CA1070953A - Agglomerates for use in a blast furnace and method of making the same - Google Patents
Agglomerates for use in a blast furnace and method of making the sameInfo
- Publication number
- CA1070953A CA1070953A CA244,332A CA244332A CA1070953A CA 1070953 A CA1070953 A CA 1070953A CA 244332 A CA244332 A CA 244332A CA 1070953 A CA1070953 A CA 1070953A
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- CA
- Canada
- Prior art keywords
- sio2
- iron ore
- pellets
- cao
- agglomerates
- Prior art date
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B1/00—Preliminary treatment of ores or scrap
- C22B1/14—Agglomerating; Briquetting; Binding; Granulating
- C22B1/24—Binding; Briquetting ; Granulating
- C22B1/242—Binding; Briquetting ; Granulating with binders
- C22B1/243—Binding; Briquetting ; Granulating with binders inorganic
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Geology (AREA)
- Environmental & Geological Engineering (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Life Sciences & Earth Sciences (AREA)
- Inorganic Chemistry (AREA)
- Manufacturing & Machinery (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Manufacture And Refinement Of Metals (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Agglomerates for use in a blast furnace containing a cementitious material and formulated to maintain the CaO to SiO2 ratio in a range of from 1.2 to 1.9 and the slag forming ratio in a range of from 13 to 19% which when charged in a blast furnace assume an improved drafting aspect in the smelting reaction zone, The as-formed discrete moist agglomerates are to be cured without the necessity of a powder matrix prior to introduction to the furnace, whereby an adequate crushing strength is imparted to the agglomerate product to reduce handling difficulties resulting from the production of fines.
Agglomerates for use in a blast furnace containing a cementitious material and formulated to maintain the CaO to SiO2 ratio in a range of from 1.2 to 1.9 and the slag forming ratio in a range of from 13 to 19% which when charged in a blast furnace assume an improved drafting aspect in the smelting reaction zone, The as-formed discrete moist agglomerates are to be cured without the necessity of a powder matrix prior to introduction to the furnace, whereby an adequate crushing strength is imparted to the agglomerate product to reduce handling difficulties resulting from the production of fines.
Description
~L~7~g53 This inven-tion relates to a charge composi-tion for smelting furnaces, and a method of forming non-fired agglomerates such as pellets or briquettes therefrom which show excellent reducibility and do not in-terfere with the proper smelting reac-tions and temperatures.
In order to facilitate the smelting of a charge including finely divided iron ore while preventing the iron ore fines from being scattered about in the smelting furnace and also insuring satisfactory smelting reactions, it has been the prior art practice to form the charge composition into agglomerates such as pellets or briquettes prior to charging to the smelting furnace.
The utilization of pellets or briquettes in a smelting furnace such as a blast furnace is itself not without problems for the reason that the pellets formed from the charge composition are frequently found to possess an inadequate crushing strength for convenient handling from a travelling grate system to a fur~
nace smelting system as well as for the smooth smelting procedure in which the charged pellets are forced to move downward under the action oE gravity and the weight of the pellets accumulated there- `
ZO on, thereby an excessive amount of fines tend to be formed in the~
smelting furnace.
One approach to the impartment of an adequate crushing strength to the pellets or briquettes is that the finely divided iron ore is mixed with limestone, coal and bentonite as a binder, and the mixture after, if necessary, moistened with a proper amount of water is agglomerated and then fired at elevated temp-eratures as high as several hundreds of centigrades to produce indurated pellets or briquettes. This method, however, has a drawback resulting from the provision of the firing step which ~;
makes complicated the process for producing iron from the iron ore and consequently increases the production cost of iron.
Another drawback is that the gas discharged from the firing step .. , ~ , .
contains poisonous compounds such as Sx and N0x which must be removed, or otherwise they would pollute the enviror~ment of air.
Many attempts have been made to eliminate the firing step from the processing operation for the production of indurated pellets or briquettes and a technique has been proposed to provide non-fired pellets or briquettes having a crushing strength large enough to prevent an unacceptably excess amount of fines from being produced from the pellets or briquettes during the travelling and smelting opera-tions. For example, the finely divided ore is blended with a binder having a hydraulic property such as Portland cement clinker, then the blend after moistening is formed into discrete molst pellets, and then the pellets are cured as buried in a power matrix which rnay be identical in material to the used ore. This agglornerate composition of non-fired pellets and the process for making same, however, have much to be desired, which is explained in more detail below.
The prior art non-fired pellets, though having a crushing strength not inferior to that of the fired pellet at ordinary temperature, are found to have inferior reducibility and interfere with the proper smelting reactions and temperatures, because of the scabbing and premature softening phenomena of the pellets occurring as the temperature of the smelting zone ranges from 500 to 1,400C. I`he prior art process for making non-fired pellets, on the other hand, as, for example, disclosed in Japanese Patent Publication No. Sho 46-32324, includes a curing step in which the as-formed discrete moist pellets are buried in ore powders as a matrix to avoid the adhesion of the pellets with one another until the cementitious material is completely cuxed n After the curing has been completed, it is necessary to rid the pellets of the ore powder by means of screening. However carefully the screening is carried out, it is difficult to reduce to zero the residual amount of ore fines adhering to the surfaces of the pellets which when ~L~7~53 `
charged into a smel-ting furnace such as a blast furnace tends to reject some of the adhered ore fines. When -the reiected ore fines are accumulated in -the Eurnace, it is founcl that the degree of passability of forced air or oxygen draft is gradually decreased, and that when the amount of fines produced is excessive, the handling difficulties become serious. Furthermore, the screening operation is very time-consuming, and the necessary iron ore powder as the matrix amounts to more than 30% based on the total weight of the charge pellets.
Accordingly, the present invention has for the general object to eliminate the above-mentioned conventional drawbacks, and to provide non-firedAagglome~ates which do not interfere with the proper smelting reactions, and a process for making non-fired agglomerates without the necessity of using the powder matrix which would be otherwise necessary to prevent the adhesion of the ~ ~
pellets during the curing operation. ~-To achieve this, at first, the composition of agglomera-tes such as pellets and briquettes is formulated so that the ratio CaO/SiO2 is in a range of from 1.2 to 1.9, and the amount of slag forming rnaterial is in a range of l~/o to 19% based on the total weight of the non-fired agglomerate, whereby the drafting aspect of the indurated pellets or briquettes in the smelting zone are improved.
Secondary, the present invention has an object to improve the properties of the agglomerates by adjusting the composition, namely the CaO/SiO2 ratio and the proportion of the slag forming material through blending of iron ores.
Thirdly, the curing of the as-formed discre-te moist agglomerate such as pellets or briquettes is carried out without using any powder matrix to impart excellent high-temperature properties to the resultant cured agglomerates without sacrificing the reducibili-ty of the agglomerate charge, whereby the handling ~(:1 7C~g~3 difficulties of -the pellets or briquettes due to -the adhesion of fines are minimized.
For the purpose of fur-thermore reducing the possibility of assuming the adhering aspects of the mo:ist pellets during the curing operation with no matrix, an over-coating of an inorganic substance is applied around each of the moist pellets before subjecting the curing operation.
Fourthly, the present invention has an object to provide an improved agglomeration method which can give satis-factory agglomerates without using any powder matrix during thecuring.
These and other features of the present invention will be more readily unders-tood when the following description is read in connection with the accompanying drawings.
The present invention is characterized in that a mixture is prepared by mixing and grinding iron ore powder, additives and hydraulic binder in a proportion so as to assure high-temperature properties of resultant agglomerates, or by mixing and grinding the required total amount of water-hardening binder with parts of iron ore powder and additives and mixing the resultant mixture with the remaining amounts of iron ore powder and additives or by mixing and grinding the required total amounts of hydraulic binder and additives with a part of iron ore powder and mixing the resultant mixture with the remaining amount of iron ore powder, and then the mixture obtained above is moistened and agglomerated.
The hydraulic binder used in the present invention includes portland cement, portland cement clinker, alumina cement, alumina cement clinker, cement mixed with blast furnace slag, cement mi.xed with fly ash, cement mixed with borazon and masonry-mixed cement.
The additives used .in the present inven-tion include silica stone, beach sand, shirasu, blast furnace slag, electric ~L~709~3 furnace slag, dust from ferro-silicon production, serpentine, peridotite, limestone, quick lime,slabbed lime, dolomite, con-' verter slag, etc.
FIGURE l is a graph showing the production deficiency as a function of the type of the charge composition according to the prior art, wherein the ordinate is in production deficiency index, and the sintered iron ore is taken as a reference charge composition and has t.herefore 10 ~ unit index.
FIGURE 2 is a graph showing the degree of softening tendency of a non-fired pellet as a function of the CaO to SiO2 ratio with the amount of slag produced on smelting being taken as a para-meter, wherein the degree of softening tendency i5 expressed in terms of press~re.loss in forced air draft, and the slag ls taken as consisting of CaO, SiO2 and Al2O3. .~
PIGURE 3 is a graph showing the production deficiency i ~.
20 as a function of CaO/SiO2 in a pellet produced in accordance~with the present invention in ~;
comparison with that according to the prior art.
FIGURE 4 is a graph showing the effectiveness of iron ore blending on the pressure loss at 1,200C.
with variation of CaO/SiO2 of non-fired pellets.
FIGURE 5 is a graph showing the dependence of concentra-tion of SiO2 in the non-fired pellet on the source of iron ore for effecting equivalent CaO/SiO2 ratios.
FIGURE 6 is a graph showing a relationship between the concentration of SiO2 in an iron ore blend and ; ~
~7()953 the pressure loss of pellets made up the~efrom.
FIGURE 7 is a graph showing the dependence of -the apparent pellet density on the type of the pelletizing procedure and also the superiorlty of the two-step blending of the charye composition to the one-step blending.
In the blast-furnace process, the ixon-bearing materials including pellets and briquettes are charged into the blast furnace from the top thereof and are reduced during their downward movements in the blast furnace, and soften and melt down due to increased furnace temperature and pressure imposed thereon.
Approaching the bottom of the furnace, the particular section of the charge enters a softening zone from which it proceeds to the smelting reaction zone and is ultimately to form into both liquid iron and liquid slag. When the softening zone operates in a relatively lower temperature range and when the temperature range is relatively wider with a longer distance along the furnace, a higher pressure for the blast air mus-t be employed, or otherwise the reduction reactions would be retarded and the temperature dis-tribution would be made uneven to,result in a decrease of pro-duction efficiency. Based on this fact, it is preferred to evaluate the quality of the charge in terms of production deficiency index. In practice, the production deficiency index is determined in such a manner that at first a reference charge is processed under a certain production efficiency to measure the correspondingly necessary pressure of air blast, and then a given charge is pro-cessed with an air blast equivalent in pressure to the previously measured one to derive a production deficiency index value by dividing the former or reference production efficiency by the resultant one. As shown in Figure 1, two charge compositions of non-fired pellets have production deficiency index values as high .
: :
~L~)7~3S3 as two and higher than those of fired.pellets.
For improving the quality of non-fired pellets with respect to the produc-tion efficiency, the present inventors have conducted various experiments and have now found a mechanism of causing such increases in the production deficiency index value of the pellets. This mechanism depends upon the following two main factors~ A 'Iscabbing'l phenomenon occurs which leads to an apparent softening effect of the pellets, and (2) A Fe-silicate slag material having a relatively lower melting point is formed in a large proportion to the total charge weight to cause the premature softening effect of the pellets. :
Therefore, in order to produce non-fired agglomerates such as pellets and briquettes having excellent load-softening property, it is enough to prevent the softening due to the above factors. Regarding the softening due to the scabbing, it is natural that the scabbing phenomenon itself must be supressed, and the softening due to the factor (2) can be prevented by increasing the basicity of the whole charge so as to cause melting of FeO into the slag.
