Process for producing acicular magnetic metallic particle powder

The process for producing acicular magnetic metallic particle powder with improved magnetic properties which include the characteristic steps for producing the acicular particles of metal oxide with homogeneous and dense coats of crystalline silica and to thereby protect the particles from heat-damages during the reduction thereof under high temperatures.

BACKGROUND OF THE INVENTION 
The present invention relates to improved processes for producing acicular 
magnetic metallic particle powder, more particularly to novel processes 
for producing the same most suitable for a magnetic material used for 
magnetic recording, which has a large saturated magnetic flux density 
.delta.s (e.g. 90-200 emu/g) and a high coercive force Hc (e.g. 500-2000 
Oe) and is in such a condition that no cross linking between the particles 
exists and so the particles are substantially independent from each other. 
In recent years, a demand for a high efficiency of magnetic recording media 
has more and more increased with the progress in miniaturizing and 
lightening a reproducing apparatus for magnetic recording. Namely, it has 
been demanded to elevate a bit density, an output characteristic and 
especially a frequency characteristic of magnetic recording media. 
Therefore, a magnetic recording material must have a large saturated 
magnetic flux density and a high coercive force to satisfy the said 
demand. 
By the way, the magnetic materials conventionally employed in magnetic 
recording media are magnetic metal oxide powder such as magnetite, 
maghemite and chromium dioxide, and each of these magnetic oxide type 
powder has 70-85 emu/g of saturated magnetic flux density .delta.s and 
250-500 Oe of coercive force Hc. And it is a main factor in limiting the 
level of reproducing output and bit density that .delta.s of above 
magnetic oxide type particle powder is at most about 85 emu/g and 
generally 70-80 emu/g. Further, Co-magnetite or Co-maghemite magnetic 
powder, having been also used as a magnetic recording material, is 
characterized by a coercive force Hc as high as 400-800 Oe and on the 
contrary a saturated magnetic flux density .delta.s as low as 60-70 emu/g. 
On the other hand, instead of these magnetic oxide type powder, a 
development of magnetic non-oxide type particle powder having such 
properties as a larger saturated magnetic flux density and a still higher 
coercive force suitable for recording with both of high reproducing output 
and bit density, has been recently promoted. An acicular magnetic iron 
particle powder is one of examples having such properties as 
abovementioned. 
A process generally known in the prior art to produce the acicular magnetic 
iron particle powder comprises reducing acicular iron (III) oxide 
hydroxide particles of acicular ferric oxide particles at a temperature as 
lower as possible than 350.degree. C. in a stream of reducing gas. In the 
above-mentioned process the higher the heating temperature at the 
reduction in reducing gas, the larger the saturated magnetic flux density 
.delta.s of acicular magnetic iron particle powder becomes. It is, 
however, ascertained that a deformation of the acicularity of this 
resultant magnetic iron particle powder and a sintering between said 
particle powder remarkably proceed in that degree. And, therefore, the 
coercive force Hc of the obtained acicular magnetic iron particle powder 
becomes extremely low, since the coercive force of acicular magnetic iron 
particle powder used as a magnetic recording material largely depends on a 
shape-anisotropy thereof. An acicularity of magnetic iron particle powder 
is one of the most important properties. Accordingly, in a process for 
producing acicular magnetic iron particle powder, it is important to form 
first of all as a starting material acicular, iron (III) oxide hydroxide 
particles or acicular ferric oxide particles having superior acicularity. 
After the formation of said particles, there arises a problem how to keep 
this acicularity in reducing the same while heating to produce acicular 
magnetic iron particle powder. 
The shape of the particle is especially sensitive to the heating 
temperature, and the particle growth is so remarkable, particularly in 
reducing atmosphere, that the unit particle grows over the original size 
of the particle itself and the external shape of the particle itself is 
gradually deformed to cause a modification of the shape and a sintering 
between the particles. As the result, the coercive force is lowered. Thus, 
there has been arisen a serious difficulty that the condition for 
obtaining acicular magnetic iron particle powder having a high coercive 
force Hc, namely maintaining the low heating temperature when reducing, 
conflicts with the condition for obtaining acicular magnetic iron particle 
powder having a large saturated magnetic flux density .delta.s, namely 
maintaining the high heating temperature when reducing. 
On the other hand, the following processes are provided to prevent the 
sintering of magnetic particles in a process for producing magnetic oxide 
type particles such as acicular magnetic ferric oxide by means of laying 
SiO.sub.2 on the particles (Japanese patent application laying open Nos. 
83100/73 and 41299/74). 
(A) A process comprising adjusting a suspension containing acicular iron 
(III) oxide hydroxide particles or acicular ferric oxide particles to an 
acid region, pH 4.0-6.5, and adding water-soluble silicate thereinto; 
(B) A process comprising adjusting a suspension containing acicular iron 
(III) oxide hydroxide particles or acicular ferric oxide particles to over 
about pH 12 and adding water-soluble silicate thereinto while treating 
said particles through a hydrolysis of water-soluble silicate under 
oxidizing atmosphere. 
However, the inventors of the present invention found that the 
above-mentioned processes (A) and (B) did not remove the aforementioned 
difficulty in producing acicular magnetic iron particle powder of 
non-oxide type. That is, as the result of thorough research, it is found 
that these known processes for laying SiO.sub.2 on magnetic particles have 
following defects. 
In the process (A), as soon as water-soluble silicate is added into the 
suspension, an immediate precipitation in the form of SiO.sub.2 arises and 
it is easy to form a mixture of ferric oxide particles and SiO.sub.2 
particles. Accordingly, SiO.sub.2 particles become to lie on the surface 
of ferric oxide particles unevenly, and the absorbability between 
SiO.sub.2 particles and ferric oxide particles is weak. And, in the 
process (B), in the case that sodium silicate is used as water-soluble 
silicate, water-soluble sodium silicate is hydrolyzed to produce Na.sub.2 
Si.sub.2 O.sub.5 as shown in formula (1) below, and then Na.sub.2 Si.sub.2 
O.sub.5 is decomposed by the dissolved oxygen or oxygen gas to form a 
precipitation of SiO.sub.2 particles as shown in formula (2) below. The 
resultant precipitated SiO.sub.2 particles lie on the magnetic particles. 
EQU 2Na.sub.2 SiO.sub.3 + H.sub.2 O .revreaction. Na.sub.2 Si.sub.2 O.sub.5 + 
2NaOH (1) 
EQU na.sub.2 Si.sub.2 O.sub.5 + 1/2O.sub.2 .fwdarw. 2SiO.sub.2 + 2NaO (2) 
However, this process for laying prepared SiO.sub.2 particles on iron (III) 
oxide hydroxide particles through hydrolyzing water-soluble sodium 
silicate results in that the obtained SiO.sub.2 particles laid on the 
magnetic particles are uneven and coerse similarly in the process (A) 
because of taking long time for the hydrolysis. 
Further, in the both of said processes (A) and (B), the amount of SiO.sub.2 
actually laid on acicular iron (III) oxide hydroxide particles is just a 
little comparing with that of added water-soluble silicate calculated as 
the amount of SiO.sub.2. 
In fact, as shown in each example in the Japanese patent application laying 
open Nos. 83100/73 and 41299/74 the amount of SiO.sub.2 laid on acicular 
iron (III) oxide particles equals to 2-3 wt.% of added water-soluble 
silicate calculated as the amount of SiO.sub.2. This is because that the 
resultant SiO.sub.2 precipitate is in the suspended state in the solution 
without being laid on acicular iron (III) oxide hydroxide therein to form 
a mixture of acicular iron (III) oxide hydroxide particles and SiO.sub.2 
particles, which are undesirable from the economical point of view, since 
most of the resultant SiO.sub.2 particles do not lie on the surface of 
iron (III) oxide hydroxide particles and are in the state of the 
uneffective suspension. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide novel processes for 
producing acicular magnetic metallic particle powder which has the 
remarkably improved magnetic properties, namely, the both of high .delta.s 
and Hc values and commercially valuable use as high-power and -density 
magnetic recording media. 
Another object of this invention is to provide economically advantageous 
processes for producing the above-mentioned magnetic product wherein total 
times of the process are effectively shortened. 
Other objects of this invention will become more apparent from the 
following description. 
The foregoing and other objects of this invention will be accomplished by 
the processes of this invention which are summarized below. 
The process of this invention for producing acicular magnetic metallic 
particle powder comprises the steps of: 
preparing an aqueous suspension of acicular particles of at least one metal 
oxide selected from the group consisting of ferric oxide, iron (III) oxide 
hydroxide and each of these ones containing Co and/or Ni therein while 
adjusting the pH of the suspension at a desired value higher than 8; 
adding an amorphous water-soluble silicate into the suspension as fully 
agitated under non-oxidizing atmosphere and to thereby coat homogeneously 
and densely the acicular particles of metal oxide with the amorphous 
silicate; 
water-washing, collecting and drying the resultant particles and to thereby 
obtain the acicular particles of metal oxide as homogeneously and densely 
coated with the crystalline silica; and 
subsequently heating the same in a stream of a reducing gas at temperatures 
between 400.degree.-800.degree. C. and to thereby obtain acicular magnetic 
metal particle powder. 
