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By Robert Stefanko

13.1-SUBSIDENCE AND GROUND MOVEMENT ROBERT STEFANKO 13.1.1-SUBSIDENCE AND FAILURE This treatment of subsidence and ground movement will be confined to subsidence control for limiting surface damage by assessing the significant parameters affecting ground movements during mining to optimize mining techniques. The material will be equally applicable to evaporite mining as well as coal but will not deal with block caving or similar systems where failure is deliberately induced and surface protection is not necessary or provided. When an underground opening is established, the original equilibrium is disturbed with resultant stress concentrations. While many factors are involved in opening stability, the span (W) undoubtedly is one of the most important factors in failure. Assuming it is relatively small, the overlying rock strata can bridge across the opening and little if any movement or convergence of top and bottom will occur. However, as the span increases, a point is reached where the stress in the overlying rock strata exceeds some strength value of the rock, and the top breaks. If the opening span is limited to some subcritical value (-We) and/or is at great depth (D), a pseudo-arch will form, achielring stability before rupture occurs to the surface. The boundary of this arch is thought to approximate an ellipse in form with the major axis vertical and equal to four times W, although there has been little field research to substantiate this theory. However, if the width of this opening is increased at this horizon to some critical value (W,) or the same span is created in a shallower seam, the overlying strata ill progressively rupture to the surface and the characteristic subsidence trough is formed (Fig. 13-1) The area of the surface affected is much greater than the area of the seam extracted, depending on the "angle of draw" (a), which is the angle between a vertical line from the edge of the opening and another line extended to a point at which subsidence tails out to zero. This angle has been found to be about 35° in Europe but is rather academic, being a function of instrument precision in detecting subsidence. Since the subsidence effect is so small at any point beyond n 25° angle, this latter may be considered the practical limit of subsidence. Further- more, indications are that the angle of draw varies with depth and nature of the strata.' In a field study over a 1,000-ft-deep potash mine in New Mexico, the maximum angle of draw was found to be 51.5°.2 So far, only a single opening has been considered. However, the width of the excavation applies to any complex mining system of entries, rooms and longwalls as long as the seam has been fully extracted and the width represents the distance from one solid rib to another. It will be noted that the amount of vertical displacement varies from point to point on the surface (s), but the maximum subsidence for a given trough occurs at the center (S). This latter value may not he equal to the maximum possible subsidence (Sm) which occurs only if a critical or super- critical (+W,) width has been exceeded (Fig. 13-1). Substantial surface damage can result when subsidence occurs. Factors affecting the amount and type of ground movement are: thickness and properties of the seam, angle of draw, width of excavation, depth and type of overburden, inclinations of strata and surface, and the amount of support left in the gob. In Europe,

Jan 1, 1973

By C. P. Wa1ters

This paper is written primarily from the standpoint of the miner and consumer of Missouri fireclays in refractory products, principally firebrick used in the linings of blast furnaces, hot metal transfer cars, the recuperators of glass melting tanks, and other allied uses involving contact with liquid metals and reactive slags. For these uses the manufacturing plant requires forecast of certain physical characteristics, as well as chemical characteristics, of the raw clays as a determination of practicability of mining, treatment such as pre -firing or calcining, and final usage area. These methods and results may leave something to be desired from an academic view, but are used my operation as an on-running guide to seeking and mining raw material. The high quality fireclays of Missouri are generally differentiated by the user as the softer, somewhat plastic, "bond" clays and the so-called flint clays suitable for a fired or unfired aggregate portion of a brick composition. The semi-plastic "bond" clays are mined in a producing area of northeast Missouri including Montgomery, Audrain, Callaway, Monroe, and Lincoln counties. In this geographic area the clays are the members of the cheltenham formation and occur in rather large lenses in which the usable members may approach 15 meters in thickness and an area of 6o to 8o acres. Overburden is dependent on the surface topography and, in our operations, varies from 8 meters to 25 meters. It can be up to 36 meters in the prairie uplands, but this is rarely strip mined. There was former mining by shaft and drift methods in which the overlying coal was also worked. These clays are primarily used as a binding material for other clay aggregates and is characterized by its lower refractoriness-cone 3o (165o c) to cone 321/2 (172o c), in terms of pyrometric cone equivalent, and an alumina - Al2o3 - content of plus or minus 31%. Physically, these clays are readily slaked by water to a fine grain size and exhibit a degree of

