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By T. Fraser, F. F. Giese, R. L. Sutherland

Under a cooperative agreement between United States Bureau of Mines and the Truax-Traer Coal Company, some operating-scale experiments have been made with the cyclone thickener in the preparation plant of the Shamrock No. 1 mine at Kayford, West Virginia. These tests were made with a 14-in. cyclone of the type developed by the Dutch State Mines in Limburg. It was applied experimentally to the thickening of solids in washery water to obtain performance data to show the range of sizes that may be recovered and other operating characteristics that would indicate the field of usefulness of this device in coal preparation practice. This is a progress report only. It covers results obtained in a series of 17 tests of the operation of what has been considered a unit of commercial size operating on suspensions of fine coal mixed with a relatively small amount of free impurities. The underflow material, after further dewatering in centrifuges, is a marketable grade of coal. The coal at Shamrock No. 1 is mined from the No. 2 Gas seam. It is made up essentially of a low ash coal suitable for byproduct use with one or more bands of splint coal of higher ash content suitable for industrial fuel. The washing plant was installed for the two-fold purpose of removing the free impurities incident to mechanized mining and to separate the run of mine coal into the two types. To accomplish these results it is necessary to make a separation at a specific gravity of about 1.4 to recover the byproduct coal and to separate the splint coal from refuse at a higher gravity. The coarse coal section of the washing plant was put into service about 10 months before the fine coal section was completed. Inadequate screening capacity in the raw coal preparation plant resulted in a portion of the fine coal being fed to the jigs. No provision for preliminary dedusting of the raw coal feed was incorporated in the design. Under the conditions in the initial operating period, separation at the desired lower gravity was not realized, apparently because of the presence of an excessive amount of fine solids in suspension in the wash water. Difficulty in wetting the fine coal may also have been a contributing factor. Additional facilities for the removal of the fine solids from the wash water was indicated. Information then available on the Driessen cone for this purpose was studied and a cooperative agreement with the Bureau of Mines for experimental work with a unit of standard size as developed by the Dutch State Mines was arranged. During the time required for construction of the cone it was found the addition of a small amount of wetting

Jan 1, 1950

By S. Oglesby, A. L. Thomas

Until recently, probably the best means of sampling airborne dusts has been the impinger method. Dust-laden air is drawn into a sampling tube, and the particulate matters separated from the air and collected in the liquid in the impinger. A sample of the suspension is prepared on a slide and counted through a projection microscope. This method is simple and reliable, but it does suffer drawbacks. Since the collected particles must be counted visually, results of the dust count are not immediately available at the time the sample is taken. This seriously impedes studying the effectiveness of dust inhibitors, such as water sprays, or making adjustments in equipment. Greatest disadvantage of all, however, is the tedious task of counting particles in the sample. With the growing desirability of taking more samples, the effort involved in making these counts is a real factor in the cost of dust control. Finally, it is generally recognized that impinger dust counts can be related to health hazard only for specific kinds of dust and under similar conditions. This is because the rather complex mechanism of dust damage depends so much on size distribution of the dust. Counts determined by impinger samples cover a limited size range that does not necessarily corre-

Jan 1, 1960

By J. W. Leonard, B. A. Donahue

A method is described for incorporating coal petrography into mining and preparation plant quality control based on conventional analyses. Complete analyses are made of each of the uniform and relatively distinct petrographic bands in a coal face. With this information it is possible to develop petrographic standardization curves. These curves permit diverse coal characteristics to be rapidly monitored by application of a few standard coal tests. Single seam quality control is discussed in this paper. Inquiries continue to be made about coal petrography, and specifically, about whether coal petrography should be applied more extensively in the coal industry. Coal petrography is the study of the distinct physical, optical, and chemical increments which make up the organic rock mixture which we know as coal. Petrography can be applied by any coal company or at any coal mine where conventional coal analytical facilities are available and where additional resources can be allocated for an increased number of determinations. This first paper of a two part series includes (1) a proposed conventional approach to single seam petrography based on the development of petrographic standardization graphs and (2) some informative and supplemental interrelationships involving coal characteristics developed as a result of this research. In the second part of this series, to be published at a later date, those petrographic characteristics of bands taken from many different coal seams located over wide geographic areas which can be closely related to each other will be considered. These findings will be examined looking toward the development of a comprehensive, conventional approach to coal petrography which can be used as a common basis for multi-rank petrographic standardization graphs. Implicit to the understanding of coal petrography is the well established fact1,2 that the individual analyses of distinct petrographic bands often differ widely from the average analysis which is used to characterize a whole coal seam. In fact, a distinct and minor banded increment of coal in a high volatile seam may actually be a medium or low volatile coal with an analysis similar to the average analyses of seams located in distant coal fields. During the mining of coal, much degradation is caused which tends to randomly scatter coal bands. The degraded, scattered, and physically separated constituents of distinct petrographic bands are processed side by side in the coal preparation plant as distinct petrographic fractions with particles grouped according to a narrow size range (which derives in part from common hardness), specific gravity, and-or surface chemistry. Tile data obtained from numerous analyses made on each of the bands of any given coal seam, although diverse, can be readily interrelated. Indeed, these interrelationships can be developed into a series of nomographs or petrographic standardization graphs to serve as a basis for developing a conventional petrographic* program. Thus, by analyzing one property (for example, ash) of a selected petrographic fraction collected or separated from the flow of coal in a preparation plant, it should be possible to estimate the other physical, chemical, and thermal properties of this fraction by referring to the petrographic standardization graphs. It follows that any or all petrographic fractions in the total coal flow originating from a single coal seam can be rapidly monitored for many properties on the basis of a single analysis run on each fraction. In recent years coal petrographic research has concentrated heavily on that phase of this work which involves microscopic reflectance measurements made at many minute and selected points on a coal briquette surface.3,4 Thus, the percentage of the total number of reflectance measurements that fall into each of a consecutive series of arbitrarily chosen reflectance classes are used to characterize coal. This procedure is analagous to using the face of a coal briquette to represent the reflectance characteristics present at the face of a coal seam. Moreover, by analyzing petrographic fractions of known and uniform optical properties it is possible to closely relate reflectance to other coal characteristics. For example, reflectance has long been known to be related to such

