View source for Direct reduced iron

{{Short description|Iron metal made from ore without use of a blast furnace}}
[[File:Hot-briquetted iron.JPG|thumb|Hot-briquetted iron]]
'''Direct reduced iron''' ('''DRI'''), also called '''sponge iron''',<ref>{{cite web |url=http://www.businessdictionary.com/definition/direct-reduced-iron-DRI.html |title=What is direct reduced iron (DRI)? definition and meaning |publisher=Businessdictionary.com |access-date=2011-07-11 |archive-date=2018-01-04 |archive-url=https://web.archive.org/web/20180104205144/http://www.businessdictionary.com/definition/direct-reduced-iron-DRI.html |url-status=dead }}</ref> is produced from the direct [[reduction (chemistry)|reduction]] of [[iron ore]] (in the form of lumps, pellets, or fines) into iron by a [[reducing atmosphere|reducing gas]] which contains elemental [[carbon]] (produced from [[natural gas]] or [[coal]]) and/or [[hydrogen]].  When hydrogen is used as the reducing gas no carbon dioxide is produced. Many ores are suitable for direct reduction.

Direct reduction refers to solid-state processes which reduce iron oxides to metallic iron at temperatures below the melting point of iron. Reduced iron derives its name from these processes, one example being  heating iron ore in a furnace at a high temperature of {{convert|800|to|1200|C}} in the presence of the reducing gas [[syngas]], a mixture of hydrogen and [[carbon monoxide]], or pure hydrogen.<ref>{{cite web|url=http://metallics.org.uk/dri/|title=Direct reduced iron (DRI)|date=14 November 2019|publisher=International Iron Metallics Association}}</ref>[[File:DRI evolution.svg|400px|thumb|Production of direct-reduced iron and breakdown by process]]
== Process ==
Direct reduction processes can be divided roughly into two categories: gas-based and coal-based. In both cases, the objective of the process is to remove the [[oxygen]] contained in various forms of iron ore (sized ore, concentrates, pellets, mill scale, furnace dust, etc.) in order to convert the ore to metallic iron, without melting it (below {{convert|1200|C}}).

The direct reduction process is comparatively energy efficient. [[Steel]] made using DRI requires significantly less fuel, in that a traditional blast furnace is not needed. DRI is most commonly made into steel using [[electric arc furnace]]s to take advantage of the heat produced by the DRI product.<ref>R. J. Fruehan, et al. (2000). [https://www1.eere.energy.gov/manufacturing/resources/steel/pdfs/theoretical_minimum_energies.pdf Theoretical Minimum Energies to Produce Steel (for Selected Conditions)]</ref>

=== Benefits ===
Direct reduction processes were developed to overcome the difficulties of conventional [[blast furnace]]s. DRI plants need not be part of an integrated steel plant, as is characteristic of blast furnaces. The initial capital investment and operating costs of direct reduction plants are lower than integrated steel plants and are more suitable for developing countries where supplies of high grade coking coal are limited, but where steel scrap is generally available for recycling.{{Cn|date=December 2022}} Many other countries use variants of the process.

Factors that help make DRI economical:

* Direct-reduced iron has about the same iron content as [[pig iron]], typically 90–94% total iron (depending on the quality of the raw ore) so it is an excellent feedstock for the electric furnaces used by [[steel mill#Minimill|mini mill]]s, 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 DRI that is transported hot, directly from the reduction furnace, into an electric arc furnace, 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.
* 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. The use of natural gas also produces greenhouse gases.
* 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 a natural gas source as it is more cost effective to ship the ore rather than the gas.
* To [[fossil fuel phase-out|eliminate fossil fuel use]] in iron and steel making, [[renewable hydrogen]] gas can be used in place of [[syngas]] to produce DRI and eliminate production of greenhouse gases.<ref>{{cite web |title = Steel making today and tomorrow |url = http://www.hybritdevelopment.com/steel-making-today-and-tomorrow | archive-url = https://web.archive.org/web/20201220220736/http://www.hybritdevelopment.com/steel-making-today-and-tomorrow | archive-date = 20 December 2020}}</ref>

=== Problems ===

Direct reduced iron is highly susceptible to [[oxidation]] and [[rusting]] if left unprotected, and is normally quickly processed further to steel.<ref>{{Cite web |title=Direct Reduced Iron (DRI) - Cargo Handbook - the world's largest cargo transport guidelines website |url=https://www.cargohandbook.com/Direct_Reduced_Iron_(DRI)#:~:text=Directly%20reduced%20iron%20is%20highly,processed%20to%20create%20wrought%20iron. |access-date=2022-06-18 |website=www.cargohandbook.com}}</ref> The bulk iron{{Page needed|date=December 2022}} can also catch fire (it is [[pyrophoric]]).<ref>{{Citation |last1 = Hattwig |first1 = Martin |last2 = Steen |first2 = Henrikus |title = Handbook of explosion prevention and protection |pages = 269–270 |publisher = Wiley-VCH |year = 2004 |url = https://books.google.com/books?id=ws3q0zBGLV8C&pg=PA269 |isbn = 978-3-527-30718-0 |postscript =.}} (dead link 24 October 2019)</ref> Unlike [[blast furnace]] [[pig iron]], which is almost pure metal, DRI contains some [[Silicon dioxide|siliceous]] [[gangue]] (if made from scrap, not from new iron from direct reduced iron with natural gas), which needs to be removed in the steel-making process.

