Technical Field

The present invention relates to a process and a system for the dry granulation of powdered ceramic mixture.

Background Art

With reference to the preparation and processing of the raw materials, with application e.g. in the ceramic sector, the use is known of wet granulation methods.

Specifically, the wet granulation process involves wetting the powdered ceramic mixtures previously obtained by dry grinding, and the subsequent drying of the granulate thus obtained in order to have a predefined humidity level suitable for the subsequent processing steps (pressing, extrusion, casting, etc.).

The use is also well known of wet grinding methods and subsequent spray drying (atomization) of the raw materials.

In particular, in such case the raw materials are wet ground in special rotary mills to obtain a liquid and viscous mixture (slip). The liquid mixture thus made is then introduced into suitable spray dryers (atomizers), which are able to produce a uniform powder with constant humidity, called atomized powder, ready for the subsequent processing steps.

The benefits are also however known of the so-called dry preparation of raw materials compared to conventional wet preparation processes.

In fact, it is known that dry preparation allows obtaining, among other benefits, a strong reduction of heat and electricity consumption, reduced gas emissions, low water consumption and a strong reduction in the amount of chemical additives normally used in the processing steps (e.g., deflocculants, toughening agents, etc.).

In particular, the dry preparation of agglomerated powders in the production of pressed tiles, as an alternative to wet granulation and atomization processes, has been the subject of extensive research over the last decades and numerous solutions have been studied and put forward over the years.

For example, appropriate granulators/agglomerators have been devised which, thanks to extremely accurate incoming material dosing systems and systems for measuring the level of humidity, are able to ensure a homogeneous and constant supply of water to the powdery particles of the ceramic system and are able to produce granules with a similar morphology to an agglomerate of fine particles, such therefore as to facilitate the subsequent processing steps.

The known solutions and processes for the dry preparation are however susceptible to further upgrading, aimed in particular at further reducing the presence of air in the processed raw materials and improving their fluidity.

It is in fact known that the presence of a high percentage of air in the dry ground material compared to atomized material in many cases involves greater pressing difficulties both in terms of production and quality.

For example, it can involve: lower press production output; greater raw material waste; difficulty in obtaining large sizes due to an uneven distribution of the powder within the mold cavity; surface defects caused by the presence of fine powder not properly agglomerated and having less humidity.

Description of the Invention

Therefore, the main aim of the present invention is to devise a process and a system for the dry granulation of powdered ceramic mixture which allow obtaining granulate with a reduced percentage of air.

Another object of the present invention is to devise a process and a system for the dry granulation of powdered ceramic mixture which allow obtaining granulate with high fluidity index.

Another object of the present invention is to devise a process and a system for the dry granulation of powdered ceramic mixture which allow minimizing the presence of fine powders in the obtained granulate.

Another object of the present invention is to devise a process and a system for the dry granulation of powdered ceramic mixture which allows overcoming the mentioned drawbacks of the prior art within the ambit of a simple, rational, easy, effective to use solution which also has an extremely reduced processing cost. The aforementioned objects are achieved by the present process for the dry granulation of powdered ceramic mixture according to the characteristics described in claim 1.

The aforementioned objects are also achieved by the present system for the dry granulation of powdered ceramic mixture according to the characteristics described in claim 10.

Brief Description of the Drawings

Other characteristics and advantages of the present invention will become better evident from the description of a preferred, but not exclusive, embodiment of a process and a system for the dry granulation of powdered ceramic mixture, illustrated by way of an indicative, but non-limiting example in the accompanying drawings, in which:

Figure 1 is a schematic view of a possible embodiment of a system according to the invention;

Figure 2 schematically illustrates the details of a possible embodiment of the system compacting means according to the invention;

Figure 3 is a block diagram which schematically illustrates the steps of the process according to the invention;

Figure 4 is a table illustrating an example of possible technical characteristics of the granulate material obtained by means of the system and the process according to the invention, compared with the technical characteristics of a granulate material obtained by means of a wet granulation/agglomeration process without drying of known type;

Figure 5 is a table illustrating an example of grain size of the granulate material obtained by means of the system and the process according to the invention, compared with the grain size of a granulate material obtained by means of a wet granulation process without drying of known type.

Embodiments of the Invention

With particular reference to such figures, globally indicated with reference numeral 1 is a possible embodiment of a system for the dry granulation of powdered ceramic mixture. In this regard, it is specified that different embodiments of the system 1 cannot be ruled out from that illustrated in the figures.

The system 1 can e.g. be used for the dry granulation of ceramic mixture powder obtained by dry grinding for the production of pressed tiles, as an alternative to the known processes of wet granulation and atomization.

In particular, the system 1 comprises feeding means 2 of a powdered material to be processed P.

