CA1099518A

CA1099518A - Fired iron-ore pellets having macro pores and process for producing the same

Info

Publication number: CA1099518A
Application number: CA273,060A
Authority: CA
Inventors: Isao Fujita; Mamoru Onoda; Fumikazu Kawaguchi; Tadao Tsutaya; Yoshimichi Takenaka; Ryo Watanabe
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1976-03-03
Filing date: 1977-03-03
Publication date: 1981-04-21
Also published as: FR2343054A1; DE2709327B2; AU2273977A; DE2709327A1; SE7702318L; DE2709327C3; AU499367B2; FR2343054B1; SE435937B; SE435937C

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to fired iron-ore pellets and a process for producing the same. The process includes the steps of: crushing iron ore to be pelletized; adding carbonace-ous materials of grain sizes of 0.1 to 3 mm in diameter to the thus crushed iron ore in an amount of up to 4% by weight for mixing therewith; pelletizing a mixture thus prepared; and firing the pellets thus obtained; thereby providing pellets, in each of which there are intentionally dispersed macro-pores of sizes of 0.1 to 0.3 mm in diameter, at a ratio of up to 25% to the entire pores contained therein. Pellets produced by this process demonstrate superior properties in both high temperatures and in non-temperatures and have a high gas efficiency as well as being free of retardation of reduction even in an elevated tempera-ture range in a blast furnace. These pellets are also free of the softening and sticking phenomena found in the prior art thereby improving the productivity of the blast furnace and reducing the coke ratio.

Description

1 BACKG~OUND OF THE IN~'ENTION
1) Field of the Invention This invention relates to fired iron-ore pellets and a process for producing the same, and more particularly to pellets for use in the production of pig iron in a blast furnace, which present a prominent reducing property as well as excellen-t softening and sticking properties in a high temperature range, that is, the pellets of the present invention possess excellent behavior characteristics and properties in a high temperature range.

2) Description of the Prior Art Prior art pellets as a charge into a blast furnace are classified in~o a few types, i.e., acid pellets and self-flu~ing pellets containing limestones and so on. These pellets are produced particularly in an attempt to increase a tumbler index and improve a reducibility of pellets which is stipulated in Japanese Industrial Standards ~JIS M-8713), by improving the compressive strength and porosity of pellets. However, the pellets are not necessarily satisfactory in their behavior characteristics of reduction in a high tempera~ure range in a lower part of a blast furnace (in a belly of the furnace and in the lower part thereof), because of hindered gas flow into the inner portions of the pellets. This is caused by the fact that the sizes of the pores in the pellets are less than 0.1 mm, and that the metallic iron in the outer peripheral portions of pellets forms shell layers. As a result, the reduction proceeds reluctantly and low melting point slags are formed in the pellets, so that the pellets are softened, and pellets themselves are stuc~ to each other resul-ting in various pxoblems in a blast furnace.

~ , . .

With an increase in the demand for preventing air pollution, the recovery amount o~ ~arious kinds of dusts skemming from respective steps of iron manufacturing process is increased due to the provision of highly efficient dust collectors.
Recently, various kinds of dusts have been reused. For instance, dusts from blast furnaces, converters and the like are used as raw materials for pellets. These dusts more or less contain carbon and are of a munute grain, so that there tends to be pro-duced micro-pores in fired pellets~ In addition, these dusts also contain a great amount of impurities which in turn lower the quality of iron. Furthermore, these dusts also lower the co~pressive strength and reduction degree of pellets, as stipulated in JIS, and are not desirable as raw materials for pellets.
Yet furthermore, under a reducing atmosphere in a high temperature range, metallic iron in the outer shells of the pellets provides a compacted layer therein, while the formation of a liquid~slag phase in the inner portions of pellets is accelerated, thereby causing clogging of pores therein with the resultant retardation of reduction. In addition, pellets are of a spherical shape and contact each other in surface-contacting relation, and ~end to be contracted to a considerable degree under a load because of the formation of metallic iron and excluding of slag from pellets with the result that there arises a tendency to foxm large clogs in the furnace.
Accordingly, the diffusion of gases into the interior of the pellets in a blast furnace is hindered thereby leading to the increased consumption of fuel.
In this manner, when prior art fired pellets charged in a blast furnace descend into a high temperature zone, the reduction reluctantly proceeds, with the accompanying acceleration of the softening and sticking phenomena of pellets to form large clogs of pellets. This brings about a non-uniform flow of gas, hanging, slip,brokan tuyere, and the like, causing various problems in the blast furnace operation.

SUMMARY OF THE INVENTION
The present invention is directed to reasonably avoiding the shortcomings experienced with prior art fired pellets.
It is accordingly, the first object of the present invention to provide fired iron-ore pellets having macro-pores for use in a blast furnace, and a process for producing the sameO
It is the se~ond object of the present invention to provide fired iron~ore pellets having characteristics and properties superior to those of prior art pe~llets not only in a high temperature ranye but also in a room temperature range, and a process for producing the same~
It is the third object of the present invention to provide fired iron-ore pellets which present a high gas efficiency and are free of retardation of reduction even in an elevated temperature range in a blast furnace, thereby insuring high reduction degree, and a process for producing the same.
It is the fourth object of the present invention to provide fired iron-ore pellets which are free of the sotening and sticking phanomena in a high temperatuxe range in a blast furnace, and which reduce problems in operation, such as a non-uniform flow of gas, hanging, 51ip and the like r thereby improving the productivity of a blast furnace, and reducing the coke ratio, in addition to a process for producing the same.
The first aspect of the present invention which attains the aforesaid objects is characterized in that macro-pores of sizes of 0.1 to 3 mm in diameter are intentionally dispersed in each of the pellets in a ratio of up to 25% tothe entire pores contained therein.
The second aspect of the present invention is character-ized in that, according to the first aspect of the present inven-tion, the pellets provide slag structures containing MgO.
The third aspect of the present invention is charac-terized in that, according to the second aspect of the invention, MgO is added in an amount of up to 3~ by weight.
The fourth aspect of the present invention is character-ized in that, according to the first aspect of the inventionl thepreferable range oE ratios of macro-pores to the entire poxes in each of pellets is between 5 and 25%.
The fifth aspect of the present invention is character-ized in that, according to the first aspect of the invention, the most preferable range of ratios of the macro-pores to the entire pores in each of pellets is between 15 and 25%.
The sixth aspect of the present invention provides a process for producing fired iron-ore pellets, in which the raw materials for iron-ore pellets are crushed, pelletized and fired, the aforesaid process being characterized in that carbonaceous materials of grain sizes of O.l to 3 mm in diameter are added to the raw materials for mixing, after the crushing of the raw materials.
The seventh aspect of the present invèntion is charac-terized in that, according to the sixth aspect of the present invention, carbonaceous materials of grain sizes of O.l to 3 mm in diameter, and flux containing MgO are added to the raw materials for iron-ore pellets, for mixing therewith.
The eighth aspect of the present invention is character-ized in that according to the seventh aspect, flux containing MgO
is added to raw materials for pellets in an amount of up to 3~ by weight.
The ninth aspect of the present invention is 1~)9~Sl~ !

1 characterized in that, accordiny to the sixth aspect of the invention, macro-pGres of size o~ 0.1 to 3 mm in diameter are intentionally dispersed in each of fired iron-ore pellets at ratios of up to 25% to the entire pores contained in each of pellets.
The tenth aspect of the present invention is characterized in that, according to the ninth aspect of the present invention, a preferable range o~ ratios of macro-pores in each o~ fired iron-ore pellets is between 5 and 25~.

