Abstract:
A device for preparing moulding sand. The device includes a mixer configured to operate under at least a partial vacuum and at least one feeder to introduce components to be mixed. An inlet in the mixer is of sufficient size to allow rapid introduction of the components into the mixer from the feeder. The combination of large inlet opening and a pressure differential between the mixer and the feeder significantly reduces the time required to fill the mixer. In addition, the presence of the pressure differential made possible by the vacuum in the mixer reduces the likelihood that airborne contaminants resident in the mixer will migrate outside the mixer. Optional moveable conveyor chutes minimize contact of airborne contaminants with adjacent equipment, while optional mixing tools facilitate the introduction of powdered additives into the mixer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 371 of PCT/DE01/02259 filed on Jun. 16, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a device for the preparation of mould sand using a mixing process in a mixer, wherein the preparation takes place at least partially under vacuum. 
     The preparation of sand for the manufacture of moulds is intended to produce the correct mixing ratio of grain sizes and of proportions of quartz sand, binding agent, coal dust, optionally other additives as well as recycled and new sand, to homogenise the mixture and, in this context, to coat the grains with the binding agent to a large extent, to adjust the correct moisture content, to adjust the correct temperature of the mould sand and finally to convey the ready-prepared sand to the places of use. 
     Recycled sand generally has an increased temperature, for example, of between 100° C. and 140° C. Because sand temperatures above approximately 50° C. can present considerable problems to the moulding equipment, and at excessively high temperatures, uncontrollable evaporation losses over the distance between the mixer and the moulding equipment can lead to fluctuations in moisture in the ready sand, the sand must be cooled in this case. Conveyor belt coolers, through which the sand passes continuously as a result of vibrating or agitating movements of a screen mesh, are mostly used for this purpose. 
     An alternative cooling method has been proposed in DE 295 24 03 C2. This cooling method provides the concurrent preparation and cooling of clay-bound foundry moulding sands in a vacuum mixer. In this context, the individual components are initially placed into the mixer. After a short prehomogenizing process, the temperature and moisture of the mixture are measured and the necessary quantity of water is added. Finally, during the preparation process, the pressure in the mixture is gradually reduced. As soon as the pressure corresponds to the vapour pressure curve of water, the water in the sand begins to boil and removes the necessary evaporation heat from the sand. As a result, an extremely effective cooling method is achieved in an economical manner. According to the disclosure of DE 199 45 569, alongside the cooling effect on the mould sand, the described cooling of mould sand under vacuum also leads to an increase in quality of the mould sand prepared. Accordingly, DE 199 45 569 suggests preparation under vacuum even for recycled sand which has already been cooled. 
     It has been shown that the best mould sand quality can be achieved with the assistance of vacuum preparation. However, the known stages of the method and the devices and/or peripheral equipment used and their method of operation are only suitable to a limited extent or not at all for use in a fully automated foundry moulding plant. According to experience, error-free and above all economically optimized operation is not possible with the known method. 
     This is because, among other factors, the filling and emptying of the mixer is very time-consuming. For the ventilation of the mixer, a mixer cover is provided, in all known versions, which, in the closed condition, must be vacuum-tight to allow vacuum operation, and which is opened for the purpose of charging the mixer. In this context, the cover is generally connected to the mixer in a rotatable manner by means of a rotating axis. The cover can be designed in such a manner that it is rotated outwards or inwards in order to open the container. If it is rotated inwards, the closing mechanism must press the cover outwards against the sealing surface of the mixer with considerable force during vacuum operation. In order to manufacture the closing mechanism in an economical manner, the mixer cover must therefore be very small, because only a small force need then be applied by the closing mechanism. 
