METHOD FOR THE DIRECT REDUCTION OF IRON ORE

Abstract

The invention relates to a process for direct reduction of iron ore to sponge iron, wherein the iron ore passes through a reduction zone for reducing the iron ore to sponge iron. The reduction zone is subdivided into a pre-reduction zone supplied with a first reduction gas and into an end-reduction zone supplied with a second reduction gas. The first reduction gas has a different gas composition compared to the second reduction gas. A first reduction gas has a hydrogen proportion at least 5% by volume higher compared to the second reduction gas.

Claims

1. A process for direct reduction of iron ore to sponge iron, wherein the iron ore passes through a reduction zone for reducing the iron ore to sponge iron, wherein the reduction zone is subdivided into a pre-reduction zone supplied with a first reduction gas and into an end-reduction zone supplied with a second reduction gas, wherein the first reduction gas has a different gas composition compared to the second reduction gas, wherein a first reduction gas having a hydrogen proportion at least 5% by volume higher compared to the second reduction gas is used.

2. The process as claimed in claim 1, wherein the first reduction gas has a hydrogen proportion of at least 55% by volume.

3. The process as claimed in claim 2, wherein the first reduction gas consists of hydrogen.

4. The process as claimed in claim 3, wherein the first reduction gas is heated to a temperature between 500 and 1200 C.

5. The process as claimed in claim 4, further comprising using a second reduction gas having at least one of (i) a higher proportion of at least one compound or mixture of carbon and hydrogen and (ii) at least one compound or mixture of carbon and oxygen compared to the first reduction gas.

6. The process as claimed in claim 5, wherein the second reduction gas comprises at least one of (i) at least one compound or mixture of carbon and hydrogen and (ii) at least one compound or mixture of carbon and oxygen in a proportion of at least 55% by volume.

7. The process as claimed in claim 6, wherein the second reduction gas comprises at least one of (i) at least one compound or mixture of carbon and hydrogen and (ii) at least one compound or mixture of carbon and oxygen in a proportion of at least 70% by volume.

8. The process as claimed in claim 7, wherein as a result of the carbon-containing compound or mixture the second reduction gas brings about a carburizing of the pre-reduced iron ore in the end-reduction zone.

9. The process as claimed in claim 8, wherein the carbon content of the sponge iron after passage through the end-reduction zone is in the range from 0.5% by weight to 3.5% by weight.

10. The process as claimed in claim 9, wherein the second reduction gas is heated to a temperature between 700 and 1300 C.

11. The process as claimed in claim 10, wherein the sponge iron passes through a cooling zone arranged downstream of the reduction zone which is supplied with a cooling gas.

12. The process as claimed in claim 11, wherein the reduction zone above the cooling zone is arranged in a shaft furnace and the iron ore passes through the shaft furnace in a vertical direction.

13. The process as claimed in claim 11, wherein the reduction zone comprises a pre-reduction zone and an end-reduction zone comprising at least one fluidized bed reactor in each case.

14. The process of claim 13 wherein the cooling zone comprises one or more fluidized bed reactors.

Description

[0024] The invention is more particularly elucidated with reference to the following exemplary embodiments in conjunction with the figures. The sole FIG. 1 shows an example of a process according to the invention with reference to a schematic representation of a shaft furnace.

