EP1615758A1

EP1615758A1 - Method and apparatus for thermal conditioning of wet ceramic products

Info

Publication number: EP1615758A1
Application number: EP03798970A
Authority: EP
Inventors: George Vekinis; Ioannis Gyllis
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-10-01
Filing date: 2003-10-01
Publication date: 2006-01-18
Also published as: GR1004709B; WO2004030882A1; GR20020100429A; AU2003264909A1

Abstract

The present method and apparatus for thermal conditioning of wet ceramic materials or articles before drying, enables the crack-free and rapid drying of ceramic materials or articles, improves quality and reduces overall production time and cost, even of clay products with a high concentration of high plasticity clay. The method offers a significant increase in productivity of a factory without major restructuring or changes to the factory or the production method. The method is characterised by multi-level irradiation of wet ceramic materials or articles using small, localised sources of electromagnetic radiation so that the temperature of the water inside the wet ceramic materials or articles is raised uniformly and quickly to near the boiling point before they enter the drier, keeping the surface temperature of the ceramic materials or articles lower that the centre without drying.

Description

METHOD AND APPARATUS FOR THERMAL CONDITIONING OF WET CERAMIC PRODUCTS

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and apparatus for thermal conditioning of ceramic materials and products to assist and accelerate their drying during the production process.

BACKGROUND OF THE INVENTION

Manufacturing of clay-based traditional (earthenware) ceramics is generally a three stage process. The first stage consists of forming (dry or wet) wet tiles, bricks, crockery, fixtures and others by manual or automatic pressing or extrusion. After forming, at the second stage, the articles are dried in temperature-controlled air and in humidity- controlled environments and then, at the third and final stage, they are fired in large kilns or furnaces at temperatures from about 850°C to 1300°C, thereby consolidating the clay by joining some of the components into a hard (usually glassy) and relatively strong mass. Glazing and decorations are usually applied prior to the firing process.

During the traditional production process, drying of the wet bricks, tiles, wares and other ceramic products is usually carried out by very slow heating in successively drier atmospheres, starting with a relative humidity of 100% and reducing gradually until the total water content of the ceramic products is less than about 5%. The overall mechanism of drying is based on, and controlled by, the laws of diffusion, namely heat diffuses slowly inwards, heats up the internal water and promote its diffusion towards the surface where it finally evaporates to the surroundings from where it must be removed by air convection. In the conventional drying process, these two mechanisms, heat diffusing inwards and water diffusing outwards, are acting against one another, setting up thermal stresses in the process.

A number of physical mechanisms can damage the ceramic material or article during drying: - Sudden "thermal shock" experienced by the material or article as it enters the drier can result in cracking, spalling etc.

- Steep thermal gradients between the centre of the article and its surface, can result in stresses, which also can cause internal or external cracking.

- Rapid and non-homogeneous drying of the surface can result in inhomogeneous shrinkage, setting up sub-surface damage which can appear during the firing stage.

Thus, in order to avoid these problems, the drying process is gradually and extremely slow and can take up to 72 hours or more. The wet article enters the drier at a room- temperature region at a relative humidity of 100%, and thereafter gets heated up gradually until it reaches a region where the temperature is over 100°C and the relative humidity about 0%. After a few hours at this region, the article is sufficiently dry for further processing and firing. Thus, due to the time consuming drying process, the floor space of the drier region in a ceramics factory occupies a very large portion of the total factory's space. Therefore, it is clear that the largest proportion of the drying time and drier space is actually used to "pre-heat" the wet articles to avoid damage and only a small fraction of the total drying time and drier space is actually dedicated to drying, namely the removal of the water.

The greatest problems are encountered when the wet articles to be dried contain a large proportion of "high plasticity" clay (which is necessary for high-quality, smooth surface products), which does not allow easy diffusion of water outwards or heat inwards. In such cases, and in order to facilitate drying, coarse and highly abrasive raw materials, such as sand etc., are blended into the high plasticity clay, which results, however, in lower quality products and higher erosion of the clay preparation and product-shaping equipment. Since these low-plasticity "mixers" have to be transported from afar, the transport costs of these low-plasticity "mixers" add even more to the already high, overall production cost.

