Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
1999-08-02
2001-08-07
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
C075S387000, C075S490000
Reexamination Certificate
active
06270550
ABSTRACT:
This invention relates to the direct reduction of pelletized iron concentrates or lump iron ore in a shaft furnace using a reducing gas.
Known and industrially proven technologies for such direct reduction utilize countercurrent flow of charge materials and reducing gas, and utilize external reformers or gasifiers and sulfur cleaning equipment to produce the reducing gas. This equipment is expensive and requires special catalysts, typically nickel based, for its operation, and thus has significant impact on the economics of the operation.
Attempts have accordingly been made to eliminate such external reforming equipment from the direct reduction process by effecting reforming within the furnace itself, with iron from the charge itself acting as a catalyst.
The concept of the countercurrent AREX (trademark) shaft furnace process as described in U.S. Pat. No. 5,064,467, is to partially oxidize a mixture of natural gas and process gas in a single pass through the system. Preheated streams of a mixture of recycled and natural gas, and oxygen enriched air, are mixed in an external combustion chamber and partially react there. During the conversion of natural gas by enriched air, heat and reducing gas are produced and the products of partial combustion exit the chamber with a temperature about 1000-1100° C. This gas is injected to the reforming zone of the shaft furnace where the reforming reactions occur by the catalytic action of the metallic iron component of the hot direct reduced iron that is present. The reducing process gas which is generated passes in countercurrent through the charge for the preheating and reduction of the iron oxides which are being fed down through the shaft furnace.
Difficulties which may occur with this process are:
a) the likelihood of burden material softening and sticking in the shaft since the reducing gas entering the metallization zone is at high temperature;
b) lower product quality (due to sticking) and difficulties in controlling the carbon content in the iron produced;
c) the increase (in comparison with conventional processes) of the volume of reducing gas used in the process due to lower concentration of reducing agents (CO and H
2
);
d) the increase (in comparison with conventional processes) of reducing gas volume due to the necessity to compensate for the heat of natural gas conversion in metallization zone.
A method for virgin iron production described in Russian Patent No. 739120 utilizes the concept of natural gas self reforming in the metallization zone without preliminary partial oxidation.
In this process, prereduction and even partial metallization (up to 40-60% metallization) is carried out in a prereduction zone in flow of a rich fuel gas produced in external burners in which the air consumed is about 0.3-0.5 of stoichiometric value. Final metallization occurs in a metallization zone in cocurrent flow with a preheated (up to 600-800° C.) mixture of natural gas and recycled gas. The material temperature in this zone drops from 900-1000° C. to 700° C.
Disadvantages of this process are:
a) the necessity to incorporate a carbon monoxide remover to remove carbon monoxide from the recycled gas since a significant amount of this gas is created during the process by reactions of water vapour with carbon and carbon dioxide with carbon. This also increases the total reducing gas volume entering the metallization zone;
b) rich fuel gas flows from the prereduction zone to the final metallization zone, which decreases the reduction potential of the reducing gas and requires a greater gas volume;
c) gas losses occur from the prereduction zone which cause an increased requirement for reducing gas for metallization;
d) no means is disclosed to control the carbon content in the final product.
These difficulties with prior art techniques for direct reduction in shaft furnaces with natural gas self reforming in the metallization zone are believed to render such processes non-competitive compared with conventional processes using external reforming of natural gas. Reduction of fuel usage and recycled gas volume for the metallization step in known processes using natural gas reforming in the metallization zone is difficult because of resulting lack of heat in the metallization zone and lowering of the quality of the iron produced, the significant volume of carbon monoxide produced, losses of reducing gas to the prereduction zone, and passage of rich gas to the metallization zone.
The present invention provides a method for effecting direct reduction of iron oxides in the form of lumps or pellets, comprising passing an ore burden downwardly successively through prereduction and metallization zones of a shaft furnace while passing a flow of rich fuel gas produced by external partial combustion upwardly in counter current through the prereduction zone, and passing a flow of reducing gas in cocurrent through the metallization zone in an independent flow that has not been subject to external combustion, wherein an intermediate zone is provided forming a narrowed throat between the prereduction and metallization zones, and wherein the reducing gas is at a boundary between the intermediate and metallization zones, whereby passage of rich fuel gas from the prereduction zone to the metallization zone is inhibited.
REFERENCES:
patent: 4179282 (1979-12-01), Rubio
patent: 4834792 (1989-05-01), Becerra-Novoa
patent: 5064467 (1991-11-01), Dam G. et al.
patent: 739120 (1980-06-01), None
patent: 173 18 22 (1992-05-01), None
patent: 1756362 (1992-08-01), None
Blank Michael
Gordon Iakov
Andrews Melvyn
Hatch Associates Ltd.
Shlesinger & Arkwright & Garvey LLP
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