     If the cover is opened outwards, the closing mechanism can be designed to be weaker and can therefore be manufactured more economical ly, because the necessary pressing force can be generated by the pressure difference between the mixing container and the surroundings alone. However, with this version, the design must take into consideration that sufficient rotational clearance remains above the cover so that the cover can be opened without it coming into contact with any objects. Accordingly, dosage funnels or dosage devices must be attached at an appropriate distance above the mixer opening. This distance necessarily increases in proportion with the size of cover. However, when filling the mixer, it is important to ensure that the sealing surface of the charging opening remains as a free from contamination as possible, in order to guarantee a vacuum-tight closability. However, the probability of contamination of the sealing surface increases considerably as the distance between the dosage funnel and filling opening and/or the dropping height of the substance to be charged increases. For this reason, it is currently assumed, that a generic mixer cannot be manufactured economically with a large charging opening. Accordingly, the known mixers all possess only a relatively small opening in the pressure casing of the mixer, and, in the case of the equipment known so far, the mixture is added only in a very fine flow. This results in a very long charging time and therefore also a very low plant performance. If the mixer is loaded too quickly, an excess air pressure rapidly builds up as a result of compression of the air in the mixer. This excess pressure generally leads to the emission from the mixer of dust-like components of the mixture, which can, for example, also be deposited on very sensitive machine components, such as cogwheels and gaskets. This means that the plant must be cleaned more frequently which, once again, is associated with higher costs and undesired interruptions of operation. For this reason, it has generally been considered that, on the one hand, the charging rate cannot be further increased, because larger input openings cannot be realized in an economical manner. And, on the other hand, it has been considered that a larger charging rate will lead to the disadvantages described and must therefore be avoided. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide a method and a device for preparation of mould sand, which can be used economically without errors, and provides mould sands of uniform temperature and uniformly high quality in an economical manner and which, in comparison with the known mixers, has an increased charging rate. 
     With regard to the method, this object is achieved by the material to be charged being added at least at times in a volume flow of at least 100-800 l/s through an opening with a diameter of at least 150 mm, preferably at least 300 mm, and particularly preferably at least 500 mm. 
     In this context, the pressure difference between the ambient pressure and the pressure in the mixing chamber of the mixer is preferably used either as the exclusive or predominant drive for at least one feeder process for water or for a mixture component or for accelerating the feeder process. According to the invention, therefore, the dosage devices and charging devices are coupled with the mixing unit in terms of the design and method. As a result, the vacuum prevailing in the mixer can be used; for example, to accelerate the charging processes, and even during the charging phase, to improve the distribution of the additives and the fluids to be added. In particular in combination with a large charging opening, the consistent use of pressure difference can achieve a significant reduction in charging time. Moreover, this method causes no additional costs, because the evacuation device necessary for the preparation method is already present. 
     Particularly preferred is a method in which at least a portion of the quality-determining mixture components is fed into the mixer during the charging and/or mixing process. The quality-determining mixture components are the additives already mentioned, such as, for example, bentonite, coal dust, etc., which are added to the recycled sand, in order to adjust the quality of the prepared mould sand. The vacuum pressure in the mixer, which is used to suck in the components to be filled, also effectively prevents the emission of dust-like mixture components from the mixer and their being deposited, for example, on sensitive machine parts. 
     A particularly expedient version of the method according to the invention additionally provides that the individual mixture components are introduced into the mixer one after another according to a predetermined sequence. However, for special cases of application, it may be advantageous if the water is only introduced into the mixer after the other mixture components have been introduced into the mixer essentially concurrently. As a result, after the other mixture components have been introduced, it is possible to determine the residual moisture and temperature of the recycled sand and from this, to calculate the appropriate quantity of water to be added. 
     In particular, in order to be able to mix the water to be added as well as possible with the mixture, a preferred version of the method provides that at least one portion of the water to be added is introduced directly into the mixture with the assistance of a preferably rotating feeder device. In this context, by rotating is meant rotating relative to the mixer, so that it is irrelevant whether the feeder device rotates or the feeder device remains stationary and the mixer rotates around the stationary feeder device, or whether both the mixer and the feeder device rotate. This direct method of feeding, that is, below the mixture level, can enable a very good mixing of the water with the mixture. 
     A particularly effective version of the method provides that at least one portion of the water is introduced into the mixture via a feeder device, which is coupled with a mixing tool or even integrated into a mixing tool. This is particularly advantageous if the mixer already provides a mixing tool. Moreover, as a result of this stage of the method, the water can be mixed directly with the filling substance. 
     It is also particularly expedient if the quality-determining mixture components, such as e.g. bentonite and coal dust, are introduced into the mixer below the filling level of the mixture. This measure also guarantees a very good mixing of the quality-determining mixture components with the mixture in the mixer. 
     The quality-determining mixture components are preferably introduced centrally and directly inside the vertically and tangentially flowing mixture bed. This further enhances miscibility. 