The another process of this invention for producing acicular magnetic 
metallic particle powder comprises the steps of: 
preparing an aqueous suspension of acicular particles of at least one metal 
oxide selected from the group consisting of ferric oxide, iron (III) oxide 
hydroxide and each of these ones containing Co and/or Ni therein while 
adjusting the pH of the suspension at a desired value higher than 8; 
adding an amorphous water-soluble silicate into the suspension as fully 
agitated under non-oxidizing atmosphere and to thereby coat homogeneously 
and densely the acicular particles of metal oxide with the amorphous 
silicate; 
neutralizing the resultant amorphous silicate on the particles by adding an 
acid into the suspension and to thereby obtain the acicular particles of 
metal oxide as homogeneously and densely coated with the crystalline 
silica; 
water-washing, collecting and drying the resultant particles, and 
subsequently heating the same in a stream of a reducing gas at 
temperatures between 400.degree.-800.degree. C. and to thereby obtain 
acicular magnetic metal particle powder.

DETAILED DESCRIPTION 
Under the aforementioned situations in the art, we have carried out 
researches for years for producing acicular magnetic metallic particles in 
which reduction of acicular metal oxide particles can be conducted at a 
high temperature while without accompanying any decrease of Hc values of 
resultant magnetic metallic particles. 
As a result of our researches, it has been found that in the case where 
acicular metal oxide particles are more evenly coated with homogeneous and 
dense silica (SiO.sub.2) coats, the particles can be effectively protected 
from the aforementioned heat-damages during the reduction thereof at 
higher temperatures and further the very even and dense silica (SiO.sub.2) 
coats on the particles can be easily formed by previously coating the 
particles with amorphous water-soluble silicate under the critical 
conditions as excluded a hydrolytic and oxydative decomposition of 
amorphous silicate into uneven and rough silica grains and thereafter 
followed by converting the silicate coats on the particles into even and 
dense silica (SiO.sub.2) coats. 
Based on these findings, we have accomplished the process of this invention 
briefly summarized above. 
First of all, the most important process in the present invention, namely 
the process for coating the surface of acicular particles of metal oxide 
with amorphous water-soluble silicate under the condition that the 
amorphous water-soluble silicate is not hydrolyzed nor decomposed by 
oxidation, will be described with reference to the detailed data. 
For preventing the hydrolysis and the oxidative decomposition of 
water-soluble silicate, it is necessary to treat the particle of metal 
oxide with the amorphous water-soluble silicate aqueous solution 
efficiently in short time in an alkaline region under non-oxidizing 
atmosphere. This treatment of the particles under the non-oxidizing 
atmosphere prevents the reaction of formula (2) and accordingly prevents 
the hydrolytic reaction of formula (1). 
FIG. 1-A shows the relation between a pH value of the suspension containing 
acicular iron (III) oxide hydroxide particles and a viscosity thereof. In 
FIG. 1-A, the number 1 shows the viscosity of the suspension obtained by 
dispersing acicular iron (III) oxide hydroxide particles in water. The 
curve A shows the relation between the pH value of the suspension and the 
viscosity thereof after suspending the acicular iron (III) oxide hydroxide 
particles in water and adding caustic soda to adjust the pH value. The 
curve B shows the relation between the pH value of the suspension and the 
viscosity thereof when the sodium silicate is added into the suspension of 
the curve A under non-oxidizing atmosphere to adjust the pH value. As 
shown in the curve A, the viscosity of the suspension including no sodium 
silicate scarcely lowers in the region of the pH value higher than 8, 
while as shown in the curve B the viscosity of the suspension after being 
added with sodium silicate suddenly lowers in that region and to obtain a 
suspension with lower viscosity. Therefore the suspension of the curve B 
enables the amorphous sodium silicate and the particles of metal oxide to 
disperse evenly therein and to thereby obtain the particles homogeneously 
and finely coated with the sodium silicate. 
Accordingly, in the suspension of the curve B in FIG. 1-A the starting 
material particles are effectively treated in short time without causing 
the hydrolysis of the silicate if treating with the amorphous 
water-soluble silicate and under non-oxidizing atmosphere. Furthermore, it 
is possible to hold the starting particles in a fully dispersed state in 
the amorphous water-soluble silicate aqueous solution, so the surface of 
particles can be evenly and fully coated with the amorphous water-soluble 
silicate. 
Similarly, FIG. 1-B showing the relation between a pH value and a viscosity 
of the suspension containing particles coated with sodium silicate, which 
is prepared by suspending acicular iron (III) oxide hydroxide particles 
including Co in water and then adding sodium silicate thereinto, indicates 
the variation both of the pH and the viscosity when sodium silicate is 
added under non-oxidizing atmosphere. From FIG. 1-B it is found that the 
suspension has the low viscosity at the pH value higher than 8 and to 
obtain an even dispersion of the amorphous sodium silicate and the 
suspending particles and also to obtain the particles homogeneously and 
densely coated with the sodium silicate. 
The number 1 in FIG. 1-B indicates the viscosity of the suspension when 
admixing and dispersing the starting material particles of metal oxide 
into water. 
By the way, the viscosity in FIG. 1-A and FIG. 1-B was measured by the use 
of a Stormer's viscometer. 
The amorphous silicate coats on the metal oxide particles obtained in the 
above-mentioned manner are converted to the crystalline silica coats by 
the subsequent steps of water-washing, collecting and drying the resultant 
particles or by the neutralization with a suitable acid such as H.sub.2 
SO.sub.4 and HCl. 
By the way, said water-washing may be applied to the particles precipitated 
and collected by decantation or the particles recovered and collected by 
the filtration under a reduced pressure. Of course, in the case of said 
neutralization with the acid the water-washing is necessary after the 
neutralization to eliminate the resulting unnecessary salt. 
FIG. 2-A shows the variation of pH value and viscosity when H.sub.2 
SO.sub.4 is added into the suspension containing acicular iron (III) oxide 
hydroxide particles coated with sodium silicate to neutralize said sodium 
silicate and to thereby obtain the particles coated with silica. From FIG. 
2-A it is found that the suspension has a low viscosity in the region of 
the pH value of lower than 5 after the neutralization. This neutralizing 
reaction is formulated in formula (3). 
EQU Na.sub.2 SiO.sub.3 + H.sub.2 SO.sub.4 .fwdarw. SiO.sub.2 + Na.sub.2 
SO.sub.4 + H.sub.2 O (3) 
as the above neutralizing reaction proceeds in short time, finely divided 
particles of silica is produced and to thereby obtain the acicular 
particles coated therewith. Further, said silica particles evenly coat the 
surface of the acicular particles because the surfaces are evenly coated 
with the amorphous water-soluble silicate. 
Furthermore, as most of the amorphous water-soluble silicate added into the 
suspension coat the surface of acicular iron (III) oxide hydroxide 
particles (or acicular ferric oxide particles) in advance and said 
silicate is converted to silica on those particles, so free particles of 
silica scarcely exists and all the particles of silica coat the surface of 
metal oxide particles. 
FIG. 2-B shows the variation of pH value and viscosity similarly to FIG. 
2-A but the starting material particles are acicular iron (III) oxide 
hydroxide containing Co. As is the same with FIG. 2-A, the viscosity of 
the suspension lowers when the pH value decreases lower than 5 by the 
neutralization. 
By the way, in the case that the starting acicular particles of metal oxide 
contain such metal as Co and/or Ni other than Fe, the suspension 
containing said particle becomes an acidic suspension with the pH value 
lower than 5 due to the neutralizing reaction of the formula (3), in which 
arising the dissolution of Co and/or Ni metal contained in said particle. 
This dissolution was studied through experiments by using the suspension 
with pH4 and the results are shown in FIG. 3. From the line A in FIG. 3 it 
is found that in the case of the particles having been coated with no 
silica Co-dissolution is increased in a straight line in proportion to the 
rise of the temperature of the suspension. On the contrary, as is shown in 
the curve B, it is surprising that in the case of the particles having 
been coated with silica Co-dissolution is decreased in proportion to the 
rise of the temperature of the suspension. Therefore, in the present 
invention the dissolution of Co and/or Ni can be prevented by carrying out 
the neutralizing reaction at a high temperature and a stable component 
composition of the resultant metallic particles can be obtained. In this 
case, not similarly to the aforementioned cases of the prior art, free and 
rough particles (grains) of silica are not produced within the suspension 
and all the fine particles of silica can be tightly formed only on the 
surface of metal oxide particles as their coating layers. Thus, the 
metallic particles obtained after accomplishing reduction step can retain 
the acicularity of the starting particles and therefore can be provided 
with the both of high .delta.s and Hc values. 
