Mining

MINING

Mining is the process of taking minerals or coal from the earth. Most substances that we get from the earth we obtain by mining. Mining provides iron for making airplanes, automobiles, and refrigerators. Mines also supply salt for food; gold, silver, and diamonds for jewelry; and coal for fuel. We mine stone for buildings, phosphate for fertilizer, and gravel for highways.

Some substances can be mined more cheaply than others because they are found at the earth's surface. Some lie far beneath the surface and can be removed only by digging deep underground. Other mined substances are found in oceans, lakes, and rivers.

People have mined the earth for thousands of years, perhaps first in Africa. Prehistoric people in Europe dug pits and tunnels to get flint, a hard stone used to make tools and weapons. Sometime after 3500 B.C., people were mining tin and copper. They combined these metals to make bronze, a hard alloy (mixture of metals) that made better tools and weapons. The ancient Romans probably were the first people to realize that mining could make a nation rich and powerful. Merchants traded valuable stones and metals and brought riches to the Roman Empire. The Romans took over the mines of every country they conquered.

The Roman Empire ended in the A.D. 400's. For about a thousand years, few advancements were made in mining. During the 1400's, coal, iron, and other materials were mined in Europe, especially in Germany, Sweden, and France. Mining also developed in South America. The Inca Indians and other tribes of South America used metals to make tools, jewelry, and weapons.

Mining began in what is now the United States during the early 1700's. French explorers mined lead and zinc in the valley of the Mississippi River. In the mid-1800's, miners began to dig up large amounts of coal in Pennsylvania. At about the same time, thousands of people rushed to California hoping to find gold. In the West, the gold rush led to the discovery of copper, lead, silver, and other useful minerals.

Most substances obtained by mining are minerals. A mineral is made of materials that were never alive. Thus, coal, petroleum, and natural gas are not considered true minerals because they were formed from the remains of plants that lived long ago. Coal, however, is obtained by mining. Because this article discusses the recovery of mined substances, it includes both minerals and coal. For information on drilling for oil and natural gas, see the World Book articles PETROLEUM and GAS.

Kinds of mining

There are many methods of mining. Each is based on where and how a coal or mineral deposit is found. Some deposits lie at or near the earth's surface, and others are far underground. Some minerals are found as a compact mass, and others are widely scattered. Minerals also vary in hardness and in the ease with which the mineral-bearing material called ore can be separated from the surrounding rocks. Some minerals occur in such large bodies of water as oceans and seas, and are obtained by pumping. For a discussion of the methods of mining a particular mineral, see the World Book article on that mineral. For details on coal-mining methods, see COAL (How coal is mined).

Today, most mines are highly mechanized. Hydraulically powered drills bore holes in the ore. Large machines dig and load the ore, and trains, trucks, and conveyors transport it. In underground mines, high-speed elevators called skips carry ore to the surface.

Surface mining methods are used when deposits occur at or near the surface of the earth. These methods include placer mining, dredging, open-pit mining, strip mining, and quarrying.

Placer mining is a way of obtaining gold, platinum, tin, and other so-called heavy minerals from gravel and sand deposits where nearby water supplies are plentiful. The exact technique used depends on the size and kind of deposit. On a small scale, panning may be used to recover gold and other minerals from streams. On a larger scale, miners use a form of placer mining called sluicing. In this method, the mineral-bearing gravel and sand are shoveled into the upper end of a slanting wooden trough called a riffle box. In the box, they are washed by water. The valuable minerals are heavier than the sand and gravel, and settle in grooves on the bottom of the box. The gravel and sand are washed away. The mineral-bearing gravel and sand may also be moved directly from a deposit into the riffle box by the force of water shooting out through a large nozzle called a giant. This process is called hydraulicking.

Dredging is used especially where mineral-bearing sand and gravel layers are exceptionally thick. In dredging, a pond or lake must be formed so that a large, bargelike machine called a dredge can be floated. An endless chain of buckets is attached to a boom (long beam) at the front end of the dredge. The buckets dip into the water when one end of the boom is lowered. They dig up the mineral-bearing sand and gravel and move the material to a bin on the deck of the dredge. The material is taken from the bin and washed in much the same way as in placer mining. After the valuable minerals are collected, the sand and gravel are put on a conveyor belt and dumped back into the pond behind the dredge. By digging forward while disposing of the waste sand and gravel to the rear, the pond and the dredge move ahead as the deposit is mined.

When mining some kinds of loose gravel deposits, machines called draglines or slacklines are used. These machines have a scoop attached to a high boom. The scoop is pulled back and forth along the deposit to gather material, which is put into a separating bin.

Open-pit mining is used to recover valuable minerals from large, thick orebodies (beds or veins of ore) lying close to the surface. First, the miners must remove the overburden--that is, the layer of rock and other material that covers the deposit. Then they use explosives to break up great masses of ore-bearing hard rock. The miners mine the deposit in a series of horizontal layers called benches. Miners build a road connecting the benches up the sides of the pit. Trucks or trains haul the ore up the road and out of the pit.

Strip mining is a method of obtaining coal and such minerals as phosphate that lie flat near the earth's surface. Strip mining around hills or mountains is called contour mining or collar mining. Strip mining on flat terrain is called furrow mining. In this method, miners cut a furrow and cast the overburden into a ridge parallel to the cut called a spoil. They break the ore up with machines or explosives and then use shovels or machines to load the ore into trucks or railroad cars. After removing all the ore from the first cut, the miners cut a new furrow, casting the spoil into the previous cut.

In the past, strip mining had a bad reputation because it caused great destruction in the mined areas. This was especially true in mountainous areas, where strip mining would destroy vegetation on mountainsides and lead to mud slides and severe soil erosion.

Today, mining companies in the United States must plan for reclamation of the land before they can begin mining. Reclamation is a process in which the land is restored as closely as possible to its original state. In many cases, the reclaimed land is more valuable than it was before mining. For example, the lakes created in the final cuts of some mined areas have provided excellent fishing and water sports.

Quarrying is a way of mining a deposit that lies at the surface of the earth with little or no overburden. Such rocks and minerals as limestone, gypsum, and mica are produced from quarries. Sand and gravel used for making concrete and large stones used for building are also mined in quarries. Miners have several methods of quarrying. Hard minerals are drilled or are blasted with explosives. Sand and gravel are simply shoveled onto trucks or trains and shipped. Building stones such as marble and granite are sold in natural blocks or pieces. To free these blocks, miners saw, channel, or cut them on four sides, and wedge them free from parent rock. Then the miners hoist the blocks onto trucks or trains.

Underground mining methods are used when the deposit lies deep beneath the earth's surface. First, the miners drive (dig) an opening into the mine. A vertical opening is called a shaft. A passage that is nearly horizontal, dug into the side of a hill or mountain, is called an adit. In coal mining, it is called a slope. From these main passages, miners dig systems of horizontal passages called levels. A wide variety of mining methods are available for removing the ore. Types of underground mining include (1) room-and-pillar mining, (2) longwall mining, (3) sublevel stoping, (4) cut-and-fill mining, (5) block caving, and (6) sublevel caving.

Room-and-pillar mining is a method of recovering ore from horizontal or nearly horizontal orebodies. Miners excavate the orebody as completely as possible, leaving parts of the ore as pillars to support the hanging wall (the rock above the ore vein). Room-and-pillar mining is the most widely used method of underground mining in the United States. Materials commonly mined using this method include coal and the minerals lead, limestone, potash, salt, and uranium.

