Method of preparing an aluminum-containing iron oxide nucleus

Details Method of preparing an aluminum-containing iron oxide nucleus Method of preparing an aluminum-containing iron oxide nucleus

2001-08-31

2003-01-21

Bell, Mark L. (Department: 1755)

Adhesive bonding and miscellaneous chemical manufacture

Surface bonding means and/or assembly means therefor

With bending, folding, winding, or wrapping means

C106S404000, C106S418000, C106S439000, C106S442000, C106S474000, C423S632000

Reexamination Certificate

active

06508290

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a method of preparing an aluminium-containing iron oxide nucleus with an &agr;-FeOOH crystal structure from FeCl
2
. This nucleus is a suitable starting material for the preparation of iron oxide yellow and for use as a yellow coloring pigment.
Synthetic iron oxides are usually prepared by the Laux method, the Penniman method, the precipitation method, the neutralisation method or the roasting method (Ullmann's Encyclopedia of Industrial Chemistry, 5
th
edition, 1992, Vol. A20, pages 297 et seq.). The iron oxides thus obtained are generally employed as pigments.
Basically two processes are known for preparing fine-particle &agr;-FeOOH (needle width between 5 and 30 nm):
the acid process and
the alkaline process.
In the acid process an iron (II) component, an iron salt dissolved in water, is introduced initially and an alkaline component is added to it in metered quantities and with intensive mixing; the alkaline component is generally an alkali metal or alkaline earth metal compound dissolved or suspended in water or an ammonia solution. The quantity of alkaline component added is generally between 15% and 70% of the stoichiometrically required quantity. The pH after its addition is within the slightly acid range.
When the addition of the alkaline component is over the material is oxidized with an oxidant, usually atmospheric oxygen. The reaction is carried out at temperatures between 20 and 50° C. At considerably higher temperatures there is a danger of undesirable magnetite forming. The end of the reaction can be recognized by a sharp drop in pH and redox potential. When the reaction is over the properties of the product obtained (generally described as the nucleus) are determined and the product—if suitable—is processed immediately to form an &agr;-FeOOH pigment.
The alkaline process differs from the acid process in the quantity of alkaline component. In the alkaline process it is at least about 120% of the stoichiometrically required quantity and generally considerably more. The temperatures at which the reaction is carried out may be slightly above those used in the acid process as there is less danger here of magnetite forming.
The alkaline process basically produces relatively long-needled &agr;-FeOOH crystallites with a length to width ratio of 10:1 to 30:1. As these crystallites also contain very few dendrites the process is particularly suitable for preparing &agr;-FeOOH as a starting material for magnetic tapes.
Nuclei prepared by the alkaline process cannot be employed directly—or only to a limited extent—for making (&agr;-FeOOH pigments for use in paints and lacquers, as all the other coloring metals present in the industrially used Fe component become incorporated during the process. These metals (particularly Mn, Cr, Cu, Ni) have a considerably adverse effect on color properties, thereby restricting the use of nuclei thus prepared for the production of coloring pigments.
In the preparation of iron oxide pigments it is preferable to start with an &agr;-FeOOH nucleus and then to coarsen it (build it up) in the acid substance, thereby diminishing the introduction of coloring metals. Furthermore the build-up must only take place at pH levels below about 4, as coloring metals are introduced to an increasing extent at higher pH levels. Moreover, the particle shape of the (&agr;-FeOOH considerably affects the color properties, the viscosity of the lacquer and the requirements for the binder.
Short-needle &agr;-FeOOH particles are necessary in order to obtain the desired low viscosity in the lacquer and little binder requirements. These can be made by intensively grinding long-needle &agr;-FeOOH particles. A more favourably priced alternative is direct preparation of short-needle &agr;-FeOOH particles.
Modifying additives are required in order to guide the particle shape of the &agr;-FeOOH nucleus and thus that of the pigment built up therefrom towards a low length to width ratio. The use of B, Al, Ga, Si, Ge, Sn or Pb as 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, achieved through an appropriate pigment build-up procedure and addition of the modifiers listed above. Although the specification describes the use of FeCl
2
it does not mention the exact conditions for preparing an (&agr;-FeOOH nucleus from FeCl
2
. However since FeCl
2
clearly differs from FeSO
4
, especially in the nucleus-forming phase, the conditions under which a good pigment is obtained with FeSO
4
cannot be transferred to FeCl
2
.
An object of the invention was to provide a method of preparing a short-needle &agr;-FeOOH nucleus by the precipitation process in a simple and cost-effective way. In a further step the &agr;-FeOOH nucleus is built up to form an &agr;-FeOOH pigment.
The object is solved by the method of the invention.
SUMMARY OF THE INVENTION
The invention relates to a method of preparing aluminium-containing iron oxide nuclei with an &agr;-FeOOH crystal structure with an aspect ratio (AR) of 2100 to 3600 and a BET surface area of 50 to 150 m2/g using FeCl
2
, comprising the steps of
a) initially adding 4-13 mol %, based on the total iron, of an Al component to an iron (II) chloride solution with a total Fe content of 20-100 g/l, and an Fe (III) content of 0.1 to 10 mol % Fe (III) (based on the total Fe)
b) heating the mixture to a precipitation temperature between 30 and 60° C.,
c) adding a precipitating agent with an active ingredient content of 2-10 equivalents per liter, to the mixture and the molar ratio of Fe+Al to precipitating agent is 20-80% of the stoichiometric quantity,
d) oxidizing the precipitated suspension by an oxidant at a speed such that the oxidation rate is 2-50 mol %/h of the iron to be oxidized.
DETAILED DESCRIPTION OF THE INVENTION
In the context of the present invention the aspect ratio is the product of the BET surface area and the mean crystallite size, determined from the 110 reflex of the &agr;-FeOOH by X-ray crystallography.
The Al-containing &agr;-FeOOH nucleus obtained after oxidation may optionally by used to prepare iron oxide yellow pigments without further separation, when its properties have been tested.
The process preferably uses
a) an iron (II) chloride solution with a total Fe content of preferably 40-65 g/l,
b) a precipitation temperature between 35 and 50° C.,
c) a precipitating agent with an active ingredient content of 4-8 equivalents per liter,
c) a molar ratio of Fe+Al to precipitating agent of 30-60% of the stoichiometric quantity, and
The Al-containing &agr;-FeOOH nucleus obtained after oxidation is separated if appropriate.
It is more preferred to proceed as follows:
Initial chemicals:
FeCl
2
solution with an Fe content of 55 g/l Fe, including 1.5 mol % 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
Proportion of Fe+Al/precipitating agent: 35-40%
Reaction conditions:
Temperature: 44° C.
Oxidation speed: 30-35 mol % Fe (II)/h
AlCl
3
(as an aqueous solution) is preferably employed as the Al component. The use of Si or Ti as nucleus modifiers, in their chloride form, is also possible but entails a greater industrial outlay on production.
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 may be used as precipitating agents in an aqueous solution or an aqueous slurry.
The oxidant used may be atmospheric oxygen, oxygen, ozone, H
2
O
2
, chlorine, nitrates of alkali metals or alkaline earth metals or NH
4
NO
3
.
If the iron (II) chloride solution employed contains quite large quantities of precipitable coloring metals at pH levels below 4, these may be precipitated by adding an alkaline component to the iron (II) chloride solution up to pH 4. The solid formed may be separated from the remaining clear, purified solution by sedimentation, filtering or centrifuging. Not only are the undesirable coloring metals rem

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