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Extraction Of Metals

Definition of metals

Metals are defined chemically as elements which have the following general qualities [1]:

  1. They conduct electricity
  2. They have a metallic lustre
  3. They are malleable and ductile.
  4. They form cations.
  5. They have basic oxides.

Occurrence In universe

In seawater and in earth’s crust, metals are usually present in the form of ores or minerals. The occurrence of metals in nature is closely associated with their position in the reactivity series. Most metals are in oxide form due to the presence of oxygen in abundance in nature. Considering this fact, the following main states of metals are present in nature [2].

  1. Least reactive metals usually occur in form of free metal. Examples are Gold, Silver and copper.
  2. Metals from middle order of reactivity series are found in form of oxides, Sulphides and carbonates. Examples are Zinc, Iron and Lead.
  3. Highly reactive metals never occur in Free State. They always occur in form of salts. Examples are Potassium, Sodium, Calcium, Magnesium and Aluminium.

What is metallurgy:

Metallurgy is branch of science dealing with Extraction of metals from natural sources and then modifying them for using in various industries [3]. Principles involving alloy synthesis also belong to this class of science. Majorly metallurgy involves commercial synthesis of some metals instead of synthesis in laboratories. This branch of science is being used from many thousand years ago. In initial years, iron metal was optimized to be extracted by different methods. The branch of metallurgy dealing with extraction of metals from salts is called “extractive metallurgy”. This is done by processing minerals containing metals.

Factors for deciding method of extraction:

  • Presence of metal to be extracted in reactivity series.
  • Nature of salt from which metal is to be extracted.
  • Mining cost
  • Nature of metal deposit
  • Location of metal deposit

What is metal ores?

Ores are collection of minerals containing one or more metals [4] [5].  These metals are economically and conveniently extracted from these ores by following certain extraction techniques. The reason behind presence of metals in form of ores and minerals is that, most of metals being highly reactive, cannot exist in free form [6]. Ores have usually high traces of metals to be extracted and that’s why these ores are of major importance in metal industry. Table below shows common ores of some metals and their extraction procedures. Bauxite, for example is an ore of aluminium containing around 60 %- 70 % of aluminium and it is usually extracted from this ore [7].

Extraction Of Metals 1

Classification of metals on basis of extraction methods:

With respect to techniques by which these metals are being extracted, metals are divided into following main classes:

  • Class 1:

Class 1 includes metals with highly electropositive nature. Mainly these metals are alkali metals. They are extracted by electrolysis. Source of extraction is molten or fused form [8].

  • Class 2:

This class includes electropositive metals which are alkaline earth metals. Electrolysis is method used with these cases of metals. Metal in form of molten form is used.

  • Class 3:

Electrolysis and reduction in used for reactive metals with high valency.

  • Class 4:

Roasting and reduction methods are used for extracting heavy metals. Examples are iron, copper, nickel and cobalt.

  • Class 5:

Amalgamation and cyanide process of extraction are used for less reactive metals. Common example is the silver.

Major Steps in extraction:

Extraction of metals from their natural sources is basicallya broad term which includes several steps in it. These steps start from exploraton of metal reservoirs to final refining of metals. Major factors involved in deciding all these processes are low cost and phhysical properties of metal to be extracted.These Steps are explained one by one in coming paragraphs:

Extraction Of Metals 2
  • Exploration of ores:

Major techniques involved in Exploration of high quality ore deposits to get desired metals are as follows [9]:

  • Geological methods:

These are basically collection and presentation of geological data to explre and find some ore. In some cases these methods are combined with modern technologies to get desired level of quality ore. These include following [10]:

  1. Mapping
  2. Stripping
  3. Trenching
  • Geophysical methods:

These are types of analytical techniques which involve physical nature of ores. These methods are basically used for detecting ores which are difficult to be explored by chemical techniques.Main types of geophysical  methods as follows:

  1. Magnetic surveys
  2. Electromagnetic radiation
  3. Induced Polarization
  4. Radioactivity
  5. Elactrical Conductivity
  6. Density
  • Geochemical methods:

These types of explorations are done by chemically analyzing rocks, sediments in lakes and soil of area to be explored. In this type of exploartion methods, the sample is further analyzed and refined in small scale at laboratory to get exat percentage of metal to be extracted and further detected. This exloration is of following main types:

  1. Rock sampling
  2. Others
  3. Seismic explorations

Seismic methods involve estimation of the shapes and physical properties of Earth’s subsurface layers. This method is done by detecting the  return of  sound waves that are propagated through the Earth [11].

  • Hyperspectral technologies:

Unmanned ariel system to detect presence of ores is widely used method nowadys for ore detection. These methods area lso known as remote sensing and they in combination with geophyical methods, have gained much importance in this industry [12].

  • Drilling and blasting:

Drilling and blasting is of formost importance for further mining of ore. Methods to get ores out from reservoirs from drilling are as follows:

  1. Down hole hammers
  2. Turbodrills
  3. In hole Drilling motors
  4. Jet Drilling systems
  • Mining:

Mining is basically an excavation produced in earth to extract minerals and ores. This industry has its history from very ancient times and together with engineering technologies, it has widely been used nowadays [13].

