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By A. M. Kivari

Operations at the cement plant of the Permanente Corporation, in the hills about 45 miles south of San Francisco and 12 miles west of San Jose, are interesting to the members because of the adoption of modern hydrometallurgical methods for handling and beneficiating raw materials. Closed-circuit grinding, classification, hy-droseparation, flotation, agitation, and thickening are familiar terms at this cement plant. These methods make it possible to process refractory limestone into suitable materials for the mallufacture of modern cements. Construction of the plant, having originally a design capacity of 7000 bbl. of cement per day, was started in June 1939, and the first clinker was ground on the following Christmas Day. At present the capacity is about 12,000 bbl. of finished cement per 24 hr., most of the production being low-heat cement. † About 3I30 tons of limestone, 620 tons of clay and I00 tons of gypsum are required daily for this output. Occasionally iron oxide is added to the clay to obtain the desired composition. This, together with the gypsum, is purchased from outside sources. up to the present time, the ccrporation has produced low-heat, normal, moderate-heat, moderate-sulphate-resisting, high-early-strength, oil-well, modihed-oil-well, and plastic cements. Most of the present production is for Shasta Dam, near Red- ding, Calif., the total demand being 5,800,000 bbl. of cement. Following are some of the cement require ments of the Shasta Dam specifications: Per Cent Chemical composition: Loss in ignition.......... 2.25 Insoluble residue......... 0.75 SO3.................... 2.25 MgO................... 5.'' Uncombined lime in clinker (CaO)......... I.25 Percentage of iron oxide to alumina ratio....... I.5 Compound composition: Tricalcium silicate (3CaO.SiO2)........... Not more than 35 Dicalcium silicate (zCaO.SiO2)........... Not less than 40 and not more than 65 Tricalcium aluminate (3CaO.Al2O3).......... Not more than 7 Tetracalcium aluminofcr- rite (4CaO.Al²O³.Fe²O³) Not more than 20 Fineness: The average specific surface in the bin sampled shall be not less than 1800 sq. cm. per gram and no individual sample less than 1600 sq. centimeters. Compressive strength: The average compressive strength in pounds per square inch of three mortar cubes composed of one part cement and 2.75 parts fine testing sand, by weight, shall be equal to or greater than the following:

Jan 1, 1942

By R. M. Hardgrove

Pulverized coal was first used for firing cement kilns about 45 years ago, with such success that it has continued in general use. Based on cost, pulverized coal is usually the most economical fuel. In some localities natural gas or oil is cheaper, but 85.5 per cent of all cement kilns at present are fired with pulverized coal. The pulverized-coal flame is well suited to kiln conditions. Its soft, luminous nature gives it a high radiating property and the length of the flame can be readily varied to suit conditions in a kiln of any length. Ash and slag cause most of the dificulties incident to the burning of coal. In a. cement kiln, however, the ash, being similar in composition to the raw mix and being deposited uniformly in the mix and clinker, occasions no difficulty as long as it remains constant in quality and quantity. Until 6 years ago almost all cement kilns using pulverized coal employed the bin system. The coal was dried and pulverizeti in a separate plant and delivered into a bin at the kiln. Thus the bin system requires driers, storage-type pulverizers, conveying systems, individual kiln bins, pulverized-coal feeders and primary air fans. The direct-fired system requires only a pulverizer and a blower. Fig. I shows typical plants. The first installations of direct-fired pulverizers were of the impact type but fineness of pulverization was both inadequate and variable. About 1927 a few direct-fired installations employing tube-type pulverizers were made, which were much more satisfactory. As more efficient pulverizers were developed from 1934 on, equipment for direct firing was installed at a large number of plants, until now 196 of about 900 kilns in use in the United States are direct-fired. Most of these replaced antiquated bin systems. Process of Burning Cement Clinkers The process of burning cement clinkers is fundamentally one of heat transfer. In boilers and other heat-exchange apparatus the area of the surface receiving heat from the gases is the best unit for comparing output, This seems to be equally true for kilns. The inside kiln surface is the heat-receiving surface and its area is used as the unit of kiln size in the following comparisons. Heat input is based on British thermal units in the fuel. E. G. Bailey,' in his work on boiler-furnace heat absorption, has used the unit "B.t.u. available." This is the practical calorific power of the fuel, plus the heat in the air used for combustion. In other words, it is the heat available for useful work in the furnace or kiln. The unit of kiln output is barrels per hour. The composition of the raw mix, the percentage of water in the slurry of wet plants and hardness of burning all affect the output. These conditions, however, are hard to evaluate and probably have less effect on the output than irregularities of raw feed and clinker discharge and inexpert control of combustion. Wet vs. Dry Plants The study presented here covers 88 plantsl of which 41 are wet and 47 dry. Of

