Conservation Science

be carbon or more likely in the presence of just sufficient air, carbon monoxide
gas, CO.

(1)

The reducing agent will combine with the oxygen or sulfur part of the ore and
free the metal. The unwanted material forms a slag, which may be in the form of
some silicate or oxides of other metals found associated with the metal being
extracted. The whole process can be represented by the following simple
equation:

(2)

Oxygen from the air must be prevented from mixing with the metal or else it will
immediately react to produce fresh oxide. Additionally, the presence of oxygen
may exhaust the reducing agent. This indicates that the design of the furnace
required to heat the ore/reducing mixture, must be such that there is a reasonably
large region in which no oxygen is present. This is where the reaction in
Equation (2) takes place and the atmosphere is said to be reducing. The tem-
perature at which the metal is produced varies from metal to metal and the rates
of the reaction will be faster if the reactants are in the liquid phase, i.e.the metal
and slag are molten. This will depend on the melting point of metal. Table 1
indicates the melting of some of the common metals and the temperatures
required to reduce the ores to the metal.
One of the main limitations of the early metallurgists was that they could not
generate particularly high temperatures in their furnaces. Metals with low-
melting points such as tin, lead and copper were produced in large quantities
much earlier than the higher melting point ones such as iron. In reality, the pro-
ductionof wrought iron was carried out virtually by solid-state reduction with

Orereducing agent heat metal slag gas

2CO 2 2CO

124 Chapter 6

Table 1Typical ores from which some common metals are extracted

Reducing Metal Melting
Temperature Point
Metal Chief Ores Chemical Formula (°C) (°C)

Copper Malachite CuCO 3 · Cu(OH) 2 800 1083
Chalcotite Cu 2 S
Chalcopyrite CuFeS 2
Lead Galena PbS 800 327
Tin Cassiterite SnO 2 600 232
Iron Haematite Fe 2 O 3 1000 1535
Limonite Fe 2 O 3 ·3H 2 O
Magnetite Fe 3 O 4
Siderite FeCO 3