Method and device for discontinuous gravimetric dosing of bulk material

Abstract

The invention relates to a method and a device for the discontinuous gravimetric dosing of bulk material of a batch C from a storage container (9) into a collection, mixing, or reaction container (21) by means of a dosing scale (1). The dosing scale (1) is pressurized with a process gas (18) during filling from the storage container (9) and dosing into the collection, mixing, or reaction container (21). To ensure high dosing accuracy and simultaneously meet increased containment requirements, the following method steps are carried out with the method and device according to the invention: a) Filling the dosing scale (1) from the storage container (9) up to a first fill level, b) Carrying out a pressure equalization between the interior of the dosing scale (1) and the environment of the dosing scale (1), c) Determine a first weight value G ieof batch C by weighing at the first fill level in the dosing scale (1), d) generating an overpressure in the dosing scale (1) compared to the ambient pressure by feeding a process gas (18) into the dosing scale (1), e) Dosing batch C from the dosing scale (1) into the collection, mixing or reaction container (21). f) Carrying out a pressure equalization between the interior of the dosing scale (1) and the environment of the dosing scale (1), g) Determining a second weight value G iz of batch C by weighing, with a second fill level in the dosing scale (1) after dosing has been completed, h) Determine the weight G of batch C by calculating the difference between the weight values G ie and G iz , i) Generating an overpressure in the dosing scale (1) compared to the ambient pressure by feeding a process gas into the dosing scale (1).

Description

Field of TechnologyThe invention relates to a method for the discontinuous gravimetric metering of bulk material of a batch from a storage container into a collection, mixing or reaction container by means of a metering scale.The invention further relates to an apparatus for the discontinuous gravimetric metering of bulk material of a batch from a storage container into a collection, mixing or reaction container by means of a metering scale mounted on force transducers, having a weighing container and a metering device as well as an evaluation and control device, wherein the metering scale is connected to the storage container via a filling line for the purpose of feeding it and to a collection, mixing or reaction container for the purpose of metering the bulk material.Prior ArtIn industrial production and processing processes, bulk materials are frequently added to the process in gravimetrically metered amounts. For this purpose, the bulk material stored in a container is usually fed batchwise to a metering and weighing device and, after weighing, is fed to a collecting, mixing or reaction container. Precise compliance with the batch quantity is a prerequisite for obtaining high-quality products at the end of the process engineering processes.In order to minimize undesired emissions into the environment during weighing and metering of the bulk material and to protect the bulk material from property-modifying influences from the environment, the use of fully or largely closed systems is known. The aim is thus pursued of meeting containment requirements, that is to say both to prevent bulk material particles from escaping from the metering and weighing device and to prevent undesired substances from entering the metering and weighing device. For example, in the production of lithium ion batteries, different active materials such as lithium, manganese, nickel, cobalt, etc. are metered in and mixed, which are classified as harmful to health depending on the concentration. At the same time, these materials sometimes have highly hygroscopic properties, with the risk that their chemical properties change disadvantageously when moisture is absorbed from the environment and consequently the quality of the end product suffers. Closed systems remedy this, but have the disadvantage that they reinforce the influence of pressure fluctuations during the filling of the metering and weighing device on the measurement result, which in turn impairs the metering accuracy.EP 3 491 347 B1 discloses a dosing and weighing device for bulk materials which has a housing which is designed to be air-tight towards the outside, apart from the inlet and outlet opening. The housing contains a weighing surface with a weight sensor for detecting the charge weight and a pressure difference sensor that detects the pressure difference between areas located upstream of the weighing surface and areas located downstream. For weighing and metering, the weighing surface is moved alternately between a weighing position, in which the product flow between inlet opening and outlet opening is interrupted, and a metering position, which allows the product flow between inlet opening and outlet opening. If a pressure difference is detected by the differential pressure sensor, a control unit mathematically corrects the measured value detected by the weight sensor as a function of