WO2005093159A1 - Method and tower for batching the aggregates in a plant for producing bituminous concretes and related production plant - Google Patents

Abstract

A method of batching the aggregates to be introduced into a mixer in a plant for producing bituminous concretes comprises the steps of, for each aggregate to be batched, filling a weighing hopper with the aggregate to be batched and operating discharge of the hopper into the mixer in a batching cycle, through a computerized control system, until the hopper weight that is detected by the computerized control system has decreased by a value that has been previously established to correspond to the aggregate amount to be introduced into the mixer. A mixing tower (12) according to the method comprises a mixer (19) and, for each aggregate, a hopper (18) provided with weighing means and fed with the aggregate to be batched. The computerized control system in a batching cycle operates discharge of the hopper (18) into the mixer (19) until the hopper weight that is detected by the computerized control system has decreased by a value that has been previously established to correspond to the aggregate amount to be introduced into the mixer. Also described is a plant (10) equipped with such a mixing tower.

Description

"METHOD AND TOWER FOR BATCHING THE AGGREGATES IN A PLANT FOR PRODUCING BITUMINOUS CONCRETES AND RELATED PRODUCTION PLANT"

The present invention relates to a method and a tower for batching the aggregates for producing bituminous concretes, and to a plant for asphalt production.

It is known that asphalts are substantially formed of several different components, such as gravel, sand, filler, binder (generally bitumen) and other substances (cork, cellulose, rubber latex, resins, etc.) adapted to give the base product its main features or to enhance said features. For each particular application (road paving, airport strips, embankment coating, ground insulation, sports grounds, etc.) there is a requirement for asphalts having physical features that are quite different from each other but that, once they have been formulated and established by a designer, must be compulsorily complied with during the production step. For instance, there is a typical feature distinguishing and joining the most different types of asphalt; this feature is the particle-size curve, i.e. the function defining the sizes and mutual proportioning of the different lithic components. The non-respect of this parameter nullifies the functional character of the product and/or greatly decreases the product duration in time. The above brings about the need for the batching plants of the different components to be sufficiently precise, with very narrow tolerances .

In modern discontinuous-cycle plants the production process is completely automatic, except for a very first step. Usually, the aggregates follow a route of their own starting with selection of the suitable particle sizes for the formulation to be produced, among the crushed-stone heaps present in the yard, followed by the drying and heating steps, a hot regrading (if necessary), storage in a waiting silo or silo, weighing of the different fractions and introduction into the mixer. At the same time, through other routes and with suitable means, batching of all the other components of the formulation takes place and then the subsequent introduction into the mixer according to the times and sequences provided by the automatic operation of the plant.

One of the most critical components to be batched is represented by the aggregates, mainly due to the number and variety of aggregate elements to be batched.

As above mentioned, to this aim the aggregates follow a particular route of their own. Usually, a mechanical means, generally a mechanical shovel is first used, to pick up the aggregate materials from the storage heaps and separately charge the different particle sizes into the respective hoppers of the feeders, also referred to as pre-batchers . When charging of the feeders has been completed, the true production step can begin; the operator starts the automatic control system of the plant recalling the type or number of the formulation to be produced from the central memory of the management computer, inputs the different variable parameters and starts production.

Upon command of the control system the aggregates move from the feeders to the drying plant and are then sent on top of the so-called mixing tower to be batched and introduced into the mixer together with the other formulation elements .

In the mixing tower some vertically-set areas, from top to bottom, can be identified for discharge of each area into the subsequent one by gravity, i.e. the hot-aggregate selecting area, the area of the waiting hoppers, the weighing area, the mixing area and, finally, the area for discharge of the finished product. The area of the waiting hoppers consists of a sheet metal body, in the form of a parallelepiped, internally divided by partitions so as to form as many funnel-shaped sectors as there are aggregate fractions to be distinguished. In a first type of plant of the known art, the automatic system simultaneously operates all the pre-batchers necessary to compose the formulation, so that the aggregates are mixed and on coming out of the drier they must be regraded. To this aim, the aggregates are conveyed to a vibrating screen disposed in the selecting area on top of the mixing tower.

