Method and plant for producing hollow microspheres made of glass

Abstract

A process and a plant produce hollow microspheres made of glass. According to the process an aqueous suspension is prepared from starting materials containing glass powder and water glass, feedstock particles having a diameter between 5 μm and 300 μm, in particular between 5 μm and 100 μm, being produced from the suspension. The feedstock particles are mixed with a pulverulent release agent made of aluminum hydroxide in an intensive mixer. The mixture of feedstock particles and release agent is subsequently introduced into a firing chamber of a furnace. The feedstock particles expand in the firing chamber, at a firing temperature which exceeds the softening temperature of the glass powder, to form the hollow microspheres.

Claims

1. A method for producing hollow microspheres made of glass, which comprises the steps of: preparing an aqueous suspension of starting substances containing glass powder and water glass; producing firing material particles having diameters lying between 5 micrometers and 300 micrometers from the aqueous suspension of starting substances; mixing the firing material particles with a powdered release agent made of aluminum hydroxide in an intensive mixer; and introducing a mixture of the firing material particles and the powdered release agent into a firing chamber of a kiln, the firing material particles expanding to form the hollow microspheres in the firing chamber at a firing temperature which exceeds a softening temperature of the glass powder.

2. The method according to claim 1, wherein the glass powder which is used for production of the aqueous suspension of starting substances is ground to a fineness of D.sub.97≤47 μm.

3. The method according to claim 1, wherein the powdered release agent made of the aluminum hydroxide is present with a fineness of D.sub.90≤35 μm.

4. The method according to claim 1, wherein a batch of the firing material particles and the powdered release agent is mixed intensively for a mixing time of from 10 seconds to 10 minutes.

5. The method according to claim 4, which further comprises producing the batch of the firing material particles and the powdered release agent with a release agent proportion of from 5 mass % to 25 mass %.

6. The method according to claim 4, which further comprises producing the batch of the firing material particles and the powdered release agent with a release agent proportion of 17 mass %.

7. The method according to claim 1, which further comprises using an Eirich intensive mixer for mixing the firing material particles with the powdered release agent.

8. The method according to claim 1, which further comprises carrying out the expanding in a rotary kiln.

9. The method according to claim 1, which further comprises carrying out the expanding in a shaft kiln.

10. The method according to claim 1, wherein the diameters lie between 5 micrometers and 100 micrometers.

11. The method according to claim 1, wherein the glass powder which is used for production of the aqueous suspension of starting substances is ground to a fineness of D.sub.97≤35.

12. The method according to claim 1, wherein the powdered release agent made of aluminum hydroxide is present with a fineness of D.sub.90≤5 μm.

13. The method according to claim 1, wherein a batch of the firing material particles and the powdered release agent is mixed intensively for a mixing time of about 5 minutes.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a schematically simplified representation of a plant for producing hollow microspheres made of glass, having an intensive mixer for mixing firing material particles with a powdered release agent made of aluminum hydroxide, and having a firing kiln configured as a rotary kiln, into which the mixture of firing material particles and release agent is introduced, so that the firing material particles are expanded to form the desired hollow microspheres;

(2) FIG. 2 is an illustration according to FIG. 1 of an alternative embodiment of the plant, in which the firing kiln is configured as a shaft kiln; and

(3) FIGS. 3-5 are microscopy images of hollow microspheres which are produced by the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) Parts and structures which correspond to one another are always provided with the same references in all the figures.

(5) Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a plant 1 for producing hollow microspheres M made of glass, i.e. for producing hollow glass spheres, the typical diameters of which lie in a range of between 7 and 500 micrometers.

(6) The plant 1 contains a first silo 2 as a storage container for firing material particles G, and a second silo 3 has a storage container for powdered release agent T made of aluminum hydroxide (Al(OH).sub.3). The plant 1 furthermore contains an intensive mixer 5 for mixing the firing material particles G with the release agent T, as well as a firing kiln configured as a rotary kiln 6 for expansion of the firing material particles G to form the desired hollow microspheres M.

