Deposits mined by open-pit techniques are generally divided into horizontal layers called benches. The thickness (that is, the height) of the benches depends on the type of deposit, the mineral being mined, and the equipment being used; for large mines it is on the order of 12 to 15 metres (about 40 to 50 feet). Mining is generally conducted on a number of benches at any one time. The top of each bench is equivalent to a working level, and access to different levels is gained through a system of ramps. The width of a ramp depends on the equipment being used, but typical widths are from 20 to 40 metres (65 to 130 feet). Mining on a new level is begun by extending a ramp downward. This initial, or drop, cut is then progressively widened to form the new pit bottom.
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The walls of a pit have a certain slope determined by the strength of the rock mass and other factors. The stability of these walls, and even of individual benches and groups of benches, is very important—particularly as the pit gets deeper. Increasing the pit slope angle by only a few degrees can decrease stripping costs tremendously or increase revenues through increased ore recovery, but it can also result in a number of slope failures on a small or large scale. Millions of tons of material may be involved in such slides. For this reason, mines have ongoing slope-stability programs involving the collection and analysis of structural data, hydrogeologic information, and operational practices (blasting, in particular), so that the best slope designs may be achieved. It is not unusual for five or more different slope angles to be involved in one large pit.
As a pit is deepened, more and more waste rock must be stripped away in order to uncover the ore. Eventually there comes a point where the revenue from the exposed ore is less than the costs involved in its recovery. Mining then ceases. The ratio of the amount of waste rock stripped to ore removed is called the overall stripping ratio. The break-even stripping ratio is a function of ore value and the costs involved.
The first step in the evaluation and design of an open-pit mine is the determination of reserves. As was explained above, information regarding the deposit is collected through the drilling of probe holes. The locations of the holes are plotted on a plan map, and sections taken through the holes give a good idea of the ore body’s vertical extent. From these vertical sections the tentative locations of the benches are selected. However, since the deposit is to be mined in horizontal benches, it is also convenient to calculate the ore reserve in horizontal sections, with the thickness of each section equal to the height of a bench. These horizontal sections are divided along coordinate lines into a series of blocks, with the plan dimensions (i.e., the length and width) of each block generally being one to three times the bench height. After the grade of each block has been determined, the blocks are assembled into a block model representation of the ore body. (This model must be significantly larger than the actual ore reserve in order to include the eventual pit that must be dug to expose the ore body.)
Economic factors such as costs and expected revenues, which vary with grade and block location, are then applied; the result is an economic block model. Some of the blocks in the model will eventually fall within the pit, but others will lie outside. Of the several techniques for determining which of the blocks should be included in the final pit, the most common is the floating cone technique. In two dimensions the removal of a given ore block would require the removal of a set of overlying blocks as well. All of these would be included in an inverted triangle with its sides corresponding to the slope angle, its base lying on the surface, and its apex located in the ore block under consideration. In an actual three-dimensional case, this triangle would be a cone. The economic value of the ore block at the apex of the cone would be compared with the total cost of removing all of the blocks included in the cone. If the net value proved positive, then the cone would be mined. This technique would be applied to all of the blocks making up the block model, and at the end of this process a final pit outline would result.
The largest open-pit operations can move almost one million tons of material (both ore and waste) per day. In smaller operations the rate may be only a couple of thousand tons per day. In most of these mines there are four unit operations: drilling, blasting, loading, and hauling.
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In large mines rotary drills are used to drill holes with diameters ranging from 150 to 450 mm (about 6 to 18 inches). The drill bit, made up of three cones containing either steel or tungsten carbide cutting edges, is rotated against the hole bottom under a heavy load, breaking the rock by compression and shear. An air compressor on the drilling machine forces air down the centre of the drill string so that the cuttings are removed. In smaller pits holes are often drilled by pneumatic or hydraulic percussion machines. These rigs may be truck- or crawler-mounted. Hole diameters are often in the range of 75 to 120 mm (about 3 to 5 inches).
Holes are drilled in special patterns so that blasting produces the types of fragmentation desired for the subsequent loading, hauling, and crushing operations. These patterns are defined by the burden (the shortest distance between the hole and the exposed bench face) and the spacing between the holes. Generally, the burden is 25 to 35 times the diameter of the blasthole, depending on the type of rock and explosive being used, and the spacing is equal to the burden.
There are a number of explosives used, but most are based on a slurry of ammonium nitrate and fuel oil (ANFO), which is transported by tanker truck and pumped into the holes. When filled with ANFO, a blasthole 400 mm (about 16 inches) in diameter and 7.5 metres (about 25 feet) deep can develop about one billion horsepower. It is incumbent upon those involved in the drilling and blasting to turn this power into useful fragmentation work. To achieve the proper fragmentation, a series of blastholes is generally shot in a carefully controlled sequence.
The object of blasting is to fragment the rock and then displace it into a pile that will facilitate its loading and transport. In large open pits the main implements for loading are electric, diesel-electric, or hydraulic shovels, while electric or mechanical-drive trucks are used for transport. The size of the shovels is generally specified by dipper, or bucket, size; those in common use have dipper capacities ranging from 15 to 50 cubic metres (20 to 65 cubic yards). This means that 30 to 100 tons can be dug in a single “bite” of the shovel. The size of the trucks is matched to that of the shovel, a common rule of thumb being that the truck should be filled in four to six swings of the shovel. Thus, for a shovel of 15-cubic-metre capacity, a truck having a capacity of 120 to 180 tons (four to six swings) should be assigned. The largest trucks have capacities of more than 350 tons (about 12 swings) and are equipped with engines that produce more than 3,500 horsepower; their tire diameters are often more than 3 metres (10 feet). Because of their high mobility, very large-capacity wheel loaders (front-end loaders) are also used in open-pit mines.
As pits became deeper—the deepest pits in the world exceed 800 metres (2,600 feet)—alternate modes of transporting broken ore and waste rock became more common. One of these is the belt conveyor, but in general this method requires in-pit crushing of the run-of-mine material prior to transport. For most materials a maximum angle of 18° is possible. To transport directly up the sides of pit walls, special conveying techniques are under development.
After loading, waste rock is transported to special dumps, while ore is generally hauled to a mineral-processing plant for further treatment. (In some cases ore is of sufficiently high quality for direct shipment without intermediate processing.) In some operations separate dumps are created for the various grades of sub-ore material, and these dumps may be re-mined later and processed in the mill. Certain dumps can be treated by various solutions to extract the contained metals (a process known as heap leaching or dump leaching).
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