Mercuric sulphides are converted to mercuric oxides by roasting which when subjected thermolysis at tube.
Reduction process is used for converting metal – oxides into metal. Metal – chloride can also be reduced directly. The reduction reaction chosen depends on the chemical reaction or reactivity of metals. Generally reduction by heat, chemical reduction or electrolytic reduction processes are utilized.
Reduction by heat:
Metals that are unreactive, like Hg, can be reduced from their ores by heating them. Mercury ore cinnabar is actually mercury
sulphide. This can be heated at 300°C so that S is removed as SO2 and HgO is obtained. Hg is a very unreactive
metal. HgO dissociates into Hg and oxygen soon.
The reaction is shown below.
Steel (iron) can be produced by reduction of Iron oxide (Fe2O3) by coal (carbon) in
blast furnace.
Chemical reduction:
Various reducing agents are used for different metal – oxides to obtain free metals. Carbon, Al, Na, Ca are some reducing
agents that are put in use.
Reduction by carbon:
Oxides of Zn, Fe, Ni, Sn, Pb are reduced by heating them with carbon. Metal – oxide is mixed with coke, a source of carbon,
and heated in a furnace. Carbon reacts with oxygen and free metal is obtained.
Reduction of ZnO with carbon on heating gives pure metal and CO gas.
Example below shows how Zn is obtained from ZnO on reduction with coke.
Reduction by Al
This process is called the Thermite process. Al is more reactive than carbon. Some metal – oxides that cannot be reduced by
coke are reduced by Al. Al itself attracts oxygen from the metal – oxide and becomes aluminum oxide and this frees the
metal. Mn and Cr metal oxides are extracted and reduced by Al. Example below shows what happens when manganese dioxide is heated
with aluminum powder.
Electrolysis of fused NaCl separates Na metal and Cl2 gas bubbling out of the Carbon anode.
Reduction by electrolysis:
Highly reactive metal – oxides and metal – chlorides are not easy to be reduced by chemical reactions. Metals such as
Na, K, Mn, Ca have to be freed from their ores by electrolytic processes. These metals are so reactive that they themselves are
powerful reducing agents. Molten metal – oxides or chlorides form the electrolyte in an electrochemical cell. The cathode
of the cell provides the electrons needed for the metal to free itself from the metal – oxide or metal – chloride
bonds. In the electrolysis, metals atoms get deposited on the cathode electrodes, which then have to carefully removed and
stored.