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By R. Pazhianur

Cathodic Protection using impressed current was employed to reduce the wear of the mill wall during ball mill grinding. Wear rates were determined from weight-loss measurements made on four, I-inch diameter, mild steel coupons installed flush with the interior surface of the mill wall. Grinding tests were conducted on quartz, pyrite and quartz-pyrite mixtures at pH values of 4.6, 7.1 and 9.2. Cathodic protection was shown to reduce mill wear by 40-60010. An examination of the coupon surfaces using scanning electron microscopy showed that cathodic protection greatly reduced pitting corrosion. The presence of a sulfide mineral, such as pyrite, was found to increase the wear rate significantly, possibly due to galvanic interactions. It is also found that the wear rate is significantly higher than the sum of abrasive wear and the corrosive wear, due to a synergistic effect.

Jan 1, 1996

By Nick Barton, Khosrow Bakhtar

Two plate load tests of 1 m2 area were per- formed in a pressure tunnel at the Rocky Mountain Pumped Hydro Project. Reaction for the 250 tons loading in this 10.5 meter diameter tunnel was provided by deeply anchored, diverging rock bolts, eight per load frame. Pressure build up to 2.5 MPa was provided by circular flatjacks. Rock deformation was recorded at depths of approximately 0.3, 0.9, 1.2, 1.8 and 3.3 meters using axially located multiple position borehole extensometers. The tests were performed in representative units of massive shale and thin to medium bedded siltstone. Stress was built up in four equal steps to 2.5 MPa, held constant for one week for creep monitoring, then unloaded, reloaded and unloaded over a period of one day. The most deformable unit was the massive shale which exhibited 1.3 mm of permanent deformation at the surface after the first loading and unloading cycle, which included 0.36 mm of creep. After the second loading-unloading cycle, the permanent deformation had increased to 2.6 mm, following a maximum deformation at the surface of approximately 3.6 mm. At the maximum stress of 2.5 MPa, the tangent modulus averaged 0.4 GPa when loading, and 0.3 GPa when unloading. The thin to medium bedded siltstone exhibited only 0.17 mm of permanent deformation at the surface after the first loading-unloading cycle, which included only 0.05 mm of creep. After the second loading-unloading cycle, the permanent deformation had increased to 0.27 mm, following a maximum deformation at the surface of approximately 0.53 mm. At the maximum stress of 2.5 MPa, the tangent modulus averaged 6.2 GPa when loading and 21.9 GPa when unloading.

Jan 1, 1986

By D. E. Munson

The Waste Isolation Pilot Plant (WIPP) Project was established in 1975 by the Department of Energy (DOE) under Public Law 193 to develop the necessary technology for safe disposal of radioactive wastes from the defense programs of the U.S. The technology development involves extensive theoretical, numerical, and laboratory programs, as well as construction of the WIPP facility in southeastern New Mexico for in situ testing and waste storage demonstration. If after the demonstration, the facility is found to comply with regulations of the Environmental Protection Agency, then the facility will become a repository for Transuranic (TRU) waste. The underground portion of the facility is 655 m beneath the ground surface in massive natural salt deposits of the Salado formation, as shown in Figure 1.

Jan 1, 1991

By J. C. Motz

With ore grades continually decreasing and finer mill feed requirements, it becomes increasingly important to have the capability to handle the larger tonnages economically. With the 10 ft. Standard capable of reducing 1,633 metric tons per hour (1,800 short tons per hour) to 80 percent passing 50 MM (2 inches) and the 10 ft. Short Head following to produce a mill feed of all minus 12 MM (1/2 inch) at a rate of 608 metric tons per hour (670 short tons per hour), we feel significant savings will be realized in installation and operating costs. The largest concentration of cost in the secondary and tertiary crushing stations is the building itself that houses the process equipment. An example of the savings to be attained can be seen from an examination of a 40,820 metric ton (45,000 short ton) per day crushing plant. A total of nine 7 ft. Symons Cone crushers is used to generate 12 MM (1/2 inch) ball mill feed. The average center distance between units is 6.1 M (20 feet) and allowing a 6.1 M (20 feet) nominal distance from the centerline to the wails, the building becomes 61 M (200 feet) long. For this production rate, four 10 ft. Symons Cone crushers would be required. With the 10 ft. crushing plant requiring a center distance spacing of 7.6 M (25 feet), the building length totals approximately 38 M (125 feet). Construction costs vary depending on location, but a reduction in building length from 61 M (200 feet) to 38 M (125 feet) will result in estimated savings of between $2,500,000 and S4,000,000. While the 10 ft. Symons Cone has been involved in a number of feasibility studies for some time, the Process Machinery Division of Rexnord commenced detail design work on the 10 ft. Cone crusher in 1972. For comparison purposes, the 10 ft. and 7 ft. capacities are given in Table 1.

