Direct Reduction: Process Description

 

Courtesy of Midrex Direct Reduction Corporation.

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In a direct reduction process, lump iron oxide pellets and/or lump iron ore, are reduced (oxygen removed) by a reducing gas, producing direct reduced iron (DRI). If the cooling stage is omitted, the DRI can be immediately briquetted into hot briquetted iron (HBI). The reducing gas can be generated externally to the reduction furnace, or can be generated from hydrocarbons introduced into the reduction zone of the furnace. In the former case, the reducing gas is produced from a mixture of natural gas (usually methane) and recycled gas from the reducing furnace. The mixture is passed through catalyst tubes where it is chemically converted to a gas that is rich in hydrogen and carbon monoxide. Examples of processes that use variations of this general procedure include Midrex and HYL. When the reducing gas is generated from hydrocarbons in the reduction zone of the furnace, it is typically a rotary kiln furnace that uses hydrocarbon fuels (primarily coal, but sometimes oil and natural gas) without prior gasification in the reduction chamber. Examples include the ACCAR and SL/RN processes.

Glossary

DRI – also known as direct reduced iron, is a virgin iron source that is relatively uniform in composition, and virtually free from tramp elements. It is used increasingly in electric furnace steelmaking to dilute the contaminants present in the scrap used in these processes. It has an associated energy value in the form of combined carbon, which has a tendency to increase furnace efficiency. For captive DRI production facilities, there is the added advantage that the delivery of hot DRI to the furnace can reduce energy consumption 16 to 20%.

Processes
Midrex – The charge is fed in continuously from the top of the furnace, passing uniformly through the preheat, reduction, and cooling zones of the furnace. The reducing gas consists of about 95% combined hydrogen plus carbon monoxide. It is heated to a temperature range of 1400° to 1700°F and is fed in from the bottom of the furnace, below the reducing section. The gas flows countercurrent to the descending solids. At the top of the furnace, the partially spent reducing gas (approximately 70% hydrogen plus carbon monoxide) exists and is recompressed, enriched with natural gas, preheated to 750°F, and transported to the gas reformer. The reformer reforms the mixture back to 95% hydrogen plus carbon monoxide, which is then ready for re-use by the direct reduction furnace. In the cooling zone, the cooling gases flow countercurrent to the DRI. At the top of the cooling zone, the cooling gases exit, are sent to recycling, then return to the bottom of the cooling zone. The cooled direct reduced iron (DRI) is discharged through the bottom of the furnace, after which it is screened for removal of fines, and treated to minimize the danger of spontaneous ignition during extended storage. The reduced fines are briquetted to produce a usable DRI product.

HYL – The HYL process uses reformed natural gas to reduce lump ore and fixed pellets in fixed-bus retorts. Prior to the gas reformer, natural gas is mixed with excess steam (over and above stoichiometric requirements) and is then passed over nickel-based catalysts. The excess steam server to prevent carbon formation and promote catalyst life. After the reformer, the water vapor in the reformed gas is removed by quenching, to achieve a hydrogen-rich reducing gas. The HYL process utilized four reactors in the reducing section. The reduction of the charge occurs in an initial and main reduction stage, while the third stage is used for cooling, carburization, and the final adjustment of metallization. The charge reduction takes place at temperatures above 1800°F, the advantages of this being that the reduction efficiency is raised, and the result is a more stable product with a reduced pyrophoric tendency. The HYL process uses cold reducing natural gas for product cooling as well as carburization. Product cooling occurs at temperatures of around 1020°F, during which time carbon is deposited to form a cementite (Fe2C) shell that retards reoxidation.

ACCAR – The Allis-Chalmers Controlled Atmosphere Reactor (ACCAR) was designed to produce highly metallized DRI in an intricately ported rotary kiln. The liquid and/or gaseous fuels are injected under the bed, and air above it. The charge (e.g. coal, lump ore, and/or iron oxide pellets) is heated to reduction temperature by counter-flowing hot gas. The carbon and carbon monoxide reducing gases are formed from the hydrocarbons present in the reduction zone, and initiate the reduction. Additional liquid and/or gaseous fuel introduced through the kiln shell ports near the product end of the kiln brings about the final degree of reduction. The product is then discharged into a rotary cooler which is externally spray-cooled. Magnetic separation is used to separate the DRI and coal ash, followed by screening to separate the coarse and fine product.

SL/RN – The charge, preheated to 1800°F by counter-flowing freeboard gases, usually consists of lump ore (or pellets), coal, recycled char, and flux if sulphur need to be removed from the coal. Reduction is brought about by reducing gases generated from hydrocarbons present in the reduction section. In order to raise kiln efficiency, the preheat zone is usually limited to 40 to 50% of total kiln length. To ensure a uniform temperature throughout the reduction zone, and to prevent accredation formation due to overheating in hot zones, all the coal is introduced at the feed end of the kiln. As the charge moves into the reduction zone, reduction begins when it has reached roughly 1650°F. After reduction, the solids are discharged into a sealed rotary cooler, where water is sprayed on the cooler shell to reduce the temperature of the solids to about 200°F in a non-oxidizing atmosphere. The cooled material is then separate into DRI, DRI fines, and non-magnetics by a series of screens and magnetic separators. The fines are briquetted to be used later with the DRI.

 

 

The Making, Shaping, and Treating of Steel, 10th Edition, Association of Iron and Steel Engineers, 1985.

Davis, C.G., J.F McFarlin, and H.R. Pratt, “Direct Reduction Technology and Economics”, Ironmaking and Steelmaking, Vol. 9, No. 3