Abstract:
The invention provides an installation and method for the continuous production of materials using exothermically hardening binders, such as gypsum or cement, comprising a mixing and feeding station and a calibrating unit or press and also a control system for operating the installation. This installation is distinguished by the fact that the distance between the feeding station and the entrance to the calibrating unit or press on the one hand and the distance between the feeding station and the end of the calibrating unit or press on the other hand, taking into account the duration of the mixing and forming time, corresponds to the ratio between the period of time from the beginning of mixing to the beginning of stiffening and the period of time from the beginning of mixing to the end of hydration, and by the fact that the rate of advance of the continuous installation is controlled in such a way that the exothermically hardening binder reaches its maximum temperature shortly before leaving the calibrating unit or press.

Description:
BACKGROUND TO THE INVENTION 
     The invention relates to an installation for the continuous production of materials using exothermically hardening binders, such as gypsum or cement, comprising a mixing and feeding station and a calibrating unit or press and also a control system for operating the installation. 
     PRIOR ART 
     It is known that sheet-form materials containing gypsum, for example gypsum plaster board, can be produced in a continuous installation. To this end, plaster of paris (a sulfatic binder essentially consisting of CaSO 4 .1/2H 2  O) is made up with water and additives into a free-flowing slurry, applied to a continuously advancing, endless sheet of cardboard, covered with a second endless sheet of cardboard and formed at a forming station into an endless board surrounded on all sides by cardboard. After the binder has hardened, this endless board is cut into individual boards of the required length. For a given rate of advance, the distance between the forming station and the cutting station has to be gauged in such a way that complete hardening of the binder is possible, otherwise regular further processing and the production of boards of uniform quality are not guaranteed. For a given rate of advance, the length of the installation is dependent upon the hardening time of the binder. Since this hardening time, which will be defined in more detail hereinafter, is subject to considerable variation, the distance between the molding station and the cutting station gauged according to the rate of advance must correspond in the interests of safety to the longest hardening time to be expected. The result of this is that, where the binder hardens in a relatively short time, the greater capacity of the installation which is theoretically possible in that case,is not reached. This gives rise to economic disadvantages. 
     DE-AS 22 07 799 issued 9-14-1972 describes a process and an installation for the continuous production of gypsum-based components by pouring the gypsum into a straight, longitudinally extending molding channel, cutting or sawing the &#34;strand&#34; of gypsum thus formed into individual lengths and aftertreating the cut or sawn lengths. Although this publication refers to an automated process, it does not mention any particular control system. As a result, it is not possible with this process and installation to overcome the disadvantages mentioned in the foregoing. 
     For the sake of completeness, it is also mentioned that the book by Erich Probst entitled &#34;Handbuch der Betonsteinindustrie&#34; 1951, pages 33 to 39, contains information on the setting and hardening of gypsum whilst &#34;Zementtaschenbuch&#34; 1970/71, pages 64 et seq., contains information on the setting of cement. However, neither of these publications contains any indication as to how the above-mentioned disadvantages in the continuous production of materials using exothermically hardening binders, such as gypsum or cement, can be eliminated. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to design an installation of the type mentioned at the beginning in such a way that it operates optimally in terms of an economical production. 
     EXPLANATION OF THE INVENTION 
     According to the invention, this object is achieved in that the distance between the feeding station and the entrance to the calibrating unit or press on the one hand and the distance between the feeding station and the end of the calibrating unit or press on the other hand, taking into account the mixing and forming time, corresponds to the ratio between the period of time from the beginning of mixing to the beginning of stiffening and the period of time from the beginning of mixing to the end of hydration and in that the rate of advance of the continuous installation is controlled in such a way that the exothermically hardening binder reaches it maximum temperature shortly before leaving the calibrating unit or press. 
     In this way, maximal output is obtained for the shortest possible length of the installation as a whole. 
     More particularly, the installation according to the invention may be designed in such a way that at least two temperature sensors situated at a distance from one another in the feed direction of the installation are arranged as actual-value indicators in the region of the calibrating unit or press. The arrangement may be such that the temperature sensors are preferably situated at the end of the calibrating unit or press, the second or last temperature sensor immediately preceding the end of the calibrating unit or press in the feed direction of the installation. 