Jan 1, 1942

By John R. Green

Rotary kilns, varying in construction, are used in a variety of processes. Products differ and operating conditions vary accorcling to economic requirements. All of these variables influence the degree of instrumentation and type of control necessary or profitable to install. Products fired in rotary kilns are the results of combined temperature and time treatment and the interrelation of these two factors should be kept constantly in mind in the perusal of this paper. Measurements of Kiln Temperature The securing of proper temperatures within the rotary kiln, either of the charge or of the kiln lining in the high fire zone, are basic objectives of kiln operation; therefore the measurement of these high-temperature conditions merits first consideration. For these measurements, radiation pyrometers, properly designed and protected, have proved adequate, particularly where records ox automatic control are desired, while Optica1 pyometers may serve as portable indicators. Light-sensitive devices or photoelectric cells in this service are capable of producing records as well as certain forms of automatic control. High-temperature measuring devices for rotary-kiln work should have the measuring unit protected from dust, as a continued accumulation of deposit would lower the readings. Fig. 1 shows a typical application of radiation equipment to a rotary kiln, mounted far enough away from the sighting hole through the kiln hood to be protected From puffs or stingouts and with a sighting tube having a series of settling chambers, to prevent dust from reaching the lens. Although dust conditions inside the kiln itself usually fluctuate considerably and have their effect upon the readings at that moment, such effects are not cumulative and show as a continuous fluctuation in the temperature indicated, provided the radiation unit and recorder are responsive to such slight and rapid temperature fluctuations. The ambient temperature to which a measuring unit is subjected fluctuates from possibly 0" to over zoo°F., therefore these changes should have no effect upon the final reading or upon the life of the measuring unit. A very narrow sighting angle is desirable, so that measurement of temperature may be made as far as possible into the high fire zone without effect of the flame temperature. There should some sighting means on the measuring unit, so that the proper field may be viewed. Measurement of wall temperature taken at a point where the charge has just left the wall is less affected by dusting conditions than when taken elsewhere. Factors influencing Temperature Table I gives an outline of the factors that produce kiln temperatures and comments on the possibility of measurement and control. Of greater number and of varied influence are the factors that control when, where and how the heat is delivered to the kiln, the type of flame and the rate of

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By J. E. Wilson, F. P. Lebar

TYPICAL of southwestern Wyoming are coal structures that dip from 4° to 17°. Those at the Reliance and Winton mines of the Union Pacific Coal Co. average 91/2 and 15°, respectively, and dip almost directly to the west. By using a hoist for main haulage, a method of mining has been developed whereby four places are driven on the pitch of the seam. Slightly upgrade from the strike at intervals of 300 ft. a two-entry system is driven to the north and to the south. In both fields, which are nongassy, an exhaust system of ventilation is used. Slope, manway, and lower or haulage entries are placed on the intake. Two air-courses and upper or breaking entries are placed on the return. A retreat system of mining is used exclusively. From the opening of its first coal mine, in the year 1868, until 1923, when the Joy loader was introduced into The Union Pacific Coal Company's operations, slopes, aircourses, and manways were driven by hand. Development work was conducted on the night shift or off shift, and the main hoist was used for haulage. The slope was driven down the pitch with crosscuts turned at intervals of 75 ft. and entries at 230 ft. Connections to the workings above were made by driving up the pitch 75 ft. Mule haulage was used to furnish cars to the hand loaders driving planes up the pitch. A sheave was placed at the upper end of the plane with a I¼-in. hemp rope dou- bling around it and having me end attached to the car and the other to the mule. The mulc, pulling downhill, raised to the working face a small wooden car of one-ton capacity. With the introduction of mechanical loading, mule haulage and small wooden cars disappeared and auxiliary hoists, electric locomotives, and steel cars with a capacity of 4 tons were substituted. Development work was done during two or three shifts of the 24-hr. period. Driving of haulage slopes down the pitch by hand continued. Crosscut intervals were changed to Ioo ft. and the crosscuts were driven with shaking conveyors. With the use of a 90° swivel, aircourses and manways were driven up the pitch. Entry intervals were changed from 230 ft. to 300 feet. Mobile loaders of the Joy type were first employed by this company in the year 1923 at Hanna, Wyo. In 1928, two 4-BU Joy loaders were transferred from Hanna to Winton No. I mine, where they were used in driving slopes and entries. At the time of transfer, the machines were about worn out and were not suited to the steep pitch, therefore frequent breakdowns occurred. However, experience justified the belief that the mobile loader was fitted for driving slopes in this field. During 1937 a unit was installed in Reliance No. 7 mine, and during 1939 one in Winton No. 7½ mine. With the introduction of this type of mobile loader, a new era of development began. Speed undreamed of in hand-loading days became a reality. The cost of development down the pitch has been reduced until it

