Method for manufacturing agglomerates of fired pellets

Abstract: A method for manufacturing agglomerates of fired pellets comprising the steps of: the first pelletization step of adding and mixing fluxes to fine iron ores containing 30 to 95 wt.% of those of 0.125mm or less in particle size to form a mixture and to pelletize the mixture into green pellets; the second pelletization step of adding powder cokes containing 80 to 100 wt.% of those of 1 mm or less in particle size to the green pellets, in amount of 2.5 to 4.0 wt.% to the fine iron ores, to prepare, through pelletization, green pellets coated with the powder cokes; and the sintering step of charging the green pellets coated with the powder cokes into a grate type sintering machine to manufacture the agglomerates of fired pellets. And furthermore, in another method for manufacturing agglomerated of fired pellets, fine iron ores containing 10 to 80 wt.% of those of 0.044mm or less in particle size and powder cokes containing 20 to 70 wt.% of those of 1mm or less in particle size are used.

Description: The present invention relates to a method for manufacturing agglomerates of fired pellets fitted for materials used for a blast furnace or a direct reduction furnace, and more particularly, to conditions on materials used for manufacture of the agglomerates of fired pellets and conditions on pelletization of the materials. As materials used for a blast furnace or a direct reduction furnace, agglomerates of fired pellets, which are made from fine iron ores by pelletization and by sintering are well known. Consumption of these fired pellets are increasing in amount year by year, various research and development on these fired pellets has been performed. For example, a method is disclosed in JP-A-61 106 728 U.S-A-4 722 750 corresponds, wherein: (a) To fine iron ores mainly composed of those of 5mm or less in particle size, fluxes are added, and the fine iron ores are pelletized, as the first step pelletization, into green pellets; (b) the green pellets are coated on their surface, as the second step pelletization, with solid fuels such as powder cokes, powder chars, fine powder coals and powder oil cokes to prepare mini-pellets of 3 to 9 mm in particle size, providing that the addition ratio of the solid fuels is 2.5 to 3.5 wt.% to the fine iron ores; (c) the mini-pellets are sintered, through a grate type sintering machine equipped with zones for drying, igniting, sintering and cooling, to prepare blocky agglomerates of mini-pellets; (d) the agglomerates of mini-pellets manufactured by sintering are composed of mini-pellets combined on their surface through work of calcium ferrite. This method, however, allows the following difficulties to remain still unsettled: (1) The yield is low, and, consequently, the productivity is low. (2) The strength of the agglomerates of mini-pellets is not satisfactory for the operation of a blast furnace and a direct reduction furnace. US-A-4 504 306 refers to a method of producing agglomerates well suited for use in an iron producing blast furnace from a fine iron ore as a principle raw material, according to which drum pelletizers are used in a two-stage pellet production in which the shell is made of solid fuels. It is an object of the present invention to provide a method for manufacturing agglomerates of fired pellets, enabling the productivity to be good enough and the strength to be strong enough for the operation of a blast furnace and a direct reduction furnace. In accordance with the present invention, a method is provided for manufacturing agglomerates of fired pellets comprising the steps of: adding fluxes including quick limes to fine iron ores, 30 to 95 weight % of the fine iron ores having a particle size of 0.125 mm or less, and the amount of quick limes being 1.0 to 2.5 weight % based on the weight of fine iron ores; mixing the fluxes and fine iron ores to produce a mixture; pelletizing the mixture to form green pellets; adding powder cokes to the green pellets in an amount of 2.5 to 4.0 weight % based on the weight of fine iron ores in the green pellets, 80 to 100 weight % of the powder cokes having a particle size of 1 mm or less; pelletizing the green pellets and the powder cokes to produce green pellets coated with the powder cokes; charging the coated green pellets into a grate type sintering machine; and sintering the coated green pellets to produce agglomerates of fired pellets, said agglomerates containing 0.5 to 5.0 weight % SiO₂; said step of pelletizing the mixture to form green pellets includes pelletizing the mixture of fluxes and fine iron ores using a disc type pelletizer; and said step of pelletizing the green pellets and the powder cokes includes pelletizing the green pellets and the powder cokes using a drum type pelletizer. Furthermore, a method is provided for manufacturing agglomerates of fired pellets comprising the steps of: adding fluxes including quick limes to fine iron ores, 10 to 80 weight % of the fine iron ores having a particle size of 0.044 mm or less, and the amount of quick limes being 1.0 to 2.5 weight % based on the weight of fine iron ores; mixing the fluxes and fine iron ores to produce a mixture; pelletizing the mixture to form green pellets; adding powder cokes to the green pellets in an amount of 2.5 to 4.0 weight % based on the weight of fine iron ores in the green pellets, 20 to 70 weight % of the powder cokes having a particle size of 0.1 mm or less; pelletizing the green pellets and the powder cokes to produce green pellets coated with the powder cokes; charging the coated green pellets into a grate type sintering machine; and sintering the coated green pellets to produce agglomerates of fired pellets, said agglomerates containing 0.5 to 5.0 weight % SiO₂; said step of pelletizing the mixture to form green pellets includes pelletizing the mixture of fluxes and fine iron ores using a disc type pelletizer; and said step of pelletizing the green pellets and the powder cokes includes pelletizing the green pellets and the powder cokes using a drum type pelletizer. The object and the other objects and advantages of the present invention will become more apparent from the detailed description to follow, taken in conjunction with the appended drawings. Fig. 1 is a graphic representation showing relation of blend ratio of 0.125 mm or less fine iron ores contained in those used of 8 mm or less in particle size, to reduction index of obtained agglomerates of fired pellets, according to a method of the present invention; Fig. 2 is a graphic representation showing relation of blend ratio of 0.125 mm or less fine iron ores contained in those used of 8 mm or less in particle size, to shatter index of the obtained agglomerates of fired pellets, according to the method of the present invention; Fig. 3 is a graphic representation showing relation of blend ratio of 1mm or less powder cokes contained in those, used for coating green pellets, of 5mm or less in particle size, to yield of the obtained agglomerates of fired pellets, according to the method of the present invention; Fig. 4 is a graphic representation showing relation of blend ratio of lmm or less powder cokes contained in those of 5mm or less in particle size, to productivity of the obtained agglomerates of fired pellets, according to the method of the present invention; Fig. 5 is a graphic representation showing relation of quick lime addition amount to fine iron ores, to yield of the obtained agglomerates of fired pellets, according to the method of the present invention; Fig. 6 is a graphic representation showing relation of quick lime addition amount to fine iron ores, to the shatter index, according to the method of the present invention; Fig. 7 is a graphic representation showing relation of blend ratio of 5mm or less green pellets in particle size contained in those used, to the yield, according to a preferred mode of the method of the present invention. Fig. 8 is a graphic representation showing relation of blend ratio of 5mm or less green pellets contained in those used, to the productivity, according to a preferred mode of the method of the present invention. Fig. 9 is a graphic representation showing relation of blend ratio of 5mm or less green pellets contained in those used, to the shatter index, according to a preferred mode of the method of the present invention. Fig. 10 is a graphic representation showing relation of SiO₂ content in the obtained agglomerates of fired pellets, to reduction index of the obtained agglomerates of fired pellets, according to the method of the present invention; Fig. 11 is a graphic representation showing relation of SiO₂ content in the obtained agglomerates of fired pellets, to reduction degradation index, according to the method of the present invention. Fig. 12 is a graphic representation showing relation of SiO₂ content in the obtained agglomerates of fired pellets, to the shatter index according to the method of the present invention; Fig. 13 is a graphic representation showing relation of SiO₂ content in the manufactured agglomerates of fired pellets, to the yield, according to the method of the present invention. Detailed description of the invention Now, the method for manufacturing fired pellets of the present invention will be described. 1.0 to 2.5 wt.% quick limes were added and mixed, as a flux, to fine iron ores containing 30 to 95 wt.% of those of 0.125mm or less in particle size. Subsequently, a mixture thus prepared, was pelletized, by means of a disc type pelletizer, into 3 to 13mm green pellets (the first pelletization). Further, powder cokes containing 80 to 100 wt.% of those of lmm or less in particle size were added to the green pellets, in amount of 2.5 to 4.0 wt.% to the fine iron ores, and the green pellets were pelletized again, by means of a drum type pelletizer into the green pellets coated with the powder cokes (the second pelletization). The green pellets coated with the powder cokes were charged into a grate type sintering machine to manufacture agglomerates of fired pellets composed of fired pellets combined in plurality. Terms "Reduction index", "shatter index" and "reduction degradation index" herein contained, have meanings as defined herebelow throughout in this specification. (1) Reduction index (RI): The reduction index was measured by a method specified in JIS (Japanese Industrial Standards), which comprises: reducing the fired pellets in an amount of 500g charged into an experimental electric furnace by means of a reducing gas comprising 30 vol.% CO and 70 vol.% N₂ at a temperature of 900°C for 180 minutes, and measuring the reduction index of the fired pellets. (2) Shatter index (SI+₅): The shatter index was measured by a method specified in JIS, which comprises: dropping the fired pellets in an amount of 20 Kg four times from a height of 2 m onto an iron plate, sieving the thus dropped fired pellets through a 5-mm mesh screen, and measuring the ratio of particles on the screen. (3) Reduction degradation index (RDI): The reduction degradation index was measured by a method specified by the Ironmaking committee of the Iron and Steel Institute of Japan, which comprises: reducing the fired pellets in an amount of 500g charged into an experimental electric furnace by means of a reducing gas comprising 30 vol.% CO and 70 vol.% N₂ at a temperature of 550°C for 30 minutes, receiving the thus reduced fired pellets in a drum, rotating the drum by 900 revolutions, sieving the fired pellets taken out from the drum through a 3-mm mesh screen, and measuring the ratio of particles under the screen. Particle Size of Fine Iron Ores Particle size of fine iron ores will be described in detail herebelow. The following conception occurred to those engaged in research and development: (A) If blend ratio of powdery fine iron ores increases and fine iron ores to be used become smaller on average in particle size, then reduction index of fired pellets will be increased because many macro-pores are formed in each body of the fired pellets to be obtained when the fine iron ores are pelletized into green pellets. (B) If fluxes are added to fine iron ores and the fine iron ores are pelletized into green pellets, then agglomerates of fired pellets will be strengthened in their shatter index because the green pellets, thus pelletized into, become high both in strength and density. Based on this conception, an experiment was carried out wherein blend ratios of fine iron ores having various distribution of their particle sizes were varied to pelletize green pellets into agglomerates of fired pellets, and reduction indexes and shatter indexes of the agglomerates of fired pellets were checked. Fig. 1 of the drawing shows graphically relation of blend ratio of 0.125mm or less fine iron ores contained in those of 8mm or less in particle size, to reduction index of obtained agglomerates of fired pellets. Fig. 2 graphically shows relation of blend ratio of 0.125mm or less fine iron ores included in those of 8mm or less in particle size, to shatter index of the obtained agglomerates of fire pellets. As shown in Fig. 1, because macro-pores contained in each body of fired pellets increase as the blend ratio of 0.125mm or less in particle size are increasing, reduction index of the agglomerates of fired pellets is improved. When the blend ratio of fine iron ores is 30 wt.% or more, the reduction index is high enough to be well more than 75%. As shown in Fig. 2, if the blend ratio of 0.125mm or less fine iron ores is 30 wt.% or more, the density and strength of the green pellets are increased so high as to allow the shatter index of the obtained agglomerates of fired pellets to show more than 85%. However, if the blend ratio becomes 95 wt.% or more, green pellets get apt to be melted through excessive heating and to form glassy slag, this resulting in rapid deterioration of the shatter index. From the results of the experiment, it became apparent that if powder iron ores consisting of 30 to 95 wt.% of those of 0.125mm or less in particle size and of the rest of those more than 0.125mm are used, then the reduction index and the shatter index of the agglomerates of fired pellets will be by far improved. The range of 50 to 95 wt.% of powder iron ores of 0.125mm or less is more preferable. Powder Cokes Powder cokes to be added at the step of the second pelletization will now be explained about. The concept thereof was made as shown herebelow. (A) If particle size becomes relatively fine, powder cokes will be allowed to coat the surface of green pellets fully and uniformly. (B) If the green pellets are sintered, in good condition, in a sintering machine, improvement in yield and productivity of the fired pellets will be able to be attained. According to this way of thinking, an experiment was carried out, wherein green pellets were coated with various particle sizes of powder cokes and various blend ratios thereof to manufacture agglomerates of fired pellets, and shatter indexes and productivities of the agglomerates of fired pellets corresponding to the variation were checked. Fig. 3 graphically shows relation of blend ratio of 1mm or less powder cokes contained in those of 5mm or less in particle size, to the yield of the obtained agglomerates of fired pellets. Fig. 4 graphically shows relation of blend ratio of lmm or less powder cokes contained in those of 5mm or less in particle size, to the shatter index of the obtained agglomerates of fired pellets. In this experiment, fine iron ores used were of 8mm or less in particle size, green pellets of 3 to 13mm, and the powder cokes were added in amount of 3.5 wt.%. As seen from Fig. 3, the more the blend ratio of lmm or less powder cokes becomes, the better green pellets get coated and sintered, this resulting in improving the yield. If the blend ratio is 80 wt.% or more, the yield is high enough to show 75% or more. As seen from Fig. 4, the productivity also increases, as the blend ratio is going up. In the range of 80 wt.% or more of the blend ratio, the productivity is good enough to mark 1.5 T/H/M² or more. Consequently, the blending ratio of lmm or less powder cokes ranges preferably 80 to 100 wt.%. To further improve the yield and the productivity, it is more preferable to keep the blending ratio of 1mm or less powder cokes in the range of 90 to 100 wt.%. The amount of powder cokes for coating the green pellets are recommended to be 2.5 to 4.0 wt.% to the amount of fine iron ores. If the amount of the powder cokes for coating is less than 2.5 wt.%, it is impossible to sinter the green pellets into fired pellets of high shatter index in a short time, namely, efficiency in sintering the green pellets in a sintering machine cannot be raised. Contrarily, if the amount of the powder cokes for coating is over 4.0 wt.%, the temperature at the time of sintering the green pellets rises excessively so high that the agglomerates of fired pellets become too dense in their texture. The Second Pelletization The reasons for a drum type pelletizer being fitted for coating green pellets with powder cokes will be explained herebelow. In a pelletizer of drum type, its inclined drum rotates and, therefore, green pellets can be pushed out, almost equally regardless of thier particle sizes, through the end of the drum. Consequently, the green pellets are discharged almost without difference in their retention time in the pelletizer. Due to this performance, in a case, for example, that 3 to 13mm green pellets in particle size are coated with powder cokes, the green pellets are allowed to be successfully covered without dispersion of coating amount. Even in the case of using large size green pellets, there is no shortage of coating amount. Therefore, even in the lower layer portion where larger green pellets in particle size are easy to gather when charged into a sintering machine, the sintering works so well that there is no occurence of deterioration either in yield of the agglomerates of fired pellets, or in productivity due to prolonging sintering time. If powder cokes are coated with by means of a disc type pelletizer which is customarily used, time during which green pellets stay in the disc pelletizer is different, depending on their particle sizes. Due to the difference of the retention time, coating amount of power cokes per unit weight of green pellets are dispersed, and, thus, shortage of coating amount covering green pellets occurs. Owing to this, in the lower layer portion which is easy to allow large size green pellets to gather in charging them into the sintering machine, the sintering does not work well. This results in deterioration either in yield of the agglomerates fired pellets or in productivity thereof because of sintering time becoming longer. Addition of Quick Limes According to the method of the present invention, fine iron ores were pelletized by use of a disc type pelletizer and only with addition of fluxes, and, thereafter, coating with powder cokes was made. From this performance, it became apparent that this method was so good for pelletization of fine iron ores that green pellets could be obtained from fine iron ores with addition of quick limes in small amount. But, owing to this addition amount being small, there remained the possibility of deteriorating the yield and the shatter index. In this connection, an experiment was carried out wherein various amount of quick limes were added to manufacture fired pellets by means of sintering green pellets pelletized through the addition of quick limes to fine iron ores. Fig. 5 graphically shows relation of quick lime addition amount to fine iron ores, to yield of the agglomerates of fired pellets. Fig. 6 graphically shows relation of quick lime addition amount to shatter index of the agglomerates of fired pellets. In this experiment, fine iron ores were of 8mm or less in particle size, green pellets of 3 to 13mm, and powder cokes were added in amount of 3.5 wt.%. As shown in Fig. 5, the more the addition amount of quick limes to fine iron ores increases, the better the yield of the obtained agglomeretes of fired pellets is improved. When the addition amount is 1.0 wt.% or more, the yield marks 75% or more. In the case that the addition amount is over 2.5 wt.%, it can be admitted that the yield becomes 85% or more, but the growth of the yield is smaller in proportion, i.e. the increase of quick lime addition amount, after all, extends aspects of demerits. As recongnized from Fig. 6, as the addition amount is going up, the shatter index increases. If the addition amount is 1.0 wt.% or more, the shatter index gets well over 85%. In the case that the addition amount is 2.5 wt.% or more, the shatter index becomes well over 90%, but the growth of shatter index is smaller in proportion. Judging from the results, to maintain the yield of the obtained agglomerates of fired pellets 75% level or more and, at the same time, the shatter index more than 85%, and still to allow the addition amount of quick limes to be as small as possible, wherein the quick lime addition amount ranges 1.0 to 2.5 wt.%. Note that fluxes together with quick limes are, of course, added to fine iron ores so as to keep CaO/SiO₂ ratio 1.0 to 2.5. Particle Size of Green Pellets If blend ratio of small green pellets increases and green pellets to be used become relatively small, yield of agglomerates of fired pellets can be expected to be improved, since sintering of green pellets are well performed. But, if blend ratio of small green pellets become excessive, at the time of sintering, permeability among the green pellets is deteriorated so much that, owing to long time being required for the sintering, the productivity is deteriorated. Furthermore, because the green pellets are apt to be melted when excessively heated, they form glassy slag. Consequently, this results in deterioration of the shatter index. Beside that, this increases melted texture portion. Therefore, there further remains danger of deteriorating reduction index and reduction degradation index of the agglomerates of fired pellets. In this connection, an experiment was carried out, wherein particle sizes and blend ratios of green pellets were varied, and the green pellets were coated with powder cokes to manufacture agglomerates of fired pellets. Fig. 7 graphically shows relation of blend ratio of 5mm or less green pellets included in those used to yield of the obtained agglomerates of fired pellets. Fig. 8, also, graphically shows relation of blend ratio of 5mm or less green pellets included in those used to productivity of the obtained agglomerates of fired pellets. Fig. 9, also, graphically shows relation of blend ratio of 5mm or less green pellets included in those used to shatter index of the agglomerates of fired pellets. In this experiment, 8mm or less fine iron ores in particle size were used and 3.5 wt.% powder cokes were added. As shown in Fig. 7, the more the blend ratio of 5mm or less green pellets in particle size increases, the better the sintering performance of the green pellets becomes, and, thus, the yield of the agglomerates of fired pellets is improved. If the blend ratio is 15 wt.