Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2002-01-14
2004-07-27
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S140000, C423S142000, C423S144000
Reexamination Certificate
active
06767529
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for producing magnetite particles.
Particulate magnetites produced from aqueous solutions by a precipitation process have been known for a long time. The production of magnetite by precipitation of iron(II) sulfate using an alkaline component and subsequent oxidation by air has already been described in U.S. Pat. No. 802,928. The production of magnetites by the precipitation process is likewise described in numerous subsequent publications.
The production of magnetites by the precipitation process with the addition of silicon is described in JP-A 51 044 298. Pure precipitated magnetites without added foreign elements can be produced batchwise according to DE-A 3 209 469 or continuously according to DE-A 2 618 058. Normally, the iron(II) salt used is FeSO
4
.
It is possible, however, to use any soluble iron(II) salt for the production of a magnetite by the precipitation process. The use of FeCl
2
, as described in DE-A 3 004 718, is particularly suitable here. The use of FeSO
4
or of FeCl
2
has the advantage that both substances can be obtained very economically in large quantities as waste products of the iron-working industry. Suitable precipitating agents, besides the most frequently used sodium hydroxide, are CaO or CaCO
3
(DE-A 3 004 718), ammonia (DE-A 2 460 493) or Na
2
CO
3
, MgCO
3
or MgO (EP-A 0 187 331). Air is generally used as the oxidizing agent. However, oxidation processes using nitrates (DD-A 216 040 and DD-A 284 478) are also described.
The magnetites have been used primarily for producing all kinds of pigments. The particular advantage of magnetites over organic pigments and carbon black is their far better resistance to weathering, so that pigments containing magnetite can also be used outdoors. Precipitated magnetites are also usually employed for coloring concrete moldings such as concrete paving stones or concrete roofing tiles.
Magnetites have also been used for a long time in electrophotography for the production of toners. Magnetites which have been obtained by the precipitation process are preferably employed for producing toners for photocopiers using one-component toners. The magnetic toners used for this purpose must have several properties. With the progressive development and improvement of photocopiers and printers, ever greater demands are placed on the magnetic toners and consequently on the magnetite used in them. The most recent generation of printers achieves a resolution of more than 400 dpi (dots per inch), for which it was necessary to develop finely divided toners having a very narrow particle-size distribution. The result of this was that the magnetites used for this purpose must likewise possess a very narrow particle-size distribution. A specific particle size is also necessary in order to ensure a homogeneous distribution of the magnetite particles in the final toner. The magnetites must themselves have an electrical resistance high enough to stabilize the latent image during electrostatic transfer. Furthermore, coercivity, saturation magnetization and, above all, the remanent magnetization must all be in the correct relationship to the field strengths prevailing in the machine.
It is an object of the present invention to provide a technically particularly simple and therefore also economic process for producing magnetites having a coercivity of 80 to 130 Oe (=6.366 kA/m to 10.345 kA/m) and an octahedral particle shape for the production of magnetic toners. This object can be achieved by the process according to the invention.
SUMMARY OF THE INVENTION
The invention relates to a process for producing a magnetite having a coercivity of 6.366 kA/m to 10.345 kA/m (=80 to 130 Oe) and an octahedral particle shape by
a) placing an alkaline component in the form of an aqueous solution under a protective gas,
b) heating the alkaline component to a temperature of 50° C. to 100° C.,
c) adding an iron(II) component in the form of an aqueous solution whereby the molar ratio of iron(II) component to one equivalent of alkaline component is 0.38 to 0.45, while maintaining the temperature at 50° C. to 100° C.,
d) treating the suspension obtained in c) with an oxidizing agent, whereby the rate of oxidation is 20 to 50 mol. % Fe(II)/h until the iron compound has an Fe(III) content of more than 65 mol. %,
e) after the oxidation in d), again adding an Fe(II) component in the form of an aqueous solution to the suspension obtained in d) whereby the molar ratio of Fe(II) in c) to one equivalent of total alkaline component used is 0.47 to 0.49,
f) treating the suspension obtained in e) with an oxidizing agent, whereby the rate of oxidation is 20 to 50 mol. % Fe(II)/h until the iron compound has an Fe(III) content of more than 65 mol. %,
g) filtering the suspension, and washing, drying and grounding the residue.
DETAILED DESCRIPTION OF THE INVENTION
Surprisingly, it has been found that by increasing the ratio of Fe(II) to alkaline component to 0.38 to 0.44, at temperatures of 50° C. to 100° C., preferably at 70° C. to 90° C., and by rapid oxidation (up to 50 mol. % Fe(II)), a magnetite which is usable in standard toners is obtained. Owing to the high rate of oxidation, the process is particularly economical because the plants can be utilized particularly efficiently.
The magnetites thus obtained can be used not only in magnetic toners, but also for coloring paper, plastic, paints, fibres and concrete, and in dyes.
Magnetites having a coercivity of 80 to 130 Oe (=6.366 kA/m to 10.345 kA/m) and an octahedral particle shape are obtainable by the following process according to the invention, wherein
a) an alkaline component in the form of an aqueous solution is placed under a protective gas,
b) the alkaline component is heated to a temperature of 50° C. to 100° C., preferably to 70° C. to 90° C.,
c) an iron(II) component in the form of an aqueous solution is added in a quantity such that the molar ratio of iron(II) component to one equivalent of alkaline component is 0.38 to 0.45, the temperature being maintained at 50° C. to 100° C., preferably 70° C. to 90° C.,
d) the suspension obtained in c) is treated with an oxidizing agent, with a rate of oxidation of 20 to 50 mol. % Fe(II)/h being established, until the iron compound has an Fe(III) content of more than 65 mol. %,
e) after the oxidation in d), an Fe(II) component in the form of an aqueous solution is again added to the suspension obtained in d) in a quantity such that the molar ratio of Fe(II) used in c) to one equivalent of total alkaline component used is 0.47 to 0.49,
f) the suspension obtained in e) is again treated with an oxidizing agent, with a rate of oxidation of 20 to 50 mol. % Fe(II)/h being established, until the iron compound has an Fe(III) content of more than 65 mol. %, and then
g) filtered and the residue is washed, dried and ground.
The alkaline component preferably contains an alkali metal hydroxide, an alkaline-earth metal hydroxide, an alkaline-earth metal oxide, an alkali metal carbonate, MgCO
3
or ammonia
The iron (II) component preferably contains a water-soluble Fe(II) salt, more preferably an iron sulfate or iron dichloride. It is also possible, however, to use other water-soluble Fe(II) compounds, in particular if they are available at comparable prices.
The oxidizing agent preferably contains atmospheric oxygen, pure oxygen, H
2
O
2
, chlorine, alkali metal chlorates (for example, NaOCl, NaClO
3
, NaClO
4
) or nitrates. Atmospheric oxygen, pure oxygen, H
2
O
2
or sodium nitrate are more preferred.
A particularly preferred embodiment of the process according to the invention is described in more detail below.
Sodium hydroxide solution containing 300 g NaOH per liter of water is placed in a batch-operated stirred-tank reactor, with stirring under a flow of protective gas.
The solution is then heated to a temperature of between 50° C. and 100° C., preferably between 70° C. and 90° C. When this temperature has been attained, metering of the Fe(II) component is commenced. The Fe(II) component i
Akorli Godfried R.
Bayer Aktiengesellschaft
Bos Steven
Denesvich Jill
Eyl Diderico Van
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