Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-01-07
2002-04-30
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S446000, C075S492000, C075S505000, C266S144000, C266S147000, C266S156000
Reexamination Certificate
active
06379420
ABSTRACT:
The invention relates to a method for producing a hot CO— and H
2
-containing reducing gas serving for the reduction of fine-grained metal ore, in particular iron ore, wherein the reducing gas is formed in a gasification zone by a gasification of carbon carriers, in particular coal, taking place under the supply of oxygen and subsequently is cooled down to a reducing-gas temperature favorable to the reduction process, and a plant for carrying out the method.
A method of the initially described kind is known f.i. from EP-A-0 594 557. With this known method, pig iron or a steel pre-product are won by smelting from at least prereduced sponge iron in a meltdown gasifying zone under the supply of carbon carriers and oxygen-containing gas, and a CO— and H
2
-containing reducing gas is generated. The reducing gas forming in the meltdown gasifying zone exhibits a temperature in the range of 1000 to 1200° C. At this temperature, the released hydrocarbon compounds are decomposed. At the same time, the CO
2
and H
2
O contents drop to below 6% CO
2
and 4% H
2
O on account of these temperatures, since they are converted to CO and H
2
.
For utilization in a reduction reactor, this very hot reducing gas has to be cooled prior to introduction into the reduction reactor. In accordance with EP-A-0 594 557 f.i., a spray cooler is provided to that end. The portion of the reducing gas thus cooled is admixed to the reducing gas exiting the melt-down gasifying zone. Such routinely effected cooling of the reducing gas by cooled reducing gas of the same type to roughly 700 to 900° C. prevents the occurrence of incipient melting of the ore particles in the reduction zone during ore reduction, but without causing a decrease in the reduction potential of the reducing gas.
Yet it is disadvantageous that the reducing gas thus cooled is thermodynamically unstable; from the carbon monoxide, carbon dioxide and carbon form in accordance with the Boudouard equilibrium, just as in accordance with the heterogeneous water-gas equilibrium a reaction of carbon monoxide with hydrogen to water and carbon takes place, which reaction is also exothermic, like the reaction described first. This leads to an increase in temperature of the reducing gas and hence to an increase in temperature of the reactor material. There will be formation of agglomerates. Hereby, not only the reduction process is affected but the yield of reactor material from the reduction zone as well. U.S. Pat No. 5,185,032 describes a method in which the hot reducing gas formed in a melter gasifier is cooled to a temperature of 900 to 950° by injection of water.
FR-A-2 236 951 discloses a method in which the hot reducing gas formed in an electric furnace is fed into a reduction shaft located directly above the electric furnace and upon entry into the reduction shaft is cooled by blowing in water, water vapor, carbon dioxide, hydrocarbons or other cooling media to prevent agglomeration of metal-oxide-containing material in the reduction shaft. The content in CO
2
and H
2
O of the thus cooled reducing gas is relatively high.
FR-A-766 167 describes a method in which the hot reducing gas formed in a melting aggregate is fed directly into a reduction chamber, and, in doing so, is cooled in the dome area of the melting aggregate, i.e. even before it is fed into the reduction chamber, either by feeding spent reducing gas after removal of carbonic acid or by feeding a mixture of carbonic acid or water vapor and coal, so as to prevent agglomeration of the charge material in the reduction chamber.
The invention aims at avoiding these disadvantages and difficulties and has as its object to provide a method of the initially described kind and a plant for carrying out the method, enabling a reducing gas to be produced lying in a temperature range that is favorable to the reduction of the metal ore, hence lying below the temperature at which instances of incipient melting and fouling may occur in the at least partially reduced metal ore. Moreover, the H
2
O/CO
2
— content of the reducing gas is to be optimized and, further, a chemical attack on the metallic materials of the gas-carrying systems, that is, reactors and gas conveying ducts, built-in structures etc., is to be avoided.
With a method of the initially described kind this object is achieved in that by the addition of H
2
O and/or CO
2
— in order to prevent the Boudouard and heterogeneous water-gas reaction and a resultant heating of the reducing gas and thus of the metal ore—a reducing gas which has been subjected to a cooling operation that does not effect an addition of H
2
O/CO
2
to the reducing gas is converted to a reducing gas that is thermodynamically more stable at the reducing-gas temperature.
By selectively adding H
2
O and/or CO
2
, the thermodynamically conditioned decomposition of the reductants CO and H
2
is selectively influenced or prevented. In the reducing gas, ranges of concentration are adjusted at which the Boudouard and heterogeneous water-gas reaction, which is strongly exothermic, is suppressed, so that an interfering temperature increase in the reducing gas cannot take place. At the same time, the degree of oxidation of the reducing gas is controlled and the chemical attack on metallic materials suppressed by this method.
Advantageously, amounts of H
2
O and/or CO
2
are added until the Boudouard and heterogeneous water-gas equilibrium of the reducing gas at the temperature favorable to the reduction process is almost attained.
Preferably, cooling of the reducing gas can be effected by feeding cooling gas of the same type and/or top gas.
Suitably, the addition of H
2
O is effected by feeding water vapor and the addition of CO
2
is effected by feeding a CO
2
-containing gas.
In accordance with a preferred embodiment, feeding of CO
2
into the reducing gas can at least partially be effected in that a reducing gas reacted in the reduction process, so-called top gas, is fed into the reducing gas. Other CO,-containing gases, f.i. from a CO
2
-purification, may also be employed.
To attain intensive cooling of the reducing gas, cooled reducing gas of the same type is advantageously admixed to the reducing gas, as is known per se from the prior art, and H
2
O and/or CO
2
are added into the cooled reducing gas of the same type.
A plant for carrying out the method, comprising at least one reduction reactor having a conveying duct for metal ore and a reducing-gas duct running into it, comprising a gasification reactor having feed ducts for carbon carriers and oxygen-containing gases running into it and the reducing-gas duct departing from it, and comprising a cooling means which is provided in the reducing-gas duct and does not effect an addition of H
2
O/CO
2
to the reducing gas, is characterized in that a CO
2
source and/or H
2
O source is (are) flow-connected with the reducing-gas duct conducting a reducing gas which has been subjected to cooling.
Advantageously, the reduction reactor is provided with a top-gas discharge duct carrying off reacted reducing gas from which a branch duct departs that is flow-connected with the reducing-gas duct.
Another preferred embodiment is characterized in that from the reducing-gas duct a reducing-gas recycle duct via a scrubber and a compressor runs into the reducing-gas duct again, but viewed in the gas flow direction at a position upstream of the branching-off point of the reducing-gas recycle duct, particularly upstream of the position of a dedustifying means provided in the reducing-gas duct, and that a CO
2
source and/or H
2
O source is connected with the reducing-gas recycle duct. A reactor comprising a stationary fluidized bed, a Venturi fluidized bed, a circulating fluidized bed or a cascade of cyclones may be provided as the reduction reactor.
The invention will now be described in greater detail with reference to an exemplary embodiment represented schematically in the drawing, wherein the Figure schematically represents an advantageous embodiment of a plant according to the invention.
Reference numeral
1
designates a preheating reac
Brunnbauer Günther
Kastner Walter Rainer
Kepplinger Leopold Werner
Mizelli Herbert
Wurm Johann
Andrews Melvyn
Ostrolenk Faber Gerb & Soffen
Voest-Alpine Industriean-lagenbau GmbH
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