     For some cases of application, it may also be advantageous if at least one portion of the quality-determining mixture components is first mixed with air, and this air/solid mixture is then introduced into the mixer, i.e. preferably below the filling level. After preparation of the mould sand, the mixer must necessarily be ventilated again, that is, the pressure difference between the mixing container and the ambient pressure must be equalized. This can be achieved, for example, simply by opening the container cover. However, a method, in which the mixing chamber is ventilated via a feeder which terminates in the mixing chamber below the filling level of the mixture, is particularly preferred. As a result, the mould sand is compressed to a lesser extent. By contrast, if the equalization air is fed in above the sand level, a kind of pressure cushion will be formed on the surface of the sand as a result of the pressure difference prevailing above and below the filling substance, which leads to definite, temporary compression at least of the uppermost layer of sand. 
     Of course, it is also possible to use the feeder, which is provided for feeding the quality-determining mixture components, for ventilation and/or pressure equalization. With regard to the device, the object mentioned initially is achieved by a device for the preparation of mould sand with a mixer, which has a vacuum chamber or is arranged in a vacuum chamber, which can be closed in an essentially vacuum-tight manner, with devices for feeding the components to be mixed, at least one mixing tool as well as a device for removing the ready mixture, wherein a closable feeder connection for the components to be mixed exists or can be manufactured between the mixing container and the outside, wherein the feeder opening has a cross-sectional area of at least 0.25 m 2 , preferably at least 0.4 m 2  and particularly preferably at least 0.5 m 2 . 
     In principle, the feeder opening can have any cross-sectional form, round or rectangular forms being preferred, however. 
     Feeding preferably takes place through the feeder opening either exclusively through the pressure difference between the ambient pressure and the pressure in the mixing chamber of the mixer, or feeding is at least accelerated by this pressure difference. 
     Through this at least one closable feeder connection, the pressure difference between the ambient pressure and the pressure in the mixing chamber of the mixer can be exploited as a driving force. If the feeder connection is opened, as a result of the vacuum pressure existing in the mixing chamber, material to be supplied is drawn into the mixing container from the outside. In general, an additional pump is not necessary for this purpose. The feeder therefore requires no additional energy and is, moreover, essentially maintenance-free. 
     Particularly preferred is a version in which an essentially vacuum-tight, closable filling opening of the mixer can be connected via an essentially vacuum-tight intermediate space to the outlet opening of at least one dosage device which is preferably designed as a dosage weighing device. Recycled sand, for example, can be introduced into the mixer through this opening. For this purpose, the mixer must first be placed under a vacuum. Following this, the filling opening of the mixer is opened, so that the mixing chamber is connected to the essentially vacuum-tight intermediate space. After this, the outlet opening of at least one feeder device is opened so that the charging materials from the feeder device are first conveyed into the intermediate space and then into the mixing chamber. This charging takes place very quickly because the pressure in the mixing chamber of the mixer and in the intermediate space is significantly lower than the pressure in the feeder device. Particularly preferably, the filling opening of the mixer and/or the outlet opening of the feeder device have a cover with lateral cheeks, which, with the assistance of the lateral cheeks, forms a kind of conveyor chute in the opened state. With the assistance of this conveyor chute, the supplied filling materials can be conveyed from the outlet opening of the feeder device at a high flow rate directly into the inlet opening of the mixer. Particularly preferably, the inlet opening of the mixer and also the outlet opening of the feeder device have a cover with lateral cheeks, each of which therefore form a conveyor chute in the opened state. 
     For particular cases of application, it could be advantageous if a further movable chute component is provided which can be moved independently of one of the covers. For the purpose of feeding in this case, the cover of the inlet opening of the mixer can first be opened, preferably by means of a control device, then the movable chute component can be brought into its functional position and finally the cover of the feeder device can be opened. The three chutes are then preferably arranged in such a manner that they form a passage for the substance to be filled and guarantee a rapid and targeted charging of the mixer with the substance to be filled. The chutes are preferably arranged in such a manner that they project into the openings and thereby prevent the substances to be filled from coming into contact with the edges of the opening. Contact of this kind could, under some circumstances, impair the sealing function of the cover of the inlet opening of the mixer. 
