The present invention relates to a microgranulate comprising tin dioxide, particularly for glassmaking. The invention also relates to a mixer for the production of such microgranulate and to the associated method for obtaining the microgranulate.

In the field of glassmaking, it is known to use tin dioxide (Sn0 ₂ ) to improve the quality of the glass. In particular, tin dioxide allows to reduce significantly the forming of air bubbles in the glass, which during subsequent processing can cause severe defects, such as cracks or laminations. For this very reason, tin dioxide is used widely in the production of so-called "technical" glass (or "high tech" glass) used in high- technology fields that require an extremely reduced presence of defects. Tin dioxide is added in small percentages to the mixture that will form the glass; after a step for homogenizing the various components, including the silica granules, in an appropriate mixer, the mixture is melted in a furnace and, by way of the fluid phase reaction among the various substances, becomes glass.

Tin dioxide has a very fine particle size distribution, since it is a powder constituted by microcrystals with dimensions usually comprised between 0.5 μιη and 4 μιη. For these reasons, its use in glassmaking can entail drawbacks, such as:

(i) the forming of stable agglomerations of tin dioxide (so-called stones) which do not melt into the glassy mass, do not react with the other substances of the glass and remain in the glass, giving rise to defects known as inclusions. In view of the high level of quality that is required in fields where high-tech glass is used, the presence of inclusions in a glass sheet or in an article made of glass makes these items unusable, with a consequent severe economic damage;

(ii) adhesion of the tin dioxide to the walls of containers, instruments and equipment in which it is transported or processed, a phenomenon which causes difficulties during processing, typically the clogging of instruments and equipment, with consequent halting of production and loss of product.

In order to overcome these drawbacks, a new form of use of tin dioxide, i.e., as granules instead of powder, has therefore been considered.

However, mixers (granulators) of the known type used to perform rolling of the granules along at least two directions, such as the V-type mixer-granulator (of the discontinuous type, periodically loaded and unloaded by means of appropriate openings) and the inclined rotating disk mixer (loaded and unloaded continuously), both having a single adjustable rotation, were not suitable for granulation of tin dioxide since they were subject to some drawbacks. In particular, mixers of this type produced particles with defects that were unacceptable for the purposes for which the product according to the present invention must be used, such as for example excessive size, uneven and/or excessively thick coatings, limited compactness, insufficient resistance of the coating. Furthermore, with traditional mixers drawbacks such as the progressive forming of deposits of the inner walls occurred.

The aim of the present invention is to provide a solution so that the tin dioxide can be used without the limitations and drawbacks that are known in the background art. Within this aim, an object of the invention is to provide a new form for tin dioxide that allows to avoid the problems of the forming of inclusions in the glass and of the high adhesiveness of the tin dioxide powder, at the same time ensuring good results in terms of quality of the manufactured glass.

Another object of the present invention is to provide a method for preparing the new form for tin dioxide described herein.

Another object of the invention is to provide a mixer for preparing the new form for tin dioxide described herein.

Another object of the invention is to provide a new form for tin dioxide that is effective, particularly in glassmaking, and the production of which is relatively easy and at competitive costs.

This aim, as well as these and other objects that will become better apparent hereinafter, are achieved by a microgranulate constituted by microparticles with dimensions comprised between 50 μιη and 800 μιη, wherein each microparticle is constituted by an inner core constituted by silicon dioxide and an outer coating constituted by tin dioxide, and wherein the tin dioxide is present in a quantity comprised between 10% and 85% by weight on the total weight of the microparticle.

Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the microgranulate according to the invention, of the corresponding method for its preparation and of the mixer for its preparation, illustrated by way of nonlimiting example in the accompanying drawings, wherein:

Figure 1 is a schematic view of a mixer according to the invention in a first condition of motion thereof;

Figure 2 is a schematic view of the mixer according to the invention in a second condition of motion thereof;

Figure 3 is a schematic view of a cleaning device adapted to be associated with the mixer according to the invention;

Figure 4 is a schematic view of the application of the device of Figure 3 to the mixer according to the invention;

Figure 5 is a view of a clump breaking device adapted to be associated with the mixer according to the invention;

Figure 6 is a view of the mixer according to the invention, with the clump breaking device of Figure 5 associated therewith;

Figure 7 is a schematic side view of the mixer according to the invention;

Figure 8 is a schematic view of the process for obtaining the microgranulate according to the present invention.

As is known to the person skilled in the art, the expression "granulate" is commonly used to indicates granular (or granulated) matter, i.e., a set of solid particles. In the context of the present invention, the expression "microgranulate" references the dimensions of the particles according to the invention, which are usually comprised between 50 μιη and 800 μιη.

As mentioned, a first aspect of the present invention relates to a microgranulate, each microparticle of which is constituted by a core, arranged in the internal portion of such microparticle, constituted by silicon dioxide, and a coating, which covers the core and is in contact with the outside, constituted by tin dioxide. Each microparticle contains tin dioxide in a percentage by weight on the total weight of the microparticle comprised between 10% and 85%; the complement to 100% of the weight of the microparticle is constituted by silicon dioxide.

The percentage ratio indicated above between the weight of the tin dioxide and the weight of the silicon dioxide is an important characteristic for the properties of the particles of the resulting microgranulate. The lower limit of 10% by weight of tin dioxide is due to the fact that below this percentage the resulting granule would have an excessively thin and therefore insufficiently stable coating; the upper limit of 85% by weight of tin dioxide is due to the fact that above this percentage the resulting granule would have an excessively thick coating, which would tend to decrease by self-grinding, generating granules (or powder) of pure tin dioxide, a highly undesirable phenomenon.

In a preferred embodiment of the microgranulate according to the invention, each microparticle can comprise 50% to 65% by weight on the total weight of the microparticle of tin dioxide. In an even more preferred embodiment, each microparticle can comprise 55% to 60% by weight on the total weight of the tin dioxide microparticle. It is also preferable for the thickness of the coating constituted by tin dioxide to be 15% - 20% with respect to the diameter of the inner core of the microparticle, constituted by silicon dioxide. An outer coating thickness exceeding the indicated maximum limit would in fact reduce the capacity of the coating to adhere to the core, weakening the structure of the resulting particles. A thickness of the coating lower than the indicated minimum limit would instead decrease the "transport capacity" of the core, reducing the effectiveness of the microgranulate.

The silicon dioxide (or silicic anhydride Si0 ₂ ), commonly known as "silica" or "silica sand", is a crystalline white powder with abrasive and thermal insulation properties. It is also one of the main components of glass and therefore its use in the particles of the microgranulate according to the invention does not alter the formulation of the mixture that feeds the furnaces for glassmaking.

While tin dioxide, as mentioned, has the characteristic of adhering very easily to the materials and to the media with which it makes contact, silicon dioxide is instead characterized by high lubricity properties, which eliminate the problems linked to difficulties in handling tin dioxide. This leads to a substantial improvement in glass quality, especially for technical glass, significant economic savings and an increase in productivity.

Tin dioxide is capable of adhering to the silicon dioxide without requiring the addition of any additive or binding agent. The particles that form the microgranulate according to the invention, therefore, do not contain additives and/or binding substances in order to allow the adhesiveness of the tin dioxide coating to the silicon dioxide core. Furthermore, the morphology of silicon dioxide, which is different from that of tin dioxide, prevents the forming of agglomerations, with the consequent substantially total disappearance of so-called inclusions.

In the particles of the microgranulate, the silicon dioxide that is present in the inner core is in close contact with the tin dioxide: the intimate contact facilitates the reaction between these substances, allowing the production of glass at lower temperatures than those commonly applied when the glass is prepared by adding tin dioxide powder to the mixture of the substances that will form the glass, including silica. This entails a further improvement in the quality of the resulting glass and a significant energy saving.

Furthermore, the silicon dioxide conveys the tin dioxide in the glass production bath, facilitating its dispersion and melting and at the same time preventing the forming of agglomerations of tin dioxide: this occurs by way of the fact that the tin dioxide powder is never present in the free state but adheres to the silicon dioxide granule until it is dissolved completely in the glass production bath.

Another aspect of the invention relates to a process for preparing the microgranulate described herein by utilizing a mixer that has been developed specifically for this process and therefore also is part of the invention. Since the description of the process must necessarily refer to the structurally components of the mixer, such mixer will be described in detail hereinafter with reference to Figures 1 to 7.

