Process for producing iron oxide

In a process for producing an iron oxide having magnetite as a main component by heating to reduce a powder of an oxide or a hydrated iron oxide comprising an iron oxide as a main component in an atmosphere for reduction, an improvement characterized in that said atmosphere for reduction is formed by passing an industrial liquefied nitrogen gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing an iron oxide used 
for a magnetic recording medium. More particularly, it relates to a 
process for producing a magnetite powder which imparts superior magnetic 
characteristics by reducing an iron oxide powder without sintering under 
high chemical stability even at high temperature. 
2. Description of the Prior Arts 
Magnetite powder has been used for a magnetic recording medium. Usually, 
the magnetite powder has been produced by reducing acicular goethite 
powder with hydrogen gas. 
When the magnetite powder is used for the magnetic recording medium, 
excellent magnetic characteristics are required especially higher coercive 
force and intensity of magnetization are required. 
Heretofore, in order to produce magnetite (Fe.sub.3 O.sub.4) by heating an 
iron oxide (Fe.sub.2 O.sub.3) in an inert gas, a high temperature such as 
higher than 1000.degree. C. has been required. When an iron oxide powder 
is used as the starting material, a sintering has been caused and a 
magnetite having desired magnetic characteristics could not be obtained. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for producing 
an iron oxide comprising magnetite as a main component which has excellent 
magnetic characteristics. 
It is another object of the present invention to provide a process for 
producing an iron oxide comprising magnetite as a main component which has 
excellent magnetic characteristics without any sintering by heating at 
relatively lower temperature. 
The foregoing and other objects of the present invention have been attained 
by producing an iron oxide having magnetite as a main component which is 
used for a magnetic recording medium by heating to reduce a powder of an 
oxide or a hydrated oxide comprising an iron oxide as a main component in 
an atmosphere passing an industrial liquefied nitrogen gas separated from 
air. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The inventors have studied and have found that the magnetite powder can be 
obtained by heating it only at about 600.degree. C. in an atmosphere 
passing an industrial liquefied nitrogen gas and moreover, the magnetite 
had superior coercive force (Hc) and intensity of magnetization (sigma S) 
in comparison with those of the product obtained by reducing with hydrogen 
gas. 
The starting materials in the process of the present invention include 
hydrated iron oxides such as goethite and lepidocrosite; .alpha.-iron 
oxides obtained by dehydrating the hydrated iron oxide; and hydrated iron 
oxides and .gamma.-iron oxides which incorporate a small amount of a Si or 
Al component as a sintering proofing agent, or a small amount of a Zn or 
Ni component for controlling a size or shape of the particle; or a small 
amount of a Co or Mn component for improving the coercive force of the 
product. 
The particles of the magnetite obtained by a reduction of the hydrated iron 
oxide is usually has a length of 0.1 to 2.mu. preferably 0.2 to 1.mu. and 
an acicular ratio of 2 to 40 preferably 5 to 20. 
The aqueous solution of ferrous ion can be produced by dissolving a ferrous 
compound such as ferrous chloride, ferrous sulfate, ferrous nitrate etc. 
in water. A concentration of the ferrous compound is from a saturated 
concentration to 0.5 wt.% preferably 5 to 40 wt.% especially 10 to 30 
wt.%. 
The base is preferably sodium hydroxide, carbonate or bicarbonate or 
potassium hydroxide, carbonate or bicarbonate or ammonium hydroxide. 
The concentration of the base is usually 1 to 40 wt.% preferably 5 to 30 
wt.%. 
The oxidizing agent can be alkali chlorates, air, oxygen, ozone and alkali 
nitrates. The oxidizing agent is added at a ratio of more than a 
stoichiometric amount for converting a ferrous compound into a ferric 
compound. The oxidizing agent can be added before, during or after the 
mixing of the aqueous solution of ferrous ion with the base, since the 
oxidation is performed after forming ferrous hydroxide. That is, the 
oxidizing agent can be mixed with the base or a slurry of ferrous 
hydroxide. The temperature for the oxidation is usually in a range of 
0.degree. to 80.degree. C. preferably 5.degree. to 60.degree. C. 
especially 20.degree. to 50.degree. C. 
The conventional air bubbling oxidation method can be also employed. 
The preparation of a hydrated iron oxide can be modified as desired. 
In accordance with the process of the present invention, the reducing 
reaction is satisfactorily performed even at a relatively lower 
temperature of about 600.degree. C. to obtain the magnetite powder having 
excellent magnetic characteristics. 
It is important to pass an industrial liquefied nitrogen gas into the 
atmosphere whereby a small amount of impurities in the industrial 
liquefied nitrogen gas can be used for the reduction. 
