Copper extraction

Currently, the most common source of copper ore is the mineral chalcopyrite (CuFeS₂), which accounts for about 50% of copper production. The focus of this article is on the process of copper extraction from chalcopyrite ore into pure metal. Processes for other minerals are mentioned.

For economic and environmental reasons, many of the byproducts of extraction are reclaimed. Sulfur dioxide gas, for example, is captured and turned into sulfuric acid — which is then used in the extraction process.

Mining

To a large extent, copper oxide and sulfides are naturally separated in nature. As such, once mined the processing of the ore generally does not need to separate the oxides and sulfides. For an extensive discussion of orebody formation in general, see Ore genesis.

Primary orebody formation

Copper orebodies are formed when hydrothermal solutions bring copper dissolved from deep underground to cool near surface environments where the copper and associated metals precipitate as minerals in veins and disseminations within the rock. Copper is usually deposited as copper sulfide minerals or in some environments as native copper metal.

The most common copper minerals in the primary hydrothermal zone are:

Bornite Cu₅FeS₄
Chalcopyrite CuFeS₂

Oxidation

Orebodies exposed to weathering within the regolith become oxidised by atmospheric oxygen carried within rain water and surface waters which penetrate the weathered rock. Prolonged weathering of primary sulphide minerals converts them to soluble sulphate minerals, and with carbon dioxide carbonate minerals are formed.

Weathered sulphide mineralization is converted to a gossan or iron cap, composed of weathered oxide minerals of primary sulphides. Generally this is composed primarily of iron oxides with some copper oxide and copper carbonates. The iron in the oxidized zone remains as hydrous iron oxides (goethite, limonite, etc.) within the gossan.

The most common copper minerals in the oxidized zone are:

Atacamite Cu₂Cl(OH)₃
Azurite Cu₃(CO₃)₂(OH)₂
Cuprite Cu₂O
Chrysocolla CuSiO₃(H₂O)₂
Malachite Cu₂CO₃(OH)₂
Native Copper Cu
Tenorite CuO

Secondary orebody enrichment

Beneath the oxidized zone, some dissolved copper is precipitated as secondary or supergene copper minerals. This enriches the sulfides, making a secondary enrichment, or transitional zone. The secondary enrichment replaces iron in the minerals with more copper, further enriching the ore.

The most common copper minerals in the secondary enrichment zone are:

Chalcocite Cu₂S
Covellite CuS

Reclamation and regulation

The requirements for companies or individuals to reclaim copper mines varies from country to country around the world. Regulations also vary from state to state within the United States. For instance, the State of New Mexico requires all hardrock mines to be reclaimed to a self-sustaining ecosystem after mining has been completed. Financial assurance is held by the state until the reclamation has been completed.

Hydrometallurgical extraction

Oxide ores

Oxide ores are readily leached by sulfuric acid, usually using a heap leach or dump leach process in combination with solvent extraction and electrowinning technology (SX-EW). Commonly sulfuric acid is used as a leach for copper oxide, although it is possible to use water. There have been examples where froth flotation was used to concentrate malachite. In general froth flotation is not used to concentrate copper oxide ores, as the cost of leaching is cheap when compared to the cost of grinding and flotation. The implication of this is that copper oxides are more economic to process than copper sulfides.

Secondary ores

Secondary sulfides - those formed in secondary enrichment - are resistant (refractory) to sulfuric leaching. High grade secondary sulfides may be concentrated using froth flotation, and subsequently smelted to recover the copper, or else they can be leached using a bacterial oxidation process to oxidize the sulfides to sulfuric acid, which also allows for simultaneous leaching with sulfuric acid. As with oxide ores, solvent extraction and electrowinning technologies are used to recover the copper from the pregnant leach solution.

Pyrometallurgical extraction

The following is a process of copper extraction from chalcopyrite ore into pure metal. While oxide ores can be processed using Pyrometallurgical techniques, Hydrometallurgical methods are more cost effective.

The copper ore is crushed and ground before it is concentrated to between 20 and 40% copper in a flotation process. The next major step in production uses pyrometallurgical processes to convert the copper concentrate to 99% pure copper suitable for electrochemical refining. These high temperature processes first roast the concentrate, then smelt it in a furnace, oxidise and reduce the molten products to progressively remove sulfur, iron, silicon and oxygen to leave behind relatively pure copper.

Concentration

All copper sulfide ores are concentrated using the froth flotation process. Ground ore is mixed with xanthate reagents (for example, pine oil), which reacts with the copper sulfide mineral to make it hydrophobic on its surface. (Besides xanthates, dithiophosphates and thionocarbamates are commonly used.)