Jan 1, 1968

By J. W. Leomrd, B. A. Donahue

Results of research are presented examining the extent to which the analytical characteristics of the relatively distinct coal bands from a variety of coal seams can be related to each other. This paper dis-cusses an approach for developing a practical coal petrographic quality control program based on conventional analyses, most of which is part of the standard A.S.T.M. procedure. The work is a final follow-up to Part I of this series which was prepared by the authors as an approach to applying conventional coal petrography to single coal seams. Recent published work by the authors entitled Petrography for Coal Mining and Coal Prepration: Part I dealt with interrelating various chemical and physical properties of coal1 measured using conventional analyses made on distinct petrographic bands taken from single coal seams. Since most coal production facilities process coal from a single coal seam or from very closely related coal seams in the same area,2 emphasis on interrelating the properties within a single seam appeared appropriate. The distinct petrographic bands were analyzed on the assumption that such differentiated data would be more representative of a heterogeneous coal seam than the single analytical value commonly used to characterize each property of an entire seam.34 Effort was directed at demonstrating the extent to which the interrelated chemical and physical properties 5,6 could be developed into nomographs or petrographic standardization graphs. Thus, one analysis, determined on a series of petrographic fractions separated from a single sample, was used to estimate numerous other properties in each fraction by referring to the previously established petrographic standardization graphs. This conventional approach to coal petrography was undertaken as a suggested feasible means by which a few coal analyses could be employed to develope a more penetrating knowledge of the properties of coal from any given seam in order to monitor more extensively its performance at the point of utilization. Such procedures can support development of the type of in- formation commonly sought through automated testing and through the use of computers.7 The broad knowledge which can be developed through these procedures is intended for application in the generation of an analytical profile or broad characterization of coal. These estimates were not intended as replacements for individual coal analytical tests. In this expanded second part of the research program, distinct petrographic bands from nine coal seams in the Central Appalachian Region were physically and chemically analyzed to elucidate the extent to which this concept of conventional petrography could be broadened for application to numerous coal seams. In presenting this second phase of work, the relationships developed are presented individually and not in a connecting series of nomographs or petrographic standardization graphs as in the previous work, thus leaving open the combinations of possibilities to individual interpretation and application. MATERIAL AND EXPERIMENTAL WORK Nine coal seams, representing a wide range of rank, from the Central Appalachian coal fields were used in this study. The distinct petrographic bands from the Kittanning, Pond Creek, Jawbone, Tiller, Poca-hontas No. 3, No. 2 Gas, Eagle, Winifrede, and Pittsburgh coal seams were separated by carefully removing a portion of each band at the face of the seam. The following were determined: ash, sulfur, free swelling index, heating value, bulk specific gravity, volatile matter, Hardgrove Grindability Index, and Gieseler Plastometer measurements. Determinations, where procedures were available, were carried out using ASTM standard procedures.8 Bulk specific gravity was determined using a kerosene volume displacement procedure modified from a method applied by Headlee and McClelland of the West Virginia Geological survey,9 Those bands with a bulk specific gravity greater than 1.60, which is generally above the practical specific gravity cleaning range of bituminous coals, were excluded and no analyses were performed. Much of the initial organization of this second phase of work was developed through the extensive use of a computerized statistical monitoring program (see Ref. 4). However, in order to achieve the closest possible interpretation of results, the final organization of data proceeded mainly from exhaustive trial

Jan 1, 1968

By J. M. Schopt

COAL petrology is the study of the lithologic composition and texture of coal and includes megascopic as well as microscopic differentiation. Coal petrography is a quantitative study, principally (but not exclusively) microscopical in nature. The distinction between coal petrologic and coal petro-graphic studies usually is not important, but it is important to recognize that these studies differ materially from other types of petrology that deal primarily with minerals. The petrologic entities* of coal are less clearly distinguished than most minerals and the basis of distinction is not always clearly expressed. The relation of coal petrology to geologic, mining, and technologic problems is most evident when results of coal petrologic studies can be stated quantitatively. The chemical and physical composition of coal and the determination of its quality and properties are continuing problems. The source of these problems lies in the heterogeneity of coal and in its almost complete insolubility in ordinary solvents at normal temperatures. The empirical and comparative character of standard chemical coal analytic values is one of the consequences resulting from heterogeneity and insolubility of coal. Nevertheless, these coal analytic methods are highly technical and diversified. Coal petrol.ogy employs a group of similar analytical methods which provide information about coal that the other methods cannot supply. The heterogeneity of coal is most clearly demonstrated by means of petrologic studies. The properties utilized in coal petrologic studies are related to texture (size, form, and association), hardness, light reflectance, and light absorption and transmission (color). Size relations are most evident but are commonly overlooked. Coal is classed chemically as a mixture, and chemical analyses thus show the composition of the sample as a whole. Geologically coal is an organic clastic deposit, derived from diverse plant fragments and particles having a size range comparable to that of a conglomerate or breccia. The mixture of different materials that compose coal, large and small, can be realized only by microscopic study. This is because the fundamental cellular units of plants, from which coal is formed, are microscopic in size. The kinds of altered plant particles and fragments in coal have specific differences in hardness, reflectance, and color that signify differences in composition. These differences must be taken into account in evaluating the aggregate properties of coal. Terminology and Texture Full discussion of terms applied by various petro-logists in analysis of coal is beyond the scope of this article. For convenience the simpler types of petrologic entities that are defined without reference to lithologic occurrence or texture will be called coal components. Fig. 1 lists reactive and relatively inert

Jan 1, 1957

By M. Schmellenkamp

By comparing two longwall operations, one begun in 1956 and the other in 1960, the author is able to demonstrate the increases in production and performance made possible by longwall mining. These achievements have been brought about by continuous development of longwall mining equipment and associated roof supports. Because of this progress, longwall mining is able to provide, under proper conditions, high production per man-shift, remarkable cost savings, dependable roof control, safe working conditions and truly continuous production. It may seem odd that the title of this paper is not simply "Longwall Mining" but instead "Progress in Longwall Mining." However, the word "Progress" definitely has its place in longwall mining. If it had not been for progress in the development of longwall mining equipment and roof supports, the longwall mining method would not be able to compete in production and performance with modem coal mining machines. The longwall mining method was practiced at the beginning of the century and there were several successful operations in coal fields in Illinois. In those early days of longwall mining, the coal was undercut by hand 2 to 2.5 ft at the bottom of the seam and packwalls were built in the gob to support the roof. The roof eventually subsided and the weight of the subsiding roof was used to ride the face and break the undercut coal. Utilizing natural weight to soften the coal face is still practiced in modern longwall mining; however, the packwalling method has been replaced by the caving method and the roof is now supported by yielding steel roof supports and forepoling steel headers. The purpose of these yielding-type roof supports is to provide a safe working area for the crew along the entire longwall face, to permit continuous mechanical mining across the prop free face, and to provide a strong resistance for the roof by forming an even breaking line at the gob for the roof to cave. Roof supports and associated forepoling headers should be kept as close as possible to the face in order to prevent a caving between face and supports, especially under friable roof. This means that the coal should be extracted in small slices, allowing only a narrow roof area to be exposed and unsupported. The coal planer with its relatively high cutting velocity of 75 ft per min provides such an extraction of coal and has proved its high performance under difficult mining conditions. Since 1951, several longwall faces in southem West Virginia and Pennsylvania which have been equipped with the coal planer and friction-type manual roof supports have been successfully operated. Compared to today's longwall mining, these longwall faces required such a large crew, primarily to handle the roof supports, that the actual high production per shift was charged with too high a labor cost, thereby decreasing the tons per man. Yet, even then the longwall faces outperformed the conventional mining system under the same conditions. In order to demonstrate the progress that has been made in the development of longwall mining, a comparison will be made between a longwall face in Arkansas which was installed in 1956 and a modemized longwall face started in 1960 in southern West Virginia. LONGWALL MINING IN 1956 The 320-ft longwall face was developed in a 32-in. thick coal seam near Greenwood, Ark. The method of mining the 320-ft coal block was the advancing system in which three entries on either side of the face were driven ahead of the advancing longwall face. The face was equipped with a coal planer and a Panzer conveyor; timbering was done with wooden timbers and cribs. The roof supports were set without any pattern. The crew to operate the planer and to handle the roof supports (timbers and cribs) consisted of 15 men per shift. During a period of approximately eight monthsof single shift operating time, the average tonnage produced in this relatively low seam amounted to about 263 tons of clean coal per shift. To show the development in the coal plow from then until now, it should be pointed out that the standard plow was used in this operation. The plow was equipped with rigid bottom bits which could not be adjusted if the plow started to climb, thereby leaving bottom coal to be recovered by pick hammers end causing delays in production. The height of the plow

