Method for producing composite oxides for use ad starting materials for ferrites

Abstract: A method for producing an oxide useful as a starting material for soft ferrites comprised mainly of iron, zinc and at least one metal selected from manganese, magnesium, nickel and copper is described. In the method, while chlorides of metals other than zinc are roasted for oxidation to produce a composite oxide, zinc oxide is added during the roasting. This method ensures addition of zinc oxide at high yield without use of zinc chloride having a high vapor pressure. The resultant oxide for ferrites has desired ratios of metal constituents and contains spinel ferrites which are intermediate products with zinc oxide being uniformly dispersed. The use of the starting oxide leads to omission of a calcination step and a powder mixing step in the production process of ferrites. Ferrites with good quality can be obtained by direct sintering. An alternating method using zinc chloride instead of zinc oxide is also described which can provide an equipment oxide for ferrites.

Description: This invention relates to a method for producing ferrites and more particularly, to an improved method for producing composite oxides used as starting materials for high-quality ferrites wherein chlorides or oxides of metals constituting soft ferrites are provided as a starting material. In general, the industrial method for production of ferrites comprises mixing compounds, such as oxides or carbonates of metals such as iron, manganese, magnesium, nickel, copper, zinc and the like used to constitute ferrites, at predetermined molar ratios, subjecting the mixture to calcination (preliminary sintering), pulverizing, granulating and molding, and finally sintering at high temperatures. However, this method has the problem that it requires a calcination step wherein relatively high temperature and long time are used, which has influences on the quality of the product such as the incorporation of impurities during the production process, non-uniformity in quality when viewed microscopically, and the need for a long production time and a great amount of energy. In general, the iron oxides used as starting material and forming a major proportion of the composition for ferrite are obtained by subjecting hydrochloric acid pickling waste liquors generated in the hot-rolling process of steel sheets or coils to high temperature oxidation roasting by a spray roasting furnace. After mixing, these metal oxides are heated at the time of the calcination or sintering and reacted to finally form a single phase having a spinel structure through an intermediate formation phase such as of ZnFe 2 O 4 , ZnMn 2 O 4 , MnFe 2 O 4 or the like. In the above-stated series of production steps for the ferrite, the uniform dispersion of the respective oxides by mixing greatly influences the magnetic characteristics of a final ferrite product. Accordingly, an improved method has been demanded, while taking an economy into consideration. With respect to the phase change at the time of sintering, the intermediate formation phases such as ZnFe 2 O 4 and MnFe 2 O 4 are volumetrically expanded at 1.5 to 2% and 0.3 to 0.5%, respectively, during their formation. Accordingly, if the calcination step is omitted, a press molding is imparted with cyclic stress owing to expansion and subsequent shrinkage during the sintering. As a result, the sintered products finally have a reduced sintering density, involve defects such as deformation and cracks, and as a result suffer deterioration in the accuracy of their dimensions and their strength. An improved method for producing ferrites over the prior techniques has been proposed in which a mixed solution of chlorides of metals constituting a ferrite is used as a starting material and roasted for oxidation (Japanese Patent Publication No. 47-11550). However, when all the starting materials are used in the form of metal chlorides, a metal chloride, such as zinc chloride, having a high vapor pressure is vaporized during roasting. This presents the problem that such a metal cannot reach the intended concentration in a product obtained from the bottom of the roasting furnace. To avoid this, Japanese Patent Publication No. 63-17776 proposes a method wherein, among metals constituting a ferrite, only metals whose chlorides are low in vapor pressure are provided as a mixed solution of the chlorides. This mixed solution is roasted for oxidation to obtain mixed oxides and/or partially reacted composite oxide. The oxides thus obtained are then mechanically mixed with oxides of metals whose chlorides have high vapor pressures. The mixture is directly sintered, without calcination, to obtain a ferrite. In this technique, in the case of the ingredient Zn, its oxide having a size of from 0.1 to 1 µm has to