Hollow glass microspheres and method for producing the same

A method for producing hollow glass microspheres includes preparing an aqueous suspension of starting materials including finely ground glass and waterglass. Combustible particles are produced from the suspension and are mixed with an AlO(OH)-containing pulverulent release agent. The mixture of combustible particles and release agent is introduced into a combustion chamber of a furnace where it is expanded at a combustion temperature which exceeds the softening temperature of the finely ground glass, to form the hollow microspheres. Hollow glass microspheres produced according to the method are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2017 219 692.0, filed Nov. 6, 2017; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention pertains to hollow glass microspheres and to a method for producing them.

Hollow glass microspheres, being hollow, spherical particles having typical diameters in the sub millimeter region (around 1 to 1000 micrometers), are much in use as lightweight aggregates in composite materials and lightweight concrete. Other areas for use of these hollow glass microspheres (HGM) include medicine, the consumer goods industry, and the oil and gas industry. Hollow microspheres are at least substantially in a state of monocellular expansion, meaning that they have a glass wall which is thin (in comparison to the sphere diameter) and which surrounds a single large, central, and spherical cavity (with the diameter of this central cavity being only slightly less than the sphere diameter). The glass wall of a hollow microsphere of this kind may, however, include further cavities (bubbles) with a substantially smaller diameter.

HGM should be distinguished from expanded glass particles, which are likewise frequently employed as lightweight aggregates. Expanded glass particles may likewise have a spherical or spheroidal outer contour, but differ critically from the aforementioned hollow microspheres in their multicellular structure. The volume of expanded glass particles is therefore filled by a foamlike glass matrix which surrounds a multiplicity of cavities, with each of these cavities being small in comparison to the particle size.

Expanded glass particles are customarily produced by expansion of green-particle pellets (or combustible material), formed from finely ground glass, waterglass, and an expandant, in a rotary tube furnace. In order to prevent the fused combustible particles sticking to the furnace wall or to other combustible particles, a release agent is generally introduced into the furnace together with the combustible material. Examples of release agents used include kaolin and finely ground clay.

Hollow glass microspheres and expanded glass particles may in principle be produced from the same or similar starting materials. From a process engineering standpoint, however, the production of hollow glass microspheres is substantially more difficult to manage than the production of expanded glass particles. This is so in particular because, in order to produce hollow microspheres, the green particles have to be melted to a much greater degree than for expanded glass production, so that the bubbles which form at the start of the expansion process unite to form the large central cavity and are therefore able to displace the glass matrix to the outer margin of the sphere.

With the greater melting of the glass matrix, however, there is a considerable increase in the propensity of the particles to stick. Moreover, the risk increases of the melted particles being crushed or abraded away by contact with other particles or with the furnace wall during the expansion process. To date, therefore, it has generally not been possible, or at least not on an industrial scale (production scale), to use rotary tube furnaces in order to produce hollow glass microspheres, despite the fact that the use of rotary tube furnaces would in itself be advantageous, due to their robustness, the high achievable throughput, and the comparatively low cost and effort of their operation. In particular, the tendency of the particles to stick cannot usually be adequately managed by using the conventional release agents.

To date, therefore, hollow glass microspheres have customarily been produced in vertical furnaces (also referred to below as “shaft furnaces”), in which either the green particles are expanded in an upwardly directed flow of hot gas, and then discharged with the gas flow from the upper end of the vertical furnace (in accordance, for example, with U.S. Pat. No. 3,230,064 A), or the green particles are expanded in falling (in accordance, for example, with U.S. Publication No. 2007/0275335 A1).

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide hollow glass microspheres and a method for producing the same, which overcome the hereinafore-mentioned disadvantages of the heretofore-known microspheres and methods of this general type and which enable effective production of hollow glass microspheres.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing hollow glass microspheres by:preparing an aqueous suspension of starting materials including finely ground glass and waterglass,producing combustible particles from the suspension,mixing the combustible particles with a pulverulent release agent containing AlO(OH),introducing the mixture of combustible particles and release agent into a combustion chamber of a furnace, andexpanding the combustible particles in the combustion chamber at a combustion temperature which exceeds the softening temperature of the finely ground glass, to form the hollow microspheres.