Longwall mining is also used to dig ore from horizontal seams. Miners use a machine to cut or break ore from a single long face called a longwall. Hydraulic roof supports hold up the hanging wall above the miners. As the miners dig farther into the vein, the supports advance with them, and the hanging wall behind them collapses.

Sublevel stoping is used in orebodies with a steep dip. A dip is the angle the orebody makes with the horizontal. Miners develop sublevels between the main levels and drill and blast the ore from both the sublevels and main levels. As the ore is removed, empty chambers called stopes form. The ore, broken in large vertical slices, falls to the bottom of the empty stopes. There, it is recovered for transport out of the mine.

Cut-and-fill mining is a method of removing ore from vertical veins in horizontal slices, starting at the stope's bottom and advancing upwards. After miners excavate a slice of ore, they fill the stope with waste material called gangue or waste sand from ore-processing plants. This material supports the walls and provides a working platform from which to mine the next ore slice.

Block caving is a way of mining such ores as copper and iron when they are scattered throughout the waste material. In this method, the miners dig levels, dividing the orebody into large sections or blocks. Then they undercut each block with a horizontal slot. The pressure of the overlying rock and ore causes the ore above the slot to cave in. Large machines transport the ore to vertical or inclined openings called ore passes.

Sublevel caving is used in large, steeply dipping orebodies. Miners divide the orebody into sublevels 25 to 50 feet (7.5 to 15 meters) apart. Each sublevel is developed with a network of drifts (horizontal passages) that penetrate the complete ore section.

From the sublevel drifts, the miners drill deep holes into a fan-shaped pattern in the ore section immediately above. The blasting of one fan breaks the ore, causing it to cave into the drift. There, it is loaded and transported to the ore passes.

Pumping methods are used to recover minerals that occur in large bodies of water or that can be changed into liquid form. The waters of the ocean and of some lakes, including Great Salt Lake in Utah, contain huge amounts of mineral elements. Miners often obtain these minerals by pumping the water into plants where it is treated. Pumps move large amounts of seawater through precipitators (separators) in order to remove the minerals. Most of the magnesium used today is obtained by this method.

Pumping is sometimes used to get salt from beds beneath the surface of the earth. Mine workers drill holes and circulate water underground to dissolve the salt and form a saltwater solution called brine. The brine is then pumped to the surface and taken to a factory. There, the water is evaporated, and the salt forms a solid again.

The Frasch process, another pumping method, is often used in mining sulfur, a mineral that melts easily. The miners bore holes in a buried sulfur bed and inject superheated water. The sulfur then melts and forms a liquid. The miners force the liquid sulfur to the surface by pumping compressed air into the holes. After the sulfur cools, it becomes a solid again and can be stored until needed.

The mining industry

In the late 1990's, the annual value of U.S. mining production was about $55 billion. In Canada, the value of mineral production in U.S. dollars was about $14 billion a year. These figures do not include oil and natural gas production because they are not obtained by traditional mining methods. But economists often include oil and natural gas in mining statistics.

The United States mining industry employs about 535,000 people and offers a wide variety of careers for professional, skilled, and semiskilled workers. Management in the mining industry consists mainly of business people and engineers. These people manage mines, smelters, and refineries, and direct the search for new deposits. They also try to improve mining methods.

Many universities and colleges offer training for specialized careers in mining industry engineering. A geological engineer or geologist guides the search for mineral deposits and estimates their value. A mining engineer designs the mine and directs the mining process. A metallurgist or metallurgical engineer directs beneficiation (milling, smelting, and refining) of minerals so the company can sell or use them. The mining industry also employs computer specialists, surveyors, and scientists, including chemists and physicists.

Among the workers in mines are people skilled in operating and maintaining various kinds of mining machinery. Other workers include mechanics, electricians, truckdrivers, and laborers. Some people with mining experience are federal or state mine inspectors. They help enforce laws that promote miners' health and safety. Others with mining experience may be involved in the sale of mining equipment, materials, and supplies.

For more on mining careers, contact the American Institute of Mining, Metallurgical and Petroleum Engineers, based in New York City.

Contributor: William Hustrulid, Ph.D., Head, Mining Research and Development, LKAB, Kiruna, Sweden.

Additional resources

Bates, Robert L. The Challenge of Mineral Resources. Enslow, 1991. Mineral Resources A-Z. 1991.

Gregory, Cedric E. Rudiments of Mining Practice. Gulf Pub., 1983.

Shepherd, Robert. Ancient Mining. Elsevier, 1993.

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COAL MINING

Coal is a black or brown rock that can be ignited and burned. As coal burns, it produces useful energy in the form of heat. People use this heat to warm buildings and to make or process various products. But the main use of the heat from coal is the production of electricity. Coal-burning power plants supply more than half the electricity used in the United States and nearly half that is used throughout the world. Another major use of coal is the production of coke, a raw material in the manufacture of iron and steel. In addition, the coke-making process provides raw materials used to make such products as drugs, dyes, and fertilizers.

Coal was once the main source of energy in all industrial countries. Coal-burning steam engines provided most of the power in these countries from the early 1800's to the early 1900's. Since the early 1900's, petroleum and natural gas have become the leading sources of energy in much of the world. Unlike coal, petroleum can be easily made into gasoline and the other fuels needed to run transportation equipment. Natural gas has replaced coal as a source of heat for some applications. However, people are rapidly using up the world's supplies of petroleum and natural gas that can be removed from the ground economically. If the present rates of use continue, little may remain of these supplies by about 2050. By contrast, the world's supply of coal can last more than 250 years at the present rate of use. Increased use of coal, especially for producing electricity, could relieve a shortage of gas and oil.

Historically, the burning of coal has been a major cause of air pollution. But since the 1970's, air pollution from coal burning has declined despite increases in coal consumption. This is due partly to the use of air pollution control systems by utilities and industries, as required by the United States Clean Air Act of 1970. It is also due to better coal-cleaning procedures and the use of coals with low sulfur content.

In the past, few jobs were harder or more dangerous than that of an underground coal miner. During the 1800's, many miners had to work underground 10 or more hours a day, six days a week. Picks were almost the only equipment they had to break the coal loose. The miners shoveled the coal into wagons. In many cases, children as young as 10 years of age hauled the coal from the mines. Women worked as loaders and haulers. Over the years, thousands of men, women, and children were killed in mine accidents. Thousands more died of lung diseases from breathing coal dust.

Today, machines do most of the work in coal mines. Mine safety has been improved, work hours have been shortened, and child labor is prohibited. The death rate from U.S. mine accidents has dropped greatly since 1900. However, coal mining remains a hazardous job.

This article discusses how coal was formed, where it is found, its uses, and how it is mined. The article also discusses the cleaning and shipping of coal, the coal industry, and the history of the use of coal.

COAL/How coal was formed

Coal developed from the remains of plants that died400 million to 1 million years ago. For this reason, it is often referred to as a fossil fuel. The coal-forming plants probably grew in swamps. As the plants died, they gradually formed a thick layer of matter on the swamp floor. Over the years, this matter hardened into a substance called peat. In time, the peat deposits became buried under sand or other mineral matter. As the mineral matter accumulated, some of it turned into such rocks as sandstone and shale. The increasing weight of the rock layers and of the other overlying materials began to change the peat into coal. Coal, sandstone, and other rocks formed from deposited materials are called sedimentary rocks.

The first stage in the formation of coal produces a dark brown type of coal called lignite. Lignite develops from buried peat deposits that have been under pressure. The pressure results from the weight of the overlying materials and from movements within the earth's crust. As the pressure increases, lignite turns into subbituminous coal. Under greater pressure, subbituminous coal turns into a harder coal called bituminous coal. Intense pressure changes bituminous coal into anthracite, the hardest of all coals. Bituminous coal is also known as soft coal; anthracite, as hard coal.

Anthracites are the oldest coals in most cases, and lignites are the youngest. Some anthracites began to form as long as 400 million years ago. Some lignites developed within the last 1 million years.

The greatest period of coal formation occurred during a time in the earth's history called the Carboniferous Period, from about 360 million to 286 million years ago. During the Carboniferous Period, swamps covered much of the earth. Tall ferns and other treelike plants grew in the swamps and produced huge amounts of peat-forming matter after they died.

Today's plentiful deposits of bituminous coal developed largely from the vast peat deposits formed during the Carboniferous Period. It took about 3 to 7 feet (0.9 to 2.1 meters) of compact plant matter to produce a bed of bituminous coal 1 foot (0.3 meter) thick.

Plant materials are still accumulating in some parts of Maine, the Okefenokee Swamp in Georgia and Florida, and other swampy locations. These materials could eventually develop into coal.

Coal beds are also called coal seams or coal veins. Present-day seams range in thickness from less than 1 inch (2.5 centimeters) to 400 feet (120 meters) or more. The thickest seams are subbituminous coals and lignites. Many coal deposits consist of two or more seams separated by layers of rocks. These formations were produced by new coal-forming swamps developing over buried ones. Each new swamp became buried and developed into a separate seam of coal.

Some coal beds lie nearly parallel to the earth's surface. Other beds have been tilted by earth movements and lie at an angle to the surface. Most of the deepest beds consist of anthracites or bituminous coals. In many cases, earth movements have uplifted deep anthracite and bituminous beds to a position nearer the surface. Such movements also account for coal seams in hills and mountains.

COAL/Where coal is found

Coal is found on every continent. Deposits occur as far north as the Arctic and as far south as Antarctica. Some coal deposits occur off ocean coastlines. However, deep underwater deposits have little value at this time because they are difficult to mine.

Coal deposits that can be mined profitably are called coal reserves. In most cases, a coal seam must be at least 24 inches (61 centimeters) thick for mining engineers to class it as a reserve. Some long-range estimates of coal reserves include beds 12 to 24 inches (30 to 61 centimeters) thick. But such thin beds would probably be mined only after more productive deposits were exhausted. Most estimates of coal reserves include only tested deposits. The reserves may actually be somewhat larger or smaller than the estimates.

To estimate coal reserves, mining engineers drill into the ground in suspected coal-bearing areas. A drill brings up samples of the rock formations in the order in which they occur. The depth and thickness of a coal seam can thus be estimated. By taking a number of such samples, engineers can estimate the extent of a particular deposit. A large area of tested reserves is called a coal field.

World coal reserves. No reliable estimates exist for the total amount of coal that lies beneath the earth's surface. The world's proved recoverable reserves of coal total over 1.1 trillion short tons (1 trillion metric tons). This figure represents the amount of coal that can be profitably recovered from known deposits with current technology. Most of the proved recoverable reserves are in Australia, China, Germany, India, Indonesia, Poland, Russia, South Africa, and the United States.

Location of U.S. and Canadian reserves. About half of all U.S. coal reserves lie in the eastern half of the nation, from the Appalachian Highlands to the eastern edge of the Great Plains. The rest of the reserves are in the western part of the country, especially the Rocky Mountain States, the northern Great Plains, and Alaska. The eastern reserves include nearly all the nation's anthracite deposits. They also include more than four-fifths of its bituminous deposits. The western reserves include almost all the subbituminous coal and lignite in the United States.

Canada's coal reserves consist chiefly of bituminous coal. The nation also possesses large fields of subbituminous coal and lignite. However, these deposits are smaller than the bituminous deposits. More than 95 percent of Canada's reserves are in the country's western provinces--British Columbia, Alberta, and Saskatchewan.

COAL/The uses of coal

The way in which coal is used depends on its chemical composition and moisture content. Coal is often referred to as a mineral. But unlike a true mineral, coal has no fixed chemical formula. All coal consists of certain solids and moisture. The solids are composed chiefly of the elements carbon, hydrogen, nitrogen, oxygen, and sulfur. However, coal varies widely in the amount of each element it contains as well as in the amount of moisture it contains. No two deposits of coal are exactly alike in their makeup.

Coal is usually classified according to how much carbon it contains. Coal can thus be grouped into four main classes, or ranks: (1) anthracites; (2) bituminous coals; (3) subbituminous coals; and (4) lignites, or brown coals. The carbon content of the coals decreases down through the ranks. The highest-ranking anthracites contain about 98 percent carbon. The lowest-ranking lignites have a carbon content of only about 30 percent. The amount of moisture in the coals can be as low as less than 1 percent, in anthracites and bituminous coal, and as high as 45 percent, in lignites. High-moisture subbituminous and lignite coals have a lower heating value than do anthracites and bituminous coals. Heating value refers to the amount of heat that is produced by a given amount of coal when it is burned.

Bituminous coals are by far the most plentiful. They are also the most widely used of the major ranks of coals. They have a slightly higher heating value than do anthracites and are the only coals suited to making coke. Anthracites are slow to ignite. They also burn too slowly to be suitable for industrial purposes such as the generation of electricity. Anthracites are also the least plentiful of the four ranks of coals. About 2 percent of the coal in the United States is anthracite.

Coal as a fuel

Coal is a useful fuel because it is abundant and has a relatively high heating value. However, coal has certain impurities that limit its usefulness as a fuel. These impurities include sulfur and various minerals. As coal is burned, most of the sulfur combines with oxygen and forms a poisonous gas called sulfur dioxide. Most of the minerals turn into ash. The coal industry refers to ash-producing substances in coal as ash even before the coal is burned.

Coal known as low-sulfur coal can be burned in fairly large quantities without adding harmful amounts of sulfur dioxide to the air. Medium- and high-sulfur coals can cause serious air pollution if burned in large quantities without proper safeguards.

The United States Department of Energy (DOE) classifies sulfur content according to the weight of the sulfur in a sample of coal that can produce 1 million British thermal units (Btu's) of heat. Such a sample is low-sulfur coal if it contains 0.60 pounds (0.272 kilograms) or less of sulfur, medium-sulfur coal if its sulfur content is 0.61 to 1.67 pounds (0.277 to 0.758 kilograms), and high-sulfur coal if it contains 1.68 pounds (0.763 kilograms) or more of sulfur.

Some of the ash produced by burning powdered coal may also escape into the air. Like sulfur dioxide, such fly ash can contribute to air pollution. However, devices have been developed to trap fly ash in smokestacks and so prevent it from polluting the air.

Coal is used as a fuel chiefly in the production of electricity. Electric power plants use more than three-fourths of the coal mined in the United States.

Electric power production. Most electric power plants are steam-turbine plants. All nuclear power plants and almost all plants fueled by coal, gas, or oil are steam-turbine plants. They use high-pressure steam to generate electricity. The steam spins the wheels of turbines, which drive the generators that produce electricity. Steam-turbine plants differ mainly in how they create heat to make steam. Nuclear plants create the heat by splitting uranium atoms. Other plants burn coal, gas, or fuel oil. Steam-turbine plants produce about 70 percent of the electricity used in the United States. Coal-burning plants account for most of this output. See ELECTRIC GENERATOR; ELECTRIC POWER; TURBINE.