be mechanically mixed in a subsequent step. This leads to non-uniformity in the composition of the mixture, causing the magnetic characteristics of a final product to be degraded. This method is pronouncedly improved over the method using the respective powdered oxides as starting materials, but the improvement is not satisfactory with respect to uniform dispersion. As stated above, in the known methods for producing composite oxides used as starting materials for soft ferrites, metal chlorides, such as that of Zn, having a high vapor pressure cannot be roasted in a high temperature roasting step simultaneously with other metal chlorides. After the roasting, an additional mechanical mixing step is required, thus leaving room for a further improvement with respect to the uniform dispersion or the incorporation of impurities. An object of the invention is to provide a method for producing directly sinterable starting materials for soft ferrites such as Fe-(Mn, Mg, Ni)-Zn ferrites wherein main metal elements constituting the soft ferrite are roasted in the same roasting furnace at the same time. A powdery composite oxide is collected from the bottom of the furnace and subsequent steps of calcination and powder mixing can be omitted in contrast with ordinary production processes. Another object of the invention is to provide a method wherein uniform dispersion of zinc is ensured, and the resultant ferrite product (such as a ferrite core) may have improved characteristic properties such as the magnetic and electrical characteristics. A further object of the invention is to provide a method for producing ferrite products wherein an intermediate phase (as will be conventionally produced in a calcination and a sintering steps) is preliminarily produced so that the volumetric expansion ascribed to the formation of the intermediate phase at the time of sintering without calcination is suppressed. In this way a ferrite product is finally obtained which has a high density and a high dimensional accuracy, without involving any deformation or cracking. A still further object of the invention is to provide a method wherein powdery composite oxides for use as starting materials for high-quality ferrites are produced efficiently in an economical manner. In order to achieve the above objects, the present inventors made intensive studies on a method for producing composite oxides useful as starting materials for ferrites inexpensively and efficiently and, as a result, found that the problems involved in the volatilization at the time of roasting of ingredients having a high vapor pressure, such as chlorides of metals including zinc, and the problems of non-uniformity at the time of mechanical mixing of additives and poor economy could be solved. The present invention is accomplished based on the above finding. According to the present invention there is provided a method for producing a starting oxide for soft ferrites which comprises iron, zinc and at least one metal selected from manganese, magnesium, nickel and copper, the method comprising adjusting a mixed chloride aqueous solution of iron chloride and at least one chloride selected from the chlorides of manganese, magnesium, nickel and copper to a pH of from 0.2 to 7.0; adding zinc oxide and/or a zinc compound capable of ready conversion into zinc oxide to the pH adjusted mixed chloride aqueous solution to obtain a suspension; and feeding quickly the suspension to a furnace and roasting for oxidation at high temperatures to obtain an oxide useful as a starting material for soft ferrites. More particularly, the present invention provides a method for producing oxides used as starting materials for soft ferrites containing as main components iron, zinc and at least one metal selected from the group consisting of manganese, magnesium, nickel and copper. The pH of a mixed aqueous solution of the iron chloride and the at least one member selected from chlorides of manganese, nickel, magnesium and copper may be adjusted to a value in the range of from 0.2 to 7.0. Thereafter, zinc oxide and/or a zinc compound readily convertible into zinc oxide is dispersed in the mixed solution and oxidized by roasting. For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only to the accompanying drawings, wherein: Fig. 1 is a graphical representation of the relation between the residual amount of chlorine and the temperature; Fig. 2 is a flow chart showing a production process of a starting composite oxide for Mn-Zn ferrite according to the invention; Fig. 3 is a schematic view of an oxidation roasting furnace for starting oxides for soft ferrites according to the invention; Figs. 4 and 5 are, respectively, flow charts of prior art production processes of soft ferrites. Most industrially mass-produced soft ferrites are generally composed