Jan 1, 1978

By Fitzhugh T. Lee

Results from the first part of a two-part investigation of the stability of highwalls in open-pit coal mines in the Paleocene Fort Union Formation of the western Powder River Basin of Wyoming and Montana indicate that these highwalls are subject to time-dependent deformation. Field investigations and laboratory physical properties tests of coal and overburden rocks suggest that several factors influence highwall stability. Some of these factors are rebound of overconsolidated rocks, desiccation, water, orientation and spacing of fractures, and strength and deformation properties. Factors of safety for a typical highwall in the study area (calculated by the finite element method) may be less than 1.0 when open fractures are present and the highwall has degraded. Although it is concluded that most open-pit highwalls in the Fort Union Formation within the study area have adequate short-term stability, these highwalls do deteriorate and become progressively less stable. Because of this, postmining failures are common and stability conditions could become critical if mining were delayed and then resumed after a period of several months. The second part of the investigation will utilize field measurements of rock-mass properties and instrumentation of actively mined highwalls to obtain data for comparison with the results of the initial investigation. Because the height of highwalls will increase as the more shallow coal is exhausted, these data will also be used to predict the behavior of slopes higher than those presently found in the western Powder River Basin.

Jan 1, 1977

By William D. Cassity, Batric Pesic

Introduction Gold mining on an industrial scale is becoming increasingly dependent on low-grade refractory sulfide ores that contain large amounts of pyrite (FeS2) and arsenopyrite (FeAsS). Gold is usually present as sub-microscopic particles dispersed evenly throughout the sulfide matrix. Without some form of preoxidation, efficient recovery of this finely disseminated gold through cyanide leaching is hampered because the surface of the sulfide particle becomes passivated and impervious to penetration of the cyanide. Traditional methods for recovery of refractory gold from arsenopyrite ores include roasting and autoclave leaching. A modem alternative method is to preoxidize the mineral using the bacterium Thiobacillus ferrooxidans prior to attempting to re- cover the gold through cyanidation. The bioleaching of arsenopyrite ores presents a problem in the mobilization of large quantities of arsenic, present both in the +5 and +3 ionic states. Dissolved arsenic species pose a dual problem: they are -1 to the bioleaching process itself through inhibition of bacterial activity, and they also pose an environmental threat. The arsenate anion (AsO2) has been shown to be more toxic to living organisms, including Thiobacillus ferrooxidans, than the arsenate anion (AsO43) (Collinet and Morin, 1989). There is some disagreement as to the fate of arsenite in bioleaching solution. Mandl, Matulova, and Docekalova (1992) reported that dissolved arsenite concentration was constant during a long-term bioleaching study of chalcopyrite using Thiobacillus ferrooxidans. Torma and Oolman (1992) report that Thiobacillus ferrooxidans has been found capable of oxidizing As3+ to As5+. The solid products of arsenopyrite/pyrite bioleaching include jarosite (MFe3(SO4)2(OH)6 where M can be K+, Na+, NH4+ or H3O+), scorodite (FeAsO4.2H2O), ferric hydroxides, and ferric hydroxysufites (Carlson et. al., 1992; Van Breemen, 1982; Lazaroff et. al., 1982). The particular species formed is a function mainly of pH. Little study has been performed on the effect of solid products on bioleaching. Pesic and Kim (1993) showed that Thiobacillus ferrooridans cells served as a nucleation sites for jarosite particles, which grew rapidly and eventually killed the cell. Most bioleaching studies are performed at low pH&apos;s (<2.0) under uncontrolled conditions. By studying bioleaching of arsenopyrite under controlled pH conditions and at higher pH&apos;s (2.0 to 3.0), the distribution of iron and arsenic between the solid and liquid phases can be controlled, and their effects studied. The objective is to develop a bioleaching process that operates at high efficiency while keeping amounts of dissolved heavy metals low. Pesic, Stohok, and Torma (1993) demonstrated the usefulness of this approach in the leaching of cobaltite (CoAsS) concentrates. Materials and Methods The focus of this study was the bioleaching of arsenopyrite ore by Thiobacillus ferrooxidans. The parameters studied included the effects of pH, and the effects of arsenate (AsO43 and arsenite (AsO2). pH studies included pH set initially but not controlled, and pH controlled continuously with NaOH or LiOH. Bioleaching results from two strains of Thiobacillus ferrooxidans with different adaptation histories were compared. Bioleaching experiments were conducted in 150 ml stirred glass reactors using a strain of Thiobacillus ferrooxidans adapted to arsenopyrite ore, supplied by the U.S. Bureau of Mines. These bioreactors were innoculated with stock cells grown on arsenopyrite ore for 14-16 days. Typical elemental breakdown for this ore was: As, 9.90%; Fe, 18.60%, Stot, 9.26%; Suifide, 7.50%; SiO2, 25.90%. Mineralogically, the ore contained both arsenopyrite and pyrite. Bioleaching efficiency was measured by withdrawing bioleach solution samples at selected intervals, filtering, and measuring the amount of dissolved cobalt, iron, and arsenic by atomic absorption spectroscopy. Because the precipitation of iron and arsenic at higher pH&apos;s would lead to unreliable results, a 100 mg tracer of cobaltite concentrate from the Blackbird mine in southern Idaho was added to solution. Dissolution of the cobaltite tracer was used to indicate bioleaching of the bulk arsenopyrite ore. Cobalt has been shown to be stable in solution even when iron and arsenic were precipitated (Pesic, Storhok, and Tonna, 1993). The pa-

