EP0118253A1

EP0118253A1 - Fine particles of ferromagnetic metal and process for producing the same

Info

Publication number: EP0118253A1
Application number: EP84301141A
Authority: EP
Inventors: Masashi Ushiyama; Takayoshi Yoshizaki
Original assignee: Chisso Corp
Current assignee: JNC Corp
Priority date: 1983-02-22
Filing date: 1984-02-22
Publication date: 1984-09-12
Also published as: DE3461871D1; EP0118253B1; JPH0348243B2; JPS59153810A

Abstract

Fine particles of ferromagnetic metal having a metal firmly adhered thereon are provided, which are produced by adding to an aqueous suspension of iron a-oxyhydroxide, an aqueous solution of a compound such as salt, metal salt, oxide, of a metal which is different from iron and exclusive of alkali metal (including an acidic or alkaline aqueous solution) and an aqueous solution of a ferrous salt, followed by oxidation reaction, filtration, water-washing and reduction.

Description

This invention relates to fine particles of ferromagnetic metal and a process for producing the same. More particularly it relates to fine particles of ferromagnetic metal obtained by adding a compound such as a salt of a metal which is different from iron and exclusive of alkali metal (which metal will hereinafter be referred to as a "different kind metal") to an aqueous suspension of iron α-oxyhydroxide, to adhere the different kind metal onto the iron 2-oxyhydroxide, followed by reduction, and such a process for producing the fine particles.
Fine particles of ferromagnetic metal iron having a high coercive force and a high magnetic flux density have recently been noted for elevating the performance of cassette tapes for audio and for developing video tapes for compact YTR (video tape recorders). In brder to elevate the performances of these tapes, it has been attempted to make magnetic powder finer. However, as magnetic powder is made finer, such a tendency is observed that the coercive force is enhanced, but the dispersibility, weather resistance, orientativity, etc. of magnetic powder become inferior. Further, when the starting iron a-oxyhydroxide is made finer, the resulting particles are liable to collapse at the time of being reduced on heating. Thus, for the purpose of improving various characteristics of magnetic powder, taking into account of processing the powder into paint, of relationship with magnetic head, etc., a process of adding a different kind metal to iron a-oxyhydroxide has been developed.
As for the process of adding a different kind metal, the following processes have been known:

(1) a process of adding a salt or the like of a different kind metal together with ferrous hydroxide at the time of preparing iron a-oxyhydroxide to coprecipitate them, or carrying out oxidation reaction in a state where a salt or the like of a different kind metal is dissolved in the reaction liquid this process hereinafter being referred to as "dope process"; and
(2) a process of suspending in water, powder or a wet cake of iron a-oxyhydroxide formed, and if necessary, making the resulting suspension acidic or alkaline, and thereafter adding an aqueous solution of a salt of a different kind metal, followed, if necessary, by neutralizing the suspension for precipitation and adhesion (this process hereinafter being referred to as "adhesion process").