On the basis of this discovery, the present invention contemplates to improve the properties of the charged pellets in the softening zone of the smelting furnace by suppressing the softening tendency of the non-fired pellet attributable to these factors. As is well known in the art, the scabbing material is iron which is formed on the surfaces of the pellets when the pellet is heated to about 900C. in a CO atmosphere, and the occurrence of this scabbing phenomenon is limited to that area of the surface of the pellet which is occupied with the exposure surfaces of iron ore particles contained in the pellets.
According to the present invention, the part of the softening tendency of a pellet ascribable to the first-named factor or ''scabbing" phenomenon is suppressed by minimizing the : ' ` ~7~953 propor-tion of -the sum of the exposed areas of iron ore particles at the surface of the pellet. By reference to Figure 2 it is to be understood that a high quality iron ore designated by "A"
analyzing about 2~0O/o SiO2, when formulated with 9% cementitious binder provides a pressure loss value of more than 900 mmH2O
which may be considered -to depend upon that part of the softening tendency which is a-ttributable to the "scabbing". This is mainly because (l) those constituents of the charge composition which are to form slag materials occupy so small a volume corresponding to a weight of 10.6% that the swm of the exposed areas of iron ore particles is relatively large as compared with the entire surface of the pellet, and (2) the CaO to SiO2 ratio is as high as 1.6, which is unfavorable for formation of slag materials in -the reducing reaction zone, so that it is made more difficult to suppress -the increase o~ the sum of the exposed areas of iron ore particles. ~With such an iron ore, therefore, the present invention provides means of adjusting for the concentration of slag-forming constituents as well as for the CaO/SiO2 ratio in the slag, na.mely, the basicity. It is to be noted that although the slags pro-duced in the well known blast furnaces are generally found tocontain MgO besides CaO, Al2O3 and sio2, the term "slag-forming constituents" herein used is to exclude MgO, because it does not contribute to the slag formation so long as the temperature is-maintained at a low level of about l,000C.
With reference to Figure 2, as the basicity is decreased with increase in the amount of SiO2 added, the pressure loss of a resulting charge composition containing the iron ore "A" is decreased, reaching a minimum value of about 500 mm~I20 at a basicity of about 1.3, and then increased to more than g00 mm~I2O, as indicated by the dot-and-dash curve. The reason why -the pressure loss increases as the basicity is decreased from 1.3 is that the solubility of FeO in the Fe-Silicate slag increases with ~7~353 incrcase of -the amount oE low-meltiny po:int slag which constitutes the other part of the softening tendency which is ascribed to the second-named factor as mentioned above. For this reason, accord-ing to the present invention, while maintalniny -the basicity constant at a level of 1.3, the concentration of the slay-forming constituen-ts i9 increased to more than 13.0'~o in order to insure ,;
that the pressure loss value falls in a range below 200 r~I20 as indicated by the oblique lines. This range is confined with the acceptable range for the typical conventional fired pellets.
In the case of the iron ore "~", the increase of weight proportion of the produced slag is effected by previous addition of additional granulated blast furnace,slag to the charge composition prior to the pelletizing procedures.
In the case of another type of iron ore designated by "B" analyzing about 5% SiO2, a charge composition formulated by adding 9% cementitious binder to the fines of iron ore "B"
provides non-fired pellets having a softening tendency of more than 1,000 mm~I2O in pressure loss, as shown by the solid curve in Figure 2. This i9 because the amount of slag-forming con-stituents is as high as 15% based on the total weight of the charge, and because the basicity,is as low as 0.9. Such a ~, situation promotes the reaction of the slag-formable constituents with FeO at relatively lower temperatures of a range in which the reducing reaction zone operates, thereby causing a relatively larger amount of Fe-Silicate slag to be formed and therefore causing a large,amount of the so-called low-melting po~nt slag.
In this connection, it is to be noted that the occurrence of the "scabbing" phenomenon is negligible because the exposure of the iron ore particles out of the pellet surfaces is suppressed as the low-melting point slag is produced in excess. With this type of iron ore, according to the present invention, a CaO-source material or materials is or are formulatecl into a charge _ g _ ~7~9S3 composition prior to -the pelletizing procedures to account for the adjustment of basicity CaO/SiO2. As the amount of the CaO-source materlal added is i~creased to increase the basicity from 0.9, the pressure loss of the pellet charge is decreased entering the acceptable range below 200 mmll20 at a basicity of 1.2. The basicity may be :-increased to as high as 1.9 to e,ffect acceptable results, and there is no need to increase the basicity from 1.9 as the possibility of encountering situations requiring the . 10 adjustment of basicity to more than 1.9 is very small in : practice. In addition thereto, the solid and dashed curves indicate that the pellet charge with a basicity of more than 1.9 tends to deteriorate the drafting aspect. The charge composition consisting of iron ore "B" and 9%
ce~ent.itious binder provides slag-forming constituents in an amount of about 15U/o which is more than enough to account for ' the suppression of occurrence of the scabbing phenomenon.
~In this respect, an additional increase in the concentration of the slag-forming constituents caused by the adjustment of the basicity is not intended, but the concentration of slag-forming constituents in a charge,composition of the present '' invention may be increased to as large as 19% inasmuch as the pressure loss of the resulting pellet charge falls in the ~' acceptable range below 200 mmH20.
It will be appreciated from the foregoing that the ' ,~
present invention contemplates to formulate a charge composition ~;:
prior to the pelletizing operation as having a basicity in a . range of from 1.2 to 1.9 by addition of additional CaO-and/or SiO2-so'urce materials or material, and as having slag-forming :
constltuents in a range4of from 13% to 19% by addition of an additional slag-source material.
.. . . . .
~7~353 As a b:inder having a hydraulic property, use may be made of Portland cement, Portland cement clinker, alumina cement, . ..
alumina cemen-t clinker, blast furnace slag mi.xed cement, fly ash-mixed cement, pozzolan-mixed cement and masonry-mixed cement. As a CaO-source ma-terial, use may be made of lirnestone, hydro cake, and dolomiteO As a SiO2-source material, use may be made of seashore sand, periodotite, serpentine, fire brlck chips and acid slag. As an additive for increasing slag percentage, use may be made of the above-mentioned materials and blast furnace slag or blast furnace granulated slag.
For exemplary purposes, a number of formulations based on the principle of the invention will be next illustrated which may be used .in the pelletizing process of the inventi.on to be described later.
xample I
Use was made of two iron ores, the one (A) of the ores analyzing a lower content of SiO2 being supplied fro.m the magnetiteseries after concentrated by magnetic separation, and the other ore (B) from the hematite group after pulverized~ The iron ores A and B were mixed with Portland cement, limestone, white sand and blast furnace granulated slag in the following proportions.
- 11 - ' .
Table 1 Blas-t furnace charge composition IronPoxtland Lime- White Slag Basicity Final Sample Orecement stone Sand added ~CaO/SiO2) slag No. (%) (%) (%) (%~ (%) (%) . ~
1 88.3 (Ore A) , 9.0 - 2.7 - 1.23 13.0
In order to facilitate the smelting of a charge including finely divided iron ore while preventing the iron ore fines from being scattered about in the smelting furnace and also insuring satisfactory smelting reactions, it has been the prior art practice to form the charge composition into agglomerates such as pellets or briquettes prior to charging to the smelting furnace.
The utilization of pellets or briquettes in a smelting furnace such as a blast furnace is itself not without problems for the reason that the pellets formed from the charge composition are frequently found to possess an inadequate crushing strength for convenient handling from a travelling grate system to a fur~
nace smelting system as well as for the smooth smelting procedure in which the charged pellets are forced to move downward under the action oE gravity and the weight of the pellets accumulated there- `
ZO on, thereby an excessive amount of fines tend to be formed in the~
smelting furnace.
One approach to the impartment of an adequate crushing strength to the pellets or briquettes is that the finely divided iron ore is mixed with limestone, coal and bentonite as a binder, and the mixture after, if necessary, moistened with a proper amount of water is agglomerated and then fired at elevated temp-eratures as high as several hundreds of centigrades to produce indurated pellets or briquettes. This method, however, has a drawback resulting from the provision of the firing step which ~;
makes complicated the process for producing iron from the iron ore and consequently increases the production cost of iron.
Another drawback is that the gas discharged from the firing step .. , ~ , .
contains poisonous compounds such as Sx and N0x which must be removed, or otherwise they would pollute the enviror~ment of air.
Many attempts have been made to eliminate the firing step from the processing operation for the production of indurated pellets or briquettes and a technique has been proposed to provide non-fired pellets or briquettes having a crushing strength large enough to prevent an unacceptably excess amount of fines from being produced from the pellets or briquettes during the travelling and smelting opera-tions. For example, the finely divided ore is blended with a binder having a hydraulic property such as Portland cement clinker, then the blend after moistening is formed into discrete molst pellets, and then the pellets are cured as buried in a power matrix which rnay be identical in material to the used ore. This agglornerate composition of non-fired pellets and the process for making same, however, have much to be desired, which is explained in more detail below.
The prior art non-fired pellets, though having a crushing strength not inferior to that of the fired pellet at ordinary temperature, are found to have inferior reducibility and interfere with the proper smelting reactions and temperatures, because of the scabbing and premature softening phenomena of the pellets occurring as the temperature of the smelting zone ranges from 500 to 1,400C. I`he prior art process for making non-fired pellets, on the other hand, as, for example, disclosed in Japanese Patent Publication No. Sho 46-32324, includes a curing step in which the as-formed discrete moist pellets are buried in ore powders as a matrix to avoid the adhesion of the pellets with one another until the cementitious material is completely cuxed n After the curing has been completed, it is necessary to rid the pellets of the ore powder by means of screening. However carefully the screening is carried out, it is difficult to reduce to zero the residual amount of ore fines adhering to the surfaces of the pellets which when ~L~7~53 `
charged into a smel-ting furnace such as a blast furnace tends to reject some of the adhered ore fines. When -the reiected ore fines are accumulated in -the Eurnace, it is founcl that the degree of passability of forced air or oxygen draft is gradually decreased, and that when the amount of fines produced is excessive, the handling difficulties become serious. Furthermore, the screening operation is very time-consuming, and the necessary iron ore powder as the matrix amounts to more than 30% based on the total weight of the charge pellets.
Accordingly, the present invention has for the general object to eliminate the above-mentioned conventional drawbacks, and to provide non-firedAagglome~ates which do not interfere with the proper smelting reactions, and a process for making non-fired agglomerates without the necessity of using the powder matrix which would be otherwise necessary to prevent the adhesion of the ~ ~
pellets during the curing operation. ~-To achieve this, at first, the composition of agglomera-tes such as pellets and briquettes is formulated so that the ratio CaO/SiO2 is in a range of from 1.2 to 1.9, and the amount of slag forming rnaterial is in a range of l~/o to 19% based on the total weight of the non-fired agglomerate, whereby the drafting aspect of the indurated pellets or briquettes in the smelting zone are improved.
Secondary, the present invention has an object to improve the properties of the agglomerates by adjusting the composition, namely the CaO/SiO2 ratio and the proportion of the slag forming material through blending of iron ores.
Thirdly, the curing of the as-formed discre-te moist agglomerate such as pellets or briquettes is carried out without using any powder matrix to impart excellent high-temperature properties to the resultant cured agglomerates without sacrificing the reducibili-ty of the agglomerate charge, whereby the handling ~(:1 7C~g~3 difficulties of -the pellets or briquettes due to -the adhesion of fines are minimized.
