Plasticitv of Coals J
Its Measurement and Relation to Quality of Coke Produced R. E. BREWER
AND
R. G. ATKINSON, Pittsburgh Experiment Station, U. S. Bureau of Mines, Pittsburgh, Pa.
Determinations of plastic properties of bituminous coking coals by revised Davis plastometer, Agde-Damm, and Layng-Hathorne test methods show closer correlation of data than has been obtained heretofore. It is shown that definite form of apparatus and procedure of test are important governing factors in this better duplicability of results. The comparative merits and limitations of data from each revised method are discussed. It is concluded that the most complete and reliable information upon the plastic properties of bituminous coking coals is given by the modified Agde-Damm test for the preplastic range and by the modified Davis plastometer for the plastic range. The modified Layng-Hathorne test, while showing some improvement over the older procedure, is less satisfactory because of uncontrollable variables. Plasticity data from the revised AgdeDamm and plastometer tests enable a fairly reliable prediction of the strength of a coke to be expected from the carbonization of a bituminous coking coal.
gas, observed during the heating of the coal charge under controlled conditions. The literature is replete with references to and publications upon methods of measuring the plastic properties of bituminous coals. No attempt will be made here to give a complete bibliography. Original publications upon the better-known test methods, some later important applications of these and modified procedures for measuring plasticity and related properties of coals, and certain interpretations of the significance of plasticity characteristics in explaining the behavior of a coal on heating have appeared (1, 4-9, 16-26). It seems desirable here to deviate from the usual orthodox classifications of test methods according to the type of apparatus employed or the names of the investigators and follow instead a classification based upon the properties of the coal.
Methods for Measuring Plastic Properties of Coals Penetrometer instruments in which a needle moves vertically or laterally through the heated coal mass, apparatus measuring resistance to shear caused by movement of a stirring device within the coal, and extrusion instruments depend on changes in viscosity to characterize the plastic properties of coal. Dilatometer methods, which measure the linear expansion or contraction of a test sample, frequently under a known load, have been widely used t o define the plastic properties from swelling and softening observations on the heated coal. The gas-flow principle, which depends on variations in the void spaces of the test sample during heating through the plastic stage, has also been widely applied. The principles of the three distinct methods just cited for measuring the plastic properties of bituminous coals have been extensively applied in the Davis plastometer, Agde-Damm, and Layng-Hathorne test methods, respectively. These methods with some changes in the two latter tests (11, 12) were used in the Bureau of Mines laboratory on the first thirty-two coals (11, 13, 15) tested in the Bureau of MinesAmerican Gas Association survey of American coals. Comparison of the plasticity temperature and pressure data found on these coals frequently showed poor correlation between these test methods on certain coals. The objects of the present study were to determine as far as possible the reasons for this lack of agreement, to revise the operating technic in each test so as to obtain better correlation of data, and t o apply the new data in a practical prediction of coke quality. The three test methods as now employed in the Bureau of Mines laboratory are described below. MODIFIEDDAVISPLASTOMETER TEST. This test method as now used makes a continuous measurement of the viscosity of the coal while it is heated through the plastic state. Figure 1 shows a vertical section through the instrument.
B
ITUMINOUS coking coals, when heated a t a moderate rate in the absence of air, undergo complex and continuous changes in chemical composition and physical character. Iluring carbonization, most bituminous coals, except those bordering on the lignites and semianthracites, show evidence of softening, coalescence, swelling, fluidity, and finally hardening of the coal substance. There are marked distinctions in the degree to which each of these plastic characteristics is shown by different bituminous coking coals. Even for a given coal, the magnitude of these observations is affected greatly by the particular conditions under which such properties are determined. Moreover, complete interpretation of observed plasticity data is complicated by the transient character of the chemical and physical changes involved. Direct applications of plasticity data as absolute indices of the behavior of a coal in gasification, in coke-making, or in combustion are, therefore, made difficult. The aggregate information gained from laboratory tests, however, when certain related temperature, pressure, and expansion (or contraction) data are combined and suitably weighed, furnishes a valuable guide for more complete interpretations of the probable nature of the mechanisms of softening, fusion, coalescence, solidification, and accompanying phenomena. In the Bureau of Mines survey of the gas- and coke-making properties of coal the plastic properties of bituminous coals have been determined from measurements of viscosity, of expansion and contraction, and of resistance to flow of an inert