According to the dope process, the resulting iron a-oxyhydroxide has greatly varied particulate forms, depending on the kind, quantity and addition manner of the metal compound added., and all the characteristics of fine particles of metal obtained by heating and reduction are also much different from those in the case where no different kind metal is added. Further, depending on the quantity of the additive, branched particles may be formed or the acicularity of particles of iron a-oxyhydroxide may be lost, although this varies depending on the kind of the compound of the different kind metal added. The dope process is promising as a proces for improving the characteristics of the fine particles of metal, but the adhesion process is rather preferable for improving the objective characteristics of the fine particles.
The adhesion process includes a process of suspending in water, powder or a wet cake of iron a-oxyhydroxide and then adding an aqueous solution of a salt or the like of a different kind metal, to effect the adhesion; and a process of adding an acid or an alkali to the above aqueous suspension to once sufficiently disperse the agglomerate of iron a-oxyhydroxide, followed by adding an aqueous solution of a salt or the like of a different kind metal, and adding an alkali or an acid for neutralization to adhere the compound of the different kind metal, and thereafter filtering the resulting material. These processes, however, have drawbacks that the adhesion of the metal compound onto iron a-oxyhydroxide is weak and the resulting particles are liable to collapse at the time of heat-reduction step. In order to strengthen the adhesion of the metal compound onto iron a-oxyhydroxide and to reduce the collapse of particles at the time of reduction step, it is necessary that after the aqueous solution of a salt or the like of a different kind metal is added, the resulting mixture be subjected to heat treatment at 70°C or higher, preferably 90°C or higher. However, although the collapse of particles at the time of reduction step is reduced by the heat treatment, degradation of magnetic characteristics due to the effect of the coexistent anions is observed. The coexistent anions can be removed to a certain extent by water-washing after the heat treatment, but several tens-% of their amount prior to the water-washing remain in iron a-oxyhydroxide, hence it is impossible to entirely remove the effect of anions. On the other hand, if water-washing is carried out without the heat treatment, it is inevitable that a portion of the metal compound to be adhered flows away. Further, according to these processes, only limited metals such as Ni, Co, Ca, Mn, Mg, etc. can be adhered.
It is an object of the present invention to provide a process for producing fine particles of ferromagnetic metal wherein the drawbacks of the prior art may be overcome, that is, wherein almost all metals in an optional quantity can be firmly adhered even at low temperatures; anions which cause degradation of magnetic characteristics can be removed; and collapse of particles at the time of reduction step can be scarcely observed, so as to produce fine particles of ferromagnetic metal.
The present invention provides a process for producing fine particles of ferromagnetic metal by adding an aqueous solution of a compound of a metal which is different from iron and exclusive of alkali metal (this metal hereinafter being referred to as a "different kind metal"), to an aqueous suspension of iron a-oxyhydroxide, to adhere said compound of a different kind metal onto particles of iron a -oxyhydroxide, followed by reduction, characterised in that the method comprises adding a ferrous salt together with said compound of a different kind metal to said aqueous suspension of iron a-oxyhydroxide to effect the adhesion.
The present invention further provides fine particles of ferromagnetic metal obtained by adding an aqueous solution of a compound of a metal which is different from iron and exclusive of alkali metal (this metal hereinafter being referred to as a "different kind metal"), to an aqueous suspension of iron a-oxyhydroxide, to adhere said compound of a different kind metal onto particles of iron a-oxyhydroxide, followed by reduction, characterised in that the fine particles of ferromagnetic metal, are obtained by adding a ferrous salt together with said compound of a different kind metal to said aqueous suspension of iron a-oxyhydroxide to effect the adhesion.
Preferably the present invention provides a process for producing fine particles of ferromagnetic metal, which comprises adding to an aqueous suspension of iron a-oxyhydroxide, an aqeous solution of a compound such as salt, metal acid salt, oxide, of a different kind metal from iron (including an acidic or alkaline aqueous solution) and an aqueous solution of a ferrous salt, followed by oxidation reaction, filtration, water-washing, drying and reduction, and the thus produced fine particles of ferromagnetic metal.
The above aqueous suspension of iron a-oxyhydroxide may include a slurry formed by preparation of iron a-oxyhydroxide, or a suspension obtained by suspending in water, a wet cake or dried powder of iron a-oxyhydroxide obtained from the above slurry. This aqueous suspension may be acidic, alkaline or neutral. Thus, unlike the prior art, the present invention has a merit that even the slurry formed by preparation or iron a-oxyhydroxide can be used as it is.
Examples of the above different kind metai are Ni, Co, Mg, Ca, Sr, Ba, Ti, Cr, Mo, Mn, Cu, Ag, Zn, Cd, Al, Si, Sn, Pb, Bi, Nb and Sm. One or more kinds of the metals can be used.
As for said different kind metal, alkali metals are excluded, because they dissolve in an aqueous solvent in large quantities and hardly deposit on iron a-oxyhydroxide.
The above aqueous solution of a compound such as salt of a different kind metal added may be used in the form of a neutral, acidic or alkaline aqueous solution. In this case, which is choiced among the neutral, acidic and alkaline aqueous solutions, can be decided taking into account the solubility of the salt or the like of a different kind metal in the aqueous solutions, as well as whether or not a uniform adhesion occurs to a desired extent when the solutions are added to the aqueous suspension of iron a-oxyhydroxide. These regulations of the solutions can be achieved by dissolving an acidic or alkaline substance in the solutions.
As to the amount of the different kind metal added, the atom/molecule ratio (i.e. a ratio of the number of atoms to that of molecules; this applies to the ratio hereinafter mentioned) of "different kind metal (M)/iron a-oxyhydroxide (FeOOH)", relative to iron a-oxyhydroxide, based on the amount of the metal adhered, is preferably in the range of about 0.005 to 0.4, more preferably 0.01 to 0.2. If the ratio is too large, the resulting iron a-oxyhydroxide having the metal adhered thereon is liable to agglomerate.
As to the amount of the above Fe(II) added, the atom/ molecule ratio of "Fe(II)/α/FeOOH" relative to iron a-oxyhy- aroxide, based on the amount of the metal adhered, is preferably in the range of about 0.0025 to 0.3.
The ratio is not limited to the above range, but if the ratio is too small, the adhesion strength of the added different kind metal onto iron. a-oxyhydroxide is insufficient depending on the amount ratio of Fe(II) to the different kind metal added at the same time, as mentioned later. If the ratio is too large, the iron a-oxyhydroxide resulting from the adhesion is liable to agglomerate.
Most of iron of the above ferrous salts and different kind metals are adhered in nearly 100% of the addition amount. Thus, in this case, it is possible to select the addition amount of these salts (in a literal sense) as an equivalent to the adhesion amount. However, some of the different kind metals are easily soluble in an alkaline aqueous solution, and have various percentages adhesion depending on their kind and pH, but it is possible to easily determine the addition amount in which a desirede adhesion amount is given, through experiments.
The amount ratio of the compound such as salt of the different kind metal and the ferrous salt to be added is preferably 0.5 or higher in terms of the atom ratio of "Fe(II)/ different kind metal (M)" (i.e. the ratio of the respective numbers of the atoms; this applies to those hereinafter mentioned), based on the amount of the metalsadhered. This ratio is not limited to the above range, but if the ratio is too small, the adhesion strength of the compound of the different kind metal onto iron a-oxyhydroxide is insufficient.
After addition of the salt or the like of a different kind metal, the reaction mixture may, if necessary, be adjusted to an appropriate pH, whereby a part or most part of the different kind metal precipitates or gels around iron a-oxyhydroxide .or singly in the form of a compound such as hydroxide, its hydrate, oxide, etc. together with ferrous ions. However, the precipitate or the like at this stage is not yet adhered onto iron a-oxyhydroxide, or even if adhered, it has a low adhesion strength; hence the particles are liable to collapse at the time of reduction step. Thus the oxidation process by air or an oxidizing agent is carried out. Presumably, the precipitated Fe(II) is oxidized through the oxidation reaction and deposited or epitaxialized onto the surface of iron a-oxyhydroxide in the form of iron a-oxyhydroxide, and at that time, involves the different kind metal therein to effect a strong adhesion of the different kind metal onto iron a-oxyhydroxide. And the particles thus obtained hardly collapse at the time of reduction step.
Embodiments of the present invention will now be described by way of example only.

