Steel is used in a wide variety of manufactured products, such as skyscrapers, automobiles and bridges. Ironmaking is the first step in the steelmaking process. The coke oven/blast furnace process, which produces pig iron for steelmaking, requires additional energy to prepare the raw iron ore as sinter and pellets. Also, large amounts of carbon are consumed and emitted as carbon dioxide.
Alternative processes can avoid some of these issues but are limited by low production capacities and raw-material restrictions. Therefore, the U.S. steel industry would benefit from the development of a low-capital-cost process, which is scalable to large capacities that can take advantage of the availability of inexpensive iron-ore concentrate and can use fuels that significantly reduce potentially harmful emissions.
Recently, efforts have been undertaken to improve the process. This project summary offers a look into one such effort.
The project goal is to develop an entirely new transformational process for alternate ironmaking based on the direct gaseous reduction of iron-oxide concentrates in a flash-reduction process. The ultimate objective is to significantly increase energy productivity and reduce environmental emissions, especially CO2, versus the conventional blast-furnace ironmaking route.
The novel flash ironmaking process uses gaseous reducing agents such as natural gas, hydrogen, other syngas or a combination thereof. The proposed technology is to be applied to the production of iron as a feed to the steelmaking process, eventually replacing the blast furnace and other alternative ironmaking processes. The process can also be part of continuous direct steelmaking.
One of the core technical problems facing the U.S. steel industry is how to develop a new technology to produce steel from iron ore. An ideal process would replace the blast furnace and coke oven; would use domestic iron ores, especially concentrates that the U.S. has in abundance; and would greatly reduce energy requirements. It should also be a high-intensity process requiring much less capital investment than the blast-furnace/coke-oven combination, and it must be capable of producing 5,000-10,000 tons of metal per day so that it can support existing steel mills.
The fundamental question regarding the feasibility of adapting the flash furnace to ironmaking concerns the speed of reaction. Can iron-oxide concentrates be completely reduced in the few seconds of residence time typically available in a flash furnace?
A multidisciplinary research team comprised of the American Iron and Steel Institute, ArcelorMittal USA, TimkenSteel, United States Steel Corporation and Berry Metal Company, which fabricated a bench reactor (pictured), was established.
Research performed under the AISI/DOE Technology Roadmap Program (DE-FC36-97ID13554) at the University of Utah using iron-ore concentrates (about 30 µm size) show that 90-99% reduction can be achieved within 2-7 seconds of residence time at temperatures of 1300°C (2372°F) or higher. This was verified by additional laboratory-scale testing. Thus, the question of whether the reduction rate of concentrate particles is fast enough for a flash-reduction process was resolved conclusively in the affirmative.
The studies also established that because of the high temperature and lack of contact between the iron-ore particles in flash furnaces, sticking and fusion of the particles are eliminated. Sticking is a crucial problem that has beset other alternative ironmaking processes and eliminated them as a replacement for the blast furnace.
Additional large-scale laboratory tests supported by AISI under the CO2 Breakthrough Program (2008-2011) included successful operation of an oxyfuel burner as a supply source of both heat and reducing agent in an industrial operation. Detailed material and energy balances showed that flash ironmaking technology can use any of the three possible reductants/fuels or combination thereof. A natural-gas-fired flash smelter will generate only 39% of the CO2 of a blast furnace. Carbon-dioxide emissions using coal and hydrogen are 69% and 4%, respectively. The flash ironmaking process also eliminates the iron-ore pelletizing and indurating steps and, of course, eliminates the need for coke and coke ovens.
As mentioned, a major advantage of flash ironmaking over existing ironmaking processes that use shaft or fluidized-bed furnaces is the elimination of sticking and particle fusion at high temperatures. The ability to use ore fines provides a cost advantage over processes that require ore to be agglomerated into pellets for ironmaking. The fine particles also cut the furnace’s processing time to seconds. This translates to a smaller system for the same output, reducing both capital costs and operating costs. Other potential benefits include improved refractory life and ease of feeding raw materials into the vessel.
The large-scale bench reactor was successfully commissioned in November 2015 (shown). University of Utah researchers are running experimental tests to address equipment issues related to running the large-scale bench reactor at temperature for several hours.
For more information: Contact David Forrest, AMO Technology Manager, Department of Energy, 1000 Independence Ave. SW, Washington, D.C.; tel: 202-586-5725; e-mail: David.Forrest@ee.doe.gov; web: http://energy.gov/eere/amo/advancedmanufacturing-office