Method of preparing gamma ferric hydroxyoxide powder

A method of preparing gamma ferric hydroxyoxide in which ferrous ions are reacted in an alkaline solution to produce a suspension of ferric hydroxide, the resulting suspension is treated with an oxidizing gas in a first oxidizing step at a relatively high oxidation rate, sufficient to cause nucleation of gamma ferric hydroxyoxide to occur, and then the oxidation rate is reduced in a second oxidizing step to cause crystal growth to occur on the nuclei formed in the first oxidizing step. The resulting product has magnetic and physical characteristics making it extremely suitable for use in the manufacture of magnetic recording media.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing gamma ferric 
hydroxyoxide by means of at least two oxidation steps differing in 
intensity, to produce a raw material for magnetic powders used to make 
magnetic recording media. 
2. Description of the Prior Art 
When a magnetic powder such as gamma ferric oxide is prepared, there can be 
numerous starting materials including acicular alpha ferric hydroxyoxide 
(geothite) or acicular gamma ferric hydroxyoxide (lepidocrocite). These 
materials are dehydrated, reduced and then oxidized to form the desired 
gamma ferric oxide. In this particular case, the gamma form has an 
advantage compared with the alpha form in that it has superior dispersion 
characteristics because it produces crystals with no branches and it can 
produce a magnetic powder having a high residual magnetic flux density. 
However, the use of gamma ferric hydroxyoxide as a starting material 
requires severe conditions of formation as compared with the alpha form so 
that a method of manufacturing magnetic powders utilizing the gamma form 
as the starting material is not widely employed. 
The lepidocrocite is produced by generating ferrous hydroxide by the 
reaction of an alkali metal hydroxide such as sodium hydroxide with a 
ferrous salt such as ferrous chloride in aqueous alkaline solution by 
passing an oxidizing gas into the aqueous alkaline solution. It is known 
that the gamma ferric hydroxyoxide is produced through an intermediate 
product which is generated by the process known as green rust. 
Upon the production of seed crystals, the usual technique would be to grow 
crystals from such seeds to a desired size. Consequently, it is preferable 
that the seed crystal be as small as possible so that the ultimate crystal 
size can be small. However, when lepidocrocite is formed, it is not 
usually possible to provide a suitably small sized seed crystal if high 
speed oxidation is carried out, as the crystal under these conditions 
becomes vein-like in shape and appearance and is not entirely suitable for 
transformation into ferric oxide usable in magnetic recording media. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method for preparing gamma 
lepidocrocite suitable for making a magnetic powder having improved 
magnetic characteristics. The improved material produced according to the 
present invention has an improved crystalline shape. 
In general, the method of the present invention makes use of a suspension 
of ferrous hydroxide obtained by the reaction of ferrous ions and alkali 
in an aqueous solution. This suspension is subjected to relatively high 
rate oxidation by passing an oxygen containing gas into the solution. The 
oxidation is carried out at a speed sufficient to cause nuclei of gamma 
ferric hydroxyoxide to be produced. The end point of the first part of the 
reaction can be determined by the ratio of ferric ions to total amount of 
iron in solution, and when this reaches a value in the range from 0.3 to 
0.48, preferably 0.35 to 0.47, the initial oxidation at a relatively rapid 
rate is terminated and a slower oxidation is commenced, at an oxidizing 
rate of 1/5 to 1/50 of the original oxidation rate. In the second stage, 
at which the oxidation rate is insufficient to increase the amount of new 
nuclei formed significantly, the previously formed nuclei are grown into 
crystals of a desirable shape very suitable for transformation into gamma 
ferric oxide for magnetic recording purposes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As mentioned previously, upon synthesizing lepidocrocite, it is well known 
that this material is generated through an intermediate product which is 
known as green rust and is produced in the oxidizing process by passing an 
oxidizing gas into a solution containing the hydroxide. The present 
inventors discovered that when the above-mentioned hydroxide was oxidized, 
with high speed oxidizing gas injection, at a certain oxidizing state, the 
crystals of green rust coexisted with the fine or microcrystals of 
lepidocrocite, and the microcrystals of lepidocrocite consisted of 
particles of relatively uniform particle size, with a finely defined 
rectangular shape. It was discovered, however, that if the high rate 
oxidation is continued beyond this point, the shape of the lepidocrocite 
was significantly changed to be vein-like. 
