Kiln and method for firing substantially flat ceramic articles

Abstract

A method and kiln (1) for firing substantially flat ceramic articles (2); the kiln (1) comprises: a firing chamber (3) with a firing zone (FZ) and a pre-heating zone (PZ); a conveyor device (6) to convey a plurality of substantially flat ceramic articles (2) along a given path (P) through the firing chamber (3); and a heating system (7) configured to heat, by convection, the pre-heating zone (PZ) and, by radiation, the firing zone (FZ) so as so as to fire the ceramic articles (2) and obtain finished ceramic products (T).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority from Italian patent application no. 102024000003460 filed on February 19, 2024, the entire disclosure of which is incorporated herein by reference.TECHNICAL SECTOR

[0002] This invention relates to a kiln and method for firing substantially flat basic ceramic articles. In particular, this invention finds advantageous but not exclusive application in the firing of substantially flat basic ceramic articles to obtain tiles or ceramic slabs.BACKGROUND OF THE INVENTION

[0003] In the field of manufacturing substantially flat ceramic articles, in particular ceramic slabs and tiles, it is known to fire the basic ceramic articles at a high temperature. The articles are obtained by pressing a semi-dry mixture (possibly followed by a decoration phase) inside a kiln, typically a tunnel kiln, along which the ceramic articles are transported on a conveyor, typically consisting of a series of ceramic rollers.

[0004] The ceramic article firing cycle is designed with great precision and involves heating the ceramic articles from the kiln inlet, holding them inside a firing chamber at a predefined temperature for sufficient time to fire them, and a subsequent controlled cooling before reaching the kiln outlet.

[0005] More specifically, the kiln is normally divided into a pre-heating zone, a firing zone proper, downstream of the pre-heating zone, and a cooling zone, located downstream of the firing zone, to reduce the temperature of the ceramic articles coming from the firing zone.

[0006] The known kilns are typically heated by a series of gas burners, typically using methane gas, organised into groups of burners and arranged above and below the advancement plane of the ceramic articles to heat, respectively, the pre-firing zone and the proper firing zone, up to the firing temperature so as to fire the ceramic articles in transit through the various zones of the kiln and obtain finished ceramic products, such as finished ceramic slabs or tiles.

[0007] For the success of the firing process, it is essential to control the environmental conditions inside the kiln very precisely, especially in terms of temperature and pressure. Specifically, it is essential to control the conditions within the firing zone of the kiln very precisely and, as far as possible, to ensure that, at least in this zone, the presence of combustion waste products is minimised, the temperature remains constantly at the firing temperature chosen for the type of ceramic article to be fired, and the pressure remains substantially constant and equal to or slightly above atmospheric pressure.

[0008] Some of the main drawbacks of the known methods and kilns for firing substantially flat ceramic articles are related precisely to the need to meet these requirements in the firing zone. More specifically, many of these drawbacks are linked to the objective difficulty of controlling the combustion gas flow and thus the amount of unburnt gas, which, especially in the firing zone, could (if it were to reach the ceramic articles) damage the ceramic articles or in any case compromise their aesthetic appearance.

[0009] This is combined with the drawbacks of the high consumption of non-renewable resources, resulting in the emission of CO 2 and all the other combustion waste products, with all the associated environmental and economic problems.

[0010] It is clear that the consumption of non-renewable resources increases as the temperatures that must be reached increase, and therefore, even in this case, a substantial part of this consumption derives from the need to maintain a constant temperature in the firing zone (which represents a section of the kiln having a certain length) that is as uniform as possible and equal to precisely the maximum temperature planned for the firing cycle.

[0011] The purpose of this invention is to provide a kiln and method for firing substantially flat basic ceramic articles, which allow the drawbacks of the prior art to be overcome, at least in part, and which are, at the same time, both easy and cheap to make.SUMMARY

[0012] According to this invention, a kiln and method for firing substantially flat ceramic articles are provided according to what is claimed in the independent claims that follow and, preferably, in any one of the claims depending directly or indirectly on the independent claims.

[0013] The claims describe preferred embodiments of this invention forming an integral part of this description.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to better understand this invention, a number of non-limiting embodiments thereof will now be described by way of example with reference to the attached drawings, in which: Figure 1 is a lateral and schematic view of a kiln for firing basic ceramic articles in accordance with this invention; Figure 2 is a schematic, cross-section representation of the kiln in Figure 1 at the firing zone; Figure 2A is a schematic, cross-section representation of the kiln in Figure 1 at the firing zone according to an additional variant of the embodiment in Figure 1; Figure 3 is a schematic, cross-section representation of the kiln in Figure 1 at the pre-firing zone according to a first variant of the embodiment of the invention; Figures 4A, 4B, and 4C represent perspective views of different possible embodiments of the electric heaters illustrated, in use, in Figure 2; and Figure 5 represents perspective views of various embodiments of the electric heaters illustrated, in use, in Figure 2A. DETAILED DESCRIPTION

[0015] In Figures 1, 2, 2A and 3, reference number 1 denotes, as a whole, a kiln for firing substantially flat ceramic articles 2. In particular, this discussion will refer to the firing of substantially (but not necessarily) flat basic ceramic articles 2 (henceforth, for brevity, only "ceramic articles 2") to obtain final ceramic products T, more precisely ceramic slabs, more precisely tiles.

[0016] The ceramic articles 2 are generally obtained by pressing a ceramic powder (a semi-dry mixture, especially having a moisture content of between 5% and 7%) mainly based on silica. According to some non-limiting embodiments, the ceramic articles 2 comprise additional inorganic oxides such as, for example, the oxides of Magnesium, Sodium Zirconium and Potassium.

[0017] In the context of this description, the term "second" component does not imply the presence of a "first" component. These terms are in fact used as tags to improve clarity and should not be understood in a limiting way.