The technic now used in making a test is to charge 18 grams of
0- to 20-mesh coal into the retort, which is then assembled by screwing in place the head containing the thermocouple well. The retort is placed in the furnace and rotated continuously at constant speed (2 r. . m.) by means of a motor-driven chain attached to the sprocfet on the outer shaft. The resistance, or
torque, necessary to prevent rotation of the inner shaft, caused by the coal charge moving against the rabble arms, is measured by the position of the indicating dial. The scale is calibrated in kilogram-centimeters (inch-pounds, or more yoperly, poundinches) directly with weights. Free rotation o the dial is pre443
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FIGURE 1. PLASTOMETER
m= *
0.0001”
vented by means of tension springs not shown in Figure 1. The coal charge is heated a t a rate of 4.8’ * 0.2” C. per minute. Temperatures and resistances are observed and recorded at 2- to 5-minute intervals within the plastic temperature range. “True” temperatures of the coal are obtained by adding experimentally predetermined corrections found from temperature measurements taken in the center of the retort to those temperatures observed regularly in the thermocouple well. When the temperature reaches the plastic range, the coal sample becomes viscous, and resistance to rotation develops rapidly. With higher temperatures the resistance usually decreases because the coal becomes more fluid. As the heating is continued further, chemical changes in the coal cause it to become more and more viscous and finally to solidify. When solidification occurs the coke formed is quickly broken up and resistance to rotation practically ceases, thus indicating the end of the plastic range. MODIFIED AGDE-DAMM TEST. This “expansion test” indicates the plastic properties of a coal by measuring the linear expansion and contraction which a small cylindrical briquet of coal undergoes while being heated to 500’ C. Figure 2 shows a vertical section of the assembled apparatus ready for the test. Two coal briquets are formed by compressing 0.7-gram samples of 0- to 60-mesh coal in 7.6-em. glass test tubes of 0.8-cm. bore a t 10.3 kg. per sq. cm. (146 lb. per sq. inch). The lengths of the finished briquets (approximately 1.8 cm.) are then carefully measured and the tubes inserted in the 1.1-cm. holes of the copper heating block. One briquet is allowed to expand freely during the test. The second briquet supports a total weight of 500 grams, represented by the assembly shown in Figure 2. The micrometer distance gage is set arbitrarily to register the measured length of this coal column. The changes in length of the column during a test are indicated directly by the gage readings. The coal charge is heated a t a rate of 4.8” * 0.2’ C. per minute. Temperatures are measured by thermocouples in the hollow-rod plunger and copper block. Gage and temperature readings are taken and recorded a t 5-minute intervals up to 270’ C. and at 2minute intervals thereafter, until the end of the run at 500” C. I t is usually advisable to take additional readings at 1-minute intervals near the critical temperature points (where the change on the distance gage is from contraction to expansion, or vice versa) and elsewhere in the test where the rate of gage movement is rapid. A small electric buzzer is used to tap the gage before each reading, except when the rate of change is quite rapid.
1
177.8 mm.
FIGURE 2. MODIFIED AGDE-DAMM APPARATUS
Two predetermined sets of corrections are applied to eliminate the effect of the thermal expansion of the instrument and to obtain the “true” coal temperatures. Corrections for the thermal expansion of the instrument from room temperature
NOVEMBER 15, 1936
ANALYTICAL EDITION
445
TABLEI. SUMMARY OF PLASTIC RANGE TESTS
-
Coal
Coal Bed
;; (%itburgh 70
Millers Creek
33 34 23
38 39
39A 39B 41 40
coal 33 coal28
7 0 9 coal 33 13009 coal 23 PoGhontas No. 4 36 280% 0 7 coal coal 23 70 coal36 { 3 0 $ coal 23 Upper Cedar Grove 80% coal 39 1 2 0 7 coal 41 70% coal 39 3 0 7 coal41 Beotley Lower Cedar Grove
{
coal 40 30% coal 41
Modified Agde-Damm Dry Free Mineralex,pan- ReMattersion sistFree Initial Initial swelling ance Fixed contrac- exprtn- coeffi- develCarbon tion sion cient ops
%
62.6 62.0
c.
c.
315 326
391 409
60.1
360
65.1 84.2
372 439
62.4 66.6
312 335
Above 490 rlbove 500 Above 500 409 419
68.4 60.9 65.3
346 308 331
67.4 81.3 62.7 67.1 68.6
C.
2.4 1.0
-
Modified Davis Plastometer
Maximum fluidity Ko.-cm. In.-lb. C.
...... 403
1.6
...
...
(1.4)
431
Maximum resistance In.-lb. C.
KO.-cm.
..
21.2
Res1stance ends
c.
Modified Layng-Hathorne Maximum resistance, Plastic mm. range water
c.
. . . . . . . ...
.....
...
475
122 37 31
(18.4)
470
1.0
No resistanoc
391-435 None
1.0 1.0
Slight intermittent resistance
None
Slight intermittent resistance
.....
