Bingham Canyon Copper
Finding Chalcopyrite at "The Richest Hole on Earth"
by Steve Voynick, Rock&Gem Contributing Editor
All photos courtesy of Kennecott Utah Copper Corporation
Chalcopyrite, or copper iron sulfide, is among the best-known copper minerals. Chalcopyrite crystals have unevenly faced tetrahedrons that are striated in different directions, a metallic luster and a brassy-gold color somewhat less yellow than pyrite. Crystals usually tarnish to iridescence or even to deep blues, purples or blacks. Among all metal sulfide minerals, only pyrite exceeds chalcopyrite's crustal abundance. Chalcopyrite, however, is the most abundant copper mineral.
Chalcopyrite is a popular mineral among collectors. Fine specimens, such as those from Arizona, Missouri and Mexico, may have well-formed tetrahedrons measuring as many as four or five inches across. Chalcopyrite's true claim to fame, however, is not its visual appeal but that it is the major ore of copper. When it comes to sheer size in chalcopyrite deposits, nothing approaches the deposit at Bingham Canyon, Utah.
For 90 years, Kennecott Utah Corporation's Bingham Canyon Mine has been the granddaddy of all copper mines. Just how big is Bingham Canyon? Whether you're talking about metal production or the actual size of the mine, Bingham Canyon is simply the largest copper mine in the world.
To begin with a few production statistics, the Bingham Canyon Mine has produced more copper than any other mine in history--about 14.5 million tons of the metal. Bingham Canyon is primarily a copper mine, but it has also yielded a bonanza in byproduct metals. These include 18.5 million troy ounces (about 620 tons) of gold, 157 million troy ounces (nearly 5,000 tons) of silver, 610 million pounds of molybdenum and significant amounts of platinum and palladium. The cumulative value of Bingham Canyon metals far exceeds the total worth of the Comstock Lode and the California and Klondike gold rushes combined. With production statistics like that, it's no wonder that the Bingham Canyon Mine has been nicknamed "the Richest Hole on Earth."
|The Bingham Canyon open pit stretches for 2.5 miles across the rim and is the largest manmade excavation on Earth
Bingham Canyon is also the biggest hole on earth. As the largest of all man-made excavations, Bingham Canyon is more than a half-mile deep. If the world's tallest building, the 1,454-foot-high Sears Tower, were placed at the bottom of the pit, it would reach only halfway to the mine's rim. The Bingham Canyon open pit stretches more than 2.5 miles across at the rim and covers 2,000 acres. Its immensity makes it the only manmade feature visible to the naked eye from the orbiting space shuttle.
Nestled in the Oquirrh Mountains 20 miles southwest of Salt Lake City, Bingham Canyon sits atop what may be the greatest single metal deposit ever discovered. The Bingham Canyon story, which combines geology, history, technological innovation and modernization, is a classic in American mining.
The oldest rocks at Bingham Canyon--sandstones, quartzites and limestones--were originally deposited as sediment by the shallow seas that covered the region 300 million years ago (in the late Paleozoic Era). Much later, between 60 and 135 million years ago, extensive folding and faulting of the sediments created the Oquirrh Mountains.
Just 30 to 40 million years ago, massive igneous intrusions initiated the process of mineralization. Extreme pressure forced superheated, mineral-rich solutions into fractured intrusive and adjacent sedimentary rock. Upon cooling, the mineralized solutions deposited enormous quantities of metals throughout a broad section of igneous and sedimentary rock that is now known as the Bingham Stock.
Bingham Canyon is not presently a source of notable mineral specimens. The Bingham Stock is a porphyry deposit, meaning that copper minerals--primarily chalcopyrite--are present in very low grades and disseminated throughout the granite-like host rock as tiny grains, seams and fracture coatings. Other metals associated (in smaller quantities) with the chalcopyrite include the following: gold, silver, lead, molybdenum, platinum and palladium.
In relatively recent geologic time, erosion exposed the upper part of the Bingham Stock, permitting surface and near-surface oxidation to enrich many outcrops. In the 1860s, prospectors searching the rugged Bingham Canyon area discovered outcrops of high-grade lead carbonate, or cerussite. In 1873, the persistent prospectors found gold and silver in the outcrops, triggering a mining boom. Underground mining continued for 20 years as miners pursued rich, oxidized pockets and veins of gold- and silver-bearing cerussite and two copper carbonates, azurite and malachite.
If Bingham Canyon ever produced notable mineral specimens, it was during the early underground mining period. The oxidized ores mined during the 1870s and 1880s were so rich that they could be smelted immediately, with no need for prior concentration.
By the early 1890s, underground mining had exhausted the high-grade, oxidized ores. An enormous amount of mineralization remained, but it consisted only of deep, unoxidized sulfides so low in grade that mining them was not economical. The problem of low-grade ores was not unique to Bingham Canyon. Miners across the West were rapidly running out of high-grade, direct-smelting ores, and many mines and entire districts were closing. The subsequent drop in metal production had national implications, for American industry, embarking on broad programs of industrialization, mechanization and electrification, now needed huge quantities of base metals, especially copper. But if low-grade, porphyry copper deposits, such as that at Bingham Canyon, were to supply the national demand for copper, a great technological advancement was necessary.