of about 70 wt% of Fe 2 O 3 , 15 to 30 wt% of M 2+ O wherein M represents Mn, Mg, Ni or the like, and 0 to 15 wt% of ZnO. Of these, Zn exhibits a high vapor pressure when used as a chloride. When zinc chloride is spray roasted along with other metals at the same temperature level, a large amount of zinc chloride is exhausted from the top of a furnace, with a considerable lowering in yield of zinc in the oxide collected from the bottom of the furnace. As shown in Fig. 4, in the conventional production of Mn-Zn ferrites, iron oxide, manganese oxide and zinc oxide are mixed at predetermined molar ratios, calcined at 800 to 1000°C, pulverized, granulated, molded and sintered. In this method, uniform mixing is not satisfactorily attained and the resultant product does not have good magnetic characteristics. In addition, calcination at 800 to 1000°C is necessary, leading to high production costs and contamination with impurities at the time of pulverizing. Fig. 5 shows an improvement of the above method wherein iron chloride and manganese chloride are used as starting materials for roasting. However, zinc chloride is not roasted for oxidation at the same time, such a step of mixing with zinc oxide is essentially included, thus presenting a problem on non-uniformity. The dechlorination and the promotion of the spinel formation reaction of the Starting oxides are described. Aqueous solutions of FeCl 2 and MnCl 2 were mixed at a predetermined molar ratio, after which the mixture was spray roasted at 800°C. The resultant oxide composition had a composition of Fe 2 O 3 :Mn 2 O 3 = 75.3:24.7 (wt%) and a residual chlorine content was 0.30%. The mixed oxides was heated in a stream of air at a temperature ranging from 200 to 1000°C for 40 minutes. The variation in the residual amount of chlorine is shown in Fig. 1. In order to reduce the residual amount of chlorine to not greater than 0.20 wt%, the lower limit of the treating temperature should be 300°C. On the other hand, over 1000°C, it is unfavorable that sintering of the powder proceeds excessively and it takes a long time for a subsequent pulverizing step. The results of an experiment in which zinc oxide is mixed with an aqueous solution of a chloride of a metal (such as iron, manganese, nickel, magnesium and the like) followed by roasting the mixture of ingredients at the same time is described below Guaranteed hydrochloric acid reagent was dissolved in hot pure water at 80°C to prepare hydrochloric acid aqueous solutions having the following values of pH. Sample 1: pH = -0 Sample 2: pH = 0.1 Sample 3: pH = 0.2 Sample 4: pH = 0.4 Sample 5: pH = 1.0 sample 6: pH = 2.0 400 g of guaranteed zinc oxide (ZnO) was added to one liter of each solution (kept at 80°C) and agitated by means of a stirrer, followed by filtration and a determination of the dependence on acidity of the amount of dissolved zinc oxide in the respective pH solutions from the amount of remaining zinc oxide. When the pH exceeds 0.2, inclusive, the amount of zinc oxide dissolved in the acidic solution is significantly reduced. More particularly, when the pH value is not less than 0.2, the amount of the dissolved zinc oxide is found to be not larger than 5 g/liter. Accordingly the adjustment of the pH of the chloride solution to not less than 0.2 makes it possible to roast for oxidation the chloride solution admixed with zinc oxide without significant dissolution of zinc oxide in the chloride solution. With regard to the upper limit of the pH, when the pH is larger than 7.0, metal ions form hydroxides and settle. Accordingly, the pH is preferably not larger than 7.0. In Fig. 2 an aqueous solution of mixed chlorides of iron and other metals such as manganese, magnesium, nickel and copper is provided as a starting material. The free hydrochloric acid in the chloride solution is neutralized, and to the neutralized solution a powder and/or an aqueous slurry of zinc oxide is added to obtain a slurry. The thus obtained slurry is immediately fed to a spray roasting furnace before any metal substitution reaction of Zn with metals, such as Fe and Ni, which have an ionization tendency smaller than Zn) can occur and simultaneously roasted for oxidation. An aqueous solution 11 of mixed chlorides of Fe and Mn is neutralized in a neutralization vessel 12 by means of a neutralizing agent 13 to a pH of not less than 0.2, preferably not less than 0.7. When zinc oxide 15 is added into a mixer 16 at a pH of less than 0.2, a large amount of zinc oxide is converted into a corresponding chloride. In this case, when roasted in a spray roasting furnace 18, zinc chloride (which has a high vapor pressure) is volatilized during the roasting, so there is the problem that the content of zinc in a product 21 collected from the bottom of the roasting furnace 18 is lower than the intended concentration. A neutralized or pH-adjusted chloride mixed solution 14 is subsequently introduced into a mixer 16, to which a powder or an aqueous slurry of zinc oxide is added, thereby obtaining a zinc oxide slurry 17 of the aqueous solution of the Fe and Mn mixed chlorides. The zinc oxide 15 fed to the mixer 16 is preferably used in the form of a powder so that the concentration of the mixed chloride aqueous solution is not lowered. Of course, zinc oxide may be fed in the form of an aqueous slurry. With the aqueous slurry, the concentration should preferably be as high as possible so that the amount of fuel in the roasting furnace 18 is saved. The zinc oxide slurry 17 of the Fe and Mn mixed chloride aqueous solution is immediately fed to the spray roasting furnace 18, before any metal substitution reaction between zinc suspended in the slurry and metals having a smaller ionization tendency (Fe in the case of Mn-Fe ferrites) can proceed, and is roasted to ensure oxidation at the same time. The zinc oxide slurry 17 should preferably be fed to the roasting furnace 18 within 30 minutes, preferably within 5 minutes, after the mixing. The zinc oxide slurry 17 of the Fe and Mn mixed chloride aqueous solution is sprayed in the spray roasting furnace 18 and roasted for oxidation under high temperature conditions of 850 to 900°C by combustion of fuel 19 supplied to the roasting furnace 18. During the course of the roasting for oxidation, Fe and Mn undergo the spinel reaction with zinc oxide, thereby efficiently producing a very uniform reaction product (an intermediate to a spinel single phase) such as ZnFe 2 O 4 or ZnMn 2 O 4 . Combustion exhaust gas, hydrogen chloride and evaporated water vapor 20 are exhausted from the top of the roasting furnace 18. To the thus obtained composite oxide powder 21 are added trace additives 22 such as SiO 2 , CaCO 3 and the like, followed by mixing, molding and direct high temperature sintering in a sintering furnace 23. A ferrite sintered core product 24 which has good magnetic characteristics and a high dimensional accuracy is thereby obtained without any deformation and cracking. As described above, in the production of a powdery composite oxide by mixing a solution of a chloride of iron (iron being a main constituent element for ferrites) and a solution of manganese chloride and spray roasting the mixture, the free hydrochloric acid in the Fe and Mn chloride aqueous solutions is neutralized. Then, zinc oxide is suspended in the neutralized chloride aqueous solution to obtain a slurry. Immediately, the slurry is fed to the roasting furnace and spray roasted therein to ensure homogeneous mixing, oxidation and spinel conversion of Fe-Mn-Zn by the single roasting step, while omitting mechanical mixing and calcination of zinc-containing starting material as is performed in ordinary processes. The thus obtained soft ferrite product has good magnetic characteristics and dimensional accuracy. The present invention will now be illustrated by way of an example. Example 1 3000 liters of a hydrochloric acid pickling waste liquor of steel sheets which had been concentrated to an extent that 25 g of iron was contained in each 100 ml was charged with 250 kg of metallic manganese, followed by heating to 80°C to completely dissolve the metallic manganese. The solution had a pH of not larger than 0.1. Ammonia was added to the solution in a storage tank 33 in Fig. 3 so that the pH was adjusted to 4.0, followed by passage to an agitation vessel 35 by means of a feed pump 34. On the other hand, 125 kg of zinc oxide was added from the storage tank 32 to the agitation vessel 35 through the feed pump 34 and sufficiently agitated and mixed. The mixture was then sprayed from a top of a spray roasting furnace 31 kept at 810°C by a spraying device 36 and a spraying nozzle 37, so that the solution was oxidized and roasted to obtain a product 39 at the bottom of the furnace. X-ray diffraction revealed that the resulting oxides consisted of Fe 2 O 3 , Mn 2 O 3 , spinel and a small amount of ZnO, and that no chloride was detected. The composition of Fe, Mn and Zn was determined by chemical analysis so that the total of Fe, Mn and Zn was 100 wt%. The deviations in the composition of Fe, Mn and Zn are as shown in Table 1, from which it will be seen that as compared with expected concentrations prior to the spray roasting, the oxides for soft ferrites are only slightly different. 