(1) A wet cake or powder of iron a-oxyhydroxide is supended in water. It is preferable to sufficiently disperse iron .a-oxyhydroxide by adding an acid such as acetic acid to the aqueous suspension to make its pH 4 or less. An aqueous solution (usually a neutral or acidic aqueous solution) of a salt or the like of a different kind metal is added to the above suspension, followed by adding an aqueous solution of a ferrous salt. An aqueous solution of a basic substance such as NaOH, ammonia, Na₂CO₃, etc. or ammonia gas is then added in an amount of a chemical equivalent or more to the ferrous salt and the salt of a different kind metal added, taking into account the amount in which the added acid in neutralized, which relation is expressed by an equation, "(amount of base added) ≧ (acid-neutralizing amount) + (amount of ferrous salt added) + (amount of metal salt added". Successivly, an oxygen-containing gas such as air is blown therein, or an oxidizing agent is added to carryout oxidation reaction, followed by filtration, water-washing, drying, heat-reduction to obtain fine particles of metal. As for said oxidizing agent, persulfate, perchlorate, hydrogen peroxide are exemplified.
(2) In an alternative embodiment, an aqueous solution of a salt or the like of a different kind metal (this solution may contain an acid or a base) is added to a slurry formed by preparation of iron a-oxyhydroxide, and at the same time with or before or after this addition, an aqueous solution of a ferrous salt is added. At that time, as to the addition speed of the aqueous solution of themetal salt and the aqueous solution of the ferrous salt, a lower speed is preferable. The metal salt or the like and the ferrous salt may be added in the form of a mixed solution theref, so long as precipitate is not formed, or an aqueous solution of the ferrous salt may be added, followed by adding an aqueous solution of the metal salt or the like, whereby fine particles of metal having almost the same powder magnetic characteristics are formed. However, in these cases, fine granules are liable to be formed; thus a process of addding an aqueous solution of the metal salt or the like, followed by adding an aqueous solution of the ferrous salt is preferred. After addition of the aqueous solution of the ferrous salt, an oxygen-containing gas such as air is blown in the mixture or an oxidizing agent is added to carry out oxidation reaction. The oxidation is followed by filtration, water-washing, drying, and heat-reduction to obtain fine particles of metal. This process (2) also yields fine particles of metal having the same magnetic characteristics as those in the case of the process (1) wherein adhesion is effected using the above aqueous suspension of the wet cake or the like.