Jan 1, 1942

By W. H. Moore, E. T. Powell

IN the early days of mining in the Anthracite field, only the thicker and better seams of coal were mined, because of the limited mining and coal-cleaning facilities, therefore many of the thinner and less productive seams were permitted to remain unmined. With improvements in mining methods, the introduction of modem preparation plants, and the increased market demand for small-size coal, the thinner and more laminated seams have now become assets. In many instances these seams overlie the mined-out beds of the thicker and better seams, from which all recoverable coal was removed by the conventional breast-and-pillar method of mining. Some collieries in certain sections of the Anthracite field are now relying on these thinner and laminated seams. For the purpose of illustration, a large area in the Hickory Swamp-Ridge Basin of the western middle coal field has been selected, where as early as 1898 the No. 4, or Little Buck Mountain, seam was mined and pillars removed. The No. 5, or Buck Mountain, seam lying on a comparatively light pitch and with intervening strata of but 55 ft. of rock was not mined until 1935. Farther east, in the same basin the Nos. 8 and 9 seams, which are two splits of the Mammoth seam, have been mined and robbed and the workings have been abandoned since 1896. The overlying No. 9½ seam, or the upper split of the Mam- moth, and the No. 9¾ or 4-ft. seam remained virgin, with only an interval of 50 ft. of rock separating them, and with 40 to so ft. of rock strata separating the No. 9½ seam from the completely mined area of the Nos. 8 and 9 seams. These seams lie on a pitch of 45° on the south side of the basin and on 75° on the north side of the basin. Mining has been carried on in the 9½ and 9¾ seams for 3 years on both dips of the basin, for a distance of 3500 ft. In some sections the overlying strata of the thicker seams previously mined have settled without any apparent breaks in the upper strata, as indications have shown in the top split of the Mammoth; while in other areas the roof and bottom of this top split of the Mammoth have settled unevenly, thereby causing the bottom and roof to break into blocks. In attempting to mine over a large area of the No. 5 or Buck Mountain seam, a similar settlement has been encountered; the roof of the seam has been broken and a considerable amount of timbering has been necessary in order to make the mining of the seam possible. The conventional breast-and-pillar method, using shaker conveyors, has been used in the No. 5 seam, and a slant-breast method in the mining of Nos. 9½ and 9¾ seams. Mining OF Light-pitch Seams The No. 4 seam was mined and robbed by the conventional breast-and-pillar system. In this area the seam pitched from 5° to 25° on the north dip, averaged 8 ft. in thickness and had a slate bottom and a

