THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CVIII. FRIDAY, AUGUST 28, 1911. No. 2800. Weathering of the Pittsburg Coal Bed at the Experimental Mine near Bruceton, Pa.* By HORACE C. PORTER and A. C. FIELDNER. The experimental mine of the Bureau of Mines is an opening in the Pittsburg coal at a point where the bed outcrops on a hillside, a mile south of Bruceton, Pa., or about 13 miles south of Pittsburg. During the driving of the two nearly horizontal entries of this mine an unusual opportunity was presented to study the variation in the character of the coal at increasing depth below the surface and distance from the outcrop. Accordingly, during the summer of 1911, when the entries had been driven about 620 ft., such a study was made by taking samples at different distances from the outcrop, analysing and comparing these in various ways, and making observations of the physical character of the coal at the several points. All of the samples were subjected to a complete chemical analysis. Certain ones also were compared with respect to ease of oxidation, tendency to liberate inflammable gases, and relative inflammability of the dust obtained by pulverising. Two 15-ton samples were compared as to their steaming qualities in a boiler fur- nace. The results of the analyses and tests are pub- lished in this paper as a contribution to the effect of weathering on coal beds, with especial reference to decrease of fuel value and the relation between the appearance and the chemical composition of weathered coal. The results have demonstrated that indications of weathering, such as yellowish coatings of iron hydrate PER CENT SV„TV0DNI3HI 20 15 10 100 600 700 200 300 400 500 distance erqm outcrop, eeet Fig. 1.—Curve showing Variation in Total Moisture Content of Samples as received. or a dull appearance of the surfaces, do not always signify a material change of the chemical composition or heating value of the coal itself. The chemical analyses show that changes in com- position have occurred in the coal for a distance of about 50 ft. from the outcrop. The analytical data serve as a basis for certain deductions as to the nature of these changes. Several points of similarity become evident between weathered coal of this character and the cretaceous coals and lignites. On the other hand, certain dissimilar properties of the two render it alto- gether doubtful whether a true metamorphosis or reversion of the bituminous coal to the lignitic type could ever take place through the agency of weather- ing. The analyses also show that the composition and heating value of the unweathered coal, computed on the moisture and ash free basis are fairly constant. ' In addition to the usual analyses, special tests were made in order to show the relative oxygen consuming power of the coal samples and their power of liberating inflammable gases, because these properties are known to vary with the nature of the coal, and have a bearing on mining operations. As the direct union of freshly broken coal with oxygen lowers the oxygen content of mine air in places where ventilation is inadequate, and as the continuous escape of inflammable gas from broken * From Technical Paper 35, U.S. Bureau of Mines. coal tends to increase the danger of explosions, it is of interest to determine to what extent, if any, this behaviour of coal is affected by proximity to the out- crop and consequent weathering. Samples were taken at different points in the mine and put in 5-gallon glass bottles, the coal being crushed so as to pass a | in. screen; 201b. were placed in each bottle as quickly as possible after the coal had been broken down, and the bottle was sealed before it left the mine. By admitting air to the bottles in measured quantities daily and draw- ing off the air and gases, the progress of oxidation of the coal and of the liberation of inflammable gas was followed. The samples thus tested were taken at 5 ft., 50 ft., and 620 ft. from the outcrop. It is known that the degree of inflammability of coal dust is lowered by weathering of the dust itself.* Frazer, Hoffman, and Schollf have shown also that samples of exposed coal from country banks are less inflammable when powdered than the usual mine sample of the same type of coal. In the present investigation, three samples, taken at the outcrop, 50 ft. in and 620 ft. 16 Sulphur Content of Samples on Moisture-free Basis. Fig. 2.—Curves showing Variation in Ash and 100 600 700 200 300 400 500 DISTANCE EROM OUTCROP. EEET 14 ggl2 2g n R 1° gk 55 fa -es * S5 • ,85 Fig. 3.—Curve showing Variation in Carbon Content of Samples on Moisture and Ash-free Basis. 