(7) The firing material particles G stored in the first silo 2 are approximately spherical particles, the diameters of which lie in the range of between 5 micrometers and 300 micrometers. The firing material particles G are in this case produced by a spray-drying process. To this end, (recycled) glass powder, water glass and a blowing agent as starting substances are prepared with water to form a suspension (slurry).

(8) In order to produce a lower fraction of the firing material particles G (i.e. to produce firing material particles G with diameters which tend to be smaller) in tests a particularly fine glass powder was used for the preparation, which is present for example with a particle size distribution that is characterized by the following D values: D.sub.10=2.274 μm, D.sub.50=9.342 μm, D.sub.90=24.13 μm and D.sub.97=34.95 μm, respectively 10, 50, 90 and 97% by volume of the particles having a diameter for each D value which is less than or equal to the value respectively indicated. A glass powder used experimentally in order to produce an upper fraction of the firing material particles G (to produce firing material particles G which tend to be larger) was characterized by the following D values: D.sub.10=3.195 μm, D.sub.50=14.44 μm, D.sub.90=35.63 μm and D.sub.97=46.60 μm. The particle size distributions above were determined by measuring a random sample of the respective glass powder with a particle size analyzer from the company Beckman Coulter.

(9) The suspension is particulated and dried in a spray tower to form the firing material particles G with the desired diameters. Optionally, classification is carried out after the spraying process, the fraction with the desired diameters being selected and fed to the silo 2. The firing material particles G may, however, be produced in a different way, for instance with the aid of a granulator plate and subsequent drying of the granulate, or with the aid of a compacting device, the resulting granulate being broken down to the desired size after drying. A subsequent classification step is typically also provided in the alternative embodiments.

(10) The release agent T stored in the second silo 3 is for example the powdered product Apyral 40CD from the company Nabaltec, which is marketed as a flame retardant. The release agent T is distinguished by an aluminum hydroxide content of 99.5%. The particle size distribution of the powder is characterized by the values D.sub.10=0.6 micrometers, D.sub.50=1.3 micrometers and D.sub.90=3.2 micrometers.

(11) The intensive mixer 5 is configured as an Eirich intensive mixer. The intensive mixer 5 contains an essentially cup-shaped mixing container 10, which is mounted rotatably about its longitudinal axis 11, which stands obliquely to the vertical. A mixing tool 12, rotatable counter to the mixing container 10, is arranged parallel to the longitudinal axis 11, eccentrically in the mixing container 10. The mixing container 10 can be charged through a closable top opening 15, and can be emptied through a likewise closable centrally arranged bottom opening 16. For example, an Eirich intensive mixer is used which has a power input of from 10 to 20 kilowatts per 100 kg of mixing material (preferably about 15 kilowatts per 100 kg of mixing material) and a circumferential speed at the outermost point of the stirring tool of at least 30 meters per second.

(12) In the conventional way, the rotary kiln 6 contains an elongated hollow-cylindrical rotary tube 20 made of refractory steel, in the interior of which a firing chamber 21 is formed. The rotary tube 20 is mounted rotatably about its longitudinal axis 23, which is arranged slightly inclined relative to the horizontal. The rotary tube 6 is for the most part housed by a stationary enclosure 25, so that an annular gap is formed between the rotary tube 20 and the enclosure 25. The annular gap is fired by means of a plurality of gas-operated burners 26 which are arranged distributed over the length of the rotary tube 20, so that the firing chamber 21 is heated indirectly via the lateral surface of the rotary tube 20.

(13) During operation of the plant 1, firing material particles G and release agent T are dosed from the two silos 2, 3 to a mixing trough 30 arranged below the silos 2, 3, so that in the latter there is a premix of firing material particles G and release agent T with a release agent proportion of 10 mass %. The adjustment of the desired mass ratio may, for example, be carried out by means of a balance, although it is also possible for the adjustment to be carried out volumetrically, for example by means of rotary valves or conveying screws assigned to the silos 2, 3. Through the mixing trough 30, the premix of firing material particles G and release agent T is conveyed into the mixing container 10 of the intensive mixer 5.