According to the preferred processes of the present invention, when one kind or a combination of two kinds or more of salts or the like of different kind metal other than alkali metals, which are soluble in water or an aqueous solution of an acid or a base, is used, it is possible to adhere the metal(s) in an optional amount onto iron a-oxyhydroxide. The bond between the metal compound adhered, which compound has often been changed from the metal compound such as metal salt added, and iron a-oxyhydroxide is so strong that flowing ,away of the metal compound due to water-washing is not observed. Further, anions which adheres weakly to the iron a-oxyhydroxide and cause degradation of magnetic characteristics of the iron α-oxyhydroxide are selectively removed. Collapse of particles at the time of reduction step is scarcely observed. Thus, a remarkable improvement in the magnetic characteristics as compared with conventional adhesion processes is observed.
Iron a-oxyhydroxide prepared according to conventional processes is observed to contain very fine particles which are much smaller than average size particles and cause degradation of transfer characteristics and extension of Hc(coercive force) distribution i.e. SFD(switching field distribution). According to the preferred processes of the present invention, very fine particles are combined with larger particles and disappear; hence particles of iron a-oxyhydroxide having the metal compound adhered thereon after oxidation reaction has a uniform particle size. The thus combined very fine particles do not separate from the larger particles even in the reduction reaction, to give fine particles of metal well retaining the particulate form of iron a-oxyhydroxide after adhesion.
The present invention will be further described by way of Examples, but it is, of course, not limited thereto.

