Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-03-02
2003-03-18
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
Reexamination Certificate
active
06534032
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a process for the production of iron oxide, starting from FeSO
4
or FeCl
2
and an alkaline component.
The production of iron oxide yellow (hereinafter also referred to as &agr;-FeOOH or goethite) has long been known. A detailed description of the prior art can be found for example in Ullmann's Encyclopedia of Industrial Chemistry 5
th
edition, 1992, Vol. A20, p. 297 et seq.
The method according to Penniman-Zoph (U.S. Pat. Nos. 1,327,061; 1,368,748), the precipitation method and the Laux method are used to produce iron oxide yellow. The raw materials used are metal cuttings and iron(II) sulfate in the Penniman-Zoph method, iron(II) sulfate or iron(II) chloride and an alkaline component (sodium hydroxide solution, ammonia, Ca(OH)
2
, etc.) in the precipitation method and ferrous metal and nitrobenzene in the Laux method.
For the first two methods a so-called seed, namely a relatively finely divided goethite, first has to be produced. This process step is usually performed in separate reactors. The seed production itself is performed at temperatures of 30 to 50° C.
In the conventional method of seed production it is not possible to precipitate all of the iron(II) and convert it to &agr;-FeOOH by oxidation since undesirable(black) magnetite is produced at a pH greater than 6.5. Magnetite formation is dependent on temperature, oxidation rate and pH. Magnetite is preferably formed at elevated temperatures, at a relatively low oxidation rate and at a pH greater than 6.5. At a pH of 8, corresponding to complete iron(II) precipitation, magnetite is always formed. In fact at temperatures above 50° C. this is the only phase formed.
However, with the alkaline precipitation method (using a very high NaOH excess) it is possible to obtain a pure &agr;-FeOOH phase even at elevated temperatures. Depending on the NaOH excess, more or less pronounced long needles are obtained. This method generally does not produce any products that can be used as colored pigments, primarily because all of the manganese contained in the iron raw material is precipitated out at the same time. Manganese, especially in sizeable quantities, leads to an undesirable brown tint. This alkaline precipitation method is therefore described for the production of &agr;-FeOOH as a starting material for magnetic acicular iron oxides. The alkaline precipitation method leads to a very finely divided product. The BET surface area of an &agr;-FeOOH of this type is between 40 and 120 m
2
/g, which already results in a brown tint because of its very fine-particle character. The presence of color-imparting metals such as Ni, Cr, Cu, Mn is of no relevance in the starting materials for acicular magnetic iron oxides; in fact these elements are commonly used as an additive to regulate the magnetic properties.
The various iron(II) raw materials that can be used for seed production also differ in their magnetite-forming tendency.
Iron(II) chloride, a raw material that would be preferable to use since it is very inexpensive to obtain, forms comparatively large amounts of magnetite. In this case magnetite formation at a pH of 6.5 begins at only 39° C. In uncooled reactors magnetite formation can therefore not be prevented with this raw material.
Although iron(II) sulfate does not form magnetite under these conditions, it is significantly more expensive and is usually sold as a solid (FeSO
4
.7H
2
O), which means that expensive equipment is required to dissolve the salt.
The two process steps of seed production and pigment manufacture that are required in the Penniman-Zoph method and the precipitation method increase the manufacturing costs of iron oxide yellow pigments produced by these methods. The required plants are also very large, since the production rates for both methods are only around 1-2 g pigment per liter per hour during the pigment forming stage.
The production of iron oxide yellow by the Laux method requires particularly pure ferrous metal, as otherwise the color quality of the iron oxide yellow falls markedly. Since these raw materials are very expensive, this method for iron oxide yellow production is no more cost-effective than the Penniman-Zoph method or the precipitation method.
A method for producing iron oxide yellow pigments by the precipitation method in an alkaline environment is known from U.S. Pat. No. 2,558,304. According to the method described there, it should be possible to produce pale yellow to deep brown iron oxide yellow pigments. In this case a solution of an iron(II) salt (FeSO
4
or FeCl
2
are mentioned) is added to an alkaline component consisting of a hydroxide of the alkalis or alkaline earths. At the end of precipitation the alkali is present in an amount that is at least 115% of the theoretically required amount. Precipitation is performed at temperatures below 40° C. A precipitation at elevated temperature is not described, in fact, according to U.S. Pat. No. 2,558,304, it is favorable to perform the reaction at lower temperatures, for example at 25° C. Oxidation to &agr;-FeOOH is then performed with air, whereby oxidation times of 15 minutes to 30 hours are required. Typical concentrations of the iron salts are around 100 g Fe per liter. The concentration of the alkaline component is likewise in the area of 100 g/l in cases where NaOH is used. This method delivers very finely divided, in some cases even transparent iron oxide yellow pigments.
The restriction of the reaction temperature to 40° C. hinders the technical feasibility because it means that oxidation can only be performed very slowly. If oxidation is performed more quickly and in relatively large reactors (~100 m
3
volume), the reaction heat generated (120 kcal/mol equivalent to 502 kJ/mol FeOOH) can no longer be dissipated without cooling.
The method described can therefore only be performed with very complex and expensive cooling or in small reactors or with long oxidation times. All three factors make this method uneconomical if inexpensive iron oxide yellow pigments are to be produced.
Also, numerous publications on the production of iron oxide yellow by the precipitation method in an alkaline environment as a starting material for acicular magnetic iron oxides are known. In this instance the color quality of the iron oxide yellow is irrelevant. DE-A 2 455 158 is an example, in which the production of an iron oxide yellow in an alkaline environment by oxidation of an aqueous solution of an iron(II) salt with an alkali or alkaline-earth metal nitrate or chlorate is described. The color properties of the products obtained are not described, and they are also irrelevant for the application described. Since the method uses relatively expensive oxidizing agents, it is unsuitable for the production of a particularly inexpensive iron oxide yellow. If an iron(II) salt with more than 4 mol % Fe(III) and/or one with more than 0.7 wt. % Mn (based on on the Fe content of the iron(II) salt) is used, no suitable colored pigments are obtained. Yellow pigments for coloring concrete components, plastics, paper or similar media therefore cannot be produced by methods in which feedstocks of this type are used since black magnetite is formed if too much Fe(III) is present or the yellow pigment becomes dark brownish if the manganese content is too high.
An object of the present invention is therefore to provide a method that allows iron oxide yellow to be produced by a very simple and inexpensive method from iron raw materials that are available at a reasonable price.
This object may be achieved by the process of the present invention as described hereinafter.
SUMMARY OF THE INVENTION
The present invention relates to a process for the production of an iron oxide yellow by
a) adding FeSO
4
or FeCl
2
with an Fe(III) content of 0.0 to 4.0 mol % Fe(III) and a Mn content of up to 0.7 wt. %, based on the Fe content, to a solution or suspension of an alkaline component that has been heated to a temperature of between 42 and 75° C., and
b) oxidizing the suspension fo
Akorli Godfried R.
Bos Steven
Roy Thomas W.
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