4a. The extraction of copper from copper ores

The rock-face telltale signs of green-turquoise-blue colour of	copper containing mineral ores in south-west Ireland.
The ruins of early 19th century engine houses of copper	mines in south-west Ireland, long since disused.

Bingham Canyon open-cast mine is one of the largest of its kind in the world.

It is also known as the Kennecott Copper Mine and is southwest of Salt Lake City in Utah, USA.

• 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 of its position in the reactivity series of metals, less reactive copper can be extracted using carbon in a smelting furnace. Copper is well 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)  (equation with state symbols)
  • 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)   (equation with state symbols)
  • The copper oxide is reduced to copper because of oxygen loss.
  • The carbon acts as the reducing agent – the 'oxygen remover', gains oxygen and gets oxidised.
  • 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 sulfide ores ...
  • These include chalcocite/chalcosine = copper(I) sulfide Cu₂S and covellite = copper(II) sulfide 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 sulfide 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) sulfide + oxygen ==> copper + sulfur dioxide
      • Cu₂S + O₂ ==> 2Cu + SO₂
        • Cu₂S(s) + O₂(g) ==> 2Cu(s) + SO₂(g)  (equation with state symbols)
      • 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) sulfide + oxygen ==> copper + sulphur dioxide
      • CuS + O₂ ==> Cu + SO₂
        • CuS(s) + O₂(g) ==> Cu(s) + SO₂(g)  (equation with state symbols)
• 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) sulfate solution.
    • Using displacement with scrap iron to displace–extract copper from a solution of a copper salt
    • iron + copper(II) sulfate ==> iron(II) sulphate + copper
    • Fe + CuSO₄ ==> FeSO₄ + Cu
      • iron + copper(II) ion ==> iron(II) ion + copper
      • The fully balanced symbol ionic equation is ...
      • Fe(s) + Cu²⁺(aq) ==> Fe²⁺(aq) + Cu(s)
      • Again, to fully understand what is happening, you can think of it as two 'half–reactions'
        • Fe ==> Fe²⁺ + 2e^–     (the oxidation half equation, electron loss, iron atom is oxidised)
        • Cu²⁺ + 2e^– ==> Cu       (the reduction half equation, electron gain, copper ion is reduced)
        • The electron loss and gain cancel out, so you don't see electrons in the full equation.
        • So, although it seems a very different chemical change from the copper ore reduction ore using
      • Iron (Fe) is the reducing agent (electron donor)  and the copper(II) ion (Cu²⁺) is the oxidising agent (electron remover or acceptor).
    • Oxidation-reduction theory for this displacement reaction:

• 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).

TOP OF PAGE and metal extraction index

4b. The future for copper mining and other sources of metals?

How is copper extracted by phytomining and bioleaching?

• Introduction to alternative biological methods of extracting metals:

  • Such methods are needed because the Earth’s resources of metal ores are limited.

  • Rich high grade ores will be more rapidly used up than low grade ores, limiting sustainability - unless we find ways of using low grade ores or waste from mining high grade ores.

  • Traditional large scale copper mining is damaging to the environment and produces huge amounts of waste, but these methods have less impact on the environment and can make good use of waste containing residual copper, but the processes of bioleaching and phytomining are unfortunately slow.

  • Finite reserves of copper ores are becoming scarce and new ways of extracting copper from low-grade ores include phytomining, and bioleaching.

    • Low grade ores only contain small concentrations of copper and would not normally be considered economic to mine and process, as would waste material from processing high grade copper ores.

  • These two methods avoid traditional mining methods of digging, moving and disposing of large amounts of rock - they make finite copper resources last longer and reduce the impact on the environment.

  • Phytomining uses plants to absorb metal compounds and the plants are then harvested and burned to produce ash that contains a more concentrated raw material of the metal compounds. From this ash the metal can then be extracted.

  • Bioleaching uses bacteria to produce leachate solutions that contain metal compounds. The metal compounds can be processed to obtain the metal e.g. copper can be obtained from solutions of copper compounds by displacement using scrap iron or by electrolysis (experiments, hopefully you will have done).

• 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) or waste material from the mining operations, are being researched to limit the environmental impact of traditional mining.

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

  • Phytomining Phytoextraction ('mining with plants',

    • Extracting copper in this way is a commercial example of phytoextraction.

    • Phytomining uses growing plants in soil to absorb metal compounds.

    • Such plants cannot always get rid of the copper ions and they build up in leaves.

    • These plants naturally absorb copper compounds through their roots as they feed on the nutrients around them and because they can't always get rid of the excess of certain metals like copper, this results in higher concentrations of these copper compounds in the plant tissues e.g. leaves.

    • The plants are then cropped (harvested), dried and burned in a furnace to produce an ash that contains the metal's soluble compounds which can be extracted.

    • The combustion process concentrates the metal in the ash.

    • The bigger the plant and the faster it grows, the greater the yield of metals like copper.

