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Explosives - ANFO (Ammonium Nitrate - Fuel Oil)

Mining is the search for, extraction, and beneficiation and processing of solid minerals from the earth. The kinds of minerals extracted from the earth vary widely. For thousands of years, these and other minerals have provided the raw materials with which human civilizations have been built.

The vitality of the U.S. economy depends on key mineral resources. In the course of a lifetime, each American will use 3.5 million pounds of minerals, metals, and fuels. Every year, 46,000 pounds of new minerals including 7,500 pounds of coal energy must be provided for every person in the U.S. to maintain a high standard of living. Low-cost coal is used to generate a large portion of the nation's electricity supply, helping to keep U.S. electricity costs among the lowest in the world and thereby enhancing the competitiveness of U.S. industry. The contribution that the mining industry has made to the economic health, well being, and security of the U.S. throughout its history is unquestioned.

Surface mining and underground mining are the prevailing mining methods. The method selected depends on a variety of factors including the nature and location of the deposit as well as the size, depth, and grade of the minerals. Both surface and underground mining are widely used in the extraction of coal. In 2000, the total amount of coal produced was 1.07 billion tons. Of this, 373 million tons or 35 percent came from underground mines and the remaining 699 million tons or 65 percent came from surface mines. Of the 1.3 billion crude metal ore produced in the US in 2000, 1.2 billion tons or 92 percent came from surface mining. Most of the industrial minerals in the US are extracted by surface mining. In 2000, the total amount of crude industrial ore mined in the US was 3.2 billion tons. Of this, 3.1 billion or 96 percent came from surface mines.

Bingham Canyon near Salt Lake City, Utah, is probably the best-known example of a surface mine in the U.S. It is also the largest surface mine in the US, measuring approximately 2 miles in diameter and a little more than a half mile deep. Conical in shape and used for near-surface ore bodies, these mines feature a series of benches winding down into the pit. The benches are used as working areas and haul roads. The rock is drilled, blasted and loaded into large haul trucks that take it to a processing facility.

Underground mining is used when mineralization is deep beneath the surface and/or when ore grade or quality is sufficient to justify more targeted mining. In order to get to the ore body, a vertical shaft, horizontal adit, or inclined passageway must be drilled remove ore and waste and provide ventilation. Once the ore body is exposed, several levels of horizontal tunnels called drifts and crosscuts are created to provide access to mining areas called stopes. The area actually being mined at any given time is called the face. Broken rock is hauled from the face by trains, loaders, or trucks that go directly to the surface, or to the shaft where it is hoisted to the surface and sent to a processing facility.

There are seven general phases of the mining process. These are: 1) mine exploration; 2) design; 3) construction, which includes mine site preparation; 4) extraction operations in either underground or surface mines; 5) beneficiation operations consisting of crushing and grinding, separations, solvent extraction/electrowinning; 6) processing, which consists of smelting and/or refining depending on the mineral and the final product; and 7) reclamation (closure and post-closure). In each of these stages, there are a variety of equipment and materials used.

Exposives and Coal Mining

Coal is extracted principally in two ways: surface mining and underground mining. The mining method used depends primarily on the depth of the coal bed from the surface and the surrounding terrain. Coal beds deeper than 100 to 200 feet or on hilly terrain are usually mined by underground methods, while those at lesser depths are surface mined. Surface mining accounts for about 60 percent of the total U.S. coal production of 1 billion tons/year. A large surface mine can be three miles long and a mile wide. Underground mining accounted for 430 million short tons in 1998, about 40 percent of total US coal production that year.

Coal fields in the eastern U.S. are characterized by relatively thin seams of deeply buried coal. Eastern coal generally has a high heating value. Conversely, relatively thick seams of shallow reserves characterize the coalfields in the western U.S. Western coal generally has a low heating value. These deposit characteristics also greatly influence the mining method used. Underground mining is more frequently used on the thinner seams of the eastern coalfields. However, surface mining is most frequently used in the West. These deposits require relatively low-volume, shallow digging to expose thick coal seams for recovery.

In general, where favorable coal seam conditions exist, surface mining is the least expensive and most productive method of taking coal from the ground. Used when the coal seam is relatively close to the surface, it can result in the removal of as much as 95 percent of the total coal from a particular deposit.

Area surface mining is done on relatively flat land under which the coal is buried at roughly uniform depth. In this method, the overburden from a 100- to 200-foot-wide cut is used to fill the mined-out area of the preceding cut. Contour surface mining follows coal beds lying in hillsides. Excavation begins at a location where coal and surface elevations are the same. It proceeds towards the center of the hill or mountain until the overburden becomes too thick to remove economically. Mountain top removal mining is used to recover coal buried at or near the summit of a large hill or mountain by entirely removing the elevated area.

Most surface mines follow the same basic steps to produce coal. Bulldozers clear and level the mining area; topsoil is removed and stored for later use in the reclamation process; holes are drilled through the overburden, loaded with explosives and discharged, shattering the rock in the overburden; power shovels or draglines clear away the overburden until the coal is exposed. The dragline has a large bucket suspended from the end of a boom, which may be as long as 300 feet. The bucket, which is suspended by cables, is able to scoop up to 250 tons of overburden as it is dragged across the excavation area. The dragline is one of the largest land-based machines in the world. Smaller shovels then scoop up the coal and load it onto trucks, which carry the coal to the preparation plant.