With the objects of the invention in view, there are also provided hollow glass microspheres having a glass wall surrounding a central cavity, which is obtainable by the method described above.

According to the method, for the production of hollow glass microspheres, an aqueous suspension is prepared of starting materials including finely ground glass and waterglass, this suspension being referred to below as “starting suspension”. The starting suspension is optionally admixed with an expandant (also referred to as “blowing agent”; e.g., soda niter or sugar). From the starting suspension, combustible particles (“green particles”) are produced, with diameters of preferably between 1 micrometer and 700 micrometers, more particularly between 20 micrometers and 200 micrometers. The lower limit of the above range figures refers in this case, for example, to the d10of the respective particle size distribution. The upper limit refers, for example, to the d90of the respective particle size distribution. The “dx” (where x=10, 50, 90, etc.) of the particle size indication, in this case and hereinafter, means that x % of the particles have a size of not more than dx. For example, then, the d50indicates the mean particle size in respect of which 50% of the particles are smaller. The combustible particles are mixed with a pulverulent release agent, after which the mixture of combustible particles and release agent is introduced into a combustion chamber of a furnace.

In the combustion chamber, where the prevailing combustion temperature exceeds the softening temperature of the finely ground glass, the combustible particles, finally, undergo expansion to form the hollow microspheres. In this case, in a typical manifestation of the method, the combustible particles undergo an increase in their diameter, or expansion, of 25% to 70%. In total, the diameter of the hollow microspheres resulting from the expansion process, in typical sizing, is between around 2 micrometers and 1000 micrometers, preferably between 7 micrometers and 600 micrometers.

Employed in accordance with the invention is a release agent which includes aluminum oxide hydroxide, AlO(OH) (CAS No.: 1318-23-6). This compound, which is also referred to as “aluminum metahydroxide”, is employed in this case in particular in the γ-AlO(OH) variant, which corresponds to the mineral boehmite.

The invention is based on extensive experiments by the applicant which have shown that by using AlO(OH) as release agent, the sticking tendency of the green particles and also of the hollow microspheres formed from them can be suppressed in a particularly effective way, so making it possible in particular for the hollow microspheres to be produced in industrial rotary tube furnaces (in other words on the production scale) with sufficient effectiveness.

In one suitable embodiment of the invention, the release agent is formed exclusively—apart from commonplace impurities in the order of magnitude of up to at most 1 to 2 wt %—of AlO(OH). Preferably, however, the release agent, as well as AlO(OH), also includes aluminum hydroxide (Al(OH)3; CAS No.: 21645-51-2, also called “aluminum orthohydroxide”). Particularly good results with regard to the quality of the hollow microspheres, and more particularly a high sphericity and low bulk density, have been achieved in advantageous embodiments of the invention employing, as release agent, a mixture which is formed exclusively—again, apart from commonplace impurities in the order of magnitude of at most 1-2 wt %—of AlO(OH) and Al(OH)3.

In advantageous embodiments of the mixed release agent, the fractions of AlO(OH) and Al(OH)3are selected preferably in such a way,that the fraction of Al(OH)3in the mixture of combustible particles and release agent is between 6 wt % and 30 wt %, preferably between 8 wt % and 25 wt %, andthat the fraction of AlO(OH) in the mixture of combustible particles and release agent is between 1 wt % and 25 wt %, preferably between 2 wt % and 12 wt %, more particularly between 4 wt % and 10 wt %.