Meanwhile, with the change of the specific gravity during the reducing 
process from oxide to metal, the volume of the produced acicular metallic 
particle is gradually shrinking, and therefore the starting particles must 
be chosen in due consideration to this shrinkage of the particle volume. 
Accordingly, it is important that the acicularity and the size of starting 
particles are chosen previously to be adapted to the magnetic properties 
of final product to be produced. The inventors of the present invention 
have long been engaged in the production of acicular metal oxide particles 
and have already developed various processes to obtain metal oxide 
particles having superior acicular crystals (average axis ratio; more than 
15:1 or still further more than 20:1). Several preferred processes are 
described as below. 
(1) Iron (III) oxide hydroxide particles having superior acicularity 
(average axis ratio; more than 15:1) can be produced by the process 
comprising preparing a ferrous salt solution, adding an alkali in an 
amount of more than stoichiometrically equivalent to that of ferrous salt 
into said solution at a temperature between 40-55.degree. C. under 
non-oxidizing atmosphere and to thereby obtain a solution having a pH of 
higher than 11 and containing a ferrous precipitate therein, thereafter 
oxidizing said ferrous precipitate by introducing an oxidizing gas into 
the resultant solution as maintained at a temperature lower than 
50.degree. C., and subsequently water-washing, collecting and drying the 
resultant particles of iron (III) oxide hydroxide. 
In the abovementioned process for producing iron (III) oxide hydroxide 
particles with superior acicularity, if ferrous hydroxide is precipitated 
at a temperature of lower than 40.degree. C., it is impossible to obtain 
iron (III) oxide hydroxide particles having superior acicularity (average 
axis ratio; more than 15:1) in the following steps. At a temperature of 
higher than 55.degree. C., granular megnetite particles come to be blended 
into the ferrous hydroxide precipitate. When the oxidizing reaction is 
carried out at a temperature of higher than 50.degree. C., granular 
magnetic particles come to be blended into the iron (III) oxide hydroxide 
precipitate. 
(2) Ferric oxide particles having superior acicularity can be produced by 
heating the iron (III) oxide hydroxide particles obtained in the preceding 
process (1). To be more precise, acicular .alpha.-hematite particles are 
prepared by heating and dehydrating the iron (III) oxyde hydroxide 
particles obtained in the process (1), acicular magnetite particles are 
prepared by heating and reducing the same, and acicular maghemite 
particles are prepared by heating and reoxidizing the same. These oxide 
particles have also superior acicularity. Therefore, it these acicular 
particles are used as the starting material of the present invention, 
acicular magnetic iron particle powder having superior magnetic properties 
can be obtained. 
(3) Furthermore, in the process (1), if the ferrous salt solution contains 
0.01-1 atomic % of Cr based on the amount of Fe therein, iron (III) oxide 
hydroxide particles having superior acicularity and including Cr can be 
obtained. Cr included in the acicular particles of iron (III) oxide 
hydroxide is known to effect a restraint upon the growth of the acicular 
particles and to thereby prevent the deformation of the particles and the 
sintering between the particles during the reducing thereof as heating in 
the reducing gas. 
Therefore, in the case that the acicular iron (III) oxide hydroxide 
particles including Cr or the acicular ferric oxide particles including 
Cr, which is prepared by the heat-treatment of the former are used as 
starting materials, the superior magnetic properties such higher saturated 
magnetic flux density and higher coercive force can be obtained as shown 
in FIG. 6 in comparison with those of the particle including no Cr. This 
is because of the synergetic effects of Cr and silica with the restraining 
effect upon the growth of the metallic particles. 
The synergetic effects as mentioned above is not remarkable where the 
Cr-content is less than 0.01 atomic % based on the amount of Fe-content. 
While more than 1 atomic % of Cr-content has a restraining effect upon the 
growth of particles as well, less than 1 atomic % of Cr-content is fully 
enough to satisfy the objects of the present invention. 
(4) Iron (III) oxide hydroxide particles having a superior acicularity 
(average axis ratio; more than 20:1) and containing Co and/or Ni can be 
produced by the process comprising, similarly to the process (1), 
preparing a ferrous salt solution with containing 0.1-10 atomic % of Co 
and/or Ni based on the amount of Fe component therein, adding an alkali in 
an amount of more than stoichiometrically equivalent to that of the 
ferrous salt into said ferrous salt solution at a temperature between 
40-55.degree. C. under non-oxidizing atomosphere and to thereby obtain a 
solution having a pH value of higher than 11 and containing a ferrous 
precipitate therein with homogeneously containing Co and/or Ni, thereafter 
oxidizing said ferrous precipitate by introducing an oxidizing gas into 
the resultant solution as maintained at a temperature lower than 
50.degree. C., then water-washing, collecting and drying the resultant 
particles of iron (III) oxide hydroxide. In this process (4), if the 
amount of Co- and/or Ni-content based on the Fe-content is less than 0.1 
atomic %, it is impossible to improve the magnetic properties of acicular 
magnetic Fe-Co or Fe-Co-Ni alloy particle powder. 
If the amount of Co- and/or Ni-content is more than 10 atomic %, the 
acicularity of the magnetic alloy particles decays. 
(5) Ferric oxide particles containing Co and/or Ni and having superior 
acicularity can be produced by heating the particles obtained in the 
preceding process (4). To be more precise, acicular hematite particles 
containing Co and/or Ni are prepared by heating and dehydrating the iron 
(III) oxide hydroxide particles obtained in the process (4), acicular 
magnetite particles containing Co and/or Ni are prepared by heating and 
reducing the same, and acicular maghemite particles containing Co and/or 
Ni are prepared by reducing and reoxidizing the same. These metal oxide 
particles have also superior acicularity, which therefore very appropriate 
to be used as the starting materials of the present invention. 
And also these particles in the process (4) and (5) may contain 0.01-1 
atomic % of Cr based on the Fe-content therein similarly in the process 
(2). 
Needless to say, the starting materials of the present invention are not 
limited to each acicular particles obtained by said processes (1)-(5) but 
may be chosen from the metal oxide particles with an appropriate 
acicularity and a proper size, which may be produced by various other 
processes, on considering the shrinkage of the particle volume during the 
reducing step. 
As mentioned before, convertionally in the art acicular magnetic iron 
particle powder is obtained by heating acicular iron (III) oxide hydroxide 
particles or acicular ferric oxide particles at a temperature as low as 
possible (lower than 350.degree. C.) under the atmosphere of reducing gas 
with a high partial pressure. Therefore, it takes a long time and a large 
amount of reducing gas until the ferric oxide particles are reduced to the 
acicular magnetic iron particles. This is the result of paying regard to 
the acicular of the starting material particles. 
On the contrary, by the process of present invention acicular magnetic 
metallic particle powder having superior acicularity can be obtained in 
short time. 
FIG. 4-A shows the magnetic properties of the resultant particle powder in 
the case that acicular iron (III) oxide hydroxide particle powder is 
reduced at some temperatures from 350.degree. C. to 500.degree. C. in a 
stream of H.sub.2 gas for 2 hours. The curve A in FIG. 4-A indicates that 
the saturated magnetic flux density .delta.s increases with the rise of 
the reducing temperature. On the contrary, the curve B indicates that the 
coercive force Hc decreases with the rise of the reducing temperature. 
Thus, in conventional processes acicular magnetic iron particle powder was 
used to be produced at a reducing temperature as low as possible. 
FIG. 5-A shows the magnetic properties of the produced particle powder 
obtained by reducing acicular iron (III) oxide hydroxide particle powder 
coated with 4.5 mol % of silica in a stream of H.sub.2 gas at each 
temperature between 400.degree. C.-800.degree. C. for 2 hours. 
In FIG. 5-A, curves A and B are related to the saturated magnetic flux 
density .delta.s, and the coercive force Hc respectively, showing that 
both properties tend to increase with the rise of the reducing 
temperature, in a quite different way from the conventional process as 
shown in FIG. 4A. 
FIG. 6 shows the magnetic properties of the particle powder produced by 
reducing acicular iron (III) oxide hydroxide particle powder coated with 
4.5 mol % of silica and containing 0.3 atomic % of Cr in H.sub.2 stream at 
each reducing temperature between 400.degree. C.-800.degree. C. for 2 
hours. In FIG. 6, curves A and B are related to the saturated magnetic 
flux density .delta.s and the coercive force Hc respectively. As seen from 
the figure, the coercive force Hc does not lower even at a reducing 
temperature of 800.degree. C., thereby the fact indicating an excellent 
synergetic effects provided by the combination of silica coats and Cr. 
FIG. 7-A shows the magnetic properties of the particle powder produced by 
reducing acicular iron (III) oxide hydroxide particle powder in H.sub.2 
stream at a reducing temperature of 350.degree. C. for each reducing 
period between 1-14 hours. In FIG. 7-A, curves A and B are related to the 
saturated magnetic flux density .delta.s and the coercive force Hc, and 
the both describe their change at each reducing time as showing that long 
times are required in order to attain their values to maximum ones because 
of the low reducing temperature. 
