Process for the preparation of bright, color-pure iron oxide red pigments

A bright, color-pure iron oxide red pigment is produced by an improved process by precipitation of iron(II) salts in aqueous solution with an alkaline precipitating agent, oxidation at temperatures of from 20.degree. C. to 100.degree. C. and annealing of the resulting precipitation products under air to form iron oxide red pigments, wherein the improvement comprises before adding the alkaline precipitating agent, the iron(III) content of the aqueous solution is adjusted to 1 to 15 mol %, based on the total iron content of the iron salt solution.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the preparation of bright, 
color-pure iron oxide red pigments by the precipitation of iron(II) salts 
with alkali in aqueous solution, oxidation at temperatures of from 
20.degree. C. to 100.degree. C., followed by annealing of the resulting 
precipitation products in air to iron oxide red pigments, and the use of 
these iron oxide red pigments. 
There are basically four known processes for the preparation of synthetic 
iron oxide red. One is the oxidation of iron oxide black Fe.sub.3 O.sub.4 
(T. C. Patton, Pigment Handbook, Volume 1, John Wiley and Sons, 1973). 
Iron oxide red pigments are used in building materials and lacquers. 
Bright, color-pure iron oxide red pigments are desirable in particular for 
use in lacquer systems. The color shade of the pigment in the lacquer is 
determined according to DIN 6174 (equivalent ISO/DIN 7724, 1-3 drafts). 
The red content, (a*)AS determined in the lacquer is taken as a measure of 
the color purity of the iron oxide red pigment. 
The color shade of the iron oxide red pigment obtained after annealing is 
influenced by the choice of starting material, namely needle-shaped 
goethite or isometric magnetite. The use of needle-shaped goethite results 
in red pigments which have an undesirable yellow tinge. Finely divided 
isometric magnetites give rise to an iron oxide red with a purer color on 
annealing. Finely divided magnetites, however, can only be obtained by the 
aniline process, using aromatic nitro compounds as oxidizing agents. 
Commercial iron(II) salts of the kind obtained as waste liquors in steel 
pickling works and titanium dioxide factories are normally used as the raw 
material for obtaining precipitated magnetite as the starting material for 
iron oxide red pigments. These waste liquors normally contain a small, 
variable proportion of iron(III). Precipitation is carried out, for 
example, with NaOH, Na.sub.2 CO.sub.3, MgO, MgCO.sub.3 or NH.sub.3. The 
oxidizing agent used for precipitating the magnetite may be oxygen, air, 
chlorates, nitrates, peroxides, etc. 
Three processes are known for the precipitation of isometric magnetites, 
the one-stage process, the two-stage process and the iron(III) chloride 
process. 
In the one-stage process (Winnacker-Kuchler, Volume 2, page 171, Munich 
1970), iron oxide black is prepared in one step from iron(II) salts and 
alkalies with gassing at temperatures above 50.degree. C. and pH values 
above 4.5. The iron(II) salt used in the process should be as far as 
possible free from iron(III) constituents (DE-PS No. 2,919,854, page 8, 
lines 19 et seq.). 
In the first step of the two-stage process (Winnacker-Kuchler, Volume 2, 
page 171, Munich 1970), a yellow nucleus is precipitated at an acid (pH ph 
&lt;5) or an alkaline pH above 12 with alkalies at temperatures from room 
temperature to 100.degree. C. under oxidizing conditions. This 
time-consuming step is followed by the second stage of the process, 
consisting of reacting the yellow nucleus with a further quantity of 
iron(II) salt and alkalies to form iron oxide black. 
The iron(III) chloride process for the precipitation of iron oxide black is 
technically particularly complicated. It starts with mixtures of iron(III) 
salt and iron(II) salt used in approximately stoichiometric proportions 
corresponding to that of the magnetite, i.e. about 1/2 to 2/3 of the 
mixture is of the expensive iron(III) salt. The iron(III) used in this 
process is preferably iron(III) chloride (JP No. 56-60 789), which is 
highly corrosive. This salt is precipitated with alkalies to form 
brownish-black iron oxides. 
