The present invention relates to a method and an apparatus for the production of tissue paper.

As is known, the term “tissue paper” indicates crepe paper of the type of toilet paper and similar products having a grammage of less than 100 g/m ² , the production process of which has peculiar technical problems.

In the state of the art, the conventional method of producing tissue paper is carried out using a single-cylinder machine that comprises a dryer cylinder (also known as a “Yankee cylinder” or “Yankee dryer”), and involves drying the paper in contact with the dryer cylinder, usually aided by a hot-air thermal hood with the interposition of a liquid product constituted by a liquid mixture of chemical products, also known as a “coater”.

After drying, the paper is released from the surface of the dryer cylinder by way of a creping blade or rasp, also known as a “creping doctor”, and then it is conveyed to an apparatus for winding to be wound into a spool.

Normally, the liquid mixture is sprayed on the surface of the cylinder using hydraulic nozzles.

Usually, the liquid mixture that is distributed on the surface of the cylinder comprises the following products, mixed together before being dispensed:

- a chemical release product based on vegetable, mineral or synthetic oils, in order to facilitate the release of the sheet of paper from the dryer cylinder;

- a chemical adhesion product based on polymeric chains, for maintaining the adhesion of the sheet of paper on the surface of the dryer cylinder;

- a chemical modifier product based on mono-ammonium phosphate salts or diammonium phosphate salts, for protecting the dryer cylinder and modifying the hardness of the chemical adhesion product.

It is likewise known to vary the percentage of the chemical products in the mixture and the overall quantity of mixture dispensed, in order to seek to optimize the quality of the paper produced, to facilitate its release and to overcome drawbacks such as the overheating of the cylinder and the creation of sparks owing to the friction of the creping blade on the surface of this cylinder.

In particular, for obtaining a product of acceptable quality, it is essential that the paper, after drying, has a certain humidity (which generally needs to be comprised between 4% and 6%) and a grammage as uniform as possible along all the width. The humidity and the grammage of the paper obtained depend partially on the composition and on the quantity of the liquid product dispensed on the cylinder; this product is in fact diluted in water, part of which is evaporated by the Yankee cylinder by contact, and a minimal part is absorbed by the paper.

The quality of the paper obtained also depends on the efficacy with which the creping blade removes the paper from the surface of the cylinder, and this efficacy depends in turn on the composition and on the quality of the liquid product dispensed on the cylinder.

Currently the dosage of the chemical products for adhesion and release is entrusted to the experience of the operator, by manual actuation: the operator controlling the characteristics of the paper downstream of the cylinder decides, on the basis of their experience and training, whether and how to vary the quantity and composition of the mixture dispensed. Optionally, in some conventional apparatuses, the operator is aided by detection devices that detect one or more process parameters (such as the vibration of the creping blade or the overall humidity of the paper downstream of the cylinder), but the choice of the dosage is still entrusted to the operator. A first problem that is found with the prior art just described is represented by the incidence of human error: actually the quality of the product depends on the experience of the operator; this problem is exacerbated by the fact that with multiple shifts the operators change and therefore the errors mount up and the characteristics of the paper produced change from shift to shift.

Another problematic aspect is represented by the fact that the transverse dimension of the sheets of tissue paper usually varies from 3 to 6 meters and therefore such sheets have, downstream of the cylinder, a certain variability of grammage over the width, which entails a corresponding variability of humidity over the width: the thicker zones are more humid, while the thinner zones are less humid.

When the differences over the width exceed a certain threshold, the product is not up to the required standard and consequently there are production discards.

In the state of the art, this cannot even be corrected by the operator because the dispensing of the chemical products occurs uniformly along the full extension of the cylinder.

The patent document CN102978991 describes a solution that tries to overcome this latter problem by having, for dispensing the products on the cylinder, a series of hydraulic nozzles each of which dispenses all the products already mixed, and the flow of which can be varied individually for each nozzle. In this manner it is possible to vary the quantity of mixture dispensed in a non-uniform manner along the width of the cylinder. However, this solution does not make it possible to vary the ratio between the chemical products in the mixture in the individual zones, and therefore the dosage ratio of the products will always be the same along all of the width.

