Most mineral deposits are just curiosities. Those that make mines are extraordinary and rare. Some people insist on reclamation of a mine soon after it is mined out. For some kinds of mines, such as a coal mine, it is obvious when the mineral is gone and the mined land can be reclaimed. However, metal mines, particularly large copper mines, can have many lives and be “mined out” many times. For these mines, wholesale reclamation such as filling in an open pit is uneconomic, wasteful, and unsound environmental practice, because it takes less to to reopen an old mine than to construct a new one. This story concerns the large copper deposits of southern Arizona and New Mexico; mines which produce about 66% of the nation’s domestically mined copper; mines which have many lives.
To understand how a mineral deposit can have many lives, we must appreciate the difference between mineralization and ore. “Ore” is that part of the mineralization which can be extracted at a profit. The term “ore” is purely economic and at any given time, it depends on the price of the commodity, the current technology, the costs of extraction and beneficiation, and the regulatory climate. A particular volume of mineralization can be classified as ore or removed from that classification. Government regulation is adding more and more cost, thereby decreasing “ore”, the amount of mineralization than can be made available to us.
The history of each large copper mine in the Southwest is unique, but their stories are similar. This essay, a composite of those histories, concerns the Copper King mine, a fictional name based on a medium-sized “porphyry” copper deposit. Commonly, large mineral deposits experience mining in some form over long periods of time, more than 125 years so far for the Copper King. The ability to provide raw materials over such a long time span is possible because different parts of the deposit are mined in response to changing economy and technology.
The Copper King
For hundreds of years, aboriginal people mined small quantities of the decorative green and blue oxide minerals and native copper at the Copper King mine site. Eventually, however, they could no longer extract material using their primitive tools. For them, the site became mined out.
Modern recorded activity began in the 1870s when turquoise attracted attention of local ranchers. Turquoise was mined with hand tools from small pits on veins, but production was spotty due to the remoteness of the mine and frequent attacks by aborigines. As time progressed, prospectors found the rich veins of chalcocite (a copper sulfide) which also were exploited with hand tools and primitive machines. Activity remained on a small scale, however, because only the richest mineralization could pay for the cost of transportation to the distant smelters by pack mule and wagon. Chalcocite ore mined at this time had a copper content of 30% copper per ton of rock and it was soon exhausted. The mine was mined-out.
In the 1890s, a newly constructed railroad, part of the transcontinental system, provided a more economical means of getting bulk ore to market. Lower transportation costs made lower-grade mineralization “ore” and allowed several mining companies to exploit the rich veins by costlier underground mining methods. The rock mined at this stage had a copper content of 4% to 10% copper per ton. Some companies prospered; others did not. Eventually, one company bought out all others and consolidated the mining camp into one operation for more efficient production. Underground mining of veins continued until the early 1920s, providing copper for the war effort in World War I. After the war, copper prices plunged and the mines closed. The remaining veins were not rich enough or numerous enough to support the cost of mining and processing. Again the mine was mined out.
During the ensuing years, geologists were at work in other areas creating ideas about disseminated mineral deposits, those with mineralization dispersed throughout the rock rather just confined to veins. Mining companies developed equipment enabling bulk mining of large, low-grade deposits with copper grades of 1% or less.
Floatation and Smelting
One key advance was the development of concentrating low-grade ore minerals by flotation extraction milling. This is a process where the mined rock is crushed to a consistency of talcum powder, and then transferred to large tanks which contain a stirring arm, much like a slow-speed blender or food processor. This same equipment is now used in some paper pulp mills. Within the tank, a mixture of powdered rock, water and pine oil is injected with air to form bubbles. The rock material, or gangue, sinks to the bottom of the tank. The ore minerals become attached to the bubbles through chemical attraction and surface tension, and float to the top where they are skimmed or “floated” off. The skimmed material, called concentrate, contains about 30% copper produced from rock originally containing 1% copper or less. The concentrate is dried, then sent to a smelter. Some by-product metals, such as molybdenum, lead or zinc, can be extracted in the concentrator through separate circuits. Other metals, such as gold and silver remain with the copper and are extracted in the refinery.
In the late 1940s, geologists thought these “new” techniques could be applied to the Copper King. During the 1950s and early 1960s, the low-grade disseminated chalcocite and chalcopyrite (CuFeS2) mineralization was explored with over 1000 drill holes. Finally, in the mid-1960s, the drilling had delineated mineralization of sufficient quantity and grade that it could be classified as ore based on new bulk mining and milling techniques. The exploration work and new technology lead the Copper King Mining Company to justify expenditure of several hundred million dollars for construction of an open pit mine, a concentrator, and purchase of equipment including large trucks and power shovels. Mining of this new, lower-grade chalcocite and chalcopyrite began in the 1960s.
