US3850715A

US3850715A - Method for cooling heat bloated inorganic articles

Info

Publication number: US3850715A
Application number: US00267457A
Authority: US
Inventors: A Jebens; R Westergaard
Original assignee: Sentralinstitutt for Industriell Forskning
Current assignee: Sentralinstitutt for Industriell Forskning
Priority date: 1970-05-15
Filing date: 1972-06-29
Publication date: 1974-11-26
Anticipated expiration: 1991-11-26

Abstract

A method for producing an element of inorganic foam material from pellets, granules or powder of for instance a clay containing material, glass powder or aggregates thereof slag, flotation waste and similar expandable material. The element is built up by depositing a number of layers successively. Each layer is bloated before the next layer is deposited. After two or more layers have been bloated, a forced cooling is performed from the bottom part of the bloated material for the remaining bloating process. The temperature difference between the top and bottom parts rises to between 80*C and 600*C and preferably 450*C. The forced cooling is carried out by draughting cooling air past the bottom part.

Description

[ NOV. 26, 1974 [22] Filed:

[ METHOD FOR COOLING HEAT BLOATED INORGANIC ARTICLES [75] Inventors: Aksel Jebens, Hosle; Richard Hoegh Westergaard, Oslo, both of Norway [73] Assignee: Sentralinstitutt for industriell forskning, Forshingsveien, Norway June 29, 1972 [21] Appl. No.: 267,457

Related U.S. Application Data [63] Continuation-impart of Ser. No. 142,992, May 13,

197i, abandoned.

[30] Foreign Application Priority Data 2,693,018 ll/l954 Czarnccki...... 264/66 X 3,351,687 ll/l967 Thome ct alm 264/(16 X 3,723,593 3/1973 Ono 264/66 X Primary Examiner-Edward G. Whitby Attorney, Agent, or FirmWender'oth, Lind & Ponack [57] ABSTRACT A method for producing an element of inorganic foam material from pellets, granules or powder of for instance a clay containing material, glass powder or aggregates thereof slag, flotation waste and similar expandable material. The element is built up by depositing a number of layers successively. Each layer is bloated before the next layer is deposited. After two or more layers have been bloated, a forced cooling is performed from the bottom part of the bloated material for the remaining bloating process. The temperature difference between the top and bottom parts rises to between 80C and 600C and preferably 450C. The forced cooling is carried out by draughting cooling air past the bottom part.

10 Claims, 10 Drawing Figures METHOD FOR COOLING HEAT BLOATED INORGANIC ARTICLES This application is a continuation-in-part of Ser. No. 142,992, filed May 13, 1971, now abandonded.

This invention relates to a method for producing elements of inorganic foam materials, in particular the invention relates to cooling of r such elements, produced for instance from a clay containing material, glass powder or aggregates thereof, slag, flotation waste and similar expandable materials. Such elements are especially Y applicable as building or insulation materials, and are produced by charging e.g., pellets, granules or powder in a form, on a conveyor or in a conveyed form. The pellets may natively contain, or may be admixed with, an expanding agent which either alone or by reaction with the material liberates gas as the material is melting, whereby a high-viscous foam is formed, which is stabilized by cooling.

Production of such foam elements by successive depositions of pellet layers and heating in a form during continuous heat supply, is previously known in principle, i.a., from the U.S. Pat. Nos. 1.877.137 and 1.882.082. The present invention concerns such production-and aims at reducing the cooling time.

A particular practicable foam element of the type which can be produced by these methods, is foam clay, and in the following, the invention will be described with reference to such materials only, because it will be understood, that any inorganic material which can be melted together and expanded similar to clay, can be used.

In the patents referred to above, it is a requirement that the material be annealed and that the cooling be performed very slowly in order to avoid cracks in the blocks due to thermal tensions, whether the word block is to be interpreted as a singly built block, as a long sheet or string of foam clay being discharged from the kiln, or as units in applicable sizes which have been cut from this sheet or string, or as minor elements formed by cutting or dividing the units. An exception is, how ever, mentioned in U.S. Pat. No. 1.845.350, where rapid cooling is allowed down to cherry red colour when the material is still plastic in its inner part. The present'invention aims i.a. at improving this cooling process.