Bituminous coals have long been the preferred coals for electric power production because they are the most plentiful coals and have the highest heating value. Subbituminous coals and lignites have the lowest heating value. However, nearly all the subbituminous coal and about 90 percent of the lignite in the United States have a low sulfur content. On the other hand, about 50 percent of the nation's bituminous coal has a medium- or high-sulfur content. To meet federal and state pollution standards, power plants are burning more subbituminous coal and lignite. However, these coals cause problems for industry because they quickly lose their moisture, break up, and become dusty. This dustiness makes them difficult to handle and transport.

Other uses of coal as a fuel. In parts of Asia and Europe, coal is widely used for heating homes and other buildings. In the United States, natural gas and fuel oil have almost entirely replaced coal as a domestic heating fuel. However, the rising cost of oil and natural gas has led some factories and other commercial buildings to switch back to coal. Anthracites are the cleanest-burning coals, and so they are the preferred coals for heating homes. However, anthracites are also the most expensive coals. For this reason, bituminous coals are often preferred to anthracites for heating factories and other commercial buildings. Subbituminous coals and lignites have such a low heating value that they must be burned in large amounts in order to heat effectively. As a result, they are seldom used for domestic heating.

In the past, coal also provided heat for the manufacture of a wide variety of products, from glass to canned foods. Since the early 1900's, manufacturers have come to use natural gas in making most of these products. Coal is used mainly by the cement and paper industries. However, some industries have switched back to coal to avoid paying higher prices for natural gas.

Coal as a raw material

Many substances made from coal serve as raw materials in manufacturing. Coke is the most widely used of these substances. Coke is made by heating bituminous coal to about 2000 °F (1100 °C) in an airtight oven. The lack of oxygen prevents the coal from burning. The heat changes some of the solids in the coal into gases. The remaining solid matter is coke--a hard, foamlike mass of nearly pure carbon. It takes about 11/2 short tons (1.4 metric tons) of bituminous coal to produce 1 short ton (0.9 metric ton) of coke. For an illustration of the coke-making process, see the Coke article in the print version of The World Book Encyclopedia.

The coal used to make coke is called coking coal. To be suitable for coking, the coal must have various characteristics, such as a low-sulfur content and a specified amount of ash. Only certain types of bituminous coals have all the necessary characteristics.

About 90 percent of the coke produced in the United States is used to make iron and steel. Most coking plants are a part of steel mills. The mills burn coke with iron ore and limestone to change the ore into the pig iron required to make steel. It takes about 900 pounds (410 kilograms) of coke to produce 1 short ton (0.9 metric ton) of pig iron. For a description of the role of coke in the iron-making process, see the World Book article IRON AND STEEL (Raw materials).

The coke-making process is called carbonization. Some of the gases produced during carbonization turn into liquid ammonia and coal tar as they cool. Through further processing, some of the remaining gases change into light oil. Manufacturers use the ammonia, coal tar, and light oil to make such products as drugs, dyes, and fertilizers. Coal tar is also used for roofing and for road surfacing.

Some of the gas produced during carbonization does not become liquid. This coal gas, or coke oven gas, burns like natural gas. But coal gas has a lower heating value and, unlike natural gas, gives off large amounts of soot as it burns. Coal gas is used chiefly at the plants where it is produced. Coal gas provides heat for the coke-making and steel-making processes.

Gas can be produced from coal directly, without carbonization, by various methods. Such methods are known as gasification. The simplest gasification method involves burning coal in the presence of forced air or steam. The resulting gas, like coke oven gas, has a low heating value and produces soot. It is used chiefly in some manufacturing processes. Coal can be used to make high-energy gas and such high-energy liquid fuels as gasoline and fuel oil. But the present methods of producing these fuels from coal are costly and complex. The section The coal industry discusses how researchers are working to develop cheaper and simpler methods.

COAL/How coal is mined

Coal mines can be divided into two main groups: (1) surface mines and (2) underground mines. In most cases, surface mining involves stripping away the soil and rock that lie over a coal deposit. This material is known as overburden. After the overburden has been removed, the coal can easily be dug up and hauled away. Underground mining involves digging tunnels into a coal deposit. Miners must go into the tunnels to remove the coal.

Surface mining is usually limited to coal deposits within 100 to 200 feet (30 to 61 meters) of the earth's surface. The more overburden that must be removed, the more difficult and costly surface mining becomes. Most coal deposits deeper than 200 feet are mined underground. Surface mines produce about 60 percent of the coal mined in the United States. Underground mines produce the rest.

Surface mining

Nearly all surface mining is strip mining-that is, mining by first stripping away the overburden. Many coal seams are exposed on the sides of hills or mountains. In some cases, these seams are mined from the surface without removing any overburden. Miners use machines called augers to dig out the coal. This method of surface mining is known as auger mining.

Strip mining depends on powerful machines that dig up the overburden and pile it out of the line of work. The dug-up overburden is called spoils. In time, a strip mine and its spoils may cover an enormous area. The digging up of vast areas of land has caused serious environmental problems in the past. As a result, the U.S. government now requires that all new strip-mined land be reclaimed-that is, returned as closely as possible to its original condition. Strip mining thus involves methods of (1) mining the coal and (2) reclaiming the land.

Mining the coal. Most strip mines follow the same basic steps to produce coal. First, bulldozers or loaders clear and level the vegetation and soil above the mining area. Many small holes are then drilled through the rocky overburden to the coal bed. Each hole is loaded with explosives. The explosives are set off, shattering the rock in the overburden. Giant power shovels or other earthmoving machines then clear away the broken rock. Some of these earthmovers are as tall as a 20-story building and can remove more than 3,500 short tons (3,180 metric tons) of overburden per hour. After a fairly large area of coal is exposed, explosives may be used again. Coal-digging machines then scoop up the coal and load it into trucks. The trucks carry the coal from the mine.

Although most strip mines follow the same basic steps, strip-mining methods vary according to whether the land is flat or hilly. Strip mining can thus be classed as either (1) area mining or (2) contour mining. Area mining is practiced where the land is fairly level. Contour mining is practiced in hilly or mountainous country. It involves mining on the contour-that is, around slopes.

In area mining, an earthmover digs up all the broken overburden from a long, narrow strip of land along the edge of the coal field. The resulting deep ditch is referred to as a cut. As the earthmover digs the cut, it piles the spoils along the side of the cut that is away from the mining area. The piled spoils form a ridge called a spoil bank. After the cut is completed, the coal is dug, loaded into trucks, and hauled away. The earthmover then digs an identical cut alongside the first one. It piles the spoils from this cut into the first cut. This process is repeated over and over across the width of the coal field until all of the coal has been mined. The spoil banks form a series of long, parallel ridges on the land that can later be leveled.

Area mining is impractical where coal seams are embedded in hills. If a seam lies near the top of a hill, an earthmover may simply remove the hilltop and so expose the coal. If a seam lies near the base of a hill, it must be mined on the contour.

In contour mining, an earthmover removes the shattered overburden immediately above the point where a seam outcrops (is exposed) all around a hill. The resulting cut forms a wide ledge on the hillside. The spoils may be stored temporarily on the hillside or used to fill in the cuts. After the exposed coal has been mined and hauled away, the earthmover may advance up the slope and dig another cut immediately above the first one. However, the depth of the overburden increases sharply with the rise of the slope. After the first or second cut, the overburden may be too great for a coal company to remove profitably. But if the seam is thick enough, a company may dig an underground mine to remove the rest of the coal.

Reclaiming the land. The chief environmental problems that strip mining can cause result from burying fertile soil under piles of rock. The rocks tend to give off acids when exposed to moisture. Rainwater runs down the bare slopes, carrying acids and mud with it. The runoff from the slopes may wash away fertile soil in surrounding areas and pollute streams and rivers with acids and mud.

The first step in reclaiming strip-mined land is to reduce the steep slopes formed by the spoils. The spoil banks created by area mining can be leveled by bulldozing. The spoils from contour mining can be used to fill in the cuts in the hillsides. As much topsoil as possible should then be returned to its original position so that the area can be replanted.

Mining companies now reclaim all strip-mined land, much of which has been turned into farms and recreation areas. In 1977, the U.S. Congress passed a law requiring mine owners to reclaim all the land they use for strip mining after 1978. In every case, the mine owners must restore the land as nearly as possible to its original condition. Many older strip-mined lands have not been reclaimed. But since 1977, the government has collected over $3 billion in fees from coal producers to reclaim these older mines.

Auger mining. A coal auger is a machine shaped like an enormous corkscrew. It bores into the side of a coal outcrop on a slope and twists out the coal in chunks. Contour mines often use augers when the overburden in a slope is too great to remove. An auger can penetrate the outcrop and recover coal that could not otherwise be mined. Some augers can bore 200 feet (61 meters) or more into a hillside.

Companies often employ auger mining to mine outcrops of high-quality coal that cannot be mined economically by other methods. However, auger mining can recover only a small portion--as little as 15 percent--of the coal in a seam. The method is best used in combination with contour mining.

Underground mining

Underground mining is more hazardous to workers than surface mining. The miners may be injured or killed by cave-ins, falling rocks, explosions, and poisonous gases. To prevent such disasters, every step in underground coal mining must be designed to safeguard the workers.

Underground mining generally requires more human labor than surface mining. But even so, underground mines are highly mechanized. Machines do all the digging, loading, and hauling in nearly all the mines. Nonmechanized mines produce only about 1 percent of the coal mined underground in the United States.

In most cases, miners begin an underground mine by digging two access passages from the surface to the coal bed. One passage will serve as an entrance and exit for the miners and their equipment. The other passage will be used to haul out the coal. Both passages will also serve to circulate air in and out of the mine. As the mining progresses, the workers dig tunnels from the access passages into the coal seam.

Underground mines can be divided into three main groups according to the angle at which the access passages are dug into the ground. The three groups are (1) shaft mines, (2) slope mines, and (3) drift mines. Some mines have two or all three types of passages.

In a shaft mine, the access passages run straight down from the surface to the coal seam. The entrance and exit shaft must have a hoist. Most mines under more than 700 feet (210 meters) of cover are shaft mines. In a slope mine, the access passages are dug on a slant. They may follow a slanting seam or slant down through the cover to reach the seam. Drift mines are used to mine seams of coal that outcrop in hills or mountains. The access passages for these mines are dug into a seam where the coal bed outcrops on a slope.

Two main systems of underground mining are used: (1) the room-and-pillar system and (2) the longwall system. Each system has its own set of mining techniques. Either system may be used in a shaft, slope, or drift mine. The room-and-pillar system is by far the more common system of underground mining in the United States. The longwall system is more widely used elsewhere, especially in European countries.

The room-and-pillar system involves initially leaving pillars of coal standing in a mine to support the overburden. Miners may begin a room-and-pillar mine by digging three or more long, parallel tunnels into the coal seam from the access passages. These tunnels are called main entries. In most cases, the walls, or ribs, of coal separating the main entries are 40 to 80 feet (12 to 24 meters) wide. Cuts are made through each wall every 40 to 80 feet. The cuts thus form square or rectangular pillars of coal that measure 40 to 80 feet on each side. The coal dug in building the entries is hauled to the surface.

The pillars help support the overburden in the main entries. But in addition, the entry roofs must be bolted to hold them in place. To bolt the roof, the miners first drill holes 3 to 6 feet (0.9 to 1.8 meters) or more into the roof. They then anchor a long metal bolt into each hole and fasten the free end of each bolt to the roof. The bolts bind together the separate layers of rock just above the roof to help prevent them from falling. The miners must also support the roof in all other parts of the mine as they are developed.

A conveyor belt or a railroad track is built in one of the main entries to carry the coal to the access passages. A railroad may also transport the miners along the main entries. At least two main entries serve chiefly to circulate air through the mine. The mine may also need such facilities as water drainage ditches, gas drainage pipes, compressed air pipes, water pipes, and electric power cables. These facilities are built into the main entries and later extended to other parts of the mine.

After the main entries have been constructed, the miners dig sets of subentries at right angles from the main entries into the coal seam. Each set of subentries consists of three or more parallel tunnels, which serve the same purposes as the main entries. Cuts are made through the walls separating these tunnels, forming pillars like those between the main entries. At various points along each set of subentries, the miners dig room entries at right angles into the seam. They then begin to dig rooms into the seam from the room entries.

As the miners enlarge a room, they leave pillars of coal to support the overburden. A room is mined only a certain distance into the seam. When this distance is reached, the miners may remove the pillars. The room roof collapses as the pillars are removed, and so they must be removed in retreat-that is, from the back of the room toward the front. The miners' exit from the room thus remains open as the roof falls. Pillars are also sometimes removed from entries. Like room pillars, they must be removed in retreat to protect the miners.

All room-and-pillar mining involves leaving some pillars in place. Room-and-pillar mines differ, however, in their mining methods. Mechanized room-and-pillar mines use two main methods: (1) the conventional mechanized method and (2) continuous mining.

The conventional mechanized method produces about 10 percent of the coal mined underground in the United States. This method was more widely practiced during the 1930's and 1940's than it is today. During the 1930's, it largely replaced the earlier method of digging coal by hand. Since about 1950, continuous mining has increasingly replaced the conventional method.

The conventional method involves five main steps. (1) A machine that resembles a chain saw cuts a long, deep slit, usually along the base of the coal face. (2) Another machine drills a number of holes into the face. (3) Each hole is loaded with explosives. The explosives are set off, shattering the coal. The undercutting along the bottom of the face causes the broken coal to fall to the floor. (4) A machine loads the coal onto shuttle cars, scoops, or a conveyor. (5) Miners bolt the roof that has been exposed by the blast.

A separate crew of miners carries out each of the five steps. After a crew has completed its job on a particular face, the next crew moves in. The miners can thus work five faces of coal at a time. But there are frequent pauses in production as the crews change places.

Continuous mining accounts for about 60 percent of the output of U.S. underground coal mines. The method uses machines called continuous miners. A continuous miner gouges the coal from the coal face-that is, the coal exposed on the surface of a wall. One worker operating a continuous miner can produce about 2 short tons (1.8 metric tons) of coal per hour. The machine automatically loads the coal onto shuttle cars or a conveyor belt, which carries it to the railroad or conveyor in the main entries.

A continuous miner can usually dig and load coal much faster than the coal can be hauled out of a mine. The machine can work faster than the haulage, roof-bolting, ventilation, construction, and drainage systems can be completed. As a result, a continuous miner must frequently be stopped to allow the other mine systems to catch up.

The longwall system of underground mining involves digging main tunnels or entries like those in a room-and-pillar mine. However, the coal is mined from one long face, called a longwall, rather than from many short faces in a number of rooms.

A longwall face is about 300 to 700 feet (91 to 210 meters) long. The miners move a powerful cutting machine back and forth across the face, plowing or shearing off the coal. The coal falls onto a conveyor belt. Movable steel props support the roof over the length of the immediate work area. As the miners work the machine farther into the seam, the roof supporters are advanced. The roof behind the miners is allowed to fall. After a face has been dug out 4,000 to 6,000 feet (1,200 to 1,800 meters) into the seam, a new face is developed and mined. This process is repeated over and over until as much coal as possible has been removed from the seam.

