HOLLOW GLASS MICROSPHERES AND METHOD FOR PRODUCING SAME

Abstract

Hollow glass microspheres are produced. An aqueous suspension is prepared of starting materials of finely ground glass and waterglass. Firing material particles are produced from the suspension and are mixed with a pulverulent release agent. The mixture of firing material particles and release agent is introduced into a firing chamber of a furnace where it is expanded at a firing temperature which exceeds the softening temperature of the finely ground glass, to form the hollow microspheres. The release agent is Al(OH).sub.3 and dehydroxylated kaolin.

Claims

1. A method of producing hollow glass microspheres, the method comprising: preparing an aqueous suspension of starting materials of finely ground glass and waterglass; producing firing material particles from the suspension; forming a mixture by mixing the firing material particles with a pulverulent release agent containing Al(OH).sub.3 and dehydroxylated kaolin; introducing the mixture of firing material particles and release agent into a firing chamber of a furnace; and expanding the firing material particles in the firing chamber at a firing temperature which exceeds a softening temperature of the finely ground glass, to form the hollow microspheres.

2. The method according to claim 1, wherein: a fraction of Al(OH).sub.3 in the mixture of firing material particles and release agent is between 7 wt % and 30 wt %; and a fraction of dehydroxylated kaolin in the mixture of firing material particles and release agent is between 2 wt % and 15 wt %.

3. The method according to claim 2, wherein: the fraction of Al(OH).sub.3 in the mixture is between 10 wt % and 25 wt %; and the fraction of dehydroxylated kaolin in the mixture is between 5 wt % and 10 wt %.

4. The method according to claim 1, wherein at least 90% of the Al(OH).sub.3 particles in the release agent have a particle diameter of less than 4 micrometers.

5. The method according to claim 4, wherein at least 90% of the Al(OH).sub.3 particles in the release agent have a particle diameter of less than 3.5 micrometers.

6. The method according to claim 1, wherein at least 90% of the dehydroxylated kaolin particles in the release agent have a particle diameter of less than 5 micrometers.

7. The method according to claim 6, wherein at least 90% of the dehydroxylated kaolin particles in the release agent have a particle diameter of less than 4 micrometers.

8. The method according to claim 1, which comprises mixing the firing material particles with the pulverulent release agent in an intensive mixer.

9. The method according to claim 1, wherein the furnace is a rotary tube furnace.

10. The method according to claim 9, wherein the furnace is a directly heated rotary tube furnace.

11. The method according to claim 1, which comprises setting the firing temperature to a value between 800 C. and 980 C.

12. The method according to claim 11, which comprises setting the firing temperature to a value between 830 C. and 940 C.

13. A multiplicity of hollow glass microspheres, each microsphere comprising a glass wall surrounding a central cavity and having been obtained by the method according to claim 1.

14. A release agent, comprising Al(OH).sub.3 and dehydroxylated kaolin in pulverulent form, wherein the Al(OH).sub.3 and the dehydroxylated kaolin, in a mixture with firing material particles formed from an aqueous suspension of starting materials of finely ground glass and waterglass act as a release agent in the production of hollow microspheres.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0036] FIG. 1 is a greatly simplified schematic illustration of a plant for producing hollow glass microspheres according to the invention; and

[0037] FIG. 2 in a representation in accordance with FIG. 1, shows an alternative embodiment of the plant, in which the combustion furnace is implemented as a shaft furnace.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a plant for producing hollow glass microspheres, having a mixer for mixing firing material particles with a pulverulent release agent composed of Al(OH).sub.3 in a mixture with dehydroxylated kaolin, and also having a combustion furnace, implemented as a rotary tube furnace, 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

[0039] FIG. 1 shows a plant 1 for producing hollow glass microspheres M, i.e., for producing hollow glass spheres whose typical diameters are predominantly, for example, in a range of between 40 and 350 micrometers.

[0040] The plant 1 comprises a first silo 2, which forms a reservoir vessel for firing material particles G, and also a second silo 3, which forms a reservoir vessel for pulverulent release agent T. Additionally, the plant 1 comprises a mixer 5 for mixing the firing material particles G with the release agent T, and also a combustion furnace, implemented as a rotary tube furnace 6, for expanding the combustion particles G to form the desired hollow microspheres M.

[0041] The firing material particles G stored in the first silo 2 are approximately spherical particles whose diameters are, preferably, approximately in the range between 20 micrometers and 200 micrometers (m). The firing material particles G are produced preferably by spray granulation. Starting materials for that process, comprising finely ground glass, waterglass, and an expander (e.g., soda niter, sugar, or glycerol), are used to prepare a highly mobile suspension (slip) with water, and this suspension is sprayed in a spraying tower in order to form the firing material particles G. The firing material particles G are subsequently dried. Drying is followed optionally by classifying, where the fraction having the desired diameters is selected and supplied to the silo 2.