1.7 1.0
403 416
0.2 1.2
(0.2) (1.0)
434 437
30.0 30.0
(26.0) (26.0)
464 467
490 490
384-431 385-432
195 143
425 405 418
1.0 1.2 1.0
421 406 417
0.5 0.9 0.9
(0.4) (0.8) (0.8)
435 440 439
39.2 46.1 94.5
(34.0) (40.0) (82.0)
485 479 483
494 490 489
379-434 395-444 384-448
169 88 106
340 410 334 338
418 454 413 413
1.1 1.0 1.3 1.3
420 447 415 418
1.6 2.1 0.6 1.2
(1.4) (1.8) (0.5)
(1.0)
440 469 440 443
94.5 82.9 47.2 66.8
(82.0) (72.0) (41.0) (58.0)
485 503 467 472
491 515 478 487
383-447 445-525 414-447 404-451
116 190 199 178
344
414
1.3
414
2.3
(2.0)
443
86.4
(75.0)
480
487
400-..
to 500’ C. were predetermined from gage readings in blank runs in which a cylindrical block of quartz was substituted for a coal briquet of like dimensions. These readings are subtracted from gage readings at corresponding temperatures observed in regular tests. Temperature measurements made in the center of the coal charge in separate runs gave data to correct the lower plunger and higher block temperatures regularly observed in test runs. When the charge of coal reaches a temperature of about 300’ C., the individual particles soften and rather rapid contraction takes place. As the temperature rises, this contraction continues with increasing rate to the point of initial rapid expansion at which a very sharp change in direction of the expansion-temperature curve is noted. Shortly after rapid expansion occurs, the coal with continued heating frequently becomes so fluid that it will no longer support the weight of the plunger, which may sometimes even sink to the bottom of the test, tube. Upon completion of the test a t 500” C., the free expansion sample is removed from the furnace, allowed to cool, and its length again measured. The net linear change during the test is expressed as the “swelling coefficient.” The significance of the expansion and contraction characteristics shown by the coal under these test conditions will be pointed out in the discussion of results. MODIFIEDLAYNG-HATHORNE TEST. A detailed description of the apparatus and general operating technic have been given in an earlier publication (18). The procedure has been modified to use a 3-cm. column of 0- to 20-mesh coal, supported in the electric furnace and heated under controlled rates as previously described (12). Pressures in the apparatus necessary to force a stream of purified nitrogen at 20 cc. per minute through the charge and corresponding temperatures at the wall of the glass tube opposite the center of the charge are read and recorded a t 2- to 5-minute intervals during the test. Predetermined corrections found from temperature measurements taken in the center of the coal charge are subtracted from the observed temperatures to obtain the “true” coal temperatures. When the temperature is reached at which the coal softens, the resistance to gas flow increases markedly until a maximum is reached. The resistance then drops to approximately its initial value. The significance of the resistance-temperature curve obtained in this manner is discussed below.
.
145
Coals Tested The bituminous coals tested in this investigation are current coals under study in the B. M.-A. G. A. survey of American coals. For ready comparison of the plasticity data with the gas-, coke-, and by-product-making properties of these coals, as published (14),or to be published, the coals have been aspigned the same numbers. Table I gives the coal number, the name of the coal bed, the per cent of dry, mineral-matterfree fixed carbon designating rank (d), and plasticity data by the three revised test methods. The recorded plasticity data represent averages of repeat tests (two or more) by each method upon each coal. Coals 28 and 23 used in blending coals 33 and 36 were second lots of coals 28 and 23 used earlier in the survey (11). Shortage of sample of coal 28 prevented plasticity determinations by the plastometer and gas-flow methods. Except for coals 23 and 41, the coals and their blends are all of high-volatile A rank.
