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By Roy C. Bartholomae, Jeffrey Shawn Peterson

1. INTRODUCTION In the early 1980s, the U.S. Department of the Interior’s Bureau of Mines Pittsburgh Research Center required that a large reverberation acoustic facility be constructed. This facility was designed to determine the sound power level emissions of mining equipment, especially continuous miners and jumbo drills. A contract for a conceptual design of a reverberation chamber was awarded to Wyle Laboratories of Huntsville, AL, which had extensive experience in the design, construction, and accreditation of acoustical reverberation chambers. The following standards were referenced as guidelines for the design of the chamber: ANSI S1.31-1980, “Precision Methods for the Determination of Sound Power Levels of Broadband Noise Sources in Reverberation Rooms” and ANSI S1.32-1980: “Precision Methods for the Determination of Sound Power Levels of Discrete-Frequency and Narrowband Noise Sources in Reverberation Rooms.” Specific requested criteria included that the room be large enough to accommodate typically large mining equipment and that there be adequate acoustical modal density in the frequency range of interest. It was also required that the room dimensions would support broadband acoustical measurements down to the 100 Hz third octave band. With the closing of the Bureau of Mines in 1996, this reverberation chamber lay dormant for a number of years. However, this facility is now part of the National Institute for Occupational Safety and Health’s (NIOSH’s) Pittsburgh Research Laboratory (PRL). One of NIOSH’s missions is to decrease noise induced hearing loss among the nation’s mineworkers. To this end, PRL researchers are refurbishing and instrumenting this reverberation chamber to facilitate assessment of the various engineering controls being developed to decrease the sound power level emissions of mining equipment. Early on, this work has sought to bring the chamber up to the level of ISO 3743-21 standard for engineering grade measurements for broad-band noise, with a longer range goal of improving the lab to meet the requirements of ISO 37412standard for broadband precision grade measurements. 2. DESIGN OF THE REVERBERATION CHAMBER The nominal room dimensions are 18.3 meters long by 10.3 meters wide with a height of 6.7 meters. The chamber volume is 1,286 cubic meters. These dimensions are important, as few reverberation facilities are large enough to test typically huge mining equipment. The floor is poured concrete with a sealer to make it acoustically reflective. The walls are built of hollow concrete block, 16 inches long x 8 inches high by 12inches thick. Each block has two hollow cores. The hollow block cores were completely filled with concrete with horizontal and vertical steel reinforcing bars 16 inches on center. This was done to supply a

By R. Pallini

The Franklin Research Center (FRO, a Division of The Franklin Institute was contracted by the U.S. Department of Energy to investigate the application of elastomeric materials for the purpose of smoothing loads on dragline thrust rollers. This report covers the design, fabrication, and demonstration of a portable- device for measuring roller circle track deviations and roller loadings. The results of this work are documented in this report. During December 1981, a field test group visited the Falkirk Mine, Underwood, North Dakota for the purpose of demonstrating the use of the designed and built device. The data collected indicates the practicality and usefulness of such a device for determining actual dragline roller circle roller loads, while the dragline is in service. The device was useful in determining roller load peaking that may be a contributing Factor in the premature failure of these thrust rollers. The device could be used for field evaluation of elastomeric cushioning materials used to eliminate these load peaks.

Jan 1, 1982

By Joseph A. DeCarlo

Coal is one of our basic sources of energy (heat, power, light) and an essential raw material for most metallurgical processing. More than two-thirds of the electricity currently produced by thermal powerplants in the United States is generated by coal, and it is expected that coal will continue to be the major fuel used for this application at least through the 1970's. Coke produced from bituminous coking coal is the principal fuel of iron blast furnaces, and unless ironmaking methods change greatly, coal will continue to maintain a dominant role in iron-ore processing and steel manufacturing for many years. Export markets are expected to increase also, adding to the overall increased demand for coal. Accompanying the increased total requirement, however, will be an ever-increasing demand for higher quality coals, specifically, coals of low sulfur content. Except for special-purpose coals such as those used for the production if coke and ceramics where impurities in the fuel contaminate the end product, the amount of sulfur in coal has been only a minor consideration in the selection of a particular coal for fuel. Major considerations, particularly for the coals used to generate electric power, have been availability and cost. An increased national concern for control of air pollution, however, particularly that resulting from the emission of sulfur oxides in stack gases to the atmosphere, has led to more rigid specifications on coal quality, and the sulfur content of coals will become increasingly more important in the future.