(14) As an alternative to the exemplary representation, the mixing trough 30 may also be omitted, in which case firing material particles G and release agent T are respectively dosed separately into the intensive mixer 5, so that the desired mixing ratio is produced therein.

(15) The mixing process is carried out batchwise, a batch of the premix respectively being subjected to a mixing process. The premix of the release agent T and firing material particles G is homogenized for a mixing time of 5 minutes in the intensive mixer 5. After the end of the mixing process, the mixture of firing material particles G and release agent T is extracted from the mixing container 10 through the bottom opening 16. The mixture is stored in a buffer container (not explicitly represented) connected between the intensive mixer 5 and the rotary kiln 6.

(16) From the buffer container, the mixture of firing material particles G and release agent T is fed continuously to a charging device (not explicitly represented here) of the firing chamber 21 of the rotary kiln 6 (this is indicated by an arrow 31). In the firing chamber 21, during operation of the plant 1, a firing temperature which exceeds the softening temperature of the glass powder, and is for example 960° C.) is generated by means of the burners 26, at which temperature the firing material particles G expand successively to form the desired hollow microspheres M in a period of time from about 4 to 10 minutes. The hollow microspheres M are extracted from the firing chamber 21 and, after a cooling and sorting step, fed to a product reservoir (not represented here). The release agent T is separated from the hollow microspheres M by screening or air separation. The release agent oxidized during the firing process to form aluminum oxide (Al.sub.2O.sub.3, corundum) is optionally partially reused, in which case the release agent may then in particular be admixed to the firing material particles G in the conventional way at the start of the firing process. Furthermore, the hollow microspheres M, again by screening or air separation, are separated from multicellularly (in foam fashion) expanded particles (i.e. particles having a plurality of cavities) which occur in a small extent besides the hollow microspheres M during the firing process. These multicellularly expanded particles are either disposed of as rejects or delivered for a different use.

(17) FIG. 2 shows an alternative embodiment of the plant 1. In contrast to the first embodiment, in this case the expansion process is carried out in a shaft kiln 40 rather than in a rotary kiln.

(18) The shaft kiln 40 comprises a shaft-like, elongated firing chamber 41 oriented vertically in terms of its longitudinal extent, which is enclosed by a double wall 42 made of steel, which is thermally insulated outward. Cooling air K is guided in a cooling gap 43 formed by the double wall 42. The firing chamber 41 is widened in stages upward.

(19) The shaft kiln 40 is assigned a gas-operated burner 45, which is used to generate a hot gas flow H directed from the bottom upward in the firing chamber 41. To this end, the hot gas generated by the burner 45 is fed through a hot gas line 46 to the firing chamber 41 as the hot gas flow H. Approximately halfway up the firing chamber 41, i.e. in the region of the above-described cross-sectional widening, a number of (for example six) additional gas-operated burners 47 are arranged, which are positioned distributed in the manner of a ring around the circumference of the firing chamber 41.

(20) Above the firing chamber 41, according to FIG. 2 there follows a region which is used as a cooling trap 50 and has a further widened cross section relative to the cross section of the upper section of the firing chamber 41. As an alternative, the firing chamber 41 and the optionally provided cooling trap 50 can be configured with a uniform cross section over their entire height.

(21) Lastly, the shaft kiln 40 contains a charging device, which is formed here by a firing material line 55. The firing material line 55 is fed through the double wall 42 and opens into the lower section of the firing chamber 41. The firing material line 55 is fed from the buffer container downstream of the intensive mixer 5 (this is indicated by the arrow 56). The firing material line 55 extends, in particular, sloping downward in the charging direction, so that the firing material slides into the firing chamber 41 without active conveying (merely under the effect of the force of gravity). Optionally, however, the charging device may also comprise means for actively conveying the firing material, for example a compressed-air system or a conveying screw.