[0018] With particular reference to Figure 1, the kiln 1 for firing ceramic articles 2 (which advantageously is a roller kiln, as will be further explained below) comprises: a firing chamber 3, which, in turn, has an opening 4A at the input station 4, through which, in use, the ceramic articles 2 enter the firing chamber 3; another opening 5A at the output station 5 through which, in use, the final ceramic products T exit the firing chamber 3; a conveyor device 6 for conveying a plurality of ceramic articles 2 along a given path P extending through the firing chamber 3 in a forward direction A from the input station 4 to the output station 5; and a heating system 7 configured to provide heat to at least part of the firing chamber 3 so as to fire the ceramic articles 2 passing through it and obtain the final ceramic products T.

[0019] Advantageously but without imposing limits, the kiln 1 is a tunnel kiln and comprises: two side walls 8 facing each other to delimit the firing chamber 3 on the side, and a vault wall 9 and a back wall 10 facing each other to delimit the firing chamber 3 at the top and bottom. Advantageously but without imposing limits, these walls 8, 9 and 10 are made of insulating and refractory material, for example, masonry in the cases illustrated.

[0020] Advantageously but without imposing limits, the firing chamber 3 comprises: at least one pre-heating zone PZ, downstream of the input station 4, wherein the ceramic articles 2 are gradually pre-heated from an input temperature (which is variable from the room temperature up to at most about 300°C) C up to a temperature of about 1100°C; and a firing zone FZ proper, arranged immediately (i.e., with no gap) downstream of the pre-heating zone PZ along the given path P, wherein the ceramic articles 2 are fired at a firing temperature, which varies from about 1100°C to about 1400°C; in particular, from about 1200°C to about 1300°C. Advantageously but without imposing limits, there is also a cooling zone RZ, located downstream of the firing zone FZ, and designed to allow a controlled reduction in temperature of the ceramic articles 2 passing through it.

[0021] According to some advantageous but non-limiting embodiments, such as the one schematically illustrated in Figure 1, the cooling zone RZ is divided into a first cooling section immediately (i.e. without a gap) downstream of the firing zone FZ for the so-called rapid cooling of the ceramic articles from the above-mentioned firing temperature up to a temperature of at least approximately 600°C; a second cooling zone for cooling the ceramic articles 2 from a temperature of approximately 600° to a temperature of approximately 450°C, and a third cooling zone, at the end of which the finished ceramic products T will be at a temperature that is essentially equal to the room temperature.

[0022] More specifically, according to some advantageous but non-limiting embodiments, the kiln 1 comprises a cooling system 11 (only schematically illustrated in Figure 1 and known, so not explained in detail here) configured to feed a controlled flow of cold air into the firing chamber 3 (in particular the cooling zone RZ) to induce the gradual cooling of the now-fired ceramic articles 2 before they leave the firing chamber 3.

[0023] Advantageously but without imposing limits, the kiln 1 (more specifically, the firing chamber 3) also comprises a pair of insulating separating bulkheads 12, essentially vertical, which extend within the firing chamber 3 transversely to the forward direction A at the ends 13a and 13b of the firing zone FZ to limit (or at least obstruct) the flow of air from the outside to the inside of the firing zone FZ (see, for example, Figure 1). In this way, it is possible to substantially reduce the gas flow from the pre-heating zone PZ or the cooling zone RZ to the firing zone FZ in order to try to facilitate the maintenance of constant ambient conditions in that zone FZ. More specifically, it tries to maintain a temperature in the firing zone FZ substantially equal to the above-mentioned firing temperature and a pressure substantially equal to atmospheric pressure, thus contributing to improving the performance of the kiln 1 in terms of the aesthetic quality of the final ceramic products T obtained (as will be better explained below).

[0024] More advantageously but without imposing limits, the insulating separating bulkheads 12 are made of an insulating and refractory material, for example, in the case illustrated in Figure 1, in masonry. Even more advantageously but without imposing limits, these bulkheads 12 are provided with relative openings 14a and 14b, aligned with each other along the given path P, and configured to allow ceramic articles 2, passing along the given path P, to pass through them. In particular, these openings 14a and 14b are sized so as to allow the passage of the ceramic articles 2 while compromising as little as possible the insulation of the firing zone FR with respect to the rest of the firing chamber 3.

[0025] Even more advantageously but without imposing limits, the firing chamber 3 has a length of at most about 600m (in particular, from about 20m to about 600m), the pre-heating zone PZ accounts for about 40% of the overall length of the firing chamber 3, and the firing zone FZ proper for about 20% of the overall length of the firing chamber 3.

[0026] Advantageously, the conveyor device 6 comprises (in particular, consists of) a series of rollers, advantageously made of ceramic material, parallel to each other transversally to said forward direction A and arranged in succession along said given conveyor path P so as to define a conveyor plane 15 intended, in use, to receive the ceramic articles 2 and move them along the given path P.

[0027] Advantageously, the above-mentioned heating system 7 is configured to heat by convection the pre-heating zone PZ so as to impose in said zone PZ a temperature increasing from the above-mentioned input temperature of the ceramic articles 2 (in particular, from a temperature varying from the room temperature that is about 20°C, up to about 300°C) up to about 1110°C (in particular, up to a firing temperature varying from about 1100°C to about 1400°C) and to heat the firing zone FZ by radiation so as to impose in said firing zone FZ a substantially constant firing temperature. Specifically, this firing temperature ranges from about 1100°C to about 1400°C; in particular, from about 1200°C to about 1300°C.

[0028] Still more advantageously but without imposing limits, the above-mentioned heating system 7 is configured to heat the pre-heating zone PZ by convection from the above-mentioned input temperature varying from about 20°C to about 300°C up to the above-mentioned firing temperature and to heat the firing zone FZ by radiation so that in this zone FZ the temperature is constantly maintained equal to the above-mentioned firing temperature (both along the forward direction A and transversally to this forward direction A).

[0029] Still more advantageously but without imposing limits, the kiln 1 comprises a first, convective heating module 16, arranged in the pre-heating zone PZ and configured to transfer, by convection, a hot air flow within the pre-heating zone PZ; and a second radiating heating module 17 arranged in the firing zone FZ, comprising at least one electric heater 18a, 18b and configured to transfer, by radiation, heat within the firing zone FZ.

