Generally, ores with a higher percentage of iron are more valuable. If ore has more than 54% iron, it is classifi ed as a high-grade ore and requires no further benefi ciation other than sizing. Ores grading less than 54% iron are considered low-grade and require upgrading to become a marketable product. High-grade iron ores are marketed in two sizes. The first, which is ore greater than 8 mm in size, is called “lump ore.” Ore that is less than 8 mm is called “fine ore.”
iron ore consists of rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and carbonates, and vary in colour from dark grey, bright yellow, and deep purple, to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), gothite, limonite, or siderite. Hematite is also known as “natural ore.” In addition, iron
ore is the raw material used to make pig iron, which is one of the main raw materials for making steel.
There are two aspects to iron ore demand: quantity and quality. Since the major trade item is in mineral rather than metallic form, there are many chemical and physical variants of iron ore, but they all serve the same purpose: providing the iron component of steel (98%), and to a lesser extent (2%), its non-metallurgical uses as iron oxide in the production of pigments, electronics, heavy media,
abrasives, and construction.
Therefore, steel production is the driving force for almost all iron ore demand. However, technological changes at all stages from iron ore mining to the production of finished steel have been major factors in determining the quantities and properties of the iron ore demanded. There are two technologies used to produce steel: basic oxygen furnaces (BOF), which are charged with molten blast furnace iron and ferrous scrap at the integrated steel mills; and electric arc furnaces (EAF), which are charged with scrap and/or
direct reduced iron (DRI) at the mini-mill plants.
Iron ore pelletizing (IOP) is the second largest consumer of bentonite after foundry sands. In standard IOP, iron ore is ground and then mixed with small amounts of bentonite that bind the grains, allowing further processing (agglomeration) into balls or pellets by the tumbling and induration effect using straight grate processes. These are then sintered in rotary kilns to obtain a hard outer surface.
About 25% of world iron ore output is pelletized. The other basic forms of iron ore used in metal production include lump ore prepared by crushing and screening, and sinter produced from natural or screened fines. Bentonite absorbs the water, functions as a binder, and enhances the strength of the pellets. On the downside, bentonite adds unwanted silica to the blast furnace, which increases the demand for flux and coke. The Canadian iron ore industry is largely supplied with bentonite from European producers.
In Brazil, some ore that contains practically no other minerals can grade as high as 68% Fe, but the crude ore mined in Canada grades between 30 and 44% Fe. Therefore, Canadian mines crush and grind the ore and then use gravitational and magnetic concentration methods to produce concentrates with an iron content of about 65%.
Depending on grain size, the concentrate is then shipped as is, or agglomerated into balls about a centimetre in diameter and fi red to produce hard iron ore pellets. Steel companies take the pellets and coke made from coal and load them into blast furnaces where the minerals are reduced to metallic iron. Unpelletized concentrate received at steel plants is sintered before being charged to the blast
As noted above, the chemical composition of iron ore consists of oxygen and iron bonded together intomolecules. To convert it to metallic iron, the iron ore must be smelted or sent through a direct reduction process to remove the oxygen. Oxygen-iron bonds are strong and to remove the iron from the oxygen, a stronger elemental bond must be presented to attach to the oxygen. Carbon is
used because the strength of a carbon-oxygen bond is greater than that of the iron-oxygen bond at high temperatures. Thus, the iron ore must be powdered and mixed with coke to be burnt in the smelting process.
However, it is not entirely as simple as that because carbon monoxide is the primary ingredient of chemically stripping oxygen from iron. Thus, the iron and carbon smelting must be kept in an oxygen-defi cient reduced state to promote the burning of carbon to produce carbon monoxide (and not carbon dioxide).