For the purpose of fur-thermore reducing the possibility of assuming the adhering aspects of the mo:ist pellets during the curing operation with no matrix, an over-coating of an inorganic substance is applied around each of the moist pellets before subjecting the curing operation.
Fourthly, the present invention has an object to provide an improved agglomeration method which can give satis-factory agglomerates without using any powder matrix during thecuring.
These and other features of the present invention will be more readily unders-tood when the following description is read in connection with the accompanying drawings.
The present invention is characterized in that a mixture is prepared by mixing and grinding iron ore powder, additives and hydraulic binder in a proportion so as to assure high-temperature properties of resultant agglomerates, or by mixing and grinding the required total amount of water-hardening binder with parts of iron ore powder and additives and mixing the resultant mixture with the remaining amounts of iron ore powder and additives or by mixing and grinding the required total amounts of hydraulic binder and additives with a part of iron ore powder and mixing the resultant mixture with the remaining amount of iron ore powder, and then the mixture obtained above is moistened and agglomerated.
The hydraulic binder used in the present invention includes portland cement, portland cement clinker, alumina cement, alumina cement clinker, cement mixed with blast furnace slag, cement mi.xed with fly ash, cement mixed with borazon and masonry-mixed cement.
The additives used .in the present inven-tion include silica stone, beach sand, shirasu, blast furnace slag, electric ~L~709~3 furnace slag, dust from ferro-silicon production, serpentine, peridotite, limestone, quick lime,slabbed lime, dolomite, con-' verter slag, etc.
FIGURE l is a graph showing the production deficiency as a function of the type of the charge composition according to the prior art, wherein the ordinate is in production deficiency index, and the sintered iron ore is taken as a reference charge composition and has t.herefore 10 ~ unit index.
FIGURE 2 is a graph showing the degree of softening tendency of a non-fired pellet as a function of the CaO to SiO2 ratio with the amount of slag produced on smelting being taken as a para-meter, wherein the degree of softening tendency i5 expressed in terms of press~re.loss in forced air draft, and the slag ls taken as consisting of CaO, SiO2 and Al2O3. .~
PIGURE 3 is a graph showing the production deficiency i ~.
20 as a function of CaO/SiO2 in a pellet produced in accordance~with the present invention in ~;
comparison with that according to the prior art.
FIGURE 4 is a graph showing the effectiveness of iron ore blending on the pressure loss at 1,200C.
with variation of CaO/SiO2 of non-fired pellets.
FIGURE 5 is a graph showing the dependence of concentra-tion of SiO2 in the non-fired pellet on the source of iron ore for effecting equivalent CaO/SiO2 ratios.
FIGURE 6 is a graph showing a relationship between the concentration of SiO2 in an iron ore blend and ; ~
~7()953 the pressure loss of pellets made up the~efrom.
FIGURE 7 is a graph showing the dependence of -the apparent pellet density on the type of the pelletizing procedure and also the superiorlty of the two-step blending of the charye composition to the one-step blending.
In the blast-furnace process, the ixon-bearing materials including pellets and briquettes are charged into the blast furnace from the top thereof and are reduced during their downward movements in the blast furnace, and soften and melt down due to increased furnace temperature and pressure imposed thereon.
Approaching the bottom of the furnace, the particular section of the charge enters a softening zone from which it proceeds to the smelting reaction zone and is ultimately to form into both liquid iron and liquid slag. When the softening zone operates in a relatively lower temperature range and when the temperature range is relatively wider with a longer distance along the furnace, a higher pressure for the blast air mus-t be employed, or otherwise the reduction reactions would be retarded and the temperature dis-tribution would be made uneven to,result in a decrease of pro-duction efficiency. Based on this fact, it is preferred to evaluate the quality of the charge in terms of production deficiency index. In practice, the production deficiency index is determined in such a manner that at first a reference charge is processed under a certain production efficiency to measure the correspondingly necessary pressure of air blast, and then a given charge is pro-cessed with an air blast equivalent in pressure to the previously measured one to derive a production deficiency index value by dividing the former or reference production efficiency by the resultant one. As shown in Figure 1, two charge compositions of non-fired pellets have production deficiency index values as high .
: :
~L~)7~3S3 as two and higher than those of fired.pellets.
For improving the quality of non-fired pellets with respect to the produc-tion efficiency, the present inventors have conducted various experiments and have now found a mechanism of causing such increases in the production deficiency index value of the pellets. This mechanism depends upon the following two main factors~ A 'Iscabbing'l phenomenon occurs which leads to an apparent softening effect of the pellets, and (2) A Fe-silicate slag material having a relatively lower melting point is formed in a large proportion to the total charge weight to cause the premature softening effect of the pellets. :
Therefore, in order to produce non-fired agglomerates such as pellets and briquettes having excellent load-softening property, it is enough to prevent the softening due to the above factors. Regarding the softening due to the scabbing, it is natural that the scabbing phenomenon itself must be supressed, and the softening due to the factor (2) can be prevented by increasing the basicity of the whole charge so as to cause melting of FeO into the slag.
On the basis of this discovery, the present invention contemplates to improve the properties of the charged pellets in the softening zone of the smelting furnace by suppressing the softening tendency of the non-fired pellet attributable to these factors. As is well known in the art, the scabbing material is iron which is formed on the surfaces of the pellets when the pellet is heated to about 900C. in a CO atmosphere, and the occurrence of this scabbing phenomenon is limited to that area of the surface of the pellet which is occupied with the exposure surfaces of iron ore particles contained in the pellets.
According to the present invention, the part of the softening tendency of a pellet ascribable to the first-named factor or ''scabbing" phenomenon is suppressed by minimizing the : ' ` ~7~953 propor-tion of -the sum of the exposed areas of iron ore particles at the surface of the pellet. By reference to Figure 2 it is to be understood that a high quality iron ore designated by "A"
analyzing about 2~0O/o SiO2, when formulated with 9% cementitious binder provides a pressure loss value of more than 900 mmH2O
which may be considered -to depend upon that part of the softening tendency which is a-ttributable to the "scabbing". This is mainly because (l) those constituents of the charge composition which are to form slag materials occupy so small a volume corresponding to a weight of 10.6% that the swm of the exposed areas of iron ore particles is relatively large as compared with the entire surface of the pellet, and (2) the CaO to SiO2 ratio is as high as 1.6, which is unfavorable for formation of slag materials in -the reducing reaction zone, so that it is made more difficult to suppress -the increase o~ the sum of the exposed areas of iron ore particles. ~With such an iron ore, therefore, the present invention provides means of adjusting for the concentration of slag-forming constituents as well as for the CaO/SiO2 ratio in the slag, na.mely, the basicity. It is to be noted that although the slags pro-duced in the well known blast furnaces are generally found tocontain MgO besides CaO, Al2O3 and sio2, the term "slag-forming constituents" herein used is to exclude MgO, because it does not contribute to the slag formation so long as the temperature is-maintained at a low level of about l,000C.
With reference to Figure 2, as the basicity is decreased with increase in the amount of SiO2 added, the pressure loss of a resulting charge composition containing the iron ore "A" is decreased, reaching a minimum value of about 500 mm~I20 at a basicity of about 1.3, and then increased to more than g00 mm~I2O, as indicated by the dot-and-dash curve. The reason why -the pressure loss increases as the basicity is decreased from 1.3 is that the solubility of FeO in the Fe-Silicate slag increases with ~7~353 incrcase of -the amount oE low-meltiny po:int slag which constitutes the other part of the softening tendency which is ascribed to the second-named factor as mentioned above. For this reason, accord-ing to the present invention, while maintalniny -the basicity constant at a level of 1.3, the concentration of the slay-forming constituen-ts i9 increased to more than 13.0'~o in order to insure ,;
that the pressure loss value falls in a range below 200 r~I20 as indicated by the oblique lines. This range is confined with the acceptable range for the typical conventional fired pellets.
In the case of the iron ore "~", the increase of weight proportion of the produced slag is effected by previous addition of additional granulated blast furnace,slag to the charge composition prior to the pelletizing procedures.
In the case of another type of iron ore designated by "B" analyzing about 5% SiO2, a charge composition formulated by adding 9% cementitious binder to the fines of iron ore "B"
provides non-fired pellets having a softening tendency of more than 1,000 mm~I2O in pressure loss, as shown by the solid curve in Figure 2. This i9 because the amount of slag-forming con-stituents is as high as 15% based on the total weight of the charge, and because the basicity,is as low as 0.9. Such a ~, situation promotes the reaction of the slag-formable constituents with FeO at relatively lower temperatures of a range in which the reducing reaction zone operates, thereby causing a relatively larger amount of Fe-Silicate slag to be formed and therefore causing a large,amount of the so-called low-melting po~nt slag.
In this connection, it is to be noted that the occurrence of the "scabbing" phenomenon is negligible because the exposure of the iron ore particles out of the pellet surfaces is suppressed as the low-melting point slag is produced in excess. With this type of iron ore, according to the present invention, a CaO-source material or materials is or are formulatecl into a charge _ g _ ~7~9S3 composition prior to -the pelletizing procedures to account for the adjustment of basicity CaO/SiO2. As the amount of the CaO-source materlal added is i~creased to increase the basicity from 0.9, the pressure loss of the pellet charge is decreased entering the acceptable range below 200 mmll20 at a basicity of 1.2. The basicity may be :-increased to as high as 1.9 to e,ffect acceptable results, and there is no need to increase the basicity from 1.9 as the possibility of encountering situations requiring the . 10 adjustment of basicity to more than 1.9 is very small in : practice. In addition thereto, the solid and dashed curves indicate that the pellet charge with a basicity of more than 1.9 tends to deteriorate the drafting aspect. The charge composition consisting of iron ore "B" and 9%
ce~ent.itious binder provides slag-forming constituents in an amount of about 15U/o which is more than enough to account for ' the suppression of occurrence of the scabbing phenomenon.
~In this respect, an additional increase in the concentration of the slag-forming constituents caused by the adjustment of the basicity is not intended, but the concentration of slag-forming constituents in a charge,composition of the present '' invention may be increased to as large as 19% inasmuch as the pressure loss of the resulting pellet charge falls in the ~' acceptable range below 200 mmH20.
It will be appreciated from the foregoing that the ' ,~
present invention contemplates to formulate a charge composition ~;:
prior to the pelletizing operation as having a basicity in a . range of from 1.2 to 1.9 by addition of additional CaO-and/or SiO2-so'urce materials or material, and as having slag-forming :
constltuents in a range4of from 13% to 19% by addition of an additional slag-source material.
.. . . . .
~7~353 As a b:inder having a hydraulic property, use may be made of Portland cement, Portland cement clinker, alumina cement, . ..
alumina cemen-t clinker, blast furnace slag mi.xed cement, fly ash-mixed cement, pozzolan-mixed cement and masonry-mixed cement. As a CaO-source ma-terial, use may be made of lirnestone, hydro cake, and dolomiteO As a SiO2-source material, use may be made of seashore sand, periodotite, serpentine, fire brlck chips and acid slag. As an additive for increasing slag percentage, use may be made of the above-mentioned materials and blast furnace slag or blast furnace granulated slag.
For exemplary purposes, a number of formulations based on the principle of the invention will be next illustrated which may be used .in the pelletizing process of the inventi.on to be described later.
xample I
Use was made of two iron ores, the one (A) of the ores analyzing a lower content of SiO2 being supplied fro.m the magnetiteseries after concentrated by magnetic separation, and the other ore (B) from the hematite group after pulverized~ The iron ores A and B were mixed with Portland cement, limestone, white sand and blast furnace granulated slag in the following proportions.
