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 SiO₂ content of said fine grains to be at least 50 wt% of the total SiO₂ content of the mix, and sintering of the thus controlled mix.

Description: Title An Improved Raw Materials Mix And Placess For Producing Self-Fluxing, Sintered Ores, And Improved Self-Fluxing, Sintered Ores This inveiltion relates to an improvecl raw materials mix for producing self-fluxing, sintered ores, and moic particluarly, to an improved process for producing lowslag, silltered ores which are resistant to disintegration, and the improved self-fluxing sintered ores resultinng from the process. The process is based on the finding that the SiO2 content of the fine grains of the new materials mix is important to the production of improved self-fluxing, sintered ores. Sintereing bv a common Dwight-Lloyd sinter machille generally comprises preparing a raw materilas mix of iran ore, limestone, silica, miscellaneous other materials and coke, agglomerating the mix in the presence of and charging the resulting agglomerate into the sinter machine. In that machine, the surface layer of tic 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 tic surface layer and ending with the bottom layer. In the production of sintered ores which flux themselves and have a CaO/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 an improved quality in terms of shatter index and degradation index after reduction at 550 C (RDI) are factors always to be kept in mind. In order to improve the RDI value which expresses, as a weight percentage of graiiis below 5 mm in side, the ease with which the sintered ore disintegrates 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 tbe coke content of the mix or its slag content (expressed as the sum of CaO andSiO2) 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 tiie 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 mote slag to be charged into the blast furnace, thus inecreasing 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 aboe, 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 S102 content of the sintered ore in the range of from about 5.6 to about 6.o wt%, inclusive. Certain proposals have been made for decreasing the SiO2 content of sintered ore without increasing the coke content. 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). Hoever, the technique proposed uses essentially low firing temperatures and requires the higher range of firing temperature to be retained for a lengttiy period of time; this inevitably lowers the product 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.G wt%, and, therefore, the technique is not an effective method of lowcrinrr the SiO2 content of the sintcred ore. The high blast furnace slag volume also denotes an increased furnace fuel consumption, and, therefore, with the curreiit concern over energy saving, ;l reduction ill tlie blast furnace slag volume is an important task for the industry. 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. More specifically, the process of this invention for producing a self-fluxing sintered ore is based on the finding that SiOo 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 sucli SiO, can bind and agglomerate the coarse grains of tile iron ore to give a strength which is such tliat the resulting sintered ore will not disintegrate under the load (the weight of furnace charge) when it is charged into a blast furnace. 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 aiid said mix containing not more than 5,.4 et% of SiO2 in terms of sintered ore product, i.e. sintered ore product from which coke, combined COO and combined water have been removed by sintering of tiie mix, said process being characterized by controlliiig the SiO, content of said fine grains to be at least 50 wt% of tie total Sit, 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, silie and return fines, at least 25 wtr,c 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 CO2 and combined water have been removed by sintering of the mix, said process being characte ised b controlling the SiO2 content of said fine grains to a value in the range of from 2.4 to 3.0 wt%, inclusive, o the 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 (the exprcssioii "mix(dry)" refers to a raw material mix which has been dried so as to be freed of water other than combined water); ( a process according to (2) above wherein the sum of tie CaO and SiO" contained in said fine grains is at least 4.0 wtc,c, of said mix; (4) a process according to (2) or (3) above, wherein tile SiO2 c 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 CaO to (SiO2-Al2O3) contained in said fine grains is whithin the range of form 0.5:1 to 2.0: :1, inclusive; (5) a process according to (4) above, wherein the weight ratio of CaO to (SiO2-Al2O3) contained in said fine grains is within the range of from 1.0:1 to l.b: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 trains below 1 mm in size and said mix contaiiiing not more than 5.4 wt% of SiO2 in terms of "sintered ore product" (said sintered ore product being a product from wich 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 SiO, 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 siye 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 asicity (CaO/SiO2) ratio) of said fine grains being not more than 1.3: 1; and a self-fluxing sintered iron ole which contains 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 (CaO/SIO2 ratio) below 1.3:1. The invention defined above provides a novel process for producing a sintered ore with a reduced slag content (CaO 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 invenion is tliat the process of the invention produces a sintered ore the SiO" content of which is below 5.4 wt% without adverse effects on the quality and productivity of the ore. 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 siye 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 tlie process of this invention. FIG. 3 shows the relationship of the same parameters for another raw material mix used in the process of this invent ion FIG. 4 shows the relationship of the RDI value aiid the ratio of the Sit, content of fine grains below 1 mm in size to the SiO2 content of the total mix. FIGS. 5, 7 and 9 respectively show the productivity (a), Liie sintering time (b), the sinter tu sintcr cake ratio (c), the coke consumption (d), the shatter iiidex (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 tiie resulting sintered ore. Preferred embodiments of the invention are now described in detail. 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 tlie silltered ore are determined by the bond, it becomes very important to form a suitable bond. however, witli the current sintering process using suction through the sinter bed as is typicall performed with a Dwight-Lloyd sintering 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. 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. In general, a raw materials mix for sintering is about 6% water, which accelerates the formation of pseudoparticles that increase the gas permeability of the sinter bed. Each pseudo-particle comprises a coarse grain core about 1 to 5 mm in size wich is surlounded ly fine grains below 1 mm in siye adhering to the core. Our studies suggest that the formation of the melt in question starts at that protion 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, tlie coarse grains do not fuse completely and the resulting melt is not a uniform mixture of fine and coa@se grains but contains a higher proportion of the initially fused fine grains en it has coagulated and formed slag bonds. Therefore, to form a required amount of melt quickly, it is necessary that the dine grains below 1 mm in size surrounding a coarse grain should contain as many starting poilits as possible which melt at low temperatures. The relt 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 usiilg a raw material which contains a large proportion of SiO" and CaO in only fine gfllins. However no such iron ore is found in nature, and, therefore, a practical method is to add fine grains of silica, serpeiitite, peridotite and Ni-slag as SiO C? sources and fine grains of limestone as a Ca() source. Other effective materials are slag from a blast furnace, slag from a converter, and returned fines which contain large proportions of SiO2 und CaO2. 