Coal samples, room-temperature

The only published work on the diffusion of gas in coals of different rank appears to be that of Bolt and Innes (2) who studied the diffusion of carbon dioxide from eleven samples of coal at 38°C. They found the diffusion coefficient to range from 3.5 to 9.2 x 10 8 sq. cm./sec., with no apparent correlation with coal rank. Diffusion data on coals of different rank at temperatures higher than 38°C. have only been reported by the present authors (6). It has been shown (7) that the diffusion of inert or noble gases from coal above room temperatures can be rigorously analyzed by using simple diffusion theory, and that true diffusion parameters of the micropore systems can be obtained. In this paper our measurements on the unsteady state release of argon from coals of various rank, over a temperature range, are reported. 

Proton nmr spectra of fractions A, B and C and all bottoms products were recorded on a Varian HA lOOnmr spectrometer using a solution of the sample dissolved in pyridine-d5. Spectra were run at room temperature with tetra methyl silane (TMS) as an internal standard, with a sweep width of 0 to 1000 cps from TMS. Fraction D and the whole coal were only partly soluble in pyridine and it was therefore not possible to get representative spectra from them. 

Helium and mercury densities were determined on the 6-8 mesh fraction. The larger mesh size was used to avoid the possibility that mercury would not penetrate the space between particles in the mercury density measurements. The coal was placed in a calibrated density tube, evacuated at room temperature for one hour, and then heated at 100°C. in vacuo for 2 hours. The weight of the coal after this treatment was used to compute the densities. Helium densities were determined at 30°C. by the method of Rossman and Smith (11). Mercury densities were determined by admitting mercury at an absolute pressure of 1140 torr to the coal sample after evacuation, following the helium density measurement. 

For each run, coal samples of approximately 50 grams were dried, at 100°C. for 4 hours and weighed after cooling in a desiccator for % hour. In the early experiments, coal and solvent were mixed in the autoclave, and runs were performed. It was found that the time necessary for the autoclave and mixture to be heated from room temperature to reaction temperature was 1 hours. When extraction fraction vs. time was plotted, it showed that at higher temperatures more than 80% of the total possible extraction of coal dissolved before the system reached the reaction temperature. Consequently, the data obtained in the first 2 hours were incorrect. 

The mixture in the autoclave is homogeneous. This was insured by keeping the Magne Drive stirrer always at a steady 1500 r.p.m. It was experimentally proved that even distribution of coal and tetralin in the autoclave is a fair assumption because the fraction extracted from the last portion of mixture, which remained in the autoclave and was taken after the system cooled to room temperature and the autoclave was opened, was found to be very close to the yield obtained from the last sample taken from the system through the sample lines at reaction temperature. 

Usually, the first moisture value to be obtained on a coal sample is the air-dry loss moisture. This moisture loss occurs during an attempt to bring the coal sample into equilibrium with the atmosphere in the sample preparation room. The practice of using temperatures above room temperature may accelerate oxidation but shortens the time needed for air drying hence, temperatures above 40 to 50°C (104 to 122°F) are not recommended for air drying. 

In the procedure (ASTM D-2639), the sample is air dried prior to preparation and the temperature should not exceed 15°C (59°F) above room temperature, and drying should not be continued to the extent that oxidation of the coal occurs and the plastic properties of the coal are not altered by oxidation. The apparatus is then immersed in the heating bath and a known torque applied to the stirrer. During the initial heating no movement of the stirrer occurs, but as the temperature is raised, the stirrer begins to rotate. With increasing temperature, the stirrer speed increases until at some point the coal resolidifies and the stirrer is halted (Figure 7.4). The plastic properties of the sample are then measured by the resistance to motion of the fluid mass in the plastometer. 