The eleventh aspect of the present invention is characterized in that, according to the ninth aspect o~ the invention, the most preferable range of ratios of macro-pores in ~ach of ~ired iron-ore pellets is between 15 and 25~.
BRIEF DESCRIPTIO~ OF TH~ nRAWINGS
Fig. 1 is a plot showing the relationship between macro-porosity and a reduction degree at a high temperature 1250C, of pellets according to the invention;
Fig. 2 is a plot showing the relationship between macro-porosity and a reduction degree stipulated in JIS (Japanese Industrial Standards);
Fig. 3 is a plot showing the relationship of macro-porosity versus a softening temperature;
~ ig. 4 is a plot showing the relationship between macro-porosity and a melting down temperature;
Fig. 5 is a plot showing the relationship between macro-poxosity and compressive strength;
Fig. 6 is a plot showing the relationship between macro-porosity and amounts of carbonaceous materials added;
Fig. 7 is a plot showing the relationship between an amount of coke breeze which is gi~en in terms of amount of ~995~L13 dolomite added, and reduction degree at a high temperature 1250C;
Fig. 8 is a plot showing the relationship between an amount of coke breeze and softening temperatuxe, also between the same and melting down temperature;
Fig. 9~is a plot showing the relationship between an amount of coke breeze and compressive strength; and Fig. 10 is a plot showing the relationship between.
the ratio of macro-pores to the entire pores in pellets, and amount of FeO contained therein The following symbols are used in Figures 1 to 6:

SYMBOL CARBONACEOUS MATERIAL GRAIN SIZE (mm) .
(not included) o coke breeze0.1 - 0.5 O coke breeze0.5 - 1.0 coke bree7.e 1 - 2 coke breeze 2 - 3 X brown coal 1 - 2 !
The following symbols are used in Figures 7 to 10:

SYMBOL A~lOUNT OF DOLOMITE (%) O
0 3.0 6.0 X g O

DETAILED DESCRIPTION OF THE PREFERRED EMBODI~NTS
The present invention is directed to providing a reasonable solution to the aforesaid shortcomings in properties and characteristics of prior art fired iron-ore pellets. More particularly, fired iron-ore pellets are provicled in which macro-pores of sizes of 0~1 to 3 mm in diameter are intentionally '~;`

~ 9~8 . . .

dispersed at a ratio of up to 25% to the entire pores. In addition, a process is provided for producing the aforesaid fired iron ore pellets, in which ~arbonaceous materials of sizes of 0.1 to

3 mm in diameter are added to raw materials for pellets ~or mixing therewith in an amount of up to 4%~ and then a mixture thus prepared is pelletized and fired.
It should be noted however that the present invention is not directed to providing pellets, in each of which several macro-pores are formed naturally, but to providing pellets, in each o~ which macro-pores are intentionally di~persed.
The reason why the upper limit of grain sizes (in diameter of carbonaceous materials is set to 3 mm is that if the upper limit is exceeded, then there arises a difficulty in pelleti-zing. In addition, the reason why the grain sizes of carbonaceous materials are set to the size of macro-pores ~0.1 to 3 mm) is that carbonaceous materials are burnt out in the course of ~0 - 6a -;~

~39~Sgl ~
1 firing, so tha-t the~e remains in -the pellets, macro-pores of the same sizes as those of the carbonaceous materials.
In other words, according to the present invention, carbonaceous materials of relatively rough or large sizes are uniformly mixed with the raw ma-terial for the pellets in a suitable amount beorehand, and then a mixture thus prepared is fired in a firing stepr thereby intentionally Eorminy macro-pores in each of the pellets uniformly, while ore grains around the pores are tightly sintered together due to the heat produced by burning the carbonaceous materials, thereby imparting desired properties to the pellet products The inventors have conducted the following tests and experiments to determine the relationship between the quality or properties of the pellet products and the types and amounts of carbonaceous materials which affect a volumetric ratio of macro-pores produced to the entire pores as well as the sizes o macro-pores. Pellets used in these tests were prepared according to the following procedure: Coke breeze of sizes of 0.1 to 3 mm in diameter and brown coal of sizes of 0.1 to 2 mm were added as carbonaceous materials to hematite ore [grain of sizes of up to 44 ~...... 60 to 95~, grains of sizes of up to 10 ~u ... 15 to Z5~];
and then bentonite and water were added so as to hand-pelletize the materials into green pellets of sizes of 11 to 13 mm, after which the green pellets were charged in an Elema furnace for firing under the conditions of a given temperature, duration of heating and oxygen partial pressure. Then, the various kinds of propertie.s of the pellets were measured or determined.
Figs. 1 to 6 show the summary of such test results, in which there was established the relationship of macro-porosity to a softening temperature, melting down temperature, high g9~