[0025] FIG. 1 elucidates the invention using the example of a shaft furnace (10). Iron ore (io) is introduced at the upper end of the shaft furnace (10). The produced sponge iron (si) is with-drawn at the lower end of the shaft furnace (10). The shaft furnace (10) has a reduction zone (11) comprising a pre-reduction zone (12) and an end-reduction zone (13) and optionally a cooling zone (14) arranged in it. The reduction zone (11) is arranged above the optional cooling zone (14). The cooling zone (14) is not mandatory if hot employment of the hot sponge iron directly exiting the reduction zone (11) is possible and/or the second reduction gas (23) introduced into the end-reduction zone (12) comprises at least one carbon-containing compound or mixture which not only further reduces the pre-reduced iron ore by reaction in the end-reduction zone (13) of the reduction zone (11) but can also simultaneously achieve sufficient carburization to allow supply to the subsequent processes with the required carbon content. The first reduction gas (22) as well as the second reduction gas (23) are passed through the iron ore in the reduction zone (11) in countercurrent and thus counter to a direction of motion of the iron ore. The second reduction gas (23) is passed through a gas heater (33) and heated to a temperature of up to 1300 C. before introduction. The second reduction gas (23) comprises a make-up gas gas (NG) from a source comprising at least one compound or mixture of carbon and hydrogen and/or at least one compound or mixture of carbon and oxygen, wherein it is preferable to employ natural gas having a very high proportion of hydrocarbon-containing compounds or mixtures, methane (CH.sub.4). The make-up gas (NG) may be mixed with a reformed gas (RG) which is worked up from the process gas (40) discharged from the reduction zone (11) of the shaft furnace (10). The discharged process gas (40) may be composed of unused reduction gas from any gaseous reaction products. The discharged process gas (40) may comprise hydrogen (H.sub.2), at least one compound or mixture of carbon and oxygen (CO, CO.sub.2) and/or at least one hydrogen-containing compound (H.sub.2O) and unavoidable impurities. The discharged process gas (40) may be supplied to a first process step in which at least one compound or mixture of the process gas and/or at least portions of the unavoidable impurities are separated and/or removed, for example in a unit for process gas purification and dedusting in which at least a portion of the unavoidable impurities are removed from the discharged process gas (40). In a further process step the process gas may be passed through a unit, for example through a condenser, and correspondingly cooled so that the steam (H.sub.2O) present in the process gas is condensed and thus removed from the process gas. The condensing and discharging of the condensate dehumidifies the process gas. A portion of the dehumidified process gas or the entirety of the dehumidified process gas, shown as a dashed line, may be used as gas (sub)stream a) for firing the gas heater (32, 33). Should insufficient dehumidified process gas be available an appropriate fuel gas for firing the gas heater (32, 33) is provided in whole or in part. If a portion of the dehumidified process gas or the entirety of the dehumidified process gas is not provided for firing the gas heater (32, 33) it is possible to separate carbon dioxide (CO.sub.2) from the dehumidified process gas in a further process step, for example in a scrubber. The separated carbon dioxide may be employed as cooling gas (24) or a portion of the cooling gas (24) in an optional cooling zone (14). However, the process gas freed of carbon dioxide may alternatively also be used, in whole or in part, shown as a dashed line, as gas (sub)stream b) for firing the gas heater (32, 33). Should insufficient gas (sub)stream b) be available an appropriate fuel gas for firing the gas heater (32, 33) is provided in whole or in part. In addition or as an alternative the process gas/reformed gas (RG) freed of carbon dioxide may also be returned to the direct reduction in a further process step by mixing it with the make-up gas (NG), in particular before the mixture is heated to a temperature between 700 and 1300 C. in the gas heater (33). It is possible, optionally and thus indicated by a dashed line, to additionally admix the hot reduction gas with oxygen (O.sub.2) to increase the reactivity of the second reduction gas (23) in the end-reduction zone (13) of the reduction zone (11) and thus the heat input.

[0026] The first reduction gas (22) introduced into the pre-reduction zone (12) of the reduction zone (11) compared to the second reduction gas (23) has a hydrogen proportion at least 5% by volume higher compared to the second reduction gas (23) and especially has a hydrogen proportion of at least 55% by volume. It is particularly preferable when the first reduction gas (22) consists of hydrogen (H.sub.2). The first reduction gas (22) may be heated to a temperature between 500 and 1200 C. in a gas heater (32) before introduction into the pre-reduction zone (12).

