Copper extraction techniques



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 &mdash; which is then used in the extraction process.

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 supergene 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
Most high grade copper sulfide ores, containing about 0.25% to 5% copper metal (the rest being unwanted rock), are concentrated using the froth flotation process. Ground ore is mixed with xanthate reagents (or other reagents of the thiol class), which react 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 chemicals (the 'collector chemical') and introduced to a water bath (aeration tank) containing surfactant. Air is constantly forced through the slurry and the hydrophobic copper sulfides particles latches onto and rides the air bubbles to the surface, where it forms a froth and is skimmed off. These skimmings are generally re-processed (cleaned) to reach a high purity 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 methylisobutyl carbinol or, for short, MIBC, an alcohol.

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 (CuFeS2) 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 froth (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.

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) + SiO2 (s) → FeO.SiO2 (l)


 * In a parallel reaction the iron sulfide is converted to slag:
 * 2FeS(l) + 3O2 + 2SiO2 (l) →	 2FeO.SiO2(l) + 2SO2(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:


 * 2Cu2S + 3O2 → 2Cu2O + 2SO2


 * Cu2S + 2Cu2O → 6Cu + SO2

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


 * 2FeS + 3O2 → 2FeO + 2SO2


 * 2FeO + 2SiO2 → 2FeSiO3

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 refined by electrolysis. The anodes cast from processed blister copper are placed into an aqueous solution of 3-4% copper sulfate and 10-16% sulfuric acid. Cathodes are thin rolled sheets of highly pure copper. A potential of only 0.2-0.4 volts is required for the process to commence. At the anode, copper and less noble metals dissolve. More noble metals such as silver and gold as well as selenium and tellurium settle to the bottom of the cell as anode mud, which forms a saleable byproduct. Copper(II) ions migrate through the electrolyte to the cathode. At the cathode, copper metal plates out but less noble constituents such as arsenic and zinc remain in solution. 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 (a standard that specifies the purity and maximum electrical resistivity of the product).