Jan 1, 1995

By M. De Ladurantaye-Noël, M. Laliberte

Selenium is widely present in the environment at usually low concentration. It is mostly present in the environment at higher oxidation states, i.e., as selenites and selenates (+4 and +6 oxidation states respectively). It is rarely present as selenides (-2 oxidation state) and almost never found as the element (0 oxidation state). Selenites and selenates are generally soluble in water, as such they cannot easily be removed from water by precipitation. Selenites can form insoluble surface complexes with metal oxides such as ferric hydrous oxide and manganese hydroxide, and can therefore be removed that way. However, it is not so common to find selenites in natural environments as it will quickly oxidize to selenate in aerobic conditions. Elemental selenium can also be produced from selenites or selenates by using a reduction process; the end product is a colloid that can be destabilized to be removed from water using the right chemistry. Because of their toxicity, selenides need to be handled with care, and if present, it can form insoluble metal salts or, at low pH, be emitted as H2Se, a noxious and highly toxic gas. The most favorable selenium forms for its removal are therefore selenite and elemental selenium. Organo-selenium species play a crucial part in understanding selenium toxicity. There are a great number of these species. Basically, any organic species that contains sulfur has an equivalent that contains selenium. Of particular interest is the substitution of sulfur by selenium in the two sulfur-bearing amino acids, cysteine and methionine, resulting in selenocysteine and selenomethionine. Methylseleninic acid (CH3SeO2H) is another organo-selenium specie which has been detected in water treated by biological processes and which is biologically active.

Feb 1, 2023

By B. J. Scheiner

The Bureau of Mines, Department of Interior, has been investigating the use of polyethylene oxide (PEO) as a flocculant for a variety of mineral wastes. As part of this investigation, the hydrodynamics of the flocculation process were studied. It was determined that to obtain optimum dewatering results, the factors of energy input into the stirrer, polymer concentration, and polymer dosage must be considered. Lt was found that at a given polymer concentration, there was an optimum stirrer speed and polymer dosage. With excess PEO, the amount of water removed in the trommel decreased, but no evidence for steric stabilization was observed.

