# Patent

## Title
Method of producing ferrite magnet

## Abstract

A magnetic oxide obtained by mixing a given amount ranging from 0.20 to 76.0% by weight of at least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powder substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550.degree. to 1500.degree. C., and a process for producing the same.

## Background
The present invention relates to a magnetic oxide suitable for use in a wide range of technical fields, e.g., those of toners and carriers for electrophotography, electric resistance elements and magnetic fluids, which magnetic oxide is produced by mixing a magnetization control material of Ca, Ti, Sn, Si or Mg with hematite, hematite+magnetite, or magnetite to obtain a mixed powder and sintering the mixed powder so that it exhibits a given saturation magnetization. Also, the present invention relates to a process for producing the above magnetic oxide.

A single phase magnetite powder as one of the magnetic oxides has been utilized in a wide range of technical fields, e.g., those of magnetic fluids, electric resistance elements and toners and carriers for electrophotography. It is highly desired in the art to mass-produce a single phase magnetite powder exhibiting any predetermined saturation magnetization at a lowered cost.

The following three general methods are known for producing such a magnetite, and it is know that they have respective drawbacks as described below. alkaline and coprecipitated. production, although fine particles each having a relatively high purity are readily obtained by coprecipitation. or steam to thereby be reduced. atmosphere, e.g., either high pressure steam or hydrogen and carbon monoxide, so that danger accompanies the mass production, and that large scale equipment is required. difficult to stably produce fine quality powder for a prolonged period of time.

The magnetite powders produced by the above conventional processes have a drawback in that their saturation magnetization values are so high as compared with those of the conventional spinel ferrite that it is unfeasible to attain control of the saturation magnetization by composition regulation with the result that the conventional magnetite powders cannot be employed in use in which the saturation magnetization values inherent in magnetite powder are unacceptable. Magnetite powder having a saturation magnetization value inherent therein (fixed value specified in the second experimental example described later, e.g., up to 92 emu/g) has only partly been employed in use in carriers for electrophotographic development and the like. Moreover, the above magnetite powder having the saturation magnetization value inherent therein as it is cannot replace ferrite or the like in uses in which it has been employed, and, when the replacement is to be made, it is required to alter the circuit or device to be employed disadvantageously.

Taking into account the advantages and disadvantages of the above processes, various proposals have been made with respect to the production of magnetite powder for use in magnetic toners for electrophotography, etc.

For example, Japanese Patent Publication Nos. 238,580/1987, 39,498/1990 and 51,505/1990 disclose processes for producing magnetite useful as a carrier component of an electrophotographic developer, especially a two-component developer composed of a toner and a carrier. Each of these processes comprises providing magnetite powder (or globular magnetite particles) as a starting material, granulating the same into globular grains through, for example, mixing with a binder, followed by heating (sintering), and applying a resin coating to the resultant globular magnetite particles according to the conventional technique.

The use of a material called soft ferrite as carrier particles is known (U.S. Pat. No. 3,929,657, etc.). In this connection, it is known that the carrier particles each composed of the above ferrite not only are excellent in magnetic properties but also do not require a resin coating layer, so that their durability is excellent. Nothing that ferrite particles have resistance variations depending on the control of sintering atmosphere, even with the same composition, attempts have been made to enlarge resistance variation ranges by changing the sintering atmosphere. As an example thereof, there can be mentioned an attempt described in Japanese Patent Publication No. 37782/1987. This comprises providing an Mg ferrite as a starting material and increasing the amount of ferric oxide (Fe.sub.2 O.sub.3) to more that 53% by mole so as to enlarge its resistance variation range.

The Inventor's primary object, in view of the above prior art, is to easily mass-produce a magnetic oxide, in particular, a magnetic oxide exhibiting any desired saturation magnetization at a lowered cost by mixing a magnetization control material of Ca, Ti, Sn, Si or Mg with hematite, hematite+magnetite, or magnetite to obtain a mixed powder, further mixing a substance having a carbon-to-carbon single or double bond with the mixed powder and sintering the obtained mixture.

Another object of the present invention is to provide a process for mass-producing the above magnetic oxide of fine quality at a lowered cost and safely by simple equipment and operation, as compared with the above conventional processes.

A further object of the present invention is to provide a process for mass-producing the above magnetic oxide having any desired saturation magnetization value.

According to the present invention, there is provided a magnetic oxide obtained by mixing a given amount ranging from 0.20 to 76.0% by weight of at least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, hematite+magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powdery substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550.degree. to 1500.degree. C., and also provided is a process for producing the same.

In the above magnetic oxide and the process for producing the same, preferred conditions for embodying the present invention are as set forth below with respect to each magnetization control material.

In the employment of Ca compound

(1) Starting material: hematite or hematite+magnetite

(2) Starting material: magnetite

In the employment of Ti compound

(3) Starting material: hematite+magnetite 1200.degree. to 1450.degree. C.

(4) Magnetite: 47.3 to 98.8% by weight

In the employment of Sn compound

(5) Hematite or hematite+magnetite: 24.0 to 99.2% by weight

(6) Magnetite: 24.0 to 99.2% by weight

In the employment of Si compound

(7) Hematite (or hematite+magnetite): 35 to 99.5% by weight

(8) Magnetite: 35 to 99.5% by weight

In the employment of Mg compound

(9) Starting compound: hematite (or hematite+magnetite)

(10) Starting compound: magnetite

(11) Starting compound: hematite (or hematite+magnetite)

(12) Starting compound: magnetite

FIG. 1 is an explanatory view of the steps taken in the first embodiment of the present invention;

FIG. 2 is a view of an exemplary heating curve suitable for sintering in the first embodiment;

FIG. 3 is an explanatory view of the saturation magnetization referred to in the present invention;

FIG. 4 is an explanatory view of the steps taken in the second embodiment of the present invention;

FIG. 5 is a view of an exemplary heating-cooling curve suitable for the second embodiment of the present invention;

FIG. 6 is an explanatory view of the steps taken in the third embodiment of the present invention;

FIG. 7 is a view of an exemplary heating-cooling curve suitable for the third embodiment of the present invention;

FIG. 8 is an explanatory view of the steps taken in the fourth embodiment of the present invention;

FIG. 9 is a view of an exemplary heating-cooling curve suitable for the fourth embodiment of the present invention;

FIG. 10 is an explanatory view of the steps taken in the fifth embodiment of the present invention;

FIGS. 11(a), 11(b) and 11(c) are explanatory view of the ranges of saturation magnetization control attained in the fifth embodiment of the present invention;

FIGS. 12(a) and 12(b) are explanatory views of the grain strengths attained in the fifth embodiment of the present invention; and

FIG. 13 is a view of an exemplary heating curve suitable for the fifth embodiment of the present invention.

Preferred embodiments of the present invention will now be described with reference to the following Experimental Examples.

First Embodiment

Referring to FIG. 1, hematite, hematite+magnetite, or magnetite is blended with 1.43 to 64.0% by weight of Ca to form a mixed powder in the blending step 1. The term "Ca" is employed herein for simplification of the description of the specification, and means the Ca moiety, expressed in % by weight, of organic and inorganic compounds each containing Ca, such as CaO, CaCO.sub.3, Ca(OH).sub.2, CaCl.sub.2 and CaF.sub.2. The sum of the weight percentage of hematite, hematite+magnetite, or magnetite and the 1.43 to 64.0% by weight of this Ca is 100% by weight (that is, for example, the weight percentage of the "O" moiety of CaO is not contained n the value of 100% by weight). The terminology "hematite+magnetite" employed herein means a mixture (mixed powder) of hematite and magnetite. In particular, the magnetite as a starting material may be either of: companies), and off-specification particle size (recovered item) so as for the same to have the given particle size.

The product grains described later are those each composed of, for example, 10.sup.3 to 10.sup.7 starting powder particles (1-3 .mu.m) gathered into a globular form having a diameter of 50 to 100 .mu.m. Thus, any once produced magnetite powder product (off-specification item) can be pulverized to thereby easily obtain the desired starting material powder.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0% by weight of a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid substance). For example, 2% by weight of PVA (polyvinyl alcohol) as a binder and 1% by weight of a polycarboxylate as a dispersant are added to hematite powder as a starting material, and further water is added for granulation into globular grains. The amount of waster added is in the range of 30 to 70% by weight. When the amount of water was less that 30% by weight, the viscosity of a slurry obtained by milling was too high to be granulated into globular grains. On the other hand, when the amount of waster exceeded 70% by weight, the slurry concentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 is wetly crushed in an attrition mill to thereby prepare a slurry having a mixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, the slurry is agitated in an attrition mill for 1 hr, and subjected to hot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4 are heated at 1200.degree. to 1450.degree. C., provided that those of magnetite may be heated at 550.degree. to 1450.degree. C., in an atmosphere of an inert gas such as nitrogen gas for 2 hr to thereby obtain powder in which single phase magnetite and another magnetic and/or nonmagnetic phase are coexistent. The value of saturation magnetization of the powder can be regulated by the amount of Ca added, so that it is feasible to produce a powdery magnetic oxide having any desired saturation magnetization by changing the amount of Ca added (see Tables 1 to 4). When hematite is present in part of the hematite or magnetite powder, during the 1200.degree. to 1450.degree. C. sintering step 5, not only is the thermal transition from hematite to magnetite effected in the inert gas (in weakly reducing atmosphere) but also the organic substance mixed is brought by the heating in the inert gas into the state of incomplete combustion, in which the hematite is deprived of oxygen during the thermal decomposition of the organic substance to thereby be reduced to markedly promote the conversion to magnetite. In particular, with respect to magnetite, a magnetic oxide having any desired saturation magnetization can be obtained by the sintering at temperatures as relatively low as 550.degree. to 1450.degree. C. This is because there is no need for newly making conversion into magnetite (reduction of hematite), so that the strength required for handling, etc. can be obtained by bonding or mild sintering of the starting material magnetite in the form of a vast plurality of grains.

In the disintegration step 6, the powder in which single phase magnetite and nonmagnetic phase are coexistent, obtained in the sintering step 5, is crushed into a product. According to the above steps, hematite, hematite+magnetite, or magnetite is mixed with the above Ca to obtain a mixed powder, and this mixed powder is further mixed with a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid substance) and water, milled well, hot-air dried, granulated into globular grains and sintered in an inert gas at 1200.degree. to 1450.degree. C., provided that the sintering of magnetite is conducted at 550.degree. to 1450.degree. C., so that the powder in which magnetite and nonmagnetic phase are coexistent (magnetic oxide powder) can be produced. Thus, it has become feasible to safely mass-produce the magnetic oxide powder having any desired saturation magnetization at a lowered cost. Descriptions wilt be made in order below.

Table 1 shows sintering experiment results (hematite) of the present invention. In the experiment, Ca was blended with hematite powder in the amount specified in the Table, and mixed with water to obtain a slurry having a powder concentration of 50% by weight. The slurry was agitated in an attrition mill for 1 hr, and dried at 110.degree. C. 1.0% by weight of polyvinyl alcohol was added to the obtained powder. The mixture was mixed in a mortar, and sifted through a 425-.mu.m-mesh standard sieve to obtain grains. 0.5 g of the resultant grains were put in a cylindrical mold having a diameter of 12.5 mm, and shaped under a molding pressure of 1 t/cm.sup.2. The molding was heated at 1100.degree. to 1500.degree. C. for 2 hr in nitrogen gas. The saturation magnetization of each sample after the heating was measured by means of an vibrating sample magnetometer. single phase magnetite was effected, the saturation magnetization of each of the samples was 92 emu/g. magnetite was incomplete so as for hematite (.alpha.-Fe.sub.2 O.sub.3) to remain with the result that the saturation magnetization values of Sample Nos. 1 to 8 were 80 emu/g or less to show the inappropriateness of the heating temperature of 1100.degree. C. On the other hand, when the heating was conducted at 1500.degree. C., the conversion to magnetite was incomplete so as to form wustite (FeO) with the result that the saturation magnetization values of Sample Nos. 33 to 40 were as low as 87 emu/g or less to show the inappropriateness of the heating temperature of 1500.degree. C. Therefore, it has been found that the appropriate heating temperature is in the range of 1200.degree. to 1450.degree. C., the amount of Ca added was increased from 0% by weight. Sample Nos. 11, 19 and 27 of Table 1 containing Ca in an amount of 1.43% by weight exhibited a slight change in saturation magnetization as compared with the sample not containing Ca. The above value of 1.43% by weight was taken as the lower limit of the amount of Ca to be added. On the other hand, the upper limit of the amount of Ca to be added was taken as 10.65% by weight at which the saturation magnetization attributed to the nonmagnetic phase formed by the amount of Ca increased from 1.43% by weight was 20 to 10 emu/g or which was just below the value at which melting would occur during the sintering. Therefore, the amount of Ca to be added to hematite was appropriately in the range of 1.43 to 10.65% by weight.

It has been found from the above experimental results that powder (magnetic oxide) having an arbitrary saturation magnetization in which magnetite and nonmagnetic phase are coexistent can be formed by sintering at 1200.degree. to 1450.degree. C. for 2 hr a mixed powder obtained by mixing 1.43 to 10.65% by weight of Ca with hematite.

Table 2 shows sintering experiment results regarding hematite+magnetite. In the experiment, Ca was blended in the amount specified in the Table with powder of 1:1 mixture of hematite and magnetite, and treated and measured in the same manner as described above with respect to Table 1. magnetization of each of the samples was 92 emu/g. magnetite was incomplete so as for hematite (.alpha.-FE.sub.2 O.sub.3) to remain with the result that the saturation magnetization values of Sample Nos. 1 to 8 saturation magnetization values of Sample Nos. 1 to 8 were 67 emu/g or less to show the inappropriateness of the heating temperature of 1100.degree. C. On the other hand, when the heating was conducted at 1500.degree. C., the conversion to magnetite was incomplete so as to form wustite (FeO) with the result that the saturation magnetization values of Sample Nos. 33 to 40 were as low as 87 emu/g or less to show the inappropriateness of the heating temperature of 1500.degree. C. Therefore, it has been found that the appropriate heating temperature is in the range of 1200.degree. to 1450.degree. C. 1200.degree. to 1450.degree. C., the amount of Ca added was increased from 0% by weight. Sample Nos. 11, 19 and 27 of Table 2 containing Ca in an amount of 1.43% by weight exhibited a slight change in saturation magnetization as compared with the sample not containing Ca. The above value of 1.43% by weight was taken as the lower limit of the amount of Ca to be added. On the other hand, the upper limit of the amount of Ca to be added was taken as 10.65% by weight at which the saturation magnetization attributed to the nonmagnetic phase formed by the amount of Ca increased from 1.43% by weight was 20 to 10 emu/g or which was just below the value at which melting would occur during the sintering. Therefore, the amount of Ca to be added to hematite+magnetite was appropriately in the range of 1.43 to 10.65% by weight.

It has been found from the above experimental results that powder (magnetic oxide) having an arbitrary saturation magnetization in which magnetite and nonmagnetic phase are coexistent can be formed by sintering at 1200.degree. to 1450.degree. C. for 2 hr a mixed powder obtained by mixing 1.43 to 10.65% by weight of Ca with hematite+magnetite.

Table 3 shows sintering experiment results regarding magnetite. In the experiment, Ca was blended with magnetite powder in the amount specified in the Table, and treated and measured in the same manner as described above with respect to Table 1. saturation magnetization of each of the samples was 91 to 92 emu/g. converted to hematite (.alpha.-FE.sub.2 O.sub.3) with the result that the saturation magnetization values of Sample Nos. 1 to 12 were as low as 86 emu/g to show the inappropriateness of the heating temperature of 500.degree. C. The cause of the phenomenon of the above conversion has been found to be the partial oxidation of the magnetite to hematite by a trace (for example, on the order of 10 ppm) of inevitable O.sub.2 contained in the inert gas (nitrogen gas in this embodiment). On the other hand, when the heating was conducted at 1500.degree. C., the magnetite was converted to wustite (FeO) with the result that the saturation magnetization values of Sample Nos. 85 to 96 were as low as 86 emu/g or less to show the inappropriateness of the heating temperature of 1500.degree. C. Therefore, it has been found that the appropriate heating temperature is in the range of 550.degree. to 1450.degree. C. 550.degree. to 1450.degree. C., the amount of Ca added was increased from 0% by weight. Sample Nos. 15, 27, 39, 51 63 and 75 of Table 3 containing Ca in an amount of 1,434 by weight exhibited a slight change in saturation magnetization as compared with the sample not containing Ca. The above value of 1.43% by weight was taken as the lower limit of the amount of Ca to be added. On the other hand, the upper limit of the amount of Ca to be added was taken as 64.0% by weight at which the saturation magnetization attributed to the nonmagnetic phase formed by the amount of Ca increased from 1.43% by weight was as low as 20 to 10 emu/g as observed with respect to Sample Nos. 24, 36, 45 and 57 of Table 3 or which was just below the value at which melting would occur during the sintering. Therefore, the amount of Ca to be added to magnetite was appropriately in the range of 1.43 to 64.0% by weight.

It has been found from the above experimental results that powder (magnetic oxide powder) having an arbitrary saturation magnetization in which magnetite and nonmagnetic phase are coexistent can be formed by sintering at 550.degree. to 1450.degree. C. for 2 hr a mixed powder obtained by mixing 1.43 to 64.0% by weight of Ca with magnetite.

FIG. 2 illustrates an exemplary heating curve suitable for the present invention. The curve shows an exemplary course of heating in which heating is conducted at T.degree.C. for 2 hr. In particular, the heating is so performed that the temperature is elevated at a rate of 200.degree. C./hr from room temperature to T.degree.C., which is held for 2 hr. Thereafter, the temperature is lowered to room temperature at a rate of 200.degree. C./hr. Herein, T.degree.C. is the heating temperature specified in Tables 1 to 3.

FIG. 3 illustrates the saturation magnetization referred to in the present invention. In particular, it is an explanatory view of the measurement of the saturation magnetization shown in Table 1 to 3. The axis of abscissa indicates the intensity of applied magnetic field (H Oe) while the axis of ordinate indicates the corresponding intensity of magnetization (M emu). The intensity of magnetization (Ms emu) of powder in which magnetite and nonmagnetic phase are coexistent is measured by the use of an vibrating sample magnetometer while keeping the application of, for example, 15 kOe magnetic field as specified in FIG. 3. The saturation magnetization is calculated by the following formula specified in the figure:

The values calculated by the above formula are shown as the saturation magnetization .delta. s in Tables 1 to 3.

In the above embodiment, the saturation magnetization of magnetite is regulated only through the nonmagnetic phase created by the addition of Ca. In this connection, the saturation magnetization may further finely be regulated by the formation of an oxide phase (hematite, calcium peroxide, etc.), for example, on the surface of the magnetite powder.

Second Embodiment

The production of magnetic oxides having desired saturation magnetization values by mixing Ti compounds with magnetite or magnetite+hematite will now be described referring to FIGS. 4 and 5 and Tables 5 to 7. magnetite     47.3-98.8% by 9.8-98.8% by weight      weight hematite      none        0.0-79.0% by weight Ti compound   1.2-52.7% by 1.2-52.7% by weight      weight ______________________________________ (Value for Ti compound: in terms of Ti)

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0% by weight of a compound having --C--C or --C.dbd.C-- in its molecules (liquid or solid substance). For example, 2% by weight of PVA (polyvinyl alcohol) as a binder and 1% by weight of a polycarboxylate as a dispersant are added to the mixed powder as a starting material, and further water is added for granulation into globular grains. The amount of water added is in the range of 30 to 70% by weight. When the amount of water was less that 30% by weight, the viscosity of a slurry obtained by milling was too high to be granulated into globular grains. On the other hand, when the amount of water exceeded 70% by weight, the slurry concentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 is wet milled in an attrition mill to thereby prepare a slurry having a mixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In thus step, the slurry is agitated in an attrition mill for 1 hr, and subjected to hot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4 are heated at 550.degree. to 1450.degree. C. (with respect to magnetite) in an atmosphere of an inert gas such as nitrogen gas for 2 hr to thereby form magnetite and another magnetic and/or nonmagnetic phase. The ratio of the nonmagnetic phase can be controlled by regulating the amount, in terms of Ti, of Ti compound added, so that it is feasible to produce a magnetic oxide having any desired saturation magnetization by changing the amount of Ti compound added (see FIG. 5). When hematite is present in part of the magnetite powder, during the 1200.degree. to 1450.degree. C. sintering step 5, not only is the thermal transition from hematite to magnetite effected in the inert gas (in weakly reducing atmosphere) but also the organic substance mixed is brought by the heating in the inert gas into the state of incomplete combustion, in which the hematite is deprived of oxygen during the thermal decomposition of the organic substance to thereby be reduced so as to markedly promote the conversion to magnetite.

In the disintegration step 6, the powder in which magnetite and nonmagnetic phase are coexistent, obtained in the sintering step 5, is crushed into a product.

According to the above steps, magnetite or magnetite+hematite is mixed with the Ti compound to obtain a mixed powder, and this mixed powder is further mixed with a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid substance) and water, milled well, hot-air dried, granulated into globular grains and sintered in an inert gas at 550.degree. to 1450.degree. C. (for only magnetite as the starting material) or 1200.degree. to 1450.degree. C. (for magnetite+hematite), so that the powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent (magnetic oxide powder) can be produced. Thus, it has become feasible to safely mass-produce the magnetic oxide powder having any desired saturation magnetization at a lowered cost.

Table 4 shows sintering experiment results (TiO.sub.2 : 0% by weight). PVA (polyvinyl alcohol) was added as a binder in the amount specified in Table 4 to hematite powder, and 1% by weight of a polycarboxylate as a dispersant and further water were added and agitated for granulation into grains. The resultant grains were sintered at the temperature specified in the Table, and the qualitative analysis thereof by powder X-ray diffractometry was conducted. Results are shown in Table 4. The following has been found from the results. without the addition of PVA (Sample Nos. 1 to 8), even if the heating temperature was changed. The X-ray diffraction pattern shows the presence of the phase of .alpha.-FE.sub.2 O.sub.3 or FeO. temperature was varied, single phase magnetite (Fe.sub.3 O.sub.4 only) (Sample Nos. 13 to 15) was obtained at heating temperatures ranging from 1200.degree. to 1450.degree. C. .alpha.-FE.sub.2 O.sub.3 was coexistent at heating temperatures of 1150.degree. C. or below (Sample Nos. 9 to 12) and FeO was coexistent at heating temperatures of 1500.degree. C. or above (Sample No. 16). Therefore, it is requisite that the heating temperature be in the range of 1200.degree. to 1450.degree. C. (Sample Nos. 13 to 15). was varied between 0.1 and 3.0% by weight. Single phase magnetite was obtained without exception (Sample Nos. 17 to 22).

It has been found from the above experimental results that single phase magnetite is formed as long as the amount of PVA added to hematite powder is 0.1% by weight (4% by weight) and the sintering is conducted at temperatures ranging from 1200.degree. to 1450.degree. C.

Tables 5 and 6 show experimental example results (magnetite plus Ti) of the present invention. In this experimental example, 0.0 to 84.4% by weight, in terms of Ti of powdery titanium oxide (TlO.sub.2) was mixed with magnetite powder, 1.0% by weight of polyvinyl alcohol was added, and further water was added and mixed to obtain a slurry having a powder concentration of 50% by weight. The slurry was agitated in an attrition mill for 1 hr, and granulated by means of a spray dryer. The thus obtained grains were heated at 400.degree. to 1500.degree. C. in nitrogen gas for 2 hr. The oxygen concentration of the nitrogen gas was measured by means of a zirconia oxygen concentration meter. The saturation magnetization of each heated sample was measured by means of the vibrating sample magnetometer (see FIG. 3 relating to the first embodiment). The material phase was identified by the powder X-ray diffractometry. The collapse test of each sample was conducted by the use of a microcompression tester (MCTM-500 manufactured by Shimadzu Corp.), and the corresponding grain strength was calculated according to the following Hiramatsu's formula. grains)}/{.pi..times.(Diameter of grain).sup.2 }

(1) Heating temperature range

As shown in Table 5, the formation of hematite (.alpha.-FE.sub.2 O.sub.3) attributed to the oxidation by trace O contained in nitrogen gas was not detected when the heating was performed at 500.degree. C. (Sample Nos. 1 to 12). However, the grains strength Pa was on the order of E6 (meaning 10.sup.6), so that the strength of globular grains formed by joining of a vast plurality of magnetite powder particles was weak to a practical disadvantage. On the other hand, it was found that, when the heating was performed at 1500.degree. C. (Sample Nos. 85 to 96), wustite (FeO) was formed, so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained. Further, it was found that, when a Ti compound is mixed with magnetite as a starting material and the saturation magnetization is arbitrarily regulated by the formation of magnetite and another magnetic and/or nonmagnetic phase, the assurance of a strength required for actual handling by joining or mild sintering is satisfactory because it is not necessary to newly reduce the concerned magnetite for conversion to magnetite, so that desirable saturation magnetization regulation can be made even if the heating is performed at a temperature as low as 550.degree. C.

The above demonstrates that the suitable heating temperature range is from 550.degree. to 1450.degree. C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added (Sample Nos. 13 to 84) within the heating temperature range of 550.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 90 to 0 emu/g as the above amount was increased from 1.2 to 52.7% by weight. When the amount of Ti added exceeded 52.7% by weight, the saturation magnetization was decreased to about 0 emu/g to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Ti, of Ti compound added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound with magnetite and effecting the heating at 550.degree. to 1450.degree. C. (provided that 0.2 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

FIG. 5 shows exemplary saturation magnetization curves as functions of the amount of Ti added. The saturation magnetization curves are drawn in a graph in which the axis of abscissa indicates the weight percentage, in terms of Ti, of a Ti compound added while the axis of ordinate indicates the corresponding saturation magnetization value. They represent experimental example results shown in Table 3. In particular, the saturation magnetization curves are line segments formed by plotting data shown in Table 3 with respect to each of the heating temperature range of 550.degree. to 600.degree. C., heating temperature of 800.degree. C. and heating temperature range of 1100.degree. to 1450.degree. C. in a graph in which the axis of abscissa indicates the weight percentage of Ti added while the axis of ordinate indicates the corresponding saturation magnetization value emu/g and connecting the plots.

Once the saturation magnetization curves of FIG. 5 have been established, therefore, not only can the weight percentage of Ti to be added for obtaining the desired saturation magnetization emu/g be determined but also the saturation magnetization emu/g can contrarily be determined with respect to any certain weight percentage of Ti, to thereby ensure production of a magnetic oxide powder having a desired saturation magnetization.

Table 6 shows results of another experimental example (1:1 mixture of magnetite and hematite, Ti) according to the present invention. It shows experimental data obtained by varying the amount of Ti added as specified in the Table while the proportion of magnetite powder to hematite powder was kept at 1:1. In practice, 0.0 to 84.4% by weight, in terms of Ti, of titanium oxide (TiO.sub.2) was mixed with a mixture of magnetite and hematite powders whose proportion was kept at 1:1, and granulated in the same manner as specified in Table 5. The resultant grains were heated at 1100.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the samples were measured in the same manner as specified in Table 5, and results are shown in Table 6.

(1) Heating temperature range

Table 6 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 12) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 37 to 48) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added (Sample Nos. 13 to 36) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 87 to 0 eum/g as the above amount was increased from 1.2 to 52.7% by weight. When the amount of Ti added exceeded 52.7% by weight, the saturation magnetization was decreased to about 0 emu/g to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Ti, of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound with a 1:1 mixture of magnetite and hematite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of substance having a carbon-to carbon single or double bond is also added).

Table 7 shows results of an additional experimental example (1:3 mixture of magnetite and hematite, Ti). It shows experimental data obtained by varying the amount of Ti added as specified in the Table while the proportion of magnetite powder to hematite powder was kept at 1:3. In practice, 0.0 to 84.4% by weight, in terms of Ti, of powdery titanium oxide (TiO.sub.2) was mixed with a mixture of magnetite and hematite powders whose proportion was kept at 1:3 in the same manner as specified in Table 6, and granulated in the same manner as specified in Table 5. The resultant grains were heated at 1100.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the samples were measured in the same manner as specified in Table 5, and results are shown in Table 7.

(1) Heating temperature range

Table 7 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 12) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 37 to 48) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added (Sample Nos. 13 to 36) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 87 to 0 emu/g as the above amount was increased from 1.2 to 52.7% by weight. When the amount of Ti added exceeded 52.7% by weight, the saturation magnetization was decreased to about 0 emu/g to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Ti, of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound with a 1:3 mixture of magnetite and hematite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

Table 8 shows results of a further experimental example (1:4 mixture of magnetite and hematite, Ti). It shows experimental data obtained by varying the amount of Ti added as specified in the Table while the proportion of magnetite powder to hematite powder was kept at 1:4. In practice, 0.0 to 84.4% by weight, in terms of Ti, of powdery titanium oxide (TiO.sub.2) was mixed with a mixture of magnetite and hematite powders whose proportion was kept at 1:4 in the same manner as specified in Table 6, and granulated in the same manner as specified in Table 5. The resultant grains were heated at 1100.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the samples were measured in the same manner as specified in Table 5, and results are shown in Table 8.

(1) Heating temperature range

Table 8 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 12) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 37 to 48) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Ti added (% by weight)

Upon noting the amount, in terms of Ti, of powdery titanium oxide added (Sample Nos. 13 to 36) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 87 to 0 emu/g as the above amount was increased from 1.2 to 52.7% by weight. When the amount of Ti added exceeded 52.7% by weight, the saturation magnetization was decreased to about 0 emu/g to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Ti, of Ti compound to be added is in the range of 12 to 52.7% by weight

Therefore, it has been found that the suitable amount, in terms of Ti, of Ti compound to be added is in the range of 1.2 to 52.7% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound with a 1:4 mixture of magnetite and hematite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

Detailed descriptions on the heating curves and the saturation magnetization measurements are omitted in view of the similarity to and obviousness from those for FIGS. 2 and 3 relating to the first embodiment of the present invention.

Third Embodiment

The production of magnetic oxides having desired saturation magnetization values by mixing Sn compounds with hematite, hematite+magnetite, or magnetite will now be described referring to FIG. 6 and Tables 9 to 11.

Referring now to FIG. 6, hematite, hematite+magnetite or magnetite and an Sn compound are weighed and blended in the following proportions in the blending step 1. hematite           24.0-99.2% by weight hematite + magnetite 24.0-99.2% by weight, or magnetite          24.0-99.2% by weight together with      0.8-87.6% by weight. Sn compound (in terms of Sn) ______________________________________

Thus, a mixed powder is obtained.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0% by weight of a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid substance). For example, 2% by weight of PVA (polyvinyl alcohol) as a binder and 1% by weight of a polycarboxylate as a dispersant are added to the mixed powder as a starting material, and further water is added for granulation into globular grains. The amount of water added is in the range of 30 to 70% by weight. When the amount of water was less than 30% by weight, the viscosity of a slurry obtained by milling was too high to be granulated into globular grains. On the other hand, when the amount of water exceeded 70% by weight, the slurry concentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 is wet crushed in an attrition mill to thereby prepare a slurry having a mixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, the slurry is agitated in an attrition mill for 1 hr, and subjected to hot-air drying by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4 are heated at 1200.degree. to 1450.degree. C. (with respect to hematite or hematite+magnetite) or 550.degree. to 1450.degree. C. (with respect to magnetite) in an atmosphere of an inert gas such as nitrogen gas for 2 hr to thereby form magnetite and another magnetic and/or nonmagnetic phase. The ratio of the nonmagnetic phase can be controlled by regulating the amount, in terms of Sn, compound added, so that it is feasible to produce a magnetic oxide having any desired saturation magnetization by changing the amount of Sn compound added. When hematite is present in part of the magnetite powder, not only is the thermal transition from hematite to magnetite effected in the inert gas (in weakly reducing atmosphere) but also the organic substance mixed is brought by the heating in the inert gas into the state of incomplete combustion, in which the hematite is deprived of oxygen during the thermal decomposition of the organic substance to thereby be reduced so as to markedly promote the conversion to magnetite.

In the crushing step 6, the powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent, obtained in the sintering step 5, is crushed into a product.

According to the above steps, hematite, hematite+magnetite, or magnetite is mixed with the Sn compound to obtain a mixed powder, and this mixed powder is further mixed with a compound having --C--C-- or --C.dbd.C-- in its molecules and water, milled well, hot-air dried, granulated into globular grains and sintered in an inert gas at 550.degree. to 1450.degree. C. (with respect to magnetite) to 1200.degree. to 1450.degree. C. (with respect to magnetite) or 1200.degree. to 1450.degree. C. (with respect to hematite or hematite+magnetite), so that the magnetic oxide powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent can be produced. Thus, it has become feasible to safely mass-produce the magnetic oxide powder having any desired saturation magnetization at a lowered cost. Descriptions will be made in order below.