In another advantageous embodiment of the invention, the release agent, as well as AlO(OH) and Al(OH)3, also includes dehydroxylated kaolin, more particularly in a fraction which is between 3 wt % and 10 wt %, preferably between 4 wt % and 7 wt %, of the overall mixture of combustible particles and release agent. In this case the release agent used preferably includes a three-component mixture containing only—again, apart from commonplace impurities in the order of magnitude of at most 1-2 wt %—AlO(OH), Al(OH)3and dehydroxylated kaolin. The term “dehydroxylated kaolin” is used as a generic term, embracing metakaolin and calcined (anhydrous) kaolin. Metakaolin is customarily produced by heating kaolin to temperatures between 650° C. and 750° C. Calcined (anhydrous) kaolin is obtained by heating kaolin to temperatures above 900° C.—see, for example, EP 1 715 009 A2.

Embodiments of the method that have proven particularly useful are those in which the Al(OH)3fraction selected in the release agent is coarser than the AlO(OH) fraction.

In a useful development of the invention, the AlO(OH) used for the release agent is selected or conditioned in such a way that at least 90% of the AlO(OH) particles in the release agent have a particle diameter of less than 1 micrometer (d90=1 μm), preferably less than 0.7 micrometer (d90=0.7 μm). Having been found experimentally to be particularly suitable, and hence also preferred, in this case is a product which contains 99.7% AlO(OH) in a particle size distribution of d10=0.2 μm, d50=0.35 μm and d90=0.6 μm.

In a useful development of the invention, the Al(OH)3used optionally for the release agent is selected or conditioned in such a way that at least 90% of the Al(OH)3particles in the release agent have a particle diameter of less than 4 micrometers (d90=4 μm), preferably less than 3.5 micrometers (d90=3.5 μm). Having been found experimentally to be particularly suitable, and therefore also preferred, in this case is a product containing 99.5% Al(OH)3in a particle size distribution of d10=0.6 μm, d50=1.3 μm and d90=3.2 μm.

In a useful development of the invention, the dehydroxylated kaolin used optionally for the release agent is selected or conditioned in such a way that at least 90% of the dehydroxylated kaolin particles in the release agent have a particle diameter of less than 5 micrometers, preferably less than 4 micrometers. Having been found experimentally to be particularly suitable, and therefore also preferred, in this case are products in which the dehydroxylated kaolin particles have a mean particle size of 3 μm.

The combustible particles are produced preferably by spray granulation. Alternatively, the combustible particles are produced by granulation in an intensive mixer, more particularly in an Eirich intensive mixer.

In one useful embodiment of the method, the combustible particles, before being fed to the combustion chamber, are mixed with the pulverulent release agent in an intensive mixer. This mixing in the intensive mixer produces a particularly dense and homogeneous distribution of the release agent on the surface of the combustible particles, and therefore—in comparison to other kinds of mixing of combustible particles and release agent—allows a saving to be made in release agent, without any need to accept an increase in agglomeration during the combustion process.

An intensive mixer is a mixer in which the mixing procedure is carried out at a power input of at least about 2 kilowatts per 100 kilograms of mixture, or one whose mixing tool in the mixing procedure moves at a peripheral velocity of at least 15 meters per second relative to the mixing vessel. The intensive mixer used in accordance with the invention preferably features a power input of at least 5 kilowatts per 100 kilograms of mixture, more particularly at least 10 kilowatts per 100 kilograms. One preferred embodiment uses an Eirich intensive mixer to mix the combustible particles with the release agent. Within the context of the invention, however, it is also possible in principle to carry out the method using a “horizontal” Lodige plowshare mixer or with an Ekato mixer, which is characterized by a conical mixing vessel. Before being introduced into the combustion chamber, the mix of combustible particles and release agent is preferably mixed intensively for a mixing time of 1 to 10 minutes, more particularly for around 2 to 5 minutes.

The furnace employed for the expansion process is preferably a rotary tube furnace. Employed more particularly in this case is a rotary tube furnace which is heated directly (that is, from the inside) by flaming, and which, due to its rational mode of operation and of the high combustion temperatures (which are comparatively easy to attain) is advantageous for the production of hollow glass microspheres. A decisive step forward in this case is that with the method of the invention it is possible to utilize the advantages of the directly heated rotary tube furnace without any overheating of the combustible particles and of the hollow microspheres formed from them. An alternative to this is to use a rotary tube furnace heated indirectly (again, preferably, by flaming). In the latter case, the supply of heat into the combustion chamber is accomplished from outside through the outer surface of the rotary tube. A further alternative within the method of the invention is to use a shaft furnace (vertical furnace), in which the combustible particles are expanded in an ascending stream of hot gas. In this variant method as well, the use of the release agent of the invention has been found to result in a substantial reduction in the sticking tendency, and to make an advantageous contribution to the formation of the hollow spheres.