[0030] Advantageously, the heating, by radiation, in the firing zone FZ ensures a more uniform distribution of heat, favouring the maintenance of substantially constant ambient conditions in terms of temperature and pressure, with obvious advantages in terms of the aesthetic appearance of the final ceramic product T. It should be added that the use of an electric heater 18a, 18b in this zone FZ of the firing chamber 3 allows a considerable reduction in the consumption of non-renewable substances and in the production of CO 2 and other combustion waste products compared to the known kilns 1, with consequent environmental and economic advantages.

[0031] According to some advantageous but non-limiting embodiments, such as, for example, the one illustrated in Figure 2, the second radiant heating module 17 comprises (in particular, consists of) at least one upper electric heater 18a (in particular, a plurality of upper electric heaters 18a, each) arranged above the conveyor device 6; at least one second lower electric heater 18b (in particular, a plurality of upper electric heaters 18b, each) arranged below the conveyor device 6; and a first electric power supply (not visible in the attached figures and known) for supplying power to the various electric heaters 18a, 18b. More specifically (advantageously but without imposing limits), in this case, each electric heater 18a, 18b is a radiant panel comprising: an insulating support 19, advantageously but without imposing limits made of refractory material; a plurality of electrically conductive elements 20, which are supported by the support 19, have electric terminals 21 at their ends configured to receive electric current from the aforementioned first electric power supply and are configured to transfer heat by Joule effect to the firing zone FZ (see Figure 2 and Figures 4A and 4B).

[0032] More specifically, advantageously but without imposing limits, in this case, the above-mentioned first electric power supply is configured to transfer an alternating electric current having an intensity ranging from approximately 10A to approximately 200A to each electrically conductive element 20 so as to heat these electrically conductive elements 20 and transfer heat by Joule effect to the firing zone FZ.

[0033] Even more advantageously but without imposing limits, each electric heater 18a, 18b is configured to receive a voltage varying between about 55V and about 220V; more advantageously but without imposing limits, the above-mentioned first electric power supply is configured to transfer a voltage varying between about 55V and about 220V to each electrically conductive element 20; in particular, at the above-mentioned electrical terminals 21 of each electrically conductive element 20. Advantageously but without imposing limits, these electrically conductive elements 20 are wire-like elements arranged on the support 19 forming a resistive coil (see Figures 4A and 4B).

[0034] Even more advantageously but without imposing limits, these electrically conductive elements 20 comprise (in particular, consist of) a material chosen from iron-chromium-aluminium alloy (FeCrAI alloy), nickel-based alloy, tungsten-based alloy or molybdenum-based alloy; even more advantageously but without imposing limits, these electrically conductive elements 20 are made of Kanthal ™< .

[0035] According to some advantageous but non-limiting embodiments, in this case, each electric heater 18a, 18b is a radiant panel having a power output ranging from about 1 to about 20kW.

[0036] Advantageously but without imposing limits, the upper electric heater 18a (in particular, each of the upper electric heaters 18a) is installed on the vault wall 9 of the kiln 1 with the electrically conductive element 20 facing the inside of the firing zone FZ.

[0037] Even more advantageously but without imposing limits, in this case, at least in the firing zone FZ, the vault wall 9 is mounted movable on the side walls 8 so that it can be moved away from the side walls 8 in order to open the kiln 1 (in particular, to access the firing chamber 3) and intervene for maintenance and replacement of the upper electric heaters 18a. While the lower electric heater 18b (more specifically, each of the lower electric heaters 18a) is installed (more specifically, rests) on the base wall 10.

[0038] Advantageously, the use of this type of electric, so-called radiant panel, heaters 18a, 18b, essentially planar is suitable for the firing of substantially flat ceramic articles 2 as it allows for a better, in particular more homogeneous, heat distribution in the firing zone FZ.

[0039] According to some advantageous but non-limiting embodiments, such as those illustrated in Figures 1 and 2, the kiln 1 also comprises a substantially horizontal protection layer 22 arranged between the conveyor device 6 (in particular, between the conveyor plane 15) and the lower electric heater 18b (in particular, and the plurality of lower electric heaters 18b) to protect the (that is, each) lower electric heater 18b in the event of unintentional damage to the ceramic articles 2 or the rollers of the conveyor device 6, which could unintentionally cause the ceramic material to fall onto the lower electric heaters 18b damaging them. In fact, damage to one (or more than one) electric heater 18b could jeopardise the firing step, but its replacement requires machine downtime, with all the productivity disadvantages this entails.

[0040] Advantageously but without imposing limits, the protection layer 22 comprises (in particular, consists of) one or more plates of conductive material, so as to physically protect the lower heaters 18b without unduly compromising their energy efficiency.

[0041] Alternatively to what has been said about the second heating module 17, according to some advantageous but non-limiting embodiments, such as the one illustrated in Figure 2A, the second heating module 17 comprises: a first plurality of radiating tubular elements 23a made of silicon carbide that are mounted between the side walls 8, above the conveyor device 6 (in particular, the conveyor plane 15); a second plurality of radiating tubular elements 23b made of silicon carbide that extend between the walls 8 below the conveyor device 6 (in particular, the conveyor plane 15); and a second electric power supply (not visible in the attached figures and known) in connection with the radiating tubular elements 23a, 23b, by means of electrical terminals 24 delimited by protection sleeves 25 made of ceramic material and / or alumina (see Figure 5), and configured to supply power to these tubular elements 23a, 23b so as to heat them up to a temperature of at least about 600°C; in particular between 600°C and 2000°C (more specifically, between 1100°C and 1650°C), thereby rendering them incandescent. Specifically, advantageously but without imposing limits, the radiating tubular elements 23a, 23b are configured to transfer heat by radiation to the firing zone FZ. Advantageously but without imposing limits, in this case, each plurality of radiating tubular elements 23a ,23b has a power output ranging from about 1 to about 20kW.

[0042] The variant of the heating module 17 equipped with radiating tubular elements 23a, 23b is easier to produce than the solution with electric, so-called radiant panel, heaters 18a, 18b, since the radiating tubular elements 23a, 23b are installed by inserting them directly into special housings made in the side walls 8 of the kiln 1 and are also easier to replace in the event of damage, for example in the event of the unintentional breakage of some ceramic articles 2.