     A particularly preferred version of the device according to the invention provides that the mixing chamber of the mixer is arranged in a pressurized container and that, inside the pressurized container but outside the mixing chamber, a closable air feeder is provided. The pressurized container is advantageously connected via appropriate seals to the mixing chamber arranged in the pressurized container. These seats necessarily allow the passage of air, but are intended to hold the components of the mixture in the mixing chamber as firmly as possible. It is not desirable for mixture to escape from the mixing chamber into the pressurized container because contamination of the seals and movable drive components and bearings can occur there. If the mixing chamber is then rapidly loaded with the substance to be filled without vacuum, the pressure in the mixing chamber will rise very rapidly. The seals generally used between the mixing chamber and the pressurized container are, however, not capable of sustaining their sealing function under such an abrupt rise in pressure. It can therefore occur that material from the mixing chamber, which then has a higher pressure than the pressurized chamber, enters the pressurized chamber. As a result of the closable air feeder arranged according to the invention inside the pressurized container but outside the mixing chamber, the pressure in the pressurized container outside the mixing chamber can be increased by the air feeder during the charging process, so that the pressure in the pressurized container is higher than pressure in the mixing chamber. In this way, any transfer of material from the mixing chamber into the pressurized chamber is prevented. 
     Particularly preferred is a version in which a control unit is provided which opens the air feeder when the mixture components are supplied and closes the air feeder when the vacuum container is closed in a vacuum-tight manner. This control unit is preferably automated, so that, depending on the stage of the method, it is possible to evacuate the pressurized container and also to allow a build-up of pressure in the pressurized container, in order to counteract any abrupt rise in pressure in the mixing chamber as a result of the feeding of mixture components. It is self-evident that the air feeder described can be used outside the mixing chamber but inside the vacuum chamber even with known mixers. Even if the known mixers do not use the pressure difference between the pressurized chamber and the surroundings as a driving force, and consequently, the abrupt rise in pressure resulting from the charging process is significantly smaller, a transfer of material from the mixing chamber into the pressurized chamber will be prevented even with known mixing containers. 
     A further particularly preferred version of the present invention provides that a feeder device for water is arranged in such a manner that the water is supplied through or along a preferably eccentrically arranged mixing tool with mixing paddles and fed into the mixture essentially in the region of the ends of the mixing paddles. Also in this case, according to the invention, the pressure difference between the mixing chamber and the outside environment is exploited. If water is to be fed into the mixture, a valve merely has to be opened. As a result of the vacuum pressure prevailing in the mixing chamber, the water is sucked through the feeder device directly into the mixture. The arrangement of the feeder device along a mixing tool has the advantage that the water can be fed directly into the mixture at different positions. In this context, the fluid outlet openings in the water feeder device are preferably arranged at different depths below the level of the mixture. Adequate mixing can therefore be achieved extremely quickly. 
     Particularly preferably, the feeder device for water has a dosage weighing device, the dosage weighing device and mixer being connected by a preferably at least partially resilient pipe, which can be closed by a valve, the valve being preferably arranged on the cover of the mixer. 
     The so-called quality-determining mixture components are preferably fed with the assistance of a feeder lance, if possible below the level of the mixture. In this context, the outlet opening of the feeder lance is, if possible, oriented tangentially to the direction of flow of the mixture and preferably points in the direction of flow. This guarantees that, as a result of the flow of mixture emphasized by the rotation of the mixture, the quality-determining mixture components, which are sucked into the mixing chamber as a result of the vacuum pressure prevailing in the mixing chamber, are drawn along with the mixture in the direction of flow and are rapidly and effectively mixed with the latter. 
     Further advantages, features and possible applications of the present invention are explained with reference to the following description of preferred versions and the associated figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in: 
         FIGS. 1   a ) and  1   b ) a lateral view of the arrangement of a feeder device and a feeder opening of the mixer in an opened and in a closed position, 
         FIG. 2  a lateral view of a vacuum mixer with a partial sectional view, 
         FIG. 3  a detailed view of  FIG. 2 , 
         FIGS. 4   a ) and  4   b ) a schematic representation of the connection between a fluid dosage weighing device and mixing container, 
         FIGS. 5   a ) and  5   b ) a lateral view and a top view of the feeder elements for fluids in the mixing chamber, 
         FIG. 6  a schematic representation of the feeder for the quality-determining components and 
         FIG. 7  a lateral view of the arrangement of an alternative feeder device and a feeder opening of the mixer. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1   a ) and  1   b ) show the outlet region of a solids weighing device  10  and the inlet region of the mixer  1 . The solids weighing device  10  is used for determining the quantity of recycled sand to be supplied or optionally also of other mixture components. In  FIG. 1   a ), both the mixer  1  and the solids weighing device  10  are closed, while  FIG. 1   b ) shows the conveyance position between the mixer  1  and the solids weighing device  10 . 
