Process for preparing ferromagnetic particles comprising metallic iron

A process for preparing ferromagnetic iron particles comprising metallic iron as the major component by reduction of particles of an iron oxide under heating, characterized in that the iron oxide particles are provided with a first coating layer containing at least one metal compound chosen from compounds of aluminum, zinc and alkaline earth metals at the surfaces and a second coating layer containing at least one silicon compound thereon before the reduction, whereby the coated particles are prevented from sintering and breaking upon reduction so as to give ferromagnetic particles of metallic iron having excellent magnetic characteristics.

The present invention relates to a process for preparing ferromagnetic 
particles comprising metallic iron. More particularly, it relates to a 
process for preparing ferromagnetic particles of metallic iron having 
excellent magnetic characteristics while preventing the particles from 
sintering and breaking. 
In general, ferromagnetic particles comprising metallic iron as the major 
component have better magnetic characteristics than ferromagnetic 
particles of iron oxide such as Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 
O.sub.3 and are used as recording elements for magnetic recording media 
such as magnetic recording tapes. While the ferromagnetic particles of 
metallic iron are usually prepared by reduction of needle-shaped particles 
of an iron oxide such as .alpha.-FeOOH or .alpha.-Fe.sub.2 O.sub.3 under 
heating, the heat treatment of the iron oxide particles for reduction 
tends to cause sintering between the particles, partial melting of each 
particle, formation of micropores, etc., whereby the evenness of the 
particle size, the needle-shape of the particles and the density of the 
particles become inferior so that the magnetic characteristics and the 
mechanical strength of the ferromagnetic particles are markedly 
deteriorated. 
As a result of an extensive study to overcome the said problem on the heat 
treatment of particles of an iron oxide for reduction, it has been found 
that the provision of those particles with a first coating layer of at 
least one metal compound chosen from aluminum, zinc and alkaline earth 
metals at the surfaces and a second coating layer of at least one silicon 
compound thereon before the said heat treatment can prevent them from 
sintering and breaking, whereby ferromagnetic particles of metallic iron 
of excellent magnetic characteristics are obtained. 
According to the present invention, there is provided a process for 
preparing ferromagnetic particles comprising metallic iron as the major 
component by reduction of particles of an iron oxide under heating, 
characterized in that the iron oxide particles are provided with a first 
coating layer containing at least one metal compound chosen from compounds 
of aluminum, zinc and alkaline earth metals at the surfaces and a second 
coating layer containing at least one silicon compound thereon before the 
reduction step, whereby the coated particles are prevented from sintering 
and breaking during reduction so as to give ferromagnetic particles of 
metallic iron having excellent magnetic characteristics. 
The iron oxide particles to be reduced may be particles of .alpha.-FeOOH, 
.alpha.-Fe.sub.2 O.sub.3, .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, etc. 
Among them, .alpha.-FeOOH particles, particularly containing nickel are 
favorable, because they have an even size and scarecely contain branched 
particles and, when reduced under heating as such or after being 
dehydrated under heating to .alpha.-Fe.sub.2 O.sub.3, can be effectively 
prevented from sintering and breaking so as to give ferromagnetic 
particles of metallic iron having excellent magnetic characteristics. 
For preparation of .alpha.-FeOOH particles containing nickel, nickel 
hydroxide may be added to an aqueous suspension of ferrous hydroxide and 
oxidized with gaseous oxygen in an alkaline medium, optionally followed by 
controlling the pH so as to coprecipitate ferrous hydroxide and nickel 
hydroxide. In an alternative way, a water-soluble nickel salt may be added 
to an aqueous suspension of ferrous hydroxide, optionally followed by 
controlling the pH, whereby ferrous hydroxide and nickel hydroxide are 
coprecipitated. In another alternative way, an alkali may be added to an 
aqueous solution of a water-soluble iron compound containing a 
water-soluble nickel compound so that ferrous hydroxide and nickel 
hydroxide are coprecipitated. The amount of the nickel component (Ni) in 
the .alpha.-FeOOH particles may be such that the atomic ratio of the 
nickel component and the iron component (Fe) therein is 0.001-0.15:1. 