In a second type of plants on the contrary, each feeder is provided to contain an already well defined particle-size class that can be used for several formulations. Drying of the aggregates and conveyance of same to the mixing tower takes place individually for each particle size necessary for a particular formulation and in the selecting area on top of the tower there is only a distributor sequentially starting the different particle sizes in the preestablished waiting sector to avoid earlier mixing of said particle sizes. Therefore production of the concrete can only start when the waiting sectors have been all filled. Both types of plants have advantages and disadvantages. For instance, in known plants of the first type, to keep the screen bulkiness and complexity within reasonable limits, the standard number of selections is usually four, while the maximum number is generally no more than seven and only for the most sophisticated plants . The waiting sectors must necessarily be provided with overflow channels due to the fact that, for different reasons, there is no level indicator of sufficient precision and also due to the fact that it is unthinkable to provide feeding of the re-grading screen in such a manner that distribution in the underlying silos will be always equal to the picked-up amounts. Theoretically, in the second type there is no limit to the number of selections and types of the aggregates that can be used, even if use of a single fractionated container makes it difficult to enlarge the plant and prevents a true economy of scale in producing the components . There are also difficulties in measuring filling of the different silos during the sequential introduction of the aggregates. In modern known plants batching of the different components takes place by weight. To this aim, close to the mixer there are hoppers equipped with weighing instruments (generally loading cells). Feeding of these hoppers occurs during the mixing time, so that at the end of one mixing cycle another cycle can immediately begin.

In known plants, for weighing the aggregates a hopper is used that is positioned on the vertical of the mixer and under the discharging mouths of the waiting silos. During the mixing period (about 30 seconds) the waiting silo mouths open and close, one at a time and in succession, so that each of them introduces the amount of aggregates provided by the formulation into the underlying hopper.

The system automatism carries out control of the weighing operations taking place in succession and closure of the shutter of the concerned silo when the weight required by the formulation has been reached. The sum of the individual weighing operations gives the total amount of the aggregates that will be introduced into the mixer. When, at the end of the mixing time, the shutter of the mixer opens and closes after letting the concrete fall down, the shutter of the hopper weighing the aggregates in turn opens and discharges its contents into the mixer to begin a new cycle.

During the mixing step also the other elements composing the formulation will be discharged within the times and delays provided by the automatism of the mixer plant. This known technique for weighing the aggregates involves several problems affecting operation of the whole plant and the quality of the final product.

Different trouble elements in weighing the exact desired amount of aggregates of each type may even give rise to important deviations from the preestablished formulation.

For example, when the weighing hopper detects that the preestablished aggregate weight has been reached, the control system closes the discharge shutter of this aggregate from the corresponding section of the waiting silo. However, between the shutter and underlying weighing hopper a space is necessarily present that, upon closure of the upper shutter, still contains an undefined amount of falling aggregates that have not yet been weighed. This involves an unavoidable batching in excess.

Use of a lower weighing hopper receiving the discharged material from all waiting silos also involves the impossibility of the waiting silos being standardized, because since they all must discharge into said weighing hopper, it is preferable for them to be made as the "segments" of the same silo, also in view of reducing the paths between the shutters and the weighing hopper and limiting the above mentioned excess weighing error. Therefore, if waiting sections are wished to be added so as to be able to treat a greater number of aggregates, it will be necessary to replace the whole silo by one of a different shape and greater size if the available spaces for each aggregate are not to be reduced. It is a general aim of the present invention to obviate the above mentioned drawbacks by providing a method, a tower and a plant enabling high accuracy in making a formulation, while being of easy and cheap accomplishment. It is a further aim to solve the problem concerning the non- standardization of the waiting silos. In addition, the method, tower and plant in accordance with the invention not only enable the weighing problems typical of known aggregate-weighing systems to be solved, but they also reach the further purpose of solving the problems present in known plants due to the impossibility of precise level indicators being provided and to the requirement of outflow channels discharging the hot aggregates to the ground, which will bring about an important energy waste (note that more than 90% of the thermal and electric energy consumed by the plant is used for heating/drying the aggregates) .