     An inlet connecting piece  2  is arranged at the upper side of the mixer  1 . This inlet connecting piece  2  is sealed in a vacuum-tight manner through the container cover  3  with the assistance of the lever arm  5 , which is driven, for example, by a hydraulic cylinder. It can be seen that the container cover  3  provides one lateral cheek  4  at both of its lateral outer edges. 
     The solids weighing device  10  also has an outlet flap  11 , which provides lateral cheeks  11 ′ at both of its lateral outer edges. This flap can be opened or closed via the lever  12 . 
     Moreover, this version has a conveyor chute  13 . The conveyor chute  13  also has lateral cheeks  13 ′ at both of its lateral outer edges. The conveyor chute  13  can be moved with the assistance of the parallel guide  14  and the lifting drive  15  into the intermediate space between the solids weighing device  10  and the mixer  1 . As a result of the lateral cheeks, the outlet flap  11 , filling flap  3  and conveyor chute  13  provide an essentially U-shaped cross-section, the lateral cheeks forming the two arms of the U-shape. 
     The conveyor chute  13  is arranged in such a manner that, in the extended position, when the outlet cover  11  of the solids weighing device  10  is open, together with the outlet flap  11  and the lateral cheeks  11 ′,  14 ′, it forms a channel with an essentially rectangular cross-section. 
     This channel is further extended by the open filling cover  8  with its lateral cheeks  4 , so that, in the conveying position, the situation shown in  FIG. 1   b ) is produced. In this position, the material is guided from the solids weighing device  10  directly into the mixer  1 . The conveyor chutes in this arrangement form a kind of channel, so that the edges of the opening are covered and cannot come into contact with the material to be filled. 
     The entire range of movement of the flaps  3 ,  11  and of the conveyor chute  13  is surrounded by a housing  6  and/or  6 ′. In the version shown, the housing is designed in two parts; and the two housing parts  6 ,  6 ′ are connected to one another via a flexible preferably sealing connection  7 . The charging process is as follows. First, the two covers  3 ,  11  of the mixer  1  and the solids weighing device  10  are closed. If the mixer is to be charged with materials which are located in the solids weighing device  10 , the cover  3  of the mixer  1  is first opened. Next, the conveyor chute  13  is moved into the region between the solids weighing device  10  and the mixer  1 . This has not hitherto been possible because, in the extended state, the conveyor chute  13  is located within the rotational range of the filling cover  3  of the mixer  1 . If the outlet flap  11  of the solids weighing device  10  is then opened, the materials from the solids weighing device will be filled directly and rapidly into the mixing chamber of the mixer  1  via the channel formed by the outlet flap  11 , inlet cover  3  and conveyor chute  13 . In this manner, the mould sand from the solids weighing device  10  is conveyed into the mixer  1  without significant material loss and without substantial emissions of dust, through a large cross-section and in a short time. 
     As shown in the diagrams, additionally arranged in the housing  6 ,  6 ′ are air nozzles  8 ,  9  which direct an air-flow onto the seal of the inlet cover  3  and the drive mechanism of the inlet cover  3 , so that, after each filling process, those places are fanned on which a deposit of sand may have a negative effect, in order to guarantee a secure and sealed closure of the inlet cover  3 . 
     The inlet cover  3  of the mixer  1  according to the invention does not have any particularly complicated design for the sealing elements. Rather, it is simply pressed by the vacuum pressure prevailing in the mixer  1  against the opening of the mixer  1 , so that the opening or the cover  3  merely needs to be surrounded by a sealing ring. However, this version of the inlet cover  3  necessarily requires a certain distance between the solids weighing device  10  and the mixer  1 , because the cover  3  requires sufficient clearance for rotation. As described, this distance is bridged by the material guiding channel, which is formed from the flaps  3 ,  11  and the conveyor chute  15  and lateral cheeks  4 ,  11 ′,  13 ′. According to the invention, the charging time for a mixer of this kind is reduced from approximately 30-40 seconds, which is entirely standard for mixers available on the market, to less than 10 seconds. 
     The mixing chamber  16  of a vacuum mixer  1  is normally arranged in a vacuum chamber  17 . The basic structure is shown in  FIG. 2  with further detail shown in FIG.  3 . The vacuum chamber  17  is sealed via flexible seals  18  from the mixing chamber  16 . In this context, the seal  18  is used only to prevent the entry of mixture material from the mixing chamber  16  into the vacuum chamber  17 . The drive unit for the mixer is generally arranged in the vacuum chamber  17  but outside the mixing chamber  16 . For this reason, the reliable function of the seal  18  is extremely important, as otherwise the intermediate space  17  would have to be cleaned frequently, as otherwise the drive unit could be destroyed as a result of solid materials of the mixture. The charging phase, in particular, is a very critical moment for the seal  18 . Even with the conventional mixers, an abrupt rise in pressure occurs as a result of the filling process, so that repeated functional failures of the seal  18  occur. This problem is further intensified by the charging process as described with reference to  FIGS. 1   a ) and  1   b ). According to the invention, the mixing container is under vacuum at the beginning of the charging process, so that the abrupt rise in pressure in the mixing container is even more pronounced during charging. In order to prevent the entry of dust into the intermediate space  17 , a sliding ring seal may, for example, be used. However, since this causes very high costs, a version according to the invention provides a closable air feeder  19 . This air feeder, which in  FIGS. 2 and 3  is designed as a pressure fan, is capable of increasing the pressure in the intermediate space  17  at the beginning of the charging process. In this context, the rise in pressure in the intermediate chamber  17  should approximately correspond to or even exceed the abrupt rise in pressure in the mixing chamber  16 . 
       FIG. 3  shows structural details of the seal  18 . Approximately at the start of the charging process, the valve  21  is opened, so that the pressure fan  19  introduces air into the intermediate space  17  between the pressurized chamber wall  17 ′ and the mixing chamber wall  16 ′. The air introduced flows in the direction of the arrow through the gap seals  18 ,  22  into the mixing chamber  16 . As a result of this measure, the emission of dust or material from the mixing chamber  16  into the intermediate space  17  is effectively prevented. 
     It is evident that the air feeder does not necessarily require a pressure fan  19  or a similar device; for some cases of application, it may be adequate if only a closable opening is provided as the air feeder, which is simply opened at the beginning of the charging process, so that the pressure in the vacuum chamber and/or the intermediate space  17  and the mixing chamber  16  rises in an approximately synchronized manner. 
     At the start of the preparation of the mould sand under vacuum, the air feeder must be switched off again or closed. 
       FIGS. 4   a ) and  4   b ) show the charging of the mixer with the necessary quantity of mixing water. Between 0.5 and 4% mixing water are normally added to the mixture. The exact quantity of water to be added is determined by measuring the residual moisture of the recycled sand before placing in the mixer or even in the mixer. The residual moisture of the recycled sand and therefore also the quantity of mixing water to be added depends on the preliminary thermal loading of the recycled sand. Moreover, it must be borne in mind that the vacuum-cooling process also consumes a certain quantity of water because, as described above, it is based upon the removal of evaporation heat, so that an additional quantity of water must be added, which evaporates during the vacuum phase. 
       FIG. 4   a ) shows a conventional arrangement. A weighing container  25  is shown which is suspended by means of a carrier structure  24  on a weighing cell  23 . The weighing cell  23  measures the weight of the weighing container  25  including the carrier structure and content of water. When the valve  26  is opened, the water leaves the weighing container  25  via an outlet pipe  27  and flows into an inlet pipe  30 . The inlet pipe  30  is connected rigidly to the pressurized container of the mixer. The inlet pipe  30  and outlet pipe  27  are expediently surrounded by a pressure-resistant but flexible sleeve  29 . To allow the water to be added very quickly, water is drawn from the weighing container  25  and the quantity is determined via the weight loss, which is detected by the weighing cell  23 . 
     The pressure difference between the mixing chamber and the surroundings or, in this case, the weighing container  25  can also be used advantageously in the context of the water supply, in order to significantly accelerate the charging process. This is possible in a similar manner to the charging of mould sand described with reference to  FIGS. 1 and 2 , for example, if the mixing water is supplied while the mixing container is under vacuum. However, with the arrangement shown in  FIG. 4   a ), this is only possible subject to other disadvantages. 