1. The first embodiment of the process of this i1vention is a process for producing self-fluxing, sintered ores using a raw materials mix comprising at least 25 wt% of finc 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 tlle SiO2 content of said fine grains to be at least 50 wt% of the total SiO content of the mis, and wintering the thus controlled mix. 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 siiitering process (see FIG. 1 and Table 1) be compared with the mix used in the first embodiment of the process of this iiivcntioii (see FIG. 2, Table 2 and FIG. 3 and Table -3). FIGS. 1, 2 and 3 each show the relationship of the weight of SiO2 C? and CaO for each grain size (as percentage of tie total content of each size) to the basicity (CaO/SiO2 ratio by weight) for eacli 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 1.55:1. 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. Table 1 - Comparative Example 1 EMI9.1 <tb> <SEP> 1 <tb> Srl <SEP> s,i;.c; <SEP> < o <SEP> )¯ <SEP> 0.5 <SEP> 1.0 <SEP> ¯ <SEP> 2.0 <tb> <SEP> paimctc1 <SEP> ?1 <SEP> O.''5 <SEP> 1.0 <SEP> 1.0 <tb> <SEP> (I) <SEP> a <SEP> t <SEP> ( <SEP> 4 <SEP> ) <SEP> l <SEP> l <SEP> 10 <SEP> . <SEP> 515.9 <SEP> 10.5 <SEP> I. <tb> <SEP> I <tb> - <SEP> CaO <SEP> coiitent <SEP> 5. <SEP> s, <SEP> 7. <SEP> '1 <SEP> 70.7 <tb> ss <SEP> l'or <SEP> ewlell <SEP> (c) <tb> <SEP> c: <SEP> 0.8'15 <SEP> 1.1100 <tb> <SEP> Id <SEP> G,o <SEP> G.o <tb> <SEP> I <SEP> or¯eacll <SEP> ( <SEP> 'c <SEP> ) <SEP> ¯ <SEP> . <SEP> ¯ <tb> <SEP> fraill <SEP> size <SEP> c <SEP> 0.7c, <SEP> 5 <SEP> O.GSOO <SEP> 0.9000 <SEP> 1.0374 <tb> <SEP> basiit. <tb> <SEP> (crt/sib <tb> <SEP> , > <SEP> 1- <SEP> 1 <SEP> 1.42 <SEP> 1.2 <SEP> 1.61 <tb> <SEP> foi- <SEP> cacri' <SEP> 1' <SEP> " <tb> <SEP> CtiO <SEP> content <SEP> lS <SEP> as <SEP> l <tb> I <SEP> jcceiitae <SEP> of <SEP> r <SEP> 12.7 <SEP> 13.0 <SEP> 1; > , <SEP> 271 <tb> Cau <SEP> nf <SEP> mix <SEP> t <SEP> ('r) <tb> I <SEP> SifJ, <SEP> content <SEP> as <SEP> a <tb> <SEP> pei'entage <SEP> I)e-cníaXre <SEP> of <SEP> Ii <SEP> 1 .0 <SEP> 12.3 <SEP> 17.G <SEP> 20.3 <tb> <SEP> SilO" <SEP> of <SEP> mix <SEP> ('e) <tb> Table 1 - Comparative Example 1 (Continued) EMI10.1 <tb> X <SEP> - <SEP> ).0 <SEP> l0.0 <tb> <SEP> rrain <SEP> to <SEP> total <SEP> - <SEP> 1cs <tb> <SEP> mctcru"fl) <SEP> tC <SEP> I- <SEP> average <SEP> imoi <SEP> I <tb> <SEP> )a <SEP> w <SEP> te <SEP> -. <tb> <SEP> Ivel <SEP> ame <tb> <SEP> weight <SEP> ( <SEP> eG <SEP> ) <SEP> a <SEP> 35.0 <SEP> 5.4 <SEP> grain <tb> <SEP> size <tb> <SEP> 2.07'iiiun <tb> <SEP> li <tb> <SEP> C, <SEP> cotent <SEP> b <SEP> 6.0 <SEP> 0 <SEP> o.s <SEP> 6,9 <tb> <SEP> for <SEP> cach <SEP> ( > <SEP> --e <SEP> S--L <tb> <SEP> raiii <SEP> sic <SEP> c <SEP> 2,10UO <SEP> O <SEP> G.1 <SEP> s,o <tb> <SEP> ¯ <SEP> . <SEP> . <SEP> . <SEP> <tb> <SEP> Si(J,, <SEP> content <SEP> d <SEP> 4. <SEP> 5.11 <SEP> 33? <tb> <SEP> fo]¯¯eictl <SEP> (t'b) <SEP> ¯ <SEP> .¯ <SEP> i <tb> <SEP> I-ain <SEP> sire <SEP> e <SEP> I) <tb> <SEP> ¯ <SEP> . <SEP> . <SEP> . <SEP> ¯ <SEP> ¯ <SEP> ¯ <SEP> ¯ <tb> 1 <SEP> busic: <SEP> ity <tb> <SEP> (CaO/SiO,) <SEP> f <SEP> O <tb> <SEP> fo <SEP> cacli- <SEP> 1.4 <SEP> 1. <tb> <SEP> graii <SEP> size <tb> <SEP> C <SEP> t <SEP> o <SEP> ll <SEP> t <SEP> e <SEP> ll <SEP> t <SEP> t <SEP> S <SEP> t <tb> <SEP> CaO <SEP> content <SEP> ns <SEP> u <tb> <SEP> Ca(J <SEP> of <SEP> mix <SEP> 111i: <tb> <SEP> Si(,,, <SEP> content <SEP> as <SEP> a <SEP> I <tb> <SEP> per-entage <SEP> of <SEP> h <SEP> j2.5 <SEP> 2.G <SEP> 100 <SEP> 44,c, <tb> <SEP> pel <SEP> centaOc <SEP> of <SEP> h <SEP> 32. <SEP> <SEP> <SEP> . <SEP> () <SEP> lûû <SEP> 4h, <SEP> 9 <tb> <SEP> SiO <SEP> of <SEP> mix <SEP> () <tb> Notes: c = a x b/100 e = a x d/100 f = bid = c/c g = c/6.8516 x 100 11 = e/5.1106 x i()O Tbale 2 - Example 1 EMI11.1 <tb> <SEP> grabi <SEP> S\iiizC) <SEP> size <SEP> l <SEP> O <SEP> - <SEP> ¯ <SEP> 2.0 <SEP> ¯ <tb> <SEP> size <tb> <SEP> (rmu > <tb> <SEP> for <SEP> ({C)) <SEP> a <SEP> 1. <SEP> o1;25 <SEP> 117.9 <tb> <SEP> o <SEP> ¯. <SEP> , <tb> <SEP> ic <SEP> igllt <SEP> a <SEP> 1 <tb> <SEP> 10.7 <SEP> 15.0 <SEP> 117.9 <tb> <SEP> CaO <SEP> content <SEP> for <SEP> 1) <SEP> 5.19 <SEP> 6.05 <SEP> /.l5 <SEP> 10.10 <tb> <SEP> each <SEP> etCll <SEP> 8.0 <SEP> c'J(Lin <tb> <SEP> I <SEP> 0.s <SEP> ().()1)I <SEP> 1.O, <SEP> 1 <tb> <SEP> d <SEP> 5.6 <SEP> .. <SEP> 07 <tb> <SEP> ii) <SEP> colltcnt <SEP> fol' <tb> <SEP> each <SEP> 1 <SEP> ill <SEP> size('c) <SEP> ---------------- <tb> <SEP> e <SEP> g <tb> <SEP> basicity <SEP> (CaO/SiO,) <tb> <SEP> size <SEP> each <SEP> grain <SEP> f <SEP> O <SEP> 0.91 <SEP> 1.1)1 <SEP> 1. <SEP> i <tb> <SEP> size <SEP> I <tb> <SEP> ---¯-- <SEP> 9 <tb> I <SEP> Cl() <SEP> c <SEP> oiiteiit <SEP> as <SEP> a <tb> <SEP> percentage <SEP> )erCclitlbe <SEP> of <SEP> CaO <SEP> Lr <SEP> 112.7 <SEP> 13.0 <SEP> 1 <SEP> ') <SEP> j <SEP> 10.2 <tb> of <SEP> mix <SEP> ( & ) <SEP> t" <SEP> ) <SEP> I <tb> <SEP> I <SEP> I <tb> <SEP> 510 <SEP> content <SEP> IS <SEP> a <SEP> ci <tb> <SEP> percentage <SEP> of <SEP> 5i02 <SEP> 1 <SEP> i1 <SEP> lob.9 <SEP> 15.1s <SEP> 1.2 <SEP> J4.1 <tb> <SEP> of <SEP> mix <SEP> e) <SEP> I <tb> Table 2 - Example 1 (Continued) EMI12.1 <tb> t <SEP> \ <SEP> gi-abi <SEP> size <SEP> 10.ON <SEP> to <SEP> t <SEP> l()-û¯ <SEP> | <SEP> total <SEP> e <SEP> less <tb> <SEP> (mln > <SEP> j <SEP> tian <tb> <SEP> .0 <SEP> uvciage <SEP> 1 <SEP> <tb> <SEP> pa <SEP> i'amc <SEP> te <SEP> r <SEP> 2. <SEP> | <tb> <SEP> slVel'l'rC <SEP> i <SEP> { <tb> <SEP> ]-aiil <tb> <SEP> weighl <SEP> (,G-) <SEP> 1 <SEP> 911. <SEP> 7 <SEP> ¯ <SEP> ¯ <SEP> size <SEP> Z <tb> <SEP> [' <SEP> . <SEP> U.vnun <tb> <SEP> CtO <SEP> c <SEP> onzent <SEP> i <SEP> o <SEP> r <SEP> ( <SEP> " <SEP> ) <SEP> 1; <SEP> 1 <SEP> 5 <SEP> . <SEP> aq <SEP> n <tb> <SEP> etch <SEP> 1 <SEP> lin <SEP> } <tb> <SEP> c <SEP> r <SEP> o <SEP> ( <SEP> 5 <tb> <SEP> SiO <SEP> eontellt <SEP> for( <SEP> ) <SEP> d <SEP> 4. <SEP> D) <SEP> O-56 <SEP> 1 <SEP> t;. <SEP> 19 <tb> <SEP> eacll <SEP> gl*tin <SEP> sizc <SEP> S <SEP> . <tb> <SEP> t)tsie <SEP> i <SEP> X <SEP> \ <SEP> (CtO/ > ;ito <SEP> ) <tb> <SEP> 1'01' <SEP> eacil <SEP> gra <SEP> ¯ <SEP> i' <SEP> l. <SEP> )tJ <SEP> O <SEP> 1. <SEP> 9 <SEP> 1()w <tb> <SEP> sizc <tb> <SEP> (,tO <SEP> eu <SEP> ellt <SEP> tS <SEP> t <SEP> . <SEP> ¯ <SEP> &verbar; <tb> <SEP> j)e <SEP> l <SEP> c <SEP> elt <SEP> e <SEP> ol <SEP> C10 <SEP> (r <SEP> ,n <SEP> .8 <SEP> o <SEP> 10() <SEP> /l <SEP> l <SEP> . <SEP> (3 <tb> <SEP> oi <SEP> IlliN <SEP> ((e) <tb> <SEP> SiiOs,, <SEP> contcllt <SEP> tS <SEP> a <tb> <SEP> percentlfre <SEP> of <SEP> riOo <SEP> i1 <SEP> 9Û <SEP> .6 <SEP> . <SEP> t) <SEP> 1V() <SEP> -v <SEP> " <SEP> . <SEP> 5 <tb> <SEP> l <tb> Notes: c = a x b/100 e = a x d/100 f = b/d = c/e g = c/6.6255 x 100 h = e/4.9106 x 100 Table 3 - Example 2 EMI13.1 <tb> <SEP> I <tb> <SEP> I <SEP> "l'i:lli <SEP> izo <tb> <SEP> I <SEP> I <SEP> U <SEP> <SEP> (r!ni) <SEP> ll!lll <SEP> 0.23 <SEP> U.?) <SEP> -. <tb> <SEP> Iii--1iiic1 <SEP> I- <SEP> 1. <SEP> fi <tb> <SEP> "-.L1rI) <SEP> (',. <SEP> 10.1 <SEP> (J <SEP> (I <SEP> 1-j <SEP> . <SEP> U <SEP> 17. <tb> <SEP> C;I(, <SEP> c;o::tciii <SEP> iC99 <tb> <SEP> LI <SEP> I <SEP> ( <SEP> ) <tb> <SEP> z.Ii <SEP> i-aii <SEP> izc- <tb> <SEP> C <SEP> 1 <SEP> ii <SEP> ¯ <SEP> I. <SEP> f)})(I <SEP> I <SEP> 1. <SEP> 7]j <SEP> It <tb> <SEP> d <SEP> j <SEP> 5. <SEP> 1 <SEP> ;. <SEP> 1 <tb> <SEP> i <SEP> l <SEP> ' <tb> i <SEP> 0C? <SEP> C <SEP> Oi <SEP> I.Cìl; <SEP> i <SEP> or <SEP> d <SEP> . <SEP> I <SEP> -- <SEP> t <SEP> . <SEP> -) <SEP> I <SEP> -3 <tb> I <SEP> tlcll <SEP> < 3 <SEP> lil1 <SEP> size <SEP> V <tb> <SEP> I <tb> <SEP> - <SEP> 1 <SEP> 1 <SEP> 1.O <tb> : <SEP> size <tb> ! <SEP> jjU <SEP> co!:tnr <SEP> aj <SEP> j <SEP> j <tb> Lsizc <SEP> I <SEP> < <tb> <SEP> l <SEP> 1,lt) <SEP> C <SEP> 01 <SEP> 1 <SEP> ont <SEP> ;iK <SEP> t <SEP> i <SEP> 1G.1 <SEP> i <tb> <SEP> LLt <SEP> Of <SEP> cez <SEP> e <SEP> 12.7 <SEP> 12. <SEP> C) <SEP> 1G.1 <SEP> 27.-' <tb> <SEP> I <tb> <SEP> ;I <SEP> L <SEP> I <tb> f <SEP> = <SEP> , <SEP> !i <SEP> u <SEP> ' <SEP> l <SEP> t <SEP> 'i <SEP> ,L <SEP> I <tb> <SEP> iCL'e-1I'L'L <SEP> 01. <SEP> ±102 <SEP> 1 <SEP> .7 <SEP> 17.1 <SEP> ) <SEP> 1 <tb> <SEP> i' <SEP> ir. <SEP> \C,o')lj <tb> Table 3 - Example 2 (Continued) EMI14.1 <tb> <SEP> "'J'a) <SEP> size <SEP> 5.0 <SEP> 1() <tb> <SEP> lotal <SEP> ei-ae <tb> <SEP> tns <SEP> tcl- <SEP> mm) <SEP> ) <SEP> ) <SEP> aveJac <SEP> In: <tb> <SEP> 2.0 <SEP> 5.0 <tb> <SEP> avci-aLe <tb> <SEP> wei±rjlt <SEP> s) <SEP> a <SEP> 95. <SEP> } <SEP> 1i1.7 <tb> <SEP> weiiPlt <SEP> () <SEP> a <SEP> 55.0 <SEP> 1 <SEP> I <tb> <SEP> S1%C' <tb> <SEP> iOI <SEP> coiitcnt <SEP> elell <SEP> :1 <SEP> );. <SEP> \\{.) <SEP> I <SEP> ---------- <SEP> (rcj <SEP> 5 <tb> <SEP> each <SEP> ({r) <tb> I <SEP> L';'in <SEP> size <SEP> e <SEP> l.995(i <SEP> ±1 <SEP> (,.4') <SEP> (,-4 <SEP> <SEP> 9 <tb> <SEP> SlO.) <SEP> coiitcnt <SEP> d <SEP> 4.3 <SEP> i <SEP> 1i <SEP> (3 <SEP> i <SEP> ) <tb> ; <SEP> loi--caclt <SEP> (i <SEP> ) <SEP> 1--- <SEP> --1- <SEP> , <tb> <SEP> 2 <SEP> ".TS-T,.i <SEP> 0.1575 <SEP> 4.t't7 <SEP> 2.3512 <tb> <SEP> ', <SEP> sic <SEP> it- <SEP> (cut <SEP> l <SEP> u <tb> <SEP> each <SEP> grain <SEP> - <SEP> f <SEP> 1.35 <SEP> 1.37 <SEP> 1. <SEP> l'i <tb> <SEP> I <SEP> <tb> ' <SEP> ba0 <SEP> con;cnt <SEP> as <tb> percentage <SEP> of <SEP> r3t! <SEP> sr <SEP> 91.1 <SEP> O <SEP> 1()tis <SEP> Ill. <SEP> 17 <tb> of <SEP> iuis <SEP> (?) <tb> .iO <SEP> content <SEP> as <SEP> < 1 <SEP> 100 <SEP> j <tb> J)erc'entage <SEP> of <SEP> SiiOu, <SEP> 11 <SEP> )''.1 <SEP> o. <SEP> 3 <SEP> l <SEP> 100 <tb> <SEP> of <SEP> mix <SEP> ( <SEP> ¯ <SEP> (·) <tb> Notes: c=a x b/100 e = a x d/100 f = b/d = @/c g = c/6.4225 x 100 h = e/4.6879 x 100 As FIG. 1 shows, the 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 uniformly distributed over the six ranges. In addition, as Table 1 (above) shows, both the CaO and SiO2 contens, as percentages of the CaO and SiO2 contents of the mix, are below 50% in the case of gram sizes below 1 mm. On the other hand, as FIG 2 shows, the mix for the process of this invention is characteriyed by a CaO distribution for grain sizes nelow 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 siye 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, tne grains of the mix below 1 mm in siye 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%. PIG. 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 ill size is at least 50% of tile total SiO2 content of ihe mis, aiid that the SiO2 content of the mix is 5,2% iii terms of the SiO,, content of the sintered ore product. The mix is characterized in that grains below 1 mm in siye have a higher SiO2 content and a lower besicity (CaO/SiO2) than grains below 1 mm in siye in the conventional mix (sec FIG 1). It follows that, as shows in table 3 (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). As described in the foregoing, the SiO content of tiie mix to be used iii tiie process of tliis: invention is so controlled that it is not more than 5.4% as a 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 elimillated 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: Y = aX + b ................ (1) in which: Y is the SiO2 content (wtz%) of the sintered ore product, X is the SiO2 content (wt%) of the mix, and a aiid 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. the mix for use in the process of this invention contains at least 25%, preferably from 25 to 60%, of fine grains below 1 mlii in size. lf the content of fine grains below 1 mm in size is lower than 25%, not enougli slag bonds are formed by sinterbig to provide a strong sintered ore. lf the content of fine grains below 1 mm in size exceds 60%, the gas permeability of the sinter bed is decrcased 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. 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 @, from which one can understand that the RDI value is greatly and suddenly improved when the SiO2 content of mix graines below 1 mm in siye exceds 50% of the total SiO2 content of the mix. This is probbly because an uncreased reactive area of the SiO2-containing mix causes rapid and uniform formation of SiO2 slag, the basic component of the slag hond which binds the particles or iron oxide together and groverns the strength of the sintered @@@ product. As a result, a low-slag sintered ore containing not more than 5.4% of SiO2 is produced. The following are illustrative methods of contolling the SiO2 content of the sintered ore product to be not greater than 5.4% and also of controoling the SiO2 conteiit 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 mixt 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) sccolld, decreasing the size of returned fines contain- ing moi-e 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. 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 characteriyed by lowering the SiO2 content of the mix to not more than 3.4 wt% in terms of sintered ore product, increasing the SiO2 content of said dine grains of the mix below 1 mm in size to be between 2.4 and 3.4 weight % of the raw material mix (dry), and controlling the weight ratio of CaO to SiO2 basicity of said fine grains to be below 1. 1, alld sintering the thus controlled mix. Teherefore, 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 SiO1, 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. Thus, tile next point to be considered is the composition of the fine grains of the mix below 1 nun iii size. As we have already revealed, the lower the basicity (CuO/SiO, ratio) of fine grains of tile mix below 1 mm in siye, the higher the reduction strength of the sintered ore, and this effect is particularly conspicous when tiie fine grains have a basicity below 1.0:1. Accordingly, two raw materials mixes with the same average composition will provide sintered ores ol' different reduction strengths if the i'ine grains below 1 mm in siye have different basicities. For example, a mix in wliich the fine grains below 1 mm ill size have a higher basicity provides a sintered ore of hingher reduction strength than a mis in which such fine grains have a lower basicity. however, our studies have also shown that not only is the basicity of fine grains below 1 mm in size, a important factor, but also the absolute amounts of SiO" and GaO contained in suc fine grains. The above discussion shows that the amounts of fine grains of a mix below 1 mm ill size and their constituents, especially SiO2 and CaO, have an important effect on the (lualit of tic sintered ore product. We have also found that most of tlie SiO,, and CaU ill coarse grains larger than 1 mm in siye either remains unreacted or is reacted but mostly confined within tlie 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 