An improved raw materials mix and process for producing self-fluxing, sintered ores

Abstract

  A process is disclosed for producing self-fluxing, sintered ores using a raw materials mix comprising at least 25 wt% of fine grains below 1 mm in size and not more than 5.4 wt% of SiO₂ in terms of sintered ore product. The process is characterized by control of the Si0₂ content of said fine grains to be at least 50 wt% of the total Si0₂ content of the mix, and sintering of the thus controlled mix.

Description

[0001] This invention relates to an improved raw materials mix for producing self-fluxing, sintered ores, and more particularly, to an improved process for producing low-slag, sintered ores which are resistant to disintegration, and the improved self-fluxing sintered ores resulting from the process. The process is based on the finding that the Si0₂ content of the fine grains of the new materials mix is important to the production of improved self-fluxing, sintered ores.

[0002] Sintereing by a common Dwight-Lloyd sinter machine generally comprises preparing a raw materilas mix of iron ore, limestone, silica, miscellaneous other materials and coke, agglomerating the mix in the presence of water, and charging the resulting agglomerate into the sinter machine. In that machine, the surface layer of the sinter bed is ignited in an ignition furnace, and suction is applied downwardly of the sinter bed for a period of about 20 minutes during which the whole thickness of the sinter bed is sintered, starting from the surface layer and ending with the bottom layer.

[0003] In the production of sintered ores which flux themselves and have a Ca0/SiO₂ level in the range of from 1.0:1 to 2.0:1 generally from 1.3:1 to 1.8:1, a high productivity, a low fuel consumption and all improved quality in terms of shatter index and degradation index after reduction at 550°C (RDl) are factors always to be kept in mind. In order to improve the RDI value which expresses, as a weight percentage of grains below 3 min in size, the ease with which the sintered ore disintegrate when subjected to a reducing atmosphere in a blast furnace at a temperature in the range of from 400 to 600°c according to the conventional technique, either the coke content of the mix or its slag content (expressed as the sum of Ca0 and SiO₂) is increased. The first method, i.e. increasing the coke content, is effective in improving the RDI value, but lowers the gas permeability of the sinter bed, thus decreasing the productivity and reducibility while increasing coke consumption. The second method, i.e. increasing the slag content (expressed as the sum of CaO and SiO₂), achieves the intended purpose, but calls for more slag to be charged into the blast furnace, thus increasing the blast furnace fuel consumption. Usually, blast furnaces are charged with about 300 kg of slag per ton of pig, and this amount is far greater than the amount necessary for furnace operations. As mentioned above, such excess slag is primarily due to the high content of SiO₂ carried with the furnace charge, especially itsmajor ingredient, i.e., the sintered ore. A small SiO₂ content of sintered ore denotes a low strength and a low yield of sintered ore. To avoid this problem, it has been necessary to keep the SiO₂ content of the sintered ore in the range of from about 5.6 to about 6.0 wt%, inclusive.

[0004] Certain proposals have been made for decreasing the SiO₂ content of sintered ore without increasing the coke content.

[0005] One such proposal recommends burning a large amount of a gaseous fuel (coke oven gas) in an ignition furnace or heating furnace to replace the usual solid fuel (coke). However, the technique proposed uses essentially low firing temperatures and requires the higher range of firing temperature to be retained for a lengthy period of time; this inevitably lowers the productivity of the process. In addition, past records of the technique For reducing the SiO₂ content in terms of the sintered ore show that the SO₂ content cannot be made lower than about 5.6 Wt%, and, therefore, the technique is not an effective method of lowering the SiO₂ content of the sintered ore. The high blast furnace slag volume also denotes an increased furnace fuel consumption, and, therefore, with the current concern over energy saving, a reduction in the blast furnace slag volume is an important task for the industry.

[0006] We have now surprisingly found that, in a raw materials mix for sintering which consists of coarse and fine grains, the SiO₂ content of the fine grains is critical to the production of an improved self-fluxing, sintered ore.

[0007] More specifically, the process of this invention for producing a self-fluxing sintered ore is based on the finding that SiO₂ contained in the fine grains of the raw materials mix melts easily in the sintering operation to form a melt, and that even a small amount of such SiO₂ can bind and agglomerate the coarse grains of the iron ore to give a strength which is such that the resulting sintered ore will not disintegrate under the load (the weight of furnace charge) when it is charged into a blast furnace.

[0008] According to our invention, we provide:

(1) a process for producing a self-fluxing, sintered ore using a raw material mix comprising iron ore, limestone, silica and coke, at least 25 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 5.4 wt% of SiO₂ in terms of sintered ore product, i.e. sintered ore product from which coke, combined C0₂ and combined water have been removed by sintering of the mix, said process being characterized by controlling the SiO₂ content of said fine grains to be at least 50 wt% of the total SiO₂ content of the mix, and sintering the thus controlled mix;

(2) a process for producing.self-fluxing sintered ores using a raw materials mix comprising iron ore, limestone, silic and return fines, at least 25 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 5.4 wt% of SiO₂ in terms of sintered ore product, i.e. sintered ore product from which coke, combined C0₂ and combined water have been removed by sintering of the mix, said process being characterised by controlling the SiO₂ content of said fine grains to a value in the range of from 2.4 to 3.0 wt%, inclusive, of the mix (dry), and the basicity (CaO/SiO₂ ratio) of said fine grains to be not more than 1.3:1 and sincering the thus controlled mix (the expression "mix(dry)" refers to a raw material mix which has been dried so as to be freed of water other than combined water);

(3) a process according to (2) above wherein the sum of the Ca0 and SiO₂ contained in said fine grains is at least 4.0 wt% of said mix;

(4) a process according to (2) or (3) above, wherein the SiO₂ content of said fine grains minus the Al₂O₃ content of said fine grains is within the range of from 1.8 to 2.4 wt%, inclusive, based on the mix, and the weight ratio of Ca0 to (SiO₂-Al₂O₃) contained in said fine grains is within the range of from 0.5:1 to 2.0:1, inclusive;

(5) a process according to (4) above, wherein the weight ratio of Ca0 to (SiO₂-Al₂O₃) contained in said fine grains is within the range of from 1.0:1 to 1. 5: l, inclusive;

(6) a composition consisting of a raw materials mix for producing a self-fluxing, sintered ore, said mix comprising iron ore, limestone, silica and coke, at least 25 wt%of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 5.4 wt% of SiO₂ in terms of "sintered ore product." (said sintered ore product being a product from which coke, combined CO₂ and combined water have been removed by sintering of the mix), said mix being characterised by an SiO₂ content of said fine grains of at least 50%of the total SiO₂ content of said mix;

(7) a composition consisting of a raw materials mix for producing a self-fluxing, sintered ore, said mix comprising iron ore, limestone, silica, coke and return fines, at least 25 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 5.4 wt% of SiO₂ in terms of "sintered ore product" ( as defined in (6) above), said mix being characterized by an SiO₂ content of said fine grains between 2.4 and 3.0 wt% of said mix(dry), and the basicity (CaO/SiO₂ ratio) of said fine grains being not more than 1.3:1; and a self-fluxing sintered iron ore which contains not more than 5.4 weight % of SiO₂ and in which fine grains below 1 mm in size account for at least 23% of the ore, characterized in that said fine grains have a basicity (CaO/SiO₂ ratio) below 1.3:1.

[0009] The invention defined above provides a novel process for producing a sintered ore with a reduced slag content (Ca0 plus SiO₂) without increasing the RDI value of the ore, such sintered ore of low slag content requiring less thermal energy for melting in a blast furnace. A further feature of the invention is that the process of the invention produces a sintered ore the SiO₂ content of which is below 5.4 wt% without adverse effects on the quality and productivity of the ore.

[0010] Reference is now made to the accompanying drawings, in which:

FIG. 1 shows the relationship of the weight percentage of CaO and SiO₂ to the basicity (CaO/SiO₂ by weight).for six ranges of grain size of a raw materials mix used in the conventional sintering process.

FIG. 2 shows the relationship of the same parameters for a raw materials mix used in the process of this invention.

FIG. 3 shows the relationship of the same parameters for another raw materials mix used in the process of this invention.

FIG. 4 shows the relationship of the RDI value and the ratio of the SiO₂ content of fine grains below 1 mm in size to the Si0₂ content of the total mix.

FIGS. 5, 7 and 9 respectively show the productivity (a), the sintering time (b), the sinter tu sinter cake ratio (c), the coke consumption (d), the shatter index (e), and the RDI value (f) obtained in the practice of the process of this invention, and in various comparison examples.

FIG. 6 shows the SiO₂ content and CaO content of fine grains of the mix (of this invention) below 1 mm in size in relation to the RDI value of the resulting sintered ore.

FIG 8 showsthe SiO₂ content, the Al₂O₃ content, and the CaO content of fine grains of the mix (of this invention) below 1 mm in size in relation to the RDI value of the resulting sintered ore.

[0011] Preferred embodiments of the invention are now described in detail.

[0012] In general, particles of self-fluxing sintered ore having a CaO/SiO₂ level in the range of from 1.0:1 to 2.0:1, inclusive, mostly from 1.3:1 to 1.8:1, inclusive, agglomerate together by the action of "slag bond" wherein the grains of iron are bonded by means of a melt. Since the quality and productivity of the sintered ore are determined by the bond, it becomes very important to form a suitable bond. However, with the current sintering process using suction through the sinter bed as is typically performed with a Dwight-Lloyd wintering machine, the sintering reaction at high temperatures finishes so soon. that it is necessary to form the required melt within a short period of time.

[0013] In accordance with this invention, we have found that the quantity and quality of the fine grains of a raw materials mix for sintering below 1 mm in size are important factors, and that the proper control of these two factors accelerates the formation of the above melt, leading to a reduction in the SiO₂ content of the resulting sintered ore.

[0014] In general, a raw materials mix for sintering is about 6% water, which accelerates the formation of pseudo- particles that increase the gas permeability of the sinter bed. Each pseudo-particle comprises a coarse grain core about 1 to 5 mm in size which is surrounded by fine grains below 1 mm in size adhering to the core. Our studies suggest that the formation of the melt in question starts at that portion of the surrounding fine grains which melts at low temperatures. The quantity of initially fused grains gradually increases by melting adjacent coarse grains, but because the sintering materials stays in a high temperature range only for a short period of time, the coarse grains do not fuse completely and the resulting melt is not a uniform mixture of fine and coarse grains but contains a higher proportion of the initially fused fine grains when it has coagulated and formed slag bonds. Therefore, to form a required amount of melt quickly, it is necessary that the fine grains below 1 mm in size surrounding a coarse grain should contain as many starting points as possible which melt at low temperatures. The melt basically consists of SiO₂, CaO and iron oxides, and, since the greater part of the sintering material is made up of iron ore as the source of iron oxides, the fine grains surrounding a coarse grain unavoidably contain a large proportion of iron oxides. Accordingly, the more SiO₂ and CaO sources which are present in the fine grains, the more easily is a melt is formed. This may be achieved by using a raw material which contains a large proportion of SiO₂ and Ca0 in only fine grains. However no such iron ore is found in nature, and, therefore, a practical method is to add fine grains of silica, serpentite, peridotite and Ni-slag as SiO₂ sources and fine grains of limestone as a CaO source. Other effective materials are slag from a blast furnace, slag from a converter, and returned fines which contain large proportions of SiO₂ and CaU. 1. The first embodiment of the process of this invention is a process for producing self-fluxing, sintered ores using a raw materials mix comprising at least 25 wt% of fine grains below 1 mm in size and not more than 5.4 wt% of "SiO₂ in terms of sintered ore product", as defined above, said process being characterized by controlling the SiO₂ content of said fine grains to be at least 50 wt% of the total SiO₂ content of the mix, and sintering the thus controlled mix.

[0015] The first embodiment of the process of this invention is now described in detail with reference to the accompanying drawings and the following tables. Let the mix used in the conventional sintering process (see FIG. 1 and Table 1) be compared with the mix used in the first embodiment of the process of this invention (see FIG. 2, Table 2 and FIG. 5 and Table 5). FIGS. 1, 2 and 3 each show the relationship of the weight of SiO₂ and Ca0 for each grain size (as percentage of the total content of each size) to the basicity (CaO/SiO₂ ratio by weight) for each grain size. The grain size scale of each figure is divided into six ranges, namely: -0.25 mm, 0.25~0.5 mm, 0.5~1 mm, 1~2 mm, 2~5 mm, and 5~10 mm. The premix has an average basicity (CaO/SiO₂) of about 1.33:1. Tables 1, 2 and 3 (below) each show the grain size distribution of the mix and the contents of CaO and SiO₂, respectively, for each grain size as percentages of the CaO and SiO₂ contents of the mix.

[0016] As FIG. 1 shows, tile mix for the conventional process contains more CaO in the range of from 1 to 2 mm and of from 0.25 to 0.5 mm than in the other ranges, but its SiO₂ content is unifromly distributed over the six ranges. In addition, as Table 1 (above) shows, both the Ca0 and SiO₂ contents, as percentages of the Ca0 and SiO₂ contents of the mix, are below 50% in tile case of gram sizes below 1 mm. On the other hand, as FIG 2 shows, the mix for the process of this invention is characterized by a CaO distribution for grain sizes below 1 mm which is similar to that obtained with the mix of the conventional process and yet contains more SiO₂ in grains below 1 mm in size than does the conventional mix. Furthermore, the mix for the process of this invention is controlled so that is contains not more than 5.4 wt% of SiO₂ in terms of the SiO₂ content of the sintered ore product. In consequence, the grains of the mix below 1 mm in size have a low basicity (CaO/SiO₂). As Table 2 shows, the SiO₂ content for grain sizes below 1 mm, as a percentage of the total SiO₂ of the mix, is higher than 50%.

[0017] FIG. 3 illustrates another mix to be used in the process of this invention. The mix is such that the SiO₂ content of fine grains below 1 mm in size is at least 50% of the total SiO₂ content of the mix, and that the SiO₂ content of the mix is 5.2% in terms of the SiO₂ content of the sintered ore product. The mix is characterized in that grains below 1 mm in size have a higher SiO₂ content and a lower basicity (CaO/SiO₂) than grains below 1 mm in size in the conventional mix (see FIG 1). It follows that, as shows in Table 5 (above), the SiO₂ content of grains below 1 mm in size as compared with the total SiO₂ content of the mix is at least 50% as with the mix the characteristics of which are shown in Table 2 (above).

[0018] As described in the foregoing, the SiO₂ content of the mix to be used in the process of this invention is so controlled that it is not more than 5.4% as converted to a value for sintered ore product. Such conversion is necessary for determining accuratly the SiO₂ content of the mix because some portions of the mix are eliminated as gas and dust in the course of sintering. No general conversion formula can be set because the type and amount of the ingredients to be eliminated from the mix vary slightly with the composition of the mix and the sintering conditions, but it can be approximated by the following relationship:

in which: Y is the SiO₂ content (wt%) of the sintered ore product, X is the SiO₂ content (wt%) of the mix, and a and b are constants, generally 1.1 and not more than 0.2, respectively, which canbe determined empiracally on the basis of actual records of sintering operations.

[0019] The mix for use in the process of this invention contains at least 25%, preferably from 25 to 60%, of fine grains below 1 mm in size. If the content of fine grains below 1 mm in size is lower than 25%, not enough slag bonds are formed by sintering to provide a strong sintered ore. If the content of fine grains below 1 mm in size exceeds 60%, the gas permeability of the sinter bed is decreased but this will not sacrifice the productivity of the process of this invention if the agglomeration operation is enhanced by longer agglomeration or if a binder such as quicklime or bentonite is added.

[0020] Sintered ores were produced from themix described above with varying proportions of SiO₂ in grains below 1 mm in size relative to the total SiO₂ of the mix, and the RDI values of the products were plotted in FIG 4, from which one can understand that the RDJ value is greatly and suddenly improved when the SiO₂ content of mix grains below 1 mm in size exceeds 50% of the total SiO₂ content of the mix. This is probably because an increased reactive area of the SiO₂-containing mix causes rapid and uniform formation of SiO₂ slag, the basic component of the slag bond which binds the particles or iron oxide together and governs the strength of the sintered are product. As a result, a low-slag sintered ore containing not more than 5.4% of SiO₂ is produced.

[0021] The following are illustrative methods of controlling the SiO₂ content of the sintered ore product. to be not greater than 5.4% and also of controlling the SiO₂ content of mix grains below 1 mm in size to be at least 50% of the total SiO₂ content of the mix:

(1) first controlling the SiO₂ content of the mix to be not more than 5.4% in terms of the SiO₂ content of the sintered ore product, and at the same time decreasing the CaO content of the mix so that its basicity (CaO/SiO₂) does not vary, and then decreasing the grain size of SiO₂ sources such as silica, dunnite, serptentite, Miferma and other high SiO₂ content ores to below 1 mm;

(2) second, decreasing the size of returned fines containing more SiO₂ than ordinary ores to below 1 mm; and

(3) third, using a large proportion of ores which contain more SiO₂ than ordinary ores, and which comprise many fine grains below 1 mm in size.

[0022] 2. The second embodiment of the process of this invention is a process for producing a low slag-content sintered ore using a raw materials mix for a self-fluxing sintered ore comprising at least 25 weight % of fine grains below 1 mm in size, said process being characterized by lowering the SiO₂ content of the mix to not more than

CaO tu SiO₂ basicity of said fine grains to be below 1.3:1, and sintering the thus controlled mix.

[0023] Therefore, the sintered ore produced by the second embodiment of this invention is a self-fluxing (basic) sintered ore which contains not more than 5.4 wt% of SiO₂, and the fine grains below 1 mm in size which account for at least 25% of the ore are characterized by having a basicity (CaO/SiO₂) below 1.3:1.

[0024] Thus, the next point to be considered is the composition of the fine grains of the mix below 1 mm in size. As we have already revealed, the lower the basicity (CaO/SiO₂ ratio) of fine grains of the mix below 1 mm in size, the higher the reduction strength of the sintered ore, and this effect is particularly conspicuous when the fine grains have a basicity below 1.0:1. Accordingly, two raw materials mixes with the same average composition will provide sintered ores of different reduction strengths if the fine grains below 1 mm in size have different basicities. For example, a mix in which the fine grains below 1 mm in size have a higher basicity provides a sintered ore of higher reduction strength than a mix in which such fine grams have a lower basicity.

[0025] However, our studies have also shown that not only is the basicity of fine grains below 1 mm in size, an important factor, but also the absolute amounts of SiO₂ and Ca0 contained in such fine grains.

[0026] The above discussion shows that the amounts of fine grains of a mix below 1 mm in size and their constituents, especially SiO₂ and CaO, have an important effect on the quality of the sintered ore product. We have also found that most of the SiO₂ and CaO in coarse grains larger than 1 mm in size either remains unreacted or is reacted but mostly confined within the coarse particles, thus failing to perform the function of a "bond". However, CaO which forms slag more easily than SiO₂ will form some slag in the later stage of the sintering reaction. Therefore, by properly controlling the SiO₂ and CaO contents of fine grains below 1 mm in size, sintered ores can be produced without impairing their quality and productivity even if the SiO₂ and Ca0 contents of coarse grains larger than 1 mm in size are decreased. In other words, the necessary and sufficient requirement for high quality and productivity of sintered ore is that the proper conditions of the amount and constituents (SiO₂ and Ca0) of fine grains of a raw materials mix for sintering below 1 mm in size should be satisfied. By reducing the SiO₂ and Ca0 contents of coarse grains more than 1 mm in size which have not been involved in the formation of a "bond", and thereby decreasing the total SiO₂ and Ca0 contents of the raw matcrial, a low-slag sintered ore can be prepared which contains not more than 5.4 wt% of SiO₂ but which has previously been difficult to produce commercially due to low quality and productivity. However, it is substantially impossible in practice selectively to eliminate SiO₂ and CaO from only the coarse grains of the raw material which are more than 1 mm in size. Therefore, as a practically feasible method, the SiO₂ and CaO contents of the coarse grains are reduced not directly but indirectly by decreasing the SiO₂ and Ca0 contents of the total mix, and then compensating for the required amounts of SiO₂ and CaO in the fine grains below 1 mm in size. Therefore, a 40 kg-pot test was conducted to determine the quantitative relationship between the SiO₂ and Ca0 contents of fine grains below 1 mm in size and the quality of the sintered ore product. The results of the test are shown in FIG 6 of the drawings.

[0027] FIG 6 is a graph plotting the RD1 values of sintered ore products produced by varying the SiO₂ and CaO contents of fine grains of a raw materials mix below 1 mm in size. On the x-axis, the weight of SiO₂ contained in fine grains below 1 mm in size is plotted as a percentage of the components of the mix (dry). This factor is defined by the following formula (the factor will hereunder be referred to as [SiO₂]in -1 mm):

wherein A is the percentage by weight of the line grains below 1 mm in size contained in the mix, and B is the percentage by weight of SiO₂ contained in the fine grains below 1 mm in size. On the y-axis, the weight of CaO contained in fine grains below 1 mm in size is plotted as a percentage of the components of the mix(dry). The RD1 values of the resulting sintered ore products are indicated by numerals in the graph. The basicity . (CaO/SiO₂ ratio) of the fine grains below 1 mm in size is shown as a solid line sloping upwards to the right, and the sum of the Ca0 and SiO₂ contained in the fine gmins below 1 mm in size is shown as a dotted line sloping upwards to the left. As is clear from FIG 6, the area of RDI <40 is hatched and it enclosed the region where [SiO₂] in -1 mm is at least 2.4 and the CaO/SiO₂ ratio of fine grains below 1 mm in size is not more than 1.3:1. In particular, the region where [SiO₂] in -1 mm is at least 3.0 and the CaO/SiO₂ ratio of fine grains below 1 mm in size is not more than 1.0:1 is characterized by very desirable RDI values (<30). Therefore, to keep the RD1 value of a sintered ore product within a desired range, it is necessary that the [SiO₂] in -1 mm should be higher than a certain value and that the CaO/SiO₂ ratio of line grains below 1 mm in size should not exceed a given value.

[0028] Since the sintered ore produced by the process of this invention is of low SiO₂ content, the total SiO₂ content of the mix should naturally be smaller than that of the conventional mix, and this requirement unavoidably constitutes a limit to the increase in the level of [SiO₂] in - 1 mm. The region where [SiO₂] in -1 mm exceeds 5.0 in FIG 6 is obtainable only in a laboratory by selecting only iron ores which arc extremely low in SiO₂ content, blending them with fine powders of SiO₂ sources and sintering the resulting mix. In commercial operations where selection of such ores is difficult, there is little possibility of obtaining the stated range.

[0029] Furthermore, the strength at ordinary temperatures (shatter index) of a sintered ore is directly correlated with the sum of the CaO and SiO₂ contained in fine grains below 1 mm in size, and, therefore, the value of the sum cannot be made excessively low. With the level of SiO₂ in -1 mm of the mix being at least 2.4, if the sum of CaO and SiO₂ contained in fine grains below 1 mm in size is smaller than 4.0, the sintered ore product has a tendency to exhibit low strength at ordinary temperatures (shatter index), making it necessary to implement separate provisions for increasing the shatter index by, for instance, incorporating more coke in the mix.

[0030] The following conclusion can be drawn from the above discussion: the technique of this invention can be easily implemented within the hatched area of FIG 6 where the [SiO₂] in -1 mm value is at leat 2.4 and the CaO/SiO₂ ratio of fine grains below 1 mm in size is not more than 1.3:1, especially in the dotted area where the [SiO₂] in -1 mm value is between 2.4 and 3.0, the CaO/SiO₂ ratio of fine grains below 1 mm in size is not greater than 1.3:1 and the sum of CaO and SiO., contained in the fine grains below 1. mm in size is at least 4.0. It is to be noted again that the mix for use in the process of this invention generally contains from 23 to 60 wt% of fine grains below 1 mm in size.

[0031] It is well known that the reduction strength of a intered ore will usually decrease with an increase in the Al₂O₃ content of the ore. In contrast with this statistical fact, some cases are observed in which an increase in the Al₂O₃ content docs not necessarily result in a low reduction strength and, therefore, the relationship between the Al₂O₃ content and the reduction strength has not been altogether clear. On the basis of our understanding that a slag bond is formed of relatively fine grains and that most of the coarse grains remain in the unmelted ore, we have carried out experiments varying the Al₂O₃ content of fine grains only rather than the AL₂O₃ content averaged by both fine and coarse grains, and we have found that there is an inverse relationship between the Al₂O₃ content of fine grains below 1 mm in size and the reduction strength of the sintered ore product. We have also found that the inconsistency which has been observed in some cases between the average Al₂0₃ content of the sintered ore and its reduction strength can be explained by the Al₂O₃ content of fine grains below 1 mm in size. Accordingly, it is necessary to control not only the average Al₂O₃ content of a sintered ore but also the Al₂O₃ content of fine grains below 1 mm in size. What is more, a raw material for sintering should not be composed of crushed iron ores having a high Al₂O₃content, and this is important for-ensuring all acceptable reduction strength in the sintered ore.

[0032] The above discussion shows that the proportions of fine grains of a raw material for sintering which arc below 1 mm in size and of their constituents, especially Si0₂, CaO and Al₂O, have an important effect on the quality of the sintered ore product. We have also found that most of the SiO₂ , CaU and Al₂0₃ in the coarse grains larger than 1 nun in size either remains unreacted or is reacted but mostly confined within the coarse grains, thus failing to perform the function of a"bond". However, CaO which more easily forms slag than SiO₂ and Al₂O₃ will form some slag in the later stage of the sintering reaction. Therefore, by properly controlling the SiO₂ Ca0 and Al₂O₃ contents of fine grains below 1 mm in size, sintered ores with quality and productivity unimpaired can be produced even if the SiO₂ and CaO contents of coarse grains larger than 1 mm in size are decreased, or even if the Al₂O₃ content of said coarse grains is increased. In other words, as stated above, the necessary and sufficient requirement for high quality and productivity of the sintered ore is that the proper conditions with respect to the amount and constituents (SiO₂, CaO and Al₂O₃) of fine grains of a raw materials mix for sintering below 1 mm in size should be satisfied. By reducing the SiO₂ and CaO contents of coarse grains larger than 1 mm in size which have not been involved in the formation of a "bond", and therby decreasing the total SiO₂ and CaO contents of the raw material, a low-slag sintered ore can be prepared which contains not more than 5.4 wt% of SiO₂ but which has previously been difficult to produce commercially due to low quality and producivity. However, it is substantially impossible in practice selectively to eliminate SiO₂ and Ca0 from only the coarse grains of the raw material below 1 mm in size. Therefore, as a practically feasible method, tne SiO₂ and CaO contents of the coarse grains are reduced not directly but indirectly by decreasing the SiO₂ and CaO contents of the total raw material, and then compensating for the required amounts of SiO₂ and CaO in the fine grains below 1 mm in size. Accordingly, a 40 kg-pot test was conducted to determine the quantitative relationship between the SiO₂, Ca0 and Al₂O₃ contents of fine grains below 1 mm in size and the quality of the sintered ore product. The results of the test are shown in FIG 8 of the accompanying drawings.

[0033] FIG 8 is a graph plotting the RDI values of sintered ore products produced by varying the Si0₂, Ca0 and Al₂0₃ contents of fine grains of a raw materials mix which are below 1 mm in size. We have plotted on the x-axis the weight of the SiO₂ contained in fine grains below 1 mm in size minus the weight of Al₂0₃ contained in the fine grains as a percentage of the components of the mix(dry). This factor is defined by the following formula (the factor will hereunder be referred to as [SiO₂ - Al₂0₃ in -1 mm):

wherein A is the precentage by weight of the fine grains below 1 mm in size contained in the mix, B is the percentage by weight of Si0₂ contained in the fine grains below 1 mm in size, and C is the percentage by weight of Al₂O₃ contained in the fine grains below.1 mm in size.

[0034] On the y-axis, we have plotted the weight of the Ca0 content of the fine grains below 1 mm in size as a percentage of the components of the mix (dry). The RDI values of the resulting sintered ore products are indicated by numerals in the graph of Fig 8. The CaO/(SiO₂ - Al₂O₃) values for fine grains below 1 mm in size are shown as dotted lines. As is clear from FIG 8, the area of RDI 40 is hatched and it encloses the region where [SiO₂ - Al₂O₃] in -1 mm is at least 1 .8 and the CaO/ (SiO₂ - Al₂O₃) ratio-of fine grains below 1 mm in size is not greater than 2.0: 1. ]n particular the region where the SiO₂ -Al₂O₃] in -1mm value is at least 2.4 and the CaO/(SiO₂ - Al₂0₃) ratio or fine grains below 1 mm in size is not greater than 1.8:1 is characterized by very desirable RDI values (<30). The refore,to keep the RDI value of a sintered ore product within a desired range, it is necessary that the [SiO₂ - Al₂O₃] in ―1 mm value should be higher than a certain level and that the CaO/(SiO₂ - Al₂O₃) ratio of fine grains below 1 mm in size should not exceed a given value.

[0035] Now, the change in the Al₂0₃ content of a raw material for sintering is generally smaller than that in the SiO₂ content, and furthermore, in practice iron ores having an extremely low content of Al₂0₃ are not generally available in large quantities, Therefore, it is unavoidable that the level of SiO₂ - Al₂0₃ in -1 mm must be increased by increasing the SiO₂ content of fine grains below 1 mm in size. However, since the sintered ore produced by the process of this invention is of low SiO₂ content, the total SiO₂ content of the raw materials mix should naturally be smaller than that of the conventional mix, and this requirement unavoidably constitutes a limit to the available increase in the level of SiO₂ - Al₂O₃] in -1 mm. The region where [SiO₂ - Al₂O₃) ] i n -1 mm exceeds 2.4 in FIG 8 is obtainable only in a laboratory by selecting only iron ores which are extremely low in SiO₂ and Al₂O₃ contents, blending them with fine powders of SiO₂ sources and sintering the resulting mix. In commercial operations where selection of such iron ores is difficult, there is little possiblity of obtaining the stated range.

[0036] Furthermore, the strength at ordinary temperatures (shatter index) of the sintered ore is directly correlated with the sum of CaO and SiO₂ contained in fine grains below 1 mm in size, and, therefore, the value of the sum cannot be made excessivley low. With the level of SiO₂ - Al₂O₃] in -1 mm of a raw materials mix being at least 1.8, if the value of the CaO/(Sio₂ - Al₂O₃) ratio of the fine grains below 1 mm in size is smaller than 1.0:1, the sum of the Ca0 and SiO₂ contained in said fine grains decreases, and the sintered ore product has a tendency to exhibit a low strength at ordinary temperatures (shatter index), making it necessary to implement separate provisions for increasing the shatter index by, for instance, incorporating more coke in the mix. It is to be mentioned here that the level of the CaO/(SiO₂ - Al₂0₃) ratio of fine grains below 1 mm in size should not be below O.5:1 because, otherwise, sintered ore which is very low in strength at ordinary temperatues is produced.

[0037] The following conclusion can be drawn from the above discussion: the technique of this invention can be easily implemented within the hatched area of FIG 8 Where the [SiO₂ - Al₂0_ ] in -1 mm values are at least 1.S and the Cao/ Sio₂ - Al₂0₃) ratio of fine grains below 1 mm in sizeare not greater than 2.0:1 especially in the dotted area Where the [ SiO₂ -Al₂O₃]in -1 mm values are between 1.Sand 2.4and the CaO / (Sio₂ -Al₂ O₃) rates between 0.5:1 preferably 1.0:1 and 2.0:1.

[0038] Preferred embodiments of the process of this invention are described hereunder in greater detail with reference to the following examples.

[0039] Three different raw materials mixes each comprising iron ores, limestone, silica, coke and return fines were agglomerated in the presence of water , and the resulting agglomerates were charged into a 40 kg test pot at. a negative pressure of 1700 mm II₂0 to produce three different sintered ores. The description of the ingredients of each mix and the grain size distribution of each ingredientare shown in Table 4 (for Comparative Example 1), Table 5 (for Example 1) and Table6

Example 2). The average basicity (CaO/SiO₂) of each raw materials mix was about 1.35:1. The raw materials mix used in Example 1 according to one embodiment of the process of this invention incorporated fine grains below 1 mm in size) of silica containing at least 90% of Si0₂so that the Si0₂ content of the mix was not more than 5.4%in terms of sintered ore product.

[0040] The raw materials mix used in Example 2 according to another embodiment of this invention likewise incorporated fine grains (below 1 mm in size) of silica containing at least 90%of Si0₂,but it contained a smaller amount of silica and limestone so that the Si0₂ content of the mix was not more than 5.2% in terms of sintered ore product. Tables 4,5 and 6 arc keyed to Table1, 2 and 3, respectively. The compositions of the principal ingredients of each mix are identified in Table 8 below.

[0041] The test. results are shown in Table 7 (below) and

of the drawings. The process of this invention (Examples 1 and 2) was slightly more productive than the conventional process(Comparative Example 1) due_' to a shortersintering period and a higher ratio of sinter to sinter cake. The process of thisinvention consumed slightlyless coke than the conventional process. This means that, although the process of this invention achieved almost the same results of sintering as in the conventional technique with respect to moisture content, coke content, productivity, sinterins time, sinter to sinter cake ratio, coke consumption and shatter index, it greatly reduced the RDI vaiue of the sintered ore product (as in Example 1), or reduced the SI0₂content of the sintered ore product to less than 3.2% without greatly increasing its RDI value (as in Example 2) .

[0042] As described in the foregoing, the process of this invention is comparable with, or even superior to, the conventional sintering process with respect to productivity, coke consumption, shatter index and other factors while it can greatly reduce the RDI value of the sintered product, or reduce the Si0₂content of the.sintered product to below 5.4% without greatly increasing its RDI value.

[0043] Comparative Examples 2 and 3 and Examples 3to 7 of this invention are described hereunder. Sevendifferent raw materials mixes each comprising iron ores, limestone, silica, coke and return fines were aggolomerated in the presence of water, and the resulting agglomerates were charged into a 40 kg test pot at a negative pressure of 1700 mmll₂0 to produce seven different sintered ores. The description of the ingredients of each mix and the composition of each ingredient are shown in Tables 9 to 12 (Comparison Example 2 and Examples 3 to 5) and Tables 19 to 22 (Comparison example 3 and Examples 6 and 7). The proportions of the ingredients of the mixes are indicated in Table 13 (Comparison Example 2 and Examples 5 to 5) and in Table 23(Comparison Example 3 and Examples 6 and 7). Tables s 14 to 17 arc keyed to the data on the proportions of ingredients set forth in Table 13 for Comparative Example 2 and Examples 3 to 5, respectively, and each table shows the SiO₂ and Ca0 contents of the raw materials mix and the sintered ore product. In each of Tables 14 to 17, the data on the Si0₂ content, the CaO content and the CaO/SiO₂ratio of the mix are classified under coarse grains largerthan 1 mm in size and fine grains below 1 mm in size.

[0044] Tables 24 to 26are keyed to the data on the proportions of ingredients set forth in Table 23 for Comparative Example 3 and Examples 6 and 7, respectively, and each table shows the SiO₂, Ca0 and Al₂0₃contents and-the Ca0/Si0₂ratios of the raw materials mix and sintered ore product, and the (Si0₂ -Al₂0₃) contents and Ca0/(Si0₂ - Al₂0₃)ratios of fine grains (below 1 mm in size) of the mix. In each of Tables 24 to 26, the data on the Si0₂ content, Ca0 content and Al₂0₃ content of the mix are classified under'coarse grains" (above l mm in size) and "fine grains" (below 1 mm in size), respectively.

[0045] As mentioned above in connection with the raw material mix to be used in the process of this invention, the relationship between Y which is the Si0₂ content (wt%) of the sintered ore produce and X which is the Si0₂ content (wt%) of the raw materials mix can be approximated by the following formula:

wherein aand bare constants, generally 1.1 and not more than 0.2, respectively, which can be determined empirically on the basis of actual records of sintering operations.

[0046] As Tables 13 and 14 show, the raw materials mix prepared in Comparative Example 2 is such that the amount of silica containing at least 90%of Si0₂is simply decreased to lower the Si0₂content in terms of sintered ore product from the ordinary range of 5.6 t.o 6.0 wt% down to 5.4 wt%. As a result, the level of [Si0₂ in -1 mm of the mix is 2.25 and the Ca0/Si0₂ratio of fine grains below 1 mm in size is 1.20:1. Therefore, a simple reduction of the Si0₂ content gives a level of [Si0₂]in -1 mm which is below 2.4.

[0047] As shown in Tables 13 and 15, the raw materials mix prepared in Example 3 is such that not only is the amount of silica which is added lowered but the grain size is also decreased to below 1 mm so as thereby to decrease the Si0₂content in terms of sintered ore product down to 5.4 wt%. As a result, the [Si0₂]in - 1 mm value is increased to 2.77 and the Ca0/Si0₂ratio of fine grains below 1 mm in size is decreased to 0.95:1.

[0048] In Example 4 (shown in Tables s 13 and 16), the raw materials mix contains both return fines a part of which is crushed to a size below 1 mm and relatively course grains of other iron ores, so that the Si0₂content in terms of sintered ore product is decreased to 5.4 wt%. The mixis characterized by an [Si0₂] in -1 mm value which is as high as 2.97 and a Ca0/Si0₂ratio of fine grains below 1 mm in size as high as 1.25:1.

[0049] In Example 5 (illustrated in Tables 13 and 17), the raw materials mix is such that not only is the amount of silica added decreased to 0.8 wt% but also the grain size is decreased to below 1 mm and also it incorporates partially crushed iron ores containing a higher proportion of Si0₂than for the mix as a whole, so that the Si0₂ content in terms of sintered ore is decreased to 5.0 wt%. Inconsequence, the [Si0₂]in - 1 mm of the mix is 2.55 and the Ca0/Si0₂ ratio of fine grain below 1 mm in size is 1.12:1. The results obtained by sintering the raw material mixes prepared in Comparative Example 2 and Examples 3 to 5, respectively, are shown in Table 18 and illustrated in the graph of FIG. 7.

[0050] As shown in Table 18 and FIG 7, the raw material mix of Comparative Example 2, which was prepared by simply reducing the SiO₂ content, provided a sintered ore having an excessively high RDI value which could only be produced after an extended sintering period and in a low sinter to sinter cake rato. The mix consumed a large amount of coke as it was sintered. In contrast, the raw materials mix of Example 3, which was prepared by not only decreasing the amount of silica added but also by reducing its grain size to below 1 mm, provided a sintered ore having a desired RDI value (below 40), which could be produced with high productivity and in a short sintering period, consuming less coke, although the sintered ore product contained 5.4 wt% of SiO₂ which was less than the ordinary values between 5.6 and 6.0 wt%.

[0051] The raw materials mix of Example 4, which contained return fines with somewhat more SiO₂ than the intended sintered one product and part of which was crushed to a size below 1 mm, provided a sintered ore having a high shatter index and an RDI value below 40. Such ore could be produced in a high sinter to sinter cake ratio, consuming a small amount of coke.

[0052] The raw materials mix of Example 5, prepared by not only decreasing the amount of silica added to 0.8 wt% but also by reducing its grain size to below I mm and by incorporating partially crushed iron ores which were relatively high in SiO₂ content, provided a sintered ore having a desired value of RDI below 40 without sacrificing the productivity and sinter to sinter cake ratio and without increasing the coke consumption, although the resulting sintered ore had its SiO₂ content decreased to 5.0 wt%.