[0027] After exiting the reduction zone (11)/the end-reduction zone (13) the sponge iron enters the optional cooling zone (14). The sponge iron has a temperature in the range from 500 to 800 C. In the cooling zone (14) cooling gas (24) is also passed through the sponge iron counter to the direction of motion of the sponge iron. Unconsumed cooling gas, together with any gaseous reaction products, is discharged again as process gas (25). A certain proportion of the cooling gas (24) may enter the end-reduction zone (13). A certain proportion of the second reduction gas (23) may likewise enter the cooling zone (14). Mixtures of cooling gas (24) and reduction gas (23) can therefore occur at the transition between the end-reduction zone (13) and the cooling zone (14). The cooling gas (24) especially comprises a carbon-containing compound or mixture, preferably carbon dioxide (CO.sub.2) or methane. Hydrogen (H.sub.2) may, if required, be ad-mixed with the cooling gas (24), as a result of which the cooling gas (24) undergoes the Bosch reaction in the presence of the hot sponge iron as catalyst in the cooling zone (14). Hydrogen (H.sub.2) and carbon dioxide (CO.sub.2) in the cooling gas thus react according to the reaction

CO.sub.2+2H.sub.2.fwdarw.C+2H.sub.2O

to afford steam (H.sub.2O) and carbon (C), wherein the carbon is deposited on the sponge iron serving as catalyst. The steam with other gaseous reaction products is discharged as process gas (25) from the cooling zone (14) of the shaft furnace (10). The deposited carbon then diffuses into the interior of the sponge iron and forms cementite (Fe.sub.3C). This effect increases the carbon content of the sponge iron (si) to 2.0% by weight to 4.5% by weight. The sponge iron (si) carburized and cooled in this way may be withdrawn in the lower region of the shaft furnace (10) and sent for further processing in the known manner of steel production.

[0028] The particularly preferred process mode for direct reduction of iron ore (io) to sponge iron (si) provides for the use of hydrogen (H.sub.2) as the first reduction gas (22) which after heating to a temperature between 500 and 1200 C. is introduced into the pre-reduction zone (12) of the reduction zone (11) in a shaft furnace (10). When using hydrogen (H.sub.2) as the first reduction gas (22) the reaction of the iron ore to pre-reduced iron ore in the pre-reduction zone (12) is substantially based on

Fe.sub.2O.sub.3+3H.sub.2.fwdarw.2Fe+3H.sub.2O.

[0029] As the second reduction gas (23) of the particularly preferred process mode provides for natural gas as the make-up gas (NG) which, after heating to operating temperature between 700 and 1300 C., is mixed with oxygen (O.sub.2) if required and introduced into the end-reduction zone (13) of the reduction zone (11) of the shaft furnace (10). When using a make-up gas composed of natural gas (NG) without supply of additional oxygen the reaction is the reaction of the pre-reduced iron ore to sponge iron in the end-reduction zone (13) is substantially based on

3Fe.sub.2O.sub.3+4CH.sub.4.fwdarw.2Fe.sub.3C +2H.sub.2+6H.sub.2O+CO.sub.2+CO.

[0030] A cooling gas (24) composed of carbon dioxide (CO.sub.2) and hydrogen (H.sub.2) may be introduced into the cooling zone (14) and the sponge iron (si) cooled to a temperature below 100 C.

[0031] As shown in FIG. 1 after its dehumidification the process gas (40) discharged from the shaft furnace (10) above the reduction zone (11) is supplied in its entirety as fuel gas/as a portion thereof to the gas heater (33), as indicated by a dashed line, and is not supplied to the make-up gas (NG) and mixed therewith.

[0032] The particularly preferred configuration allows variable optimal adjustment of a direct reduction process with hydrogen (22) and natural gas (23) in variable mixing ratios of 0% to 100% in terms of CO.sub.2 emission, efficiency and the availability of the reduction gases.

[0033] Alternatively and not shown here the invention may also be performed in a cascade of fluidized bed reactors. At least one fluidized bed reactor in each case forms a pre-reduction and end-reduction zone of a reduction zone and depending on the circumstances and if hot employment is not possible at least one further fluidized bed reactor may be used in the cascade as a cooling zone. The iron ore would thus successively pass through the first and the second fluidized bed reactor of the reduction zone and optionally a third fluidized bed reactor of the cooling zone while undergoing stepwise conversion into sponge iron. If required, the last fluidized bed reactor can effect cooling of the sponge iron using cooling gas. The principle substantially corresponds to that of a shaft furnace but distributed over a plurality of fluidized bed reactors instead of a shaft. The number of fluidized bed reactors can be interconnected as required.