The sulfide ore is crushed and ground to increase the surface area of the ore for subsequent processing. The powdered ore is mixed with pine oil (the 'collector chemical') and introduced to a water bath (aeration tank) containing surfactant. Air is constantly forced through the slurry and the hydrophobic mix of copper and pine oil latches onto and rides the air bubbles to the surface, where it forms a froth and is skimmed off. These skimmings are cleaned of the collector chemical and surfactant, leaving copper concentrate. The remainder is discarded as tailings, or processed to extract other elements.

An example collector chemical is potassium amyl xanthate.
An example frother chemical is sodium oleate, a component of some soaps.

To improve the process efficiency, limestone is used to raise the pH of the water bath, causing the collector to ionize more and to preferentially bond to chalcopyrite (Cu Fe S₂) and avoid the pyrite (FeS₂) - iron exists in both primary zone minerals.

The product from this froth flotation process is known as copper concentrate. When the foam (which is between 20 and 40% copper) is dried it is known as copper concentrate. Copper concentrate may be treated by either hydrometallurgical methods or sintered before pyrometallurgical methods are used to produce copper metal. Copper concentrate is sometimes traded either via spot contracts or under long term contracts as an intermediate product in its own right.

Roasting

In the roaster, the copper concentrate is partially oxidised to produce calcine and sulfur dioxide gas. The stoichiometry of the reaction which takes place is:

2CuFeS_2(s) + 3O_2(g) → 2FeO_(s) + 2CuS_(s) + 2SO₂_(g)

As of 2005, roasting is no longer common in copper concentrate treatment. Direct smelting using the Flash Smelting or El Teniente furnace is now used.

Smelting

The calcine is then mixed with silica and limestone and smelted at 1200°C (in an exothermic reaction) to form a liquid called copper matte. This temperature allows reactions to proceed rapidly, and allow the matte and slag to melt, so they can be tapped out of the furnace. In copper recycling, this is the point where scrap copper is introduced.

Several reactions occur.

For example iron oxides and sulfides are converted to slag which is floated off the matte. The reactions for this are:

FeO_(s) + SiO_{2 (s)} → FeO.SiO_{2 (l)}

In a parallel reaction the iron sulfide is converted to slag:

2FeS_(l) + 3O₂ + 2SiO_{2 (l)} → 2FeO.SiO_2(l) + 2SO_2(g)

The slag is discarded or reprocessed to recover any remaining copper.

Conversion to blister

The matte, which is produced in the smelter, contains around 70% copper primarily as copper sulfide as well as iron sulfide. The sulfur is removed at high temperature as sulfur dioxide by blowing air through molten matte:

CuS_(l) + O_2(g) → Cu_(l) + SO_2(g)

In a parallel reaction the iron sulfide is converted to slag:

2FeS_(l) + 3O₂ + 2SiO_{2 (l)} → 2FeO.SiO_2(l) + 2SO_2(g)

The end product is (about) 98% pure copper known as blister because of the broken surface created by the escape of sulfur dioxide gas as the copper ingots are cast. By-products generated in the process are sulfur dioxide and slag.

Reduction

The blistered copper is put into an anode furnace (a furnace that makes anodes) to get rid of most of the remaining oxygen. This is done by blowing natural gas through the molten copper oxide. When this flame burns green, indicating the copper oxidation spectrum, the oxygen has mostly been burned off. This creates copper at about 99% pure. The anodes produced from this are fed to the electrorefinery.

Electrorefining

The copper is then put into sheets which are refined by electrolysis. The copper anodes are placed into a solution of copper sulfate and sulfuric acid. The copper then migrates across the solution to the cathode, also made of copper to maintain purity. The reactions are:

At the anode: Cu_(s) → Cu²⁺_(aq) + 2e^-

At the cathode: Cu²⁺_(aq) + 2e^- → Cu_(s)

Copper cathode is 99.99% copper in sheets of dimensions: 96 cm x 95 cm x 1 cm, with a mass of about 100 kg. It is a true commodity, deliverable to the metal exchanges in New York, London and Shanghai. The chemical specification for electrolytic grade copper is ASTM B 115-00.

External links

The Copper Development Association's copper production page.
CCU-1c Copper Concentrate
Copper Processing
National Pollutant Inventory - Copper and copper compounds fact sheet
University of Pittsburgh School of Engineering Chemical and Petroleum Engineering Department, Froth Flotation Lab notes.