is there ore in the ground, page-15

A brief outline on what to look for in Iron Ore Assay results


Iron Ore Contaminants and Parameters


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 4 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 volatilization 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 (rough guide only)

Hematite

Fe (%) >58
SiO2 (%) <5
P (%) <0.1
S (%) <0.05
Al2O3 (%) <2
LOI (%) 7 - 10