How do mining companies extract Copper in the mining process?
Depending on the configuration and depth of the copper deposit, mining operations use either open pit or underground mining methods:
Open Pit Mining
Open-pit mining starts from the earth's surface and maintains exposure to the surface throughout the extraction period. A pit or oval shape is excavated over time to access the copper ore deposits and this excavation usually has stepped, or benched, side slopes and can reach depths as low as 400 meters. The excavation is achieved using large loading typically shovels, loaders and excavators and hauling equipment, typically large dump trucks, rail cars or conveyors. This equipment is organized into an efficient sequence of drill, blast, muck, load, and haul rock material. In the beginning stages of the open pit, the soft overburden is removed before the ore can be excavated. The design of the pit and the rock characteristics determine the slope of the pit walls. If the slope angle exceeds certain conditions, the pit wall can fail and a rock slide can make the pit unsafe to work in. Pit operators are constantly reviewing pit wall slope stability to ensure safe working conditions.
Underground mining
Under certain circumstances open pit mining can become prohibitively expensive and underground mining may be considered. A major factor in the decision to operate by underground mining rather than open pit mining is the strip ratio or the ratio of ore to waste. In situations where excess waste needs to be removed, surface mining is no longer attractive. The objective of underground mining is to extract the ore below the surface of the earth safely, economically, and with as little waste as possible. The entry from the surface to an underground mine may be through an adit, or horizontal tunnel, a shaft or vertical tunnel, or a declined shaft or ramp. A typical underground mine has a number of roughly horizontal levels at various depths below the surface and these spread out from the access to the surface. Ore is mined in stopes, and rock material that is left in place to support the ceiling is called a pillar and can sometimes be recovered in the final stage of extraction. An underground vertical opening driven from an upper level downwards is called a winze; and a raise if it was made by driving upward from a lower level.
Mechanised underground mining is highly dependent on the operation of underground equipment and labour intensive underground mining is highly dependent on a large number of people working underground with smaller hand held equipment. In mechanized mining, rubber-tired vehicles powered by diesel or electric power, rail haulage, and multiple drill units are organized into sequence to drill blast and muck both ore and waste rock for processing. This equipment is designed to fit within the confines of underground tunnel sizes. In order to protect miners and their equipment much attention is paid to mine safety. Mine ventilation provides fresh air underground and at the same time removes noxious gases from blasting as well as dangerous dusts and diesel fumes that might cause lung disease. Ground fall (rocks falling from the roof of a tunnel or stope) is a common safety hazard and roof support is accomplished with timber, concrete, or steel supports or, most commonly, with roof bolts, which are long steel rods used to bind the exposed roof surface to the rock behind it.
Typically underground mines are developed in elevation levels with a plan to mine the copper ore body in a sequence that allows for safe ground support and efficient extraction of rock material.
Ore processing
Once the ore is hauled above ground or outside the open pit, it is sent for processing. Processing of sulphide copper deposits and oxide copper deposits are quite different.
Sulphide Ore processing
Sulphide ore is sent through a series of crushers and grinders to reduce the ore particle size to a powdery fineness. This is usually achieved with a primary crusher and then grinding. Often semi-autogenous mills ("SAG") and ball mill sequences utilize steel balls in a rotating cylinder to crush the ore during repeated cylinder rotations. Once a certain particle size is achieved, the ore is further concentrated and then flushed with water to be further processed by flotation. In the flotation process the ore particle is added to a chemical reagent mix in varying conditions of Ph, together with air bubbles to create a froth. The flotation cells recover the copper from sulfide ores by chemically attaching the copper to a chemical bubble and overflowing or "skimming" the froth (bubbles with copper) for further processing. The froth is concentrated and dried to create a copper concentrate powder that contains between 24% and 40% copper content. The concentrate is then shipped to a smelter for further purification.
Oxide ore processing
Oxide ore is most common in open pit operations and once the ore leaves the pit is placed on a heap leach pad in a large stack or pile. Under the pile is a pre- constructed impervious liner in order to collect solution. The heap leach pad is sprayed with a mild sulphuric acid solution so that the copper can be dissolved. Pads have a series of pipes and hoses dispensing a diluted sulfuric acid solution to the ore. Copper is dissolved and flows to a pond at the bottom of the pad where is it collected on the liner. This process can take several months. The leached solution is pumped to the Solvent Extraction ("SX") circuit, where the copper is stripped using organic solutions; the stripped liquid is then returned to the leach pads for re-use. The strip solution is cleaned using dilute acid and flotation and is filtered prior to the electro-winning ("EW") phase, where the copper is plated using electrolysis on cathodes square sheets of copper. This cathode copper is then transported for sale to end users.
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