Jan 1, 1942

By A. Lee Barrett

SO commonplace that they are seldom noticed, mining-machinr, bits have a definite and important bearing on the cost of coal production. At the average mine many thousands of bits are used during the year. The actual cost represented by the purchase of the bits used, as well as that of sharpening and handling them, represents a considerable sum of money. From the point of view of indirect costs they are important, as the lack of good bits at a mining machine can seriously impair production. Good bits tend to ensure an adequate supply of coal. Classes OF Bits Mining-machine bits may be divided into three general classes, based on shape and method of use: (I) chisel, (2) pick-point and (3) one-use. The chisel and pick-point bits are shown in Fig. I. The so-called "chisel" bits were made of steel of various sizes and were sharpened to a wide chisel cutting edge. Few of this type of bit are used, except in places where coring is very bad. Sometimes chisel bits are used with standard bits to help cut out the core. The "pick-point" bit is widely used today. Normally it is made from ½ by I-in. steel, is about 4½ in. long when new, has a rake angle of about 25°, a front clearance of about 23O, a side clearance of approximsteji 8".' There are many variations in the shape of this bit, depending on the ideas of the individual that may be dressing the dies of the bit machine. This bit has been generally used for many years and at present, in one form or another, is by far the most popular bit used in coal mines. This bit usually is made from 0.85 carbon steel, although in recent years some alloy-steel bits have made their appearance. At present, carbon-steel bits are frequently tipped in. hard facing materials. A third class of bits might be called the L'one-use" bit; that is, a small bit that is used once and then thrown away. These bits have two or three points, are usually made from 0.85 carbon steel and sometimes from higher alloys. Recently some. of them have been tipped with hard facing material. Most of them were used at first with special

Jan 1, 1942

By John S. Johnson

The purpose of this article is to describe several types of borehole lining in common use, and especially to offer a relatively new means of reducing the expense of maintaining boreholes where they are subjected to corrosive action of water flowing either inside or outside of their casing. In the Anthracite region of Pennsylvania, boreholes are used for many purposes, the principal ones being: openings between the surface and underground workings through which are passed steam, water and compressed-air lines, ropeways from surface hoists to underground rope haulages, conduits for electric power and signal lines, water-discharge lines from underground pumping stations, and inlets to mine workings for flushing material. Linings for Boreholes The inadequacy of steel pipe for use in boreholes has long been recognized. Various substitute materials have been tried, including screw-joint cast-iron pipe (Fig. I). Cast-iron pipe, while very effective as a discharge line for acid water (mine water) being pumped to the surface, is not so effective when serving as a conduit for silt and crushed rock being flushed underground for filling mine voids. Terra cotta pipe (Fig. 2) has been used in boreholes, and lasts a long time if the glazed inner surface of the pipe remains intact. Its installation in a borehole is very difficult and is recommended only where it is certain that there will be no subsidence of the surrounding territory. Accessibility in order properly to join each section of the pipe is essential to a satisfactory job. In large water holes, where it is possible to lower a man into the hole for sealing the joints of the pipe, glazed terra cotta pipe has been used successfully for delivering mine water from pumps in the mines. For large holes used for flushing refuse into mine workings where the surface wash is shallow, standard paving brick have been used to form a tube through the wash to the bedrock. These brick are laid radially, backed by common brick and cobbles laid in cement. It is obvious that the shaft through the wash must be large enough to permit a man to work in it. The flushing material is carried through bare rock from the juncture at bedrock to the openings underground. This plan has been used particularly where a cast-iron surface line is laid upon the surface to carry the flushing material from the breaker to the hole, and down the hole continuously, with no opening to the air at the top of the hole, thereby eliminating the necessity of an attendant at this point (Fig. 3). Another similar type of borehole lining is constructed of hexagonal pieces of vitrified brick (Fig. 4). This type requires a steel drive pipe through the surface wash large enough to provide a finished tube of the desired size surrounded by a brick wall about 4 in. thick. The special bricks are laid in cement, six to a circle, around a lubricated pipe used as a form, and lowered in sections inside the large drive pipe; the bottom section is seated in a socket drilled in the solid rock, having the same inside