[0043] Advantageously but without imposing limits, the radiating tubular elements 23a, 23b have a diameter of approximately 12mm to approximately 75mm and a length of approximately 775mm to approximately 2150mm, depending on the dimensions of the kiln 1 in which they are to be installed.

[0044] Even more advantageously but without imposing limits, in this case, the above-mentioned second electric power supply is configured to transfer an alternating electric current having an intensity ranging from approximately 10A to approximately 200A to each radiating tubular element 23a, 23b so as to heat them to the above temperatures and transfer heat by radiation to the firing zone FZ.

[0045] Even more advantageously but without imposing limits, each radiating tubular element 23a, 23b is configured to receive a voltage varying between about 55V and about 220V; more advantageously but without imposing limits, the above-mentioned second electric power supply is configured to transfer a voltage varying between about 55V and about 220V to each radiating tubular element 23a, 23b; in particular, at the above-mentioned electrical terminals 24 of each radiating tubular element 23a, 23b.

[0046] According to some advantageous but non-limiting embodiments not illustrated, in this case (that is, when the second heating module 17 comprises radiating tubular elements 23a, 23b), the kiln 1 (in particular, the firing chamber 3) also comprises an upper conductive layer (not illustrated in the attached figures) arranged between the plurality of upper radiating tubular elements 23a and the conveyor device 6 and a lower conductive layer arranged between the plurality of lower radiating tubular elements 23a and the conveyor device 6. These conductive layers, when provided, are configured to receive heat by radiation from the radiating tubular elements 23a, 23b and transmit it to the conveyor device 6, then to the conveyor plane 15. The presence of these conductive layers allows for a more homogeneous diffusion of heat, reducing the concentrated heat zones that could arise in the firing zone FZ by using discrete heating elements such as the radiating tubular elements 23a, 23b described above.

[0047] Advantageously but without imposing limits, the kiln 1 (in particular, the second heating module 17) also comprises a power modulator that can be operated to vary the magnitude, that is the voltage of the power supplied from the power source to the radiant panel electric heaters 18a, 18b and / or to the radiating tubular elements 23a, 23b.

[0048] Advantageously but without imposing limits, the kiln 1 also comprises at least one control unit CU configured to control the operation of the second heating module 17, for example by acting on the aforementioned power modulator, to ensure that substantially constant temperature and pressure conditions are maintained within the firing zone FZ. Even more advantageously but without imposing limits, the kiln 1 comprises at least one temperature detector (not visible in the attached figures and known, which advantageously comprises at least one thermocouple) arranged within the firing zone FZ to detect the temperature within the firing chamber 3, and the control unit CU is configured to control the actuation of the second heating module 17, for example by acting on the aforementioned power modulator, at least according to what is detected by this temperature detector.

[0049] Advantageously, the use of one or more electric heaters 18a, 18b, or one or more radiating tubular elements 23a, 23b allows, electric firing being perfectly oxidising, better aesthetic results to be obtained; in fact, various types of glazes when fired by radiation (more advantageously by means of electric heating systems) develop a higher gloss than they would develop when fired by convection.

[0050] According to some advantageous but non-limiting embodiments, such as, for example, the one schematically illustrated in Figure 1, the first convective heating module 16 comprises (in particular, consists of) at least one (known) burner 26 configured to burn a combustion mixture in order to heat the pre-heating zone PZ; at least one feeding device (not illustrated and known) for feeding a fuel mixture to (that is, at least part of) the burners 26; and a second feeding device (also known and not described in detail) for feeding an oxidizer, which advantageously but without imposing limits (comprises) is substantially ambient air (with about 21% oxygen), to the (that is, at least part of the) burners 26 so as to form the combustion mixture together with the fuel.

[0051] According to some advantageous but non-limiting embodiments, the (that is, each) burner 26 is made in accordance with the Italian patent No. 102020000010738 of the same applicant.

[0052] Even more advantageously but without imposing limits, the kiln 1 comprises a plurality of burners 26, advantageously but without imposing limits arranged in groups of burners 26, arranged (that is, mounted on the walls 8, 9, 10 of the kiln 1) on several levels, some above and some below, or only above, or only below, at the given path P to allow uniform distribution of heat within the firing chamber 3.

[0053] In detail, the burners 26 of this plurality of burners may all be of the same or different types, and as will be further explained below, they may differ in operation, fuels used and / or structure.

[0054] Advantageously but without imposing limits, the above-mentioned fuel mixture feeding device is configured to feed a fuel mixture comprising (in particular, consisting of) a fuel chosen from the following: methane gas, diesel, liquefied petroleum gas (LPG), hydrogen or a combination thereof, to (that is, to at least part of) the burners 26 as will be better described below.

[0055] For example, according to some advantageous but non-limiting embodiments, the fuel mixture comprises (in particular, consists of) at least one of either hydrogen or methane gas. In other words, the combustion mixture comprises (in particular, consists of) one part methane gas and one part hydrogen; even more specifically, the fuel mixture comprises up to about 60%, in particular up to about 50%, hydrogen. In contrast, according to other embodiments, the combustion mixture consists of only one of either hydrogen or methane.

[0056] According to other advantageous but non-limiting embodiments, the combustion mixture comprises (in particular, consists of) at least one of either methane gas or liquefied petroleum gas (LPG). In other words, the combustion mixture comprises (in particular, consists of) one part of methane gas and liquefied petroleum gas (LPG). In contrast, according to other embodiments, the combustion mixture consists of only one of either methane gas and / or diesel and / or liquefied petroleum gas.

[0057] It is understood that the fuel mixture can be formed from any number of fuels.

[0058] According to some advantageous but non-limiting embodiments, the kiln 1 comprises a mixing device (not visible in the attached figures and known) placed upstream of the fuel mixture feeding device and in fluidic connection with the fuel mixture feeding device and configured to mix two or more fuels together upstream of the fuel mixture feeding device so as to form the fuel mixture and the (each) fuel mixture feeding device comprises (in particular, consists of) a single fuel mixture supply line fluidically connected to the burner 26 (or a group of burners 26).