Jan 1, 1987

By Paul Lenchuk

The concrete block industry can trace its beginnings to the early 18th century when most of the experimental work was being carried out in England. In America, the concrete masonry industry got started shortly after the Civil War and has had sometimes a spectacular, but always a steady growth since then. From a few hundred thousand concrete masonry units made in the late 19th century, the industry today produces approximately 3.3 billion concrete masonry units making it the largest industry in the United States for masonry wall materials. There were 3500 concrete masonry plants manufacturing these units after World War II. This nwnber decreased to 1500 by 1970 although the production of concrete masonry units increased by over 100% in the last 25 years. One of the reasons for this increase in production by less than half of the plants which once existed, is the tremendous strides made in automation of production machinery. The concrete masonry plant of today is probably one of the most automated small businesses in existence. Within a few years it will be possible to push a button and the entire manufacturing process will begin with no human hands touching the units until they are laid up in a wall at the manufacturing plant and are trans-ported to the job site. No other concrete product has reached this state of manufacturing sophistication and it is one of the reasons why the price of a concrete masonry unit is not much different than it was 2S years ago. Because of automation, it is safe to say that concrete masonry will remain a highly competitive building material for many years to come. The problems that beset the continued use of this material are not related to the cost of manufacturing and delivering the product. What are some of the problems as well as some of the opportunities and is the golden age of opportunity knocking rather than a wolf in disguise? The use of concrete masonry has grown each year because a larger unit was provided at less cost than other competitive materials. Concrete masonry has had severe competition from the clay products industry as well as masons who laid the product in the wall and who resisted concrete masonry for many years. Even today

Jan 1, 1971

By Andrew W. Jenike

The first significant studies of problems related to the handling and storage of bulk solids were reported at the end of the nineteenth century. Early work originated from the necessity to store large quantities of grain and was concerned mainly with pressures affecting the structural design of silos and bins. The Janssen formula (about 1898) and the work of Ketchum [1] form the highlights of that period. In more recent years, progress in mechanization, increasing plant capacities, and rising costs of labor have focused the attention of engineers on the problem of gravity flow of bulk solids. A great deal of work has been done with models, and flow patterns obtained in small scale bins have been described in several publications, for example [2, 3]. An attempt to develop a theory of flow of bulk solids was reported in 1954 [4, 5, 61 and presented to the in a paper [7] in the same year. AIME recommended the continuation of this work and, as a result, Engineering Foundation granted a sum of money to set up a Bulk Solids Flow Laboratory. Two and a half years ago the project gained the support of the University of Utah and a laboratory was set up on the campus of the University. This project is now in progress, it has received the support of the American Iron and Steel Institute and of Kaiser Engineers, Division of Henry J. Kaiser Company. The results obtained to date are summarized in this report.

Jan 1, 1959

By J. A. Marts, R. C. Gilmore, Jr. Bogin, S. Saki, J. F. Brune, J. W. Grubb

"Researchers at the Colorado School of Mines have studied the influence of headgate side ventilation controls near the longwall start-up room on the formation of explosive gas zones (EGZs) in underground coal longwall bleeder ventilated gobs. In a project funded by the National Institute for Occupational Safety and Health (NIOSH) researchers developed a Computational Fluid Dynamics (CFD) model to study the formation of methane-air mixtures in the gob, start-up room, and bleeder entries. The relative change in size and location of EGZs are examined in response to ventilation controls in the headgate side bleeder entries near the start-up room. Modeling suggests that adjustments to the ventilation controls can be made to minimize the size of the EGZ. However the EGZ may form in or around active working areas regardless of the ventilation control settings. Research found regulators on the crosscuts into start-up room and first entry inby can force air into to crosscuts outby the start-up room causing more air to flow through the gob. INTRODUCTION Underground longwall mine ventilation systems may operate with either a bleeder ventilated gob or a progressively sealed gob. Bleeder ventilated gobs must meet the standards for safe operation of the bleeder system by examining bleeder system effectiveness. As defined in the United States Mine Safety and Health Administration (MSHA) Program Policy Letter (PPL) No. P13-V-12 (MSHA 2013), the pillared area or gob is “any area where a reduction in pillar size has occurred during retreat mining” and includes the “caved and broken material, the interstices within the caved material, and the open spaces surrounding the caved material.” The PPL further states that the intended purpose of ventilating the gob is “to protect active workings from the hazards of methane accumulations …” The requirements for operating a bleeder system as stated in The United States Code of Federal Regulations (CFR 2013) Title 30 §75.334(b) is to pass ventilation air though the gob “and to continuously dilute and move methane-air mixtures [from the gob] away from active workings and into a return air course …” where the concentration cannot exceed 2 percent methane (CFR 30 § 75.323). Further clarification of the bleeder system as described in the PPL states the following: “A bleeder system includes the area from which pillars are wholly or partially recovered, bleeder entries, bleeder connectors, and all associated ventilation control devices that control the air movement through the area. Bleeder entries are air courses designed and maintained as part of the bleeder system.”"