[0042] In the embodiment of the plant 1 that is shown, the mixer 5 is implemented as an Eirich intensive mixer. The mixer 5 in this case comprises a substantially cup-shaped mixing vessel 10, which is mounted rotatably about its longitudinal axis 11, which is inclined relative to the vertical. A mixing tool 12, which is rotatable counter to the mixing vessel 10, is arranged eccentrically in the mixing vessel 10, in parallel to the longitudinal axis 11. The mixing vessel 10 can be charged by way of a closable lid opening 15 and can be emptied via a likewise closable and centrally disposed base opening 16. In exemplary sizing, the mixer 5 in this embodiment has a power input of 10 to 20 kilowatts per 100 kg mixture (preferably about 15 kilowatts per 100 kg mixture) and a peripheral velocity at the outermost point of the stirring tool of at least 30 meters per second. In alternative embodiments, however, the plant 1 may also include a different kind of mixer, such as a drum mixer, for example.

[0043] The rotary tube furnace 6 conventionally comprises an elongated, hollow-cylindrical rotary tube 20 made from steel which is resistant to high temperatures, with a firing chamber 21 formed in the interior of the tube. The rotary tube 20 is mounted rotatably about its longitudinal axis 23, which is arranged with a slight incline relative to the horizontal. As shown, the rotary tube furnace is designed as a directly heated rotary tube furnace. The firing chamber 21 in this case is fired directly with a gas-operated burner 26, which is disposed at the output end of the rotary tube 20.

[0044] In the operation of the plant 1, firing material particles G and release agent T are metered from the two silos 2, 3 onto a mixing chute 30 which is disposed beneath the silos 2, 3, so that at that point there is a premix composed of firing material particles G and release agent T, with a specified release agent fraction. The desired mass ratio is set by means of a balance, for example. Alternatively, the setting is performed volumetrically, by means of conveying screws or star wheels assigned to the silos 2, 3, for example. Via the mixing chute 30, the premix of firing material particles G and release agent T is conveyed into the mixing vessel 10 of the mixer 5.

[0045] Alternatively to the exemplary representation, there may also be no mixing chute 30, in which case firing material particles G and release agent T are each metered separately into the mixer 5, so that the desired mixing ratio is generated there.

[0046] The mixing procedure takes place batchwise, with one batch of the premix being subjected to a mixing procedure in each case. The premix of release agent T and firing material particles G is homogenized in the mixer 5 for a mixing time of 1 to 10 minutes. After the end of the mixing procedure, the mixture of firing material particles G and release agent T is discharged from the mixing vessel 10 via the base opening 16. The mixture is optionally stored in a buffer vessel (not shown explicitly) which is placed between the mixer 5 and the rotary tube furnace 6.

[0047] From the mixing chute 30 or the optional downstream buffer vessel, the mixture of firing material particles G and release agent T is supplied continuously, by means of a charging facility which is not shown explicitly here, to the firing chamber 21 of the rotary tube furnace 6 (indicated by an arrow 31). In the firing chamber 21, in the operation of the plant 1, the burner 26 is used to generate a specified firing temperature, at which the firing material particles G undergo successive expansion to form the desired hollow microspheres M within a period of around 1 to 15 minutes.

[0048] The hollow microspheres M produced are discharged from the firing chamber 21 and, after a cooling and sorting step, are supplied to a product reservoir (not shown here). The release agent T is separated from the hollow microspheres M by sieving or pneumatic classifying. Optionally, again by sieving or pneumatic classifying, the hollow microspheres M are separated from particles which have undergo multicellular (foamlike) expansion (that is, particles having a plurality of large cavities), which may be formed during the firing process alongside the hollow microspheres M. These multicellularly expanded particles are either discarded as rejects or supplied for an alternative use.

[0049] FIG. 2 shows an alternative embodiment of the plant 1. In contrast to the first embodiment, the expansion process here is carried out not in a rotary tube furnace but instead in a shaft furnace 40.

[0050] The shaft furnace 40 comprises a firing chamber 41 which is extended in the manner of a shaft and aligned vertically with respect to the longitudinal extent, this chamber 41 being surrounded by a double jacket 42 of steel that is insulated thermally with respect to the outside. Cooling air K is guided in a cooling gap 43 which is formed by the double jacket 42. Toward the top, the firing chamber 41 is widened in a steplike manner.