[0053] As Tables 23 and 24 show, the raw materials mix prepared in Comparative Example 3 is such that the amount of silica containing at least 90% of SiO₂ is simply decreased to lower the SiO₂ content in terms of sintered ore product from the ordinary range of 5.6 to 6.0 wt% down to 5. 4 wt%. As a result the level of [SiO₂- Al₂O₃] in -1 mm of the mix is 1.64 and the weight ratio of CaO/(SiO₂ - Al₂O₃) of fine grains below 1 mm in size is 1.34:1. Therefore, a simple decrease in the SiO₂ content gives a level of [SiO₂ - Al₂O₃] in -1 mm which is below 1.8.

[0054] As shown in Tables 23 and 25, the raw materials mix prepared in Example 6 contains both return fines, part of which is crushed to a size below 1 mm, and relatively coarse grains of other ores so that the SiO₂ content in terms of sintered ore product is decreased to 5.4 wt%. The mix is characterized by an [SiO₂ - Al₂O₃] in -1 mm value which is as high as 2.03 and a weight ratio of CaO/(SiO₂ - Al₂O₃) of fine grains below 1 mm in size which is as high as 1.83:1.

[0055] In Example 7 illustrated in Tables 23 and 26, the raw materials mix is such that not only is the amount of silica added decreased to 0.7 wt% but also its grain size is lowered to below 1 mm and further it incorporates partially crushed iron ores containing a higher proportion of SiO₂ than for the mix as a whole, so that the SiO₂ content in terms of sintered ore is decreased to 5.0 wt%. In consequence, the [SiO₂ - Al₂O₃] in -1 mm of the mix is 1.83 and the weight ratio of CaO/(SiO₂ - Al₂O₃) of fine grains below 1 mm in size is 1.29:1.

[0056] The results obtained by sintering the raw materials mix prepared in Comparative Example 3 and Examples 6 and 7 are shown in Table 27 and illustrated in the graph of FIG 9. As is clear from Table 27 and FIG 9, the raw materials mix of Comparative Example 3, which was prepared by simply lowering the SiO₂ content, provided a sintered ore having a relatively high RDI value which could only be produced after extended sintering and in a low sinter to sinter cake ratio. The mix consumed a large amount of cake as It was sintered.

[0057] 

[0058] As described in the foregoing, the process of this invention is comparable with, or even superior to, the conventional sintering technique with respect to productivity, coke consumption, shatter index and other factors, while it can lower the SiO₂ content of the sintered ore to below 5.4 wt% and decrease the slag content (SiO₂ plus CaO) of the ore without greatly increasing the level of RDI. Accordingly, the process can greatly curtail the amount of slag charged into a blast furnace, yielding an appreciable decrease in the blast furnace fuel consumption.

Claims

1. A process for producing a self-fluxing sintered ore using a raw materials mix comprising iron, ore, limestone, silica and coke, at least 25 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 5.4 wt% of SiO₂ in terms of "sintered ore product" (said sintered ore product being a product from which coke, combined CO₂ and combined water have been removed by sintering of the mix), said process being characterized by controlling the SiO₂ content of said fine grains to be at least 50 wt% of the total SiO₂ content of the raw materials mix, and sintering the thus controlled mix.

2. A process for producing a self-fluxing sintered ore using a raw materials mix comprising iron ore, silica, limestone, coke and return fines, at least 25 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 3.4 wt% of SiO₂ in terms of "sintered ore product" (as defined in Claim 1 above), said process being characterized by controlling the SiO₂ content of said fine grains to be between 2.4 and 3.0 wt% of the raw materials mix (dry) and the basicity (CaO/SiO₂ ratio) of said fine grains to be not more than 1.3:1, and sintering the thus controlled mix.

3. A process according to Claim 2, wherein the sum of the Ca0 and SiO₂ contained in said fine grains is at least 4.0 wt% of said mix.

4. A process according to Claim 2 or Claim 3, wherein the SiO₂ content of said fine grains minus the Al₂O₃ content of said fine grains is between 1.8 and 2.4 wt% of the raw materials mix, and the weight ratio of CaO to (SiO₂ - Al₂O₃) contained in said fine grains is between 0.5:1 and 2.0:1.

5. A process according to Claim 4, wherein the weight ratio of Ca0 to (SiO₂- Al₂O₃) contained in said fine grains is between 1.0:1 and 1.8:1.

6. A composition consisting of a raw materials mix for producing a self-fluxing, sintered ore said mix comprising iron ore, limestone, silica and coke, at least 23 wt% of said mix consisting of fine grains below 1 mm in size and said mix containing not more than 3.4 wt% of SiO₂ in terms of "sintered ore product" (said sintered ore product being a product from which coke, combined CO₂ and combined water have been removed by sintering of the nix), said mix being characterized by an SiO₂ content of said fine grains of at least 50% of the total SiO₂ content of said mix.

7. A composition consisting of a raw materials mix for producing a self-fluxing, sintered ore, said mix comprising iron ore, limestone, silica, coke and return fines, at least 25 wt of said mix consisting of fine grains bel ow 1 mm in size, and not more than 5. wt% of SiO₂, in terms of "sintered ore product" (as defined in Claim 6 above), said mix being characterized by an SiO₂ content of said fine grains between 2.4 and 3.0 wt%, of said mix (dry), and the basicity (Ca0/SiO₂ ratio) of said fine grains being not more than 1.3:1.

8. A self-fluxing sintered iron ore which corr- tains not more than 5.4 weight % of SiO₂ and in which fine grains below 1 mm in size account for at least 25% of the ore, characterized in that said fine grains have a basicity (Ca0/SiO₂, ratio) below 1.3:1.

Drawing