The longwall system originated in Europe. Underground mines in Europe are much deeper, on the average, than U.S. underground mines. The pressure of the overburden becomes intense in an extremely deep mine. Longwall mining relieves the pressure by allowing the roof to cave in throughout most of a mine. In a European longwall mine, the roof remains in place only over the main entries, over the longwall face, and over two tunnels leading to the face. The mines can thus recover up to 90 percent of the coal in a seam.

Mine safety laws in the United States require longwall mines to have fully developed subentries as well as main entries. Thus U.S. longwall mining includes some of the main features of the room-and-pillar system. One kind of longwall mining is called the retreating longwall system. This type of mining uses the room-and-pillar system to reach and expose the long coal face. Longwall equipment then mines the coal. These mines are much more productive than room-and-pillar mines because less coal is left in place.

Longwall mines produce about 30 percent of the coal mined underground in the United States. However, more and more U.S. mines are adopting longwall techniques. A few American mines have adopted another variation of the longwall method called shortwall mining. A shortwall face is only about 150 to 200 feet (46 to 61 meters) long, and it is mined with continuous-mining machines rather than with longwall equipment. This system, which was developed in Australia, is suited to coal seams whose structure prevents them from being divided into long faces.

COAL/Cleaning and shipping coal

Some coal is shipped to buyers exactly as it comes from the mine without any processing. In the coal industry, such coal is called run-of-mine coal. It ranges in size from fine particles to large chunks. About 10 percent of the coal sold by U.S. companies is run-of-mine coal.

The two largest users of coal, the electric power industry and the coking industry, have definite quality requirements for the coal they buy. Much run-of-mine coal does not meet these requirements because it is the incorrect size or contains unacceptable amounts of impurities. As a result, mining companies sort their coal according to size and clean the coal to remove impurities. The companies sort about 50 percent of their coal without cleaning it, and both sort and clean 40 percent of the coal.

Companies sort coal by crushing large pieces and passing the coal through a screening device. The following section describes the cleaning process.

Cleaning coal. Mining companies clean coal in preparation plants. Most large coal mines have a preparation plant on the mine property. The plants use a variety of machines and other equipment to remove the impurities from coal.

Ash and sulfur are the chief impurities in coal. The ash consists chiefly of mineral compounds of aluminum, calcium, iron, and silicon. Some of the sulfur in coal is also in the form of minerals, especially pyrite, a compound of iron and sulfur. The rest is organic sulfur, which is closely combined with the carbon in coal. Run-of-mine coal may also contain pieces of rock or clay. These materials must be removed in addition to the other impurities.

Preparation plants rely on the principle of specific gravity to remove impurities. According to this principle, if two solid substances are placed in a solution, the heavier substance will settle to the bottom first. Most mineral impurities in coal are heavier than pure coal. As a result, these impurities can be separated from run-of-mine coal that is placed in a solution. The entire coal-cleaning process involves three main steps: (1) sorting, (2) washing, and (3) dewatering.

Sorting. Large pieces of pure coal may settle to the bottom of a solution faster than small pieces that have many impurities. Therefore, the pieces must first be sorted according to size. In many preparation plants, a screening device sorts the coal into three sizes--coarse, medium, and fine. Large chunks are crushed and then sorted into the three main batches according to size.

Washing. The typical preparation plant uses water as the solution for separating the impurities from coal. Each batch of sorted coal is piped into a separate washing device, where it is mixed with water. The devices separate the impurities by means of specific gravity. The heaviest pieces--those containing the largest amounts of impurities--drop into a refuse bin. Washing removes much of the ash from coal. But the organic sulfur is so closely bound to the carbon that only small amounts can be removed.

Dewatering. The washing leaves the coal dripping wet. If this excess moisture is not removed, the heating value of the coal will be greatly reduced. Preparation plants use vibrators, spinning devices called centrifuges, and hot-air blowers to dewater coal after it is washed.

In most cases, the separate batches of coal are mixed together again either before or after dewatering. The resulting mixture of various sizes of coal is shipped chiefly to electric power companies and coking plants. All coking plants and many power companies grind coal to a powder before they use it. They therefore accept shipments of mixed sizes. Some coal users require coal of a uniform size. Preparation plants that supply these users leave the cleaned coal in separate batches.

Shipping coal. Most coal shipments within a country are carried by rail, barge, or truck. A particular shipment may travel by two or all three of these means. Huge cargo ships transport coal across oceans, between coastal ports, and on large inland waterways, such as the Great Lakes.

Barges provide the cheapest way of shipping coal within a country. But they can operate only between river or coastal ports. Trucks are the least costly means of moving small shipments of coal short distances by land. Much coal, however, must be shipped long distances over land to reach buyers. Railroads offer the most economical means of making such shipments. About two-thirds of the coal shipped from mines in the United States goes by rail.

Many large shipments of coal in the United States are delivered to electric power companies and coking plants by unit trains. A unit train normally carries only one kind of freight and travels nonstop from its loading point to its destination. A 100-car unit train may carry 10,000 short tons (9,100 metric tons) or more of coal. To meet the need for low-sulfur coal, more and more power plants east of the Mississippi River are importing subbituminous coal from the West. Unit trains help speed such long-distance shipments.

A 273-mile (439-kilometer) underground pipeline carries coal from a mine in Arizona to a power plant in Nevada. The coal is crushed and mixed with water to form a slurry (soupy substance) that can be pumped through the pipeline. The coal and power industries favor building other such pipelines in the United States.

In the past, nearly all coal shipments consisted of anthracite, bituminous coal, or subbituminous coal. It costs as much to ship a given amount of lignite as it costs to ship the same amount of a higher-ranking coal. But lignite has the lowest heating value of the four ranks. It therefore could not formerly compete with the higher-ranking coals in distant markets. Lignite was used chiefly by power plants built in the lignite fields. Conveyor belts or small railways carried the coal from the mines to the plants. But a growing need for low-sulfur coal and improvements in coal preparation technology have increased the amount of lignite shipped in the United States. Some is shipped by rail from mines in the Western United States to plants in the Midwest.

COAL/The coal industry

In most countries, the central government owns all or nearly all the coal mines. The major exceptions are Australia, Canada, Germany, South Africa, and the United States. In these countries, all or nearly all the coal mines are privately owned. In each of these countries, however, the central government regulates certain aspects of the coal industry.

Australia and the United States are the leading coal exporters in the world. The other leading exporters include Canada, China, Indonesia, Poland, and South Africa. Japan purchases approximately 30 percent of the world's coal exports--far more than any other country.

This section of the article deals chiefly with the coal industry in the United States. However, much of the information in this section also applies to the coal industry in other countries.

Coal producers. The United States has about 3,500 active coal mines and about 3,000 coal-mining companies. Most of the companies are small, independent firms that own and operate one or two small mines. All the small companies together supply less than a third of the coal mined in the United States. The 30 largest coal companies in the United States produce about two-thirds of the nation's coal. Some of the companies are independently owned, but many are owned by corporations outside the coal industry. The chief outside owners of coal-mining companies include oil companies, railroads, and ore-mining firms.

Steel companies and electric utilities in the United States also own coal mines. These companies produce coal chiefly for their own use. Their mines are known as captive mines.

