Method for coating the surface of inorganic particles, particularly titanium dioxide pigment particles

Abstract

The invention relates to a method for coating the surface of inorganic solid particles in an aqueous suspension. The untreated particles, particularly TiO.sub.2 base material, are made into an aqueous suspension and subsequently disagglomerated. According to the invention, a disagglomerated suspension of untreated particles is fed (recirculated) from an intermediate vessel (vessel) in a cyclic process. The intermediate vessel contains a high-speed agitator preferably having a minimum peripheral speed of 15 m/s or a specific agitator capacity P/V of at least 30 W/m3. A pipeline mixer (e.g. inline disperser) based on the rotor/stator principle is furthermore installed in the circuit. The water-soluble precursor compounds of the coating substances, and equally any necessary pH-controlling substances, are metered into the pipeline mixer. This leads to surface coatings with greater smoothness (low specific surface area according to BET), improved density (low sulphuric-acid solubility), and less coating substance precipitated separately (improved gloss).

Claims

1. A method for coating titanium dioxide particles in an aqueous suspension with at least one coating substance, comprising: providing a circuit containing a vessel in fluid communication with a pipeline mixer; wherein the vessel is equipped with a high-speed agitator displaying a peripheral speed of at least 15 m/s or a specific agitator capacity of at least 30 W/m.sup.3; wherein the pipeline mixer is based on the rotor/stator principle; metering a water-soluble precursor compound of the at least one coating substance into the suspension solely in the pipeline mixer; and circulating the aqueous suspension of titanium dioxide particles through the pipeline mixer and to the vessel more than once.

2. The method of claim 1, wherein the at least one coating substance includes at least one coating selected from the group consisting of silicon dioxide and aluminum oxide.

3. The method of claim 2, wherein the at least one coating substance includes silicon dioxide and the suspension is maintained at a temperature of about 85 to about 95 C. during precipitation of silicon dioxide onto the particles.

4. The method of claim 2, wherein the at least one coating substance includes aluminum oxide and the suspension is maintained at a temperature of about 45 to about 55 C. during precipitation of aluminum oxide onto the particles.

5. The method of claim 2, wherein a sufficient amount of a water soluble precursor compound of silicon dioxide is metered into the pipeline mixer to form a dense skin on the particles.

6. The method of claim 1, further comprising maintaining the temperature of the suspension in a desired range by heating or cooling the vessel.

7. The method of claim 1, wherein the coated, inorganic particles display a pH value of less than about 6.5 at the end of the process.

8. The method of claim 1, further comprising metering pH-controlling substances into the suspension via the pipeline mixer.

9. The method of claim 8, wherein titanyl chloride is used as the pH-controlling substance.

10. The method of claim 8, wherein the at least one coating substance includes at least one coating selected from the group consisting of silicon dioxide and aluminum oxide.

11. The method of claim 10, wherein the at least one coating substance includes silicon dioxide and the suspension is maintained at a temperature of about 85 to about 95 C. during precipitation of silicon dioxide onto the particles.

12. The method of claim 10, wherein the at least one coating substance includes aluminum oxide and the suspension in the pipeline mixer is maintained at a temperature of about 45 to about 55 C. during precipitation of aluminum oxide onto the particles.

13. The method of claim 10, wherein a sufficient amount of a water soluble precursor compound of silicon dioxide is metered into the pipeline mixer to form a dense skin on the particles.

14. The method of claim 8, further comprising maintaining the temperature of the suspension in the vessel in a desired range using heating or cooling.

15. The method of claim 1 wherein the aqueous suspension is added to the circuit at the vessel.

16. The method of claim 1 wherein the aqueous suspension is added to the circuit at the pipeline mixer.

17. The method of claim 16 wherein the inorganic solid particles are first deagglomerated by passing the aqueous suspension through a mill prior to its addition to the circuit.

18. The method of claim 17 wherein no dispersing aid is added to the suspension.

19. A method for coating titanium dioxide particles in an aqueous suspension with at least one coating substance, comprising: providing a circuit containing a vessel in fluid communication with a pipeline mixer; wherein the vessel is equipped with a high-speed agitator displaying a peripheral speed of at least 15 m/s or a specific agitator capacity of at least 30 W/m.sup.3; wherein the pipeline mixer is based on the rotor/stator principle; circulating the aqueous suspension of particles through the pipeline mixer and to the vessel more than once; and metering a water-soluble precursor compound of the at least one coating substance and at least one pH-controlling substance into the suspension solely in the pipeline mixer.