    • The ash is dissolved in hydrochloric acid or sulfuric acid and the copper can be extracted by electrolysis,

    • in which the copper ions are reduced and deposited on a negative copper cathode electrode.

    • Cu²⁺(aq)  +  2e^-  ===> Cu(s)

       or more cheaply by displacement of the copper with scrap iron. e.g.

    • iron + copper(II) sulfate ==> iron(II) sulfate + copper

    • Fe(s)  +  CuSO₄(aq) ===>  FeSO₄(aq) +  Cu(s)

    • In this extraction process, the copper metal is precipitated out of the copper sulfate solution, the iron dissolves in the reaction forming iron sulfate solution.

      • In terms of oxidation and reduction the ionic redox equation is:

        • Fe(s)  + Cu²⁺(aq)  ===>  Fe²⁺(aq)  +  Cu(s)

      • You should be able to work out the two half equation:

      • iron atoms are oxidised by losing 2 electrons: Fe  ==>  Fe²⁺  +  2e^-

      • copper ions are reduced by gaining 2 electrons: Cu²⁺  +  2e^-  ==> Cu

  • Bioleaching 'bacteria extraction' (bacterial extraction with 'rock eating bacteria'!)

    • Apparently 10% of copper in the US comes from bacteria which live off the surrounding rocks.

    • You can use low grade ores or waste material from the higher ore grade mining operations.

    • Bioleaching uses bacteria (bacterial microorganisms) with dilute sulfuric acid to produce leachate solutions that contain soluble copper compounds that can be processed to extract the copper.

    • Some bacteria naturally absorb copper compounds as they chemically interact with the surrounding mineral rocks to as an energy source and free the metal from the ore to form copper ions.

    • ie it is a copper leaching effect with respect to the surrounding rock material as they break down ores like chalcopyrite (CuFeS₂).

    • From the acidic bacterial discharges you can produce solutions (blue leachates of copper ions), which contain soluble copper compounds in commercially viable concentrations.

      • The result is an acidic solution of dilute sulfuric acid, copper(II) sulfate and iron(II) sulfate from which you extract the copper.

    • Again, the copper can be extracted by electrolysis or more cheaply by displacement of the copper with scrap iron - as described in phytomining above.

      • Or you can electrolysis, the copper ions are reduced and deposited on a negative copper cathode electrode.

      • Cu²⁺(aq)  +  2e^-  ===> Cu(s)

  • Advantages and disadvantages of phytomining and bioleaching to extract copper

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

    • In phytomining the plants are slow to grow and in bioleaching the biochemistry is relatively slow.

    • However, less energy is needed - smaller carbon footprint - recycling copper only uses ~15% of the energy that is required to extract and purify copper from its naturally occurring ore

    • There is less damage to the environment - lower impact as low grade ores/waste does not have to be mined in the same way - wildlife habitats are less affected.

    • A good economic environmentally way of treating waste from metal ore mining.

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

    • 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.

    • These rapidly developing and becoming increasingly industrialised countries will experience increased consumer demands for all the e.g. electrical products that we in the West take for granted.

    • social, economic and environmental impacts of exploiting metal ores and recycling

• 'Advanced' technical notes on bioleaching (NOT for GCSE students)

  • 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 result is an acidic solution of dilute sulfuric acid, copper(II) sulfate and iron(II) sulfate.

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

4C. Purification of Copper by Electrolysis (extraction from ore above)

• The impure copper from a smelter is cast into a block to form the positive anode. The cathode is made of previously purified copper. These are dipped into an electrolyte of copper(II) sulphate solution. 
• When the d.c electrical current is passed through the solution electrolysis takes place.  The copper anode dissolves forming blue copper(II) ions Cu²⁺.
• These positive ions are attracted to the negative cathode and become copper atoms. The mass of copper dissolving at the anode exactly equals the mass of copper deposited on the cathode. The concentration of the copper(II) sulphate remains constant.
• Any impurities present in the impure copper anode fall to the bottom of the electrolysis cell tank. This 'anode sludge' is not completely mineral waste, it can contain valuable metals such as silver. If these valuable metals can be recovered, their sale would help the economics of the process.
• See section above for extraction of impure copper from an ore.

Raw materials for the electrolysis process:

• Impure copper from a copper smelter.

• Electrolyte of aqueous copper(II) sulphate.

• A pure copper cathode.

Electrolysis is using d.c. electrical energy to bring about chemical changes at the electrolyte connections called the anode and cathode  electrodes.

An electrolyte is a conducting melt or solution of ions which carry the electric charge as part of the circuit.

Scrap copper can be recycled and purified this way too ,and is cheaper than starting from copper ore AND saves valuable mineral resources.