Discussion of Results
A study of the data summarized in Table I obtained by the three revised test methods shows, in general, much closer agreement on a given coal than was found by these methods as previously employed. The better checks are partly due to the correction of temperature data to obtain the “true” temperature of the test samples. The previous objections to the necessity for preparation of a special 0- to 100-mesh coal sample for the Agde-D8mm test and the use of sized fractions of 20to 40-mesh coal and coal-electrode carbon mixture for the Davis and Layng-Hathorne methods, respectively, have been eliminated by the use of representative 0- to 60- and 0- to 20mesh coal samples. Slower rotation of the Davis retort gives results in accordance with the known fact that fluidity of a coal must result before pronounced coke formation occurs. The changed procedure-i. e., using a 3-cm. column of 0- to 20-mesh coal without electrode carbon-in the revised Layng-Hathorne test gives somewhat better correlation of data with that found by the other two test methods. The values of the maximum resistances developed, however, in successive tests by this method are still not closely duplicable and the temperature-pressure relationships are not as sharply defined as those shown by the other two tests. In general, the Davis plastometer gives much more com-
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plasticity curves. Lack of space, however, prevents graphic representation of all the data. The characteristic features of the temperaturepressure relationships are, therefore, presented for three typical coals. Figures 3, 4, and 5 are for coals 38,40, and 41, respectively. E Inspection of t,he curves shows that shortly 0 2.03 after the initial rapid expansion temperature E is reached in the Agde-Damm test, a sudden 1.781 change from expansion to apparent contrac-8 tion of the coal charge is frequently noted. 1.52 This reversal takes place at a temperature ap1.279 p r o x i m a t i n g that of maximum fluidity as 2 observed with the plastometer and is charac1.02 teristic of all coals showing an appreciable degree of fluidity, as in the case of coals 38 and 40. This sudden contraction is due, in part, to a portion of the coal working out around the plunger, thus leaving less FIGURE3. PLASTIC PROPERTIES OB BLENDOF 70 PER CENTALMAAND 30 PER sumorting. coal substance. Occasionallv. a CENTPOCAHONTAS COALS se&d slFght expansion, aided possibly ' in part by further action of the fluxing material, is observed. plete information in the plastic range. Minor limitations of Because of the many uncontrollable variables affecting the this method lie in the difficulties of determining accurately movement of the plunger after the temperature of initial small changes during the period of greatest fluidity and exrapid expansion, it is evident that much significance cannot tremely high resistances, above 63.4 kg.-cm. (55 inch-pounds) be attached to this frequently designated "final expansion shown by certain coals. The critical temperature points for temperature" of the Agde-Damm test except, perhaps, for a such coals in the present work were checked by a smaller, tworabble-arm retort. Extrapolation of temperature-resistance few less fusible coals. The plastometer curves show that when fusion of the coal curves constructed from data by the four-rabble-arm instrutakes place the resistance to stirring increases sharply for a ment showed excellent agreement in every instance with that short time and then decreases as the zone of high fluidity is predicted from test data by the two-rabble-arm instrument. approached. Apparently, this initial rise and fall of the reIt is hoped that a stronger drive now planned for the foursistance always occur to a greater or less degree. The only rabble-arm retort will eliminate its present minor limitations. explanation the authors can offer for this at present is that Reference to the data in Table I shows that the Agdeevidently the coal when first fused is sticky and contains conD a m test gives an indication of softening (initial contracsiderable solid material which imparts a high viscosity to the tion temperature) at a temperature considerably lower than mass; then when melting becomes more complete, the visany observed plasticity shown by the other two test methods. cosity is lowered. Following this initial hump portion of the Proof that softening and incipient fusion do occur in coal a t curve, a range of high fluidity is next observed. This may be temperatures considerably below the initial expansion temdefined as the distance between the new appearance of low reperatures has been given by Thiessen and Sprunk (27). The sistance and the change to increasing marked resistance temperature at which rapid contraction is observed in the caused by the beginning of solidification. This range of high Agde-Damm test is interpreted a t that point where the coal fluidity is related to the character of the cokes produced, as particles begin to soften appreciably and the pressure of the shown below. plunger presses coal into the void spaces of the charge. The plastometer curve for coal 38 is unique in the fact that Table I illustrates the very close agreement between "the there are two distinct maximum points near the end of the temDerature of initial expansion" in the Agde-Damm apparatus a n d the temperature where resistance develops in the plastometer. As was pointed out by Audibert and Delmas (S), the swelling or intumescence, as observed here in the AgdeDamm test, is caused by imprisoned gas bubbles and occurs after individual coal particles fuse together and impede the escape of gas. It is also this fusing together of the coal particles that gives rise to the initial resistance to stirring observed in the plastometer test. This has been 6 demonstrated by stopping a plastometer test 1.78 5" C. below the point a t which initial resistance 2 develops, quenching the retort to room tempera1.522 8 ture, a n d r e m o v i n g the coal charge. It was found that only a very slight or incipient state 1.27! 