The industrial liquefied nitrogen gas is obtained by compressing and 
cooling air to form a liquid air and distilling nitrogen and special 
impurities from the liquid air. The special impurities include methane 
(CH.sub.4) and carbon monoxide (CO) which impart reducing property at high 
temperature. The industrial liquefied nitrogen gas comprises a small 
amount (ppm) of such special impurities for reduction as shown in the 
following table. The special impurities for reduction impart the important 
function for the process of the present invention. It has been confirmed 
that free oxygen is formed in the reducing reaction and is finally 
discharged as oxygen. It is preferable to use the industrial liquefied 
nitrogen gas containing H.sub.2, CH.sub.4 and CO at each concentration of 
2 to 100 ppm preferably 5 to 50 ppm respectively. 
TABLE 
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Industrial liquefied 
Pure nitrogen gas 
nitrogen gas 
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Purity higher than 99.9995% 
higher than 99.999% 
O.sub.2 less than 0.5 ppm 
less than 10 ppm 
CO.sub.2 less than 1 ppm 
more than 2 ppm 
H.sub.2 " " 
CH.sub.4 " " 
CO " " 
Nitrogen oxides 
less than 0.1 ppm 
" 
Moisture D.P. lower than 
D.P. lower than 
-70.degree. C. -70.degree. C. 
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In accordance with the process of the present invention, a reaction 
temperature in the process for reducing an iron oxide, can be set at 
relatively lower temperature of about 600.degree. C. whereby a sintering 
is not caused. Moreover, it produce a magnetite powder having higher 
coercive force and higher intensity of magnetization in comparison with 
those of the conventional process using hydrogen gas. 
Therefore, magnetic recording media obtained by using the magnetite powder 
of the present invention or the .gamma.-iron oxide powder obtained by 
oxidizing the magnetite had excellent electromagnetic convertible 
characteristics. 
The industrial liquefied nitrogen gas is used instead of hydrogen gas, and 
accordingly, the cost for the production is remarkably lowered and the 
industrial advantages are remarkable.

The present invention will be further illustrated by certain examples and 
references which are provided for purposes of illustration only and are 
not intended to be limiting the present invention. 
EXAMPLE 1 
In a quartz boat, 10 g. of acicular hydrated iron oxide containing 1.2 wt.% 
of SiO.sub.2 component which had a specific surface area of 73.9 m.sup.2 
/g. (measured by BET method) and an average length of 0.35.mu. was charged 
and the port was set in a reducing furnace. An industrial liquefied 
nitrogen gas comprising impurities for reduction (CH.sub.4, CO etc.) was 
fed at a rate of 0.5 liter per minutes into a reducing furnace which was 
heated at about 600.degree. C. for 1 hour and then the furnace was cooled 
to obtain a magnetite powder. The magnetic characteristics of the 
resulting magnetite are shown as Sample A-1 in Table 1. 
REFERENCE 1 
In accordance with the process of Example 1 except using hydrogen gas 
instead of nitrogen gas and reducing at 400.degree. C., the test was 
carried out. The magnetic characteristics of the resulting magnetite are 
shown as Sample C-1 in Table 1. 
As it is clearly found, from the data of Samples A-1 and C-1, Sample A-1 
(Example 1) had superior coercive force and intensity of magnetization to 
those of Sample C-1 (Reference 1). 
EXAMPLE 2 
In accordance with the process of Example 1 except using acicular hydrated 
iron oxide containing 0.73 wt.% of SiO.sub.2 component and containing 1.3 
wt.% of Zn component (based on Fe) which had a specific surface area of 
56.0 m.sup.2 /g. (measured by BET method) and an average length of 
0.4.mu., instead of the acicular hydrated iron oxide the test was carried 
out. The magnetic characteristics of the resulting magnetite are shown as 
Sample A-2 in Table 1. 
REFERENCE 2 
In accordance with the process of Example 2 except using hydrogen gas 
instead of nitrogen gas and reducing at 400.degree. C., the test was 
carried out. The magnetic characteristics of the resulting magnetite are 
shown as Sample C-2 in Table 1. 
As it is clearly found, from the data of Samples A-2 and C-2, Sample A-2 
(Example 2) had superior coercive force and intensity of magnetization to 
those of Sample C-2 (Reference 2). 
In Examples, the reaction temperature was 600.degree. C. whereas in 
References, the reaction temperature was 400.degree. C. because when 
nitrogen gas was used, it was not reduced to form the magnetite at 
400.degree. C. whereas when hydrogen gas was used, metallic iron was 
formed at 600.degree. C. and the product could not be compared with the 
magnetite. 
TABLE 1 
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Hc Sigma S Sigma R 
Square 
Sample (Oe) (emu/g) (emu/g) 
ratio 
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A-1 423 82.4 37.5 0.455 
C-1 391 80.5 36.3 0.451 
A-2 442 83.6 40.5 0.484 
C-2 426 81.2 39.0 0.480 
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