Copper extraction: Encyclopedia II - Copper extraction - Pyrometallurgical Extraction

Copper extraction - 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.

Copper extraction - 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.

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.

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 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.

Copper extraction - 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) + 2SO_2(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.

Copper extraction - Smelting

The calcine is then mixed with silica and limestone and smelted at 1200°C (in an exothermic reaction) to form a liquid called matte. 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)

Copper extraction - 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.

Copper extraction - 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.

Copper extraction - Electorefining

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 99.97% 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.

Adapted from the Wikipedia article "Pyrometallurgical Extraction", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki