Carbon Reduction Method for Extracting Metals from their Oxides Chemistry Tutorial

Key Concepts

• An activity series for metals places the metals in order of their activity (or reactivity) from most active (or reactive) to least active (or reactive).
• The more active (or reactive) a metal is, the more stable the compound it forms and the greater the energy required to produce the metal from its ore.

  Very active metals (Group 1, Group 2 and aluminium) require electrolysis to extract the metal while very inactive metals like gold and silver can occur in nature as the element so require no energy to extract the metal.

• Transition metals can often be extracted from their ores by heating the ore with a suitable reducing agent in a process known as thermal reduction.

  most reactive elements (Group 1, Group 2, aluminium)

  → → → → → → → → → → → → → → (transition metals)

  least reactive elements (Noble Metals)

  potassium

  >

  calcium

  >

  sodium

  >

  magnesium

  >

  aluminium

  >

  zinc

  >

  iron

  >

  nickel

  >

  tin

  >

  lead

  >

  copper

  >

  silver

  >

  gold

  electrolysis

  thermal reduction of oxide

  native elements

• Carbon can be used as reducing agent for transition metal oxides because carbon is more active than the transition metals in the ores:

  most reactive elements → → → → → → → → → →

  → → → → → → → → → → → →

  least reactive elements

  potassium

  >

  calcium

  >

  sodium

  >

  magnesium

  >

  aluminium

  carbon

  >

  zinc

  >

  iron

  >

  nickel

  >

  tin

  >

  lead

  >

  copper

  >

  silver

  >

  gold

• The general equation for the reactions in which metal oxides are reduced by heating with carbon:

  metal oxide + carbon → metal + carbon dioxide^*
  or
  metal oxide + carbon → metal + carbon monoxide^**

• Metal is reduced from a positive oxidation state in the metal oxide to the metallic element.

  Carbon is oxidized from elemental carbon to an oxide of carbon in which carbon is in a positive oxidation state (has a positive oxidation number).

Example: Extracting Lead from Cerussite

1. Mining : digging or blasting to obtain the lead ore from the surrounding rock
2. Crushing : mined lead ore is crushed into very small pieces
3. Grinding : crushed lead ore is ground into a powder
4. Concentrating by flotation (also known as froth flotation or as ore flotation).
   Concentrating the ore by froth flotation increases the percentage of lead in the ore by removing many of the minerals that do not contain lead.

   • A collector oil, such as an organic xanthate or thiophosphate, is added to the powdered ore and adheres strongly to the cerussite particles making them water repellant.
     This collector oil does not adsorb so strongly on other minerals such as silicate minerals so these particles will not repel water.
   • This mixture is fed into a water bath containing a foaming agent such as crude cresol or pine oil (soap is not a suitable foaming agent in this case).
   • Jets of air are forced through the water bath.
   • Water repellant cerussite particles stick to the foam bubbles and float to the surface making a froth.

     Gangue (waste) falls to the bottom and is removed.

   • The froth which contains the lead is skimmed off the surface and the now enriched or concentrated ore is taken away for roasting.

     The water bath mixture is recycled.

5. Thermal Decompostion of the Cerussite to Produce Lead(II) Oxide:

   The lead(II) carbonate that makes up the mineral cerussite is not very stable.

   On heating, lead(II) carbonate decomposes into lead(II) oxide and carbon dioxide:

   PbCO_3(s) → PbO_(s) + CO_2(g)

   The lead(II) oxide produced in this way can then be reduced to metallic lead using heat and carbon because lead is less active than carbon.

6. Reduction of Lead Using Carbon and Heat:

   On heating with carbon, the carbon is oxidized to carbon dioxide (or carbon monoxide) while lead is reduced from a positive "ion" in lead(II) oxide to the neutral metallic element:

   Chemical Reaction	2PbO	+	C	→	2Pb	+	CO2
   Pb : Oxidation State	+2				0			Oxidation state has decreased. Lead has been reduced.
   C : Oxidation State			0				+4	Oxidation state has increased. Carbon has been oxidized.