Jan 1, 1977

By Yang Song, Li Shaokang, Wang Zhiqiang, Wang Jinjun, Li Guofeng, Su Meixuan

CaCO3, as a component widely existing in natural minerals, is a common impurity. In recent years, triboelectric separation has been applied to the separation of minerals containing CaCO3. However, the CaCO3’ s tribocharging mechanism is still unclear and the charging effect of CaCO3 is not obvious. In order to study the effect of triboelectric charging of CaCO3 particles under different conditions, according to the single particle frictional collision charge-mass ratio formula derived from the condenser model, an experimental platform for particle tribocharging was built in this paper. Then, the formula of charge-to-mass ratio (Q/m) was derived based on the analysis of the particle force. Particle image velocimetry (PIV) was used to record particle motion trajectories to calculate the Q/m of one particle by this formula. In the experiment, the airflow velocity, tribocharge material, and moving distance of the particles were varied to investigate the variation of the particle charge-mass ratio results. The results show that CaCO3 was negatively charged when tribocharging with mental. The particle’s Q/m is positively correlated with the airspeed. When the metal is triboelectrically charged with the insulator, the metal with a smaller work function leads to the insulator particle with a larger Q/m. As the moving distance of the particles increases beyond 230 mm, the particle charge ratio decreases. The particle charge-mass ratio is consistent with the trend of the charge-mass ratio deduced from the capacitance model.

Jun 3, 2023

By E. R. Spencer

An assessment of U.S. western hard-rock miners? noise exposures was conducted as part of a multi-year National Institute for Occupational Safety and Health (NIOSH) survey of noise exposures in each sector of the mining industry. Noise from selected mining equipment and operator noise exposures were measured, analyzed, and tabulated for dissemination to the participating sites and are being used to direct NIOSH research and interventions to address the greatest noise hazards. Eighty-two noise dosimeter measurements were obtained, along with time-motion studies as the miners operated hard-rock mining machines. Ninety-six percent of the operators had daily noise doses that exceeded the Mine Safety and Health Administration?s permissible exposure level. The average gold miner dosages while operating the following equipment were: haul trucks - 261%, load-haul-dumps (LHDs) - 235%, single boom drills - 221%, bolters - 214%, and dual boom drills - 163%. The worst exposure level was a silver miner with a daily dose of 873%. Time-motion data showed that this miner?s exposure accumulated most rapidly while operating a jack-leg drill. These results will be used to help prioritize noise control development by NIOSH and other partners. Disclaimer: The findings and conclusions in this report have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be construed to represent any agency determination or policy.

Jan 1, 2009

By W. W. Bristol

The Jerritt Canyon Joint Venture project is located approximately 50 miles (80 km) north of Elko. Nevada, U.S.A. and produces between 250 and 300 thousand ounces of gold annually. The Joint Venture is 70% owned by Independence Mining Company and 30% owned by Meridian Gold Company. Independence Mining Company is the operator. The Jerrin Canyon Joint Venture started operations in 1980 mining a series of open pit mines. Since the early 1990's, the discovery of deeper ores has required the joint venture to develop and mine a significant portion of its annual production utilizing underground mining methods. Underground production started with the West Generator Mine in 1993 followed by the Murray Mine in late 1994. Presently, two additional underground projects are being developed. Underground mining at Jerritt Canyon is budgeted to produce 128 thousand ounces in 1997. This paper documents the major transition/redevelopment of the Jerritt Canyon mining methods.

Jan 1, 1997

By J. A. Rusnak, M. M. Gadde, S. S. Peng

In situ stresses are one of the key inputs to a successful underground excavation design. Because of the complex mechanisms involved in arriving at the current state of stress at a point in the Earth’s crust, often the only reliable way of estimating in situ stresses is by actual measurement. In the United States coal mines, overcoring relief method is by far the most popular technique used for estimating the virgin stress field. Typically, for stress measurement with overcoring, a vertical borehole is drilled into the roof or floor of an underground coal mine opening to a certain distance and change in borehole diameter is monitored at multiple points along the length of the hole when this borehole is overcored. For the measured stresses to represent the pre-mining stress field, the measurement points should be located beyond the influence zone of the mine opening. Actual data from several US coal mines have been collected to check if the measured stresses by overcoring represent true in situ stresses. The data indicate that most of the stress measurements were within the excavation influence zone. A three-dimensional numerical modeling methodology was applied to one of the collected cases to correct the measured stresses for the excavation influence. The modeling approach demonstrated the usefulness of integrating numerical modeling into a stress measurement program to estimate ‘true’ in situ stresses.

