November, 1930
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
121 1
It will be obvious that in this test, since no channels mere used, it was necessary to connect the fume hood tightly with the top of the unit, and to arrange suitable bag filters and exhausters to collect the black made.
channels is that of a cooling agent, and it has been known that this has a lower limit of desirability, but the present results show this effect more clearly. On complete removal of the channels the yields and properties (Table IV) are not so much diminished as might have T a b l e IV--Effect of Presence of C h a n n e l s been expected from other considerations. Although an enNo. 1501 No. 1505 tirely different type of black might have been expected in this MISCELLANEOUS DATA: case, that which was actually produced must apparently be Channel height, mm. 70 NO channels regarded merely as an inferior grade of channel black. In Combustion, per cent 400 400 Channel temperature, C. 520 413a other words, while the presence and cooling action of the chanYield. Ibs. ner M cu. ft. 3.00 1.65 nels do cont,rol the yield and quality of the black the limit Color valui, per cent std. 90 90b Oil adsorption, cc. per 100 grams 110 105 of variation of properties is such (compared with the properIodine adsorption, my. per gram 20.0 15.0 Acetone extract, per cent 0 0.7 ties of blacks made by other processes) that it cannot be PROPERTIES I N RUWJEK: definitely said to control the type of black made. FurtherMax. modulus 500 per cent elongation, kg. per sq. cm. 214 179 281 Ultimate tensile at max. modulus, kg. per sq. cm. 274 more, from the data presented it does not appear that the 670 610 Ultimate elongation at max. modulus, per cent type of the black is appreciably affected by the atmosphere Max tensile kg. per sq. cm. 304 295 Cure’ to max: modulus, min. at 140’ C. 105 105 surrounding the flames-i. e., the per cent excess air-for Abrasion resistance, per cent std. 103.5 92.0 similar temperature conditions. It must therefore be cona This is the temperature in the burner house, and corresponds t o 405’ C. in the corresponding test with channels. cluded that the type of the black is determined by conditions 6 Approximate value. within the flame itself-i. e., such factors as size, shape, temDiscussion perature of the gaseous layers, temperature of the luminous One of the facts shown most directly by the present results carbon particles, etc. Certain of these factors are not suscep is at carbon black equal in quality to that made from tible to direct investigation. Certain of the others have been methane may be made from hydrocarbons as high as propane, the subject of special studies, which have not, however, made although this presumably cemes to be true with the higher possible general conclusions as to exact cause of difference in liquid hydrocarbons. The close agreement between various the type of black produced by various luminous flames. maxima observed for propane with the operating conditions Acknowledgment of the commercial channel process using methane shows the The authors wish to express their appreciation to the Genessential identity of the basic flame reactions of the two gases. The function of the channels is clearly shown to be mainly eral Atlas Carbon Company for permission to publish these a cooling effect by the very large change in yields and proper- results and to C. ,J, Wright, vice president of this company, ties observed when the channels are superheated to G50° C. whose interest and support have made it possible to carry on (Table 11). The fart that with further cooling to tempera- this work. Literature Cited tures below the optimum the yield is not much greater, while the properties are lower in general, is not easily interpreted. (1) Chamberlin and Rose, Am. Inst. Chem. Eng., Preprint (June 19, 1999). I t has always been generally assumed that the function of the (2) Neal and Perrott, Bur. Mines, Bull. 192 (1922).
The Slacking of Coal and Its Proper Interpretation’ S. W. Parr and D. R. Mitchell fiNIVERSITY OF
ILLINOIS, URBANA,ILL.
LACKING tests on coal are carried out by bringing the coal into moisture equilibrium with the air, drying for 23 hours, submerging in water for 1 hour, drying, and &merging again-repeating thus the routine through eight cycles ( I ) . After each drying the amount passing through a l/rinch (G-mm.) sieve is weighed and the cumulative percentage after eight cycles is taken as the slacking index. It has been proposed to use this factor as an indication of rank, beginning with the lignites and extending up through the sub-bituminous into the true bituminous, supposedly giving a measure of lignitic character to any coal having slacking proclivities. This interpretation of the phenomenon of “slacking” is altogether misleading and erroneous. If slacking were a fixed and definite function, the case would be different, but it is not.
