Process for preparing yellow iron oxide pigments with CaCo

1. Iron oxide pigment having L*, a* and b* values, measured in full shade in CIELAB units, of

L*=58 to 62,

a*=7 to 13, and

b*=43 to 50,

a Ca content of from 10 to 6500 mg/kg iron oxide pigment, a particle length to width ratio of from 3:1 to 30:1, an isoelectric point of from 6 to 9 and a water-solubles content, based on the iron oxide pigment, of from 0.01 to 2.1% by weight.

2. Process for preparing iron oxide pigments according to claim 1 , comprising introducing as an initial charge an aqueous solution of an Fe(II) component, adding an α-FeOOH yellow nucleus, then carrying out oxidation and at the same time as or immediately after the beginning of oxidation and adding calcium carbonate, as an aqueous suspension, as powder or as granules as precipitant.

3. Process for preparing iron oxide pigments according to claim 1 , comprising:

a. introducing as an initial charge an aqueous solution of an Fe(II) component having an Fe concentration of from 55 to 220 g/l, based on the aqueous solution introduced,

b. adding an α-FeOOH yellow nucleus,

c. setting a temperature of from 55° C. to 85° C.,

d. carrying out the oxidation to form further α-FeOOH, where

1) the temperature during oxidation is from 55° C. to 85° C., and

2) the oxidation rate is from 0.2 to 10 mol % Fe(III) formed/h;

3) during or immediately after the beginning of oxidation calcium carbonate and/or dolomite, are added as an aqueous suspension, as powder or as granules, as precipitant, in a concentration of from 80 to 500 g/l based on the suspension added;

4) the pH at the beginning of oxidation is raised at a rate of from 0.05 to 1.0 pH/h by continuous addition of the precipitant until the pH rise end point is reached, the pH rise end point being situated at a pH of from 2.4 to 5.2.

4. Process for preparing iron oxide pigment according to claim 2 , characterized in that the α-FeOOH yellow nucleus has a needle length of from 0.1 to 2 μm and a length to width ratio of from 30:1 to 5:1, the α-FeOOH yellow nucleus being prepared such that either an alkaline precipitant selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, alkaline earth metal hydroxides, alkaline earth metal carbonates, ammonia, dolomite and combinations thereof is used or an Fe(II) component is introduced as an initial charge and heated, the component not introduced initially being added to the initial charge component, and then oxidation is carried out.

5. Process for preparing iron oxide pigment according to claim 2 , characterized in that the α-FeOOH yellow nucleus has a needle length of from 0.1 to 2 μm and a length to width ratio of from 30:1 to 5:1, the α-FeOOH yellow nucleus being prepared such that:

a. an alkaline precipitant such as alkali metal hydroxides, alkali metal carbonates, alkaline earth metal hydroxides, alkaline earth metal carbonates, ammonia, dolomite and combinations thereof is used in a concentration, based on the nucleus-forming reaction, of from 1.25 to 15 equivalents/l,

b. an Fe(II) component having an Fe concentration of from 21 to 150 g/l is used, the amount of Fe(II) components being from 20% to 83% or from 125% to 500% of the stoichiometry of the alkaline precipitant,

c. either the alkaline precipitant or the Fe(II) component is introduced as initial charge and heated to a temperature of from 30° C. to 60° C.,

d. the component not introduced initially is added to the initial charge component after the temperature determined in step c) is reached,

e. then oxidation is carried out, where

1) the oxidation rate is from 5 to 50 mol % Fe(III)/h,

2) the precipitation time is from 15 to 95 minutes.

6. Process for preparing yellow iron oxide pigments according to claim 2 , characterized in that atmospheric oxygen, oxygen, ozone, hydrogen peroxide, chlorine, chlorates (I) to (VII) or nitrates are used as oxidizing agents.

7. A process for colouring building materials comprising providing yellow iron oxide pigment according to claim 1 .

8. A process for colouring plastics or paper or for preparing colorant preparations comprising providing yellow iron oxide pigment according to claim 1 .

9. A process for preparing colorants which come into contact with foods comprising providing yellow iron oxide pigment according to claim 1 .

10. A process for preparing heterogeneous catalysts comprising providing yellow iron oxide pigment according to claim 1 .

11. A process for preparing red iron oxide pigments comprising providing yellow iron oxide pigment according to claim 1 .