Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1066483
This invention relates to a process for the
production of fine particles of magnetite suitable for use
in magnetic tapes and ink and the like. More particularly,
the invention relates to a process which involves reacting
ferrous and ferric salts, and ammonia under controlled
conditions in an aqueous medium to cause precipitation of
spheroidal, fine particles of magnetite.
It is known to prepare particles of magnetite by
subjecting iron powder to partial oxidation or by subject-
ing particles of hematite to partial reduction. The
magnetite particles thus produced are too coarse for use
in magnetic inks and must be pulverized by some means.
The means conventionally used for doing so is wet grinding
in a ball mill. However, grinding must be continued for a
very long time, frequently as long as 1,000 hours, to
decrease the state of subdivision of the particles to that
suitable for use in magnetic ink. Because of the prolonged
grinding required, the cost of the grinding operation is
considerable and usually accounts for a substantial portion
of the overall cost of production of the magnetite particles.
It is an object of the present invention to
provide a process by which spherDidal magnetite particles
suitable for use in magnetic inks and tapes can be produced
but which does not involve grinding the particles.
It is another object to provide a process by which
the size of magnetite particles can be controlled within the
range of 200 to 500A by simple and inexpensive means.
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1066483
These and other objects may be accomplished by
a process which involves providing solutions containing
ferrous ions, ferric ions, and free ammonia; mixing said
solutions in proportions necessary to neutralize any free
acid; maintaining the temperature of the ~ixture at at
least 50C to cause a reaction therein with resulting
formation of finely divided particles of magnetite; and
recovering said magnetite particles.
The process of the invention is described below
with reference to the drawings in which:
Figure 1 is a graph showing the effect of the
mixing sequence of ammonia and aqueous
solutions of ferrous and ferric sulphate
on the settling rate of magnetite
precipitated from the resulting slurry;
and
Figure 2 is a graph showing the effect of the
Fe~+~/Fe~ molar ratio on the settling
rate of magnetite from a slurry of ferrous
hydroxide and ammonia to which is added
an aqueous solution of ferric sulphate.
As starting materials for the process of the
subject invention, solutions containing free ammonia and
dissolved sàlts of di- and tri-valent iron are required.
Three separate solutions may conveniently be used for the
purpose, one a solution which contains ferrous ions (for
the sake of brevity referred to below as solution 1), a
solution which contains ferric ions (solution 2) and a
solution which contains free ammonia (solution 3). Solutions
(1) and (2) may be derived from the combination of iron
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values and one or more of the common acids in which the
iron values are soluble. The acid may, for example, be
sulphuric acid, hydrochloric acid and acetic acid.
The first step of the process involves mixing
solutions (1), (2) and (3) in such quantities that all free
acid in solutions (1) and (2) are neutralized by the free
ammonia in solution (3). In addition, it is preferred
that solutions (1) and (2) be combined in such proportions
that the ferric to ferroùs molar ratio in the resulting
mixture be in the range of 3:1 to 1:1 and more perferably
2:1. It is also preferred that solutions (1), t-2) and (3)
be so combined that the concentration of soluble iron values
in the mixture be in the range of 10 to 30 g.p.l.
The most convenient method for determining the
correct amount of solution (3) which must be combined with
solutions (1) and (2) to neutralize all free acid is to
monitor the pH of the resulting mixture. All free acid
will be neutralized when the pH of the mixture is the same
or higher than the pH of the iron salt resulting from the
reaction between the iron values and the acid in solutions
(1) and (2). For example, should the acid in solutions (1)
and (2) be sulphuric acid, a sufficient quantity of
solution (3) should be added to increase the pH to at
least 5.~ which is the pH of iron sulphate in solution.