  • Surface mining:

In Surface mining, rocks and soil covering the ores and mineral deposits,a re removed. It accounts to two third of world’s solid minerals. Surface mining is of following main types [14].

  1. Open Pit mining
  2. Strip mining
  3. Contour mining
  4. Dredging mining
  5. Hydraulic mining
  • Underground mining:

It is used to extract ore from if it below the surface of the earth, economically and with as little waste as possible. Underground mining is  of following two maintypes.

  1. Room-and-Pillar mining
  2. Long wall mining
  3. In situ Mining or leaching
    • Extraction of water soluble salts
    • Brine Extraction
    • Sulphur Extraction
    • Bore hole mining
  • Crushing and milling

Metal , after being taken out of reservoirs, are then subjcted to further crushing and milling procedures.This is done in order to get  easier to process ore.

  • Separation from ore:

After being mining out, the ore is further processed to get separated from attacched rocks, soil and some salts. This is performed by using various techniques depending on the type of compound to be extracted and the extent to which it is bounded with the metal to be extracted.

  • Gravity Separation:  This method involves specific gravity of ore to be refined [15].
  • Magnetic separation: This method involves use of magnetic field for purifying ore. Ore which are electromagnetic in  nature involve this procedure for its extraction.
  • Froth Flotation:  This method involves physical properties of metal in presence of oil and water.Leaching:In this procedure solid ore is dissolved in a regent which makes impuriteis to settle down after reaction.
  • Liquation: This is used for ores having low melting point then impurities.
  • Extraction of crude metal from ore

Calcination

This process involves heating of substance in limited supply of oxygen. During this process,  thermal energy is used to change the chemical nature of the substance [16]. Carbonates are usually converted into oxides by this method. The metal to  be extracted is first converted into oxides to get its pure form by further procedure.  

  • Roasting

In this method, elevated temperature is used to react a gas with metal on that temperature. Heat is constantly being given throughout the reaction. However the higher temperature limit should not exceed boiling point of metal to be extracted.

  • Refining and purification of metal
  • Oxidative refining
  • Distillation
  • Liquidation
  • Electrolytic refining
    • Distribution

Types of metallurgy

  • Hydro-metallurgy:

This type of metal extraction involves use of aqueous solution. Major steps in this procedure are as follows.

  1. Leaching: Firstly metals are dissoluted in aqueous solutions or in a solvent.
  2. Concentration: After leaching, the liquor needs to be further concentrated. In this method, other impurities are also removed from solution. This is mainly done by precipitation and cementation.
  3. Purification: After concentration, the metal is further purified.
  4. Metal recovery: Metal is further recovered in final step by procedures such as electrolysis, gaseous reduction and precipitation.
  • Pyrometallurgy
    1. Calcination
    2. Roasting
    3. Refining
  • Electrometallurgy
    1. Electrolysis: Electrolysis procedures are sometimes used both for recovering and refining metals. These steps are Electrowining and Electrorefining.
Extraction Of Metals 3

Conclusion:

As the figure indicates, the method of extracting some metal depends on physical and chemical nature of that metal. Metals which are present on highest position in the reactive series are difficult to extract and thus they are extracted by electrolysis. While others which are lower in the series are easier to extract. Thus, for these metals, reduction by carbon is done as initial step of final extraction.

REFERENCES

  1. Duffus, J.H. (2002) Heavy Metals—A Meaningless Term? IUPAC Technical Report, 74, 793-807.
  2. https://www.excellup.com/classten/scienceten/metalsten5.aspx
  3. https://www.britannica.com/science/metallurgy/Brass
  4. https://www.usgs.gov/faqs/how-do-we-extract-minerals?qt-news_science_products=0#qt-news_science_products
  5. https://ec.europa.eu/eurostat/web/environmental-data-centre-on-natural-resources-old/natural-resources/raw-materials/metal-ores
  6. https://www.jagranjosh.com/general-knowledge/minerals-and-ores-natural-materials-1456201467-1
  7. https://www.chemguide.co.uk/inorganic/extraction/introduction.html
  8. https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/5412
  9. https://www.nap.edu/read/10318/chapter/5#22
  10. https://newpacificmetals.com/mining-101/types-of-mineral-exploration
  11. Mondol, Nazmul & Bjørlykke, Knut. (2010). Seismic Exploration. 10.1007/978-3-642-02332-3_17.
  12. Remote Sens. 2019, 11(18), 2084; https://doi.org/10.3390/rs11182084
  13. http://www.cienciaviva.pt/img/upload/Introduction%20to%20mining.pdf
  14. https://www.convergencetraining.com/blog/what-is-surface-mining
  15. https://www.kullabs.com/classes/subjects/units/lessons/notes/note-detail/5412
  16. https://www.toppr.com/guides/chemistry/general-principles-and-processes-of-isolation-of-elements/extraction-of-crude-metal-from-concentrated-ore/
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