gcse 1. Introduction to the Extraction of Metals method related to the activity of reactivity series of metals igcse KS4 science chemistry revision notes revising

1. Introduction to the extraction of metals

What methods can use in extracting metals from mineral ores?

The Earth's crust contains many different rocks. Rocks are a mixture of minerals and from some we can make useful substances.
A mineral can be a solid metallic or non-metallic element or a compound found naturally in the Earth's crust.
The simplest definition of an ore is a mixture of a metal containing mineral and other materials ('minerals') from the surrounding rocks, which can be described as impurities with respect to what you want from the ore.
The metal ore, a mineral or mixture of minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its compounds, in it to be worth digging out! Ores are often oxides, carbonates or sulphides. They are all finite resources so we should use them wisely!
- Any ore must contain enough of the metal to make it worthwhile to mine and then extract the metal.
- High grade ores will therefore be the most economical to exploit BUT over time the better quality-richer sources will decrease, especially with the power of the global economy and powerful and growing economies of Brazil, China, India and other Asian economies.
- Since the majority of metals are found combined with non-metals like oxygen (oxide ion) or sulfur (sulfide ion) or the carbonate ion, chemical reactions are needed to free the metal from its mineral source.
In order to extract a metal, the ore or compound of the metal must undergo a process called reduction to free the metal i.e. the positive metal ion gains negative electrons to form the neutral metal atom, or the oxide loses oxygen, to form the free metallic atoms.
- The chemical that removes the oxygen from an oxide is called the reducing agent i.e. carbon, carbon monoxide or sometimes hydrogen.
- Detailed REDOX notes on the metal reactivity page, and on this page where appropriate.
Generally speaking, the method of extraction depends on the metals position in the reactivity series
The reactivity series of metals can be presented to include two non-metals, carbon and hydrogen, to help predict which method could be used to extract the metal.
- lower Pt Au Ag Cu (H) Pb Sn Fe Zn (C) Al Mg Ca Na K higher in series
- RULE: Any element higher in the series can displace any other lower element
Metals above carbon in the reactivity series cannot usually be extracted with carbon or carbon monoxide.
So, metals more reactive than carbon are usually extracted by electrolysis of the purified molten ore or other suitable compound.
- e.g. aluminium from molten aluminium oxide or sodium from molten sodium chloride.
- The ore or compound must be molten or dissolved in a solution in an electrolysis cell to allow free movement of ions (electrical current). Theory given in the appropriate sections.
- Because these reactive metals cannot be obtained by relatively cheap carbon reduction methods, their extraction tends to be more costly due to more specialised stages in the extraction process, more energy is needed (maybe costly electricity) and more costly specialist chemicals like a more reactive metal or chlorine (remember carbon-coke is relatively cheap e.g. as used in the blast furnace extraction of iron).
Metals below carbon can be extracted by heating the oxide with carbon or carbon monoxide. The non-metallic elements carbon will displace the metals less reactive than carbon in a smelter or blast furnace e.g. iron or zinc and metals lower in the series.
- Therefore metals like iron, copper, tin, lead, zinc can readily be extracted by reaction-reduction of their e.g. oxides using cheap carbon (i.e. coke made from coal).
- Metals below hydrogen will not displace hydrogen from acids. Their oxides are easily reduced to the metal by heating in a stream of hydrogen, though this is an extraction method rarely used in industry. In fact most metal oxides below carbon can be reduced when heated in hydrogen, even if the metal reacts with acid.
Some metals are so unreactive that they do not readily combine with oxygen in the air or any other element present in the Earth's crust, and so can be found as the metal itself. For example, a metal, most frequently found as the metal is gold (and sometimes copper and silver) and no chemical separation or extraction is needed. In fact all the metals below hydrogen can be found as the 'free' or 'native' element, though they occur mainly as compounds combined with non-metals like oxygen (oxide ion) or sulfur (sulfide ion) or the carbonate ion.
Other methods are used in special cases using the displacement rule. A more reactive metal can be used to displace and extract a less reactive metal but these are costly processes since the more reactive metal also has to be produced in the first place! See Titanium or see at the end of the section on copper extraction.
Sometimes electrolysis is used to purify less reactive metals which have previously been extracted using carbon or hydrogen (e.g. copper and zinc). Electrolysis is also used to plate one metal with another.
The demand for raw materials does have social, economic and environmental implications e.g. conservation of mineral resources by recycling metals, minimising pollution etc.
Historically as technology and science have developed the methods of extraction have improved to the point were all metals can be produced. The reactivity is a measure of the ease of compound formation and stability (i.e. more reactive, more readily formed stable compound, more difficult to reduce to the metal).
- The least reactive metals such as gold, silver and copper have been used for the past 10000 years because the pure metal was found naturally.
- Moderately reactive metals like copper, iron, lead, tin have been extracted using carbon based smelting for the past 2000-3000 years.
- BUT it is only in the last 200 years that very reactive metals like sodium or aluminium have been extracted by electrolysis.
- In other words, our exploitation of metal mineral resources as developed and expanded as the technology of metal extraction has also developed. This technology is increasingly looking at ways of extracting metals from low grade ores which were previously uneconomic to use.
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WHERE NEXT? Other associated KS4 Science GCSE/IGCSE chemistry web pages on this site

Metallic Bonding - structure and properties of metals

Extra Electrochemistry-electrolysis theory, experiment designs and links to all electrolysis sections

Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel 360 Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10) also useful revising and introduction to metal extraction for A level AS/A2/IB chemistry students

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