Table 9 shows experimental example results (hematite, Sn) according to this embodiment. In this experimental example, 0.8 to 87.6% by weight, in terms of Sn of powdery tin oxide was mixed with hematite powder, 1.0% by weight of polyvinyl alcohol was added, and further water was added and mixed to obtain a slurry having a powder concentration of 50% by weight. The slurry was agitated in an attrition mill for 1 hr, and granulated by means of a spray dryer. The thus obtained grains were heated at 1100.degree. to 1500.degree. C. in nitrogen gas for 2 hr. The oxygen concentration of the nitrogen gas was measured by means of the zirconia oxygen concentration meter. The saturation magnetization of each heated sample was measured by means of the vibrating sample magnetometer. The material phase was identified by the powder X-ray diffractometry.

(1) Heating temperature range

Table 9 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 10) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (sample Nos. 41 to 50) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn of powdery tin oxide added (Sample Nos. 11 to 40) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 8 emu/g as the above amount was increased from 0.8 to 87.6% by weight. When the amount of Sn added exceeded 76.0% by weight, the saturation magnetization became extremely low to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Sn, of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound with hematite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added). FIG. 7 shows an exemplary saturation magnetization curve as a function of the amount of Sn added. The saturation magnetization curve is drawn in a graph in which the axis of abscissa indicates the weight percentage, in terms of Sn, or Sn compound added while the axis of ordinate indicates the corresponding saturation magnetization value. It represents experimental example results shown in Table 9. In particular, the saturation magnetization curve is a line segment formed by plotting data shown in Table 9 with respect to the heating temperature range of 1200.degree. to 1450.degree. C. in a graph in which the axis of abscissa indicates the weight percentage of Sn added while the axis of ordinate indicated the corresponding saturation magnetization value emu/g and connecting the plots.

Once the saturation magnetization curve of FIG. 7 has been established, therefore, not only can the weight percentage of Sn to be added for obtaining a desired saturation magnetization emu/g be determined but also the saturation magnetization emu/g can contrarily be determined with respect to any given weight percentage of Sn, to thereby ensure production of a magnetic oxide powder having a desired saturation magnetization.

Table 10 shows results of another experimental example (1;1 mixture of hematite and magnetite, Sn) according to this embodiment. It shows experimental data obtained by varying the amount of Sn added as specified in the Table while the proportion of hematite powder to magnetite powder was kept at 1:1. In practice, 0.0 to 87.6% by weight, in terms of Sn, of powdery tin oxide was mixed with a 1:1 mixture of hematite and magnetite powders, and granulated in the same manner as specified in Table 9. The resultant grains were heated at 100.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the samples were measured in the same manner as specified in Table 9, and results are shown in Table 10.

(1) Heating temperature range

Table 10 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 10) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 41 to 50) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn, of powdery tin oxide added (Sample Nos. 11 to 40) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 9 emu/g as the above amount was increased form 0.8 to 87.6% by weight. When the amount of Sn added exceeded 76.0% by weight, the saturation magnetization became extremely small to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Sn, of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0% by weight, in terms of Sn, of a Sn compound with a 1:1 mixture of hematite and magnetite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

Table 11 shows results of an additional experimental example (magnetite, Sn) according to this embodiment. In this experimental example, 0.0 to 87.6% by weight, in terms of Sn, of powdery tin oxide was mixed with magnetite powder, and granulated in the same manner as specified in Table 9. The thus obtained grains were heated at 500.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the samples were measured in the same manner as specified in Table 9, and results are shown in Table 11.

(1) Heating temperature range

Table 11 demonstrates that, during the heating at 500.degree. C. (Sample Nos. 1 to 10), the magnetite is oxidized by trace amount of O.sub.2 contained in nitrogen gas to form hematite (.alpha.-FE.sub.2 O.sub.3) while the heating at 1500.degree. C. (Sample Nos. 81 to 90) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained. When an Sn compound is mixed with magnetite as a starting material and magnetite and another magnetic and/or nonmagnetic phase is formed to attain arbitrary regulation of the saturation magnetization, there is no need for newly reducing the magnetite to achieve conversion to magnetite, so that it is satisfactory to obtain the strength required for handling, etc., by bonding or mild sintering. Thus, it has been found that desirable saturation magnetization regulation can be achieved at a temperature as low as 550.degree. C.

(2) Amount of Sn added (% by weight)

Upon noting the amount, in terms of Sn, of powdery tin oxide added (Sample Nos. 11 to 80) within the heating temperature range of 550.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 8 emu/g as the above amount was increased from 0.8 to 87.6% by weight. When the amount of Sn added exceeded 76.9% by weight, the saturation magnetization became extremely low to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Sn, of Sn compound to be added is in the range of 0.8 to 76.0% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound with magnetite and effecting the heating at 550.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

As described above, in the second and third embodiments, Ti and Sn compounds are individually mixed with magnetite, magnetite+hematite or hematite, and the resultant mixed powder is mixed with a substance having a carbon-to-carbon single or double bond and sintered to thereby produce a powder comprising magnetite and another magnetic and/or nonmagnetic phase and having an arbitrary saturation magnetization value. Therefore, a magnetic oxide having any desired saturation magnetization can easily be mass-produced at a lowered cost. In particular, a magnetic oxide comprising magnetite and another magnetic and/or nonmagnetic phase and having an arbitrary saturation magnetization value can easily be produced at a lowered cost by a simple process in which a large volume of magnetite, magnetite+hematite or hematite is mixed with a Ti or Sn compound, further mixed with an organic substance and directly brought into the sintering step 5.

Fourth Embodiment

As shown in FIG. 8, 0.5 to 65% by weight, in terms of Si, of a powdery Si compound was weighed and mixed with a member selected from among hematite, hematite+magnetite and magnetite powders in the blending step 1.

In the mixing step 2, the thus mixed powder is mixed with 0.1 to 4.0% by weight of a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid). For example, 2% by weight of polyvinyl alcohol as a binder and 1% by weight of a polycarboxylate as a dispersant are added to the mixed powder, and further water is added for granulation into globular grains. The amount of water added is in the range of 30 to 70% by weight. When the amount of water was less that 30% by weight, the viscosity of a slurry obtained by milling was too high to be granulated into globular grains. On the other hand, when the amount of water exceeded 70% by weight, the slurry concentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 is wet crushed in an attrition mill to thereby prepare a slurry having a mixed powder concentration of about 50% by weight.

In the granulating step 4, globular grains are formed. In this step, the slurry is agitated in an attrition mill for 1 hr, and hot-air dried by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4 are heated at 1200.degree. to 1450.degree. C. or 550.degree. to 1450.degree. C. in an atmosphere of an inert gas (such as nitrogen gas) for 2 hr to thereby form magnetite and another magnetic and/or nonmagnetic phase. The value of the saturation magnetization can be controlled by regulating the amount, in terms of Si, of Si compound added, so that it is feasible to produce a magnetic oxide having any desired saturation magnetization by changing the amount of Si compound added (see Tables 13 to 15). When hematite is present in part of the hematite or magnetite powder, not only is the thermal transition from hematite to magnetite effected in the inert gas (in weakly reducing atmosphere) but also the organic substance mixed is brought by the heating in the inert gas into the state of incomplete combustion, in which the hematite is deprived of oxygen during the thermal decomposition of the organic substance to thereby be reduced so as to markedly promote the conversion to magnetite,

In the crushing step 6, the powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent, obtained in the sintering step 5, is crushed into a product.

According to the above steps, 35 to 99.5% by weight of hematite, hematite+magnetite, or magnetite is mixed with 0.6 to 65% by weight, in terms of Si, of an Si compound to obtain a mixed powder, and this mixed powder is further mixed with 0.1 to 4.0% by weight of a compound having --C--C-- or --C.dbd.C-- in its molecules and still further water, milled well, hot-air dried, granulated into globular grains and sintered in an inert gas at 1200.degree. to 1450.degree. C. (with respect to hematite or hematite+magnetite) or 550.degree. to 1450.degree. C. (with respect to magnetite only), so that the magnetic oxide powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent can be produced. Thus, it has become feasible to safely mass-produce the magnetic oxide powder having any desired saturation magnetization at a lowered cost. Descriptions will be made below.

Table 12 shows sintering experiment results (SiO.sub.2 : 0% by weight) According to this embodiment. In this experiment, PVA (polyvinyl alcohol) was added as a binder in an amount specified in the Table to hematite powder, and further 1% by weight of a polycarboxylate as a dispersant and water were added and mixed. The mixture was granulated, and sintered at a temperature T specified in the Table. After the sintering, a qualitative analysis by powder X-ray diffractometry was conducted. Results are shown in the Table. The following has been found from the results. temperatures from only hematite powder having no PVA added thereto (Sample Nos. 1 to 8). The X-ray diffractometry showed the presence of the phase of hematite (.alpha.-FE.sub.2 O.sub.3) or wustite (FeO). temperature was varied, single phase magnetite (Fe.sub.3 O.sub.4 only) was obtained at heating temperatures ranging from 1200.degree. to 1450.degree. C. (Sample Nos. 9 to 12), and wustite (FeO) was coexistent at heating temperatures of 1500.degree. C. or above (Sample Nos. 16).

Therefore, it is requisite that the heating temperature be in the range of 1200.degree. to 1450.degree. C. (Sample Nos. 13 to 15). added was varied between 0.1 and 3.0% by weight. In any case, single phase magnetite was obtained (Sample Nos. 17 to 22).

From the above experimental results, it has been found that completely single phase magnetite can be obtained whenever PVA is added to hematite powder in an amount of 0.1 to 3% by weight (4% by weight) and whenever the sintering performed by heating at 1200.degree. to 1450.degree. C.

Table 13 shows experimental example results (hematite) according to this embodiment. In this experimental example, 0.0 to 79.5% by weight, in terms of Si, of powdery silicon oxide was mixed with hematite powder, and further mixed with water to obtain a slurry having a powder concentration of 50% by weight. The slurry was agitated in an attrition mill for 1 hr, and dried at 110.degree. C. 1.0% by weight of polyvinyl alcohol was added to the obtained pellet. The mixture was mixed in a mortar, and sifted through a 425-.mu.m-mesh standard sieve to obtain grains. 0.5 g of the resultant grains were put in a cylindrical mold having a diameter of 12.5 mm, and shaped under a molding pressure of 1 t/cm.sup.2. The molding was heated at 1100.degree. to 1500.degree. C. for 2 hr in nitrogen gas. The oxygen concentration of the nitrogen gas was measured by the use of a zirconia oxygen concentration meter. The saturation magnetization of each sample after the heating was measured by means of a vibrating sample magnetometer (see FIG. 3 of the first embodiment). Results are shown in Table 13.

(1) Heating temperature range

Table 13 demonstrates that the heating at 1100.degree. C.(Sample Nos. 1 to 10) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 41 to 50) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added (Sample Nos. 11 to 40) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 20 emu/g as the above amount was increased from 0.5 to 65.0% by weight (see FIG. 9 referred to later). When the amount of Si added exceeded as small as 20 emu/g or less to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Si, of Si compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite+nonmagnetic phase can be formed by mixing 0.5 to 65% by weight, in terms of Si, of an Si compound with hematite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

FIG. 9 shows an exemplary saturation magnetization curve as a function of the amount of Si added in this embodiment. The saturation magnetization curve is drawn in a graph in which the axis of abscissa indicates the weight percentage, in terms of Si, of Si compound added while the axis of ordinate indicates the corresponding saturation magnetization value. It represents experimental example results shown in Table 13. In particular, the saturation magnetization curve is a line segment formed by plotting data shown in Table 18 with respect to the heating temperature range of 1200.degree. to 1450.degree. C. in a graph in which the axis of abscissa indicates the weight percentage of Si added while the axis of ordinate indicates the corresponding saturation magnetization value emu/g and connecting the plots.

Once the saturation magnetization curve of FIG. 9 has been established, therefore, not only can the weight percentage of Si to be added for obtaining a desired saturation magnetization emu/g be determined but also the saturation magnetization emu/g can contrarily be determined with respect to any given weight percentage of Si, to thereby ensure production of a magnetic oxide powder having a desired saturation magnetization.

Table 14 shows results of another experimental example (hematite+magnetite) according to this embodiment. In the experimental example, 0.0 to 79.5% by weight, in terms of Si, of powdery silicon oxide was mixed with a 50% by weight: 50% by weight mixture of hematite and magnetite powders (hematite+magnetite), and the resultant mixed powder was mixed with water, milled, granulated, molded and heated in the same manner as specified in Table 13 to thereby obtain samples. The samples were measured, and results are shown in Table 14.

(1) Heating temperature range

Table 14 demonstrates that the heating at 1100.degree. C. (Sample Nos. 1 to 10) causes hematite (.alpha.-FE.sub.2 O.sub.3) to remain while the heating at 1500.degree. C. (Sample Nos. 41 to 50) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained.

Therefore, it has been found that the suitable heating temperature range is from 1200.degree. to 1450.degree. C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added (Sample Nos. 11 to 40) within the heating temperature range of 1200.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 20 emu/g as the above amount was increased from 0.5 to 65.0% by weight. When the amount of Si added exceeded 65% by weight, the saturation magnetization became as small as 20 emu/g to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, in terms of Si, of Si compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 0.5 to 65% by weight, in terms of Si, of a Si compound with hematite+magnetite and effecting the heating at 1200.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

Table 15 shows results of a further experimental example (magnetite) according to this embodiment. In this experimental example, 0.0 to 79.5% by weight, in terms of Si, of powdery silicon oxide was mixed with magnetite powder and further mixed with water in the same manner as specified in Table 13 to obtain a slurry having a powder concentration of 50% by weight. The slurry was agitated in an attrition mill for 1 hr, and dried at 110.degree. C. 1.0% by weight of polyvinyl alcohol was added to the obtained pellet. The mixture was mixed in a mortar, and sifted through a 425-.mu.m-mesh standard sieve to obtain grains. 0.5 g of the resultant grains were put in a cylindrical mold having a diameter of 12.5 mm, and shaped under a molding pressure of 1 t/cm.sup.2. The molding was heated at 500.degree. to 1500.degree. C. for 2 hr in nitrogen gas. The oxygen concentration of the nitrogen gas was measured by the use of a zirconia oxygen concentration meter. The saturation magnetization of each sample after the heating was measured by means of a vibrating sample magnetometer (see FIG. 3 of the first embodiment). Results are shown in Table 15.

(1) Heating temperature range

Table 15 demonstrates that, during the heating at 500.degree. C. (Sample Nos. 1 to 10), the magnetite is oxidized by trace O.sub.2 contained in nitrogen gas to form hematite (.alpha.-FE.sub.2 O.sub.3) while the heating at 1500.degree. C. (Sample Nos. 81 to 90) forms wustite (FeO), so that the single phase magnetite (Fe.sub.3 O.sub.4 only) cannot be obtained. When an Si compound is mixed with magnetite as a starting material and magnetite and another magnetic and/or nonmagnetic phase is formed to attain arbitrary regulation of the saturation magnetization, there is no need for newly reducing the magnetite to achieve conversion to magnetite, so that it is satisfactory to obtain the strength required for handling and other practical operation by bonding or mild sintering. Thus, it has been found that desirable saturation magnetization regulation can be achieved at a temperature as low as 550.degree. C.

Therefore, it has been found that the suitable heating temperature range is from 550.degree. to 1450.degree. C.

(2) Amount of Si added (% by weight)

Upon noting the amount, in terms of Si, of powdery silicon oxide added (Sample No. 11 to 80) within the heating temperature range of 550.degree. to 1450.degree. C. set in section (1) above, it was found that the saturation magnetization was continuously decreased from 91 to 16 emu/g as the above amount was increased from 0.5 to 65.0% by weight. When the amount of Si added exceeded 65% by weight, the saturation magnetization became as small as 16 emu/g or less to thereby cause the magnetic oxide to lose practicability.

Therefore, it has been found that the suitable amount, terms of Si, of Si compound to be added is in the range of 0.5 to 65% by weight.

The above demonstrates that desirable single phase magnetite and another magnetic and/or nonmagnetic phase can be formed by mixing 0.5 to 65% by weight, in terms of Si, of an Si compound with magnetite and effecting the heating at 550.degree. to 1450.degree. C. (provided that 0.1 to 4.0% by weight of a substance having a carbon-to-carbon single or double bond is also added).