Jan 1, 2006

By T. Faulkner, S. A. Richards

"The Scale-Distance formula is one of the primary means of determining the maximum amount of explosives that can be safely detonated at one time by a single delay. This paper describes a method of applying the Scale-Distance formula to the entire mining area using gridded surfaces. Using this technique, the operator can better determine the economic impact of nearby structures on the mine plan. INTRODUCTION As the surface mining of coal and stone encroaches near populated areas, precautions must be taken to minimize blast vibration damage to protected structures. The common method employed to minimize vibration damage is not to reduce the total amount of explosive in the hole (which would negatively affect fragmentation), but rather to introduce delays to reduce the amount of explosive detonated at any point in time. The Scale-Distance formula, an industry-accepted standard, may be used to calculate the maximum weight of explosive that can be detonated at one time without damaging the structure. This information may then be used to determine the number of delays required for each hole. Detonators make up a large part of the cost of blasting. At roughly $15 per detonator an additional detonator in the hole can be significant. It is normal to set off hundreds of holes at in one shot. An additional detonator in each hole can cost additional thousands of dollars per shot. The number of detonators is also a function of the depth of the holes and the hole spacing. Being able to accurately forecast the economic impact of blasting near structures improves a company’s ability to make informed decisions on how to approach property owners. Operators basically have three options when dealing with property owners: 1. Buy the property and blast without restriction 2. Negotiate with property owner for a waiver to allow unrestricted blasting 3. Comply with the legal blasting restrictions This paper proposes a method for the engineer to forecast for all points in the proposed mining area the impact of nearby structures upon the blast design. Being able to create a contour map and ultimately estimate the number of delays required for the entire forecasted mining area, the engineer can create accurate economic forecasts of the cost of blasting. The objective is to reduce the level of uncertainty in the forecast of the blast component portion of the forecast."

Jan 1, 2016

By W. A. McKinney

The U.S. Bureau of Mines, Kennecott Copper Corp., and New Mexico Institute of Mining and Technology are cooperating on a research project to improve extraction of copper from previously leached waste dumps that are in part compacted and impervious to leach solutions. The field work is being done at the "J" leach dump, Chino Mines Division, Kennecott Copper Corp., Santa Rita, N. Mex. The laboratory percolation leaching tests were made at the U.S. Bureau of Mines Salt Lake City Metallurgy Research Center on drill core samples from the "J" dump to determine how much copper might be leached if permeability could be restored by explosive heaving or other means. The core samples from nine drill holes on "J" dump contained from 0.06 to 0.14 percent copper. Copper extractions ranging from 15 to 3 percent were obtained in 75 days by percolation leaching of minus-1-inch material with dilute sulfuric acid. The tests results indicate that about 2'7 percent of the copper remaining in the "J" dump could be extracted under ideal leaching conditions.

Jan 1, 1970

By M. Brändström, R. Dube, J. Moyo, T. Perkins, G. Bermudez, L. Bartsch, C. Smith, R. Sorensen

The Kennecott Copperton concentrator is currently the bottleneck for the Kennecott operation and several projects have been initiated to increase throughput. This increase in grinding circuit throughput has resulted in a coarser grind size which has impacted flotation circuit recovery. Metso Outotec 300m3 forced air tank flotation cells are currently utilized in the rougher flotation circuit at the Copperton concentrator. Higher overall recoveries have been observed on the newer Outotec tank cells compared to the original 85m3 self-aspirated flotation cells they replaced. However, coarse particle flotation response on these large, forced air cells did not show encouraging results when compared to smaller previously installed cells. This coarse recovery loss has been attributed to the longer froth transport duration and transport distance on large cells (100m3 and larger). Cell manufacturers addressed the risks of additional coarse particle losses in large cells by installing concentric circumferential launders and crowders (also called center launders). These are designed to reduce froth transport time and distance in the last cells in rougher stages where low residual floatable minerals causes a lack of froth mobility. The center launders also increase froth crowding, which also helps to increase froth transport velocity and increases copper and molybdenum recoveries. A trial set of the center launder retrofits were installed on the last three tank cells in one of the parallel rougher flotation rows at Copperton. The launder retrofits were installed at the end of October 2020 and monitored until March 2021. This six-month trial showed statistically significant improvements in copper and molybdenum flotation recoveries through the rougher circuit.