It is clear, therefore, that the conventional drying method used in factories producing ceramic materials or articles is very time-consuming. Consequently, dryers for ceramic articles, such as bricks, tiles, etc., need to be very large, at least large enough to ensure complete drying of the materials or articles in accordance with the factory's productivity.

In addition, in the heavy clay industry, drying plays the most crucial role in determining the overall productivity of a factory, due to the fact that traditional dryers occupy the majority of the floor space of the factory. And since heavy clay factories are often situated very near or even inside urban areas, the cost of this floor space can be very significant.

The problem can be exacerbated further, if there are major fluctuations in environmental conditions without a concommitant optimisation in production conditions. Changes in environmental temperature and humidity can increase the thermal gradients in the articles and can substantially increase the time needed for complete drying and the number of damaged articles.

Therefore, it is clear, that any reduction in overall production time and overall floor space in a ceramic factory will offer very significant cost reductions, productivity increases and production flexibility.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an improved method for thermal conditioning of wet ceramic products, which overcomes the deficiencies of the prior art.

A further aspect of the present invention is to provide an improved apparatus for thermal conditioning of wet ceramic products, more particularly for carrying out the method of the present invention.

In accordance with the above objects of the present invention, a method for thermal conditioning of wet ceramic materials or articles is provided wherein the wet ceramic materials or articles receive electromagnetic radiation of such intensity and in such manner so as to raise the temperature of the water inside of the ceramic material or article up to or close to the boiling point of water without increasing the surface temperature of said ceramic material or article and without any drying of the materials or articles.

In accordance with a second embodiment of the present invention, there is provided an apparatus for thermal conditioning of wet ceramic materials or articles, more particularly for carrying out the method of the present invention, which comprises at least one preferred tubular modular unit, wherein said at least one modular unit comprises means of electromagnetic radiation, so that the ceramic materials or articles moving through said apparatus receive electromagnetic radiation of sufficient intensity and in such a way as to provide sufficient thermal energy to heat up the water inside the materials or articles to or close to the boiling point of water, without heating the surface of the materials or articles and without drying them.

Moreover, according to another embodiment of the present invention, it is provided an apparatus for thermal conditioning of wet ceramic materials or articles, more particularly for carrying out the method of the present invention, which comprises at least one suitable shaped, non-movable processing chamber, wherein said at least one chamber comprises adjustable means of electromagnetic radiation, so that any ceramic materials or articles placed within said chamber receive electromagnetic radiation of sufficient intensity and in such a way as to provide sufficient thermal energy to heat up the water inside said materials or articles to or close to the boiling point of water, without heating the surface of the materials or articles and without drying them.

Further preferred embodiments of the present invention are defined in dependent claims 2 to 6, 8 to 10 and 12 to 17.

Other objects and advantages of the present invention will become apparent to those skilled in the art in view of the following detailed description taken in conjuction with the accompanying drawings, wherein like reference numbers refer to similar parts throughout the drawings, and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a cross-sectional view of a preferred arrangement of a conditioner for a continuous feeding ceramic plant, e.g. a modern brick factory according to the present invention ; Fig. 2 shows a cross-sectional view of a preferred module according to the present invention.

DETAILED DESRIPTION OF THE INVENTION In accordance with one aspect of the present invention a "thermal conditioning" stage is provided between the forming and drying stages.

By the term "thermal conditioning" it is meant "thermal preparation of the formed wet article in such a way as to increase the internal temperature without increasing the surface temperature substantially, and thereby to reduce the internal thermal gradients during later drying". This reduction in internal thermal gradients eliminates the damage and final waste and enables rapid overall drying of the articles, even if they comprise a large amount of "high plasticity" clay.

According to the thermal conditioning method of the present invention, the rapid, well distributed pre-heating is carried out by irradiation of only the inside of the wet ceramic articles using penetrating electromagnetic radiation which "couples" with the water molecules inside the wet ceramic material or article and then absorbs heat energy.