As the metal compound, there may be used any one chosen from aluminum 
compounds such as aluminum sulfate, aluminum nitrate, aluminum chloride 
and sodium aluminate, zinc compounds such as zinc sulfate, zinc nitrate, 
zinc chloride, zinc hydroxide and zinc oxide, and alkaline earth metal 
compounds such as alkaline earth metal sulfate, alkaline earth metal 
nitrate, alkaline earth metal chloride, alkaline earth metal hydroxide and 
alkaline earth metal oxide. Examples of the alkaline earth metal are 
magnesium, calcium, etc. The amount of the metal compound may be such that 
the weight ratio of the metal component (Me) therein to the iron component 
(Fe) in the iron oxide may be from 0.0001 to 0.05. When it is less than 
the lower limit, no material effect is produced. When it is more than the 
higher limit, unfavorable influences are given on the magnetic 
characteristiscs. 
As the silicon compound, there may be used sodium orthosilicate, sodium 
metasilicate, potassum metasilicate, waterglass, silicic sol, silica, 
silicone oil, etc. The amount of the silicon compound may be such that the 
weight ratio of the silicon component (Si) therein to the iron component 
(Fe) in the iron oxide may be from 0.001 to 0.1, preferably from 0.003 to 
0.02. When it is less than the lower limit, no significant effect is 
produced. When it is more than the upper limit, the saturation 
magnetization (.sigma.s) of the ferromagnetic particles of metallic iron 
as the ultimate product tends to be lowered. 
For the formation of the first coating layer containing the metal compound 
on the surfaces of the iron oxide particles, there may be adopted various 
procedures, of which a typical example comprises dispersing the iron oxide 
particles in an aqueous solution of the metal compound so as to make the 
particles of the metal compound adsorb onto the iron oxide particles. 
Another typical procedure comprises adding an alkali to an aqueous 
dispersion of the iron oxide particles containing the metal compound to 
produce hydroxides of iron and the metal, and blowing carbon dioxide gas 
therein or adding an acid therein for neutralization, optionally followed 
by collecting the resulting particles and heating them in the air. 
The formation of the second coating layer containing the silicon compound 
may be carried out in substantially the same manner as above. 
After the formation of the first coating layer and/or the second coating 
layer, the resulting iron oxide particles may be heated at a temperature 
of 150.degree. to 600.degree. C. By such heat treatment, the metal 
ocmponent and/or the silicon compound are converted into forms not readily 
soluble into an aqueous medium, and the coating layers thereby become 
dense. 
The iron oxide particles provided with the first coating layer and the 
second coating layer are then subjected to heat treatment in a reductive 
atmosphere such as hydrogen, usually at a temperature of 300.degree. to 
600.degree. C., for reduction.

Practical and presently preferred embodiments of the invention are 
illustratively shown in the following Examples. 
EXAMPLE 1 
To a suspension of .alpha.-FeOOH particles (average long axis, 0.5.mu.; 
axis ratio, 20/1) (10 g) in water (800 ml), a mixture of 1 N NaOH aqueous 
solution (100 ml) and an aqueous solution (10 ml) containing aluminum 
sulfate (0.01 mol/liter) was added, and carbon dioxide gas was blown 
therein while stirring to make a pH of 6 to 8. The precipitated particles 
were collected, washed with water and dried to give .alpha.-FeOOH 
particles having aluminum hydroxide deposited on the surfaces. The 
particles were heated in an electric furnace at 300.degree. C. for 2 hours 
for dehydration to obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a 
first coating layer of aluminum oxide at the surfaces. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were dispersed in 
water (800 ml), 1 N NaOH aqueous solution (50 ml) and an aqueous solution 
(10 ml) containing sodium orthosilicate (1 mol/liter) were added thereto, 
and carbon dioxide gas was blown therein while stirring to make a pH of 
not more than 8, whereby silicic acid sol was deposited on the surfaces of 
the particles. The particles were collected, washed with water and dried 
to obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating 
layer of aluminum oxide and a second coating layer of silicic acid. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing aluminum and silicon. 