Mar 2, 2022

By John E. Urosek, Mark L. Mott, Gary E. Smith

One function of Mine Safety and Health Administration (MSHA), Pittsburgh Health Technology Center's Ventilation Division is to provide assistance to MSHA enforcement and industry personnel for maintaining adequate fresh air to all areas of an underground mine. In fulfilling this objective, total mine venti1ation pressure-air quantity distribution on investigations are conducted to measure the air pressures and quantities necessary to describe and analyze the existing ventilation systems. Various ventilation parameters are calculated from these measured pressures and quantities. Some of these parameters are: pressure drop in intake and return airways, air leakage, percentages of air reaching active sections, average regulator drops, and fan pressures. The purpose of this paper is to present and compare values of ventilation parameters calculated from investigations at twenty underground coal mines.

Jan 1, 1987

By R. B. Tippin

Great Salt Lake Minerals & Chemicals Corporation (GSL), a subsidiary of Gulf Resources & Chemical Corporation, is located on the east side of the Great Salt Lake, approximately 20 miles from Ogden, Utah. GSL processes brine from the north arm of the lake to produce potash, salt cake, salt and magnesium chloride brine. Potash fertilizer, in the form of K2SO4 is GSL's primary product. The GSL Sulfate of Potash Complex The GSL potash complex (depicted in Figure I) is divided into a solar evaporation pond system and the process plants. The solar,evaporation system consists of a 17,000-acre (68.8 Km2) ponding operation where the various salts deposit during the summer evaporation season, and a harvesting operation which recovers the deposits during the remainder of the year. The harvest is either preconcentrated in the flotation plant or processed directly in the chemical plant where sulfate of potash (SOP) is produced. The chemistry of the lake brine results in a solar deposit con¬taining primarily kainite (KCl-MgSO4.3H20), schoenite (K2SO4?MgSO4^ 6H2O), and halite (NaCI), with some carnallite (KCl.MgCl2.6H20) and epsomite (MgSO4.7H20). Other minor minerals may be deposited, depending upon the weather during the evaporation season. Each year, the solar deposit in the evaporation ponds will greatly depend upon the summer weather conditions, which have in the past proven to be quite unpredictable. Weather may alter both the quantity of the solar deposit and the composition of this deposit. During, a "short" evaporation season, the tonnage deposited decreases, but depending upon the short-term weather, the potash mineral content

Jan 1, 1977

The methods used by Newmont Gold Company for deposit modeling are illustrated by examples taken from current operations and advanced projects. These methods vary depending on a number of factors, including geology, mining and processing methods, economic feasibility, and availability of historical information. It is argued that successful modeling requires understanding these factors, knowing the strengths and weaknesses of the various modeling techniques, and being flexible in selecting the appropriate methods.

Jan 1, 1997

By Hugo T. Dummett

I would like to share some observations about the mining business and place the ongoing discussion in the context of our changed world. The mining industry practices its trade in a universe defined by prices, laws and governments. Prices, as in what the industry is paid for what it produces, and the costs of goods and services. Laws that have to be transparent and fair. And governments that the industry prefers to be stable and supportive of the industry, and certainly not corrupt. However, the day-to-day realities of the mining business in 2000 are not what they were 10 years ago. Several events have occurred that have resulted in a less populated workplace that is obliged to work much smarter and faster. These include real declining prices for most mineral commodities; low and diminishing returns to company shareholders; massive restructuring of the industry through mergers, partnerships and alliances; gold’s changed role from reserve currency to just another commodity; the growing importance of native title and the startling imperatives of “sustainable development.” In addition, the “borderless world” now means that resource companies will need to pass the test of being responsible corporate guests who respect and enhance the social, moral and societal values of their host countries.