Jan 1, 2015

By LeRoy D. Loy

The coal mining industry must be able to compete with alternate sources of fuel for the utility and industrial customers. If the industry is unable to control transportation costs, either due to government interference, or for other reasons, then mining costs must be kept down. Australia, South Africa, Canada, (and later Colombia) will take larger shares of the export market if mining costs are not kept competitive. The government&apos;s environmental regulations must be met without incremental increases in mining costs. Entrepreneurial instincts must be retained by the industry to provide for their investment in the future. What are the Problems with Existing Research? Government funding for coal mining research has been reduced or virtually eliminated. BCR (Bituminous Coal Research) is now a membership- supported organization, but its primary focus has been on health and safety-oriented research. BCR, however, is now attempting to widen its scope and conduct other types of research. There are almost no funds in EPRI&apos;s (Electric Power Research Institute) program for coal mining research. Large mining companies do some research for their own purposes and needs, but this is proprietary and therefore results are not shared with the remainder of the industry. The mining industry as a unit is not undertaking productivity (systems) oriented research in any common or cohesive manner.

Jan 1, 1983

By J. A. Hartlage

Four years ago, Ashland Chemical Company introduced a new copper solvent extraction reagent for hydrometallurgical utilization. The reagent, an alkylated 8-hydroxyquinoline, is called Kelex 100 ®(1). Since the time of introduction, the reagent has been investigated by several workers for its solvent extraction utility in the separation and purification of transition metals (2-7). In the following text, those factors which are essential to the optimum performance of Kelex 100 on concentrated copper solutions will be discussed. To date, the reagent has been found useful in active concentrations of from 1 to 20% Kelex 100. In order to achieve the best performance for Kelex 100, it has been found that a phase modifier is essential. Various alcohols and phenols have been evaluated in order to improve phase performance and stripping characteristics of the solvent system. Modifiers and diluents are especially important when solvent activities between 3 and 20% Kelex 100 are employed. A 15% Kelex 100 solvent, for example, containing 15% nonyl phenol in Escaid 100 (Exxon) can be utilized to remove greater than 33 grams per liter of copper from an aqueous feed without interstage neutralization, and in the same process scheme the reagent can be stripped to less than 1 gram per liter barren organic producing a pregnant electrolyte containing greater than 40 grams per liter copper and less than 120 grams per liter sulphuric acid. The unusually efficient stripping of Kelex is accomplished by protenation of the extractive molecule during the stripping process.

Jan 1, 1974

By M. du Plessis, J. P. Gouvea

Mesh based areal support is commonly applied in South-African deep-level gold mines. It forms part of an integrated support system that can cater for both rockfall and rockburst incidents. The application has been successfully applied over many years. Operations have different geotechnical conditions and practical considerations, which has led to the development of various types of mesh and integrated mesh-based support systems (i.e. meshing and lacing versus welded or chain-link mesh sheets). Typically rolls or sheets of mesh is pinned with rockbolts to the surface of an exposed excavation. Applications vary from hangingwall coverage in a conventional stope gully to complete excavation coverage in footwall drives of conventional mines or on-reef drives in massive mechanised mines. It is however critical to understand the behaviour of the rockmass and apply suitable support systems.

Apr 26, 2022

In addition to capital and operating costs, the accurate estimation of ore reserves is a key component to the successful outcome of any feasibility study. In recent years, there have been many project failures caused by the improper estimation of ore reserves. The accurate estimation of an ore reserve relies heavily on an understanding of the geology and the underlying assumptions unique to the estimation techniques being used. These assumptions are reviewed, and the shortcomings of the estimation techniques and their possible impact on a given project&apos;s economics are discussed. The latest developments in geostatistics in overcoming existing reserve-estimation problems are presented.

Jan 1, 1998

By Jerry Dolph

In September 1855, 55-year-old Noah Spencer Kellogg and his $3 jackass named Bill discovered what was known at the time as a “blow out” of high-grade lead and silver ore. The deposit was located on the side of a North Idaho mountain high above the South Fork of the Coeur d’Alene River. The ore was later assayed at 41,141 g/t (1,200 oz/st) silver. Kellogg named his discovery the Bunker Hill Lode, after the June 17, 1775 Revolutionary War battle of Bunker Hill. A thriving pioneer settlement sprang into existence around the resulting mine’s workings and was eventually— and appropriately — named Kellogg. The peak rising hundreds of feet above the mine’s adit was named “Jack-ass Peak” after Noah Kellogg’s hairy companion.