EXAMPLE 2 
In the same manner as in Example 1 except that the dehydration was carried 
out at 500.degree. C. and the reduction was carried out at 400.degree. C., 
the operations were effected to give ferromagnetic particles of metallic 
iron containing aluminum and silicon. 
EXAMPLE 3 
To a suspension of .alpha.-Fe.sub.2 O.sub.3 particles (average long axis, 
0.5.mu.; axis ratio, 20/1) (9 g) in water (800 ml), a mixture of 1 N NaOH 
aqueous solution (100 ml) and an aqueous solution (10 ml) containing 
aluminum sulfate (0.01 mol/liter) was added, and carbon dioxide gas was 
blown therein while stirring to make a pH of 6 to 8. The precipitated 
particles were collected, washed with water and dried and heated in an 
electric furnace at 250.degree. C. for 2 hours to obtain particles of 
.alpha.-Fe.sub.2 O.sub.3 having a first coating layer of hydrated aluminum 
oxide at the surfaces. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were dispersed in 
water (800 ml), 1 N NaOH aqueous solution (50 ml) and an aqueous solution 
(10 ml) containing sodium orthosilicate (1 mol/liter) were added thereto, 
and carbon dioxide gas was blown therein while stirring to make a pH of 
not more than 8, whereby silicic acid sol was deposited on the surfaces of 
the particles. The particles were collected, washed with water and dried 
to obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating 
layer of hydrated aluminum oxide and a second coating layer of silicic 
acid. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing aluminum and silicon. 
EXAMPLE 4 
.alpha.-Fe.sub.2 O.sub.3 particles having a first coating layer of aluminum 
oxide obtained in Example 1 (9 g) were dispersed in a solution of silicone 
oil (dimethylpolysiloxane; "KF-96" manufactured by Shinetsu Kagaku Kogyo 
K.K.; 100 c.s.) (0.4 g) in methylethylketone (800 ml). The dispersion was 
filtered, and the collected particles were dried. The dried particles were 
reduced by heating in an electric furnace at 500.degree. C. in a stream of 
hydrogen at a rate of 1 liter/minute for 2 hours to give ferromagnetic 
particles of metallic iron containing aluminum and silicon. 
EXAMPLE 5 
To a suspension of .alpha.-FeOOH particles (average long axis, 0.5.mu.; 
axis ratio, 20/1) (10 g) in water (800 ml), a mixture of 1 N NaOH aqueous 
solution (100 ml) and an aqueous solution (10 ml) containing zinc sulfate 
(1 mol/liter) was added, and carbon dioxide gas was blown therein while 
stirring to make a pH of 7 to 8. The precipitated particles were 
collected, washed with water and dried to give .alpha.-FeOOH particles 
having zinc hydroxide deposited on the surfaces. The particles were heated 
in the air at 300.degree. C. for 2 hours for dehydration to obtain 
particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating layer of zinc 
oxide at the surfaces. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were dispersed in 
water (800 ml), 1 N NaOH aqueous solution (50 ml) and an aqueous solution 
(20 ml) containing Na.sub.4 SiO.sub.4 (1 mol/liter) were added thereto, 
and carbon dioxide gas was blown therein while stirring to make a pH of 7 
to 8, whereby silicic acid sol was deposited on the surfaces of the 
particles. The particles were collected, washed with water and dried to 
obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating layer 
of zinc oxide and a second coating layer of silica. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing zinc and silicon. 
