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The average grade of copper ores in the 21st century is below 0.6% copper, with a proportion of economic ore minerals being less than 2% of the total volume of the ore rock. Thus, all mining operations, the ore must usually be beneficiated (concentrated).
Leaching is a process widely used in extractive metallurgy where ore is treated with chemicals to convert the valuable metals within the ore, into soluble salts while the impurity remains insoluble. These can then be washed out and processed to give the pure metal; the materials left over are commonly known as tailings.
In-situ leach mining involves pumping of a lixiviant into the ore body via a borehole, which circulates through the porous rock dissolving the ore and is extracted via a second borehole. The lixiviant varies according to the ore deposit: for salt deposits the leachate can be fresh water into which salts can readily dissolve.
While in care and maintenance and with no active mining, Black Rock Minerals re-processed existing stockpiles through a small process plant and heap leach. Copper levels of 3.5 milligrams per litre were recorded in the local drinking water, well above the permitted level of 0.002 milligrams per litre.
A number of byproducts are recovered from American copper mining. In 2013, American copper mining produced 28,500 metric tons of molybdenum, worth about $700–800 million, which was 47% of total US production. [3] In 2014, copper mining produced about 15 metric tons of gold, worth $600 million, which represented 7% of US gold production. [4]
A metallurgical assay is a compositional analysis of an ore, metal, or alloy, usually performed in order to test for purity or quality. Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals are assayed by an assay office.
Porphyry copper deposits represent an important resource and the dominant source of copper that is mined today to satisfy global demand. [6] Via compilation of geological data, it has been found that the majority of porphyry deposits are Phanerozoic in age and were emplaced at depths of approximately 1 to 6 kilometres with vertical thicknesses on average of 2 kilometres. [6]
The widest application of stamp mills, however, seems to have occurred in Roman mining, where ore from deep veins was first crushed into small pieces for further processing. [3] Here, the regularity and spacing of large indentations on stone anvils indicate the use of cam-operated ore stamps, much like the devices of later medieval mining.