eveii if the SiO2 and CaO 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 CaO) of fine grains of a raw materials mix for sintering below 1 mm in size should be satisfied. By reducing tlie SiO,, arid CaO coiltents of coarse grains more than 1 uun in size which have not beeii involved in the formation of a "bond", and thereby decreasing the total SiO2 and CaO contents of the raw material, a low-slag sintered ore can 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 selectivity to eliminate SiO2 and CaO from only the coarse grains of the law 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 CaO contents of the totalmix, and then compensating for the required amounts of SiO2 and CaO in the fine grains below 1 mm in size. Therefore, a a 40 kg-pot test was contucted to determine the quantitative relationship between the SiO2 and CaO contents of fine grains below 1 mm iii size and tlic quality of tiie sintered ore product. The results of the test are shown in FIG 6 of the drawings. FIG 6 is a graph plotting the RDI values of sintered ore products produced by varying the SiO2 and CaO contents of fine grains of raw materials mix below 1 mm in size. On the x-axia, the weight of SiO2 contained in flue grains below 1 mm in size is plotted as a percentage of the components of the mix (dry). This factor is defined by tile following formula (tile factor will lere- under be referred to as [SiO2]in -1 mm): A x B 100 wherein A is the percentage by weight of the fine 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 RDI values of the resulting sintered ore products are indicated by numerals iii the graph. The basicity (CaO/SiO2 ratio) of tile fine grains below 1 jiun iii ize is shown as a solid line slopping upwards to the right, and the sum of the CaO and SiO2 contained in tlie fine grains below 1 mm in size is shown as a dotted line slopping 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 iii 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 RDi 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 sine grains below 1 mm in size should not exceed a given value. 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 inercase in the level of [SiO2] in -1 mm. The region where [SiO2] in -1 mm exceeds 3.0 in FIG 6 is obtainable only in a laboratory by selecting only iron ores which are 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 obtain- ing the stated range. Furthermore, the strength at ordinary temperatures (shattter 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 imp lenient separate provisions for increasing the shatter index by, for instance, incorporating more coke in the mix. 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 SiO2 contained in the dine grains below 1 mm in size is at least 4.0. It is to be noted again tiiat the mix fOl' use in the process of this invention generally contains from 25 to 60 wt% @ @@ fiiie grains below 1 mm iii size. It is well known that the reduction strength of a intered ore will usually decrease with an increase in the Al2O3 content of the area. In contrast with this statistical fact, some cases are observed in which an increase in the Al2O3 content does not necessarily result ill a low reductioll strength and, tilerelore, the relation- ship 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 remaiii in tile unmelted ore, we have carried out experiments varying tile Al2O3 content of fine grains only rather than the Al2O3 content averaged by both fine and coarse graiiis, 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 Al2O3 content of the sintered ore and its reduction strength can be explained by the Al2O3 content of fine grains below 1 nnn 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 mole, a raw uiatei-ial for sintering should not be composed of crushed iron ores having a high Al2O3 content, and this is important for ensuring an acceptable reduction strength iu tile sintered ore. The above discussion shows that the proportions of fine grains of a raw material for sintering which are below 1 mm in size and of their contituents, especially SiO2, CaO and Al12O3, have an important effect on the quality of the sintered ore product. We have also found that most of the SiO2, CaO and Al2O3 in the coarse grains larger than 1 mm in size either remains unreacted or is reacted but mostly confined within the coarse grains, thus failing to perform @@e function of a"bond". However, CaO which more easily forms slag than SiO2 and Al2O3 will foi-ni some slag in tlie later stage of the sintering reaction. Therefore, by properly controlling the SiO2, CaO and Al2O3 contents of fine grains below 1 mm in size, sintered ores with quality and productivity unimpared 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 pruductivity 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 lowslag sintered ore can be prepared which contains not more than 5.4 wt% of SiO2 but which has previuosly been difficult to produce commercially de to low quality and productivity. However, it is substantially impossible in practice selectively to elimiante SiO2 and CaO from only the coarse grains of the raw material below 1 mm in size. Therefore, as a practically feasible method, the SiO2 and CaO contents of the coarse grains are reduced not diercted 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 iii size. Accordingly, a 40 kg-pot test was conducted to determine the quantitative relationship between the SiO2, CaO 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. FIG 8 is a graph plotting the RDI values of sintered ore products produced by varying the SiO2, CaO and Al 203 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 A120¯ 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 - Al2O3, in -1 mm): A x (B - C) 100 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 SiO2 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. On the y-axis, we have plotted the weight of the CaO 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 ill the graph of Fig 8. The CaO/(SiOO - A 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 tile region where [SiO2 - Al2O3] in -1 mm is at least 1 .8 and the CaO/ (SiOt, - Al2O3) ) ratio-of fine grains below 1 miii in size -, not greater than 2.0:1. In particular the region where the SiO2 -Al2O3] in -1mm value is at least 2.4 and the CaO/(SiO2 - Al2O3) ratio of fine grains below 1 mm in size is not greater than 1.8:1 is characterized by very desirable RDi values ( < 30). Therefore,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. Now, the change in the Al2O3 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 Al2O3 are not generally available in large quantities. Therefore, it is unavoidable that the level of SiO2 - Al2O3 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 in the raw materials mix should maturally 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] in -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 in 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 possibility of obtaining the stated range. 