Developments In Direct Reduced Iron
Developments In Direct Reduced Iron (DRI)
Direct reduced iron (DRI) is produced through the solid state reduction of iron oxides derived from iron ore or electric arc furnace (EAF) fines for the use as a virgin iron source in the EAF or basic oxygen furnace (BOF) processes. Virgin iron sources are needed in the EAF process to dilute the residuals (Cu, Ni, Cr, Mo, or Va) present from previous steel making operations in the scrap steel used as the primary raw material used in the EAF. DRI is produced in many different processes using several different fuels and different feed stocks. The DRI processes use several reaction vessels including shaft furnaces, rotary hearth furnaces, fluidized bed reactors, moving bed reactors, and rotary kilns. The fuels used in the processes are primarily coal or natural gas, these fuels are used to create a reducing atmosphere and elevated temperature to create a more favorable reaction. The quality of the DRI is measured by the amount of metallic iron (Fe or Fe3C) is present in the product. This number is called metallization and is reported in a percentage of the total mass of the product. The products of the DRI processes are either pelletized into DRI or briquetted into hot briquetted iron (HBI).
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The reduction of the iron ore or EAF fines takes place in several reactions that reduce the iron oxides to metallic iron (eq. 1 and 2). The reducing gases are produced by combusting natural gas or a carbon source then adding the heated combustion products to the reaction vessels.
Fe2O3 + 3H2(g) → 2Fe + 3H2O (eq. 1)
Fe2O3 + 3CO(g) → 2Fe + 3CO2 (eq. 2)
Hydrogen and carbon monoxide are formed from combusting natural gas, while only carbon monoxide is formed when coal or coke is combusted. The difference between the processes is how the feed stock is exposed to the combustion gases.
The most common type of reaction vessel is the shaft furnace where the feed stock is fed into the top of the furnace then dropped through the vessel to be exposed to the reducing gasses then extracted from the bottom of the furnace. The originator of using the shaft furnace for DRI production is the Midrex corporation and is known as the Midrex Process. The Midrex process uses a reformed natural gas as the reduction gas and requires the use of pelletized iron ore as a feed stock. The natural gas is combusted in a reforming vessel then fed into the shaft furnace where it is mixed with additional natural gas and oxygen to create some more chemical energy for the process (figure 1). The flue gas is then fed through a post combustion chamber and the energy extracted from post combustion is used to preheat the feed gas. This gas is then fed to a bag house where it is scrubbed before being reintroduced to the atmosphere.
This process creates primarily DRI pellets due to the feed stock being pelletized iron ore and there not being a need to change the geometry through briquetting. A fault of the Midrex process is its dependence on pelletized ore. The process requires the feed stock to contain no more than 3% fines. The DRI pellets produced have a high metallization of 95% on average, making it the highest quality DRI. The use of the post combustion allows the Midrex process to gain the majority of the available energy and is what has made it favorable for the majority of worldwide DRI production
A faster production method and one that has more flexibility than the shaft furnace is the rotary hearth furnace or the Fastmet process. The rotary hearth furnace is a continuous operation that feeds material into a rotating furnace that passes the material through the reducing atmosphere then removes the material near when it completes the full rotation (figure 2). Unlike the Midrex process the Fastmet process uses fines as a feedstock. This allows for the recycling of EAF fines and mill dust that would be otherwise disposed of as a hazardous material. The Fastmet process uses a carbon reducing agent and oxygen burners. The carbon can be from numerous sources such as coal, carbon bearing wastes, and coke. The process is rarely run using coke due to the high cost of the coke and the process being capable of running on lower quality carbon mixes. The process requires a pelletizing or a briquetting operation due to the feed stock being fines. The entire process requires between six and twelve minutes to complete. The rotary hearth furnaces produce a direct reduced iron with a metallization ranging from 85-92% depending on the quality of the feed stock.
Fluidized bed reactors are a batch reactor that introduces the material into the reactor then the reducing gasses are fed into the bottom of the vessel with enough pressure to float the feed material. This floating of the feed material allows for all of the surface area of the material to be exposed to the reducing gasses. There are several forms of fluidized bed reactors being used in the market today. The difference between the reacting vessels is the number of reacting vessels and the type of fuel used to creating the reducing gases. The first type of fluid bed reactor is the Finmet process utilizing iron ore fines or EAF dust as a feed material and natural gas as a reducing fuel. This process uses up to a four stage reactor with the progressive stages using a higher gas velocity and a lower reaction time to gain between 91% and 93% metallization (figure 3). The Finmet process requires a briquetting operation because the feed stock is fines but because of needing a high purity natural gas fuel it produces a very low residual HBI.
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The next two processes, Circofer and Circored, both feature a two stage fluidized bed reactor with the first stage being a short retention time vessel with a high gas velocity and the second stage a long retention time with a low gas velocity (figure 4). The difference between the two is that the Circofer process uses metallurgical coal for a fuel while the Circored process uses natural gas. Both of these reach an average 92% metallization and feed pellets.
Hsysla steel developed a moving bed reactor to create DRI (figure 5). The Hyl process feeds lump iron into the process and a high hydrogen content reformed natural gas. The high hydrogen is created by reforming with a nickle-based catalyst. The Hyl process uses an elevated temperature and pressure to increase the processing time for the reactions. The high hydrogen and the elevated temperature and pressure create a high quality DRI with 93% average metallization.
The final reactor vessel is the Allis Chalmers controlled Atmosphere Reactor (ACCAR). The ACCAR uses a counter flow rotary kiln. The rotary kiln process uses a low quality but highly reactive coal to create reducing gasses (figure 6). The rotary kiln produces a DRI with a 92% metallization. The reacting vessel does not utilize any of the post combustion in the process but post combustion vessels have been added to the process to create enough energy to power the entire facility and add some back to the grid.
With DRI being produced in many different processing methods there are some key features that set some appart from the others. The rotary kiln, shaft furnace, and the moving bed reactor vessels produce the highest metallization. The rotary hearth furnace has the fastest process time. The most popular on the market right now is the Midrex shaft furnace with nearly 60% of the market share of DRI production (table 1). DRI is a material that EAF operations have come to depend on and will continue to increase use due to the high cost of pig iron and the continued recycling of scrap steel.
Table 1) Comparative summary of processes as of 2008
Low quality Coal
Pellets and Energy
Fluid bed reactor
Moving bed reactor
Pellets or briquettes
two phase fluid bed
Pellets or briquettes
two phase fluid bed
Pellets or briquettes
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