Apparatus for producing reduced iron and compact drying

Details Apparatus for producing reduced iron and compact drying Apparatus for producing reduced iron and compact drying

2001-09-17

2003-04-22

Andrews, Melvyn (Department: 1742)

Metallurgical apparatus

With means to preheat gas

By remote heat exchanger

C266S145000, C266S155000, C266S160000, C266S175000

Reexamination Certificate

active

06551548

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for producing reduced iron which comprises (mixing a reducing agent and iron oxide, agglomerating the mixture, drying the compacts (pellets or briquettes) in a dryer, and reducing the dried compacts in a high temperature atmosphere in a reducing furnace. This invention also relates to a method for drying compacts, which method is applied to the apparatus.
2. Description of the Related Art
To produce reduced iron, the first step is to mix an iron ore powder, a coal powder, a limestone powder, and a binder, and to compress and agglomerate the mixture to form wet compacts called green compacts. Then, the wet compacts are dried to some degree to form dry compacts. The dry compacts are heated to a high temperature in a reducing furnace, where the iron oxide in the iron ore is reduced with the coal (a reducing agent) to form reduced iron compacts.
An example of a conventional apparatus for producing reduced iron is explained by way of FIG.
6
. At an upper portion of a circular rotary hearth type of reducing furnace
1
, there are provided a device
2
for introducing compacts (dry compacts), and an off-gas duct
3
or discharging a hot off-gas, a residual form of a gas used in reduction. Inside the furnace
1
, a discharger
4
is provided for discharging reduced compacts P (reduced iron compacts). On a circumferential sidewall of the furnace
1
, a plurality of burners
5
is provided for generating a reducing hot gas.
Powders of coal (a reducing agent), iron ore, etc. as raw materials are mixed with a binder, and the mixture is fed to a pelletizer or a briquetter
6
, where compacts (wet compacts) are formed. The resulting compacts are sent to a dryer
7
, where the compacts are dried at about 120 to 150° C. to become dry compacts the dry compacts are supplied to the rotary hearth of the reducing furnace
1
via the introducing device
2
.
In the reducing furnace
1
, fuel and combustion air are fed to the burners
5
, which generate a high temperature hot gas. The hot gas turns in the direction of a dashed arrow, and during this motion, exerts a reductive action on the compacts, an object to be treated, in a high temperature atmosphere. A hot off-gas discharged through the off-gas duct
3
is primarily cooled by a primary cooler
8
of a water spray type, and then brought to a heat exchanger
9
, where the off-gas exchanges heat with the combustion air. Further, the off-gas is secondarily cooled by a secondary cooler
10
of a water spray type to about 300° C., for example. Then, the cooled off-gas is conveyed to the dryer
7
to dry the compacts. Then, the off-gas is passed through a dust collector
11
, where it is cleaned, and then dissipated into the air.
When the rotary hearth inside the reducing furnace
1
makes nearly one rotation in the direction of a solid arrow in
FIG. 6
, the reduced compacts P are discharged from the screw type discharger
4
. The compacts are delivered to a portable container
13
by a discharge chute
12
, and then transported to a subsequent step.
In the dryer
7
of the foregoing conventional apparatus for producing reduced iron, the hot off-gas discharged from the reducing furnace
1
and cooled by the water spray type of primary cooler
8
and secondary cooler
10
is used as a heat source for drying the compacts (wet compacts) at a temperature of about 120 to 150° C. That is, the compacts are dried with the hot off-gas which is very rich with steam. Hence, during the unstable state which occurs immediately after initiation of operation of the apparatus, moisture is condensed onto the surfaces of the wet compacts. As a result, sticking of the wet compacts to each other occurs, whereupon the wet compacts may lump together, becoming large masses. In a situation such as that immediately after the start of operation, the properties, such as temperature and flow rate, of the hot off-gas discharged from the reducing furnace
1
are not stable, so that drying in the dryer
7
is unstable. This may cause the problem of wet compact lumping.
The wet compacts, which have been treated in the dryer
7
during such an unsteady operation, may have moisture remaining in the compacts. If such wet compacts are rapidly heated in the reducing furnace
1
in a subsequent step, surface portions of the compacts may peel off, or the compacts may rupture.
The heating gas in a dryer like the above-mentioned dryer
7
, or in a dryer using hot air from a heat exchanger or the like as a heat source for drying compacts (wet compacts), may cause compact rupture, or the formation of a combustible gas from coal in the compacts, if the temperature of the heating gas is too high. To avoid these risks, the maximum temperature of the heating gas is set at 200° C. or lower. Depending in the moisture content, etc. of the heating gas, however, a higher gas temperature than that may be set. Since a conventional dryer uses a heating gas whose temperature has been set to be somewhat low, it has posed the problem of taking considerable time for drying compacts.
In an apparatus for producing reduced iron, which uses coal as a reducing agent, volatile matter (hereinafter referred to as VM), such as CO, CH
4
, H
2
O, CO
2
, and N
2
, occurs from coal, if the temperature of th heating gas is too high. At a high oxygen concentration, therefore, the coal may catch fire. Once VM develops in the dryer, the VM cannot be utilized as a heat source in the reducing furnace in the subsequent step. This poses the disadvantage that the thermal efficiency of the reducing furnace is lowered. If the temperature of the heating gas is the sulfuric acid dew point (120° C.) or lower, on the other hand, corrosion will be induced because of dew formation in piping, etc. inside the dryer.
As described above, temperature control for the heating gas is of vital importance in efficiently drying compacts (wet compacts) in a dryer. There has been an intense demand for the realization of a dryer capable of producing stable drying conditions.
SUMMARY OF THE INVENTION
The present invention has been proposed in light of these circumstances. It is an object of this invention to provide an apparatus for producing reduced iron, which can perform highly efficient, stable drying in a dryer and produce high quality reduced iron stably; and also to provide a method for drying compacts which is applied to the apparatus.
A first aspect of the present invention, as a means of attaining the above object, is a method for drying compacts, the method being applied to an apparatus for producing reduced iron by mixing and agglomerating a powder of a reducing agent and a powder of iron oxide in a pelletizer to form compacts or in a briquetter to form briquettes, drying the compacts (pellets or briquettes) in a dryer, and reducing the dried compacts in a high temperature atmosphere in a reducing furnace, wherein
a temperature range of a heating gas supplied to the dryer is set based on the following equation:
Sulferic acid dew point <T
g
<100/40·C
H2O
+200
where T
g
denotes the temperature (° C.) of the heating gas, and CH
H2O
denotes a moisture concentration (vol %) in the heating gas.
a temperature range of a heating gas supplied to the dryer is set based on the following equation:
Sulfuric acid dew point≦T
g
≦100/40·C
H2O
+200
where T
g
denotes the temperature [° C.] of the heating gas, and C
H2O
denotes a moisture concentration [vol %] in the heating gas.
According to the above aspect of the invention, a high gas temperature adapted for the moisture concentration (moisture content) in the heating gas can be set. Thus, the drying can be shortened, and highly efficient, stable drying can be performed, so that high quality reduced iron can be produced stably. Furthermore, the temperature of the heating gas on the exit side of the dryer is at a high temperature which is above the sulfuric acid dew point. Thus, acid corrosion of piping, etc. minimally occurs.
In the method for drying compacts a

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