Shaft furnace for reducing ores

A shaft furnace for reducing ores with a gas, wherein the furnace is constructed to destroy the plug-flow state of ores in or above a plug-flow zone where a reduction ratio of the ores is 50 to 70%, in order to prevent the clustering by changing the plug-flow zone into a shear-flow zone. Further, the shaft furnace for reducing ores with a gas is so constructed that a hydraulic radius R (cm) of the furnace in the reduction zone in which the reduction ratio is greater than 50% is selected to be, EQU R.ltoreq.120 exp(8.6-0.009T) wherein T denotes the temperature (.degree.C.) of the reducing gas, in order to prevent the clustering.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a shaft furnace for directly reducing the 
ores by the counter-flow of ores which descends in the furnace and a 
reducing gas which rises in the furnace. 
2. Description of the Prior Art 
The production of reduced iron by the shaft furnace of this type has long 
been placed in the industrial applications. To increase the productivity 
by the shaft furnace of this system, it is necessary to essentially 
increase the rate of reduction reaction. For this purpose, the reduction 
temperature may be elevated or the reaction pressure may be increased. As 
for the method of increasing the reacting pressure, i.e., as for the 
operation system under elevated pressure, a pilot plant has been already 
developed and there has been reported that the reduction time was 
remarkably reduced by the operation under the pressure of 4 kg/cm.sup.2 G. 
With the operation of the system under high pressure, however, a specially 
designed sealing mechanism must be employed to cope with the increased 
pressure, requiring increased cost for the construction of facilities. 
On the other hand, the method of increasing the productivity by raising the 
reduction temperature is advantageous as compared with the high-pressure 
operation system because it does not require increased cost for the 
construction of the aforementioned facilities. This method, however, 
presents such a problem that the burden materials are often clustered with 
each other due to elevated temperature. 
That is, ores which are the burden materials accumulate and gradually 
descend through the shaft furnace. In a high-temperature zone at the lower 
portion of the reduction zone, therefore, the clustering develops such 
that ores adhere with each other due to metallurgical change as a result 
of the pressure of ores (pressure which generates in the ore layer due to 
the self weight of the ores) and reduction of ores, giving rise to the 
occurrence of deflection in the flow of reducing gas as well as the 
occurrence of hanging. In extreme cases, therefore, it becomes difficult 
to continue the operation, presenting a serious problem in producing the 
reduced iron with the shaft furnace of this type. 
To avoid the clustering, it has been attempted to use ore pellets admixed 
with small amounts of limestone (refer to the specification of U.S. Pat. 
No. 3,957,486). In this case, however, limitation is imposed on the ores 
which are to be processed. 
Furthermore, to prevent the clustering with the conventional methods, it 
was often attempted to provide agitating wings, baffle plates or burden 
feeders at the lower portion of the furnace to destroy the clustering 
(refer, for example, to U.S. Pat. No. 3,558,118). 
Thus, there had been no effective method for preventing the clustering in 
the reduction zone. 
In view of such circumstances, the inventors of the present invention have 
conducted research in relation to the phenomenon of clustering, and have 
previously announced the results in a paper entitled "Reduction Properties 
of Raw Materials for Direct Reduction Shaft Furnace", Iron and Steel, 
1977, Vol. 63, No. 14, pp. 2269-2277, by Dentaro Kaneko et al., and in a 
paper entitled "Clustering Phenomena during Iron Oxide Reduction in Shaft 
Furnace", Iron and Steel, 1978, Vol. 64, No. 6, pp. 681-690, by Dentaro 
Kaneko et al. 
In a series of these studies, it was clarified that the clustering which 
takes place during the reduction process of a shaft furnace is a sintering 
phenomenon by diffusion of solid metallic iron that is formed by the 
reduction, and that if microstructurally considered, the clustering 
develops and grows by the intertwining of fibrous metallic iron that is 
formed by the reduction. It was further clarified that the clustering is 
caused by the three principal factors; i.e., reduction temperature, 
compressive load which acts upon the burden raw materials, and properties 
of the burden raw materials. At the same time, various effects by these 
factors upon the clustering were also clarified. 
