WO2020065512A1

WO2020065512A1 - Plant for producing flowable ceramic material in the form of powder for making ceramic tiles

Info

Publication number: WO2020065512A1
Application number: PCT/IB2019/058071
Authority: WO
Inventors: Emilio Benedetti; Giuseppe Cavani
Original assignee: L.B. - Officine Meccaniche - S.P.A.
Priority date: 2018-09-24
Filing date: 2019-09-24
Publication date: 2020-04-02

Abstract

Plant for producing flowable ceramic material in the form of granules that are usable for making ceramic tiles comprises a mixer for preparing a paste from a mixture of raw materials, these raw materials comprising clays, inerts and fluxes according to a defined ceramic mixture recipe of the ceramic tiles and an abrasive machine for removing portions of paste from said paste exiting said mixer to obtain ceramic granules.

Description

Plant for producing flowable ceramic material in the form of powder for making ceramic tiles

The invention relates to a plant for producing flowable ceramic material in the form of powder, in particular ceramic granules suitable for manufacturing ceramic tiles by pressing, in particular porcelain stoneware tiles.

In order to obtain highly compact ceramic tiles, i.e. with low water absorption and high resistance to breakage, in particular like porcelain stoneware tiles, it is necessary to choose suitable raw materials, for example substantially devoid of iron, reduce the dimension of the particles by milling to increase the active surface of the components of the raw materials, mix the milled raw materials so as to obtain even distribution of the various components and a more intimate contact between the components, to promote vitrification during firing. The production of ceramic granules after milling enables the ceramic tiles to be formed by pressing. Further, the ceramic granules show greater flowability than fine ceramic powders that are on the other hand difficult to handle by the conveyor devices in a production plant precisely because of the great specific surface thereof.

On the basis of the quantity of water used, two known processes for preparing raw materials for making ceramic tiles can be distinguished, which are indicated respectively as“wet” process and“dry process”.

In the wet process, the dosed raw materials are milled with a great quantity of water, for example in a cylindrical mill, to obtain a liquid suspension, known as slip, which is sprayed in an atomizer. In the latter, the drops of sprayed slip are hit by a current of hot air that evaporates water from the drops so as to obtain an atomized powder, i.e. roundish ceramic granules, measuring on average between 0.2 and 0.9 mm and having a residual water content of around 4-7%. The atomized powder is then used to supply a forming press where a pressed semifinished product is formed that is subsequently fired in a horizontal kiln, for example a roller kiln and a ceramic tile is thus produced.

During atomization, a large part of the water used as a milling vehicle is removed, this requiring great thermal energy and thus high fuel consumption.

Further, in order to permit an effective air-drops heat exchange, the atomizer has large dimensions so that the path of the slip drops is sufficient to extract the necessary water content; it is thus necessary to provide ample space for installing an atomizer that is not always available, this entailing plant complications. In the dry process, the raw materials are milled in a roller or hammer mill according to a set recipe and subsequently granulated in a rotating drum by adding a definite quantity of water, generally rather limited if compared with the water used in the wet process.

Although this “dry” technique has remedied various drawbacks of the atomization process, proving to be a valid alternative thereto, dry mixing of the raw materials can produce zones of ceramic material that are not sufficiently homogeneous, that are detectable only after the ceramic tiles have been fired, having a different water absorption feature and thus porosity. This is not desirable in ceramic tiles in general and in particular in porcelain stoneware tiles, which are required to have great resistance to flexure, which can on the other hand be reduced because of these zones of less compactness.

A traditional plant for producing ceramic tiles provides an extrusion for forming ceramic tiles starting from a paste prepared by wetting a mixture of milled clays. Nevertheless, this forming plant implements a method that does not allow types of ceramic tiles having low porosity - zero porosity like vitrified stoneware to be obtained. In fact, porcelain stoneware tiles formed by extrusion are obtained by wetting an atomized powder, with subsequent high energy consumption as mentioned above.

One object of the invention is to improve known techniques for producing flowable ceramic material in the form of powder for making ceramic tiles, in particular tiles having low water absorption, like Bib, Blla or also Bla tiles, for example porcelain stoneware tiles, according to the UNI EN 1441 1 standard.