Example 1

An aqueous solution of FeSO₄ (0.34 mol/ℓ) (38ℓ) was added to an aqueous solutuion of NaOH (6 mol/ℓ)(12.6ℓ), and air was passed through the mixed solution at a flow rate of 20ℓ/min. while keeping the temperature of the solution at 40°C, to prepare iron α-oxyhydroxide, which was then filtered and washed with water to obtain a wet cake (2,500g) (1,000g, as dry weight of iron α-oxyhydroxide), followed by adding water (24ℓ), stirring for one hour, adding acetic acid so as to give a pH of 3.5, stirring for 30.minutes, adding an aqueous solution of NiSO₄ (0.5 mol/ℓ) (1,124 mℓ) and then an aqueous solution of FeSO₄ (0.5 mol/ℓ)(1,124 mℓ), stirring for 15 minutes, adding an aqueous solution. of NaOH (5 mol/ℓ)(1,400 mℓ), stirring for 30 minutes, blowing air at a flow rate of 0.2ℓ/min. to carry out oxidation reaction for 5 hours while maintaining the temperature at 25°C, thereafter filtering, water-washing, drying, and reducing the resulting material on heating, to obtain fine particles of ferromagnetic metal.
Nearly 100% of the added Ni was retained on iron a-oxyhydroxide and no Ni was detected in the filtrate and the washing liquid. SO
and Na flowed away through filtration and water-washing and were scarcely observed in iron a-oxyhydroxide. Very fine particles which had been present in the iron a-oxyhydroxide prior to the adhesion were not observed in the Ni-adhered iron a-oxyhydroxide and had a uniform particle size. The Ni-adhered iron a-oxyhydroxide (500g) was reduced by hydrogen at a flow amount of 20ℓ/min., at a reduction temperature of 500°C for 5 hours to obtain fine particles of ferromagnetic metal retaining a good particulate form of the Ni-adhered iron a-oxyhydroxide. The magnetic characteristic values of the thus prepared magnetic powder were as follows: Hc:l,5880e, σs:159 emu/g, and σr/σs:0.53 Further,the characteristic values of an oriented sheet obtained by using this magnetic powder were as follows: Hc:1,463Oe, Br:2,984 gauss, Br/Bm:0.846, and SFD:0.432. Thus, He was higher by about 1000e as compared with fine particles of metal bbtained according to a process of Comparative examples mentioned later, and also SFD was improved.

Examples 2 - 10

Magnetic powder was prepared in the same manner as in Example 1 except that the kind and amount of the salt of the different kind metal added and the atomic ratio of the different kind metal to Fe(II) were varied. As in the case of Example 1, nearly 100% of the added metal was retained on the iron a-oxyhydroxide, and anions and Na⁺ were scarcely observed; thus, fine particles of ferromagnetic metal well retaining the particulate form of the adhered iron a-oxyhydroxide were obtained.
The characteristic values of the fine particles thus obtained are shown in Table listed later, together with their preparation conditions.

Example 11

An aqueous solution of NiSO₄ (0.5 mol/ℓ)(1,290 mℓ) was added at a flow rate of 20 mℓ/min. to a reaction slurry of iron a-oxyhydroxide prepared as in Example 1, followed by adding an aqueous solution of FeSO₄ (0.5 mol/ℓ) (1,290 mℓ) at a flow rate of 20 mi/min., stirring for 30 minutes, and carrying out oxidation reaction for 6 hours, while passing air at a low amount of 0.2 ℓ/min. to obtain a Ni-adhered iron a-oxyhydroxide as in the case of Example 1. Reduction was carried out as in Example 1 to obtain fine particles of ferromagnetic metal well retaining the particulate form of the Ni-adhered iron oxyhydroxide.
The characteristic values of the thus obtained fine particles are shown in Table listed later, together with their preparation conditions.

Examples 12 - 17

Example 11 was repeated except that the kind and amount of the salt of the different kind metal and the reduction temperature were varied. Metal-adhered iron a-oxyhydroxides similar to that of Example 11 were formed, and fine particles of ferromagnetic metal well retaining their particulate form.
The characteristic values of the thus obtained fine particles are shown in Table listed later together with their preparation conditions.

Comparative example 1

Water (12 ℓ) was added to the same wet cake of iron a-oxyhydroxide (1,250g; dry weight 500g) as in Example 1, followed by stirring for one hour, adding acetic acid so as to give a pH of 3.5, stirring for 30 minutes, adding an aqueous solution of NiS04 (0.5 mol/ℓ) (562 mℓ), stirring for 30 minutes, adding a 28% aqueous ammonia so as to give a pH of 9.5, stirring for 30 minutes, raising the temperature up to 90°C, aging for one hour, filtering, drying and reducing under the same conditions as in Example 1.
The resulting Ni-adhered iron a-oxyhydroxide contained 0.32% of SO₄ ^2-, and collapse of particles due to reduction was observed. The characteristic properties of the magnetic powder were as follows: Hc:1,4650e, σs:155 emu/g and σr/ σs:0.51. Further, SFD of the oriented sheet was 0.612, i.e. notably inferior to those of Examples.