FIG. 8A and FIG. 9-A show the magnetic properties of the particle powder 
produced by reducing acicular iron (III) oxide hydroxide particle powder 
coated with 4.5 mol % of silica in H.sub.2 stream at each reducing 
temperature of 400.degree. C., 450.degree. C., and 600.degree. C. for each 
period between 1-8 hours. FIG. 8-A is related to the coercive force Hc, 
and FIG. 9-A is related to the saturated magnetic flux density .delta.s. 
In FIG. 8-A and FIG. 9-A, each curve A, B and C represents the magnetic 
properties of the powder at a reducing temperature of 600.degree. C., 
450.degree. C. and 400.degree. C. respectively, showing that both Hc and 
.delta.s reach their maximum values in shorter time and their values 
increase more as the reducing temperature becomes higher. 
FIG. 10-A and FIG. 11-A show the magnetic properties of the particle powder 
produced by reducing each acicular iron (III) oxide hydroxide particle 
powder coated with 0-12 mol % of silica in H.sub.2 stream at a reducing 
temperature of 600.degree. C. for 2 hours and for 6 hours, respectively. 
FIG. 10-A is related to the coercive force Hc, and FIG. 11-A is related to 
the saturated magnetic flux density .delta.s. In both figures, the curves 
A and B shows the magnetic properties at reducing times of 6 hours and 2 
hours, respectively. As seen from the figures, the magnetic properties Hc 
and .delta.s are influenced by the temperature and reducing times depend 
on the amount of silica coats. This fact indicates that the coercive force 
Hc and the saturated magnetic flux density .delta.s can be simultaneously 
increased if the reducing temperature and times are properly selected in 
correlation with the amount of silica coats. 
On the other hand, FIG. 4-B shows the magnetic properties of the particle 
powder produced by reducing acicular iron (III) oxide hydroxide particle 
powder containing Co having 2.7 atomic % Co against Fe in H.sub.2 gas 
stream at each temperature between 330.degree. C.-500.degree. C. for 2 
hours. In the figure, curve A indicates that the saturated magnetic flux 
density .delta.s increases with the rise of reducing temperature, contrary 
to which, curve B indicates that the coercive force Hc decreases with the 
rise of reducing temperature. Thus, in conventional process, acicular 
iron-alloy magnetic particle powder containing Fe-Co as its main 
components is obliged to be produced under as low reducing temperature as 
possible. 
FIG. 5-B shows the magnetic properties of the particle powder produced by 
reducing acicular iron (III) oxide hydroxide particle powder coated with 
4.5 mol % of silica and containing 2.7 atomic % of Co based on Fe 
component contained therein in H.sub.2 stream at each reducing temperature 
between 350.degree. C.-800.degree. C. for 2 hours. In the figure, curves A 
and B are related to the saturated magnetic flux density .delta.s and the 
coercive force Hc respectively, and both properties tend to increase with 
the rise of reducing temperature. This phenomenon is quite different from 
the conventional example shown in FIG. 4. 
FIG. 7-B shows the magnetic properties of the particle power produced by 
reducing acicular iron (III) oxide hydroxide particle powder containing 
2.7 atomic % of Co based on Fe component contained therein in H.sub.2 
stream at a reducing temperature of 330.degree. C. for each period between 
1-14 hours. In the figure, change of the saturated magnetic flux density 
.delta.s and coercive force Hc according to each reducing period are 
indicated by curves A and B respectively, and showing that long times are 
required for both properties to attain their values to maximum ones 
because of low reducing temperatures. 
FIG. 8-B and FIG. 9-B show the magnetic properties of the particle powder 
produced by reducing acicular iron (III) oxide hydroxide particle powder 
containing 2.7 atomic % of Co based on Fe component contained therein in 
H.sub.2 stream at each reducing temperature of 400.degree. C., 500.degree. 
C. and 650.degree. C. for each reducing period between 0.5-8 hours. FIG. 
8-B and 9-B are related to the coercive force Hc and the saturated 
magnetic flux density .delta.s, respectively. 
In the figures, curves A, B and C are representing the magnetic properties 
measured at a temperature of 650.degree. C., 500.degree. C. and 
400.degree. C. respectively, and showing that the maximum values thereof 
can be attained in shorter times and the values increase more as the 
reducing temperature becomes higher. FIG. 10-B and 11-B shows the magnetic 
properties of the particle powder produced by reducing the acicular iron 
(III) oxide hydroxide particle powder which contains 2.7 atomic % of Co 
based on the amount of Fe-content therein and is coated with 0-12 mol % of 
silica calculated the amount of SiO.sub.2 based on the amount of the total 
metal, at the reducing temperature of 650.degree. C. for 2 and 8 hours in 
a stream of H.sub.2 gas. FIG. 10-B and 11-B represent the coercive force 
Hc and the saturated magnetic flux density .delta.s respectively. In FIG. 
10-B and 11-B, curves A and B represent the magnetic properties of the 
particle reduced for 8 and 2 hours respectively. As shown by curves A and 
B in FIG. 10-B and 11-B, the properties of Hc and .delta.s are influenced 
by the temperature and the reducime time depended on the amount of silica 
coat, which indicates that the coercive force Hc and the saturated 
magnetic flux density .delta.s can be increased simultaneously if the 
reducing temperature, the time and the amount of silica coat are 
appropriately selected. 
Followingly, the various conditions for carrying out the process of the 
present invention will be mentioned. 
In the present invention, iron (III) oxide hydroxide particles mean 
acicular .alpha.-, .beta.-, .gamma.- iron (III) oxide hyroxide particles, 
and acicular ferric oxide particles mean acicular hematite particles, 
magnetite particles, and maghemite particles. (Same can be applied to 
those containing Co and/or Ni). As water-soluble silicate used in the 
present invention, sodium silicate and potassium silicate are employed. In 
the present invention, water-soluble silicate is added into a suspension 
containing the starting material particles at a pH value higher than 8 
because an effective treatment in short time is desired so as not to cause 
the hydrolytic reaction and also because the starting material particles 
must be dispersed fully at a low viscosity to coat the surface of 
particles with the amorphous water-soluble silicate homogeneously and 
fully. Where pH is lower than 8, as is obvious from the FIG. 1-A or 1-B, 
the viscosity increases and the treatment cannot be carried out 
effectively as the hydrolytic reaction easily occurs and thereby bringing 
about difficulties in dispersing the particles homogeneously and fully. 
In order to prevent the progress of the hydrolitic reaction, the step of 
the addition of silicate in the present invention is carried out under the 
non-oxidizing atmosphere. In the case that the amorphous water-soluble 
silicate coat is less than 1 mol % calculated as the SiO.sub.2 amount 
based on the Fe amount, the object of the present invention can not be 
enoughly satisfied and in the case more than 15 mol %, the saturated 
magnetic flux density decreases due to the lowering of the purity although 
the superior acicularity can be obtained. 
In the present invention, H.sub.2 SO.sub.4 , HCl, etc. are employed as an 
acid for the neutralization. 
In the present invention, the reducing reaction does not smoothly progress 
if the reducing temperature in a reducing gas is lower than 400.degree. 
C., and if the temperature is higher than 800.degree. C. the reducing 
reaction radically progresses bringing about the deformation of the 
acicular particles and the sintering between the particles. Moreover, as 
highly refined arrangements and skilled techniques are required for the 
reduction at a temperature as high as 800.degree. C. in a reducing gas, 
which is not desirable from the economical and industrial viewpoints. 
Considering the progress of the reducing reaction, the shape of the 
particles, the sintering between the particles, the industrial materials 
and the industrial arrangements, the heating temperatures higher than 
450.degree. C. and less than 700.degree. C. are preferred. In the case 
that the starting materials contain Cr, it is sometimes preferable to 
raise the reducing temperature up to around 800.degree. C. 
Followingly, the advantages of the present invention will be summarized 
below. 
In the present invention, it is possible to obtain acicular magnetic 
metallic particle powder which keeps the shape of the starting material 
and, for example, is substantially independent to each other without no 
cross linking between the particles. Acicular magnetic metallic particle 
powder thus obtained can be used as the magnetic recording material having 
high reproducing output and a high bit density which nowadays is most 
wanted as it has a large saturated magnetic flux density .delta.s, e.g. 
90-200 emu/g and a high coercive force from 500 Oe 2000 Oe. 
Further, in the present invention, as acicular magnetic metallic particle 
powder can be obtained by a reduction at a reducing temperature higher 
than 400.degree. C. in short time, it is economical and industrially 
desirable from the viewpoints of the arrangements and the manufacture. 
Other advantages of the process according to the present invention will be 
understood by those skilled in the art from the following examples. 