Jan 1, 1966

By Joseph A. DeCarlo

Coal is one of our basic sources of energy (heat, power, light) and an essential raw material for most metallurgical processing. More than two-thirds of the electricity currently produced by thermal powerplants in the United States is generated by coal, and it is expected that coal will continue to be the major fuel used for this application at least through the 1970's. Coke produced from bituminous coking coal is the principal fuel of iron blast furnaces, and unless ironmaking methods change greatly, coal will continue to maintain a dominant role in iron-ore processing and steel manufacturing for many years. Export markets are expected to increase also, adding to the overall increased demand for coal. Accompanying the increased total requirement, however, will be an ever-increasing demand for higher quality coals, specifically, coals of low sulfur content. Except for special-purpose coals such as those used for the production if coke and ceramics where impurities in the fuel contaminate the end product, the amount of sulfur in coal has been only a minor consideration in the selection of a particular coal for fuel. Major considerations, particularly for the coals used to generate electric power, have been availability and cost. An increased national concern for control of air pollution, however, particularly that resulting from the emission of sulfur oxides in stack gases to the atmosphere, has led to more rigid specifications on coal quality, and the sulfur content of coals will become increasingly more important in the future.

Jan 1, 1966

By K. Thirumalai

This study forms a part of Bureau of Mines continuing research on thermal fragmentation of rocks. The potential of internal heating techniques for rock fragmentation is discussed. A concept of rock fragmentation with rapid inter¬nal heating of a localized inner volume of rock is advanced The energy required for rock fragmentation is made available by creating a required vol¬ume of localized heating. The fragmentation of rock using the proposed method is analyzed and qualitatively compared with the results of experiments with Charcoal granite, Dresser basalt, and Sioux Quartzite. High frequency dielec¬tric heating below 100 MHz is used in the study as an example to examine the working of the concept. The selection of operating conditions for the experi¬mental heating is explained. Fragmentation of Charcoal granite and Dresser basalt test rock blocks is achieved for localized heating below 550° C. The heated volume required for fragmentation is observed to be less than 2 percent of the test block volume and the electrical energy consumption is in the order of 3.5 to 6 5 kwhr/m3 The method offers a possibility for in situ fragmenta¬tion of rocks, favorable for internal heating. The method is highly dependent on rock properties governing internal heat generation The efficiency of dielectric heating in the experimental study is observed to increase with increasing dielectric loss of the rock material, ranging from about 10 percent in Sioux. Quartzite to about 50 percent in Dresses basalt

Jan 1, 1970

By R. V. Ramani

The overall objective of this study is to contribute to the reduction of the quartz dust hazard in underground coal mines through a better under-standing of the sources and generation of airborne quartz dust. This research project has included extensive experiments in operating mines for (1) documenting the amount of respirable dust generated, (2) measuring the quartz levels in host materials and airborne dust, (3) analyzing the size consist of the mined product, and (4) establishing the relationship between these various parameters. The results of the study indicate that the amount of dust airborne is a very small fraction of the total (<15 µm) and respirable (<7 µm) dust generated. The quartz contents in the total and respirable airborne dust are generally higher than those in the host material. Quartz content increases with decreasing size of airborne dust. Also, it appears that rock breaks into a different size distribution than coal. The source of quartz for the airborne dust appears to be the roof and the floor material rather than the coal seam itself. No general relationship is found between the Hardgrove Grindability Index and either the quartz content in the airborne dust or the amount of respirable dust. Extended descriptions of the experimental design, the experiments, data analyses and results are presented in this volume. The raw data, the data management computer program and the sampling and safety plan are presented in Volume 2.

Jan 1, 1987

By Terry J. Close

At the request of the Idaho Congressional Delegation, the U.S. Bureau of Mines Western Field Operations Center investigated the mineral resources of the 140,000-acre Red Mountain roadless area, located northeast of Lowman, in central Idaho. The area includes U.S. Forest Service Red Mountain RARE II area (4-063). The highly mineralized road less area is underlain mainly by granitic rocks which are transected by a northeast-trending fault system. Associated with the granitic rocks and fault system are gold and black sand mineral resources. At least 1,395 ounces of gold and 858 ounces of silver have been recovered from six lode and placer mines within the roadless area. A total of 525 tons of columbium-tantalum, 2,049 tons of euxenite, 83.5 tons of columbite, and 54,862 tons of ilmenite concentrates have been produced from a nearby black sand mineral placer. Mine workings and facilities in the roadless area consist of two dismantled mills, 15 open and caved underground excavations totaling about 2,900 ft of workings, seven large pits and quarries, over 166 smaller dozer-and hand-dug prospect pits and trenches, and in excess of 66 drill holes. The mineralized sites in the roadless area are covered by about 690 current (1992) mining claims. About 130 individuals are holding the claims. Also within the roadless area are four patented lode mining claims. Twenty-five of the 54 mineralized sites identified during the study of the roadless area have, or likely have, mineral resources. One has gold and three have black sand mineral resources that are marginal or subeconomic. There also are two hot springs with thermal water resources, some veins that could be sources of decorative quartz, and dimension stone which was quarried in the past and could be again.

Jan 1, 1993

By Open-Hearth Steel

We have a very high-powered organization up here this morning, headed by Kenneth C. McCutcheon, general superintendent of the Ashland Division of the American Rolling Mill Company, and L. A. Lambing, superintendent of the open-hearth and Bessemer departments of Jones & Laughlin Steel .corporation of Aliquippa, Pa., representing the open-hearth men. Also, we have P. F. Dolan, assistant general manager of the Maryland plant of Bethlehem Steel Company, of Sparrows Point, and Harry E. McDonnell, superintendent of blast furnaces of Weirton Steel Company of Weirton, W. Va., representing the blast-furnace section.

Jan 1, 1940

By C. H. Hopper

DIAMOND drills have played an important part in the development of mines for many years. Recently. they have also entered the production field arid the use of diamond drills m the primary breaking of ores has become a big branch of the industry. The writer has estimated roughly that the total diamond drilling in Canada during 1942 was 5,625,000 feet, of which 2,250,000 feet, or 40 per cent, was for blast-holes. This accounts for 6,750,000 tons of ore broken, using the conservative factor of 3 tons per foot of hole. Many consider that the use of diamond drills for blast-holes is the most important advance in mine practice of the past decade, and the industry owes a great deal to the staff of Noranda Mines, Limited, for the pioneering they have done in the development of long-hole mining methods and for publishing their results. A great many mines have followed Noranda&apos;s lead. At the Waite-Amulet, Aldermac, Flin Flon, Canadian Malartic, Hoyle, and Matachewan Consolidated mines, most of the ore is broken by long-hole blasting. While the mines were developing new methods, the diamond-drill industry was developing mechanically-set bits and better machines. Through the effective use o smaller, cheaper diamonds, as well as more efficient setting, bit costs per foot are only one-half what they were ten years ago. Few people realize that a diamond bit is relatively inexpensive. The common EXT coring bit, for example, contains some 200 small diamonds, weighing altogether about 2.70 carats which, at $3.40 per carat, cost $9.20. After use, the equivalent of about 100 of these diamonds may be recovered. Therefore the true cost of the bit is only $7.00, made up of $4.60, the cost of the diamonds lost, $2.00 for setting, and $0.40 tax. The trend in machines has been toward specialization for the different classes of rock in blast-hole work. Light, high-speed machines have been developed for soft ores and higher powered machines for hard ores.

Jan 1, 1943

By H. Allenius

Today, most requests for quotations force all bidders to follow the bid document to the letter. The choice is then made based on price since it is the only difference in technically identical offers. However, clients allow and even encourage suppliers to come up with alternatives. Overall economy again, is much more than low investment costs. Operation costs may affect the economy of a project far more than a million saved in investment. There is almost always the choice: Should we go for the cheapest solution or should we go for the most economical solution? We strongly recommend the study of the long-term effects on investments as they often surpass any apparent savings in capital expenditure. With high operating costs, time is never on our side. Almost all relevant inputs and outputs in a concentrator have a value that can be expressed in a suitable currency. Unit prices may differ substantially from one operation to the other, but everything has a price tag and globalization has made it possible to move both goods and services to almost any place in the world. Even a rather simple exercise can give a good indication on which process improvements are worth pursuing when investing in new systems or equipment. The ones that do not qualify, as money earners, will naturally appear as clearly. No ?rocket science? is required, just process knowledge, practical experience, an open mind and some common sense and the reward may be the difference of having a good or an excellent project. The model we have developed can be made as detailed as required, but we do not recommend too much detail, as it sometimes tends to distract work and obscure the important issues. Two different process solutions compared in a simplified example illustrate the importance of evaluating a project based on its long-term effects rather than focusing on the investment costs only.