0 of fusion had resulted in the coal. However, 1025 when the test was continued until 10" C. above 2 the initial resistance temperature, the coal had 0.76 completely fused. With the foregoing generalizations in mind, a more complete picture of the plastic characterTemperature, "C istics of the a t e e n coals would be gained by L PLASTIC PROPERTIES OF LOWERCEDARGROVECOA FIGURE4. detailed studies of their temperature-pressure L
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ANALYTICAL EDITION
441
val temperature range (the difference between initial expansion and initial cont r a c t i o n temperatures obtained by the modified Agde-Damm test); (2) decrease in the plastometer plastic range; and (3) decrease in length of the plastometer high fluidity temperature range. The first two of these general relationships were pointed out by Davis and co-workers (IO) who suggested that “a long contraction interval E 0 connotes poor coking power” and that 2.54 “both high-rank, strongly coking coals and 2.29 E low-rank, weakly coking coals have short 3 plastic ranges and that the ranges of coals 2.030 lying in between tend to increase with 8 c coking power.” The present paper does 1.78: not include a sufficiently large number of 5 1.523 high-rank and low-rank coking coals to prove conclusively this second relation1.27 ship. The evidence in Table I1 and Figure 6, taken in conjunction with the following discussion of coals 33, 28, 34, and 23, however, strongly supports the statement Temperature, “C that the strength of the coke produced inFIGURE 5 . PL.4STIC PROPERTIES O F BECKLEY COAL creases with decrease in the length of the plastic range as determined by the plastomeplastic range. This coal consists of a blend and evidently ter. The relative ranks of the fifteen coals may be-obtained by comparing their percentage of dry, mineral-matter-free each coal in the blend forms coke more or less separately. From the limited information available a t the present time it fixed carbon recorded in Table I. is thought that these two solidification points indicate a weakIt will be seen that coal 33 is a low-ranking high-volatile A coal. ening of the coke structure due to the wide difference in the When blended with 70 per cent of coal 28 (a high-ranking highproperties of the coals used in this blend. It is well known volatile A coal) it should give in coal 35 a coal capable of that the strength of a coke from a blend is dependent upon producing a stronger coke. This was found to be the case, the character of the entering coals. Weak cokes may result, Coal 33 has a “shatter-tumbler index” of 47.25, while coal 35 shows 64.25. Likewise, a stronger coke should be produced from however, from blends showing only one solidification point. coal 34 (a blend of 70 per cent of coal 33 and 30 per cent of coal 23, The Layng-Hathorne curves show that pressure usually a high-ranking low-volatile coal) than from coal 33 alone. The starts to develop in this test a t temperatures somewhat below “shatter-tumbler index” on coal 34 is 65.7. Coal 36 and its the point a t which fusion is indicated by the other instruments. blends, coal 37 (a blend of 80 per cent of coal 36 and 20 per cent of coal 23), and coal 38 (a blend of 70 per cent of coal 36 and 30 Furthermore, the point of maximum resistance in this test per cent of coal 23) show some interesting relations. The remay occur approximately where the coal is most fluid, as spective “shatter-tumbler indices” are 56.85, 69.2, and 64.3. shown by coals 38 and 40, or may occur elsewhere, as in coal At first thought one might expect that the coke from coal 38 41. A study of the factors influencing the Layng-Hathorne would be stronger than that from coal 37. More complete study of the data in Tables I and 11, however, shows coal 23 to have (1) test showed that both the magnitude of the maximum presa dry, mineral-matter-free fixed-carbon content of 84.2 per cent sure developed and the temperature a t which this occurs are (near the upper limit for bituminous coals), (2) an extremely greatly affected by uncontrollable variables in the method. high “initial contraction temperature” and no “initial expansion From a study of a large number of Layng-Hathorne curves temperature” under 500’ C. by the Agde-Damm test, and (3) nearly negligible resistance by the Davis plastometer. Referthe authors believe that the test is not capable of giving any ence to the Davis plastometer temperature-resistance curve for reliable information which is not given to a fuller extent by the coal 38, Figure 3, illustrates rather strikingly that one may other tests. here be dealing with what amounts to two quite different coals instead of a homogeneous blend. The two definite maximum Correlation of Plastic Properties of Coals with resistances 30.0 and 39.2 kg.