The present inventin was accomplished based upon the results of such 
studies and various experiments and simulation tests performed in relation 
to the formation of cluster, reduction conditions of ores in the furnace, 
flowing conditions in the furnace, and the like. 
SUMMARY OF THE INVENTION 
The fundamental object of the present invention therefore is to provide a 
shaft furnace for reducing ores, which does not develop the clustering. 
More specifically, the present invention is to provide a shaft furnace for 
reducing ores, having such an internal-furnace construction and/or an 
furnace construction per se that will not develop clustering, and to 
provide a shaft furnace for reducing ores, having a furnace construction 
so as to maintain a range of the compressive load that does not develop 
clustering. 
In order to attain the above-mentioned objects, a first embodiment of the 
present invention deals with a shaft furnace for reducing ores with a gas, 
wherein a mechanism is provided to destroy the plug-flow state of ores in 
a cluster forming plug-flow zone where a reduction ratio of the ores is 50 
to 70%, or in a plug-flow zone just above the cluster forming plug-flow 
zone, in order to prevent the clustering by changing the plug-flow zone 
into a shear-flow zone. 
A second embodiment of the present invention deals with a shaft furnace for 
reducing ores with a gas at a temperature of higher than 900.degree. C., 
wherein a hydraulic radius R (cm) of the furnace in the reduction zone 
where the reduction ratio is greater than 50% is selected to be, 
EQU R.ltoreq.120 exp (8.6-0.009T) 
wherein T denotes the temperature (.degree.C.) of the reducing gas, in 
order to prevent the clustering. 
A third embodiment of the present invention deals with a shaft furnace for 
reducing ores with a gas, wherein a mechanism is provided to destroy the 
plug-flow state of ores in a cluster forming plug-flow zone where a 
reduction ratio of the ores is 50 to 70%, or in a plug-flow zone just 
above the cluster forming plug-flow zone, and a hydraulic radius R (cm) of 
the furnace in the reduction zone where the reduction ratio is greater 
than 50% is slected to be, 
EQU R.ltoreq.120 exp (8.6-0.009T) 
wherein T denotes the temperature (.degree.C.) of the reducing gas, in 
order to prevent the clustering. 
A fourth embodiment of the present invention deals with a shaft furnace of 
the above-mentioned third embodiment, wherein the shaft furnace is the one 
which reduces the ores at a temperature as high as 900.degree. C. or more.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, below is mentioned in detail the internal-furnace construction of a 
shaft furnace for reducing ores, which is capable of preventing the 
clustering. 
In order to find major causes for forming cluster, the inventors of the 
present invention have observed the flowing state of ores in the furnace, 
and have confirmed the fact that the neighboring ore particles cease the 
movement relative to each other in a reduction zone where the reduction is 
effected to 50 to 70% and creates a plug-flow by which the particles of 
ores descend maintaining the same surfaces relative to each other or 
maintaining the state of point-to-point contact. The inventors have 
further confirmed that as the reduction ratio proceeds to 50 to 70% or 
more, the ore particles descend in plug-flow, maintaining the same 
surfaces or point-to-point contact and undergo the solid-phase sintering 
to form the cluster. 
The present invention, therefore, is to provide a shaft furnace in which 
the plug-flow which is a cause of cluster is destroyed in a reduction 
ratio range of 50 to 70% where the formation of cluster is started, by 
establishing a shear-flow zone so that ore particles will revolve or 
acquire linear motion relative to each other to change the contacting 
surfaces or points among the particles, in order to prevent or restrain 
the formation or growth of cluster. 
To examine the descending state of ores in the shaft furnace, a model of 
three-dimensional shaft furnace 1 m in diameter and about 5 m in height (3 
m for cylindrical portion and 2 m for conical portion) was constructed as 
shown in FIG. 1. Pellets having a particle size of 12 mm were introduced 
into the model furnace to measure the pressure on the central line of the 
model furnace, i.e., to measure a ratio K of horizontal pressure 
.sigma..sub.H to vertical pressure .sigma..sub.V. The results were as 
shown in FIG. 2, from which it will be learned that the pressure ratio K 
is close to unity at the upper portion of the furnace, gradually decreases 
toward the middle portion, and converges to a Rankin's constant (0.218) 
which is found when the angle of friction in the pellet layer is 
.phi.i=40.degree. in the lower portion of the furnace. The results of FIG. 