A further object is to provide a plant for producing flowable ceramic material in the form of powder that enables the advantages of intimate mixing to be maintained that are obtainable with wet milling whilst nevertheless reducing overall energy consumption of the plant.

A still further object is to provide a plant that enables a flowable ceramic material to be obtained in the form of powder, i.e. granules having a substantially uniform composition of each grain of powder.

According to the invention, a plant is provided for the production of flowable ceramic material in the form of powder, i.e. granules, as defined by claim 1.

Owing to the invention, it is possible to produce a ceramic powder that is sufficiently flowable to supply a ceramic press by reducing the quantity of process water and thus the energy necessary for removal thereof. Owing to the plant according to the invention, the ceramic granules that are produced have a greater degree of homogeneity of the composition that the homogeneity that is obtainable with the known dry process.

Also owing to the invention, it is possible to obtain a ceramic granule having humidity, apparent density, granulometry and flowability features that are comparable with those of an atomized powder of known type obtained through atomization.

The invention can be better comprised and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non-limiting example, in which:

Figure 1 is a flow diagram of a first embodiment of a plant according to the invention; Figure 2 is a flow diagram of a second embodiment of a plant according to the invention. With reference to the Figures, the Figures show a plant 1 , 100 according to the invention for producing a flowable ceramic material in the form of powder, in particular granules, which can be used directly for making ceramic tiles. In particular, the flowable ceramic material obtained by the plant 1 , 100 can be used for making ceramic tiles that are classifiable as Bib, Blla or also Bla according to claim UNI EN 1441 1. The plant 1 , 100 includes a mixer for preparing a paste, which comprises a mixture of raw materials in water vehicle, the raw materials being selected and dosed according to a formulation or recipe of a ceramic mixture for ceramic tiles, in particular porcelain stoneware ceramic tiles. The raw materials thus comprise clays, inerts and fluxes.

The obtained paste has a water content that is such as to display“plastic” behaviour, which is due to the presence of clays in the mixture. On the basis of the steps applied to obtain the paste, it can be obtained in the form of clumps of rather small dimensions, which can have average dimensions of around 1 cm.

The paste is then subsequently granulated in an abrasive or brushing machine with a granulation technique that scrapes the paste. For example, in the abrasive machine, the paste is made to interact with a scratching drum or with a brush that removes pellets from the paste, thus obtaining ceramic granules. If the paste is in the form of clumps, granulation by scratching reduces the clumps to granules or pellets of smaller dimensions.

The still plastic ceramic granules can also be given a round shape by rolling so as to obtain rounded granules. This can occur in a screen, in particular a rotating screen, in which the granules generated by scratching traverse a perforated drum, which in addition to giving a round shape to the granules retains a larger fraction of the granules generated by the scratching of the clumps of paste; this larger fraction is recovered by a line 2, 102 called the“absc recovery”, where“absc” stands for“above screen”, which indicates that the recovered material comprises granules having a dimension that is above a maximum desired value, i.e. a dimension exceeding a set threshold. The recovery line 2, 102 shows the largest screened fraction entering the abrasive machine.

Subsequently, the rounded and selected granules are dried in a dryer, for example a fluid bed dryer, to obtain the flowable ceramic material in the form of granular powder, which is suitable for supplying a ceramic press for forming ceramic tiles.

Alternatively, after granulation by scratching, the paste pellets can be dried in the dryer without the step of rounding the ceramic granules. This occurs when the screen downstream of the abrasive machine is a flat screen or a tilted screen, i.e. a screen that does not perform the function of giving a spherical shape to the granules but only of retaining granules of an excessive dimension with respect to a maximum desired dimension possibly generated by the abrasive machine.

The plant 1 , 100, comprises another recovery line for recovering fine powder 3, 103 that collects the fine powder that may have been produced during scratching in the abrasive machine and the subsequent screening in the screen downstream of the abrasive machine. This fine powder is captured by an air filter present in the dryer and the fine powder recovery line 3, 103 conveys the fine powder inside the mixer, where the paste is prepared and which is located upstream of the abrasive machine. The powder captured by the filter of the dryer is collected during the aspiration of air necessary for removing humidity inside the fluid bed dryer. This powder is returned to the mixer to lower the humidity of the paste being formed inside the mixer.