-om. (26 and 34 inch-pound) at 465” Quality of Coke Produced and 485” C., respectively, and the character of the curve between these points suggest that, during coke formation, coal 36 Table I1 and Figure 6 show certain general relations between plasticity data on eleven of the coals and the strength of their cokes. The coals have been arranged in the table in TABLE11. RELATIONS OF PLASTICITY DATATO COKESTRENQTH the order of increasing average cumulative percentages of their 45.7-cm. (18-inch) retort, 900” C. cokes retained on a Coke Tests 45.7-Cm. Retort, Mi$:,-d Cumulativ; % a t 9000 C. D ~ Modified ~ ~Davis, 3.81-cm. (1.5-inch) screen in the shatter test and on a 2.54ShatterInitial P1astometer cm. (1-inch) screen in the tumbler test. These averages for tumbler ConHigh Coal Shatter Tumbler indices, traction Plastic fluidlty the shatter-tumbler tests will, hereafter, be referred to as the No. indices indices average Interval range range “shatter-tumbler indices.” Because of lack of numerical c. c. c. data for the plastic properties of the slightly fusing and non97 87 53.4 56.85 19 36 60.3 84 97 59.9 15 49.1 70.7 39 fusing coals 34, 23, and 33 and for coal 28 used for blending, 87 14 56.4 63.7 72 39A 71.0 83 64.25 12 72 58.0 35 70.5 these coals were omitted from Table 11. The relation of 79 64.3 12 73 59.8 68.8 38 plastic properties of these coals to the strength of their cokes 79 11 63 57.2 64.4 40 71.6 84 6 9 . 2 74 7 6 2 . 7 7 5 . 7 37 will be discussed in connection with coals 35, 36, 37, and 38. 71 60.3 69.95 78 6 79.6 39B 73 70 4 6 4 . 0 70.15 40B 7 6 . 3 A study of the data in Table 11, graphically presented in 70.5 69 75 3 63.7 40A 77.3 Figure 6, shows the following general relations: The coke 68 44 1 74.4 66.7 41 82.1 strength increases with (1) decrease in the contraction inter0
INDUSTRIAL AND EXGINEERING CHEMISTRY
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Hathorne tests have resulted in the development of improved methods for measuring the plastic properties of coking coals. Test results by the revised methods show better agreement and afford more complete information of the relationships of plasticity to other properties of coals and their cokes than has been possible heretofore. It is concluded from this study that plasticity measurements by the Davis plastometer method as now modified give more reliable and complete information over the plastic range than can be obtained by the other two methods. Data from the modified Agde-Damm test method provide valuable knowledge on the preplastic temperature range, but have limited significance, except for the less fusible coals, in the plastic range. Measurements of plasticity by the modified Layng-Hathorne method, while showing some improvements, furnish but little reliable information that cannot be more accurately and completely provided by the Davis plastometer method. The quality of coke produced is predictable with a fair degree of certainty from the plasticity data on the coal carbonized. The strength of the coke has been shown t o increase with decreases, in the Agde-Damm “Contraction interval,” in the length of the “plastic range” and the length of “high fluidity” as observed with the Davis plastometer.
Aclcnowledgment The authors are indebted to J. D. Davis and A. C. Fieldner FIGURE6. RELATIONOF PLASTIC PROPERTY TEMPERATURE for many helpful suggestions during the course of the work. CHARACTERISTICS OF COALTO STRENGTH OF COKE Acknowledgment is made\to H. M. Cooper for the analyses of the coals from which the dry, mineral-matter-free fixed carbon designating the ranks of the coal were calculated. had almost completely coked before coal 23 had started. The net result was the production of a coke of weaker structure from Literature Cited coal 38 (the 70:30 blend) than from coal 37 (the 80:20 blend). The value of the maximum resistances, 30.0 kg.-cm. (26 inchAgde, G., and von Lyncker, L., Brennstgff-Chem., 10, 86-7 pound) for coals 36 and 37, and the tendency toward the forma(1929). tion of a second maximum of 9.2 kg.-cm. (8 inch-pound) at Am. Soc. Testing Materials, “Tentative Specifications for 475” C. by coal 37 adds support to the above interpretations for Classification of Coals by Ranks,” Vol. 35, Part I, pp. 847-53 coal 38. Apparently, then, coal 38, the blend of 70 per cent of (1935). coal 36 and 30 per cent of coal 23, has so much of this latter Audibert, E., and Delmas, L., Fuel, 6, 131-40, 182-9 (1927); slightly fusing coal that the resulting coke is somewhat weaker Audibert, E., Ibid., 8, 225-43 (1929). than coke formed from coal 37, the 80:20 blend of these same Baum, K., and Heuser, P., Glilckauf, 66, 1497-1502, 1538-44 coals. (1930); Fuel, 10, 51-64 (1931). Reference to Table I1 and Figure 6 would suggest that the relation between the Davis plastic range on coal 40 and the “shatter-tumbler index” of its coke is out of line with that for coals of nearly similar properties. Table I shows also that, of all the coals studied, this coal develops the highest maximum resistance by the modified Layng-Hathorne method. However, the temperature range data for this coal as observed by “the contraction interval” in the Agde-Damm test and by “high fluidity” in the plastometer method show good correlations with the “shatter-tumbler index” of the coke produced. It may be pointed out that the yield of coke was lower and the yield of tar higher a t 900’ C. on coal 40 than would be expected upon comparing these yields with those for coals immediately above and below it in Table 11. Furthermore, as may be seen in Figure 4, the evidence of a second maximum in the temperature-pressure curve would indicate a second fusion caused perhaps by a portion of the tar being released a t about 470” C. These peculiarities for coal 40 may explain in part the lower plastic range than would be expected upon comparison with coals of similar composition. The other coals in Table I1 show no important exceptions in the general correlations between coke strength and plastic properties.