- 11 - ' .
Table 1 Blas-t furnace charge composition IronPoxtland Lime- White Slag Basicity Final Sample Orecement stone Sand added ~CaO/SiO2) slag No. (%) (%) (%) (%~ (%) (%) . ~
1 88.3 (Ore A) , 9.0 - 2.7 - 1.23 13.0
2 84.3 (Ore A) 9.0 - 1.6 5.1 1.27 17.0
3 85u9 (Ore B) 9.0 2.5 _ _ 1.25 17.4
4 87~ (Ore B) 9.0 3.6 - _ 1.54 18.3 85.9 (Ore B) 9.0 5.1 - _ 1.81 19.0 .. . - _ ~
10 6 91.0 (Ore A) 9.0 - - _ 1.62 10.8 7 91.0 (Ore B) 9.0 - - - 0.95 15.0 . .. .. _ _ _ Note: Samples No. 6 and No. 7 are in -the prior ar-t.
In -this example, -the process for forming non-fired pellets from the charge composition was carried out in the follow-ing manner: (1) Each of these composition samples was grounded in a batch type ball mill for ten minutes; (2) The grounded mixture is formed into discrete moist pellets by using a laboratory pelle-tizer of the tire type (550 mm in diame-ter with 200 mm long) operated at a speed o~ 30 r.p.m., and (3) The moist or green pellets were cured a-t ordinary temperature for 7 days and then dried prior to the reduc-tion test.
Reduction test:- Using these charge compositions of indurated pellets, the production deficiency index was computed based on the da-ta obtained by the reduction test, and -the results are shown in Figure 3, wherein-the solid circle marks are for the ore A, marks, x; for ore B, and the blank circle marks are for the prior art.
It is evidenced from Figure 3, tha-t the pellets of the present invention are far superior to those of the prior art in respect to the load softening property, and, -therefore, in the ` ~7~953 non-interference with the proper smelting reactions and tempera-tures, as the air drafting aspect of the pellets charged in a blast furnace is improved by suppressing the scabbing and/or by decreasing the amount of low-melting point slag produced therein.
It is known that the activity of the SiO2 added in the charge composition differs with different types of natural material or by-products which are employed as the source of SiO2 and with the particle size thereof, but the present inventors have now found that the SiO2-source g~gue material dispersed in iron ores exhibits a very high activity of SiO2. This finding of the invention leads to an effectiveness of blending a type ground iron ore having SiO2 in a relatively high concentration with another type of ground iron ore having E'e in a relatively higher concentration and SiO~ in a relatively lower concentration than those of the former type iron ore. This blending method provides several advantages, one of wnich is to minimize the amount of addition of additional high-active and accordingly expensive SiO2-source material, another one which is to avoid the complexity of the iron ore processing operation due to the provision of a step :~;
of adding such SiO2-source material to formulate a charge com-position, or otherwise of a step of decreasing the particle size of a SiO2-source material which is to be added thereto, and still another one which is to prevent the amount of slag-formable con-stituents from being increased in a resulting charge composition which would be otherwise effected by the addition of the additional SiO2~source material.
In order to materialize such an effectiveness of the blending, an experiment has been made using four types of iron ore designated A', B', C' and D' analyzing Fe and SiO2 in contents as shown in Table 2 below ~7~i3 Table 2 Sample Ore To-tal amount of SiO2 content ~o. Fe contained ~%) (/O) .. ~
l~' Ilig~-~?e iron-ore 69.0 0.59 B' I~liddle-SiO2 red iron ore 62.7 4.78 C' High-SiO2 magneti-te 64.~6 8. 54 D' Middle-SiO2 Limonite 57.5 5.96 _ . ~
In this experiment, these four types of iron ore were processed in the single or blended form wi-th 9% Portland cement and either of additional SiO2-source and CaO-source materials to produce a number of specimens of non-fired pellets with variation of CAO/SiO2 in a manner similar -to that described in connec-tion with Example I, and the results are shown in Figure 4, wherein the ordinate is in pressure loss at 1200C., and the abscissa is in CaO/SiO2 or ~asicity. The three dashed curves represent the use of iron ore in the single form, with -the blank circles being assigned to a combination of iron ore A' and additional SiO2, with the dotted blank circles to a combina-tion or iron ore B' and additional CaO, and with the solid circles to a combination of iron ore C' and additional CaO. I'he solid triangle mark represents a blend of 40/O ore A' and 51% ore B'. A solid curve associated with the semi-solid circles represent a blend of ore A' and ore C' in a varied ratio of from 57%/3~% to 68%/23% to account for varia-tion or the basicity. Another solld curve associated with blank square marks represents a blend of ore A' and ore D' in a varied ratio of from 46%/45% to 45%/44% with addition of limestone in an amount of 0% to 2% to account for variation of -the basici-ty. With this formulation of the charge compositions of non-fired pellets, the xesultan-t concentra-tion of SiO2 in the pellet is varied with the different six schemes as -- 1~ --7~953 shown in F.igure 5, wherein the same marks are emp]oyec1 to denote -the identical schemes to those of Figure ~. :
It will be understood from Figures 4 and 5 that the activity of SiO2 supplied from any one of the iron ores B', C' and D' is higher than that of SiO2 supplied from the added SiO2-source ma-terial. In more detail, in -the case of the high-Fe ore A', the adjustment of CaO/SlO2 to more than 1.2 with addition of a SiO2-source material effects no establishment of the pressure loss value at a level lower than 200 mmH2O. On the other hand, the iron ores B' and C' having SiO2 in high contents result in the rapid decreases of pressure loss values -to less than-200 mmH2O
with increase of the basicity up to not less than 1.2. In the latter connection, it is to be noted tha-t the iron ores B' and C' with respective additional CaO-source materials prov:ide slag-forming constituents (SiO2-~A12O3-~CaO) in an amount of 18% and 21 -23% respectively. In this respect, therefore, a charge composition which is formulated from the iron ore A and an additional SiO2-source material must be further s~upplied with both of SiO2-source and CaO-source.materials to increase the amount of the slag-form-ing constituents to more than 16%, as the charge compositioninitially contains -the slag-formable constituents in an amount of about 11% to 12%. When the high-Fe type of iron ore is given as a raw material for the charge composition of non-fired pellets, therefore, it is preferred to utilize the blending technique of -:~
the invention as applied to the charge compositions A' + B', ~ '.
A' -~ C', and A' + D', whereby the pressure loss values are set in :~ :
below-200 mmH2O provided that the basicity is adjusted to not less than 1.2, although the resulting concentrations of SiO2 in the non-fired pellets produced by the blending are almost equal to that of SiO2 in the pellet produced from the iron ore A' and a SiO2 -source material without accounting for adjustment of the amount of slag-formable constituents.
~7~g53 In proportioning two or more iron ores to be blended, it is preferred that a concentration of SiO2 not Less than 1.5%
is resulted in the blend prior to -the mixing and grinding pro-cedure in which a 9% hydraulic binder is added and in which the basiclty is adjusted to not les9 -than 1.2%, as shown in Figure 6, wherein the adjustment of the basicity is effected by use of limestone. By taking into account the amount of slag which is to be produced when the resultant pellets are subjected to the smeltingreactions in a furnace, it is preferred to adjust the concentration of SiO2 in the blend to less than 7%.
EXAMPLE II
The four iron ores A', B', C', and D' were used in combination with Portland cement clinker as a hydraulic hinder, limestone as a CaO-source material and seashore sand as a SiO2-source material to prepare seven charge compositions as shown in Table 3 below, in w~ich samples No. 1 through No. 4 have the blending aspect of the invention, while samples No 5 through No. 7, though satisfying the requirement of the basiclty of the invention, lack the blending aspect so that the amount of the slag-forming constituents in the samples No. 6 and No. 7 is as 21 high as about 18% and 22% respectively.
- 16 _ ~L~7~9~i3 -Table 3 .
_. ~
Sampl~ Iron ore Binde~ Additive Factor No. Type Ratio sio2 % Lime- Sand -Sl ~
(%) (%) stone (% (%) _ ._ _ _ ___ ~ ~ ~
1 A'+B' 40~0 51~1 2 ~02 9~0 0 0 13 ~70 1~30 4~54 2 A'-~C' 68~0 23~0 2~72 9~0 0 0 13~29 1~56 4~55 3 A'+D' 46~0 45~0 3~33 9~0 0 0 13~15 1~25 4~91 4 A1+DI 45~0 44~0 3~33 9~0 2~0 0 13~87 1~52 4~69 ~_ ~_~ _ . . . . ~ _ , .. _. _ A' 88~8 0~77 9~00 2a2 11~73 1~31 4~49 6 B' 87~5 4~78 9~03~5 0 19~33 1~54 6~08 ;
7 Cl 81~0 8~54 9~010 0 22~39 L.31 9~15 ~ . , These charge composi-tions were formed in-to non--fired pellets in a manner similar to -that shown in Example I, and the pellets were then subjected to the crushing tes-t and the load ~ .
softening test. The results are shown in Table 4 below.
Table 4 Crushing strength Pressure loss Concentration :
Sample Iron ore of pellet af-terat 1200C of SiO in No. 7 days' curing (mn~I20) pelle~ :;
(kg/P) : :
. 1 A'+B' 220 86 4 ~ 54 2 A'+CI 239 78 4 ~ 55 3 Al +DI 169 133 4~91 4 Al +DI 170 80 4~69 Al 199 303 4~ 49 6 B' 205 67 6~08 7 CJ 227 60 9~15 .
~ 17 ~
~7C~9~3 As is evident from these Tables 3 and ~, the blending aspec-t of -the invention is very effective for rni.nimizing the con-cen-tration of Si02 in the pellet while still preserving the improved high--temperature properties thereof as defined by a pressure loss value lower than 200 mn~l20 according to the load so~tening testO
The present invention has been described in connection with the principle of formulating the charge composition which is to be formed into non-fired pellets or brique-ttes, but it further-more concerns a novel method of pelleti.zing the charge compositionwhich method is particularly adapted for use on the compositional framework, that is, the requirernents of control for the ratio in a range of from 1.2 to 1.9 and for the amount of slag-forming constituents in a range of from 13% -to 19%.
As is ]cnown in the art, the as-formed discrete moist or "green" pellets are subsequently to be buried in a powder which serves as a matrix, until the curing of the cementitious material used as a binder ln the pellets has been completed. The use of such a matrix gives rise to two main advantages, one of which is to avoid the adhesion of the pellets with one another during the curing procedure, and another advantage which is to preven-t the deforrnation of the pellets and to decrease the percentage of crushed pellets as the pressure applied to a pellet is made uniform over the enti.re surface area thereof despite the fact that the pellet is l.ocated under an accumulation of pellets.
When the pel.lets are accumulated without using the matrix, as indicated by the experiments conducted by the present inventors, .
the green pellets produced by the conventional method and having a crushing strength of less than 1.0 kg. per pellets at maximum tends upon accumulation at as high a level as more than two meters to gradually deform with progressive increase in -the area of mutually contacting surfaces of the pellets. ~s a result, a _ 18 _ 9'7~953 number of huge blocks are formecl, and -the percentage of cracked pellets is increased.
With -the blocked charge material, .i-t is made more difficult to perform the travelling from the pelletizing system to -the smelting system, and furthermore divid.ing and screening operations are required to perform prior to introduction into the furnace, whereby an excessive amoun-t of fines are produced.
