Copper can be extracted from sulfide ores (usually higher grade ores) by direct smelting.
The earliest evidence of copper smelting occurs in Serbian artefacts dating from around 5000 BC.
Copper can be extracted from oxide ores using electrolysis (electrowinning) for low grade ores, or by the carbon reduction method of smelting for higher grade ores.
The process of extracting copper from higher grade sulfide ores involves:
Example: Extraction of Copper From Chalcopyrite (CuFeS2)
Mining : digging or blasting to obtain the copper ore from the surrounding rock
Crushing : mined copper ore is crushed into very small pieces
Grinding : crushed copper ore is ground into a powder
Concentrating by flotation (also known as froth flotation or as ore flotation).
Mining produce ores containing 2% or less of copper.
Concentrating the ore by froth flotation can result in ores with up to 35% copper.
A collector oil, such as an organic xanthate or thiophosphate, is added to the powdered ore and adheres strongly to the chalcopyrite particles making them water repellant.
This collector oil does not adsorb so strongly on other minerals such as silicate minerals so these particles will not repel water.
This mixture is fed into a water bath containing a foaming agent such as crude cresol or pine oil (soap is not a suitable foaming agent in this case).
Jets of air are forced through the water bath.
Water repellant chalcopyrite particles stick to the foam bubbles and float to the surface making a froth.
Gangue (waste) falls to the bottom and is removed.
The froth which contains the copper is skimmed off the surface and the now enriched or concentrated ore is taken away for roasting.
The water bath mixture is recycled.
The concentrated (enriched) ore can contain up to 35% copper and a number of impurities such as antimony, arsenic, and lead.
Roasting : reduces impurities in the copper ore and produces calcine (a mixture of products)*
Concentrated (enriched) ore is heated to between 500oC and 700oC in air.
Some impurities in the ore, such as arsenic and antimony, are oxidized and form volatile gases which can be removed:
4As + 3O2(g) → 2As2O3(g)
4Sb + 3O2(g) → 2Sb2O3(g)
Roasting the ore containing chalcopyrite, CuFeS2, also produces sulfur dioxide gas and a mixture of compounds called calcine.
One of the reactions in the formation of calcine is shown below:
2FeO(s) + Cu2S(s)
Calcine, a mixture of solids including copper oxides, sulfides and sulfates, can then be smelted.
eg, copper(I) oxide can be produced from copper(I) sulfide:
2Cu2S + 3O2(g) → 2Cu2O + 2SO2(g)
Smelting with Fluxes*: converts the calcine to matte (a mixture of copper sulfides and iron sulfides)
Calcine is heated to over 1200oC with fluxes such as silica (SiO2) and limestone (CaCO3).
Calcine melts and its compounds react with the fluxes.
Any copper(I) oxide present will be converted into copper(I) sulfide during the smelting process because copper has a higher affinity for sulfur than it does for oxygen.
for example: Cu2O + FeS → Cu2S + FeO
Impurities form a slag which floats on the surface and is easily removed.
One of the reactions to remove iron impurities is shown below:
(FeO.SiO2 can also represented as FeSiO3)
After the slag is removed, the resulting product, called matte, is a mixture of copper sulfides (mostly Cu2S) and impurities such as iron sulfides.
This matte, containing about 40% copper, can then be fed to a converter.
Conversion of Matte into Blister Copper (impure copper)
Air is blown through the molten matte which converts iron(II) sulfide to iron(II) oxide and sulfur dioxide
2FeS + 3O2 → 2FeO + 2SO2(g)
Iron(II) oxide, FeO, slag is skimmed off.
Relatively pure copper(I) sulfide, Cu2S, accumulates in the bottom of the converter.
Air is blasted through this copper(I) sulfide to reduce the copper and oxidize the sulfur to sulfur dioxide: