Production of steel through the conventional blast furnace route requires consumption of coking coal which has limited reserves in India. As an appropriate alternative to this scarcity, the evolution of a technology for the reduction of iron ore using non coking Coal, which is abundantly available in the country, was contemplated.
The other reason for the popularity of DRI was its suitability to replace scrap as metallic feed in electric steelmaking (EAF/IF), which constitutes ~30% of the world’s steel production. The nonavailability of consistent quality scrap at a reasonable price necessitated the search for an alternative, for use in secondary steel sectors. DRI is not only a substitute for steel scrap as a feed material in EAF/IF but also a more suitable melting stock for good-quality-steel production. DRI has been recognized as a high-quality, cheaper, and high-purity charge material worldwide. In comparison with scrap (residual elements, which cannot be removed during steelmaking, vary between 0.13% and 0.73%), the use of DRI offers consistency in composition and size, as well as low residual elements (0.02%) that apparently improves the steel quality. DRI has specially been found an excellent alternative feed for mini steel plants, allowing them to use lower grades of scrap for the rest of the charge or to produce higher grades of steel.
Iron Ore and non-coking coal are the prime raw materials for the production of sponge iron. These are charged into a rotary kiln in requisite proportion along with some dolomite which acts as a desulphurized. Coal plays a dual role in the process by acting as a reductant as well as a fuel for providing heat to maintain the requisite temperature inside the kiln at 850-10500C. The process reduction occurs in solid-state. The crucial factor in this reduction process is the controlled combustion of coal and its conversion to carbon monoxide to remove oxygen from the iron ore.
The overall production process inside the kiln requires a duration of 10-12 hours during which the iron ore is reduced and discharged to a rotary cooler for cooling below 1000C, to prevent reoxidation and effect a better separation of non-magnetic from sponge iron.
The process of sponge iron making aims to remove the oxygen from iron ore and when the same occurs, the departing oxygen causes micropores in the ore body making it porous. . The final product when observed under a microscope, resembles a honeycomb structure looking “spongy” in texture and hence the name sponge iron. As the metallurgical process involves a solid state reduction, sponge iron is also called Direct Reduced Iron (DRI)
The quality of sponge iron is primarily ascertained by the percentage of metallic content (iron) present in the product. The properties of sponge iron typically produced at TSLP are shown alongside
Sponge iron manufacture is highly sensitive to raw material characteristics. Therefore, it is essential to examine the chemical and physical characteristics of raw materials, both individually and in combination. The primary raw materials for the production of sponge iron are iron ore and non-coking coal. Several tests are conducted in the company’s laboratory to ascertain their suitability in a rotary kiln
Coal: Important factors determining coal quality, besides the chemical properties such as fixed carbon,
ash content, volatile matter, etc., are some of its metallurgical properties viz. reactivity and ash fusion
Coals of higher reactivity are preferred as they permit the operation of the furnace at lower temperatures with higher kiln output. Generally, coals with higher fixed carbons are preferred, since they provide more carbon for reduction and are of higher calorific value. Ash content should be as low as possible, as it affects the kiln throughput and heat requirement adversely. High moisture also reduces kiln productivity and increases the energy requirement. Since sulfur of coals is picked up by DRI, it should be as low as possible. Further, the melting characteristics of ash need to be considered while selecting coals for DRI production.
The entire coal requirement of the company has been linked to the best sources of the subsidiary coal companies of Coal India Limited and also imports from South Africa.
Iron Ore: Since DRI making is a solid-state reduction process, the gangue in the ore is retained in the product. Due to oxygen removal, there is a reduction in weight by about 30% and the percentage of gangue material goes up by about 1.4 times. Hence, it is very important to choose a higher grade of iron ore input. In all DR processes, the only noteworthy chemical change that takes place is the removal of oxygen from the iron oxide in the charged iron ore. Since no melting or refining occurs, all impurities in the ore feed get concentrated in the reduced product. As a result, in any iron ore chosen for the DR process, the total iron content should be as high as possible and the gangue content (silica plus alumina) should not only be a minimum, but the gangue should have a composition that is acceptable to the user of the final product.
The phosphorus content of the iron ore is extremely important and should be as low as possible (preferably below 0.03%) because there is no removal of phosphorus during the reduction process.
Like phosphorus, the sulphur content of the iron ore has to be low (below 0.02%), since during reduction there could be some pickup of sulfur. The sulphur content of many iron ores available in the world is quite low (0.01–0.02). Apart from this, to ensure a better kiln campaign life and output, the iron ore is made to undergo a series of other tests viz. shatter, tumbler & abrasion indices, reducibility, etc.
As for the consumption of kilns in the Gamharia plant, the entire requirement is sourced from the company’s captive iron ore mine near Barajamda. The Joda plant being in the proximity of the country’s best iron ore sources, the entire iron ore for Joda is sourced from Tata Steel’s iron ore mines. In cases of exigency, there are several other iron ore mines in the vicinity which supplies quality iron ore at competitive prices.
Dolomite: Dolomite acts in the process as a desulphurise, removing sulphur from the feed mix during the reduction process. As mentioned, since most iron ores contain very low sulphur (0.01–0.02%), the sulfur pickup in DRI is almost fully from the charged coal; as even the best non-coking coals generally contain 0.5–0.6% sulphur. Dolomite should contain a minimum of 28% CaO, 20% MgO and a maximum of 6% acid insoluble. It is mixed in small proportion along with other raw materials before charging into the kiln. The entire requirement of dolomite is met from the dolomite quarries in Bhutan and Chattisgarh.
There are many factors that have established the popularity of DRI over the years, to a point where it is no more an alternate to scrap but a substitute material…some of its distinct advantages are:
Sponge iron produced at TSLP is distinct due to its high metallic iron content and consistent chemical and physical characteristics. This provides secondary steel makers a flexibility in preparing their furnace charge to produce finer quality steels than what is possible using steel scrap only.
Some of the features of the sponge iron produced at TSLP are: