Method of producing ferromagnetic metal powder by gaseous reduction of silicon compound-coated raw material

A method of producing ferromagnetic iron powder comprises the steps of dipping iron compound powder in a solution in which silicone oil is dissolved, removing solvent from the iron compound powder, reducing the iron compound powder in a reducing atmosphere at an elevated temperature and forming oxide layers on whole surfaces of the individual particles of the iron compound powder. Ferromagnetic iron powder produced according to this method has both a high coercive force and a high remanence ratio.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing ferromagnetic metal 
powder, especially composed mainly of iron, and more particularly to a 
method of producing ferromagnetic metal powder usable as a high density 
magnetic recording medium for a magnetic tape, a magnetic drum, magnetic 
disc or the like. 
2. Brief Description of the Prior Art 
Many methods of producing ferromagnetic metal powder have been investigated 
heretofore. These methods can be roughly classified as follows: 
I. method comprising the steps of dissolving a compound of ferromagnetic 
metal into water and adding thereto reducing agent containing tetrahydride 
borate ions or hypophosphite ions, 
Ii. method comprising the step of reducing an oxalate, a formate, a oxide 
or a oxyhydroxide of ferromagnetic metal in a reducing atmosphere at an 
elevated temperature, 
Iii. method comprising the steps of preparing an aqueous solution in which 
a compound of ferromagnetic metal is dissolved and electrolyzing the 
solution, and so on. 
The metal powder for a magnetic recording medium of high density is desired 
to be composed of particles of high coercive force and high remanence 
ratio having needlelike shapes and uniform size. However, it is difficult 
to obtain such needlelike shaped and uniform-sized metal powder by the 
above-mentioned prior art method. Namely, according to the above-mentioned 
prior art method (i), we can obtain a powder composed of only 
sphere-shaped particles, dielike-shaped particles or chain-sphere-shaped 
particles which are formed by linking the globe-like or dielike-shaped 
particles together. According to the above-mentioned prior art method 
(iii), the shape of the obtained particles of the powder is only 
dendritic. Furthermore, in the above-mentioned prior art (ii), although 
somewhat needlelike shaped particle powder can be obtained when the 
needlelike shaped oxalate, formate, oxide or oxyhydroxide is used as 
starting material, it is difficult to obtain uniform-sized metal powder 
having needlelike shapes because the particles link together or break in 
the reducing process at an elevated temperature. As mentioned above, it is 
very difficult to obtain needle-like shaped ferromagnetic powder according 
to the prior art. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method to 
produce ferromagnetic metal powder. It is another object of the present 
invention to provide an improved method to produce ferromagnetic iron 
powder. It is still another object of the present invention to provide a 
method of producing needlelike-shaped and uniform sized ferromagnetic iron 
powder having both a high coercive force and a high remanence ratio. 
On our various investigations concerning the method of producing needlelike 
ferromagnetic metal powder, it was found that the needlelike ferromagnetic 
metal powder could be obtained according to the method in which the 
above-mentioned prior art method (ii) was improved. The present invention, 
which is based on our above-mentioned discovery, provides a method for 
producing needlelike-shaped ferromagnetic metal powder having a high 
coercive force and a high remanece ratio, by improving the above-mentioned 
prior art method (ii) which comprises the step of reducing a metal 
compound such as a oxalate, a formulate, a oxide or a oxyhydroxide of 
ferromagnetic metal at an elevated temperature. 
In the method of the present invention, there also may be used as a 
starting raw material, oxalate, formate, oxide or oxyhydroxide of 
ferromagnetic metal or their mixture, among which iron compound is 
preferable because of the high value of the magnetization in the obtained 
metal, and among which the oxide and the oxyhydroxide are preferable too 
because of the facility of obtaining needlelike shaped powder. 
Furthermore, another reason why the oxide is preferable is that the number 
of atoms made free at the reducing step is few whereby the particles are 
hard to break down. Accordingly, the iron oxide and the iron oxyhydroxide, 
especially the iron oxide, are more preferable for the starting material 
in the method of the present invention. Among the iron oxide usable as the 
starting material, magnetite and ferric oxide such as maghemite are 
desirable because of the facility of obtaining needlelike shaped powder. 
It is to be desired that the particles of the raw material are needlelike. 
In addition, the iron oxide containing Co, Ni, Al, Cr, Mg, B or the like 
may be used as the starting material to improve corrosion-resistance 
property. 
In this invention, the above-mentioned starting material powder preferably 
composed mainly of iron compound is reduced by heating it in a reducing 
atmosphere after the starting material powder is coated with a solution of 
inorganic silicon compound or a solution of organic silicon compound, 
which has a boiling point of higher than 250.degree. C., and then the 
solvent of the solution is removed by drying. The drying step may be 
omitted when the solvent can be removed by natural drying, in another 
step. As the inorganic silicon compound or organic silicon compound, there 
is usually used a silicate such as sodium silicate or the like listed in 
Table 3 or silicon oil, respectively. The latter is used more usually. 
