GB2074708A

GB2074708A - Heat treatment of ceramic products

Info

Publication number: GB2074708A
Application number: GB8112544A
Authority: GB
Original assignee: PALOHEIMO Oy
Current assignee: PALOHEIMO Oy
Priority date: 1980-04-25
Filing date: 1981-04-23
Publication date: 1981-11-04
Also published as: FI801334A; IT1205242B; FI64562B; FI64562C; DE3116435A1; IT8121136A0

Abstract

In a tunnel kiln wet shaped articles 2 arranged in vertical layers and horizontal rows travel from left to right through drying zone 6 and pre-heating zone 5 to firing zone 4. They then return in the opposite direction, and are cooled by the transfer of heat to the incoming articles. Alternate conveying rollers 3 rotate in opposite directions and each roller has a varying diameter to contact only the articles which are to be driven in the corresponding direction. Each article is transferred laterally at the end of the firing zone to engage with the rollers rotating in the opposite sense. Hot and cold articles travelling in opposite directions thus alternate in the horizontal plane as well as in the vertical plane. In an alternative arrangement the firing zone is at the centre and wet articles travel from both ends of the tunnel and emerge from the opposite end after cooling. <IMAGE>

Description

SPECIFICATION Method for the heat treatment of ceramic products In Europe, the manufacture of ceramic prod ucts pressed from a wet, plastic clay composi tion is carried out by the "wet pressing method", which is characterized in that the ceramic products thus obtained are mechani cally so weak that they must be dried as individual pieces before they are piled up on firing trolleys and taken to firing kilns on the trolleys. Drying is always a separate proce dure, which is completely separate from the firing.

Mainly in the U.S.A., a so-called semi-dry method is also used, in which the products are formed from "semi-dry" clay by means of great mechanical forces. Owing to their me chanical strength, these semi-dry products can be piled immediately on firing trolleys and taken to the firing kiln for drying and firing. In this case, when designing the length and structure of the firing kiln, it has been taken into consideration that some drying must take place place before the firing.

Both methods have considerable deficien cies, such as high consumption of energy and long duration.

By the method according to the present invention, great savings of energy and time are achieved in the manufacture of ceramic products, as well as great savings in invest ment in structures, and the main characteris tics of the invention are given in accompany ing Claim 1.

The kiln according to the invention can be used for the drying and firing of products modeled from a wet, as well as from semi-dry, clay composition.

The drying and firing processes have been combined so that the drying takes place while each product is separate from the others, and the firing takes place likewise. Thus the drying and firing take place in the same channel. The firing gases used for firing the ceramic prod uct flow through the channel and emerge from the front end of the drying section.

Furthermore, the kiln according to the in vention has been constructed so that it oper ates as an effective heat exchanger. This has been achieved so that the products travel simultaneously in the kiln from its front end to the firing zone and from the firing zone to the front end of the kiln. This means that adjacent to each product traveling to the firing zone there travels in the opposite direction a prod uct with a greater heat charge, coming from the firing zone.

The traveling of the above-mentioned prod ucts in opposite directions has been arranged either so that the kiln is a tunnel, and wet, modeled products are fed in from each end of the kiln, in the middle of the tunnel there is a firing zone, and the fired product is withdrawn from the opposite end, or so that the tunnel is closed at one end and at the closed end there is situated the firing zone, the products travel ing from the front end to the firing zone, changing direction in the firing zone and traveling back to the front end. The travel direction of layers of products can be opposite, or products adjacent laterally and vertically can travel in opposite directions. The latter method is the most effective in terms of heat transfer.

The firing kiln according to the invention has an advantage not only in its low consumption of heat but also in its speed and in the even quality of the products manufactured. Owing to the small size of the apparatus and the separation of the products, the drying and firing are sped up considerably.

This is also because the transfer of heat takes place rapidly and directly from one product to another by radiation. In experiments, for example, brick pipes intended for underground drainage ditches were dried and fired in two hours. Nowadays a minimum of 40 hours, generally about 75-85 hours, are required for this process.

One or several firing layers may be situated in the firing kiln according to the invention.

The conveyors can be rollers, mats, nets, etc.

The most advantageous is an alternative in which the structure allows the products to travel maximally close to each other, and the conveyor element itself need not pass through the firing kiln.

The firing kiln according to the invention is described in greater detail with reference to the accompanying drawings: Figure 1 is a longitudinal section of a firing kiln in which the firing zone is at one end of the kiln and the products are conveyed by means of rollers.

Figure 2 is cross-section ll-ll of the kiln.

Figure 3 is a thermal curve illustrating the operation of the kiln.

Figure 4 shows the rollers of the kiln, in a cross section seen from the side.

Figure 5 shows the rollers of the kiln, in section V-V.

Figure 6 is a top view of a kiln in which the drying zone is at an angle of 90 .

Figure 7 is a longitudinal section of an alternative form of kiln Fig. 1 depicts a longitudinal cross section of a firing kiln 1. The opening for feeding and withdrawal is on the left 8. At the other end 7 of the furnace there is a firing zone and the end of the kiln itself is closed. The products 2 travel on top of the conveyor rollers 3 via the drying zone 6 to the heat-raising zone 5 and to the actual firing zone 4. In the firing zone a suitable pushing member transfers the product over the distance of one product width laterally, as described in connection with Fig.

5. After the lateral transfer, the products travel back in the opposite direction, yielding heat to the products in the zone 5 intended for raising the heat, and later to the products in the drying zone 6. Every second conveyor roller 3 rotates in this case in a different direction.

The travel of the products to be fired can also be arranged so that the products of one layer first travel from the drying 6 to the firing 4, are then transferred by a lift device to the next lowest layer, and travel back in this layer.

In this case, the layers are in pairs and the travel directions of the products in the layers are opposite.

Fig. 2 depicts cross section ll-ll of the firing kiln 1. The products 10 and 11 travel on top of the conveyor rollers 3, half of the products 10 are traveling to the firing zone and half 11 are returning from the firing zone, simultaneously yielding a charge of heat.

Fig. 3 depicts a drying and firing curve 9 of the firing kiln, i.e. the temperature prevailing in the kiln. The temperature is, of course, adjustable according to the clay composition and the product.

Fig. 4 depicts one alternative for conveyor rollers 3. Every second roller rotates in a different direction and only every second supports and touches the product to be conveyed.

Fig. 5 depicts section V-V of the conveyor roller example. The conveyor roller 3 touches and conveys the pipes 1 5 emerging from the firing zone. The conveyor roller does not touch the pipes 1 6 traveling to the firing zone, but the pipes pass over the roller. In the firing zone there occurs a lateral transfer of the pipes in such a manner that a suitable transfer member transfers the pipes 1 6 to the place of the pipes 15, and as a result the direction of conveyance is reversed and the return movement of the transferred pipes begins.

Fig. 6 depicts a plan view of a kiln in which the drying section 6 joins the rest of the kiln at an angle of 90 . By means of this solution, effective air circulation 12 can be achieved.

When a tubular product is involved, it is important in rapid drying that air flows both inside and outside the pipe. The same air circulates around the pipes coming from firing in the lower layer, and thereby heat transfer is enhanced. By means of a suitable pushing member 1 3 the products are transferred after the drying to the heat-raising zone and at the same time onto another conveyor. At the rear end 14 of the firing zone the products are transferred one layer lower, in which the re turn movement takes place.

Fig. 7 shows an embodiment in which the firing zone 4 is arranged in the middle of the tunnel kiln, which is constructed symmetrically. The wet formed articles 2 enter at both ends of the kiln and are in each case withdrawn after firing from the opposite end. The articles travelling in opposite directions are arranged to travel as close as possible to one another so that the hot articles from the firing zone give up heat to dry the incoming articles before they are fired.

The kiln according to the invention is provided with normal kiln devices, such as control and regulation devices. Likewise, blowers which make the circulation of air more effective can be connected to the kiln, especially to the drying section of the kiln and in general to those sections in which the temperature is below 800 C.

Claims

1. A method for the heat treatment of ceramic products in a tunnel-like firing kiln, characterized in that, after modeling, the wet products travel to the firing zone separate from each other, and the hot products emerging from the firing zone travel in the reverse direction in relation to the wet ones, adjacent to the wet products and/or above or below them, yielding their thermal energy to the wet products, thereby drying them before their entering the firing zone and thereby also cooling themselves.

2. A method according to Claim 1, characterized in that the products to be fired are transferred in the firing zone or immediately thereafter in the lateral or vertical direction in order to change their travel direction.

3. A method according to any of the above claims, characterized in that the products to be fired are conveyed in one or several layers and the conveying takes place on rollers of which every second rotates in a different direction, and that the rollers have been profiled in the longitudinal direction in such a manner that each product travels in a controlled manner forwards and that the products touch only every second roller, every second roller rotating in the same direction.

4. A method according to any of the above claims, characterized in that the firing zone is situated in the middle of the firing kiln and the furnace is symmetrical, and after the modeling the products are fed into the firing kiln while wet from both ends of the kiln and are withdrawn at the opposite end, and the products are placed in the kiln in such a manner that the products traveling in opposite directions are maximally close to each other.

5. A method of heat treatment of ceramic products in a tunnel kiln in which the wet shaped articles are conveyed through a drying zone to a firing zone and fired articles are conveyed from the firing zone through the drying zone close to the wet articles in order to transfer heat from the fired articles to the wet articles.