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Ideally,
iron ore contains only iron and oxygen. In nature, this is rarely
the case. Typically, iron ore contains a host of deleterious elements
which are unwanted in modern steel. Table below shows the desirable
levels to look for when assessing iron ore projects.
Silica
Iron
ore typically contains silicates, usually in the form of quartz.
Silica is undesirable because silicon does not bond with carbon
during the smelting process and can remain in the iron after it is
refined. Historically, siliceous iron ore created wrought iron, a
malleable and strong form of iron used by blacksmiths throughout history.
Modern
steelmaking techniques generally use lime and other fluxes to help
remove the silica from the molten iron ore, and form a slag on the
surface of the molten metal. This slag can then be removed.
Phosphorus
Phosphorus
is a deleterious metal because it makes steel brittle, even at
concentrations of as little as 0.5%.
Phosphorus
cannot be easily removed by fluxing or smelting, and so iron ores
must generally be low in phosphorus to begin with. The iron pillar of
India which does not rust, however, is protected by a phosphoric composition.
Phosphoric
acid is used at a rust converter because phosphoric iron is less
susceptible to oxidation.
Alumina
Alumina
(Al2O3) is generally present in iron ores as clay. This is usually
removed by washing the iron ore, and by fluxing. However, again, iron
oxide deposits must be relatively low in alumina in order to be
considered ore.
Sulphur
Sulphur
is unwanted because it produces undesirable sulphur dioxide gases in
the flue emissions from a smelter and interferes with the smelting process.
Loss
on Ignition (LOI)
Whilst
it is desirable to have low contaminant levels of the elements
mentioned above, it is considered the opposite for an LOI measure.
Essentially, the LOI is a measure of the water content of the ore,
which evaporates when the ore is fed into a blast furnace.
A
typical iron ore analysis should include an LOI determination at
1000ºC, normally undertaken by Thermogravimetric Analyser (TGA).
This allows for an addition of the oxides, generated at the ignition
temperature and the LOI, to arrive at a total (oxides plus LOI). The
LOI is due to the loss of water from hydrated minerals (goethite,
gibbsite and kaolinite), decomposition of carbonates (calcite,
siderite and dolomite) and the volatilisation of organic compounds.
The LOI may be offset to some extent by the weight gain due to
oxidation of reduced iron and manganese mineral phases.
Desirable
Contaminant and Grade Levels of Dee Laa Iron Ore Standard
|
Iron Ores |
Fe % |
SiO2 % |
P % |
S % |
Al3O2 |
LOI |
|
Hermatite/Magnetite |
> 55 |
5 max |
0.1 max |
0.5 max |
2 max |
7-10 |
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