Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-08-16
2002-05-28
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S505000, C075S506000, C075S010380, C075S010410, C075S484000
Reexamination Certificate
active
06395057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for production of directly reduced iron in a multi-stage furnace.
2. Discussion of the Background
Production of directly reduced iron takes place in a direct reduction process by reduction of iron oxide with solid or gaseous reducing agents. A carbon carrier, which reacts with carbon dioxide and forms the reduction gas CO at higher temperatures, for example, serves as a solid reducing agent.
A process of this kind can be carried out, for example, in a rotary hearth furnace, i.e. in a furnace with a rotatable annular furnace bottom, which is lined with refractory material on the top side and is enclosed by a casing. Burners, which penetrate the casing and heat the interior of the casing to the required reaction temperature of over 1000° C., are mounted on the top of the casing.
The iron oxide is spread together with the reducing agent at a specific point on the rotary hearth and is introduced by the rotation of the rotary hearth into the interior of the casing, where it reacts with the reducing agent because of the high temperatures and is present as directly reduced iron after about one revolution of the rotary hearth. In this process iron oxide and reducing agent after charging on to the refractory lining of the rotary hearth must first be heated to the required reaction temperature before the actual reduction reaction can begin. This takes place in the area bordering on the charging zone of the rotary furnace in the direction of rotation by heat transfer from the hot waste gases of the burners to the charged materials.
Because of the low thermal conductivity of the charged materials, the heating-up phase takes a considerable time before the required reaction temperature is achieved within the charged material layers. The longer the heating-up phase, the lower is the productivity of the rotary hearth furnace, because the heating rate determines the speed of rotation of the rotary hearth.
The reduction process depends on the concentration of the reduction gases, which are in contact with the ore. However, the gas composition in the individual furnace zones can hardly be affected, because the entire furnace consists only of a single process space. In the conventional processes the diffusion of the CO from the reducing agent to the ore and CO
2
from the ore to the reducing agent thus cannot be affected.
From a certain degree of metallisation onwards the speed of the reduction process diminishes in such a way that the process is usually interrupted when a degree of metallisation of 85-95% is achieved. Uneconomical extension of the process time would be required to reduce the remaining oxides.
The document DE 1 225 673 relates to a process for dry reduction of iron ore in a multi-stage furnace, which has several stages one above the other. The iron ore is charged to the top stage and gradually transferred to the lower stages. In the lower stages (reduction stages) a reducing gas is fed in order to reduce the iron ore. In the upper stages the iron ore is preheated to the required reduction temperature by combustion of the rising reducing gas. Before introduction into the multi-stage furnace a solid reducing agent can be mixed with the iron ore. Part of the reducing gas from at least one of the upper reduction stages is removed and fed into at least one of the lower reduction stages.
A process for production of sponge iron in a multi-stage furnace, which has several stages one above the other, is already known from document U.S. Pat. No. 2,089,782. The iron ore is charged to the top stage and gradually transferred to the lower stages. Solid reducing agent is charged to one of the stages underneath. The iron ore is reduced in the lower stages (reduction stages). The thermal energy required for the reduction is supplied by an electrically heated melt provided under the bottom stage of the multi-stage furnace. In the upper stages the iron ore is preheated by combustion of the reducing gas rising from the reduction stages.
SUMMARY OF THE INVENTION
Consequently the task of the present invention is to propose an alternative process for production of directly reduced iron.
According to the invention this problem is solved by a process for production of directly reduced iron in a multi-stage furnace which has several stages one above the other, a high temperature prevailing in the lower stages and in which
ore is continuously introduced into the multi-stage furnace and deposited on the top stage and gradually transferred to the lower stages;
reducing agent is deposited on the topmost stage and/or on one of the stages underneath it;
a gas containing oxygen is fed into the lower stages and reacts with the reducing agent to form reducing gas, the reducing gas reacting with the ore to form directly reduced iron;
the directly reduced iron is discharged together with residues of reducing agents in the area of the bottom stage of the multi-stage furnace.
An important advantage of the invention is that the process space is subdivided into different zones, the solids move continuously from the top downwards and the gases from the bottom upwards. By subdividing the process space into different zones the process conditions can be measured and controlled in the different zones or even for each stage and selectively .
Solid, liquid or gaseous reducing agents come into consideration as reducing agents.
In this process fine-grained ore can be charged and caking avoided by selective process control and continuous circulation. This is particularly advantageous, if ash-forming reducing agents are used. The separation of the ash of the reducing agent from the iron can be easily carried out. This separation can take place, for example, in the hot stage by screening. After partial cooling below 700° C. it is possible on the other hand to separate the directly reduced iron via magnetic separators from the ash and excess reducing agent. Hence this process can be used, because the continuous agitation in the multi-stage furnace prevents caking of the iron. The directly reduced iron is accordingly produced in fine-grained form and is easily picked up by the magnetic separators. The quality of the directly reduced iron obtained in this way is independent of the quantity of residues of the reducing agent.
The iron obtained can subsequently be processed into pellets or briquettes or introduced directly into a melting furnace (electric furnace etc.) and further processed.
If required, the reducing agent residues produced are burnt in burners with any unused reducing agents and the resulting heat fed to the furnace.
Accordingly a less expensive reducing agent which has a relatively high ash content can be used and/or work carried out with a relatively high excess of reducing agent.
In cases in which it is necessary to work with an excess of reducing agents, it is advantageous to treat the residues in order to separate the unused reducing agents and reuse them. This can be done e.g. by screening the residues, if the unused reducing agents are present in a sufficiently coarse form. The unused reducing agents can be introduced directly into the multi-stage furnace.
However, the charge of reducing agents can also be divided among several stages.
It is thus possible for coarse-grained reducing agents (1-3 mm) to be introduced at a higher point into the multi-stage furnace and fine-grained reducing agents (<1 mm) added at a lower point. Consequently discharge of dust with the exhaust gases is largely avoided and the reaction accelerated by the fine reducing agent particles introduced lower down.
The charging of coarser particles reduces the consumption of reducing agents, because the small particles are consumed faster via waste gases in the upper stages than is necessary for reduction of the iron ore.
According to a preferred embodiment the ore is dried and possibly preheated by the hot gases in the multi-stage furnace before it is fed into the multi-stage furnace and comes into contact with the reducing agent. The ore is preferably
Frieden Romain
Hansmann Thomas
Solvi Marc
Mai Ngoclan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Paul Wurth S.A.
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