| Extraction
of metals
Francine
Taylor-Campbell, Contributor
 |
| Students
of Williams Knibb Memorial High School, in Trelawny, take notes during day two
of The Gleaner's Youth Link CSEC Seminar, in Montego Bay, on Wednesday, April
12, 2006. - Photo by Tashieka Mair/Freelance Photographer | MAIN
POINTS The
reactive metals are found in nature as compounds called ores. These compounds
contain the metal in combination with oxygen, sulphur or chlorine along with other
impurities. Metals that are not very reactive are usually found in their free
state that is, uncombined, for example gold. To
extract a metal Xx+ from its ore the process of reduction is used, that is the
metal ions gain electrons to become atoms. Xx+
+ xe ======= X For
the process of reduction, a reducing agent such as carbon or hydrogen is generally
used. The reducing agent works by displacing the metal from its ore. However,
for very reactive metals, for example sodium and potassium, the process of electrolysis
is used to supply electrons to the molten ore to extract the metal. In
the extraction of aluminium, the ore (bauxite) is purified and dissolved in molten
cryolite (Na3AlF6) from which it produces
the ions A13+
and O2-. The aluminium oxide/ cryolite solution is then
electrolysed in a cell using graphite (carbon) electrodes. Aluminium ions (+ve)
move towards the cathode where they gain electrons and form molten aluminium.
Al3+
(l) + 3e ===== Al (l) This molten aluminium is collected at the bottom of the
cell. The oxide ions (-ve) move towards the anode where they lose electrons to
form oxygen which is collected.
2O2-(l)
======= O2 (g) + 4e The
extraction of iron is different from that of aluminium. This is because iron is
not as reactive, hence a reducing agent such as carbon monoxide can be used to
extract it from its ore. In
this process, the ore (haematite) is mixed with limestone (CaCO3)
and coke (carbon) and heated in a blast furnace. The reaction occurs in different
stages: 1.
The carbon burns to form carbon dioxide C
(s) + O2 (g) ======= CO2 (g) 2.
The carbon dioxide reacts with more coke (carbon) to form carbon monoxide which
is the reducing agent in this method. CO2
(s) + C (s) ======= 2CO (g) 3.
The carbon monoxide reduces the haematite to iron which is collected at the bottom
of the furnace as molten iron. Fe2O3
(s) + 3CO (g) ====== 2Fe (s) + 3CO2 (g) Impurities
in the ore, mainly silica, are removed by reacting with calcium oxide formed from
the decomposition of limestone. CaCO3
(s) ======= CaO (s) + CO2 (g) CaO
(s) + SiO2 (s) ======= CaSiO3 (l)
slag or calcium silicate This
slag is also collected at the bottom of the furnace. This
reaction is facilitated because the calcium oxide is basic while silica is acidic.
The
slag produced is used for road building and also as a fertiliser. The iron produced
is impure, containing carbon, which is removed by reacting it with oxygen. From
this, steel and other types of iron are produced. Francine
Taylor-Campbell is an independent contributor. | 