The mixer according to the invention comprises a cylinder 1 that is elongated and inclined with an adjustable inclination, in order to turn the circle both on its longitudinal axis 2 (main rotation) and on its horizontal axis that passes through its center of gravity 3 and forms with it a variable angle (secondary rotation). The two rotation rates are adjustable independently.

In this manner the granules contained in the cylinder are made to roll on two planes that are mutually almost perpendicular, the first one being perpendicular to the axis of the cylinder (main rotation) and the second one being parallel thereto (secondary rotation), as required by the process. The adjustment of the inclination and of the two rates allows balancing of the two movements, making the coating uniform on the entire surface of the granules.

The two rotation rates are calculated as a function of the diameter and length of the cylinder. Preferably, the mass contained in the granulator is made to travel longitudinally along the cylinder at a speed of approximately 1 m/s (longitudinal speed). Preferably, moreover, the mass contained in the granulator is made to rotate on the walls of the cylinder with a peripheral speed, measured along its circumference, of approximately 0.3 m/s. The setting of the two speeds as described here offers the advantage of allowing to maintain the same operating conditions in mixers having a different volume. As is evident to the person skilled in the art, however, the speed values given here are to be considered as indications and may vary according to the percentage of tin dioxide and to the physical characteristics of the granules, such as the particle size distribution, the crystalline shape and the specific surface.

The cylinder can be made to rotate by applying directly the nominal speeds (longitudinal and peripheral): it is in fact not necessary to apply programmed variations of such speeds, but the inverters that drive the gearmotors normally use a short acceleration ramp in order to reduce the mechanical stresses on the cylinder during rotation start.

Preferably, the diameter/length ratio of the cylinder can be comprised between approximately 1/5 and approximately 1/4, which is a good compromise between granulation efficiency and the structural strength of the cylinder proper. In a more preferred embodiment, the cylinder can have a diameter/length ratio of 1/5; in another more preferred embodiment, the cylinder can have a diameter/length ratio of 1/4. The wall of the cylinder must have the lowest possible thickness, preferably approximately 1 mm.

The mixer is of the discontinuous type and is loaded and unloaded from an opening arranged on one of its ends 4.

The mixer according to the invention is provided with a cleaning device with whip-like percussion elements and with a floating internal compacting clump breaking device.

The cleaning device is shown in Figures 3 and 4 and allows to keep the inner surface of the cylinder 1 clean, avoiding the forming of deposits that would prevent control of the thickness of the coating of the granules, leading to the forming of irregular and poorly coated granules.

The cleaning device, which allows to keep the inner surface of the cylinder free from deposits, is constituted by a plurality of whips 5, preferably shaped like a bicorn, made for example of steel wire, and connected to a support 6 provided with a helical spring 7 that is fixed to a base element 8. The supports 6 are periodically lifted and released with a snap action by a plurality of pins 9 arranged in a helical pattern and fixed to the surface of a shaft 10, which is made to rotate by a gearmotor 11. The release of the support 6 causes the percussion of the whip on a portion of the outer surface of the cylinder 1 of the mixer.

The whips strike sequentially the outer surface of the cylinder 1, which, being thin (for example 1 mm, as mentioned), is easily made to vibrate, causing the internal deposits to detach.

The cleaning device is effective not so much due to the action of the force applied by the percussions but because of their regularity. The rotation rate of the shaft 10 is adjusted automatically as a function of the rotation rate of the cylinder 1 of the mixer about its own longitudinal axis, so that the entire outer surface of the cylinder 1 is affected by the cleaning.

The mixer according to the invention is further provided with a floating internal compacting clump breaking device, as shown in Figure 5 and in Figure 6.

The device is constituted by a helical element 15, made of spring steel wire, which is connected to a shaft 16 by means of spokes 17 which are fixed to the helical element with eyelets so as to leave the helical element a little freedom of motion with respect to the shaft 16. The shaft 16 has a plurality of segments joined by rings 18. Conveniently, in order to increase the effectiveness of the clump breaking device, in particular its compacting action, the edge of the helical element can have a zigzag shape, as shown in Figure 5.

Figure 6 shows the operation of the clump breaking device within the cylinder 1 of the mixer according to the invention.

The floating clump breaking device moves freely in the cylinder 1 and is rotated by way of the friction between the helical element and the inner wall of the cylinder 1. The segmentation of the shaft 16 further allows the helical element to adapt better to the inner surface of the cylinder 1 during motion, preventing it from jamming.