Jan 1, 1963

By H. Rees

This paper supplements and brings up to date a report prepared earlier this year and presented at the Eleventh Pan American Railway Congress in Mexico City. Most of the original report is contained in this paper in order to acquaint the reader with the project background. On December 1, 1961, construction of this locomotive was completed. Stationary testing started immediately, and a host of problems in various areas were uncovered. Many of these had no relation to the use of coal in a direct fired gas turbine. All of them emphasized the problems which plagued previous projects of this kind. In 1944 the Locomotive Development Committee of Bituminous Coal Research, Inc., was formed to study the problems of combustion, coal handling, and fly ash removal which would have to be solved before coal could be used to fire a gas turbine. By 1947 this committee was composed of the chief executive officers of nine eastern railroads and five eastern coal companies. All of the members of this committee were vitally concerned in a coal fired gas turbine because coal cost only one-fourth as much per heat unit as fuel oil. A number of research projects were started to develop and test components on a small scale. This included work at John Hopkins University, Battelle Memorial Institute, Institute of Gas Technology at Chicago, Purdue University, and Southern Research Institute. By 1946 a pilot plant had been installed at the Dunkirk, New York plant of Alco Products Div. of American Locomotive Co. The U. S. Bureau of Mines made available to the Locomotive Development Committee (hereafter called LDC) a Houdry process gas turbine plant, originally destined for a Russian oil refinery. The full scale pilot plant was used to evaluate the results obtained from the small scale studies under actual gas turbine conditions. Operation of this plant disclosed a number of problems which had not been discovered in the small scale tests. By 1951 four 250 hr tests had been conducted with the Houdry gas turbine to test out a variety of equipment. The major problem concerned the removal of abrasive fly ash from the products of combustion so that wear on turbine blades would not be excessive. The first 250 hr test showed that the original fly ash separating equipment was inadequate. Large ash particles passing through the turbine blading were nicking the leading edges of the first row blades. At the conclusion of the second 250 hr test, erosion was so severe that the first row of stator and the first row of rotor blades were replaced. At the end of the third 250 hr test, it was necessary to replace the first three rows of stator blading. The fourth test was used to test out a new design fly ash separator developed by LDC and named the Dunlab tube. This device gave more encouraging results than any of the others tested. Early enthusiasm for overcoming major difficulties had resulted in placing an order in 1946 with Allis-Chalmers Mfg. Co. for a 3750 hp regenerative gas turbine. The gas turbine, together with associated combustion and ash removal equipment, coal handling equipment, generators and gear reduction unit was installed at Dunkirk by 1951. A 750 hr test was made in 1952 with the LDC-Allis Chalmers gas turbine power plant. At the end of this test it was necessary to replace the first four rows of both rotor and stator blading. Rows 5 and 6 did not require replacement. In 1953 the American Locomotive Co. joined with LDC and assumed an active role in equipment development and manufacture. During 1954 LDC-Alco operated the rebladed gas turbine 300 hr to determine if a new fly ash separator was adequate. Performance of the improved separator was considerably better, but trouble was experienced with clogged blowdown lines from the Dunlab tubes. When these lines clogged it permitted the passage of abrasive particles. In 1955 a total of 1421 hr was run. This included 243 hr of cycle operation on Union Pacific coal from the D. O. Clark mine at Superior, Wyoming. During the 243 hr a total of 370 tons of coal was consumed. The purpose of this test was to determine if U.P. coal was suitable for use in a coal fired turbine. It was the unofficial opinion that it was as good or better than the eastern coals that had been tested.

Jan 1, 1964

By J. Hudy

An investigation has been made to determine the applicability of the Cendrex X-ray instrument for the measurement of the ash contents of washed bituminous coal products. The results obtained for selected products from five preparation plants in the northern Appalachian region are described. Emphasis of the study was placed on low-ash bituminous coals suit-able for utility or metallurgical markets. There was good agreement between the results obtained by conventional ash determination methods and by the Cendrex for all the products tested. A principal disadvantage of coal in competition with other fuels has been its inconsistent quality, especially with respect to ash content. Mine rnechanization and production from coalbeds of inferior quality have accentuated this failing. Present quality control methods in coal preparation plants are inadequate because (1) control samples taken over a production shift only serve to give the average quality of the product, (2) conventional sampling and analysis data are usually not available at the time a shipment is scheduled, and (3) the plant operator does not have the use of such data when the coal is being processed so that operating conditions may be adjusted for correcting variations in quality. Thus, the development and use of continuous-control monitoring instruments is of prime importance. The Federal Bureau of Mines arranged to investigate an instrument using x-rays for rapid continuous ash determinations. Developed by the Dutch State Mines and first described in 1958,' this instrument, which has found widespread acceptance in Europe, is designed to provide ash analysis of any desired coal feed or product stream with only a five-minute time lag between sample collection and analytical results. With limit controls, this unit could be used to feed back signals to preset the separating gravity of coal washers, control the ratio of various coals needed to produce an acceptable blend, and permit the sorting of a washed product into two qualities, one which is below and the other which is above a prescribed ash content. This unit can also be used discontinuously for rapid determination of coal samples taken in the usual manner. THEORY Di jkstra and Sieswerda of the Dutch State Mines studied the effect of chemical composition and ash content on the absorption coefficient of coal with the transmitted radiation method.' They found that with an X-ray wavelength of 1.79 A and values calculated for the absorption coefficients of ash and combustibles, the absorption coefficient of coal gave a linear relationship when plotted against the ash content. Thus, a method of measuring the absorption coefficient of coal could also be used to measure the ash content. However, practical attempts to measure the ash content of coal by comparing the intensities of the transmitted X-rays failed. The absorption coefficient of coal can also be obtained by irradiating a layer of coal with x-rays and measuring the intensity of the diffused reflection (backscatter method). The relationship between the diffused reflection and the ash content is actually hyperbolic but approximates a straight line over a narrow range. A measuring circuit that balanced the difference in intensity of the reflected radiation from a standard surface against that from a coal sample was used. By plotting this difference of reflected radiation against the ash content, a curve which approximated a straight line over a range of 8 percentage points of ash was obtained. DESCRIPTION OF EQUIPMENT The apparatus performs two distinct functions: (1) the preparation of the sample to a suitable size consist and moisture content for delivery to the x-ray element, and (2) the measurement of the ash content by the x-ray element. In a continuous operation the coal sample is delivered to a conditioner where it is dried to a maximum of 1% surface moisture and pulverized in a high speed mill to -48-mesh. This material is fed, at a rate of 250-300 gpm onto a rotating disk, shown in Fig. 1, where it is mixed with material already present. The mixture is rotated on the disk and

