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4. The extraction of copper from copper ores & purification by electrolysis

Doc Brown's GCSE/IGCSE O Level KS4 science–CHEMISTRY Revision Notes

 Mining of Minerals and Methods of Extracting of Metals 

How do we extract copper from its copper carbonate or copper sulphide ores? What are the raw materials for extracting copper? How do we purify impure copper obtained from smelting copper mineral ores? The chemistry of carbon reduction of copper oxides, sulfides or carbonates are described. How can we purify copper by electrolysis? The electrolytic purification of the impure copper is described with the electrode equations. How copper can be extracted from ores by phytomining (phytoextraction) and bioleaching using 'rock eating' bacteria! Scroll down for revision notes on extraction procedures and theory which should prove useful for school/college assignments/projects on ways of extracting metals from their ores.

Equation note: The equations are often written three times: (i) word equation, (ii) balanced symbol equation without state symbols, and, (iii) with the state symbols (g), (l), (s) or (aq) to give the complete balanced symbol equation.

Metal extraction index

extract1extract2

1. Introduction to Metal Extraction

2. Extraction of Iron and Steel Making

3. Extraction of Aluminium and Sodium

4. Extraction of copper & purification by electrolysis (this page)

5. Extraction of Lead, Zinc, Titanium and Chromium

6. Economic & environmental Issues and recycling

 

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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 SO42–.

            • The optimum conditions for these bacteria is pH 2–3 and 20oC to 55oC. 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.

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

(c) doc b

  • 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.

The redox details of the electrode processes:
  • 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.
  • At the positive (+) anode, the process is an oxidation, electron loss, as the copper atoms of the positive anode electrode dissolve to form blue copper(II) ions.

copper atoms ==> copper ions + electrons

Cu ==> Cu2+ + 2e

Cu(s) ==> Cu2+(aq) + 2e

  • The negative () 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.

copper ions + electrons ==> copper atoms

Cu2+ + 2e ==> Cu

Cu2+(aq) + 2e ==> Cu(s)

The NEW 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

  • USES OF COPPER

  • (c) doc bCOPPER, 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.

 

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WHERE NEXT? Other associated KS4 Science GCSE/IGCSE chemistry web pages on this site


Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10), useful revising and introduction to metal extraction for A level AS/A2/IB chemistry students


keywords formula equations copper extraction purification: CuCO3 ==> CuO + CO2 2CuO + C ==> 2Cu + CO2 CuFeS2 2CuFeS2 + 4O2 ==> Cu2S + 3SO2 + 2FeO Cu2S + O2 ==> 2Cu + SO2 CuS + O2 ==> Cu + SO2 Fe + CuSO4 ==> FeSO4 + Cu Cu ==> Cu2+ + 2e– Cu2+ + 2e– ==> Cu


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Extracting and smelting copper ores and purifying copper by electrolysis

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