Detailed descriptions on the heating curbed for 2-hr heating at the heating temperature T.degree.C. (heating temperature specified in Tables 12 to 15) and the saturation magnetization relating to the above embodiment are omitted in view of the similarity to and obviousness from those form FIGS. 2 and 3 relating to the first embodiment of the present invention.

Besides the above embodiment, it has been found that the saturation magnetization may further finely be regulated by controlling the sintering conditions so as to form an oxide phase (hematite calcium peroxide, etc.), for example, on the surface of the powder. This is applicable according to necessity although additional operation is required.

As described above, in the forgoing embodiment, an Si compound and a substance having a carbon-to-carbon single or double bond are mixed with hematite, hematite+magnetite, or magnetite, and sintered to thereby produce a powder comprising magnetite and another magnetic and/or nonmagnetic phase and having an arbitrary saturation magnetization value. Therefore, a magnetic oxide having any desired saturation magnetization can easily be mass-produced at a lowered cost. In particular, a powder (magnetic oxide) comprising magnetite and another magnetic and/or nonmagnetic phase and having an arbitrary saturation magnetization value can easily be produced at a lowered cost by a simple process in which a mixed powder comprising a large volume of hematite, hematite+magnetite, or magnetite, an Si compound and a substance having a carbon-to-carbon single or double bond is directly brought into the sintering step 5.

Fifth Embodiment

Referring to FIG. 10, hematite or hematite+magnetite is blended with 0.20 to 18.50% by weight of Mg or alternatively magnetite is blended with 0.20 to 26.20% by weight of Mg, optionally together with 0.20 to 10.00% by weight of Mn (the upper limit of the total of the former two is 18.50% by weight while that of the latter is 26.20% by weight) to thereby form a mixed powder in the blending step 1. The term "Mg" means the Mg moiety, expressed in % by weight, of organic and inorganic compounds each containing Mg, such as MgCO.sub.3, Mg(OH).sub.2, MgCl.sub.2 nH.sub.2 O and MgO. Likewise, the term "Mn" means the Mn moiety expressed in % by weight of organic and inorganic compounds each containing Mn, such as MnO.sub.2, Mn.sub.3 O.sub.4, MnOOH, MnCO.sub.3, Mn.sub.2 O.sub.3 and MnO. The terms "Mg" and "Mn" are employed for brevity in this specification. 0.20 to 18.50% by weight or 0.20 to 26.20% by weight of Mg optionally together with 0.20 to 10.00% by weight of Mn is blended with the balance (q.s. ad 100%) of hematite, hematite+magnetite, or magnetite (that is, for example, the weight percentage of the "O" moiety of MgO is not contained in the value of 100% by weight). The terminology "hematite+magnetite" employed herein means a mixture (mixed powder) of hematite and magnetite. In particular, the magnetite as a starting material may be either of: companies), and off-specification particle size (recovered item) so as for the same to have the given particle size.

The product grains described later are those each composed of, for example, 103 to 107 starting powder particles (1-3 .mu.m) gathered into a globular form having a diameter of 50 to 100 .mu.m. Thus, any once produced magnetite powder product (off-specification item) can be pulverized to thereby easily obtain the desired starting material powder.

In the mixing step 2, the above mixed powder is mixed with 0.1 to 4.0% by weight of a compound having --C--C-- or --C.dbd.C-- in its molecules (liquid or solid). For example, 2% by weight of polyvinyl alcohol as a binder and 1% by weight of a polycarboxylate as a dispersant are added to the mixed powder, and further water is added for granulation into globular grains. The amount of water added is in the range of 30 to 70% by weight. When the amount of water was less than 30% by weight, the viscosity of the slurry obtained by milling was too high to be granulated into globular grains. On the other hand, when the amount of water exceeded 70% by weight, the slurry concentration was too low to be granulated into dense globular grains.

In the wet-milling step 3, the mixture obtained in the mixing step 2 is wet crushed in an attrition mill to thereby prepare a slurry having a mixed powder concentration of about 50% by weight.

In the granulating step 4, the powder is formed into globular grains. In this step, the slurry is agitated in an attrition mill (attritor) for 1 hr, and hot-air dried by a spray dryer to obtain globular grains.

In the sintering step 5, the grains obtained in the granulating step 4 are heated at 550.degree. to 1500.degree. C. in an atmosphere of an inert gas (such as nitrogen gas) for 2 hr to thereby obtain powder in which single phase magnetite and nonmagnetic phase are coexistent. The value of saturation magnetization of the powder can be regulated by the amount of Mg added, so that it is feasible to produce a powdery magnetic oxide having any desired saturation magnetization by changing the amount of Mg added (see Tables 16 to 18). When hematite is present in part of the hematite or magnetite powder, during the 550.degree. to 1500.degree. C. sintering step 5, not only is the thermal transition from hematite to magnetite effected in the inert gas (in weakly reducing atmosphere) but also the organic substance mixed is brought by the heating in the inert gas into the stage of incomplete combustion, in which the hematite is deprived of oxygen during the thermal decomposition of the organic substance to thereby be reduced to markedly promote the conversion to magnetite. The mixing of Mn improves the grain strength of the powder (see Table 10 and FIG. 12).

In the disintegration step 6, the powder in which magnetite and another magnetic and/or nonmagnetic phase are coexistent, obtained in the sintering step 5, is disintegrated into a product.

According to the above steps, hematite, hematite+magnetite, or magnetite is mixed with the above Mg, optionally together with Mn for improving the grain strength of the powder, to obtain a mixed powder, and this mixed powder is further mixed with a compound having --C--C-- or --C.dbd.C-- in its molecules and water, milled well, hot-air dried, granulated into globular grains and sintered in an inert gas at 550.degree. to 1500.degree. C., so that the magnetic oxide powder in which magnetite and nonmagnetic phase are coexistent can be produced. Thus, it has become feasible to safely mass-produce a magnetic oxide powder having any desired saturation magnetization and a satisfactory grain strength at a lowered cost. Descriptions will be made below.

Table 16 shows sintering experiment results (hematite+Mg) according to this embodiment. In the experiment, 0.00 to 40.00% by weight, in terms of Mg, of powder of magnesium oxide (MgO) was blended with hematite powder, and mixed with water to obtain a slurry having a powder concentration of 50% by weight. 1.0% by weight of polyvinyl alcohol was added to the slurry, and agitated in an attrition mill for 1 hr. The slurry was spray dried by the use of a spray dryer to obtain grains. The thus obtained grains were heated at 500.degree. to 1500.degree. C. in nitrogen gas for 2 hr. After the heating, the saturation magnetization of each sample was measured by means of a vibrating sample magnetometer. The grain strength of each sample was determined by measuring with the use of a micro-compression tester (MCTM-500 manufactured by Shimadzu Corp.) and calculating according to the following Hiramatsu's formula. grain)}/{.rho..times.(Diameter of grain).sup.2 } as small as less than 1E7 Pa to thereby cause the magnetic oxide to fall to serve practical use. Thus, 500.degree. C. is unacceptable. On the other hand, when the heating temperature was 550.degree. C., the grain strength was not smaller than 1E7 Pa, so that the magnetic oxide would serve practical use. Therefore, it has been determined that 550.degree. C. is an appropriate lower limit of the heating temperature. magnetite was so incomplete as to form wustite (FeO) with the result that the saturation magnetization began to become as small as 86 emu/g or less as noted with respect to Sample No. 91. Although satisfactory saturation magnetization can be obtained at higher temperatures, it has been determined that 1500.degree. C. is an appropriate upper limit of the heating temperature from the viewpoint of the practical heating limit of electric furnaces. 33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetization began to decrease within the heating temperature range of 550.degree. to 1500.degree. C., a saturation magnetization of 90 to 82 emu/g was obtained at 550.degree. to 1100.degree. C.

Thus, it has been determined that 0.20% by weight is an appropriate lower limit of the amount of Mg added. On the other hand, the amount of Mg added at which the saturation magnetization was at least 10 emu/g was 18.50% by weight (Sample Nos. 18, 28, 38, 48, 58, 68, 78, 88 and 98) at 550.degree. to 1500.degree. C. Thus, it has been determined that 18.50% by weight is an appropriate upper limit of the amount of Mg added.

It has been found from the above experimental results that a powder (magnetic oxide) comprising magnetite and another magnetic and/or nonmagnetic phase admixed together and having an arbitrary saturation magnetization as indicated by the hatched portion in FIG. 11(a) described later can be formed by mixing 0.20 to 18.50% by weight of Mg with hematite and sintering the resultant mixed powder at 550.degree. to 1500.degree. C. for 2 hr.

Table 17 shows results of another sintering experiment (hematite+magnetite+Mg) according to this embodiment.

In the experiment, 0.00 to 40.0% by weight, in terms of of powder of magnesium oxide (MgO) was blended with a 1:1 mixture of hematite and magnetite powders, treated in the same manner as specified in Table 16, and measured. as small as less that 1E7 Pa to thereby cause the magnetic oxide to fall to serve practical use. Thus, 500.degree. C. is unacceptable. On the other hand, when the heating temperature was 550.degree. C., the grain strength was not smaller than 1E7 Pa, so that the magnetic oxide would serve practical use. Therefore, it has been determined that 550.degree. C. is an appropriate lower limit of the heating temperature. magnetite was so incomplete as to form wustite (FeO) with the result that the saturation magnetization began to become as small as 86 emu/g or less as noted with respect to Sample No. 91. Although satisfactory saturation magnetization can be obtained at higher temperatures, it has been determined that 1500.degree. C. is an appropriate upper limit of the heating temperature from the viewpoint of the practical heating limit of electric furnaces. 33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetization began to decrease within the heating temperature range of 550.degree. to 1500.degree. C., a saturation magnetization of 61 emu/g was obtained at 550.degree. to 1100.degree. C. while a saturation magnetization of 90 to 82 emu/g was obtained at 1200.degree. to 1500.degree. C. Thus, it has been determined that 0.20% by weight is an appropriate power limit of the amount of Mg added. On the other hand, the amount of Mg added at which the saturation magnetization was at least 10 emu/g was 18.50% by weight (Sample Nos. 18, 28, 38, 48, 58, 68, 78, 88 and 98) at 550.degree. to 1500.degree. C. Thus, it has been determined that 18.50% by weight is an appropriate upper limit of the amount of Mg added.

It has been found from the above experimental results that a powder (magnetic oxide) comprising magnetite and another magnetic and/or nonmagnetic phase admixed together and having an arbitrary saturation magnetization as indicated by the hatched portion in FIG. 11(b) described later can be formed by mixing 0.20 to 18.50% by weight of Mg with hematite+magnetite and sintering the resultant mixed powder at 550.degree. to 1500.degree. C. for 2 hr.

Table 18 shows results of a further sintering experiment (magnetite+Mg) according to this embodiment. In the experiment, 0.00 to 40.0% by weight, in terms of Mg, of powder of magnesium oxide (MgO) was blended with magnetite, treated in the same manner as specified in Table 16, and measured. as small as less than 1E7 Pa to thereby cause the magnetic oxide to fail to serve practical use. Thus, 500.degree. C. is unacceptable. On the other hand, when the heating temperature was 550.degree. C., the grain strength was not smaller than 1E7 Pa, so that the magnetic oxide would serve practical use. Therefore, it has been determined that 550.degree. C. is an appropriate lower limit of the heating temperature. magnetite was so incomplete as to form wustite (FeO) with the result that the saturation magnetization began to become as small as 86 emu/g or less as noted with respect to sample No. 91. Although satisfactory saturation magnetization can be obtained at higher temperatures, it has been determined that 1500.degree. C. is an appropriate upper limit of the heating temperature from the viewpoint of the practical heating limit of electric furnaces. 33, 43, 53, 63, 73, 83 and 93) at which the saturation magnetization began to decrease within the heating temperature range of 550.degree. to 1500.degree. C., a saturation magnetization of 91 to 82 emu/g was obtained at 550.degree. to 1500.degree. C. Thus, it has been determined that 0.20% by weight is an appropriate lower limit of the amount of Mg added. On the other hand, the amount of Mg added at which the saturation magnetization was at most about 10 emu/g was 26.20% by weight (Sample Nos. 19, 29, 39 and 49) at 550.degree. to 1100.degree. C. and 19.50% by weight (Sample Nos. 58, 68, 78, 88 and 98) at 1200.degree. to 1500.degree. C. It has been determined, taking the above two values into account, that 26.20% by weight is an appropriate upper limit of the amount of Mg added.

It has been found from the above experimental results that a powder (magnetic oxide) comprising magnetite and another magnetic and/or nonmagnetic phase admixed together and having an arbitrary saturation magnetization as indicated by the hatched portion in FIG. 11(a) described later can be formed by mixing 0.20 to 26.20% by weight of Mg with magnetite and sintering the resultant mixed powder at 550.degree. to 1100.degree. C. for 2 hr.

FIGS. 11(a), 11(b) and 11(c) are explanatory vies of the scope of variation of saturation magnetization by regulation. These are summaries of the above-mentioned experimental results of Tables 16 to 18, made for easy understanding. The axis of abscissa indicates the heating temperature, the axis of ordinate the saturation magnetization by regulation.

FIG. 11(a) shows the scope of variation of saturation magnetization by regulation, based on Table 16 relating to hematite+Mg. 1100.degree. C., the magnetic oxide can have an arbitrary saturation magnetization ranging from 40 to 10 emu/g as indicated by the hatched portion in the figure by changing the amount of Mg added within the range of 0.20 to 18.50% by weight, as described with respect to Table 16. 1500.degree. C., the magnetic oxide can have an arbitrary saturation magnetization ranging from 90 to 10 emu/g as indicated by the hatched portion in the figure by changing the amount of Mg added within the range of 0.20 to 18.50% by weight, as described with respect to Table 16.

FIG. 11(b) shows the scope of variation of saturation magnetization by regulation, based on Table 17 relating to hematite+magnetite+Mg. 1100.degree. C., the magnetic oxide can have an arbitrary saturation magnetization ranging from 60 to 10 emu/g as indicated by the hatched portion in the Figure by changing the amount of Mg added within the range of 0.20 to 18.50% by weight, as described with respect to Table 17.

FIG. 11(c) shows the scope of variation of saturation magnetization by regulation, based on Table 18 relating to magnetite+Mg. 1100.degree. C., the magnetic oxide can have an arbitrary saturation magnetization ranging from 90 to 10 emu/g as indicated by the hatched portion in the Figure by changing the amount of Mg added within the range of 0.20 to 26.20% by weight, as described with respect to Table 18.

Table 19 shows results of a still further sintering experiment (hematite+Mg+Mn) according to this embodiment. In the experiment, 3.60% by weight, in terms of of powder of magnesium oxide (MgO) was blended with hematite powder, and further 0.00 to 10.00% by weight, in terms of Mn, of manganese oxide (Mn.sub.3 O.sub.4)was added thereto. The resultant mixture was treated under the same conditions as specified in Table 16. gradually increased from 13E8 Pa of Sample No. 16 of Table 16 not containing Mn to 17E8 Pa in accordance with the increase of the amount of Mn added as shown in Table 19, thus demonstrating the contribution of the addition of Mn to the grain strength. The saturation magnetization was also increased from 27 emu/g to 39 emu/g in accordance with the increase of the amount of Mn added. The increase of the saturation magnetization is caused by the incorporation of Mn which itself is magnetized. was gradually increased from 36E9 Pa of Sample No. 66 of Table 16 not containing Mn to 52E9 Pa in accordance with the increase of the amount of Mn added as shown in Table 19, thus demonstrating the contribution of the addition of M to the grain strength. The saturation magnetization was also increased from 67 emu/g to 80 emu/g in accordance with the increase of the amount of Mn added. The increase of the saturation magnetization is caused by the incorporation of Mn which itself is magnetized.

FIGS. 12(a) and 12(b) are explanatory views of the effect of the addition of Mn on the grain strength of the Mg blend according to this embodiment. These are graphs of the results of sintering experiment shown in Table 19, prepared for easy understanding. The axis of abscissa indicates the amount of Mn added, % by weight, while the axis of ordinate indicates the grain strength Pa.

FIG. 12 (a) shows the relationship between the amount of Mn added and the grain strength with respect to the magnetic oxide produced by sintering a powder obtained by mixing hematite with 3.60% by weight of Mg and further with Mn at 550.degree. C. in N.sub.2 for 2 hr as specified in Table 19. This graph demonstrates the effect of the addition of Mn in increasing the grain strength of the sintered magnetic oxide.