Jan 1, 2000

By M. Ahrens

The paper describes the main drive systems of an iron ore pelletizing plant and related important design aspects. It is shown how today?s drive systems provide the desired flexibility for the process and lead to high energy efficiency and system reliability. With respect to electric drives expansion projects have many technical issues in common with green-field installations. However, there are aspects where important, distinct differences occur. It is shown which considerations for selecting and sizing drive systems for such a plant as well as for integrating them into an overall plant control concept need to be taken into account. Besides the design of the drive systems a key aspect for such an expansion project is the network interaction. Harmonics levels at different points in the plant network need to be checked carefully with simulations during the design phase to avoid problems later-on during operation. The power factor compensation and the harmonic filtering of the new production line cannot be designed independently of the already existing plant. The paper highlights areas where problems could occur and which points need to be paid attention to when a plant is expanded.

Jan 1, 2008

By James R. Dunn

The nature of water and clay in carbonate rocks is currently being investigated at Rensselaer Polytechnic Institute by various thermal and sorption techniques. Two basic concepts 'led to the present research. (1) To explain the expansion and contraction on wetting and drying of some unsound argillaceous carbonate rocks, it was postulated that the pore space around each clay particle is such that when water is sorbed by the clay, force is exerted against the pore walls. (2) The clays in expansive rocks are rejected from the growing dolomite crystals during their growth, and form a more or less connected network, thereby becoming "wettable," or available to water. New quantitative cold differential-thermal-analysis (electrocalorimetric) equipment was designed and constructed to check the "limited space" and "wettable clay" concepts and to determine the thermal nature of the transition of water to ice in rocks. This DTA equipment can detect the release of heat in freezing of less than 0.001 per cent water in a rock. Twenty-three rock cores (nine sound and fourteen unsound) and 4 procelain cylinders for standards were studied. In the nine sound rocks and in all the procelain cylinders, the rapid freezing of from 50 to 100 per cent of the water was readily detectable after 7 to 10° C of supercooling. No further freezing could be detected to -20° C. However, in eleven of the fourteen unsound rocks, no freezing was detected to as low as -40° C; in the other three, less than 50 per cent of the water froze. As a check of the DTA measurements, adsorption isotherms were determined for all materials. Calculations of theoretical freezing points for sorbed water again demonstrated that the least sound rocks had the most nonfreezable water. "Frost" sensitivity in these carbonate rocks is, therefore, probably not the result of the force of expansion of water to ice. The disruptive force is believed to result from expansion of sorbed, ordered water on clay surfaces. The relationship between the unfilled rock pores after 24-hour water saturation and the water sorbed at 85 per cent humidity at 30° C (i.e., the theoretical nonfreezable water delineates the sound and unsound carbonate rocks.

Jan 1, 1966

The Mine Safety and Health Administration (MSHA) has conducted studies to evaluate dust control technologies on deep cut continuous mining sections. These surveys have been conducted in different mines using various mining methods, face ventilation systems and dust control systems. The surveys typically consist of respirable dust sampling combined with a time study of mining activities. Measurements of dust control and operational parameters are also made. Critical elements identified for controlling respirable dust in deep cut mining systems include controlling intake dust, dust collector capacity, face ventilation quantities and velocities, positioning of workers, work practices and selection of equipment. This paper discusses how the dust surveys are conducted in deep cut mining sections and how these surveys are used to identify specific dust generating sources.

Jan 1, 1999

By Kan Aketa

The Japanese iron and steel industry has been developed remarkably, supported by the increase of steel demand. In particular, progress in various technical aspects of the ironmaking process, including burden materials, equipment, and operation has greatly contributed to the increase in productivity and the reduction of the fuel rate in blast furnaces. With the recent rise in the price of petroleum, the conversion of the energy source from oil to coke has resulted in a temporary increase in the fuel rate of blast furnaces.

Jan 1, 1982

By John R. Marks

The first 0.7 m3 (1 cu yd) diesel load- haul-dump vehicle (LHD) arrived at the Homestake Gold Mine in Lead, SD in I969 to muck a ventilation drift. Others soon followed. Mechanization has subsequently changed stoping methods, manpower requirements, ventilation practice, the definition of minable reserves, and it has impacted safety and health. These have all had a direct bearing on the economics of producing gold ore from deep underground. Mechanized mining Before mechanization, production quotas at Homestake were met by several open cut-and-fill (OCF) mining methods. OCF mining relied on jackleg and wagon drilling, electric and compressed air slushing and cribbed raises brought up through the fill. Even square- set stoping was used until about eight years ago. The mine became 100% mechanized in 1988. The two current stoping methods are mechanized cut-and-fill (MCF) and vertical crater retreat (VCR). In 1992, MCF accounted for 54’S, VCR for 44% and development/uphole for 2% of the 1.4 Mt (1.55 million st) of underground ore produced. These methods are typically ramp- based and use diesel-powered LHDs, trucks, pneumatic drill jumbos and utility vehicles. Electric LHDs and rubber-tired compressed air muckers have also been used in VCR scrams. Pfarr (1991) and Mitchell (1980) provide good overviews of current production methods.