[0059] According to other advantageous but non-limiting embodiments, the fuel mixture feeding device comprises a feeding duct for each fuel of the fuel mixture, fluidically connected to the burner 26 (or to a group of burners 26).

[0060] In both cases, advantageously but without imposing limits, the (or each) fuel feeding device comprises a motorised, e.g., electrically operated, control valve (not visible in the attached figures and known) arranged along the (or each) fuel mixture feeding duct or downstream of the (or the various) feeding ducts that can be operated (suitably opened) to regulate the quantity (flow rate - that is, the quantity by weight in the unit of time) of the fuel mixture to be fed to the burner 26 (or to each group of burners 26), and thus the quantity (flow rate) of the fuel mixture comprised in the combustion mixture.

[0061] Similarly, in accordance with some non-limiting embodiments that are not illustrated, the oxidizer feeding device also comprises (in particular, consists of at least) one oxidizer feeding duct, fluidically connected to the burner 26 (or to each group of burners 26) and a control valve, advantageously electrically operated and advantageously arranged along the (or each) oxidizer feeding duct and able to be operated (that is, suitably opened) to regulate the quantity (flow rate) of the oxidizer to be fed to the burner 26 (or each group of burners 26) and thus the quantity (flow rate) of the oxidizer that constitutes the combustion mixture together with the fuel mixture.

[0062] Advantageously but without imposing limits (according to some embodiments not illustrated), the kiln 1 further comprises at least one temperature detection device 27, advantageously comprising at least one thermocouple, for detecting the temperature within the firing chamber 3, in particular in the pre-heating zone PZ, and a control unit CU (which may be the same as the one described above) that is configured to control the actuation of the control valve of the fuel mixture feeding device and / or the mounted control valve of the oxidizer feeding device so as to vary the quantity of fuel mixture and / or the quantity of oxidizer mixture according to the temperature detected by the temperature detection device 27, so as to ensure controlled temperature conditions, in particular, always between 300°C and 1100°C, within the pre-heating zone PZ.

[0063] Alternatively or in addition, according to some advantageous but non-limiting embodiments, there is also a flow meter (known, for example comprising a calibrated flange, and not described in detail) mounted along the inlet duct of the fuel feeding device or of the oxidizer feeding device respectively and configured to estimate a flow rate of the fuel or oxidizer mixture passing through that inlet duct. In this case, the control unit CU is configured to regulate the actuation of the fuel feeding device or of the oxidizer feeding device (in particular, the actuation of the control valve of the fuel mixture feeding device and / or the mounted control valve of the oxidizer feeding device) also depending on what is detected by this flow meter so that the temperature within the pre-heating zone PZ (in particular, along the pre-heating zone PZ) remains within a defined range (thus, in particular, as close to the optimum temperature as possible); in particular, always between 300°C and 1100°C.

[0064] According to other advantageous but non-limiting variants of this invention, the kiln 1 comprises: an identification unit as well configured to estimate a quantity related to the density of the fuel mixture, so as to estimate the type of fuel mixture being fed by each fuel mixture feeding device, and a control assembly (which according to some advantageous but non-limiting embodiments is comprised in - or is the same as - the control unit CU and) that is configured to actuate the fuel mixture feeding device and / or the oxidizer feeding device (that is, vary the actuation of the respective control valves) as a function of the temperature detected by the temperature detection device 27, and / or according to the type of fuel mixture detected by the identification unit and / or the flow rate of the fuel mixture and / or the flow rate of the oxidizer estimated by the flow rate measuring devices, for example as shown in the patent 102021000023858 of the same applicant.

[0065] According to still other advantageous but non-limiting embodiments, the (that is, each) burner 26 additionally comprises a combustion unit (not visible in the attached figures) and is equipped with at least one ignition device, also known, to trigger the combustion of the combustion mixture.

[0066] Even more advantageously, according to certain non-limiting embodiments, which are particularly advantageous when one of the two fuels is substantially liquefied petroleum gas (so-called LPG), the (that is, each) burner 26 also comprises a mixing duct (not visible in the attached figures) that is arranged upstream of said combustion unit and is structured and sized to mix the fuel mixture and the oxidizer so as to generate a combustion mixture. By doing so, that is pre-mixing the combustion mixture with the oxidizer before the combustion unit, it is also possible to obtain, using liquified petroleum gas, performance comparable to that obtained with gas burners (for example, methane gas), while at the same time obtaining a reduction in energy and operating costs, thanks to the use of liquefied petroleum gas, which is much easier to obtain and less expensive than other gas fuels, as explained in more detail in title no.102020000022663 of the same applicant.

[0067] Alternatively or in combination with the foregoing in relation to the first heating module 16, according to some advantageous but non-limiting embodiments such as that illustrated schematically in Figure 3, the first heating module 16 comprises (in particular, consists of) at least one electric heater 28 comprising, in turn: a tubular casing 29 having, at one end, an inlet duct 30 for introducing a gas comprising (in particular, consisting of) ambient air within the tubular casing 29; at least one electric heating element (not visible in the attached figures) extending within the tubular casing 29 and able to be operated to heat the gas fed via the duct 30; and a tubular outflow element 31 extending from the tubular casing 29, on the opposite side from the inlet duct 30, is configured to be traversed by the gas leaving the electric heater. More advantageously but without imposing limits, this outflow element 31 comprises at least one leading outlet 32 for introducing at least part of the heated gas to the firing chamber 3 (in particular, to the pre-heating zone PZ). Even more advantageously but without imposing limits, the leading outlet has a through hole with an equivalent diameter (i.e. the diameter of a circle with the same area as the through hole) of less than or equal to approximately 25 mm (in particular, less than or equal to approximately 20 mm). In use, this sizing of the outlet 32 ensures that the back pressure of the gas leaving increases, allowing the speed at which the aforementioned gas, once heated, is fed to the firing chamber 3 to be increased.