Jan 1, 1969

By D. W. Loucks

There is plenty of evidence to indi-cate that at least one of man's chief interests in life is to make himself as comfortable as possible. If you doubt this, just watch the fellow next to you for the next half hour trying to find the most comfortable position that a hard chair has to offer. Comfort, however, does not always mean an easy chair. To some, it may mean a wealth of money; to another, freedom from worry. But to most of us, it means first of all a comfortable atmosphere in which to live, and to a great many of us it probably also means freedom from that annoying task of firing the furnace. Today more than ever before. automatic heat is one improvement that is placed high on everyone's list. Perhaps this is because automatic heating is becoming relatively cheaper. Perhaps it is because of a good publicity campaign on the part of the oil and gas men or maybe it is just that we are getting lazier day by day. At any rate, almost every issue of Better Homes and Gardens, House Beautiful, or your other favorite home magazine carries an article extolling the virtues of this or that automatic heating system. If I were to ask you to name the first thing that came to your mind when I said automatic heat, you would prob-ably say either gas furnace or oil burner. Or if you had just been studying heating systems, you might possibly say heat pump. But chances are you would not mention anything about coal, and yet coal is the most common source of the greatest automatic heat of them all. I say this because coal is the fuel used almost universally by the district heating industry in producing and delivering to certain heavily populated areas heat ready to use at the touch of a valve or the click of a thermostat. Although the industry is over a half century old, it has not experienced the widespread development of other utility industries because of certain limitations which I believe you will realize from the next few minutes discussion. District Heating Operations We may define district heating as any operation where two or more buildings are heated from a central heating plant. The method of heat transfer may be hot water or in some cases warm air, but generally the medium of heat transfer is steam. So universally is steam used that the industry is frequently referred to as the district steam industry. The Allegheny County Steam Heating Co. which operates the district heating system in downtown Pittsburgh is a subsidiary of the Du-quesne Light Co. Although organized in 1912 primarily as a means of securing the electric load of downtown buildings, the service has now become so valuable and so popular that it is no longer considered a necessary adjunct to the electric business but rather a separate business standing on its own feet. Fig 1 shows the layout of the plants and distribution system of downtown Pittsburgh. Two generating plants, one known as the Stanwix and the other as Twelfth Street, supply the area. Each has two boilers with capacity totaling 1,350,000 lb per hour. The Stanwix Plant is supplied coal by truck. The coal is pulverized at the plant and burned as powdered fuel. Coal is supplied to the Twelfth Street Plant also by truck but the boilers arc stoker fired. Over 1 1/2 miles of tunnel house a portion of our main lines, but it requires over twelve miles of pipeline, ranging in size from 32 down to 1 in. in diameter, to supply all our customers. The distribution system consists of two systems in a sense, one high and one low pressure with certain interconnections between the two. Our high pressure system supplies steam up to 125 Ib to some but not all customers, while the low pressure system operates in the range of 10 to 20 psi. Note that the two plants are tied together through large steam mains and that the system to some extent is a loop system, making it possible to have a portion of the line shut, down without interrupting service to any customer. Fig 2 conveys a picture of the extent to which steam service is used in the downtown triangle. The black area indicates the buildings which now use district steam. The dotted area indi-

Jan 1, 1950

By A. C. Fieldner, W. A. Selvig

Ash in coal has always becri regarded as an undesirable substance, as the heat content of a coal dccreases in direct proportion to its ash contcnt. It represents so much inert materid that has to be transported, handled when the coal is burned, and finally disposed of. W. R. Roberts, in an address bcfore the Engineers' Society of Western Pennsylvania, Pittsburgh, on Sept. 29, 1925, said that on an annual output of five hundred rnillion tons of coal, a reduction of 3 per cent. of ash means fifteen million tons of refusc and a decrcase of $30,000,000 in freight charges. Ash also often prevents effieient utilization of the coal through the formation of clinker causcd by the mcltirig of the ash constituents when subjected to heat. The mineral constituents of coal may be of such nature as to make the coal unsuited for speeifie uses as for instance, high-sulfur coal for making metallurgical coke. While the usual proximate analysis of coal gives the amount of ash, it gives no information as to the nature and coniposition of the ash nor of the minera1 constituents from which the ash is produced. For the efficient washing of coal to remove ash-forining constituents it is desirable to know the composition and distribution of the ash-fonning constituents. Obviously such information is also desirable for the most efficient utilization of coal. Nature and Composition of Coal Ash Coal ash may be defined as the inorganic residue remailning after cornplete ignitior of coal. It is dcrived from the mineral constituents of thc coal. The ash-forming constituents may be roughly classified as inherent impurities and extraneous impurities. The former are mixed intimately with the coal substance, and arc derived either from the original coal-forming material or from external sources such as sedimentation and preeipitation while the vegetal coal-forming plant remains accumulated. The latter are present in the coal bed as partings, bands or nodules of pyrite! slate, shale, calcit,e, bone, etc. These are formed either during the laying down of the coal bed, or subsequently. Flag-

Jan 1, 1927

By S. W. Parr

The extent to which geological carbonization has taken place in the process of coal formation is a fundamental factor in all considerations relating to classification, oxidation, deterioration, spontaneous combus tion, and especially in all considerations relating to destructive distillation having as an objective the production of marketable coke and by-products. Moreovcr, it may be readily shown that a striking relationship exists between the geological decomposition or geological carbonization processes and those artificially produced by heat. This may be well illustrated by bringing again to view some of the charts more commonly mct with 25 years ago, as in Fig. 1.' In this figurc the line of geological decomposition by decay, pressure, etc., because of its relation to the line of carbonization by destructive distillation, becomes of special significance when we attempt to formulate any system of classification which shall be reasonably scientific and at

Jan 1, 1927

By A. L. Bailey, B. A. Landry

All who are even generally aware of the tremendous rate of increase in coal washing operations must realize the growing importance of the float-and-sink test. I believe it is conservative to estimate that, in the past decade, the dollar volume of float-and-sink testing has increased tenfold. It is a simple matter of economy, then, to examine the factors that determine the cost of adequate float-and-sink testing. When the Coal Preparation Section of the Bureau of Mines entered upon a greatly expanded program of such work in connection with the synthetic liquid fuels investigations, it seemed advisable to examine these factors experimentally. The principal consideration that differentiates float-and-sink test sampling from general purpose sampling, is that the original particle size must be preserved. Therefore, the total cost of the test will be directly affected by any standard that might be proposed to limit sample bulk reduction at any given particle size. For this reason, the relationship of sample size to variability of results was the first factor to be studied experimentally. Of course, the matter is rendered complex by the circumstance that the float-and-sink test, not a simple analytical measurement but a process test, comprehends a number of more-or-less independent items of fundamental data; and as shown in the report, the' variability of the samples differs with respect to these different items. This condition and the wide variety of situations in which float-and-sink test data are used, in combination with other factors, to study complex process operations, indicate the difficulty of setting up fixed standards for float-and-sink sampling and testing. At this stage at least, it is the intent rather to obtain experimental data on the principal Factors involved so that the reader may arrive more intelligently at a procedure adapted to his problem. The authors of this paper have presented experimental data showing the relationship between size of sample and particle size for different variability tolerances with respect to percentage of sink. In further studies, data are being collected to appraise also the variability of the samples with respect to float-ash content and size consist. The scope of this work will be broadened to cover particle sizes up to 4 in., and a third series of tests has yielded similar data for a much cleaner type of raw coal. Thus, the further studies will make available a fairly comprehensive meas- ure of variability with respect to size consist, percentage of sink, percentage of middlings, and percentage of ash in the float, for three coals ranging from 3.87 to 17.68 pct in refuse content (heavy sink material) and from 4.18 to 17.52 pct in middlings. Introduction At present there are no published standards for float-and-sink test sampling. During the rapid expansion of float-and-sink test work, varying procedures have been based on adaptations of the ASTM standards for sampling coal for analyses. For this reason, a special study of gross sample reduction has been undertaken to determine the limits for this step in the operation where no reduction in particle size is to be made before testing. Float-and-sink tests are made whenever a thorough study of coal characteristics is desired. The tests may be made on samples from coal-cleaning units such as jigs or tables, or coal samples may be tested which are taken from a loading boom, railroad car, or the coal seam itself. The resultant gross sample may be large and pose a problem of sample reduction. The question is, then, how much can the sample be reduced and still fall within preassigned limits of accuracy of the original gross sample of coal? Coal from channel samples may be crushed to liberate impurities and then separated into various gravity fractions from which washability curves are drawn; from these curves, it is possible to determine the cleaning characteristics of the coal. However, coal samples from coal-cleaning units cannot be

