The list of commodities is extensive, although the relative proportions mined underground and on the surface vary from mineral to mineral. Massive tonnages of hard coal and lignite are also produced from surface mines, although the terminology used – open cast or open cut, rather than open-pit – indicates that the technology and engineering concepts used are often markedly different from those employed in the hard rock environment.
Open-pit mining differs from quarrying (with the possible exception of iron ore) in that the valuable mineral constitutes only a small proportion of the total tonnage of rock produced. In quarrying, the rock itself is the valuable commodity, with virtually all of the raw rock that is won being processed (often only by crushing and screening) to give saleable products. In open-pit mining, on the other hand, the metal content of the ore produced may be only fractions of a percent, meaning that a huge proportion of the rock produced is effectively waste material.
In addition, the ore body geometry may mean that large tonnages of barren rock also have to be mined and transported in order to access the ore in which the valuable mineral (or minerals) makes up only a small proportion. This has to be recovered from the rock matrix through both physical (crushing and grinding) and/or chemical processes such as flotation, meaning that processing costs are higher. This is reflected in metal commodity prices, with a ton of copper or zinc being valued much more highly than a ton of granite or basalt.
High performance drilling is key to successful open-pit mining, with two principal areas of operation: blasthole drilling, and drilling presplit holes that help to enhance pit wall stability. These operations have different requirements, in terms of both their aim and the equipment used.
Blasthole drilling is an integral part of the production process, and needs large, heavy drill rigs that can produce high meterage of often large diameter holes. These are then charged with bulk explosives, such as ANFO or emulsions, to produce broken rock that can be handled by the loading shovels. Drilling is carried out on a specifically defined grid system, taking into account the relationships between the hole diameter, the burden (the distance between each row of holes) and the spacing (the distance from hole to hole along the bench).
Two main changes have occurred in open-pit mining technology over the past 100 years. The most obvious has been the increase in the scale of operations as higher capacity equipment has been developed. For instance, in the 1920s, a mining shovel with a 5 m3 bucket was exceptional. Today, large open-pits use rope shovels or hydraulic excavators with bucket sizes of ten times that capacity.
The other big change which has occurred is the evolution of transport systems. Mines have moved from rail-bound to haul truck and, in an increasing number of cases, to in-pit primary crushing of the ore (but not the waste rock) followed by belt conveyor transportation out of the pit. In addition, in tandem with the growth in loading-shovel capacities, haul trucks have increased in size with the top-of-the-range 45 tons capacity hauler of the 1950s having been replaced by the 400 tons hauler that takes three-or four-pass loading from the current generation of shovels.
Open-pit mining, also known as opencast mining, is a surface mining technique that extracts minerals from an open pit in the ground. This surface mining technique is used when mineral or ore deposits are found relatively close to the surface.
Open-pit mines are used when deposits of commercially useful ore or rocks are found near the surface. To create an open-pit mine, the miners must determine the location of the ore. This is done through drilling of probe holes in the ground, then plotting each hole location on a map. Sophisticated geophysical logging services can assist in this task by helping to build very detailed sub-surface models. This valuable information is used to ensure that operations are targeted in the correct locations. Blast holes are then drilled in a very specific pattern and benches are blasted. A deep open-pit mine can extend down many hundreds of meters in a stepped fashion.
One of the several advantages of open-pit mining are the powerful trucks and shovels which can be used to move large volumes of rock — the restrictions on size are not as severe as with underground mining. Additionally, set-up and production costs tend to be lower than other methods, which also means that lower grades of ore are economic to mine.
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