Therefore, the water inside the wet article is heated up much more and quicker than the water on its surface. During irradiation, the environment is kept at room-temperature and a relative humidity of 100% ensuring that drying does not start and no water is removed from the surface of the materials or articles. In practice, this is achieved by the use of a large number of intense, compact and specially located sources of electromagnetic radiation which are activated in short pulses according to a pattern which depends on the material under processing.

Immediately following this conditioning process, the materials or articles enter the drier where drying takes place. In the case of such internally heated wet articles, according to the method of the present invention, the two diffusion mechanisms - heat diffusion and water diffusion - are both acting outwards during drying instead of one against the other and no thermal stresses are created in the process. Refering now to Fig.1 , the materials or articles in a "green" wet state from the extruder enters the apparatus (1) from the left entrance (2) into the entrance chamber (3) which may incorporate custom-designed openings to ensure no leaks of electro-magnetic radiation. The wet green articles move continuously on wagons or other suitable carrying system. The electro-magnetic radiation is applied on the green articles moving through the chamber made up of preferably tubular or other shape modular units (4) by the use of a large number of intense radiation generators (5). The wet green bodies are irradiated continuously or intermitently until they exit the chamber at the other side (6) with a very high internal temperature. The atmosphere inside the chambers is kept at room temperature and a relative humidity of 100%. A ventilator (7) may be fitted to the apparatus if necessary, depending on the material or article being processed.

The total maximum output power of the electromagnetic power sources is greater than about 0.1 kW, supplied by one or more generators, and the frequency of electromagnetic radiation to which the wet ceramic material or article is subjected, is between 10MHz and 60GHz.

The sources of electromagnetic radiation are controlled by an electronic computer, so as to enable optimum thermal conditioning according to the type of product, the moisture content, the internal temperature, the total product load and other factors, information on which is provided by suitable sensors within the apparatus (1) or the materials or articles. Moreover, uniformity of thermal conditioning is enhanced by the use of computer- controlled field electromagnetic stirrers, wave guides and air circulators within the apparatus (1).

The wet articles are of any material used in ceramic production such as earthenware, clay or other material which is extruded, moulded, or pressed. Each module unit (4) is designed according to the application. A preferred configuration is shown schematically in Fig. 2. The wagons or other means used to carry the wet articles move between two screens (8) which may also support internal generators (9). The position of the external generators (10) depends on the application, see e.g. an alternative arrangement of the external generators (10) in Fig. 2. The modular units (4) are joined together by any mechanical or other joining or welding method to any length required according to the needs of the factory and the prevailing production conditions .

Further, the sources of electromagnetic radiation (5, 9, 10), such as generators, are placed in adjustable slots on the tubular modular units (4) enabling positional optimisation for thermal conditioning of any type of wet ceramic material or article according to the prevailing production and environmental conditions.

According to another alternative (not shown), the conditioner comprises at least one suitable shaped, non-movable processing chamber, and said chamber comprises adjustable sources of electromagnetic radiation (5, 9, 10), so that any ceramic materials or articles placed within said chamber (4) receive electromagnetic radiation of sufficient intensity and in such a way as to provide sufficient thermal energy to heat up the water inside said materials or articles to or close to the boiling point of water, without heating the surface of the materials or articles and without drying them.

Due to the use of the thermal conditioning method of the present invention, the very long and time-consuming drying of wet ceramic materials or articles in a conventional drier is reduced drastically. The internally heated conditioned articles can enter directly into a high temperature drying region at up to 100°C without the danger of any thermal gradients or sudden drying cracking the material. The present method can be implemented on wet ceramic materials or products that either move continuously between the shaping process to the dryer or remain in a chamber. Moreover, the present method can be implemented in-series with the existing production line without any changes, or at least with minor changes to the existing equipment in any ceramic factory. For example, in a brick factory, drying of wet bricks can take up to 48 hours in a traditional drier. The bricks are produced in a "green" wet state by an extruder and fed or placed in a continuous or semi-continuous drier. Up to 85% of the time spent in the drier is used for heating the "green" articles until completely dry. The thermal conditioning method of the present invention pre-heats the centre of the "green" articles, thereby immediately initiating the diffusion of water vapour from the centre to the surface, greatly accelerating the drying process.

While preferred embodiments have been shown and described obviously minor modifications in design and construction can be effected in the invention without departing from the spirit and scope thereof, as defined in the appended claims.

Claims

1. A method for thermal conditioning of wet ceramic materials or articles wherein said wet ceramic materials or articles receive electromagnetic radiation of such intensity and in such manner so as to raise the temperature of the water inside of the ceramic material or article up to or close to the boiling point of water without increasing the surface temperature of said ceramic material or article and without any drying of the materials or articles.

2. The method according to claim 1, wherein said thermal conditioning stage is carried out between the forming and drying stages.

3. The method according to any preceding claim, wherein the total maximum output power of the electromagnetic power sources is greater than about 0.1 kW, supplied by one or more generators.

4. The method according to any preceding claim, wherein the frequency of electromagnetic radiation to which the wet ceramic material or article is subjected, is between 10MHz and 60GHz.

5. The method according to any preceding claim, wherein the wet articles are of any material used in ceramic production such as earthenware, clay or other material which is extruded, moulded, or pressed.

6. The method according to claim 2, wherein a thermal conditioning apparatus is placed in series with the extruder or the press used for shape-forming the wet ceramic articles and the dryer, and is placed between the extruder or press and the dryer which later dries the materials or articles.

7 . An apparatus for thermal conditioning of wet ceramic materials or articles, more particularly for carrying out the method of claim 1, comprising at least one preferred tubular modular unit (4), wherein said at least one modular unit (4) comprises means of electromagnetic radiation (5, 9, 10) so that the ceramic materials or articles moving through said apparatus (1) receive electromagnetic radiation of sufficient intensity and in such a way as to provide sufficient thermal energy to heat up the water inside the materials or articles to or close to the boiling point of water, without heating the surface of the materials or articles and without drying them.

8 . The apparatus according to claim 7, wherein said modular units (4) are joined together by any mechanical or other joining or welding method to any length required according to the needs of the factory and the prevailing production conditions.

9. The apparatus according to claim 7, wherein said means of electromagnetic radiation comprises sources of electromagnetic radiation (5, 9, 10), such as generators, which are placed in adjustable slots on the preferred tubular modules(4) enabling positional optimisation for thermal conditioning of any type of wet ceramic material or article according to the prevailing production and environmental conditions.

10. The apparatus according to any preceding claim, wherein said at least one modular unit (4) comprises means which allows the movement through it of rolling platforms or wagons upon which the ceramic materials or articles are placed.

11. An apparatus for thermal conditioning of wet ceramic materials or articles, more particularly for carrying out the method of claim 1, comprising at least one suitable shaped, non-movable processing chamber (4), wherein said at least one chamber (4) comprises adjustable means of electromagnetic radiation (5, 9, 10), so that any ceramic materials or articles placed within said chamber (4) receive electromagnetic radiation of sufficient intensity and in such a way as to provide sufficient thermal energy to heat up the water inside said materials or articles to or close to the boiling point of water, without heating the surface of the materials or articles and without drying them.

12. The apparatus according to claim 11, wherein said means of electromagnetic radiation comprises sources of electromagnetic radiation (5, 9, 10), such as generators.

13. The apparatus according to any preceding claim, wherein said sources of electromagnetic radiation (5, 9, 10) are controlled by an electronic computer, so as to enable optimum thermal conditioning according to the type of product, the moisture content, the internal temperature, the total product load and other factors, information on which is provided by suitable sensors within the apparatus (1) or the materials or articles.

14. The apparatus according to any precending claim, wherein uniformity of thermal conditioning is enhanced by the use of computer-controlled field electromagnetic stirrers, wave guides and air circulators within the apparatus (1).

15. The apparatus according to any preceding claim, wherein the total maximum output power of the electromagnetic power sources (5, 9, 10) is greater than about 0.1 kW, supplied by one or more generators.

16 . The apparatus according to any preceding claim, wherein the frequency of electromagnetic radiation to which the wet ceramic material or article is subjected, is between 10MHz and 60GHz.

17 . The apparatus according to any preceding claim, wherein said apparatus (1) is incorporated within any existing ceramic drier used in ceramic production factories.