The present invention is based upon a method by which lepidocrocite of 
improved shape can be obtained, and accordingly magnetic powders having 
superior magnetic characteristics, particularly residual flux density, and 
squareness ratio, can be manufactured by using the improved lepidocrocite 
as a starting material. 
In accordance with the present invention, the oxidizing process is 
conducted in at least two stages, in the first of which an 
oxygen-containing gas is used under relatively high oxidation rates, and 
in the second stage, a relatively low rate oxidizing process is carried 
out. This high rate and low rate oxidation processes can be adjusted by 
changing the amount of oxygen in the oxygen-containing gas, changing the 
agitating speed, or the like. In accordance with the present invention, 
the high rate oxidation has a reaction velocity sufficient to generate 
nuclei of lepidocrocite while the low speed oxidation does not generate 
any significant amounts of new nuclei, but provides conditions more 
conducive to proper crystal growth on the existing nuclei. 
In the first stage of oxidation, a condition is achieved such that the fine 
crystals of green rust coexist with the fine crystals of lepidocrocite. 
The progress of the reaction is monitored by determining the amount of 
ferric ion present as the reaction progresses, and the oxidation process 
is switched from the high rate to the low rate process when the ratio of 
ferric ions to total iron dissolved in the solution reaches a 
predetermined value. The high speed oxidation is maintained until the 
ratio of hydroxyl ions to ferrous ions is smaller than 1 and the ratio of 
ferric ions to total iron is in the range of 0.3 to 0.48, preferably 0.35 
to 0.47. When this condition is achieved, the second stage of low rate 
oxidation begins, using an oxidation rate corresponding to about 1/5 to 
1/50 of the high rate of the first stage. In the second stage, the fine 
crystals of green rust and the fine nuclei of gamma lepidocrocite are 
grown into larger crystals of the desired product. These crystals exhibit 
a central rectangular shape and do not evidence any significant vein 
shape. 
When seed crystals of the gamma lepidocrocite thus prepared are treated 
with green rust provided from a separate source and containing no nuclei 
of gamma lepidocrocite, and the mixture is subjected to a third stage of 
low rate oxidizing process under the same conditions as in the second 
stage, crystals of the desired gamma ferric hydroxyoxide can be readily 
grown under easily controlled conditions. 
Turning to the drawings, in FIG. 1, a solid line curve 1 represents the 
programming of gas injection oxidation according to the method of the 
invention. From the starting time, t.sub.0, the high rate oxidation is 
carried out until a time t.sub.1 when the suspension has a ratio of ferric 
ions to total iron represented by the point P. At this stage, the low rate 
oxidation is initiated as indicated by the change in slope of the curve. 
As mentioned, the point P occurs when the ratio is in the range from 0.3 
to 0.48 and preferably from 0.35 to 0.47. The desired seed crystals are 
obtained at the second stage of low rate oxidation or green rust which has 
been prepared separately is added to the solution at a time t.sub.2 when 
the ratio of ferric ions to iron is at a value Q. Then a third stage 
oxidation commences, at a rate substantially that used between the time 
intervals t.sub.1 and t.sub.2. 
The broken line curve 1' in FIG. 1 represents the progress of the reaction 
in the normal prior art method where a relatively slow rate second 
oxidation step is not employed. 
The following specific examples of the invention have been added for 
purposes of clarity. 
EXAMPLE 1 
A homogeneous mixer with a volume of 10 l was charged with 973.7 g of 
FeCl.sub.2.4H.sub.2 O, 196 g of NaOH and 7 l of deionized water. The 
mixture was stirred such that the rotational velocity of the blade of the 
mixer was about 10,000 rpm. Thereafter, a mixture of about 50% oxygen and 
50% nitrogen was fed into the mixer at a rate of about 25 l/min. to carry 
out a high rate oxidation in 5 minutes and 25 seconds at an initial 
temperature of 30.degree. C. This gas mixture was changed to 100% nitrogen 
to stop the oxidation. At this time, the ratio of ferric ions to iron in 
solution was 0.47. The mixture or suspension was then transferred to a 
circular type ventilating agitator and a mixed gas containing 10% oxygen 
and 90% nitrogen was supplied thereto at the rate of 15 l/min. These low 
rate oxidation conditions were continued at a temperature of 30.degree. C. 
while the propeller of the agitator was rotated at a rotational speed of 
1050 rpm. The precipitate thus obtained was substantially only gamma 
ferric hydroxyoxide, and this was confirmed by an X-ray diffraction test. 