FIG. 12(b) shows the relationship between the amount of Mn added and the grain strength with respect to the magnetic oxide produced by sintering a powder obtained by mixing hematite with 3.60% by weight of Mg and further with Mn at 1300.degree. C. in N.sub.2 for 2 hr as specified in Table 19. This graph demonstrates the effect of the addition of Mn in increasing the grain strength of the sintered magnetic oxide.

FIG. 13 shows an exemplary heating temperature curve according to this embodiment. The curve is employed in the experiments relating to the above-mentioned Table 16 and FIGS. 12(a) and 12(b). In the sintering, the heating is conducted in an atmosphere of N.sub.2 at a rate of 200.degree. C./hr to a given temperature, which is held for 2 hr. Thereafter, the cooling is conducted at a rate of 200.degree. C./hr.

As described above, in the foregoing embodiment, an Si compound and a substance having a carbon-to-carbon single or double bond are mixed with hematite, hematite+magnetite, or magnetite, and sintered to thereby produce a powder comprising magnetite and nonmagnetic phase admixed together and having an arbitrary saturation magnetization value. Therefore, a magnetic oxide having any desired saturation magnetization can easily be mass-produced at a lowered cost. In particular, a powder (magnetic oxide) comprising magnetite and another magnetic and/or nonmagnetic phase admixed together and having an arbitrary saturation magnetization value can easily be produced at a lowered cost by a simple process in which a mixed powder comprising a large volume of hematite, hematite+magnetite, or magnetite, an Si compound and a substance having a carbon-to-carbon single or double bond is directly brought into the sintering step 5. __________________________________________________________________________ Firing Experiment Results (Hematite) of the Present Invention Amt. of Ca Heating   Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  1100 N.sub.2 80     found  found  none 2   0.71  "    "    82     found  found  none 3   1.43  "    "    80     found  found  none 4   3.55  "    "    75     found  found  none 5   4.97  "    "    72     found  found  none 6   7.10  "    "    68     found  found  none 7   10.65 "    "    64     found  found  none 8   14.30 "    "    60     found  found  none 9   0.00  1200 N.sub.2 92     none   found  none 10  0.71  "    "    88     none   found  none 11  1.43  "    "    88     none   found  none 12  3.55  "    "    86     none   found  none 13  4.97  "    "    80     none   found  none 14  7.10  "    "    77     none   found  none 15  10.65 "    "    molten immeasurable immeasurable immeasurable 16  14.30 "    "    "      "      "      " 17  0.00  1300 N.sub.2 92     none   found  none 18  0.71  "    "    88     none   found  none 19  1.43  "    "    87     none   found  none 20  3.55  "    "    86     none   found  none 21  4.97  "    "    81     none   found  none 22  7.10  "    "    77     none   found  none 23  10.65 "    "    molten immeasurable immeasurable immeasurable 24  14.30 "    "    "      "      "      " 25  0.00  1450 N.sub.2 92     none   found  none 26  0.71  "    "    88     none   found  none 27  1.43  "    "    87     none   found  none 28  3.55  "    "    85     none   found  none 29  4.97  "    "    80     none   found  none 30  7.10  "    "    76     none   found  none 31  10.65 "    "    molten immeasurable immeasurable immeasurable 32  14.30 "    "    "      "      "      " 33  0.00  1500 N.sub.2 85     none   found  found 34  0.71  "    "    85     none   found  found 35  1.43  "    "    87     none   found  found 36  3.55  "    "    80     none   found  found 37  4.97  "    "    77     none   found  found 38  7.10  "    "    molten none   found  found 39  10.65 "    "    "      immeasurable immeasurable immeasurable 40  14.30 "    "    "      "      "      " __________________________________________________________________________ __________________________________________________________________________ Firing Experiment Results (Hematite + Magnetite) of the Present Invention Amt. of Ca Heating   Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  1100 N.sub.2 67     found  found  none 2   0.71  "    "    66     found  found  none 3   1.43  "    "    65     found  found  none 4   3.55  "    "    64     found  found  none 5   4.97  "    "    61     found  found  none 6   7.10  "    "    56     found  found  none 7   10.65 "    "    51     found  found  none 8   14.30 "    "    47     found  found  none 9   0.00  1200 N.sub.2 92     none   found  none 10  0.71  "    "    89     none   found  none 11  1.43  "    "    88     none   found  none 12  3.55  "    "    86     none   found  none 13  4.97  "    "    80     none   found  none 14  7.10  "    "    77     none   found  none 15  10.65 "    "    molten immeasurable immeasurable immeasurable 16  14.30 "    "    "      "      "      " 17  0.00  1300 N.sub.2 92     none   found  none 18  0.71  "    "    88     none   found  none 19  1.43  "    "    88     none   found  none 20  3.55  "    "    86     none   found  none 21  4.97  "    "    81     none   found  none 22  7.10  "    "    75     none   found  none 23  10.65 "    "    molten immeasurable immeasurable immeasurable 24  14.30 "    "    "      "      "      " 25  0.00  1450 N.sub.2 92     none   found  none 26  0.71  "    "    88     none   found  none 27  1.43  "    "    88     none   found  none 28  3.55  "    "    85     none   found  none 29  4.97  "    "    82     none   found  none 30  7.10  "    "    76     none   found  none 31  10.65 "    "    molten immeasurable immeasurable immeasurable 32  14.30 "    "    "      "      "      " 33  0.00  1500 N.sub.2 87     none   found  found 34  0.71  "    "    85     none   found  found 35  1.43  "    "    86     none   found  found 36  3.55  "    "    80     none   found  found 37  4.97  "    "    77     none   found  found 38  7.10  "    "    molten none   found  found 39  10.65 "    "    "      immeasurable immeasurable immeasurable 40  14.30 "    "    "      "      "      " __________________________________________________________________________ __________________________________________________________________________ Firing Experiment Results (Magnetite) of the Present Invention Amt. of Ca Heating   Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  500  N.sub.2 86     found  found  none 2   0.71  "    "    85     found  found  none 3   1.43  "    "    84     found  found  none 4   3.55  "    "    83     found  found  none 5   4.97  "    "    80     found  found  none 6   7.10  "    "    77     found  found  none 7   10.65 "    "    72     found  found  none 8   14.30 "    "    69     found  found  none 9   21.30 "    "    60     found  found  none 10  35.50 "    "    44     found  found  none 11  56.80 "    "    18     found  found  none 12  64.00 "    "     9     found  found  none 13  0.00  550  N.sub.2 91     none   found  none 14  0.71  "    "    90     none   found  none 15  1.43  "    "    90     none   found  none 16  3.55  "    "    87     none   found  none 17  4.97  "    "    85     none   found  none 18  7.10  "    "    82     none   found  none 19  10.65 "    "    77     none   found  none 20  14.30 "    "    74     none   found  none 21  21.30 "    "    65     none   found  none 22  35.50 "    "    45     none   found  none 23  56.80 "    "    19     none   found  none 24  64.00 "    "    10     none   found  none 25  0.00  600  N.sub.2 92     none   found  none 26  0.71  "    "    90     none   found  none 27  1.43  "    "    90     none   found  none 28  3.55  "    "    87     none   found  none 29  4.97  "    "    84     none   found  none 30  7.10  "    "    83     none   found  none 31  10.65 "    "    76     none   found  none 32  14.30 "    "    74     none   found  none 33  21.30 "    "    65     none   found  none 34  35.50 "    "    45     none   found  none 35  56.80 "    "    20     none   found  none 36  64.00 "    "    10     none   found  none 37  0.00  800  N.sub.2 91     none   found  none 38  0.71  "    "    91     none   found  none 39  1.43  "    "    89     none   found  none 40  3.55  "    "    86     none   found  none 41  4.97  "    "    85     none   found  none 42  7.10  "    "    77     none   found  none 43  10.65 "    "    60     none   found  none 44  14.30 "    "    47     none   found  none 45  21.30 "    "    20     none   found  none 46  35.50 "    "     2     none   found  none 47  56.80 "    "     1     none   found  none 48  64.00 "    "     0     none   found  none 49  0.00  1100 N.sub.2 91     found  found  none 50  0.71  "    "    91     found  found  none 51  1.43  "    "    89     found  found  none 52  3.55  "    "    87     found  found  none 53  4.97  "    "    84     found  found  none 54  7.10  "    "    76     found  found  none 55  10.65 "    "    62     found  found  none 56  14.30 "    "    45     found  found  none 57  21.30 "    "    19     found  found  none 58  35.50 "    "     2     found  found  none 59  56.80 "    "     1     found  found  none 60  64.00 "    "     0     found  found  none 61  0.00  1200 N.sub.2 92     none   found  none 62  0.71  "    "    91     none   found  none 63  1.43  "    "    90     none   found  none 64  3.55  "    "    87     none   found  none 65  4.97  "    "    85     none   found  none 66  7.10  "    "    74     none   found  none 67  10.65 "    "    molten immeasurable immeasurable immeasurable 68  14.30 "    "    "      "      "      " 69  21.30 "    "    "      "      "      " 70  35.50 "    "    "      "      "      " 71  56.80 "    "    "      "      "      " 72  64.00 "    "    "      "      "      " 73  0.00  1450 N.sub.2 92     none   found  none 74  0.71  "    "    90     none   found  none 75  1.43  "    "    90     none   found  none 76  3.55  "    "    87     none   found  none 77  4.97  "    "    84     none   found  none 78  7.10  "    "    75     none   found  none 79  10.65 "    "    molten immeasurable immeasurable immeasurable 80  14.30 "    "    "      "      "      " 81  21.30 "    "    "      "      "      " 82  35.50 "    "    "      "      "      " 83  56.80 "    "    "      "      "      " 84  64.00 "    "    "      "      "      " 85  0.00  1500 N.sub.2 86     none   found  none 86  0.71  "    "    86     none   found  none 87  1.43  "    "    84     none   found  none 88  3.55  "    "    80     none   found  none 89  4.97  "    "    78     none   found  none 90  7.10  "    "    molten immeasurable immeasurable immeasurable 91  10.65 "    "    "      "      "      " 92  14.30 "    "    "      "      "      " 93  21.30 "    "    "      "      "      " 94  35.50 "    "    "      "      "      " 95  56.80 "    "    "      "      "      " 96  64.00 "    "    "      "      "      " __________________________________________________________________________ ______________________________________ Firing Experiment Results (TiO.sub.2 : 0 wt. %) of the Present Invention Amt.              Results of qual. Sam-       of PVA   Heating  anal. of powder ple        added    temp.    X-ray diffractometry No.        (wt. %)  T (.degree.C.) Fe.sub.3 O.sub.4 .alpha.-Fe.sub.2 O.sub.3 FeO ______________________________________ Comp. Ex. 1      0        800    --    .smallcircle. -- "       2      "        1000   --    .smallcircle. -- "       3      "        1100   --    .smallcircle. -- "       4      "        1150   .smallcircle. .smallcircle. -- "       5      "        1200   .smallcircle. .smallcircle. -- "       6      "        1400   .smallcircle. .smallcircle. -- "       7      "        1450   .smallcircle. .smallcircle. -- "       8      "        1470   .smallcircle. --     .smallcircle. Comp. Ex. 9      2.0      800    .smallcircle. .smallcircle. -- "       10     "        1000   .smallcircle. .smallcircle. -- "       11     "        1100   .smallcircle. .smallcircle. -- "       12     "        1150   .smallcircle. .smallcircle. -- Invention 13     "        1200   .smallcircle. --     -- "       14     "        1400   .smallcircle. --     -- "       15     "        1450   .smallcircle. --     -- Comp. Ex. 16     "        1500   .smallcircle. --     .smallcircle. Invention 17     0.1      1300   .smallcircle. --     -- "       18     0.2      "      .smallcircle. --     -- "       19     0.5      "      .smallcircle. --     -- "       20     1.0      "      .smallcircle. --     -- "       21     2.0      "      .smallcircle. --     -- "       22     3.0      "      .smallcircle. --     -- ______________________________________ __________________________________________________________________________ Experimental Examples of the Present Invention (Magnetite, Ti) Amt. of Ti Heating   Satn.  Grain Results of powder X-ray Sample added temp. Atmo- magnetization strength diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) (Pa) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  500  N.sub.2 86     15E6 none found none 2   0.60  "    "    85     15E6 none found none 3   1.20  "    "    84     17E6 none found none 4   3.00  "    "    83     12E6 none found none 5   6.20  "    "    80     13E6 none found none 6   9.50  "    "    77     14E6 none found none 7   13.00 "    "    72     13E6 none found none 8   16.70 "    "    69     12E6 none found none 9   28.60 "    "    60     11E6 none found none 10  37.50 "    "    44     12E6 none found none 11  52.70 "    "    28     13E6 none found none 12  84.40 "    "    9      11E6 none found none 13  0.00  550  N.sub.2 91     15E8 none found none 14  0.60  "    "    91     15E8 none found none 15  1.20  "    "    90     17E8 none found none 16  3.00  "    "    86     12E8 none found none 17  6.20  "    "    81     13E8 none found none 18  9.50  "    "    75     14E8 none found none 19  13.00 "    "    70     13E8 none found none 20  16.70 "    "    64     12E8 none found none 21  28.60 "    "    45     11E8 none found none 22  37.50 "    "    36     12E8 none found none 23  52.70 "    "    20     13E8 none found none 24  84.40 "    "    1      11E8 none found none 25  0.00  600  N.sub.2 92     23E8 none found none 26  0.60  "    "    91     25E8 none found none 27  1.20  "    "    90     25E8 none found none 28  3.00  "    "    86     22E8 none found none 29  6.20  "    "    81     22E8 none found none 30  9.50  "    "    74     24E8 none found none 31  13.00 "    "    70     24E8 none found none 32  16.70 "    "    64     22E8 none found none 33  28.60 "    "    47     21E8 none found none 34  37.50 "    "    35     20E8 none found none 35  52.70 "    "    20     25E8 none found none 36  84.40 "    "    1      21E8 none found none 37  0.00  800  N.sub.2 92     45E8 none found none 38  0.60  "    "    91     48E8 none found none 39  1.20  "    "    85     47E8 none found none 40  3.00  "    "    78     42E8 none found none 41  6.20  "    "    66     41E8 none found none 42  9.50  "    "    58     44E8 none found none 43  13.00 "    "    51     49E8 none found none 44  16.70 "    "    42     42E8 none found none 45  28.60 "    "    21     45E8 none found none 46  37.50 "    "    3      43E8 none found none 47  52.70 "    "    1      40E8 none found none 48  84.40 "    "    1      41E8 none found none 49  0.00  1100 N.sub.2 91     15E9 none found none 50  0.60  "    "    91     16E9 none found none 51  1.20  "    "    87     17E9 none found none 52  3.00  "    "    77     14E9 none found none 53  6.20  "    "    58     13E9 none found none 54  9.50  "    "    35     14E9 none found none 55  13.00 "    "    20     13E9 none found none 56  16.70 "    "    2      11E9 none found none 57  28.60 "    "    2      11E9 none found none 58  37.50 "    "    2      12E9 none found none 59  52.70 "    "    1      13E9 none found none 60  84.40 "    "    0      12E9 none found none 61  0.00  1200 N.sub.2 92     25E9 none found none 62  0.60  "    "    91     25E9 none found none 63  1.20  "    "    86     23E9 none found none 64  3.00  "    "    78     22E9 none found none 65  6.20  "    "    57     22E9 none found none 66  9.50  "    "    37     25E9 none found none 67  13.00 "    "    20     24E9 none found none 68  16.70 "    "    1      23E9 none found none 69  28.60 "    "    0      21E9 none found none 70  37.50 "    "    0      20E9 none found none 71  52.70 "    "    0      26E9 none found none 72  84.40 "    "    0      21E9 none found none 73  0.00  1450 N.sub.2 92     23E9 none found none 74  0.60  "    "    91     24E9 none found none 75  1.20  "    "    87     25E9 none found none 76  3.00  "    "    78     22E9 none found none 77  6.20  "    "    56     26E9 none found none 78  9.50  "    "    37     24E9 none found none 79  13.00 "    "    21     23E9 none found none 80  16.70 "    "    1      22E9 none found none 81  28.60 "    "    0      21E9 none found none 82  37.50 "    "    0      26E9 none found none 83  52.70 "    "    0      25E9 none found none 84  84.40 "    "    0      21E9 none found none 85  0.00  1500 N.sub.2 85     20E9 none found found 86  0.60  "    "    82     21E9 none found found 87  1.20  "    "    80     21E9 none found found 88  3.00  "    "    69     20E9 none found found 89  6.20  "    "    49     19E9 none found found 90  9.50  "    "    30     22E9 none found found 91  13.00 "    "    14     21E9 none found found 92  16.70 "    "    1      22E9 none found found 93  28.60 "    "    0      18E9 none found found 94  37.50 "    "    0      16E9 none found found 95  52.70 "    "    0      17E9 none found found 96  84.40 "    "    0      19E9 none found found __________________________________________________________________________ __________________________________________________________________________ Experiment Examples of the Present Invention (1:1 Magnetite + Hematite, Ti) Amt. of                    Satn. mag- Amt. of Amt.         magnet- netite hematite of Ti Heating  ization Grain Results of powder X-ray Sample added added added temp. Atmo- .delta.s strength diffractometry No. (wt %) (wt %) (wt %) T (.degree.C.) sphere (emu/g) (Pa) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.2 O.sub.4 FeO __________________________________________________________________________ 1   50.00 50.00 0.00 1100 N.sub.2 91   15E9 found found none 2   49.70 49.70 0.60 "    "   91   16E9 found found none 3   49.40 49.40 1.20 "    "   87   17E9 found found none 4   48.50 48.50 3.00 "    "   77   14E9 found found none 5   46.90 46.90 6.20 "    "   58   13E9 found found none 6   45.25 45.25 9.50 "    "   35   14E9 found found none 7   43.50 43.50 13.00 "    "   20   13E9 found found none 8   41.70 41.70 16.60 "    "   2    11E9 found found none 9   35.70 35.70 28.60 "    "   2    11E9 found found none 10  31.30 31.30 37.40 "    "   2    12E9 found found none 11  23.70 23.70 52.60 "    "   1    13E9 found found none 12  7.80 7.80 84.40 "    "   0    12E9 found found none 13  50.00 50.00 0.00 1200 N.sub.2 92   25E9 none found none 14  49.70 49.70 0.60 "    "   91   25E9 none found none 15  49.40 49.40 1.20 "    "   86   23E9 none found none 16  48.50 48.50 3.00 "    "   78   22E9 none found none 17  46.90 46.90 6.20 "    "   57   22E9 none found none 18  45.25 45.25 9.50 "    "   37   25E9 none found none 19  43.50 43.50 13.00 "    "   20   24E9 none found none 20  41.70 41.70 16.60 "    "   1    23E9 none found none 21  35.70 35.70 28.60 "    "   0    21E9 none found none 22  31.30 31.30 37.40 "    "   0    20E9 none found none 23  23.70 23.70 52.70 "    "   0    26E9 none found none 24  7.80 7.80 84.40 "    "   0    21E9 none found none 25  50.00 50.00 0.00 1450 N.sub.2 92   23E9 none found none 26  49.70 49.70 0.60 "    "   91   24E9 none found none 27  49.40 49.40 1.20 "    "   87   25E9 none found none 28  48.50 48.50 3.00 "    "   78   22E9 none found none 29  46.90 46.90 6.20 "    "   56   26E9 none found none 30  45.25 45.25 9.50 "    "   37   24E9 none found none 31  43.50 43.50 13.00 "    "   21   23E9 none found none 32  41.70 41.70 16.60 "    "   1    22E9 none found none 33  35.70 35.70 28.60 "    "   0    21E9 none found none 34  31.30 31.30 37.40 "    "   0    26E9 none found none 35  23.70 23.70 52.60 "    "   0    25E9 none found none 36  7.80 7.80 84.40 "    "   0    21E9 none found none 37  50.00 50.00 0.00 1500 N.sub.2 85   20E9 none found found 38  49.70 49.70 0.60 "    "   82   21E9 none found found 39  49.40 49.40 1.20 "    "   80   21E9 none found found 40  48.50 48.50 3.00 "    "   69   20E9 none found found 41  46.90 46.90 6.20 "    "   49   19E9 none found found 42  45.25 45.25 9.50 "    "   30   22E9 none found found 43  43.50 43.50 13.00 "    "   14   21E9 none found found 44  41.70 41.70 16.60 "    "   1    22E9 none found found 45  35.70 35.70 28.60 "    "   0    18E9 none found found 46  31.30 31.30 37.40 "    "   0    16E9 none found found 47  23.70 23.70 52.60 "    "   0    17E9 none found found 48  7.80 7.80 84.40 "    "   0    19E9 none found found __________________________________________________________________________ __________________________________________________________________________ Experiment Examples of the Present Invention (1:3 Magnetite + Hematite, Ti) Amt. of                    Satn. mag- Amt. of Amt.         magnet- netite hematite of Ti Heating  ization Grain Results of powder X-ray Sample added added added temp. Atmo- .delta.s strength diffractometry No. (wt %) (wt %) (wt %) T (.degree.C.) sphere (emu/g) (Pa) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.2 O.sub.4 FeO __________________________________________________________________________ 