The combustion process is carried out preferably at a combustion temperature of between 800° C. and 1150° C., preferably between 850° C. and 1000° C. An advantage of the AlO(OH)-containing release agent of the invention over other release agents in this case is that a comparatively high combustion temperature can be set, without any agglomeration (or at any rate without substantial agglomeration) of the combustible particles.

One special embodiment of the invention are the hollow glass microspheres obtainable by the above-described method of the invention. Analysis has shown that with this method, during the expansion process, aluminum oxide from the release agent becomes integrated in the melted glass wall of the hollow microspheres. As a result of this, the glass wall has a two-phase construction. While an inner region of the glass wall is formed of a lime-soda-silicate glass of low aluminum content, the aluminum fraction in an outer layer of the glass wall is markedly increased. The outer layer has a thickness, for example, of approximately 1 micrometer, and thus generally has a lower radial extent than the inner region of the glass wall. As a result of the increased aluminum fraction in the outer layer of the glass wall, the chemical stability of the glass wall is increased significantly by comparison with the lime-soda-silicate glass of the inner region.

Another embodiment of the invention is the use of a release agent which includes AlO(OH), preferably in a mixture with Al(OH)3and also, optionally, with dehydroxylated kaolin, in the production of hollow glass microspheres.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the figures of the drawings, in which structures and parts that correspond to one another are provided consistently with the same reference symbols, and first, particularly, toFIG. 1thereof, there is seen a plant1for producing hollow glass microspheres M, i.e., for producing hollow glass spheres having typical diameters which are predominantly, for example, in a range of between 40 and 350 micrometers.

The plant1includes a first silo2as reservoir vessel for combustible particles G, and also a second silo3as a reservoir vessel for pulverulent release agent T. Additionally, the plant1includes a mixer5for mixing the combustible particles G with the release agent T, and also a combustion furnace, implemented as rotary tube furnace6, for expanding the combustion particles G to form the desired hollow microspheres M.

The combustible particles G stored in the first silo2are approximately spherical particles whose diameters are, for example, approximately in the range between 20 micrometers and 200 micrometers. The combustible particles G are produced preferably by spray granulation. Starting materials for that process, including 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 combustible particles G. The combustible particles G are subsequently dried. Drying is followed optionally by classifying, where the fraction having the desired diameters is selected and supplied to the silo2.

In the embodiment of the plant1that is shown, the mixer5is implemented as an Eirich intensive mixer. The mixer5in this case includes a substantially cup-shaped mixing vessel10, which is mounted rotatably about its longitudinal axis11, which is inclined relative to the vertical. A mixing tool12, which is rotatable counter to the mixing vessel10, is disposed eccentrically in the mixing vessel10, in parallel to the longitudinal axis11. The mixing vessel10can be charged by way of a closable lid opening15and can be emptied through a likewise closable and centrally disposed base opening16. In exemplary sizing, the mixer5in 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 plant1may also include a different kind of mixer, such as a drum mixer, for example.

The rotary tube furnace6conventionally includes an elongated, hollow-cylindrical rotary tube20made from steel which is resistant to high temperatures, with a combustion chamber21formed in the interior of the tube. The rotary tube20is mounted rotatably about its longitudinal axis23, which is disposed with a slight incline relative to the horizontal. As shown, the rotary tube furnace is constructed as a directly heated rotary tube furnace. The combustion chamber21in this case is fired directly with a gas-operated burner26, which is disposed at the output end of the rotary tube20.