A further problem in known plants for producing bituminous concretes resides in that, in order to avoid an excessive and unacceptable extension in height of the mixing tower, the space (and therefore the available volume) between the weighing hopper sized for receiving the discharged materials from all waiting silos and the underlying mixer, is limited in height. In known plants the direct admission to the mixer of wet milled asphalt to be recycled that in this manner comes into contact with the overheated aggregates already present in the mixer, causes a sudden and uncontrollable formation of vapor. This phenomenon gives rise to a vapor "explosion" in the mixer and, as a result, to an anomalous escape of dusty gases from the hopper case and entrainment of even coarse powders towards the surrounding metal walls. Due to the nature of the materials forming the powders, clogging takes place in longer or shorter periods of time. Also condensing and dripping phenomena on the walls of the waiting silos occur. All that is due to the limited spaces that are available for gas expansion, to an inappropriate insulation degree and to the difficulties in removing the amount of vapors formed in a short period of time. Another aim of the present invention is to solve these problems as well, enabling creation of a suitable free volume for gas expansion over the mixer, without at the same time greatly increasing the tower height or reducing the useful sizes of the different tower components. In view of the above aims, in accordance with the invention, a method has been devised of batching the aggregates to be introduced into a mixer in a plant for producing bituminous concretes, which comprises the steps of, for each aggregate to be batched, filling a weighing hopper with the aggregate to be batched and operating discharge of the hopper into the mixer in a batching cycle, through a computerized control system, until the hopper weight that is detected by the computerized control system has decreased by a value that has been previously established to correspond to the aggregate amount to be introduced into the mixer.

Still in accordance with the invention a tower for mixing the ingredients of a production formulation for a plant producing bituminous concretes has been conceived, which comprises a mixer and means for batching the aggregates to be introduced into the mixer in accordance with the formulation, characterized in that the batching means for each aggregate of the formulation comprises a hopper provided with weighing means and fed with the aggregate to be batched, and a computerized control system that in a batching cycle operates discharge of the hopper into the mixer until the hopper weight that is detected by the computerized control system through the weighing means has decreased by a value that has been previously established to correspond to the amount of that aggregate to be introduced into the mixer.

Also herein claimed in accordance with the invention is a plant equipped with such a mixing tower. For better explaining the innovative principles of the present invention and the advantages it offers over the known art, a possible embodiment applying said principles will be described hereinafter by way of example, with the aid of the accompanying drawings . In the drawings : - Fig. 1 is a diagrammatic view of a plant for producing bituminous concretes, made in accordance with the invention;

- Fig. 2 shows a partial view, but in more detail, of the mixing tower of the plant seen in Fig. 1. With reference to the drawings, shown in Fig. 1 is a diagram of a plant, generally denoted at 10, for producing bituminous concretes. Plant 10 comprises an area 11 for preparation of the aggregates and an area or tower 12 for measuring or batching and mixing the formulation components . The area 11 for example comprises feeders 13 receiving the aggregates of different particle sizes (picked up by a bucket from storage heaps 22) and sending them to a known drier 14. The hot aggregates coming out of the drier are lifted by a known elevator 15 (a bucket elevator, for example) for reaching the top of the mixing tower in the area 12.