2 teaches the following fact. Namely, when the pressure ratio is K=1 in 
the upper portion of the furnace, the pressure of the ore layer in the 
upper portion of the furnace does not yet reach the yielding state 
(.tau.=C+.sigma. tan .phi.i, where .tau. represents a shear pressure, C a 
constant which represents cohesion, and .sigma. a normal pressure) of 
Mohr-Coulomb, but remains in a pressure state which is close to 
hydro-static state. In such an unyielding state, the ore layer does not 
undergo the deformation or destruction; the ores descend in the plug-flow 
state maintaining the same surfaces or point-to-point contact state. In 
the lower portion of the furnace, on the other hand, the pressure of the 
ore layer is converged to the Rankin's constant (K=0.218) which indicates 
Mohr-Coulomb's criterion of yielding. Therefore, the ore layer is under 
such a pressure that develops deformation or destruction, whereby the ores 
descend in a shear-flow in which the ore particles are caused to revolve 
or move linearly. Near the walls of the furnace, the pressure ratio K 
converges to a value close to the Rankin's constant (K=0.218) even in the 
upper portion of the furnace, thereby establishing the shear-flow. When 
these conditions were observed using a model of two-dimensional shaft 
furnace having the same shape in cross section, it was recognized as shown 
in FIG. 3 that a plug-flow zone PF was formed downwardly protruding from 
the upper portion to the middle portion of the furnace, and a shear-flow 
zone SF was formed beneath the plug-flow zone PF. The results of FIG. 3 
were in good agreement with the measured results of pressure of FIG. 2. 
In the plug-flow zone, the ores are not permitted to move relative to each 
other but are permitted to descend in a static manner, resulting in 
creating conditions for forming the cluster. Below are mentioned the 
reduction conditions in the furnace and the development of clustering. 
FIG. 4 is a diagram showing the change of shrinkage ratio and reduction 
ratio of ore layer when high-grade oxidized pellets of an iron grade of 
68.4% are reduced under the conditions of a reduction temperature of 
910.degree. C. and an ore pressure of 2 kg/cm.sup.2. As will be obvious 
from FIG. 4, there exists a strong correlation which serves as an 
indication of cluster formation between the shrinkage ratio and the 
reduction ratio of the ore layer. In a first period in which the reduction 
quickly proceeds from 0% to 50-70%, the ores are swollen while Fe.sub.2 
O.sub.3 is reduced to Fe.sub.3 O.sub.4 which is then reduced to FeO. In 
this period, metallic iron is less formed. In other words, no cluster is 
formed in this period of reduction and swelling. 
In a second period in which the reduction ratio proceeds from 50-70% to a 
nearly saturated state of 90-98%, the ore layer is quickly shrunk, unlike 
that of the first period. At the same time, the reduction reaction of FeO 
to Fe is promoted, resulting in permitting the formation of fibrous 
metallic iron to be remarkably promoted. During the step of such rapid 
shrinkage, the fibers of metallic iron intertwine with each other as the 
reduction proceeds, whereby the cluster is allowed to rapidly grow, being 
assisted by the intertwining. The degree of shrinkage is reduced in a 
third period where the reduction ratio reaches a nearly saturated state of 
90 to 98%. Even in the third period, however, the cluster continues to 
grow. 
Therefore, as will be obvious from the aforementioned description, if the 
zone in which the reduction ratio reaches 50 to 70% resides in the 
plug-flow zone PF, the formation of cluster is further promoted. 
Namely, in the plug-flow zone PF, the ores descend at a constant speed 
maintaining the same surfaces or point-to-point contact. Therefore, the 
fibrous metallic iron grows with the reduction maintaining the intertwined 
state. The degree of intertwining is further promoted by the increase in 
the contact areas caused by the plastic deformation as a result of the 
softening of ores, whereby the cluster having high degree of bonding is 
formed. 
