Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-08-31
2003-01-21
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S404000, C106S418000, C106S439000, C106S442000, C106S474000
Reexamination Certificate
active
06508877
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for the production of an aluminum-containing iron oxide crystallization nucleus with an &agr;-FeOOH crystal structure from FeCl
2
. This nucleus is suitable as starting material for the production of iron oxide red (&agr;-Fe
2
O
3
) via &agr;-FeOOH as intermediate product.
Synthetic iron oxides are normally produced by the Laux process, Penniman process, precipitation process, neutralization process or roasting process (Ullmann's Encyclopedia of Industrial Chemistry, 5
th
Ed., 1992, Vol. A20, p. 297 ff). The iron oxides obtained in this way are generally used as pigments.
For the production of finely particulate &agr;-FeOOH (needle width between 5 and 30 nm) two processes are known:
the so-called acid process and
the so-called alkaline process
In the acid process an iron (II) component, such as an iron salt dissolved in water, is used as starting material and an alkaline component, such as an alkali or alkaline earth metal compound dissolved or suspended in water or also ammonia solution, is added thereto while mixing thoroughly. The amount of alkaline component that is added is generally between 15% and 70% of the stoichiometrically required amount. The pH after the addition of the alkaline component is in the weakly acidic range.
After completion of the addition of the alkaline component the reaction mixture is oxidized with an oxidizing agent, such as atmospheric oxygen. The reaction is carried out at temperatures between 20° C. and 50° C. At significantly higher temperatures there is the danger of the formation of undesirable magnetite. The end point of the reaction can be recognized by a sharp drop in pH and redox potential. After completion of the reaction the properties of the resultant product (generally termed crystallization nucleus) are determined and if appropriate the product is directly processed further into &agr;-FeOOH pigment.
The alkaline process differs from the acid process by the amount of alkaline component that is added. In the alkaline process the amount of added component is at least 120% of the stoichiometrically necessary amount, and is usually significantly greater. The temperatures at which this reaction is carried out may be somewhat higher than the temperatures employed in the acid process, since the danger of the formation of magnetite is not as high in this case.
In the alkaline process relatively long needle-shaped &agr;-FeOOH crystallites are usually obtained with a length to width ratio of 10:1 to 30:1. Since these crystallites furthermore have a very low dendrite content this process is particularly suitable for the production of &agr;-FeOOH as starting product for magnetic tapes.
For the production of &agr;-FeOOH pigments for use in paints and lacquers, the crystallization nuclei produced by the alkaline process cannot be used directly or can only be used to a limited extent since in this process all chromophoric metals present in the Fe component are incorporated. These metals (in particular Mn, Cr, Cu, Ni) significantly impair the color properties and thus restrict the use of crystallization nuclei produced in this way as color pigments.
In order to produce iron oxide yellow pigments an &agr;-FeOOH crystallization nucleus is preferably used and this is then coarsened (built up) in the acid, as a result of which the incorporation of the chromophoric metals is reduced. If it is desired to produce a particularly color-pure iron oxide red (&agr;-Fe
2
O
3
) from &agr;-FeOOH by calcination, the &agr;-FeOOH serving as starting material should contain only minor amounts of chromophoric metals. Furthermore, the build-up may take place only at a pH of less than ca. 4 since at higher pH values the chromophoric metals are incorporated in increasing amounts. In addition the particle shape of the &agr;-FeOOH considerably affects the color properties, the viscosity of the coating composition and the need for binders.
In order to achieve a desirable low viscosity in the coating composition and a low binder requirement, short-needle &agr;-FeOOH particles are necessary. These may be produced from long-needle &agr;-FeOOH particles by intensive grinding. A more cost-effective alternative is to produce short-needle &agr;-FeOOH particles directly.
In order to control the particle shape of the &agr;-FeOOH crystallization nucleus and thus the shape of the pigment built up therefrom, so as to obtain a low length to width ratio, modifying additives are necessary. The use of B, Al, Ga, Si, Ge, Sn or Pb as crystallization nucleus modifiers is known from U.S. Pat. No. 4,620,879. This patent specification describes an iron oxide yellow with a particularly low silking index, which is achieved by an appropriate procedure for the pigment build-up and by adding the aforementioned modifiers. However, this patent specification does not state how to prepare an &agr;-FeOOH crystallization nucleus for the production of a particularly good (color-pure) &agr;-Fe
2
O
3
pigment.
An object of the present invention is to provide a process for the simple and inexpensive production of a short-needle &agr;-FeOOH crystallization nucleus according to the precipitation process. This &agr;-FeOOH crystallization nucleus should allow the formation of an (&agr;-FeOOH pigment. From this subsequently isolated &agr;-FeOOH pigment an &agr;-Fe
2
O
3
red pigment should be finally produced by annealing.
This object is achieved by the process according to the invention. If iron (II) chloride is used instead of iron (II) sulfate and 3 to 16 mole % of aluminum, based on Fe is added thereto, then a very finely particulate &agr;-FeOOH with an aspect ratio of 2100 to 3100 is obtained. In the present context the aspect ratio denotes the mathematical product of the BET surface and mean crystallite size that has been determined by X-ray analysis of the 110 reflection of the &agr;-FeOOH.
SUMMARY OF THE INVENTION
The invention relates to a process for the production of aluminum-containing iron oxide crystallization nuclei having an &agr;-FeOOH crystal structure with an aspect ratio of 2100 to 3100 by using FeCl
2
, containing the steps of
a) initially adding an Al component, while stirring, in amounts of 6 to 20 mole % based on total Fe, to an iron II chloride solution with a total Fe content of 20-100 g/l, preferably 40-65 g/l, and a Fe III content of 0.1−10 mole % Fe III (based on total Fe),
b) heating this mixture to a precipitation temperature between 30° C. and 60° C., preferably between 35 and 50° C.,
c) adding a precipitating agent with an active constituent content of 2-10 equivalents per liter, preferably 4-8 equivalents per liter, to the mixture, and the molar ratio Fe+Al to precipitating agent is 20%-80% of the stoichiometric amount, preferably 30% to 60% of the stoichiometric amount,
d) then oxidizing the precipitated suspension with an oxidizing agent at an oxidation rate of 2-50 mole %/hr. of the iron to be oxidized.
The Al-containing &agr;-FeOOH crystallization nucleus obtained after the oxidization may if desired be used without further isolation, after checking the properties, for the production of iron oxide red pigments via &agr;-FeOOH as intermediate.
DETAILED DESCRITION OF THE INVENTION
The following procedure is preferably employed:
Starting chemicals:
FeCl
2
solution with an Fe content of 55 g/l Fe, of which 1.5 mole % is Fe III
AlCl
3
solution
NaOH solution with an NaOH content of 300 g/l=7.5 equivalents NaOH/l
Al/Fe ratio: 12-13
Ratio Fe+Al/precipitating agent: 30-40%
Reaction conditions:
Temperature: 34° C.
Oxidation rate: 30-35 mole % Fe II/hr.
AlCl
3
(as aqueous solution) is preferably used as Al component. The use of Si or Ti in the form of their chlorides as crystallization nucleus modifier is also possible, but involves a greater technical expenditure in the production.
Suitable precipitating agents include NaOH, KOH, Na
2
CO
3
, K
2
CO
3
, Mg(OH)
2
, MgO, MgCO
3
, Ca(OH)
2
, CaO, CaCO
3
, NH
3
or secondary or tertiary aliphatic amines in aqueous solution or as an aqueous s
Bayer Aktiengesellschaft
Bell Mark L.
Gil Joseph C.
Manlove Shalie
Roy Thomas W.
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