The different aggregates are sorted out (by means of a sorting device 16 known by itself) into a plurality of storage or waiting silos 17. As a person skilled in the art knows, the sorting device can be any device of known type, depending on the modes of supplying the aggregates from the area 11. For example, if the aggregates have already been selected in the area 11 and individually sent to the tower to the area 12, the device 16 will be a mere distributor sequentially conveying the different aggregates into the corresponding silos 17. If on the contrary a regrading is necessary on arrival at the tower, the device 16 will be a suitable separating screen. In both cases, silos 17 will each receive one type of aggregate being one component of the formulations to be prepared. The number of silos 16 will be at least the same as the number of the aggregate components that are wished to be kept separate. Each silo 17 has a discharge opening provided with a shutter at the lower part thereof for carrying out discharging, upon command, into an underlying weighing hopper 18 of its own. All hoppers in turn have discharge shutters operated for pouring the aggregates into the underlying known mixer 19. In other words, each particle-size fraction of aggregates is provided with two waiting silos 17, 18 vertically superposed on each other.

The other components of the formulation reach the mixer through batching systems 20 (generally weight measuring systems), that are known by themselves and are not herein further described as they can be easily envisaged by a person skilled in the art. These batching systems will be designed to add components such as bitumen, filler and several additives provided by the formulation, into the mixer . The mixer has an outlet 21 for ensilage, upon command, of the finished product that will be sent to the use yard. Except for the operation of charging the aggregates from the storage heaps 22 (which operation is typically carried out manually) operation of the plant is managed by a known and proper electronic control system 23 with a suitably programmed command computer 24.

Shown in greater detail in Fig. 2 is that portion of the mixing tower 12 that is designed to batch the aggregates. For the sake of simplicity one silo alone 17 is fully shown with the related weighing hopper 18 associated therewith. In any case, all pairs of silos 17 with the underlying hopper 18 are identical although it is not excluded that they may have features different from each other, and the description of only one of them is sufficient for full understanding of the invention.

All hoppers 18 of the different silos 17 discharge into a conveying funnel 25 carrying the aggregates into the underlying mixer 19. The distributor device 16 fills each silo 17 with the assigned aggregate.

Advantageously, each silo 17 is suspended on a weighing system 26 (a loading cell, for example) that, after subtracting the tare, gives the net weight of the aggregate contained in the silo and therefore the level reached by the aggregate contained therein.

Each silo 17 is provided with a mechanism of its own 27 operated for discharge of the silo contents to the associated hopper 18 of a lower capacity. The weighing hopper 18 under the silo is in turn provided with a weighing system of its own 28 (a loading cell as well, for example) and with a mechanism 29 operated for discharge of the contents to funnel 25. Both weighing systems and discharge mechanisms of each pair are connected to the control system 23, 24.

In use, each silo 17 filled by the distributor device 16, is operated by the control system to feed the respective lower hopper. The weighing system 28 allows the control system to stop feeding of silo 17 when the lower hopper 18 reaches a weight corresponding to a preestablished filling condition (maximum load) . In this step it is not important whether after closure of shutter 27 an excess weight is obtained due to the fact that some material was still falling when closure of shutter 27 was operated.

When the hopper (or hoppers) 18 is/are full, the control system can open the discharge shutter (or shutters) 29 and measure the amount of aggregates discharged into the mixer as the weight difference from the starting weight. In this way, when the aggregate weight provided by the particular execution formulation is reached, the control system will close shutter (or shutters) 29 and the tolerance from the provided weight practically will only depend on the driving speed for closing shutter. Thus, the tolerance can be reduced to a minimum.