To examine the reduction which advances in the shaft furnace, therefore, 
the reduction ratio in the shaft furnace as shown in FIG. 1 was analyzed 
using a finite element method with the help of a computer under ordinarily 
employed operation conditions (iron grade of ore pellets 66%, reduction 
gas composed of 50% of H.sub.2, 30% of CO, the remainder being H.sub.2 O, 
CO.sub.2, CH.sub.4, N.sub.2, and temperature of reduction gas at a blowing 
nozzle of 770.degree. C.). The results were as illustrated in FIG. 5, 
which further indicates a boundary line between the plug-flow zone PF and 
the shear-flow zone SF of FIG. 3. As will be understood from FIG. 5, when 
the shaft furnace is operated under ordinary operation conditions, the 
zone in which the reduction ratio lies from 50 to 70% in the shaft furnace 
is included in the plug-flow zone where the clustering easily takes place. 
To avoid the formation of clustering, therefore, it is necessary to provide 
a mechanism for destroying the plug-flow state in or just above the zone 
where the reduction ratio lies from 50 to 70%, so that the plug-flow zone 
PF stays above the zone of the reduction ratio of 50 to 70%. By so doing, 
the zone in which the reduction ratio lies from 50 to 70% creates a shear 
flow such that the ore layer descends, being revolved relative to each 
other or being linearly moved relatively to each other, and the formation 
of cluster is prevented. 
Below is mentioned, with reference to FIG. 6(a), means for expanding the 
shear-flow zone SF when the zone of 50 to 70% reduction exists in the 
plug-flow zone PF. 
FIG. 6(b) illustrates a method in which burden feeders 1 are arrayed not 
only in the lower reduction zone but also in the zone of 50 to 70% 
reduction, to lift up the shear-flow zone SF to a position which is 
considerably above the zone of 50 to 70% reduction in order that the 
plug-flow zone PF stays in the upper portion of the shaft furnace 2. 
FIG. 6(c) shows a method in which a conical baffle plate 3 is installed 
above the zone of 50 to 70% reduction, to expand the shear-flow zone SF up 
to the upper portion of the zone of 50 to 70% reduction. The burden 
feeders or buffle plate may further be installed in the lower zone of the 
furnace as denoted by 1', 3' in FIGS. 6(b) and 6(c), to more effectively 
restrain the development of clustering phenomenon. 
According to an embodiment shown in FIG. 7, agitating devices 6, 
6-consisting of many agitating wings 4, 4' attached to drive shafts 5, 
5-are rotated in the shaft furnace 2 around the zone of 50 to 70% 
reduction, in order to lift up the shear-flow zone SF above the zone of 50 
to 70% reduction, so that the plug-flow zone PF is limited to the upper 
portion of the shaft furnace 2. 
In the aforementioned embodiments, means 1, 3 and 6 for positively 
disturbing the flow of ore layer were provided in the shaft furnace 2 to 
expand the shear-flow zone SF. Referring to FIGS. 8(a) and 8(b), on the 
other hand, further embodiments are shown in which the cross-sectional 
areas of the shaft furnace 2' may be stepwisely changed in the zone of 50 
to 70% reduction to expand the shear-flow zone SF, so that the 
cross-sectional areas on the zone of 50 to 70% reduction are reduced or 
increased by a predetermined ratio to expand the shear-flow zone SF while 
imparting radial acceleration to the flow of ores in the furnaces. 
The construction itself of the shaft furnace for reducing ores, which is 
capable of preventing the clustering, will now be described, with the 
principle of the present invention being first discussed. 
(I) Relation between the reduction ratio in the furnace and the shrinkage 
ratio of ore layer: 
As mentioned earlier, the clustering takes place by the intertwining of 
fibrous metallic iron which forms in the step of forming metallic iron. 