Jan 1, 1942

By Paul L. Burkhart

THOUGH at an earlier period brief studies had been made by the Lehigh Navigation Coal Company Inc., it was not until 1924 that J. B. Warriner, then general manager, called for a comprehensive study of timber treatment, treating solutions, and related operations. Because of the impossibility of procuring accurate costs and other pertinent treating data, the American Wood Preserving Co. agreed to treat a car of mine timber at the Port Reading plant of the Reading Railroad. Fig. I shows the treating data collected at the treating plant on this timber, which was installed in two of the Lehigh mines in 1925. Table i.—Results of Timber Survey in 1925 before Treated Timber Was Introduced Length of tunnels and gangways ft..... 522,987a Length of tunnels and gangways timbered, ft.................................. 224.880 Number of steel mine sets in use......... 2,412 Number of timber sets in use............ 63,834 Number of timber sets with evidence of rot 50: 419 Number of timber sets with evidence of squeeze..... ...................... I3,415 Percentage of all timber sets showing evidence of failure: BY wet rot........................ 6.0 By dry rot........................ 73.0 By squeeze....................... : Average life of wood set in Panther Valley, years............................... 5.3 • 99.05 miles. In the latter part of 1925, a survey of the tunnels and gangways of the company's Panther Valley mines was made to determine the proportion of gangways or tunnels timbered, cause of timber failures, approximate life of the average wood set and the approximate percentage of treated timber that it would be profitable for the company to use. Results of this survey appear in Table I. Because of the high percentage of timber that gave evidence of failure by rot, as noted in Table I, and a further study of the life of the sections where timbering was necessary, it was deemed advisable to fill 25 per cent of the yearly gangway timber requirement with treated timber. Quantities and Treatments OF Treated Timber Purchased In the early days of the program, none of the large treating companies would accept orders for small quantities of treated timber and the company was compelled to state the quantity of timber it would require at each operation in each month of the year. Table 2 is an example of the tabulation given to the various treating companies to assist them in making a yearly bid on the minimum quantity of timber required by the company during the year 1931. A bill of timber was calculated for each car, the weight of which would be between the minimum and maximum weight permitted for railroad-car loading. The treating company was paid for timber upon delivery at the operation specified. As extra cars of timber were required, they were ordered from the treating company, and payment was required On their receipt, on the basis of the piece or tonnage

Jan 1, 1942

By D. L. McElroy, J. L. Schroder

Although the earliest use of rubber-tired haulage was in Illinois in 1936, the first unit of this type of equipment used in West Virginia was shipped into the state in 1938. All units placed in West Virginia mines have been of the shuttle-car type. The Katherine Coal Mining Co., at Lumberport, W. Va., was the second company in the United States to install shuttle cars and the Gay Coal and Coke Co., at Mt. Gay, W. Va., was the first company in the country to install the thin-bed type of shuttle car. On Oct. 8, 1940, nine companies in the state were using 41 shuttle cars, of which 10 cars at three mines were of the thin-bed type. All shuttle cars of the low type were in the southern fields and all of the high type were in northern West Virginia. At that time several units were in the process of delivery. The data in this paper were collected in the field by Mr. Schroder in connection with graduate work on mechanical loading of coal in West Virginia by mobile loaders. In the course of his field work he visited five mines in the state that were using shuttle cars. All of these mines were in high-volatile coal beds of strong structure. The more pertinent data obtained at the mines visited are given in Table I. The maximum distance of shuttle-car haul is the distance between the transfer station and the farthermost place. Tonnage per loading unit per foot of bed thickness is to some degree a measure of relative efficiency, especially if the different operations are working the same bed or have somewhat similar conditions. The thin-bed operation is well up in the higher bracket of the afore-mentioned column, which is as

Jan 1, 1942

By C. S. Allen

With increased coal production and mechanization of coal mines many transportation problems arise. The main objective is to bring the coal to the tipple or dump it as quickly as possible. Larger and faster haulage locomotives or two locomotives connected in tandem have been adopted to solve the problem. Motor equipment, gears and control systems of locomotives to be placed on these hauls are carefully studied to meet these requirements, by an analysis of motor characteristic curves with the corresponding tractive effort curves, in order to determine the correct load for maximum efficiency and to determine the approximate schedule. Control equipment is usually considered to be the controller and resistance. Sometimes accelerating curves are calculated in order to obtain smooth starting in the least possible time without slipping the wheels. Train control is very important, but why limit this to acceleration ? Deceleratien-or-retardation of the locomotive is a part of the train control and should be given as much consideration as the control for acceleration. Types or Braking Common braking systems for mine locomotives utilize screw-type mechanical brakes, dynamic braking and the air brake. For gathering and haulage locomotives, the srrew-type mechanical brake is regarded as standard. A brake of this kind cannot be applied without much effort and lapse of time. Motormen, like other people, follow the path of least resistance, and therefore have their brakes ready for instant application—which means that the shoes are virtually dragging. Even then, a motorman bucks and plugs the motors to gain time and avoid as much physical effort as possible. "Bucking of motors" consists in turning the reverse cylinder of the controller to parallel position corresponding to the opposite direction of travel while the locomotive is in motion, without applying power from an external source. When this occurs, both motors tend to act as series generators, because the armatures are revolving and there is residual magnetism in the field circuit. Owing to slight differences in motor manufacture, one of these will generate a slightly higher potential and will overcome the generated voltage of the second and tend to drive it as a motor in the reverse Direction.-This action retards the locomotive but is very severe on the segments of the reverse cylinder and on the motor commutators. "Plugging motors" is the application of power to the motors corresponding to the opposite direction of travel. In order to "plug" the motors it is necessary first to "buck,, them (which reverses the connections to the motors) and then to apply power. Theoretically, this should be done only in an emergency, but locomotives in mine service are frequently thus treated., Dynamic braking is retardation of the locomotive by the motors acting as generators, which take no power from the line.