Jan 1, 1993

By C. D. Litton

This paper describes two new gas diagnostic techniques which have been used successfully to determine: 1) the combustion status of underground mines which have been sealed due to fire (Littol1 Ratio); and 2) the location of fires in abandoned, underground mines or waste banks (Kim Ratio). The Litton Ratio makes use of the rate of change of CO, calculated on both an air-free and methane-free basis, from mine gas samples and compares these values with those expected from a normal, combustion-free environment. The Kim Ratio uses the concentrations of total hydrocarbons, C2 ?C5, less the methane, to arrive at ratio values which increase as combustion intensity increases. The details of these two new techniques, their application to real situations, and their limitations will be discussed.

Jan 1, 1989

By T. Gupta, T. Ghosh

"In recent years, the demand for Rare Earth Elements (REE) have grown rapidly due to increasing demand and global supply shortage. The supply deficit of these critical elements has encouraged the search for new sources of REEs, possibly Coal and Coal byproducts. Coals from certain parts of the world can be rich in REEs, and can approach a total concentration of 1000 ppm. Two Alaskan coal samples were investigated for the effect of size and density on the concentration of REEs on three selected size fractions. Flotation tests were conducted for finer fractions. Additionally Bottom ash and Fly Ash samples from a power plant have been examined for REE concentration. The results show that the upgrade potential for REEs on ash basis from whole coal basis ranges from 2:1 to 4:1 for Wishbone Hill and Healy coal samples respectively. Flotation of the finer fraction for both coal samples, conducted under similar conditions, reveal higher concentration of REE’s in the tailings. Both coal samples have a comparatively higher LREE content than HREE. Trends of REE content of the power plant products on ash basis revealed that Fly Ash has a slightly higher concentration of both LREE and HREE than Bottom Ash. Keywords: Rare Earth Elements, Coal, Coal Ash INTRODUCTION Rare Earth Elements (REEs) and their compounds are vital components in industrial, automotive, petrochemical, defense and medical applications. Dysprosium in combination with other elements is used in laser materials and nuclear control applications. Neodymium is present in magnets that are used in green power generation such as the manufacture of wind turbines and also in hybrid cars. Europium, Terbium and Yttrium are used for devices that have visual displays such as computer screens and laptops [1, 2]. REEs pose fewer environmental problems of disposal and recycling than conventional heavy metal counterparts [1]. Rare-earth metals are key ingredients for radar systems, avionics, modern national defense and satellites [2-8]. In the last 25 years China has been mining, processing and refining Rare Earth elements at affordable prices thus gaining a virtual monopoly in the market. The many fold rise of the prices of REE elements over the last few years and frequent disruptions in supply has forced various nations to invest in indigenous extraction of REEs by increasing production from previously productive sites and to expand the production from new alternate sources including coal and coal byproducts [9]. Coal and Coal beds have been found to be significantly enriched with certain critical metals and can be a potential source for both Light Rare Earth Elements (LREE) and Heavy Rare Earth Elements (HREE) [10]. Moreover, coals with high REE concentrations have been discovered in Pavlovka and Rakovka coal deposits in Russia, Appalachian deposits in USA, Sichuan Basin deposits in China and others across the globe [10-13]. A recent study from China looked at how different trace elements (including numerous REEs) partition in various size fractions of Antaibao coal from China. The finding indicates that the REEs, phosphorous, and thorium all have their high concentrations in the particle sizes 0.5 to 3 mm, and 6 to 25 mm. REEs are also often present in the form of phosphates such as monazite in coal [13, 14]. The objective of the study is to identify the concentration of REEs in Alaskan coals and to investigate the effect of density and size in the distribution of REEs in coal and coal byproducts."

Jan 1, 2016