The precipitation processes mentioned above yield magnetites which, on 
annealing, give rise to iron oxide red in the form of medium-fine to 
coarse iron oxide red pigments. These processes have the disadvantage that 
they do not yield the coloristically valuable iron oxide red pigments in 
the bright, color-pure range. 
In order to obtain bright, color-pure iron oxide red pigments by the 
annealing of precipitated magnetite, it is necessary to use a 
precipitating magnetite which has a crystallite size of about 30 to 50 nm 
determined by X-ray and a specific surface area S.sub.BET of from 25 to 60 
m.sup.2 /g. Magnetites having a particle size or crystallite size in this 
range cannot be obtained by the three precipitation processes mentioned 
above. 
Various procedures are known for enabling products in the required range of 
crystallite sizes and BET surface areas to be obtained from the 
precipitation of iron oxide black. These procedures include reduction in 
the reaction temperature and lowering of the pH during precipitation, but 
these only give rise to a non-uniform precipitation product of isometric 
magnetite and needle-shaped components such as .alpha.-FeOOH and 
.gamma.-FeOOH. When these precipitated iron oxides are annealed, they give 
rise to .alpha.-Fe.sub.2 O.sub.3 of non-uniform particles and with a 
disturbing yellow tinge. 
BRIEF DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a cost effective, 
technically elegant process for the preparation of isometric iron oxide 
red pigments which does not have the disadvantages of the processes 
described above. 
Such a process has now surprisingly been found, in which magnetites 
obtained by precipitation in the presence of iron(III) ions give rise to 
bright, color-pure iron oxide red pigments when annealed. The particle 
size of these products can be controlled by means of the iron(III) 
content. 
DETAILED DESCRIPTION 
In contrast to the iron(III) chloride process described above, the process 
according to the invention does not require the use of expensive, 
corrosive iron(III) chloride. 
The present invention thus provides a process for the preparation of 
bright, color-pure iron oxide red pigments by the precipitation of 
iron(II) salts with alkali in aqueous solution and oxidation at 
temperatures of from 20.degree. to 100.degree. C. and annealing of the 
resulting precipitation products in air to form iron oxide red pigments, 
wherein said solution is adjusted to an Fe(III) content of from 1 to 15 
mol %, based on the total iron content of the solution, before addition of 
the alkaline precipitating agent. The oxidizing agent used in this process 
is preferably air. 
At the beginning of the process, the iron(III) content required for 
controlling the particle size of the magnetite is produced in an iron(II) 
salt solution in which the iron(II) salts are preferably sulphates and/or 
chlorides obtained from steel pickling works and/or titanium dioxide 
production. Adjustment of the iron(III) content is preferably carried out 
by oxidative gassing of the acid iron(II) salt solution, e.g. an iron(II) 
sulphate solution at a pH of below 3. The iron(III) content increases with 
increasing gassing time. The iron(III) content is preferably produced by 
oxidation with air at a pH of below 3. This preliminary gassing is 
advantageously carried out with the aid of a gassing stirrer in a stirrer 
vessel at temperatures of about 20.degree. to 100.degree. C., although 
other reactors suitable for gas/liquid reactions, such as bubble columns, 
loop reactors, radiators, etc. could equally well be used. 
A clear, pigment-free solution having the desired iron(III) content is 
obtained within a short time. This solution is surprisingly found to be 
free from unwanted, precipitated basic iron(III) sulphates. It has not 
been clarified in what form the trivalent iron is present in the solution. 
The iron(III) content required for preparing the magnetites varies within 
the range of from 1 to 15 mol % of iron(III), based on the total iron 
content. 
The preliminary gassing is followed by precipitation and oxidation to 
magnetite. The precipitating agents used are preferably NaOH, NH.sub.3, 
Na.sub.2 CO.sub.3, MgO and/or MgCO.sub.3. Exceptionally high yields are 
obtained when precipitation is carried out with 0.5 to 1.3 equivalents, 
preferably 0.8 to 1.3 equivalents, of the alkaline precipitating agent and 
the oxidizing agent used is preferably air. The reaction temperature is in 
the range of from 20.degree. to 100.degree. C., preferably above 
70.degree. C. The precipitation products are washed free from salt, dried 
and then annealed at 700.degree. C. to 1100.degree. C., preferably at 
800.degree. C. to 950.degree. C., so that bright, color-pure iron oxide 
red pigments are obtained after grinding. 
