Electricity in collieries

 The many advantages of electricity as a medium of power supply, its economy and ease of generation and transmission, its convenience of application and its versatility led to its early adoption in the mining industry and to its increasing use throughout the years.

Before considering in detail the various applications of electricity to mining, it is desired to consider briefly other arms of power available for use in mines and the reasons why electricity is rapidly replacing these. As far as underground plant is concerned, the only two main forms of power employed are electricity and compressed air. Compressed air is used in places where, because of gassy conditions, electricity cannot be used. The transmission of energy in the form of compressed air over long distances underground is, however, extremely uneconomic compared with electricity and the quantity, which can be so transmitted, is limited by the size of mains, which can be installed in roadways underground. There are also considerable difficulties in operating large-power consuming units such as modern cutter loaders by compressed air.

At the surface of the mine, a third source of power is available, namely, steam. Although many collieries are completely electrified underground, steam still provides a considerable portion of the surface power requirements, particularly for the larger units, such as winding engines, main ventilating fans and ^reciprocating or turbo-compressors.

In general, electrical transmission and utilization is much more efficient and much easier in application than other means. The amount of electrical energy required at a colliery depends primarily upon the extent to which the colliery is electrified and upon the tonnage produced.

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ELECTRICITY INTRODUCED INTO MINES

The mining industry played an important part in the development of engineering generally. Mining power problems played an important part in the development of the steam engine. Later mining engineers recognized the advantage of electricity as a form of power, which could be conveniently and economically transmitted long distances underground. The rate of the use of electricity in the mines grew rapidly. Now the advantages of electric power are well known. Three or four voltages are used for the economic transmission and utilization of electricity in mines. The incoming supply may be 6,000 or 11,000 volts. The internal transmission voltage, which is also used for large machines, may be 3,300 volts. The general utilization voltage is between 400 and 650 volts; and lighting is usually carried out at 110 or 220 volts. Transformers are used to change the voltage.

In general, electrical transmission and utilization is much more efficient and much easier in application than other means.

The amount of electrical energy required at a colliery depends primarily upon the extent to which the colliery is electrified and upon the tonnage produced.

The consumption of electricity in terms of units per ton raised varies considerably throughout the coal-fields, depending largely upon the depth of the shafts (where electric wind­ing is practised), the amount of pumping and the extent to which electrically-driven compressors are employed.

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THE SEDIMENTARY ROCKS

These are the rocks laid down in beds on the surface of the earth. They are classified to origin:

(1) mechanical or clastic sediments: gravel, sand, sandstone, shale, etc.;

(2) organic: limestone, peat, coal, etc.;

(3) chemical: salt deposits, some limestones and ironstones. The mechanical sediments may be further subdivided ac­cording to the size of their constituent particles.

A rock is not generally all of one grade, but is usually made up of portions of different grades, giving combinations like silty stones and sandy shales.

The organic and chemical sediments are subdivided according to their compositions, e.g., calcareous (e.g., limestone), carbonaceous (e.g., coal), siliceous (flint), saline (e.g., rock salt).

Pebbles are frequently fragments of rocks as distinct from sand grains, which are usually simple minerals. Their size and shape depend in part on the rock from which they were derived, and in part on the treatment they have received. Pebbles of homogeneous granite are often nearly spherical, while pebbles of a metamorphic rock, such as a slate or schist, are generally flattened parallel to the cleavage planes. Again beach pebbles are, as a rule, better rounded than river pebbles, and glacial boulders are typically subangular. When commented to make hard rocks, pebbly deposits are known as conglomerates, and if the fragments are angular they are called sedimentary breccias to distinguish them from fault breccias. Most sand grains consist of quartz with lesser amounts of feldspar and muscovite. 

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