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 nun in size is smaller than 1.0: :1, the sum of the GaO and SiO" contained ill said fine grains decreases, and tlle sintered ore product has a tendency to exhibit a low strength at ordinary temperatures (shatter index), making it necessary to impleiiient separate provisions for increasing the shatter index by, for instance, incorporating more coke ill the mix. It is to be meiitioned here that the level of the CaO/(SiO2 - Al2O3) ratio of fine grains below 1 mm in size should not be below 0.5: :1 because, otherwise, sintered ore which is very low in strength at ordinary temperatues is produced. The following conclusion can be drawn from the above discussion: tile technique of this invention caii be easily implemented which the hatched area of FIG 8 where the [SiO2 - Al2O3] in -1 mm values are at least 1.8 and the CaO/(Sio" - Al2O3) ratio of fine grains below 1 mm In size are not greater than 2.0:1 especially in the dotted area where the [SiO2 - Al2O3] in -1 mm values are between 1.8 and 2.4 and the CaO/(SiO2 - Al2O3) rates between 0.5:1 preferably 1.0:1 and 2.0:1. Preferred embodiments of the process of this invention are described hereunder in greater detail with reference to the following examples. Three different raw materials mixes each comprising iron ores, limestone, silica, coke andf return fines were agglomerated in the presence of water, and the resulting agglomerates were charged ilito a 40 kg test pot at i negative pressure of 1700 mm H2O to produce three different sintered ores. The description of the ingredients of cach mix and the grain size distribution of each ingredient are shown in Table 4 (for Comparative Example 1), Table 5 (for Example 1) and Table 6 (for Example 2). The average basicity (CaO/SiO2) of each raw materials mix was about 1.55:1. The raw materials mix uscd iii Example 1 according to one embodiment ol' the process of this invention incorporated fine grains below 1 mm in size) of silica containing at least 90% of SiO2 so that tiie SiO2 content of tiie mix was not more than 5.4% in terms of sintered ore product. The raw materials mix used in Example 2 according to another embodiment of this iiiveiition likcwise incorporated fine grains (below 1 flint in size) of silica containing at least 90% of SiO2, but it contained a smaller amount of silica and limestone so that the SiO2 content of the mix was not more than 5.2% in terms of sintered ore product. Tables 4,5 and 6 are keyed to Tables 1, 2 aiid 5, respectively. The compositions of the principal ingredients of each mix are identified in Table 8 below. Table 4 (Comparative Example 1) Description and Proportions of Ingredients EMI28.1 <tb> <SEP> Description <SEP> of <SEP> Propor- <SEP> grain <SEP> size <SEP> (mm) <tb> <SEP> Ingredients <SEP> tions <tb> (%) <SEP> 10¯5 <SEP> 5¯2 <SEP> 2¯1 <SEP> 1¯0.5 <SEP> 0.5¯0.25 <SEP> ¯0.25 <tb> <SEP> M <SEP> B <SEP> R <SEP> 45 <SEP> 5% <SEP> 30% <SEP> 15% <SEP> 15% <SEP> 10% <SEP> 25% <tb> <SEP> Mt. <SEP> Newman <SEP> 24.5 <SEP> 10 <SEP> 30 <SEP> 15 <SEP> 15 <SEP> 15 <SEP> 15 <tb> <SEP> Robe <SEP> River <SEP> 15 <SEP> 10 <SEP> 30 <SEP> 20 <SEP> 20 <SEP> 10 <SEP> 10 <tb> <SEP> scale <SEP> 3 <SEP> 17 <SEP> 32 <SEP> 20 <SEP> 16 <SEP> 8 <SEP> 7 <tb> <SEP> silica <SEP> 1.7 <SEP> - <SEP> 13 <SEP> 27 <SEP> 25 <SEP> 15 <SEP> 20 <tb> <SEP> limestone <SEP> 10.8 <SEP> - <SEP> 20 <SEP> 30 <SEP> 18 <SEP> 16 <SEP> 16 <tb> <SEP> total <SEP> of <SEP> new <tb> <SEP> 100 <SEP> - <SEP> - <SEP> - <SEP> - <SEP> - <SEP> <tb> <SEP> materials <SEP> (N) <SEP> 100 <SEP> I <SEP> - <SEP> - <SEP> <tb> <SEP> return <SEP> fines <SEP> 20 <SEP> <SEP> - <SEP> 60 <SEP> 20 <SEP> 20 <SEP> 5 <SEP> <tb> <SEP> coke <SEP> breeze <SEP> 3.4 <SEP> - <SEP> 30 <SEP> 20 <SEP> 15 <SEP> 12 <SEP> 23 <tb> CaO in - 1 mm 41.9% SiO2 in - 1 mm 44.9% (SiO2 content of sintered ore 5.6%) proportion of return fines = R x 100 N+R proportion of coke breeze = C x 100 N + R + C Table 5 (Example 1) Description and Proportions of EMI29.1 <tb> <SEP> Propor- <SEP> grain <SEP> size <SEP> (mm) <tb> <SEP> Description <SEP> of <tb> <SEP> (%) <SEP> 10¯5 <SEP> 5¯2 <SEP> 2¯1 <SEP> 1¯0.5 <SEP> 0.5¯0.25 <SEP> -0.25 <tb> <SEP> M <SEP> B <SEP> R <SEP> 45 <SEP> 5% <SEP> 30% <SEP> 15% <SEP> 15% <SEP> 10% <SEP> 25% <tb> <SEP> Mt. <SEP> Newman <SEP> 25.2 <SEP> 10 <SEP> 30 <SEP> 10 <SEP> 15 <SEP> 15 <SEP> 15 <SEP> 15 <tb> <SEP> Robe <SEP> River <SEP> 15 <SEP> 10 <SEP> 30 <SEP> 20 <SEP> 20 <SEP> 10 <SEP> 10 <tb> scale <SEP> 3 <SEP> 17 <SEP> 32 <SEP> 20 <SEP> 10 <SEP> 8 <SEP> 7 <tb> <SEP> silica <SEP> 1.4 <SEP> - <SEP> - <SEP> - <SEP> 30 <SEP> 30 <SEP> 40 <tb> limestone <SEP> 10.4 <SEP> - <SEP> 20 <SEP> 30 <SEP> 18 <SEP> 18 <SEP> 10 <tb> <SEP> total <SEP> of <SEP> new <SEP> 100 <SEP> - <SEP> - <SEP> - <SEP> - <SEP> - <SEP> <SEP> materials <SEP> (N) <tb> <SEP> return <SEP> fines <tb> <SEP> 20 <SEP> - <SEP> 60 <SEP> 20 <SEP> 10 <SEP> 5 <SEP> 5 <tb> (R) <tb> <SEP> coke <SEP> breeze <tb> <SEP> 3.4 <SEP> - <SEP> 30 <SEP> 20 <SEP> 15 <SEP> 12 <SEP> 23 <tb> (C) <tb> CaO in - 1 mm 41.9% SiO2 in - 1 mm 52.5% (SiO, content of sintered ore 3.4% proportion of returnn fines = R x 100 N + proportion of coke breezer = C x 100 N + 11 + C Table 6 (Example 2) Description and Proportions of Ingredients EMI30.1 <tb> <SEP> Propor- <SEP> grain <SEP> size <SEP> (mm) <tb> <SEP> Description <SEP> of <tb> <SEP> tions <tb> <SEP> Ingredients <tb> (%) <SEP> 10¯5 <SEP> 3¯2 <SEP> 2¯1 <SEP> 1¯0.5 <SEP> 0,5¯0.25 <SEP> -0.25 <tb> <SEP> M <SEP> B <SEP> R <SEP> 45 <SEP> 5% <SEP> 30% <SEP> 15% <SEP> 15% <SEP> 10% <SEP> 25% <tb> Mt <SEP> Newman <SEP> 26.0 <SEP> 10 <SEP> 30 <SEP> 15 <SEP> 15 <SEP> 15 <SEP> 15 <tb> <SEP> Robe <SEP> River <SEP> 15 <SEP> 10 <SEP> 20 <SEP> 20 <SEP> 20 <SEP> 10 <SEP> 10 <tb> scale <SEP> 3 <SEP> 17 <SEP> 32 <SEP> 20 <SEP> 16 <SEP> 8 <SEP> 7 <tb> silica <SEP> 1.1 <SEP> - <SEP> - <SEP> - <SEP> 30 <SEP> 30 <SEP> 40 <tb> <SEP> limestone <SEP> 9.9 <SEP> - <SEP> 20 <SEP> 30 <SEP> 18 <SEP> 16 <SEP> 16 <tb> total <SEP> of <SEP> new <tb> <SEP> 100 <SEP> - <SEP> - <SEP> - <SEP> - <SEP> - <SEP> <tb> materials <SEP> (N) <tb> <SEP> return <SEP> fines <tb> 20 <SEP> - <SEP> 60 <SEP> 20 <SEP> 20 <SEP> 10 <SEP> 5 <SEP> 5 <tb> <SEP> (R) <tb> coke <SEP> breeze <tb> 3.4 <SEP> - <SEP> 30 <SEP> 20 <SEP> 15 <SEP> 12 <SEP> 23 <tb> <SEP> (C) <tb> CaO in -1 mm 41.7% SiO2 in -1 mm 50.2% (SiO2 content of sintered o@e 3.2% Proportion of return fines = R x 100 N + R proportion of coke breeze = C x 1@ N + R + C The rest results are shown in Table 7 (below) and @ @G 5 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 shorter sintering period and a higher ratio of sinter to sinter cake. The process of this invention consumed slightly less coke than the conventional process. This means that, although the process of this invention achieved almost the same results of sintering as in the conventioal technique with respect to moisture content, coke content, productivity, sintering time, sinter to sinter cake ration, coke consumption and shatter index, in gretly reduced the RDI value of the sintered ore product (as in Example 1) or reduced the SiO2 content of the sintered one product to less than 5.2% without greatly increasing its RDI value (as in Example 2). Table 7 - Test Rasults EMI31.1 <tb> sinter <SEP> mois- <SEP> produc- <SEP> 10 <tb> coke <SEP> ing <tb> <SEP> ture <SEP> tivity <SEP> sinter <tb> <SEP> ratio <tb> <SEP> (%) <SEP> (%) <SEP> (t/H.mê) <SEP> (min) <SEP> (%) <tb> <SEP> Comp <tb> <SEP> 6.0 <SEP> 3.4 <SEP> 1.32 <SEP> 22.5 <SEP> 80.4 <tb> Ex <SEP> 1 <tb> <SEP> Ex <SEP> 1 <SEP> 6.0 <SEP> 3.4 <SEP> 1.39 <SEP> 21.0 <SEP> 80.9 <tb> <SEP> Ex <SEP> 2 <SEP> 0.0 <SEP> 7.4 <SEP> 1.39 <SEP> 21.3 <SEP> 81.5 <tb> Table 7 - Test Results (Continued) EMI32.1 <tb> <SEP> coke <SEP> shatter <SEP> symbols <tb> RDI <tb> <SEP> consumptions <SEP> index <SEP> used <SEP> in <tb> <SEP> (kg/t-s) <SEP> (%) <SEP> (%) <SEP> FIG <SEP> 5 <tb> Copm <tb> <SEP> Ex <SEP> 1 <SEP> 47.1 <SEP> 86.6 <SEP> 32.1 <SEP> X <tb> <SEP> Ex <SEP> 1 <SEP> 46.9 <SEP> 86.9 <SEP> 30.8 <SEP> O <tb> <SEP> Ex <SEP> 2 <SEP> 46.5 <SEP> <RTI ID=32.1> 87.5 <SEP> 32.9 <SEP> @ <tb> Table 8 unit : wt% EMI32.2 <tb> <SEP> T.Pe <SEP> PeO <SEP> 2 <SEP> Al2O <tb> <SEP> Fe, <SEP> Fuzz <SEP> O, <SEP> I'eO <SEP> Fet O <SEP> ) <SEP> | <SEP> SiO;, <SEP> 510 <tb> <SEP> 1 <SEP> fi <SEP> 77 <SEP> 11 <SEP> O.