The National Coal Association (NCA) works to promote the interests of coal producers. The NCA is jointly sponsored by the producers and the firms that supply them with equipment, technical advice, and transportation. The association tries to increase efficiency within the industry, to encourage favorable legislation, and to inform the public about the industry. The National Independent Coal Operators Association represents the smaller coal producers.

Mineworkers. Most large coal-mining companies have a full-time staff of professional workers, including engineers, lawyers, and business experts. They also employ electricians, mechanics, and construction workers. Skilled miners, however, provide the labor on which the industry depends. Underground mining requires more miners than does surface mining. The United States has about 105,000 coal miners. About two-thirds of them work in underground mines.

Mechanization has helped miners become more productive. In 1950, each coal miner in the United States produced, on the average, about 7 short tons (6.4 metric tons) of coal daily. Today, the production rate averages about 32 short tons (29 metric tons) per miner per day. On the average, a strip miner produces more than twice as much coal as does an underground miner.

Increased mechanization has also made miners' jobs more specialized. The job of most miners is to operate a certain type of machine, such as a continuous miner or a power shovel. A beginning miner must work as an apprentice for a specified period to qualify for a particular job. Mine supervisors must have a license from the department of mining in their state. Generally, the licenses are granted to miners who have two to five years' experience and who pass a written examination.

Most mining engineering jobs call for a college degree in engineering. If the job is directly related to mine safety, it may also require a state engineering license called a P.E. (professional engineer) certificate. Some mining engineering jobs require a P.E. certificate only. The states grant P.E. certificates to applicants who meet certain educational requirements, have on-the-job experience, and pass a written examination. In some states, applicants must have an engineering degree. Other states require only a high school education.

Labor unions. About 45 percent of all coal miners in the United States belong to the United Mine Workers of America (UMW). The UMW was organized in 1890. At that time, the nation's coal miners lived and worked under miserable conditions. The mines were dangerously unsafe, and the miners earned barely enough to live on. Most miners and their families lived in company towns, which were owned and run by the mining companies. In many company towns, the housing and other facilities were far from adequate. Frequently, miners were not paid in cash. Instead, the mining companies gave them coupons that could be exchanged for goods at company-owned stores or used to pay rent on a company-owned house. The store prices and rents were unreasonably high in many cases, and some miners were always in debt to the mining companies.

During the first half of the 1900's, the UMW did much to improve the wages and working conditions of American coal miners. Through strikes and hard bargaining, the union forced the mining companies to grant the miners increasingly favorable work contracts. The UMW owed much of its success to the vigorous leadership of John L. Lewis, who headed the union from 1919 to 1960. During Lewis' long term as UMW president, the union had the overwhelming support of its members.

Although the UMW is still important, its influence has declined. This change partly reflects a steadily improving mine safety record. Also, increased mechanization and the closing of inefficient mines has reduced the total number of jobs.

This change is also due to the rapid growth of strip mining. Strip mining requires fewer miners than does underground mining. It also requires a different type of miner. Strip miners are chiefly heavy-machine operators. Unlike underground miners, they have little need for traditional mining skills. Some strip miners are members of the UMW. But many belong to various building trades unions or to no union.

The UMW has also lost influence among its members. Many UMW members feel that their contracts with the mining companies are still far from satisfactory. The miners want better health and retirement benefits and stricter mine safety measures. During the 1970's, small groups of miners frequently took matters into their own hands and went out on wildcat strikes, which did not have the approval of union leaders.

Mine safety.

Since 1900, more than 100,000 workers have been killed in coal mine accidents in the United States. Many more have been injured or disabled. Because of this extremely high accident rate, more and more aspects of mine safety have been brought under government regulation.

The federal government and the governments of the coal-mining states set minimum health and safety standards that the coal companies and miners have to follow. To make sure that all miners know their responsibilities, the companies must give every new miner a course in mine safety. The improvements in mine safety have greatly reduced the death and injury rates from mine accidents. In the early 1900's, about 3.5 miners per 1,000 were killed in mine accidents annually. The annual death rate has dropped to about .5 today--an improvement of about 85 percent.

Mine safety involves four main types of problems. They are (1) accidents involving machinery, (2) roof and rib failures, (3) accumulations of gases, and (4) concentrations of coal dust.

Accidents involving machinery kill or injure more U.S. coal miners in a typical year than any other kind of mining accident. Most strip mine accidents involve machinery. The machines in underground mines must often operate in cramped, dimly lit spaces. Thus, the miners must be doubly alert to prevent accidents.

Roof and rib failures can be prevented in many cases if a mining company carries out a scientific roof support plan. The federal government requires all U.S. mining companies to draw up such a plan for any new mine. The government must then approve the plan before mining is begun. Mining engineers make a roof support plan after studying all the rock formations surrounding the coal bed. The plan deals with such matters as the number of pillars that must be left standing, entry widths, mine geometry, and the number of roof bolts that must be used.

Accumulations of gases. Certain gases that occur in underground coal mines can become a serious hazard if they accumulate. Methane and carbon monoxide are especially dangerous. Methane is an explosive gas that occurs naturally in coal seams. It is harmless in small amounts. However, a mixture of 5 to 15 percent methane in the air can cause a violent explosion. Carbon monoxide is a poisonous gas produced by the combustion of such fuels as coal and oil. Blasting in an underground mine may produce dangerous levels of carbon monoxide if the mine is improperly ventilated.

The air vents in a mine normally prevent harmful gases from accumulating. A powerful fan at the surface circulates fresh air through the mine. The circulating air forces polluted air to the surface. As an added precaution against methane, federal law requires all underground mines to have automatic methane detectors. A mine is required to shut down temporarily if a detector shows a methane accumulation of more than 2 percent.

Concentrations of coal dust. Anyone who breathes large amounts of coal dust over a period of years may develop a disease called pneumoconiosis or black lung (see BLACK LUNG). The disease interferes with breathing and may eventually cause death. Thousands of coal miners have been victims of the disease. In addition, high concentrations of coal dust are explosive. A mixture of coal dust and methane is especially dangerous.

Proper ventilation removes much of the coal dust from the air in a mine. However, mines must also use other dust control measures. In the United States, federal law requires that underground mines be rockdusted. In this process, the miners spray powdered limestone on all exposed surfaces in the mine entries. The limestone dilutes and coats the coal dust and so lessens the chance of an explosion. Mines use water sprays to hold down the dust along a face that is being mined.

Government regulation. State departments of mining have traditionally set and enforced safety standards for American coal mines. In the past, the U.S. Bureau of Mines had this responsibility at the federal level. On occasion, Congress has made urgently needed standards a matter of law, as in the Federal Coal Mine Health and Safety Act of 1969. This act strengthened the safety standards for mine ventilation, coal dust concentrations, roof supports, and mining equipment. The regulation of coal dust has helped reduce the occurrence of black lung among miners. In addition, the act established a federal black lung benefits program. This program provides financial and medical benefits to miners already disabled by black lung.

The Mine Safety and Health Administration enforces federal mine safety standards. This agency routinely inspects the mines for safety. The United States Department of the Interior, the United States Environmental Protection Agency (EPA), and state environmental protection agencies regulate the environmental aspects of U.S. coal-mining activities.

Coal research is sponsored by the U.S. Department of Energy, the U.S. Department of the Interior, the EPA, and several coal and oil companies. The goals of most coal research are (1) to find ways to burn more coal without increasing air pollution and (2) to develop economical methods of converting coal into liquid fuels and synthetic natural gas.

