Fluidized Bed Reduction

Fluidized-bed processes currently are used extensively in industrial applications. Process variations include fluidized-bed combustion, gasification, heat treatment, and catalytic reactions. Attractive characteristics of fluidized-bed processes include high, uniform heat transfer between gas and solid and high reaction rates due to the large exposed surface area of the fluidized-bed solid material. Fluidized-bed oxidation of metals and reduction of metal oxides are possible, but defluidization caused by sticking, as well as the pyrophoricity of reduced powders, has restricted industrial usage.

This article describes the processes of fluidized-bed reduction of iron oxides that were used from 1958 to 1970. Oxide reduction, atomization, electrolytic deposition, and carbonyl processes have replaced the fluidized-bed process for current iron powder production.

Typically, in the fluidized-bed process a granular material is kept in fluid motion by using a fluidizing gas. In many cases, the fluidizing gas reacts chemically with the granular solid. Thus, in fluidized-bed oxidation, the fluidizing gas typically is air or oxygen. In fluidized-bed reduction, the fluidizing gas contains hydrogen or carbon monoxide, or a mixture of the two.

Principles of Fluidized Reduction Processes. Many of the principles of fluidized-bed reduction were developed as a by-product of research aimed at direct ironmaking, that is, bypassing the reduction of iron ores in a blast furnace and replacing this procedure with continuous fluidized-bed reduction of iron ore. Iron processed in this manner is referred to as direct-reduced iron. To a great extent, these principles also apply to the reduction of high-purity iron ore or iron oxide, where the reduced iron is used in metallurgical applications without further processing.

For direct-reduced iron production, ore feed in the form of pellets is preferred (Ref 2). Fully oxidized pellets, such as hematite (Fe2O3) pellets, should be used, because magnetite (Fe3O4) forms a relatively compact iron shell during reduction that strongly resists gas diffusion into the pellets, thus increasing reduction time (Ref 3). Pellets made from rich magnetite also disintegrate during reduction because of the high degree of swelling (Ref 4).

Sticking of the particles during reduction is perhaps the most serious problem encountered in fluidized-bed reduction of metal oxides, because it results in defluidization of the bed. The susceptibility of iron ore to sticking is a complex relationship between percent reduction, bed temperature, and gas velocity (Ref 5). By reducing low-gangue ore, Agarwal et al. (Ref 5) found that the susceptibility to defluidization is highest between about 620 and 730 °C (1150 and 1350 °F) for products reduced more than 90%. Below about 620 °C (1150 °F), the tendency to defluidize is minimal, but the resulting product is usually pyrophoric. At temperatures above 700 °C (1300 °F), the tendency to defluidize decreases.

Similar findings were observed by Bondarenko et al. (Ref 6) who established that the zone of fluidization expands by using larger pellets and increasing gas velocity and temperature from 550 to 900 °C (1020 to 1650 °F). Evidently, high temperature decreases surface activity of reduced particles, and sticking is decreased.

The extremely high surface area of direct-reduced iron contributes to rapid reoxidation (pyrophoric iron). The reoxidation of direct-reduced iron (the reaction of iron with oxygen and water) is a rapid process in which spontaneous ignition of iron powder may occur.

The tendency of direct-reduced iron to reoxidation depends on reduction temperature. The higher the temperature, the less the degree of reoxidation. Raw material composition also has an effect on the oxidation or degree of reactivity (Ref 7). According to Jensen (Ref 8), the presence of cementite (Fe3C) has a passivating effect on oxidation at temperatures up to 150 °C (330 °F). At higher temperatures, this material oxidizes faster due to cementite decomposition. As discussed in Ref 8, high-temperature reoxidation may be prevented successfully by:

• Passivation during the manufacture of the product

• Keeping the product dry prior to use

• Avoiding contact with hot objects or other sources of thermal energy above about 200 °C (390 °F)

Advantages of reduction in a fluidized bed include:

• Effective interaction of gases and solids in the bed and high rates of heat and mass transfer between the solid and fluid allow high velocity of reactions.

• Fluidized-bed reduction is an isothermic process, which allows a narrow temperature range to be maintained.

• Gas recirculation is possible.

• Reduction in a fluidized bed can be converted into a fully automatic, continuous process.

Disadvantages include:

• Not all iron ore materials can be reduced in a fluidized bed because of the tendency of these particles to stick together.

• Particle size is limited from about 6.5 mm (0.25 in.) to a few microns, or about 0.001 in.