S
Slacking of Lignite
For example, if a lignite is thoroughly dried and then thoroughly soaked with water, it thoroughly disintegrates. The explanation is comparatively simple. The lignites are peculiar in that they not only have a high moisture con1 Received July 31. 1930. Presented before the Division of Gas and Fuel Chemistry a t the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930.
tent, but this free moisture is distributed uniformly throughout the texture of the material, so that in discharging this moisture there remains a considerable volume of capillary space corresponding to the high percentage content of water normally present in the freshly mined material. This is, moreover, consistent with the facts developed by Porter and Ralston ( S ) , who maintain that the “heat of wetting is probably to be explained by assuming that each particle of the dried coal acts as a sponge of a tissue permeated with microscopic pores.‘’ The suggestion is further made (2, p. 27) that the heat may be produced by two processes: (1)the mere penetration of the liquid into minute capillaries,” or ( 2 ) “the formation of a colloidal condition.’’ I n any event, upon the accession of water energy is developed and no argument is needed to show that either capillary or colloidal forces are ample to bring about the rupture and disintegration of the substances when water is added to the dried material. The slacking of a lignite, therefore, is due to the wetting and not to the drying process, and the disintegration is substantially 100 per cent after one complete cycle. Characteristics of Coals Having Slacking Tendencies Let us turn now t o certain bituminous coals that are said to show slacking tendencies after storage in the open for
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several weeks or perhaps after 2 or 3 months’ exposure to the weather. Quite a number of coals from both the Illinois and Iowa fields may be so characterized. The first notable fact about these coals, when subjected to the slacking test, is that even after the full eight cycles have been completed the cumulative percentage of fine material is relatively low, so low in fact as to raise the question as to how much of it may have been produced by abrasion due to the handling, and especially the sizing and sifting process, which should be made a t the end rather than after each cycle (Table I). Table I-Comparative Slacking T e s t s of Duplicate S a m p l e s of Illinois Coal, S h o w i n g Wide Divergence as between Eight Screenings or O n e Screening (Samples from Vermilion County Coal Bed. 6; approximate moisture 15 per cent) TOTALFINESTHROUGH INCH STEVE SAMPLE Screened after Screened only at end each cycle of 8th cycle
Per cent 59 A 59 B 69 C 59 D 60 A 60 B 60 C 60 D Mine average
Per cent
28.4 24.6
..
16:s 13.4 1615 14.3 15.2
27.8
T e s t s of Illinois Coal, S h o w i n g Divergence in Duplicate S a m p l e s THROUGH DIFMAX. l/4-INCH FERENCE DIFFERAPPROX. SIEVE IN ENCEIN COAL SCREENED DUPLI- SAMPLES COAL BED SAMAFTER CATE FROM SAME BED MOISTUREP L E ~ EACHCYCLESAMPLES MINE
a b l e 11-Slacking
COUNTY
Per cenl Williamson
2
Per cent
Per cent
This matter of variation in the same bed, and even in the same set of duplicates, is shown in Table 11. Coking Properties and Slacking Test
One other fact casts discredit on the slacking test as a measure of lignitic character. Both the Iowa and Illinois coals charged with the slacking propensity are strongly coking, and this is a property entirely incompatible with the true lignites. I n semi-plant experiments at the University of Illinois, on car lots from both Iowa and Illinois, these coals coked in a standard 12-inch oven, under proper conditions as to time (4 hours) and temperature (700” to 900” C.) produced a high-grade coke which by the shatter test is shown to equal in strength standard coke made from true coking coals for blast furnaces (Table 111). This in itself would be sufficient to put the slacking test into the discard as an index of lignitic symptoms. Tests of Coke f r o m Illinois a n d Iowa Coal S h o w i n g Absence of Lignitic Charactera COKE DESCRIPTION SHATTER INDEX 1127 Iowa 70.8 1146 Illinois 75.2 a The shatter index is the per cent of coke remaining on a 2-inch square mesh screen after being subjected t o the drop test.
