The iron pelletizing process is often associated with low grade iron ore following a range of liberation techniques to prepare and make suitable for the blast furnace.
In summary, pelletizing is achieved by combining the iron ore with water and a specific binder which is then rolled up in drum to produce relatively uniform pellets which can then be easily handled in downstream processes.
The major raw material in the iron and steel industry is iron ore which can be classified as high grade and low grade in terms of its Fe content. High grade iron ores, which can directly be used in a blast furnace, are not abundant in earth’s crust to supply the need of iron and steel industry. The exploitation of low grade iron ores is possible after enrichment. Low grade iron ores contain considerable amounts of impurities including compounds of silica, alumina, calcium, and magnesium.
Hence the concentration process is required based on crushing, grinding for adequate liberation before the implementation of separation techniques. Liberation can mostly be achieved at considerable fine particle sizes. For this reason, the concentrate obtained is not suitable to be charged directly into the blast furnace or the direct reduction plant without converting it into suitably sized agglomerates. The most commonly employed agglomeration technique is pelletizing by which a mixture of iron ore, water, and binder is rolled up in a mechanical disc or drum to produce agglomerates (green or wet pellets).
Figure 1: Iron pellets
The pelletizing process is a process which contains numerous sub-processes, or process segments. On the way from the mine to a final product, the iron ore goes through the following main process segments:
1) The iron ore is crushed and the waste rock is removed. About 85 % of the particles should be less than 44 μm (in length, width, or height). It is then possible to extract the valuable mineral, which is magnetite
2) Water is added to the magnetite to make magnetite slurry
3) Additive material (dolomite or olivine, depending on the product) is added to the slurry
4) Most of the water is then removed from the slurry by use of press filters. The
water content after this filtering is about 9% of the weight.
5) After the filtering, binder (bentonite or organic binder) is added
6) Green pellets are made by the use of balling drums. When leaving the drum, the pellets are screened, and all pellets with too small a diameter are fed back to go through the drum once more. Oversized pellets are crushed and recycled. The rest are the correctly sized pellets which form the drum’s output.
7) The on size pellets are transported on a conveyor to the drying process, where they are dried by hot air flowing through the bed.
8) The pellets are fired (1250-1300°C) and then cooled down to about 200°C.
Several of the above listed items describe process segments which may be operated with classical control techniques, and some of them like item ii), iii), and v) are already automatic in most pelletizing plants today. Clearly some process segments depend highly on a well-functioning preceding segment. If the particle size is too large, the fines are too dry, or the drying is not working, no pellets can be produced.
A less dramatic situation is when some segments are working sub optimally. If for instance the process in which the binder is added gives too many varying outputs, then the balling drum operation will suffer. Therefore, the iron ore pelletizing process may be described as a chain of several sub-processes which may depend highly upon each other’s performance.
The market demand is of course the most important aspect to be taken into account when evaluating in which direction product development should be conducted. Important parameters for the price of the pellets includes, among others, purity and size of the pellets. The size is specified by the buyer in terms of a nominal diameter ± some small variance. The nominal diameter may vary from one iron mill to another, but a quite normal value is between 10 − 12 mm.
Crushed pellets, which will always be present to some extent, is highly undesirable for the iron mill as it complicates their process. The amount of such material is thus required to be at a minimum. It also represents an unacceptable economic loss for the pellets manufacturer, as it is not possible to recycle this material into an earlier stage of the process, and therefore has to be delivered as a low quality product. Crushed pellets are normally a result of too high moisture content in the green pellets, which is caused by a badly controlled cold process. However, a small amount of crushed pellets due to the transport and handling will also always be present. Figure 2 shows an overview of the iron ore pelletizing process.
Figure 2: Overview of the iron ore pelletizing process