     For further processing the temperature measurements supplied by the temperature sensors, the installation may be designed in such a way that a transducer, more particularly in the form of a differential amplifier, is provided for forming the actual value from the difference between the temperature measurement supplied by the last temperature sensor in the feed direction of the installation and the temperature measurement or the temperature measurements supplied by one or more temperature sensors preceding the last temperature sensor. In this connection, the actual value is preferably further processed in such a way that, in the case of a positive difference exceeding a predetermined value specific to the installation, the rate of advance of the continuous installation is reduced; in the case of a negative difference exceeding a predetermined value specific to the installation, the rate of advance of the continuous installation is increased and, in the case of an absolute difference below a predetermined set value, the rate of advance of the continuous installation is left unaltered. 
     Finally, it is possible to design the installation in such a way that several temperature sensors are arranged at a right angle to the feed direction, the average value of their temperature measurements being formed before the actual value is determined therefrom. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some embodiments of the invention are described in detail in the following with reference to FIGS. 1 to 3 of the accompanying drawings, wherein: 
     FIG. 1 diagrammatically illustrates an installation for the continuous production of materials using exothermically hardening binders for explaining the basic principle of the invention and the temperature profile in the material passing through the installation from which the material formed as product is produced. FIG. 2 shows part of the installation illustrated in FIG. 1 which is provided with temperature sensors. FIGS. 2a to 2c show temperature profiles which can occur in the installation shown in FIG. 2. FIG. 3 shows one embodiment of a complete installation according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description refers by way of example to the use of gypsum as binder for explaining the invention, although the invention is by no means limited to the use of gypsum. 
     To enable the invention to be better understood, the fundamentals of the setting mechanism of exothermically hardening binders will first be discussed in brief with reference to the example of gypsum. The production of gypsum-based materials is based on the processes of dehydration and rehydration. When gypsum dihydrate (CaSO 4 .2H 2  O) is calcined, sulfatic binders consisting essentially of semihydrate (CaSO 4 .1/2H 2  O) are formed through dehydration under suitable calcining conditions. 
     When the binder is mixed with water, the semihydrate passes into solution to saturation concentration and dihydrate is again formed through hydration, its solubility amounting to only about one quarter of that of the semihydrate, so that dihydrate separates in crystalline form from the solution oversaturated with dihydrate. This process continues until the semihydrate has been completely converted into dihydrate, a polycrystalline, stable solid being formed in this way. 
     In practice, the time aspect of the hydration process is characterized by the following characteristic quantities: beginning of stiffening, end of stiffening and end of hydration. Of these characteristics, the beginning of stiffening and the end of stiffening denote a certain consistency of the gypsum mixture whilst the end of hydration characterizes the end of the reaction by which the semihydrate is converted into the dihydrate. These time characteristics are each counted from the &#34;beginning of sprinkling&#34;, i.e. from the point in time at which the binder and the water come into contact. 
     The reaction of the binder with water to form the dihydrate is an exothermic reaction, i.e. heat is released. Because of this, it is possible, by evaluating the hydration temperature-time curve, to determine the end of hydration, i.e. as the period of time from the beginning of sprinkling to the maximum of the temperature curve. In laboratory studies, this measurement is usually made under adiabatic conditions to prevent heat from flowing off and hence to prevent falsification of the temperature-time curve. For industrial processes, it is generally sufficient to determine the temperature profile under the particular conditions prevailing. Through the dissipation of heat to the surrounding atmosphere, the temperature maximum shifts towards shorter times. However, this may be ignored for the purpose in question here or may be compensated within an automatic control system. 
     For any binder based on calcium sulfate semihydrate, the parameters beginning of stiffening and end of hydration bear a constant ratio to one another. In general terms, it may be said that the period of time from the beginning of sprinkling to the beginning of stiffening and the period of time from the beginning of sprinkling to the end of hydration are in a ratio of 1:2.5. Although these setting times of the binder may be varied within wide limits, for example through the calcining conditions prevailing during production of the binder and/or by the addition of additives (accelerators, retarders), the ratio in question nevertheless remains substantially constant even under those conditions. In the case of special binders (for example synthetic gypsum produced by certain processes), this ratio may assume a different value, although even that value remains substantially constant, irrespective of the additives. 