20. The method of claim 19, wherein the at least one coating substance includes at least one coating selected from the group consisting of silicon dioxide and aluminum oxide.

21. The method of claim 20, wherein the at least one coating substance includes silicon dioxide and the suspension is maintained at a temperature of about 85 to about 95 C. during precipitation of silicon dioxide onto the particles.

22. The method of claim 20, wherein the at least one coating substance includes aluminum oxide and the suspension is maintained at a temperature of about 45 to about 55 C. during precipitation of aluminum oxide onto the particles.

23. The method of claim 20, wherein a sufficient amount of a water soluble precursor compound of silicon dioxide is metered into the pipeline mixer to form a dense skin on the particles.

24. The method of claim 19, further comprising maintaining the temperature of the suspension in a desired range by heating or cooling the vessel.

25. The method of claim 19, wherein the coated, inorganic particles display a pH value of less than about 6.5 at the end of the process.

26. The method of claim 19, wherein titanyl chloride is used as the pH-controlling substance.

27. The method of claim 19, further comprising recirculating the aqueous suspension through the circuit following the application of the at least one coating and metering a water-soluble precursor compound of a second coating substance into the suspension in the pipeline mixer.

28. The method of claim 19 wherein the aqueous suspension is added to the circuit at the vessel.

29. The method of claim 19 wherein the aqueous suspension is added to the circuit at the pipeline mixer.

30. The method of claim 29 wherein the inorganic solid particles are first deagglomerated by passing the aqueous suspension through a mill prior to its addition to the circuit.

31. The method of claim 29 wherein no dispersing aid is added to the suspension.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a flow chart of a preferred embodiment of the method according to the invention;

(3) FIG. 2 is a transmission electron microscope image of the pigment according to Example 1;

(4) FIG. 3 is a transmission electron microscope image of the pigment according to Example 3;

(5) FIG. 4 is a transmission electron microscope image of the pigment according to Reference Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(6) The present invention can be better understood by the following discussion of the manufacture and use of certain preferred embodiments. All data disclosed below regarding size, time, temperature, amount of components, concentration in % by weight, etc. are to be interpreted as also including all values lying in the range of the respective measuring accuracy known to the person skilled in the art. Unless otherwise stated, technical grades of the various materials were used in the preferred embodiments. The term substantially free is intended to connote that the particular material is not detected (i.e. is below the detection limit) using standard commercial tests and methodologies used in the industry as of the earliest priority date of this application.

(7) The method according to the invention is based on an aqueous suspension of untreated, inorganic solid particles, which are also referred to as base material below. Suitable in this context are fine inorganic solids with a particle size in the region of roughly 0.001 m to 1 mm, preferably 0.1 to 1 m, that are processed in aqueous suspensions, such as pigments (titanium dioxide, color pigments, effect pigments, etc.), fillers, extenders, titanates, iron, nickel or other metallic particles.

(8) Open to consideration as the coating are oxides, hydroxides, phosphates and similar compounds of the familiar elements Si, Ti, Al, Zr, Sn, Mn, Ce and other elements. Here and below, the term oxide is also to be taken to mean the respective hydroxide or hydrous oxide. In particular, the coatings involved are inorganic.

(9) In a special embodiment of the invention, untreated titanium dioxide pigment particles (TiO.sub.2 base material) are used. TiO.sub.2 base material manufactured by either the sulfate process or the chloride process can be used. The TiO.sub.2 base material can have an anatase or rutile structure. Rutile is preferred. The TiO.sub.2 base material is customarily doped with familiar elements, such as Al, to improve the photostability of the TiO.sub.2 pigment. In the chloride process, for example, such a quantity of AlCl.sub.3 is oxidized together with TiCl.sub.4 that the TiO.sub.2 base material displays roughly 0.5 to 2.0% by weight Al, calculated as Al.sub.2O.sub.3. During titanium dioxide production by the sulfate process, the hydrolyzed titanyl sulfate is mixed with calcining additives, such as potassium hydroxide or phosphoric acid, and subsequently calcined. The TiO.sub.2 base material from the sulfate process customarily contains roughly 0.2 to 0.3% by weight K and 0.05 to 0.4% by weight P, each calculated as the oxide.

(10) The untreated particles, particularly TiO.sub.2 base material, are made into an aqueous suspension. A dispersant is customarily added to the suspension. Suitable dispersants are known to the person skilled in the art. For example, sodium silicate or sodium hexametaphosphate is used with preference as the dispersant when disagglomerating TiO.sub.2 base material in sand mills. The dispersant concentration is customarily in the region of 0.05 to 5.0 kg/mt TiO.sub.2.

(11) The pH value of the suspension is likewise customarily set as a function of the particle type and the dispersant. When disagglomerating TiO.sub.2 base material from the chloride process, for example, the pH value is set to values of roughly 9 to 12, or to values of roughly 2 to 5. The temperature of TiO.sub.2 base material suspensions is customarily roughly 40 to 80 C. Customarily, the suspension is subsequently disagglomerated, e.g. in agitator mills, such as bead mills or sand mills, or in ultrasonic mills.

(12) A preferred embodiments of the method according to the invention for the surface-coating (post-treatment) of inorganic particles is illustrated schematically in FIG. 1. In this preferred embodiment, the disagglomerated, aqueous particle suspension (2) is fed (recirculated) from an intermediate vessel (vessel) (1) in a cyclic process (3). The intermediate vessel contains a high-speed agitator (4). The high-speed agitator has a minimum peripheral speed of 15 m/s or a specific agitator capacity P/V of at least 30 W/m.sup.3, where P=agitator capacity and V=volume of the vessel. The high-speed agitator is, for example, based on the rotor/stator principle and is preferably a jet stream mixer. A jet stream mixer permits intensive mixing on the macro and micro scale. In addition, toothed-disc mixers or high-speed pitched-blade agitators are also suitable.

(13) A pipeline mixer (e.g. inline dispersing mixer) (5) is furthermore installed in the circuit (3), it likewise being based on the rotor/stator principle. The water-soluble precursor compounds of the coating substances (6), and equally any necessary pH-controlling substances (7), are metered into the pipeline mixer (5). Local concentration, pH, viscosity and temperature gradients in the suspension are minimized in this way. In addition, pH sensors (8) are integrated in the circuit (3) and the vessel (1).

(14) Sodium silicate or potassium silicate solution is customarily used as the water-soluble precursor compound for coating the particles with silicon dioxide. It is furthermore also possible to use organometallic compounds, such as alkoxysilanes, as precursor compounds for coating with SiO.sub.2. Silicon dioxide can be precipitated onto the particle surface in the form of a porous coating or a dense skin.

(15) According to the preferred embodiment of the invention, the particles can be coated with the quantities of silicon dioxide customarily used, e.g. with roughly 1 to 20% by weight SiO.sub.2 for TiO.sub.2 pigment particles.

(16) Water-soluble aluminum salts, such as sodium aluminate, aluminum sulfate, aluminum chloride, etc., are customarily used as the precursor compound for coating with aluminum oxide. The person skilled in the art is familiar with such compounds, particularly from the extensive patent literature on the surface-coating (post-treatment) of titanium dioxide.

(17) The particles can be coated with the quantities of aluminum oxide customarily used, e.g. with roughly 0.5 to 10% by weight Al.sub.2O.sub.3 for TiO.sub.2 pigment particles.

(18) Once the suspension has passed through the pipeline mixer (e.g. inline disperser), it is pumped through the intermediate vessel and recirculated in the circuit. Thanks to the improved mixing, the high shear forces introduced by the jet stream mixer in the intermediate vessel likewise help to minimize the local concentration, pH, viscosity and temperature gradients in the suspension.

(19) Different conditions for precipitating the coating substance can be realized. For example, kinetically controlled precipitation of silicon dioxide from sodium silicate can be realized to form a dense skin. The pH-controlling substances are metered into the pipeline mixer (e.g. inline disperser) to this end, and SiO.sub.2 precipitation takes place after a delay during the continuous input of shear energy in the cyclic process. Agglomeration of the TiO.sub.2 particles during the adjustment of the pH value necessary for precipitation is minimized by the high, continuous input of shear energy.

(20) In contrast, precipitation of Al.sub.2O.sub.3 at a fixed pH value, for example, takes place directly after addition of the pH-controlling substances and mixing in the pipeline mixer (e.g. inline disperser).

(21) Use of the preferred method according to the invention additionally makes it possible to operate the coating process with a temperature profile. In this process, the intermediate vessel, for example, has a double jacket, via which the vessel can be heated with steam, or cooled with cooling water or via a heat exchanger. Where appropriate, the temperature of the feed line can also be controlled by means of a heat exchanger.

(22) In a special embodiment of the invention, the dense SiO.sub.2 skin is precipitated onto the particles at a temperature of roughly 85 to 95 C., preferably at roughly 90 C., and the subsequent Al.sub.2O.sub.3 layer at a temperature of roughly 45 to 55 C., preferably at roughly 50 C.

(23) Following precipitation, the suspension is set to a pH value of roughly 5 to 7, pumped off, and the coated particles are separated from the suspension, washed if appropriate, dried and fine-milled by familiar methods. At the end of the procedure, the particles preferably display a pH value of less than 6.5.

(24) The preferred method according to the invention particularly differs from the known methods from the prior art in that a high amount of shear energy is input, not only during addition of the precursor compound and homogenization of the slurry, but also during precipitation of the coating substance, because the slurry is continuously recirculated. This particularly relates to the precipitation of SiO.sub.2 layers, whose formdense or porousis extensively determined by the reaction kinetics.

(25) Compared to the known methods from the prior art, the preferred method according to the invention offers the following advantages:

(26) Improved homogenization of the suspension is possible, owing to the increased shear energy input that can be realized by the high-speed agitator in the intermediate vessel and the pipeline mixer based on the rotor/stator principle (e.g. inline disperser) in the cyclic system. It is furthermore possible to minimize high viscosities that, under other circumstances, lead to thickening of the suspension and corresponding processing difficulties. The high input of shear energy can additionally lead to the disintegration of particle agglomerates that are also formed as a result of the pH profile in the course of the post-treatment process.

(27) With the help of the cyclic operating mode according to the invention, it is possible to realize temperature profiles during post-treatment, e.g. in that the temperature is lowered within a short time during post-treatment. A fixed temperature throughout the process is not absolutely necessary.

(28) Furthermore, two different precipitation zones can be realized. While kinetically controlled precipitation occurs after a delay following metering and mixing, precipitation at a constant pH value can take place immediately after metering in the dispersing head of the pipeline mixer (e.g. inline disperser).

(29) The preferred method according to the invention makes it possible to build up separate, consecutively precipitated layers on the particle surface.

(30) Above and beyond this, the use of two intermediate vessels, installed in parallel, makes it possible to realize an alternating operating mode (pendulum operation mode), this improving the formation of separate layers

(31) In an alternate embodiment of the invention, the solid particles in the suspension are disagglomerated prior to their introduction into the circuit by passing the suspension through a mill, e.g. an agitator mill such as bead mill and sand mill an ultrasonic mill. In this embodiment, the suspension is added to the circuit immediately prior to the pipeline mixer instead of to the vessel. The suspension then passes through the pipeline mixer where the water-soluble precursor compound of a coating substance is metered into the suspension as described above.

(32) The subsequent steps of the method are analogous to those described above. Once the suspension has passed through the pipeline mixer (e.g. inline disperser), it is pumped through the intermediate vessel and recirculated in the circuit. Precipitation of the coating substance is likewise realized by metering pH controlling substances into the pipeline mixer. Finally, the suspension is set to a pH value of roughly 5 to 7, pumped off, and the coated particles are separated from the suspension, washed if appropriate, dried and fine-milled by familiar methods

(33) An additional advantage of this alternate embodiment of the invention is that the need for a dispersing agent can be avoided.

EXAMPLES

(34) The invention is explained in more detail below on the basis of examples, although these are not to be interpreted as a limitation of the invention. The quantities indicated refer to the TiO.sub.2 base material in each case.

Example 1

(35) 224 liters of a suspension of titanium dioxide base material from the chloride process with a solids content of 450 g/l were put into an intermediate vessel (1) with high-speed agitator (4). The suspension was pumped round the circuit (3), through the inline disperser (5) and the vessel (1), at a rate of 500 l/h for 40 min. The suspension was heated to 90 C. during the cyclic process, and the pH value set to 10 by adding NaOH (7).

(36) 2.0% by weight SiO.sub.2 in the form of sodium silicate solution (corresponding to 17.7 liters with an active substance concentration of 115 g/l) was subsequently added (6) within 20 min. Pumping was subsequently continued for a further 20 min., during which time the SiO.sub.2 skin formed on the particle surface.

(37) To lower the pH value, 0.2% by weight TiO.sub.2 in the form of titanyl chloride solution (corresponding to 1.5 liters with an active substance concentration of 140 g/l) was subsequently added (7), followed by 30% HCl, in such a way that a pH value of 7.5 was achieved after 100 min. and a pH value of 4 after 120 min.

(38) The suspension was subsequently cooled to 50 C. within approx. 20 min. 3.0% by weight Al.sub.2O.sub.3 in the form of sodium aluminate solution (corresponding to 10.4 liters with an active substance concentration of 293 g/l) was subsequently added (6) within 30 min., during simultaneous addition (7) of 30% HCl, in such a way that the pH value remained constant at roughly 4.

(39) Finally, the pH value was set to a value in the range from 5 to 8 by adding 0.2% by weight Al.sub.2O.sub.3 in the form of sodium aluminate solution, as well as NaOH.

(40) The suspension was pumped off, filtered, washed, dried and milled in a microniser.

Reference Example 1a

(41) The procedure of Example 1 was repeated, but with the difference that there was no recirculation and that the water-soluble precursor compounds were added directly to the intermediate vessel (1).

Reference Example 1b

(42) Same as Reference Example 1a, but with the difference that a propeller mixer was installed in the intermediate vessel (1), instead of the jet stream mixer.

(43) The TiO.sub.2 pigments obtained were examined under the transmission electron microscope (FIG. 2), and the specific surface area (BET), sulphuric-acid solubility and gloss (HMG) were tested (Table 1).

(44) TABLE-US-00001 TABLE 1 Sulphuric-acid BET solubility Gloss (HMG) Example 1 9.0 m.sup.2/g 10.0% by weight 76 Reference Example 1a 9.5 m.sup.2/g 16.2% by weight 74 Reference Example 1b 10.0 m.sup.2/g 16.0% by weight 72

Example 2

(45) Same as Example 1, but with the difference that 2.8% by weight SiO.sub.2, instead of 2.0% by weight, and 2.3% by weight Al.sub.2O.sub.3, instead of 3.0% by weight, were added in the form of the corresponding precursor compounds.

Reference Example 2

(46) Same as Example 2, but with the difference that there was no recirculation, that the water-soluble precursor compounds were added directly to the intermediate vessel (1), and that a propeller mixer was installed in the intermediate vessel (1), instead of the jet stream mixer.

(47) The TiO.sub.2 pigments obtained were examined under the transmission electron microscope, and the specific surface area (BET) and sulphuric-acid solubility were tested (Table 2).

(48) TABLE-US-00002 TABLE 2 BET Sulphuric-acid solubility Example 2 9.4 m.sup.2/g 5.9% by weight Reference Example 2 11.1 m.sup.2/g 8.2% by weight

Example 3

(49) Same as Example 2, but with the difference that the process was implemented at a consistent temperature of 80 C.

Reference Example 3

(50) Same as Example 3, but with the difference that there was no recirculation, that the water-soluble precursor compounds were added directly to the intermediate vessel (1), and that a propeller mixer was installed in the intermediate vessel (1), instead of the jet stream mixer.

(51) The TiO.sub.2 pigments obtained were examined under the transmission electron microscope (Example 3=FIG. 3; Reference Example 3=FIG. 4), and the specific surface area (BET) and sulphuric-acid solubility were tested (Table 3).

(52) TABLE-US-00003 TABLE 3 BET Sulphuric-acid solubility Example 3 12.2 m.sup.2/g 8.2% by weight Reference Example 3 15.1 m.sup.2/g 12.6% by weight

Test Methods

(53) Specific surface area according to BET (Brunauer, Emmett, Teller):

(54) The BET surface is measured with a Tristar 3000 from Messrs. Micromeritics in accordance with the static volumetric principle.

(55) Sulphuric-Acid Solubility:

(56) A suspension of 500 mg pigment in 25 ml concentrated sulfuric acid (96% by weight) is kept at 175 C. for 60 min. Following filtration, the dissolved TiO.sub.2 in the filtrate is determined by means of ICP atomic emission spectrometry. The lower the concentration of dissolved TiO.sub.2, the more dense the SiO.sub.2 skin on the pigment surface.

(57) Gloss (HMG):

(58) The pigment is dispersed in Alkydal F26 X 60% from Bayer in an automatic muller. A sample of the suspension with a PVC of 27% is applied to a glass plate with a film applicator. After the drawdown has dried, the gloss) (20) is measured with a haze-gloss reflectometer.

(59) The coating of the titanium dioxide particles can be visualized with the help of transmission electron microscopy (TEM).

(60) Compared to methods of the prior art, the preferred method according to the invention leads to surface coatings with greater smoothness (low specific surface area according to BET) and improved density (low sulfuric-acid solubility). In addition, less coating substance is precipitated separately (improved gloss).

(61) The above descriptions of certain embodiments are made for the purpose of illustration only and are not intended to be limiting in any manner. Other alterations and modifications of the invention will likewise become apparent to those of ordinary skill in the art upon reading the present disclosure, and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.