[0051] Assigned to the shaft furnace 40 is a gas-operated burner 45, which is used to generate a hot gas stream H, within the firing chamber 41, that is directed from bottom to top. For this purpose, the hot gas generated by the burner 45 is supplied via a hot gas line 46 to the firing chamber 41 as hot gas stream H. At approximately half the height of the firing chamber 41, specifically in the region of the above-described cross-sectional widening, there are a number (six, for example) of additional gas-operated burners 47, which are positioned in a crownlike distribution around the periphery of the firing chamber 41.

[0052] Adjoining the firing chamber 41 at the top, according to FIG. 2, is a region which serves as a cold trap 50 and which has a cross section widened further relative to the cross section of the upper portion of the firing chamber 41. Alternatively, the firing chamber 41 and also the optional cold trap 50 may be implemented with a uniform cross section over the whole of their height.

[0053] The shaft furnace 40, finally, comprises a charging facility, formed in this case by a combustibles line 55. The combustibles line 55 is passed through the double jacket 42 and opens into the lower portion of the firing chamber 41. The combustibles line 55 is fed from the mixer 5 or from an optionally downstream buffer vessel (indicated by the arrow 56). The combustibles line 55 runs in particular with a descent in the charging direction, so that without active conveying (merely under the action of gravity) the combustible material slides into the firing chamber 41. Optionally, however, the charging facility may also comprise means for the active conveying of the combustible materialfor example, a compressed air system or a conveying screw.

[0054] In the operation of the plant 1, in the exemplary embodiment above, the homogeneous mixture of firing material particles G and release agent T is conveyed continuously by means of the combustibles line 55 into the firing chamber 41, where it is captured by the hot gas stream H and carried upward.

[0055] In the lower portion of the firing chamber 41, a temperature is generated of around 650 C., for example, at which the firing material particles G are first of all preheated. The firing chamber 41 is additionally heated by the burners 47, and so the temperature in the upper portion of the firing chamber 41 is increased to the firing temperature which exceeds the softening temperature of the finely ground glass. The expansion of the firing material particles G to form the hollow microspheres M takes place here in brief flame contact at approximately 1400 C.

[0056] The expanded hollow microspheres M are supplied, finally, to the cold trap 50, where they are quenched by supply of cooling air K. Finally, the hollow microspheres M are isolated from the hot gas stream via a solids separator, and, optionally after a sorting step, they are supplied to a product reservoir (again not shown here). The entrained release agent T is separated in turn from the hollow microspheres M by means of a cyclone.

EXAMPLE 1 (INVENTION)

[0057] 91 wt % of finely ground used glass (d.sub.9750 m), 7 wt % of sodium silicate and 2 wt % of soda niter were admixed with water to produce a highly mobile slip, which was subsequently granulated in a spraying tower. For the present example, the fine particle fraction of the sprayed granules was employed, this fraction being discharged from the spraying tower with the air stream and deposited in a downstream cyclone. The firing material particles thus obtained have a particle size distribution of d.sub.1030 m, d.sub.5080 m and d.sub.90175 m.

[0058] The dried firing material particles were mixed for five minutes in an Eirich intensive mixer with the release agent, composed of Al(OH).sub.3 (particle size distribution: d.sub.10=0.6 m; d.sub.50=1.3 m; d.sub.90=3.2 m; purity: 99.5% and metakaolin (particle size distribution: d.sub.10=1 m; d.sub.50=2 m; d.sub.90=10 m) in the following proportions:

[0059] 70 wt % firing material particles

[0060] 30 wt % release agent (20 wt % Al(OH).sub.3 and 10 wt % metakaolin)

[0061] This mixture was subsequently expanded in a directly heated rotary tube furnace (production scale). In this and all the experiments described below, the firing temperature was varied during the progress of the experiment, until hollow microspheres were produced (at the firing temperatures stated; in the case of inventive example 1, at a firing temperature of 816 C.).

[0062] This experiment produced hollow microspheres with good product quality in the fractions having sphere diameters in the 40-90 m and 90-180 m ranges. Hollow microspheres of the fraction having sphere diameters in the 180-300 m range, however, were not fully foamed. No agglomeration was observed.

EXAMPLE 2 (INVENTION)

[0063] The firing material particles produced in the same way as for inventive example 1 were again mixed for five minutes in an Eirich mixer with the release agent, which again consisted of Al(OH).sub.3 (as in inventive example 1) and metakaolin (as in inventive example 1) in the following proportions:

[0064] 70 wt % firing material particles

[0065] 30 wt % release agent (25 wt % Al(OH).sub.3 and 5 wt % metakaolin)

[0066] This mixture was subsequently expanded in a directly heated rotary tube furnace (production scale) at a firing temperature of 780-840 C.

[0067] In this experiment, hollow microspheres with good product quality were obtainable in the fractions having sphere diameters in the 40-90 m, 90-180 m, and 180-300 m ranges. No agglomeration was observed.

