Method and Device for Producing Dispersions

Abstract

A method and device for producing a finely divided dispersion of solids having a mean particle size of 10 nm to 10 µm, in which at least two flows of a predispersion are sprayed by means of pumps, preferably high-pressure pumps, through one nozzle each into a grinding chamber enclosed by a reactor housing onto a collision point, wherein the grinding chamber is flooded with the predispersion and the finaly divided dispersion is removed from the grinding chamber by the overpressure of the predispersion continuing to flow into the grinding chamber.

Description

EXAMPLES

Analytical Method

The mean secondary-particle size was determined with the Zetasizer 3000 Hsa produced by Malvern.

Example of Alox: Aluminum Oxide Predispersion.

36 kg of fully demineralized water are introduced into a 60 1 stainless steel batch tank. 16.5 kg of type C aluminum oxide (supplied by Degussa AG) are sucked in with the aid of an Ystral dispersion and suction mixer (at 4500 rpm) and coarsely predispersed. A pH of 4.5 is established and maintained by adding 50-percent-strength acetic acid during sucking in. After the powder is introduced, the dispersion is completed using an Ystral Type Z 66 rotor/stator continuous homogenizer having four processing rings, a stator slot width of 1 mm and a rotational speed of 11,500 rpm. During this 15-minute dispersion at 11,500 rpm, the pH is adjusted and maintained at a pH of 4.5 by adding further 50-percent-strength acetic acid. A total of 570 g of 50-percent-strength acetic acid was needed and a solids concentration of 30 wt. % was established by adding 1.43 kg of water.

Example of SiO₂: Silicon Dioxide Predispersion

53 kg of fully demineralized water and 80 g of 30%-strength KOH solution are introduced into a 60 1 stainless-steel batch tank. With the aid of an Ystral dispersion and suction mixer (at 4500 rpm), 8 kg of AEROSIL® 90 powder are sucked in and coarsely predispersed. After introducing the powder, the dispersion is completed using an Ystral Type Z 66 rotor/stator continuous homogenizer having four processing rings, a stator slot width of 1 mm and a rotational speed of 11,500 rpm. During this 15-minute dispersion at 11,500 rpm, the pH is adjusted to and maintained at a pH of 9.5 by adding further KOH solution. In this process, a further 96 g of KOH solution was used and an abrasive-body concentration of 12.5 wt. % was established by adding 2.8 kg of water.

Example of Alox 1: Aluminum Oxide Dispersion—Dispersion in the Flooded Grinding Chamber (In Accordance with the Invention)

The predispersion is ground using a Model HJP-25050 high-pressure homogenizer Ultimaizer system supplied by Sugino Machine Ltd, but with a three-jet chamber instead of the two-jet chamber incorporated in the Ultimaizer system. (The Ultimaizer system is used only as a high-pressure pump.) The three-jet chamber divides the predispersion, which is at high pressure, into three subflows that are each decompressed via a diamond (alox 1) nozzle or an alox 2 monocrystalline corundum (colourless sapphire) nozzle having a diameter of 0.25 mm. The three dispersion jets emerging at a very high velocity meet at a collision point, in which process the desired dispersion/grinding effect is achieved. The collision point is tetrahedrally surrounded by sapphire balls (three base balls each of 8 mm and an upper ball of 10 mm). Since all three liquid jets are situated on a common imaginary plane, the angle with respect to the adjacent beam is 120° in each case. 250 MPa is chosen as the pressure for the grinding of the aluminum oxide predispersion. The dispersion can then be cooled without difficulty with the aid of a conventional heat exchanger. The mean particle size of the particles in the dispersion is 51 nm.

The example of alox 2 is performed analogously to alox 1,but using sapphire as nozzle and ball material. The mean particle size of the particles in the dispersion is 55 nm.

Example of SiO₂ 1: Silicon Dioxide Dispersion—Dispersion in the Flooded Grinding Chamber (In Accordance with the Invention)

The predispersion is ground with a Model HJP-25050 Ultimaizer system high-pressure homogenizer supplied by Sugino Machine Limited, but using a three-jet chamber instead of the two-jet chamber incorporated in the Ultimaizer system. (The Ultimaizer system is used only as a high-pressure pump.) The three-jet chamber divides the predispersion, which is at high pressure, into three subflows that are each decompressed via a nozzle having a diameter of 0.25 mm. The three dispersion jets emerging at very high velocity meet at a collision point, in which process the desired dispersion/grinding effect is achieved. The collision point is tetrahedrally surrounded by polycrystalline Si₃N₄ balls (three base balls each of 8 mm and an upper ball of 10 mm). Since all three liquid jets are situated on a common imaginary plane, the angle with respect to the adjacent jet is 12020 in each case. 250 MPa is chosen as the pressure for grinding the silicon dioxide predispersion. The dispersion can then be cooled without difficulty with the aid of a conventional heat exchanger. The mean particle size of the particles in the dispersion is 163 nm.