1   25.00 75.00 0.00 1100 N.sub.2 91   15E9 none found none 2   24.90 74.50 0.60 "    "   91   16E9 none found none 3   24.70 74.10 1.20 "    "   87   17E9 none found none 4   24.25 72.75 3.00 "    "   77   14E9 none found none 5   23.45 70.35 6.20 "    "   58   13E9 none found none 6   22.63 67.87 9.50 "    "   35   14E9 none found none 7   21.75 65.25 13.00 "    "   20   13E9 none found none 8   20.83 62.47 16.70 "    "   2    11E9 none found none 9   17.85 53.55 28.60 "    "   2    11E9 none found none 10  15.63 46.87 37.50 "    "   2    12E9 none found none 11  11.83 35.47 52.70 "    "   1    13E9 none found none 12  3.90 11.70 84.40 "    "   0    12E9 none found none 13  25.00 75.00 0.00 1200 N.sub.2 92   25E9 none found none 14  24.90 74.50 0.60 "    "   91   25E9 none found none 15  24.70 74.10 1.20 "    "   86   23E9 none found none 16  24.25 72.75 3.00 "    "   78   22E9 none found none 17  23.45 70.35 6.20 "    "   57   22E9 none found none 18  22.63 67.87 9.50 "    "   37   25E9 none found none 19  21.75 65.25 13.00 "    "   20   24E9 none found none 20  20.83 62.47 16.70 "    "   1    23E9 none found none 21  17.85 53.55 28.60 "    "   0    21E9 none found none 22  15.63 46.87 37.50 "    "   0    20E9 none found none 23  11.83 35.47 52.70 "    "   0    26E9 none found none 24  3.90 11.70 84.40 "    "   0    21E9 none found none 25  25.00 75.00 0.00 1450 N.sub.2 92   23E9 none found none 26  24.90 74.50 0.60 "    "   91   24E9 none found none 27  24.70 74.10 1.20 "    "   87   25E9 none found none 28  24.25 72.75 3.00 "    "   78   22E9 none found none 29  23.45 70.35 6.20 "    "   56   26E9 none found none 30  22.63 67.87 9.50 "    "   37   24E9 none found none 31  21.75 65.25 13.00 "    "   21   23E9 none found none 32  20.83 62.47 16.70 "    "   1    22E9 none found none 33  17.85 53.55 28.60 "    "   0    21E9 none found none 34  15.63 46.87 37.50 "    "   0    26E9 none found none 35  11.83 35.47 52.70 "    "   0    25E9 none found none 36  3.90 11.70 84.40 "    "   0    21E9 none found none 37  25.00 75.00 0.00 1500 N.sub.2 85   20E9 none found found 38  24.90 74.50 0.60 "    "   82   21E9 none found found 39  24.70 74.10 1.20 "    "   80   21E9 none found found 40  24.25 72.75 3.00 "    "   69   20E9 none found found 41  23.45 70.35 6.20 "    "   49   19E9 none found found 42  22.63 67.87 9.50 "    "   30   22E9 none found found 43  21.75 65.25 13.00 "    "   14   21E9 none found found 44  20.83 62.47 16.70 "    "   1    22E9 none found found 45  17.85 53.55 28.60 "    "   0    18E9 none found found 46  15.63 46.87 37.50 "    "   0    16E9 none found found 47  11.83 35.47 52.70 "    "   0    17E9 none found found 48  3.90 11.70 84.40 "    "   0    19E9 none found found __________________________________________________________________________ __________________________________________________________________________ Experiment Examples of the Present Invention (1:4 Magnetite + Hematite, Ti) Amt. of                    Satn. mag- Amt. of Amt.         magnet- netite hematite of Ti Heating  ization Grain Results of powder X-ray Sample added added added temp. Atmo- .delta.s strength diffractometry No. (wt %) (wt %) (wt %) T (.degree.C.) sphere (emu/g) (Pa) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.2 O.sub.4 FeO __________________________________________________________________________ 1   20.00 80.00 0.00 1100 N.sub.2 91   15E9 found found none 2   19.90 79.50 0.60 "    "   91   16E9 found found none 3   19.80 79.00 1.20 "    "   87   17E9 found found none 4   19.40 77.60 3.00 "    "   77   14E9 found found none 5   18.80 75.00 6.20 "    "   58   13E9 found found none 6   18.10 72.40 9.50 "    "   35   14E9 found found none 7   17.40 69.40 13.00 "    "   20   13E9 found found none 8   16.70 66.60 16.70 "    "   2    11E9 found found none 9   14.30 57.10 28.60 "    "   2    11E9 found found none 10  12.50 50.00 37.50 "    "   2    12E9 found found none 11  9.50 37.80 52.70 "    "   1    13E9 found found none 12  3.10 12.50 84.40 "    "   0    12E9 found found none 13  20.00 80.00 0.00 1200 N.sub.2 92   25E9 none found none 14  19.90 79.50 0.60 "    "   91   25E9 none found no)ne 15  19.80 79.00 1.20 "    "   86   23E9 none found none 16  19.40 77.60 3.00 "    "   78   22E9 none found none 17  18.80 75.00 6.20 "    "   57   22E9 none found none 18  18.10 72.40 9.50 "    "   37   25E9 none found none 19  17.40 69.40 13.00 "    "   20   24E9 none found none 20  16.70 66.60 16.70 "    "   1    23E9 none found none 21  14.30 57.10 28.60 "    "   0    21E9 none found none 22  12.50 50.00 37.50 "    "   0    20E9 none found none 23  9.50 37.80 52.70 "    "   0    26E9 none found none 24  3.10 12.50 84.40 "    "   0    21E9 none found none 25  20.00 80.00 0.00 1450 N.sub.2 92   23E9 none found none 26  19.90 79.50 0.60 "    "   91   24E9 none found none 27  19.80 79.00 1.20 "    "   87   25E9 none found none 28  19.40 77.60 3.00 "    "   78   22E9 none found none 29  18.80 75.00 6.20 "    "   56   26E9 none found none 30  18.10 72.40 9.50 "    "   37   24E9 none found none 31  17.40 69.40 13.00 "    "   21   23E9 none found none 32  16.70 66.60 16.70 "    "   1    22E9 none found none 33  14.30 57.10 28.60 "    "   0    21E9 none found none 34  12.50 50.00 37.50 "    "   0    26E9 none found none 35  9.50 37.80 52.70 "    "   0    25E9 none found none 36  3.10 12.50 84.40 "    "   0    21E9 none found none 37  20.00 80.00 0.00 1500 N.sub.2 85   20E9 none found found 38  19.90 79.50 0.60 "    "   82   21E9 none found found 39  19.80 79.00 1.20 "    "   80   21E9 none found found 40  19.40 77.60 3.00 "    "   69   20E9 none found found 41  18.80 75.00 6.20 "    "   49   19E9 none found found 42  18.10 72.40 9.50 "    "   30   22E9 none found found 43  17.40 69.40 13.00 "    "   14   21E9 none found found 44  16.70 66.60 16.70 "    "   1    22E9 none found found 45  14.30 57.10 28.60 "    "   0    18E9 none found found 46  12.50 50.00 37.50 "    "   0    16E9 none found found 47  9.50 37.80 52.70 "    "   0    17E9 none found found 48  3.10 12.50 84.40 "    "   0    19E9 none found found __________________________________________________________________________ __________________________________________________________________________ Other Experimental Examples of the Present Invention (Hematite, Sn) Satn. Amt.            magnet- of Sn Heating     ization Results of powder Sample added temp.       .delta.s X-ray diffractometry No. (wt %) T (.degree.C.) Atmosphere (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00 1100 N.sub.2 86   found  found none 2   0.80 "    "      85   found  found none 3   1.58 "    "      84   found  found none 4   3.98 "    "      83   found  found none 5   8.04 "    "      75   found  found none 6   16.44 "    "      69   found  found none 7   44.04 "    "      41   found  found none 8   64.74 "    "      26   found  found none 9   76.00 "    "      16   found  found none 10  87.60 "    "      8    found  found none 11  0.00 1200 N.sub.2 92   none   found none 12  0.80 "    "      91   none   found none 13  1.58 "    "      90   none   found none 14  3.98 "    "      87   none   found none 15  8.04 "    "      82   none   found none 16  16.44 "    "      76   none   found none 17  44.04 "    "      47   none   found none 18  64.74 "    "      28   none   found none 19  76.00 "    "      21   none   found none 20  87.60 "    "      9    none   found none 21  0.00 1300 N.sub.2 92   none   found none 22  0.80 "    "      91   none   found none 23  1.58 "    "      90   none   found none 24  3.98 "    "      86   none   found none 25  8.04 "    "      82   none   found none 26  16.44 "    "      74   none   found none 27  44.04 "    "      47   none   found none 28  64.74 "    "      25   none   found none 29  76.00 "    "      21   none   found none 30  87.60 "    "      8    none   found none 31  0.00 1450 N.sub.2 92   none   found none 32  0.80 "    "      91   none   found none 33  1.58 "    "      90   none   found none 34  3.98 "    "      87   none   found none 35  8.04 "    "      85   none   found none 36  16.44 "    "      75   none   found none 37  44.04 "    "      47   none   found none 38  64.74 "    "      29   none   found none 39  76.00 "    "      21   none   found none 40  87.60 "    "      9    none   found none 41  0.00 1500 N.sub.2 86   none   found found 42  0.80 "    "      85   none   found found 43  1.58 "    "      84   none   found found 44  3.98 "    "      83   none   found found 45  8.04 "    "      75   none   found found 46  16.44 "    "      68   none   found found 47  44.04 "    "      39   none   found found 48  64.74 "    "      22   none   found found 49  76.00 "    "      15   none   found found 50  87.60 "    "      2    none   found found __________________________________________________________________________ __________________________________________________________________________ Other Experimental Examples of the Present Invention (1:1 Hematite + Magnetite, Sn) Satn. Amt.            magnet- of Sn Heating     ization Results of powder Sample added temp.       .delta.s X-ray diffractometry No. (wt %) T (.degree.C.) Atmosphere (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00 1100 N.sub.2 86   found  found none 2   0.80 "    "      85   found  found none 3   1.58 "    "      84   found  found none 4   3.98 "    "      83   found  found none 5   8.04 "    "      75   found  found none 6   16.44 "    "      69   found  found none 7   44.04 "    "      39   found  found none 8   64.74 "    "      22   found  found none 9   76.00 "    "      16   found  found none 10  87.60 "    "      4    found  found none 11  0.00 1200 N.sub.2 92   none   found none 12  0.80 "    "      91   none   found none 13  1.58 "    "      90   none   found none 14  3.98 "    "      87   none   found none 15  8.04 "    "      85   none   found none 16  16.44 "    "      75   none   found none 17  44.04 "    "      47   none   found none 18  64.74 "    "      29   none   found none 19  76.00 "    "      21   none   found none 20  87.60 "    "      9    none   found none 21  0.00 1300 N.sub.2 92   none   found none 22  0.80 "    "      91   none   found none 23  1.58 "    "      90   none   found none 24  3.98 "    "      87   none   found none 25  8.04 "    "      85   none   found none 26  16.44 "    "      74   none   found none 27  44.04 "    "      47   none   found none 28  64.74 "    "      29   none   found none 29  76.00 "    "      20   none   found none 30  87.60 "    "      9    none   found none 31  0.00 1450 N.sub.2 92   none   found none 32  0.80 "    "      91   none   found none 33  1.58 "    "      90   none   found none 34  3.98 "    "      86   none   found none 35  8.04 "    "      85   none   found none- 36  16.44 "    "      74   none   found none 37  44.04 "    "      46   none   found none 38  64.74 "    "      29   none   found none 39  76.00 "    "      20   none   found none 40  87.60 "    "      10   none   found none 41  0.00 1500 N.sub.2 86   none   found found 42  0.80 "    "      85   none   found found 43  1.58 "    "      84   none   found found 44  3.98 "    "      83   none   found found 45  8.04 "    "      75   none   found found 46  16.44 "    "      69   none   found found 47  44.04 "    "      39   none   found found 48  64.74 "    "      22   none   found found 49  76.00 "    "      16   none   found found 50  87.60 "    "      4    none   found found __________________________________________________________________________ __________________________________________________________________________ Other Experimental Examples of the Present Invention (Magnetite, Sn) Satn. Amt.            magnet- of Sn Heating     ization Results of powder Sample added temp.       .delta.s X-ray diffractometry No. (wt %) T (.degree.C.) Atmosphere (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00 500  N.sub.2 86   found  found none 2   0.80 "    "      85   found  found none 3   1.58 "    "      84   found  found none 4   3.98 "    "      83   found  found none 5   8.04 "    "      75   found  found none 6   16.44 "    "      70   found  found none 7   44.04 "    "      39   found  found none 8   64.74 "    "      22   found  found none 9   76.00 "    "      14   found  found none 10  87.60 "    "      3    found  found none 11  0.00 550  N.sub.2 92   none   found none 12  0.80 "    "      91   none   found none 13  1.58 "    "      90   none   found none 14  3.98 "    "      87   none   found none 15  8.04 "    "      85   none   found none 16  16.44 "    "      75   none   found none 17  44.04 "    "      47   none   found none 18  64.74 "    "      29   none   found none 19  76.00 "    "      21   none   found none 20  87.60 "    "      9    none   found none 21  0.00 600  N.sub.2 92   none   found none 22  0.80 "    "      91   none   found none 23  1.58 "    "      90   none   found none 24  3.98 "    "      87   none   found none 25  8.04 "    "      85   none   found none 26  16.44 "    "      74   none   found none 27  44.04 "    "      47   none   found none 28  64.74 "    "      29   none   found none 29  76.00 "    "      20   none   found none 30  87.60 "    "      9    none   found none 31  0.00 800  N.sub.2 92   none   found none 32  0.80 "    "      91   none   found none. 33  1.58 "    "      90   none   found none 34  3.98 "    "      86   none   found none 35  8.04 "    "      85   none   found none 36  16.44 "    "      74   none   found none 37  44.04 "    "      46   none   found none 38  64.74 "    "      29   none   found none 39  76.00 "    "      20   none   found none 40  87.60 "    "      10   none   found none 41  0.00 1100 N.sub.2 92   none   found none 42  0.80 "    "      91   none   found none 43  1.58 "    "      90   none   found none 44  3.98 "    "      87   none   found none 45  8.04 "    "      83   none   found none 46  16.44 "    "      75   none   found none 47  44.04 "    "      47   none   found none 48  64.74 "    "      28   none   found none 49  76.00 "    "      20   none   found none 50  87.60 "    "      9    none   found none 51  0.00 1200 N.sub.2 92   none   found none 52  0.80 "    "      91   none   found none 53  1.58 "    "      90   none   found none 54  3.98 "    "      87   none   found none 55  8.04 "    "      82   none   found none 56  16.44 "    "      74   none   found none 57  44.04 "    "      47   none   found none 58  64.74 "    "      26   none   found none 59  76.00 "    "      21   none   found none 60  87.60 "    "      8    none   found none 61  0.00 1300 N.sub.2 92   none   found none 62  0.80 "    "      91   none   found none 63  1.58 "    "      90   none   found none 64  3.98 "    "      87   none   found none 65  8.04 "    "      85   none   found none 66  16.44 "    "      74   none   found none 67  44.04 "    "      47   none   found none 68  64.74 "    "      29   none   found none 69  76.00 "    "      20   none   found none 70  87.60 "    "      9    none   found none 71  0.00 1450 N.sub.2 92   none   found none 72  0.80 "    "      91   none   found none 73  1.58 "    "      90   none   found none 74  3.98 "    "      87   none   found none 75  8.04 "    "      82   none   found none 76  16.44 "    "      74   none   found none 77  44.04 "    "      47   none   found none 78  64.74 "    "      26   none   found none 79  76.00 "    "      21   none   found none 80  87.60 "    "      8    none   found none 81  0.00 1500 N.sub.2 86   none   found found 82  0.80 "    "      85   none   found found 83  1.58 "    "      84   none   found found 84  3.98 "    "      83   none   found found 85  8.04 "    "      75   none   found found 86  16.44 "    "      69   none   found found 87  44.04 "    "      39   none   found found 88  64.74 "    "      22   none   found found 89  76.00 "    "      16   none   found found 90  87.60 "    "      4    none   found found __________________________________________________________________________ ______________________________________ Firing Experiment Results (SiO.sub.2 : O wt. %) of the Present Invention Amt. Sam-       of PVA   Heating  Results of powder ple        added    temp.    X-ray diffractometry No.        (wt. %)  T (.degree.C.) Fe.sub.3 O.sub.4 .alpha.-Fe.sub.2 O.sub.3 FeO ______________________________________ Comp. Ex. 1      0         800   --    .smallcircle. -- "       2      "        1000   --    .smallcircle. -- "       3      "        1100   --    .smallcircle. -- "       4      "        1150   .smallcircle. .smallcircle. -- "       5      "        1200   .smallcircle. .smallcircle. -- "       6      "        1400   .smallcircle. .smallcircle. -- "       7      "        1450   .smallcircle. .smallcircle. -- "       8      "        1470   .smallcircle. --     .smallcircle. Comp. Ex. 9      2.0       800   .smallcircle. .smallcircle. -- "       10     "        1000   .smallcircle. .smallcircle. -- "       11     "        1100   .smallcircle. .smallcircle. -- "       12     "        1150   .smallcircle. .smallcircle. -- Invention 13     "         200   .smallcircle. --     -- "       14     "        1400   .smallcircle. --     -- "       15     "        1450   .smallcircle. --     -- Comp. Ex. 16     "        1500   .smallcircle. --     .smallcircle. Invention 17     0.1      1300   .smallcircle. --     -- "       18     0.2      "      .smallcircle. --     -- "       19     0.5      "      .smallcircle. --     -- "       20     1.0      "      .smallcircle. --     -- "       21     2.0      "      .smallcircle. --     -- "       22     3.0      "      .smallcircle. --     -- ______________________________________ __________________________________________________________________________ Experimental Examples of the Present Invention (Hematite) Amt. of Si Heating  Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  1100 N.sub.2 86     found  found none 2   0.50  "    "   85     found  found none 3   0.90  "    "   84     found  found none 4   2.40  "    "   83     found  found none 5   4.90  "    "   75     found  found none 6   10.40 "    "   69     found  found none 7   31.50 "    "   39     found  found none 8   51.20 "    "   22     found  found none 9   65.00 "    "   16     found  found none 10  79.50 "    "   4      found  found none 11  0.00  1200 N.sub.2 92     none   found none 12  0.50  "    "   91     none   found none 13  0.90  "    "   90     none   found none 14  2.40  "    "   87     none   found none 15  4.90  "    "   83     none   found none 16  10.40 "    "   75     none   found none 17  31.50 "    "   47     none   found none 18  51.20 "    "   28     none   found none 19  65.00 "    "   20     none   found none 20  79.50 "    "   9      none   found none 21  0.00  1300 N.sub.2 92     none   found none 22  0.50  "    "   91     none   found none 23  0.90  "    "   90     none   found none 24  2.40  "    "   87     none   found none 25  4.90  "    "   82     none   found none 26  10.40 "    "   74     none   found none 27  31.50 "    "   47     none   found none 28  51.20 "    "   26     none   found none 29  65.00 "    "   21     none   found none 30  79.50 "    "   8      none   found none 31  0.00  1450 N.sub.2 92     none   found none 32  0.50  "    "   91     none   found none 33  0.90  "    "   90     none   found none 34  2.40  "    "   87     none   found none 35  4.90  "    "   85     none   found none 36  10.40 "    "   74     none   found none 37  31.50 "    "   47     none   found none 38  51.20 "    "   29     none   found none 39  65.00 "    "   20     none   found none 40  79.50 "    "   9      none   found none 41  0.00  1500 N.sub.2 86     none   found found 42  0.50  "    "   85     none   found found 43  0.90  "    "   84     none   found found 44  2.40  "    "   83     none   found found 45  4.90  "    "   75     none   found found 46  10.40 "    "   69     none   found found 47  31.50 "    "   39     none   found found 48  51.20 "    "   22     none   found found 49  65.00 "    "   16     none   found found 50  79.50 "    "   4      none   found found __________________________________________________________________________ __________________________________________________________________________ Experimental Examples of the Present Invention (Hematite + Magnetite) Amt. of Si Heating  Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  1100 N.sub.2 86     found  found none 2   0.50  "    "   85     found  found none 3   0.90  "    "   84     found  found none 4   2.40  "    "   83     found  found none 5   4.90  "    "   75     found  found none 6   10.40 "    "   69     found  found none 7   31.50 "    "   39     found  found none 8   51.20 "    "   22     found  found none 9   65.00 "    "   16     found  found none 10  79.50 "    "   4      found  found none 11  0.00  1200 N.sub.2 92     none   found none 12  0.50  "    "   91     none   found none 13  0.90  "    "   90     none   found none 14  2.40  "    "   87     none   found none 15  4.90  "    "   85     none   found none 16  10.40 "    "   75     none   found none 17  31.50 "    "   47     none   found none 18  51.20 "    "   29     none   found none 19  65.00 "    "   21     none   found none 20  79.50 "    "   9      none   found none 21  0.00  1300 N.sub.2 92     none   found none 22  0.50  "    "   91     none   found none 23  0.90  "    "   90     none   found none 24  2.40  "    "   87     none   found none 25  4.90  "    "   85     none   found none 26  10.40 "    "   74     none   found none 27  31.50 "    "   47     none   found none 28  51.20 "    "   29     none   found none 29  65.00 "    "   20     none   found none 30  79.50 "    "   9      none   found none 31  0.00  1450 N.sub.2 92     none   found none 32  0.50  "    "   91     none   found none 33  0.90  "    "   90     none   found none 34  2.40  "    "   86     none   found none 35  4.90  "    "   85     none   found none 36  10.40 "    "   74     none   found none- 37  31.50 "    "   46     none   found none 38  51.20 "    "   29     none   found none 39  65.00 "    "   20     none   found none 40  79.50 "    "   10     none   found none 41  0.00  1500 N.sub.2 86     none   found found 42  0.50  "    "   85     none   found found 43  0.90  "    "   84     none   found found 44  2.40  "    "   83     none   found found 45  4.90  "    "   75     none   found found 46  10.40 "    "   69     none   found found 47  31.50 "    "   39     none   found found 48  51.20 "    "   22     none   found found 49  65.00 "    "   16     none   found found 50  79.50 "    "   4      none   found found __________________________________________________________________________ __________________________________________________________________________ Experimental Examples of the Present Invention (Magnetite) Amt. of Si Heating  Satn. Sample added temp. Atmo- magnetization Results of powder X-ray diffractometry No. (wt %) T (.degree.C.) sphere .delta.s (emu/g) .alpha.-Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 FeO __________________________________________________________________________ 1   0.00  500  N.sub.2 86     found  found none 2   0.50  "    "   85     found  found none 3   0.90  "    "   84     found  found none 4   2.40  "    "   83     found  found none 5   4.90  "    "   75     found  found none 6   10.40 "    "   70     found  found none 7   31.50 "    "   39     found  found none 8   51.20 "    "   22     found  found none 9   65.00 "    "   14     found  found none 10  79.50 "    "   3      found  found none 11  0.00  550  N.sub.2 92     none   found none 12  0.50  "    "   91     none   found none 13  0.90  "    "   90     none   found none 14  2.40  "    "   87     none   found none 15  4.90  "    "   85     none   found none 16  10.40 "    "   75     none   found none 17  31.50 "    "   47     none   found none 18  51.20 "    "   29     none   found none 19  65.00 "    "   21     none   found none 20  79.50 "    "   9      none   found none 21  0.00  600  N.sub.2 92     none   found none 22  0.50  "    "   91     none   found none 23  0.90  "    "   90     none   found none 24  2.40  "    "   87     none   found none 25  4.90  "    "   85     none   found none 26  10.40 "    "   74     none   found none 27  31.50 "    "   47     none   found none 28  51.20 "    "   29     none   found none 29  65.00 "    "   20     none   found none 30  79.50 "    "   9      none   found none 31  0.00  800  N.sub.2 92     none   found none 32  0.50  "    "   91     none   found none 33  0.90  "    "   90     none   found none 34  2.40  "    "   86     none   found none 35  4.90  "    "   85     none   found none 36  10.40 "    "   74     none   found none 37  31.50 "    "   46     none   found none 38  51.20 "    "   29     none   found none 39  65.00 "    "   20     none   found none 40  79.50 "    "   10     none   found none 41  0.00  1100 N.sub.2 92     none   found none 42  0.50  "    "   91     none   found none 43  0.90  "    "   90     none   found none 44  2.40  "    "   87     none   found none 45  4.90  "    "   83     none   found none 46  10.40 "    "   75     none   found none 47  31.50 "    "   47     none   found none 48  51.20 "    "   28     none   found none 49  65.00 "    "   20     none   found none 50  79.50 "    "   9      none   found none 51  0.00  1200 N.sub.2 92     none   found none 52  0.50  "    "   91     none   found none 53  0.90  "    "   90     none   found none 54  2.40  "    "   87     none   found none 55  4.90  "    "   82     none   found none 56  10.40 "    "   74     none   found none 57  31.50 "    "   47     none   found none 58  51.20 "    "   26     none   found none 59  65.00 "    "   21     none   found none 60  79.50 "    "   8      none   found none 61  0.00  1300 N.sub.2 92     none   found none 62  0.50  "    "   91     none   found none 63  0.90  "    "   90     none   found none 64  2.40  "    "   87     none   found none 65  4.90  "    "   85     none   found none 66  10.40 "    "   74     none   found none 67  31.50 "    "   47     none   found none 68  51.20 "    "   29     none   found none 69  65.00 "    "   20     none   found none 70  79.50 "    "   9      none   found none 71  0.00  1450 N.sub.2 92     none   found none 72  0.50  "    "   91     none   found none 73  0.90  "    "   90     none   found none 74  2.40  "    "   87     none   found none 75  4.90  "    "   82     none   found none 76  10.40 "    "   74     none   found none 77  31.50 "    "   47     none   found none 78  51.20 "    "   26     none   found none 79  65.00 "    "   21     none   found none 80  79.50 "    "   8      none   found none 81  0.00  1500 N.sub.2 86     none   found found 82  0.50  "    "   85     none   found found 83  0.90  "    "   84     none   found found 84  2.40  "    "   83     none   found found 85  4.90  "    "   75     none   found found 86  10.40 "    "   69     none   found found 87  31.50 "    "   39     none   found found 88  51.20 "    "   22     none   found found 89  65.00 "    "   16     none   found found 90  79.50 "    "   4      none   found found __________________________________________________________________________ ______________________________________ Firing Experiment Results (Hematite + Mg) of the Present Invention Amt. of Mg Heating        Satn.    Grain Sample added     temp.    Atmos- magnetization strength No.   (wt %)    (.degree.C.) phere .delta.s (emu/g) (Pa) ______________________________________ 1     0.00      500      N.sub.2 40       12E6 2     0.10      "        "     41       15E6 3     0.20      "        "     41       14E6 4     0.80      "        "     35       13E6 5     1.60      "        "     29       13E6 6     3.60      "        "     27       12E6 7     13.20     "        "     16       14E6 8     18.50     "        "     10       14E6 9     26.20     "        "     5        12E6 10    40.00     "        "     1        11E6 11    0.00      550      N.sub.2 40       12E8 12    0.10      "        "     41       13E8 13    0.20      "        "     41       12E8 14    0.80      "        "     35       13E8 15    1.60      "        "     29       14E8 16    3.60      "        "     27       13E8 17    13.20     "        "     16       15E8 18    18.50     "        "     10       14E8 19    26.20     "        "     5        15E8 20    40.00     "        "     1        13E8 21    0.00      600      N.sub.2 40       15E8 22    0.10      "        "     41       15E8 23    0.20      "        "     41       16E8 24    0.80      "        "     35       15E8 25    1.60      "        "     28       14E8 26    3.60      "        "     27       16E8 27    13.20     "        "     16       15E8 28    18.50     "        "     11       16E8 29    26.20     "        "     5        15E8 30    40.00     "        "     1        17E8 31    0.00      800      N.sub.2 40       25E8 32    0.10      "        "     41       26E8 33    0.20      "        "     41       26E8 34    0.80      "        "     35       26E8 35    1.60      "        "     29       25E8 36    3.60      "        "     26       28E8 37    13.20     "        "     16       26E8 38    18.50     "        "     10       25E8 39    26.20     "        "     4        27E8 40    40.00     "        "     1        23E8 41    0.00      1100     N.sub.2 40       16E9 42    0.10      "        "     41       15E9 43    0.20      "        "     40       16E9 44    0.80      "        "     35       16E9 45    1.60      "        "     29       17E9 46    3.60      "        "     24       17E9 47    13.20     "        "     16       17E9 48    18.50     "        "     10       16E9 49    26.20     "        "     5        17E9 50    40.00     "        "     1        17E9 51    0.00      1200     N.sub.2 92       23E9 52    0.10      "        "     92       24E9 53    0.20      "        "     90       25E9 54    0.80      "        "     87       26E9 55    1.60      "        "     81       25E9 56    3.60      "        "     67       26E9 57    13.20     "        "     32       26E9 58    18.50     "        "     21       23E9 59    26.20     "        "     6        25E9 60    40.00     "        "     1        27E9 61    0.00      1300     N.sub.2 92       30E9 62    0.10      "        "     92       34E9 63    0.20      "        "     90       34E9 64    0.80      "        "     87       35E9 65    1.60      "        "     81       35E9 66    3.60      "        "     67       36E9 67    13.20     "        "     32       37E9 68    18.50     "        "     21       36E9 69    26.20     "        "     6        36E9 70    40.00     "        "     1        35E9 71    0.00      1400     N.sub.2 92       33E9 72    0.10      "        "     92       34E9 73    0.20      "        "     90       34E9 74    0.80      "        "     87       35E9 75    1.60      "        "     82       35E9 76    3.60      "        "     67       36E9 77    13.20     "        "     31       37E9 78    18.50     "        "     20       36E9 79    26.20     "        "     6        36E9 80    40.00     "        "     1        35E9 81    0.00      1450     N.sub.2 92       36E9 82    0.10      "        "     92       37E9 83    0.20      "        "     90       36E9 84    0.80      "        "     87       37E9 85    1.60      "        "     81       37E9 86    3.60      "        "     66       36E9 87    13.20     "        "     32       37E9 88    18.50     "        "     20       36E9 89    26.20     "        "     6        37E9 90    40.00     "        "     1        37E9 91    0.00      1500     N.sub.2 86       40E9 92    0.10      "        "     85       42E9 93    0.20      "        "     82       40E9 94    0.80      "        "     77       41E9 95    1.60      "        "     70       42E9 96    3.60      "        "     65       41E9 97    13.20     "        "     25       43E9 98    18.50     "        "     16       43E9 99    26.20     "        "     3        41E9 100   40.00     "        "     1        40E9 ______________________________________ ______________________________________ Firing Experiment Results (Hematite + Magnetite + Mg) of the Present Invention Amt. of Mg Heating        Satn.    Grain Sample added     temp.    Atmos- magnetization strength No.   (wt %)    (.degree.C.) phere .delta.s (emu/g) (Pa) ______________________________________ 1     0.00      500      N.sub.2 60       11E6 2     0.10      "        "     61       12E6 3     0.20      "        "     61       15E6 4     0.80      "        "     55       12E6 5     1.60      "        "     49       11E6 6     3.60      "        "     37       11E6 7     13.20     "        "     26       15E6 8     18.50     "        "     20       12E6 9     26.20     "        "     7        14E6 10    40.00     "        "     3        14E6 11    0.00      550      N.sub.2 62       15E8 12    0.10      "        "     62       15E8 13    0.20      "        "     61       14E8 14    0.80      "        "     52       15E8 15    1.60      "        "     45       17E8 16    3.60      "        "     40       16E8 17    13.20     "        "     31       13E8 18    18.50     "        "     18       13E8 19    26.20     "        "     7        16E8 20    40.00     "        "     3        17E8 21    0.00      600      N.sub.2 62       17E8 22    0.10      "        "     62       15E8 23    0.20      "        "     61       16E8 24    0.80      "        "     52       14E8 25    1.60      "        "     44       15E8 26    3.60      "        "     40       17E8 27    13.20     "        "     31       15E8 28    18.50     "        "     20       15E8 29    26.20     "        "     6        14E8 30    40.00     "        "     3        15E8 31    0.00      800      N.sub.2 62       30E8 32    0.10      "        "     62       36E8 33    0.20      "        "     61       35E8 34    0.80      "        "     53       36E8 35    1.60      "        "     44       34E8 36    3.60      "        "     40       38E8 37    13.20     "        "     32       35E8 38    18.50     "        "     20       36E8 39    26.20     "        "     7        37E8 40    40.00     "        "     3        33E8 41    0.00      1100     N.sub.2 62       17E9 42    0.10      "        "     62       15E9 43    0.20      "        "     61       16E9 44    0.80      "        "     55       16E9 45    1.60      "        "     44       17E9 46    3.60      "        "     41       17E9 47    13.20     "        "     32       17E9 48    18.50     "        "     21       16E9 49    26.20     "        "     7        17E9 50    40.00     "        "     2        17E9 51    0.00      1200     N.sub.2 92       22E9 52    0.10      "        "     92       24E9 53    0.20      "        "     90       25E9 54    0.80      "        "     87       25E9 55    1.60      "        "     81       25E9 56    3.60      "        "     67       26E9 57    13.20     "        "     32       24E9 58    18.50     "        "     21       25E9 59    26.20     "        "     6        25E9 60    40.00     "        "     1        28E9 61    0.00      1300     N.sub.2 92       31E9 62    0.10      "        "     92       33E9 63    0.20      "        "     90       34E9 64    0.80      "        "     87       35E9 65    1.60      "        "     81       34E9 66    3.60      "        "     67       35E9 67    13.20     "        "     32       37E9 68    18.50     "        "     21       36E9 69    26.20     "        "     6        36E9 70    40.00     "        "     1        35E9 71    0.00      1400     N.sub.2 92       33E9 72    0.10      "        "     92       34E9 73    0.20      "        "     90       34E9 74    0.80      "        "     87       35E9 75    1.60      "        "     82       35E9 76    3.60      "        "     67       36E9 77    13.20     "        "     31       37E9 78    18.50     "        "     20       36E9 79    26.20     "        "     6        36E9 80    40.00     "        "     1        35E9 81    0.00      1450     N.sub.2 92       38E9 82    0.10      "        "     92       36E9 83    0.20      "        "     90       36E9 84    0.80      "        "     87       37E9 85    1.60      "        "     81       37E9 86    3.60      "        "     66       36E9 87    13.20     "        "     32       37E9 88    18.50     "        "     20       36E9 89    26.20     "        "     6        37E9 90    40.00     "        "     1        36E9 91    0.00      1500     N.sub.2 86       41E9 92    0.10      "        "     85       40E9 93    0.20      "        "     82       40E9 94    0.80      "        "     77       41E9 95    1.60      "        "     70       42E9 96    3.60      "        "     65       41E9 97    13.20     "        "     25       43E9 98    18.50     "        "     16       40E9 99    26.20     "        "     3        41E9 100   40.00     "        "     1        41E9 ______________________________________ ______________________________________ Firing Experiment Results (Magnetite + Mg) of the Present Invention Amt. of Mg Heating        Satn.    Grain Sample added     temp.    Atmos- magnetization strength No.   (wt %)    (.degree.C.) phere .delta.s (emu/g) (Pa) ______________________________________ 1     0.00      500      N.sub.2 65       11E6 2     0.10      "        "     64       12E6 3     0.20      "        "     64       11E6 4     0.80      "        "     58       11E6 5     1.60      "        "     50       13E6 6     3.60      "        "     44       11E6 7     13.20     "        "     34       14E6 8     18.50     "        "     25       15E6 9     26.20     "        "     10       11E6 10    40.00     "        "     6        14E6 11    0.00      550      N.sub.2 92       15E8 12    0.10      "        "     92       15E8 13    0.20      "        "     91       16E8 14    0.80      "        "     89       15E8 15    1.60      "        "     82       18E8 16    3.60      "        "     72       16E8 17    13.20     "        "     34       17E8 18    18.50     "        "     25       17E8 19    26.20     "        "     10       16E8 20    40.00     "        "     6        17E8 21    0.00      600      N.sub.2 92       14E8 22    0.10      "        "     92       15E8 23    0.20      "        "     91       15E8 24    0.80      "        "     89       14E8 25    1.60      "        "     82       16E8 26    3.60      "        "     72       17E8 27    13.20     "        "     34       15E8 28    18.50     "        "     25       16E8 29    26.20     "        "     10       15E8 30    40.00     "        "     6        16E8 31    0.00      800      N.sub.2 92       22E8 32    0.10      "        "     92       26E8 33    0.20      "        "     91       25E8 34    0.80      "        "     88       26E8 35    1.60      "        "     82       24E8 36    3.60      "        "     72       26E8 37    13.20     "        "     33       25E8 38    18.50     "        "     25       28E8 39    26.20     "        "     11       27E8 40    40.00     "        "     6        23E8 41    0.00      1100     N.sub.2 92       17E9 42    0.10      "        "     92       15E9 43    0.20      "        "     91       16E9 44    0.80      "        "     88       16E9 45    1.60      "        "     83       18E9 46    3.60      "        "     72       17E9 47    13.20     "        "     33       17E9 48    18.50     "        "     25       17E9 49    26.20     "        "     11       17E9 50    40.00     "        "     7        17E9 51    0.00      1200     N.sub.2 92       23E9 52    0.10      "        "     92       24E9 53    0.20      "        "     90       26E9 54    0.80      "        "     87       25E9 55    1.60      "        "     81       26E9 56    3.60      "        "     67       27E9 57    13.20     "        "     33       24E9 58    18.50     "        "     21       25E9 59    26.20     "        "     7        25E9 60    40.00     "        "     1        28E9 61    0.00      1300     N.sub.2 92       35E9 62    0.10      "        "     92       33E9 63    0.20      "        "     90       34E9 64    0.80      "        "     87       35E9 65    1.60      "        "     81       34E9 66    3.60      "        "     65       35E9 67    13.20     "        "     32       37E9 68    18.50     "        "     21       35E9 69    26.20     "        "     6        36E9 70    40.00     "        "     1        35E9 71    0.00      1400     N.sub.2 92       33E9 72    0.10      "        "     92       34E9 73    0.20      "        "     90       34E9 74    0.80      "        "     87       35E9 75    1.60      "        "     82       35E9 76    3.60      "        "     65       36E9 77    13.20     "        "     31       37E9 78    18.50     "        "     20       37E9 79    26.20     "        "     6        36E9 80    40.00     "        "     1        35E9 81    0.00      1450     N.sub.2 92       38E9 82    0.10      "        "     92       36E9 83    0.20      "        "     90       36E9 84    0.80      "        "     87       37E9 85    1.60      "        "     81       36E9 86    3.60      "        "     66       36E9 87    13.20     "        "     33       36E9 88    18.50     "        "     21       37E9 89    26.20     "        "     6        38E9 90    40.00     "        "     1        37E9 91    0.00      1500     N.sub.2 86       40E9 92    0.10      "        "     85       40E9 93    0.20      "        "     82       40E9 94    0.80      "        "     76       41E9 95    1.60      "        "     70       42E9 96    3.60      "        "     65       41E9 97    13.20     "        "     23       43E9 98    18.50     "        "     16       40E9 99    26.20     "        "     2        41E9 100   40.00     "        "     1        41E9 ______________________________________ ______________________________________ Firing Experiment Results (Hematite + Mg + Mn) of the Present Invention Mg = 3.60 wt % Amt. of Mn Heating        Satn.    Grain Sample added     temp.    Atmos- magnetization strength No.   (wt %)    (.degree.C.) phere .delta.s (emu/g) (Pa) ______________________________________ 1     0.00       550     N.sub.2 27       14E8 2     0.10      "        "     27       14E8 3     0.20      "        "     27       14E8 4     0.50      "        "     28       16E8 5     1.00      "        "     29       17E8 6     2.00      "        "     31       17E8 7     3.00      "        "     32       17E8 8     4.00      "        "     33       17E8 9     6.00      "        "     35       17E8 10    8.00      "        "     37       17E8 11    10.00     "        "     39       17E8 12    0.00      1300     N.sub.2 67       36E9 13    0.10      "        "     67       36E9 14    0.20      "        "     67       37E9 15    0.50      "        "     68       40E9 16    1.00      "        "     69       45E9 17    2.00      "        "     71       48E9 18    3.00      "        "     72       50E9 19    4.00      "        "     73       51E9 20    6.00      "        "     75       52E9 21    8.00      "        "     77       52E9 22    10.00     "        "     80       52E9 ______________________________________