     In the manufacture of sheet-form materials based on gypsum, a calibrating unit or press is frequently used, as for example in the production of gypsum plaster boards, glass-fiber-reinforced gypsum boards or gypsum-bonded boards by a semi-dry process according to U.S. Pat. No. 4,328,178. The general rule in the prior art is that calibration or pressing may begin at the latest with the beginning of stiffening and may be ended at the earliest after the end of stiffening and, better still, after the end of hydration. It is only in this way that the material is able to develop its maximum wet strength. 
     For the production processes mentioned above, it is more appropriate to use continuous installations which provide for higher output levels compared with batch-type installations and in which mixing and forming can be carried out continuously with considerable advantage in terms of production technology. One such installation is diagrammatically illustrated by way of example in FIG. 1. It consists of a suitable mixing/forming station 1 and of a calibrating unit or press 2. This calibrating unit or press 2 in turn consists of a lower circulating belt 3, to which the mixture is applied, and of an upper circulating belt 4. On their sides facing one another, both belts are supported by support systems 5 and 6 in such a way that a calibrating or compression gap 7 corresponding to the required panel or board thickness is formed and optionally comprises a compression element 8 on that side facing the mixing and forming unit. 
     In view of the already mentioned, constant time relationship between the beginning of stiffening and the end of hydration, there is an optimum according to the invention in regard to the length of the production installation. For a certain rate of advance and for certain length ratios of the installation which will be explained hereinafter, the ratio of the residence times of the mixture from the beginning of mixing to the beginning of the calibrating and compression gap on the one hand and from the beginning of mixing to the end of the calibrating and compression gap on the other hand corresponds to the above-mentioned ratio between the setting times (for example 1:2.5), so that, under the rule representing the prior art, according to which the calibrating or pressing operation should begin at the latest with the beginning of stiffening and should end at the earliest with the end of stiffening or end of hydration, the maximum wet strength of the material is reached. 
     To that end, the length L 1  in FIG. 1 from the point of application of the mixture to the beginning of the calibrating and compression gap is selected equal to the product of the rate of advance and the period of time between the beginning of mixing and stiffening whilst the length L 2  from the point of application of the mixture to the end of the calibrating and compression gap is selected equal to the product of the rate of advance and the period of time between the beginning of mixing and the end of hydration. In this connection, it is emphasized that the mixing and forming time is negligible in relation to the setting times, as is generally the case in continuous installations. Where the mixing and scattering time is no longer negligible in relation to the beginning of stiffening (represented by way of example in FIG. 1 by a screw conveyor 1&#39; ), both the length L 1  in FIG. 1 and also the length L 2  have to be shortened by an amount which is calculated as the product of the mixing and forming time and the rate of advance. In this case, the lengths in question become the lengths L 1  &#39; and L 2  &#39; (cf. FIG. 1). 
     If the rate of advance of the installation is denoted v, the period of time from the beginning of mixing to the beginning of stiffening t A , the period of time from the beginning of mixing to the end of hydration t H  and the duration of the mixing and forming time t M , the distances in question may be represented as follows: 
     
         L.sub.1 =v·t.sub.A 
    
     
         L.sub.2 =v·t.sub.H 
    
     
         L.sub.1 &#39;=v·(t.sub.A -t.sub.M) 
    
     
         L.sub.2 &#39;=v·(t.sub.H -t.sub.M) 
    
     When the installation is operated under the described conditions, the calibrating and pressing operation begins with the beginning of stiffening of the mixture and is ended with the end of hydration. In this case, the temperature distribution in the direction of advance which is shown at the bottom of FIG. 1 is obtained in the installation. 
     However, the binders discussed in this example generally show variations of the order of some minutes in regard to the end of hydration. In addition, the setting times can be influenced, for example by the ambient temperature and, in cases where organic additives, for example woodchips, are used, by their water-extractable constituents and by other factors. 
