prather et al., 1979 Raymond and Gooley, 1979 Russell and Rimmer, 1979 Jones et al., 1992). Generally, no single method yields a complete analysis of the mineral matter in coal and it is often necessary to employ a combination of methods. 

Scanning electron microscopy with an energy-dispersive x-ray system accessory has been used to identify the composition and nature of minerals in coals and to determine the associations of minerals with each other. Examinations can be made on samples resulting from ashing techniques or whole coal. With this technique it is possible to identify the elemental components and deduce the mineral types present in coal samples. Computerized systems to evaluate scanning electron microscopy images have been developed and are useful in characterizing the minerals in coal mine dusts and in coal. 

Some analysts prefer to use dry ashing techniques, ashing up to 20 g sub-samples in silica basins.7 Care is then needed to make sure that losses onto siliceous ash residue or by adsorption onto the basin, which are serious problems for copper and cobalt especially, are circumvented. This may be done by extracting the residue at least twice with dilute nitric acid under infrared lamps. If the residue is finally diluted to ca. 25 ml with dilute nitric acid, numerous trace elements may be determined. For some elements, however, preconcentration and sensitivity enhancement by solvent extraction may still be necessary. Details of... 

Described below are two wet-ashing techniques for water and wastewater samples prior to AAS. 

Petroleum oils often contain suspended or colloidal inorganic materials. In the case of used lubricating oils small particles of metals are present. In many cases these suspended solids are of sufficiently small particle size that efficient breakdown occurs in the flame and a simple dilution procedure may be used. However, where it is suspected that this is not the case then it is recommended that an ashing technique is used to prepare the sample. 

The actual decomposition of bonded ligands can be different from what we assume in the above calculation (weight loss), and corresponding bonding density values will be very approximate if based on the data from thermogravimetry or ashing techniques. The formation of trihydroxysilane is highly doubtful, and active silica dehydration could be observed at temperatures above 600°C [69]. 

The volatility of Po was recognized as a problem in sample preparation early on, where losses begin at temperatures above lOO C, with 90% loss by 300°C. This problem necessitates wet ashing techniques wherever possible in sample preparation. 

Low temperature ashing techniques were employed to reduce the oxidation and decomposition of the minerals which would occur under normal ashing conditions. Mineralogical analyses were performed using X-ray powder diffraction, infra-red spectroscopy, optical petrography, and scanning electron microscopy. Elemental determinations were performed using atomic absorption spectroscopy. 

Low results obtained by DPASV for cadmium were suspected to be due to an incomplete decomposition of the organic matter in addition, losses of volatile fractions were assumed with the dry ashing technique used (450°C, open system, instead of high pressure digestion). The set was consequently withdrawn. 

Three alternate techniques yielded quantitative recovery of cadmium from various matrices acid digestion (wet oxidation), ashing in the presence of sulfuric acid (wet ash), and a scaled down wet ashing procedure (mini-ash). The mini-ash technique is designed for small samples and is well suited for HVAA analysis. 

Selenium has been reported to be completely lost from organic samples when subjected to dry ashing techniques (11). As part of the Trace Metals Project, ashing studies were carried out which confirmed the loss for dry ashing of petroleum matrices. When ashing aids such as sulfur or magnesium oxide were used, some selenium was retained in the ash, but the recovery was not complete. 

A variety of techniques are available for the determination of trace elements in crude oils, and chemical methods of analysis have been summarized by Milner and McCoy. Most geochemical studies of metals in petroleum have used emission spectrography of petroleum ashes because of the multielement nature of the method. " In recent years techniques such as polarograplWfcolorimetric analysis, X-r fluorescence, ESR, flame atomic absorption, and flameless atomic absorption have been used for the analysis of crude oils. Many of the techniques require preconcentration of the metals, usually by ashing techniques and consequently involve the risk of loss of volatile compounds or contamination by reagents. For many elements at very low concentrations (<1 ug/g) the risk of contamination is very high. Most of the applications cited above involve the determination of one or a few specific elements and are not suitable for multielement analysis. 

The ashing of calcified tissues employing the LTA technique has the principal advantage that losses of the volatile elements occur to a much lesser extent than is the case with conventional ashing techniques. It is also probable that the micro-structure remains unchanged during ashing by LTA. However, the method is slow and amenable to small sample amounts only. 

The ground samples of enamel and dentin can often be used directly for measurements by NAA, GF-AAS and other techniques which employ solid sampling. It is, however, necessary to dissolve the samples for measurements by other instrumental analytical techniques. Enamel contains very low amount of organic matter and can be dissolved using a single mineral acid. It is usually necessary to eliminate the organic matter from dentin when various dry or wet ashing techniques can be used. 

Clegg, M.S., Keen, C.S., Loennerdal, B. and Hurley, L.S. (1981). Influence of ashing techniques on the analysis of trace elements in animal tissue.l Wet ashing, Biol. Trace Elem. Res. 3(2), 107-115... 

---