Process for producing haematite pigments

Abstract

Process for producing a haematite pigment having an a* value greater than 20, especially greater than 25, CIELAB units measured as a full shade in alkyd resin to DIN EN ISO 787-25:2007, by reacting iron sulfate with oxygen in the presence of a pigment seed, characterized in that a) water comprising i) at least one pigment seed selected from the group consisting of FeOOH and Fe.sub.2O.sub.3 and ii) iron sulfate forms an initial charge and b) at least one alkaline earth metal carbonate, especially CaCO.sub.3, MgCO.sub.3, CaMg(CO.sub.3).sub.2 or mixtures thereof and c) oxygen-containing gas
are added to the initial charge, where at least 95% by weight of the total amount of iron sulfate used in the process is present in the initial charge a) prior to addition of components b) and c).

Claims

1. A process for producing a haematite pigment having an a* value greater than 20 CIELAB units measured as a full shade in alkyd resin to DIN EN ISO 787-25:2007, comprising the steps of (1) providing water comprising i) at least one pigment seed selected from the group consisting of FeOOH and Fe.sub.2O.sub.3 and ii) iron sulfate to form an initial charge (a) and (2) adding at least one alkaline earth metal carbonate (b) and oxygen-containing gas (c) to the initial charge, wherein at least 95% by weight of the total amount of iron sulfate used in the process is present in the initial charge (a) prior to addition of components (b) and (c).

2. The process according to claim 1, wherein the iron sulfate content in the initial charge (a) prior to the addition of component (b) is 20 to 200 g/l.

3. The process according to claim 1, wherein the content of an FeOOH and/or Fe.sub.2O.sub.3 seed in the initial charge prior to the addition of component (b) is 1 to 20 g/l.

4. The process according to at claim 1, wherein component (b) is added to the initial charge at a temperature of 75 to 100° C.

5. The process according to claim 1, wherein component (b) is added to the initial charge in the form of an aqueous suspension.

6. The process according to claim 1, wherein the pigment seed in the initial charge is FeOOH with a BET surface area of 130 to 200 m.sup.2/g or Fe.sub.2O.sub.3 with a BET surface area of 40 to 200 m.sup.2/g.

7. The process according to claim 1, wherein the particle size of the pigment seed is not more than 100 nm.

8. The process according to claim 1, wherein the haematite pigment obtained has an iron oxide content (Fe.sub.2O.sub.3) of greater than 95% by weight, based on the pigment.

9. The process according to claim 1, wherein component (b) is added to the initial charge in the form of an aqueous suspension with a content of 100 to 400 g/l CaCO.sub.3.

10. The process according to claim 1, wherein component (b) is CaCO.sub.3, MgCO.sub.3, CaMg(CO.sub.3).sub.2 or mixtures thereof.

11. The process according to claim 1, wherein the pigment seed in the initial charge (a) is α-FeOOH with a BET surface area of 130 to 200 m.sup.2/g.

12. The process according to claim 1, wherein the haematite pigment obtained has an a* value greater than 25 CIELAB units.

13. The process according to claim 1, wherein the haematite pigment obtained has an iron oxide content (Fe.sub.2O.sub.3) of greater than 99% by weight, based on the pigment.

Description

EXAMPLES

[0061] Test Methods

[0062] Testing of the Colour Values in Full Shade

[0063] The colour values in full shade were ascertained to DIN EN ISO 787-25:2007 using the test paste described below.

[0064] 5 g of a thixotroped long-oil alkyd resin (WorléeKyd P 151) were applied to the lower part of a plate-type paint dispersion machine with plate diameter 240 mm, and the respective iron oxide red pigment was processed with the test paste to give a coloured paste with a pigment volume concentration (PVC) of 10%.

[0065] The test paste contains 95% by weight of alkyd resin (Worléekyd P151 from Worlée-Chemie GmbH, DE) and, as thixotroping agent, 5% by weight of Luvotix HAT (Lehmann & Voss & Co KG, DE). This is done by stirring the Luvotix into the alkyd resin that has been preheated to 70 to 75° C. and incorporating it at 95° C. until dissolution. The cooled paste is then rolled in a three-roll mill to free it of bubbles.

[0066] The red pigments were weighed out according to

[00001]$m_P=\frac{\mathrm{PVC}*m_b*{\rho }_p}{\left(100-\mathrm{PVC}\right)*{\rho }_b}$

[0067] m.sub.P=mass of the iron oxide red pigment

[0068] PVC=pigment volume concentration

[0069] m.sub.b=mass of binder

[0070] ρp=pigment density

[0071] ρb=binder density

[0072] The finished paste was transferred to a paste plate and analysed by colorimetry using a Datacolor 600 colorimeter using the d/8° measurement geometry and the D65/10° illuminant with specular reflection (CIELAB colour space to DIN 5033 Part 7).

Comparative Experiment 1 (Analogous to CN102976413 A, Embodiment 1)

[0073] 260 g of an α-FeOOH seed with a BET surface area of 105 m.sup.2/g (Ref. 1) are initially charged together with 1580 g of water in the reactor (for lack of other details in CN102976413, an α-FeOOH seed with a BET surface area obtained by customary methods in the order of magnitude of about 100 m.sup.2/g is assumed). The pH is adjusted to 4.0 with H.sub.2SO.sub.4 and the mixture is heated to 70-73° C. 500 g of FeSO.sub.4 (calculated as anhydrous iron sulfate), 11 333 g of water and a further 90 g of the seed are added to the initial charge thus obtained. On attainment of a temperature of T=75° C., the supply of air is commenced (50 l/h, pH 2.0-2.5). Then a total of 2400 g of solid FeSO.sub.4 (calculated as anhydrous iron sulfate) and 1573 g of solid CaCO.sub.3 were added uniformly over a period of 20 h. For that purpose, 60 g of iron sulfate (calculated as anhydrous iron sulfate) and 39.3 g of CaCO.sub.3 are added every 30 min. The air was supplied in the first 20 min of the addition at about 50 l/h, and for the rest of the time 75 l/h was introduced. After 20 h, air was introduced for a further 10 min and then the temperature was adjusted to 66° C. The pH was kept in the range of 2.8-3.5 and 766 g of FeSO.sub.4 (see above) and 756 g of CaCO.sub.3 (see above) were added uniformly over a period of 10 h. The air was added at a rate of 60 l/h.

[0074] Subsequently, the reaction mixture is cooled, and the solids obtained are filtered off by washing the sediment 3× with demineralized water and dried at 140° C. The gypsum formed was not removed. The analysis of the pigment obtained can be found in Table 1.

INVENTIVE EXAMPLES

Example 1

[0075] 1a) Production of a Yellow Seed with a BET of 100 m.sup.2/g

[0076] A jacketed 30 litre glass laboratory reactor with Intermig stirrer, temperature measurement by means of a PT100 thermocouple, EMF, and pH measurement, heated by means of a Julabo SL thermostat, and controlled by means of a Siemens PCS7 process control system, was initially charged with 16 643 cm.sup.3 of FeSO.sub.4 solution with c=100 g/l sparged with N.sub.2 at 300 l/h, and heated up to 50° C. while stirring at n=800 min.sup.−1 within 50 minutes. On attainment of the target temperature, the nitrogen sparging was ended and the metered addition of 2500 cm.sup.3 of NaOH, c=320 g/l, was commenced while sparging with air at 500 l/h, with a target pH of 5.0, keeping the temperature constant. A metering time of 402 minutes was followed by the further oxidation time under air at 500 l/h. Thereafter, the target pH was reset by means of FeSO.sub.4 solution at the end of the experiment.

[0077] For the analysis data, 1 l of the finished seed suspension was washed by demineralized water on a suction filter (MN 218) to <200 μS/cm and dried in an air circulation drying cabinet at 90° C. overnight, comminuted and then passed through a 2 mm screen and analysed. 1b) Pigment production In a 30 litre reactor, demineralized water, iron sulfate solution and the α-FeOOH seed produced in Example 1a) having a BET surface area of 98 g/m.sup.2 were combined. The resultant suspension (17 I) has an FeSO.sub.4 concentration of 59.8 g/l and an α-FeOOH seed concentration of 3 g/l (calculated as Fe.sub.2O.sub.3).

[0078] The suspension is heated up to 85° C. As soon as the temperature has been attained, the air supply via a sparging coil is started at an air rate of 65 l/h, and 3.554 l of a CaCO.sub.3 suspension (conc.=200 g/l) are added over a period of 6.5 h.

[0079] After 6.5 h, the reaction is ended. The Fe.sup.2+ content as a measure of the conversion of the reaction is checked by means of cerimetry. The resultant haematite sediment is washed 3 times, filtered off and dried at 140° C. in an oven. The CaSO.sub.4 formed as a by-product is not separated off. The analysis of the pigment can be found in Table 1.

TABLE-US-00001 TABLE 1 Seed STY CIELab* colour values Example Seed BET[m.sup.2/g] [g/l/h] L* a* b* Ex. 1 α-FeOOH 98 3.4 40.2 21.7 19.2 Comp. Ex. 1 α-FeOOH 105 3.5 61.3 15.1 42.5

[0080] By comparison with the process known from CN102976413, the process according to the invention leads to coloristically distinctly improved reds (higher a* value).

Example 2a

[0081] Production of a Haematite Seed Analogously to Example 2 of WO2013045608

[0082] The haematite seed was produced according to Example 2 of WO'608, choosing the total amount of nitric acid such that it corresponds to a calculated starting concentration not of 4.5% by weight but of 7% by weight in the reaction mixture. A seed having a BET surface area of 98 m.sup.2/g was obtained.

Example 2b

[0083] In a 30 liter reactor, demineralized water, iron sulfate solution and seed from Example 2a) are combined so as to result in 17 l of a suspension having an FeSO.sub.4 concentration of 108 g/I and a haematite seed concentration of 8 g/l (calculated as Fe.sub.2O.sub.3).

[0084] The suspension is heated up to 85° C. As soon as the temperature has been attained, the air supply via a sparging coil is started at an air rate of 250 l/h, and 6.6691 of a CaCO.sub.3 suspension (concentration 200 g/l) are added over a period of 7.5 hours.

[0085] After 7.5 h, the reaction is ended. The Fe.sup.2+ content is checked by means of cerimetry. The resultant haematite sediment is washed 3 times, filtered off and dried at 140° C. in an oven. The CaSO.sub.4 formed as a by-product (about 25% by weight) is not separated off. Space-time yield (STY)=5.4 g/l/h. The analysis of the pigment can be found in Table 2.

TABLE-US-00002 TABLE 2 Colour values (full Seed Fe.sup.2+ content shade) Experiment Seed BET[m.sup.2/g] after 7.5 h [g/l] Yield.sup.1) [%] L* a* b* Example 2 α-Fe.sub.2O.sub.3 98 0.0 100 36.3 28.9 19.8 .sup.1)determined via Fe.sup.2+ content

Example 3

[0086] In a 30 liter reactor, demineralized water, iron sulfate solution and seed from Example 2a) are combined so as to result in a 17 I suspension having an FeSO.sub.4 concentration of 108 g/l and a seed concentration of 8 g/l (calculated as Fe.sub.2O.sub.3).

[0087] The suspension is heated up to 85° C. As soon as the temperature has been attained, the air supply via a sparging coil is started at an air rate of 125 l/h, and 6.669 l of a CaCO.sub.3 suspension (conc.=200 g/l) are added over a period of 7.6 hours.

[0088] The Fe.sup.2+ content is checked by means of cerimetry. The resultant haematite sediment is washed 3 times, filtered off and dried at 140° C. in an oven. The CaSO.sub.4 formed as a by-product (about 15% by weight) is not separated off. STY=5.4 g/l/h (Fe.sub.2O.sub.3)

[0089] The analysis of the pigment can be found in Table 3.