Another drawback of this solution derives from the fact that the quantity of product dispensed by each individual hydraulic nozzle is varied by varying the pressure of the product exiting from the hydraulic nozzle, by way of a respective pump for each nozzle. As is known, varying the pressure in a hydraulic nozzle also varies the width of the jet and the result of this is that increasing the pressure creates wider jets and therefore zones of overlap between the jets of adjacent nozzles, in which there is an excess of chemical products. Conversely, by decreasing the pressure, narrower jets are created which are not sufficient to cover the entire useful width of the cylinder, thus creating, between one jet and the next, “dry” zones that are not reached by the chemical products.

The aim of the present invention is to provide a method for the production of tissue paper that solves the above technical problem, compensates for the drawbacks and overcomes the drawbacks of the prior art, by allowing a more effective adjustment of the dispensing of the chemical products on the surface of the cylinder.

Within this aim, an object of the present invention is to provide a method for the production of tissue paper that makes it possible to correct the variability of humidity of the paper produced across the width.

Another object of the invention is to provide a method for the production of tissue paper that makes it possible to minimize the technical problems deriving from the friction of the creping blade on the surface of the cylinder.

Another object of the invention is to provide a method for the production of tissue paper that decreases the impact of human error and of the abilities of the operator on the quality of the product obtained.

Another object of the invention is to provide a method for the production of tissue paper that is economically advantageous in terms of energy costs and chemical products and water used, and possibly also of reduction of production discards.

This aim and these and other objects which will become better apparent hereinafter are all achieved by a method according to claim 1, optionally comprising one or more of the characteristics of the dependent claims.

This aim and these and other objects which will become better apparent hereinafter are also achieved by an apparatus according to claim 7, optionally provided with one or more of the characteristics of the dependent claims.

Further characteristics and advantages of the invention will become better apparent from the detailed description of a preferred, but not exclusive, embodiment of a method and an apparatus for the production of tissue paper, illustrated by way of non-limiting example with the aid of the accompanying drawings wherein:

Figure 1 is a schematic diagram of an apparatus for the production of tissue paper, according to the invention;

Figure 2 is a more detailed schematic diagram of part of the apparatus of Figure 1;

Figure 3 is a detail, in side view, of an atomizer nozzle of the apparatus according to the invention;

Figure 4 is a series of diagrams that show, from the top down: a humidity profile of the paper detected by a scanner downstream of the cylinder, in an apparatus according to the invention; a dispensing profile of the chemical release product modeled on the basis of the humidity profile; a dispensing profile of the mixture of chemical adhesion product and chemical modifier product dispensed for release, modeled on the basis of the humidity profile; the profile of the resulting mixture on the surface of the cylinder;

Figure 5 is a schematic diagram of part of a possible embodiment of an apparatus according to the invention;

Figure 6 is a more detailed perspective view of part of the apparatus of Figure 5;

Figure 7 is a cross-sectional view taken along a vertical plane of part of the apparatus of Figure 5;

Figure 8 is a schematic diagram of part of the apparatus of Figure 5;

Figure 9 is a diagram that shows a humidity profile of the paper and a dispensing profile of the nozzles, at various sectors of the paper.

With reference to the figures, the method for the production of tissue paper 90 having a grammage of less than 100 g/m ² using a single-cylinder machine 10 is provided preferably by the apparatus described below and generally designated by the reference numeral 1.

Generally, the apparatus 1 for the production of tissue paper comprises a single-cylinder machine 10 which comprises a dryer cylinder 2 (i.e. a dryer cylinder of the type also known as a “Yankee cylinder” or “Yankee dryer”) which has a work surface 21 that has a useful width L that is adapted to be covered by the paper 90.

Preferably, the cylinder 2 is, in a known manner, surmounted by a hot air hood 211 which is per se known in the technical sector (they are known as “Yankee hoods”).

This type of single-cylinder machines 10 for the production of tissue paper are widely known in the sector, in all their implementation details, and therefore it will not be described further in the description. Here we clarify only that the term “useful width” L means the width of the work surface 21, which should be understood as the surface portion of the cylinder 2 that is effectively covered by the paper 90 in the drying step, therefore such useful width L can vary according to the width of the paper being in production. At the sides of the work surface 21, the surface of the cylinder 2 presents edge zones 22a, 22b which remain uncovered during the drying step, as they are not covered by the paper 90. In practice the overall width of the edge zones 22a, 22b is equal to the difference between the width of the paper 90 being in production and the total width of the cylinder 2.