     The variations in the setting behavior of the binders may be reduced in part by homogenizing different batches. In addition, the setting times may be standardized by the addition of additives in quantities measured to eliminate the variations. However, this requires above all exact control of the binders and continuous and expensive monitoring of production, experience having shown that variations in setting attributable to measuring and metering errors cannot be avoided. 
     It would be possible to select the lengths L 1  and L 2  of the installation in such a way that, even when the period of time from the beginning of mixing to the beginning of stiffening is very short, calibration or pressing take place at the right time and, even when the period of time from the beginning of mixing to the end of hydration is the longest which can be expected, setting within the calibrating unit or press is guaranteed. Such an approach would increase the capital cost of an installation of the type in question and would preclude economically favorable operation. 
     The interrelationships explained in the foregoing also apply correspondingly to other exothermically hardening binders, for example cement and modified sulfatic binders, in whose case the time factor of the hardening process also has to be taken into account in production. 
     The present invention obviates the disadvantages mentioned in the foregoing and, for a very small capital investment, provides for economically optimal operation through an independent control system for an installation for the continuous production of sheet-form materials using exothermically hardening binders which takes into account variations in the setting times of the binder used and always provides for optimal utilization of the installation. 
     According to the invention, the rate of advance in an installation of the type shown in FIG. 1 is controlled in such a way that the maximum temperature of the setting mixture is reached at the end of the calibrating unit or press. This function is performed by an automatic control system which acts, for example through a suitable circuit, on the drive motors of the calibrating unit or press together with the mixing and feeding station. Two continuously measuring temperature sensors 9, 10 are arranged in the vicinity of the calibrating unit or press 2 as the signal generators of the automatic control system. 
     These temperature sensors are preferably situated in the end part of the calibrating unit or press 2, as shown by way of example in FIG. 2 where the temperature sensors are denoted by the references 9 and 10. In this embodiment, the second or last temperature sensor 9 is arranged immediately in front of the end of the constant calibrating or compression gap 7. 
     For temperature measurement, temperature sensors 9, 10 measuring without contact may be arranged outside the calibrating unit or press 2 or, alternatively, temperature sensors measuring by contact, such as for example resistance thermometers or thermocouples, may be arranged in the fixed supporting system for the calibrating units or presses. 
     Instead of the individual temperature sensors 9 and 10 in FIG. 2, it would even be possible to arrange several temperature sensors transversely of the direction of advance for averaging purposes in order thus to neutralize possible variations over the width of the board. 
     The measurements of the temperature sensors converted into an electrical signal are continuously compared with one another in an automatic control system. The difference between the last measurement (in the direction of advance) and the measurement provided by the preceding temperature sensor or the preceding temperature sensors is used as the manipulated variable for regulating the rate of advance of the installation. This difference is compared with a predetermined set value. 
     The speed of advance of the installation is preferably regulated on the basis of the following criteria: 
     (a) If the last measurement (in the direction of advance of the board 3) is larger than the measurement(s) of the preceding temperature sensor(s) by a difference Δθ set  to be defined according to the particular parameters of the installation (FIG. 2a:+Δθ), the setting mixture has not reached its maximum temperature and the resulting control signal slows down the rate of advance of the board. 
     (b) If, by contrast, the measurement of the last temperature sensor (in the direction of advance of the board) is smaller by the difference Δθ set  than the measurement(s) previously obtained, the maximum temperature has already been reached and the rate of advance of the board is accelerated by a corresponding control signal (FIG. 2b). 
     (c) Within these limits, i.e. where the last two sensors record the same temperature or in the case of a temperature difference smaller than the predetermined value Δθ set , no control signal is released and the rate of advance of the board remains unchanged (FIG. 2c). 
     The control circuit may be designed, for example, in such a way that, as shown in FIG. 3, the two temperature sensors 9 and 10 are connected to a transducer 11 which forms the difference between the measured temperatures as the actual value. This actual value is compared with a set value by a set value adjuster 13 in a control signal generator 12 and, depending on the result, a control signal is delivered to the final control element 14 which either leaves the drive motor 15 for the board 3 rotating at the same speed or increases or reduces its rotational speed.