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Occurrence and Extraction of Metals: Extraction of Metals and Extraction of Aluminum

Extraction of Metals

Reduction of Concentrated Ores by Other Methods

Reduction by Precipitation

  • Noble metals like silver and gold are extracted from their concentrated ores by dissolving metal ions in the form of their soluble complexes.
  • The metal are regenerated by adding a suitable reagent.

For example, Concentrated argentite ore is treated with a dilute solution of sodium cyanide to form a soluble complex

This solution is decanted off and treated with zinc to precipitate silver,

Electrolytic Reduction

  • Active metals like sodium, potassium and aluminum etc. are extracted by the electrolysis of their fused (molten) salts.
  • For example, sodium is obtained by the electrolysis of fused sodium chloride (Down՚s process) .

Refining of Metals

  • Metals produced are generally impure except by electrolytic reduction method.
  • The impurities may be in the form of other metals, unreduced oxide of the metal, non-metals like carbon, silicon, phosphorus, Sulphur etc. and flux or slag.

These Metals may be refined by following methods:

Liquation

  • Easily fusible metals like tin, lead etc. are refined by this process.
  • The impure metal is poured on the sloping hearth of a reverberatory furnace (Fig) and heated slowly to a temperature little above the melting point of the metal.
Illustration: Liquation

The pure metal drains out leaving behind infusible impurities.

Poling

  • Poling involves stirring the impure molten metal with green logs or bamboo.
  • The hydrocarbons contained in the pole reduce any metal oxide present as impurity.
  • Copper and tin are refined by this method (Fig) .
Illustration: Poling

Distillation

  • Volatile metals like zinc and mercury are purified by distillation.
  • The pure metal distils over, leaving behind non-volatile impurities.

Electrolytic Refining

  • A large number of metals like copper, silver, zinc, tin etc. are refined by electrolysis.
  • A block of impure metal is made the anode and a thin sheet of pure metal forms the cathode of the electrolytic cell containing suitable metal salt solution which acts as an electrolyte (fig) .
Illustration: Electrolytic Refining
  • On passing the current pure metal deposits at the cathode sheet while more electropositive impurities are left in solution.
  • Less electropositive metals do not dissolve and fall away from the anode to settle below it as anode mud.
  • For example, In the electrolytic refining of crude copper (blister copper) , a large piece of impure copper is made anode and a thin piece of pure copper is made the cathode.
  • An acidified solution of copper sulphate is used as an electrolyte.
  • On passing an electric current of low voltage through the solution copper (II) ions obtained from copper sulphate solution go to the cathode where they are reduced to the free copper metal and get deposited.

(at Cathode)

An equivalent amount of the metal from the anode dissolves into the electrolyte as ions.

(at anode) Extraction of .

Extraction of Aluminum

  • The ore of aluminum from which it is profitable to extract the metal is bauxite.
  • The bauxite is first freed from silica and iron (III) oxide impurities by dissolving it in sodium hydroxide (iron (III) oxide remains undissolved) and then precipitating aluminum hydroxide by seeding the solution with a little aluminum hydroxide.

Aluminum oxide is now obtained by heating the hydroxide

  • It is dissolved in molten cryolite, , and electrolyzed at about , using a number of graphite blocks as anodes and a graphite lined bath as the cathode.
  • Aluminum is discharged at the cathode and collects at the bottom of the molten electrolyte as a liquid.
  • It can be tapped off from there and allowed to solidify.