% or more, the yield is 78% or more. The productivity is, as seen in Fig. 8, maintaining the level of 1.5 T/H/M² or more so far as the blend ratio of the green pellets is 40 wt.% or less, while the productivity goes down to less than 1.5 T/H/M² when the blend ratio is over 40 wt.%, since in this range, owing to deterioration of permeability, sintering time becomes long. With respect to the shatter index of the agglomerates of fired pellets, as shown in Fig. 9, the more the blend ratio of 5mm or less green pellets becomes, the more the shatter index is deteriorated, since glassy slag of the green pellets increase in proportion with the increase of the blend ratio. If the blend ratio is over 40 wt.%, the shatter index is less than 90%. Accordingly, in order to keep the yield 78% or more, the productivity 1.5T/H/M² level or more and the shatter index more than 90%, it is preferable to use green pellets consisting of 15 to 40 wt.% of 5mm or less green pellets in particle size and the rest of those of more than 5mm in particle size. 20 to 30 wt.% of 5mm or less is more preferable. SiO ₂ Content in Agglomerates of Fired Pellets According to the method of the present invention, fine iron ores are pelletized by use of a disc type pelletizer and only with addition of fluxes, and, thereafter, coating with powder cokes is made, and, resultantly, this method is good for the pelletization enough to form good spherical green pellets. Therefore, from the performance of this method, it was found that, during the process of sintering green pellets, SiO₂ contained in fine iron ores and CaO contained in fluxes reacted each other, although the SiO₂ content was small, to form slag and thereby to allow the fine iron ores to one another be combined and well agglomerated. In this connection, agglomerates of fired pellets of various SiO₂ contents were manufactured experimentally from green pellets which had been prepared from fine iron ores having various SiO₂ contents. In this experiment, relations of SiO₂ content in agglomerates of fired pellets, respectively, to reduction index, reduction degradation index, yield, and shatter index were pursued. Fig. 10 graphically shows relation of SiO₂ content in obtained agglomerates of fired pellets to their reduction index. Fig. 11 graphically shows relation of SiO₂ content in the obtained agglomerates of fired pellets to their reduction degradation index. Fig. 12 graphically shows relation of SiO₂ content in the obtained fired pellets to their shatter index. Fig. 13 graphically shows relation of SiO₂ content in the obtained agglomerates of fired pellets to their yield. The reduction index of the agglomerates of fired pellets, as shown in Fig. 10, goes down as the SiO₂ content in the agglomerates of fired pellets is increasing. The reduction index, however, maintains the level higher than 80% in the SiO₂ content range of 0.5 to 5.0 wt.%. If the SiO₂ content is over 5.0 wt.%, the reduction index remarkably goes down. The reduction degradation index of the agglomerates of fired pellets, as seen from Fig. 11, shows good mark of less than 30 % in the SiO₂ content range of 0.5 to 5.0 wt.%. If the SiO₂ content is less than 0.5 wt.%, the reduction degradation index is deteriorated, while if the SiO₂ content is over 5.0 wt.%, the reduction degradation index becomes worse over 30%. Furthermore, as shown in Fig. 12, the shatter index of the agglomerates of fired pellets keeps the level enough to be more than 85% also in the SiO₂ content range of 0.5 to 5.0. wt.%. If the SiO₂ content is less than 0.5 wt.%, the shatter index rapidly declines. With respect to the yield of the agglomerates of fired pellets, as shown in Fig. 13, the yield increases as the SiO₂ content is going up, and the yield satisfies the level of being well more than 75% even in the SiO₂ content range of 0.5 to 5.0 wt.%. If the SiO₂ content is lowered less than 0.5 wt.%, the yield rapidly declines. Judging from these results, in order to keep the reduction index of more than 80% and the reduction degradation index of 30% or less without deterioration of the yield and the shatter index, the SiO₂ content of the agglomerates of fired pellets ranges from 0.5 to 5.0 wt.%. 1.0 to 4.0 wt.% of the SiO₂ content is preferable. Example 1 To fine iron ores consisting of 40 wt.% powdery fine iron ores and 60 wt.% coarse grain iron ores, quick limes of 0.5 to 5.0 wt.% as a flux and binder were added. Furthermore, limestones as another flux were added so as to control CaO/SiO₂ ratio of agglomerates of fired pellets within the range of 1.0 to 2.5. Subsequently, the fine iron ores to which the quick limes and the limestones were mixed and pelletized by a disc type pelletizer into green pellets of 3 to 13mm in particle size with water content of 8 to 9 wt.