Direct reduction of iron (DRI) – ‘Sponge iron’

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.