Such fines not only lntroduce handling difficulties but also contamina-te the working environment. Moreover, the forma-tion of the crackings in the pellets leads to a decrease in the crushing . .
strength of the pellets themselves which also facili-tates the `
production of fines to impair the production efficiency because of the additional handling difficulties and the accumulation of the ~;
fines in the furnace. Such a situation will take on a new con- .
ditioning aspect of the furnace. . ` :
The present inventors have made many attempts to over~
come such problems and have now found that when the crushing strength of the green pellets is increased and at the same time when the plasticity is decreased, the curing of the pellets may ~ :;
be performed in accumulation, whereby remarkably reducing the mutual adhesion of the pellets which would be otherwise effected and simultaneously availing of the rapidly decreased proportion of cracked pellets after the curing operation. For example, in the case of pellets having an average diameter of about 15 mm, a crushing strength of 2.0 kg-P at the as-formed state and a .:
porosity of 30/O permit for an accumulation of 2 meters high.
In order to impart to the as-formed moist pellets adequate crushing stxength and porosity, it is found important to control the size and distribution of the particles of a given charge composition and the amount of water contained in the pellet, although many other factors affect the crushing strength and plasticity of the green pellets. With regard to this, it is -- 19 _ 1~7~3g~3 preferred to increase the density oE the charge composition prior to the pelle-tizing procedures, and then to adjust the amount of water in the green pellet to as low a level as possible while still sa-tisfying the pellet-formable moisture range.
According to an embodiment of the present invention, the process for pelletizing a charge compo.sition comprises pro-portioning a granulated iron ore, a hydraulic binder and one or more additives to adjust the basiclty CaO/SiO2 ln a range of 1.2-1.9 and the amount of slag-forming constituents in the range of 13%-19%, mixing all of these materials at a time, and then grinding the mixture to obtain as uniform a distribution of the, particle sizes as possible, and then forming the thus-ground mlx~ure into discrete moist pellets having such a crushing strength and such a porosity as specified above. The performance of grinding of all the ingredients of a charge composition in the mixed form facilitates the minimization of the distribution range of ~ualities of the individual pellets which in turn leads to an increase of the average crushing strength level. A further important aspect of this grinding procedure is to sustain the capillarity in the pellet as the compactness of the particles is improved to result in almost regular openings of correspondinyly ' ' ' smaller size, thereby contributing an addi-tional very effective ' factor to the improvement of the crushing strength. In addition thereto, the improved compactness assists in minimizing the '~
'necessary amount of water for moistening the ground mlxture which in turn causes a decrease in the plasticit~ of the green pellet and the porosity thereof.
According to another em'bodiment of the inven-tion, the grinding operation is divided into two successive steps when the nature of given raw-'materials for the charge composition does not permit the single-step grinding operation -to provide the desired properties of the moist pellets in the as-formed state. An ~L~7~5i3 example of the two-step gxlnding opera-tion is such that, in the first step, the total amount of the hydraulic binder, a fraction of the granulated ore and a fraction of the additive are ground in the mixed form to produce a powdery mixture which serves as a matrix for the remaining ore and additive-in the second step. In this example, it is preferred that the sum of the first fractions of the ore and additive -taken in the first step is in a range of from a one-fold to five-fol.d excess in relation to the total amount of the binder. As another example, 10 in the first step, the sum of the total amounts of the binder and additive and a fraction of the granulated ore are ground in the mixed form to produce a powdery mixture which serves as a matrix for the remaining ore in the second step. In this second example, it is preferred that the first fraction of the ore used in -the first step is in a range of from a 3/4-fold to 4-fold ex-cess in relation to the sum of the total arnounts of the binder and.
additive. It is evident from Figure 4 that the apparent density of the green pellets produced in the two-step grinding operation is relatively higher when the first step is operated wi-th a mixture of the fractionated ingredients in -the proportion :
identif.ied as preferable.
In some cases where no sufficient area of the plottage for the curing of the green pellets can be secured and therefore it cannot be helped to use a number of vertical curing containers such as hoppers or bins, it is required -that the percentage of pellets mutually adhered after the curing be reduced to zero, or otherwise, even when the percen-tage is very small, the forma-tion of a scaffold of pellets or the uneven downward advance thereof in the container would be resulted as the pellets are successively cut out from the bottom of the container, thereby introducin~ a handling difficulty. The present inventors have made many at-tempts -to fulfill -the requirement, and have now :Eound that - 2~1 _ ~07~i3 the applicat:ion of an adherent coating.of a very small -thic1cness preferably of not larger than 0.2 mm is very ef~ective when a suitable organic material is selected for employment as the material o~ the coating. By the term "suitable", it is meant that the coating does not adversely affect the high-temperature properties of the resultant pellets, since the volume of the coating, though having as small a thickness as 0.2 mm, amounts to about 5% based on the volume of the bare pellet having a diameter of 15 mm. Experiments conducted by the present inventors indicate lQ that as the suitable inorganic ma-terial, use may be made of a powdery mixture of the same iron ore and additive as those of .
the charge composition, a powdery iron ore or ores having a basicity of 1.2-1.9, or a powdery material selected Erom the group of limestone, slaked lime, dolomite, blast furnace sl.ag and converter slag. . .
A coating apparatus which is found as suitable for good ~ .
results with the above-identified coating material is of the .
continuously operated rotary drum type. The reason for this is that although the coating operation for -the strongly adhered coating takes no more than 4 minutes, the coating thickness depends on the retention time so,that when the retention time varies with different green pellets, the degree of cohesiveness of the coating to the pellet surface varies with the individual pellets to produce a bad influence on the high-temperature - properties thereof. The utiliza-tion of an apparatus such as a pan pelletizer in which the retention and discharge depends on the pellet size distribution is undesirable, because the larger the diameter of a green pellet, the shorter the retention time, while the smaller the diameter, the pellet stays in -the apparatus for a period longer than necessary. As a result, the range of thick-nesses of the coatings is extended wider than that effected by the use of the rotary drum type coating apparatus.
~7~53 It i9 impor-tant to control the amoun-t of water with which the inorganic coating material is moistened before the application to the green pellets, although the moist coating material may be mixed with the green pellets either before or at charging into -the coating apparatus. It is found that the acceptable moisture range is - 30% based on the amount of water ~ -contained in the green pellets. When the inorganlc coating material is moistened with an amount of water less than the lower limit of the range, namely, for the green pelle-ts contain-ing 8% water, less than 5.5%, some of the water in pellet is rapidly transferred to the as-applied coating with the result that the coated green pellets tend to form crackings therein wi-th decrease in the crushing strength of the green pellets. On the other hand, when the coating material is moistened with an amount of water more than the upper limit of the range, namely, for the equivalent green pellets to the above, more than 10%, a fraction of all of the moist coating material is formed into pellets by itself, and the coating operation is made more difficult to perform because of the high plasticity théreof.
It will be seen from the foregoing description that the present invention provides a'process for making non-fired or cold pellets c~f high improved characteristics for use in a smelting furnace and particularly blast furnace from a charge composition of which the ingredients are proportionated to account for adjustment of the basicity CaO/SiO2 and the slag-forming constituents, characterized as including a step of grinding the ingredients, in all, or fractionated amounts thereof with the successive stages, to produce a powdery mixture of such a nature that the green pellets made therefrom can be cured in an accumulation of more than 2 meters without causing excessive deformation thereof and the formation of crackings therein to splinters and without using a powc~ery matrix which would be ~(~7~1~35,3 otherwise necessary for the huring of -the green pellets in the curing operation, whereby giving advantages that the production of Eines from the cured pellets is minimized in the tra~elling and smelting procedures to minimize the handling difficulties and environrnen-tal pollution, and tha-t the process is economical because of the high production ef~iciency. When the vertical curing containers are used, the pelletizing process may comprise an additional step of coating the green pel:Lets by use of a continuously operated rotary drum. It is, of course, desirable to utilize the coating step in producing the coated yreen pellets which are to be cured in yards. In the latter connection, the :: .
cu-tting-out operation of the cured pellets followed by the travelling operation can be carried out very easily.
The present invention will now be fur-ther illustrated in and by the following examples of the pelletizing process. They are not intended to limit it in any manner.
EXA~PLE III
For accumulation curing in yard, a charge composition was made from the raw materials shown in Table 5 below to result in the constituents shown in Table 6 below.
Table 5 _ ,,,,, . _ . , - - I
Crypoilog iron ore Furnace dust Cement clinker Limestone .
~30.1% (by weight) 4.5 8.7 6.7 ... _ . ., . .. _ .. _ ._ Table 6 T.Te FeO sio2 A123 ~aO CaO/Sio2 slag(sio2tcao~Al2o3) ._ . ._. .. .. _~ _ .. . _ ~ .. _ _. _. _ _.. _ _ .. __ _ ~56.0 7.6 0., 9.5 1.2~ 17.
.
~7~953 The charge composltion was divicled into three e~ual parts and -the parts were subjected to different processes, A,B
and C.
In process A, all the raw materials were mixed and ground in a single step. In process B, the total amounts of the cement clinker and limestone were mixed with an equal amount of the ore thereto, and the mixture was ground -to produce a powdery mixture which was then mixed with the remaining ore and the total amount of the furnace dust. The latter mixture was ground to perform the second step of the grinding operation. In process C, all the raw materials were subjected to the mixing alone. The three specimens of the charge composition obtained from the processes A, B and C were pelletized under the same conditions by use of a pan pelletizer of 5 meters in diameter and the green pellets were accumulated in a height of 2 meters until the curing was completed. The results are shown in Table 7 below.
It is evident from Table` 7 that the green and cured pellets according to process A of the invention are superior in crushing strength, cracking percentage and mutual adhesion to those of the process C of the prior art, and that the pellets according to the process B employing the two'step mixing and grinding procedure of the lnvention are further improved over those of the process A particularly in cracking percentage and mutual adhesion.
., Table 7 . ~
A B C
Particle size prior -to pelletizing83.2 85.1 78~9 (-44~m%) .
~. _ . . __ __ _ _ _ _ . , : Green pellet:
Crushing strength (Kg/P~ 3.9 5.0 2,0 ~ .
Dropping strength (frequency) 5~3 6.7 4.5 Porosity (%~ 26,4 25.3 30,2 .
~:' Moistur~ (%~ 8.5 7.5 9.0 _ . ,_, Crushing strength of the cured ~ .
pellets after a day or days:
1 day 35 40 15 2 days 71 75 43 6 days 100 110 64 10 days 150 150 138 :
_ . . __ .
Percentage of cracked pellets 7,5 4.3 18.0 ~ . .
Mutual adhesion of pellets Slight Very Strong slight _ ... . . _ ...
Shutter test* after days passed 2 days (frequency) 1 1 3 : .
3 days ( " ) 3 1 5 : ~ :
4 days ( " ) 7 2 13 ~
.. -. ,............................... _ ~
Pressure loss according to the : :
load softening test (mmH2O) 160 160 190 Note: * Using a shutter test machine, a block of pellets adhered with another and having a weight of 40 kg was dropped from a height of 2 meters in a frequency such that the block was split into the individual discrete pellets.
2~ - :
~ ",''~, j ' .
. :
~7~53 EXAMPLE IV
For the coating of green pellets to be cured in a hopper, the as-formed green pelLets just after discharge from procoss s oE Example I[I were mixed wi-th a moist coating ~aterial shown in Table 8, and the mixture was thrown into a continuous type rotary dru~ of 1 meter in diameter and 3.5 meters long.
After the duration of a time period of 2 minutes, the pellets were discharged therefrom ln the as-coated state, and then accumulated in a height of 3 meters in the hopper, while per-forming continuous cutting-out operation. The effective curing period in the hopper was about 30 days, and the cut-out masses of the pellets were further indurated in other curing facilities for 10 days. The results are shown in Table 9.