Summary and Conclusions Critical studies of the apparatus and procedures formerly used in the Agde-Damm, Davis plastometer, and Layng-
Bunte, K., Bruckner, H., and Sandjana, Jal, I b i d . , 14, 350-63 (1935). Coffman, A. W., and Layng, E. T., IND.ENG.CHEM.,20, 165-70 (1928). Damm, P., GZuckauj, 64, 1073-80, 1105-11 (1928); Fuel, 8, 163-77 (1928). Davies, R. G., and Mott, R. A., Ibid., 12, 294-303 (1933). Davis, J. D., IND. EKG.CHEM.,Anal. Ed., 3,43-5 (1931). Davis, J. D., Jung, F. W., Juetter, B., and Wallace, D. A., IND. ENG.CHEM.,25,1269-74 (1933). Fieldner, A. C., and Davis, J. D., Monograph, 5, U. S. Bur. Mines, 7-8, 102-3,106-11 (1934). Fieldner, A. C., Davis, J. D., Thiessen, R., Kester, E. B., and Selvig, W. A,, U. S. Bur. Mines, B u l l . 344, 14-19 (1931). Fieldner, A. C., Davis, J. D., Thiessen, R., Selvig, W. A., Reynolds, D. A., Jung, F. W., and Sprunk, G. C., U. S. Bur. LMines,Tech. P a p e r 570 (1936). Fieldner, A. C., Davis, J. D., Thiessen, R., Selvig, W. A., Reynolds, D. A., Sprunk, G. C., and Holmes, C. R., Ibid. (in press). Fieldner, A. C., Davis, J. D., Thiessen, R., Selvig, W. A., Reynolds, D. A., Sprunk, G. C., and Jung, F. W., I b i d . , 571 (1936). Foxwell, G. E., Fuel, 3, 122-8 (1924). Gieseler. K., Gliickauf,70, 178-83 (1934). Jung, G., I b i d . , 71, 1141-8 (1935). Kaats, L., and Richter, H. E., Gas u. Wasserfach, 78, 221-9 (1935). Layng, E. T., and Coffman, A. W., IXD.Eso. CHEM, 19, 924-5 (1927). Layng, E. T., and Hathorne, W. S., Ibid., 17, 165-7 (1925). Lessing, R., J. SOC.Chem. I n d . , 31, 465-8 (1912); See also Cas World, 56, 842-9 (1912); Orig. Corn. 8th Intern. Congress A p p l . Chenz., 10, 195-8 (1912); J. Gas Lighting, 118, 855-61 (1912); Trans. Inst. Gas Engrs., 1912.242;Fuel, 2,152-5,186-90 (1923).
NOVEMBER 15, 1936
AIC‘ALYTICAL EDITION
(23) . , Lum. J. H.. and Curtis. H. A.. IND.ENG.CHEM..Anal. Ed... 7., 327-33 (1935). (24) Pieters’ H‘ J’s Koopmans’ H” and Hovers’ J’ T’’ 13, 82-6 (1934). Pitts(25) Porter, H. C., proc, Srd Intern, conf. Bituminous burgh, Vol. I, 613-30 (1931).
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w’
449
(26) Porter. H. C.. IND. EXG.CHEM.. 27. 962-6 (1935). (27) Thiessen, R.,and Sprunk, G. C.’, Fuel, 13, 116-25 (1934). RECEIWDAugust 3, 1936. Presented before the Division of Gas and Fuel Chemistry a t the 9Znd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936. Published by permission of the Director, U. S. Bureau of Mines. (Not subject to copyright.)
Hexanitrato Ammonium Cerate as a Proposed Reference Standard in Oxidimetrv J G. FREDERICK SMITH, V. R. SCLLIVAN.
4YD
GERALD FRANK, University of Illinois, Urbana, Ill.