The layer of ore pellets undergoes the plastic deformation due to the 
softening of ores accompanying the progress of reduction reaction and due 
to the increase in the pressure, and is hence shrunk. The shrinkage of the 
ore layer means that contacting surfaces among the ore particles are 
increased, causing the cohesion among the ores to be increased and the 
clustering to be grown. 
The relation between the reduction ratio in the furnace and the shrinkage 
ratio of ore layer is as mentioned earlier in detail with reference to 
FIG. 4. 
(II) Relation between the shrinkage ratio and the cluster strength: 
As will be obvious from the foregoing description, the shrinkage of ore 
layer is very intimately related to the development of clustering. 
However, so far as the formed cluster is brittle, it is crushed again by 
the normal pressure and shear pressure generated in the ore layer during 
the descending step, and presents almost no problem for the operation. 
What becomes a problem for the operation is the cluster which has a high 
degree of bonding (or which has a high clustering strength). 
For the purpose of convenience, therefore, the cluster strength CS is 
defined as follows: 
EQU CS=(W's/Ws).times.100 
where Ws denotes the weight (g) of the cluster on a sieve when it is taken 
out as a specimen, and W's denotes the weight (g) of the cluster on a 
sieve after it is introduced into a steel cylinder having an inner 
diameter of 120 mm and a length of 700 mm and is rotated at 30 rpm for 5 
minutes. 
The relation between the thus defined cluster strength and the shrinkage 
ratio is as shown in FIG. 9. 
Referring to FIG. 9, a characteristic feature is that when the shrinkage 
ratio of ores is smaller than about 15%, the cluster strength CS is zero, 
that is, the cluster does not substantially have strength. As the cluster 
strength CS exceeds 15%, the cluster strength rapidly increases in 
proportion to the increase in the shrinkage ratio. 
Accordingly, if the shrinkage ratio of ore layer is restrained to be 
smaller than 15%, the cluster can be practically neglected even if it is 
formed. 
(III) Relation between the pressure of the ore layer and the shrinkage 
ratio: 
It was clarified in (II) above that the development of cluster can be 
almost all prevented if the shrinkage ratio of the ore layer is smaller 
than 15%. Below is mentioned how the pressure created by the weight of the 
ore layer would affect the shrinkage in the shaft furnace. 
FIG. 10 is a diagram which shows the change in shrinkage ratios of the ore 
layer when high-grade oxidized pellets of an iron grade of 68.4% are 
reduced at a reduction temperature of 960.degree. C. under the pressures 
of 0.3, 1.0 and 2.0 kg/cm.sup.2. 
As will be obvious from FIG. 10, the shrinkage ratio is smaller than 10% 
when the pressure .sigma..sub.V is 0.3 kg/cm.sup.2, smaller than about 15% 
when the pressure .sigma..sub.V is 1.0 kg/cm.sup.2, but reaches as great 
as 30% when the pressure .sigma..sub.V is 2.0 kg/cm.sup.2. 
Therefore, if the pressure in the furnace is maintained to be smaller than 
1.0 kg/cm.sup.2, the shrinkage ratio of the ore layer can be restrained to 
be smaller than about 15%. In that event, even if the cluster is formed, 
it is so brittle that no problem is presented for the operation. 
As mentioned earlier, the clustering phenomenon develops with the increase 
in the reduction temperature. In other words, the clustering phenomenon 
develops depending upon the reduction temperature. Using the 
aforementioned oxidized pellets (iron grade of 68.4%), therefore, the 
reduction was effected at various reduction temperatures and under various 
pressures to find critical pressures (pressures under which the shrinkage 
ratio becomes about 15%) at each of the reduction temperatures. The 
results were as shown in FIG. 11, in which the region on the right side of 
a curve A is a zone where the clustering takes place. Calculation 
indicates that the curve is given by, 
EQU .sigma..sub.C =exp (8.6-0.009T) 
where 
.sigma..sub.C is a critical pressure (kg/cm.sup.2), 
T is a temperature (.degree.C.) of the reduction gas. 