Pollution control. In 1977, Congress passed a law requiring all U.S. electric power plants built since 1971 to meet federal pollution standards by 1982. These standards, which were further tightened in the Clean Air Act of 1990, prohibit the burning of medium- or high-sulfur coals without a means of controlling sulfur dioxide pollution. Medium- and high-sulfur coals make up more than a third of all U.S. coal reserves. These resources can be used for electric power production only after ways are found to control sulfur dioxide pollution.

Cleaning removes some of the sulfur from coal. But it does not remove enough from high-sulfur and some medium-sulfur coals to meet air quality standards. Sulfur dioxide can be controlled to some extent by devices called scrubbers. A scrubber absorbs sulfur dioxide fumes as they pass through a plant's smokestacks.

Researchers are now working to develop new processes for using coal to produce power. These processes will make coal use more efficient and safer for the environment. They include fluidized-bed combustion. In this process, crushed coal is burned in a bed of limestone. The limestone captures sulfur from the coal and so prevents sulfur dioxide from forming. The heat from the burning coal boils water that is circulated through the bed in metal coils. The boiling water produces steam, which may be used to produce electricity.

Coal conversion. To turn coal into a high-energy fuel, the hydrogen content of the coal must be increased. Bituminous coals have the highest hydrogen content of the four ranks of coal. On the average, they consist of about 5 percent hydrogen. The hydrogen must be increased to about 12 percent to produce a high-energy liquid fuel and to about 25 percent to produce synthetic natural gas.

The process of converting coal into a liquid fuel is called coal hydrogenation or liquefaction. Various methods of coal hydrogenation have been developed. In the typical method, a mixture of pulverized coal and oil is treated with hydrogen gas at high temperatures and under great pressure. The hydrogen gradually combines with the carbon molecules, forming a liquid fuel. This process can produce such high-energy fuels as gasoline and fuel oil.

Coal can easily be turned into low-energy gas by the carbonization and gasification methods described in the section The uses of coal. Low-energy gas can also be produced from unmined coal. This process, called underground gasification, involves digging two widely spaced wells from ground level to the base of a coal seam. The coal at the bottom of one well is ignited. Air is blown down the second well. The air seeps through pores in the seam, and the fire moves toward it. After a passage has been burned between the two wells, the air current forces the gases up the first well. Compared with natural gas, low-energy gas made from coal has limited uses. Low-energy gas must be enriched with hydrogen for its heating value to equal that of natural gas.

The present methods of obtaining high-energy fuels from coal cost too much for commercial use. Hydrogen is expensive to produce. In addition, most fuels made from coal contain unacceptable amounts of sulfur and ash. Researchers are trying to develop cheaper methods of coal conversion. In 1980, Congress passed the Energy Security Act, which provides federal funding for coal conversion research and for the construction of synthetic fuel plants. In 1984, two commercial-sized coal gasification plants began operating in the United States. However, both plants rely heavily on the federal government for financial support.

COAL/History of coal use

No one knows where or when people discovered that coal can be burned to provide heat. The discovery may have been made independently in various parts of the world during prehistoric times. The Chinese were the first people to develop a coal industry. By the A.D. 300's, they were mining coal from surface deposits and using it to heat buildings and smelt metals. Coal had become the leading fuel in China by the 1000's.

Commercial coal mining developed more slowly in Europe. During the 1200's, a number of commercial mines were started in England and in what is now Belgium. The coal was dug from open pits and used mainly for smelting and forging metals. But most Europeans regarded coal as a dirty fuel and objected to its use.

Wood, and charcoal made from wood, were the preferred fuels in Europe until the 1600's. During the 1600's, a severe shortage of wood occurred in western Europe. Many western European countries, but especially England, then sharply increased their coal output to relieve the fuel shortage.

Developments in England. During the 1500's, English factories burned huge quantities of charcoal in making such products as bricks, glass, salt, and soap. By the early 1600's, wood had become so scarce in England that most factories had to switch to coal. By the late 1600's, England produced about 80 percent of the world's total coal output. It remained the leading coal producer for the next 200 years.

Charcoal had also been widely used in England as a fuel for drying malt, the chief ingredient in beer. Brewers tried using coal for this process. But the gases it produced were absorbed by the malt and so spoiled the flavor of the beer. The brewers found, however, that the undesirable gases could be eliminated if they preheated the coal in an airtight oven. They thus developed the process for making coke. About 1710, an English ironmaker named Abraham Darby succeeded in using coke to smelt iron. Coke then gradually replaced charcoal as the preferred fuel for ironmaking.

The spread of the new ironmaking process became part of a much larger development in England--the Industrial Revolution. The revolution consisted chiefly of a huge increase in factory production. The increase was made possible by the development of the steam engine in England during the 1700's. Steam engines provided the power to run factory machinery. But they required a plentiful supply of energy. Coal was the only fuel available to meet this need.

During the 1800's, the Industrial Revolution spread from England to other parts of the world. It succeeded chiefly in countries that had an abundance of coal. Coal thus played a key role in the growth of industry in Europe and North America.

Developments in North America. The North American Indians used coal long before the first white settlers arrived. For example, the Pueblo Indians in what is now the Southwestern United States dug coal from hillsides and used it for baking pottery. European explorers and settlers discovered coal in eastern North America during the last half of the 1600's. In the 1700's, a few small coal mines were opened in what are now Nova Scotia, Virginia, and Pennsylvania. The mines supplied coal chiefly to blacksmiths and ironmakers. Most settlers saw no advantage in using coal as long as wood was plentiful. Wood and charcoal remained the chief fuels in America throughout the 1700's.

The Industrial Revolution spread to the United States during the first half of the 1800's. By then, coal was essential not only to manufacturing but also to transportation. Steamships and steam-powered railroads were becoming the chief means of transportation, and they required huge amounts of coal to fire their boilers. As industry and transportation grew in the United States, so did the production and use of coal. By the late 1800's, the United States had replaced England as the world's leading coal producer.

The United States led in coal production until the mid-1900's. Its demand for coal then declined as the use of petroleum and natural gas increased. The Soviet Union surpassed the United States in coal production from the late 1950's through the late 1970's. During the 1980's and 1990's, China usually ranked first, and the United States usually ranked second.

Recent developments. The growing scarcity of petroleum and natural gas has led to a sharp rise in the demand for coal. As a result, coal production in the United States increased greatly in the 1970's, 1980's, and 1990's. The increased output was used mainly to produce electric power. Today, electric power can be produced more cheaply from coal than from either natural gas or fuel oil.

Contributor: Joseph W. Leonard, III, M.S., Former Mining Engineering Foundation Professor, University of Kentucky.

Critically reviewed by the National Coal Association

Questions

How was coal formed?

What is by far the chief system of underground coal mining in the United States?

What is the main use of coal?

Why are coal miners more productive today than they were in the past?

How is coal usually classified? What are the four main classes of coal?

What are preparation plants? Why are they needed?

Why did many countries in western Europe sharply increase their coal output during the 1600's?

What is strip mining? What environmental problems can it cause? How can these problems be prevented?

Why has coal mining become a less dangerous occupation than it was in the past?

Why have more and more power plants switched from bituminous coal to subbituminous coal or lignite?

Additional resources

Level I

Dineen, Jacqueline. Coal. Enslow, 1988.

Hansen, Michael C. Coal: How It Is Found and Used. Enslow, 1990.

Level II

National Research Council Staff. Coal: Energy for the Future. National Academy Pr., 1995.

Shifflett, Crandall A. Coal Towns: Life, Work and Culture in Company Towns of Southern Appalachia, 1880-1960. Univ. of Tenn. Pr., 1991.

Speight, James G. The Chemistry and Technology of Coal. 2nd ed. Marcel Dekker, 1994.

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