EXAMPLE 6 
To a suspension of .alpha.-FeOOH particles (average long axis, 0.5.mu.; 
axis ratio, 20/1) (10 g) in water (800 ml), an aqueous solution (10 ml) 
containing magnesium sulfate (0.01 mol/liter) was added, and 1 N NaOH 
aqueous solution (50 ml) was added thereto while stirring. The 
precipitated particles were collected, washed with water and dried to give 
.alpha.-FeOOH particles having magnesium hydroxide deposited on the 
surfaces. The particles were heated in an electric furnace at 300.degree. 
C. for 2 hours for dehydration to obtain particles of .alpha.-Fe.sub.2 
O.sub.3 having a first coating layer of magnesium oxide at the surfaces. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were dispersed in 
water (800 ml), 1 N NaOH aqueous solution (50 ml) and an aqueous solution 
(10 ml) containing sodium orthosilicate (1 mol/liter) were added thereto, 
and carbon dioxide gas was blown therein while stirring to make a pH of 
not more than 8, whereby silicic acid sol was deposited on the surfaces of 
the particles. The particles were collected, washed with water and dried 
to obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating 
layer of magnesium oxide and a second coating layer of silicic acid. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing magnesium and silicon. 
EXAMPLE 7 
In the same manner as in Example 6 except that an aqueous solution (10 ml) 
containing calcium sulfate (0.01 mol/liter) was used in place of an 
aqueous solution (10 ml) containing magnesium sulfate (0.01 mol/liter), 
the operations were effected to give ferromagnetic particles of metallic 
iron containing calcium and silicon. 
EXAMPLE 8 
To a suspension of .alpha.-FeOOH particles (average long axis, 0.5.mu.; 
axis ratio, 20/1) (10 g) in water (800 ml), an aqueous solution (4 ml) 
containing magnesium sulfate (0.01 mol/liter) and an aqueous solution (10 
ml) containing Na.sub.4 SiO.sub.4 (1 mol/liter) were added, and 1 N NaOH 
aqueous solution (50 ml) was added thereto while stirring, whereby 
.alpha.-FeOOH particles having magnesium hydroxide deposited on the 
surfaces were produced. Then, carbon dioxide gas was blown therein while 
stirring to make a pH of 6 to 8, whereby silicic acid sol was deposited on 
the surfaces of the particles. The particles were collected, washed with 
water and dried, followed by heating in the air at 300.degree. C. for 2 
hours for dehydration to obtain particles of .alpha.-Fe.sub.2 O.sub.3 
having a first coating layer of magnesium oxide and a second coating layer 
of silica. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing magnesium and silicon. 
EXAMPLE 9 
To a suspension of .alpha.-Fe.sub.2 O.sub.3 particles (average long axis, 
0.5.mu.; axis ratio, 20/1) (9 g) in water (800 ml), an aqueous solution (3 
ml) containing calcium nitrate (0.01 mol/liter) and an aqueous solution 
(10 ml) containing Na.sub.4 SiO.sub.4 (1 mol/liter) were added, and 1 N 
NaOH aqueous solution (50 ml) was added thereto while stirring, whereby 
.alpha.-Fe.sub.2 O.sub.3 particles having calcium hydroxide deposited on 
the surfaces were produced. Then, carbon dioxide gas was blown therein 
while stirring to make a pH of 6 to 8, whereby silicic acid sol was 
deposited on the surfaces of the particles. The particles were collected, 
washed with water and dried to obtain particles of .alpha.-Fe.sub.2 
O.sub.3 having a first coating layer of calcium hydroxide and a second 
coating layer of silicic acid. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace at 500.degree. C. in a stream of hydrogen 
at a rate of 1 liter/minute for 2 hours to give ferromagnetic particles of 
metallic iron containing calcium and silicon. 