The breakthrough began in 1898 when Daniel Jackling, a metallurgical engineer, and Robert Gemmell, a mining engineer, proposed new mining and milling methods that would revolutionize metal mining. Jackling and Gemmell believed that low-grade, porphyry copper ores could be mined profitably. They figured the key was to combine massive surface-mining methods with a then unproved milling process: flotation separation.
In 1903, Jackling formed the Utah Copper Company and built a 300-ton-per-day mill at Bingham Canyon. Three years later, he began using steam shovels in the nation's first mechanized open pit mine. Jackling then constructed another mill to provide the first large-scale demonstration of the flotation separation process.
Flotation separation uses the tendency of oil-covered mineral particles, particularly those of metal sulfides, to adhere to oil bubbles. In the process, finely ground ore slurry is mixed with an oily reagent, then vigorously agitated and aerated in tanks. Mineral particles adhere to the rising bubbles and float off as an oily froth, while particles of nonmineralized rock, called gangue, fall to the bottom and are discarded as tailings.
At Bingham Canyon, the effectiveness of large-scale flotation separation exceeded Jackling's hopes. Although Bingham Canyon ore contained a mere 0.6-percent copper--a ton yielded only 12 pounds of the metal--inexpensive flotation separation produced a smelter-ready concentrate containing 23 percent copper and byproduct metals.
Mining engineers throughout the West widely copied the combination of mechanized, large-scale open-pit mining and flotation separation, enabling a new generation of large surface mines to supply ample amounts of copper for national industrialization. Even so, no other mine grew as rapidly as Bingham Canyon. Kennecott Utah Copper Corporation acquired the mine property in the 1930s and stepped up expansion. During World War II, Bingham Canyon alone provided more than one-third of the prodigious amount of copper used by the Allies in the war effort.
By 1980, the huge Bingham Canyon Mine had 2,500 miners who drilled and blasted 370,000 tons of ore and overburden every day. An additional 5,200 employees worked in haulage, concentrating, smelting and refining to produce 300,000 tons of pure copper each year.
By then, Bingham Canyon had become an antiquated, labor-intensive, inefficient mine. Saddled by high costs of production and environmental compliance, Bingham Canyon could longer compete with producers of cheap, foreign copper. When copper prices plummeted in 1982, Bingham Canyon incurred huge annual operating losses. Soon afterwards, Bingham Canyon shut down for the first time in eighty years. The old mine still had one big asset, though. That was its world-class copper deposit, with enough reserves for 30 more years of mining--if the mine could regain market competitiveness.
Over the next decade, in testimony to its confidence in the size and quality of the remaining chalcopyrite ore body, Kennecott Utah Copper poured $1.5 billion into upgrading its mining, concentrating, smelting and refining operations. Today, the rejuvenated Bingham Canyon Mine is among the most safe, clean and efficient major mining operations anywhere.
At Bingham Canyon, the process of recovering copper from chalcopyrite begins when miners drill and blast the in situ ore and overburden. Detonation of large patterns of 55-foot-deep, 12-inch-diameter drill holes, each loaded with a half-ton of explosive, breaks the solid rock. Electric shovels, some lifting nearly 100 tons of material in a single bucket "bite," fill 240-ton-capacity haulage trucks that move the broken rock to the crusher.
After crushing, a five-mile-long conveyor belt moves the ore to the concentrator, where huge grinding mills reduce it to the consistency of face powder. Flotation then separates the gangue from the metalliferous particles, which float off as a 28-percent concentrate of copper along with lesser amounts of silver, gold, lead, molybdenum, platinum and palladium. A selective flotation step separates the molybdenite (molybdenum disulfide) from the chalcopyrite.
The filtered concentrate slurry is piped 17 miles to the smelter, where it is dried, and then injected along with oxygen into a flash smelting furnace to oxidize the iron and sulfur. The oxidized iron is skimmed off, while the sulfur dioxide gas is captured and sent to an on-site acid plant for conversion to valuable sulfuric acid--a million tons of it each year.
Left behind is a molten copper sulfide called matte. The 70-percent-copper matte is water-quenched to form a sand-like solid, then injected, with oxygen, into a flash-converting furnace that produces molten, 98.6-percent-pure copper. This copper is then cast into 700-pound anode plates and shipped by rail to the refinery.
At the refinery, the anode plates are pressed flat and interleaved with stainless steel cathode blanks. Automated robotic vehicles place the prepared anodes in cells containing an acidic electrolyte. When the cells are electrified, the anodes slowly dissolve, freeing copper ions that are deposited on the cathode as 99.9-percent-pure copper.
Impurities and precious metals settle to the bottom of the electrolytic cells as "slimes." A chlorination leaching process recovers the gold and silver, which is melted in induction furnaces and poured into 400-troy-ounce gold bars and 1,000-troy-ounce silver bars. Each year, the precious metals refining plant pours 500,000 troy ounces of gold and 4 million troy ounces of silver--surpassing the production of many large primary gold mines.
Bingham Canyon is a half-hour drive southwest of Salt Lake City. If you're ever in the area, it's certainly worth a visit. The Kennecott Utah Copper Corporation maintains an overlook with an unforgettable view of the vast open pit, and distributes publications explaining the mine's history and operations. An hour or two at Bingham Canyon will leave visitors with a good idea of the effort behind American copper mining. The huge open pit illustrates just what lengths some people will go to for a little chalcopyrite.
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