In any case, it is also possible to precisely establish the minimum amount possibly fallen in excess, through measurement of the weight difference of the hopper between before opening of shutter 29 and after the new closure of shutter 29. Substantially, in accordance with the method of the present invention, the amount of aggregates of each individual fraction to be introduced into the mixer is established by subtraction from the silo or lower hopper 18. The upper silo 17 can feed the lower silo subsequently to weighing, to restore the provided level, cycle by cycle, if necessary, or the provided level can be restored on reaching a weight fixed as the minimum one for the requirements of the current production. Although the upper hopper can advantageously be much bigger than the corresponding lower silo, after a certain number of batches the upper silo must be filled up with the material so that delivery may go on. To establish when said filling up is to be carried out and the amount of same, the control system advantageously uses the weighing system 26 with which each silo can be provided. The system using the weight of silo 17 or a signal issuing therefrom and proportional, is surely more precise than the system using simple mechanical minimum- and maximum-indicators and is surely less expensive and less delicate than detecting systems of the "radar" type proposed in some plants of the known art to detect the surface level of the aggregates contained in the silo. After suitable calibration, the weighing system 26 of each silo 17 enables the computerized control system 23, 24 to stop introduction of material into the silo when a maximum filling level is reached and to maintain the amount of aggregate in the silo within a preestablished minimum and maximum interval. Unlike a banal maximum- and minimum- detector, a detection by weight enables an indication of the silo contents instant by instant, which will allow systems of proportional feedback control to be implemented for treatment of the aggregates disposed upstream and for carrying out filling of the silo. For instance, in the case of a plant with a screen for regrading, the control device can increase or reduce and even stop, if necessary, the speed of extraction of the pre-batcher/pre-batchers cooperating in supplying the silo in question. If the aggregate supply takes place sequentially, on reaching the maximum level provided in the silo, the control system can stop the corresponding feeder and start it again on reaching the minimum level so that the desired level can be restored. At all events, in most conditions an excessive filling of the waiting silos can be avoided, as well as the necessity to use the overflow channels for discharging the already dried aggregates to the ground and the resulting energy consumption. At this point it is apparent that the intended purposes have been reached. The batching system of the invention is precise and does not suffer the unreliabilities typical of the systems of the known art, such as those due to the material discharged from the silo but still moving towards the weighing hopper.

It is also apparent that, by virtue of the invention, the plant can be provided with as many modular units (formed of pairs consisting of waiting silo and weighing hopper) as there are selections of aggregate. All the silo-hopper pairs can be identical with each other and this offers the construction advantage that a certain number of standard parts can be stored and the plant can be then manufactured exactly as desired by the customer in a short period of time and with an undeniable economy of scale.

In addition, the large free volume over the mixer (obtained through elimination of the hopper for weighing all the aggregates as used in the known art) avoids vapor pressures being formed, which pressures cause clogging and condensate, as present in the known art. The free space can be also advantageously used for known suction means to evacuate the residual gases.

The possibility of avoiding or limiting use of overflow channels allows a great reduction in energy consumption in the plant.

Another advantage as compared with the known art is the possibility of carrying out introduction of all the aggregates into the mixer either simultaneously or at different times and/or in sequence, by merely intervening in a suitable manner on the control system. This allows working improvements and/or optimization both in terms of quality of the product and in terms of production times, since it is possible to select the best sequence for introducing the different aggregates into the mixer. For instance, an introduction sequence promoting mixing can be selected.

Obviously, the above description of an embodiment applying the innovative principles of the present invention is given by way of example only and therefore must not be considered as a limitation of the scope of the patent rights herein claimed.

For instance, shapes and sizes of the different elements can change depending on specific requirements, as well as the stages for treating the aggregates and the other different components in the formulation.

The towers 12 and therefore the different elements 16, 17, 18, 19 can be enclosed in an environment separated from the outside by means of panels that can be insulated and in any case adapted to create a big environment where gas expansion can be allowed. This environment can be constantly maintained under a vacuum by known systems, such as an exhaust fan, and the gases can be conveyed to filtering systems.

Claims

1. A method of batching the aggregates to be introduced into a mixer in a plant for producing bituminous concretes, comprising the steps of, for each aggregate to be batched, filling a weighing hopper with the aggregate to be batched and operating discharge of the hopper into the mixer, in a batching cycle, through a computerized control system, until the hopper weight that is detected by the computerized control system has decreased by a value that has been previously established to correspond to the aggregate amount to be introduced into the mixer.