     In the case of the arrangement shown in  FIG. 4   a , the vacuum pressure in the mixing container exerts a drawing force on the valve  26  via the inlet pipe  30  with diameter D. This drawing force depends on the momentary pressure in the mixing container and has a disadvantageous effect on the measuring accuracy of the weighing cell  23 . Even the filling of the weighing container  25  with water, during a process phase, in which no water is supplied to the mixing chamber, cannot be metered accurately, because the changing pressure in the mixing chamber is also always exerted on the weighing the cell  23 . The particular version shown in  FIG. 4   b ) provides that the valve  26  is not arranged in the outlet pipe  26  but in and/or on the inlet pipe  30 . In this case, the sleeve  29  is necessarily above the valve  26  and not, as in the case of conventional plants, below the valve  26 . Moreover, this arrangement has the advantage that the falsifying influence of the mixing chamber pressure on the weighing cell  23 , on the one hand, occurs only while the valve is open, and on the other hand, the pressure acts on the weighing cell  23  only via the significantly smaller cross-section d′ of the outlet pipe. 
     As a result of this arrangement, the weighing container  25  can reliably be filled with the desired quantity when the valve  26  is closed. The weighing error while the valve is open can easily be corrected by means of a tare correction. 
     For particularly accurate dosages, the tare correction can be carried out with the assistance of the dosage computer  31  and the dosage meter  33 . The dosage meter  33  registers the current pressure in the mixing chamber and passes this value to the dosage computer  31 . The dosage computer  31  calculates the drawing force exerted by the mixing chamber on the weighing cell  23  and corrects the weighing result, so that the mixing water can be metered very accurately. 
     The duration of filling can be considerably reduced by exploiting the pressure difference between the mixing chamber and the ambient pressure. For example, the cross-section d′ of the outlet pipe can be reduced, so that the falsifying influence of the drawing force can be further reduced. As a result, the rate of filling is necessarily increased, but this is more than compensated by the vacuum-filling process. 
     Introducing the mixing water under vacuum has the additional advantage that the water is immediately finely distributed and spreads in a nebulous manner in the mixing chamber. 
     The thorough mixing of the mixing water with the mixture can be further improved and above all accelerated if the mixing water is supplied via a device, as shown in  FIGS. 5   a ) and Sb). A mixing shaft  34  with mixing tools  35  is provided in the mixer  1 . The mixing shaft  34  is mounted outside the container in a bearing  32 . Above the bearing, a rotary connection  31  is connected to the inlet pipe  30 . The water flowing from the dosage device, preferably from the; water weighing device described with reference to  FIG. 4   b ), in the direction of the arrow, is guided via the rotary connection  31  into the longitudinal bore hole  33  of the mixing shaft  34 . The longitudinal borehole  33  is connected at different heights by pipes or hoses  36  to outlet nozzles  37 . Subject to the vacuum prevailing in the mixing container, the water is sucked through the supply and distribution system described directly into the mixture, without pumping or other conveying devices being necessary. The method according to the invention even allows the processing of recycled condensation water from a thermal exchange unit of the vacuum cooling process. Condensation water is generally contaminated with fine particles, so that there can be no question of charging this water when using pumps or conventional nozzles, because a pump will wear very rapidly as a result of the fine particles and the nozzles can frequently become clogged. According to the present invention, however, this water can be reused directly without prior costly cleaning processes. 
     An alternative version of the present invention is shown in FIG.  6 . In this case, powdered additives are successfully utilized by exploiting the pressure difference (principle of suction conveyance) between the mixing container and the atmosphere. 
     These additives, often also referred to as quality-determining mixture components, are normally blown into the mixer under pressure. However, for this purpose, appropriate pressure stores must be provided for the pumping air. In addition to the undesirable additional space requirement, the consumption of expensive compressed air is not to be ignored. Moreover, the vacuum cooling process cannot be implemented while the additives are being supplied, because the supply of additives under pressure is also necessarily associated with an increase in pressure in the mixing container. Furthermore, the charge of compressed air in the mixing chamber can have disadvantageous consequences. In addition to restricting the sealing function of the seal  8 , as described in the context of  FIGS. 2 and 3 , the charge of air can also delay the uniform thorough mixing of the mixture with the mixing water and the additives. 