Comparative example 2

Ni was adhered as in Comparative example 1, followed by filtering, washing with water (60ℓ), drying and reducing under the same conditions as in Example 1.
In spite of water-washing after adhesion, the resulting iron a-oxyhydroxide contained 0.16% of SO₄ ^-2, collapse of particles due to reduction was observed; Hc was reduced; and SFD was inferior.
The characteristic values and preparation conditions of the thus obtained fine particles are shown in Table listed later.

Comparative example 3

Comparative example 1 was repeated except that the added salt of the different kind metal was changed to Al₂(SO₄)₃.
Al flowed away during filtration and water-washing; only about 30% of the amount of the metal added was retained in the resulting iron a-oxyhydroxide; and SO₄ ^2- was observed in an amount of 0.30%.
The characteristic values and preparation conditions of the thus obtained fine particles are shown in Table listed below.

Claims

1. A process for producing fine particles of ferromagnetic metal by adding an aqueous solution of a compound of a metal which is different from iron and exclusive of alkali metal (this metal hereinafter being referred to as a "different kind metal"), to an aqueous suspension of iron a-oxyhydroxide, to adhere said compound of a different kind metal onto particles of iron a -oxyhydroxide, followed by reduction, characterised in that the method comprises adding a ferrous salt together with said compound of a different kind metal to said aqueous suspension of iron α-oxyhydroxide to effect the adhesion.

2. A process according to Claim 1, wherein said aqueous suspension of iron a-oxyhydroxide is a slurry formed by preparation reaction of iron a-oxyhydroxide.

3. A process according to Claim 1, wherein said aqueous suspension of iron a-oxyhydroxide is a suspension obtained by suspending in water, a wet cake or dry powder obtained from a slurry formed by preparation reaction of iron a-oxyhydroxide.

4. A process according to any foregoing claim, wherein the amount of said different kind metal added is in the range of 0.005 to 0.4 in terms of an atom to molecule ratio of Fe(II)/iron a-oxyhydroxide (α -Fe00H) based on the amount of the metal adhered.

5. A process according to Claim 4, wherein the amount of said different kind metal added is in the range of 0.01 to 0.2 in terms of an atom to molecule ratio of Fe(II)/iron a-oxyhydroxide ( a-FeOOH) based on the amount of the metal adhered.

6. A process according to Claim 4, wherein the amount of said ferrous salt added is in the range of 0.0025 to 0.3 in terms of an atom to molecule ratio of Fe(II)/iron a-oxyhydroxide (α-FeOOH) based on the amount of the metal adhered.

7. A process according to any foregoing claim, wherein the ratio of the amount of said different kind metal (M) added, to that of said ferrous salt added is 0.5 or more in terms of the atomic ratio of Fe(II)/M.

8. A process according to any foregoing claim, wherein said different kind metal is at least one member selected from the group consisting of Ni, Co, Mg, Ca, Sr, Ba, Ti, Cr, Mo, Mn, Cu, Ag, Zn, Cd, Al, Si, Sn, Pb, Bi, Nb and Sm.

9. Fine particles of ferromagnetic metal obtained by adding an aqueous solution of a compound of a metal which is different from iron and exclusive of alkali metal (this metal hereinafter being referred to as a "different kind metal"), to an aqueous suspension of iron a-oxyhydroxide, to adhere said compound of a different kind metal onto particles of iron a-oxyhydroxide, followed by reduction, characterised in that the fine particles of ferromagnetic metal, are obtained by adding a ferrous salt together with said compound of a different kind metal to said aqueous suspension of iron a-oxyhydroxide to effect the adhesion.

10. Fine particles according to Claim 9, wherein said aqueous suspension of iron a-oxyhydroxide is a slurry formed by preparation reaction of iron a-oxyhydroxide.

11. Fine particles according to Claim 9, wherein said aqueous suspension of iron a-oxyhydroxide is a suspension obtained by suspending in water, a wet cake or dry powder obtained from a slurry formed by preparation reaction of iron a-oxyhydroxide.