1 temperature reduction degree, JIS reduction degree, compressive strength, and amount of carbonaceous materials added. In this respect, the term, "a high temperature reduction degree" as used herein is defined as the degree of reduction obtained ~hen the product pellets are reduced to FeO (Wustite) and then the pellets are reduced at a given temperature of 1250C under an atmosphere of CO:N2 = 30:70. In addition, the term "a softening temperature" is defined as the temperature when the pellets are heated under a load of 0.12 kg/cm2 and an atmosphere of CO:N2 = 30:70 and exhibit a contraction percentage of 10~, while"a melting temperature" is defined as a temperature, when the pellets contract abruptly and then start dropping off a vessel~
The test results reveal ~hat certain metallurgical properties of the pellets, such as high temperature reduction degree, JIS reduction degree, softening temperature, melting down temperature, are increased or enhanced, with an increase in macro- !
porosity. The macro~porosity of over 5% presents marked improvements in these properties. In addition, a particularly marked improvement is noted in a melting down temperature at a macro-porosity of over 15%. It can be seen from this that this tendency is independent of the types or kinds of carbonaceous materials. From the viewpoint of high temperature properties, it is desirable to define the macro-porosity of pellets to at least above 5%, and it is preferable to set a macro-porosity to above lS~. Turning to the physical properties of the pellets, as is apparent from Fig. 5, the compressive strength of the pellet products remains equal to those of the prior art pellets, as far as macro-porosity is up to 25~. However, when the macro-porosity exceeds 25%, the compressive strength of the pellets is sharply reduced. Meanwhile, the compressive strengths in the.se figures ~c~s~

l are found to be relatively low, because of a simplified pelleti~
zing operation, such as hand-pelletizing. In the practical operation, it was confirmed that the compressive strengtns o~ the pellets can be increased up to 150 to 200 kg/pellet. Fo~ desired physical properties of pellets, macro-porosity should be main-tained lower than 25~. ~ccordingly, the consideration of the metallurgical and physical properties of the pelle-ts leads to a conclusion that a ratio of macro-pores to the ent:ire pores should preferably range from 5 to 25%, and desirably from 15 to 25%.

Fig. 6 shows the amounts and types of carbonaceous materials which exert a large influence on the macro-porosity of pellet products. The amounts of carbonaceous materials added and macro-pores produced provide a positive linear correlation.
In the case of coke breeze, the upper limit of 25% of macro-porosity corresponds to 4% coke bre~3ze added. On the other hand, in the case of brown coal, the above upper limit corresponds to as low as 1.3~ brown coal. This may be attributed to the fact that brown coal contains a great amount of volatile matter (46%), as compared with coke breeze, so that Eormation of pores may be accelerated due to the gassifica-tion of volatile matter which presents high reactivity. In either case, the process for pro-ducing pellets ac`cording to the present invention imposes a limitation on the amount of carbonaceous materials of up to 4%.
No particular limitation is placed on the types of carbonaceous materials, Thus, carbonaceous materials which may be employed in the present invention include, in addition to coke breeze and brown coal referred to thus far, coals such as bituminous coal, hard coal and the like, and charcoals, and inorganic or organic materials such as foaming styxol, starch and the li~e.
However, materials which contain a great amount of volatile s~
matter or tend to produce a great amoun-t of gases are not recommended because these materials tend to produce crac~ing in the pellets themselves. Meanwhile, the more the caloriic value given by burning of carbonaceous materials, the higher the sintering degree of grains around the macro-pores. As a result, an increase in calorific value leads to prevention of the lowering in strength of the pellet products. For this reason, it is pre~erable to use high calorific carbonaceous materials, such as bituminous coal, hard coal, coke breeze and the like.