Similarly, should the acid be acetic acid, solution (3)
should be added in sufficient quantity to increase the pH
to that of iron acetate in solution. In cases where the
form of the iron salt in solutions (1) and (2) is unknown
because, for example, the acid make-up of solutions (1)
and (2) is undetermined, the quantity of solution (3) with
10~483
solutions (1~ and (2) should be that required tO raise the
pH to about 7. It is possible to raise the pH to a higher
value but it is preferred not to do so because of the
added cost of handling a more basic mixture and because
of the tendency of the magnetite formed in the mixture to
oxidize to iron hydroxide.
As indicated above, it is preferred to maintain
the Fe+++/Fe++ molar ratio in the mixture resulting from
the combination of solutions (1), (2) and (3) within the
range of 3:1 and 1:1. Where the molar ratio is outside
this range, undesirably large quantities of iron hydroxide
are formed in the mixture with resulting lowering of the
magnetic properties of the end product of the process. It
should be noted, however, that the lowering of magnetic
properties is more pronounced where the ferric to ferrous
molar ratio is below 1:1 than where it is above 3:1.
In general, the overall concentration of iron in
the mixture has no effect on the formation of magnetite .
However, an iron concentration below about 30 g.p.l. favours
production of a finer magnetite product than a higher
concentration of iron. Since a finer product is generally
more useful than a coarser product, it is preferable to
maintain the concentration of iron below 30 g.p~l.
The sequence of mixing solutions (1), (2), and (3)
has a significant effect on the quantity of magnetite pro-
duced by the subject process. The mixing of ferrous
solution (1) and ferric solution (2) and the addition of
the resulting mixture to ammoniacal solution (3) causes an
immediate reaction in which substantially all iron values
present in the mixture or slurry convert to magnetite.
106~483
Even more rapid and complete conversion of the soluble iron
values to magnetite occurs when the ferrous-ferric mixture
is sprayed into ammoniacal solution (3).
The addition of ammoniacal solution (3) to a
ferrous-ferric solution (1) (2) results in the precipitation
first of brown Fe(OH)3, then the precipitation of green
Fe(OH)2 and finally, magnetite. However, conversion of
Fe(OH~3 and Fe(OH)2 to magnetite is incomplete. The
addition of ferrous solution (1) to ammoniacal solution (3)
and afterward the addition of ferric solution (2) to the
resulting ferrous ammoniàcal solution (1) (3) results first
in the precipitation of Fe(OH)2 and afterward the precipitation
of magnetite but again conversion to the latter compound is
incomplete. Similarly, the addition of ferrous solution (1)
to a combined solution (2) and (3) results in the incomplete
conversion of soluble iron values to magnetite.
The temperature of the mixture or slurry resulting
from the combination of solutions (1), (2) and (3) should be
maintained at at least 50C and preferably should be main-
tained at a temperature within the range of from about 100 to
150C. Maintaining the temperature near the upper limit of
this range results in a magnetite end product which is
denser and which has a smaller surface area than the end
product produced at a lower temperature.
The reaction which is believed to occur in the
mixture or slurry is as follows (where solutions (1) and
(~) contain sulphuric acid):
2 4 3 4 + ~N~40H ----~ Fe304 + 4(NH4)2S04 + 4H 0
(1)
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Besides this reaction, other side reactions are
also believed to take place, they being:
Fe2(S04)3 ~ 6NH40H -----------t 2Fe(OH)3 ~ 3(NH4)2S04 (2)
FeS04 + 2NH40H ----------------~ Fe(OH)2 ~ (NH4)2S04 (3)
3 4 ~ 2NH40H --~ Fe304 ~ (NH4)2S04 ~ 4H20 (4)
Fe(OH)2 + Fe2(S04)3 ~ 6NH40H -~ Fe304 + 3(NH4)2S04 ~ 4H20 (5)
As evident from the above equations, ammonia is
essential to precipitation of magnetite. The minimum
quantity of ammonia required is the stoichiometric equivalent
of the q~antity of sulphate sulphur present in the combined
solution. Thus 2 moles of ammonia are required for each
mole of sulphate sulphur present. Ferric sulphate and
ferrous sulphate solutions accordingly require 3 and 2 moles
of ammonia per mole of iron respectively. In addition, 2
moles of ammonia are required for each mole of free sul-
phuric acid present in the combined solution. As previously
indicated, the most convenient method for ensuring that
sufficient ammonia is present in the mixture or slurry
resulting from the combination of the three solutions is
by monitoring the pH thereof . When the pH is 5.4 or
higher, no further ammonia need be added.