the existing differential pressure. This procedure represents a direct intervention in the weighing result, which can prove to be problematic with regard to the calibration capability of the device.Furthermore, EP 0 644 406 A1 discloses a device for gravimetric metering of bulk materials, having a weighing container supported on load cells, which weighing container is charged with bulk material from a storage container and from which a reaction or mixing vessel is filled via a discharge device. A pressure sensor in the interior of the weighing container monitors the pressure in the weighing container. The device is controlled by means of weighing electronics on the basis of the measurement signals obtained from the load cells and the pressure sensor. In this case, measurement results are only passed on to the weighing electronics when the pressure in the weighing container has reached a specific limit pressure value, in particular the atmospheric pressure. In this way, unusable measurement results are ignored because of pressure fluctuations, which leads to an increase in the dosing accuracy. Since the measurement results remain uncorrupted, the calibration capability of the device is fundamentally given.SUMMARY OF THE INVENTIONAgainst this background, the object of the invention is to specify a method and a device for gravimetric metering of bulk materials which is distinguished by a high metering accuracy and at the same time meets increased containment requirements.This object is achieved by a method for the discontinuous gravimetric metering of bulk material of a batch C from a storage container into a collection, mixing or reaction container by means of a metering scale, wherein a process gas is applied to the metering scale during the filling from the storage container and metering into the collection, mixing or reaction container, with the following method steps:a) filling the dosing scale from the storage container up to a first fill level,b) carrying out a pressure compensation between the interior of the metering scale and the environment of the metering scale,c) determining a first weight value G ie of the batch C by weighing at the first fill level in the dosing scale,d) generating an overpressure in the dosing scale relative to the ambient pressure by feeding a process gas into the dosing scale,e) metering the charge C from the metering scale into the collecting, mixing or reaction container,f) carrying out a pressure compensation between the interior of the metering scale and the environment of the metering scale,g) determining a second weight value G iz of the batch C by weighing, at a second fill level in the dosing scale after dosing has taken place,h) Determining the weight G of the charge C by forming the difference between the weight values G ie and G iz,i) Generating an overpressure in the dosing scale relative to the ambient pressure by feeding a process gas into the dosing scale.Furthermore, this object is achieved by a device of the type described at the beginning which is designed to carry out the abovementioned method steps a) to i).Advantageous further developments are evident from the dependent claims.The invention is based on the idea of holding the bulk material to be metered while being covered with a process gas during the majority of the metering process and of checking the metering scale before and after a weighing for determining relevant weight values and opening it free to the environment of the metering scale in a time-limited manner, that is to say carrying out a pressure compensation. In this way, it is possible to avoid pressure influences that are harmful to the measurement result and to meet the naturally opposing requirements for a high accuracy of the weighing result, on the one hand, and containment requirements for protecting the operating personnel and the bulk material, on the other hand. The measurement result remains unaffected, so that any required calibration capability of the method and the device according to the invention is not impaired.Advantageously, at the beginning of the method according to the invention, it is checked whether the quantity of bulk material currently present in the weighing container is sufficient for the metering to be carried out and, if appropriate, a filling is conducted into the paths in order to support a smooth and efficient sequence of the method. The bulk material quantity of a batch or partial batch to be made at least available in a metering section is determined by the difference of a weight-related maximum fill limit F max of the metering scale and a weight-related minimum fill limit F min.In order to keep the times during which pressure equalization with the environment takes place as short as possible and in order to ensure high metering accuracy, provision is made in an advantageous development of the invention to carry out the metering of the majority of the batch until a predetermined pre-shutdown value is reached, with feed of process gas under pressure, and to meter only the remaining residual quantity with a balanced pressure ratio between the interior of the metering scale and the balance