4a. The extraction of copper from copper ores and its purification by electrolysis

• How is copper extracted? how is copper purified? What is the state of copper ore reserves?
• Copper can be extracted from copper–rich ores by heating the ores in a furnace (smelting) and the copper can be purified by electrolysis.
• However, the supply of copper–rich (high grade) ores is limited.
• Copper is extracted from its ores by chemical processes that involve heat or electricity (roasting ores in a smelter–furnace and purification by electrolysis – all the details below).
• Because its position in the reactivity series of metals, copper can be extracted using carbon in a smelting furnace. Copper is below carbon (less reactive) and so can be displaced by carbon from its compounds eg copper oxides or sulfides. However, in practice, modern copper smelters can actually manage the extraction without using carbon (coke) and then electrolysis is usually used to purify the impure copper from the smelter.

• The metal copper can be easily extracted BUT copper–rich ores are becoming scarce so new methods of extracting copper are being developed to exploit low grade ores.

  • A low grade ore is one with low concentrations of copper and research is going on to try and exploit waste material left over from processing high grade ores.

• Copper rich ores are relatively rare, so very valuable, so production costs are quite high.
  • Copper ores are concentrated by a technique known as froth flotation.
  • The ores are roasted to drive off unwanted water and convert them to a more suitable chemical form for reduction to copper metal.
  • After reduction of the ore the liquid copper can be run off from the coke fired copper smelter (furnace).
    • Below are descriptions of the extraction of copper with balanced chemical equations.
    • The balanced equations quoted below, are a simplification of what can be quite complicated chemistry, BUT they do adequately describe and illustrate the chemical processes for extracting copper from its ores.
• From copper carbonate ores* ...
  • The ore can be roasted to concentrate the copper as its oxide.
  • Water is driven off and the carbonate thermally decomposed.
  • copper(II) carbonate ==> copper oxide + carbon dioxide
  • CuCO₃ ==> CuO + CO₂   (a thermal decomposition)
    • CuCO₃(s) ==> CuO(s) + CO₂(g)
  • The oxide can be smelted by heating with carbon (coke, charcoal) to reduce the oxide to impure copper, though this method isn't really used much these days (the 'bronze age' method archaeologically!).
  • copper(II) oxide + carbon ==> copper + carbon dioxide
  • 2CuO + C ==> 2Cu + CO₂     (an oxide reduction reaction, O loss)
    • 2CuO(s) + C(s) ==> 2Cu(s) + CO₂(g)
  • The copper oxide is reduced to copper because of oxygen loss.
  • The carbon acts as the reducing agent – the 'oxygen remover'.
  • REDOX definition reminders – reduction is a process of oxygen loss (or electron gain) and oxidation is a process of oxygen gain (or electron loss).

• From copper sulphide ores ...
  • These include chalcocite/chalcosine = copper(I) sulphide Cu₂S and covellite = copper(II) sulphide CuS
    • and chalcopyrite CuFeS₂. which is one of the most important ores for the extraction of copper.
      • This can be roasted in air to produce copper(I) sulfide which is roasted again in a controlled amount of air so as not to form a copper oxide (see below).
      • 2CuFeS₂ +  4O₂ ==> Cu₂S + 3SO₂ + 2FeO
  • Copper sulphide ores can be rapidly roasted in heated air enriched with oxygen to form impure copper and this extraction process is called 'flash smelting'.
    • Nasty sulphur dioxide gas is formed, this must be collected to avoid pollution and can be used to make sulphuric acid to help the economy of the process.
    • copper(I) sulphide + oxygen ==> copper + sulphur dioxide
      • Cu₂S + O₂ ==> 2Cu + SO₂
        • Cu₂S(s) + O₂(g) ==> 2Cu(s) + SO₂(g)
      • The loss of sulfur from the copper sulfide is still a reduction change.
        • at the same time the sulfur gets oxidised to sulfur dioxide – oxygen gain.
    • or
    • copper(II) sulphide + oxygen ==> copper + sulphur dioxide
      • CuS + O₂ ==> Cu + SO₂
        • CuS(s) + O₂(g) ==> Cu(s) + SO₂(g)
• It is also possible to dissolve an oxide or carbonate ore in dilute sulphuric acid and extracting copper by ....
  • (1) using electrolysis see purification by electrolysis, or
  • (2) by adding a more reactive metal to displace it e.g. scrap iron or steel is used by adding it to the resulting copper(II) sulphate solution.
    • Using displacement with scrap iron to displace–extract copper from a solution of a copper salt
    • iron + copper(II) sulphate ==> iron(II) sulphate + copper
    • Fe + CuSO₄ ==> FeSO₄ + Cu
      • Fe(s) + CuSO₄(aq) ==> FeSO₄(aq) + Cu(s)
        • sulfate = sulphate, sulfate is now the official name for this ion.

• The copper obtained from the smelting processes described above is too impure to use, so it is purified by electrolysis (details further down the page).

• The future for copper mining? How is copper extracted by phytomining and bioleaching?

• Copper–rich ores are being depleted and traditional mining and extraction have major environmental impacts, so there are important issues involved with the future exploitation of copper ore reserves.

• Because of these issues new ways of extracting copper from low–grade ores (eg containing ~1% copper) are being researched to limit the environmental impact of traditional mining.

  • For example, copper can be extracted by phytomining, or by bioleaching.

    • Phytomining ('mining with plants', a commercial example of phytoextraction, ) Phytomining uses plants to absorb metal compounds and the plants are burned to produce ash that contains the metal's compounds. Some plants naturally absorb copper compounds through their roots as they feed on the nutrients around them. They can't always get rid of the excess of certain metals like copper, so this resulting in higher concentrations of these copper compounds in the plant tissues. The plants can then be burned to produce an ash that contains the copper compounds. The ash is dissolved in acid and the copper can be extracted by electrolysis or more cheaply by displacement of the copper with scrap iron.

    • Bioleaching (with 'rock eating bacteria'!) Apparently 10% of copper in the US comes from bacteria which live off the surrounding rocks. Bioleaching uses bacteria (bacterial microorganisms) to produce leachate solutions that contain soluble copper compounds. Some bacteria naturally absorb copper compounds as they chemically interact with the surrounding mineral rocks, ie a copper leaching effect with respect to the surrounding rock material as they break down ores like chalcopyrite (CuFeS₂). From the bacterial discharges you can produce solutions (leachates), which contain soluble copper compounds in commercially viable concentrations. Again, the copper can be extracted by electrolysis or more cheaply by displacement of the copper with scrap iron.

      • Neither phytomining or bioleaching are fast so it isn't always economical to use this technique for extracting copper, but ...

      • ... these methods might be useful in developing countries where the huge capital investment required to build complex smelting furnaces would not be available.

        • 'Advanced' technical notes on bioleaching

          • The microorganisms essentially catalyses processes that occurs naturally eg the copper sulphide minerals like chalcopyrite (CuFeS₂) are oxidised to a solution of copper(II) Cu²⁺, iron(II)/(III) Fe²⁺/Fe³⁺ and sulfate ions SO₄^2–.

          • The optimum conditions for these bacteria is pH 2–3 and 20^oC to 55^oC. These bacteria occur naturally so it is possible to spray dilute acid on low grade ores, OR, spray the dilute acid onto waste material from the mining process to try to get any remaining copper from copper bearing rocks.

          • The aerated acidified water slowly percolates through the pieces of broken rock and the colonies of the useful bacteria establish themselves quite naturally in this acidic environment.

          • The bacterial leachings are dilute and impure but, after filtration, the copper can be recovered, usually by displacement with cheap scrap iron.

• The supply of copper–rich ores is limited so it is important to recycle as much copper as possible especially as demand for copper is growing as the economies of African countries, India, China and Brazil etc. are rapidly developing and becoming increasingly industrialised with the ensuing consumer demands for all the eg electrical products that we in the West take for granted.

• The general social, economic and environmental impacts of exploiting metal ores are discussed on a separate page.