PIFFERI, Giuseppe (55 Via Ludovico Ariosto, Castellarano, Castellarano, I-42014, IT)
MORI, Gherardo (4 Vicolo Oratorio, Verona, Verona, I-37121, IT)
PIFFERI, Giuseppe (55 Via Ludovico Ariosto, Castellarano, Castellarano, I-42014, IT)
Claims
1 ). A method for realising prefabricated refractory plates to be used in a tunnel kiln comprising stages of:
- mixing powders comprising 10-35% clays, 0.1 -35% kaolins, 0.1 -80% aluminas, the values being expressed in percentage by weight; - moistening the powder mixture with water up until a powder paste is obtained having a consistency enabling the paste to be extruded;
- extruding the paste to form a plate;
- drying the extruded plate;
- firing the dried plate. 2). The method of claim 1 , wherein the powder mixture comprises 15-35% of clays, 15-35% of kaolins, 0.1 -10% of aluminas and further 15-35% of talcs and 17-38% of chamotte, the values being expressed in percentage by weight.
3). The method of claim 1 , wherein the powder mixture comprises 15-35% clays, 5-25% of kaolins, 0.1 -10% aluminas and further 0.1 -20% of chamotte and 40-60% of sillimanite, the values being expressed in percentage by weight.
4). The method of claim 1 , wherein the powder mixture comprises 10-30% of clays, 0.1 -20% of kaolins, 50-80% of aluminas and further 5-15% of zirconiums, the values being expressed in percentage by weight.
5). The method of claim 1 , wherein the stage of moistening is done using water at 15% weight with respect to the overall weight of the mixture of the powders to be moistened.
6). The method of claim 1 , wherein the stage of drying is done at a maximum temperature of 80 °C for a duration of about 24-30 hours.
7). The method of claim 1 , wherein the stage of firing is done at a maximum temperature of about 1300-1400 °C. 8). A prefabricated refractory plate (2) for realising a refractory covering in a tunnel kiln (1 ) comprising a composition as follows: AI 2 O 3 35-74, SiO 2 19-55, Fe 2 O 3 +TiO 2 <3.5, CaO-Na 2 O-K 2 O <1.5, which values are expressed in percentage by weight. 9). The plate (2) of claim 8, comprising a first composition (K) as follows: AI 2 O 3 35-37, SiO 2 52-55, MgO 7-8, Fe 2 O 3 +TiO 2 <3.5, CaO-Na 2 O-K 2 O <1.5, which values are expressed in percentage by weight. 10). The plate (2) of claim 8, comprising a second composition (S) as follows: AI 2 O 3 52-55, SiO 2 42-45, MgO 0.1 -1 , Fe 2 O 3 +TiO 2 <2, CaO-Na 2 O-K 2 O <1 , which values are expressed in percentage by weight.
11 ). The plate (2) of claim 8, comprising a third composition (P) as follows: AI 2 O 3 72-74, SiO 2 19-20, ZrO 2 6, Fe 2 O 3 +TiO 2 <0.7, CaO-Na 2 O-K 2 O <0.4, which values are expressed in percentage by weight. 12). A tunnel kiln (1 ) for masonry products, subdivisible into a first initial preheating zone (A), a second central firing zone (B) and a third final cooling zone (C), the kiln (1 ) comprising a refractory internal covering realised by the prefabricated plates (2) placed with a surface thereof facing kiln (1 ) heat, each plate (2) comprising a composition as follows: AI 2 O 3 35-74, SiO 2 19-55, Fe 2 O 3 +TiO 2 <3.5, CaO-Na 2 O-K 2 O <1.5, which values are expressed in percentage by weight.
13). The kiln (1 ) of claim 12, wherein the plates (2), located in both the first part of the preheating zone (A) of the kiln up to 700 °C and the cooling zone (C) with temperatures not above 900 °C exhibit a first composition (K) comprising: AI 2 O 3 35-37, SiO 2 52-55, MgO 7-8, Fe 2 O 3 +TiO 2 <3.5, CaO- Na 2 O-K 2 O <1.5, the numerical values being expressed in weight percentages.