A method for producing an element of inorganic foam material from pellets, granules or powder of for instance a clay containing material, glass powder or aggregates thereof, slag, flotation waste and similar expandable material. The element is built up by depositing a number of layers successively. Each layer is bloated before the next layer is deposited. After two or more layers have been bloated, a forced cooling is performed from the bottom part of the bloated material for the remaining bloating process.

By this method the foam is stabilized immediately after the burning, and a dimension-stabilized block is obtained at an early stage in the process. Removal from the form and further treatment may then be performed even before a forced cooling is started from above. This feature is especially important when charging is performed for making blocks of relatively large thickness from a material which cannot be exposed to high temperatures for a long time because of e.g., deformation caused by static pressure or chemical reactions resulting in i.a., formation of gases. Further, the cooling requirements related to the first part of the cooling kiln,

are diminished, and a reduction of the cooling period I is obtained.

Further, the invention also aims at decreasing the cooling period between the moment of solidification and the time interval in which temperature equalization takes place. This is obtained by keeping the tempera ture of the cooling kiln, immediately after the transfer of the material to the cooling kiln, so much lower than the solidifying temperature, that the difference between the solidifying temperature and the temperature of the cooling kiln is essentially higher, e.g., 2-3 times higher, than the difference between the temperature of the middle part of the block and that of the cooling kiln during the solidification, and that the temperature of the cooling kiln, prior to the solidification of the middle part of the block, is increased so much that the difference between the solidifying temperature and the temperature of the cooling kiln, becomes substantially equal to that difference between the temperature of the middle part of the block and the temperature of the cooling kiln which is to be kept during the continued cooling. During this continued cooling, the cooling rate may be determined such that the temperature distribution during said continued cooling does not result in residual tensions which will exceed those tensions that result in formation of cracks when temperature equalization at ambient temperature takes place, in other words, annealing and very slow cooling is necessary, see the patents referred to above. However, said cooling rate is preferably determined such that the temperature distribution during the continued cooling, in fact does result in residual tensions which really exceed those tensions that result in formation of cracks when temperature equalization at ambient temperature occurs, but such continued cooling process can only be performed when cutting according to U.S. Ser. No. 142,945 is performed, or when the block is equipped with channels according to U.S. Ser. No 142,993 now U.S. Pat. No. 3,758,652.

As far as the general deposition of layers is concerned, and the burning process as well, references are made to the above mentioned copending applications. Additionally shall be mentioned that a preferred feature of the present invention concerns deposition of layers of which each is 1 to 3 times the thickness of the pellets, and that each layer is heated to approximately 1',200C in less than 15 minutes simultaneously with the cooling from below. The first deposition should rather be laid on a ground plate having been preheated up to the reaction temperature of the foam material.

In the ideal case, which does not necessarily have to be practised in detail, provided that the temperature distribution established in the foam material is maintained as far as possible in fabric plants where means for the best control of the cooling rate is probably not at hand, the continued cooling is performed at a rate which is approximately proportional to the coefficient of conductivity for heat of the foam material, whereby the temperature profile established during the solidification, is maintained'approximately. If this profile may cause formation of cracks when temperature equalization takes place, one of the two precautions according to US. Ser. Nos. 142.945 and 142,993 has to be applied.

The above stated cooling rate is now to be explained further, since the cooling rate at any time after the solidification, strictly speaking, is to be kept proportional to the thermal diffusivity )t/cp. The thermal diffusivity is, however, approximately proportional to the coefficient of conductivity for heat A, because the specific heat c is changed very little and the density p is practically not changed as the temperature varies. The result is that the very same temperature distribution as has been established during the solidification, is maintained all the time, and the material is tension free until temperature equalization takes place. However, it is hereby also revealed that the material can stand a certain amount of temporary tensions during the continued cooling, which means that a somewhat higher temperature difference can be allowed, compared to the temperature difference between outer and inner parts of the block during the solidification. In addition hereto, this means that the material can be cooled at a rate which is somewhat higher than the cooling down rate corresponding to maintenance of the same temperature distribution as existed during the solidification. In other words, it is allowable to control the continued cooling, as already stated, such that temperature distribution established during the solidification, can be maintained approximately, until the block surfaces have obtained handling temperature.

In the following, the invention will be explained more comprehensive by means of a theoretical consideration.

Foam clay is viscous at high temperatures and accordingly it is tension free at such temperatures. When a foam clay block during solidification is cooled at a forced rate, the heat transport from the inner part of the block to its surfaces requires that the temperature falls relatively rapidly towards the surfaces. The cooling process can be described mathematically for the following idealized and general case: A sheet of infinite length and breadth and uniform thickness, is cooled in such a way that the temperature is the same on each side of the sheet, and falls at an even rate. If the thermal coefficient of expansion and the coefficient of conductivity for heat do not depend upon the temperature and are constant through the thickness L of the block, the temperature profile across the thickness of the block, will be parabolic. This can be calculated from the equation where the x axis is perpendicular to the sheet plane and the T axis is assumed to lie in this sheet plane. For constant values of A coefficient of conductivity for heat, p density and c specific heat and dT/dt cooling rate in degrees per unit of time, this equation gives In a realistic case, where the block has a finite breadth and length, the influence of the four edge surfaces will affect the temperature profile in such a way that the profile, at all six surfaces, will become steeper than the parabolic profile.

If, for instance, the final size of an element is half as thick as the bloated block, the continuous cooling period can, according to this formula, be reduced to one fourth if the block is treated as taught in the above mentiond U.S. Ser. Nos. 142,945 and 142,993 provided that the temperature profiles are parabolic. Due to reasons described above, the temperature profiles are steeper close to the surfaces and accordingly the cooling period can be reduced by more than one fourth by treatment according to said application.

Briefly repeated, the bloated foam material is first cooled as fast as possible with due regard to cracks caused by temporary tensions, but immediately before the middle part of the block solidifies, the cooling rate is adjusted such that an approximate parabolic temperature distribution is obtained across the thickness of the block during the last phase of the solidification. In practice is chosen, the highest possible temperature difference between the surface and the inner part of the block, because the continued cooling can be performed more rapidly, the higher this temperature difference is. The temperature difference is limited by the maximal residual tension which is acceptable in the final product, or cut elements, when temperature equalization at handling temperature has taken place, in addition hereto by temporary tensions during cutting if this is included, and during the successive temperature equalization. It is not possible to precalculate practicable and exact cooling data for the various types of clay and different block sizes. Persons skilled in this art will, however, due to the information here given, be able to control the cooling rate by their experience.

In order to decide whether the continued cooling requires precautions to prevent cracking, or not, still' further information is assumed to be desirable, and the invention will therefore be explained more comprehensively and be illustrated as follows. Assume a one dimensional block, i.e., an infinite large plate at uniform thickness. While this block is still somewhat viscous, a tension equalization will occur during the cooling, while, when the entire block has been solidified, an equalization of tensions will not take place any longer. If the block is assumed to be cooled at an even rate and uniformly from both sides, the temperature profile will be in the form of a parabola, and when the outer surfaces have reached ambient temperature, a temperature equalization will start. Before such temperature equalization takes place, cutting of the block may possibly have to be performed, se below, lest a continued contraction inwards in the block shall result in crack formations. Such crack formations are caused by a tension distribution corresponding to the temperature distribution during the solidification, with compressive stress in the surface layers, two neutral planes parallel to the surface, and tensile stress in the middle part of the block, corresponding to the sum of the compressive stresses.

With the above and other objects in view which will become apparent from the detailed description below, some preferred forms of the invention are shown in the drawings in which:

FIG. 1 is a longitudinal section of a tunnel kiln,

FIG. 2 is a cross section of a kiln embodiment shown in FIG. 1,

FIG. 3 is a cross section of another kiln embodiment shown in FIG. 1,

FIG. 4 is a part of FIG. 1, seen from below,

FIG. 5 shows a preferred way of directing coolant flows FIG. 6 illustrates utilization of heated cooling air as combustion air,

FIG. 7 is a diagram showing a cut through a one dimensional block.

FIG. 8 is a diagram illustrating different examples of cutting in one dimension, and

FIG. 9 is a partial perspective diagram illustrating some further examples of cutting.

In FIG. 1 is shown in principle a tunnel kiln 11, which will be used herein to illustrate preferred embodiments of the invention as well as some possible modifications thereof. Such kiln type belongs to the prior art, see for instance US. Pat. specification No. 2,052,324 where one example has been described, and a complete description of such kiln is consequently considered unnecessary to enable skilled readers to practice the invention. When embodiments in the following are described as applicable also in connection with batch operation, generally known kiln types may be referred to.

. In the kiln 11 is shown a hearth 12 which in this case is a movable hearth for continuous production, and which may preferably comprise a number of individually wheeled carriages 13 consistuting moulds for the articles to be produced or an endless conveyor to which side walls may be fitted. Inthe various embodiments being applicable for performing the present invention, a support 14 in each mould or on the conveyor as well as a cooling means 15 forms an essential part of the invention, said support 14 constituting the bottom of the mould or the bed on top of the conveyor on which a first of a number of layers of heat bloatable inorganic material is successively deposited to build up an article or a sheet 18 to be cut into articles 18), to the desired thickness. In the upper part of the kiln 11, heating means 16 are provided, which means may be of the oil burner type or in the form of electrical heating elements, for bloating each layer before a succeeding layer is deposited through feeders 17.

The apparatus of kiln equipment having been invented comprises the support 14 and the cooling means 15, as already mentioned. These essential parts are located in the lower part of the kiln 11 for cooling the bloated article or sheet 18 by starting to force cool the article or sheet from its bottom surface after at least two layers have been deposited and before the last layer is deposited, and may also comprise a trap (see succeeding figures) for parting or separating the cooling means 15 from the kiln atmosphere in which the material is bloated. For effectively cooling of the articles or sheet from below, in a controlled way, inlet openings and outlets are provided for conducting a cooling medium through the lower part past the support 15, see succeeding. figures.

When performing the present invention, two different main ways of conducting the cooling medium past the lower part of the article or sheet, is recommended for users, and will now be illustrated with reference to FIGS.- 2 and 3, and FIGS. 4 and 5 respectively.

In FIG. 2, which is a cross section of a kiln as shown in FIG. 1, inlet tubings 21 a and 21 b in the side walls 22 a and 22 b are shown, it being implied that a number of such inlets in most cases are preferably arranged as a row along the kiln walls below traps 23, if such are desired. The outlet tubings 24, however, are located in the bottom 25 of the moving hearth, one for each pair of inlet tubings 21 a and 21 b, and are connected to fans 26 and, when practicable, also with flaps, rather than equipping the inlets with fans. Sliding manifolds 27 a at the inlet sides and sliding manifolds 27 b below the hearth bottom are provided. Further, the bloated material 28 between side walls 29 is shown resting on a support 290. Above an insulating refractory material 291.

cooling passages 292 are shown below the support 290.

In FIG. 3 a modification of FIG. 2 has been shown, without inlet manifolds. Corresponding parts have been given corresponding numbers.

In FIG. 4 a, which is a cross section of another kiln embodiment as shown in FIG. I, one way of conducting the cooling medium is shown in principle. and will 'be explained further with reference to FIG. 4 b. Here. a number of inlet tubings 41 a are shown in the one side wall 42 a of the kiln, and a number of outlet tubings 41 b are shown in the other wall 42 b. Between the walls 42 a and 49 a narrow wave-shaped slit 43 may be arranged for separation purposes. A set of inlets may preferably be followed-by a set of outlets inthe same side wall, as will be apparent below, and the inlets communicate with the outlets through :a number of passages 44 being located between the support 490 on which the first layer is deposited and an underlying structure 491 of refractory heat insulating material. Fans 46 and flaps (not shown) may be arranged.

In FIG. 4 b, which is FIG. 4a seen from below inlet ducts 44 a and outlet ducts 44 b are shown in the

side walls

42 a and 42 b of the mould orconveyor mentioned above. Interfacing side walls of the kiln and mould are arranged in close relationship to each other, and the inlet tubings 41 a communicate with inlet ducts 44 a and outlet ducts 44 b communicate with outlet tubings 41 b. Additionally, the ducts communicate with each other through a number of passages 44 which can be arranged and directed in a number of ways without departing from the scope of the invention, since they are presupposed to be obvious for skilled persons in view of the representative examples being herein described. Each inlet duct 44 a may be part of a sliding manifold 47 a continuing in a number of passages 44 being directed from the one side to the other of the mould or conveyor and being terrninately into another sliding manifold 47 b.

Fixed manifolds

47 c and 47 d are also provided. In practice, an inlet tubing will alternately follow an outlet tubing in each side to obtain even cooling of the material having been bloated as described below.

In FIGS. 5 a and 5 b a preferred embodiment of the invention has been shown. Parts corresponding to parts in previous figures have been given corresponding numbers. It is seen from FIGS. 5 a and 5 b that only one complete manifold 57 is needed. Coolant flow is illustrated by arrows. The support 590 is preferably made from tubes with rectangular cross section, laid side by side, so that their upper walls form an interrupted support for the foam material.

In order to describe better how to utilize the apparatus having now been described, the cooling procedure will now be explained with general references to the figures, in order to further illustrate preferred embodiments of the method according to the invention.

In order to minimize mass and heat flow between the coolant system and the kiln chamber is used parting means or a small clearance shaped to obtain a long path to prevent heat transfer by radiation and convection between the moving hearth and the kiln walls. If a sand lock or other tight parting means is not used, the fans and channels are arranged to give a low pressue difference between the coolant system and the kiln chamber.

The coolant may be conducted in various ways from outside the kiln through the kiln walls, the inlets and outlets through the walls functioning as manifolds as well. The coolant may either be drawn direct through the kiln walls and under the support upon which the foam material is resting, or it may be conducted through the kiln walls and pass below the moving hearth before it is conducted to channels below the support. In the latter case, the structures of the moving health is given a certain cooling from below, which may be desirable. In this case manifolds for letting cooling air pass through the kiln walls are not needed.

In order to obtain even cooling of the support, one may operate with opposite flow directions from time to time or from place to place along a tunnel kiln. The latter principle is most convenient and will make it possible to use air first as a coolant and then as a preheated combustion air. This principle may be practiced by operating with a number of zones along the kiln, using opposite flow direction for any two neighbouring zones. Thus the flow will be constant with time.

This implies a separate manifold system in the kiln walls for each zone, but manifolds along the one or both and/or the bottom of the moving hearth need not be effected. In order to prevent a collision between coolant from two neighbouring zones it may be provided for sufficient space between the zones, sothat no channel under the support is connected with two zones at the same time.

In FIG. a is also shown an economic way of supplying combustion air to the kiln atmosphere via burners 593. After having removed cooling air at an increased temperature from the lower section, the heated coolant is conducted by. a line 594 to the upper section and used as preheated combustion air. If traps are used, a complete separation between lower and upper parts is not necessarily needed.

The forced cooling from below can preferably be performed by means of circulation channels for cooling gas which are arranged below a support plate that can be heated for receiving the first pellet layers, and which may be in the form of a relatively thin plate of heat resistant material.

In addition to using a support plate resting on blocks or the like to provide air passages the support may be made up of tubes having a square or rectangular cross section laid together to form an even support with air channels under it. The support may also be one or more blocks provided with air passages for circulation of air. The upper parts of the latter blocks may then suitably be made of a heat-conducting material while the lower parts hereof may be made of heat-insulating material. Similarly, if the support is a support plate resting on blocks, at least the upper parts of said blocks are preferably made of heat-conducting material so that the heat is removed from those parts of the support plate which are resting on the blocks. The essential features of the support are that it forms a firm, heat conducting support for the pellet layers and that it is possible to pass cooling air immediately below it to remove heat from the pellet layer. Thus the cooling air does not get in direct contact with the bloated pellet material, but passes along the underside of the bloated material to remove the heat therefrom in an efficient manner.

The passages under the support may have any desired shape as long as an efficient cooling is achieved when air is forced through the passages. Although air has been referred to as the cooling medium it should be noted that any other gas or even a liquid may be used if this is found practical.

After the first layer has been deposited it is heated to full burning temperature, and the cooling may start after two layers have been bloated. At this moment the block has a thickness ofabout 5 cm, and it is practically impossible to obtain a temperature difference of for instance 250C and still obtain that the pellet layers fuse together.

Before the block has reached a thickness of about 10 cm, i.e., before 3 to 4 layers of pellets have been bloated, partly complicated unsteady state conditions are prevailing and cannot be described as a temperature difference across the thickness of the block. With respect to the temperature differences between the top layer and the bottom layer it should, however, be noted that a great temperature difference is undesired for the first few layers since this may lead to poor sintring. A lower temperature difference is needed to obtain a. given cooling while the block is thin, than when it is thick. The reason for this is greatly due to the fact that the thermal conductivity 'for such a porous material is increasing drastically with temperature.

If the process is not to be interrupted after the build up to a thckness of 10 cm, but is to be continued for the production of thicker blocks, the temperature difference is gradually increased to utilize the invention in the best possible manner. The preferred temperature differences for different block thicknesses are as follows:

200C for a thickness of about 10 cm 400C for a thickness of about 20 cm 200 500C for a thickness of about 25 cm 250 600C for a thickness of about 30 cm These values for temperature differences are for a typical Norwegian clays, and do not necessarily apply to all types of clay.

It is difficult to increase the temperature difference as the block thickness increases above about 20 cm, and for thicknesses above 30 cm it seems impractical to try to obtain a further increase above the difference of 600C.

The most usual foam block thicknesses are between 20 and 30 cm, and the most convenient temperature differences between the top and the bottom thereof in the production are between 250 and 450C.

In FIG. 7, a cut has been shown through a one dimensional block with an approximate parabolic temperature distribution curve, where t are areas with compressive stresses, a are tension free planes and s are areas with tensile stresses. The most actual materials, for instance foam clay, stands higher compressive stress than tensile stress, and by fractures due to thermal residual tensions, such fractures appear as cracks inside the block, here shownby S. Further is given a temperature difference between inner and outer parts of the block, related to the solidifying interval and to the continued cooling, which certainly results in formations of cracks if precautions are not taken according to one of the above mentioned applications. This difference is adjusted or maintained such that the given temperature distribution is maintained approximately, until the surfaces have reached handling temperature, which moment is the preferred one for special cutting operations described in US. Ser. No. 142,945 and briefly illustrated below. A corresponding temperature difference of about one third of that illustrated, allows continued cooling according to the previously known methods referred to.

In FIG. 8 is shown different examples of cutting in the one dimension, to prevent formation of cracks which otherwise would occur because of the continued cooling being controlled in accordance with the temperature statements here given. In addition to showing the neutral planes a, it is also indicated by broken lines how the temperature distribution will gradually be after cutting, for three different cutting operations. Further is indicated by continuously drawn lines how the residual tensions are distributed in the cut elements, after the temperature equalization at 20C has occurred.

In FIG. 9 is given some other examples of cutting when a forced, continued cooling is desired.

Since dimensional stability has been obtained at an early stage, the block may, after the forced cooling in the tunnel kiln, readily be mechanically handled and stored edgewise during the continued cooling until a cutting operation takes place if such operation-has been included in the process. This increases the storing capacity.

EXAMPLE In order to illustrate the process there is given below an example of how the process may be performed as a batch operation. As raw material is used a natural occurring clay containing iron oxides (for example between 4 8 percent by weight) and some organic matter (for example corresponding to a carbon content about 0.5 percent by weight). The clay used expanded during minutes when a dry pellet (7 9 mm diam.) was brought into an oven kept at l,130 to 1,l50C, and we have found that many Norwegian clays are suitable.

The kiln used was heated by electric heating elements in the ceiling, but blocks have also been made in the same kiln using an oil burner as heat source. The top-of the kiln could be lifted up in order to unmould and remove the block. The bottom of the kiln was made of thin plates of refractory material. Under these plates were cooling channels for circulating cold air,

and under these channels was thermal insulation. Upon.

the bottom plates was a mould consisting of loose heat resisting bricks covered with slip agent in order to pre vent the block from sticking to the sides. Upon the plates inside the mould was a layer of sand preventing the block to stick to the bottom. Before the process started, the kiln was heated to 1,200C and kept at this temperature for several hours in order to establish stable conditions. Then a first layer of 7 9 mm diam. dried pellets of clay was introduced into the kiln This was done by means of a device making it possible to deposite one single layer of pellets. When the area inside the mould was covered by a pellet layer the temperature in the oven decreased considerably but increased again to about 1,200C in approximately 10 minutes. The pellets had then expanded and melted together to a foamed material of 2.5 to 3.0 cm thickness. At this moment a new layer of pellets was introduced into the kiln, and with 10 minutes intervals the procedure was repeated. In order to stabilize the pore structure forced cooling from below was started about the time when the 4th layer of pellets was fed into the kiln. When 10 layers of pellets had been fed into the kiln,

that is after 100 minutes approximately, the dimensions of the block were 60 X 80 cm and 25 30 cm thick.

The density of the block could vary between 0.30 and 0.40 g/cm. At this moment the temperature of the top of the block could be l,l l,l80C and the bottom 700 800C depending on the cooling from below. After removal of the mould the block was transferred to an electrically heated cooling chamber the cooling rate of which could be programmed.

During the first 6 hours the temperature in the cooling chamber is kept at about 680C being the solidifying temperature of the material, and when the temperature in the middle of the block is for example 50 higher than the temperature on the surface of the block, the temperature in the cooling chamber is descreased with a rather of 40C/h, corresponding to approx. 80C higher temperature in the middle of the block than on the surface. After 2.5 hours the temperature in the middle decreased from approx. 730C to 630C and it is certain that the interior of the block has solidified and the block is tension free. From now on the temperature difference between the interior'and the surface may be increased, for example to l00C or as much as. can be done wiwthout forming preliminary tensions that result in cracks, corresponding to using an increased cooling rate. To put it more precisely, it is possible to apply a higher cooling rate than the cooling rate which will maintain the block, absolutely tension free during the continued cooling. With a cooling rate of 35C/ h at this moment (630C in the middle of the block) the cooling rate during the continued cooling is. maintained proportional to the thermal conductivity of the material. This means that the cooling rate will be approximately as follows: 25 C/h at 500C; 22C/h at 4007 l'8C/h at 300C; l5C/h at 200C and 13C/ln at 100C, the temperatures referring to the middle of the block. By using the above procedure it is possible to obtaine crack-free blocks with a cooling time less than 36 hours.

In one case when this procedure was not used we could hear the block crack due to permanent tensions being built up after the block had been taken out of the cooling chamber and while the temperature equalisation was taking place. The temperature in the middle of the block was then measured immediately (through a hole made when the block temperature was 700C) and found to be 25C higher than the surface temperature. Thus a test to check whether the cooling rate is' too high, is to listen to the block as it cooles. The blocks were taken out of the cooling oven when the surface temperature was 25 to 40 C and the tempreature in the middle 70 to 100 C, and the cutting was done before the temperature difference between the interior and the surface had dropped below 50C. The cutting to precise dimensions was performed by cutting off approximately 5 cm of the four (imperfect) edges, dividing the block in two along and cutting off one or two centimeters of the top and the bottom. A blank of 80 X X 28 cm would thus give two blocks of X 25 X 25 cm.

It is of course not necessary to use exactly the cooling rate stated above, the essential point is that there will not occur preliminary tensions high enough to result in cracks, and that the quickest possible cooling may be obtained by keeping the cooling rate proportional to the heat conductivity of the material at any time. It is very easy to find out how the cooling rate shall be adjusted as a function of the temperature starting with a certain cooling rate. A block is then simply cooled with constant cooling rate and the difference between the llll middle (a thermocouple is inserted in a hole in the block) and the surface of the block is observed at the various temperatures. The mentioned temperature difference will then be approximately inversely proportional to the thermal conductivity of the material. When another block is to be cooled a cooling rate is chosen that is at any time inversely proportional to the mentioned temperature differences. Due to the variation of thermal conductivity (dependent on temperature) across the block at high temperatures, this procedure will not give an absolutely correct result, but this is very easy to adjust afterwards.

Above is described the process performed as a batch operation, but it is obvious that the principles may be applied to a continuous process. A suitable way of doing this is to build up blocks on trolleys running through a tunnel kiln and passing below several feed points in succession, until a block of desired thickness has been built up. The trolleys can have side walls with a certain slip angle functioning as a mould so that the finished block may easily be lifted up from the trolley. It is, however, proposed that there are no partition walls between the cars and that the foam material comes out of the kiln as a continuous string with a rectangular cross section. When a block shall be removed ,from a car, the string is cut, preferably at the joint between two cars. A layer of sand on the car tops and introduction of sand along the mentioned side walls will prevent the block from sticking to the car. The sand may be introduced at a number of feed points on each side of the tunnel kiln, if desired at the same points where the material to be bloated is fed into the kiln. It may be an advantage to do the cutting by way of local crushing. Somewhere, for example between the 3rd and the 4th feed point forced cooling of the trolleys is started so that the block is cooled from below. As a result of this cooling the block is sufficiently dimensionally stable to be handled so that it can be transferred to the cooling furnace. The cooling chamber may be arranged as a tunnel kiln with trolleys upon which the blocks are placed. It will be most practical that the blocks are stood upright on the trolleys. The correct cooling rate at the various temperatures is established by giving the tunnel cooling chamber a corresponding temperature profile which is maintained by circulating cooling air at correct temperature between the blocks at any place.