(IV) Relation between the pressure in the furnace and shape of furnace: 
In order to contrive the furnace shape by which the pressure .sigma..sub.V 
in the furnace becomes smaller than the aforementioned critical pressure 
.sigma..sub.C, the distribution of pressure in the furnace is first 
considered below. 
The ores can be treated as granular substances. Therefore, by using the 
Janssen's equation which is applied for the pressure distribution of 
granular substances in a vessel, the vertical pressure .sigma..sub.V is 
given by, 
##EQU1## 
where 
R: a hydraulic radius, 
.mu..omega.: a coefficient of friction between particles and walls of the 
vessel, 
h: a depth from a free surface, 
.gamma.: a bulk density 
##EQU2## 
.phi.i: an angle of friction. 
Measurement of pressure created by the ore pellets in the vessel also 
indicated that the above Janssen's equation can be employed to indicate 
the approximate average value of the pressures. Measurement indicated that 
the bulk density .gamma. was about 0.0025 (kg/cm.sup.3), the frictional 
coefficient .mu..omega. with respect to the walls was about 0.7, the angle 
of friction .phi.i was about 40.degree., and, therefore, the constant K 
was about 0.218. 
According to the above equation, the pressure .sigma..sub.V in the furnace 
becomes maximum at an infinite depth h. Therefore, a maximum pressure 
.sigma..sub.Vmax in the furnace is given by, 
##EQU3## 
Accordingly, the hydraulic radius R (cm) by which the maximum pressure 
.sigma.Vmax in the furnace does not exceed the critical pressure 
.sigma..sub.C is given by, 
EQU R.ltoreq.120.sigma.C=120 exp (8.6-0.009T) 
In the field of hydrodynamics, if the cross-sectional area of the flow is 
denoted by F, and if the whole length of the contacting portions between 
the flow and the wall forming the flow path in the cross-sectional area is 
denoted by U, i.e., if the wetted perimeter is denoted by U, the hydraulic 
radius R is given by, 
EQU R=2F/U 
With regard to the shaft furnace, the cross-sectional area in the flow of 
ores can be detected by F, and the total length of the walls of the shaft 
furnace coming into contact with the ores in the cross-sectional area can 
be denoted by U. 
By setting the hydraulic radius of the shaft furnace in accordance with the 
above-mentioned conditions, the shrinkage ratio of the ore layer can be 
restrained to be less than 15% so that the cluster strength becomes 
substantially zero. 
As mentioned already in (I) above, the cluster is formed in the lower 
reduction zone where the reduction ratio reaches 50 to 70% or more. Hence, 
the condition of the above-mentioned hydraulic radius needs to be 
satisfied at least in the lower reduction zone. 
The reason why Janssen's equation was applied for calculating the pressure 
in the furnace was because the following facts were clarified by the 
experiments conducted by the inventors. 
(a) The direction of a maximum pressure in the ore layer which flows in the 
vessel due to the gravity changes depending upon the time and place. An 
overpressure, that is, a so-called switch pressure is generated when the 
maximum pressure changes from the active state in which it is directed in 
the vertical direction to the passive state in which it is directed in the 
horizontal direction. Here, the pressure develops locally and 
instantaneously. Therefore, the average pressure which participates in the 
shrinkage of the ore layer is found by Janssen's equation by which the 
maximum pressure is presumed to work in the vertical direction. 
(b) Janssen's equation cannot be correctly applied when the furnace wall is 
tilted relative to the vertical direction. In practically designing the 
furnace on an industrial scale, however, the error can be neglected. 
Among the experiments of reduction from which the relation of FIG. 11 was 
derived, the results of the changes in shrinkage ratio of the ore layer 
when the reduction was carried out under the pressure of 2 kg/cm.sup.2 and 
at three different temperatures, i.e., 860.degree. C., 910.degree. C. and 
960.degree. C. are shown in FIG. 12. It will be understood that when the 
reduction temperature is 860.degree. C., no remarkable shrinkage takes 
place; i.e., the shrinkage ratio is smaller than about 10%. The reduced 
iron of the shrinkage ratio of about 10% was partially adhered with each 
other. The adhesion, however, was easily crushed with finger and was not 
so serious as to form the cluster. When the reduction temperature was 
higher than 910.degree. C., on the other hand, the shrinkage developed 
markedly with the progress of the reduction reaction. With the temperature 
of 900.degree. C. as a boundary, conspicuous shrinkage takes place when 
the reduction temperature is higher than 900.degree. C. Observation of the 
reduced iron having a shrinkage ratio of about 30% reduced at a 
temperature of 960.degree. C. indicated that the reduced iron was 
completely clustered. 