Table 8 Coating Operation _ . ~ I
Sample Raw Material Constituents ~ Property No, _ _ __ _ _ _ T~Fe FeO CaO SiO2 Particle Size (-~4~m%
by weight) ___. ..... . ... _...... , .. _ _ ........... .. , .
1 Powdered iron ore 64.525.7 0,2 8.0 0.02 77.6 Powdered iron ore 2 + 55,0 21,9~.3 6~8 1.22 76.0 Limestone powder , 3 Limestone powder 0,7 - 54.4 0~1 ~L637~53 ~
., '.
Table 9 .
Sample __ 2 _ :
... ,, _ _......... . '' Moisture (%) 4.5 '7.2 6.5 8.7 ~ . _ . . _ Percentage of coating weight : .
based on green pellet weight(%) 4.0 4. 4.2¦ 3.4 _ _ _ _ _ . . I - .
Coated green pellet: .
Crushing strength (kg/P) 2.0 4.9 5.2 5.0 .
Dropping strength (frequency) 1.5 6.7 6.3 6.4 Porosity (%) 30.5 26. 25.226.0 AITount of water (%) 7.2 7.6 7.8 7.5 -- . _ Crushing strength of the cured .
pellets after a day or days .
passed (kg/P) .
1 day 15 42 45 40 2 days 43 80 83 78 6 days 61 ~ 120 120 115 days 89 155 160 155 :~
.. _. .~
Percentage of cracked pellets(%)~ 85.0 0.1 0.3 0.1 ~ ~
- ... =_ . _.. _.. ~_..... _.__ . ... . ... . .. _ Mutual adhesion Slight none none none Cutting-out from hopper Bad GoodGood Good :
~ (Scaffold) .
High-temperature properties Pressure loss (mmH2O) 700 500 190 190 Quality of pellet as estimated from interference with proper I
conditioning of blast furnace ~, ~oor Poor Good Good -- 28 -- .
~ .
~7~9~3 It is to be unders-tood from the results shows above that . ' the percen-tage of water contained in the coating composition is of importance in preventing a decrease of the crushing strength and the formation of crackings of the pellets and also in facilitating minimizatlon of cutting-out difficulty of the indurated pellets from the curing hopper as the mutual-adheslon of the green pellets is prevented, and that w,hen the basicity of the coating composi-tion is controlled so as not to be within the above specified range, poor high-temperature properties of the coated pellets result despite the fact that the bare pellets without the coating have good high-temperature properties. In addition thereto, the screening operation of the indurated pellets can be omitted before -the pellets havetravelled from the pelletizing system to the smelting system, and the travelling does not suffer from the production of fines, whereby the handling difficulties which have so far been encountered in the prior art are not introduced and moreover, the drafting aspect of the furnace:is improved as the fines are not produced. Thus the present invention has accomplished a remarkable advance not only in the art but also in the economical and social aspects of the art.
, . - 29 -
10 6 91.0 (Ore A) 9.0 - - _ 1.62 10.8 7 91.0 (Ore B) 9.0 - - - 0.95 15.0 . .. .. _ _ _ Note: Samples No. 6 and No. 7 are in -the prior ar-t.
In -this example, -the process for forming non-fired pellets from the charge composition was carried out in the follow-ing manner: (1) Each of these composition samples was grounded in a batch type ball mill for ten minutes; (2) The grounded mixture is formed into discrete moist pellets by using a laboratory pelle-tizer of the tire type (550 mm in diame-ter with 200 mm long) operated at a speed o~ 30 r.p.m., and (3) The moist or green pellets were cured a-t ordinary temperature for 7 days and then dried prior to the reduc-tion test.
Reduction test:- Using these charge compositions of indurated pellets, the production deficiency index was computed based on the da-ta obtained by the reduction test, and -the results are shown in Figure 3, wherein-the solid circle marks are for the ore A, marks, x; for ore B, and the blank circle marks are for the prior art.
It is evidenced from Figure 3, tha-t the pellets of the present invention are far superior to those of the prior art in respect to the load softening property, and, -therefore, in the ` ~7~953 non-interference with the proper smelting reactions and tempera-tures, as the air drafting aspect of the pellets charged in a blast furnace is improved by suppressing the scabbing and/or by decreasing the amount of low-melting point slag produced therein.
It is known that the activity of the SiO2 added in the charge composition differs with different types of natural material or by-products which are employed as the source of SiO2 and with the particle size thereof, but the present inventors have now found that the SiO2-source g~gue material dispersed in iron ores exhibits a very high activity of SiO2. This finding of the invention leads to an effectiveness of blending a type ground iron ore having SiO2 in a relatively high concentration with another type of ground iron ore having E'e in a relatively higher concentration and SiO~ in a relatively lower concentration than those of the former type iron ore. This blending method provides several advantages, one of wnich is to minimize the amount of addition of additional high-active and accordingly expensive SiO2-source material, another one which is to avoid the complexity of the iron ore processing operation due to the provision of a step :~;
of adding such SiO2-source material to formulate a charge com-position, or otherwise of a step of decreasing the particle size of a SiO2-source material which is to be added thereto, and still another one which is to prevent the amount of slag-formable con-stituents from being increased in a resulting charge composition which would be otherwise effected by the addition of the additional SiO2~source material.
In order to materialize such an effectiveness of the blending, an experiment has been made using four types of iron ore designated A', B', C' and D' analyzing Fe and SiO2 in contents as shown in Table 2 below ~7~i3 Table 2 Sample Ore To-tal amount of SiO2 content ~o. Fe contained ~%) (/O) .. ~
l~' Ilig~-~?e iron-ore 69.0 0.59 B' I~liddle-SiO2 red iron ore 62.7 4.78 C' High-SiO2 magneti-te 64.~6 8. 54 D' Middle-SiO2 Limonite 57.5 5.96 _ . ~
In this experiment, these four types of iron ore were processed in the single or blended form wi-th 9% Portland cement and either of additional SiO2-source and CaO-source materials to produce a number of specimens of non-fired pellets with variation of CAO/SiO2 in a manner similar -to that described in connec-tion with Example I, and the results are shown in Figure 4, wherein the ordinate is in pressure loss at 1200C., and the abscissa is in CaO/SiO2 or ~asicity. The three dashed curves represent the use of iron ore in the single form, with -the blank circles being assigned to a combination of iron ore A' and additional SiO2, with the dotted blank circles to a combina-tion or iron ore B' and additional CaO, and with the solid circles to a combination of iron ore C' and additional CaO. I'he solid triangle mark represents a blend of 40/O ore A' and 51% ore B'. A solid curve associated with the semi-solid circles represent a blend of ore A' and ore C' in a varied ratio of from 57%/3~% to 68%/23% to account for varia-tion or the basicity. Another solld curve associated with blank square marks represents a blend of ore A' and ore D' in a varied ratio of from 46%/45% to 45%/44% with addition of limestone in an amount of 0% to 2% to account for variation of -the basici-ty. With this formulation of the charge compositions of non-fired pellets, the xesultan-t concentra-tion of SiO2 in the pellet is varied with the different six schemes as -- 1~ --7~953 shown in F.igure 5, wherein the same marks are emp]oyec1 to denote -the identical schemes to those of Figure ~. :
It will be understood from Figures 4 and 5 that the activity of SiO2 supplied from any one of the iron ores B', C' and D' is higher than that of SiO2 supplied from the added SiO2-source ma-terial. In more detail, in -the case of the high-Fe ore A', the adjustment of CaO/SlO2 to more than 1.2 with addition of a SiO2-source material effects no establishment of the pressure loss value at a level lower than 200 mmH2O. On the other hand, the iron ores B' and C' having SiO2 in high contents result in the rapid decreases of pressure loss values -to less than-200 mmH2O
with increase of the basicity up to not less than 1.2. In the latter connection, it is to be noted tha-t the iron ores B' and C' with respective additional CaO-source materials prov:ide slag-forming constituents (SiO2-~A12O3-~CaO) in an amount of 18% and 21 -23% respectively. In this respect, therefore, a charge composition which is formulated from the iron ore A and an additional SiO2-source material must be further s~upplied with both of SiO2-source and CaO-source.materials to increase the amount of the slag-form-ing constituents to more than 16%, as the charge compositioninitially contains -the slag-formable constituents in an amount of about 11% to 12%. When the high-Fe type of iron ore is given as a raw material for the charge composition of non-fired pellets, therefore, it is preferred to utilize the blending technique of -:~
the invention as applied to the charge compositions A' + B', ~ '.
A' -~ C', and A' + D', whereby the pressure loss values are set in :~ :
below-200 mmH2O provided that the basicity is adjusted to not less than 1.2, although the resulting concentrations of SiO2 in the non-fired pellets produced by the blending are almost equal to that of SiO2 in the pellet produced from the iron ore A' and a SiO2 -source material without accounting for adjustment of the amount of slag-formable constituents.
~7~g53 In proportioning two or more iron ores to be blended, it is preferred that a concentration of SiO2 not Less than 1.5%
is resulted in the blend prior to -the mixing and grinding pro-cedure in which a 9% hydraulic binder is added and in which the basiclty is adjusted to not les9 -than 1.2%, as shown in Figure 6, wherein the adjustment of the basicity is effected by use of limestone. By taking into account the amount of slag which is to be produced when the resultant pellets are subjected to the smeltingreactions in a furnace, it is preferred to adjust the concentration of SiO2 in the blend to less than 7%.
EXAMPLE II
The four iron ores A', B', C', and D' were used in combination with Portland cement clinker as a hydraulic hinder, limestone as a CaO-source material and seashore sand as a SiO2-source material to prepare seven charge compositions as shown in Table 3 below, in w~ich samples No. 1 through No. 4 have the blending aspect of the invention, while samples No 5 through No. 7, though satisfying the requirement of the basiclty of the invention, lack the blending aspect so that the amount of the slag-forming constituents in the samples No. 6 and No. 7 is as 21 high as about 18% and 22% respectively.
- 16 _ ~L~7~9~i3 -Table 3 .
_. ~
Sampl~ Iron ore Binde~ Additive Factor No. Type Ratio sio2 % Lime- Sand -Sl ~
(%) (%) stone (% (%) _ ._ _ _ ___ ~ ~ ~
1 A'+B' 40~0 51~1 2 ~02 9~0 0 0 13 ~70 1~30 4~54 2 A'-~C' 68~0 23~0 2~72 9~0 0 0 13~29 1~56 4~55 3 A'+D' 46~0 45~0 3~33 9~0 0 0 13~15 1~25 4~91 4 A1+DI 45~0 44~0 3~33 9~0 2~0 0 13~87 1~52 4~69 ~_ ~_~ _ . . . . ~ _ , .. _. _ A' 88~8 0~77 9~00 2a2 11~73 1~31 4~49 6 B' 87~5 4~78 9~03~5 0 19~33 1~54 6~08 ;
7 Cl 81~0 8~54 9~010 0 22~39 L.31 9~15 ~ . , These charge composi-tions were formed in-to non--fired pellets in a manner similar to -that shown in Example I, and the pellets were then subjected to the crushing tes-t and the load ~ .
softening test. The results are shown in Table 4 below.
Table 4 Crushing strength Pressure loss Concentration :
Sample Iron ore of pellet af-terat 1200C of SiO in No. 7 days' curing (mn~I20) pelle~ :;
(kg/P) : :
. 1 A'+B' 220 86 4 ~ 54 2 A'+CI 239 78 4 ~ 55 3 Al +DI 169 133 4~91 4 Al +DI 170 80 4~69 Al 199 303 4~ 49 6 B' 205 67 6~08 7 CJ 227 60 9~15 .
