Direct reduced iron pellets

Details Direct reduced iron pellets Direct reduced iron pellets

1999-12-06

2002-01-29

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Solid treating composition for liquid metal or charge

C075S320000, C075S765000, C075S766000, C075S962000

Reexamination Certificate

active

06342089

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to direct reduced iron pellets. More particularly, the present invention relates to direct reduced iron pellets that are produced with a novel binder that allows the direct reduced iron pellets to be at least partially formed from by-products of iron and steelmaking processes.
Iron oxide-containing materials used in iron and steelmaking processes, such as taconite, typically contain high concentrations of gangue in addition to the iron oxide. It is known that the iron concentration may be enriched by finely grinding the iron oxide-containing materials and then magnetically separating the iron oxide from the gangue. The refined, finely ground iron is then mixed with a binder and formed into pellets which are referred to as green pellets.
Green pellets commonly have a relatively low compressive strength that makes it difficult to handle the green pellets without the green pellets experiencing a high degree of degradation. However, firing green pellets has been found to increase the compressive strength of the pellets and harden the pellets.
Firing green pellets in a reducing atmosphere, such as a mixture of carbon monoxide and hydrogen, not only strengthens the green pellets but also converts iron oxide in the pellets to metallic iron. Pellets containing high concentrations of metallic iron are referred to as direct reduced iron pellets. The highly metallized nature and high purity of direct reduced iron pellets makes it desirable to use direct reduced iron pellets for production of steel by smelting in electric furnaces or various other techniques.
Prior art methods for producing direct reduced iron require the use of high grade iron as well as large amounts of gaseous reductants, such as natural gas, to produce a direct reduced iron product possessing a high degree of metallization. The prior art methods for producing direct reduced iron pellets also require the use of vertically oriented furnaces to reduce pellet degradation during the firing process.
For example, MacKay et al. U.S. Pat. No. 4,725,309 and Becerra-Nova et al. U.S. Pat. No. 5,078,787 disclose feeding the green pellets into the top of a vertical shaft moving bed furnace. A reducing gas, such as natural gas, is then injected into the furnace in a direction that is substantially countercurrent to the flow of pellets in the furnace. Contact between the green pellets and the reducing gas causes iron oxide in the green pellets to be metallized and thereby converts the green pellets into direct reduced iron pellets.
Bueno C. et al. U.S. Pat. No. 5,078,788 discloses a similar process for fabricating direct reduced iron pellets. However, Bueno C. et al. includes a preliminary step of cracking a heavy hydrocarbon oil to produce a methane-rich gas, which is used to reduce the iron oxide to metallic iron.
Other processes for converting green pellets into direct reduced iron pellets use coal to drive the metallization process. Attempts have been made to use rotary kiln furnaces with coal-based reduction processes because it is possible to operate rotary kiln furnaces in a cost effective manner. However, the use of rotary kiln furnaces in coal-based reduction processes has previously been undesirable because tumbling of pellets in the rotary kiln furnace causes a significant portion of the pellets to degrade into fines. The degradation of pellets into fines is undesirable because fines lead to the formation of accretions on the walls of the rotary kiln furnace. Build up of accretions eventually reduces the productivity of the rotary kiln furnace to an extent that the rotary kiln furnace must be shut down so that the accretions may be removed.
To minimize the formation of accretions in rotary kiln furnaces, prior art processes commonly use induration techniques to hardened the green pellets prior to firing. However, indurated pellets must be fired for longer periods of time to obtain desirable metallization levels because induration increases the density of the pellets. This factor alone dramatically reduces the rate at which direct reduced iron pellets may be produced using coal-based reduction processes in rotary kiln furnaces.
Coal-based direct reduced iron processes have also been used with rotary hearth furnaces. While pellet degradation is reduced in rotary hearth firing processes, rotary hearth firing processes require capital and maintenance costs that are substantially greater than rotary kiln furnaces.
Another consideration when preparing direct reduced iron pellets is the ability to fabricate the pellets using by-products from iron and steelmaking processes. Iron and steelmaking processes annually generate millions of tons of by-products that must either be recycled or disposed. These by-products are commonly referred to as reverts and include a variety of furnace dusts and sludges. While it is desirable to reuse the reverts because the reverts contain high levels of iron and carbon, directly recycling reverts in a commercially viable manner has not previously been possible because reverts typically contain small but significant concentrations of heavy metals, such as lead and zinc.
Landfilling reverts is undesirable because of uncertainty regarding potential toxic affects caused by heavy metals leaching from the landfilled material. The ability of steelmaking companies to landfill certain forms of reverts, such as electric are furnace dust, is further limited because high lead levels have caused the Environmental Protection Agency (hereinafter EPA) to list some of these reverts as hazardous waste.
Another important consideration in producing green pellets with desirable compressive strength characteristics is the selection of an appropriate binder. While it is desirable to use a strong binder that precludes pellet degradation, it is desirable to select a binder that minimizes the introduction of impurities into the pellet because any impurities introduced into the pellets must be removed at some point in the steelmaking process.
One of the most commonly used binders in the production of green pellets is bentonite. Various other materials have been used as binders in the fabrication of green pellets. For example, Ruckl, U.S. Pat. No. 3,212,877 describes a binder that is prepared from a carbonaceous material that softens but remains viscous during the firing process. Anthes et al. U.S. Pat. No. 3,326,668 discloses using fibrous material, such as peat moss, as a binder. Laydell, U.S. Pat. No. 3,811,865 discloses using tree bark as a binder. Vadis, U.S. Pat. No. 4,474,603 teaches using Gyttja as a binder.
Preparing a binder from a finely divided shale, which is obtained from washing coal, is taught in Endell, U.S. Pat. No. 3,525,784. Using a double salt of potash-magnesia as a binder is described in Adams et al., U.S. Pat. No. 3,819,360.
None of the preceding binders, when used alone or in combination, have allowed the production of direct reduced iron pellets that possess desirable compressive strength, metallization, and residual carbon levels when the direct reduced iron pellets are fabricated with a significant concentration of reverts.
SUMMARY OF THE INVENTION
The present invention includes a method for producing direct reduced iron pellets. The method includes pelletizing an agglomeration of iron oxide-containing material, internal reductant, and binder to form green pellets. The method also includes a step of firing the green pellets in a furnace to convert the green pellets into the direct reduced iron pellets.
The binder used in the present invention includes a non-combustible fibrous material. The binder has a sufficient binding capacity to maintain the integrity of the pellet throughout the firing process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention includes a process for producing direct reduced iron pellets. The direct reduced iron pellets are prepared by metallizing green pellets. The green pellets are produced from an agglomeration of iron oxide-containing material, internal reductant, and binder.
The method of the pr

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