Metallurgical apparatus – Means for treating ores or for extracting metals – By means applying heat to work – e.g. – furnace
Reexamination Certificate
2001-04-10
2003-07-15
Andrews, Melvyn (Department: 1742)
Metallurgical apparatus
Means for treating ores or for extracting metals
By means applying heat to work, e.g., furnace
Reexamination Certificate
active
06592806
ABSTRACT:
The entire disclosure of Japanese Patent Application No. 2000-203530 filed on Jul. 5, 2000 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reduced iron discharger in a rotary hearth reducing furnace for producing reduced iron by reducing, in a high temperature atmosphere, pellet-or briquette-like agglomerates which have been formed from a powdery mixture of an iron oxide powder and a reducing agent and supplied onto a rotary hearth.
2. Description of the Related Art
To produce reduced iron, the first step is, generally, to mix a powder of iron ore (iron oxide), a powder of coal (reducing agent), a powder of limestone (fluxing agent), and a binder such as bentonite, and to compress and pelletize the mixture to form wet balls called “green balls.” Then, the wet balls are dried to some degree to form dry balls. The dry balls are heated to a high temperature in a reducing furnace, where the iron oxide in the iron ore is reduced by the coal as a reducing agent to form reduced iron in the form of pellets.
An example of an apparatus for producing such reduced iron is explained by way of FIG.
7
. Powders of iron ore, coal, etc. and a binder are mixed in a mixer (not shown). The resulting mixed powder is pelletized in a pelletizer
1
to form green balls (green or raw pellets) GB. Then, the green balls GB are charged into a dryer
2
, where they are dried with an off-gas from a reducing furnace
4
(to be described later on) to form dry balls DB. The dry balls DB are supplied to the reducing furnace
4
by a pellet feeder
3
.
The interior of the reducing furnace
4
is maintained in a high temperature atmosphere upon heating by a burner
5
, and an inside off-gas is discharged from an off-gas duct
6
. Thus, the dry balls DB are preheated and heated with radiant heat from the wall of the furnace when they are passed through the interior of the reducing furnace
4
. During their passage, the iron oxide in the iron ore is reduced with the coal as the reducing agent to form reduced iron in the form of pellets. The reduced pellets are discharged to the outside by a pellet discharger
8
, and accommodated into a portable vessel
9
.
The off-gas from the off-gas duct
6
usually contains some unburned gas, and is thus burned in an after burner chamber
7
nearly completely. Then, the off-gas is cooled in a water spray primary cooler
10
, and then sent to a heat exchanger
11
, where it undergoes heat exchange. Combustion air heated by the heat exchange is sent to the reducing furnace
4
, and fed into the furnace together with fuel. On the other hand, the off-gas is cooled again in a secondary cooler
12
, and part of it is sent to the dryer
2
as drying air for the green balls GB as stated earlier. The remaining part of the off-gas is cleaned in a dust collector
13
, and released into the atmosphere via a stack
14
.
A screw discharger as shown in
FIG. 8
has been used as the pellet discharger
8
. When this discharger is used, a rotary hearth
15
is supported by a floor rail
16
disposed concentrically in a furnace chamber, and a horizontal roller
18
disposed in an inner peripheral portion of a furnace wall
17
in such a manner that a wheel
19
contacts the floor rail
16
and a side surface rail
20
of the rotary hearth
15
itself contacts the horizontal roller
18
. The rotary hearth
15
is rotated by a rotational drive system (not shown), with a space between the rotary hearth
15
and the furnace wall
17
being sealed with a water groove
21
. A discharge screw
62
having a spiral blade
62
a
is mounted across the rotary hearth
15
, with a tiny gap being kept between the discharge screw
62
and the upper surface of the rotary hearth
15
, and a shaft end portion of the discharge screw
62
is supported by a bearing
63
. The discharge screw
62
is rotated by a motor
64
in the direction indicated by an arrow
65
in the drawing. As a result, reduced iron P on the rotary hearth
15
is raked out by the spiral blade
62
a
toward a discharge port on the right side in the drawing.
With the conventional screw discharger, the reduced iron raked out from a site on the moving rotary hearth
15
in a perpendicularly lateral direction by the spiral blade
62
a
increases in amount and becomes bulky as it approaches the discharge port in the end portion of the discharge screw
62
, as shown by the symbol P in FIG.
8
. Thus, the height of the spiral blade
62
a
needs to be consistent with the amount of reduced iron at the discharge port. Hence, the entrance side of the discharge screw
62
(i.e., the side opposite to the discharge port), where the amount of reduced iron is small, faces the problem that the height of the blade made of an expensive heat resistant steel is useless. Besides, during raking-out by the discharge screw
62
, the reduced iron at a high temperature is converted into a powder or powdered under the pressure of the spiral blade
62
a
, resulting in a decreased yield.
The rotational speed of the discharge screw
62
is linked to the volume of production by the reducing furnace. That is, if the discharge screw
62
rotates in the same manner when the amount of green pellets supplied into the furnace increases, not all of the reduced iron P will be discharged, and some of the reduced iron P escapes the discharge screw
62
. To increase the volume of production, therefore, the rotational speed of the discharge screw
62
must be increased.
FIG. 9
is a graph showing the relationship between the necessary rotational speed of the discharge screw
62
, the rotational speed of the rotary hearth
15
, and the volume of production. The horizontal axis represents the volume of production (t/hr), and the vertical axis represents the screw speed (r.p.m.). As an example, the graph shows the course of the necessary rotational speed of the discharge screw
62
in response to changes in volume of production in the reducing furnace whose hearth rotational speed is
6
rotations per hour. When the hearth rotational speed is 6 rotations per hour, the corresponding screw speed is 7 rotations per minute. At this screw rotational speed, the volume producible without escape of reduced iron is up to about 45 tons per hour. To produce a greater volume, the screw rotational speed should be increased in proportion to the increase in the volume of production. When the rotational speed of the discharge screw
62
increases, the speed of the reduced iron P discharged from the reducing furnace becomes high. As a result, powdering of the high temperature reduced iron due to collision is accelerated, aggravating the aforementioned decrease in the yield.
SUMMARY OF THE INVENTION
The present invention has been proposed in light of these circumstances. It is an object of this invention to provide a reduced iron discharger in a rotary hearth reducing furnace, which involves minimal structural waste and obtains a satisfactory yield.
A first aspect of the present invention, as a means of attaining the above object, is a reduced iron discharger in a rotary hearth reducing furnace for producing reduced iron by reducing agglomerates in a high temperature atmosphere, the agglomerates being pelletized from a powdery mixture of an iron oxide powder and a reducing agent and supplied onto a rotary hearth, wherein rotary blades capable of discharging the reduced iron from a site on the rotary hearth are provided. Thus, the reduced iron discharger can serve as an apparatus which involves minimal structural waste and obtains a satisfactory yield.
A second aspect of the invention is the above-mentioned reduced iron discharger in a rotary hearth reducing furnace, wherein the blades each comprise a body member and a front end member detachably provided on the body member. Thus, when the front end portion of the blade wears, only the front end member can be replaced easily.
A third aspect of the invention is the above reduced iron discharger in a rotary heart
Fujioka Hironori
Kamikawa Susumu
Onaka Yoshimitsu
Osaka Hiromi
Sato Keiichi
Andrews Melvyn
Birch & Stewart Kolasch & Birch, LLP
Mitsubishi Heavy Industries Ltd.
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