Copper Mining in Southeastern Arizona

Copper Mining in Southeastern Arizona at the Morenci Operation

The Morenci Operation: Arizona

Copper is a metal that is typically found in ore form. It is smelted or leached out of the ore and used in myriad different applications. Copper wire and bus bar are used in electrical applications while jewelers, home builders, and even aerospace engineers have valued its high conductivity, relative beauty, and fairly low cost for decades. The Spanish came to appreciate the value of copper hundreds of years ago as they mined it using very similar techniques as are used today.

Arizona has been home to copper mines for centuries. The first copper mines in the area were under Spanish rule in the 1750's, and Arizona has become one of the world's largest copper sources. The Morenci mine, located in a desolate area approximately 150 miles southwest of Phoenix is the largest operational mine in the state, producing over 247 million tons of ore per year. This ore is processed on site and yields approximately 15,000 tons of copper and over 355,000 tons of copper cathode. The copper ore is placed on leach pads on site; so much of the leaching and mineral separation is done immediately after the ore is mined. This helps cut down on production costs.

History of the Morenci Mine

The Detroit Copper Company started mining at Morenci in 1872. Copper Queen Consolidated Mining later bought out the operation in 1885, and the company name was officially changed to Phelps Dodge in 1917. In modern times, Phelps Dodge has merged with Freeport McMoRan Copper & Gold, whose principal claim to fame is the Grasberg copper and gold mining operation in Indonesia. Morenci is currently the largest copper producing mine in North America and continues to be a major boon to the Phelps' Company copper output. This output is second only to that of Chile's Codelco Company. Phelps Dodge Morenci, Inc. was established in 1986 as a partnership between Phelps Dodge Mining Company and Sumitomo Metal Mining Arizona Inc.

The copper-bearing mountainous region in southeastern Arizona was first identified by Americans in 1865. A regiment of California Volunteers showed that the future site of the Morenci mining operation was part of a vast porphyry copper orebody extending across a dissected mountain terrain. Essentially, it was a small part or finger of a huge copper ore vein contained in the mountains near Silver City, Arizona. Besides copper ores there are many other types of mineral and metals in this deposit including sulphide and oxide ores; pyrite and chalcocite being the main sulphide minerals, and chrysocolla and malachite the predominant oxides. Molybdenite, galena and sphalerite are also present nearby.

Extraction Process

The extraction methods used at the Morenci mine are both new and old. The copper ore is extracted by planting explosives at the bottom and sides of large open pit mines and pulverizing the rock that contains the ore. This ore is then picked up by giant dump trucks and loaded onto conveyor belts to the processing area. Until 1984, the Morenci mine used a smelter to extract the copper and lead out of the raw ore. But this smelting process was environmentally harmful and also hazardous to the health of the workers. It was abandoned in favor of a hydrometallurgical process that extracts the copper and lead from the ore through two leaching and solvent processes and one electrowinning tank, instead of smelting. Some of the Morenci ore does go to a smelter in a different region of Arizona and even to smelters in other states, but there is no longer any smelting done on site. The current type of extraction that the Morenci operation uses keeps the air and ground around the mine much safer and has helped reduce the toxic emissions coming from the operation.

The highest-grade ore is conveyed to leach pads within the mining pit. This operation surrounding the higher-grade ore is the largest in the world, as two complexes were joined to form the current leaching site. There the ore is crushed and pushed through a screen, with the fine sulfide ore (~-0.5 mm) going to a complex housing the froth flotation cells that begin the copper recovery process. The lower grade or coarser ore moves to the heap leach, where the ore is mixed into to a dilute sulfuric acid solution to dissolve the copper. This solution is then subjected to a process called solvent extraction (SX). The SX process concentrates and purifies the copper leach solution so the copper can be recovered at a high electrical current efficiency by the electrowinning cells. This is accomplished by adding a chemical reagent to the SX tanks, which selectively binds with and extracts the copper. This reagent is easily separated from the copper (stripped), as the operation looks to recover as much of the reagent as possible for re-use. The concentrated copper solution is dissolved in sulfuric acid and then sent to the electrolytic cells for recovery as copper plates (cathodes).

The Morenci operation has quite a large crusher, which can output at maximum approximately 63,000 tons of ore per day. This crusher pulverizes the ore and more conveyors send it to a nearby stump-leaching site called the Stargo leaching pad. This ore is agglomerated and made ready for spreading by two mobile stacking units. The actual leaching is bacterially assisted, with air blown into alternate lifts. This is currently the most efficient process for extracting the copper and other metals like lead out of the crushed ore.

Environmental Concerns and Impacts

The process of ore extraction by smelting is quite environmentally damaging. Many of the largest copper mines in the state of Arizona are designated as Superfund Clean-up Sites, or places where the Federal Government has placed restrictions on future operations while mandating clean-up of environmental toxins and contaminants. While the Morenci mine itself no longer has a smelter, one of the mine's most utilized smelters, called the Miami smelter, is currently on the Federal Government's list of Superfund sites due to toxic metals and contaminants that have leached through into the groundwater nearby.

The hydrometallurgical process of leaching copper ore also has many environmental concerns. This process often involves radioactive chemicals and other toxic chemicals. The Philips Dodge operations are regular recipients of Notices of Violation from the U.S. Attorney General. These notices reflect violations to environmental or other previously outlined health concerns associated with the mining operation. Some other major environmental concerns for this company, including the Morenci location are:

1) Phelps Dodge has pending water quality permits intended to stop the discharge of mining toxic waste into groundwater used for public and private drinking water.