Furthermore, a more desirable result can be obtained when a solution 
containing silver ions is used with the above-mentioned silicon compound 
solution. Both of them may be used as a mixed solution or as separate two 
solutions. The starting material powder is dipped once in the former case, 
but twice in the latter case, namely, it should be dipped in a silicon 
compound solution and in a silver ions solution separately. As the silver 
compound used for the solute of the silver ions containing solution, there 
can be used many silver salts such as silver nitrate, silver sulfate and 
silver chloride. 
For the solvent of silicone oil, many organic solvents, such as ketone, 
formamide, aromatic compound can be used. Water, aqueous acidic solution 
and aqueous alkaline solution are used for solvents for inorganic silicon 
compounds. In the solution containing silver ions, water, liquid ammonia, 
strong inorganic acid or alcohol are employed as solvents. However, the 
latter can be used only for silver nitrate. 
In the method of the present invention, the amount of inorganic silicon 
compound or organic silicon compound, such as silicone oil, in the 
above-mentioned solution is more than 0.05 g/l. When the amount of silicon 
compound in the solution is less than 0/05 g/l, it is difficult to expect 
a better result than the prior art method. The preferable amount of the 
silicon compound in the solution is more than 0.1 g/l and the more 
preferable amount thereof is more than 0.2 g/l. Furthermore, the silver 
compound may be contained in the solution in amount of more than 0.02 g/l 
and preferably more than 0.05 g/l. The upper limit value thereof may be 
determined by the concentration in the saturated solution. 
For example, FIG. 1 shows the relation between the silicone oil 
concentration in the solution and the magnetic properties of the obtained 
ferromagnetic iron powder in the method of the present invention. As 
apparent from this figure, the ferromagnetic metal powder produced 
according to the method of this invention is greatly improved in the 
coercive force (curve 1) and the remanence ratio (curve 2), as compared 
with the case in which no silicone oil is used as in the prior art method. 
In addition, it is obvious that a large desirable effect can be obtained 
using even very dilute solution of the silicone oil. The detailed 
description about FIG. 1 will be read later in the description concerning 
example 1. 
Any reducing atmosphere containing reducing gas such as H.sub.2 and CO may 
be used at the heating step for reduction, additionally H.sub.2 gas and 
town gas can be obtained easily to thereby be preferable. 
The suitable ratio of H.sub.2 to H.sub.2 O, CO to CO.sub.2 or the like for 
reducing Fe.sub.3 O.sub.4, which is the rate-determining step in reducing 
every kind of iron oxide, is known well in this field or in the field of 
the iron and steel production. To the method of the present invention, the 
above-mentioned well-known suitable ratio may be applied. 
The suitable reducing temperature ranges from 250.degree. to 600.degree. C. 
in the case that the solute of the solution in which the starting material 
powder is dipped is only silicon compound, especially silicone oil or 
sodium silicate. It ranges from 200.degree. to 700.degree. C. in the case 
of using both silver salt and silicon compound as the solutes. It ranges 
preferably from 300.degree. to 550.degree. C. in the former case, and from 
250.degree. to 600.degree. C. in the latter case. 
When the heating temperature is lower than the above-mentioned lower limit 
of the suitable temperature range, the reducing phenomenon occurs 
insufficiently. On the other hand, the reduced particles stick together, 
when the heating temperature is higher than the above-mentioned upper 
limit of that range. Of course, both cases are not suitable. 
According to the composition of the reducing atmosphere and the heating 
temperature, various heating time is necessary for reduction process. The 
composition of the discharged gas of the heating atmosphere is somewhat 
different from the composition of the charged gas during the reduction 
process, but it becomes almost the same as that of the charged gas in the 
range of the compositional intrinsic fluctuation after the reduction 
process is completed. Accordingly, we can recognize the completion of the 
reduction process by detecting the discharged gas composition, and the 
necessary heating time becomes apparent naturally. 
The produced powder can become corrosion-resistant and be convenient to 
handle, by forming surface layers such as oxide layers on the surface of 
the produced metal particles according to the well-known method concerning 
the nonactivation treatment of pure iron, namely by making the particle 
surfaces nonactive. 
The ferromagnetic metal powder produced according to the aforementioned 
method has both a high coercive force and a high remanence ratio as 
compared with that produced by a conventional method. 
Other and further objects, features and advantages of the invention will be 
apparent more fully from the following description taken in connection 
with the accompanying drawings.