Figure 7 is a view of an embodiment of the cylinder of the mixer according to the invention, in which the cylinder 1 is made of a thin stainless steel plate (for example 1 mm) and is supported by two supports 20 which are fixed on a first inclined movable frame 21 rotated by a gearmotor 22 fixed on such frame.

The movable frame 21 is connected to a second frame 23 by means of two lockable hinges 24, which allow to adjust its inclination.

The second frame 23, having a horizontal axis, is in turn supported by supports 25 connected to a footing 26. The frame 23 is turned about its own axis by a gearmotor 27, which is fixed on the footing 26. The coupling between the frame and the gearmotor is provided by means of a universal joint 28.

The rotation rate of the two frames is independently adjustable by means of two frequency variators.

Figure 7 further shows the presence of the cleaning device with the whip-like percussion elements 5 and the floating internal compaction clump breaking device, generally designated by the reference digit 30.

On the bottom of the cylinder, which lies opposite the one connected to the gearmotor 22, there is an opening that is closed by the hatch 4 and through which the components of the microgranulate are introduced and from which the product is unloaded after granulation.

The electric power supply of the gearmotors for the rotation of the cylinder 1 and for the shaft of the cleaning system that are mounted on the inclined frame 21 is ensured by a rotary electrical coupling 31 that is mounted on the footing 26 at the opposite end of the gearmotor 27.

As mentioned earlier, the invention also relates to a process for preparing the microgranulate described herein by means of the rotary and oscillating mixer cited above; the process is illustrated schematically in Figure 8.

The silicon dioxide and the tin dioxide, contained in the hoppers 40 and 41, are made to merge, in the desired quantities, in an additional hopper 42, from which loading into the cylinder of the mixer is performed. At this point the method for preparing the microgranulate according to the invention provides for the following steps:

(a) introducing, from the hopper 42, the silicon dioxide in the cylinder

1 of the mixer, which is stationary in the loading position 50, through the hatch 4;

(b) rotating the cylinder 1 ;

(c) stopping the cylinder 1 in the loading position 50;

(d) introducing in the cylinder 1 a portion of the total quantity of tin dioxide that is intended;

(e) rotating the cylinder 1 ;

(f) repeating steps (c), (d) and (e) the number of times needed in order to introduce 100% of the total quantity of tin dioxide intended;

(g) stopping the cylinder 1 in the unloading position 60 and unloading the product into the hopper 70 of a belt conveyor and dosage unit 80;

(h) unloading the product from the belt conveyor and dosage unit 80 into a multifrequency screen 90 provided with a first mesh 100 and with a second mesh 1 10.

The principle underlying the granulation process resides in that the granules of silicon dioxide (silica) "roll" on a layer of tin dioxide powder deposited on the wall of the rotating and oscillating mixer; in this manner, the tin dioxide adheres to the surface of the silica granules, forming the outer coating. In order to obtain a uniform coating, the silica granules must "roll" on the tin dioxide powder along two or more spatial directions, so as to obtain particles that have a pseudo-spheroidal shape.

As described in detail previously with reference to the mixer, the cylinder 1 is inclined and rotates both on its longitudinal axis 2 and on a horizontal axis that passes through its center of gravity 3. The combination of the inclination and of the rotation of the cylinder 1 allows the granules contained inside it to move along two substantially mutually perpendicular directions and in a balanced manner, allowing the forming of a uniform coating of tin dioxide on the silica granules.

Therefore, the rotation of the cylinder 1, performed in step (e) and repeated at each successive addition of tin dioxide, allows the tin dioxide to be distributed uniformly on the silica granules that are present inside the cylinder 1. Furthermore, the same type of rotation of the cylinder 1 performed in step (b) of the process allows such silica granules to be distributed uniformly inside it.

In an embodiment of the method, the speeds of the main rotation and of the secondary rotation can be kept equal (i.e., not changed) in all the subsequent granulation steps (step (e) and subsequent repetitions). In another embodiment of the method, such speeds can be increased progressively between one granulation step and the next, utilizing the progressive increase in the stability of the granules being coated with tin dioxide. This embodiment offers the advantage of being able to reduce the time required for the granulation process.

