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14.4 Extracting Metals from Ores

GCSE Chemistry · AQA · Chapter 14: The Earth's Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how metals are extracted using carbon or electrolysis, and newer biological methods for copper (Higher tier). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain what an ore is and use the reactivity series to choose an extraction method
  • Write equations for reducing metal oxides with carbon, including iron and copper
  • Describe phytomining and bioleaching (Higher tier)
  • Describe how copper is obtained from solutions by displacement with scrap iron or by electrolysis, and how impure copper is purified by electrolysis
  • Evaluate methods of extraction and carry out calculations on ores and reacting masses

Key words

  • Ore: A rock that contains enough of a metal or metal compound to make extracting the metal worthwhile (economic).
  • Reduction: Loss of oxygen from a compound (or, Higher tier, gain of electrons).
  • Low-grade ore: An ore that contains only a small percentage of the metal compound.
  • Phytomining: (Higher tier) Using plants to absorb metal compounds from the ground; the plants are burned to give metal-rich ash.
  • Bioleaching: (Higher tier) Using bacteria to produce leachate solutions that contain metal compounds.
  • Displacement: A more reactive metal takes the place of a less reactive metal in its compound.

Ores and the reactivity series

Most metals are found as compounds in ores. Whether a rock counts as an ore depends on how much metal it contains and on the price of the metal. The method of extraction depends on the metal’s position in the reactivity series compared with carbon.

Reduction with carbon

Carbon is more reactive than iron, zinc and copper, so it removes oxygen from their oxides. The metal oxide is reduced and the carbon is oxidised.

2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g) (iron in a blast furnace)

2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

Worked example: mass of iron from an ore

How much iron can be made from 800 tonnes of iron(III) oxide, Fe₂O₃?
Mr Fe₂O₃ = 2 × 56 + 3 × 16 = 160. Mass of iron in 160 t = 112 t.
mass of iron = 800 × 112 ÷ 160 = 560 tonnes

Answer: 560 tonnes of iron

Copper from high-grade ores

Most copper still comes from high-grade ores, which are a finite resource. There are two main routes:

  • Smelting (roasting): copper sulfide ores are heated strongly in a furnace with air to give impure copper. This uses huge amounts of energy, and sulfur dioxide must be stopped from escaping because it causes acid rain.
  • Using sulfuric acid: the ore is turned into copper sulfate solution, then the copper is obtained by displacement or electrolysis (see “Getting copper from solution” below).

For an ore that contains copper carbonate, the lab version of the acid route has three steps:

1 Thermal decomposition: CuCO₃(s) → CuO(s) + CO₂(g)

2 React with dilute sulfuric acid: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

3 Filter off any unreacted oxide, then add scrap iron (or electrolyse) to obtain copper.

Purifying copper by electrolysis. Copper for electrical wiring must be very pure. Impure copper is made the positive electrode (anode) and a thin sheet of pure copper is the negative electrode (cathode), in copper(II) sulfate solution. Copper atoms leave the anode as ions; copper ions are deposited on the cathode. Impurities fall to the bottom as a sludge, which can contain valuable metals such as silver and gold.

  • anode (+): impure copper; Half equation (Higher tier) Cu(s) → Cu²⁺(aq) + 2e−; Type oxidation (loss of electrons)
  • cathode (−): pure copper; Half equation (Higher tier) Cu²⁺(aq) + 2e− → Cu(s); Type reduction (gain of electrons)

Exam tip: Copper ions are positive, so they are attracted to the negative electrode. That is where pure copper always collects.

Biological methods for low-grade copper ores (Higher tier)

The Earth’s supply of high-grade copper ores is running out. New ways of extracting copper from low-grade ores and mine waste avoid the digging, moving and disposal of large amounts of rock.

In phytomining the copper compounds are “leached” out of the ash with sulfuric acid, giving copper sulfate solution. Bioleaching is slow – it can take years to recover around half of the copper in a heap – so scientists are looking for ways to speed it up. Its share of world copper production is expected to grow as high-grade ores run out.

  • Phytomining; Advantages uses low-grade ore and mine waste; less digging and waste rock; plants can clean up polluted land; burning plants releases energy; Disadvantages very slow (plants need time to grow); needs a lot of land
  • Bioleaching; Advantages uses low-grade ore; no high temperatures, so less energy; no sulfur dioxide from smelting; Disadvantages slow; leachate is toxic and acidic and can pollute ground water

Getting copper from solution

Both methods produce a solution of a copper compound, such as copper sulfate. The copper is obtained by:

  • Displacement with scrap iron (cheap): Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Ionic equation: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
  • Electrolysis: copper ions gain electrons at the negative electrode: Cu²⁺ + 2e− → Cu

Worked example: displacement

A leachate contains 63.8 g of copper sulfate, CuSO₄. (Ar: Cu = 63.5, S = 32, O = 16, Fe = 56)
Mr CuSO₄ = 63.5 + 32 + 4 × 16 = 159.5. Moles = 63.8 ÷ 159.5 = 0.400 mol
1 : 1 ratio, so 0.400 mol Cu = 0.400 × 63.5 = 25.4 g. Iron needed = 0.400 × 56 = 22.4 g

Answer: 25.4 g of copper, using 22.4 g of iron

Higher tier: In the displacement reaction iron atoms lose electrons (Fe → Fe²⁺ + 2e−), so iron is oxidised. Copper ions gain electrons, so they are reduced. Remember OIL RIG.

Common mistakes

  • Saying aluminium is extracted with carbon. It is more reactive than carbon, so it needs electrolysis.
  • Saying phytomining plants “produce” copper. They absorb copper compounds from the soil; copper is obtained from the ash.
  • Saying bioleaching uses enzymes or plants. It uses bacteria, which produce a leachate.
  • Forgetting the final step: the copper compound must still be turned into copper by displacement or electrolysis.
  • Saying pure copper forms at the positive electrode. In purification the impure copper is the anode (+); pure copper collects on the cathode (−).
  • Calling the metal oxide “oxidised” in carbon reduction. The oxide loses oxygen, so it is reduced.

Revision questions

Try these, then check your answers below each one.

Question 1. Which metal is extracted by electrolysis of its molten compound rather than by heating with carbon? (Choose from: Aluminium, Iron, Zinc, Copper.)

Answer: Aluminium

Working: Aluminium is more reactive than carbon, so carbon cannot reduce its oxide. It is extracted by electrolysis.

Question 2. (Higher tier) In bioleaching, which organisms produce the leachate solution containing metal compounds?

Answer: bacteria

Working: Bacteria produce leachate solutions that contain metal compounds.

Question 3. Calculate the percentage by mass of copper in CuFeS₂, to 3 significant figures. (Ar: Cu = 63.5, Fe = 56, S = 32)

Answer: 34.6 %

Working: Mr = 183.5. 63.5 ÷ 183.5 × 100 = 34.6%.

Related lessons

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This lesson is section 14.4 (AGCCH1404) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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