By the late 1980s, however, mineralization grading over 0.4% copper per ton, the minimum required for the concentrator, was getting scarce and the Copper King was facing the end of its current stage of economic life. Again, it would become mined out. However, during the last years of mining, a new process was perfected: electrowinning-solvent extraction (SX/EW). This process allowed recovery of copper from even lower-grade chalcocite and, for the first time, economic extraction of copper from oxide mineralization which could not be processed by flotation. However, this method could not deal with the primary chalcopyrite.
The solvent extraction part of the SX/EW process is very similar to the natural process which formed the chalcocite and oxide mineralization. Rock is mined and placed in large, tabular heaps, usually 25 to 50 feet high, covering several acres. Slightly acidic water is sprayed on the heaps and allowed to percolate downward. The water dissolves copper in the rock. The copper-rich water is collected and piped to the extraction plant where copper is first stripped from the water using an organic solvent such as kerosene. The water is recycled. Copper-rich solution is pumped to a tank house. The tanks are like large automobile batteries, but run in reverse by applying electricity causing the copper to plate out on one of the “battery” electrodes. The SX/EW method is much less costly than the concentration process because it produces copper of sufficient purity for market without going through a concentrator or smelter. The SX/EW process is also more environmentally friendly. There are trade-offs, however. SX/EW cannot recover by-product metals such as molybdenum, gold and silver, nor can it recover copper from chalcopyrite.
SX/EW processing began during the later stages of mining at the Copper King and supplemented the concentrator ore, and continued alone after the concentrator closed. Because SX/EW can process lower-grade chalcocite and oxide minerals, it led to more geological investigation and exploration drilling which identified hundreds of millions of tons of formerly worthless rock which could now be classified as ore using the new process. Now all chalcocite, rich veins and low-grade disseminations alike, containing as little as 0.3% copper per ton could be mined. In addition, all the oxide material, which could not previously be exploited on large scale, could be mined to grades as low as 0.1% copper per ton.
High-temperature Pressure Leaching
In about 2003, a mining company perfected a method of leaching chalcopyrite in real time. This made possible the relatively inexpensive extraction of lower-grade chalcopyrite (more mineralization became “ore”) and it also eliminated the need for expensive smelting.
Under the new technology, the sulfide slurry from the concentrator is pumped into a pressure tank at 600 psi and heated to 225 C. Addition of oxygen causes the sulfides to break down according to this formula: Chalcopyrite + oxygen + water becomes aqueous copper sulfate + hematite + sulfuric acid. The reaction is written: 4 CuFeS2 + 17 O2 + 4 H2O = 4 CuSO4 + 2 Fe2O3 + 4 H2SO4.
Pyrite undergoes a similar reaction to produce hematite and sulfuric acid: 4 FeS2 + 15 O2 + 8 H2O = 2 Fe2O3 + 8 H2SO4. These reactions are similar to the natural weathering process which occurs over thousands of years. Pressure leaching does it in one hour.
By-product gold and silver, if any, stays with the hematite and can be recovered through conventional cyanidation leaching after the solids are removed from the reaction vessel. The sulfuric acid can be used to leach oxide copper ores. The aqueous copper sulfate goes to the solvent extraction – electrowinning (SX/EW) plant.
The Copper King mine will continue into the future, but what then? All mining since 1870 or any that will occur in the next few decades will have exploited only the top 1000 feet of the 7000-foot thick mineral deposit. Most of the remaining material is low-grade chalcopyrite (about 0.3% copper or less), material not economically extractable now. At that time, will the mine finally be mined out? No! New technological processes will be developed to economically extract some or much of the remaining copper.
One such process under development is the dissolution of chalcopyrite using sulfur-eating bacteria which can exist only in the environment of the sulfide mineralization. When this method is fully developed, it could allow leaching of chalcopyrite in situ (in place underground) and give the mine another life. It could also allow leaching of very low grade mine tailings left from previous mining operations.
The Copper King mine has had a long life, often marked by periods of inactivity: periods awaiting changes in economics or technology, periods awaiting a development which will turn mineralization into ore. Those who say that we should fill in open pits are short sighted because that activity may make a mine uneconomic at the outset and could make future mining based on new technology impossible. It is more environmentally sound to find ways to continue mining at an existing mineral deposit than to find and exploit a new deposit on virgin ground.
Mining is a risky business which requires huge up-front expenditures. It ultimately depends on economics. The engineering factors, such as deposit geometry and cost of equipment can be calculated with reasonable certainty. Other costs, especially that of ever-changing government regulation in the form of royalty schemes or environmental laws, make mining risky indeed and may actually waste a portion of the natural resource by drastically decreasing the amount of mineralization that can be called ore.