In general, in order to determine the rotation rates one must take into account two factors: low speeds can cause a reduced granulating effect; high speeds can produce a milling effect which can cause the destruction of the granules since they are made to fall back onto themselves.

In a preferred embodiment, the tin dioxide portion introduced in step (d) can be comprised between 20% and 25% of the total quantity to be introduced. In these conditions, the repetition of steps (c), (d) and (e) for the number of times required to introduce 100% of the tin dioxide quantity, provided by step (f) of the process, must be performed 4 or 5 times. As is evident to the person skilled in the art, the last addition relates to the portion of tin dioxide that is required to reach 100% and is not necessarily comprised between 20% and 25% of the total quantity.

The introduction of tin dioxide is performed by means of successive additions in order to prevent it from agglomerating, forming granules of tin dioxide alone, or from coating the walls of the cylinder, forming deposits. The described method in fact allows the tin dioxide powder to be "adsorbed" integrally by the moving mass in the cylinder, which is constituted initially by silicon dioxide granules and subsequently by silicon dioxide granules coated with tin dioxide with a progressively increasing thickness.

In a preferred embodiment of the process, at the end of step (f), i.e., when 100% of the tin dioxide has been introduced in the cylinder 1 by way of successive additions, it is possible to perform a step (f ') for keeping the cylinder 1 in rotation for an additional time. This step allows to consolidate the tin dioxide coating, improving its adhesion on the silicon dioxide core, and to even out the shape and dimension of the particles.

In step (h), the product is unloaded into a multifrequency screen 90 provided with two meshes 100 and 110. The first mesh 100 retains the coated granules that are excessively large (larger than 800 μιη), while the second mesh 110 (usually with a 50 μιη mesh size) is used to clean the microgranulate of any free tin dioxide powder (which has dimensions comprised on average between 0.5 μιη and 4 μιη) and of the pure tin dioxide granules (the dimensions of which are usually smaller than 50 μιη) which might agglomerate in the glass melting baths, producing defects similar to those of the powder proper. Therefore, the product that accumulates between the two meshes 100 and 1 10 of the multifrequency screen 90 corresponds to the microgranulate of interest.

Preferably, therefore, at the end of step (h) the process can further comprise a step (i) of recovering the product accumulated between the two meshes 100 and 110 of the multifrequency screen 90.

After it has been collected, the microgranulate is sent to packaging. It is also possible to also collect the material retained by the first mesh 100 (particles with size exceeding 800 μιη) and/or the material that is passed between the openings of the second mesh 110 (particles smaller than 50 μιη and tin dioxide powder), to be sent to recycling.

It is important to stress that the granulation process according to the present invention does not modify in any way the physical-chemical characteristics of the tin dioxide, such as the specific surface, the crystalline structure, the surface activity. This is made possible by the fact that the conditions in which the granulation process occurs are bland and do not alter significantly the crystalline structure of the tin dioxide, which can thus reacquire easily its original characteristics once the glass preparation conditions have been reached.

The rotating and oscillating mixer according to the invention is the only device that is capable of preparing the microgranulate according to the invention, which can then be used in glassmaking, in particular of high-tech glass.

Another aspect of the invention in fact relates to the use of the microgranulate described herein, formed by microparticles with dimensions comprised between 50 μιη and 800 μιη, constituted by an inner core constituted by silicon dioxide and an outer coating constituted by tin dioxide, in glassmaking.

In a preferred embodiment of this aspect of the invention, the microgranulate can be used in the manufacture of technical glass (so-called high-tech glass). Examples of these kinds of technical glass are liquid crystal glass, electrochromic glass, holographic glass, self-cleaning glass.

In view of the above, in practice it has been found that the invention fully achieves the intended aim, since the microgranulate described herein allows to avoid the problems linked to the forming of inclusions in the glass and to the adhesiveness of tin dioxide powder, thus allowing to obtain high- quality glass.

Furthermore, the mixer and the method according to the invention allow to prepare the microgranulate in a manner that is simple, inexpensive and without using complex devices.

The microgranulate, the mixer and the method thus conceived are susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may further be replaced with other technically equivalent elements.

In practice, the materials used, as well as the contingent shapes and dimensions, may be any according to requirements and to the state of the art.

The disclosures in Italian Patent Application No. 102015000033777 (UB2015A002157) from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.