Downstream of the dryer, the plant 1 , 100 comprises a control screen, i.e. a screen that acts to recover granular material above the threshold, which granular material has been possibly produced during maintenance of the plant 1 , 100, and of the machines or during operations of cleaning of the plant or of one of the machines that make up the plant.

An outlet port of the control screen is connected to a still further material recovery line 4, 104 that connects the control screen to the inlet port 10 - or to another dedicated inlet port - of the mixer, so that the material intercepted by the network of the control screen can be returned to the paste production cycle.

The paste prepared with the mixer has a water content, i.e. humidity, that can be comprised between 18% and 20% in weight, depending on the raw materials used. Depending on the preparation method, the paste can have lower humidity, for example between 14% and 17% in weight, in particular between 14% and 16% in weight, even more in particular about 15% in weight.

To prepare the paste, the raw materials have to be first treated according to preliminary steps that comprise dosing and milling the raw materials.

These preliminary steps in the preparation of the paste can be performed according to a “wet” technique, in which the raw materials that enter the plant have been wet-milled, as in the embodiment of the plant 1 of Figure 1 ; in this embodiment, the material that enters the plant 1 is a liquid mixture, as will be disclosed further below, from which an excess of water is separated. Or the paste is prepared according to a“hybrid” technique, as occurs in the embodiment of the plant 100 of Figure 2, in which the raw materials that enter the plant 100 have been partially wet milled and partially dry milled. In other words, preparing the paste with a hybrid technique comprises mixing in the mixer a fraction of dry-milled raw materials that enter the mixer from an inlet port 20 with the remaining fraction of wet-milled raw materials from the same inlet port 20 or from another inlet port 10.

With the hybrid technique, the wet-milled part and the dry-milled part have the same ceramic mixture recipe. In this embodiment of the same recipe, the paste is obtained in the mixer by mixing a recipe of wet-milled raw materials with the same recipe of dry- milled raw materials, dosed according to set proportions. In other words, a fraction of material having the given recipe is wet-milled and a remaining fraction of raw materials having the composition of raw materials in relation to the given recipe is dry-milled. The two fractions are then mixed together in the mixer, which can be discontinuous. The wet- milled fraction can come from a traditional wet-milled plant in which a water suspension is produced, known also as slip, comprising solid particles of the raw materials dispersed in a water vehicle. A part of slip produced is used in the plant 1 , 100 and a remaining part can be used in a traditional plant for producing atomized granules.

For example, if a ceramic mixture recipe comprises 35%“plastic” raw materials, i.e. clay raw materials and 65%“hard”,“non-plastic” raw materials, the paste is prepared by mixing 78% in weight of dry-milled recipe with 22% in weight of the same wet-milled recipe. The percentage of dry-milled recipe varies generally between about 69% and about 82% in weight. Consequently, the percentage of wet-milled recipe varies generally between about 18% and about 31 % in weight. The expert person acknowledges that these percentages of the two portions of material entering the plant 100, the dry-milled step defining a solid portion and the wet-milled step defining a liquid mixture portion, can vary on the basis of the quantity of fine material that is recovered inside the plant.

As indicated above, the paste obtained upstream of the abrasive machine can be in the form of agglomerates or lumps.

In wet preparation of the raw materials, a liquid mixture is obtained, which constitutes the material entering the plant 1 in the embodiment of Figure 1.

Subsequently, a quantity of excess water is removed from the liquid mixture in a solid- liquid separator to obtain a paste, as will be indicated below.