Jan 1, 1942

By Charles T. Holland

In connection with the problems of bumps in coal mines, much has been written concerning the manner in which roof action and methods of mining enter into the pressure effects observed but little has been written upon the relationship between the physical properties of coal and associated rocks and the cause of bumps. The question of the strength of bituminous coal, of its elasticity, and the energy it can absorb and store has been neglected to a considerable extent in connection with this problem. It is generally agreed that one of the necessary conditions for bumps is a strong coal.' Therefore, since the strength of coal is an important factor, a study of this property should be made. several investigators have conducted laboratory experiments upon the strength of coal, using small cubical or rectangular test pieces. An attempt will not be made to go into all of these investigations, but they are briefly summarized in Table I. Experiments of the U. S. Bureau of Mines and of Lawall and Holland (pp. 27 and 34 of ref. 9), however, indicate that regardless of whether the coal is a soft and weak semihituminous (Pocahontas No 3) or a hard and strong bituminous (Stockton) coal, the maximum ultimate unit stress developed in laboratory tests is higher in small cubes than in larger cubes of the Same coal.

Jan 1, 1942

By C. S. Westerberg, C. P. Heiner

Observation of the clinkering action of coal from the Castle Gate D seam in underfeed stokers over a period of years has given rise to the present investigation of the effect of bony coal on clinkering. Some very costly examples of slagging clinkers in particular cases have been of especial interest to the authors. In such cases coal analyses and ash-softening and fluid temperatures have been found of little value. In general, it was noted that slagging mas accompanied by a somewhat higher ash content of the stoker slack, and the suspicion naturally arose that foreign matter was the troublemaker. One would naturally suspect bony coal and rock impurities because of their occurrence and prevalence within the seam and the relative absence of sulphur or other impurities in the coal mined. The test results given herein indicate that the concentration of the bony coal and rock impurities in the coal determines the clinkering tendencies of the ash during combustion. Castle Gate D Seam Extending from the Utah-Colorado line near Grand Junction, Colo., westward to Greenriver, Utah, and thence in semicircular form north and westward to Castle Gate, and from there southward to Mt. Hilgard in central Utah, is a line of nearly perpendicular cliffs, composed of interbedded massive sandstone and shales, making up the coal-bearing horizon of the Mesaverde formation of Upper Cretaceous age. The cliffs face roughly concentrically toward the south and southeast and the strata, with conformably enclosed coal beds, gently dip away from the face of the cliffs and at right angles to their front line. Back of these cliffs the mountain masses rise to an altitude of from 8000 to 10,ooo ft. above sea level. Stretching away from their foot is a great shale plain, on which the towns are situated and over which the railroad lines and highways run. The Castle Gate mines enter the coal beds at their outcrop; the workings extending within a particular coal bed away from the outcrop and under the mountain mass back from the cliff line (Fig. I). Throughout the Mesaverde formation in the Castle Gate general area, there are from two to five workable coal beds. Usually not all of these are coincidentally of workable size and merchantable quality throughout any given large tract. In the Castle Gate mine area, the base of the Mesaverde horizon and coal measures is marked by a massive, cliffmaking, gray sandstone bed 60 to 80 ft. thick. Locally this is referred to as the "Castle Gate sandstone floor." Frank R. Clark, in U. S. Geol. Survey Bull. 793, designates it as the "Aberdeen sandstone." At Castle Gate the various coal beds are designated by the letters A, B, C, etc., commencing with the lowest bed.