%. To the green pellets, 3.5 wt.% powder cokes were further added and the green pellets were coated, through drum pelletization, with the powder cokes. The powdery fine iron ores, the coarse grain iron ores. the quick limes and the powder cokes used in Example 1 are indicated in the Tables 1 to 9. Next, the green pellets were charged into an endless grate type sintering machine to be laid in 400mm thick on the grate of the sintering machine. And then, the green pellets were moved through zones for drying, igniting and sintering on the grate in order, to form agglomerates of fired pellets. The yields and the shatter indexes of the manufactured agglomerates of fired pellets are shown in Table 10. As seen from Table 10, the manufactured agglomerates of fired pellets of Test Nos. 16 to 19, as Examples of the present invention, having addition amount of 1.0 to 4.0 wt.% quick limes, maintain the yields of well more than 75% and the shatter indexes of well more than 85%, and this enables to economically manufacture agglomerates of fired pellets with small addition amount of quick limes. In comparison, the manufactured agglomerates of fired pellets of Test No. 20 as one of Controls to which 0.5 wt.% quick limes were added show remarkable deterioration of the yield and the shatter indexes. With respect to the manufactured agglomerates of fired pellets of Test Nos. 21 and 22, as Controls, to which over 2.5 quick limes were added, they show good marks of well over 85% yield and well over 90% shatter indexes, but, owing to large addition amount of the quick limes, they failed to be economically manufactured. (wt.%) 0.044mm or less Over 0.044mm to 0.125mm Over 0.125mm to 0.5mm Over 0.5mm 63.86 31.07 4.48 0.59 (wt.%) T.Fe SiO₂ Al₂O₃ CaO MgO FeO 67.80 0.81 0.63 0.04 0.40 0.09 0.044mm or less Over 0.044mm to 0.125mm Over 0.125mm to 0.50mm Over 0.50mm to 1.00 10.07 11.88 16.92 10.75 Over 1.00mm to 2.00mm Over 2.00mm to 2.83mm Over 2.83mm to 8mm Over 8mm 14.36 9.41 24.14 2.47 (wt.%) T.Fe SiO₂ Al₂O₃ CaO MgO FeO 59.47 5.60 1.80 1.80 1.78 4.40 Test Nos. Blend Ratio of 0.125mm or Less (wt.%) Examples 1 30 2 40 3 60 4 80 5 95 Controls 6 20 7 100 (wt.%) 0.125mm or Less Over 0.125mm to 0.5mm Over 0.5mm to 1 mm Over 1mm 16.2 20.0 18.3 45.5 (wt.%) 3mm or More to 5mm Over 5mm to 7mm Over 7mmm to 9mm Over 9mm to 10mm Over 10mm to 13mm 7 35 39 11 8 (wt.%) 0.1mm or less Over 0.1mm to 0.5mm Over 0.5mm to 1mm Over 1mm 21.83 66.75 10.52 0.90 Test Nos. Reduction Index (%) Shatter Index SI+₅ (%) Examples 1 76.9 85.4 2 80.7 88.3 3 83.2 90.7 4 85.0 91.4 5 84.2 90.6 Controls 6 69.8 77.1 7 84.7 80.3 Test Nos. Addition Amount of Quick Limes (wt.%) Yield (%) Shatter Index (%) Examples 16 1.0 75.3 88.3 17 1.5 78.1 90.3 18 2.0 80.5 90.6 19 2.5 85.7 91.9 Controls 20 0.5 62.2 83.4 21 3.0 86.0 92.2 22 5.0 86.8 92.7 Example 2 5 kinds of fine iron ores composed of particle size distribution as shown in Table 11(a) and chemical composition as shown in Table 11(b), each, were blended as shown in Table 12 so as to allow SiO₂ amount contained in each of the fine iron ores to range 0.5 to 6.0 wt.%. Subsequently, to these fine iron ores thus blended, quick limes as a flux and binder, and limestones as a regulator of basicity, were added and mixed with the fine iron ores. The amount of the quick limes ranged 1.0 to 2.7 wt.%, and the basicity was regulated in the range of 1.8 to 2.2. The mixture of the fine iron ores with the quick limes and the limestones were pelletized, by means of a disc type pelletizer, into green pellets of 3 to 13mm in particle size with water content of 8 to 9 wt.%. Subsequently, to the green pellets, 3.5 wt.% powder cokes were added, and the green pellets were coated, through pelletization, with the powder cokes. The drum quick limes and the powder cokes used in Example 2 were same as those used in Example 1 in respect to particle size distribution and chemical composition. Next, the green pellets were charged into an endless grate type sintering machine to be laid in 400mm thickness on the grate of the sintering machine, and then, were moved through zones for drying, igniting and sintering in order, to form agglomerates of fired pellets. The SiO₂ contents in the manufactured agglomerates of fired pellets, the yields, the shatter indexes, the reduction indexes and the reduction degradation indexes of the manufactured agglomerates of fired pellets are shown in Table 13. As seen from Table 13, manufactured agglomerates of fired pellets of Test Nos. of 29 to 34, as Examples of the present invention having 0.5 to 5.0 wt.