~ 17 ~
~7C~9~3 As is evident from these Tables 3 and ~, the blending aspec-t of -the invention is very effective for rni.nimizing the con-cen-tration of Si02 in the pellet while still preserving the improved high--temperature properties thereof as defined by a pressure loss value lower than 200 mn~l20 according to the load so~tening testO
The present invention has been described in connection with the principle of formulating the charge composition which is to be formed into non-fired pellets or brique-ttes, but it further-more concerns a novel method of pelleti.zing the charge compositionwhich method is particularly adapted for use on the compositional framework, that is, the requirernents of control for the ratio in a range of from 1.2 to 1.9 and for the amount of slag-forming constituents in a range of from 13% -to 19%.
As is ]cnown in the art, the as-formed discrete moist or "green" pellets are subsequently to be buried in a powder which serves as a matrix, until the curing of the cementitious material used as a binder ln the pellets has been completed. The use of such a matrix gives rise to two main advantages, one of which is to avoid the adhesion of the pellets with one another during the curing procedure, and another advantage which is to preven-t the deforrnation of the pellets and to decrease the percentage of crushed pellets as the pressure applied to a pellet is made uniform over the enti.re surface area thereof despite the fact that the pellet is l.ocated under an accumulation of pellets.
When the pel.lets are accumulated without using the matrix, as indicated by the experiments conducted by the present inventors, .
the green pellets produced by the conventional method and having a crushing strength of less than 1.0 kg. per pellets at maximum tends upon accumulation at as high a level as more than two meters to gradually deform with progressive increase in -the area of mutually contacting surfaces of the pellets. ~s a result, a _ 18 _ 9'7~953 number of huge blocks are formecl, and -the percentage of cracked pellets is increased.
With -the blocked charge material, .i-t is made more difficult to perform the travelling from the pelletizing system to -the smelting system, and furthermore divid.ing and screening operations are required to perform prior to introduction into the furnace, whereby an excessive amoun-t of fines are produced.
Such fines not only lntroduce handling difficulties but also contamina-te the working environment. Moreover, the forma-tion of the crackings in the pellets leads to a decrease in the crushing . .
strength of the pellets themselves which also facili-tates the `
production of fines to impair the production efficiency because of the additional handling difficulties and the accumulation of the ~;
fines in the furnace. Such a situation will take on a new con- .
ditioning aspect of the furnace. . ` :
The present inventors have made many attempts to over~
come such problems and have now found that when the crushing strength of the green pellets is increased and at the same time when the plasticity is decreased, the curing of the pellets may ~ :;
be performed in accumulation, whereby remarkably reducing the mutual adhesion of the pellets which would be otherwise effected and simultaneously availing of the rapidly decreased proportion of cracked pellets after the curing operation. For example, in the case of pellets having an average diameter of about 15 mm, a crushing strength of 2.0 kg-P at the as-formed state and a .:
porosity of 30/O permit for an accumulation of 2 meters high.
In order to impart to the as-formed moist pellets adequate crushing stxength and porosity, it is found important to control the size and distribution of the particles of a given charge composition and the amount of water contained in the pellet, although many other factors affect the crushing strength and plasticity of the green pellets. With regard to this, it is -- 19 _ 1~7~3g~3 preferred to increase the density oE the charge composition prior to the pelle-tizing procedures, and then to adjust the amount of water in the green pellet to as low a level as possible while still sa-tisfying the pellet-formable moisture range.
According to an embodiment of the present invention, the process for pelletizing a charge compo.sition comprises pro-portioning a granulated iron ore, a hydraulic binder and one or more additives to adjust the basiclty CaO/SiO2 ln a range of 1.2-1.9 and the amount of slag-forming constituents in the range of 13%-19%, mixing all of these materials at a time, and then grinding the mixture to obtain as uniform a distribution of the, particle sizes as possible, and then forming the thus-ground mlx~ure into discrete moist pellets having such a crushing strength and such a porosity as specified above. The performance of grinding of all the ingredients of a charge composition in the mixed form facilitates the minimization of the distribution range of ~ualities of the individual pellets which in turn leads to an increase of the average crushing strength level. A further important aspect of this grinding procedure is to sustain the capillarity in the pellet as the compactness of the particles is improved to result in almost regular openings of correspondinyly ' ' ' smaller size, thereby contributing an addi-tional very effective ' factor to the improvement of the crushing strength. In addition thereto, the improved compactness assists in minimizing the '~
'necessary amount of water for moistening the ground mlxture which in turn causes a decrease in the plasticit~ of the green pellet and the porosity thereof.
According to another em'bodiment of the inven-tion, the grinding operation is divided into two successive steps when the nature of given raw-'materials for the charge composition does not permit the single-step grinding operation -to provide the desired properties of the moist pellets in the as-formed state. An ~L~7~5i3 example of the two-step gxlnding opera-tion is such that, in the first step, the total amount of the hydraulic binder, a fraction of the granulated ore and a fraction of the additive are ground in the mixed form to produce a powdery mixture which serves as a matrix for the remaining ore and additive-in the second step. In this example, it is preferred that the sum of the first fractions of the ore and additive -taken in the first step is in a range of from a one-fold to five-fol.d excess in relation to the total amount of the binder. As another example, 10 in the first step, the sum of the total amounts of the binder and additive and a fraction of the granulated ore are ground in the mixed form to produce a powdery mixture which serves as a matrix for the remaining ore in the second step. In this second example, it is preferred that the first fraction of the ore used in -the first step is in a range of from a 3/4-fold to 4-fold ex-cess in relation to the sum of the total arnounts of the binder and.
additive. It is evident from Figure 4 that the apparent density of the green pellets produced in the two-step grinding operation is relatively higher when the first step is operated wi-th a mixture of the fractionated ingredients in -the proportion :
identif.ied as preferable.
In some cases where no sufficient area of the plottage for the curing of the green pellets can be secured and therefore it cannot be helped to use a number of vertical curing containers such as hoppers or bins, it is required -that the percentage of pellets mutually adhered after the curing be reduced to zero, or otherwise, even when the percen-tage is very small, the forma-tion of a scaffold of pellets or the uneven downward advance thereof in the container would be resulted as the pellets are successively cut out from the bottom of the container, thereby introducin~ a handling difficulty. The present inventors have made many at-tempts -to fulfill -the requirement, and have now :Eound that - 2~1 _ ~07~i3 the applicat:ion of an adherent coating.of a very small -thic1cness preferably of not larger than 0.2 mm is very ef~ective when a suitable organic material is selected for employment as the material o~ the coating. By the term "suitable", it is meant that the coating does not adversely affect the high-temperature properties of the resultant pellets, since the volume of the coating, though having as small a thickness as 0.2 mm, amounts to about 5% based on the volume of the bare pellet having a diameter of 15 mm. Experiments conducted by the present inventors indicate lQ that as the suitable inorganic ma-terial, use may be made of a powdery mixture of the same iron ore and additive as those of .
the charge composition, a powdery iron ore or ores having a basicity of 1.2-1.9, or a powdery material selected Erom the group of limestone, slaked lime, dolomite, blast furnace sl.ag and converter slag. . .
A coating apparatus which is found as suitable for good ~ .
results with the above-identified coating material is of the .
continuously operated rotary drum type. The reason for this is that although the coating operation for -the strongly adhered coating takes no more than 4 minutes, the coating thickness depends on the retention time so,that when the retention time varies with different green pellets, the degree of cohesiveness of the coating to the pellet surface varies with the individual pellets to produce a bad influence on the high-temperature - properties thereof. The utiliza-tion of an apparatus such as a pan pelletizer in which the retention and discharge depends on the pellet size distribution is undesirable, because the larger the diameter of a green pellet, the shorter the retention time, while the smaller the diameter, the pellet stays in -the apparatus for a period longer than necessary. As a result, the range of thick-nesses of the coatings is extended wider than that effected by the use of the rotary drum type coating apparatus.
~7~53 It i9 impor-tant to control the amoun-t of water with which the inorganic coating material is moistened before the application to the green pellets, although the moist coating material may be mixed with the green pellets either before or at charging into -the coating apparatus. It is found that the acceptable moisture range is - 30% based on the amount of water ~ -contained in the green pellets. When the inorganlc coating material is moistened with an amount of water less than the lower limit of the range, namely, for the green pelle-ts contain-ing 8% water, less than 5.5%, some of the water in pellet is rapidly transferred to the as-applied coating with the result that the coated green pellets tend to form crackings therein wi-th decrease in the crushing strength of the green pellets. On the other hand, when the coating material is moistened with an amount of water more than the upper limit of the range, namely, for the equivalent green pellets to the above, more than 10%, a fraction of all of the moist coating material is formed into pellets by itself, and the coating operation is made more difficult to perform because of the high plasticity théreof.
It will be seen from the foregoing description that the present invention provides a'process for making non-fired or cold pellets c~f high improved characteristics for use in a smelting furnace and particularly blast furnace from a charge composition of which the ingredients are proportionated to account for adjustment of the basicity CaO/SiO2 and the slag-forming constituents, characterized as including a step of grinding the ingredients, in all, or fractionated amounts thereof with the successive stages, to produce a powdery mixture of such a nature that the green pellets made therefrom can be cured in an accumulation of more than 2 meters without causing excessive deformation thereof and the formation of crackings therein to splinters and without using a powc~ery matrix which would be ~(~7~1~35,3 otherwise necessary for the huring of -the green pellets in the curing operation, whereby giving advantages that the production of Eines from the cured pellets is minimized in the tra~elling and smelting procedures to minimize the handling difficulties and environrnen-tal pollution, and tha-t the process is economical because of the high production ef~iciency. When the vertical curing containers are used, the pelletizing process may comprise an additional step of coating the green pel:Lets by use of a continuously operated rotary drum. It is, of course, desirable to utilize the coating step in producing the coated yreen pellets which are to be cured in yards. In the latter connection, the :: .
cu-tting-out operation of the cured pellets followed by the travelling operation can be carried out very easily.
The present invention will now be fur-ther illustrated in and by the following examples of the pelletizing process. They are not intended to limit it in any manner.
EXA~PLE III
For accumulation curing in yard, a charge composition was made from the raw materials shown in Table 5 below to result in the constituents shown in Table 6 below.
Table 5 _ ,,,,, . _ . , - - I
Crypoilog iron ore Furnace dust Cement clinker Limestone .
~30.1% (by weight) 4.5 8.7 6.7 ... _ . ., . .. _ .. _ ._ Table 6 T.Te FeO sio2 A123 ~aO CaO/Sio2 slag(sio2tcao~Al2o3) ._ . ._. .. .. _~ _ .. . _ ~ .. _ _. _. _ _.. _ _ .. __ _ ~56.0 7.6 0., 9.5 1.2~ 17.
.
~7~953 The charge composltion was divicled into three e~ual parts and -the parts were subjected to different processes, A,B
and C.
In process A, all the raw materials were mixed and ground in a single step. In process B, the total amounts of the cement clinker and limestone were mixed with an equal amount of the ore thereto, and the mixture was ground -to produce a powdery mixture which was then mixed with the remaining ore and the total amount of the furnace dust. The latter mixture was ground to perform the second step of the grinding operation. In process C, all the raw materials were subjected to the mixing alone. The three specimens of the charge composition obtained from the processes A, B and C were pelletized under the same conditions by use of a pan pelletizer of 5 meters in diameter and the green pellets were accumulated in a height of 2 meters until the curing was completed. The results are shown in Table 7 below.
It is evident from Table` 7 that the green and cured pellets according to process A of the invention are superior in crushing strength, cracking percentage and mutual adhesion to those of the process C of the prior art, and that the pellets according to the process B employing the two'step mixing and grinding procedure of the lnvention are further improved over those of the process A particularly in cracking percentage and mutual adhesion.