-)N <SEP> 9G151 <SEP> 1.55 <SEP> ().69 <tb> <SEP> l <SEP> - <SEP> l <tb> ol-e <SEP> '?lit. <SEP> N <SEP> t <SEP> j <SEP> U.O, <SEP> b,.-9l <SEP> I <SEP> (I.3() <SEP> i <tb> <SEP> *) <SEP> T)lij <tb> <SEP> I( <SEP> 1 <tb> scrle <SEP> 72.s4 <SEP> 6.15 <SEP> .45 <SEP> 0.7 <SEP> 0. <tb> (mill <SEP> 11 <SEP> scale) <SEP> ) <tb> <SEP> 0.94 <SEP> 1, <SEP> 33.0 <SEP> 1.32 <tb> limestolle <SEP> ' <SEP> - <SEP> O..;; <SEP> I <SEP> O <SEP> (; <SEP> ) <SEP> ljSJi <SEP> (), <SEP> OS <tb> Table 8 (Continued) unit : wt% EMI33.1 <tb> jvC <SEP> C <SEP> S <SEP> l <SEP> Ct <SEP> rU <SEP> CXe*4 <SEP> <tb> <SEP> '.1 <SEP> B <SEP> it <SEP> 0.14 <SEP> O.O'i <SEP> 0 <SEP> U!J} <tb> <SEP> *9 <tb> <SEP> 01.e <SEP> Mt <SEP> N <SEP> O.O() <SEP> 0.05 <SEP> G'f <tb> <SEP> *) <SEP> l <SEP> l <tb> <SEP> it <SEP> it <SEP> 0.15 <SEP> | <SEP> 0.1" <tb> 5Cjil( <tb> (mill <SEP> scale) <SEP> ,.OS) <SEP> O.'?9 <SEP> (), <tb> <SEP> silica <SEP> 0.19 <SEP> t <SEP> U <SEP> . <SEP> 1 <SEP> e, <SEP> O <SEP> . <SEP> 0.45 <SEP> O.9'i <tb> <SEP> 1 <SEP> ins <SEP> toiie <SEP> SN. <SEP> . <SEP> t <SEP> 0. <SEP> 5() <SEP> t) <SEP> li <tb> *1 Mineracôes Brasileiras Reunidas S.A. (Brazil) *2 Mt. Newman Min. Co. Pty. Ltd. (Australia) *3 Robe River (Australia) *4 combined water As described in the foregoing, the process of this invention is compatible with, or even superior to, the conventional sintering process with respect to productivity, coke consumption, shatter index and other factors whlie it can greatly reduce the RDI value of the sintered product, or reduce the SiO2 content of the sintered product to below 5.4% without grealty increasing ist RDI value. Comparative Examples 2 and 3 and Examples 5 to 7 of this intervention are described hereunder. Seven different raw amterials mixes each comprising iron ores, dimestone, 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 mmH2O 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 Example 6 and 7). The proportion of the ingredients of the mixes are indicated in Table 13 (Comparison Example 2 and Examples 3 to 5) and in Table 25 (Comparison Example 3 and Examples 6 und 7). Tables 14 to 17 are keyed to the data on the proportions of ingredients set forth iii Table 15 for Comparative Example 2 aiid Examples 9 to 5, respectively, arid each table shows the SiO2 and CaO contents of the raw materials mix aiid the sintered ore product. In each of Tables 14 to 17, the data on the SiO2 content, the CaO content and the CaO/SiO2 ratio of the mix are classified under coarse grains largerthan 1 mm in size and fine grains below 1 mn; iii size. Tables 24 to 26 are keyed to the data on the proportions of ingredients set forth In Table 25 for Comparative Example 3 and Examples 6 and 7, respectively, and each table shows the SiO2, GaO and Al2O3 contents and' the CaO/SiO2 ratios of the raw materials mix aiirl sintered ore product, and the (SiO2 - Al2O3) contents and CaO/(SiO2 - Al2O3) ratios of fine grains (below 1 mm in size) of the mix. In each of Tables 24 to 26, the data on the SiO2 content, CaO content and Al2O3 content of the mix are classified under "coarse grains" (above 1 mm in size) and "fine grains" (below 1 mm in size), respectively. As mentioned above in connection with the raw material mix to be used in the process of this invention, the relationship betwenn Y which is the SiO2 content (wt%) of the sintered one product and X which is the SiO2 content (wt%) of the raw materials mix can be approximated by the following formula: Y = aX + b - (1) wherein a and b are 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. As Tables 13 and 14 show, the raw materials mix prepared in Comparative Example 1 is such taht 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, tile level of in -1 mm of the mix is 2.25 and the CaO/SiO2 ratio of fine grains below 1 nun iii size is 1.20:1. Therefore, a simple reduction of the SiO2 content gives a level of [SiO2] in -1 mm which is below 2.4. XS shown in Tables 13 and 15, the raw materials mix prepared iii Example 3 is such that iiot 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 SiO2 content in therms of sintered ore product down to 5.4 wt%. As a result,the [SiO2] in - 1 mm value is increased to 2.77 and the CaO/SiO2 ratio of fine grains below 1 mm iii size is decreased to 0.95:1. In Example 4 (shown in Tables 13 and 16), the raw materials mix contains both return fines a part of whiel is crushed to a size below 1 mm and relatively coarse grains of other iron 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] in - 1 mm value which is as high as 2.97 and a CaO/SiO2 ratio of fine grains below 1 mm in size as high as 1.25:1. In Example 5 (illustrated in Tables 15 und 17), the raw materials mix is such that not only is the amount of silica added decreased to 0.8 wt% but also the erain size is decl euscd to below 1 mill and also 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 2wt% in consequence, the [SiO2] in - 1 mm of the mix is 2.55 and the CaO/SiO2 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 tile graplj of FlG. 7. 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 iii a low sintcr 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 tile amount of silica added but also by reducing its grain size to below 1 mm, provided a sintered ore haviiig a desired RDI value (below 40), which could be produced with high productivity and in a short sintering period, consuming less coke, althougt the sintered ore product contained 5.4 wt% of SiO2 which was less than the ordinary values between 5.6 and 6.0 wt%. The raw materials mix of Example 4, which containes return fines with somewhat more SiO2 than the intended sintered ore 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. 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 1 mm and by incorporating partially crushed iron ores which were relatively high in SiO2 content, previded 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 tiie resultiiig sintered ore liad its SiO2 content decreased to 5.0 wt%. Table 9 unit : wt% EMI37.1 <tb> \ <SEP> ,io. <SEP> | <SEP> T. <tb> <SEP> ore <SEP> \ <SEP> I <SEP> i <SEP> : <SEP> i <SEP> ! <SEP> ' <SEP> I <SEP> . <tb> <SEP> Or= <SEP> I <SEP> e <tb> <SEP> N <SEP> 67,62; <SEP> G.35.i90.91 <SEP> ! <SEP> 1.61 <SEP> 'l <SEP> 0.67 <SEP> iio.1010.04;0 <SEP> 39 <tb> kD <SEP> CF <SEP> 82/ <SEP> ! <SEP> 65.01 <SEP> 1 <SEP> 09, <SEP> C2, <SEP> 4. <SEP> i <SEP> 1.25 <SEP> 10. <SEP> 10 <SEP> 0.091.00 <tb> <SEP> + <SEP> 13 <tb> <SEP> 3 <SEP> 61. <SEP> !Z1 <SEP> 9. <SEP> izl <SEP> 8 <SEP> . <SEP> ! <SEP> i <SEP> 13 <SEP> ' <SEP> 2. <SEP> 9q <SEP> 0. <SEP> 05 <SEP> 1 <SEP> 0.0:J <tb> <SEP> ra <SEP> l <SEP> 61.98 <SEP> 0. <SEP> 10 <SEP> 8'. <SEP> 3 <SEP> . <SEP> 2.78 <SEP> 10 <SEP> 09.0.052.33 <tb> .5 <SEP> - <SEP> I¯¯¯ <SEP> . <SEP> . <SEP> I' <tb> 5 <SEP> , <SEP> "-" <SEP> 15 <SEP> 35 <SEP> 0 <SEP> 20 <SEP> 1. <SEP> 70 <SEP> 5.50 <SEP> 2.51 <SEP> 0. <SEP> 15 <tb> <SEP> 0. <SEP> 14: <SEP> b.40 <tb> *1 Mineracoes Brasileiras Reunidas S.A. (Brazil) *2 Rio Doee common fine (Brazil) *3 Mt. Newman Min. Co. Pty. Lt (Australia) *4 Hamersley (Australia) *5 Robe River ( " ) *5 combined water Table 10 unit : wt% EMI37.2 <tb> <SEP> o- <SEP> TRI <tb> <SEP> 01111,1 <SEP> T, <SEP> E'e <SEP> E'e <SEP> C\; <tb> <SEP> + <SEP> (I4 <SEP> T.le <SEP> | <SEP> I'eO <SEP> 1:C,,O¯ <SEP> 9 <SEP> l"t;¯ <SEP> Ca() <tb> iQlilll <SEP> s;caic) <SEP> 75.r3 <SEP> S <SEP> I <tb> fllii <SEP> SC1le) <SEP> | <SEP> 73.33 <SEP> 4 <SEP> . <SEP> '9 <SEP> 2 > .59 <SEP> U.71t <SEP> 1().1, <tb> <SEP> I------------ <SEP> I <SEP> I <tb> Table 11 unit: wt% EMI38.1 <tb> <SEP> T.Fe <SEP> Fe2O3 <SEP> SiO2 <SEP> Al2O3 <SEP> CaO <SEP> MgO <SEP> CW <tb> silica <SEP> 0.94 <SEP> 1.34 <SEP> 93.00 <SEP> 1.92 <SEP> 0.45 <SEP> 0.94 <tb> Table 12 unit : wt% EMI38.2 <tb> <SEP> T.Fe2 <SEP> SiO2 <SEP> Al2O3 <SEP> CaO <SEP> MgO <SEP> CW <tb> limestone <SEP> 0.17 <SEP> 0.66 <SEP> 0.07 <SEP> 55.01 <SEP> 0.55 <SEP> 0.29 <tb> Table 13 EMI38.3 <tb> <SEP> proportions <SEP> (wt%) <tb> <SEP> Ingredients <SEP> Com. <SEP> Ex. <SEP> 2 <SEP> Ex. <SEP> 3 <SEP> Ex. <SEP> 4 <SEP> Ex. <SEP> 5 <tb> <SEP> description <SEP> new <SEP> to- <SEP> new <SEP> to- <SEP> new <SEP> to- <SEP> new <SEP> to <SEP> material <SEP> tal <SEP> material <SEP> tal <SEP> material <SEP> tal <SEP> material <SEP> tal <tb> <SEP> M <SEP> B <SEP> R <SEP> 32 <SEP> <SEP> . <SEP> . <SEP> 32 <SEP> 40 <SEP> <tb> <SEP> Rio <SEP> Doce <SEP> 9.7 <SEP> 14.7 <SEP> 6. <SEP> 7 <SEP> 8.6 <tb> CF <tb> <SEP> Mt. <tb> <SEP> 10 <SEP> 10 <SEP> 22 <SEP> 22 <tb> <SEP> Newman <tb> <SEP> Hamersley <SEP> 23 <SEP> 18 <SEP> 6 <SEP> 6 <tb> <SEP> Robe <SEP> River <SEP> 10 <SEP> 77.1 <SEP> 10 <SEP> 77. <SEP> 1 <SEP> 10 <SEP> 77. <SEP> 1 <SEP> 10 <SEP> | <SEP> 77. <SEP> 1 <tb> <SEP> scale <SEP> 3 <SEP> 3 <SEP> 3 <SEP> 3 <tb> <SEP> silica <SEP> 1.4 <SEP> 1.4 <SEP> 1.4 <SEP> 0.8 <SEP> <tb> limestone <SEP> 10.9 <SEP> 10.9 <SEP> 10.9 <SEP> 9.6 <tb> <SEP> total <SEP> of <SEP> new <SEP> 100 <SEP> 100 <SEP> 100 <SEP> 100 <tb> <SEP> rnaterials <SEP> <tb> <SEP> return <tb> fines <SEP> 19.5 <SEP> 19.5 <SEP> 19. <SEP> 5 <SEP> 19.5 <tb> <SEP> coke <tb> <SEP> breeze <SEP> 3.4 <SEP> 3.4 <SEP> 3.4 <SEP> 3.4 <tb> Table 14 Corn. Ex. 2 EMI39.1 <tb> <SEP> raw <SEP> material <SEP> mix <SEP> I <SEP> sintered <tb> <SEP> # <SEP> <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <SEP> total <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> <SEP> 59.9 <SEP> 40.1 <SEP> 100.0 <SEP> <tb> <SEP> weight <SEP> (%) <tb> <SEP> SiO2 <SEP> (%) <SEP> 2. <SEP> 55 <SEP> 2. <SEP> 25 <SEP> 4. <SEP> 80 <SEP> 5.40 <tb> <SEP> CaO <SEP> (%) <SEP> 4. <SEP> 26 <SEP> 2. <SEP> 70 <SEP> 6. <SEP> 96 <SEP> 7. <SEP> 83 <tb> <SEP> CaO/SiO2 <SEP> 1. <SEP> 67 <SEP> 1 <SEP> 1.20 <SEP> 1.45 <SEP> 1.45 <tb> Table 15 Ex 3 EMI39.2 <tb> <SEP> raw <SEP> material <SEP> mix <SEP> sintered <tb> <SEP> # <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <SEP> total <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> 57.0 <SEP> 43.0 <SEP> 100.0 <tb> <SEP> weight <SEP> (%) <tb> SiO2 <SEP> (%) <SEP> 2.03 <SEP> 2.77 <SEP> 4.80 <SEP> 5.40 <tb> <SEP> CaO <SEP> (%) <SEP> 4.33 <SEP> 2.63 <SEP> 6.96 <SEP> 7.83 <tb> <SEP> CaO/SiO2 <SEP> 2.13 <SEP> 0.95 <SEP> 1.45 <SEP> 1.45 <tb> Table 16 Ex. 4 EMI39.3 <tb> <SEP> raw <SEP> material <SEP> mix <SEP> I <SEP> sintered <tb> <SEP> # <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <SEP> total <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> <SEP> 51.2 <SEP> 48.8 <SEP> 100.0 <SEP> <tb> <SEP> weight <SEP> (%) <tb> <SEP> SiO2 <SEP> (%) <SEP> 1. <SEP> 83 <SEP> 2. <SEP> 97 <SEP> 4. <SEP> 80 <SEP> 5.40 <SEP> <tb> <SEP> CaO <SEP> (%) <SEP> 3.25 <SEP> 3.71 <SEP> 6.96 <SEP> 7. <SEP> 83 <tb> <SEP> CaO/SiO2 <SEP> | <SEP> 1.78 <SEP> | <SEP> 1.25 <SEP> | <SEP> 1.45 <SEP> | <SEP> 1.45 <tb> Table 17 Ex. 5 EMI39.4 <tb> <SEP> raw <SEP> material <SEP> mix <SEP> I <SEP> sintered <tb> <SEP> # <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <SEP> total <SEP> ore <tb> <SEP> Proportion <SEP> by <SEP> 56.2 <SEP> | <SEP> 43. <SEP> 8 <SEP> 100.0 <SEP> <tb> <SEP> weight <SEP> (%) <tb> SiO2 <SEP> (%) <SEP> 1.95 <SEP> 2.55 <SEP> 4.50 <SEP> 5.00 <tb> <SEP> CaO <SEP> (%) <SEP> 3.66 <SEP> 2.86 <SEP> 6.52 <SEP> 7. <SEP> 25 <tb> <SEP> CaO/SiO2 <SEP> 1.88 <SEP> 1.12 <SEP> 1.45 <SEP> 1.43 <tb> TABLE 1@ EMI40.1 <tb> <SEP> L <tb> <SEP> IaT'a- <SEP> iliols- <SEP> t <SEP> coke <SEP> eveluclu <SEP> tcl¯ <tb> <SEP> 'ictcr <tb> <SEP> vit' <SEP> I <SEP> tilnc <tb> <SEP> Nu. <SEP> ture <SEP> I <SEP> ivIL' <tb> <SEP> -- <SEP> | <SEP> <tb> <SEP> Corn, <SEP> Ex. <SEP> 0 <SEP> }- <SEP> J3 <SEP> 1 <SEP> 1. <SEP> JJ <SEP> a <SEP> r <tb> <SEP> Lx. <SEP> Cs. <SEP> i <SEP> 6.() <SEP> 5.)) <SEP> 1 <SEP> )-, <SEP> ) <tb> <SEP> 1 <SEP> <tb> <SEP> Ex. <SEP> " <SEP> i'j.O <SEP> a <SEP> C} <SEP> rat) <SEP> | <SEP> t", <SEP> t <tb> <SEP> Ex. <SEP> 5 <SEP> 6.o <SEP> 1 <SEP> 1.59 <SEP> <tb> <SEP> I <SEP> I <tb> <SEP> -jiara- <SEP> | <SEP> i <SEP> I <SEP> co7fc <SEP> st <SEP> shatter <SEP> 11 <SEP> I) <SEP> I <SEP> i <tb> <SEP> I <SEP> S <SEP> - <SEP> consui;tptioil <SEP> in <tb> <SEP> \ <SEP> - <SEP> tion <SEP> - <SEP> index <tb> <SEP> tion <SEP> incics <tb> <SEP> No.J <SEP> I <SEP> orc <tb> i'LUlt <SEP> rat <SEP> (;) <SEP> (f') <tb> <SEP> 'f <SEP> (Ng/t-s) <SEP> | <SEP> (110 <SEP> wC'c') <tb> from. <SEP> D) <SEP> Ex. <SEP> L1 <SEP> 9 <SEP> 75.7 <SEP> 'i7. <SEP> 5 <SEP> 1 <SEP> , <SEP> !15, <tb> <SEP> 1 <tb> Lx. <SEP> 3 <SEP> 77 <SEP> st <SEP> s <SEP> J <SEP> I, <SEP> 11 <tb> <SEP> .7 <SEP> 1 <SEP> )U.(' <SEP> 5. <SEP> 1 <tb> Lx. <SEP> N <SEP> 79.1 <SEP> Sus,5 <SEP> 55.5 <SEP> 55.9 <SEP> i <tb> <SEP> 1 <tb> 1 <SEP> <tb> <SEP> Lox. <SEP> 5 <SEP> 77 <SEP> N6. <SEP> 57.1 <SEP> i <SEP> * <SEP> ( <tb> <SEP> I <SEP> I <tb> As Tables 23 und 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 tcrms of sintered ore product from tile 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 ill 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. As shown in Tables 25 and 25, the raw materials mix prepared in Example 6 contains boht return fines, part of which is crushed to a size below 1 nun, and relatively coarse grains of other ores so that the SiOo content i terms of sintered are product is decreaced 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 1.85:1. 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 mni 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 e0 wt%. In consequence, the [SiO2 - Al2O3] in -1 mm of the mix is 1.83 and tlie weight ratio of CaO/(SiO" - Al2O3) of fine grains below 1 null in size is 1.29:1. 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 rativ. The mix consumed a large amount of coke as it was sintered. Table 27 Test Results EMI43.1 <tb> <SEP> Ia0 <SEP> i <SEP> "ltsi <SEP> 9 <SEP> s- <SEP> col' <SEP> ' <SEP> j. <SEP> i-()jiic <SEP> I <SEP> 51 <SEP> il <tb> <SEP> ile <SEP> I <SEP> ('i' <tb> <SEP> tit <SEP> -e <SEP> cOIl <SEP> en <SEP> t- <SEP> i <SEP> R <SEP> i <SEP> L <SEP> ! <tb> <SEP> ilun <SEP> N(j. <SEP> \ <SEP> (z) <SEP> (rio <SEP> ) <SEP> (t/Fill. <SEP> I <SEP> (win) <tb> 1 <SEP> S <SEP> <tb> <SEP> Coiii.Ex <SEP> 3. <SEP> G <SEP> . <SEP> 1/ <SEP> 5. <SEP> ii <SEP> i <SEP> , <SEP> 2 <SEP> :2'i.() <tb> <SEP> Ex <SEP> . <SEP> G <SEP> 25.5 <tb> Ex <SEP> G. <SEP> 7. <SEP> ) <SEP> I <SEP> . <SEP> J') <SEP> 7. <tb> <SEP> J <tb> <SEP> \ <SEP> . <SEP> t} <SEP> n <SEP> - <SEP> ol\( <SEP> s)iiiltci' <SEP> t <SEP> cr <SEP> I <SEP> S <SEP> I <SEP> c), <tb> <SEP> iie <SEP> Lel <SEP> I <SEP> O <SEP> e(} <SEP> Sll" <SEP> 1j¯ <SEP> II <SEP> D <SEP> I <tb> <SEP> Si <SEP> 111 <SEP> (i' <SEP> t <SEP> iOIl <SEP> intle,x <SEP> i <SEP> sir. <tb> <SEP> Iii <tb> <SEP> cake <SEP> (S;r/t-s) <SEP> (1 <SEP> () <SEP> mmS) <SEP> ) <SEP> ( <SEP> r, <SEP> ! <SEP> ! <SEP> (s) <SEP> I't' <tb> <SEP> liii <SEP> l <SEP> . <SEP> r <SEP> i <SEP> o <tb> Com. <SEP> Ex <SEP> 5 <SEP> 75.7 <SEP> liT.2 <SEP> '() <SEP> (1 <SEP> N < ) <SEP> 1 <SEP> ) <SEP> 1 <tb> <SEP> I <tb> Ex. <SEP> U <SEP> 79 <SEP> '15.6 <SEP> b <SEP> . <SEP> 1 <SEP> I <SEP> ! <tb> <SEP> E) <SEP> ,- <SEP> / <SEP> . <SEP> ' <SEP> 1 <SEP> 1 <SEP> so <SEP> ?s <SEP> ¯ <SEP> 9 <tb> The raws materials mit of Example @, which contained return fines having somewhat more SiO2 than the intended sintered ore product and part of which was crushed to @ 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. The raw material mix of Example 7 prepared by both lowering the amount of silica added to 0.7 wt% and decreasing its grain size to below 1 mm, and also by incorporating into the mix partially crushed