By the way, in the examples and the comparison examples the amount of 
SiO.sub.2 and Cr was measured by Si analysis of JISG1212 and atomic 
adsorption analysis respectively. And the viscosity was measured by a 
Stormer's viscometer. 
PREATION OF STARTING MATERIAL TICLES 
EXAMPLE 1 
42 l of aqueous solution containing 52 mol of FeSO.sub.4 was added into 18 
l of 15.2N NaOH aqueous solution previously placed in a reactor, while 
introducing N.sub.2 gas thereinto at a flow rate of 200 l/min.. This 
procedure was continued for 15 minutes at pH 12.2 and at a tempereture of 
48.degree. C. to form ferrous hydroxide colloid. Thereafter, air 
introduced at a flow rate of 300 l/min. into the resulting colloidal 
solution at 40.degree. C. to carry out oxidizing reaction for 20 hours, 
thereby obtaining acicular iron (III) oxide hydroxide particles. 
Subsequently, the obtained particles were washed with water, filtered, 
dried, and ground in usual manner. These resultant particles were acicular 
.alpha.-FeOOH by a chemical analysis and were acicular particles having 
0.8-1.0.mu. of long axis and 15:1 axial ratio of long axis to short axis 
by electro-microscopic observation. 
EXAMPLE 2 
20 l of aqueous suspension containing 2.6 mol of fine divided .alpha.-FeOOH 
particles as a seed and 17.4 mol of FeSO4 was heated to maintain the 
temperature of the suspension at 68.degree. C. while introducing air at a 
flow rate of 100 l/min.. 5l of 3.58-N NaOH aqueous solution was gradually 
poured into said suspension so at to control the pH value thereof at 4.0 
and to thereby obtain acicular iron (III) oxide hydroxide particles. The 
resultant particles were washed with water, filtered, dried, and ground in 
usual manner. According to a chemical analysis and electromicroscopic 
observation, these particles thus obtained were .alpha.-FeOOH particles 
which were acicular and had 0.5-0.7.mu. of long axis and 15:1 of axial 
ratio of long axis to short axis. 
EXAMPLE 3 
14 l of aqueous admixture consisting of Cr.sub.2 (SO.sub.4).sub.3 aqueous 
solution containing 0.045 mol of Cr and FeSO.sub.4 aqueous solution 
containing 15 mol of Fe was added into 6 l of 11.55-N NaOH aqueous 
solution previously placed in a reactor. And thereby the formation of 
ferrous hydroxide colloid containing Cr was carried out for 15 minutes at 
pH 12.5 and at a temperature of 45.degree. C. while introducing N.sub.2 
gas at a flow rate of 200 l/min.. Thereafter air was introduced at a rate 
of 300 l/min. into the resulting colloidal solution at 45.degree. C. to 
carry out oxydizing reaction for 14 hours, thereby obtaining acicular iron 
(III) oxide hydroxide particles containing Cr. The resultant particles 
were washed with water, filtered, dried, and ground in usual manner. 
According to a chemical analysis and electromicroscopic observation, these 
particle were acicular .alpha.-FeOOH particles containing 0.3 atomic % of 
Cr to Fe therein and having 0.5-0.6.mu. of long axis and 18:1 of axial 
ratio of long axis to short axis. 
EXAMPLE 4 
From 14 l of aqueous admixture consisting of Cr.sub.2 (SO.sub.4).sub.3 
aqueous solution containing 0.015 mol of Cr and FeSO.sub.4 aqueous 
solution containing 15 mol of Fe and 6 l of 11.55-N NaOH aqueous solution, 
acicular iron (III) oxide hydroxide particles were prepared in accordance 
with the procedures employed in Example 3. The particles obtained were 
.alpha.-FeOOH particles containing 0.1 atomic % of Cr to Fe therein and by 
electro-microscopic observation they were acicular particles having 
0.5-0.6.mu. of long axis and 15:1 of axial ratio. 
EXAMPLE 5 
500g of acicular .alpha.-FeOOH particles obtained in Example 1 was heated 
and dehydrated at 300.degree. C. in a stream of air to obtain acicular 
.alpha.-Fe.sub.2 O.sub.3 particle powder. By electromicroscopic 
observation, they were acicular particles having 0.65-0.8.mu. of long axis 
and 12:1 of axial ratio. 
EXAMPLE 6 
500g of acicular .alpha.-FeOOH particles containing Cr obtained in Example 
3 was heated and dehydrated at 300.degree. C. in a stream of air to obtain 
acicular .alpha.-Fe.sub.2 O.sub.3 particle powder containing 0.3 atomic % 
of Cr to Fe. By electromicroscopic observation, they were acicular 
particles having 0.5-0.6.mu. of long axis and 18:1 of axial ratio. 
EXAMPLE 7 
500g of acicular .alpha.-FeOOH particles obtained in Example 1 was heated 
and reduced at 350.degree. C. in a stream of H.sub.2 gas to form acicular 
magnetite particles, thereafter the magnetite were reoxidized at 
270.degree. C. in a stream of air to obtain acicular .gamma.-Fe.sub.2 
O.sub.3 particle powder. By electromicroscopic observation the resultant 
particles were acicular particles having 0.5-0.65.mu. of long axis and 8:1 
of axial ratio. 
EXAMPLE 8 
500g of acicular .alpha.-FeOOH particles containing Cr obtained in Example 
3 was heated and reduced at 400.degree. C. in a stream of H.sub.2 gas to 
produce acicular magnetite particle containing 0.3 atomic % of Cr to Fe, 
therafter the magnetite were reoxidized at 300.degree. C. in a stream of 
air to obtain acicular .gamma.-Fe.sub.2 O.sub.3 particle powder containing 
Cr. By electro-microscopic observation these were acicular particles 
having 0.45-0.55.mu. of long axis and 15:1 of axial ratio. 
EXAMPLE 9 
42 l of aqueous solution containing 60 mol of FeSO.sub.4 and 1.6 mol of 
CoSO.sub.4 was added into 18 l of 15.8-N NaOH aqueous solution previously 
placed in a reactor, while nitrogen was introduced thereinto at a flow 
rate of 200 l/min., then the formation of ferrous hydroxide colloid 
containing Co was continued for 15 minutes at pH 12.0 and at a temperature 
of 45.degree. C. Air was introduced at a flow rate of 280 l/min. into the 
resulting colloidal solution at 50.degree. C. to carry out oxydizing 
reaction for 18 hours and to thereby obtain acicular iron (III) oxide 
hydroxide particles containing Co. Thereafter the obtained particles were 
washed with water, filtered, dried, and ground in usual manner. According 
to a chemical analysis and electro-microscopic observation, the resultant 
particles were acicular .alpha.-FeOOH particles containing 2.7 atomic % of 
Co to Fe and having 1.0-1.2.mu. of long axis and 25:1 of axial ratio. 
EXAMPLE 10 
14 l of aqueous solution containing 20 mol of FeSO.sub.4 and 1.28 mol of 
CoSO.sub.4 was added into 6 l of 16.3-N NaOH aqueous solution previously 
placed in a reactor, while nitrogen was introduced thereinto at a flow 
rate of 100 l/min., then the formation of ferrous hydroxide colloid 
containing Co was continued for 10 minutes at pH 12.0 and at a temperature 
of 45.degree. C. Air was introduced at a flow rate of 100 l/min. into the 
resulting colloidal solution at 50.degree. C. to carry out oxydizing 
reaction for 18 hours and to thereby obtain acicular iron (III) oxide 
hydroxide particles containing Co. Thereafter the obtained particles were 
washed with water, filtered, dried, and ground in usual manner. According 
to a chemical analysis and electro-microscopic observation, the resultant 
particles were acicular .alpha.-FeOOH particles containing 6.4 atomic % of 
Co to Fe and having 0.5-0.7.mu. of long axis and 30:1 of axial ratio. 
EXAMPLE 11 
From 14 l of aqueous solution containing 20 mol of FeSo.sub.4 and 0.22 mol 
of CoSo.sub.4 and 6 l of 15.5-N NaOH aqueous solution, taking the same 
procedures as in Example 10, acicular iron (III) oxide hydroxide particles 
containing Co were prepared. The resultant particles were .alpha.-FeOOH 
particles containing 1.1 atomic % of Co to Fe and having 0.8-1.0.mu. of 
long axis and 20:1 of axial ratio. 
EXAMPLE 12 
From 14 l of aqueous solution containing 20 mol of FeSO.sub.4, 0.72 mol of 
CoSO.sub.4, and 0.12 mol of NiSO.sub.4 and 6 l of 16-N NaOH aqueous 
solution, taking the same procedures as in Example 10, acicular iron (III) 
oxide hydroxide particles containing Co-Ni were prepared. The resultant 
particles were .alpha.-FeOOH particles containing 3.6 atomic % of Co and 
0.61 atomic % of Ni to Fe and having 0.4-0.5.mu. of long axis and 30:1 of 
axial ratio. 