(22) During operation of the plant 1, in the present exemplary embodiment the homogeneous mixture of firing material particles G and release agent T are conveyed continuously by means of the firing material line 55 into the firing chamber 41, where they are collected and carried upward by the hot gas flow H.

(23) In this case, a temperature of for example 650° C., to which the firing material particles G, initially preheated, is generated in the lower section of the firing chamber 41. The firing chamber 41 is additionally heated by the burners 47, so that the temperature in the upper section of the firing chamber 41 is increased to the firing temperature which exceeds the softening temperature of the glass powder. The expansion of the firing material particles G to form the hollow microspheres M takes place in brief flame contact at about 1400° C. in this case.

(24) The expanded hollow microspheres M are lastly fed to the cooling trap 50, and quenched there by supply of cooling air K. Lastly, the hollow microspheres M are separated from the hot gas flow by a solids separator, and fed optionally after a sorting step to a product reservoir (again not represented here). The jointly extracted release agent T and any rejects (in particular multicellularly expanded particles) are in turn separated from the hollow microspheres M by means of a cyclone.

(25) Tests have revealed that the release effect substantially improved in comparison with conventional methods is attributable both to the use of aluminum hydroxide as the release agent T and to the intensive mixing of the firing material particles G with the release agent T, and in particular is achieved only by the combination of both method features.

(26) FIGS. 3 to 5 respectively represent microscopy images of hollow microspheres M produced according to the invention at different magnifications. The respective magnification can be seen from the scale respectively indicated on the bottom right in the figure. These hollow microspheres M were produced in a plant 1 according to FIG. 1, an Eirich intensive mixer on an industrial scale being used as the intensive mixer 5. The mixing process was in this case carried out for 5 minutes with a maximum stirrer speed of about 31.4 meters per second—here, the circumferential speed of the mixing tool 12 at its greatest diameter is indicated—with a counter-rotational speed of the mixing container 10 of 35 revolutions per minute. The mixer in this case had a power input of 2.4 kilowatts for 10 liters of mixing material.

(27) FIG. 3 and FIG. 4 show in particular the outstanding sphericity of the hollow microspheres M. It can furthermore be seen from FIG. 3 and FIG. 4 that the expanded particles produced by the proposed method are almost exclusively present as hollow spheres, i.e. in the form of spheres in which an individual central cavity is enclosed by a relatively thin wall. Virtually no multicellular particles, which have a plurality of cavities of comparable size in their core, are produced. It can in this case be seen from FIG. 4 that even the smallest particles are present as hollow spheres, which is shown below by way of example by two dimensioned hollow microspheres M with diameters of 10.6 μm and 9.5 μm, respectively. On the other hand, large hollow microspheres with diameters of between 300 and 500 micrometers were also produced by the method described above.

(28) FIG. 5 shows the relatively rough surface of the hollow sphere wall 60, which is typical of hollow microspheres M produced according to the invention. Furthermore, the bubbles 61 incorporated in the hollow sphere wall 60, or bubbles 61 attached to the hollow sphere wall 60, can be seen, the diameters of which are less by a multiple than the diameter of the central cavity.

(29) The invention is made particularly clear with the aid of the exemplary embodiments described above, but equally is not restricted to them. Rather, further configurational forms of the invention may be derived from the claims and the description above.

(30) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 plant 2 silo 3 silo 5 intensive mixer 6 rotary kiln 10 mixing container 11 longitudinal axis 12 mixing tool 15 top opening 16 bottom opening 20 rotary tube 21 firing chamber 23 longitudinal axis 25 enclosure 26 burner 30 mixing trough 31 arrow 40 shaft kiln 41 firing chamber 42 double wall 43 cooling gap 45 burner 46 hot gas line 47 burner 50 cooling trap 55 firing material line 56 arrow 60 hollow sphere wall 61 bubble G firing material particles H hot gas flow K cooling air M hollow microspheres T release agent