environment. Since the pre-shutdown value is reached with process gas being obscured, it is to be ensured by suitable parameterization of the pre-shutdown value that the actual batch quantity has not yet been exceeded when the pre-shutdown value is reached.The pre-shut-off value may be, for example, a percentage value based on the weight of the total batch. Preference is given here to pre-shutdown values of 90% or higher, preferably 95%, in particular 98%, with which the stated advantages are clearly evident.The pre-switch-off value is preferably reached as a trigger for the changeover from the coarse flow to the fine flow during the metering of a charge or last partial charge. By appropriate actuation of the discharge element, the conveying strength for metering in the fine stream can be reduced, for example, to a value which is only a fraction of the conveying strength in the coarse stream. In this way, the remaining remaining quantity of the batch can be metered with high precision. Since the residual amount to be metered is generally in the range of a few percent by weight, the fine-flow metering does not lead to any appreciable prolongation of the metering.On the other hand, metering the majority of a charge in the coarse flow enables very efficient and thus economical metering.The invention is suitable for feeding a charge to be metered to the collecting, mixing or reaction container in only one metering operation, provided that the weighing container is dimensioned sufficiently large to receive the entire charge. This allows efficient and highly accurate metering.Nevertheless, the invention is capable of dispensing a batch C to a collection, mixing or reaction container in several recursively successive metering sections, a partial batch being metered in each metering section until the amount of the entire batch is reached. This has the advantage that even relatively small batches can be metered in with relatively little machine outlay.In an advantageous development of the invention, the partial weights of the partial batches are determined individually and added when metered into partial batches. In this case, the feeding of the process gas for the weighings necessary for determining the partial weights is interrupted in each case briefly for the duration of a weighing operation and a pressure compensation is carried out. In this way, a high weighing accuracy is provided even in the case of metering in partial batches.In the aforementioned context, it has proven to be particularly advantageous to dose the individual partial batches under pressure with process gas until the pre-shut-off value is reached in the coarse flow and to dose only the last partial batch in the fine flow. On the one hand, this ensures a high conveying strength during metering and, on the other hand, the harmful influence of pressure during the weighing of the last partial batch is limited to a minimum.Without being limited thereto, the invention is explained in more detail below with reference to an exemplary embodiment shown in the drawing, wherein further features and advantages of the invention become apparent.Brief Description of the DrawingsIt shows FIG. 1 shows a schematic illustration of a device according to the invention, and FIG. 2 shows a flow chart with the sequence of a method according to the invention.DESCRIPTION OF THE EMBODIMENTSFIG. 1 shows a device according to the invention in schematic representation. The device 1 serves for metering two substances of different characteristics within the scope of the production process of a product. The following explanations apply analogously to devices in which only one or more than two substances are to be metered in.The device according to the invention has a dosing scale 1 with a weighing container 2, a discharge element 3 and a material discharge 4 for each substance. The discharge element 3 can be formed, for example, by a metering screw, a vibrating chute or the like. The dosing balances 1 are mounted on weighing cells 5, by means of which a weight value can be determined at each of the dosing balances 1 and at each time. The dosing balances 1 each form a closed system which is designed to be substantially gas-tight towards the environment, but at least dust-tight. Advantageously or optionally, the device has a differential pressure sensor 6 for each dosing scale 1 in order to be able to detect the differential pressure between the interior of a dosing scale 1 and its environment in order to monitor the pressure compensation. The measured values of the load cells 5 and of the differential pressure sensors 6 are transmitted to an electronic evaluation and control unit 8 via the data lines 7 for further processing. The evaluation and control unit 8 outputs signals for controlling the device and the method according to the invention to the different functional units such as the discharge element 3, shut-off elements, valves, flaps and the like as a function of the measured values acquired.For filling the weighing containers 2, each dosing scale 1 is assigned a storage container 9, wherein different substances can be stored in different storage