The above observation supports the conclusion of (II) above. Namely, the 
clustering becomes a problem when the reduction is performed at a 
temperature of higher than 900.degree. C. That is, the high-temperature 
reduction referred to in this invention represents the case when the 
reduction is performed at temperatures in excess of 900.degree. C. 
Further, since the properties of the raw materials affect the clustering, 
investigation was conducted in regard to the relation between the 
properties of the raw materials and the shrinkage ratio. FIG. 13 is a 
diagram illustrating the changes in shrinkage ratio of ore layers when 
four kinds of pellets, i.e., pellets having an iron grade of 68.4% 
(specimen A), pellets having an iron grade of 65.6% (specimen B), pellets 
having an iron grade of 63.8% (specimen C), and pellets having an iron 
grade of 67.0% admixed with 0.63% of CaO (specimen D), are reduced at a 
temperature of 910.degree. C. under a pressure of 2 kg/cm.sup.2. It will 
be recognized from FIG. 13 that when the raw material C of a low iron 
grade (smaller than 65%) or when special pellets C admixed with CaO are 
used, the shrinkage ratio is smaller than about 15% and the clustering is 
not substantially developed. The raw materials having low iron grades, 
however, encounter such problems that the productivity is decreased or it 
is obliged to use specially prepared pellets. Pellets having high iron 
grades (65% or more), on the other hand, develop the problem of clustering 
more frequently. Therefore, if the problem of clustering is solved by any 
means other than the iron grade, the raw materials having any properties 
can be used without developing the clustering. 
The shaft furnace which satisfies the condition of the hydraulic radius R 
when the reduction gas is blown at a temperature of 950.degree. C. is 
described below. 
According to the following equation, R.ltoreq.exp (8.6-0.009T), when 
T=950.degree. C., R.ltoreq.126 cm and .sigma..sub.C =1.05 kg/cm.sup.2. 
Therefore, as far as the shaft furnace fulfills R.ltoreq.1.26 m and 
.sigma..sub.V .ltoreq.1.05 kg/cm.sup.2, it is possible to prevent the 
clustering. 
(A) Double cylindrical shaft furnace 
A shaft furnace 2 shown in FIG. 14 consists of a double cylinder, the 
diameter of an inner cylinder 7 being 7.6 m, the diameter of an outer 
cylinder 8 being 10 m, and the height of the reduction portion being 30 m. 
The hydraulic radius R is 1.2 m, the pressure .sigma..sub.V in the 50 to 
70% reduction zone is 0.91 kg/cm.sup.2, and the pressure .sigma..sub.V in 
a bustle port 9 for blowing the reduction gas is 0.97 kg/cm.sup.2. 
With the cylindrical furnace having the same cross-sectional area and 
height as those of the above shaft furnace 2, the diameter will be 3.25 m, 
i.e., the hydraulic radius R will be 1.625 m, the pressure in the 50 to 
70% reduction zone will be 1.13 kg/cm.sup.2, and the pressure in the 
bustle port will be 1.26 kg/cm.sup.2. Therefore, there develops such a 
pressure that the shrinkage ratio becomes 10 to 15% or higher, whereby the 
cluster strength is remarkably increased to interrupt the operation as 
mentioned in (II) above. 