Jan 1, 1950

By A. L. Bailey, B. A. Landry

W. W. ANDERSON and G. E. KELLER*—We want to compliment the authors on this very thorough paper. It gives information which the coal industry has needed for some time. We hope that the additional information which the authors are collecting will he available shortly. The mixing and riffling procedure that was followed for experimental purposes is obviously not practical in routine float-and-sink testing because of the particle size degradation which would result in handling the sample so many times. It is important to obtain our tloat-and-sink fractions with a minimum amount of handling of material. A statement is made in the paper (p. 80) that "the variable most likely to affect the size of sample required to meet a given preassigned accuracy would be the state or degree of mixing of the coal." We agree that this is a large factor, but do not believe it is the most important factor. Our own opinion is that the most important single factor governing the total gross weight of sample that must be collected is the percentage of the weight of material in the smallest fraction that results from the screening and float-and-\ink operations. In other words, size of sample is governed by the total number of fractionations that must he made, and the distribution of material within the fractions. We can imagine a coal with perfect mixing, but with such a small amount of material in some float-and-sink fraction in one of the coarse sizes that a much larger sample would have to be taken than would be the case with very poorly mixed material, but with a large percentage of coarse material more evenly distributed in all float-and-sink fractions. Our own observation of many float-and-sink tests that we have run in our own organization on many types of coal is that the size of sample that must be used on fine size float and sink is governed more by the requirements for weight of material to be used for analysis in the laboratory than by weight of material necessary to obtain accurate float and sink percentage of weight values. In other words, it is our opinion that very small samples can be used for float-and-sink fractionation in the fine sizes, but that accurate analysis of the fractions will depend on a larger weight of sample being pulverized for the laboratory than is necessary to establish the float-and-sink distribution with respect to weight. A. L. BAILEY and B. A. LANDRY (authors' reply)—The authors thank Messrs. Anderson and Keller for their comments based on long experience. It is agreed that the involved mixing and riming technique used may be disadvantageous from the standpoint of degradation. Fortunately, the paper does point out that the extended riming was unrewarding in causing further mixing. Two large unknowns remain, however: (1) how much of the mixing from the presumed highly unmixed state in the bed was achieved toward the random state during blasting, loading, transportation, screening, and further transportation to the point where the gross sample was taken, and (2) how much of the mixing took place during the preparation described preceding riming. As has been pointed out by one of the authors.6 the degree of mixing has a very large effect on the size of sample required and there are still too few experimental data to show at what stage of coal handling most of the mixing occurs. The discussion states that the weight of material in a screened fraction, or in a float-and-sink fraction, is more important than the mixing factor. We do not believe that these factors are comparable in this instance inasmuch as our purpose was to give minimum sampling requirements to achieve a preassigned accuracy in the percentages of float, middlings, or sink, and nothing more. The gross sample had already been screened and no further division by screening was made or contemplated; also, it was not intended that the middlings and sink fractions would necessarily be adequate for percentage ash or other determination. In other words, the sample obtained by the method outlined is not intended for washability studies but only for preparation plant control. Further experimental work has been done, since the paper was prepared, to investigate the effect of increasingly larger top and bottom sizes on the variability of float, etc., of a double-screened coal from Western Pennsylvania. Results will be published and eventually attention is to be given to the preparation of sampling specifications. E. H. M. BADGER*—I should like the authors to explain more fully the fundamental assumptions on which their Eq 4 is based. The equation is of the form s2 = p(l - p) which is the usual expression for the (standard deviation)2 when the chance of finding a particular kind of particle in the sample is proportional to the number fraetion, p. But instead of the number fraction, the authors have used the weight fraction, WF/W. The chance of finding a particular kind of particle in the sample can only be proportional to the weight fraction, if the average ?eig?ts of all kinds of particles, that is, float, midlings, or sink, are the same. Surely a much more justifiable assumption would be that the average volumes of the particles are the same, and, if this is so, Eq 4 would not be true. This may be demonstrated as follows: Let be the weight fraction of float, middlings, or sink, dl the density of this fraction, and d2 the density of the rest of the coal. Then assuming that the average volumes of the pieces in the three classes are the same, the number fraction, p, is given by ? P = d1/l-?/d2 + ?/d1 = ?d2/d1 + ?(d2-d1) The weight fraction, w, in terms of p is given by ? = pd1/(l-p)d2 + pd1 = pd1/d2 + p(d1-d2) _____ [61

Jan 1, 1950

By J. A. Stachura

Continuous mining machinery provides the coal industry with one way to compete for a larger share of the total energy market. Various types of machines are discussed and some of the problems with continuous miners, encountered by operators, are reviewed. Equipment manufacturers are working with mine personnel to provide solutions for problems that arise. While coal production over the past 25 or 30 years has been on a horizontal plane, coal's share of the total energy market has declined. To participate more effectively in this total energy market, it is necessary to produce coal more efficiently. It is the obligation of all management, employes, and mining departments to gear the deep mining industry to the rapid progress and changing of today's modern industry. This can be accomplished in the near future with the selection of the proper type of continuous miner best suited to each operator's individual situation. In most mining operations there is tremendous incentive to undertake the continuous mining program. It can reduce the size of the mine greatly by permitting a minimum of working places; it makes pillar recovery work more efficient from the standpoint of overall cost, amount of coal recovered, and safety. The work force can be reduced materially permitting closer and more efficient supervision. It simplifies maintenance because equipment can be more readily standardized. The trend of the coal market favors the use of continuous mining machines. Although there appears to be a general feeling that continuous mining is still a relatively new program and will be slow in replacing conventional mechanical equipment, the fact is that tremendous strides have been made since the first machines were installed in 1948. This program is advancing at approximately the same rate that mobile loading machines replaced hand loading. From 1948 to 1955 there were approximately 450 continuous mining machines in service. In October 1959, a survey revealed that there were more than 700 continuous mining machines in service. Many operators have expressed a desire to undertake this program, but they feel that they could not do so at this time because of one or more of the following reasons: 1) the thickness of their coal seams, 2) seam characteristics, 3) soft bottoms, 4) bad roof conditions, 5) size consist, 6) insufficient flexibility in machines, 7) difficult ventilation problems, and 8) high maintenance costs. With the realization on coal about the same today as it was in 1948, or slightly less and since coal is still failing to participate to a greater degree in the total energy market, it is not surprising that the coal industry is desperately exploring more economical methods for deep mining. The manufacturers are aware that the coal industry is willing to invest in continuous miners if the equipment is built for maximum flexibility, will produce higher tons per man, and assure long life between overhaul programs. CONTINUOUS MINING MACHINES Before discussing details regarding the selection of a continuous miner, let us have a preview of some of the continuous mining machines which are available to the coal industry today. Jeffrey Manufacturing Co.: The machine shown is the Jeffrey 76 A.M. Colmol. This is their most widely used miner, and has been particularly successful in central Pennsylvania and in high-wall mining in western Kentucky. One of the outstanding features of this auger-type miner is its portability. The entire mining range can be changed from its lowest point to the maximum height without stopping the mining operation. Jeffrey 76 B.M. Colmol: This machine is similar to the 76 A.M. model; however, it is built bigger and stronger for a mining range of 50 1/2 to 72 in. This is the model that is now available (Fig. 2). Jeffrey has added two arms to the top row, omitted the odd arm in the bottom row, thus permitting a 50 pct larger throat opening. This eliminates one

Jan 1, 1961

By D. S. Gleason, D. E. Severson, D. R. Skidmore

Pittsburg and Midway Coal Co. has modified the German Pott-Broche process, on which patents date back to 1927, to produce on a bench scale liquid products by solution hydrogenation of coal. A continuing program of lignite solution-hydro gena-tion experiments is directed toward investigating coal solution reactions, determining favorable conditions for the solution refining of lignite by the Pott-Broche process, and investigating some of the uses for the de-ashed product obtained from lignite The German Pott-Broche process1" on which patents date back to 1927, has been modified by the Pittsburg and Midway Coal Co., a Gulf Oil subsidiary, to produce on a bench scale liquid products by solution -hydrogena-tion of coal." The objectives of the present effort are to investigate coal solution reactions, to determine favorable conditions for the solution refining of lignite by the Pott-Broche process, and to investigate some of the uses for the de-ashed product obtained from lignite. This paper is a summary of results to date in a continuing program of lignite solution-hydrogenation experiments. The coal solution reaction program has several principal aims. The first of these is to determine whether lignite can be successfully dissolved in solvents that might be practical for commercial development. The second object is to determine whether the solvents function after successive cycles of use, recovery, and reuse. It seems necessary to the economics of a potential commercial process that the solvent be recycled. Third, it is desired to learn something about the distribution of the ash constituents between cake and filtrate. The extent of ash removal is important. The nature and quantity of mineral matter passing through the filter may determine end-use marketability. For certain use applications, trace quantities of certain minerals can be objectionable, e.g., titanium and vanadium must be very low in electrode carbon for aluminum production. The Solution Reaction The coal solution Process involves an extremely complex system of chemical reactions. An initial solvent such as anthracene oil is a mixture of hundreds of different compounds with a boiling range of roughly 500" to 750°F at atmospheric pressure. The coal macro-molecule is broken down by thermal decomposition and solvent action into myriads of different compounds, some the same as those comprising the solvent. This similarity in structures opens up the possibility of production and subsequent recovery of solvent. Some solvent is inevitably lost by reaction. Regeneration of solvent was not a problem in the early German Pott-Broche plant. The coal refinery was an integral part of a petroleum refinery complex and replacement solvent was readily available. A coal refinery using lignite, however, might be isolated from other hydrocarbon processing facilities and the regenerability of solvent could be vital to the economic success of the venture. Several structural features of the solvent molecules have been cited as important to the coal solution process.'. The first of these is aromaticity of the material, the second, ability to transfer hydrogen to another molecule, as for example the ability of tetralin to transfer hydrogen and be converted to naphthalene. Finally, the presence of hydroxyl groups on aromatic rings within the molecule, i.e., phenolic character, seems beneficial. Mixtures of pure compounds have been tried by various investigators. Mixtures of o-cresol, a phenolic substance, and tetralin were found to dissolve bituminous coal better than either substance alone.3 This maximum solubility was not found with lignite." Hydrogen contributes to the reaction by hydro-genolysis and by combining with free radicals and molecular "loose ends" to stabilize the compounds formed in coal depolymerization. High boiling point, and correspondingly high molecular weight, seems to be a property which improves solution potential for coal with a given type of compound.' The maceral components of the coal appear to have an important bearing on its ease of solution. The fusain portion is quite inert to solvent action, but the an-thraxylon material dissolves quite readily.3 The hydrogenation reaction can be improved by the use of a catalyst; commercial hydrogenation catalysts having been found effective. Although cost is involved in the use of catalyst and catalyst recovery, the resulting saving in time and perhaps lowered temperature or pressure might justify their use in the solution refining process and decrease the total process costs. Apparatus and Procedure The coal solution runs were made in a 1-gal stainless steel stirred autoclave. The autoclave was provided with thermocouple wells and a transducer to permit continuous recording of temperature and pressure. The autoclave stirrer was magnetically driven, eliminating the leakage inherent with a rotating pressure seal. For runs in which a catalyst was used, the catalyst in the form of beads was placed in a wire mesh container mounted on the stirrer shaft. A control system programmed the heatup and reaction cycle. The permissible heating rate was 5°F per min because of the need to minimize thermal stress in the autoclave body. The temperature was raised at that rate until the reaction temperature was attained and then the temperature was held constant for the desired length of time. The maximum temperature seldom exceeded the average run temperature by more than 15°F.

Jan 1, 1971

By Vasily P. Muravjev, Gennady I. Razgildeev

The theory of reliability is a powerful mathematical tool for determining optimum selection of components and application of electrical power to coal mines. In order to make this tool useful, statistical material on equipment life must be gathered and all failures carefully recorded. Statistical data can be applied to new apparatus design and can be used to determine optimum configuration of network and maintenance to reduce costs. Reference is made to underground power systems in the USSR. Technical progress in the last few years has occurred in the design and application of highly productive coal mining and transporting machinery. One of the most important problems facing mining engineers is to increase the availability of the machine to obtain higher productivity. However, machine failures and breakdowns of electrical power supply components cause serious delays in the coal-winning processes. In spite of the fact that electrical equipment and the power supply contribute only a small percentage of direct mine costs, electrical failures as well as power supply delays can result in severe indirect losses. It is necessary, therefore, to consider reliability and safety in the application of electrical equipment underground. To obtain the best results, it is necessary to investigate and evaluate the reliability of the electrical mining components (starters, breakers, motors, cables, etc.) and optimal methods for minimizing maintenance, the desired result being equipment which is efficient and safe. Electrical equipment reliability directly influences economics: the lower the reliability the more frequently failures occur, causing serious working delays and possibly huge economic losses. By the same token, while increasing the number of preventive maintenance calls per unit time might result in fewer failures, thus reducing delays, the cost of making more frequent maintenance inspections will increase; this is one aspect to be discussed in detail. Obviously, failures of identical types of electrical equipment (starters with the same current and voltage, etc.) are influenced variably by different working conditions, environments, quality of the apparatus, maintenance procedures (personnel, frequency, quality), load, and other factors. Hence, the lifetime of identical types of equipment will vary depending upon usage in terms of the foregoing factors. Thus for a given type of electrical component, these factors must be considered quantitatively; the interaction of all these factors are evaluated quantitatively by a statistical method using probability theory. Equipment reliability must be quantitatively determined from a study of equipment life, since no one laboratory test can reproduce all working conditions and environmental effects. Nevertheless, reliability theory applied to electrical mining equipment is a very powerful mathematical tool for selecting as well as evaluating certain parameters. It is necessary to select several reliability parameters so that one can obtain an expression for the total overall economic situation in terms of cost, taking into consideration damage caused by electrical equipment failure and cost of maintenance. To accomplish this result, such characteristics as apparatus lifetime, breakdown idle time, and losses (damages) can be determined quantitatively on a statistical basis. For identical electrical equipment working under similar conditions, the previously mentioned reliability factors are random variables and therefore can be characterized readily by a probability density function. Investigations carried out by the authors in the Kuznetsk Basin (Kuzbas) in West Siberia, USSR, show that the reliability factors of electrical mining equipment are well described by the exponential, normal, or Erlang's distribution in most cases. Mining electrical equipment life is well-described by the exponential law. The distribution of lifetime density is