     According to the invention, the disadvantages described can be overcome by supplying the powdered additives with the assistance of a preferably stationary mixing tool  39  or its carrier arm  41 . The stationary mixing tool  39  is used primarily for guiding the material. With reference to the arrangement shown in  FIG. 6 , the mixing tool  39  assumes the additional function of cleaning the container wall of mixer  1 . The mixer  1  or the mixing chamber rotates in an anticlockwise direction as shown in  FIG. 6  from above. The mixing tool “scrapes” along the container wall thereby cleaning any unmixed mixture components from it. The mixing tool guides the mixture from the edge of the container to the middle of the container  1 . The mixing tool  39  is attached by means of a carrying arm  41 . The carrying arm  41  is designed to be hollow, so that the powdered additives, the quantity of which has been determined with the assistance of the dosage weighing device  43 , can be supplied via the feeder  42  into the hollow cavity  40  of the carrier arm. The additives are sucked into the mixing chamber as a result of the pressure difference between the mixing container and the surroundings. The hollow cavity  40  is connected to a feeder nozzle  45 , of which the outlet opening is arranged in such a manner that the additives sucked in are guided inwards in as radial a manner as possible. The version shown in  FIG. 6  exploits the suction action, which is formed in connection with the mixing tool  39 , in order to draw the additives inwards. For this purpose, the mixing tool  39  provides yet another extended region  39 ′ in the vicinity of the base, which is arranged in the direction of flow of the mixture essentially directly in front of the outlet opening of the feeder nozzle  45 . 
     As a result of this sophisticated arrangement and the exploitation according to the invention of the pressure difference, additives can be supplied simply and economically. Moreover, mixing is extremely effective and, above all, rapid. 
     The hollow tool designed for supplying additives can also advantageously be used for ventilation, that is, for pressure equalization of the mixing container, when the vacuum cooling process is completed. For this purpose, air is simply sucked in through the feeder  44  into the mixing container. Feeding the air directly into the mixture, that is, below the level of the mixture, provides the important advantage that the mixture is not temporarily compressed by the resulting pressure wave, which is not the case with the conventional mixers, and the air can therefore be mixed into the mixture. 
       FIG. 7  shows an alternative version of the charging opening of the mixer  1 . In this version, the mixer  1  does not have a cover. Only a pressure-resistant, rigid conveying funnel  46 , which surrounds the inlet opening, is provided. Above the conveying funnel, a similarly pressure-resistant but movable housing  47  is provided which is connected via a pressure-resistant, flexible connection  48  to the conveying funnel  46 . The weighing container  49  is used for the dosage of the mixture to be added. The quantity to be filled can be deduced from the weight of the weighing container  49 , which is determined via the force transducer  51 . At its lower end, the weighing container  49  provides a pressure-tight closure cap  11 , which can be opened and closed via an activation lever  52 . Additionally, closure mountings  51  are provided, which serve to hold the closure cap onto the weighing container  49  in a vacuum-tight manner. 
     This version allows the addition of the mixture under vacuum. The filling procedure is as follows. Initially, the closure cap  11  of the weighing container  49  is closed. The mixing container  1  is evacuated so that vacuum pressure prevails even inside the conveyor funnel  46  and the pressure-resistant housing  47 . The mixture is then filled into the weighing container  49 , and the quantity to be filled is determined via the pressure transducer  50 . When measuring the quantity to be filled, it should be taken into account that the pressure difference between the housing  47  and the interior of the weighing container  49  falsifies the weighing via the force transducer  50 . This must be taken into account when calculating the net weight. The weighing container  49  and the housing  47  rigidly connected to the weighing container can readily be displaced in a vertical; direction depending on the filling weight and the pressure difference. This vertical movement is allowed by the flexible connection  48 , which is clearly shown in  FIG. 7  on the left in the detailed enlargement. 
     In the next stage, the closure mountings  51 , which enclose the closure cap in the manner of a clamp, are rotated outwards around the axis  53 , as shown in  FIG. 7  in the right-hand detailed view. The closure cap is thus unlocked and can then be opened with the assistance of the activation lever  52 . In combination with the large charging opening, the pressure difference between the solids weighing device and the mixing container ensures rapid charging. Furthermore, a cover which includes the associated, necessary drive units can be dispensed with in the case of this version. Moreover, this version requires a lower structural height, because the rotational range for the mixing chamber cover is not necessary, and the closure cap of the solids weighing device can be designed in such a manner that it is immersed in the conveyor funnel or even in the mixing container opening during opening. 
     It is self-evident that all of the versions described can also be realized with smaller mixing container openings, although this necessarily means that the charging rate will be somewhat lower. Depending on the individual case of application, however, one of the versions described may be advantageous in combination with a smaller charging opening.