EXAMPLE 13 
20 l of 4.77-N NaOH aqueous solution was added into 30 l of aqueous 
solution containing 20 mol of FeSO.sub.4 and 0.73 mol of CoSO.sub.4. Air 
was introduced at a flow rate of 200 l/min. into the resulting colloidal 
solution of ferrous hydroxide containing Co at 45.degree. C. to carry out 
oxydizing reaction for 20 hours and to thereby obtain acicular iron (III) 
oxide hydroxide particles containing Co. Particles thus obtained were 
washed with water, filtered, dried, and ground in usual manner. According 
to a chemical analysis, and electro-microscopic observation, the resultant 
particles were .alpha.-FeOOH particles containing 3.52 atomic % of Co and 
Fe and having 0.6-0.7.mu. of long axis and 15:1 of axial ratio. 
EXAMPLE 14 
500g of acicular .alpha.-FeOOH particles containing Co obtained in Example 
13 was heated and dehydrated at 300.degree. C. in a stream of air to 
produce acicular .alpha.-Fe.sub.2 O.sub.3 particle powder containing 3.52 
atomic % of Co to Fe. According to electro-microscopic observation the 
resultant particles were acicular particles having 0.5-0.6.mu. of long 
axis and 12:1 of axial ratio. 
EXAMPLE 15 
500g of acicular .alpha.-FeOOH particles containing Co obtained in Example 
13 was heated and reduced at 350.degree. C. in a stream of H.sub.2 gas to 
obtain magnetite particles containing 3.52 atomic % of Co to Fe. 
Thereafter the obtained particles were reoxidized at 270.degree. C. in a 
stream of air and to thereby obtain acicular .gamma.-Fe.sub.2 O.sub.3 
particles containing Co. By electro-microscopic observation the resultant 
particles were acicular particles having 0.4-0.45.mu. of long axis and 8:1 
of axial ratio. 
EXAMPLE 16 
14 l of aqueous solution prepared by admixing Cr.sub.2 (SO.sub.4).sub.3 
aqueous solution containing 0.062 mol of Cr with an aqueous solution 
containing 20 mol of FeSO.sub.4 and 0.75 mol of CoSO.sub.4 was added into 
6 l of 16-N NaOH aqueous solution previously placed in a reactor, while 
introducing N.sub.2 gas thereinto at a rate of 100 l/min. The procedure 
was continued at 45.degree. C. and at pH 12 for 10 minutes to prepare 
ferrous hydroxide colloid containing Cr and Co. Thereafter air was 
introduced at a flow rate of 100 l/min. into the resulting colloidal 
aqueous solution at 50.degree. C. to carry out oxydizing reaction for 18 
hours, thereby producing acicular iron (III) oxide hydroxide particles 
containing Co and Cr. Subsequently, the obtained particles were washed 
with water, filtered, dried, and ground in usual manner. 
According to a chemical analysis and electromicroscopic observation, the 
resultant particles were acicular .alpha.-FeOOH particles containing 3.75 
atomic % of Co and 0.3 atomic % of Cr to Fe and having 0.4-0.5.mu. of long 
axis and 30:1 of an axial ratio. 
EXAMPLE 17 
14 l of aqueous solution prepared by admixing Cr.sub.2 (SO.sub.4).sub.3 
aqueous solution containing 0.063 mol of Cr with an aqueous solution 
containing 20 mol of FeSO.sub.4, 0.72 mol of CoSO.sub.4 and 0.12 mol of 
NiSO.sub.4 was added into 6 l of 16-N NaOH aqueous solution previously 
placed in a reactor, while introducing N.sub.2 gas thereinto at a rate of 
100 l/min. The procedure was continued at 45.degree. C. and at pH12 for 10 
min. to prepare ferrous hydroxide colloid containing Co, Ni and Cr. 
Thereafter air was introduced into the resulting colloidal aqueous 
solution at a flow rate of 100 l/min. at 50.degree. C. to carry out 
oxydizing reaction for 19 hours and to thereby obtain acicular iron (III) 
oxide hydroxide particles containing Co, Ni and Cr. The obtained particles 
were washed with water, filtered, dried, and ground in usual manner. 
According to a chemical analysis and electromicroscopic observation, the 
resultant particles were acicular .alpha.-FeOOH particles containing 3.62 
atomic % of Co, 0.60 atomic % of Ni, and 0.29 atomic % of Cr to Fe and 
having 0.4-0.5.mu. of long axis and 30:1 of axial ratio. 
PREATION OF TICLES COATED WITH SIO.sub.2 
EXAMPLE 18 
360g of acicular .alpha.-FeOOH particles obtained in Example 1 was 
dispersed in water to prepare 10 l of suspension. This suspension had a pH 
value of 7.8 and a viscosity of 4.7 poise. NaOH solution was poured into 
the suspension for adjusting the pH value thereof to 8.6. Thereafter 38.9 
g of sodium silicate (No. 3 on J.I.S. K-1408) containing 28.55 wt % of 
SiO.sub.2 was added into the suspension, followed by stirring and 
dispersing the particles while preventing oxidizing gas such as air from 
intermixing as far as possible. The resulting suspension had a pH value of 
10.2 and a viscosity of 2.7 poise. 
The obtained particles were then washed with water, filtered in usual 
manner and then dried at 110.degree. C. and to thereby obtain acicular 
iron (III) oxide hydroxide particles coated with 4.44 mol % of SiO.sub.2 
based on the amount of iron therein. Accordingly, 96% of the initial 
sodium silicate calculated as SiO.sub.2 amounts was coated on the 
particles. See TABLE I. 
EXAMPLES 19-20 
Particles coated with SiO.sub.2 were produced by the same procedures 
employed in Example 18 except the kind of starting material particles, the 
pH value of the addition of sodium silicate, and the amount of sodium 
silicate (No. 3 on J.I.S. K-1408). The results are shown in TABLE I. 
EXAMPLE 21 
2880g of acicular .alpha.-FeOOH particles obtained in Example 1 was 
dispersed in water to produce 80 l of suspension. This suspension had a pH 
value of 7.5 and a viscosity of 3.8 poise. NaOH solution was added into 
the suspension for adjusting the pH value to 8.5, then 322g of sodium 
silicate (No. 3 on J.I.S. K-1408) containing 28.55 wt % of SiO.sub.2 was 
added thereto, followed by agitating and dispersing the particles while 
preventing oxidizing gas such as air from intermixing as far as possible. 
The resultant suspension had a pH value of 10.5 and a viscosity of 2.6 
poise. 1-N H.sub.2 SO.sub.4 was added into the resultant suspension 
containing the particles coated with sodium silicate until the pH value of 
the suspension became 4.3 to neutralize said sodium silicate. This 
suspension containing the particles coated with silica was filtered and 
washed with water and then dried at 110.degree. C. in usual manner and 
thereby to obtain acicular .alpha.-FeOOH particles containing 4.45 mol % 
of SiO.sub.2 (SiO.sub.2 /Fe). Accordingly, 93% of initial water-soluble 
sodium silicate calculated as SiO.sub.2 amounts was coated on the 
particles. See TABLE I. 
EXAMPLES 22-23 
Particles coated with SiO.sub.2 were produced by the same procedures 
employed in Example 21 except the kind of starting material particles, 
concentration of the suspension, the pH value at the addition of sodium 
silicate, the amount of sodium silicate (No. 3 on J.I.S. K-1408) and the 
pH value after the neutralization. 
The results are shown in TABLE I. 
EXAMPLE 34 
712 g of acicular .alpha.-FeOOH particles containing Co and being obtained 
in Example 9 was dispersed in water to prepare 10 l of suspension. This 
suspension had a pH value of 7.5 and a viscosity of 4.5 poise. NaOH 
solution was poured into the suspension for adjusting the pH value thereof 
to 8.5. Thereafter, 76.2 g of sodium silicate (No. 3 on J.I.S. K-1408) 
containing 28.55 wt % of SiO.sub.2 was added into the suspension, followed 
by stirring and dispersing the particles while preventing oxidizing gas 
such as air from intermixing as far as pssible. The resulting suspension 
had a pH value of 10.0 and a viscosity of 2.9 poise. The obtained 
particles then washed with water, filtered in usual manner, and then dried 
at 120.degree. C. and to thereby obtain acicular iron (III) oxide 
hydroxide particles containing Co coated with 4.3 mol % of SiO.sub.2 based 
on the amount of total metal therein. Accordingly, 94% of the initial 
sodium silicate calculated as SiO.sub.2 amounts was coated on the 
particles. See TABLE II. 
EXAMPLES 35-36 
Particles coated with SiO.sub.2 were prepared by the same procedures 
employed in Example 34 except the kind of starting material particles, the 
pH value at the addition of sodium silicate, and the amount of sodium 
silicate (No. 3 on J.I.S. K-1408). The results are shown in TABLE II. 