EXAMPLE 3 (INVENTION)

[0068] The firing material particles produced in the same way as for inventive example 1 were again mixed for five minutes in an Eirich mixer with the release agent, which again consisted of Al(OH).sub.3 (as in inventive example 1) and metakaolin (as in inventive example 1) in the following proportions:

[0069] 70 wt % firing material particles

[0070] 30 wt % release agent (10 wt % Al(OH).sub.3 and 20 wt % metakaolin)

[0071] This mixture was subsequently expanded in a directly heated rotary tube furnace (production scale) at a firing temperature of 814 C.

[0072] In this experiment, hollow microspheres with good product quality were obtainable in the fractions having sphere diameters in the 40-90 m and 90-180 m ranges. Hollow microspheres of the fraction having sphere diameters in the 180-300 m range, however, were not fully foamed. Moreover, agglomeration was observed.

EXAMPLE 4 (INVENTION)

[0073] The firing material particles produced in the same way as for inventive example 1 were again mixed for five minutes in an Eirich mixer with the release agent, which again consisted of Al(OH).sub.3 (as in inventive example 1) and calcined kaolin (d.sub.10=1 m; d.sub.50=2 m; d.sub.90=10 m) in the following proportions:

[0074] 70 wt % firing material particles

[0075] 30 wt % release agent (25 wt % Al(OH).sub.3 and 5 wt % calcined kaolin)

[0076] This mixture was subsequently expanded in a directly heated rotary tube furnace (production scale) at a firing temperature of 838 C.

[0077] In this experiment, hollow microspheres with good product quality were obtainable in the fractions having sphere diameters in the 40-90 m, 90-180 m, and 180-300 m ranges. However, agglomeration was observed.

COMPARATIVE EXAMPLE 1

[0078] The firing material particles produced in the same way as for inventive Example 1 were here mixed for 5 minutes in the Eirich mixer with the release agent, which here consisted only of Al(OH).sub.3 (as in Example 1), in the following proportions:

[0079] 75 wt % firing material particles

[0080] 25 wt % release agent (Al(OH).sub.3)

[0081] In the same way as for inventive example 1, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a firing temperature of 720 C.

[0082] In this experiment it was not possible to obtain any satisfactory product quality. Besides hollow microspheres, the expanded material included a high fraction of rejects (particles having undergone multicell expansion).

COMPARATIVE EXAMPLE 2

[0083] The firing material particles produced in the same way as for inventive example 1 were mixed here for 5 minutes in the Eirich mixer with the release agent, which here likewise consisted only of Al(OH).sub.3 (as in inventive example 1), in the following proportions:

[0084] 76 wt % firing material particles

[0085] 24 wt % release agent (Al(OH).sub.3)

[0086] In the same way as for inventive example 1, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a firing temperature of 800 C.

[0087] In this experiment, it was not possible to maintain stable production of hollow microspheres. After initial production of high-quality hollow microspheres, there were increasingly agglomerates and reject particles (particles having undergone multicell expansion).

COMPARATIVE EXAMPLE 3

[0088] The firing material particles produced in the same way as for inventive example 1 were mixed here for 5 minutes in the Eirich mixer with the release agent, which here consisted of metakaolin, in the following proportions:

[0089] 75 wt % firing material particles

[0090] 25 wt % release agent (metakaolin)

[0091] In the same way as for inventive example 3, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a firing temperature of 862 C. to 930 C.

[0092] The product resulting from this experiment consisted almost exclusively of particles having undergone multicellular expansion. No agglomerates were observed.

[0093] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:

[0094] 1 plant

[0095] 2 silo

[0096] 3 silo

[0097] 5 mixer

[0098] 6 rotary tube furnace

[0099] 10 mixing vessel

[0100] 11 longitudinal axis

[0101] 12 mixing tool

[0102] 15 lid opening

[0103] 16 base opening

[0104] 20 rotary tube

[0105] 21 firing chamber

[0106] 23 longitudinal axis

[0107] 25 cladding

[0108] 26 burner

[0109] 30 mixing chute

[0110] 31 arrow

[0111] 40 shaft furnace

[0112] 41 firing chamber

[0113] 42 jacket

[0114] 43 cooling gap

[0115] 45 burner

[0116] 46 hot gas line

[0117] 47 burner

[0118] 50 cold trap

[0119] 55 combustibles line

[0120] 56 arrow

[0121] 60 cavity

[0122] 61 glass wall

[0123] 62 (outer) layer

[0124] 63 aluminum oxide particles

[0125] 64 (inner) region

[0126] G firing material particles

[0127] H hot gas stream

[0128] K cooling air

[0129] M hollow microspheres

[0130] T release agent