(B) Modified double cylindrical shaft furnace 
A shaft furnace 10 shown in FIG. 15 has an upper portion of a diameter of 4 
m, a cylindrical portion 11 of a height of 11 m, and a double cylindrical 
portion 12 like that of FIG. 14, wherein the cylindrical portion 11 and 
the double cylindrical portion 12 are connected together by a conical 
portion 13. The pressure .sigma..sub.V at the lowest portion of the 
cylindrical portion 11 is 0.93 kg/cm.sup.2, and the pressure .sigma..sub.V 
in a bustle port 14 of the double cylindrical portion 12 is 0.91 
kg/cm.sup.2. 
(C) Multi-cylindrical shaft furnace 
A shaft furnace 20 shown in FIG. 16 has cylinders 21, 22, 23 and 24 of 
diameters of 2.4 m, 4.8 m, 7.2 m and 9.6 m, that are combined in 
concentric relation with each other and in a multi-cylindrical manner. 
Hydraulic radii R of the shells are all 1.2 m, the pressure .sigma..sub.V 
in the 50 to 70% reduction zone is 0.91 kg/cm.sup.2, and the pressure 
.sigma..sub.V in a bustle port 25 is 0.97 kg/cm.sup.2. 
With a single-cylinder furnace having the same cross-sectional area as that 
of the shaft furnace 20, the hydraulic radius R will be 4.8 m, the 
pressure in the 50 to 70% reduction zone will be 1.87 kg/cm.sup.2, and the 
pressure in the bustle port will be 2.44 kg/cm.sup.2. 
(D) Parallel-type shaft furnace 
Shaft furnaces 30, 40 shown in FIGS. 17 and 18 consist of a combination of 
single cylinder containers 31, 41 having a rectangular shape in cross 
section or an elliptical shape in cross section with their longer sides 
being arrayed in parallel with each other. With reference to the cross 
section of the single cylinder chamber 31 of FIG. 19, the hydraulic radius 
R will be 1.2 m when the longer side is selected to be 6 m and the shorter 
side is selected to be 1.5 m. 
(E) Partitioned shaft furnace 
Shaft furnaces 50, 60 and 70 shown in FIGS. 20, 21 and 22 consist of a 
double cylinder 51, a square cylinder chamber 61 having a rectangular 
shape in cross section, and a cylinder chamber 71 having an elliptical 
shape in cross section, which are partitioned into nearly equal areas by 
partitioning boards 52, 62, and 72. By utilizing the partitioning boards, 
it is possible to design the shaft furnaces having great total 
cross-sectional areas yet maintaining small hydraulic radius. 
(F) Shaft furnace having divided lower portions 
A shaft furnace 80 illustrated in FIG. 23 consists of a single cylinder 81 
which is 5 m in diameter and 10 m in height, and which is located in the 
upper portion, and three small cylinders 82 which are 2.4 m in diameter 
and 20 m in height, and which are located in the lower portion. A conical 
portion 81a at the bottom of the single cylinder 81 is communicated with 
the upper portion of the small cylinders 82 via communication pipes 83. 
Here, the 50 to 70% reduction zone should be located near a branching 
portion 84, and an auxiliary bustle port 85 for blowing the reduction gas 
should also be formed in the lower portion of the single cylinder 81. 
The communication pipes 83 should have sufficient cross-sectional areas 
such that ores are not clogged therein. 
In this case, the pressure .sigma..sub.V in the lowest portion of the 
single cylinder 81 is 0.94 kg/cm.sup.2, and the pressure .sigma..sub.V in 
a bustle port 82a of the small cylinders 82 is 0.91 kg/cm.sup.2. 
As will be obvious from the foregoing description, according to the shaft 
furnace for reducing ores in the present invention, a plug-flow zone in 
which the cluster develops is changed to a shear-flow zone, so that the 
formation or growth of cluster is inhibited or restrained by the shear 
flow. Further, with the shaft furnace of the present invention which is so 
constructed that a hydraulic radius which is smaller than a predetermined 
value is maintained in a zone of reduction ratio of at least 50% or more, 
the cluster strength can be substantially brought into zero in the 
reduction zone where the cluster is formed, making it possible to 
eliminate the problem of clustering which interrupts the operation. 
The shaft furnace having the aforementioned features completely eliminates 
the problem of clustering, and enables the productivity to be further 
increased. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.