EXAMPLE 37 
5700 g of acicular .alpha.-FeOOH particles obtained in Example 9 was 
dispersed in water to prepare 80 l of suspension. This suspension had a pH 
value of 7.6 and a viscosity of 3.5 poise. NaOH solution was poured into 
the suspension for adjusting the pH value to 8.3, then 632 g of sodium 
silicate (No. 3 on J.I.S. K-1408) containing 28.55 wt % of SiO.sub.2 was 
added thereto, followed by agitating and dispersing the particles while 
preventing oxidizing gas such as air from intermixing as far as possible. 
The resulting suspension had a pH value of 10.2 and a viscosity of 2.9 
poise. 1-N H.sub.2 SO.sub.4 was poured into the resultant suspension 
containing the particles coated with sodium silicate at a solution 
temperature of 50.degree. C. until pH 4.0 was attained thereby to 
neutralize said sodium silicate. This suspension containing the particles 
coated with silica was washed with water, filtered, and dried at 
120.degree. C. in usual and to thereby obtain acicular .alpha.-FeOOH 
particles containing Co and 4.5 mol % of SiO.sub.2 (SiO.sub.2 /total 
metal). Accordingly, 95% of the initial water-soluble sodium silicate 
caluculated as SiO.sub.2 amounts was coated on the particles. See TABLE 
II. 
The concentration of Co ion in this solution was 5 ppm which was equivalent 
to about 1% of Co content of acicular .alpha.-FeOOH particles obtained in 
Example 9. 
EXAMPLES 38-48 
Particles coated with SiO.sub.2 were prepared by the same procedures 
employed in Example 37 except the kind of starting material particles, the 
concentration of the suspension, the pH value at the addition of sodium 
silicate, the amount of sodium silicate (No. 3 on J.I.S. K-1408) the 
neutralizing temperature, and the pH value after the neutralization. The 
results are shown in TABLE II. 
EXAMPLES 49 
1140 g of acicular .alpha.-FeOOH particles containing Co and Cr and being 
obtained in Example 16 was dispersed in water to prepare 16 l of 
suspension. This suspension had a pH value of 7.5 and a viscosity of 4.5 
poise. 
NaOH solution was added into the suspension to attain pH 8.8. Thereafter, 
126 g of sodium silicate (No. 3 on J.I.S. K-1408) containing 28.55 wt % of 
SiO.sub.2 was added into the suspension, followed by stirring and 
dispersing the particles while preventing oxidizing gas such as air from 
intermixing as far as possible. The resulting suspension had a pH value of 
10.0 and a viscosity of 2.9 poise. 1-N H.sub.2 SO.sub.4 was added into the 
resulting suspension containing the particles coated with sodium silicate 
at a solution temperature of 50.degree. C. until pH 4.8 was attained to 
neutralize said sodium silicate. This suspension containing the particles 
coated with silica was washed with water, filtered, and dried at 
120.degree. C. in usual manner and to thereby obtain acicular 
.alpha.-FeOOH particles containing 4.5 mol % of SiO.sub.2 (SiO.sub.2 
/total metal). Accordingly, 96% of the initial water-soluble sodium 
silicate calculated as SiO.sub.2 amounts was coated on the particles. The 
result is shown in TABLE II. 
EXAMPLE 50 
1140 g of acicular .alpha.-FeOOH particles containing Co, Ni and Cr and 
being obtained in Example 17 was dispersed in water to prepare 16 of 
suspension. This suspension had a pH value of 7.5 and a viscosity of 4.5 
poise. 
NaOH solution was added into the suspension to attain pH 8.5. Thereafter, 
126 g of sodium silicate (No. 3 on J.I.S. K-1408) containing 28.55 wt % of 
SiO.sub.2 was added into the suspension, followed by stirring and 
dispersing the particles while preventing oxidizing gas such as air from 
intermixing as far as possible. The resulting suspension had a pH value of 
9.5 and a viscosity of 3.0 poise. 
1-N H.sub.2 SO.sub.4 was added into the resulting suspension containing 
particles coated with sodium silicate at a solution temperature of 
50.degree. C. until pH 4.8 is attained to neutralize said sodium silicate. 
This suspension containing the particles coated with silicate was washed 
with water, filtered and dried at 120.degree. C. in usual manner and 
thereby obtain acicular .alpha.-FeOOH particles containing 4.4 mol % 
SiO.sub.2 (SiO.sub.2 /total metal). Accordingly, 96% of the initial 
water-soluble sodium silicate calculated as SiO.sub.2 amounts was coated 
on the particles. The result is shown in TABLE II. 
REDUCTION TREATMENT PROCESS 
EXAMPLE 51 
300 g of the resultant particle powder obtained in Example 18 was reduced 
in a reductor by introducing H.sub.2 gas at a flow rate of 3l/min. and at 
600.degree. C. for 3 hours. The reduced acicular metallic iron were 
preliminarily dipped in toluene solution so as not to bring about a 
radical oxidation thereof when said particles were taken out in the air, 
thereafter, said particles were coated with stable oxidized film by 
evaporating said toluene from the surfaces of the particles. The physical 
properties of acicular magnetic iron particles thus obtained are shown 
TABLE III. 
EXAMPLES 52-88 
Acicular magnetic iron particle powder was produced by the same procedures 
employed in Example 51 except the kind of particles to be reduced, the 
reduction temperature, and the reduction time. The physical properties of 
the obtained particle powder are shown in TABLE III. 
EXAMPLE 89 
300g of the resultant particle powder obtained in Example 34 was reduced in 
a reductor by introducing H.sub.2 gas at a flow rate of 3l/min. and at 
600.degree. C. for 4 hours. The reduced acicular Fe-Co alloy particles 
were preliminarily dipped in toluene solution so as not to bring about a 
radical oxidation thereof when said particles were taken out in the air, 
thereafter said particles were coated with stable oxidized film by 
evaporating said toluene from the surface of the particles. The physical 
properties of acicular magnetic Fe-Co alloy particle powder thus obtained 
are shown in TABLE IV. 
EXAMPLES 90-120 
Acicular magnetic Fe-Co or Fe-Co-Ni particle powders were obtained 
according to the same procedures employed in Example 89 except the kind of 
particles to be reduced, the reduction temperature, and the reduction 
time. The physical properties of the obtained particle powder are shown in 
TABLE IV. 
EXAMPLE 121 
300g of the resultant particle powder obtained in Example 49 was reduced in 
a reductor at 800.degree. C. for 3 hours by introducing H.sub.2 gas at a 
flow rate of 3l/min. The reduced acicular Fe-Co-Cr alloy particles were 
preliminarily dipped in toluene solution so as not to bring about a 
radical oxidation thereof when said particles were taken out in the air, 
and then after evaporation of toluene, stable oxidized film was formed on 
said particle surface. The physical properties of acicular magnetic 
Fe-Co-Cr alloy particles thus obtained are shown in TABLE IV. 
EXAMPLE 122 
300g of the resultant particle powder obtained in Example 50 was reduced in 
a reductor at 800.degree. C. for 3 hours by introducing H.sub.2 gas at a 
flow rate of 3l/min. The reduced acicular Fe-Co-Ni-Cr alloy particles were 
preliminarily dipped in toluene solution not to cause a radical oxidation 
thereof when said particles were taken out in the air, then after 
evaporation of said toluene stable oxidized film was formed on particle 
surface. The magnetic properties of acicular magnetic Fe-Co-Ni-Cr alloy 
particles thus obtained are shown in TABLE IV. 
COMISON EXAMPLES 1-15 
The resultant particles of Example 1 or 9 were reduced to acicular magnetic 
iron particles without coating the surface thereof with silica, by the 
same procedure of Example 51 except the reducing temperature and the 
reduction time. The physical properties of the obtained particles are 
shown in TABLE V. 
TABLE I 
______________________________________ 
preparation 
of particle coated with SiO.sub.2 
starting 
example particle (g) 
No. (a) (b) (c) (d) (e) (f) (h) (i) 
______________________________________ 
18 1 360/10 8.6 38.9 10.2 -- 4.44 96 
19 2 " 8.2 84.5 11.1 -- 6.74 97 
20 7 " 9.4 17.2 9.7 -- 1.90 93 
21 1 2880/80 8.5 32.2 10.5 4.3 4.45 93 
22 1 360/10 9.0 13.3 9.8 4.5 1.44 91 
23 1 " " 29.5 10.1 4.3 3.16 90 
24 1 " " 55 9.9 4.5 6.22 95 
25 1 " 8.8 74.5 10.5 3.9 7.88 89 
26 1 " 9.0 100 11.6 4.1 10.35 
87 
27 2 " 8.4 41.2 9.9 4.3 4.46 91 
28 3* 1440/40 8.2 161 10.5 4.2 4.41 90 
29 4* 360/10 8.3 41.2 10.2 4.0 4.70 96 
30 5 320/10 8.5 35.7 10.8 4.2 4.61 94 
31 6* " 8.7 " 10.2 3.9 4.08 96 
32 7 " 8.3 " 9.8 4.0 4.03 95 
33 8* " 8.5 " 10.2 4.1 4.08 96 
______________________________________ 
note: 
*containing Cr 
(a) example no. 