Jan 1, 1942

By H. J. Hager, P. H. Haskell

For the past 20 years Alabama has probably led all other coal-producing districts in the proportion of coal prepared by wet washing. All rail mines, with one exception, and a high percentage of truck mines now wash all coal under 3 or 4 in. Most of this coal has been cleaned by jigs, though within the past few years one or more wet-table plants have been installed for cleaning some of the coal from practically all the major seams. These tables supplement either single or multicell jigs usually for one of the following purposes: (I) to wash the finer sizes of raw coal, (2) to rewash the finer sizes of washed coal after they leave the jigs, (3) to re-treat jig middling products. The main purpose of this paper is to present cleaning data and operating factors involved in the preparation by tables of the finer sizes of raw coal. The specific information and data pertain to the Mary Lee seam of coal as extracted at three of the mines of the Alabama By-Products Corporation, which produces coal for both byproduct coke-oven consumption and the general steam market. The Mary Lee seam has long been recognized by coal-preparation authorities as presenting a very difficult ash-reduction problem. Washing characteristics and physical structure of the seam vary widely. Its thickness ranges from 32 in. with no rock parting to 100 in. with from one to three partings. Over the entire field the bed contains bands of bone and rash. These are distributed throughout the seam cross section. A portion of this bone or bony coal occurs as definite bands I in. or more thick; however, some bone is interlocked as layers within the coal and consequently breaks into flakes when the coal is broken. This material is relatively high in ash and its specific gravity varies widely. It has been observed that in most cases in the washing of Mary Lee coal from 10 to 18 per cent of the raw coal is within +0.10 specific gravity of the attempted point of separation. Therefore with the average wet jig, only with extreme difficulty can a low-ash product be obtained without loss of much marketable coal in the refuse. Without entering into the controversial question as to whether modern washing equipment can efficiently handle simultaneously all sizes up to a given maximum, suffice it to state that the comparatively old-type jigs in service at the Alabama By-Products Corporation's mines could not clean the he sizes satisfactorily. Therefore, in 1931 that company installed six diagonal-deck tables at the Barney mine, which were installed as an addition to a complete plant cleaning all coal below minus 3 in. The primary washed product is shipped as steam coal and another portion may be prepared as washed nut for domestic trade. These tables are washing 9 tons per hour of raw coal sized to ¼ in. to o. The washed table coal is mixed with the 3 to ¼-in. jig product to constitute a 3 in. to o steam coal.

Jan 1, 1942

By David R. Mitchell

This paper is limited primarily to a description of dry coal-cleaning processes in which air currents are used to effectuate a separation between coal and refuse. Processes depending mainly on differences in the coefficient of friction, resilience and shape are not described unless the use of air is necessary for the proper functioning of the process. The air cleaning of coal is of primary importance to the coal industry of the United States. From figures presented in the 1941 edition Mechannual, it is estimated that at least 12 million tons of bituminous coal was cleaned in the United States during 1940 by machines using air as the separating medium. One of the current problems confronting the coal-preparation engineer is the dry cleaning of small sizes and slacks. Progress in machine development for the cleaning of small sizes has been slow in recent years in the United States of America; it has been much more rapid in Britain and on the European continent. This paper does not attempt an exhaustive survey of all these developments, and only machines and processes that have found considerable commercial application are cited. Further, no critical analysis has been made of the theory involved in the separation of coal from refuse by processes using air as the separating medium other than that necessary for an understanding of particular machine designs and applications. The possibility of using air for the separation of coal from refuse was recognized early by mining men and many attempts were made to develop commercial machines. The ingenuity of the early experimenters is to be marveled at in view of their lack of knowledge of the theoretical principles involved. As in other arts, practice has largely preceded theory. Yet, science and practice play a game of leap frog, now one ahead, then the other; the careful experimenmenter governs his efforts along practical lines by adapting his experiments to the limits prescribed by scientific laws. By first studying developments and the theory involved, it is not only possible to adapt processes to new uses but also to reduce materially operating cost and to build larger units without costly failures. Although many inventors have applied for patents for separating minerals by the use of air currents, the literature in regard to coal is meager. Berrisford2 describes briefly some of the early attempts to clean coal without the use of water. Several of the examples cited by him are reproduced herein. Fig. I shows a machine used for grinding and; izing, in which rollers running in a pan with a perforated bottom and fan opening on the side crushed the coal, which was then carried by the air currents to a settling chamber. The heaviest particles dropped out first and the light ones farthest away, as shown. In the year 1864, a machine was made that could be used as a sizer or to separate dirt from coal with a sized feed (Fig. 2).

Jan 1, 1942