Jan 1, 2014

By Leonard Kellogg

Introduction The Mine Manager or Superintendent is a man faced with a difficult problem. He must direct the activities of his mine in such a manner that, over a period of time, the greatest possible profit will be realized. In this respect, a mine is no different from any other business organized for profit. What makes the mine manager&apos;s task so difficult then? The profit of a mine is determined by -among other things -the grade of ore, the market price of the metal or metals produced, and the cost to operate the mine. The mine manager cannot do much about the first two of these, but he has considerable control over the operating cost. This cost will depend in large measure on the nature of the programme on which he plans to operate his mine. This operating plan may be broad or specific, on a long-range basis or on a month to month basis. The development of the plan, however, is made very difficult by the existence of so many factors which must be considered. These may be (1) factors requiring decisions based on the best judgment and interpretation of engineering and geological information available, or (2) items which must be considered but which are largely non-controllable. To be more specific, examples of such items would be: (1).-(a) The amount and type of development work to be done in relation to amount of ore to be mined. (b) Mining methods to be followed and type of equipment to be used. © Amount of exploration diamond drilling to be done if the orebody is not clearly defined. (2).-(a) Rock type and degree of hardness. (b) Width, depth, and pitch of veins. © Amount of work requiring timbering. (d) Faults encountered. (e) Water and gas conditions met with.

Jan 1, 1952

By Benjamin A. Allou

La seule production de columbo-tantalite réalisée en Côte d?Ivoire est effectuée dans les éluvions et alluvions du gisement d?Issia, localisé au Centre- Ouest de la Côte d?Ivoire. Même si un lien direct avec les roches granitiques est suspecté, aucune attention particulière n?a été accordée à l?étude de la minéralisation primaire en relation avec l?évolution des granites et des pegmatites associées. Ces granites, qui sont en intrusion dans les roches sédimentaires du Protérozoïque inférieur, comprennent : le granite orienté à deux micas (Lobo), le granite orienté à muscovite dominante (Bitapia) et le granite porphyroïde non orienté à muscovite dominante (Issia). Les résultats de cette étude indiquent que les pegmatites à Nb-Ta-Be-Li sont liées au granite de type Issia. Les observations de terrain et les analyses sur roche totale des granites montrent que: (1) les trois principaux granites de la région sont de type- S et peralumineux dans leur composition, (2) ils ont été formés pendant la période tardi- à post-Éburnéenne, (3) le granite d?Issia fut le dernier à se mettre en place. Les pegmatites liées au granite d?Issia ont été classées en quatre types : Apegmatites stériles; B- pegmatites à Be; C- pegmatites zonées à Nb-Ta-Be; et Dpegmatites zonées à Be-Nb-Ta-Li. Les pegmatites zonées comprennent les zones suivantes: I- zone à aplite; II- zone graphique; III- zone à albite, microcline, béryl et columbo-tantalite; IV - zone à microcline massive; V- zone à quartz. Ces pegmatites présentent une zone d?altération métasomatique externe. La minéralisation de Ta, Nb et Be est liées à la zone III des pegmatites de types C et D, et celle de Li est associée aux greisens qui peuvent être observés n?importe où dans les pegmatites de type D. Les données de terrain et de laboratoire indiquent que le granite d?Issia est la roche parentale des pegmatites à éléments rares de la région. Les pegmatites minéralisées sont distribuées seulement autour du granite d?Issia et les patrons des terres rares des granites et ceux des pegmatites à éléments rares sont très similaires. Les analyses de la composition totale montrent un lien génétique entre le granite d?Issia et les pegmatites à éléments rares. Il est suggéré que les pegmatites ont été formées durant le transport et la cristallisation fractionnée du magma.