70 100 600 700 200 300 400 500 DISTANCE FROM OUTCROP, FEET 5^75 OM in, were submitted to test for inflammability in the Frazer laboratory apparatus, so as to form an approxi- mate estimate of the distance to which this particular weathering effect had penetrated into the coal bed. In view of the prejudice among consumers against “ rusty ” coal, or lumps of coal having yellow coatings of iron compounds, it was deemed of value to deter- mine by actual test in a boiler furnace the comparative evaporative power of such a coal, and of a clean, normal sample from the same seam. No essential differences in evaporative power, furnace efficiency, or burning qualities could be detected. The investigation described in this paper, although not primarily concerned with sampling, shows incident- ally the necessity of averaging a large number of samples from the same mine in order to obtain a truly * Chamberlin, R. T., “ Notes on Explosive Mine Gases and Dusts ” : Bull. 26, Bureau of Mines, p. 63. Taffanel, J., Premitre Series Travaux de Laboratoire,, Station d'essais de Litvin, Comit6 Central des Houill&res de France, p. 47 (see also Cinquitme Series des essais sur VInflammability des Poussitres). t Frazer, J. C. W., Hoffman, E. J., and Scholl, L. A. junr., “A Laboratory Study of the Inflammability of Coal Dust ” : Bull. 50, Bureau of Mines, 1913. representative ash content. The probable error of such a determination should be less than 1 per cent. Figs. 1 to 5 present curves plotted to show the follow- ing items :—Total moisture content of samples as received, variation in ash and sulphur content on moisture-free basis, variation in carbon content on moisture and ash free basis, variation in oxygen and hydrogen content on moisture and ash free basis, varia- tion in volatile matter content and in calorific value on moisture and ash free basis. The analytical results presented in the charts show clearly that there has been a considerable alteration of the coal substance in the first 40 ft. from the outcrop, the degree of alteration decreasing with distance from the outcrop. Between 40 and 75 ft. the alteration is slight, and beyond 75 ft. it is not perceptible at all by the methods of analysis employed. The alteration mani- fests itself in a decrease of calorific value (16-3 per cent, at 5 ft., 6-6 per cent, at 20 ft., etc.) and an increase of oxygen (amounting to 10 per cent, of the coal at 5 ft.) with a corresponding decrease of carbon and hydrogen. Beyond the zone of weathering (75 ft.) the variation in the largely adventitious constituents, moisture and ash, is not to be regarded as,having a relation to the outcrop. The percentage of sulphur in the weathered parts of the bed is less than half of that in the fresh coal, pro- bably because of oxidation of pyrites to iron sulphate and the leaching effect of surface waters. The fact that half of the original sulphur, however, is still present in the outcrop coal after long exposure and weathering, and that both weathered and unweathered coal contained only traces of sulphate sulphur, indicates either a remarkably slow rate of oxidation of pyrites or the pre- ss5 Is16 20 18 HHyc Qxyg ft'2 OHIO 100 200 300 400 500 600 700 DISTANCE FROM OUTCROP, FEET Fig. 4.—Curves showing Variation in Oxygen and Hydrogen Content of Sampaes on Moisture and Ash-free Basis. sence of some other form of sulphur, such or organic sulphur, which is more stable towards oxidising influences. The results of washing experiments indicate that the sulphur in the weathered coal is practically all in the form of organic sulphur. The computed figures for organic sulphur in the unweathered coal are of the same magnitude as in the outcrop coal, a relation that may indicate that this form of sulphur is not affected by the weathering reactions. Referring to figs. 1 to 5 more in detail, there is to be noted :—(1) The high percentage of total moisture near the outcrop, and also at 125 ft., its presence at the latter point being caused possibly by infiltration of water in the mine along an intruding clay seam; (2) the remarkable agreement of the curves for carbon, hydrogen, oxygen, and calorific value in coming approxi- mately to a horizontal direction at the same point, namely, 60ft. from the outcrop; (3) the constant heat- ing value of the unweathered coal substance free from moisture and ash, as shown by the horizontal direction of the calorific value curve, which conforms with similar data obtained from other mines and beds in the Appala- chian field; and (4) the well-defined, though slight, dips in the carbon and calorific value curves, and the corre- sponding rise in the oxygen curve at 200 to 300 ft. from the outcrop indicating, a slightly inferior average quality of coal at these points. The moisture retained by the coal after air drying is