2. A method as claimed in claim 1, in which the aggregate filling the weighing hopper is discharged thereinto from a waiting silo that, through the computerized control system, is operated to discharge the aggregate into the hopper after a batching cycle and until a preestablished weight of the weighing hopper has been reached, before a new batching cycle.

3. A method as claimed in claim 2, in which the waiting silo is in turn submitted to weighing to operate, through the computerized control system, discharge of new aggregate into said waiting silo upon reaching a weight of the silo corresponding to an amount that has been previously established to be the minimum one.

4. A method as claimed in claim 2, in which discharge of new aggregate into said waiting silo is stopped by the computerized control system upon reaching a weight of the silo corresponding to an amount that has been previously established to be the maximum one.

5. A tower for mixing the ingredients of a production formulation for a plant producing bituminous concretes, comprising a mixer and means for batching the aggregates to be introduced into the mixer in accordance with the formulation, characterized in that the batching means, for each aggregate of the formulation, comprises a hopper provided with weighing means and fed with the aggregate to be batched and a computerized control system that in a batching cycle operates discharge of the hopper into the mixer until the hopper weight that is detected by the computerized control system through the weighing means has decreased by a value that has been previously established to correspond to the aggregate amount to be introduced into the mixer.

6. A tower as claimed in claim 5, characterized in that, for each weighing hopper, it comprises a waiting silo that, through the computerized control system, is operated to discharge the aggregate into the hopper after a batching cycle and until a preestablished weight detected by the weighing means of the hopper has been reached, before a new batching cycle.

7. A tower as claimed in claim 6, characterized in that it comprises further weighing means of the waiting silo to operate, through the computerized control system, means for discharging new aggregate into said waiting silo when a weight of the silo detected by said further weighing means has been reached, which weight corresponds to an amount that has been previously established to be the minimum one.

8. A tower as claimed in claim 7, characterized in that the means for discharging new aggregate into said waiting silo is shut down by the control system when a weight of the silo detected by said further weighing means has been reached, which weight corresponds to an amount that has been previously established to be the maximum one.

9. A tower as claimed in claim 8, characterized in that the means for discharging new aggregate into said waiting silo is operated by the control system based on the silo weight as detected by said further weighing means, to modulate the aggregate amount in the silo between said amounts that have been previously established to be the maximum and minimum ones .

10. A tower as claimed in claim 5, characterized in that it comprises a plurality of waiting silos disposed in side by side relationship, each silo being designed to receive a preestablished particle-size class of aggregate, and each silo being disposed on top of a respective weighing hopper, all hoppers discharging into the mixer upon command.

11. A tower as claimed in claim 10, characterized in that it comprises distributor means conveying said plurality of aggregates to the silos, based on the preestablished particle-size class and arriving thereat from drying means.

12. A tower as claimed in claim 11, characterized in that the distributor means comprises a selecting screen for the preestablished particle-size classes to be sent to the silos.

13. A tower as claimed in claim 11, characterized in that the distributor means comprises a distributor sequentially selecting a corresponding silo of the plurality to send thereto the already pregraded aggregates being supplied.

14. A tower as claimed in claim 11, characterized in that the pairs formed of a silo and corresponding weighing hopper are all identical with each other to form a modular system for batching the aggregates.

15. A plant for producing bituminous concretes, comprising a mixing tower obtained as claimed in anyone of the preceding claims.

16. A plant as claimed in claim 15, characterized in that it comprises means for drying the aggregates, which means is supplied from aggregate feeders and sends the hot aggregates to the mixing tower.

17. A plant as claimed in claim 16, characterized in that the tower comprises a distributor selecting a silo from a plurality of waiting silos of the tower to send to said selected silo, the aggregates being supplied from the drying means and already pregraded for that silo by grading means upstream of the drying means .