., Table 7 . ~
A B C
Particle size prior -to pelletizing83.2 85.1 78~9 (-44~m%) .
~. _ . . __ __ _ _ _ _ . , : Green pellet:
Crushing strength (Kg/P~ 3.9 5.0 2,0 ~ .
Dropping strength (frequency) 5~3 6.7 4.5 Porosity (%~ 26,4 25.3 30,2 .
~:' Moistur~ (%~ 8.5 7.5 9.0 _ . ,_, Crushing strength of the cured ~ .
pellets after a day or days:
1 day 35 40 15 2 days 71 75 43 6 days 100 110 64 10 days 150 150 138 :
_ . . __ .
Percentage of cracked pellets 7,5 4.3 18.0 ~ . .
Mutual adhesion of pellets Slight Very Strong slight _ ... . . _ ...
Shutter test* after days passed 2 days (frequency) 1 1 3 : .
3 days ( " ) 3 1 5 : ~ :
4 days ( " ) 7 2 13 ~
.. -. ,............................... _ ~
Pressure loss according to the : :
load softening test (mmH2O) 160 160 190 Note: * Using a shutter test machine, a block of pellets adhered with another and having a weight of 40 kg was dropped from a height of 2 meters in a frequency such that the block was split into the individual discrete pellets.
2~ - :
~ ",''~, j ' .
. :
~7~53 EXAMPLE IV
For the coating of green pellets to be cured in a hopper, the as-formed green pelLets just after discharge from procoss s oE Example I[I were mixed wi-th a moist coating ~aterial shown in Table 8, and the mixture was thrown into a continuous type rotary dru~ of 1 meter in diameter and 3.5 meters long.
After the duration of a time period of 2 minutes, the pellets were discharged therefrom ln the as-coated state, and then accumulated in a height of 3 meters in the hopper, while per-forming continuous cutting-out operation. The effective curing period in the hopper was about 30 days, and the cut-out masses of the pellets were further indurated in other curing facilities for 10 days. The results are shown in Table 9.
Table 8 Coating Operation _ . ~ I
Sample Raw Material Constituents ~ Property No, _ _ __ _ _ _ T~Fe FeO CaO SiO2 Particle Size (-~4~m%
by weight) ___. ..... . ... _...... , .. _ _ ........... .. , .
1 Powdered iron ore 64.525.7 0,2 8.0 0.02 77.6 Powdered iron ore 2 + 55,0 21,9~.3 6~8 1.22 76.0 Limestone powder , 3 Limestone powder 0,7 - 54.4 0~1 ~L637~53 ~
., '.
Table 9 .
Sample __ 2 _ :
... ,, _ _......... . '' Moisture (%) 4.5 '7.2 6.5 8.7 ~ . _ . . _ Percentage of coating weight : .
based on green pellet weight(%) 4.0 4. 4.2¦ 3.4 _ _ _ _ _ . . I - .
Coated green pellet: .
Crushing strength (kg/P) 2.0 4.9 5.2 5.0 .
Dropping strength (frequency) 1.5 6.7 6.3 6.4 Porosity (%) 30.5 26. 25.226.0 AITount of water (%) 7.2 7.6 7.8 7.5 -- . _ Crushing strength of the cured .
pellets after a day or days .
passed (kg/P) .
1 day 15 42 45 40 2 days 43 80 83 78 6 days 61 ~ 120 120 115 days 89 155 160 155 :~
.. _. .~
Percentage of cracked pellets(%)~ 85.0 0.1 0.3 0.1 ~ ~
- ... =_ . _.. _.. ~_..... _.__ . ... . ... . .. _ Mutual adhesion Slight none none none Cutting-out from hopper Bad GoodGood Good :
~ (Scaffold) .
High-temperature properties Pressure loss (mmH2O) 700 500 190 190 Quality of pellet as estimated from interference with proper I
conditioning of blast furnace ~, ~oor Poor Good Good -- 28 -- .
~ .
~7~9~3 It is to be unders-tood from the results shows above that . ' the percen-tage of water contained in the coating composition is of importance in preventing a decrease of the crushing strength and the formation of crackings of the pellets and also in facilitating minimizatlon of cutting-out difficulty of the indurated pellets from the curing hopper as the mutual-adheslon of the green pellets is prevented, and that w,hen the basicity of the coating composi-tion is controlled so as not to be within the above specified range, poor high-temperature properties of the coated pellets result despite the fact that the bare pellets without the coating have good high-temperature properties. In addition thereto, the screening operation of the indurated pellets can be omitted before -the pellets havetravelled from the pelletizing system to the smelting system, and the travelling does not suffer from the production of fines, whereby the handling difficulties which have so far been encountered in the prior art are not introduced and moreover, the drafting aspect of the furnace:is improved as the fines are not produced. Thus the present invention has accomplished a remarkable advance not only in the art but also in the economical and social aspects of the art.
, . - 29 -
Claims (10)
1. Non-fired iron-bearing agglomerates adapted for use in a blast furnace and of which the ingredients are proportioned to provide a ratio CaO/SiO2 in a range of from 1.2 to 1.9 and the total amount of slag-forming constituents in a range of from 13% to 19% based on the total weight of said agglomerates.
2. A method of making non-fired agglomerates from an iron ora powder and a binder with a proper amount of water, which comprises a step of adding to a mixture of said iron ore powder and said binder at least one of CaO-source and SiO2-source materials in an amount to assure a ratio CaO/SiO2 in said agglomerates in a range of from 1.2 to 1.9 and an amount of slag-forming constituents in a range of from 13% to 19%
based on the weight of said agglomerates.
based on the weight of said agglomerates.
3. A method of making non-fired agglomerates from an iron ore powder and a binder with a proper amount of water, which comprises blending a Fe-rich low-SiO2 iron ore powder with an iron ore powder containing SiO2 or its derivatives as gangue in a blending ratio such that SiO2 content in the resultant iron ore blend is not less than 1.5%, and adding CaO
additive to the iron ore blend in such an amount to maintain the CaO/SiO2 in the range of from 1.2 to 1.9 and the amount of slag forming constituents in a range of from 13% to 19% based on the total weight of the non-fired agglomerates.
additive to the iron ore blend in such an amount to maintain the CaO/SiO2 in the range of from 1.2 to 1.9 and the amount of slag forming constituents in a range of from 13% to 19% based on the total weight of the non-fired agglomerates.
4. A process for making non-fired agglomerates, which comprises blending an iron ore powder with hydraulic binder to obtain a mixture having a CaO/SiO2 ratio in a range of from 1.2 to 1.9 and a slag forming constituent proportion in a range of from 13% to 19%, grinding and agglomerating the mixture and curing to agglomerates without using a powdery matrix of ore.
5. A process according to claim 4 in which an iron ore powder is blended with hydraulic binder and an additive to obtain the mixture.
6. A process according to claim 5, wherein the total amount of said hydraulic binder is mixed with fractions of the total amounts of said iron ore powder and said additive, the sum of said fractions being equal to from 1 to 5 times the total amount of said hydraulic binder, and the mixture is ground to produce a uniform mixture into which the remaining amounts of said iron ore powder and said additive are added.
7. A process according to claim 5, wherein the sum of the total amounts of said hydraulic binder and said additive is mixed with a fraction of the total amount of said iron ore powder, said fraction being equal to from 3/4 to 4 times the sum of the total amounts of said hydraulic binder and said addi-tive, and then the mixture is ground to produce a uniform mixture into which the remaining amount of said iron ore powder is added.
8. A process according to claim 7 which further includes a step of coating the as-formed discrete moist agglomerates with a moistened inorganic material selected from the group consist-ing of (1) a powdered ore having a CaO/SiO2 ratio of not less than 1.2, (2) a powdery mixture of a powdered ore and an addi-tive, without hydraulic bind, having a CaO/SiO2 ratio not less than 1.2 and (3) a powdery material selected from the group consisting of limestone, slaked lime, dolomite, blast furnace slag and converter slag, by use of a continuous type rotary drum, the thickness of the coating applied on the said agglom-erates being controlled to be not thicker than 0.5 mm.
9. A process according to claim 7, wherein as the inor-ganic material, use is made of a material selected from the group of an iron ore powder having a ratio CaO/SiO2 of not less than 1.2, a mixture of an iron ore powder and an additive and having a ratio CaO/SiO2 of not less than 1.2, CaO3, MgCO3 and MgCO3.CaCO3.
10. A process according to claim 8, wherein said inor-ganic material is moistened with an amount of water in the range of from 70% to 130% of the amount of water contained in the as-formed moist agglomerates.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP7143175A JPS51147401A (en) | 1975-06-14 | 1975-06-14 | A process for production of non-sintered agglomerated ores |
| JP7143275A JPS51147417A (en) | 1975-06-14 | 1975-06-14 | Method of manufacturing non-sintered briquette |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1070953A true CA1070953A (en) | 1980-02-05 |
Family
ID=26412535
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA244,332A Expired CA1070953A (en) | 1975-06-14 | 1976-01-23 | Agglomerates for use in a blast furnace and method of making the same |
Country Status (9)
| Country | Link |
|---|---|
| AU (1) | AU498700B2 (en) |
| BR (1) | BR7600418A (en) |
| CA (1) | CA1070953A (en) |
| DE (1) | DE2602628B2 (en) |
| FR (1) | FR2314254A1 (en) |
| GB (1) | GB1536033A (en) |
| IT (1) | IT1054960B (en) |
| NL (1) | NL172565C (en) |
| SE (1) | SE7600596L (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| BR8201091A (en) * | 1982-03-02 | 1983-10-25 | Fundicao Tupy S A Ind Carboqui | COMPACT NODULES PROCESS AND INSTALLATION FOR YOUR PRODUCTION AND USE OF THESE NODULES FOR THE PRODUCTION OF METAL |
| FI69408C (en) * | 1984-03-01 | 1986-02-10 | Kemira Oy | FARING REFRIGERATION FOR FUNCTIONAL RESISTANCE POROESASILIKAGRANULER MED MEKANISK HAOLLFASTHET |
-
1976
- 1976-01-19 AU AU10377/76A patent/AU498700B2/en not_active Expired
- 1976-01-21 SE SE7600596A patent/SE7600596L/en not_active Application Discontinuation
- 1976-01-23 CA CA244,332A patent/CA1070953A/en not_active Expired
- 1976-01-23 FR FR7601912A patent/FR2314254A1/en active Granted
- 1976-01-23 GB GB270176A patent/GB1536033A/en not_active Expired
- 1976-01-23 BR BR7600418A patent/BR7600418A/en unknown
- 1976-01-23 IT IT1951176A patent/IT1054960B/en active
- 1976-01-23 NL NL7600721A patent/NL172565C/en not_active IP Right Cessation
- 1976-01-24 DE DE19762602628 patent/DE2602628B2/en not_active Ceased
Also Published As
| Publication number | Publication date |
|---|---|
| AU1037776A (en) | 1977-07-28 |
| SE7600596L (en) | 1976-12-15 |
| BR7600418A (en) | 1977-05-10 |
| AU498700B2 (en) | 1979-03-22 |
| DE2602628B2 (en) | 1978-09-14 |
| DE2602628A1 (en) | 1976-12-16 |
| FR2314254B1 (en) | 1979-06-29 |
| NL172565B (en) | 1983-04-18 |
| FR2314254A1 (en) | 1977-01-07 |
| NL7600721A (en) | 1976-12-16 |
| IT1054960B (en) | 1981-11-30 |
| NL172565C (en) | 1983-09-16 |
| GB1536033A (en) | 1978-12-20 |
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