Exposives and Iron Mining

Iron is found in every state in the United States and in almost every country in the world. However, the ore must contain commercially recoverable amounts of iron in relatively large deposits or ranges if it is to be mined economically. The characteristics of iron-bearing ores vary geographically. Specifically, magnetite and hematite are the main iron-bearing ores in the Lake Superior district and in the northeastern United States, while hematite and hematite-magnetite mixtures tend to be found in ores in Alabama and the Southwest.

To be competitive, iron mining must be done on a very large scale. Surface mining is the preferred choice, although there are exceptions. Small, low-capacity mines have rapidly disappeared.

In 2000, twelve iron ore production complexes with 12 mines, 10 concentration plants, and 10 pelletizing plants were operating in Minnesota, Michigan, and six other States. The mines included eleven surface and one underground operation. Virtually all ore was concentrated before shipment. Nine mines operated by five companies accounted for 99 percent of production.

When the iron ore lies close to the surface, it often can be uncovered by stripping away a layer of dirt, sometimes only a few feet thick. The ore is mined from large open pits by progressive extraction along steps or benches. The benches provide access to progressively deeper ore, as upper-level ore is removed. After the soil and overlying rock are cleared, the ore is drilled and blasted. The portion of the ore body to be removed is first drilled in a specific pattern, and the holes are loaded with explosive mixtures and blasted. Following blasting, the fractured ore is loaded by huge electrical shovels, hydraulic excavators, or front-end loaders onto large dump trucks. The wide holes in the ground created by drilling, blasting, and ore removal are referred to as "open pits".

Exposives and Copper Mining

For nearly 5,000 years, copper was the only metal known to man. It remains one of the most used and reused of all metals. The demand for copper is due to its good strength and fatigue resistance, excellent electrical and thermal conductivity, outstanding resistance to corrosion, and ease of fabrication. Copper offers moderate levels of density, elastic modulus, and low melting temperature. It is used in electrical cables and wires, switches, plumbing, heating, roofing and building construction, chemical and pharmaceutical machinery. It is also used in alloys such as brass and bronze, alloy castings, and electroplated protective coating in undercoats of nickel, chromium, and zinc.

Copper is commonly extracted from surface, underground and increasingly, from in situ operations. In 2000, the principal mining States, in descending order, Arizona, Utah, New Mexico, and Montana, accounted for 99 percent of domestic production. Copper was also recovered at mines in three other States. Although copper was recovered at about 30 mines operating in the United States, 15 mines accounted for about 99 percent of production. At 2001 year end, four primary smelters, four electrolytic and four fire refineries, and 15 solvent extraction-electrowinning facilities were operating.

Surface mining requires extensive blasting as well as rock, soil, vegetation, and overburden removal to reach lode deposits. Benches are cut into the walls of the mine to provide access to progressively deeper ore as upper-level ore is depleted. Ore is removed from the mine and transported to beneficiation plants for milling and concentrating. The concentrate is then smelted and refined. Open-pit mining is the primary domestic source of copper.

Exposives and Crushed RockMining

Crushed rock is one of the most accessible natural resources and a major basic raw material. It is used in construction, agriculture, and other industries using complex chemical and metallurgical processes. Despite the low value of its basic products, the crushed rock industry is a major contributor to and an indicator of the economic well being of the nation.

Most crushed and broken stone is mined from open quarries; however, in many areas, factors favoring large-scale production by underground mining are becoming more frequent and prominent.

Surface mining equipment varies with the kind of stone mined, the production capacity needed, the size and shape of the deposit, estimated life of the operation, location of the deposit with respect to urban centers, and other important factors. Typically, drilling is done with tricone rotary drills, long-hole percussion drills, and churn drills. Blasting in smaller operations may be done with dynamite, but in most medium-to large-size operations, ammonium nitrate fuel oil mixture (AN-FO) is used as a low cost explosive. The rock is then extracted using power shovels or bulldozers.

Typical blasts at quarries use a 3 inch diameter hole approximately 40 feet deep. The explosive most commonly used is ANFO and the top part is stemmed with sand/gravel or drilling cuttings poured on top of the ANFO to help force the blast energy into the rock. The amounts of ANFO and stemming vary depending on the location of the hole in the blast pattern, site conditions, the hardness of the rock to be blasted, and of course, on PPV limitations for surrounding structures. Generally, a minimum amount of stemming for a 3 inch diameter, 40 foot hole would be approximately 7 feet, which would leave 33 feet of hole for ANFO. The weight of explosives in the hole would be approximately 115 pounds.

About three-quarters of the crushed stone production is limestone and dolomite, followed by, in descending order of tonnage: granite, traprock, sandstone and quartzite, miscellaneous stone, marble, slate, calcareous marl, shell, volcanic cinder and scoria.

Limestone, one of the largest produced crushed rock, is a sedimentary rock composed mostly of the mineral calcite and comprising about 15 percent of the earth's sedimentary crust. This mineral is a basic building block of the construction industry and the chief material from which aggregate, cement, lime, and building stone are made. For the purposes of this report, limestone will be used as a sample for crushed rock.

One product of limestone mining is lime. A wide range of industries use lime for a myriad of uses. It is used in many of the products and materials Americans use every day, including paper, steel, sugar, plastics, paint, and many more. The largest single use of lime is in steel manufacturing, for which it serves as a flux for removing impurities (silica, phosphorus and sulfur) in refining steel.

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Page last modified: 07-07-2011 02:51:05 ZULU