== History ==
{{Main|Bloomery}}

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.

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

== Chemistry ==
The following reactions successively convert [[hematite]] (from [[iron ore]]) into [[magnetite]], magnetite into [[ferrous oxide]], and ferrous oxide into [[iron]] by reduction with [[carbon monoxide]] or [[hydrogen]].<ref>{{Cite web|url=https://www.midrex.com/wp-content/uploads/MIdrex_Process_Brochure_4-12-18.pdf|title=MIDREX}}</ref>

{{block indent|<chem>3 Fe2O3 + CO/H2 -> 2 Fe3O4 + CO2/H2O</chem>}}
{{block indent|<chem>Fe3O4 + CO/H2 -> 3 FeO + CO2/H2O</chem>}}
{{block indent|<chem>FeO + CO/H2 -> Fe + CO2/H2O</chem>}}

[[Carburizing]] produces [[cementite]] (Fe<sub>3</sub>C):
{{block indent|<chem>3 Fe + CH4 -> Fe3C + 2H2</chem>}}
{{block indent|<chem>3 Fe + 2CO -> Fe3C + CO2</chem>}}
{{block indent|<chem>3 Fe + CO +H2 -> Fe3C + H2O</chem>}}

== Economy ==
[[India]] is the world’s largest producer of direct-reduced iron.<ref>{{cite web |date=2023 |title=2023 World Direct Reduction Statistics |url=https://www.midrex.com/wp-content/uploads/MidrexSTATSBook2023.Final_.pdf |access-date=4 September 2024 |publisher=Midrex Technologies}}</ref> 

== Uses ==
Sponge iron is not useful by itself, but can be processed to create [[wrought iron]] or steel. The sponge is removed from the furnace, called a bloomery, and repeatedly beaten with heavy hammers and folded over to remove the slag, [[redox|oxidize]] any carbon or carbide, and [[welding|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]].

=== Food ===
Hydrogen-reduced iron is used as a source of food-grade [[iron powder]], for [[food fortification]] and for [[oxygen scavenger|oxygen scavenging]]. This elemental form is not absorbed as well as ferrous forms,<ref>{{Cite journal|doi=10.1093/ajcn/82.6.1276|title=Comparison of the efficacy of wheat-based snacks fortified with ferrous sulfate, electrolytic iron, or hydrogen-reduced elemental iron: Randomized, double-blind, controlled trial in Thai women|year=2005|last1=Zimmermann|first1=Michael B.|last2=Winichagoon|first2=Pattanee|last3=Gowachirapant|first3=Sueppong|last4=Hess|first4=Sonja Y.|last5=Harrington|first5=Mary|last6=Chavasit|first6=Visith|last7=Lynch|first7=Sean R.|last8=Hurrell|first8=Richard F.|journal=The American Journal of Clinical Nutrition|volume=82|issue=6|pages=1276–1282|pmid=16332661|doi-access=free}}</ref> but the oxygen-scavenging function keeps it attractive. Purity standards for this use were established in 1977.<ref>{{Cite journal|url=https://doi.org/10.1021/jf60211a044|doi=10.1021/jf60211a044|title=Specifications for reduced iron as a food additive|year=1977|last1=Shah|first1=Bhagwan G.|last2=Giroux|first2=Alexandre|last3=Belonje|first3=Bartholomeus|journal=Journal of Agricultural and Food Chemistry|volume=25|issue=3|pages=592–594|pmid=858856}}</ref>

== See also ==
* [[Krupp-Renn Process]]
* [[Blast furnace]]
* [[Pig iron]]
* [[Steel mill]]
* [[Direct reduction]]

== References ==
;Notes
{{Reflist}}
;Bibliography
* Valipour MS, and Saboohi, Y, "[http://iopscience.iop.org/0965-0393/15/5/008 Numerical investigation of nonisothermal reduction of hematite using Syngas: the shaft scale study]", Modelling Simul. Mater. Sci. Eng. 15(5), p.&nbsp;487, 2007.
* Grobler, F. and Minnitt, R.C.A "[http://www.hannansreward.com/reports/120964-990304-Increasing-iron-role.pdf The increasing role of direct reduced iron in global steelmaking]", The Australasian Institute of Mining and Metallurgy.

== External links ==

{{Iron and steel production}}
{{Authority control}}

[[Category:Hydrogen technologies]]
[[Category:Iron Age Europe]]
[[Category:Iron]]
[[Category:Metallurgical processes]]