• Electrolysis reminders – the negative electrode (–) is called the cathode and attracts positive ions or cations e.g. Cu²⁺, and the positive electrode (+) is called the anode and attracts negative ions or anions. However in this case, the copper anode actually dissolves.
• Read the following in conjunction with the 'concept diagram' for the electrolysis of copper sulfate solution with copper electrodes, after the electrode equations.
• (i) At the positive (+ve) anode, the process is an oxidation, electron loss from copper atoms, as the copper atoms of the positive anode electrode dissolve to form blue copper(II) ions.

copper atoms ==> copper ions + electrons

oxidation half equation

Cu ==> Cu²⁺ + 2e^–   (an oxidation, electron loss)

Cu(s) ==> Cu²⁺(aq) + 2e^–

• (ii) The negative (–ve) cathode attracts the positive copper ions, this electrode process is a reduction, electron gain by the attracted copper(II) ions to form neutral copper atoms which become coated on the negative cathode electrode.

reduction half equation

copper ions + electrons ==> copper atoms

Cu²⁺ + 2e^– ==> Cu   (a reduction, electron gain)

Cu²⁺(aq) + 2e^– ==> Cu(s)

• See the concept diagram below
• Note: Reduction and Oxidation always go together, hence the use of the term redox change or reaction.
• For more on electroplating and electrolysis see ...
• Industrial Chemistry - making metals more useful
• Electrolysis of chemistry of copper solutions

• ELECTROPLATING and its applications

Concept diagram for the purification of copper by electrolysis of copper salts solution with copper electrodes

(iii) Anode sludge

You will notice a pile of dark material that gathers below the impure copper anode.

This is the residue left after the copper is oxidised, dissolves and transferred to the cathode.

In the electrolytic refining process, after the pure copper is deposited on the cathode plates insoluble impurities fall to the bottom of the cell as anode mud or sludge.

Anode sludge contains gold (Au) and other valuable metals like silver (Ag), platinum (Pt), and palladium (Pd).

These can be extracted from the anode sludge created by the electro-refining process.

In the formation of copper ore veins, copper concentrates often these precious metals and reclamation of these metals from anode slime is economically attractive and also it is environmentally friendly.

ELECTROCHEMISTRY INDEX:  1. INTRODUCTION to electrolysis – electrolytes, non–electrolytes, electrode equations   2. Electrolysis of acidified water (dilute sulfuric acid)   3. Electrolysis of sodium chloride solution (brine)   4. Electrolysis of copper(II) sulfate solution and electroplating   5. Electrolysis of molten lead(II) bromide (and other molten compounds)   6. Electrolysis of copper(II) chloride solution   7. Electrolysis of hydrochloric acid   8. Summary of electrode equations and products   9. Summary of electrolysis products from various electrolytes   10. Simple cells (batteries)   11. Fuel Cells   12. The extraction of aluminium from purified molten bauxite ore   13. Anodising aluminium to thicken and strengthen the protective oxide layer   14. The extraction of sodium from molten sodium chloride using the 'Down's Cell'   15. The purification of copper by electrolysis   16. The purification of zinc by electrolysis   17. Electroplating coating conducting surfaces with a metal layer   18. Electrolysis of brine (NaCl) for the production of chlorine, hydrogen & sodium hydroxide 19. Electrolysis calculations

4d. USES OF COPPER

• COPPER, Cu
• Copper has properties that make it useful for electrical wiring and plumbing.
  • Copper is a good conductor of electricity and heat, can be bent but is hard enough to be used to make pipes or tanks and does not react with water.
• The alloy BRASS is a mixture copper and zinc. It is a much more hard wearing metal than copper (too soft) and zinc (too brittle) but is more malleable than bronze for 'stamping' or 'cutting' it into shape.
• Copper is used in electrical wiring because it is a good conductor of electricity but for safety it is insulated by using poorly electrical conductors like PVC plastic.
• Copper is used in domestic hot water pipes because it is relatively unreactive to water and therefore doesn't corrode easily.
• Copper is used for cooking pans because it is relatively unreactive to water and therefore doesn't corrode easily, readily beaten or pressed into shape but strong enough, it is high melting and a good conductor of heat.
• Copper is also used as a roof covering and weathers to a green colour as a surface coating of a basic carbonate is formed on corrosion.
• The alloy BRONZE is a mixture of copper and tin (Sn) and is stronger than copper and just as corrosion resistant, e.g. used for sculptures.
• Iron and steel are used for boilers because of their good heat conduction properties and high melting point.
• Copper compounds are used in fungicides and pesticides e.g. a traditional recipe is copper sulphate solution plus lime is used to kill greenfly.
• Copper is alloyed with nickel to give 'cupro–nickel', an attractive hard wearing 'silvery' metal for coins.
• Many copper objects/material can be recovered from their previous use and recycled via scrap metal merchants.

See also

GCSE/IGCSE notes on TRANSITION METALS and

Advanced A Level notes on the 3d BLOCK ELEMENTS (including the 1st transition metal series)

 Doc Brown's Chemistry 

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