Table 111-Shatter
..
24:o 34.2
Vol. 22, No. 11
Per cent
17
A-49 34.5 9.9 A-49 24.6 7 Marshall 16 B-I 59.1 B-11 82.4 43.5 B-I11 38.9 42.8 Peoria 5 16 C-I c-11 36.2 15.3 c-I11 51.5 34.9 LaSalle 2 15 D-95 32.0 D-95 2.9 2 13 E-I 21.9 La Sa11e E-I1 23.4 8.1 E-I11 15.3 14.8 Franklin 6 10 F-I F-I1 11.1 4.5 F-I11 10.3 Williamson 5 8 F-IV 10.8 G-I1 4.8 1.7 6.5 G-111 2 15 H-96 LaSalle 10.0 18.7 8.7 H-96 Capital letters indicate mine from which samples were taken. Arabic numerals indicate duplicate samples. Roman numerals indicate samples taken at different places in the mine. Q
Structural Changes in Slacking of Coal
A study of the structural changes as the cycles proceed shows clearly that we have here an altogether different phenomenon. There is no textural disintegration of the sort characteristic of lignites. On the contrary, the lines of weakness, when they do develop, proceed along the bedding planes and, upon laying open such cleavages, they are seen to follow along the thin layers of fusain or mother of coal. This material, when of the soft, porous type, permits accession of moisture, which in time weakens the bond between the adjacent layers of glanz or dull coal, a bond which a t best is not over strong. Finally, after a considerable exposure to the drying and wetting process, this weakening brings about a type of disintegration which, for want of a better word, we call “slacking” but which bears absolutely no resemblance to the processes that bring about the “slacking” of a lignite. Moreover, the distribution of fusain is not by any means uniform, nor is its porosity constant in any given area. This in itself introduces such marked variables as to raise the question of value of the test for any purpose whatsoever, certainly not as evidence of the lignitic character of the deposit.
Screening Tests
One other striking illustration of the fact that the fusain layers, and not the coal, furnish the slacking property is shown by the results of screening or degradation tests made on car lots of Vermilion and Sangamon County coals, 2-inch nut size, held in open storage for six years ( 2 ) . At the end of 11/2 years the degradation (passing ‘/d-inch screen) amounted to 12.1 and 12 per cent, respectively, of the original coal not passing such a screen. After 6 years these factors became 31 and 24.7 per cent, respectively. The latter values are fairly comparable to the slacking tests carried through the usual eight cycles with one sieving. Conclusion
Obviously, therefore, slacking as related to bituminous coals is directly dependent upon the thickness, distribution, and porosity of fusain bands, rather than on the absorptive property of the coal proper. While the writers are convinced that slacking tests are inapplicable to bituminous coals as an indication of rank, they do not wish to be understood as asserting that they are without value in other connections. For example, as a qualitative test for a lignite the slacking test doubtless has value, but one cycle or, at the most, two are ample to establish this character. When applied to bituminous coals, however, the test may have a number of interpretations, such as the scope and character of the fusain layers or the behavior of the coal under open storage conditions. The eight cycles of drying and wetting may represent the ultimate degradation effect of a particular coal when stocked, The interpretation of the slacking index as determined by the eight-cycle method would seem to represent the probable limit of degradation for an extended period of storage. No mention has been made of clay bands. They are far less frequent, but when they do occur their behavior is substantially the same as that of fusain when subjected to the slacking test. Literature Cited (1) Fieldner, Proc. Am. S o 6 Testing Materials. 29, Pt. I, 399 (1929). (2) Parr, University of Illinois, Eng. Expt. Sta., Bull. 97, 35 (1917). (3) Porterrind Ralston, Bur. Mines, Tech. Pe9er 113 (lele).