Advantageously, the system 1 also comprises:

compacting means 3 adapted to compact the powder to be processed P to obtain at least one compacted element C, by means of the application of the sole force of pressure;

crushing means 4 adapted to crush said at least one compacted element C to obtain a granulate material G with predefined grain size.

In particular, through the use of such compacting means 3 and crushing means 4, the system 1 allows obtaining a granulate material G using a dry process with reduced percentages of air and high fluidity.

Specifically, the system 1 is usable for the processing of ceramic powder to be processed P to obtain any ceramic product conventionally obtainable by traditional pressing using a mold, belt pressing with or without a mold, extrusion and casting.

According to a preferred embodiment, the compacting means 3 of the system 1 make a plurality of compacted elements C, consisting of respective briquettes. Different conformations and dimensions of the compacted elements C cannot however be ruled out, as well as the realization by means of the compacting means 3 of a single compacted element C.

Preferably, in order to obtain optimal compaction, the compacting means are adapted to apply a pressure not less than 100 bars.

Advantageously, exiting from the crushing means 4 the granulate material G has the following technical characteristics:

- compression ratio of between 1.6 and 2.2;

density of between 0.8 g/cm3 and 1.2 g/cm3;

rest angle of between 35° and 60°. Specifically, with reference to a preferred embodiment, the obtained granulate material G has the following technical characteristics:

compression ratio equal to 1.8;

density equal to 1.1 g/cm3 ;

- rest angle equal to 45°.

Figure 4, in particular, shows a table indicating the technical characteristics of the granulate material G obtained using the system 1 according to the invention, compared with the technical characteristics of a granulate material obtained by means of a wet granulation process without drying of known type.

More specifically, Figure 5 shows an exemplary table indicating the grain sizes as a percentage of the granulate material obtained using the system and the process according to the invention, compared with the grain size of a granulate material obtained by means of a wet granulation process without drying of known type.

Figure 6 also shows possible values of grain size range of the granulate material obtained using the system and the process according to the invention.

With reference to the preferred embodiment shown in Figure 2, the compacting means 3 comprise two cylindrical rollers 5 side by side and counter-rotating. The powder to be processed P is introduced between the two rollers 5 for the realization of the compacted elements C.

Different embodiments of the compacting means 3 cannot however be ruled out which provide for every method having the object of reducing and/or eliminating the presence of air inside the powder of the ceramic mixture using the sole force of pure pressure.

Still according to the preferred embodiment illustrated in Figure 2, each of the rollers 5 has a respective shaped compaction surface 6 adapted to reproduce the shape of the compacted elements C.

In particular, each of the shaped surfaces 6 has a plurality of concavities 7 suitably distributed on the surfaces themselves and aligned with each other so as to mate during the rotation of the rollers 5.

Usefully, furthermore each of the rollers 5 is preferably made of special anti- wear and anti-corrosion steel. Furthermore, the feeding means 2 comprise a loading hopper 8 for loading the powder to be processed P arranged above the compacting means 3.

Usefully, the feeding means 2 also comprise a loading line 9 connected to the hopper 8 and adapted to load the powder to be processed P inside the hopper itself.

For example, such loading line 9 can consist of a special feeding screw for feeding the powder to be processed P.

At least one level probe 10, 11 is arranged inside the hopper 8 and is operatively connected to the loading line 9. In practice, such level probe 10, 11 is adapted to operate or stop the loading line 9 depending on the level of the powder to be processed P inside the hopper 8.

Preferably, as shown in Figure 1, there are two distinct probes:

- a first maximum level probe 10 adapted to stop the loading line 9 when the powder to be processed P inside the hopper 8 reaches a maximum predefined level;

- a second minimum level probe 11 adapted to start or, in any case, to suitably adjust the loading line 9 when the powder to be processed P inside the hopper 8 reaches a predefined minimum level.

Preferably, the feeding means 2 comprise a precompactor 12 arranged inside the hopper 8 and adapted to compress the powder to be processed P on the rollers 5, so as to facilitate the correct distribution inside the concavities 7.

Furthermore, the system 1 comprises sieving means 13 arranged downstream of the compacting means 3 and adapted to sieve the material exiting from the compacting means 3 to separate the compacted elements C from the waste material S.

In particular, as shown in the preferred embodiment of Figure 1, the sieving means 13 can be made up of a sieve arranged below the rollers 5.

Usefully, the system can comprise recovery means 14 for recovering the waste material S operatively connected to the sieving means 13 and to the hopper 8, adapted to move the waste material S from the sieving means 13 to the hopper 8.

In particular, as shown in Figure 1, the recovery means 14 can comprise a recovery screw 15 having an inlet connected to the sieve 13 and an elevator 16 connected to an outlet of the recovery screw 15 and having an unloading mouth 17 positioned above the hopper 8.