• Swelling of iron ore materials during reduction can result in disintegration and excessive loss of particles from the bed by the transport of material of the reaction zone (carryover).

• Pyrophoricity hinders storage and transport of reduced iron.

• A sharp temperature gradient exists near the bottom of the bed (Ref 9), which causes overheating of the grid.

• Countercurrent rates of fluid throughput are limited to the range over which the bed may be fluidized, the minimum fluidization velocity, or the carryover velocity.

• The hydrodynamics of fine materials is mismatched to the reduction process, because fluidization occurs at a gas velocity lower than that required for reduction.

Critical Fluidization Velocity. Critical velocity is the reduction gas velocity at which the fluidization of material is achieved without carryover from the reaction zone. References 10, 11, 12, 13, 14, 15, 16, 17, and 18 discuss empirical expressions for the parameters of the critical fluidization velocity. The derivations consider particle diameter, density of the solid and fluid, viscosity, and voidage.

Nu-Iron Process. Developed by the United States Steel Company in 1950 (Ref 19), the Nu-iron process uses fine iron ore (1.65 mm, or 0.065 in.) in the fluidized-bed reactor at temperatures of 600 to 700 °C (1110 to 1290 °F), which eliminates sticking and pyrophoricity. The reducing gas (73% H and 16% CO) is obtained by catalytic conversion of natural gas with steam. Before entering the reactor, iron ore is preheated to 375 °C (705 °F) by the off gas; then it is preheated to 925 °C (1695 °F) in a special furnace. The sponge iron, reduced to 90 to 95%, is briquetted and used in electric steelmaking furnaces.

The H-iron process was developed by Hydrocarbon Research Company and Bethlehem Steel Corporation. The process consists of reducing fine iron ore or mill scale (0.04 to 0.8 mm, or 0.0016 to 0.032 in.) by high-pressure hydrogen at 35 atm (515 psi) at a temperature of 540 °C (1000 °F).

Hydrogen is obtained by the water-steam or oxygen re-forming of natural gas. High hydrogen pressure is used to increase reduction velocity and decrease reactor size. To increase hydrogen utilization, three- and four-zone reactors are used, with capacities of 45 to 90 metric tons/year (50 to 100 tons/year). The reduced iron powder is annealed at 650 to 870 °C (1200 to 1600 °F) to render the powder nonpyrophoric. Depending on the reduction degree, the resulting reduced iron can be used for P/M structural parts (90 to 95% reduction degree) or in electric steelmaking furnaces (75% reduction degree).

ONIA Process. Developed in France, the ONIA (Office National Industriel d'Azote) process differs from the H-iron process. A low pressure of 5 atm (73.5 psi) is used. The composition of the reducing gas is 85 to 87% H and 14 to 16% CO. To avoid sticking, the process uses a reduction gas velocity significantly greater than the critical fluidization velocity. The sponge iron is used as a melt stock in electric furnaces and as a molding-grade powder.

Commercial Viability. As discussed in Ref 20, all of the fluidized-bed processes have been limited commercially because of sticking, high degrees of pyrophoricity due to extremely high microporosity, unsatisfactory quality of iron powder for molding applications, or a combination of these. Sticking can be eliminated by using a "semifluidized" bed in a reactor with pull-out grids (Ref 21). In this process, iron ore pellets (2 to 6 mm, or 0.08 to 0.24 in.) are reduced by the products of catalytic conversion of natural gas (35% H, 18% CO, 40% N).

Pellets are reduced in a two-stage fluidized-bed process. At first, a reduction degree of r = 30% is reached. After this stage, the fluidized bed sticks, forming a sponge cake. The second reduction stage (r = 95 to 99%) is accomplished in a semifluidized pulsating bed. Gas velocity during the impulse fluidization substantially exceeds the critical fluidization velocity, provided that the cake breaks and that there is a uniform gas supply. After cooling by nitrogen, the reduced iron is not pyrophoric because of high reduction temperatures of 800 to 870 °C (1470 to 1600 °F).

Laboratory experiments with a drum-type rotary fluidized bed also have shown methods of eliminating sticking (Ref 22). The advantages of fluidized-bed technology can be realized by use of an "improved" process that combines the fluidized bed with a mechanical or hydraulic device for breaking the sticking material. The technical feasibility to expand and develop fluidized-bed reduction processes depends on:

• Possibility to increase the temperature level of reduction with a simultaneous elimination of sticking and pyrophoricity

• Possibility to increase gas utilization efficiency

• Elimination of temperature gradient between the top and bottom of the fluidized bed

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