Mining

I

Introduction

Mining, the selective recovery of minerals and materials, other than recently formed organic materials, from the crust of the Earth. It is one of the oldest activities of mankind. It has been the major source of the materials from which tools were made, almost from the beginning of the Stone Age, 2.5 million years ago or more. From the moment when the predecessors of human beings first began to recover selected rock types from which to chip their tools, mining had begun. Initial mining simply involved digging the flints or other selected rocks from the ground. As surface deposits were depleted, the pits grew deeper, until eventually underground mining started. The oldest identified underground mine, a mine for red ochre at Bomvu Ridge in Swaziland, in southern Africa, has been dated to 40,000 bc. This significantly pre-dates the start of farming. Surface mining, of course, goes back much farther in time.

All of the materials used by modern society are either themselves made of mined materials or require mined materials in their production. It is a truism to say that if it did not grow, then somebody, somewhere, dug it out of the ground. Even the other primary industries, agriculture, fishing, and forestry, would be helpless without tools and machinery made from the products of mines. It can be argued that mining is the most basic industry of human civilization.

Mining methods are of four basic types. Firstly, materials may be mined from surface mines, open pits, quarries, or other diggings open to the atmosphere. This group constitutes by far the greatest number of mines in the world. Secondly, there are underground mines, entered through shafts or tunnels. Thirdly, there is the recovery of minerals and fuels through boreholes. Finally, there is underwater mining or dredging, which is now extending to the potential mining of the deep oceans.

Mining always involves the physical removal of materials from the crust of the Earth, frequently in huge amounts for the recovery of only small amounts of the desired product. It is therefore not possible to mine without affecting the environment, at least within the mining area. In fact, mining is considered to be one of the most important causes of environmental degradation caused by human beings. However, the modern qualified mining engineer is skilled in causing the minimum possible amount of damage and in restoring the site when mining has been completed.

Mining is normally undertaken to recover minerals or fuels. A mineral may be defined as any naturally occurring substance with a definite chemical composition and predictable and consistent properties. The most important fuels are the fossil hydrocarbons, which are not normally defined as minerals. A mineral resource is a volume of the Earth’s crust in which an abnormally high concentration of a specific mineral or fuel occurs. It becomes an ore reserve when that mineral, or the content of interest, such as a metal, can be recovered with current technology at a cost that allows a reasonable return to be made on the investment in the mine.

There is a vast variety of materials that may be recovered from such deposits. These can be classified as follows.

Metals include: the precious metals, gold, silver, and the platinum group metals; the steel industry metals, iron, nickel, cobalt, titanium, vanadium, and chromium; the base metals, copper, lead, tin, and zinc; the light metals, magnesium and aluminium; the nuclear metals, uranium, radium, and thorium; and the speciality metals, for example lithium, germanium, gallium, and arsenic.

Industrial minerals include: quartz, trona, salt, potash, asbestos, talc, feldspar, sulphur, and phosphates.

Construction materials include: sand, gravel, aggregates, brick clays and limestone, and shale for cement manufacture. Also included in this group are slate for roofing, and the polished stones, such as limestone, granite, travertine, and marble, collectively known as dimension stones.

Gemstones include: diamonds, rubies, sapphires, and emeralds.

Fuels include: coal, lignite, peat, and oil and gas (though the latter are often not thought of as being the products of mines). Uranium is often included among the fuels.

The above are only a small selection of the innumerable materials that are mined.

An ore deposit may take almost any shape. It may outcrop on the surface or be buried deep under the ground. In some of the deep South African gold mines, mining starts at depths of well over 1,500 m (5,000 ft) and goes down to more than 3,500 m (12,000 ft). Mines may recover loose, unconsolidated material, such as the sediments in a stream bed, or they may be sunk in solid rock, harder than any concrete.

As mentioned above, there are four fundamental systems of mining: surface mining (which includes quarrying), underground mining, dredge mining (which includes undersea mining), and mining through boreholes. An account of each of these follows. The fundamentals of the technology in each case remain constant but the details differ with the material being mined, the hardness of the rock, and the geometry of the deposit. There is, of course, overlap between the systems.

II

Surface Mining

Surface mining is the largest single sector of mining, being used for well over 60 per cent of all materials mined. It may be used for any material required. There are a number of names for the various types of surface mine, which generally indicate their purpose. An open pit mine is usually, but not always, for metals, a strip mine is more often than not for coal, a quarry is normally for the recovery of industrial and construction materials, while a placer mine is usually for heavy metals and minerals—frequently gold, but also platinum, tin, and other heavy minerals.

III

Open Pits

These are surface mines that take the form of large, terraced pits, going ever deeper into the earth and enlarging as they do so. The classic examples of such open pits are the diamond mines of South Africa. These mine kimberlite pipes are orebodies in the shape of a cylindrical mass of rock rising through the Earth’s crust. The mines are often more or less circular in shape.

The mining starts with drilling and blasting. The rock so broken is then loaded into trucks by huge electric power shovels, hydraulic shovels, or front-end loaders, and removed from the pit. These loading machines reach sizes such that 50 cu m (17,600 cu ft) of broken rock can be moved in one shovel-load, but more normally they range from 5 to 25 cu m (176 to 880 cu ft) per shovel-load. The trucks normally vary in size from ones that take a 35-tonne load up to ones taking as much as 220 tonnes. A modern development is to have the shovels load directly into a mobile crushing plant, from which the crushed rock is transported on conveyor belts.

The material classified as ore is transported to the recovery plant, while material classified as waste goes to the waste dumps. There may be a third category of low-grade material, which may be stockpiled for possible future treatment.

Many mines start off as surface mines and, when they reach the point where too much waste must be mined for each tonne of ore recovered, change over to underground mining methods.

IV

Strip Mines

Strip mines are usually, but not always, dug for coal or lignite. In the United Kingdom, over 10 million tonnes of coal is recovered from strip mines each year. The main difference between these mines and the open pits is that the waste material mined to uncover the coal seam, instead of being taken to remote dumping areas, is replaced in the cavity created by recent mining. These mines therefore move across the country, always filling in behind themselves and restoring the surface to look as nearly as possible as it did before mining took place. So unlike an open pit, which normally grows ever larger, a strip mine reaches its full size very quickly. On completion, the remaining hole may be made into a lake, or it may be refilled with the material that came out of the original excavation when the mine was started.

Strip mines use some of the same equipment as the open pits, particularly for the recovery of the coal, but for the removal of the overlying waste rock, the overburden, they use the largest mining equipment of all. In Germany there is a 7,500-tonne bucketwheel excavator that can mine 250,000 cu m (9 million cu ft) of soil per day. This machine is mounted on crawlers and is self-mobile. Another large machine type used mainly in strip mines is the dragline. A dragline formerly in use in the United Kingdom mined the overburden in 50 cu m (1,760 cu ft) bites.