Iron ore, its beneficiation and other related environmental issues:
Most useful metal in the world, Iron, is extracted from iron ore. It is the rock from which metallic iron can be economically extracted. Iron ore is a mineral substance which, when heated in the presence of a reductant, will yield metallic iron (Fe). The iron ore, usually, very rich in iron oxides (Fe3O4 and Fe2O3). Iron ores are mostly dark grey to rusty red in color and high specific gravity.
Types of iron ore -
Two main types of iron ore used for iron making – Magnetite (Fe3O4) and Hematite (Fe2O3). Common iron ores include:
- Hematite - Fe2O3 - 70 percent iron
- Magnetite - Fe3O4 - 72 percent iron
- Limonite - Fe2O3 + H2O - 50 percent to 66 percent iron
- Siderite - FeCO3 - 48 percent iron
Iron ore is the source of primary iron for the world's iron and steel industries. It is therefore essential for the production of steel, which in turn is essential to maintain a strong industrial base.
Almost all (98%) iron ore is used in steelmaking. Iron ore is mined in about 50 countries. The seven largest of these producing countries account for about three-quarters of total world production. Australia and Brazil together dominate the world's iron ore exports, each having about one-third of total exports.
Hematite deposits are mostly sedimentary in origin, such as the banded iron formations (BIFs). BIFs consist of alternating layers of chert (a variety of the mineral quartz), hematite and magnetite. They are found throughout the world and are the most important iron ore in the world today. Their formation is not fully understood, though it is known that they formed by the chemical precipitation of iron from shallow seas about 1.8-1.6 billion years ago, during the Proterozoic period.
Magnetite also mostly found in Banded iron formations (BIF). They are fine grained metamorphosed sedimentary rocks composed predominantly of magnetite and silica. Mining and processing of BIF formations involves coarse crushing and screening. Magnetite is beneficiated by crushing and then separating the magnetite from the gangue minerals with a magnet. This is usually so efficient that lower grade ore can be treated when it is magnetite than a comparable grade of hematite ore, especially when the magnetite is quite coarse.
Magnetic separation and flotation are the most widely accepted technologies for the upgrading of iron ore particles, but these processes result in iron concentrate with high amounts of very fine and/or interlocked silica particles.
Most of the mined out Iron ore contains lot of impurities that needs beneficiation prior to use. Inferior sources of iron ore generally required beneficiation. Research on utilization of low grade Iron ore to produce quality raw material would play a key role in future which is a fact acknowledged by the Iron and Steel industry.
Grades of Iron Ore –
Iron ore is typically classified as High grade (+65% Fe), Medium grade (+62 – 65% Fe) and Low grade (-62% Fe). Typically, the Integrated Steel Plants(ISPs) use medium/High grade Iron Ore whereas the Sponge Iron plants require only High Grade iron ore, preferably, with +67% Fe.
(i) Lumpy/Fine Ore: Iron Ore is traded in lumps (i.e. sized ore) or in fines. Production/availability of lumps is limited by virtue of the natural occurrence and also because of generation of lot of fines during crushing of large lumps present in the run-of –mines (ROM).
(ii) Natural pellet: It is a term coined by producers in some Asian counties, to designate sized iron ore used directly in Sponge Iron production.
(iii) Blue Dust: Blue Dust is the name given to naturally occurring, extremely friable, high grade Haematite Iron Ore powder.
Beneficiation -
Very low grade Iron ore cannot be used in metallurgical plants and needs to be upgraded to increase the iron content and reduce the gangue content. A process adopted to upgrade ore is called Beneficiation. Iron ore is upgraded to a higher iron content through concentration. Iron ore is being beneficiated all round the world to meet the quality requirement of Iron and Steel industries. However, each source of Iron ore has its own peculiar mineralogical characteristics and requires the specific beneficiation and metallurgical treatment to get the best product out of it. The choice of the beneficiation treatment depends on the nature of the gangue present and its association with the ore structure. Several techniques such as washing, jigging, magnetic separation, advanced gravity separation and flotation are being employed to enhance the quality of the Iron ore.
Due to the high density of hematite relative to silicates, beneficiation usually involves a combination of crushing and milling as well as heavy liquid separation. This is achieved by passing the finely crushed ore over a bath of solution containing bentonite or other agent which increases the density of the solution. When the density of the solution is properly calibrated, the hematite will sink and the silicate mineral fragments will float and can be removed.
Agglomeration of Iron Ore –
Iron Ore Fines/blue dust cannot be charged in the blast furnace directly since they block the passage for ascending gas inside the fee. So, they are agglomerated (by igniting at lower temperature causing only interfacial fusion) into larger lumpy pieces with/without addition of additives like limestone, dolomite etc. Two types of agglomerated products are commonly produced/used in the industry namely Sinter and Pellet. Accordingly the processes are known as Sintering and Pelletising respectively.
(a) Sinter : Sinter is a clinker like aggregate which is normally produced from relatively coarser fine iron ore (normally –3mm) mixed with coke breeze (-3mm), limestone dolomite fines (-3mm) and other metallurgical return wastes from the plant. Sinter is a much preferred input/raw material in blast furnaces. It improves BF operation and productivity and reduces coke consumption in blast furnace. Presently, more than 70% hot metal in the world (in India 50%) is produced through the sinter.
(b) Pellet: Pellets are normally produced in the form of globules from very fine iron ore (normally –100 mesh) and mostly used for production of Sponge Iron in gas based plants, though they are also used in blast furnaces in some countries in place of sized iron ore.
Conservation of iron ore and environmental issues -
During washing and sizing of the ore, slimes with less than 0.21 mm size are generated and discarded into the tailing pond. Huge quantity of slimes are being generated in every year during the processing of hematite ore and lost as tailings containing around 48-62% of Fe. This possess great amount of pressure on environment and on conservation issue. However beneficiation and utilization of these slimes still remains as a challenging task.
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| IRON ORE PELLETS |
The ever increasing need to utilize the slimes is being reflected in the shift in steel production from basic blast furnaces to electric arc furnace technology. In developed nations like the USA, around 40% of steel is produced in electric arc furnaces by using Iron ore pellets. Iron ore pellet is a kind of agglomerated fines which has better tumbling index as compared to that of parent ore and can be used as a substitute for the same. High-grade iron ore fines are used for the preparation of pellets (Iron content in the ore has got to be very high and gangue low to confirm to the quality of pellets. The content generally is 66% Fe, SiO2 + A12O3 – 3%, Sulfur and Phosphorus 0.05% each. The sizes of the fines should be minus 100 mesh 95% minus 325 mesh 60 – 75%). Iron ore pellets are being used for long in traditional blast furnaces in many countries where lump iron ore is not available. Pellets are ideal material as a feed to direct-reduction Iron (DRI) plants. In other words, pelletizing of iron ore concentrate produces spheres of typically 8 to 18 mm diameter. The process combines agglomeration and thermal treatment to convert the concentrate into pellets with characteristics appropriate for use in a blast furnace or direct reduction steel making process.
Advantages in using iron ore pellets instead of iron ore lumps are many – conservation of resources such as iron ore and keeping better environment are the most important among others.
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