Direct Reduced Iron (DRI)


Direct Reduced Iron (DRI)


DRIOverview

Direct-reduced iron (DRI), also called sponge iron,[1] is produced from direct reduction of iron ore (in the form of lumps, pellets or fines) by a reducing gas produced from natural gas or coal. The reducing gas is a mixture, the majority of which is hydrogen (H2) and carbon monoxide (CO) which act as reducing agents. This process of reducing the iron ore in solid form by reducing gases is called direct reduction.

The conventional route for making steel consists of sintering or pelletization plants, coke ovens, blast furnaces, and basic oxygen furnaces. Such plants require high capital expenses and raw materials of stringent specifications. Coking coal is needed to make a coke strong enough to support the burden in the blast furnace. Integrated steel plants of less than one million tons annual capacity are generally not economically viable. The coke ovens and sintering plants in an integrated steel plant are polluting and expensive units.

DRI (internal structure)2Direct Reduced Iron (DRI) is a manufactured metallic material produced by the reduction (removal of oxygen) of iron oxide at temperatures below the melting point of iron (1536° C or 2797° F). The iron oxide in either lump, conceptrate, or pellet form is reduced at 800-1050 ° C (1472-1922 ° F) by interaction with reductants (H2+CO) derived from natural gas or coal.

DRI lump, pellets, and cold-molded briquettes have an apparent density of less than 5.0 g/cm3 (312 lbs/ft3). Cold-molded briquettes are formed at a temperature less than 650° C (1202° F).

Hot Briquetted Iron (HBI) is a compacted form of DRI with enhanced physical characteristics, which make it ideal for handling, shipping, and storing as a merchant product. See HBI page for more details.

DRI fines are generated during the production and handling of DRI and HBI and can have up to 75 percent metallic content. Particle size can range from less than 6.35 mm (0.25 inch) to 12 mm (0.5 inch). Moisture content can be as much as 12 percent (by weight).


Typical Chemistry of DRI

DRI
* Depends on the iron oxide source
* Gangue is the term for the oxide minerals (i.e. CaO, Al2O3, SiO2, MgO) that remain in the DRI and are removed in the steel slag.


DRI Basics

For a description of the basic fundamentals of direct reduction and the production of DRI, click here.

(Based on the Direct Reduction Fundamentals and Applications Short Course presented by Roy Whipp, President of Whipp Technology, Inc.)


Process

Direct reduction, an alternative route of iron making, has been developed to overcome some of these difficulties of conventional blast furnaces. DRI is successfully manufactured in various parts of the world through either natural gas or coal-based technology. Iron ore is reduced in solid state at 800 to 1,050 °C (1,470 to 1,920 °F) either by reducing gas (H2+CO) or coal. The specific investment and operating costs of direct reduction plants are low compared to integrated steel plants and are more suitable for many developing countries where supplies of coking coal are limited.

The direct reduction process is energy efficient, but most competitive with the Blast Furnace (BF) when it can be integrated with Electric Arc Furnace (EAF) to take advantage of the latent heat produced by the DRI product[2], more information on this in “DRI – The EAF Energy Source of the Future?”

Factors that help make DRI economical:

  • Direct-reduced iron has about the same iron content as Pig Iron (MPI), typically 90–94% total iron (depending on the quality of the raw ore) as opposed to about 93% for molten pig DRI (HYL)2iron, so it is an excellent feedstock for the electric furnaces used by mini mills, allowing them to use lower grades of scrap for the rest of the charge or to produce higher grades of steel.
  • Hot-briquetted iron (HBI) is a compacted form of DRI designed for ease of shipping, handling, and storage.
  • Hot Direct Reduced Iron (HDRI) is iron not cooled before discharge from the reduction furnace, that is immediately transported to a waiting electric arc furnace and charged, thereby saving energy.
  • The direct reduction process uses pelletized iron ore or natural “lump” ore. One exception is the fluidized bed process which requires sized iron ore particles. Few ores are suitable for direct reduction.
  • The direct reduction process can use natural gas contaminated with inert gases, avoiding the need to remove these gases for other use. However, any inert gas contamination of the reducing gas lowers the effect (quality) of that gas stream and the thermal efficiency of the process.
  • Supplies of powdered ore and raw natural gas are both available in areas such as Northern Australia, avoiding transport costs for the gas. In most cases the DRI plant is located near natural gas source as it is more cost effective to ship the ore rather than the gas.
  • this method produces 97% pure iron.


Direct Reduction Process (HBI & DRI)
HBI-DRI Available Processes
HBI-DRI Available Processes_2

DRI/HBI Process PDF can be found here.

Problems

Directly reduced iron is highly susceptible to oxidation and rusting if left unprotected, and is normally quickly processed further to steel.[citation needed] The bulk iron can also catch fire since it is pyrophoric.[3]

History

Producing sponge iron and then working it was the earliest method used to obtain iron in the Middle East, Egypt, and Europe, where it remained in use until at least the 16th century. There is some evidence that the bloomery method was also used in China, but China had developed blast furnaces to obtain pig iron by 500 BCE.

The advantage of the bloomery technique is that iron can be obtained at a lower furnace temperature, only about 1,100°C or so. The disadvantage, relatively to using a blast furnace, is that only small quantities can be made at a time.


4000 Years Later…A History of Direct Reduction of Iron Ore.

“The modern era of direct reduction began on December 5, 1957 when the Hylsa 1M HYL Process plant first started production. Back then, total world steel production amounted to just under 300 illion tons. The electric arc furnace share of the market was barely 8%, or about 24 million tons. And of course none of that involved use of direct reduced iron. At least, not until the last month of that year.” ….more

Copyright © 1997 – Hylsa, S.A. de C.V., HYL Technology Division



Uses

Sponge iron is not useful by itself, but can be processed to create wrought iron. The sponge is removed from the furnace, called a bloomery, and repeatedly beaten with heavy hammers and folded over to remove the slag, oxidise any carbon or carbide and weld the iron together. This treatment usually creates wrought iron with about three percent slag and a fraction of a percent of other impurities. Further treatment may add controlled amounts of carbon, allowing various kinds of heat treatment (e.g. “steeling”).

Today, sponge iron is created by reducing iron ore without melting it. This makes for an energy-efficient feedstock for specialty steel manufacturers which used to rely upon scrap metal.

References

1 “What is direct reduced iron (DRI)? definition and meaning”. Businessdictionary.com. Retrieved 2011-07-11.
2 R. J. Fruehan, et al. (2000). Theoretical Minimum Energies to Produce Steel (for Selected Conditions)
3 Hattwig, Martin; Steen, Henrikus (2004), Handbook of explosion prevention and protection, Wiley-VCH, pp. 269–270, ISBN 978-3-527-30718-0.
4 Wikipedia.org



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