The fracture observation of pellets, after reduction, according to the tests given by the inventors reveals that the prior art pellets which are free of carbonaceous materials as used in the present invention provide a shell layer of metallic iron in the outer peripheral portions of the pellets, and they exhibit evidence of retardation of :eduction in the inner portions of the pellets, and that the pellets having macro-pores produced by coke breeze and the like according to the present invention are uniformly reduced up to the inner portions of the pellets, although this phenomenon is more evident in a high temperature reduction, rather than in lower temperature reduction (900 C according to JIS).
According to a preferred embodiment of the present invention, a flux containing MgO is added to the raw materials for pellets, toge~ther with carbonaceous materials, in an attempt to raise the melting point of the slags to be formed, as well as to prevent the reduction retardation in a high temperature range by dispersing macro-pores throughout the interior of each of the fired iron-ore pellets.
More specifically, carbonaceous ma-terials of sizes of 0.1 to 3 mm in diameter, and flux such as dolomite containing -- ~.0 --3S3~

1 M~O were added to the raw material -Eor pellets in a~oun~s of up to 4%, and up to 3%, respectively. In this respect, dolomite contains grains of sizes of up to 44 ~ in an amount of over 60%.
Then, a mixture thus prepared was pelle~ized and fired.
The reason why the amount of MgO is limited up to 3QO is that when the amount of MgO exceeds 3%, the meltiny down temperature of pellets is no-t raised, so that the efects of softening and contraction of the pellets are lost, ~ith the

4 accompanying lowering in reducibility.
In addition, according to the process of the present invention, carbonaceous materials and MgO flux of relatively large or rough sizes are mixed with raw ma-terials for pellets in an suitable amount, beforehand, and then a mixture is pelletized for providing green pellets. Then, the green pellets are sub]ected to a preheating firing process for burning carbonace-ous materials, thereby intentionally forming macro-pores in each of pellets in a uniformly dispersed manner, while the ash of the carbonaceous materials, gangue mineral, flux and the like are formed as slags, thereby providing tightly sintered pellets.
According to a yet another aspect of the present invention, the flux to be added to the raw material for the pellets may be carbonate, so that an endothermic reaction in a preheating process may be supplemented by heat given by the burning of carbonaceous materials.
The features of the present invention will be illustrated with reference to examples.
0, 1.5, 3.0% of coke breeze as carbonaceous material, of sizes of 0~1 to 3 mm in diameter, and 0, 3, 6, 9% of dolomite containing MgO (grains of sizes of up to 44 ~ .... 70%, grains of sizes up to 10 ~ .... 52%) were added to the raw ~g95~8 1 material ~or ~he pellets (grains of sizes of up to 44 ~ .... 60 to 70%, grains of sizes of up to 10 ,u ...~ 10 to 20~, respectively.
(meanwhile, limestone is added so as to adjust the ratio of CaO/SiO2 contained in the material ko 1.2.) Therea~ter, the raw materials wexe pelletized into green pellets of sizes of 10 to 12 mm in a tire type pelletizer, followed by preheating and firing in an Elema furnace under given conditions including temperature, firing duration, and oxygen partial pressures, thereby providing fired iron-ore pellets containing slag struc-tures and macro-pores of sizes of 0.1 to 3 mm in diameter.
Evaluation of the quality of the pellets thus prepared were made according to measurements of physical properties and metallurgical properties of pellet products, the former including the s~rength of the pellets to transportation and handling, porosities of elements affecting the strength and reducibility of pellet products, and amounts of FeO, and the I latter including reducibility at a high temperature, and softening and melting down temperatures of the pellets, because as has been described earlier, the characte~istics of the pellets in a belly of a blast furnace and in the lower portion thereof largely affect the yields of the blast furnace.
Figs. 7 to 10 shows the results of measurements of the pellet products in terms o the amount of coke breeze added, versus high temperature reduction degree, softening temperature, meltin~ down temperature, compressive strength, macro-porosity - and FeO amount. What is meant by a high temperature reduction degree is a percentage of reduction obtained when samples are reduced to FeO beforehand, and then reduced at a given temperature of 1250 C under an atmosphere of CO:N2 = 30:70. In additi.o~, the softening temperature is defined as a temperature at which the

