The relatively concentrated portion of this copper is only a small fraction of the whole, constituting an estimated 10 10 metric tons in deposits with a grade of 0. At current world mine production, this represents a million years' supply of copper theoretically available in the mineable portion of the earth's crust.
Table 4 shows some of the most common copper minerals. Some of these have long had a value in their own right, such as malachite, prized for its unusual and pleasing appearance and used for millennia in jewelry and ornaments. The barren rock, or gangue has to be separated from the sulfide minerals in order to smelt the metallic copper from the ore. By far the greatest proportion of copper is extracted from the sulfides of copper, iron and sometimes other metals.
Such ores originate from sulfur-bearing volcanic magmas, which have separated into metal sulfides and siliceous melts. The copper has concentrated almost entirely into the sulfide fraction, and if this becomes separated from the siliceous melt it can become deposited in veins or in fissures in the host rock by hydrothermal or other geological activity.
In many ores and most of those found in the Western USA the copper minerals occur as a dispersion of fine particles. Such ores are called porphyries. Where mineralized rocks become outcropped or shattered, the sulfide minerals undergo chemical changes due to air, groundwater and heat, giving rise to the other main variety of copper minerals - the oxidized ores. There is no shortage of copper resources.
In fact, copper is one of the most abundant of the metallic elements in the earth's crust. Commercially exploited deposits of copper ores are found in many parts of the world, frequently associated with mountain-building processes. Deposits occur at many locations in the western cordillera of the Americas, mainly in the United States and Chile, and in areas of the North American plains like Michigan, Ontario, Quebec and Manitoba, at sites associated with the Pre-Cambrian shield.
Copper is also found in many other countries throughout the world. In addition, huge quantities of copper are known to exist as "deep-sea nodules" scattered on the ocean floor, although high recovery costs has thus far prevented their commercial exploitation.
Chile and the USA are, respectively, the world's two leading copper-producing nations, Chile having overtaken the USA in the early s. Extraction Ores are first mechanically crushed and ground so that nearly all copper mineral particles are freed from the gangue. Flotation by the injection of air and violent agitation is carried out with the pulverized ore held in suspension in water, to which surface-active agents have been added. When processing copper through heap leaching, vast quantities of ore and overburden overburden Soil and rocks that have been moved out of the way to get to ore are called "overburden.
The overburden is separated and either hauled to the waste site or stockpiled for future site reclamation. The ore is crushed and stacked into specially designed pads. The pads are lined with synthetic or natural materials such as polyethylene or compacted clay.
Acids are introduced to copper-bearing ore, seeping through and dissolving copper and any other metals, including radionuclides. This liquid contains the dissolved copper, known as a pregnant leach solution PLS. The copper present in this liquid is collected and later processed.
A second method is in-situ leaching. Groundwater and certain geochemical conditions must be present in order to utilize this method. During in-situ leaching, rather than physically mining and removing overburden to reach copper deposits, chemicals are introduced into ore bodies using injection wells.
The PLS is then captured in production wells, collected and later processed. Once PLS is leached, it is sent to a solvent extraction plant to remove the copper. The PLS is mixed with an organic solvent that binds to the copper and chemically separates the copper from the rest of the liquid.
Then, the copper-bearing liquid is combined with another acid to precipitate the copper from the organic material. This primary crusher reduces the size of the ore from boulder to golf ball-sized rocks. Oxide ores are generally processed using hydrometallurgy.
This process uses aqueous water-based solutions to extract and purify copper from copper oxide ores at ordinary temperatures, usually in three steps: heap leaching, solvent extraction, and electrowinning. Heap Leaching is the process of using percolating chemical solutions to leach out metals. Heap leaching is very commonly used for low-grade ore, which would otherwise not be economical to send through a milling process.
Following mining, transporting, and crushing to a consistent gravel or golf ball-size, the crushed ore is piled into a heap on top of an impenetrable layer, on a slight slope. The leaching reagent dilute sulfuric acid is sprayed through sprinklers on top of the heap pile and allowed to trickle down through the heap, where it dissolves the copper from the ore. The second step is solvent extraction , in which two immiscible un-mixing liquids are stirred and allowed to separate, causing the cooper to move from one liquid to the other.
The pregnant leach solution is mixed vigorously with a solvent. The copper migrates from the leach solution into the solvent. The two liquids are then allowed to separate based on solubility, with copper remaining in solution in the solvent, and impurities remaining in the leach solution.
The leftover leach solution is then recycled, by adding additional acid and sending it back to the sprinklers in the heap leaching process. The last step is called electrowinning , a type of electrolysis. An electrical current passes through an inert anode positive electrode and through the copper solution from the previous step, which acts as an electrolyte.
At the beginning of the 20th century, the annual worldwide copper demand was at half a million tons. By World War II, this demand had multiplied by more than nine times. Continued advancements in technology have allowed the copper mining industry to keep up with worldwide demands with mining methods that are efficient and cost-effective.
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