EXAMPLE 10 
To an aqueous solution (1.5 liters) containing FeSO.sub.4.7H.sub.2 O (200 
g/liter), a solution (0.1 liter) containing NiSO.sub.4.6H.sub.2 O (114 
g/liter) and an aqueous solution (1.5 liters) containing NaOH (200 
g/liter) were added to make a suspension containing the co-precipitate of 
Fe(OH).sub.2 and Ni(OH).sub.2, of which the pH was more than 12. The 
suspension was warmed to 40.degree. C., and air was introduced therein at 
a rate of 1.6 liters/minute for 10 hours, whereby particles of 
.alpha.-FeOOH containing nickel in a needle-shape were separated out. The 
.alpha.-FeOOH particles were collected, washed with water and dried. 
Ten grams of the .alpha.-FeOOH particles were dispersed in water (0.8 
liter), 1 N NaOH aqueous solution (100 ml) and an aqueous solution (5 ml) 
containing ZnSO.sub.4 (1 mol/liter) were added thereto while stirring, and 
carbon dioxide gas was blown into the resultant mixture to make a pH of 7 
to 8, whereby particles of .alpha.-FeOOH having zinc hydroxide deposited 
thereon were precipitated. The precipitated particles were collected, 
washed with water and dried, followed by heating at 300.degree. C. in the 
air for 2 hours for dehydration. 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were dispersed in 
water (800 ml), 1 N NaOH aqueous solution (100 ml) and an aqueous solution 
(20 ml) containing Na.sub.4 SiO.sub.4 (1 mol/liter) were added thereto, 
and carbon dioxide gas was blown therein while stirring to make a pH of 7 
to 8, whereby silicic acid sol was deposited on the surfaces of the 
particles. The particles were collected, washed with water and dried to 
obtain particles of .alpha.-Fe.sub.2 O.sub.3 having a first coating layer 
of zinc oxide (Zn/Fe=4.9% by weight) and a second coating layer of silica 
(Si/Fe=2% by weight). 
The above obtained .alpha.-Fe.sub.2 O.sub.3 particles were reduced by 
heating in an electric furnace in a stream of hydrogen at a rate of 1 
liter/minute under the conditions as specified in Table 1 to give 
ferromagnetic particles of metallic iron containing nickel, zinc and 
silicon. 
TABLE 1 
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Reduction conditions 
No. Temperature (.degree.C.) 
Time (hour) 
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a 350 4 
b 400 3 
c 450 2 
d 500 1 
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COMATIVE EXAMPLE 1 
To a suspenion of .alpha.-FeOOH particles (average long axis, 0.5.mu.; axis 
ratio, 20/1) (10 g) in water (800 ml), 1 N NaOH aqueous solution (100 ml), 
an aqueous solution (10 ml) containing aluminum sulfate (0.01 mol/liter) 
and an aqueous solution (10 ml) containing sodium orthosilicate (1 
mol/liter) were added, and carbon dioxide gas was blown therein while 
stirring to make a pH of not more than 8. The precipitated particles were 
collected, washed with water and dried to obtain partilces of 
.alpha.-FeOOH having a coating layer of aluminum hydroxide and silicic 
acid at the surfaces. The particles were heated under the same conditions 
as in Example 1 for dehydration and then heated under the same conditions 
as in Example 1 for reduction to give ferromagnetic particles of metallic 
iron containing aluminum and silicon. 
COMATIVE EXAMPLE 2 
In the same manner as in Comparative Example 1 except that an aqueous 
solution (5 ml) containing zinc sulfate (1 mol/liter) was used in place of 
an aqueous solution (10 ml) containing aluminum sulfate (0.01 mol/liter), 
the operations were effected to give ferromagnetic particles of metallic 
iron containing zinc and silicon. 
COMATIVE EXAMPLE 3 
In the same manner as in Example 1 except that the treatment for 
application of the silicon compound was carried out before the dehydration 
under heating and the treatment for application of the aluminum compound 
was carried out after such dehydration, the operations were effected to 
give ferromagnetic particles of metallic iron containing aluminum and 
silicon. 