FIG. 2 is a reproduction of a photomicrograph of the gamma ferric 
hydroxyoxide at a magnification of 30,000 times from which it is apparent 
that the crystals obtained are uniform in a somewhat rectangular shape, 
and very pleasing in appearance. 
EXAMPLE 2 
To a homogeneous mixer having a volume of 10 l there was added 973.7 g of 
FeCl.sub.2.4H.sub.2 O, 196 g of NaOH and 7 l of water. The mixture was 
stirred under conditions such that the rotational speed of the blade of 
the mixer was 10,000 rpm. Thereafter, a mixed gas containing 50% oxygen 
and 50% nitrogen was fed at a rate of 25 l/min. to carry out a high rate 
oxidation at 30.degree. C. After 3 minutes, the gas was changed to a gas 
consisting of 100% nitrogen to stop the oxidation. At this time, the value 
of ferric ion to iron in the solution was 0.336. The mixture or suspension 
was transferred to a circular type ventilating agitator and a mixed gas 
containing 10% oxygen and 90% nitrogen was supplied thereto at a rate of 
15 l/min. The low rate oxidation was carried out at a temperature of 
30.degree. C. while the propeller of the agitator was rotated at a speed 
of 1050 rpm. 
FIG. 3 is a reproduction of a photomicrograph of the particles thus 
produced, with a magnification of 30,000 times. 
EXAMPLE 3 
The suspension of gamma ferric hydroxyoxide produced by Example 1 was put 
in an atmosphere of 100% nitrogen gas and removed from the treating 
vessel. Thereafter, 2 l of a solution containing green rust which had been 
made separately was poured therein and the mixture was stirred. The green 
rust containing solution was prepared by lightly oxidizing the suspension 
obtained in Example 1. Oxidation conditions consisted of passing a mixed 
gas containing 50% oxygen and 50% nitrogen at a rate of 25 l/min. to the 
solution at 30.degree. C. for 1 minute after which the gas was changed to 
100% nitrogen. Thus, a solution containing green rust but being free of 
gamma ferric hydroxyoxide crystals was obtained. The ratio of ferric ions 
to iron in the mixture was 35.5%. Thereafter, a mixed gas containing 10% 
oxygen and 90% nitrogen was supplied to the suspension at the rate of 15 
l/min. and a reaction was carried out for 2 hours at a temperature of 
30.degree. C. while the propeller was rotated at a speed of 1050 rpm. The 
crystal obtained in this way was similar to that produced by Example 1, 
but with a much larger crystal size. FIG. 4 is a reproduction of a 
photomicrograph of this crystal at a magnification of 30,000 times. 
The materials produced according to the present invention were compared 
with those produced by other types of processes as explained in the 
following examples. 
COMATIVE EXAMPLE 1 
A mixer was charged with 79.52 g of FeCl.sub.2.4H.sub.2 O and 16.0 g of 
NaOH dissolved in 800 ml of deionized water. This mixture was subjected to 
an oxidizing reaction by supplying air at the rate of 7 l/min. to the 
mixer at 25.degree. C. The precipitate produced at this time was crystals 
of gamma ferric hydroxyoxide having a veined appearance. The nature of the 
crystal was determined by an X-ray diffraction test. FIG. 5 is a 
reproduction of a photomicrograph of this crystal at a magnification of 
30,000 times. 
COMATIVE EXAMPLE 2 
A suspension was made up from 311.84 g of green rust prepared separately in 
500 ml of a dispersion liquid containing 0.18053 g/20 ml of gamma ferric 
hydroxyoxide crystals made by Comparative Example 1. The mixture or 
suspension was agitated under nitrogen gas and thereafter a mixed gas 
containing 2.5% oxygen and 97.5% nitrogen was supplied at a rate of 5 
l/min. at 30.degree. C. in a separable flask. The rotational speed of the 
propeller was 352 rpm. The precipitate thus obtained was 100% gamma ferric 
hydroxyoxide. FIG. 6 is a reproduction of a photomicrograph of the crystal 
particles obtained at a magnification of 30,000 times. 
The gamma ferric hydroxyoxide of Examples 1, 2 and 3, as well as 
Comparative Examples 1 and 2, were subjected to oxidation to form gamma 
Fe.sub.2 O.sub.3 magnetic powders and were given the specimen numbers 1 to 
5, respectively. 
The magnetic characteristics consisting of the coercive force Hc, the 
magnetizing amount .sigma..sub.S and the squareness ratio R.sub.S of the 
above magnetic powders were determined and set forth in Table 1. 