The completed sample was soaked in hexane at room temperature until the thin specimen floated off of the slide. The sample came off as a number of small pieces of various sizes from less than a millimeter across to several millimeters long. Although no residual adhesive could be observed on the pieces of coal, to insure complete removal of the adhesive the samples were immersed in a large excess of fresh solvent for several days. Then the hexane was decanted off and the specimens were stored in nitrogen at room temperature until they were used. 

Data acquisition and least square fitting of the Moessbauer spectrum were done in a microprocessor based computer (Promeda). The source used was a 220 mCi - Co Rh. All the isomer shifts in this paper are given relative to a-Fe at room temperature. The coal used in the study was IL6 (pyritic sulfur 2.0 and organic sulfur = 2.0 %) and the solvent was a SRC-II heavy distillate 850+. A specially designed reactor for Moessbauer measurements was used. The temperature stability was 1%. The stoichiometries of the pyrrhotites were determined by Moessbauer and x-ray diffraction methods (10). All the samples were also measured at low temperatures (13 K) for better resolution of all the components, using a closed-cycle helium refrigerator. 

Our objective was to measure the changes in the Moessbauer parameters as a function of temperature, solvent, H2 pressure and time. The first run consisted of a sample of about 300 mg/cm coal inside the reactor under 2 psi of N2 and no solvent. It was run between room temperature and 420° C, and maintained at 420°C for 10 minutes, long enough to obtain a Moessbauer spectrum. No evidence of transformation of FeS2 to pyrrhotite was detectable by Moessbauer spectroscopy. After rapidly returning the sample to room temperature a Moessbauer spectrum was taken, from which it was observed that the characteristic spectrum of FeS2 was retained with a very small amount of conversion to pyrrhotite. 

Materials. Samples of SRC II middle and heavy distillates were obtained from the Pittsburg and Midway Coal Mining Company. These were distilled into 50°C boiling point range fractions and labeled as follows MD-2 (150-200°C), MD-3 (200-250°C), MD-4 (250-300°C), HD-2 (300-350°C), HD-3 (350-400°C), and HD-4 (400-450°C). HD-3 and HD-4 could not be used in the apparatus because they contained solids at room temperature. 

The compositions of each coal used in this study is shown in Table I. The conditions under which these pristine samples were collected and prepared have been reported previously (12). The weathered Upper Freeport sample was exposed to air and sunlight at room temperature for four weeks, while the oxidized coal was heated... 

The samples used in this study were Witbank coal and Goonyella coal. To reduce the effect of residual water in the coal on the microimage, the coal samples were evacuated at about 10 2 Torr for approximately 24 h at room temperature and stored in a glovebox under a dry N2 atmosphere. The size of the bulk sample used was about 2.5 mm x 2.5 mm x 100 pm, which was cut artificially in order to reduce the measurement time for the 3D-SPI experiment. The specimen was placed into a 5 mm diameter high-temperature ESR tube with AFO-t powder. 

Our Mdfssbauer techniques, when used simply to supplement SEM-AIA of a coal, are quite standard. All measurements are conducted in transmission mode, with the coal sample usually at room temperature. Spectra are not obtained routinely with the sample at cryogenic ten eratures unless the coal is weathered or unusual in some other respect. In most coals the different minerals each contribute a quadrupole doublet, consisting of two similar peaks, to the spectrum (Figure 3, top). However, if the mineral is magnetically ordered, a six-peak pattern will be observed (Figure 3, bottom). Different Md ssbauer parameters are derived from the peak positions in these two-peak or six-peak patterns and the iron-bearing phase can then be identified from the values of these parameters. 

Figure 3. Room temperature Mossbauer spectra of coal samples from the Pratt seam in Alabama (top) and from an anthracite deposit in Rhode Island (bottom). In the top spectrum, peaks indicated arise from iron in the common coal minerals pyrite (P), clays (C), siderite (S), and jarosite (J), In the bottom spectrum, peaks indicated arise from iron in clays (C), and in the rare coal minerals, ankerite (A), iron metal (I), and ferric oxide (H),...

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