May 1, 2003

By Stanley Lefond

The health of the industrial minerals industry appears to be improving year by year. In fact, preliminary information suggests that only three commodities (asbestos, bromine, and sulfur-other) failed to show a production gain in 1984 over 1983. In 1983, 10 commodities failed to post gains and six remained constant. The total value of industrial mineral production in 1984 was $16.7 billion, up $1.4 billion from 1983. Cement, clays, gypsum, and crushed stone production increased. Since these are used in housing and construction, this suggests these industries are moving out of their recent doldrums. Barite and clay production also improved in 1984. Local US barite production, however, still has a long way to go to attain the 1981 high of 2.6 Mt (2.9 million st). Barite imports remain high. But they are not as high as the 1982 importation of 2 Mt (2.3 million st). Bentonite production in 1984 increased 20% in volume and 25% in value over the previous year. This suggests improvement in the drilling industry and an increase in demand for iron ore pelletizing grades. All in all, the industrial mineral industry is alive and well. And it shows signs of great improvement in the next few years. Many thanks to those industrial mineral technical committee members for their assistance in assembling this industrial minerals review: M. F. Dibble, H. W. Allen, G. H. Gallup, S. G. Ampian, D. W. L. Spry, H. G. Fleshman, and L. J. Czel.

Jan 5, 1985

By E. S. Leaver

The gold that is not recovered by a particular process is refractory to that process. A study to determine how the refractory gold is associated in the ore constituents usually indicates the needed improvement in the process or additional methods for bettering recovery. In preliminary work, segregation of the various minerals, gangue or impurities, and data showing the reactions of each in the recovery method readily confines the refractory gold to a certain part on which extensive research can be directed. In this dissecting plan, the microscope is an important aid in locating the refractory gold but should be supplemented by chemical tests.

Jan 1, 1934

By Oliver Bowles

"Asbestos" is a name applied to a group of naturally fibrous minerals. Because it has the fibrous characteristics of silk or cotton and at the same time will not burn and has other advantageous properties as well, asbestos has specialized uses for which no adequate substitutes are yet available. Asbestos furnishes a major raw material for a great variety of essential products, the manufacture of which constitutes a vast industry. Asbestos generally occurs in irregular veins scattered throughout rock masses. Most of the fiber-bearing rock is mined in huge open-pit or underground workings. The fibers are separated from the rock and sorted into groups, according to length, in large, complex mills. The longer fibers are stripped to textile plants, where they are further fiberized and prepared in special machines and are used for spinning into yarn and weaving into unburnable cloth that has many important uses. The shorter fibers, which are much more abundant than the long ones, are used in asbestos-cement building materials in heat-insulating products and for a multitude of other applications. These processes and uses are covered in considerable detail in later sections of this report.

Jan 1, 1959

By S. Chatterjee

Mining represents the first industry in the United States to have federally mandated training for its workers. Although the topics to be covered are specified in 30 CFR 48, course content is left to the discretion of the individual mine operators. Unfortunately, certain topics to be covered, such as prevention of accidents, are usually mine specific, making it very difficult to use the same course content in every mine. Further, training requirements may change from one mine to another, due to changes in the demographic characteristics of the workers, or changes in job and organizational characteristics. Therefore, if the intention of safety training is to modify the behavior of workers, it is expected that there should be some changes in the content of safety training programs in order to address the specific requirements of individual mines, or at least mines of similar worker and job characteristics. This study has been directed towards the development of a systems-based procedure for conducting a needs analysis. A special effort was made to understand the psychology of worker behavior by analyzing worker reactions to various organizational and job characteristics.

Jan 1, 1994

By T. Y. Chong, J. A. Folinsbee, M. Fullam

PJV Dome Mine has used some form of gravity concentration since start of milling operation in 1909 to recover free milling gold. Amalgamation and conventional jigs were first used and were replaced with the Knelson Concentrators in 1993. The Knelson concentrates produced were upgraded on a conventional shaking table prior to smelting and refining into gold bullion. In May 2002, a comprehensive review of the latest gravity recovery technology led to a piloting campaign aimed at further improving gravity recovery. The Knelson concentrates, subjected to intensive cyanide leaching in an Acacia pilot plant, were found to be amenable to intensive cyanide leaching with over 98% of the gold being dissolved within 24 hours. An Acacia Intensive Leach Reactor was successfully installed and commissioned in December 2002, to replace the shaking table. his paper describes pilot scale testing, installation, commissioning and optimization of the Acacia Intensive Leach Reactor. Also, the economic impacts on down stream processes are highlighted.

