About iron ore
Iron, denoted Fe, is unquestionably the world’s cheapest and most useful metal. In its natural state, iron is chemically bonded with oxygen, water, carbon dioxide or sulfur in a variety of minerals. The average iron content of the Earth's crust is between 5 and 6%, and the only more abundant and widely distributed elements are silicon, oxygen and aluminum.
Iron ore is a commercial term denoting iron-rich minerals with sufficiently high iron content to be commercially viable for exploitation. The most important ore-forming minerals are:
HEMATITE, Fe2O3 (69.94% iron); bloodstone, red iron ore. Occurs in large quantities throughout the world. GOETHITE, FeO(OH) or FeO(OH)-nH2O (62.9% iron) LIMONITE, Fe2O3.H2O (at most 62% iron); a collective term for more or less impure goethite and a mixture of hydrated iron oxides, e.g., brown iron ore. SIDERITE, FeCO3 (48.2% iron) spatic iron ore. PYRITE, FeS2 (46.55% iron) the most commonly occurring sulphide mineral in the Earth’s crust. Used in the production of sulfuric acid and gives iron-bearing pyrite residue.
The mineral seldom occurs in the pure form. It is commonly mixed with other minerals (gangue), which reduces the iron content. Concentrations of as little as 30% may be of commercial interest, provided other factors such as gangue content, the size of the deposit and accessibility are favorable.
Ores often contain impurities, undesirable chemical components such as phosphorus, sulfur, natrium and potassium (the latter two oxides are called alkalis). Other components, for example, calcium and manganese, may be considered desirable, depending on the composition of other raw materials used in the individual iron or steel producer’s processes.
Most of the iron ores on the open world market are hematites. LKAB currently mines mainly magnetite ores which are unique.
Of local interest may be mentioned two other types of ores. In the Mesabi Range of the US and Canada, taconite is mined. It is a fine-crystalline magnetite with a finely dispersed iron content. In Australia, pisolitic ore, a sedimentary mixture of hematite and goethite with a dense, round-grained structure is mined.
Upgraded iron ore
Iron ore occurs naturally in a variety of forms from solid rock masses to sand-like iron fines.
Ore mined in the natural state, crude ore, invariably requires some form of processing. Normally, before the ore is saleable as finished products, operations such as crushing, screening, dressing, concentrating and pelletizing must be performed.
As a market commodity, iron ore is sold in different forms: LUMP ORES, usually crushed to 10-25 mm; SINTER FINES, with a maximum grain size of 6 to 10 mm (sinter fines is the common name for fines and concentrates); PELLET FEED, a fine-grained concentrate less well-suited for sintering, PELLETS, ore that has been rolled into balls before sintering at 1200-1300°C in drums or on discs.
Iron production
Metallic iron is produced by removing oxygen bonded with the Fe. This is done in a smelting process with a reducing agent that has a greater capacity for binding oxygen, for example carbon or hydrogen.
Commercially, iron ore is reduced to metallic iron either in a blast furnace (the most common method) or via so-called direct reduction. The aim of LKAB's ore upgrading operations is to adapt ore products for the two process routes in the best possible way.
THE BLAST FURNACE PROCESS
In the blast furnace, iron ore is reduced and smelted to liquid crude iron. This takes place in a high stack in which a combination of ore and coke meets a blast of hot air in the lower zone of the furnace. The hot air combines with coke to generate heat and the reducing gases (CO) required in the higher zones. In the stack, the ore must allow gases to ascend so that reduction takes place evenly and continuously. Iron ore feed of the wrong structure can block the flow in the furnace and cause increased fuel consumption.
Most world pig iron production consists of low-phosphorus pig iron (<0.15% phosphorus).
Lump ores are screened for direct charging to the blast furnace. Important characteristics are: good mechanical strength, as little disintegration as possible during reduction and high reducibility. The supply of good lump ores is limited. High-grade lump ores from several mines in Australia, Brazil, India and South Africa are also used together with pellets for direct reduction.
Sinter fines is too fine-grained a material for burdening in the blast furnace and must therefore be sintered. Sintering is a process by which granular fine ore is agglomerated by heating into larger lumps. At the sinter plant fine ore is mixed with coke breeze which is ignited and combusts under the addition of air. The mixture must be relatively porous and not too densely packed or fine-grained. Normally, a size distribution of the sinter fines with maximum 20% under 0.1mm is required. Sintering is done almost exclusively on the premises of the consumer, since the material disintegrates easily during transport.
Pellets, like conventional sinter, is an agglomerate. In the pelletizing process fine-grained ore, so-called pellet feed, unsuitable for common sintering, is used. Pellet feed can be either an extremely fine-grained material produced through grinding and concentration or natural "ultrafines" (from e.g., Brazil).
Pellets are normally produced by the mining company. Not only are undesirable components separated from the raw material, but additives that can to a certain degree improve product characteristics, are added. Olivine (a magnesium silicate) is added to LKAB pellets to improve their reducibility and mechanical strength in the blast furnace.
So far, LKAB has been the world's only producer of olivine pellets. The world market is still dominated by acidic pellets, i.e., pellets from which the main resultant slag component is SiO2. Several producers make self-fluxing pellets in which SiO2 is balanced with CaO, usually through an addition of limestone.
In the pelletizing process oil and sometimes coal or natural gas are used as fuels for reaching the sintering temperature of about 1250°C that gives the pellets sufficient mechanical strength. Magnetite is oxidized to hematite in a chemical reaction which produces heat, which means that LKAB's energy consumption, and thereby carbon dioxide emissions, are very low in comparison to that of most competitors.
THE DIRECT REDUCTION PROCESS
In the direct reduction process the ore is reduced at a lower temperature and is never actually smelted. Direct reduction methods are used principally where a cheap supply of natural gas is available. Processes based on coal have met with less success.
Shaft furnace processes (Midrex and HyL), which are the most common, use a burden of pellets and lump ore that must meet somewhat different requirements than materials for blast furnace use. Here, above all, high iron content, a low content of acid components (SiO2 and Al2O3), low fines generation and good reduction characteristics in a hydrogen-rich atmosphere at low temperatures are the desired features.
Iron ore for uses other than steel
Iron ore also has uses outside the actual iron and steel industry.
About half of all iron powder for the engineering industry, for pressing, welding and cutting, is produced through the reduction of high-concentrate iron oxides or fines. The other half is produced by atomizing molten steel. The total world capacity for ore-based iron powder is about 300,000 tonnes a year. The dominant reduction method is the Höganäs method, to which LKAB has been a major supplier for decades.
Because of its high density and good heat storage capacity, magnetite is also used in the manufacture of night storage blocks. These are used, mainly in the UK, for utilizing electricity at night when tariffs are lower. LKAB is also the dominant supplier on this market.
Its high density makes magnetite suitable as a ballast material, e.g., for oil pipelines to be laid on the seabed or for oil platforms and bridge piers. This market fluctuates widely because of developments in the oil industry. The properties of iron ore make it well-suited for use as an insulation material to shield against radiation.
High-concentrate magnetite fines are used in limited quantities for washing coal washing involves immersing crushed coal in a liquid of high specific gravity in which coal floats and can be separated from the heavier rock and minerals). It is also used in the production of catalysts for ammoniac synthesis.
Other uses for LKAB’s magnetite are as a filler in polymers, for sound insulation materials for vehicles and water and sewage pipes.