In the operation of the plant1, combustible particles G and release agent T are metered from the two silos2,3onto a mixing chute30which is disposed beneath the silos2,3, so that at that point there is a premix composed of combustible particles G and release agent T, with a specified release agent fraction. The desired mass ratio is set by using a balance, for example. Alternatively, the setting is performed volumetrically, by using conveying screws or star wheels assigned to the silos2,3, for example. Via the mixing chute30, the premix of combustible particles G and release agent T is conveyed into the mixing vessel10of the mixer5.

Alternatively to the exemplary representation, there may also be no mixing chute30, in which case combustible particles G and release agent T are each metered separately into the mixer5, so that the desired mixing ratio is generated there.

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 combustible particles G is homogenized in the mixer5for a mixing time of 1 to 10 minutes. After the end of the mixing procedure, the mixture of combustible particles G and release agent T is discharged from the mixing vessel10through the base opening16. The mixture is optionally stored in a buffer vessel (not shown explicitly) which is placed between the mixer5and the rotary tube furnace6.

From the mixing chute30or the optional downstream buffer vessel, the mixture of combustible particles G and release agent T is supplied continuously, by using a charging facility which is not shown explicitly herein, to the combustion chamber21of the rotary tube furnace6(indicated by an arrow31). In the combustion chamber21, in the operation of the plant1, the burner26is used to generate a specified combustion temperature, at which the combustible particles G undergo successive expansion to form the desired hollow microspheres M within a period of around 1 to 15 minutes.

The hollow microspheres M produced are discharged from the combustion chamber21and, after a cooling and sorting step, are supplied to a product reservoir (not shown herein). 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 combustion process alongside the hollow microspheres M. These multicellularly expanded particles are either discarded as rejects or supplied for an alternative use.

FIG. 2shows an alternative embodiment of the plant1. In contrast to the first embodiment, the expansion process in this case is carried out not in a rotary tube furnace but instead in a shaft furnace40.

The shaft furnace40includes a combustion chamber41which is extended in the manner of a shaft and aligned vertically with respect to the longitudinal extent, this chamber41being surrounded by a double jacket42of steel that is insulated thermally with respect to the outside. Cooling air K is guided in a cooling gap43which is formed by the double jacket42. Toward the top, the combustion chamber41is widened in a steplike manner.

Assigned to the shaft furnace40is a gas-operated burner45, which is used to generate a hot gas stream H, within the combustion chamber41, that is directed from bottom to top. For this purpose, the hot gas generated by the burner45is supplied through a hot gas line46to the combustion chamber41as hot gas stream H. At approximately half the height of the combustion chamber41, specifically in the region of the above-described cross-sectional widening, there are a number (six, for example) of additional gas-operated burners47, which are positioned in a crownlike distribution around the periphery of the combustion chamber41.

Adjoining the combustion chamber41at the top, according toFIG. 2, is a region which serves as a cold trap50and which has a cross section widened further relative to the cross section of the upper portion of the combustion chamber41. Alternatively, the combustion chamber41and also the optional cold trap50may be implemented with a uniform cross section over the whole of their height.

The shaft furnace40, finally, includes a charging facility, formed in this case by a combustibles line55. The combustibles line55is passed through the double jacket42and opens into the lower portion of the combustion chamber41. The combustibles line55is fed from the mixer5or from an optionally downstream buffer vessel (indicated by the arrow56). The combustibles line55runs 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 combustion chamber41. Optionally, however, the charging facility may also include a device for the active conveying of the combustible material—for example, a compressed air system or a conveying screw.

In the operation of the plant1, in the exemplary embodiment above, the homogeneous mixture of combustible particles G and release agent T is conveyed continuously by using the combustibles line55into the combustion chamber41, where it is captured by the hot gas stream H and carried upward.

In the lower portion of the combustion chamber41, a temperature is generated of around 650° C., for example, at which the combustible particles G are first of all preheated. The combustion chamber41is additionally heated by the burners47, and so the temperature in the upper portion of the combustion chamber41is increased to the combustion temperature which exceeds the softening temperature of the finely ground glass. The expansion of the combustible particles G to form the hollow microspheres M takes place in this case in brief flame contact at approximately 1400° C.