[0068] Advantageously but without imposing limits, in this case, the electric heating element of the electric heater 28 comprises (in particular, consists of) an electrical wire, for example a Kanthal ™< wire or a silicon carbide wire, which extends within the tubular casing 29 and is configured to heat the gas G passing through the tubular casing 6 by the Joule effect. Even more advantageously but without imposing limits, in this case, the electric heater 28 is made in accordance with what is described under 102021000016334 of the same applicant. According to some advantageous but non-limiting embodiments, like, for example, the one illustrated in Figure 3, the kiln 1 comprises a plurality of electric heaters 28. In this case, advantageously but without imposing limits, at least some of these electric heaters 28 (for example, those arranged on the vault wall 9) are installed (that is, mounted) inclined with respect to the vertical and / or with respect to the given path P; in particular, at an angle varying between about 0° and about 60° with respect to the vertical and / or at an angle varying between about 0° and about 60° with respect to the given path P. This countercurrent arrangement (that is, orthogonal to the forward direction A) of the electric heaters 28 avoids the risk that the gas jet exiting the outlet 32 arrives directly at the ceramic articles T, with the consequent risk of damaging them.

[0069] Alternatively or in addition, according to some advantageous but non-limiting embodiments, the first heating module 16 comprises (in particular, consists of) additional radiating tubular elements that are mounted between the side walls 8, advantageously but without imposing limits, also made of silicon carbide like the above-mentioned radiating tubular elements 23a, 23b (in particular, made in accordance with what has been said above about the radiating tubular elements 23a, 23b of the second heating module 17); a high-head fan configured to inject a flow of air from the outside towards the above-mentioned pre-heating zone PZ of the firing chamber 3; and yet another electric power supply (not visible in the attached figures and known) connecting with these additional radiating tubular elements to supply power to these radiating tubular elements so as to heat them so that they can transfer heat to the air supplied by the high-head fan so as to heat, by convection, the pre-heating zone PZ, imposing the temperature varying from about 20°C to about 300°C, as mentioned above.

[0070] Advantageously but without imposing limits, the kiln 1 also comprises at least one extraction device 33 (in Figure 1, a pair of extraction devices 33), each comprising (in particular, consisting of) at least one discharge chimney for conveying (advantageously by sucking them in) the discharge fumes generated within the firing chamber 3 from the combustion to the outside of the firing chamber 3; and at least one detection unit 34 arranged immediately upstream of the firing zone FZ along the given path P (and advantageously but without imposing limits also an additional detection unit 35 immediately downstream of the firing zone FZ) and configured to detect at least the temperature and / or pressure inside the firing chamber 3. In this case, advantageously but without imposing limits, the control unit CU is configured to control the actuation of the first heating module 16 and the second heating module 17 as a function of the temperature detected by the detection unit 35 and / or to control the actuation of (that is, each) extraction device 33 as a function of the pressure detected, so as to limit undesired flows of air or fumes within the firing zone FZ and at the same time ensure optimal air and fume flow conditions in the remaining parts of the firing chamber 3.

[0071] More advantageously but without imposing limits, in this case, the control unit CU is configured to control the actuation of at least the above-mentioned motorised valve of the first heating module 16 and / or the high-head fan of the first heating module 16 and / or the power modulator of the second heating module 17 as a function of the detected temperature and / or to control the actuation of the extraction device 33 (and possibly of the additional extraction device 33) as a function of the detected pressure so as to ensure that, within the firing zone FZ, temperature and pressure conditions are respectively equal to the firing temperature and atmospheric pressure.

[0072] According to another aspect of this invention, a method for firing ceramic articles 2 is presented, which advantageously but without imposing limits, is implemented with a kiln 1 for firing ceramic articles 2 produced in accordance with any of the embodiments described above.

[0073] Advantageously, the method comprises: at least one conveying step, during which the ceramic articles 2 are conveyed along a given path P in a forward direction A from an input station 4 to an output station 5 through a firing chamber 3 of a kiln 1 comprising, in turn, at least one firing zone FZ and a pre-heating zone PZ, arranged downstream of the firing zone FZ along the given path P (as further explained above in relation to the kiln 1); a pre-heating step (at least partially) simultaneous to the conveying step, during which the ceramic articles 2 are preheated by imposing a temperature increasing from an initial input temperature (in particular, varying from room temperature - i.e. about 20°C - up to about 300°C); a firing step (at least partially) simultaneous to the conveying step and (at least partially) subsequent to the pre-heating step, during which the ceramic articles 2 are fired at a firing temperature of about 1100°C to about 1400°C within the firing zone FZ. Advantageously, during the pre-heating step the pre-heating zone PZ is heated by convection and during the firing step the firing zone FZ is heated by radiation.

[0074] More advantageously but without imposing limits, during the pre-heating step a first convective heating module 16, advantageously but without imposing limits of the type described above, arranged at the pre-heating step PZ transfers, by convection, a hot air flow within the pre-heating zone PZ and during the firing step, a second radiant heating module 17, advantageously but without imposing limits of the type described above, arranged in the firing zone FZ transfers heat by radiation within the firing zone FZ.

[0075] More specifically, when the second heating module 17 comprises at least one electric heater 18a, 18b, advantageously made according to one of the above-described embodiments (that is, comprising an insulating support 19, a plurality of electrically conductive elements 20 carried by the support 19), and a first electric power supply, during the firing step the above-mentioned first electric power supply unit supplies electric current to the electrically conductive elements 20 and these electrically conductive elements 20 transfer heat by radiation to the firing zone FZ to impose the above-mentioned firing temperature.

[0076] Alternatively, when the second heating module 17 comprises at least one plurality of radiating tubular elements 23a, 23b made of silicon carbide and mounted between the side walls 8 of the kiln 1, and a second electric power supply, during the firing step the second electric power supply supplies power through the radiating tubular elements 23a, 23b so as to heat them up to a temperature of at least about 600°C (in particular, between about 600°C and about 2000°C) and these tubular elements 23a, 23b, once heated, transfer heat by Joule effect to the firing zone FZ to impose the above-mentioned firing temperature.