The apparatus 1 also comprises, in a known manner, a creping blade 3, or rasp or creping cutter, which is configured to release the paper 90 from the work surface 21 of the cylinder 2. The creping blade 3 is positioned, in a known manner, tangentially to the surface of the cylinder 2, and thus is in contact with the entire surface of the cylinder including the edge zones 22a, 22b.

The apparatus can also comprise, upstream and downstream of the cylinder 2, any conventional apparatus necessary to the production of the paper, such as for example spooling devices for winding the paper in a spool 190 and devices to convey the paper, presses 300, etc.

In the preferred embodiments, the apparatus 1 comprises one or more vibration sensors 31, 32, 33 which are configured to detect the vibrations of the creping blade 3, i.e. at least one parameter representing the vibrations of the blade (preferably frequency and/or intensity of the vibrations).

Preferably, the vibration sensors 31, 32, 33 comprise:

- a first vibration sensor 31 positioned to detect the vibrations on a central portion of the creping blade 3 positioned in contact with the work surface 21 of the cylinder 2 within the useful width L, and

- two second vibration sensors 32, 33 positioned to detect the vibrations on two respective lateral portions of the creping blade 3 which are arranged in contact with the cylinder 2 at respective edge zones 22a, 22b outside the useful width L.

The presence of the second vibration sensors 32, 33 makes it possible to detect the vibrations of the creping blade 3 in the zones where this is in direct contact with the surface of the cylinder and not with the paper 90.

The vibration sensors 31, 32, 33 are operatively connected to an electronic control system 200 which will be better described hereinbelow.

In the preferred embodiments, which comprise the illustrated embodiment, the apparatus 1 also comprises a scanner 8 configured to detect a humidity profile and/or a grammage profile of the paper 90 along the width of the paper 90, downstream of the cylinder 2, i.e., to scan the humidity and/or of the grammage of the paper 90 transversely, detecting how the humidity and/or the grammage of the paper varies transversely.

The grammage and the humidity are correlated (they are directly proportional to each other) and therefore the scanner 8 can be configured to detect either the grammage or the humidity (preferably the humidity). However the possibility is not ruled out, for a more exact detection, of detecting both grammage and humidity. In order to carry out this function the scanner 8 can, for example, be provided with a radioactive source inside a head that scans the paper 90 along its width, or with an infrared sensor, or in another known way. The aforementioned scanner 8, when present, is also operatively connected to the electronic control system 200.

According to an optional and advantageous characteristic, the apparatus 1 comprises, as an alternative or in addition to the scanner 8 (or integrated in it), a thermographic camera or thermoscanner 66 (for example infrared) configured to detect a thermal profile of the paper 90 along the width of the paper 90, downstream of the cylinder 2 (Yankee dryer) and preferably at the spool 190. In practice the thermographic camera 66 is configured to execute a thermographic scan, transversely, of the paper 90, detecting the difference in temperature in the various zones. The Applicant has in fact observed that the cooler zones correspond to more humid zones and the warmest zones to less humid zones.

The above mentioned thermographic camera 66, when present, is also operatively connected to the electronic control system 200.

According to the invention, the apparatus 1 comprises at least two separate series 5, 6, 7 of atomizer nozzles 5a-5o, 6a-6o, 7a, 7b for dispensing the chemical products on the surface of the cylinder 2, and more precisely at least one first series 5 of first atomizer nozzles 5a-5o and a second series 6 of second atomizer nozzles 6a-6o.

The first series 5 of first atomizer nozzles 5a-5o is configured to dispense a chemical release product a on the surface of the cylinder 2 along at least all the useful width L. To this end, the atomizer nozzles 5a-5o of the first series 5 are aligned beside each other, preferably supported by a first bar 50 which optionally can move with respect to the cylinder 2, so that the jets Z of the first atomizer nozzles 5a-5o cover the entire useful width L of the cylinder 2 without overlapping.

The second series 6 of second atomizer nozzles 6a-6o is configured to dispense a chemical adhesion product b mixed with a chemical modifier product adapted to protect the surface of the cylinder and to modify the hardness of the chemical adhesion product b, on the surface of the cylinder 2 along at least all the useful width L (for convenience this mixture is generally indicated with the letter b). The atomizer nozzles 6a-6o of the second series 6 are also aligned beside each other, preferably supported by a second bar 60 which optionally can move with respect to the cylinder 2, so that the jets of the second atomizer nozzles 5a-5o cover the entire useful width L of the cylinder 2 without overlapping.