Jan 1, 1971

By W. T. Turrall, M. J. Cook

A discussion of modern developments in beneficiation of fine sizes of anthracite, this paper includes a description of the plant flowsheet, an analysis of operating results, and a summary of fundamentals of mineral separation relative to plant operation. DURING the past several years Lehigh Naviga-tion Coal Co. has installed equipment at Tamaqua and Lansford collieries for cleaning and sizing No. 4 buckwheat —3/32 + 3/64 in. and for recovering and cleaning flotation coal —20 + 200 mesh at Tamaqua colliery. Papers describing some of these installations have been presented to AIME. At Coaldale colliery, however, methods used to clean fine coal were inadequate and inefficient. Management decided upon a thorough investigation of all existing processes before building a plant to treat the combined three sizes mentioned. Investigation, carried out on company property and at other plants, entailed a study of hydrotators, hydroclassifiers, tables, spirals, flotation machines, methods of screening, and methods of dewatering. On the basis of this investigation a flowsheet was designed which necessarily excluded many processes in use, and the writers wish to emphasize that no criticism is implied by omission of processes or machines. A determining factor in selection of many machines was the policy of standardizing equipment wherever possible. Flowsheet and Operational Data The first principle to be considered in designing of any plant is efficiency of cleaning as related to laws of classification. Beneficiation of coal may be described as separation of two materials having different specific gravities. Although there is an inherent ash content even in the purest coal which slightly affects its specific gravity, it is recognized that many particles are true middlings. The percentage of these particles allowable in the final product depends upon the ultimate ash, or specifications of the consumer. Richards in his study of free settling and hindered settling classification' determined that particles of equal size and different specific gravities had unequal settling velocities. He also determined a definite size relationship for equal settling velocities with free settling and hindered settling conditions. Considering the design of machines used to clean No. 4 and No. 5 buckwheat, it may be assumed that both free settling and hindered settling conditions exist and that ratio of size for practical operations is between the established values. The essential factor is that the more efficiently and closely sized the feed is maintained, the more nearly perfect the resulting separation. Flowsheet design was governed primarily by factors related to the 1200 to 1500 tons per hr feed to the main plant. It should be stressed that because of the many sources of supply from mine and stripping, the physical characteristics of this feed varied considerably. The most significant influence was varying percentage of fines, necessitating that all machines be capable of operating under maximum peak load during periods of surge. Since cleaning practice is a wet process, about 9000 gal per min containing 14 to 15 pct solids, or 350 tons per hr, must be treated. This slurry contained all the fines, or —3/32 in., but also percentages of oversize, up to 1 % in., dependent on spills or breaks in screen jackets, a not uncommon occurrence in preparation plant practice. The flowsheet developed is shown in Fig. 1. Circled numbers represent points of sampling. Table I gives operational data including size, ash by size, composite ash, rate of flow in gallons per minute, percent solids dry weight, and average tons of solids per hour. It should be noted that these results are subject to human error in sampling practice. Detailed information reported is a representative evaluation of plant operation.

Jan 1, 1954

By R. P. Aikman, F. G. Miller, J. M. Podgursky

As part of a larger study of the mechanism of froth flotation of coal, the rates at which coal particles of various size and specific gravity react to froth flotation were investigated. Results of this study show that in flotation: I) recovery rate increases, but selectivity decreases with a decrease in particle size down to 100 mesh, and 2) recovery rate is constant for all sizes between 100M and 325M. n response to the growing demand for low-sulfur coking coal for iron making, the coal industry is continually striving to perfect methods for cleaning coal to acceptable ash and sulfur levels while maintaining maximum product yield. By present standards the industry has adequate processes for cleaning coarse coal, i.e., + 28 mesh, at almost any desired specific gravity. However, in the case of fine coal, i.e., —28 mesh, neither of the two existing processes - froth flotation or hydrocycloning — in its present state of development yields products that are comparable in ash and/or sulfur content to products normally obtained from coarse coal processes. The work outlined in this paper is part of a larger study directed toward maximum exploitation of froth flotation for fine coal processing. The concepts and data presented are now serving as a basis for designing fine-coal circuits in which improved efficiency is obtained by optimum use of froth flotation with other fine-coal processing methods. The paper includes: 1) A discussion of the value of a detailed wash-ability study as a method of assessing the problems associated with fine coal processing, 2) A general discussion of hydrocyclone principles, included to serve as a basis for understanding the mechanism of coal flotation, and 3) A detailed discussion of the mechanism of coal flotation, as well as the research leading to the optimum use of froth flotation in combination with other fine-coal cleaning methods to provide efficient fine-coal processing. Our study of the rates at which coal particles of various size and specific gravity react to flotation shows that in flotation: 1) recovery rate increases but selectivity decreases with a decrease in particle size down to 100 mesh, 2) recovery rate is constant for all sizes between 100 mesh and 325 mesh, and 3) the flotation process effectively rejects only those extremely fine particles which do not contain surface carbon. (Because of this feature, froth flotation is an excellent method for rejecting clays - the only extreme fines in - 325-mesh coal that are always carbon free.) Washability studies of flotation products are used to assess flotation efficiency. These studies show that: 1) the increase in percent sulfur in flotation products above the theoretical sulfur content as defined by the washability study is due almost entirely to flotation of fine high-specific-gravity particles that are part coal and part pyrite, and 2) in a given size range particle floatability decreases with an increase in particle specific gravity. Error curves are explained and are used to compare the mechanisms of coal flotation and hydrocycloning. These comparisons show that hydrocyclone performance is affected primarily by particle mass, i.e., particle specific gravity and size, whereas coal flotation performance is dependent on both the particle mass and the coal surface available for bubble attachment. Since the error curve is a basic tool for rating the efficiency of any coal processing system, it will be useful to quickly review the principles of error curve construction. RATING COAL-PROCESSING EFFICIENCY BY THE ERROR CURVE The organic component of coal is of lower specific gravity than its impurities, and the most effective principle for coal concentration is therefore separation on the basis of specific gravity. However, since there is no sharp dividing line between pure coal and pure ash components, specific gravity must be closely controlled if the product is to have acceptable quality at maximum yields. In turn, the basis for close control in any coal processing system is the reference standard provided by precise laboratory washability studies. In these studies, coal is carefully separated into a series of progressively higher specific gravity fractions to establish a relationship

