Civil Engineering Reference
In-Depth Information
Extraction of iron from iron ore can be simple or complicated, on a small scale
or large scale. There is quite a clear correlation today between size and efficien-
cy in the metal industry. The fact that, 250 years ago, there were handbooks on
extraction of iron for domestic needs proves that times and technologies have
changed.
The conversion of iron from ore to steel requires a long series of processes. They
begin with the breaking up of the ore, then cleaning, followed by sintering. The
iron is smelted out and reduced in a large blast furnace at 1700-1800°C. A large,
modern blast furnace can produce 1000 tons of pig iron every 24 hours. The
amount of air needed is four million cubic metres, and the cooling water is equiv-
alent to the amount a small town would use. It takes 440-600 tons of coal to pro-
duce 1 ton of iron (either charcoal or coal can be used). The amount needed can
be reduced by half if an oil spray is injected into the furnace. Carbon is used in the
process to remove oxygen from the ore by forming carbon dioxide, leaving the
iron behind. Earth kilns were once used to smelt out the iron. The ore was filled
in from above with layers of charcoal. In newer methods ore is mixed with lime
and sand. The function of the lime is to bind ash, silica, manganese, phosphorous,
sulphur and other compounds. The lime and other substances become slag from
the blast furnace, which can be used as pozzolana in the production of cement.
Steel can be made of pig iron and steel scrap. Most of the carbon in the iron is
released through different methods, e.g. oxidizing. This is done in blast furnaces
or electric arc furnaces. The latter consumes far less energy and is today used in
30-40 per cent of the world's production.
Finally the steel is rolled out to produce stanchions, beams, pipes, sheeting and
nails.
Iron and steel products that are not exposed to corrosive environments usual-
ly last for very long periods. Robust products can be recycled locally with a little
cleaning up. All steel products are well suited for recycling.
Large amounts of sulphur dioxide and dust can come from the production of
iron, while steel production releases large amounts of the greenhouse gas carbon
dioxide, as well as dust, cadmium and fluorine compounds, into the air and
water. This pollution is reduced when producing steel from waste. When pro-
ducing steel from stainless steel, there will be a release of nickel and chrome.
Arsenic is a common pollutant of iron. It is well bound in the ore, but with a
second smelting of steel scrap a good deal is released. Steel scrap is virtually
inert, but ions from iron and other metal alloys can leak into water and the earth
and damage various organisms.
Protection against corrosion
When ordinary steel is exposed to damp air, water, acids or salt solutions, it rusts.
This is hindered by coating it with zinc, tin, aluminium, cadmium, chrome or
nickel through zinc coating or galvanizing.