TABLE 1 
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Specimen No. 
Hc (Oe) .sigma..sub.S (emu/g) 
R.sub.S 
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1 330 70.1 0.42 
2 362 70.5 0.51 
3 392 73.5 0.48 
4 144 57.4 0.344 
5 240 62.3 0.46 
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As evident from the above Table, the magnetic powders provided by the 
examples of the present invention using two oxidation stages are much 
improved in magnetic characteristics as compared with those prepared by 
only high-rate gas injection oxidation as in the Comparative Examples. 
Furthermore, the magnetic powder made by using the improved lepidocrocite 
of the present invention as its raw material was used to form a magnetic 
layer for a magnetic record medium. The procedure is explained in the 
following Example. 
EXAMPLE 4 
A magnetic paint having the following composition was prepared: 
400 parts by weight of gamma Fe.sub.2 O.sub.3 (specimen 3) were combined 
with 100 parts by weight of a 
polyvinylchloride-polyvinylacetate-polyvinylalcohol copolymer (VAGH) 
together with 12 parts of a lecithin dispersion agent, and 1250 parts by 
weight of a solvent consisting of methylethylketone and cyclohexanone in a 
ratio of 1:1. 
The magnetic paint produced was coated on a film made of polyethylene 
terephthalate to produce a magnetic record medium. The coercive force, 
residual magnetic flux density, and squareness ratio of the magnetic 
record medium were measured and the results are shown in Table 2. For 
comparison, a magnetic record medium was also made up using as a starting 
material the gamma ferric hydroxyoxide produced by Comparative Example 2. 
TABLE 2 
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Hc (Oe) Br (Gauss) 
R.sub.S 
______________________________________ 
Example 4 370 1310 0.88 
Comparative 
230 1050 0.77 
Example 2 
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From the foregoing Table, it will be apparent that the magnetic record 
medium employing magnetic materials produced according to the present 
invention has superior magnetic characteristics. 
The present invention can be applied to raw materials used to provide the 
so-called alloy magnetic powder. An example of this preparation is given 
in Example 5. 
EXAMPLE 5 
The gamma ferric hydroxyoxide produced by Example 1 was dispersed into 1.5 
l of water in 3 hours, and cobalt chloride and aqueous chromium sulfate 
solutions were added to the suspension in such an amount that the atomic 
ratio of cobalt to the sum of iron and cobalt was 15 atomic % while the 
atomic ratio of chromium to the sum of iron and cobalt was 0.5 atomic %. 
The suspension was further dispersed by an agitator for 1 hour. 
Thereafter, 15.4 moles/l of ammonia water was added to the suspension and 
dispersed for 1 hour to deposit hydroxides of cobalt and chromium on the 
surface of the gamma ferric hydroxyoxide. After the completion of the 
absorption reaction, the particles were rinsed by water, dried by separate 
furnaces, dehydrated, and subjected to annealing to remove pores by 2 
hours of thermal treatment at 700.degree. C. The materials were then 
deoxidized in air to alpha ferric oxide and reduced in hydrogen at 
450.degree. C. for 3 hours to provide alloy magnetic powders. These 
magnetic powders were immersed in toluene, removed therefrom, and dried at 
room temperature. The magnetic powders thus produced had a coercive force 
of 1260 Oe, the magnetizing amount was 166 .sup.emu /g, the squareness 
ratio was 50%, and the ratio of surface area of 57.8 m.sup.2 /g. 
As described previously, the first stage of high rate oxidation is 
continued until the ratio of ferric ions to iron in solution becomes 0.3 
to 0.48. When the ratio was less than 0.3, the crystals obtained after the 
second stage oxidation became too large and had poor shape or appearance. 
When the value exceeded 0.48, the crystal was deteriorated in shape. 
FIGS. 7 and 8 are reproductions of photomicrographs of crystal particles at 
magnifications of 30,000 times where the ratio is less than 0.3 and larger 
than 0.48, respectively. It will be evident from these showings that the 
shapes of the crystals have been deteriorated. 
As described, the present invention provides improved lepidocrocites of 
rectangular shape without veins and branches. When magnetic powder and 
magnetic paint are made using the improved materials, there are superior 
dispersion properties. The magnetic characteristics are also substantially 
improved. 
The above description relates to preferred embodiments of the invention, 
but it will be apparent that many modifications and variations can be 
effected by one skilled in the art without departing from the spirit or 
scope of the novel concepts of the invention.