iron ores which were relatively hign 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%. TABLE 19 unit : wt% EMI44.1 ii <SEP> I <tb> <SEP> I <tb> <SEP> t <SEP> i <tb> <SEP> fI <SEP> B <SEP> R <SEP> ^ <SEP> 1 <SEP> iX <SEP> 0.11 <SEP> 92 <SEP> '(, <SEP> I <SEP> i' <SEP> --i <SEP> O <SEP> . <SEP> * <tb> <SEP> I <tb> <SEP> .21 <SEP> CF <SEP> , <SEP> ;, <SEP> 611.97 <SEP> I <SEP> V.lll <SEP> 0¯.7i, <SEP> | <SEP> 1 <SEP> J1 <SEP> | <SEP> 0.1() <tb> <SEP> I <SEP> I <tb> <SEP> 5 <SEP> N. <SEP> ' > 3 <SEP> N <SEP> . <SEP> 0. <SEP> j <SEP> 57.51 <SEP> S <SEP> i)¯4¯4. <SEP> 0.00 <SEP> C) <SEP> 6A <tb> <SEP> I <SEP> I <tb> Flam. <SEP> N <SEP> t; <SEP> C <SEP> , <SEP> 0. <SEP> or <SEP> 0 <SEP> . <SEP> i <SEP> "'S <SEP> L,5 <SEP> 1 <SEP> I <SEP> -. <SEP> -' <tb> <SEP> 1 <SEP> 55 <SEP> (;I <SEP> (i() <SEP> I <SEP> C) <SEP> I <tb> <SEP> - <SEP> - <SEP> - <SEP> I <tb> <SEP> II. <SEP> -'5 <SEP> 5-' <SEP> 0.17,151.79 <SEP> 5.70 <SEP> & <SEP> 0.1St <SEP> 0 <SEP> 1)1,5. <tb> *1 Mineracoes Brasileiras Reunidas S.A. (Brazil) *2 Rio Doce common fine (Brazil) *3 Mt. Newman Min. Pty. Ltd. (Australia) *4 Hamersley (Australia) *5 Robe River (Australia) @ Combined water Table 20 unit : wt% EMI45.1 <tb> <SEP> T.Fe <SEP> FeO <SEP> Fe2O3 <SEP> SiO2 <SEP> Al2O3 <SEP> CaO <SEP> MgO <SEP> CW <tb> <SEP> scale <tb> <SEP> 72.84 <SEP> 67.18 <SEP> 29.48 <SEP> 0.73 <SEP> 0.23 <SEP> 0.09 <SEP> 0.59 <SEP> 0.05 <tb> (mill <SEP> scale) <tb> Table 21 unit wt% EMI45.2 <tb> <SEP> T.Fe <SEP> Fe2O3 <SEP> SiO2 <SEP> Al2O3 <SEP> CaO <SEP> MgO <SEP> CW <tb> silica <SEP> 0.94 <SEP> 1.34 <SEP> 93.00 <SEP> 1.92 <SEP> 0.19 <SEP> 0.45 <SEP> 0.94 <tb> Table 22 unit : : wtS EMI45.3 <tb> <SEP> T.Fe <SEP> SiO2 <SEP> Al2O3 <SEP> CaO <SEP> MgO <SEP> CW <tb> limestone <SEP> 0.17 <SEP> 0.65 <SEP> 0.08 <SEP> 54.66 <SEP> 0.56 <SEP> 0.14 <tb> Table 23 EMI45.4 <tb> <SEP> proportions <SEP> (wt%) <tb> <SEP> Ingredients <SEP> Com. <SEP> Ex. <SEP> 3 <SEP> I <SEP> <SEP> Ex. <SEP> 6 <SEP> t <SEP> <SEP> Ex. <SEP> 7 <tb> <SEP> description <SEP> new <SEP> new <SEP> new <tb> total <SEP> total <SEP> total <tb> <SEP> material <SEP> material <SEP> material <tb> <SEP> M <SEP> B <SEP> R <SEP> 30 <SEP> 45 <SEP> 37 <tb> <SEP> Rio <SEP> Doce <tb> <SEP> 15.5 <SEP> 0 <SEP> 15.3 <tb> CF <tb> <SEP> Mt. <SEP> Newman <SEP> 0 <SEP> 25.5 <SEP> 20 <tb> <SEP> Hamersley <SEP> 25 <SEP> 0 <SEP> 0 <tb> Robe <SEP> River <SEP> 15 <SEP> 77. <SEP> 3 <SEP> 15 <SEP> 77. <SEP> 3 <SEP> 15 <SEP> 77.3 <SEP> <tb> <SEP> scale <SEP> 3 <SEP> 3 <SEP> 3-' <SEP> <tb> <SEP> silica <SEP> 1.4 <SEP> 1.4 <SEP> 0.7 <tb> <SEP> limestone <SEP> 10.1 <SEP> 10.1 <SEP> 9.0 <tb> tot.of <SEP> new <tb> <SEP> 100 <SEP> 100 <SEP> 100 <tb> materials <tb> <SEP> return <SEP> fines <SEP> 19.3 <SEP> 19.3 <SEP> 19.3 <tb> <SEP> coke <SEP> breeze <SEP> 3.4 <SEP> 3.4 <SEP> 3.4 <tb> Table 24 Com. Ex. 3 EMI46.1 <SEP> total <SEP> in <SEP> raw <SEP> sintered <tb> <SEP> # <SEP> 1 <SEP> mm <SEP> <SEP> < <SEP> 1 <SEP> mm <tb> <SEP> material <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> <SEP> 59.6 <SEP> 40.4 <SEP> 100.0 <SEP> <SEP> weight <SEP> (%) <tb> <SEP> SiO2 <SEP> (%) <SEP> 2.51 <SEP> 2.29 <SEP> 4.80 <SEP> 5.40 <tb> <SEP> CaO <SEP> (%) <SEP> 3.95 <SEP> 2.53 <SEP> 6.48 <SEP> 7.29 <tb> <SEP> Al2O3 <SEP> (%) <SEP> 0.93 <SEP> 0.65 <SEP> 1.60 <SEP> 1.80 <tb> CaO/SiO2 <SEP> (%) <SEP> 1.57 <SEP> 1.10 <SEP> 1.35 <SEP> 1.35 <tb> <SEP> SiO2 <SEP> - <SEP> Al2O3 <tb> <SEP> - <SEP> 1.64 <SEP> - <SEP> <SEP> - <SEP> 1.54 <SEP> - <SEP> SiO2 <SEP> - <SEP> Al2O3 <tb> Table 25 Ex. 6 EMI46.2 <SEP> total <SEP> in <SEP> raw <SEP> sintered <tb> <SEP> # <SEP> <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <tb> <SEP> material <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> <SEP> 52.5 <SEP> 47.5 <SEP> 100.0 <SEP> <SEP> weight <SEP> (%) <tb> SiO2 <SEP> (%) <SEP> 1.86 <SEP> 2.94 <SEP> 4.80 <SEP> 5.40 <tb> <SEP> CaO <SEP> (%) <SEP> 2.72 <SEP> 3.77 <SEP> 6.48 <SEP> 7.29 <tb> <SEP> Al2O3 <SEP> (%) <SEP> 0.69 <SEP> 0.91 <SEP> 1.60 <SEP> 1.80 <tb> CaO/SiO2 <SEP> (%) <SEP> .46 <SEP> @.28 <SEP> 1.35 <SEP> 1.35 <tb> <SEP> SiO2 <SEP> - <SEP> Al2O3 <tb> <SEP> - <SEP> 2.03 <SEP> - <SEP> <SEP> CaO <tb> - <SEP> 1.85 <SEP> - <SEP> <SEP> (SiO2 <SEP> - <SEP> Al2O3) <tb> Table 26 Ex. 7 EMI46.3 <tb> <SEP> total <SEP> in <SEP> raw <SEP> sintered <tb> # <SEP> 1 <SEP> mm <SEP> < <SEP> 1 <SEP> mm <tb> <SEP> material <SEP> ore <tb> <SEP> Proportion <SEP> by <tb> <SEP> 56.9 <SEP> 43.1 <SEP> 100.0 <SEP> <tb> <SEP> weight <SEP> (%) <tb> <SEP> SiO2 <SEP> (%) <SEP> 1.97 <SEP> 2.53 <SEP> 4.5 <SEP> 5.00 <tb> <SEP> CaO <SEP> (%) <SEP> 3.72 <SEP> 2.36 <SEP> 6.08 <SEP> 6.75 <tb> <SEP> Al2O3 <SEP> (%) <SEP> 0.92 <SEP> 0.70 <SEP> 1.62 <SEP> 1.80 <tb> <SEP> CaO/SiO2 <SEP> (%) <SEP> 1.89 <SEP> 0.93 <SEP> 1.35 <SEP> 1.35 <tb> <SEP> SiO2 <SEP> - <SEP> Al2O3 <tb> - <SEP> 1.83 <SEP> - <SEP> <SEP> (%) <tb> <SEP> - <SEP> 1.29 <SEP> - <SEP> (SiO2 <SEP> - <SEP> Al2O3) <tb> As described in the foregoing, the produce of this invention is comparable with, or even superior to, tiie conventional sintering technique with respect to productivity, coke consumption, shatter index and other factors, while it can lower the SiO" content of the sintcrcd 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 blust furnace, yielding an appreciable decrease in tile blast furnace fucl consumption.

Claims: CLAIMS 1. A process for producing a self-fluxing sintered ore using a raw materials mix comprising iron, orc, 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 5.11 wt% of SiO, in terms of "sintered ore product" (said sintered ore product being a product from which coke, combined COO and combined water have been removed by sintering of the mix), said process being characterized by controlling the SiO" content of sai 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 therms of "sintered ore product" (as defined in Claim 1 above), said process being char- acterized by controlling the SiO2 content of said fine grains to be between 2.4 and 5.0 wt, of tile raw materials mix (dry) and the basicity (CaO/SiOO ratio) of said fine grains to be not more than 1.5:1, and sintcring the thus controlled mix. 3. A process according to Claim 2, wherein the sum of the CaO and SiO, contained in said fine grains is at least 4.0 wt% of said mix. 4. A process according to Claim 2 or Claim 5, wherein the SiO2 content of said fine grains minus the Al2O3 content of said fine grains is betwenn 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 betwenn 0.5:1 and 2.0:1. 5. A process according to Claim 4, wherein the weight ratio of CaO to (SiO,, - Al2O3) contained in said fine grainsm is betwenn 1.0:1 ans 1.8:1. G. A composition consisting of a raw materials niis for producing a self-fluxing, sintered ore said mix comprising iron orc, 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 mix), said mix being characterized by an SiO2 content of said fine grains of at least 50% of the total SiO" content of said mix 7. A composition consisting of a rawmaterials mix der producing a self-fluxing, sintered ore, said mix comprising iron ore, limestone, silica, coke and return fines, at least 25 wt% of said mix cons. sting of fine grains below 1 mm in size, and not more than 3.4 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 betwenn 2.4 and 3.0 wt% of said mix (dry), and the basicity (CaO/SiO2 1-atio) of said fine grains being not more than 1.5:1. 8. 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 25% of tlic orc, characterized in that said flue grains have a basicity (CaO/SiO2 ratio) below 1.3:1.

Inventors: HOSOTANI, YOHZOH, SOUMA, HIDEAKI, TAKADA, TSUKASA, TASHIRO, KIYOSHI, WAJIMA, MASAMI
Applicants: NIPPON STEEL CORPORATION
Application year: 1980
Main classification: C22B1
IPC codes: C22B1