Metalliferous Mining

MINING, METALLIFEROUS, the winning of metals and their ores from the ground. Metals, in their pure state and also mechanically and chemically combined with other substances, occur all over the earth's surface. These deposits of metals and metal ores vary in extent and metal content and in their depth under the surface of the ground, which gives rise to different methods of mining. The broad classification of these methods, which is used by the American Institute of Mining and Metal lurgical Engineers, divides metalliferous mining into two main fields; open-cut mining and underground mining.

weather. Thick deposits are worked in several benches. The slope of the bank varies at different mines, but for preliminary investigations may be taken as 45° from the horizontal, giving a general slope of from 38° to 4o° for the entire side of the cut.

The shape of the ore body has an important bearing on the feasibility of open-cut mining. Due to the slope which the sides of the cut must take, a narrow ore body cannot be mined as Open-Cut Mining.—The working of metalliferous deposits which either outcrop at the surface of the ground or are covered by a shallow overburden or capping which must be removed be f ore the ore can be mined. Since the cost of removing the over burden is charged to the cost of mining, a point is reached, as the cover increases in thickness, beyond which open-cut mining does not pay and some method of underground mining must be used. Large deposits of copper and iron ores are worked by open cut mining, usually by the bench method. The depth of capping varies from a few feet up to 30o feet.

Benches.

Power-driven shovels work on benches loading the ore into cars running on a track near the edge of the bench, or on the edge of the bench above. The material is loosened by blasting, and care should be taken that the pieces are small enough to be handled easily by the shovels. The height of bank is deter mined by the thickness of the deposit, the size of the shovels used, the physical character of the ore and climatic conditions. The higher the bank the more material the shovel can handle from one position, but the greater is the danger of a slide burying the shovel and of falling rock injuring the men. The material

may stand at a high angle when dry, but slides may occur in wet economically by this method as a large flat lying deposit. If the deposit is irregular in outline it may be difficult to prevent diluting the ore with waste. Even under favourable conditions, it is seldom possible to mine the entire ore body without resort ing to underground mining for some part of it. Fig. i illustrates how side slope limits the economical depth of open-cut mining. To carry shovel working to greater depth would require the removing of too much overburden, so for deeper working some method of underground mining must be used.

The electrically operated shovel fitted with caterpillar tread is superseding the steam shovel, especially in large scale work. The electric shovel eliminates the problems of water and fuel supply, which are of especial disadvantage in cold weather. Power shovel work, due to the high cost of equipment, must be on a large scale, and consequently can be economically applied only to large ore deposits, in which case it gives a large output at a very low mining cost. Ample dump room must be available, if much capping is to be removed. No timbering is required; how ever, the surface of the ground is destroyed for further use, bad weather may seriously interfere with operations and a large outlay of capital is required.

Glory-hole Mining.

In this case a vertical connection or raise is driven to the surface from an underground haulage way. The top of this raise is broken out to form a funnel-shaped open ing, which is widened and deepened by drilling and blasting around the inside. The broken ore slides down the funnel into the raise which serves as a chute, the ore being drawn off through gates into cars on the haulage level. This is a cheap method of mining, but it is dangerous especially if pieces of the wall tend to break off and the excavation is large. A series of holes may be mined until they intersect.