Part A
Copper Mining Basics
Large open pit mine operations around the state (like Morenci, Ray and Bagdad) produce significant amounts of copper ore every year. Other types of geologic deposits have been mined from underground operations (like Jerome and Bisbee). The valuable ores of copper, gold, silver, etc. are usually bound with other elements to form ore minera.ls For example, the elements copper (Cu), iron (Fe), and sulfur (S) combine to make the copper ore mineral chalcopyrite (CuFeS2). Once mined, the ore minerals are extracted from less valuable waste rock, and the valuable elements are concentrated using several different processes (crushing, gravity separation, electrowinning, etc.). There are many indispensible uses for Arizona's economic minerals, although the value of the various mineral ores fluctuates in the free market.
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Figure 13-2. There are many important aspects in copper mining extraction and processing. The Silver Bell porphyry copper mine in Arizona (left), a Cat 785G 300-ton haul truck (middle), and bundled copper plates at the Silver Bell SX-EW plant (right). |
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Mineral resources are classified based on their economic potential. These include:
> Mineral occurrences or prospects - Deposits that are of geological interest, but may not be of economic interest
> Mineral resources - Potentially economic deposits that may or may not be feasible to extract.
> Mineral reserves - Proven or probable deposits that are both economically and technically feasible to extract. These include ore deposits.
Arizona Mining
Our civilization utilizes copper in many ways, and Arizona provides more copper than any other state in the U.S. Refer to the Wikipedia's Copper mining in Arizona web page to answer Quiz Me! questions A01 through A03.
Ore Deposits
Copper deposits are formed in several ways and are commonly found in association with many other metals (gold, silver, lead, zinc, etc.). Most of the world's copper comes from porphyry copper deposits, where the copper ore body is related to intrusive igneous rocks and processes. Copper is enriched by processes during crystallization of the granite and ore bodies form when large quantities of hydrothermal fluid passes through fractured rock within and around the intrusive. Copper can precipitate out along fractures and form a vein ore deposit or spread out through a larger volume of rock and form a disseminated ore deposit. As groundwater moves through a disseminated sulfide deposit, it dissolves the dispersed metals above the water table and releases them in a zone below the water table. The relatively low-grade, disseminated ore deposits (<1% copper) are economically viable if mined in sufficient quantity by open-pit methods. Compared to disseminated ore deposits, vein deposits typically have much higher percentages of copper, but must be mined using more expensive underground methods.
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Figure 13-3. Two major types of ore deposits: disseminated and vein (from Tarbuck and Lutgens, 2002). |
Some ore deposits form as hot, metal-rich fluids (hydrothermal fluids) migrate through rocks and fractures surrounding igneous intrusions (see Figure 13-3). As these fluids cool, various metals (Cu, Au, Ag, etc.) precipitate from solution, and combine with other elements to form ore minerals that make up some part of a larger ore body.
Disseminated Ore Deposits
> Ore minerals are spread throughout large volumes of rock.
> Economic metals in these low-grade ore deposits are present in relatively low overall concentrations (<1%).
> Extraction involves open pit mining methods.
Vein Ore Deposits
> Ore minerals are concentrated along mineral-filled fractures.
> Economic metals in these high-grade ore deposits are present in relatively high overall concentrations.
> Extraction involves underground mining methods.
Go to the Wikipedia's Porphyry copper deposit web page to answer Quiz Me! questions A04 through A06.
Mining Methods
A geologist describes an ore body by mapping the ore body and the surrounding rocks. The next step involves drilling to confirm the exact location, width, and depth of the ore body underground. Once an economically viable ore deposit has been identified, the most efficient and economical extraction method is employed. There are two main types of mining excavation processes: surface and sub-surface. Each of these has several different techniques to access the ore body. Two of the most common methods include:
Open Pit Mining - This technique is used if the ore body is large, close to the surface, and/or horizontal. In Arizona, large copper deposits are commonly associated with igneous plutons. The huge mines at Bingham Canyon, Utah and Morenci, Arizona are some of the world's largest open pit operations.
Underground Mining - A series of tunnels follow the ore body, which is usually small, deep, and/or tabular in shape. Underground mining was once more common than surface mining in Arizona. The well-known Copper Queen Mine in Bisbee is an example of an underground mine that was at its operational peak almost a century ago.