Jan 1, 1968

By T. S. Spice, H. L. Lovell, R. W. Utley

With the increased demand for lightweight aggregate, such materials have been manufactured from slags, clays, slates and, to a minor extent, the refuse of coal preparation processes. The latter source is attractive due to current interest in eliminating air pollution and improving the unsightly appearance of refuse piles as well as potential economic advantages, if a satisfactory product can be manufactured. A study has been made relating the composition of coal refuse, expressed as washability data, and the properties of a resulting aggregate. Three different refuse samples (bituminous and anthracite) were separated at gravities ranging from 1.4 to 2.96. Their chemical composition was determined. Aggregates were prepared in the laboratory from these fractions before and after ashing, at temperatures between 1250' and 1350°C. The specific gravity, expansion and compressive strength of the products were measured. The response of the refuse to bloat is related to the chemical composition of the ash to yield a glass of desirable viscosity and to the presence of a material which will form a gas in the appropriate temperature range. Carbonaceous material within the particle is a negative factor since after the glassy phase forms, oxygen cannot enter the mass thus inhibiting bloating. Precombustion of the refuse is also detrimental since it removes substances of gas-forming capability. The fraction sinking at 2.60 yielded an aggregate which had the lowest specific gravity, greatest expansion and greatest compressive strength. The purpose of this study was to seek and evaluate possible relationships between the composition of coal refuse, as expressed by washability data and factors affecting the production of a lightweight aggregate from refuse by bloating. The study was related to an intense program in the Department of Mineral Preparation at the Pennsyl- vania State University to study the possible elimination of air pollution from coal refuse fires by producing a low-carbon secondary refuse. Another phase of this program is considering possible uses of such a secondary refuse. Thus one study was to determine if a lightweight aggregate could be produced from this new refuse material and what effect the nature of refuse beneficiation would have upon the characteristics of a commercial aggregate product. It was hoped that these studies would assist in the design of a process and in establishing the optimum specific gravities at which refuse separation could be made in respect to the production of a low grade fuel from the float product and a raw material for lightweight aggregate from the sink product. The combination of producing a saleable low grade fuel and a lightweight aggregate, while at the same time minimizing disposal costs and air pollution contributions would be attractive to the coal mining industry. MECHANISM OF BLOATING Austin, Nunes and Sullivan,1 Ehlers,3 Everhart, 4 Riley7 and others, have stated that bloating is an expansion process in which two essential conditions exist simultaneously. These are: the development of a glassy phase over a relatively wide temperature range (the viscosity of this glassy phase plays a major role in the retention of the evolved gases) and the evolution of gases (SO2, SO3, CO2 and O2) from mineral decomposition in sufficient quantities at appropriate temperatures to form a cellular structure in the mass. Unless these two "type" reactions take place at the same time or in overlapping ranges of time and temperature, cellulation will not occur. In this event, the gas will escape or the melt will be too fluid to retain it; or if retained, the cells may be too large or the walls too thin for good strength in the product. The stages that occur when clays and shales are heated in an oxidizing atmosphere may be summarized as follows: 1) Drying and removal of free water to 200°C. 2) Absorbed water dehydration period (200 to 480°C). 3) Chemically-combined water dehydration period (480 to 700°C).

Jan 1, 1965

By Vard H. Johnson

IN 1943 the U. S. Bureau of Mines undertook drilling in an effort to develop new reserves of coking coal in an area near Paonia, Colo., as a part of an attempt to alleviate the shortage of known coking coal of good quality in the western United States. Geologic mapping of the area was undertaken by the U. S. Geological Survey with the purpose of first furnishing guidance in location of drillholes and later aiding in interpreting the results of the drilling. The drilling program was under the general supervision of A. L. Toenges of the U. S. Bureau of Mines. J. J. Dowd and R. G. Travis were in charge of the work in the field. Geologic mapping was started by D. A. Andrews of the Geological Survey in the summer of 1943 and was continued from the spring of 1944 to 1949 by the writer. The first few holes drilled failed to locate coking coal, but in the summer of 1944 coking coal was discovered by drilling 6 miles east of Somerset, Colo., the site of present mining. In the succeeding years, 1945 to 1948, 100 to 150 million tons of coal suitable for coking were blocked out by drilling. The ensuing discussion of the geologic controls on the distribution of coking coal in the area is based on the geologic mapping as well as the drilling done in the Paonia area, more complete descriptions of which have appeared or are in process of publication."' In order that the possible geologic controls affecting the present distribution of coking coal may be considered, it is necessary to discuss briefly the indicators of coking quality coals. Coking Coal Coal that cokes has the property of softening to form a pastelike mass at high temperatures under reducing conditions in the coke oven. This softening is accompanied by the release of the volatile constituents as bubbles of gas. After release of the contained gases and upon cooling, a hard gray coherent but spongelike mass remains that is referred to as coke. This substance varies greatly in physical properties and, to be suitable for industrial use, must be sufficiently dense and strong to withstand the crushing pressure of heavy furnace loads. Western coals have a generally high volatile content and therefore form a satisfactory coke only when they attain a rather high fluidity during the process of heating arid distillation in the coke oven. When this high degree of fluidity is developed, the volatile constituents escape and leave a finely porous coke. On the other hand, when the degree of fluidity is low the product is an excessively porous and therefore physically weak mass that is called char." Small quantities of oxygen present in coal are believed to decrease the fluidity of the material during the coking process and to favor the development of char rather than coke. In consequence, coal chemists have for some time considered the possibility of developing an index to coking qualities by inspection of chemical analyses of coals.' A formula has now been developed that does permit a rough preliminary estimate of the cokability of coal on the basis of the analysis on an ash and moisture-free basis. Coals may be eliminated as possible coking fuels if the oxygen content is greater than 11 pct. Similarly the ratio of hydrogen to oxygen must be greater than 0.5 and the ratio of fixed carbon to volatile constituents must be greater than 1.3. If the coal, on the basis of these limiting factors, appears to have possible coking qualities, the following formula permits determination of the coking index: a+b+c+d Coking index = -------- 5 a equals 22/oxygen content on ash and moisture-free basis, b equals two times the hydrogen content divided by oxygen content on moisture and ash-free basis, c equals fixed carbon/l.3 x volatile matter, and d equals the heating value on moist, ash-free basis/13,600. Coking indices higher than 1.0 suggest that the coal will coke, and indices above' 1.1 indicate good coking tendencies. Although generally usable, this formula 'is not completely satisfactory because the percentage of oxygen shown in ultimate analyses is derived only by difference; i.e., by subtracting the sum of the percentages of the constituents determined analytically from 100 pct. Although the coking index indicates the coking tendencies of coal, it is necessary to make physical tests of coke before its industrial value can be determined. The U. S. Bureau of Mines has developed a standard procedure for determining the approximate strength of coke that would be formed from a given coal. In this test one part of ground coal, mixed with 15 parts of carborundum, is baked to form a standard briquette. The weight, in kilograms, necessary to crush the briquette is termed the agglutinating index. This test determines the relative fluidity attained in the coking process by measuring the cementing strength of the coal in the briquette. A

Jan 1, 1953