containers 9. The storage containers 9 are each connected to the dosing scale 1 via a filling line 10 and a shut-off element 11. For the weighing decoupling of the dosing scale 1 from the storage container 9 or the filling line 10, a compensator 12 is integrated into the filling line 10, whereby force shunting is avoided.To vent the dosing scale 1 during filling, the dosing scale 1 is followed, with the interposition of a further compensator 12, by a venting line 13 with shut-off element 14, which vents to the environment via a filter 15.In addition, a pressure line 16 leads into the interior of the dosing scale 1, which can be alternately supplied with a process gas 18 such as air, dry gas, inert gas and the like under overpressure with respect to the environment of the dosing scale 1 via a three-way valve 17 as shut-off element or can be activated to carry out a pressure equalization 19 between the interior of the dosing scale 1 and its environment. The pressure line 16 also has a filter 20 and a compensator 12 for compensating force shunting. Alternatively, it is possible to supply or carry out the process gas 18 or the pressure equalization 19 by means of separate lines and valves.The discharge element 3 of the metering device 1 terminates in a product discharge 4 with a quick-action flap 26 which is connected via pipelines to a collecting, mixing or reaction container 21 receiving the metered bulk material. For the weighing decoupling of the dosing scale 1 with the discharge element 3 and the material discharge 4 from the collection, mixing or reaction container 21, compensators 12 are inserted into the pipelines. To vent the collection, mixing or reaction container 21 during metering, a venting line 22 is provided, which leads via the filter 14 into the environment of the metering scale 1. The metered batches are discharged from the collecting, mixing or reaction vessel 21 via a discharge line 23 with shut-off element 24.The method according to the invention for the discontinuous gravimetric metering of bulk material of a batch C is the subject matter of the flow chart shown in FIG. 2. As can be seen from the flow chart, the batch C can be metered at once in a single batch C i with the weight G i( with i=1), provided that there is sufficient bulk material in the weighing container 2. Otherwise, the bulk material can be metered in a plurality of partial batches C i with the partial weights G i( where i=1 to n, where n=number of partial batches). The weight G i or the individual partial weights G i is determined in each case by determining a first weight value G ie and a second weight value G iz, the index i referring to the number of batch C i or batch C. The index e refers to the state before the batch is metered and the index z to the state after the batch C is metered i and. Partial Batch C i. The difference between the two weight values G ie and G iz, yields the weight G i of a batch C i and the individual weight G i of a partial batch C i.Before the start of the individual method steps, the device is in a rest state, characterized by a fogging with process gas 18. the shut-off members 11 and 14 and the quick-action closure flap 26 are closed for this purpose and a substantially constant overpressure with respect to the ambient pressure is generated by feeding process gas 18 into the dosing scale 1.From this rest state, according to the method shown in FIG. 2, it is first checked whether sufficient bulk material for metering an entire batch C i or. Partial charge C i is present. For this purpose, the current weight value G akt of the charge C becomes i or. The evaluation and control unit 8 determines partial charge C i and compares it with a weight value corresponding to the maximum fill limit F max. If the test reveals that the maximum filling limit F max in the weighing container 2 has not yet been reached, the evaluation and control unit 8 issues a refill request which brings about an opening of the shut-off members 11 and 14 and leads to a filling of the weighing container 2 from the storage container 9 with simultaneous venting via the line 13. These steps are carried out with process gas 18 being covered with mist, so that the bulk material is protected from external influences. Inaccuracies caused by the fogging during the detection of the fill limit F max can be accepted since these do not have any influence on the accuracy in the determination of the batch weight G i or the individual weights G i of the partial batches C i respectively.Once the maximum filling limit F max has been reached in the weighing container 2, the shut-off elements 11 and 14 are closed again and the supply of process gas 18 is interrupted by switching the three-way valve 17 and a pressure equalization 19 between the interior of the dosing scale 1 and the environment of the dosing scale 1 is carried out by opening the line 16. The achievement of uniform pressure conditions inside and outside the dosing scale 1 is detected by the differential pressure sensor 6 and registered by the evaluation and control unit 8 and represents the trigger for determining a first weight value G ie at the point in time