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Thursday, July 14, 2011

Extraction of Iron from iron ore - Reshared Knol

Source Knol: Extraction of Iron from iron ore
by Partha Das Sharma, Hyderabad (India)

Extraction of Iron from iron ore - various basic steel:

Perhaps the most important element ever known to mankind and one of earth's most abundant, Iron (Fe), is extracted from Iron ore. The process of extracting iron - done either through ‘blast furnace’ route or by ‘direct reduction’ route - involves a number of steps.

A. Extraction of Iron using blast furnace:

Common iron ores are hematite (Fe2O3) and magnetite (Fe3O4). Since, iron is below carbon in the reactivity series, iron in the ore is reduced to iron metal by heating with carbon (coke). It is actually carbon monoxide which does the reducing in the blast furnace.

Iron ore is reduced to iron by heating them with coke (a form of carbon) in blast furnace. The iron ore contains impurities, mainly silica (silicon dioxide). Limestone (calcium carbonate) is added to the iron ore which reacts with the silica to form molten calcium silicate in the blast furnace. The calcium silicate (called slag) floats on the liquid iron.

The air blown into the bottom of the blast furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important to conserve heat energy. The coke (produced by heating coal in the absence of air) burns in the blast of hot air to form carbon dioxide; exothermic reaction releases heat. This reaction is the main source of heat in the furnace.

C + O2 = CO2

At the high temperature at the bottom of the furnace, carbon dioxide reacts with carbon to produce carbon monoxide.

C + CO2 = 2CO

It is this carbon monoxide which is the main reducing agent in the furnace to produce iron.

Fe2O3 + 3CO = 2Fe + 3CO2

In the hotter parts of the furnace, the carbon itself also acts as a reducing agent. Notice that at these temperatures, the other product of the reaction is carbon monoxide, not carbon dioxide.

Fe2O3 + 3C = 2Fe + 3CO

The temperature of the furnace is hot enough to melt the iron which trickles down to the bottom as ‘pig iron’, where it can be tapped off.

The limestone is added to convert siliceous impurities into ‘slag’ ( as calcium silicate, CaSiO3), which melts and runs to the bottom. The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron.

CaCO3 + O2 = CaO + CO2. CaO + SiO2 = CaSiO3


B. Direct reduction of iron (DRI) –

(‘Sponge iron’, another method of producing iron):

All steelmaking processes require the input of iron bearing materials as process feedstock. For making steel in a basic oxygen furnace, the iron bearing feed materials are usually blast furnace hot metal and steel scrap. A broadly used iron source is also a product known as Direct Reduced Iron ("DRI") which is produced by the solid state reduction of iron ore to highly metallized iron without the formation of liquid iron. This solid state reduction of iron ore is also called ‘sponge iron’.

Sponge iron is the product created when iron ore is reduced to metallic iron, in the presence of coal, at temperatures below the melting point of iron. The external shape of the ore is retained with 30% reduction in weight due to oxide reduction resulting in change in true density from 4.4 gm/cc to 7.8 gm/cc in this product. This paves the way for 54% reduction in volume which is manifested in pore formation through out the interior of reduced product and hence the name “Sponge Iron”. This spongy mass sometimes called a bloom. This makes for an energy-efficient feedstock for specialty steel manufacturers which used to rely upon scrap metal. The advantage of this technique is that iron can be obtained at a lower furnace temperature (only about 1,100°C or so). Only small quantities of sponge iron can be made at a time as compare to blast furnace process, is the major disadvantage.

In this method, the iron ore along with coal is charged to the top portion of the reduction zone of a rotary kiln or furnace, wherein the bed of particles which descend by gravity is reduced by a hot reducing gas largely composed by carbon monoxide (CO) and hydrogen (H2). Finally, the product sponge iron is discharged from the bottom portion of the discharge zone of the furnace and conveyed (after cooling), for example, to be melted in an electric arc furnace or to be briquetted in a briquetting machine coupled to the reduction reactor. The evolution of sponge iron as a metallic feed in electric steel making has been mainly due to reduced availability of high quality scrap and its increasing cost.

Quality of sponge iron for steel making: There are several parameters to be monitored for improving the quality of sponge iron for steel making operation, such as – (a) Size, (b) Density, (c) Unit weight, (d) Crushing strength, (e) Weather resistance, (f) Carbon contents, (g) Metallization.

(a) Size - The size of sponge iron is very important especially with regard to continuous feeding. A very fine sized material (1 mm to 2 mm) would be quickly oxidized during falling to the slag or may be lost in fume extraction system. Extremely large size (exceeding 30 mm) poses problem during continuous feeding. The size fraction less than 2 mm needs to be limited for continuous feeding.