In the hybrid cases, a part of the raw materials constitutes a water suspension, thus a liquid portion of mixture, and a remaining part constitutes a solid portion of mixture. In this case, the liquid portion and the solid portion considered overall define the quantity of mixture that it is desired to obtain according to the set recipe and which will then be indicated as a“hybrid mixture” even when the two portions are not yet mixed together. The plant 100 of Figure 2 is configured for receiving the entering“hybrid” mixture. In the cases of a hybrid mixture, the paste is obtained by mixing the solid portion with the liquid portion, for example in a continuous mixture or in a discontinuous mixture. In the embodiment in Figure 2 a discontinuous mixer is provided. In particular, the liquid portion is injected from an inlet port 10 during mixing of the solid portion inside a mixing chamber of the mixer and the paste is formed by agglomeration. The paste obtained in these cases is in the form of agglomerates or lumps.

In all the cases, the paste is prepared from the liquid mixture or from the hybrid mixture. As indicated above, the liquid mixture, i.e. the slip, is a water suspension which can be which can be obtained by wet milling of known type in the field of production of ceramic tiles, for example in a continuous or discontinuous cylindrical mill, with or without milling bodies, comprising solid particles of the raw materials dispersed in a water vehicle. The raw materials are dosed upstream of milling and during this last step the components of the mixture are also mixed intimately.

One variant of the wet technique pre-mills the“non-plastic” raw materials when dry and subsequently places these dry pre-milled materials together with the remaining raw materials of the recipe to wet mill the entire recipe, for example in the cylindrical mill. This increases the productivity of the wet mill and improves the homogeneity of the slip. Owing to the dry pre-milling of the“non-plastic” raw materials, wet milling is so much more effective as to influence also the firing of the ceramic tiles obtained from ceramic granules of the corresponding liquid mixtures. In fact, it has been established experimentally that firing such ceramic tiles can occur at a temperature that is 1 %-l .2% lower than the usual firing temperature, this permitting less consumption of firing kiln fuel. Despite the greater energy required for dry pre-milling the “non-plastic” raw materials, the overall energy balance makes this solution economic.

In the wet embodiments, the wet-milled material is supplied entering the plant 1 and preparing the paste comprises extracting water from the water suspension, i.e. removing excess water until a paste is obtained having the desired water content, i.e. the desired humidity. The water suspension is then condensed in a separator until a solid content thereof defines a paste with the desired water content.

This can occur by filter-pressing the water suspension, in a press-filter that is not shown, to separate the solid content from a liquid content of the water suspension.

Alternatively, the excess water can be removed by leaving the water suspension to decant, for example in a decantation tank, which is also not shown, and by extracting the excess water vehicle, for example by a pumping system.

In the embodiment of Figure 2, the excess water is removed by centrifugal decanting in a decanter or centrifuge decanter or said centrifuge simply exploits the centrifugal force to separate a denser liquid from a less dense liquid. The denser liquid thus defines the paste, which will be subsequently reduced into pellets or granules inside the abrasive machine. In one embodiment, the solid content or dense liquid obtained from the removal of water from the water suspension can have high humidity, for example around 20-22%. In this case, the separate solid content or the separate dense liquid is pre-mixed in the mixer with a fraction of fine material produced during scratching and recovered in the further recovery line for recovering the fine powder 3, 103, from the dryer located downstream of the abrasive machine; in this manner the paste is formed with a desired humidity content, for example around 15%. This pre-mixing is performed in the mixer located upstream of the granulation by scratching, i.e. upstream of the abrasive machine, so that the humidity content of the product obtained from the water removal step is reduced, the formation of paste clumps is assured and moreover material that is too fine that would otherwise be rejected is recovered and returned to the material production cycle in the form of paste. However the excess water is removed, the solid content that defines the paste can be mixed in the mixer located upstream of the abrasive machine, to maintain the paste homogeneous.

Through conduits that are not shown, the excess water removed from the water suspension can be reused to prepare a further water suspension in a new production cycle for producing flowable ceramic material in the form of powder.

The quantity of water delivered to the wet milling process to obtain the liquid mixture, i.e. the water suspension, with which to subsequently obtain the paste, is removed from the aqueous solution by a much lower quantity than the quantity evaporated in the atomization process, by about half. In fact, the water content of the liquid mixture, which is generally 35-38-40%, is reduced to 5-7% water content of the atomized powder in the atomization process and, for example, to 18-20% or even down to 15% of the water content of the paste in the method according to the invention. Accordingly, the energy necessary for the plant according to the invention for removing the excess water to obtain the paste is clearly less than what is necessary for the atomizer to make the water evaporate from the aqueous solution.