The values in the table show that, in the method according to the invention, the dispersion in the flooded grinding chamber results in service lives of the nozzle and ball materials that are comparable to those in a method in which the dispersion is performed in a gas-filled grinding chamber. The particle size achieved is virtually the same.

The wear of the nozzle material can easily be determined from the increasing throughput performance. With as-new nozzles, that is to say an initial nozzle diameter of 0.25 mm and the use of a three-jet chamber, a throughput of approximately 4.3 l/minute is achieved at a pressure of 250 MPa. With progressive wear, the nozzle aperture becomes increasingly greater; the throughput rises. This rise of the throughput performance is, however, limited by the performance of the high-pressure pump. For the same grinding pressure, more predispersion has increasingly to be compressed. Depending on the performance of the high-pressure pump used, the desired pressure cannot, however, be maintained from a certain throughput upwards and the performance limit of the high-pressure pump is reached. In the unit used here, this is the case at approximately 7.3 l/min.

It furthermore also has to be borne in mind that the alignment also does not always remain constant in the case of nozzle apertures that are too considerably expanded since the increase in the nozzle aperture does not occur with radial symmetry. Depending on the alignment of the normally monocrystalline nozzle material, an isotropic dependence of the wear resistance of various crystalline planes may be observed. Thus, in the case of considerably eroded diamond nozzles, hexagonal or even triangular nozzle apertures are obtained.

Since the balls are substantially subjected to stress to a lesser extent than the nozzles since, of course, most of the kinetic energy of the accelerated liquid jets is used up as fragmentation energy and/or transformed into heat at the collision point, it is sufficient for the balls to be inspected when the diamond nozzles are replaced. Incipient wear can easily be detected from a roughening of the ball surface. The balls can then be replaced as a precaution. Since such balls are used to a large extent as, for example, ball-bearing balls in the special ball bearing sector (“chemistry pumps” etc.), a timely replacement is not a large cost factor.

TABLE
Service life of nozzles/balls of the dispersing
device(&).
	Material service life
					Balls
					[service
					life of
	Substance	Material		Nozzle	nozzle
Example	dispersed	Nozzle	Balls	[h]	x]
Alox 1	AEROXIDE ®	Diamond	Sapphire	195	min.
	Alu C(#)				10(§)
Alox 2	AEROXIDE ®	Sapphire	Sapphire	55	min. 40
	Alu C
SiO2 1	AEROSIL ®	Diamond	Si3N4	350	min. 20
	90(*)
(&)Dispersion pressure 250 MPa;
(#)Degussa pyrogenically produced aluminium oxide;
(*)Degussa pyrogenically produced silicon dioxide;
(§)Service life of nozzle x at least 10: at least 10x the service life of the nozzle material, lines 2 and 3 correspondingly.

Claims

1. Method of producing a finely divided dispersion of solids having a mean particle size of 10 nm to 10 μm, in which at least two flows of a predispersion are sprayed by means of pumps, preferably high-pressure pumps, through one nozzle each into a grinding chamber enclosed by a reactor housing onto a collision point, characterized in that the grinding chamber is flooded with the predispersion and the finaly finely divided dispersion is removed from the grinding chamber by the overpressure of the predispersion continuing to flow into the grinding chamber.

2. Method according to claim 1, characterized in that the liquid phase of the predispersion is aqueous.

3. Method according to claim 1, characterized in that the predispersion contains dispersing agents and/or surfactants.

4. Method according to claim 3, characterized in that the proportion of solids in the predispersion is between 1 and 70 wt. %.

5. Method according to claim 4, characterized in that the predispersion is sprayed into the grinding chamber at a pressure of at least 50 bar.

6. Method according to claim 5, characterized in that the dispersion is cooled after leaving the grinding chamber.

7. Method according to claim 1, characterized in that the finely divided dispersion obtained after leaving the grinding chamber is sprayed into the grinding chamber several times.

8. Method according to claim 4, characterized in that organic particles, inorganic particles and/or mixtures thereof are used as solids.

9. Device for performing the method in accordance with claim 1, characterized in that a predispersion is sprayed by means of at least two nozzles each having an associated pump and feeding into a grinding chamber surrounded by a reactor housing onto a common collision point and the dispersion leaves the grinding chamber through an opening in the reactor housing.

10. Device according to claim 9, characterized in that the nozzles can be aligned with a common collision point.

11. Device according to claim 9, characterized in that the nozzles are composed of oxides, carbides, nitrides, diamond or mixtures thereof.

12. Device according to claim 9, characterized in that the nozzles have bores having a diameter of 0.5-2000 μm.

13. Device according to claim 9, characterized in that the nozzles are identical in their chemical composition with the substance to be dispersed or become identical as a result of chemical reaction under the dispersion conditions.

14. Device according to claim 9, characterized in that the collision point is surrounded by a material that is disposed in such a way that, in the event of a misalignment of the nozzles, the predispersion jet collides with said material.

15. Device according to claim 14, characterized in that the material surrounding the collision point is identical in its chemical composition to the substance to be dispersed or becomes identical as a result of chemical reaction under the dispersion conditions.