Optionally, the apparatus 1 also comprises a third series 7 of atomizer nozzles 7a, 7b which comprises at least one atomizer nozzle 7a, 7b for each one of the two edge zones 22a, 22b of the cylinder 2 which is configured to dispense on such edge region 21a, 21b a special liquid product C adapted to protect the surface of the cylinder 2 in order to prevent excessive overheating, damage, and sparks. Note that in the preferred and illustrated embodiment, the nozzles 7a, 7b of the third series are two in number, one for each one of the two edge zones 22a, 22b.

The group of the series 5, 6, 7 of atomizer nozzles 5a-5o, 6a-6o, 7a, 7b forms an apparatus for the distribution of the chemical products, which is generally indicated with the numeral 9 and which is illustrated in detail in Figure 2.

The atomizer nozzles 5a-5o, 6a-6o, 7a, 7b, of the first series 5, of the second series 6 and of the third series 7 (if any), are configured to convey the respective chemical product a, b, c to be dispensed using a vector gas V (preferably air).

Preferably, as illustrated in detail in Figure 3, each one of the atomizer nozzles 5a-5o, 6a-6o, 7a, 7b comprises a central dispensing orifice 81 for dispensing a jet of the chemical product a, b, c to be dispensed and two lateral orifices 82, 83 which are configured to dispense two flows of vector gas V which intersect so as to form, together with the jet of the chemical product a, b, c, a conical jet Z that has a preset divergence (i.e. width at a preset distance). In practice the resulting conical jet Z is composed of gas (preferably air) atomized with the chemical product which has previously been emulsified in water (since it arrives at the nozzle 5a-5o, 6a-6o, 7a, 7b already emulsified in water).

The flow rate of the central orifice 81 is adjustable by way of an internal adjustment needle so that when varying the flow rate of chemical product a, b, c dispensed, the divergence of the conical jet Z remains unchanged. In this manner, and also by varying the flow rate of the individual nozzles 5a-5o, 6a-6o, 7a, 7b in a series 5, 6, 7, the creation is avoided of zones of the surface of the cylinder where the jets overlap, also avoiding the creation of dry zones.

In the preferred embodiments, which comprise the embodiment shown in Figure 2, each series 5, 6, 7 of atomizer nozzles 5a-5o, 6a-6o, 7a, 7b is fed by a respective conduit 55, 65, 75 for feeding the vector gas V under pressure and by a respective supply conduit of liquid products 51, 61, 71. The conduits 55, 65, 75 for feeding the vector gas V under pressure are fed by a source of pressurized gas 99, for example a compressed air dispensing device.

Each one of the supply ducts liquid products 51, 61, 71 is fed by a respective chemical product feeder conduit 53, 63, 73 (originating from a respective tank of product 54, 64, 74) and by a respective water feeder conduit 52, 62, 72 (originating from a source of water 88).

Preferably, each chemical product feeder conduit 53, 63, 73 is provided with a respective variable flow-rate pump 94, 95, 96 for controlling the flow thereof, and each water feeder conduit 52, 62, 72 is provided with a respective flow-rate pump 91, 92, 93 for controlling the flow thereof. In practice, each one of the chemical products a, b, c to be dispensed are mixed (preferably emulsified) with water in the respective supply conduit for liquid products 51, 61, 71, preferably by way of a static mixer positioned at the intake of the supply conduit for liquid products 51, 61, 71 where the respective intake ducts of the chemical product 53, 63, 73 and the water supply 52, 62, 72 converge.

In this manner it is possible to measure the percentage of water and chemical product a, b, c for each series 5, 6, 7 of atomizer nozzles independently.

As previously mentioned, according to the invention, the apparatus 1 comprises an electronic control system 200 which is operatively connected to the atomizer nozzles 5a-5o, 6a-6o, 7a, 7b (of all the series 5, 6, 7 present) and is configured to selectively vary the flow rate of the individual atomizer nozzles 5a-5o, 6a-6o of the first series 5 and of the second series 6, as a function of at least one process parameter, so as to vary the ratio between the chemical adhesion product b and the chemical release product a and/or the overall quantity of chemical products dispensed along the useful width L as a function of such at least one process parameter.

The electronic control system 200 is any conventional programmable information processing device, such as a computer, an electronic controller or the like.