When the blocks have been cooled down to a suitable handling temperature they are taken out of the tunnel cooling chamber and cut to desired dimensions, as mentioned for the batch process.

It is evidently much easier to control the continuous process. In this case the conditions must be kept constant at the various points, but for the batch process it is necessary to regulate the conditions (temperature etc.) as a function of time.

Persons skilled in the art will see still further possibilities within the scope of the invention.

We claim:

1. In a method for producing an article of inorganic foam comprising depositing a number of layers of heat bloatable inorganic material successively to build up the article to the desired thickness, bloating each layer before a succeeding layer is deposited and cooling the bloated article, the improvement comprising reducing the total cooling time by starting to force cool the article from the bottom surface after at least two layers have been deposited and before the last layer is deposited and controlling the forced cooling to produce a temperature gradient between the top surface and the bottom surface of between C and 600C.

2. Process of claim 1 wherein said temperature gradient is between 250C and 450C.

3. A method as claimed in claim 1 wherein the cooling intensity is increased during the deposition so that the cooling effect is highest at the discharge of said element.

4. A method as claimed in claim 1 wherein said layers comprise pellets and each layer is l to 3 times the thickness of said pellets, and each layer is heated to approximately l,200C in less than 15 minutes simultaneously with the cooling from below.

5. A method as claimed in claim 1 wherein the first deposition is performed on a support plate heated to the bloating temperature of the foam material.

6. A method as claimed in claim 1 wherein after the bloating is complete said article is transferred to a cooling kiln wherein the temperature of said cooling kiln, immediately after the transfer of the material to said cooling kiln, is kept lower than the solidifying temperature so that the difference between the solidifying temperature and the temperature of said cooling kiln is essentially higher than the difference between the temperature of the middle part of the article and that of said cooling kiln during the solidification, and that the temperature of said cooling kiln, prior to the solidification of the middle part of said article, is increased so that the difference between the solidifying temperature and the temperature of said cooling kiln becomes substantially equal to the difference between the temperature of the middle part of said article and the temperature of said cooling kiln which is to be kept during the continued cooling.

7. A method as claimed in claim 6 wherein said continued cooling is performed at a rate which is approximately proportional to the heat conductivity of the material.

8. A method as claimed in claim 6 wherein said article is placed after said forced cooling edgewise for continued cooling.

9. In a method for producing an article of inorganic foam comprising depositing a number of layers of heat bloatable inorganic material successively on a support in a kiln to build the article to the desired thickness, bloating each layer before a succeeding layer is deposited and cooling the bloated article, the improvement comprising reducing the total cooling time by starting to force cool the article from the bottom surface after at least two layers have been deposited and before the last layer is deposited and controlling the forced cooling by controllably blowing a gaseous coolant into the kiln section located below the article being bloated to blow past the bottom part of said article through a number of passages which are separated from the bloating atmosphere, the upper wall of said passages forming a relatively thin heat conducting support for said article, said passages being directed in a substantially transverse direction to the length direction of the kiln including a movable hearth, and removing said coolant by controlled suction at an increased temperature.

10. A method as claimed in claim 9, wherein said coolant is air being controllably blown through at least one inlet tubing in the kiln wall and in slidable interaction with an inlet duct in the movable hearth, said tubings and ducts constituting manifolds which are slidably interacting with each other.

Claims

1. In a method for producing an article of inorganic foam comprising depositing a number of layers of heat bloatable inorganic material successively to build up the article to the desired thickness, bloating each layer before a succeeding layer is deposited and cooling the bloated article, the improvement comprising reducing the total cooling time by starting to force cool the article from the bottom surface after at least two layers have been deposited and before the last layer is deposited and controlling the forced cooling to produce a temperature gradient between the top surface and the bottom surface of between 80*C and 600*C.

2. Process of claim 1 wherein said temperature gradient is between 250*C and 450*C.

4. A method as claimed in claim 1 wherein said layers comprise pellets and each layer is 1 to 3 times the thickness of said pellets, and each layer is heated to approximately 1,200*C in less than 15 minutes simultaneously with the cooling from below.