Open-Pit Mining

Open-pit mining is applicable in copper, iron, coal, limestone, gravel and sand and sometimes in gold, silver, lead, zinc, and other deposits. Where general conditions indicate open-pit methods, a topographical survey is neces­sary to determine the limits and thickness of overburden. Drainage conditions and physical nature of the materials and enclosing rocks are closely studied. From these the lay-out of the pit and the mining and transportation equip­ment necessary are determined.

Most open-pits are in flat topography of moderate relief, some are on hill or mountain slopes; many have natural drainage but others become sumps.

Open-pit operations begin with the removal of the overburden over the area where mining is to start. Pit and stripping approaches are constructed. Both overburden and mineral mining begin from the top downward in successive horizontal benches until the bottom has been reached. Coal pits are usually shallow; stripping and mining are carried on simultaneously, the stripping shovel being sufficiently in advance of mining or coal loading. Thick overburdens and thick deposits, sometimes character­istic of copper and iron pits, require numerous benches; many of which may be worked simultaneously.

Mineral deposits and overburden that can be excavated without blasting are sliced in parallel cuts so as to maintain straight loading tracks, which are placed at the side of the cut. Bench heights are limited to shovel reach. On completion of a cut, loading tracks are shifted to a position for the next cut. It is not always possible to maintain straight loading tracks, and curving benches are quite common in large pits.

Surface mining methods have made it possible to mine many deposits that would be uneconomic to develop underground because of the rapid development of many types of large and efficient machinery and equipment. Although the basic concept of an open pit is simple, the planning required to develop the methods for surface mining is very complex.

The grade and tonnage of material will determine how much waste rock can be stripped, and there is often a limit to the pit that is determined by the economics of removing overburden. The ultimate pit limit and the slope of the pit walls are determined by economics, engi­neering and by the geological structure. Material that is relatively high-grade may be left unmined in some spot extending too deeply beneath the waste.

Mining is usually done by track-mounted electric shovels in the large operations and by rubber-tired diesel front-end loaders in the smaller operations. Scrapers are sometimes used in special situations. Large bucket-wheel excavators used in European coal mines have not been applied to metal mining, because this equipment is best adopted to softer bedded, relatively flat-lying beds.

Haulage is usually done by trucks, although railroads, inclined rails, and belt conveyers have been used. Each of these unloads directly into a primary crusher, and crushed material is stored in ore bins prior to delivery to the mill.⁴

Bench level intervals are determined by the type of the shovel or loader used, and these are selected on the basis of the character of the ore, and the manner in which it breaks upon blasting and supports itself on the working face. Blastholes are usually drilled vertically by self-propelled, track-mounted pneumatic rotary drills. Explosives are loaded in the holes and large volumes of ore are broken in a single blast. Sometimes the drill holes are sampled to help plan the position of the shovels in advance of mining. Blasthole control is especially desir­able when exploration data are incomplete as in older pits which have long been mined.

Stripping methods. The three basic stripping systems are the following:

1. A single stripping shovel travelling on the exposed coal seam digs and removes the overburden ahead of it and piles it in the cut from which coal has previously been removed.

2. A single dragline travelling on a bench above the coal strips overburden to widen the bench for its travel way for the next cut and also removes the high-wall bench over which it has just travelled to expose the coal seam.

3. A shovel and a dragline are used in tandem and they travel on exposed coal. The shovel works ahead h coal has previously been removed. The dragline removes the upper portion of the overburden to form another bench and casts the spoil.