The system 1 also comprises an unloading line 18 for unloading the compacted elements C, arranged at the outlet of the compacting means 3.

For example, as schematically shown in Figure 2, such unloading line 18 may comprise a chute 19 positioned exiting from the compacting means 3 and a conveyor belt 20 adapted to collect and move the compacted elements C.

Preferably, the crushing means 4 comprise a grinding mill.

Preferably, the crushing means 4 may be of the impact type, friction type or of the type of a mill which integrates both methods.

For example, the crushing means 4 can be selected from the group comprising: impact mill, disc mill, centrifugal mill, pin type mill, counter-rotating roller mill.

With specific reference to the application of the system 1 in the ceramic sector, downstream of the crushing means 4 the system 1 may have pressing means for pressing the granulate material G to obtain a semi-finished product and firing means for firing the semi-finished product thus obtained for the manufacture of a vitrified product.

In particular, the pressing means can be made up of presses and firing kilns of the type conventionally employed in the ceramic industry.

The use cannot be ruled out of humidification means of the crushed material before subsequent processing operations.

Nor can the use be ruled out of sieving means arranged downstream of the crushing means 4, and the use of the recovery and movement means to recover and move the sieved material towards the compacting means 3.

The process 100 is described below for the dry granulation of powder for ceramic mixture, shown schematically in Figure 3.

In particular, the process 100 comprises a first feeding step 110 of the powder of ceramic mixture to be processed P in the compacting means 3.

Advantageously, the process 100 also comprises:

a compaction step 120 of the powder to be processed P, by means of the compacting means 3, to obtain the plurality of compacted elements C; a crushing step 130 of the compacted elements C, by means of the crushing means 4, to obtain the granulate material G with predefined grain size.

In particular, according to a preferred embodiment, the compaction step 120 is made by means of the rollers 5 of the compacting means 3, while the crushing step 130 comprises grinding the compacted elements C.

It cannot however be ruled out the execution of the compaction step 120 by means of different compacting means 3.

Advantageously, the granulate material G obtained by means of the process 100 has the following technical characteristics:

compression ratio of between 1.6 and 2.2;

density of between 0.8 g/cm ³ and 1.2 g/cm ³ ;

rest angle of between 35° and 60°.

Specifically, with reference to a preferred embodiment, the obtained granulate material G has the following technical characteristics:

compression ratio equal to 1.8;

density equal to 1.1 g/cm3 ;

rest angle equal to 45°.

Figure 4, in particular, shows a table indicating the technical characteristics of the granulate material G obtained using the system 1 according to the invention, compared with the technical characteristics of a granulate material obtained by means of a wet granulation process without drying of known type.

More specifically, Figure 5 shows an exemplary table indicating the grain sizes as a percentage of the granulate material obtainable using the system and the process according to the invention, compared with the grain size of a granulate material obtained by means of a wet granulation process without drying of known type.

Figure 6 also shows possible values of grain size range of the granulate material obtainable using the system and the process according to the invention.

Usefully, the feeding step 110 can comprise at least a pre-compaction step 111 of the powder to be processed P, prior to the compaction step 120. Furthermore, the feeding step 110 can comprise an adjusting step 112 of the feeding of the powder to be processed P according to the level of such powder inside the hopper 8. In particular, such level of the powder to be processed P is detected by the first probe 10 and by the second probe 11.

After the compaction step 120, the process 100 can comprise a sieving step 140 of the exiting material to separate the compacted elements C from the waste material S.

Subsequently, the process 100 can provide for a recovery step 150 of the waste material S.

Finally, with specific reference to the application of the process 100 in the ceramic industry, the process 100 can comprise a forming step 160 of the granulate material G to obtain a semi-finished product A and a firing step 170 of the semi-finished product A to obtain a vitrified product B.

The presence cannot be ruled out of a humidification step for the humidification of the crushed material, following the crushing step 130.

Neither can a sieving step be ruled out following the crushing step 130, nor the presence of a recovery and movement step for the recovery and movement of the sieved material towards the compaction step 130.

It has in practice been ascertained how the described invention achieves the proposed objects.

In particular, the fact is underlined that the compaction and subsequent crushing described above allow obtaining a dry granulate with reduced percentage of air and high fluidity.

Consequently, this permits overcoming the limits which commonly affect wet granulation of conventional type, ensuring a uniform distribution of the granulate in the forming step obtained by traditional pressing using a mold, continuous and discontinuous belt pressing with and without mold, extrusion, lamination and casting.

Furthermore, the system and the process according to the invention allow making large-size ceramic products thanks to the high fluidity, compactness, low percentage of fine powder and uniform humidity similar or improved compared to atomized powder.