TABLE-US-00003 TABLE 3 Colour values (full Seed Fe.sup.2+ content shade) Experiment Seed BET[m.sup.2/g] after 7.5 h [g/l] Yield.sup.1) [%] a* b* Example 3 α-Fe.sub.2O.sub.3 98 0.0 100 36.1 28.1 18.5 .sup.1)determined via Fe.sup.2+ content

Example 4

Example 4a

[0090] Production of a Haematite Seed Analogously to Example 2 of WO2013045608

[0091] The haematite seed was produced according to Example 2 of WO'608, choosing the total amount of nitric acid such that it corresponds to a calculated starting concentration not of 4.5% by weight but of 8% by weight in the reaction mixture. A seed having a BET surface area of 120 m.sup.2/g was obtained.

Example 4b

[0092] In a 30 liter reactor, demineralized water, iron sulfate solution and seed from Example 4a) were combined so as to result in 171 of a suspension having an FeSO.sub.4 concentration of 59.5 g/l and a seed concentration of 5 g/l (calculated as Fe.sub.2O.sub.3).

[0093] The suspension is heated up to 85° C. As soon as the temperature has been attained, the air supply via a sparging coil is started at an air rate of 66 l/h, and 3.295 I of a CaCO.sub.3 suspension (conc.=200 g/l) are added over a period of 7.5 hours.

[0094] After 7.5 h, the reaction is ended. The Fe.sup.2+ content is checked by means of cerimetry. The resultant haematite sediment is washed 3 times, filtered off and dried at 140° C. in an oven. The CaSO.sub.4 formed as a by-product (about 45% by weight) is not separated off. STY=3.4 g Fe.sub.2O.sub.3/l/h

[0095] The analysis of the pigment can be found in Table 4.

TABLE-US-00004 TABLE 4 Seed Fe.sup.2+ content Colour values (full Example Seed BET[m.sup.2/g] after 7.5 h [g/l] Yield.sup.1) [%] shade) Example 4 120 6.0 88 36.6 27.5 17.9 .sup.1)determined via Fe.sup.2+ content

Example 5

[0096] In a 1.5 m.sup.3 reactor having steam heating, stirrer and pumped circulation device, 429 kg of water, 201 kg of iron sulfate solution (corresponding to 38 kg of iron sulfate, calculated in anhydrous form) and 37 kg of haematite seed suspension (corresponding to 3.7 kg of seed from Ex. 4a) are combined and heated to 85° C.

[0097] As soon as the temperature has been attained, the air supply via a sparging coil is started at an air rate of 2550 l/h, and 125 I of a CaCO.sub.3 suspension (concentration=200 g/l) are added over a period of 7.5 hours.

[0098] After 7.5 h, the reaction is ended and the suspension is cooled down. The Fe.sup.2+ content is checked by means of cerimetry. The resultant haematite sediment is washed 3 times, filtered off and dried at 140° C. in an oven. In the case of a small amount, the CaSO.sub.4 formed during the reaction was screened off with a screen (71 μm mesh) in order to obtain an iron oxide having a gypsum content of less than 1% by weight. The analysis of the pigment can be found in Table 5 (STY=3.5 g/I/h).

TABLE-US-00005 TABLE 5 Colour values (full Seed Fe.sup.2+ content shade) Experiment Seed BET[m.sup.2/g] after 7.6 h [g/l] Yield.sup.1) [%] L* a* b* Example 5 α-Fe.sub.2O.sub.3 120 37.3 27.8 18.8 Example 5 α-Fe.sub.2O.sub.3 120 0.0 100 37.2 27.6 18.6 without CaSO.sub.4 .sup.1)determined via Fe.sup.2+ content

Comparative Example (Seed)

[0099] The seed synthesis from Example 1 of DE 4235944 (=U.S. Pat. No. 5,421,878) was reworked, and the a* value of the resultant seed was determined as in the other examples (see above). Example 1 here was [0100] a) reworked as described with NaOH and [0101] b) rather than NaOH the precipitant MgCO.sub.3 was used in an equivalent amount (200.2 g/1124.8 g of water).

[0102] The colour values (L*, a* and b*) were determined by the same method as described above in full shade in the alkyd resin, and the following results were obtained:

TABLE-US-00006 Example 1 of DE 4235944 (seed) L* a* b* a) NaOH 45.9 17.4 25.9 b) MgCO.sub.3 34.1 11.9 13.5 The a* values, at 17.4 and 11.9, are well below the a* value of >20 of the present invention.