[0077] Even more advantageously but without imposing limits, the method for firing ceramic articles 2 further comprises a fume extraction step, (at least partially) simultaneous with the pre-heating step and the firing step, during which a first extraction device 33 arranged upstream of the pre-heating zone PZ along the given path P sucks in (more advantageously two extraction devices 33 placed at the ends of the firing chamber 3 suck in) the fumes contained within the firing chamber 3; and an adjustment step, (at least partially) simultaneous to the pre-heating step, the firing step and the extraction step, during which at least one detection unit 34 arranged immediately upstream of the firing zone FZ detects the temperature and / or the pressure inside the firing chamber 3 and a control unit CU controls the actuation of the first heating module 16 and / or the second heating module 17 according to the detected temperature and / or controls the actuation of the extraction device 33 (or possibly both extraction devices 33) according to the pressure detected by the (or each) detection unit 34, 35 to cause the pressure inside the firing zone FZ to be approximately equal to the atmospheric pressure.

[0078] The kiln 1 and the firing method of this invention have numerous advantages, including the following.

[0079] The combination of the two types of heating in different zones FZ and PZ of the kiln 1 allows the advantages of each of the two heating modes to be best exploited at the same time precisely where they are most useful, making the technical solution of this invention extremely versatile and suitable for firing ceramic articles 2 of various formats, from very thick ceramic articles 2, for which convection heating in the pre-heating zone PZ is extremely advantageous, up to thinner slabs, for example in the order of 3 mm, for which it is extremely advantageous to have at least one radiation heating of the ceramic articles 2 in the firing zone FZ.

[0080] More specifically, heating by radiation in the firing zone FZ reduces the consumption of non-renewable resources and the production of carbon dioxide, as well as other pollutants, with obvious environmental benefits while achieving the best results in terms of the aesthetic appearance of the final ceramic products T. By minimising the flow of fluids (air and / or unburnt gases) that could alter the pressure and / or temperature conditions in the firing zone FZ, the use of a radiant heating module 17, such as the one described above, in the firing zone FZ of the kiln 1 allows for more uniform heating than convection solutions. In addition, this type of heating in the firing zone FZ also minimises the risk of unburnt or combustion waste products falling onto the surface of the ceramic articles.

[0081] At the same time, the use of convection heating technologies has clear advantages in the pre-heating zone PZ. It has, in fact, been experimentally observed that in this zone PZ, the gas flows can (contrary to what has been said for the firing zone FZ) facilitate the diffusion of heat and the appropriate heating of the ceramic articles 2, which, especially for certain thicknesses, is favoured by the convective motions induced by the burners 26 and / or electric heaters 28.

Claims

1. A kiln (1) for firing substantially flat ceramic articles (2) comprising: a firing chamber (3), which comprises at least a firing zone (FZ) and a pre-heating zone (PZ); a conveyor device (6) to convey a plurality of substantially flat ceramic articles (2) along a given path (P), which extends through the firing chamber (3) in an forward direction (A) from an input station (4) to an output station (5); and a heating system (7) configured to supply heat to at least part of said firing chamber (3) so as to fire said plurality of substantially flat ceramic articles (2) going through said firing chamber (3) and obtain finished ceramic products (T) and configured to heat, by convection, said pre-heating zone (PZ) so as to impose a temperature increasing from a first input temperature (in particular, ranging from about 20°C to about 300°C) up to about 1100°C and, by radiation, said firing zone (FZ) so as to impose, in said firing zone (FZ), a substantially constant firing temperature; said firing temperature ranging from about 1100°C to about 1400°C; a first heating module (16) arranged at said pre-heating zone (PZ) and configured to transfer, by convection, a hot air flow into at least said pre-heating zone (PZ); a second heating module (17) arranged at said firing zone (FZ) and which is configured to transfer heat, by radiation, into said firing zone (FZ); and at least two side walls (8) parallel to and facing one another, which laterally delimit said firing chamber (3); said kiln (1) for firing being characterised in that said second heating module (17) comprises (in particular, consists of): at least one first electric heater (18a) arranged above said conveyor device (6), at least one second electric heater (18b) arranged below the conveyor device (6) and a first electric power supply; or a first plurality of radiating tubular elements (23a) made of silicon carbide, which are mounted between said side walls (8) above said conveyor device (6), a second plurality of radiating tubular elements (23b) made of silicon carbide, which extend between said side walls (8) under said conveyor device (6), a second electric power supply connected to said radiating tubular elements (23a, 23b) to supply electrical energy through said radiating tubular elements (23a, 23b) so as to heat them up to a temperature of at least about 600°C (in particular, ranging from about 600°C to about 2000°C); and in that: said first plurality of radiating tubular elements (23a) and said second plurality of radiating tubular elements (23b), when included, are configured to transfer heat, by Joule effect, to said firing zone (FZ); and said at least one first electric heater (18a) and said at least one second electric heater (18b), when included, each comprise: an insulating support (19), a plurality of conductive electric elements (20), which are carried by said support (19) and are configured to receive electric current from said first electric power supply and to transfer heat, by radiation, to said firing zone (FZ).

2. The kiln (1) for firing substantially flat ceramic articles (2) according to claim 1, comprising a pair of separating bulkheads (12), each extending crosswise to said forward direction (A) and at a relative end of said firing zone (FZ) and configured to limit the air flow from the outside towards the inside of the firing zone (FZ); said separating bulkheads (12) being provided with relative openings (13a, 13b), which are aligned with each other along said given path (P) and configured to allow said substantially flat ceramic articles (2) moving on said conveyor device (6) to go through them.

3. The kiln (1) for firing substantially flat ceramic articles (2) according to claim 1 or 2, wherein said second heating module (17) has a variable power from approximately 1kW to approximately 20 kW; in particular, when said second heating module (17) comprises said at least one electric heater (18a), at least one second electric heater (18b) and said first electric power supply, said first electric power supply is configured to transfer an alternating electric current having a variable intensity from approximately 10A to approximately 200A to each of said conductive electric elements (20); while, when said second heating module (17) comprises said first plurality of radiating tubular elements (23a), second plurality of radiating tubular elements (23b) and second electric power supply, said second electric power supply is configured to transfer an alternating electric current having a variable intensity from approximately 10A to approximately 200A to each radiating tubular element (23a, 23b).