In general, agitation of the mixture results in
a finer product; thus, the size of the final product can
be, to a certain extent, controlled by the extent to which
the mixture is agitated during the reaction.
Various so-called "surface active agents" may be
added to the mixture to control the shape and size of the
precipitated magnetite particles. Agents suitable for the
purpose are those identified by the trade marks Goulac,
Dextrin, Orzan, Tween 60, EMA-1103, Span ~5 and Aerosol C-61.
Usually a concentration of about 0.5 g.p.l. of the agent in
the mixture is sufficient for the purpose.
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1066~83
At the completion of the reaction, the magnetite
particles may be separated from liquid by filtration and
may be dried by conventional means. During drying, the
particles usually agglomerate but in most cases the
agglomerates may be broken down without difficulty. For
example, where the particles are washed with water and
dried at 100C, they agglomerate into a very brittle lump
but the lump can be easily broken down by grinding. Where
the particles are washed in acetone and dried in a vacuum,
they agglomerate into a soft product which again may be
broken down without difficulty.
The resulting magnetite particles are generally
spheroidal in shape and are within the size range of about
200 to about 500A. The particles may be somewhat contami-
nated with iron hydroxide which can be converted to mag-
netite by heat treatment in a neutral atmosphere.
EXAMPLE 1
This example illustrates the effect which the
sequence of mixing the ferrous solution (1), ferric
solution (2) and ammoniacal solution (3) has on the settling
rate of precipitated magnetite. Samples of such solutions
were mixed in various sequences to form a number of charges~
each 700 ml in volume and containing 10 g.p.l. iron. The
molar ratios of Fe ~, Fe~ and NH3 in the charges were:
Fe~/Fe~ = 2:1 and NH3/FeT = 4:1. To some charges were
added 0.5 g.p.l. EMA-1103, to others none. The charges
were maintained at a temperature of 50C for an hour each.
Figure 1 illustrates the effect of the ~lxing
sequence on the settling rate of precipitated magnatite.
The figure clearly shows that the settling rate is slowest
1066483
where the mixing sequence is solutions (1) and (2) added
to solution (3) and is fastest where the mixing sequence
is solution (1) added to a mixture of solutions (2) and
(3). It is believed that the solid product from the
former mixing sequence settles more slowly than the product
from the latter mixing sequence because the former product
is significantly finer than the latter product. Since a
finer product is preferred, the former mixing sequence is
preferred.
EXAMPLE 2
This example illustrates the effect of the
Fe+++~Fe++ molar ratio on the settling rate of precipitated
magnetite. 700 ml samples were prepared by adding various
amounts of a solution containing ferric ions to a solution
containing free ammonia and ferrous ions. The total con-
centration of iron in the samples was approximately 10 g.p.l .
and the NH3/FeT molar ratio was 4. The surface activator
EMA-1103 was added to some of the samples.
The samples were maintained at a temperature of
50C for a period of time and the settling rate of
magnetite from the samples was determined by periodically
measuring the height of the zone of clear solution above
the dark magnetite-containing slurry. The results are
shown in Figure 2.
From Figure 2 it is clear that the fastest settling
rate occurs when the Fe++ +/Fe++ ratio is 3:1 and the slowest
rate occurs when the ratio is 1:1. The slow settling rate
is believed to be due to the presence of Fe(OH)2. Mag-
netite particles precipitated from samples containing EMA-1103
settled more slowly indicating that the particles were
probably somewhat finer than magnetite particles precipitated
from samples lacking the surface activator.
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