1000 g of the starting oxides were weighed, to which 0.1 wt% of CaCO 3 and 0.01 wt% of SiO 2 were added, followed by further addition of pure water to obtain a slurry concentration of 50%. The slurry was mixed in a rotary ball mill and dried, to which PVA used as a binder was added. After granulation, a troidal core having an outer diameter of 36 mm, an inner diameter of 24 mm and a height of 10 mm was molded and sintered at 1340°C in an atmosphere of nitrogen containing 1% of oxygen. The resultant sintered core was subjected to measurement of core loss under conditions of 100 kHz, 200 mT and 100°C, and had a good magnetic characteristic of 280 mW/cm 3 as shown in Table 2. Comparative Example 1 3000 liters of a hydrochloric acid pickling waste liquor of steel sheets which had been concentrated to an extent that 25 g of iron was contained in each 100 ml was charged with 250 kg of metallic manganese, followed by heating to 80°C to completely dissolve the metallic manganese therein. After mixing 125 kg of zinc oxide, the mixture was sprayed from the top of a spray roasting furnace and roasted for oxidation to obtain a roasted product at the bottom of the furnace in the same manner as in Example 1. X-ray diffraction revealed that the resulting oxides consisted of Fe 2 O 3 , Mn 2 O 3 and spinel and that no chloride was detected. By chemical analysis, the composition of Fe, Mn and Zn was determined so that the total of Fe, Mn and Zn was 100 wt%. The deviations in the composition of Fe, Mn and Zn are as shown in Table 1, from which it will be seen that as compared with expected concentrations prior to the spray roasting, the Zn is significantly reduced with relative increases in amount of iron and manganese. 1000 g of the starting oxides were weighed, and ZnO was added in an amount to give the same content as in Example 1, 0.1 wt% of CaCO 3 and 0.01 wt% of SiO 2 were added, followed by further addition of pure water to obtain a slurry concentration of 50%. The slurry was mixed in a rotary ball mill and dried, and PVA was added as a binder. After granulation, a troidal core having an outer diameter of 36 mm, an inner diameter of 24 mm and a height of 10 mm was molded and sintered at 1340°C in an atmosphere of nitrogen containing 1% of oxygen. The resultant sintered cores were subjected to measurement of core loss under conditions of 100 kHz, 200 mT and 100°C, and had a magnetic characteristic of 1800 mW/cm 3 as shown in Table 2. The results of Example 1 and Comparative Example 1, show that the composition of zinc does not deviate from the intended composition in the present invention. This ensures formation of the starting oxides of iron, manganese and zinc for soft ferrites having good dispersability, with the possibility that articles having good magnetic characteristics can be fabricated. In Example 1, a method for producing Mn-Zn ferrites from a solution containing iron and manganese chlorides and zinc oxide has been described. The method of the present invention is readily applicable to chloride solutions containing nickel and magnesium instead of manganese. To adjust the pH ammonia has been added but the manner of the adjustment is not critical provided that the pH can be adjusted to an intended level. Analytical Results Fe Mn Zn Formulated values 68.2 22.4 9.1 Composition After Roasting in Example 1 68.9 23.0 8.1 Composition After Roasting in Comp. Ex. 1 74.9 24.8 0.3 Core Loss (100 KHz, 200 mT, 100°C) Example 1 280 mW/cm 3 Comparative Example 1 1800 mW/cm 3

Claims: A method for producing a starting oxide for soft ferrites which comprises iron, zinc and at least one metal selected from manganese, magnesium, nickel and copper, the method comprising adjusting a mixed chloride aqueous solution of iron chloride and at least one chloride selected from the chlorides of manganese, magnesium, nickel and copper to a pH of from 0.2 to 7.0; adding zinc oxide and/or a zinc compound capable of ready conversion into zinc oxide to the pH adjusted mixed chloride aqueous solution to obtain a suspension; and feeding quickly the suspension to a furnace and roasting for oxidation at high temperatures to obtain an oxide useful as a starting material for soft ferrites. A method according to claim 1 wherein the zinc oxide and/or the zinc compound capable of ready conversion into zinc oxide is added in the form of a powder or a slurry. A method according to claim 1 or 2 wherein said oxide is kept at a temperature of from 300 to 1000°C whereby the content of chlorine in the finally obtained oxide is reduced while promoting formation of a spinel phase.

Inventors: NARUTANI TETSU, TAKAKI KATASHI, TSUZAKI MASAO, YAMAZAKI MASAKATSU, YOHIKAWA FUMIAKI, YOSHIMATSU HIDEKAKU, NARUTANI, TETSU, TAKAKI, KATASHI, TSUZAKI, MASAO, YAMAZAKI, MASAKATSU, YOHIKAWA, FUMIAKI, YOSHIMATSU, HIDEKAKU, NARUTANI, TETSU, C/O RESEARCH LABORATORIES OF, TAKAKI, KATASHI, C/O MIZUSHIMA WORKS OF, TSUZAKI, MASAO, C/O TOKYO MAIN OFFICES OF, YAMAZAKI, MASAKATSU, C/O RESEARCH LABORATORIES OF, YOHIKAWA, FUMIAKI, C/O TOKYO MAIN OFFICES OF, YOSHIMATSU, HIDEKAKU, C/O RESEARCH LABORATORIES OF
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1990
Main classification: C01G49
IPC codes: C01B13, C01G49