before dosing. In embodiments according to the invention without a differential pressure sensor 6, it is also possible to wait for a certain time during which pressure compensation is expected to occur. The first weight value G ie detected by the load cells 5 is stored in the evaluation and control unit 8.Immediately thereafter, by switching the three-way valve 17 again, the pressure equalization 19 is interrupted and process gas 18 is fed under overpressure into the metering scale 1.Subsequently, the metered addition of charge C begins i or under overpressure. Partial batch C i into the collecting, mixing or reaction container 21. By activating the discharge element 3, the bulk material is conveyed in a coarse flow to the material discharge 4, where it reaches the collecting, mixing or reaction container 21 after opening the quick-action closure flap 26.During the metering, the current weight value G akt is continuously determined by the load cells 5 and the total weight G Ist of the already metered bulk material is furthermore set in relation to the desired weight G Soll of the total batch C to be metered. In this case, a check is made as to whether a predefined pre-shutdown value has already been reached. The pre-shut-off value may be a percentage value based on the weight G of the total batch C, for example 90% or 95% or 98%.As long as the pre-shut-off value has not yet been reached, it is further checked in the context of the continuous weight determination whether a weight-related minimum filling limit F min is reached in the weighing container 2. The operator parameterizes the minimum filling limit F min in such a way that, taking into account the pressure influence from the fogging, the bulk material quantity in the weighing container 2 below the minimum filling limit F min is sufficient to be able to dose the entire batch C to the end without further refilling of the weighing container 2.The check for reaching the pre-shut-off value or the minimum fill limit F min is again carried out with process gas 18 being covered, so that the bulk material is protected from external influences. Inaccuracies caused by the fogging in the detection of the pre-shutdown value or the fill limit F min can, on the other hand, be accepted since these do not influence the accuracy in the determination of the batch weight G i or. The individual weights G i of the partial batches C i have.If the continuous test leads to the result that the minimum filling limit F min is reached, a refill request is issued by the evaluation and control unit 8, which leads to a refill of the weighing container 2 from the storage container 9. Within the scope of the continuous determination of the current weight value G akt during the filling and its comparison with the maximum filling limit F max already described, a sufficient filling of the weighing container 2 for the next partial batch C i is ensured.The described process is repeated with a next partial batch C i until the continuous test during the metering reveals that the pre-shutdown value is still reached before the minimum fill limit F min in the weighing container 2 is reached. In this case, during each repetition, the partial weight G i of a partial batch C i is added to the partial weights G 1 to G i-1 of the partial batches C 1 to C i-1 determined hitherto. With a pre-shut-down value of 98%, for example, this would mean that 98% of the entire batch C has already been metered into the collection, mixing or reaction container 21 and the remaining quantity in the weighing container 2 is sufficient to be able to meter out the batch C to the end.Once the pre-shut-off value has been reached, the delivery element 3 switches over to metering the remaining charge into the fine flow. For this purpose, the supply of process gas 18 is interrupted again by switching the three-way valve 17, and a pressure equalization 19 between the interior of the dosing scale 1 and the environment of the dosing scale 1 is carried out by opening the line 16. The achievement of uniform pressure conditions inside and outside the dosing scale 1 is detected by the differential pressure sensor 6 and registered by the evaluation and control unit 25.Subsequently, in a fine flow, while maintaining the pressure balance, the batch is metered into the collection, mixing or reaction vessel 21 and a second weight value G iz is determined continuously by the load cells 5. In the evaluation and control unit 8, the weight G ist of the batch C metered into the collection, mixing or reaction container 21 is determined from the difference of the two weight values G ie and G iz and, in the case of a plurality of partial batches C i is additionally determined from the sum of the already metered individual weights G i and compared with the desired weight G soll of the batch C to be metered.When the desired weight G soll is reached, the metering in the fine flow is stopped and the pressure equalization 19 is interrupted by renewed switching of the three-way valve 17 and process gas 18 is fed under overpressure into the metering scale 1. The device is thus again in the idle state after the end of the dosing.