The person skilled in the art can select one or more process parameters as a function of which to vary the flow rate of the atomizer nozzles 5a-5o, 6a-6o, 7a, 7b according to requirements and to information in the state of the art.

In the preferred embodiments, such process parameter is at least one among:

- a parameter representing the vibrations of the creping blade 3, preferably the frequency and/or intensity of the vibrations, detected using the vibration sensors 31, 32, 33;

- the rotation speed of the cylinder 2, detected in a known manner;

- the humidity profile of the paper 90 detected along the width of the paper, after the release, downstream of the cylinder 2, by way of the scanner 8;

- the grammage profile of the paper 90 detected along the width of the paper 90, after the release, downstream of the cylinder 2, by way of the scanner 8;

- the thermal profile of the paper 90 along the width of the paper 90, detected using the thermographic camera 66, downstream of the cylinder 2, after the release; preferably such detection of the thermal profile is executed in the area where the paper is wound, at the spool 190, since the low thickness of the paper renders the detection in other areas imprecise.

In some embodiments that are particularly advantageous in terms of optimization of the distribution of the chemical products a, b, c, such at least one parameter comprises a parameter indicative of the vibrations of the creping blade 3, both the rotation speed of the cylinder 2 and at least one chemical or physical parameter indicative of the humidity detected along the width of the paper 90, after the release, such chemical or physical parameter comprising one of the following: humidity profile of the paper 90, grammage profile of the paper 90, thermal profile of the paper 90.

In these embodiments, the flow rate of the individual nozzles 5a-5o, 6a-6o is varied according to an algorithm that combines the parameter indicative of the vibrations of the creping blade 3, the rotation speed of the cylinder 2, and the chemical or physical parameter indicative of humidity detected along the width of the paper 90.

In practice, varying the flow rate of the atomizer nozzles 5a-5o of the first series 5 with respect to the flow rate of the atomizer nozzles 6a-6o of the second series 6, or vice versa, varies the ratio between the chemical adhesion product b and the chemical release product a.

Furthermore, varying the flow rates of one or more atomizers 5a-5o, 6a-6o within a series 5, 6 varies the distribution of adhesion product b or of release product a along the useful width L.

For example, it is possible to have an increase in the quantity of the chemical products dispensed as the rotation speed of the cylinder increases and/or as the vibrations of the creping blade 3 increase.

In particular, the electronic control system 200 can be configured to vary the flow rate of the individual nozzles 5a-5o, 6a-6o within a same series, based on the chemical or physical parameter indicative of the humidity detected along the width of the paper 90 (transverse profile of humidity and/or of grammage and/or of temperature) so as to, for example, increase the chemical adhesion product b in zones where the paper 90 is more humid and reduce it in zones where the paper 90 is drier.

An example of variation of the flow rate of the atomizer nozzles of the first 5 and of the second 6 series, as a function of the humidity profile of the paper 90, so as to vary the final composition of the resulting mixture on the cylinder along the width, is shown in Figures 4 and 9.

In Figure 4, M.F (“Moisture Profile”) indicates the humidity profile detected on the paper (continuous line) and the ideal or target humidity profile (dashed line), P.a indicating the chemical product a (release) dispensed by the various first nozzles 5, and P.b the chemical product b (adhesion + modifier) dispensed by the various second nozzles 6, C.T. (“Total Coating”) indicating the combination of the chemical products dispensed.

In Figure 9, M.F. indicates the humidity profile detected on the paper (PR., dot-dash line) and the ideal or target humidity profile (P.T., dashed line), and N.F. indicates the flow of chemical products dispensed by the nozzles (F.N.5: flow of product A dispensed by the nozzles of the first series 5; F.N.6: flow of product B dispensed by the nozzles of the second series), and P.S. indicates the various sectors of the paper 90, transversely.

Note that in zones where the paper produced has low humidity, the flow rate of the nozzle of the first series 5 is increased, increasing the quantity of release product a and, simultaneously, the flow rate of the nozzle of the second series 6 is decreased, reducing the quantity of adhesion and modifier product† conversely, in zones where the paper produced has high humidity, the flow rate of the nozzle of the first series 5 is decreased, reducing the quantity of release product a and, simultaneously, the flow rate of the nozzle of the second series 6 is increased, increasing the quantity of adhesion and modifier product. It is evident from Figures 4 and 9 that the ratio between the chemical products a, b, c varies along the width of the cylinder as a function of the humidity profile.

