Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-10-30
2002-12-03
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S484000
Reexamination Certificate
active
06488737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for reducing metallic oxides, particularly iron oxides, and to a device for implementing the method.
2. Discussion of the Background
The direct reduction of metallic oxides, particularly ores but also various metallic oxides to be recycled, has developed considerably in recent years.
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 and then, after the coal has been coked, 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 of the furnace and mix the coked coal with the ore as the rotating hearth rotates. After the reaction, the charge is discharged through a central shaft.
One of the disadvantages of the present state of the art is that the volatile constituents of the coal do not take part in the reduction of the metallic oxides. This method does not make it possible to obtain either a high productivity or a high degree of uniformity as regards the temperature and the material of the charge.
SUMMARY OF THE INVENTION
The objective of the present invention is to propose a method for reducing metallic oxides making a more efficient use of the reducing capacities of the volatile constituents of a carbonaceous reducing agent.
In conformity with the invention, this objective is attained by a method for the reduction of metallic oxides in a furnace with a ring-shaped rotating hearth in which a carbonaceous reducing agent and metallic oxides are deposited in a strip on a part of the said rotating hearth and are then transported in a roughly helical movement to a discharge device, characterised in that the reducing agent is preheated and mixed with the preheated metallic oxides before or during their deposition on the rotating hearth, in that, in a first reducing stage, the volatile components of the carbonaceous reducing agent (mainly methane and hydrogen) are used to initiate the reduction of the metallic oxides and in that, in a second reducing stage, carbon monoxide is used.
Unlike the methods in the present state of the art, the method according to the invention uses a part of the volatile constituents of the carbonaceous reducing agent, particularly methane and hydrogen, for their reducing capacity.
The method according to the invention makes it possible to increase the reaction rates by a mixing of the metallic oxides and the carbonaceous reducing agent by efficient use of the reducing capacities of the volatile constituents of the carbonaceous reducing agent by their forced passage through the preheated mixture that forms the furnace charge.
One of the advantages of this method lies in the fact that the volatile components, i.e. the distillation gases from the carbonaceous reducing agent, are used in a first stage to reduce the metallic oxides, whereas in known methods these gases are burnt and are used to heat the solid materials.
The metallic oxides are therefore reduced in two stages or by at least two different chemical reactions.
The first reducing stage is carried out using the hydrogen and/or methane released during the heating of the carbonaceous reducing agent. The reaction kinetics of these reactions are more favourable than those of carbon monoxide at temperatures below 900° C. The aforesaid volatile constituents are progressively released and make contact with the metallic oxides deposited on the furnace hearth under operating conditions, particularly as regards the reaction temperature, such that they participate in the reduction of the said oxides.
The metallic oxides and the reducing gases released make contact at temperatures as high as possible, but without upsetting the progress of the reduction process. The carbonaceous reducing agent is preferably preheated to a temperature up to 200° C., while the metallic oxides are preferably preheated to a temperature up to 850° C.
The two constituents are preferably preheated by means of heat recovered from the combustion gases discharged from the furnace into heat exchangers.
These operating conditions lead to an increase in the production capacity per unit surface area and to a reduction in the quantity of carbon dioxide discharged into the atmosphere per unit quantity of the reduced metallic oxides obtained.
This method also has the advantage of discharging less dust outside the furnace thanks to a control over the speed of these gases while keeping the volume of the furnace to a minimum. The metallic sponge obtained has, in bulk, a better homogeneity in the degree of reduction than products resulting from known techniques.
An excess of at least 10% of carbonaceous reducing agent is preferably used, this excess being defined with respect to the theoretical quantity necessary for the reduction of the oxides.
According to a particular form of execution, a method is proposed for the direct reduction of metallic oxides in a rotating-hearth furnace, in which, on a part called the charging zone of the hearth over a certain width of the ring, which depends on the diameter and the capacity of the furnace, a charge consisting of several layers is deposited. These layers may be deposited simultaneously or successively.
The concentrations of metallic oxides and carbonaceous reducing agent in the layers may be different. Preferably, the concentration of metallic oxides in the upper layers is greater than the concentration of metallic oxides in the lower layers. The lower layers consequently contain an excess of carbonaceous reducing agent. The concentration of carbonaceous reducing agent in the upper layers is therefore less than that in the lower layers. In such a case, there is a kind of gradient in the concentration of metallic oxides, a concentration that increases from the hearth in the direction of the upper surface of the charge. A greater quantity of volatile constituents is therefore released in the deep layers and these gases diffuse through the layers towards the upper surface of the charge, where these volatile constituents encounter a higher concentration of metallic oxides. Since the temperature of the lower layers is lower than the temperature of the upper layers, the volatile constituents of the carbonaceous reducing agent are progressively released in the lower layers and, during their diffusion towards the upper surface, encounter very hot metallic oxides. In fact, the upper layers are hotter than the lower layers, firstly because the upper layers contain a higher concentration of metallic oxides preheated to higher temperatures than the carbonaceous reducing agent and secondly because these layers are in contact with the furnace atmosphere. The volatile constituents therefore participate more effectively in the reduction of the metallic oxides.
Advantageously, the concentration of carbonaceous reducing agent in the lower layer lies between the theoretical concentration necessary for the complete reduction of the metallic oxides and a concentration of 100% by weight, preferably between 30% and 70% by weight and, in particular, preferably between 35% and 60% by weight.
The concentration of carbonaceous reducing agent in the upper layer is preferably less than 25% by weight, and, in particular, is preferably less than 16% by weight.
According to an advantageous form of execution, the charge is heated inside the furnace up to a temperature of 900-1250° C. and preferably 1050-1150° C.
It is worthwhile using the metallic oxides at a feed temperature as high as possible while avoiding the agglomeration of the metallic oxides.
Advantageously, the mixture of carbonaceous reducing agent and metallic oxides, or the charge, is turned over and progressively mixed during its residence inside the furnace.
According to another preferred form of execution, the surface of the charge is shaped by forming furrows or hummocks on it to pr
Andrews Melvyn
Sidmar N.V.
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