5~13 pelle~ products are heated under a load o~ O.12 ky/cm2 and an atmosphere of CO:N2 = 30:70, ana the contraction percent of pellets reaches 10%. The melting temperature is defined as a temperature at which the pellets contract abruptly and then start dropping off a vessel.
The results of these tests reveal that the high temperature properties of pellets as shown in Figs. 7 and 8, such as high temperature reduction degree, softening ~emperature and meltiny down -temperature of pellets containing dolomite and coke breeze, are improved as compared with those obtained from the prior art self-flu~ing pellets containing limestone. In addition, a combination of dolomite and coke breeze provides further improvements over either of these cases.
The high temperature reduction degree peaks at an amount of dolomite of 6~. However, a combination of dolomite and coke breeze in which the coke breeze is added to 3% dolomite, is superior in effect to coke breeze when used alone. Substanti-ally, constant reduction degree is obtained in a combination of coke breeze (3%) and dolomite, irrespective of the amounts of dolomite used.
In other words, although the a~dition of dolomite is effective in o'otaining the improvements in the high temperature reduction degree, the dispersion of macro-pores are laryely affected by the addition of coke breeze so that the use of coke ~reeze may reduce the amount of dolomite to be used. In other words, the quality of iron in the fired pellets may be improved.
The softening temperature and melting down temperature may be both largely improved due to the addition of dolomite.
The effect of coke breeze may be proved according to the tests on the pellets free of dolomite. It is preferable in the case of - 13 ~

s~

1 a softening temperature to use a combination of dolomite and co~e breeze which provides a higher softening temp~rature than tha~
obtained when dolomite is used alone. This tendency is enhanced with an increase in the amount of coke breeze. Particularly, the effect of coke breeze of 1.5% is most remarkable.
It follows from this that the dispersion of macro-pores due to addition of coke breeze largely affects the reduci-bility of pellets, while the softening temperature and melting down temperature are dependent on the addition of dolomite. The combined use of coke breeze and dolomite provides bet~er results for high temperature properties than either of cases of adding a single element alone. This may be considered to be due to their multiplicated or synergistic effect. Accordingly, from a view-point of the high temperature properties, pellets containing 3 to 9% dolomite and up to 3% coke breeze are much more preferable than those prior art self-fluxing pellets containing limestone.
! Turning to the physical properties, the compressive strength of the pellets remains unchanged relative to the amount of dolomite added as shown in Fig. 9. An increase in amount of coke breeze added leads to lower compressive strength in either case. ~owever, the lowering in the compressive strength in either case is not marked. Even in the case of the addition of 3% coke breeze, the compressive strenyths of 250 kg/pellet to 270 kg/pellet may be maintained which establishes that the strength is high enough for transportation and handling. The compressive strength of the pellets is governed by the amount of FeO and the porosity of the fired pellets. The addition of dolomite (MgO) and coke breeze increases the amount of FeO and porosity.