% SiO₂ content contained in the agglomerates of fired pellets, all, showed good marks of their reduction indexes and reduction degradation indexes. Contrarily, the manufactured agglomerates of fired pellets of Test Nos. 35 and 36, as Controls, having over 5.0 wt.% SiO₂ content contained in the agglomerates of fired pellets, deteriorated their reduction indexes and reduction degradation indexes, although their shatter indexes and yields were good. (wt.%) T.Fe SiO₂ Al₂O₃ CaO MgO FeO A 68.32 0.28 0.73 0.04 0.13 0.14 B 62.57 5.53 2.26 0.04 0.06 0.16 C 68.24 0.57 0.80 0.04 0.05 0.14 D 58.04 6.91 2.18 1.74 2.03 6.93 E 58.29 5.32 2.26 1.46 1.23 7.01 Test Nos. Blend ratio of Fine Iron Ores (wt.%) SiO₂ Content in Fine Iron Ores (wt.%) A B C D E Examples 29 70 - 27 - 3 0.48 30 70 - 20 5 5 0.98 31 70 - - 15 15 2.07 32 60 - - 40 - 2.88 33 40 20 - 40 - 4.03 34 20 40 - 40 - 5.10 Controls 35 10 50 - 30 10 5.54 36 - 60 - 40 - 6.02

Claims: A method for manufacturing agglomerates of fired pellets comprising the steps of: adding fluxes including quick limes to fine iron ores, 30 to 95 weight % of the fine iron ores having a particle size of 0.125 mm or less, and the amount of quick limes being 1.0 to 2.5 weight % based on the weight of fine iron ores; mixing the fluxes and fine iron ores to produce a mixture; pelletizing the mixture to form green pellets; adding powder cokes to the green pellets in an amount of 2.5 to 4.0 weight % based on the weight of fine iron ores in the green pellets, 80 to 100 weight % of the powder cokes having a particle size of 1 mm or less; pelletizing the green pellets and the powder cokes to produce green pellets coated with the powder cokes; charging the coated green pellets into a grate type sintering machine; and sintering the coated green pellets to produce agglomerates of fired pellets, said agglomerates containing 0.5 to 5.0 weight % SiO₂; said step of pelletizing the mixture to form green pellets includes pelletizing the mixture of fluxes and fine iron ores using a disc type pelletizer; and said step of pelletizing the green pellets and the powder cokes includes pelletizing the green pellets and the powder cokes using a drum type pelletizer. A method according to claim 1, characterized in that 50 to 95 weight % of the fine iron ores have a particle size of 0.125 mm or less. A method according to claim 1, characterized in that 15 to 40 % by weight of the green pellets produced by pelletizing the mixture of quick limes and fine iron ores have a particle size of 5 mm or less, and the remainder of the green pellets have a particle size greater than 5 mm. A method for manufacturing agglomerates of fired pellets comprising the steps of: adding fluxes including quick limes to fine iron ores, 10 to 80 weight % of the fine iron ores having a particle size of 0.044 mm or less, and the amount of quick limes being 1.0 to 2.5 weight % based on the weight of fine iron ores; mixing the fluxes and fine iron ores to produce a mixture; pelletizing the mixture to form green pellets; adding powder cokes to the green pellets in an amount of 2.5 to 4.0 weight % based on the weight of fine iron ores in the green pellets, 20 to 70 weight % of the powder cokes having a particle size of 0.1 mm or less; pelletizing the green pellets and the powder cokes to produce green pellets coated with the powder cokes; charging the coated green pellets into a grate type sintering machine; and sintering the coated green pellets to produce agglomerates of fired pellets, said agglomerates containing 0.5 to 5.0 weight % SiO₂; said step of pelletizing the mixture to form green pellets includes pelletizing the mixture of fluxes and fine iron ores using a disc type pelletizer; and said step of pelletizing the green pellets and the powder cokes includes pelletizing the green pellets and the powder cokes using a drum type pelletizer. A method according to claim 4, characterized in that 15 to 40 % by weight of the green pellets produced by pelletizing the mixture of quick limes and fine iron ores have a particle size of 5 mm or less, and the remainder of the green pellets have a particle size greater than 5 mm.

Inventors: NODA HIDETOSHI, SAKAMOTO NOBORU, YANAKA HIDEOMI, NODA, HIDETOSHI, SAKAMOTO, NOBORU, YANAKA, HIDEOMI, Noda, Hidetoshi c/o Patent Licence and Quality, Sakamoto, Noboru c/o Patent Licence and Quality, Yanaka, Hideomi c/o Patent Licence and Quality
Applicants: NIPPON KOKAN KK, NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1987
Main classification: C22B1
IPC codes: C22B1