(b) concentration (g/H.sub.2 Ol) 
(c) pH value before adding sodium silicate 
(d) amount of added sodium silicate (g) 
(e) pH value after adding sodium silicate 
(f) pH value after neutralizing with acid 
(g) particle coated with SiO.sub.2 
(h) SiO.sub.2 /Fe (mol %) 
(i) SiO.sub.2 coating ratio (%) 
TABLE II 
__________________________________________________________________________ 
preparation of particle 
exam- 
starting coated with SiO.sub.2 
ple particle (h) 
No. (a) (b) (c) 
(d) (e) 
(f) 
(g) 
(i) (j) 
(k) 
__________________________________________________________________________ 
34 9 712/10 
8.5 
76.2 
10.0 
-- 
-- 
2.7 4.3 
94 
35 13 " 8.3 
114.6 
10.8 
-- 
-- 
3.52 
6.6 
96 
36 15 " 9.0 
33.6 
9.5 
-- 
-- 
" 1.8 
90 
37 9 5700/80 
8.3 
632 10.2 
65 
4.0 
2.7 4.5 
93 
38 9 712/10 
8.8 
26 9.5 
55 
4.2 
" 1.4 
90 
39 9 " " 58 9.8 
50 
" " 3.1 
89 
40 9 " " 108 10.6 
" " " 6.0 
93 
41 9 " 9.0 
146 10.9 
45 
3.8 
" 7.6 
87 
42 9 " 8.9 
231 11.2 
40 
4.3 
" 11.9 
86 
43 10 " 8.5 
80 10.5 
50 
4.0 
6.4 4.3 
89 
44 11 " 8.3 
" 10.3 
" 4.1 
1.1 4.2 
88 
45 12.degree. 
" " " 10.6 
60 
4.2 
Co:3.6 
4.5 
94 
Ni:0.61 
46 13 " 8.5 
" 10.7 
50 
3.9 
3.52 
4.4 
92 
47 14 640/10 
8.9 
70 10.5 
65 
3.8 
" " 94 
48 15 " 8.5 
" 10.8 
60 
4.0 
" 4.5 
96 
49 16* 
1140/16 
8.8 
126 10.0 
50 
4.8 
3.75 
" " 
50 17*.degree. 
" 8.5 
" 9.5 
" " Co:3.62 
4.4 
94 
Ni:0.60 
__________________________________________________________________________ 
note: 
*containing Cr 
.degree.containing Ni 
(a) example No. 
(b) concentration (g/H.sub.2 Ol) 
(c) pH before adding sodium silicate 
(d) amount of added sodium silicate (g) 
(e) pH after adding sodium silicate 
(f) temperature at neutralization (.degree. C) 
(g) pH after neutralizing with acid 
(h) particle coated with SiO.sub. 2 
(i) Co/Fe (atomic %) 
(j) SiO.sub.2 /total metal (mol %) 
(k) SiO.sub.2 coating ratio (%) to the initial amount of SiO.sub.2 
TABLE III 
______________________________________ 
acicular magnetic 
exam- reducing iron particle 
ple condition acicularity 
magnetic property 
No. (a) (b) (c) (d) (e) (f) (g) 
______________________________________ 
51 18 600 3 0.60 10 165 620 
52 19 650 4.5 0.55 10 150 700 
53 20 450 5.5 0.60 8 140 515 
54 21 450 4.5 0.8 13 120 518 
55 " 500 3 " 10 138 566 
56 " 650 2 0.65 8 156 700 
57 " 700 1.5 0.5 5 165 685 
58 28* 450 4.5 0.6 17 125 512 
59 " 500 3 " 15 130 623 
60 " 650 2 " 15 150 750 
61 " 800 2 0.5 12 164 840 
62 21 400 5 0.8 12 124 505 
63 " " 6 0.65 10 126 520 
64 " " 8 0.6 " 129 520 
65 " 450 2 0.8 12 135 550 
66 " " 4 0.65 10 146 630 
67 " " 6 0.6 8 149 670 
68 " " 8 " " 152 690 
69 " 600 1 0.7 10 132 550 
70 " " 4 0.6 8 162 800 
71 " " 6 0.5 7 168 820 
72 22 " 2 0.55 8 157 653 
73 23 " " 0.7 10 154 890 
74 24 600 2 0.7 13 143 805 
75 25 " " 1.0 15 135 698 
76 26 " " " " 123 625 
77 22 " 6 0.5 6 178 512 
78 23 " " 0.6 8 175 730 
79 24 " " 0.7 13 166 1075 
80 25 " " " 15 159 1200 
81 26 " " 0.8 " 150 1195 
82 27 650 4 0.5 8 155 610 
83 28* " " 0.6 15 164 830 
84 29* " " 0.45 13 165 850 
85 30 " " 0.55 8 168 745 
86 31* " " 0.55 15 170 863 
87 32 " " 0.5 8 165 720 
88 33* " " 0.5 15 176 874 
______________________________________ 
note: 
*containing Cr 
(a) particle coated with SiO.sub.2 (example No.) 
(b) temperature (.degree. C) 
(c) time (hour) 
(d) long axis (.mu.) 
(e) long axis/short axis 
(f) saturated magnetic flux density .delta.s (emu/g) 
(g) coercive force Hc (Oe) 
TABLE IV 
______________________________________ 
acicular magnetic 
reducing 
Fe-Co alloy particle 
example condition 
acicularity 
magnetic property 
No. (a) (b) (c) (d) (e) (f) (g) 
______________________________________ 
89 34 600 4 0.7 12 140 1220 
90 35 650 6 0.6 15 155 1410 
91 36 450 8 0.6 10 130 1100 
92 37 450 " 0.8 13 108 630 
93 " 500 " " " 125 848 
94 " 600 " 0.6 11 150 1225 
95 " 650 " " " 160 1300 
96 " 700 " " 10 173 1285 
97 " 800 " 0.5 7 185 1010 
98 37 400 4 1.0 15 103 540 
99 " " 6 0.8 " 109 647 
100 " " 8 " 13 114 685 
101 " " 4 0.8 13 138 1020 
102 " " 6 " " 143 1080 
103 " " 8 0.7 " 145 1100 
104 " 650 0.5 0.8 13 125 875 
105 " " 4 0.6 10 170 1450 
106 " " 6 0.5 10 172 1490 
107 39 " " 0.8 12 175 1000 
108 40 " " " 15 138 1200 
109 41 " " 1.0 18 120 800 
110 42 " " " " 105 590 
111 39 " " 0.6 8 180 800 
112 40 " " 0.7 13 184 1190 
113 41 650 6 0.7 15 185 1480 
114 42 " " 0.8 " 144 1250 
115 43 650 4 0.4 15 165 1380 
116 44 " " 0.6 12 145 1350 
117 45.degree. 
" " 0.35 
13 173 1460 
118 46 " " 0.4 10 155 1400 
119 47 " " " 12 166 1420 
120 48 " " 0.35 10 170 1450 
121 49* 800 3 .35-0.4 
15 176 1700 
122 50*.degree. 
" " " 13 180 1500 
______________________________________ 
note: 
*containing Cr 
.degree. containing Ni 
(a) particle coated with SiO.sub.2 (example No.) 
(b) temperature (.degree. C) 
(c) time (hour) 
(d) long axis (.mu.) 
(e) long axis/short axis 
(f) saturated magnetic flux density .delta.s (em.mu./g) 
(g) coercive force Hc (Oe) 
TABLE V 
______________________________________ 
acicular magnetic iron 
compar- or Fe-Co alloy 
ison reducing particle 
example condition acicularity 
magnetic property 
No. (a) (b) (c) (d) (e) (f) (g) 
______________________________________ 
1 1 350 2 0.65 8 95 425 
2 " 400 " 0.5 " 116 350 
3 " 450 " 0.35 5 131 275 
4 " 500 " 0.3 3 154 200 
5 9 330 " 1.0 15 91 430 
6 " 400 " 0.6 8 115 375 
7 " 450 " 0.3 5 132 280 
8 " 500 " " 3 155 225 
9 " 330 " 1.0 15 85 350 
10 " " 4 0.8 13 96 500 
11 " " 6 " " 99 530 
12 " " 8 0.6 10 103 555 
13 " " 10 " " 103 580 
14 " " 12 0.5 8 107 575 
15 " " 14 " 6 110 520 
______________________________________ 
note: 
(a) starting particle (example No.) 
(b) temperature (.degree. C) 
(c) time (hour) 
(d) long axis (.mu.) 
(e) long axis/short axis 
(f) saturated magnetic flux density .delta.s (emu/g) 
(g) coercive force Hc (Oe)