(b) Density - Sponge iron after falling should have the ability to penetrate into the slag layer and reside at the slag/metal interface for effective heat transfer and chemical reaction. Sponge iron with lower density tend to float on the slag while, high density material readily penetrates into the metal. Hence, it is desirable to have the density of sponge iron in the range 4 - 6 gm/cc.

(c) Unit weight – The transition time of the sponge iron pellets through the slag is dependant on the momentum. If the pellet stays in the slag layer for too long a time, the phenomenon of slag boiling occurs. Slag fluidity is highly important. However, a heavier sponge iron pellet does not require close control in slag fluidity.

(d) Crushing strength - Sponge iron should possess good crushing strength to prevent generation of large amounts of fines.

(e) Weather resistance - Sponge iron is prone to oxidation and heat builds up in contact with atmosphere. The storage of Sponge Iron for long periods of time affects its metallization, partially due to surface re-oxidation caused by the porous structure of sponge iron pellets or lumps.

(f) Carbon contents - During continuous feeding, an active carbon — oxygen boil is necessary to shield the arcs. It has been observed that to achieve the aforesaid, sponge iron should possess a minimum of 0.60% carbon.

(g) Metallization - High metallization helps in lower power consumption but severely reduces the bath activity and results in flat bath conditions. For low metallization levels, increased carburization is required to compensate for the extra oxygen in sponge iron.

C. Basic Iron and Steel products -

(i) Pig iron – The molten iron from the bottom of the blast furnace is pig iron. It contains 3.5 - 4.5% carbon and varying amount of contamination such as, sulfur, silicon and phosphorus. Pig iron is the intermediate step on the way to cast iron and steel.

(ii) Cast Iron – Some time pig iron from the blast furnace can be used as cast iron. It is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle.

(iii) Steel - Most of the pig iron is used to make one of a number of types of steel. There isn't just one substance called steel - they are a family of alloys of iron with carbon or various other metals after removal of impurities from molten iron.

D. Removal of impurities –

Impurities in the pig iron from the Blast Furnace include carbon, sulfur, phosphorus and silicon. Sulfur is removed by reacting with magnesium (Mg) as magnesium sulfate (MgS).

Mg + S = MgS

Carbon is removed by blowing oxygen in molten iron. The impure molten iron is mixed with scrap iron (from recycling) and oxygen is blown on to the mixture. The oxygen reacts with the remaining impurities to form various oxides. The carbon forms carbon monoxide. Since this is a gas it removes itself from the iron! This carbon monoxide can be cleaned and used as a fuel gas.

Elements like phosphorus and silicon react with the oxygen to form acidic oxides. These are removed using quicklime (calcium oxide), which is added to the furnace during the oxygen blow. They react to form compounds such as calcium silicate or calcium phosphate which form a slag on top of the iron.

E. Various basic steel products used –

The various steel products used include Wrought iron, Mild steel, High carbon steel and other specialized steel.

Wrought iron - When all the carbon is removed from the molten iron to give high purity iron, it is known as wrought iron. Wrought iron is quite soft and has little structural strength. It was once used to make decorative gates and railings, but these days mild steel is normally used instead.

Mild steel - Mild steel is iron containing up to about 0.25% of carbon. The presence of the carbon makes the steel stronger and harder than pure iron. The higher the percentage of carbon, the harder the steel becomes. Mild steel is used for lots of things - nails, wire, car bodies, ship building, girders and bridges amongst others.

High carbon steel - High carbon steel contains up to about 1.5% of carbon. The presence of the extra carbon makes it very hard, but it also makes it more brittle.

F. Environmental issues related to Iron making process –

(i) Loss of landscape due to the size of the chemical plant needed.

(ii) Noise.

(iii) Atmospheric pollution from the various stages of extraction. For example: carbon dioxide (greenhouse effect); carbon monoxide (poisonous); sulphur dioxide from the sulphur content of the ores (poisonous, acid rain).

(iv) Disposal of slag, some of which is just dumped – large scale land degradation.

(v) Iron-making uses up huge amounts of coal. The coal is not used directly, but is first reduced to coke which consists of almost pure carbon. The many chemical byproducts of coking are almost all toxic.

G Environment-friendly ‘ITmk3’ process plant for production of iron nuggets by Kobe Steel :

Please refer
(http://minmetandeqip.blogspot.com/2010/01/environment-friendly-itmk3-process.html )

References:

1. http://minmetandeqip.blogspot.com/2008/03/direct-reduction-of-iron-dri-sponge.html
2. http://minmetandeqip.blogspot.com/2008/02/extraction-of-iron-using-blast-furnace.html
3. http://www.chemguide.co.uk/inorganic/extraction/iron.html
4. http://www.tatasponge.com/Operations/Technology.htm
5. http://www.gcsescience.com/ex17.htm

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Source Knol: Extraction of Iron from iron ore

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