Jan 1, 2005

By S. M. Moore

Each year more than 400 coal miners are injured (fatal and non-fatal) by rock falling from between or around roof supports. Many of these injuries could be prevented by the installation of roof screen (wire mesh). However, many coal mines may be reluctant to use roof screen because of the added cost. The goal of this study was to determine the potential saving in workers? compensation (WC) premiums that could be achieved due to a reduction in rock fall injuries after roof screening. The WC rate-setting methods utilized by IL and KY were investigated in this study. Using actual data obtained from MSHA, national and state WC bodies and individual insurance companies (e.g. average cost per injury, loss cost rate, number of injuries per year, number of injuries preventable each year with roof screening), hypothetical mines (representing two mine sizes: 67 and 150 employees) were constructed with realistic ranges for estimates of injuries and WC premium costs. Using each state?s actual WC rate-setting formulas, total savings in WC costs after a three-year period were determined. Across both states, savings in WC premiums ranged from 1.8% to 14.6% when injuries were reduced by 10% to 20%. Additionally, an economic analysis was performed for an existing mine consisting of 67 employees. For this mine, the annual cost of a roof screening program was estimated at $240,000. If the screening prevented 13 ?struck by? WC claims over a 3-year period at this mine, the reduction in WC premiums alone could pay for the entire screening program.

Jan 1, 2010

By E. Van Der Lingen, A. Paton

"The South African ferrochrome industry has been faced with various challenges during the past few years, such as influences from the market, manpower strikes, China’s control over the demand for both ferrochrome and chrome ore from South Africa, and electricity supply constraints, placing increased pressure on the local industry to improve output in order to remain globally competitive. The year 2016 brought a dramatic change in the local ferrochrome industry, being marked by higher chrome ore prices and the takeover of some idle smelters.This study investigates the methods of technology acquisition used in various parts of the ferrochrome smelter value chain throughout a business cycle, and whether there is a preference for a specific acquisition in an explicit part of the value chain. The study also considers whether companies prefer to partner with local or global institutions for collaborative development, and the methods used by companies to protect their technologies.IntroductionSouth Africa holds the world’s largest chrome ore reserves and was the world’s largest producer of ferrochrome until 2012, when China became the leading ferrochromeproducing country. South Africa’s chrome ore (chromite) supply into the Chinese market has risen significantly from 36% of China’s total chrome ore imports in 2010 to more than 70% in 2016 (Fowkes, 2014; Creamer, 2017). China controls the demand for both ferrochrome and chrome ore from South Africa (Fowkes, 2014). China’s high import rate of chrome ore has resulted in ferrochrome prices being driven down and a depressed ferrochrome market. The South African industry meets the chrome ore demands but loses out on ferrochrome beneficiation.Furthermore, the local ferrochrome industry has been threatened with the overall poor competitiveness over the past few years, resulting in four of the eight producers ceasing operations at the beginning of 2016. Challenges experienced by the producers, some of which are directly linked to production costs, include influences from the market, productivity that was affected by wildcat strikes, China’s control over the demand for both ferrochrome and chrome ore, and electricity supply issues (Fowkes, 2014, 2013; Biermann, Cromarty, and Dawson, 2012). Ferrochrome production is energy-intensive and South African producers have faced higher electricity costs, electricity supply complications, and a weaker currency exchange rate since 2009."

Oct 1, 2018

By Edward Skolnik

The concept of using fluid dense media to separate heavy ore constituents from the lighter gangue dates back to 1858, when it was patented by Sir Henry Bessemer. Its use in coal did not begin until after 1917, however, when Chance invented a process (which bears his name) in which lighter coal floats in a suspension of fine sand in water while heavier refuse sinks. Conklin used fine magnetite in suspension as a medium for floating anthracite in 1922, DuPont designed a plant employing organic liquids in 1936, and Belknap used a solution of calcium chloride (which Bessemer had suggested, among other media) in 1937. Separation of magnetic media by magnetic separators from the suspensions in which they were carried was introduced in a Butler Brothers ore concentrator pilot plant in 1937. The media first used were crushed steel and various grades of ferrosilicon. Since 1940, when American Cyanamid used magnetic separators to recover and clean magnetite in a coal washing circuit, virtually all heavy medium circuits have been of that type. As long as static baths are employed, as they are in the processes above, coal smaller than 9.5 mm or 6.4 mm cannot be cleaned economically in heavy medium. The settling velocities of the fine material are very low, and consequently the time required to separate the lighter coal from the heavier refuse becomes excessive. In addition, stray currents in the vessel and the upward velocities induced to keep media solids in suspension affect the finer solids to the degree that physical size becomes a more important factor than specific gravity.

Jan 8, 1980