Impurities in iron ores impact of sale price and company profits. This brief article gives some idea of what to look for when interpreting assay results for iron mineralisation and ores.
Impurities are common and are reported in assays along with iron grade (as Fe). It is important not to focus on iron grade without also considering the levels of impurities shown.
Even the highest grade Direct Shipping Ores (DSO) will contain impurities. Smelting is also optimised to deal with a level of impurity in an ore.
If the level of impurities in an ore are too high then the company selling the ore is penalised. This means that it will receive either a lower price. Conversely, lower than normal levels of impurity (say, for iron ores grading >64% Fe) attract a sale premium. These ores are used to blende with lower grade ores so that the blended product is standardised in grade.
Beneficiating a lower grade ore (or concentrating the ore by processing and removing one or more impurities) is an additional process and cost. Beneficiation in itself is not a bad thing, but the cost of beneficiating an ore will depend on the type of ore, its physical characteristics and the level of processing required.
Beneficiating an ore can result in sale of a premium concentrate product.
Iron ore grades can vary depending on the ore mineral constituents of the mine feed. Hematite and magnetite are the two most common ores, although hydrated iron ores (goethite) are also increasingly common.
Hematite iron ore is at its most pure when it returns a grade of 69.9% Fe. Magnetite is pure iron ore at a grade of 72.4% Fe. These ores are iron oxides, so the remainder of the % comprises oxygen. Smelting to create pure iron is one of reduction, or removing the oxygen.
[I’ve written other posts that show you how to calculate iron and mineral content from assay results so I won’t go into more detail here. These posts also go into the advantages and disadvantages (read costs) of hematite vs magnetite ores as beneficiation feeds (for more information, click on iron ore in category search to right).]
Oxygen is a major component of an iron ore but is not an impurity. It is part of the mineral ore.
Ores can also contain a volatile component, like water. This is not reported directly in an assays result. However, you can get an idea of levels by looking at the LOI figure (Loss on Ignition). A high LOI is not necessarily a bad thing as, in relative terms, the burning off of water while smelting an ore will result in the reported iron grade of the ore increasing as a proportion of the whole rock.
The main impurities in iron ores are silicon, phosphorous, aluminium and sulphur.
Example of an assay report for samples of iron. Those of a grade >63% iron are clearly Direct Shipping Ores (DSO), wheras those of lower grade would need to be beneficiated or sent to waste. Impurities are silica (SIO2), alumina (Al2O3), phosphorous (P) and sulphur (S). LOI is Loss On Ignition and represents the volatile component of the sample eg water.
Silica
Silica is the most common impurity. It can also be the easiest to separate through beneficiation. This is because of the physical contrast (eg density) between silica-rich minerals, like quartz, and iron-rich minerals.
Smelting high silicon ores results in formation of gray iron. Grey iron is brittle but easier to get a good finish. There is less shrinkage, which is why it was once preferred for castings.
Most silica is ‘slagged’ off during smelting but at higher temperatures can be reduced to the point at which it may become incorporated (alloyed) into the iron.
Silica levels in an ore should ideally be <3.5%, but for high grade ores may be less than 2%.
High silica ores can still be used but will probably be blended with high grade ores to reduce the overall impurity level. Too high and an ore needs to beneficiated.
Phosphorous
Phohorous is one of the nasty impurities in iron ore. It results in brittle iron.
Only low levels of phosphorous are tolerated. If you have more than 0.01% then the pricepaid for the ores is reduced as the ore will require blending with a low phosphorous ore to reach a tolerable grade.
It is not easy to remove phosphorous so it is preferable that ores are low in phosphorous to start.
Aluminium / alumina
Alumina is another common impurity in iron ores. This reflects its abundance in many other minerals which might also be included with the iron ore minerals.
Alumina is harder to reduce than silica and is not so easily alloyed into the smelted product. However, it’s presence increases the viscosity (or thickness of the flow) of a slag formed during the smelting process. This viscous liquid waste is harder to remove and slows down the overall smelting process.
To some extent, a higher viscosity in a furnace can be managed through addition of lime.
Sulphur
Like phosphorous, sulphur is an impurity to avoid in iron ores. High sulphur ores (>0.01-0.03%) are to be avoided as it ultimately makes iron brittle, prone to cracking and failure.
Sulphur fumes from smelting are also a source of sulphur dioxide which can interact with moisture in the atmosphere to produce sulphuric acid.