The iron oxide red pigments obtained after annealing of the magnetites are 
bright and with pure color, with particle sizes below 0.2 .mu.m and the 
color shade in the lacquer has a red content (a*) above 26 CIELAB units. 
Such iron oxide red pigments with high color purity (a*&gt;26 CIELAB units) 
are not obtainable by annealing magnetites prepared by the conventional 
method of precipitation. The color shade of the iron oxide pigment is 
determined according to DIN 6174 (equivalent ISO/DIN 7724, 1-3 drafts), 
CIELAB c/2 deg. at a 10% pigment volume concentration (PVK) in the 
lacquer. The lacquer system used is Alkydal F 48 (trade product of Bayer 
AG). 
It has further been found that the coloristic values of the end product can 
be advantageously influenced by the addition of soluble aluminium and 
phosphorous salts to the iron salt solution before precipitation. The 
addition of these compounds is advantageously carried out by adding a 
soluble aluminium salt in quantities of from 1 to 10 mol % and/or a 
soluble phosphorous salt in quantities of from 1 to 5 mol %, in each case 
based on the total iron content, to the solution before precipitation. 
It is found that the brightness (L*) of the red pigments can be increased 
in dependence upon the Al or P content. The brightness values L* are 
obtained according to DIN 6174 (equivalent ISO/DIN 7724, 1-3 drafts), in 
the color shade determination of the pigment in the lacquer. 
The present invention also relates to the use of the iron oxide red 
pigments prepared according to the invention for dyeing and coloring in 
the field of lacquers, ceramics, building materials and plastics.

The following Examples describe the production of the iron(III) content in 
the iron(II) salt solution, the precipitation of magnetite and the 
annealing to iron oxide red pigments having a red content a*&gt;26 CIELAB 
units. 
The Examples are given to illustrate the process and do not constitute any 
limitation. 
EXAMPLE 1 
An aqueous solution of 21.7 mol of FeSO.sub.4 in 22 liters of solution is 
prepared in a 30 liter container equipped with gassing stirrer. The pH is 
2.1. An iron(III) content of 0.45 mol % is determined by titration before 
oxidation is begun. The solution is heated to 70.degree. C., and air is 
introduced by way of the gassing stirrer. The table below shows how the 
Fe.sup.3+ content increases with the reaction time. For the sake of 
clarification, this relationship is also shown in graphic form in FIG. 1. 
______________________________________ 
Time/h Fe.sup.3+ /mol % 
______________________________________ 
Before starting 0.45 
after heating in air (0.5 h) 
4.0 
1 11.8 
2 18.0 
3 24.0 
4 29.2 
5 35.4 
6 40.7 
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EXAMPLE 2 
Adjustment of the iron(III) content 
1.95 liters of iron(II) sulphate solution having a pH below 3 and 
containing 300 g of FeSO.sub.4 are heated to 70.degree. C. under gassing 
with air by means of the gassing ring in a 2.5 liter glass apparatus 
equipped with cross beam stirrer, gassing ring and reflux condenser and 
heated with a heater band adjusted by a contact thermometer. An iron(III) 
content of 8.3 mol %, based on the total iron content, is established in 
1.75 hours. 
EXAMPLE 3 
Preparation of the precipitation magnetite and iron oxide red pigment 
In a 2.5 liter glass apparatus equipped with cross beam stirrer, gassing 
ring and reflux condenser and heated with a heater band controlled by 
contact thermometer, 0.6 equivalents of magnesium oxide in the form of a 
slurry of ground magnesium oxide in water are added as precipitating agent 
to 1.95 liters of iron(II) sulphate solution in which 8.3 mol % of 
iron(III) ions have been produced. The reaction mixture is heated to 
80.degree. C. and gassed. The black iron oxide obtained after 4 hours is 
washed free from salt, dried and annealed in air at 800.degree. C. for 1 
hour. The properties of the ground iron oxide red pigment are entered in 
Table 1. 
EXAMPLE 4 
The procedure is the same as in Example 3 except that one equivalent of 
magnesium oxide is used as precipitating agent. The properties of the 
ground iron oxide red are entered in Table 1. 