The expanded hollow microspheres M are supplied, finally, to the cold trap50, where they are quenched by supply of cooling air K. Finally, the hollow microspheres M are isolated from the hot gas stream through a solids separator, and, optionally after a sorting step, they are supplied to a product reservoir (again not shown herein). The entrained release agent T and also, optionally, any rejects (especially multicellularly expanded particles) are separated in turn from the hollow microspheres M by using a cyclone.

91 wt % of finely ground used glass (d97≈50 μm), 7 wt % of waterglass 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 combustible particles thus obtained have a particle size distribution of d10≈30 μm, d50≈80 μm and d90≈175 μm.

The dried combustible particles were mixed for five minutes in an Eirich intensive mixer with the release agent, composed in this case of AlO(OH) (particle size distribution: d10=0.2 μm; d50=0.35 μm; d90=0.6 μm; purity: 99.7%) in the following proportions:83.3 wt % combustible particles16.7 wt % release agent (AlO(OH))

This mixture was subsequently expanded in an indirectly heated rotary tube furnace (pilot scale). In this and all the experiments described below, the combustion temperature was varied during the progress of the experiment, until hollow microspheres were produced (at the combustion temperatures stated; in the case of inventive example 1, at a combustion temperature of 960° C.).

This experiment produced hollow microspheres which in part, however, had an non-round shape.

The combustible 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 in this case was formed of AlO(OH) (as in inventive example 1) and Al(OH)3(particle size distribution: d10=0.6 μm; d50=1.3 μm; d90=3.2 μm; purity: 99.5%) in the following proportions:83.3 wt % combustible particles16.7 wt % release agent (8.35 wt % AlO(OH) and 8.35 wt % Al(OH)3)

This mixture was expanded in the same way as for inventive example 1 in the indirectly heated rotary tube furnace (pilot scale) at a combustion temperature of 960° C.

This experiment produced hollow microspheres which overall had a rounder shape than the hollow microspheres from inventive example 1. Moreover, the hollow sphere walls visually had a more homogeneous (“more strongly vitrified”) structure. In this experiment as well, however, particles with an unround form were visible. Moreover, alongside individual hollow microspheres, there were also agglomerates.

The combustible particles produced in the same way as for inventive example 1 were mixed in this case for 20 minutes in a drum mixer with the release agent, which in this case was formed of AlO(OH) (as in inventive example 1) and Al(OH)3(as in inventive example 2), in the following proportions:70 wt % combustible particles30 wt % release agent (15 wt % AlO(OH) and also 15 wt % Al(OH)3)

In contrast to inventive examples 1 and 2, this mixture was expanded in a directly heated industrial rotary tube furnace (production scale) at a combustion temperature of 926° C.

This experiment produced hollow microspheres having good sphericity and a visually approximately homogeneous (“vitrified”) hollow sphere wall. There were no agglomerates.

The combustible particles produced in the same way as for inventive example 1 were mixed in this case in analogy to inventive example 3 for 20 minutes in the drum mixer with the release agent, which in this case was formed of AlO(OH) (as in inventive example 1) and Al(OH)3(as in inventive example 2), in the following proportions:77 wt % combustible particles23 wt % release agent (8 wt % AlO(OH) and also 15 wt % Al(OH)3)

In the same way as for inventive example 3, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a combustion temperature of 882° C.

This experiment likewise produced hollow microspheres having good sphericity and a visually approximately homogeneous (“vitrified”) hollow sphere wall. There were only sporadic agglomerates.

The combustible particles produced in the same way as for inventive example 1 were mixed in this case for 5 minutes in the Eirich mixer with the release agent, which in this case was formed of AlO(OH) (as in inventive example 1) and Al(OH)3(as in inventive example 2), in the following proportions:81.1 wt % combustible particles18.9 wt % release agent (4.7 wt % AlO(OH) and also 14.2 wt % Al(OH)3)

In the same way as for inventive example 1, this mixture was expanded in the indirectly heated rotary tube furnace (pilot scale) at a combustion temperature of 960-990° C.