It must be pointed out that the removal methods adopted by the machines provided in the plant 1 are of the mechanical type, i.e. they exploit mechanical energy and not thermal energy, as in the case of atomization, as the water is removed without supplying heat to the liquid mixture.

The saving is not only of energy but also water, in fact the excess water that is removed to obtain the paste is not lost by evaporation, as in atomization but is recovered to be used in a new paste preparation cycle in a plant 1 according to the invention.

Additives can be added to the liquid mixture, for example to adjust the rheological behaviour of the aqueous solution or of the paste or both the aqueous solution and paste that it is desired to prepare. In this description, the words“water” and“aqueous liquid” are to be considered interchangeable.

The embodiments of the plant that prepare the paste from the hybrid mixture enable a minimum quantity of water to be obtained to obtain paste, with a consequent saving in terms of energy and water consumption compared with the known atomization technique. Mixing in the mixer, for example of discontinuous type, shown in Figure 2, can be divided into two intervals of time, the first interval being the time in which the solid portion of mixture is actually mixed with the liquid portion of mixture and the paste is obtained, and the second interval being necessary for homogenizing the paste that has already been obtained. For example, in the case of a paste obtained from a hybrid mixture, the homogenization interval can also last longer than the period of time for obtaining the paste. As already indicated above, the paste obtained is reduced into granules, i.e. undergoes subsequent granulation by scratching in an abrasive machine, in which the paste that enters the machine can be in the form of small clumps and is placed in contact with a scratching drum or a brush, for example a rotating brush. The interaction of the scratching drum or of the brush with the paste produces paste pellets that are collected and define ceramic granules.

The ceramic granules can be made rounder by rolling for example in a rotating drum, such as a rotating screen, as specified above.

The still plastic, possibly rounded ceramic granules, are subsequently dried in the dryer to obtain flowable ceramic material in the form of powder.

Considering the overall expenditure of energy that is necessary for kneading and thus obtaining the paste, mixing to homogenize the paste, granulating by scratching, screening and possibly rounding the ceramic granules and drying the granules to produce flowable ceramic material in the form of powder, the expenditure nevertheless less than that of traditional atomization.

The flowable ceramic material in the form of powder obtained from the plant 1 , 100 according to the invention has features that are comparable with the features of known atomized type with the same composition of the mixture, i.e. recipe.

In Table 1 , shown below, characteristic parameters are listed of samples of the ceramic granules obtained with different embodiments of the method according to the present invention with the same recipe. Sample A is a flowable ceramic material in the form of powder obtained from a paste prepared with a hybrid technique and sample B is a flowable ceramic material in the form of powder obtained from a paste prepared with a wet technique. The Reference is the atomized powder with the same composition as the two samples A and B and is taken as a reference.

TABLE 1

The granulation and drying conditions applied are the same for the two samples A and B. As can be seen, the granulometric curves approach those of the atomized powder, in particular for sample B. Further, the flowability and the humidity of the samples enable these powder ceramic materials to be used to form ceramic tiles by pressing.

The plant 1 , 100 provides storage silos for storing the flowable ceramic material produced. From the storage silos it is possible to supply a ceramic press with the flowable ceramic material in the form of powder obtained from the plant, so as to form ceramic tiles, in particular vitrified stoneware. A set of storage silos is arranged downstream of the control screen. In the embodiment of Figure 1 , a removal line 5 is provided from the storage silos of the plant 1 , the removal line connecting the storage silos to an inlet port of the mixer, i.e. to remove from the storage silos a fraction of the ceramic granules and deliver the ceramic granules to the mixer if it is necessary to reduce the humidity of the material entering the mixer; control of the humidity of the material in the mixer is necessary to obtain the paste, which has to have a consistency contained within a certain range so that it can be scratched or brushed, i.e. so that it can be effectively scratched. The removal line may possibly not be present in the embodiment of the plant 100 of Figure 2, as in this case the reduction of the humidity in the mixer can be actuated by adding a fraction of dry milled material.