COMATIVE EXAMPLE 4 
In the same manner as in Example 5 except that the treatment for 
application of the silicon compound was carried out before the dehydration 
under heating and the treatment for application of the zinc compound was 
carried out after such dehydration, the operations were effected to give 
ferromagnetic particles of metallic iron containing zinc and silicon. 
COMATIVE EXAMPLE 5 
In the same manner as in Example 6 except that the treatment for 
application of the silicon compound was carried out before the dehydration 
under heating and the treatment for application of the magnesium compound 
was carried out after such dehydration, the operations were effected to 
give ferromagnetic particles of metallic iron containing magnesium and 
silicon. 
COMATIVE EXAMPLE 6 
In the same manner as in Example 7 except that the treatment for 
application of the silicon compound was carried out before the dehydration 
under heating and the treatment for application of the calcium compound 
was carried out after such dehydration, the operations were effected to 
give ferromagnetic particles of metallic iron containing calcium and 
silicon. 
COMATIVE EXAMPLE 7 
In the same manner as in Example 8 except that the aqueous solution of 
Na.sub.4 SiO.sub.4 was not used and the blowing of carbon dioxode gas was 
not effected, the operations were effected to give ferromagnetic particles 
of metallic iron containing magnesium. 
COMATIVE EXAMPLE 8 
In the same manner as in Example 9 except that the aqueous solution of 
Na.sub.4 SiO.sub.4 was not used and the blowing of carbon dioxide gas was 
not effected, the operations were effected to give ferromagnetic particles 
of metallic iron containing calcium. 
COMATIVE EXAMPLE 9 
In the same manner as in Example 1 except that the aqueous solution of 
aluminum sulfate was not used, the operations were effected to give 
ferromagnetic particles of metallic iron containing silicon. 
COMATIVE EXAMPLE 10 
In the same manner as in Comparative Example 1 except that the aqueous 
solution of aluminum sulfate was not used, the operations were effected to 
give ferromagnetic particles of metallic iron containing silicon. 
The ferromagnetic particles of metallic iron as prepared in the foregoing 
Examples and Comparative Examples were subjected to measurement of 
coercive force (Hc), saturation magnetization (.sigma.s), square ratio 
(.sigma.r/.sigma.s), average long axis and axis ratio. The results are 
shown in Table 2. 
TABLE 2 
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Coercive 
Saturation Average 
force magnetization 
Square long axis 
Axis 
(Oe) (emu/g) ratio (.mu.m) 
ratio 
______________________________________ 
Example 
1 1320 148 0.52 0.3 15 
2 1380 158 0.52 0.3 16 
3 1330 162 0.51 0.3 16 
4 1300 165 0.52 0.3 15 
5 1320 162 0.51 0.3 15 
6 1280 165 0.51 0.3 13 
7 1290 163 0.52 0.3 14 
8 1280 165 0.50 0.3 10 
9 1290 166 0.50 0.4 10 
10 (a) 1285 120 0.48 0.3 13 
10 (b) 1390 137 0.48 0.3 13 
10 (c) 1450 147 0.50 0.3 13 
10 (d) 1410 160 0.50 0.3 13 
Comparative 
Example 
1 1200 154 0.51 0.3 10 
2 1190 168 0.51 0.3 10 
3 580 158 0.22 0.4 3 
4 630 162 0.23 0.3 4 
5 500 163 0.24 0.4 3 
6 570 165 0.19 0.3 3 
7 500 175 0.35 0.3 3 
8 550 177 0.32 0.4 3 
9 1100 165 0.46 0.4 8 
10 1180 155 0.50 0.3 10 
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As understood from the above results, the process of this invention can 
efficiently prevent the sintering and breaking of the particles on the 
heat treatment for reduction. As a result, the produced ferromagnetic 
particles of metallic iron exhibit excellent magnetic characteristics.