Claims

Method for the discontinuous gravimetric metering of bulk material of a batch C from a storage container (9) into a collection, mixing or reaction container (21) by means of a metering scale (1), wherein the metering scale (1) is pressurized with a process gas (18) during the filling from the storage container (9) and metering into the collection, mixing or reaction container (21), comprising the following method steps: a) filling the metering scale (1) from the storage container (9) up to a first fill level, b) carrying out a pressure equalization between the interior of the metering scale (1) and the environment of the metering scale (1), c) determining a first weight value G ie of the batch C by weighing at the first fill level in the metering scale (1), d) generating an overpressure in the dosing scale (1) with respect to the ambient pressure by feeding a process gas (18) into the dosing scale (1), e) dosing the batch C from the dosing scale (1) into the collection, mixing or reaction container (21). f) carrying out a pressure equalization between the interior of the dosing scale (1) and the environment of the dosing scale (1), g) determining a second weight value G iz of the batch C by weighing, at a second fill level in the dosing scale (1) after dosing has taken place, h) determining the weight G of the batch C by forming the difference between the weight values G ie and G iz i) generating an overpressure in the dosing scale ( 1) with respect to the ambient pressure by feeding a process gas ( 18) into the dosing scale ( 1).Method according to Claim 1, characterized in that before carrying out the method step a) it is checked whether the actual fill level in the metering scale (1) has reached a maximum fill limit F max and filling from the storage container (9) up to at least the maximum fill limit F max, if the maximum fill limit F max is not reached.Method according to Claim 1 or 2, characterized in that, when carrying out process step e), the metering is effected - in a first substep up to a pre-shutdown value under overpressure by feeding in process gas (18), and - in a second substep, the remaining amount is metered after carrying out a pressure compensation with continuous weighing until the weight G i of the charge C is reached.Method according to claim 3, characterized in that the pre-shut-off value is at least 90 wt.% of the weight G of the total batch C, preferably at least 95 wt.%, most preferably at least 98 wt.%.Method according to one of Claims 1 to 4, characterized in that the entire batch C is metered in in a plurality of partial batches C 1 to C i having the individual weights G 1 to G i.Method according to Claim 5, characterized in that, in order to determine the individual weight G i of a part batch C i by weighing - a first weight value G ie of the part batch C i in the dosing scale (1) at the first fill level is determined before the dosing, and - a second weight value G iz of the part batch C i in the dosing scale (1) at the second fill level is determined after the dosing, and - the individual weight G i of the part batch C i is determined from the difference between the weight values G ie and G iz, wherein - prior to the determination of the first weight value G ie and second weight value G iz a pressure equalization with the environment of the dosing scale (1) is carried out in each case, and - after the determination of the first weight value G ie and second weight value G iz an overpressure is generated in each case by feeding a process gas (18) into the dosing scale (1).Method according to Claim 5 or 6, characterized in that, in order to determine the total weight G of the batch C, the individual weight G i of the partial batch C i is added to the sum of all the previously determined individual weights G 1 to G i-1.Method according to one of Claims 5 to 7, characterized in that before metering a part batch C i it is checked whether the sum of all individual weights G 1 to G i-1 of the part batches C 1 to C i-1 and the first weight value G ie of the part batch C i is less than the total weight G of the total batch C to be metered or greater.Method according to Claim 8, characterized in that the metering of the entire batch C i into the collecting, mixing or reaction container (21) is effected under superatmospheric pressure by feeding in process gas (18) if the sum of all individual weights G 1 to G i-1 of the batches C 1 to C i-1 and the first weight value G ie of the batch C i is less than the total weight G of the total batch C to be metered.Method according to claim 8, characterised in that the dosing of all or part of the batch C i is carried out while simultaneously weighing, while pressure compensation is carried out between the interior of the batch scale (1) and the environment of the batch scale (1) until the total weight G of the total batch C is reached, if the sum of all individual weights G 1 to G i-1 of the batches C 1 to C i-1 and the first weight value G ie of the batch C i is greater than the total weight G of the total batch C to be dosed.Device for the discontinuous gravimetric metering of bulk material, having - a metering scale (1) mounted on load cells (5) and comprising a weighing container (2) and a metering device (3), - a storage container (9) which is connected to the metering scale (1) via a filling line (10) for charging the metering scale (1), - a collecting, mixing or reaction container (21) for receiving the metered bulk material, and - an evaluation and control device (8), wherein - the metering scale (1) has a connection (16) for feeding process gas (18) under pressure and a connection (16) for carrying out a pressure compensation between the interior of the metering scale (1) and the environment of the metering scale (1), characterized in that the evaluation and control device (8) is designed to feed in process gas (18) under pressure and to carry out a pressure compensation between the interior of the metering scale (1) and the environment of the metering scale (1), A method according to any one of claims 1 to 10.

Images