Jan 1, 1996

By Norman R. Paterson

In November 1980 a combined airborne electro-magnetic (INPUT.) and magnetometer survey was conducted by Questor Surveys Limited in the State Line Area, Colorado-Wyoming. The survey was the result of discussions over a prolonged period between the authors and various investigators of the State kimberlite occurrences. As a result of these discussions and literature studies, it was decided that airborne geophysical methods would be potentially useful in the exploration for new kimberlite occurrences. The survey was designed by Paterson, Grant and Watson Limited and conducted by Questor Surveys Limited over an area in which more than 15 kimberlite bodies were known and on some of which ground geophysical surveys had been conducted. The survey was designed primarily to determine the airborne electromagnetic and magnetic responses of the known occurrences and to deter- mine whether these responses would be detectable under similar conditions in other parts of the State Line District and elsewhere.

Jan 1, 1984

By Robert E. Moore

Investigators have attempted to develop lime refractories on a scale for broad applications in the metal and ceramic industries for several decades. There was a flurry of activity in the early to mid-fifties but no commercial developments were forthcoming. The literature of that period yields little information on plant trials. A major factor suppressing past interest in lime has to have been the high success with the application of magnesic and dolomitic materials. These materials, usually tar-bonded or impregnated, have become the refractories success story of the past two decades, especially in BOF applications. It is safe to say that their development and application has essentially plateaued, and that work today is focussed on finding cheaper paths to produce MgO-based refractories. For example there are efforts underway to extract MgO from mineral magnesite and to combine natural dolomite with MgO1. Beginning in the late sixties, rather intensive research began on the development of CaO brick, especially in East and West Germany, in Poland, and other communist bloc satellites, &apos;and also in Italy and Spain. One speculates that the motivation was a regional shortage of good magnesia and dolomitic material. Of course, There has always existed the prospect of cost savings should it be possible to use raw limestone as the starting material. Quite a large literature has appeared treating of these developments which in the early seventies had already reached full plant scale trials and uses. Some of the directions followed in the research have led to novel methods of brick manufacture.

Jan 1, 1985

By Circe Verba, Scott Montross, Jon Yang

This work is a first-order study to assess the suitability of an organic acid lixiviant to extract rare earth elements (REE) from coal preparation fines refuse sourced from a Pennsylvania mine with a total REE of ~ 300 ppm. The extraction of REE using an organic acid, in this case 0.1Mcitric acid and 0.5Mtrisodium citrate solution, is compared against 0.5M(NH4)2SO4, 1MHCl, 1.2MH2SO4, and 0.5Methylenediaminetetraacetic acid (EDTA). Ashing the coal waste material prior to leaching tests with the citrate solution nominally improved REE extraction. Buffered citrate solution recovered 7% of the total REE in the as-received Isabella Fines as compared to 11% in ashed samples, whereas (NH4)2SO4 extracted 5–6%, respectively. EDTA recovered up to 33%of the total REE, most likely due to the higher coordination chelate bond. Mineral acids, however, outperformed the organic acids on ashed material (16–52%REE recovery), suggesting that organic acids may not be a suitable competitive option for REE extraction from these types of feedstocks.

Jun 15, 2021

By David A. Zegeer

It&apos;s a pleasure to be here, and an honor to be invited to address this distinguished group of professionals. Today, I&apos;d like to begin with an overview of progress in coal mine safety and health. I&apos;ll discuss some current areas of concern. And I&apos;ll make some suggestions as to where we may be headed in the future. This year marks the coal industry&apos;s 15th anniversary under the Coal Mine Safety and Health Act, which took effect in 1970. We have come a long way in those 15 years. As a coal mining man from way back, I remember the era when the Coal Mine Safety and Health Act first was passed. Some of you may remember it, too. At that time, the safety and health of coal miners was in the public eye even more than it is at present. One reason was a mine explosion that killed 78 miners when it occurred in a large underground coal mine in November 1968. That disaster raised tremendous public concern about protecting the safety of miners.

Jan 1, 1985