The oxide so produced is then treated as for the oxide-bearing ores. There is very
little iron found in copper obtained from chalcopyrite as the iron is more dif-
ficult to reduce than copper and ends up in the unwanted slag.
The tin-bearing ores are found in Devon and Cornwall with the chief
one being cassiterite or tinstone (SnO 2 ). As the ore is already in the oxide
form, roasting does not need to be carried out and the cassiterite is directly
reduced by the carbon in the fuel (charcoal or dry wood) to produce tin.
The temperature required for this reaction is approximately 600°C and the
molten tin (melting point 232°C) runs out of the bottom of the furnace into
clay or stone moulds. It is important that the ore and the fuel are well mixed as
they are loaded into the top of the furnace in order to ensure that the reaction
takes place.
The major economic ores from which iron can be extracted are haematite
(Fe 2 O 3 ), limonite (Fe 2 O 3 ·3H 2 O), magnetite (Fe 3 O 4 ) and siderite (FeCO 3 ). The
extraction of the pure metal from these ores is far more difficult than it is for
producing pure copper or pure tin. The reason for this is the oxygen is more
strongly bonded to the oxides of iron and requires more stringent reducing
conditions within the furnace to achieve this separation. The minimum tem-
perature at which the oxide is reduced is 800°C, which is well below the melt-
ing point of iron. Another consideration is that the ore is mixed with a large
amount of unwanted material, called gangue. This has to be separated from the
metal by a process of slagging whereby the unwanted material is melted by
combing with silica (sand) and is drained away from the iron. The melting point
of this slag is approximately 1200°C.
Shaft furnaces were employed in which the ore was mixed with charcoal
and the temperature was raised to approximately 1000°C by means of manually-
operated bellows. This is below the melting point of iron, so reduction
occurred within the solid phase. The charcoal combines with any air in the
furnace to produce carbon monoxide gas as in Equation (1). The following
reactions then take place between the oxides of iron and the carbon monox-
ide gas:
(7)(8)Reaction (7) occurs higher up the shaft while Reaction (8) nearer the hearth.
The end result was a mixture of iron and slag, which was subsequently ham-
mered while hot to produce a bloom of wrought iron. This was called the
bloomery process for the production of wrought iron. The wastage of iron was
very high with over 70% of available iron lost in the slag.
FeOCO Fe CO 2Fe O 23 CO 2 FeO CO 2126 Chapter 6