C
ERIC sulfate has been generally accepted as a volumetric oxidation reagent comparing favorably with potassium permanganate in versatility, accuracy, and convenience. Many of its characteristics make it a preferred reagent by comparison with permanganate. Kew developments in high-potential, reversible, internal indicators having suitable color transitions, when used in connection with ceric sulfate oxidations, have eliminated the unfavorable comparison with self-indicating permanganate reactions. The commercial availability of the double salt, ceric ammonium sulfate, Ce(S04)2,2(NH4)~S04.2Hz0, which is easily soluble in dilute sulfuric acid, has eliminated the unfavorable necessity of preparing ceric sulfate solutions from relatively impure samples of ceric oxide. The notable stability of ceric sulfate solutions, during storage under ordinary conditions and in the case of hot solutions, is a favorable circumstance*in comparison to permanganate. It would appear that the most appropriate advance in the study of new developments in ceric salt oxidimetry still to be made is that of providing a salt suitable for use as a standard of reference. This problem a t first thought might be considered almost impossible of solution. The known difficulty associated with the separation of cerium from its associated rare earth elements, praseodymium, neodymium, and lanthanum as well as thorium, has been too often experienced. Attempts have been made to utilize the double salt, ceric ammonium sulfate, as the basis of a product suitable as a standard of reference. The influence of varying concentrations of sulfuric acid and ammonium sulfate upon solutions of ceric sulfate in the attempt to prepare a double sulfate of cerium with ammonium of definite composition greatly complicates the problem. The ceric oxide or oxalate suitable in such a synthesis would have to be of high purity and therefore inaccessible. A double salt is inherently objectionable for obvious reasons of variable composition. The double ceric ammonium sulfate also is hydrated and its equivalent weight so high that a reference standard based upon its use would, if one succeeded in its preparation, be prohibitive in cost.
Advantages in Use of Complex Nitrato Cerate 1. Hexanitrato ammonium cerate is a complex salt as distinguished from a double salt (with possible variations in combining proportions) and is of high equivalent weight (548.258). The salt is not noticeably hygroscopic under ordinary atmospheric conditions. The secondary ionization of the Ce(N03)6-- ion to form ceric and nitrate ions is not pronounced but is ample for the purpose of the oxidation of divalent iron (and probably other reducing agents) as well as for the oxidation of suitable indicators and for potentiometric end-point phenomena. The product is commercially available a t reasonable cost.
2 . A product 99.6 per cent pure can be easily prepared by one crystallization starting with a low-grade (40 to 50 per cent) thorium-free mixture of ceric oxide containing lanthanum, praseodymium, and neodymium. A second crystallization of hexanitrato ammonium cerate from concentrated nitric acid in the presence of excess ammonium nitrate results in an 80 per cent yield of product of practically perfect purity. 3. The newly proposed standard of reference is easily soluble in dilute sulfuric acid, forming a solution which is stable upon storage under ordinary conditions and is perfectly stable upon digestion a t 100” C. The crystalline salt is stable a t 110” C. and is easily freed from excess nitric acid and ammonium nitrate in contact with which it is prepared. Hexanitrato ammonium cerate is soluble in water (without hydrolysis) as well as in sulfuric, nitric, perchloric, and hydrochloric acids. Pure salts of ceric sulfate and cerous chloride are easily prepared from it by digestion with sulfuric and hydrochloric acids in excess.
Factors Indicating Complex Salt Composition
It is not within the scope of this paper t o prove by physical chemical means the belief that hexanitrato ammonium cerate is a complex salt and not a double salt such as ceric ammonium sulfate. That (NH4)zCe(N03)6is ionized in solution to form ammonium ions and nitrato ceric ions, (KH4)&e(K0& F? 2NH4+ Ce(NO&--, is, however, clearly indicated. Thus, a solution of the pure acid-free salt in water is not hydrolyzed to form insoluble ceric salts as are the double salt ceric ammonium sulfate and other ceric salts. Solutions of the complex nitrato ammonium cerate in nitric acid are salted out by the addition of excess ammonium nitrate intro+, but are not similarly salted ducing the common ion (“4) out using excess nitric acid as a result of the addition of the common ion (?\TOs-). Clear solutions of the complex salt in water can be made which are more than 2.5 N , with color production about equal to 0.1 N ceric ammonium sulfate in normal sulfuric acid. A nitric acid solution of the nitrato cerate is noticeably slow in oxidizing reaction at the equivalence point when reduced by ferrous sulfate. The remarkably clean separation of the nitrato cerate from solutions containing equal concentrations of the other cerium group metals, except thorium, indicates a distinctly different composition since the other cerium group metals are known to form double salts as distinguished from complex salts. Lastly, the recrystallization of the nitrato cerate from concentrated nitric acid solutions by evaporation gives a product which tends towards a high ceric equivalency. This condition would be interpreted to indicate an impurity of H2Ce(XO3)6 in the salt (;?;H4)zCe(SO&. This tendency is eliminated by precipitation in the presence of excess ammonium nitrate.
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