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Figure 13-4. The two main modes of mining are open pit and underground. A typical open-pit mining operation (left). The small Hance underground mine (the adit is behind the tree) at Colorado River mile 77.8 in Grand Canyon NP. |
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Open Pit
Let's look at the largest open-pit excavation in the world. Use the Wikipedia's Bingham Canyon mine web page to answer Quiz Me! questions A07 through A10.
Underground
Now go to the Wikipedia's Underground mining web page to answer Quiz Me! questions A11 through A13.
Ore Processing
Copper is extracted from copper ore in one of two ways depending on the primary type of deposit (sulfide or oxide). Both processes start out the same; blasting and digging produces rock fragments that are loaded onto conveyors huge dump trucks. The ore is unloaded into pit-crushers (up to 30 feet in diameter) that crush the rock into pieces down to less than half an inch. The rock is then pulverized using a ball mill, a giant rotating drum filled with steel balls. From here, the ore extraction method can vary, depending on the type of ore. Because of their different compositions, copper sulfide and copper oxide ores are typically processed by separate methods. In a nutshell, the sulfide and oxide ore processing is somewhat similar, except mainly that smelting is necessary for sulfide ores whereas it isn't for oxide ores.
Sulfide ore
For copper sulfide ores, first step is crushing. The ore material is pulverized to ½ inch or smaller pieces in a rod mill or ball mill, (huge drums filled with steel rods or balls).
Flotation is next. The crushed material is conveyed to a flotation vat, filled with an organic solution. The copper adheres to certain chemical agents and floats to the top, whereas the waste rock sinks to the bottom. At this point in the process, the copper ore is concentrated to about 30% grade.
The sulfide concentrate mixture is then dried and sent to a smelter (flash furnace or reverberatory furnace) where it is heated at high temperatures with a flux to melt the metal to separate the copper ore from sulfur, iron, and other unwanted compounds. The slag (waste material) floats to the top and is separated from liquid copper matte, which is poured from the bottom. This copper matte is 30-70% pure copper.
The copper matte is then sent to a converter which separates the sulfur from the copper. The SO2 is removed and captured for use in leaching processes. The remaining copper is known as blister and is 98.5-99.5% pure copper.
The blister copper is then sent to an anode furnace where it is further refined and cast into anodes for use in electrolytic refining.
The final stages employ electrorefining, which involves electrochemically dissolving copper from an impure copper anode (the positive electrode) in an electrolyte containing CuSO4 and H2SO4 with subsequent electrochemical attraction and deposition of copper ions from the electrolyte onto a stainless steel or copper cathode (the negative electrode). As the electrolysis progresses, the impure copper anode dissolves away and pure copper builds up as a thicker and thicker coating on the cathode. Each anode is electrorefined until about 80%–85% of the mass has dissolved. Insoluble impurities in the anode adhere to the anode or fall to the bottom of the refining cell, where they are removed and sent to a byproduct metal recovery plant. Mature copper-loaded cathodes typically weigh 50–160 kg and are removed from the cells after 7–10 days of plating. Two or three cathode plating cycles are produced from a single anode, with each finished cathode being more than 99.9% pure copper.
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Figure 13-5. A simplified diagram showing the processing of copper sulfide ore. |
Oxide ore
As with sulfide ores, first step in processing copper oxide ores is crushing. The low-grade ore material is pulverized to ½ inch or smaller pieces in a rod mill, ball mill, or other crushers. The crushed ore is then transported to a lined leach dump or a sulfuric acid leaching tank/vat. At the leach dump, sulfuric acid is sprayed onto the crushed ore, and copper is leached out with a sulfuric acid solution. The resulting copper sulfate pregnant leach solution is then sent to the solvent extraction and electrowinning plant (SX-EW), and the waste sulfuric acid is recycled back on to the leach dump.
At the SX-EW plant, the copper sulfate leach solution is first mixed with an organic reagent solution to further concentrate the dissolved copper. This solution is then sent to the electrowinning tanks to be electrolytically refined. Again, waste sulfuric acid is recycled back onto the leach dump.
In electrowinning, an electrical current is passed from an inert anode plate (typically made out of lead) through the copper sulfate solution containing the dissolved copper ions. The copper ions are then attracted to and deposit onto the cathode.The finished cathode is more than 99.99% pure copper.
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Figure 13-6. A simplified diagram showing the processing of copper oxide ore. |
For more details about ore processing, see the Wikipedia's Copper extraction web page.
Refer to Figures 13-5 and 13-6 to answer Quiz Me! questions A14 through A19.