14). The kiln (1 ) of claim 13, wherein the plates (2) located in a zone where a consistent alkaline attack is expected exhibit a second composition (S) comprising: AI 2 O 3 52-55, SiO 2 42-45, MgO 0.1 -1 , Fe 2 O 3 +TiO 2 <2, CaO- Na 2 O-K 2 O <1 , ,the numerical values being expressed in weight percentages. 15). The kiln (1 ) of claim 14, wherein the plates (2) located in both the final part of the preheating zone (A) with temperatures above 700 °C and the first half of the firing zone (B) comprise the second composition (S). 16). The kiln (1 ) of claim 13, wherein the plates (2) located in both the second half of the firing zone (B) and in the first part of the cooling zone (C) with temperatures of above 900°C exhibit a third composition (P) comprising: AI 2 O 3 72-74, SiO 2 19-20, ZrO 2 6, Fe 2 O 3 +TiO 2 <0.7, CaO-Na 2 O-K 2 O <0.4, the numerical values being expressed in weight percentages, or the first composition (K). 17). The kiln (1 ) of claim 14, comprising a refractory tooth (3) interposed between the refractory internal covering formed by the plates (2) and a lower base (4), the tooth (3) comprising the second composition (S) or the first composition (K). |
A METHOD FOR REALISING REFRACTORY PLATES, A PLATE RELATIVE THERETO AND A TUNNEL KILN REALISED WITH THE PLATES
Technical Field
The invention relates to a method for realising prefabricated refractory plates to be used in tunnel kilns for firing masonry and ceramic items, and the like. The invention also relates to a refractory plate made according to the above method for forming internal walls of a tunnel kiln. Additionally, the invention relates to a tunnel kiln made with the above- mentioned prefabricated plates. Background Art
As is known, the use of tunnel kilns is progressively more common for firing masonry products as well as ceramic products such as tiles. The tunnel kiln is substantially a straight channel constituted by vertical walls, also called piers, by a covering, also called a vault, and by a mobile floor on wheels, typically a train of cars; the mobile floor is for moving the product from the kiln entrance to the kiln exit. During the journey from entrance to exit, the product encounters successive transversal sections, at each of which the mean temperature is kept fixed and constant over time.
In particular, the kiln is basically subdivisible into three zones: a start zone for pre-heating, a central zone for firing, and a final zone for cooling. The advantages of the use of the tunnel kilns are known and are not described herein.
One of the main characteristics of importance in a tunnel kiln is surely the type of refractory covering used for realising the internal walls. In the past, tunnel kilns were made using fired refractory bricks: traditionally an aluminous silicon with 40% alumina in the zones undergoing T>500°C and common redbrick in the zones undergoing T<500°C.
Although the use of refractory bricks represents an ideal solution in relation to the length of working life of a kiln, thanks to the high mechanical resistance and the high working temperatures of kilns thus constructed, the use thereof leads to some disadvantages and drawbacks. It is in fact necessary that the kilns be made on-site, starting with small-sized bricks which have to be stacked in an ordered fashion to form walls that are typically 2 metres-plus high, and the vault construction is very difficult, so construction times are very long and specialised workmen are required, leading to an increase in realisation costs. To obviate the above drawback, nowadays completely prefabricated tunnel kilns are increasingly widespread.
The refractory walls are constituted by prefabricated plates of a predefined height, for example, 2 metres, generally C-shaped and placed side-by-side in order to form the internal wall as well as the vault of the kiln; they are joined by support brackets and elements.
In the prior art, these plates are made using refractory concrete, based on chamotte and comprising, briefly, the following mean composition: AI 2 O 3 42, SiO 2 38, Fe 2 O 3 8, CaO 12, the values being in weight percentages. This solution too includes some disadvantages and drawbacks. The refractory concrete exhibits a mechanical resistance and a hardness that are lower with respect to fired refractory material, as well as greater heat conduction properties.
Further, the refractory concrete tends not to offer adequate resistance to eventual alkaline attacks. Indeed, in determined zones of the tunnel kiln, undesired cracks appear in the concrete plates, which have to be repaired in order to consolidate the walls, requiring a constant and continuous maintenance watch. This is due to the fact that in the kiln zone comprised between the point in which the temperature of 800 °C is first reached (in the preheating zone) and up to a temperature of about 1000°C (in the firing zone), there is a freeing of vapours from substances present internally of the clay being fired, which
vapours can chemically attack the concrete, with the resulting formation of cracks.
In particular, sodium and potassium alter the refractory concrete, forming new compounds with greater volumes. The surface of the product dilates and therefore curves towards the inside, taking on a convex shape. The surface layer is then forced to detach from the body of the product. When the altered surface crust reaches the point where it completely detaches, the sodium and potassium can continue their action on the more internal layers. This aggression, known as an alkaline attack, is a consequence of a non- sufficient temperature for guaranteeing the formation of a crystalline layer on the concrete plates which would afford greater protection of the plates. There is, therefore, a strongly felt need to have a method to hand which enables realisation of prefabricated refractory plates which are easily and quickly assemblable to realise a tunnel kiln, the plates exhibiting a high mechanical resistance and hardness; and this in the scope of a simple, economical and rational solution. Disclosure of Invention
The aim of the present invention is to provide a method for realising refractory plates for constructing a tunnel kiln, which method enables the drawbacks as described with reference to the prior art to be obviated.
The aim is attained by a method for realising refractory plates in agreement with claim 1 , thanks to a plate as in claim 8 and thanks to a tunnel kiln as in claim 12. The dependent claims delineate preferred and particularly advantageous embodiments of the method, of the plate and of the kiln of the invention. Brief description of the Drawings
Further characteristics and advantages of the invention will better emerge from a reading of the following description, provided by way of non-limiting example, with the aid of the illustrated figures of the drawings, in which: - Figure 1 is a schematic side view of a tunnel kiln;
- Figure 2A and 2B illustrate the progression of two theoretical firing curves inside the kiln;
-A-
- Figure 3 is a section view along line Ill-Ill of figure 1.
Best Mode for Carrying Out the Invention
With reference to figure 1 , 1 denotes in its entirety a tunnel kiln for production of masonry products of the present invention. The tunnel kiln 1 exhibits, in very general terms, a first initial preheating zone, denoted by A, a second central firing zone, denoted by B, and a third final cooling zone, denoted by C.
The kiln 1 comprises an internal refractory covering formed by prefabricated plates 2 made according to the present invention, In particular, each plate 2 exhibits a surface area of about 2000x700 mm and a thickness of about 100 mm. However, the size can vary according to the constructional needs and, furthermore, should the plates be destined to form the vault, where the thickness is generally smaller than that of the walls.
In the following description no detailed description of the components of the tunnel kiln will be made since they are well known in the sector.
In the preferred embodiment of the present invention, the composition of the plates 2 used for forming the internal kiln 1 wall, i.e. the flanks and the vault, is different according the exact position of each plate along the entire tunnel
1. The different compositions used are indicated in the following table, in columns K, S and P. The table also reports further values measured in the chemical-physical analyses.
The tunnel 1 is therefore divided into four zones, respectively denoted by I, II,
III and IV (figures 2A and 2B).
Zone I is entirely situated in the preheating zone A and part of the entrance to the tunnel 1 , and reaches as far as the point in which the mean temperature is about 700°C. In zone I refractory plates are used having composition K, thanks to their excellent resistance to thermal shock.
Zone Il is situated in part in the preheating zone A and in part in the firing zone B, starting from the point in which the mean temperature is about 700 °C in the preheating zone A and ending at about the first half of the firing zone B. In zone Il refractory plates having composition S are used. These plates with composition S effectively prevent any alkaline attack. Zone III is situated in the remaining part of the firing zone B and in the initial part of the cooling zone C, starting from halfway through the firing zone B and terminating in the point at which the mean temperature is about 900 °C in the cooling zone C. In zone III refractory plates 2 having composition K are preferably used, for maximum working temperatures of the kiln up to 1050°C (see figure 2A). Alternatively, refractory plates having composition P (figure
2B) are preferably used for working temperatures of more than 1050°C, typically about 1200°C, as the plates made of composition P exhibit a high mechanical resistance and an excellent resistance to thermal shock.
Finally, zone IV is entirely situated in the cooling zone C and starts from the point in which the mean temperature is about 900 °C up to the end of the tunnel 1. In zone IV refractory plates having composition K are used, thanks to their excellent resistance to thermal shock.
In the preferred embodiment of the present invention, the tooth 3 interposed between the internal refractory covering formed by the plates 2 and the lower base 4, usually present in tunnel kilns, exhibits composition S to repel alkaline attack. In the absence of alkaline attack, a tooth 3 having composition K can be used.
In the present invention, the plates 2 are made by extrusion.
The starting materials used for forming components K, S and P are respectively the following:
K S P
Clays 15-35 15-35 10-30
Kaolin 15-35 5-25 0.1 -20
Talcs 15-35 - -
Chamotte 17-38 0.1 -20 -
Aluminas 0.1 -10 0.1 -10 50-80
Sillimanite - 40-60 -
Zirconiums _ _ 5-15
The above values are expressed as percentages in weight.
Operatively the powders of the raw materials are mixed in the above- reported proportions for each component. Subsequently the mixture is moistened and a new mixture is made up until a paste is obtained having a consistency which enables it to be extruded. For example, water can be used at 15% weight with respect to the total of the mixed powders.
The paste obtained is extruded through a screw extruder of known type to form the plates 2 of the desired size. The freshly-extruded plates are dried in a dryer for about 24-30 hours at a maximum temperature of 80°C.
When dried, the plates are fired in a kiln at a maximum temperature of 1300°- 1400°C, resulting in plates ready to be mounted to form the internal walls of a tunnel kiln, in agreement with the present invention. As can be appreciated from the foregoing, the method, the plate and the tunnel kiln of the present invention enable the needs to be satisfied and the drawbacks obviated, as described in the introductory part of the present description with reference to the prior art.
The prefabricated plates, realised by extrusion, exhibit a substantially equivalent mechanical resistance to that of traditionally fired bricks, and also exhibit a high resistance to alkaline attack, thanks to the use of the composition S.
Further, the location in different zones of the tunnel kiln of plates having predetermined different compositions for each zone enables worklife duration of the kiln to be increased, as well as reducing maintenance costs therefor. Obviously an expert in the sector might make numerous modifications and variations to the method, the plate and the tunnel kiln described in order to satisfy contingent and specific requirements, without their forsaking the sought ambit of protection as it is defined in the following claims.