It is also possible to configure the electronic control system 200 to vary the ratio between water and chemical product a, b, c in each supply conduit for liquid products 51, 61, 71 (for example by acting on the pumps 91, 92, 93, 94, 95, 96) and also the pressure of the vector gas in each conduit 55, 65, 75 for feeding the vector gas V.

When the third series 7 of atomizer nozzles 7a, 7b is present, the electronic control system 200 is operatively connected to these as well and is configured to vary the flow rate of these nozzles 7a, 7b preferably as a function of the parameters indicative of the vibration of the creping blade 3 detected by the second vibration sensors 32, 33 at the respective edge zones 22a, 22b outside the useful width L, for example so as to increase the delivery of the special product c for the edge zones as the vibrations at the lateral portions of the creping blade 3 increase.

As can be seen from the foregoing description, in general terms, the method for the production of tissue paper 90 using a single-cylinder machine 10 that comprises a dryer cylinder or dryer cylinder 2 comprises: - a distribution step of at least one liquid product a, b, c on at least one work surface 21 of the cylinder 2 that has a useful width L that is adapted to be covered by the paper 90;

- a drying step of the paper 90 on the work surface 21 of the cylinder 2 with the at least one chemical product a, b, c interposed;

- a release step of the paper 90 from the work surface 21 of the cylinder 2 by way of a creping blade 3;

- a detecting step of at least one process parameter (as previously described).

The at least one liquid product a, b, c comprises at least: a chemical adhesion product b adapted to facilitate the adhesion of the paper on the work surface 21 of the cylinder 2 and a chemical release product a adapted to facilitate the release of the paper 90 from the work surface 21 of the cylinder 2.

In the distribution step, the quantity and/or the composition of the liquid product is adjusted as a function of the at least one process parameter. according to the invention, in the distribution step of the liquid product, the at least one liquid product is distributed by way of:

- a first series 5 of first atomizer nozzles 5a-5o which dispense the chemical release product a along at least all the useful width L, carrying it using a vector gas V,

- and a second series 6 of second atomizer nozzles 6a-6o which dispense the chemical adhesion product b, together with a chemical modifier product adapted to protect the surface of the cylinder and to modify the hardness of the chemical adhesion product b, along at least all the useful width L, carrying it using a vector gas V.

In the distribution step of the flow rate of the individual atomizer nozzles 5a-5o, 6a-6o of the first series 5 and of the second series 6 is selectively varied as a function of the at least one process parameter, so as to vary the ratio between the chemical adhesion product b and the chemical release product a and/or the overall quantity of chemical products dispensed along the useful width L as a function of the at least one process parameter.

Preferably, the chemical release product a is based on vegetable, mineral or synthetic oils.

Preferably, the chemical adhesion product b is based on polymeric chains.

Preferably, the chemical modifier product is based on mono ammonium phosphate salts or diammonium phosphate salts.

Figure 5 is a schematic diagram of an apparatus for the production of tissue paper, according to the invention, that comprises a single-cylinder machine 10 which comprises a dryer cylinder 2. In Figure 5, as in Figure 1 and Figure 8, for the sake of simplicity only one of the two series of nozzles is shown.

In practice it has been found that the method and the apparatus, according to the present invention, achieve the intended aim and objects in that they enable a more effective adjustment of the dispensing of the chemical products on the surface of the cylinder.

Another advantage of the method and of the apparatus, according to the invention, consists in that they make it possible to correct the variability of humidity of the paper produced across the width, by acting on the adjustment of the thickness and of the hardness of the coater on the Yankee cylinder, which entails an improvement of the humidity profile of the sheet and as a consequence of the creping.

Another advantage of the method and of the apparatus, according to the invention, consists in that they make it possible to minimize the technical problems deriving from the friction of the creping blade on the surface of the cylinder.

Another advantage of the method and of the apparatus, according to the invention, consists in decreasing the impact of human error and of the abilities of the operator on the quality of the product obtained. Another advantage of the method and of the apparatus, according to the invention, consists in being economically advantageous in terms of energy costs and chemical products and water used, and also of reduction of production discards. The method and the apparatus, thus conceived, are susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

Moreover, all the details may be substituted by other, technically equivalent elements. The disclosures in Italian Patent Application No. 102019000018056 from which this application claims priority are incorporated herein by reference.

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