4. The kiln (1) for firing substantially flat ceramic articles (2) according to claim 1 or 2 or 3, comprising a substantially horizontal protection layer (22) arranged between said conveyor device (6) and said at least one second electric heater (18b), when included (in particular, when said second heating module (17) comprises said at least one first electric heater (18a), said at least one second electric heater (18b) and said first power supply).

5. The kiln (1) for firing substantially flat ceramic articles (2) according to any of the previous claims, wherein, when said second heating module (17) comprises said at least one first electric heater (18a) and at least one second electric heater (18b), said conductive electric elements (20) comprise a material chosen from iron-chromium-aluminium alloy (FeCrAI alloy), nickel-based alloy, tungsten-based alloy or molybdenum-based alloy.

6. The kiln (1) for firing substantially flat ceramic articles (2) according to any one of the claims from 1 to 5, wherein said first heating module (16) comprises: at least one burner (26) (in particular, a plurality of burners (26), each) configured to burn a combustion mixture so as to heat said pre-heating zone (PZ) of said firing chamber (3); at least one first feeding device to feed a fuel mixture, which comprises at least one first fuel, towards said at least one burner (26) (in particular, towards each burner (26)); and a second feeding device to feed an oxidizer towards said at least one burner (26) (in particular, towards each burner (26)) so as to form, together with the fuel mixture, said combustion mixture.

7. The kiln (1) for firing substantially flat ceramic articles (2) according to claim 6, wherein said first feeding device is configured to feed a fuel mixture comprising a fuel chosen from methane gas, Diesel, liquefied petroleum gas (LPG), hydrogen or a combination thereof.

8. The kiln (1) for firing ceramic articles (2) according to any one of the preceding claims, wherein said first heating module (16) comprises at least one electric heater (28) comprising, in turn: a tubular casing (29) having, at an end, an inlet duct (30) to supply a gas comprising environment air into the tubular casing (29); at least one electric heating element, which extends inside the tubular casing (29) and can be operated to heat said gas; and a tubular outflow element (31), which extends from said tubular casing (29) on the opposite side relative to said inlet duct (30), is configured to be flown through by said gas flowing out of said electric heater (28) and comprises at least one first leading outlet (32) to lead at least part of said gas towards the outside of said tubular outflow element (31).

9. The kiln (1) for firing substantially flat ceramic articles (2) according to any one of the preceding claims, comprising: at least one extraction device (33) arranged upstream of said pre-heating zone (PZ) along said given path (P) and configured to convey the fumes contained inside said firing chamber (3) towards the outside; and at least one detection unit (34) arranged immediately upstream of said firing zone (FZ) along said given path (P) and configured to detect at least the temperature and / or the pressure inside said firing chamber (3); and a control unit (CU), which is configured to control the activation of said first heating module (16) and of said second heating module (17) as a function of the temperature detected by said detection unit (34) and / or to control the activation of said at least one suction device (33) as a function of the detected pressure.

10. A method for firing of substantially flat ceramic articles (2); the method comprises: at least one conveying step, during which the substantially flat ceramic articles (2) are conveyed along a given path (P), which extends from an input station (4) to an output station (5) through a firing chamber (3) of a kiln (1) for firing of substantially flat ceramic articles (2) comprising, in turn, at least a firing zone (FZ) and a pre-heating zone (PZ), which is arranged downstream of the firing zone (FZ) along said given path (P) and at least two side walls (8) parallel to and facing one another, which laterally delimit said firing chamber (3); a pre-heating step, which is at least partially simultaneous with said conveying step and during which said substantially flat ceramic articles (2) are pre-heated, imposing a temperature increasing from a first input temperature (in particular, ranging from about 20°C to about 300°C) up to about 1100°C inside said pre-heating zone (PZ); a firing step, which is at least partially simultaneous with said conveying step and at least partially subsequent to said pre-heating step and during which said substantially flat ceramic articles (2) are fired at a firing temperature ranging from about 1100°C to about 1400°C inside said firing zone (FZ); wherein, during said pre-heating step, a first heating module (16) arranged at said pre-heating zone (PZ) transfers, by convection, a hot air flow into at least said pre-heating zone (PZ) and said pre-heating zone (PZ) is heated by convection and, during said firing step, a second radiating heating module (17) arranged at said firing zone (FZ) transfers heat by radiation inside said firing zone (FZ) and said firing zone (FZ) is heated by radiation; and wherein said second heating module (17) comprises (in particular, consists of): at least one electric heater (18a, 18b), comprising an insulating support (19) and a plurality of conductive electric elements (20) supported by said support (19) and configured to receive electric current, and a first electric power supply to supply electric current to said conductive electric elements (20); or at least one plurality of radiating tubular elements (23a, 23b) made of silicon carbide and mounted between said side walls (8) and a second electric power supply connected to said radiating tubular elements (23a, 23b) to supply power through said radiating tubular elements (23a, 23b) so as to heat them to a temperature of at least approximately 600°C (in particular, between approximately 600°C and approximately 2000°C).

11. The method according to claim 10, comprising: a fume extraction step, which is at least partially simultaneous with said pre-heating step and with said firing step and during which at least one extraction device (33) arranged upstream of said pre-heating zone (PZ) along said given path (P) sucks the fumes contained inside said firing chamber (3); an adjustment step, which is at least partially simultaneous with said pre-heating step, with said firing step and with said fume extraction step and during which at least one detection unit (34) arranged immediately upstream of said firing zone (FZ) detects the temperature and / or the pressure inside the firing chamber (3) and a control unit (CU) controls the activation of said first heating module (16) and of said second heating module (17) as a function of the detected temperature and / or controls the activation of said at least one extraction device (33) as a function of the pressure detected by said at least one detection unit (34) so that the pressure inside said firing zone (FZ) is approximately equal to the atmospheric pressure.

12. The method according to claim 10 or 11 carried out with the kiln (1) for firing ceramic articles (2) according to any one of the claims from 1 to 9.

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