Chemistry of inorganic compounds – Carbon or compound thereof – Binary compound
Reexamination Certificate
2000-03-15
2003-09-30
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Binary compound
C423S148000
Reexamination Certificate
active
06627171
ABSTRACT:
This invention relates to a process of producing iron carbide (Fe
3
C) from granular sponge iron, which comes from an iron ore reduction plant with a carbon content of not more than 2 wt-%.
From the U.S. Pat. Nos. 5,527,379 and 5,603,748 the direct reduction of iron oxide is known, where in several fluidized beds granular, iron-oxide-containing material is brought in direct contact with hot reduction gas at temperatures of 500 to 900° C. When the reduction gas not only contains hydrogen, but also a considerable content of carbon monoxide, a product rich in Fe
3
C can be withdrawn from the last stage of the fluidized bed of the known reduction process. Practice has shown, however, that in the reduction of iron oxide to iron the steam produced greatly impedes the simultaneous formation of iron carbide as a result of the reaction of iron with Co and/or CH
4
.
It is therefore the object underlying the invention to omit the simultaneous production of iron carbide (Fe
3
C) during the direct reduction of iron oxide and the formation of sponge iron. In accordance with the invention iron carbide is produced by means of the above-mentioned process such that the sponge iron, which has a carbon content of not more than 2 wt-%, is swirled in a fluidized-bed reactor at temperatures of 500 to 800° C. with a carbonaceous gas whose water content is not more than 1.5 vol-%, and that from the reactor a product is withdrawn, whose total iron content is bound as Fe
3
C for at least 80 wt-%. Preferably, at least 85 wt-% of the total iron content of the product withdrawn are bound as Fe
3
C.
In the process in accordance with the invention, carburizing the low-carbon sponge iron is deliberately effected separate from the reduction plant. This requires a more complex apparatus than the known production of iron carbide, but the reduction plant is relieved considerably in the process in accordance with the invention. Now, the reduction plant is preferably operated with hydrogen-rich gas as reduction gas, which contains only little CO or is virtually free from CO. During the carburization a H
2
-containing gas is produced, and this hydrogen can advantageously be utilized upon separation in the reduction plant. It is recommended to form the reduction gas supplied to the fluidized bed of the last reduction stage from hydrogen for at least 80 vol-% (calculated free from nitrogen). Then, the granular sponge iron, which is supplied to the fluidized-bed reactor for carburization, usually has a carbon content of not more than 1 wt-%.
For carburizing in the fluidized-bed reactor gases rich in hydrocarbon are used, which in the reactor may also serve as fluidizing gases. As gas rich in hydrocarbon there might for instance be used methane or methane-containing natural gas. To accelerate the carburization, the fluidized-bed reactor is operated at pressures in the range from 3 to 10 bar. In the fluidized-bed reactor, the solids may form a stationary fluidized-bed, or they may be held in the state of the circulating fluidized bed. In the latter case, the reactor comprises a separator for the separation of solids, which is connected with the upper part of the reactor, and from which separated solids are recirculated to the lower part of the reactor. Per hour, at least 5 times the weight of solids is recirculated, as compared with the solids content in the reactor.
BRIEF DESCRIPTION OF THE DRAWING
Embodiments of the process will now be explained with reference to the drawing. It represents a flow diagram of the process.
From granular iron oxide, which is supplied via line
1
, there is first of all produced by means of reduction sponge iron with a carbon content of not more than 2 wt-% and preferably not more than 1 wt-%. The reduction may be effected in any manner known per se. An advantageous procedure is described in the already mentioned U.S. Pat. Nos. 5,527,379 and 5,603,748, where a drying and heating stage
2
is followed by a first reduction stage
3
and a subsequent second reduction stage
4
. In both reduction stages the reduction is effected in a fluidized bed, where hot, hydrogen-containing gas is used as reduction and fluidizing gas. The temperatures in both stages
3
and
4
lie in the range from 500 to 900° C. The first stage
3
is designed as circulating fluidized bed, to which at least in part used, H
2
-containing reduction gas from the second stage
4
is supplied through line
5
. The exhaust gas of the first stage is recirculated via line
6
to a processing plant
7
, in which there is also produced fresh gas rich in hydrogen. The gas is supplied as hot reduction gas through line
8
to the second reduction stage
4
, in which the solids preferably form a stationary fluidized bed. Preferably, the gas of line
8
comprises at least 80 vol-%, and mostly at least 90 vol-% hydrogen. A partial stream of the reduction gas of line
8
is expediently supplied directly to the first stage
3
through line
8
a
. The degree of metallization of the partly reduced ore of line
3
a
is about 50 to 80%.
From the second reduction stage, granular sponge iron with a carbon content of not more than 2 wt-% and preferably not more than 1 wt-% is withdrawn via line
10
. This sponge iron is charged into a fluidized bed reactor
11
, which is connected with a cyclone separator
12
. For carburizing the sponge iron, gas rich in hydrocarbon, which for instance chiefly consists of methane, is supplied through line
13
. This gas first of all flows into a distribution chamber
14
and then as fluidizing gas through a tuyere bottom
15
upwards through the reactor
11
. In the reactor
11
the temperatures lie in the range from 500 to 800° C. A gas-solids suspension is supplied from the upper part of the reactor
11
through the passage
16
into the separator
12
, and separated solids are recirculated to the reactor
11
through line
17
. The product withdrawn through the passage
18
from the lower part of the reactor
11
chiefly consists of iron carbide, where at least 80 wt-% of the total iron content are bound as Fe
3
C. This product is supplied to a cooling unit not represented.
Solids-containing gas leaves the separator
12
through line
20
and first gives off heat in the heat exchanger
21
. Due to the carburization, the gas of line
20
has a considerable hydrogen content, so that the H
2
content, calculated anhydrous, will be at least 10 vol-%. Expediently, there should be ensured a H
2
content in the gas of line
20
of 15 to 40 vol-% (calculated dry). For dedusting, the gas is first supplied to a filter
23
through line
22
, and is then supplied to a wet scrubbing unit
25
via line
24
. In the scrubbing unit
25
washing solution is sprayed in through line
26
, and used, solids-containing washing solution is withdrawn via line
27
. Cleaned gas is sucked in via line
29
by means of the blower
30
. It is very advantageous to at least partly separate the hydrogen content of the gas and utilize the same in the reduction plant. For this purpose, the gas is wholly or partly supplied through line
31
to a separating means
32
for separating a gas fraction rich in H
2
from the gas mixture. If desired, a partial stream of the gas coming from the blower
30
may be guided past the separating means
32
through the bypass line
33
and the opened valve
34
.
The separating means
32
may operate in a manner known per se, for instance according to the principle of pressure-swing adsorption, or may be designed as membrane separation. Furthermore, it is possible to effect a gas separation by means of deep cooling. In addition to a gas fraction rich in H
2
, which is discharged via line
36
, a residual gas is obtained from the separating means
32
, which residual gas is withdrawn via line
37
. When a purge gas is used, such as in the case of a pressure-swing adsorption plant, the latter is withdrawn via line
38
indicated in broken lines. The residual gas of line
37
is mixed with the gas of line
33
and fortified by gas rich in hydrocarbon, e.g. methane, from line
38
a
, The gas mixture, which serv
Hirsch Martin
Stroeder Michael
Weber Peter
Bos Steven
Metallgesellschaft AG
Norris & McLaughlin & Marcus
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