Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-08-31
2004-02-10
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C426S632000
Reexamination Certificate
active
06689206
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for producing yellow iron oxide pigments from iron(II) chloride and an alkaline component by the precipitation process, wherein after addition of the &agr;-FeOOH nucleus to the preprecipitated FeCl
2
the pH is 3 to 7.
The production of yellow iron oxide pigments by the precipitation process has been known for a long time. The typical course of this process is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5
th
Ed., Vol. A20, p. 297 ff. The raw material generally used is iron(II) sulfate obtained during the pickling of steel sheets, or iron(II) sulfate formed during the production of titanium dioxide by the sulfate process. Large quantities of FeCl
2
are also obtained in the manufacture of synthetic rutile for the production of TiO
2
.
However, an increased trend towards the use of hydrochloric acid as a mordant has been observable for years in the pickling industry. Due to its purity, the iron(II) chloride obtained here is particularly suitable for the production of yellow iron oxide. Furthermore, the so-called chloride process is being increasingly used worldwide for the production of titanium dioxide. Hence solutions containing iron(II) chloride, which as far as possible should be converted into a useful material, are increasingly being obtained from both sources. A common process for this conversion is the spray-roasting process, in which the iron(II) chloride or an iron(III) chloride at elevated temperatures (typically more than 1000° C.) is subjected to an oxidative hydrolysis. The end products formed here are iron oxide, typically haematite, and hydrochloric acid which, being the required useful material, is reintroduced into the pickling process. Without particular purification steps, the iron oxide obtained is suitable for the production of hard ferrites.
If soft ferrites are to be produced from such iron oxides, it is necessary to subject the iron chloride solution to additional purification operations, which makes the process significantly more expensive. As more and more iron chloride, for the most part of inferior quality, is becoming available, and the absorption capacity of the ferrite market is limited, an alternative process for the production of a useful material from this iron chloride is sought. A direct deposition or dumping of the iron chloride solutions is unacceptable for environmental reasons.
Accordingly, an object of the present invention was the conversion of iron chloride into a high-grade useful material.
In general, &agr;-FeOOH (yellow iron oxide) is produced from iron(II) salts by the precipitation process (DE-A 2 455 158) or by the Penniman process (U.S. Pat. Nos. 1,368,748, 1,327,061). In both processes a nucleus is first of all produced, onto which, in a further step, additional &agr;-FeOOH is allowed to grow relatively slowly. Unlike FeSO
4
, FeCl
2
is usually not isolated as crystalline material, because its solubility in water is significantly higher than that of FeSO
4
. It is therefore in many cases more highly contaminated than FeSO
4
, which is in fact purified by the crystallization step. In addition, the FeCl
2
solution frequently contains organic constituents, which can greatly influence the crystallization process.
As iron(II) chloride solutions from steel pickling and from TiO
2
production frequently contain interfering quantities of cations of higher valency (for example, Ti, Cr, Al, V, Si), these have to be removed by precipitation using an alkaline component (EP-A 0 911 370). The resulting iron chloride has a pH of between 2 and 4.
It has been found that many iron(II) chloride solutions obtained from steel pickling do not lead to pure needle-shaped &agr;-FeOOH when the so-called acid nucleation process is employed (see also EP-A 0 406 633, Example 1). Where FeCl
2
is used, instead of &agr;-FeOOH, in many cases &bgr;-FeOOH in the form of very long, thin needles is obtained. During the subsequent pigment formation &bgr;-FeOOH partially decomposes to form &agr;-Fe
2
O
3
, depending on the applied temperature (Chambaere, D. G. & De Grave, E., Phys. Chem. Minerals, 12, (1985), 176-184). Consequently, it not possible to use &bgr;-FeOOH as nucleus material for the production of &agr;-FeOOH pigments.
In order for the FeCl
2
from steel pickling to be usable, the nucleus has to be produced by the “alkali process” (U.S. Pat. No. 2,558,304). If one now proceeds with an “alkaline” yellow nucleus in accordance with the known process for pigment formation, where preprecipitated FeCl
2
is used, the addition of the nucleus to the iron(II) chloride produces a pH of 5-7, which at temperatures of above 50° C. leads to unwanted black magnetite.
Accordingly, it is an object of the present invention was to find a process whereby yellow iron oxide pigment can be produced from preprecipitated FeCl
2
and from a nucleus produced by the alkali process.
SUMMARY OF THE INVENTION
1. The invention relates to a process for producing a yellow iron oxide pigments comprising the steps of
I) mixing preprecipitated FeCl
2
and an &agr;-FeOOH nucleus produced by the “alkali” process, wherein
a. the Fe concentration of the FeCl
2
is between 70 and 220 g/l,
b. the Fe(III) content of the FeCl
2
is less than 8 mol % Fe(III),
c. the pH of the suspension after addition of the &agr;-FeOOH nucleus (measured at 30° C.) is 3 to 7,
II) oxidizing the suspension in a first oxidation step, wherein
d. the temperature is 20 to 45° C.,
e. the rate of oxidation in the first oxidation step is 0.5 to 10 mol % Fe(III) formed per hour,
f. the pH (measured in suspension at 30° C.) at which the first oxidation step is ends is between 1.5 and 3.0, and
III) oxidizing the suspension in a second oxidation step, wherein
g. the temperature is 55 to 85° C.,
h. the pH is increased at a rate of 0.05 to 1.0 pH/h by continuous addition of an alkaline component,
i. the end point of the rise in the pH in the second oxidation step is pH 2.4to 5.2,
j. the rate of oxidation in the second oxidation step is 0.2 to 10 mol % Fe(III) formed per hour,
k. the rate of circulation of the suspension during the reaction, based on the final volume of the batch, is between 1 and 15 times per hour.
DETAILED DESCRIPTION OF THE INVENTION
The invention preferably relates to a process where the following parameters can be adjusted:
a. the Fe concentration of the iron component used is between 85 and 130 g/l,
b. the Fe(III) content of the Fe component used is 0.1 to 2.5 mol % Fe(III),
c. the pH of the suspension after addition of the alkaline yellow nucleus (measured at 30° C.) is 4 to 6,
d. the temperature in the first oxidation step is 30 to 40° C.,
e. the rate of oxidation in the first oxidation step is 4 to 7 mol % Fe(III) formed per hour,
f. the pH (measured in suspension at 30° C.) at which the first oxidation step is carried out is between 2.4 and 2.8,
g. the temperature in the second oxidation step is 60 to 75° C.,
h. the pH in the second oxidation step is increased at a rate of 0.1 to 0.8 pH/h, by continuous addition of an alkaline component,
i. the end point of the rise in the pH in the second oxidation step is pH 2.8 to 4.2,
j. the rate of oxidation in the second oxidation step is 0.4 to 4 mol % Fe(III) formed per hour,
k. the rate of circulation of the suspension during the reaction, based on the final volume of the batch, is 1 and 3 times per hour.
The process according to the invention can contain the following steps:
Preprecipitation (not necessary if the FeCl
2
used is of high purity)
Sodium hydroxide solution is added, with intensive stirring, to an iron(II) chloride solution with an FeCl
2
content of between 50 and 450 g/l, in a quantity sufficient to establish a pH of 3 to 5. Instead of sodium hydroxide solution, other alkaline components can be used including Ca(OH)
2
, Na
2
CO
3
or ammonia. Moreover, the sedimentation behavior of the accumulating hydroxide slurry or carbonate slurry can be improved by the addition of a flocculation aid. Suitable flocculation aids include the known polya
Akorli Godfried R.
Bayer Aktiengesellschaft
Koslow C. Melissa
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