This experiment likewise produced hollow microspheres having good sphericity and a visually approximately homogeneous (“vitrified”) hollow sphere wall. In spite of the increase in the combustion temperature to 990° C., there were only a few agglomerates.

The combustible particles produced in the same way as for inventive example 1 were in this case mixed for five minutes in the Eirich mixer with the release agent, which in this case was formed of AlO(OH) (as in inventive example 1), Al(OH)3(as in inventive example 2), and calcined kaolin (particle size distribution: d10=1 μm; d50=2 μm; d90=10 μm), in the following proportions:70 wt % combustible particles30 wt % release agent (6 wt % AlO(OH) and also 19 wt % Al(OH)3and also 5 wt % calcined kaolin)

In the same way as for inventive example 3, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a combustion temperature of 828° C.

This experiment likewise produced hollow microspheres of high quality.

COMPARATIVE EXAMPLE 1

The combustible particles produced in the same way as for inventive example 1 were in this case mixed for 5 minutes in the Eirich mixer with the release agent, which in this case was formed of fine-particle Al(OH)3(particle size distribution: d10=0.3 μm; d50=0.4 μm; d90=0.8 μm; purity: 99.3%), in the following proportions:88.9 wt % combustible particles11.1 wt % release agent (Al(OH)3)

This mixture was expanded in an indirectly heated rotary tube furnace (pilot scale) at a combustion temperature of 846-970° C.

This experiment produced almost exclusively agglomerates.

COMPARATIVE EXAMPLE 2

The combustible particles produced in the same way as for inventive example 1 were mixed in this case for 5 minutes in the Eirich mixer with the release agent, which in this case was formed of Al(OH)3(as in inventive example 2), in the following proportions:75 wt % combustible particles25 wt % release agent (Al(OH)3)

In the same way as for inventive example 3, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a combustion temperature of 720° C.

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 3

The combustible particles produced in the same way as for inventive example 1 were mixed in this case for 5 minutes in the Eirich mixer with the release agent, which in this case was formed of Al(OH)3(as in inventive example 2), in the following proportions:76 wt % combustible particles24 wt % release agent (Al(OH)3)

In the same way as for inventive example 3, this mixture was expanded in the directly heated rotary tube furnace (production scale) at a combustion temperature of 800° C.

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 4

The combustible particles produced in the same way as for inventive example 1 were mixed in this case for 5 minutes in the Eirich mixer with the release agent, which in this case was formed of metakaolin, in the following proportions:75 wt % combustible particles25 wt % release agent (metakaolin)

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

The product resulting from this experiment was formed almost exclusively of particles having undergone multicellular expansion. No agglomerates were observed.

Analysis of Hollow Microspheres from Inventive Example 5

FIGS. 3 and 4show scanning electron micrographs of broken-open hollow microspheres M, produced in the experiment described in the form of inventive example 5. FromFIG. 3it is apparent that the hollow microspheres M depicted herein, with a diameter of around 70 micrometers, are formed by a single, central and spherical cavity60, surrounded by a thin glass wall61with a thickness of around 2 to 5 micrometers. FromFIGS. 3 and 4it is apparent that aluminum oxide particles63are fused within a radially outer layer62of the glass wall61, whose radial extent is around 1 micrometer. The glass wall61in this case has a two-phase construction, which is also evident in the micrograph ofFIG. 4, with this two-phase construction delimiting the outer layer62from a remaining inner region64of the glass wall61.

Through energy-dispersive x-ray spectroscopy (EDX) on the hollow microspheres M it was possible to demonstrate that the two-phase construction of the wall material, evident under the scanning electron microscope, is combined with a gradient in the chemical composition of the wall material, reflected in particular in the aluminum content. Whereas a chemical composition measured in the inner region64of the glass wall61was typical of a lime-soda glass, with an aluminum content of 1.16 wt %, the EDX measurement for the aluminum content of the outer layer62produced an estimate of around 14 wt %.

LIST OF REFERENCE SYMBOLS