EXAMPLE 5 
The procedure is the same as in Example 3 except that one equivalent of 9N 
NaOH is used as precipitating agent. The properties of the ground iron 
oxide red are entered in Table 1. 
EXAMPLE 6 
The proceudre is the same as in Example 3 except that 0.6 equivalents of 
magnesium oxide are used as precipitating agent and the iron(III) content 
is 5.6 mol %. 
The properties of the ground iron oxide red are entered in Table 1. 
EXAMPLE 7 
The procedure is the same as in Example 3 except that 1.1 equivalents of 
MgO are used as precipitating agent and 1.4 mol % of Al, based on the 
total iron content, are added in the form of Al.sub.2 
(SO.sub.4).sub.3.18H.sub.2 O before precipitation. The properties of the 
ground iron oxide red pigment are entered in Table 1. 
EXAMPLE 8 
The procedure is the same as in Example 3, except that 1.1 equivalents of 
MgO are used as precipitating agent and 2.8 mol % of Al, based on the 
total iron content, are added in the form of Al.sub.2 
(SO.sub.4).sub.3.18H.sub.2 O before precipitation. The properties of the 
ground iron oxide red pigment are entered in Table 1. 
EXAMPLE 9 
The procedure is the same as in Example 3 except that 1 equivalent of 9N 
NaOH is used as precipitating agent and 1.6 mol % of P, based on the total 
iron content, are added in the form of Na.sub.2 HPO.sub.4 before 
precipitation. The properties of the ground iron oxide red pigment are 
entered in Table 1. 
Comparison Examples: 
EXAMPLE A 
The procedure is the same as in Example 3 except that the reaction is 
carried out with an iron(II) salt solution obtained from steel pickling, 
in which the iron(III) content is less than 0.5 mol %, and precipitation 
is carried out with 1 equivalent of 9N NaOH. The properties of the ground 
iron oxide red pigment are entered in Table 1. 
EXAMPLE B 
The procedure is the same as in Example 3 except that the reaction is 
carried out with an iron(II) salt solution containing less than 0.5 mol % 
of iron(III), and precipitation is carried out with 1 equivalent of MgO. 
The properties of the ground iron oxide red pigment are entered in Table 
1. 
EXAMPLE C 
The procedure is the same as in Example 3 except that the reaction is 
carried out with an iron(II) salt solution containing less than 0.5 mol % 
of iron(III), and precipitation is carried out with 0.6 equivalents of 
MgO. The properties of the ground iron oxide red pigment are entered in 
Table 1. 
TABLE 1 
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CIELAB colour data 
Specific DIN 6174 C/2 deg 
surface 
Annealing 
10% PVK alkydal F 48 
Equivalents 
Fe.sup.3+ 
Al P area of 
tempera- 
bright- 
red yellow 
Precipitat- 
based on 
content 
added magnetite 
ture/time 
ness 
shade 
shade 
ing agent 
total iron 
[mol %] 
[mol %] 
[mol %] 
[S.sub.BET ] 
[.degree.C./min] 
[L*] 
[a*] 
[b*] 
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Example 
3 MgO 0.6 8.3 58 800.degree. C./30' 
40.3 
27.6 
21.9 
4 MgO 1 8.3 32 800.degree. C./30' 
40.4 
29.2 
21.7 
5 NaOH 1 8.3 31 800.degree. C./30' 
39.4 
27.9 
20.1 
6 MgO 0.6 5.6 35 800.degree. C./30' 
40.3 
27.0 
22.5 
7 MgO 1.1 8.3 1,4 27 800.degree. C./30' 
40.8 
26.7 
22.6 
8 MgO 1.1 8.3 2.8 29 800.degree. C./30' 
41.6 
27.1 
23.1 
9 NaOH 1.0 5.6 1.6 33 800.degree. C./30' 
41.1 
27.2 
21.7 
Comparison 
Example 
A NaOH 1 15 800.degree. C./30' 
37.9 
24.8 
17.8 
B MgO 1 15 800.degree. C./30' 
36.7 
24.0 
14.2 
C MgO 0.6 16 800.degree. C./30' 
37.0 
25.1 
15.6 
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