Metallurgical apparatus – Means for treating ores or for extracting metals – By means applying heat to work – e.g. – furnace
Reexamination Certificate
2000-07-28
2002-09-10
Andrews, Melvyn (Department: 1742)
Metallurgical apparatus
Means for treating ores or for extracting metals
By means applying heat to work, e.g., furnace
C075S484000
Reexamination Certificate
active
06447713
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an improved method for the direct reduction (generally producing what is described as DRI) of metallic oxides, particularly iron oxides, by coal making it possible to achieve a higher productivity and a lower specific consumption of coal. It also relates to an installation for implementing the method and to the metallic sponge capable of being produced.
1. Disclosure of the Invention
The invention aims firstly to increase the reaction rates by a mixing of the charge, by an increase in the efficiency of the radiation from the furnace and by an increase in the surface area for heat exchange with the furnace atmosphere, and secondly to use the reducing capacities of the volatile constituents of the coal more efficiently by their forced passage into a layer of preheated metallic oxides. Together, these operating conditions should also lead to a considerable increase in the production capacity per unit surface area and to a reduction in the production of carbon dioxide discharged into the atmosphere per unit quantity of the reduced metallic oxides obtained.
The invention also has as its objective the discharge of less dust to the gas treatment plants thanks to a control over the speed of these gases while keeping the volume of the furnace to a minimum.
The invention also aims to produce a metallic sponge having, in bulk, a better homogeneity in the degree of reduction compared with products resulting from known techniques.
2. Background Art
The direct reduction of metallic oxides, particularly of ores but also of various recycled metallic oxides, has developed considerably in recent years.
A method for producing an iron sponge with a low sulphur content is described in the document EP-0 692 543-A1.
This document indicates that a non-preheated charge is placed on a mobile hearth, the said charge consisting of superimposed layers of finely divided materials, at least one of the layers consisting mainly of iron oxides and at least one other layer being formed by a mixture of a solid carbonaceous reducing agent and a desulphurising agent. The iron oxides are mainly reduced by means of carbon monoxide from the coal, the volatile constituents of the coal taking part only partially in the reduction of the iron oxides.
Similarly, in the production of iron sponges from pellets formed from a mixture of iron oxides and coal, the iron oxides are reduced principally by means of the carbon monoxide from the coal, the volatile constituents of the coal taking part only partially in the reduction process.
A method is described in the document LU-60981-A (Société Anonyme des Minerais) for producing an iron sponge comprising the use of a continuous rotating-hearth reactor with a displacement of the material from the side to the centre, first supplied with coal alone and then with iron ore, in pellet form or broken up, preheated to the reaction temperature. Fixed scrapers cause a movement of the coal towards the centre at each rotation. After other scrapers have been used to produce a fairly thorough mixing of the heavily coked carbonaceous source and the preheated iron ore, the charge is discharged through a central shaft. The distillation gases from the coal and the reaction gases are partly or completely burnt by heating the solid materials moving in the reactor before being discharged with the entrained fine particles towards a collector located above the reactor.
This method (which is not illustrated by any drawings) does not include efficient means for the best possible use of the reducing properties of the volatile constituents of the coal, the latter being used mainly for their heating capacity by combustion outside the reaction field. Moreover, the formation of the charge by superimposed layers of materials not mixed beforehand and the use of scrapers moving the whole of the charge each time do not enable a high degree of uniformity in the charge to be obtained, either in material or in temperature, nor does it lead to a high productivity.
A method is described in the document U.S. Pat. No. 5,567,224-A (Kundrat) for producing an iron sponge in a rotating-hearth furnace using an oxidising flame located just above the upper surface of the hearth. A mixture of metallic oxides and a carbonaceous reducing agent is placed on the hearth, the said mixture passing in front of the burners during the rotation of the hearth. A second layer of the reducing agent is then charged and the heating is continued. No arrangement for stirring and/or mixing the material is provided for. The discharge takes place towards the periphery and is achieved using an Archimedean screw.
A method is described in the document U.S. Pat. No. 3,383,199-A (Schmidt) for producing an iron sponge by supplying the materials on a conveyor belt in several layers, without any indication of the means for carrying out the mixing of the constituents of each of the different layers within the layer or the mixing of the different layers with each other.
A method is described in the document U.S. Pat. No. 3,770,417-A (Kranz) for the production of an iron sponge at the same time as the production of coke based on the use of a distillation chamber placed above a heated and perforated mobile hearth. The coal, after distillation, is added to a charge of oxides in the form of a second layer in order to bring about the reduction. Scrapers mounted in the roof of the reactor stir the material and may be used to discharge it towards a central exit.
A method is described in the document U.S. Pat. No. 3,475,286-A (Kemmerer) for producing an iron sponge based on the use of a rotary furnace with a conical hearth provided with fixed scrapers mounted in the roof of the reactor, which move the material towards the centre. Provision is made for arranging the scrapers in such a way that the thickness of the bed of materials on the hearth is kept constant.
The document “The Comet Process—DRI from fines and coal” published in Steel Times, vol. 224, no 11, November 1996, page 399 describes the principle of a rotating-hearth furnace developed by the Applicant in collaboration with the Center for Metallurgical Research in Liege (Belgium).
All the methods described in these documents have the same disadvantages as those mentioned in connection with the above patent LU-60981-A, i.e. the volatile constituents of the coal participate only partially in the reduction of the metallic oxides and the methods do not make it possible to obtain a high productivity or a high degree of uniformity as regards the temperature and the material of the charge.
Characteristic Elements of the Invention
The invention relies on the observation that, in most of the methods of the present state of the art, a major part of the volatile constituents of the coal, particularly the hydrogen and the methane, are not used for their reducing capacity, this reduction being mainly carried out by carbon monoxide, whose reduction kinetics are significantly less favorable than those of hydrogen. It therefore seems that it would be particularly advantageous for the aforesaid volatile constituents to be progressively released and, by their forced passage through a layer containing the metallic oxides, for them to be put into contact with the metallic oxides under operating conditions (in particular, as regards the temperature of the metallic oxides and the successive mixtures of reagents) such that they participate in the reduction of the latter. This implies that the metallic oxide and the released reducing gases are put into contact at temperatures as high as possible, but without upsetting the progress of the reduction process. To achieve this, the coal will be preheated to a temperature of the order of 200° C., while the metallic oxides will be preheated to a temperature of the order of 800° C., both constituents being preheated by means of heat recovered from the discharged combustion gases using the same heat exchangers.
The invention concerns an improved method for the direct reduction of metallic oxides in a continuous rotating-hearth furnace, char
Andrews Melvyn
Sidmar N.V.
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