From what has been disclosed above, the plant 1 , 100 enables the prior art atomization step to be avoided and the fuel consumption to be thus reduced that is necessary for heating and removing water from the ceramic material during preparation treatments of the ceramic material to make the ceramic material suitable for forming by pressing. By reducing fuel consumption, carbon dioxide emissions into the atmosphere are further reduced, this constituting a clear advantage for the environment.

Further, the aforesaid plant 1 , 100 does not modify the milling steps upstream of the production of the flowable ceramic material and the pressing steps downstream of the production of the flowable ceramic material, this enabling the plant according to the invention to be used downstream of a wet milling plant for wet milling the raw materials, possibly using the plant to complement an existing plant that produces atomized material and is supplied by the milling plant.

In addition, the plant 100 can be arranged downstream of a dry milling plant for dry milling of the raw materials.

Claims

1 . Plant for producing flowable ceramic material in the form of granules that are usable for making ceramic tiles comprising:

a mixer for preparing a paste from a mixture of raw materials, said raw materials comprising clays, inerts and fluxes according to a defined ceramic mixture recipe of said ceramic tiles;

an abrasive machine for removing portions of paste from said paste exiting said mixer to obtain ceramic granules.

2. Plant (1 , 100) according to claim 1 , wherein said mixer comprises an inlet port (10) for receiving wet ground material.

3. Plant (1 , 100) according to claim 2, wherein upstream of said mixer a solid-liquid separator is provided for removing an excess water from a liquid blend of said raw materials until said paste having a desired humidity is obtained.

4. Plant (1 , 100) according to claim 3, wherein said separator is a centrifugal decanter.

5. Plant (1 , 100) according to claim 3 or 4, and further comprising a recovery line for recovering said excess water to reuse said excess water in a wet milling plant for milling said raw materials.

6. Plant (1 , 100) according to any one of claims 1 to 5, wherein said mixer comprises at least one further inlet port (20) for receiving powder material.

7. Plant (1 , 100) according to any one of claims 1 to 6, wherein said abrasive machine comprises a scratching drum or a brush arranged for interacting with said paste and removing by scratching pellets from said paste and thus obtaining said ceramic granules.

8. Plant (1 , 100) according to any one of claims 1 to 7, wherein downstream of said abrasive machine a screen is provided for separating said ceramic granules from granules with a dimension exceeding a set threshold size.

9. Plant (1 , 100) according to claim 8, wherein said screen is a rotating screen, comprising a rotatable drilled drum.

10. Plant (1 , 100) according to claim 8 or 9, and further comprising a recovery line (2;

102) for recovering said granules with a dimension exceeding a set threshold size, said recover line (2; 102) being conformed to take said granules with a dimension exceeding a set threshold size back to enter said abrasive machine.

1 1. Plant (1 , 100) according to any one of claims 1 to 10, wherein downstream of said abrasive machine a dryer is further provided, to dry said ceramic granules to obtain said flowable ceramic material in the fonn of a powder that is suitable for supplying a ceramic press for forming said ceramic tiles.

12. Plant (1 , 100) according to claim 1 1 , wherein a further recovery line (3; 103) is provided for recovering fine powder from said dryer to said mixer.

13. Plant (1 , 100) according to claim 1 1 or 12, wherein downstream of said dryer a control screen is provided for retaining material of dimensions exceeding a defined value.

14. Plant (1 , 100) according to claim 13, wherein a still further powder recovery line (4;

104) is provided for recovering powder from said control screen to said mixer.

15. Plant (1 , 100) according to any one of claims 1 to 14, and further comprising storage silos for storing said flowable ceramic material in the form of powder produced by said plant (1 , 100).

16. Plant (1 , 100) according to claim 15, and further comprising a removal line (5) that connects the storage silos to an inlet port of said mixer.

17. Plant (1, 100) according to any one of claims 1 to 16, wherein said plant (1 , 100) is arranged downstream of a wet milling plant for wet-milling said raw materials.

18. Plant (1, 100) according to any one of claims 1 to 17, wherein said plant (1 , 100) is arranged downstream of a dry-milling plant for dry-milling said raw materials.