## Claims

1. A process for producing magnetic oxide, comprising the steps of mixing a given amount ranging from 0.20 to 76.0% by weight of at least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powdery substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550.degree. to 1500.degree. C.

2. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite, said magnetization control material is Ca and mixed in an amount of 1.43 to 64.0% by weight, and said sintering is conducted at 1200.degree. to 1450.degree. C.

3. The process according to claim 1, wherein said starting material is magnetite, said magnetization control material is Ca and mixed in an amount of 1.43 to 64.0% by weight, and said sintering is conducted at 550.degree. to 1450.degree. C.

4. The process according to claim 1, wherein, prior to said sintering, said mixed powder is granulated into globular grains to thereby render the powder globular.

5. The process according to claim 1, wherein 47.3 to 98.8% by weight of magnetite as the starting material is mixed with 1.2 to 52.7% by weight in terms of Ti, of a Ti compound as the magnetization control material, and said sintering is conducted at 550.degree. to 1450.degree. C.

6. The process according to claim 1, wherein a mixture of 9.8 to 98.8% by weight of magnetite and 0 to 79.0% by weight of hematite as the starting material is mixed with 1.2 to 52.7% by weight, in terms of Ti, of a Ti compound as the magnetization control material, and said sintering is conducted at 1200.degree. to 1450.degree. C.

7. The process according to claim 1, wherein 24.0 to 99.2% by weight of hematite or a mixture of hematite and magnetite as the starting material is mixed with 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound as the magnetization control material, and said sintering is conducted at 1200.degree. to 1450.degree. C.

8. The process according to claim 1, wherein 24.0 to 99.2% by weight of magnetite as the starting material is mixed with 0.8 to 76.0% by weight, in terms of Sn, of an Sn compound as the magnetization control material, and said sintering is conducted at 550.degree. to 1450.degree. C.

9. The process according to claim 1, wherein 35.0 to 99.5% by weight of hematite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 1200.degree. to 1450.degree. C.

10. The process according to claim 1, wherein 35.0 to 99.5% by weight of a mixture of hematite and magnetite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 1200.degree. to 1450.degree. C.

11. The process according to claim 1, wherein 35.0 to 99.5% by weight of magnetite as the starting material is mixed with 0.5 to 65% by weight, in terms of Si, of an Si compound as the magnetization control material, and said sintering is conducted at 550.degree. to 1450.degree. C.

12. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 18.50% by weight, in terms of Mg, and said sintering is conducted at 550.degree. to 1500.degree. C.

13. The process according to claim 1, wherein said starting material is magnetite, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 26.20% by weight, in terms of Mg, and said sintering is conducted at 550.degree. to 1500.degree. C.

14. The process according to claim 1, wherein said starting material is hematite or a mixture of hematite and magnetite compounded with 0.20 to 10.0% by weight of Mn, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 18.50% by weight, in terms of Mg, and said sintering is conducted at 550.degree. to 1500.degree. C.

15. The process according to claim 1, wherein said starting material is magnetite compounded with 0.20 to 10.0% by weight of Mn, the magnetization control material is an Mg compound and mixed in an amount of 0.20 to 26.20% by weight, in terms of Mg, and said sintering is conducted at 550.degree. to 1500.degree. C.

16. A magnetic oxide obtained by mixing a given amount ranging from 0.20 to 76.0% by weight of as least one member selected from among Ca, Ti, Sn, Si and Mg magnetization control (regulation) materials with hematite, a mixture of hematite and magnetite, or magnetite as a starting material to obtain a mixed powder, adding 0.1 to 4.0% by weight of a liquid or powdery substance having a carbon-to-carbon single or double bond to the mixed powder, stirring the mixture to obtain a substantially homogeneous mixture and sintering the obtained mixture in an inert gas at 550.degree. to 1500.degree. C.