~ 14 -5~l~

1 In this respect, a lowering in compressive strength is not notable as compare~ with an increase in amount of FeO and in porosity. (See Fig. l0). This may be attributed to a larye calorific value due to the burning of the coke breeze and an improved sintering condition of grains around macro-pores in pellets.
In either case, the combined use of dolomite and coke breeze of relatively rough sizes provides their inherent advantages in combination, thereby presenting excellent pellet products. Examples given thus far refer to a combination of dolomite and coke breeze. Howevex, the present invention is by no means limited to this example. Serpentine and magnesia clinker may be used as flux as far as these contain MgO. In addition, inorganlc and organic materials such as f~aming styrol, starch and the like other than brown coal, bituminous coal, haxd coal and the like, may be used as macro-pore forming agents.
However, it is not preferable to use macro-pore forming agents which contain a volatile matter tending to produce a great amount of gases at a relatively low temperature (lower than 500C) because such agents tend to produce cracking in the pellets themselves.
The advantages of the present invention may be summarized as follows:
(l) Pellets are obtained in which macro-pores are dispersed and which provide high softening and melting down temperatures because of formation of iron oxide and slag containing MgO, and are free of retardation of reduction.
(2) Not only the porosity but also the amount of FeO in the pellet products is relatively increased, also, the sintering degree of the pellets may be improved due to the burning of the coke breeze so that desired compressive strength may be maintained.

5~8 1 (3) In the p~eheating and firing process, the burning of the coke breeze may supplement the heat in a gra~e or kiln, so that the firing temperature may be raised while shorkeniny the firing duration and hence improving the productivity of the furnace.
(4) According to the prior art, magnetite concentrate is used for producing an oxidation calorific value thereof. In contrast thereto, the present inventions find no need to use magnetite concentrate but uses hematite-based ore alone. In addition, a 1~ reduction in temperature due to the decomposition heat of the carbonate in the preheating process may be well compensated for.
(5) The addition of coke breezc may xeduce the amount of flux to be used.

(6) Coke breeze which is undersize of cokes for a blast furnace has been consumed in a sintering plant. However, coke breeze in excess may be used in a pelletizing plant.
While a description has been had of the results of certain tests and experiments, it is apparent that the present invention provides fired iron-ore pellets which are superior in high temperature properties as well as in room temperature properties. The use of pellets according to the present invention for a blast furnace may improve the gas efficiency in the fuxnace and insures a high reduction degree in a high temperature range, thereby eliminating problems, such as the softening and the sticking phenomena, non-uniform flow of gas hanging, slip and the like, thereby improving the productivity of a blast furnace, and reducing the coke ratio. In addition, carbonaceous dust stemming from an iron works may be effectively utilized.
The foregoing information and examples are presented herein for illustrative purposes only and are not intended to unduly limit the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Fired iron-ore pellets characterized in that macro-pores of sizes of 0.1 to 3 mm in diameter are intentionally dispersed in each of said pellets at ratios of up to 25% to the entire pores contained therein.

2. Fired iron-ore pellets as defined in claim 1 wherein said pellets are provided with slay structures containing MgO.

3. Fired iron-ore pellets as defined in claim 2 wherein the amount of MgO to be added to raw materials for pellets is up to 3% by weight.

4. Fired iron ore pellets as defined in claim 1 wherein the range of ratios of said macro-pores is between 5 and 25% of the entire pores contained in each of pellets.

5. Fired iron-ore pellets as defined in claim 1 wherein the range of ratios of said macro-pores is between 15 and 25% of the entire pores contained in each of pellets.

6. A process for producing fired iron-ore pellets in which raw material for pellets are crushed, pelletized and fired, characterized in that after crushing of said raw material, carbonaceous material of sizes of 0.1 to 3 mm in diameter are added thereto in an amount of up to 4% by weight.

7. A process as defined in claim 6 wherein carbonaceous material of sizes of 0.1 to 3mm in diameter and flux containing MgO are added to the raw material for the pellets.

8. A process as defined in claim 7 wherein said flux containing MgO is added in an amount of up to 3% by weight.

9. A process as defined in claim 6 wherein macro-pores of sizes 0.1 to 3 mm in diameter are produced in each of said pellets dispersed therein at ratios of up to 25 per cent of the entire pores contained therein,

10. A process as defined in claim 9 wherein the range of ratios of said macro-pores in said fired iron-ore pellets is between 5 and 25% to the entire pores contained therein.

11. A process as defined in claim 9 wherein the range of ratios of said macro-pores in each of said fired iron-ore pellets is between 15 and 25% to the entire pores contained therein.