Abstract:
A method of producing, in a continuous plant, a bituminous conglomerate containing aggregates and at least bitumen comprises the steps of drying the aggregates and mixing them with a measured amount of at least the incoming bitumen to form the bituminous conglomerate; weighing the obtained bituminous conglomerate; calculating the amount of dry aggregates present in the conglomerate as the difference between the detected weight of the obtained bituminous conglomerate and the measured amount of at least bitumen; and adjusting in real time the amount of at least the incoming bitumen depending on the calculated amount of dry aggregates, so as to keep this bitumen amount within preestablished proportions with respect to the calculated amount of dry aggregates.

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to an innovatory method and a plant applying such a method, for continuously producing a bituminous conglomerate in which exact proportions between the mixture components are adopted. 
     Known in the art of producing bituminous conglomerates is the importance of respecting precise “recipes” in the conglomerate composition. Tolerances in the recipes are usually limited; for instance, the bitumen percentage for each specific recipe is established with tolerances in the order of half a point percent and said tolerances for particular recipes can even be included between 0.05% and 0.08%. 
     By way of example, a typical recipe can be: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Value % 
                 Tolerance % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 (Dry) aggregate No. 1 
                 45 
                 +/−5 
               
               
                   
                 (Dry) aggregate No. 2 
                 15 
                 +/−5 
               
               
                   
                 (Dry) aggregate No. 3 
                 15 
                 +/−5 
               
               
                   
                 (Dry) aggregate No. 4 
                 15 
                 +/−5 
               
               
                   
                 Filler 
                 5 
                 +/−2 
               
               
                   
                 Bitumen 
                 5 
                   +/−0.3 
               
               
                   
                   
               
             
          
         
       
     
     Unfortunately, aggregates are usually wet, having a moisture content generally included between 3% and 7%. For instance, typical humidity characteristics of the aggregates on the stocks can be: 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Humidity value % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aggregate No. 1 (sand) 
                 7 
               
               
                   
                 Aggregate No. 2 (fine grit) 
                 4 
               
               
                   
                 Aggregate No. 3 (medium grit) 
                 3.5 
               
               
                   
                 Aggregate No. 4 (coarse grit) 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     The moisture content in the aggregates as well as the filler content can vary depending both on the particle sizes and, more simply, on the they position take in the storage heap. In addition, it may easily happen that the moisture degree, in the same yard and for the same material, can vary even of some points percent, during the production of one and the same product. 
     The different and variable moisture percentages of the different components give rise, as a consequence, to calculation mistakes as regards the bitumen amount to be introduced into the mixture. 
     In order to solve this problem, use of an aggregate dryer as a unit separated from the mixer has been proposed, so as to eliminate humidity before picking up the established aggregate amounts as stated by the particular recipe. 
     The above introduces a complication in the plant and often there is a preference for plants in which the whole production process takes place within the drying oven to which both bitumen and damp aggregates are conveyed to be mixed together. In fact, these plants (named “Drum Dryer”, “Drum Mixer”, “CEM” plants, etc.) have the advantage that their purchase and use is not expensive. On the other hand, aggregates are necessarily weighed when wet (usually by a weighing conveyor belt) and, even if an attempt is made to carry out a compensation of the amounts by deducting the moisture percentage detected on a sample in a laboratory, the bitumen amount to be admitted to the mixture is only theoretically calculated and this leads to a non-eliminable and often high variability in the recipe as really applied. In addition to the above, the natural filler amount present in the aggregates and resulting from the crushing process can very widely vary in percentages, ranging from 2-3% to 20-25%, from yard to yard but also in the same yard (for instance, when the aggregates do not come from the same source and/or are produced with ores of different qualities and/or qualities varying in time). This variability depends on the ore nature, since the ore can be more or less hard and friable, and also on the type of crusher used and, above all, on the presence or not of the step of washing the aggregates with water after crushing. Carrying out washing of the aggregates or not, so as to practically eliminate all excess filler, depends on technical or economical choices (if lack of water exists or the water supply is difficult, for example). 
     Since the filler amount required in the recipe is precise, when the filler amount present in the aggregates is too much, part of it will have to be necessarily discarded. Usually the filler carried along by the gases coming out of the oven reaches an appropriate filter. This therefore takes place downstream of weighing, which operation is carried out on the conveyor belt. At all events, the recovered filler is introduced into the mixing region by means of metering screws having a controlled number of revolutions to carry out a volumetric dosage. As a result, if there is an excess amount of filler, which happens very often, no control exists as regards the discarded amount and the statistical determination of this filler amount relies on laboratory tests. 
     In conclusion, the evaluation errors on the exact amount of bitumen to be introduced into the oven depend on the above estimate that, although precise, introduces an uncertainty degree into the recipe. 
     It is a general object of the present invention to obviate the above mentioned drawbacks by providing a production method and plant enabling a bituminous conglomerate to be continuously produced, with high precision and constancy in respecting the “recipe”. 
     In view of this aim, in accordance with the invention a method has been devised for production, in a continuous plant, of a bituminous conglomerate containing aggregates and at least bitumen, which method comprises the steps of drying the aggregates and mixing them with a measured amount of incoming bitumen to form the bituminous conglomerate; weighing the obtained bituminous conglomerate; calculating the amount of dry aggregates present in the conglomerate as the difference between the detected weight of the obtained bituminous conglomerate and said measured amount of bitumen; adjusting in real time at least the amount of incoming bitumen depending on the calculated amount of dry aggregates, so as to keep this bitumen amount within preestablished proportions with respect to the calculated amount of dry aggregates. 
     Still in accordance with the present invention, a continuous plant for producing bituminous conglomerate has been also devised which comprises a drier-mixer unit to which feed means sends aggregates and at least bitumen so as to form the bituminous conglomerate, characterised in that it comprises means for measuring the amounts of bitumen sent to the drier-mixer unit, means for weighing the bituminous conglomerate coming out of the unit, and a control system calculating the amount of dry aggregates present in the outgoing conglomerate as the difference between the weight of the bituminous conglomerate detected by the weighing means and the amounts of bitumen detected by the means for measuring said amounts, the control system operating the bitumen feed means so as to adjust in real time at least the amount of bitumen entering the unit, depending on the calculated amount of dry aggregates, so as to keep this bitumen amount within preestablished proportions with respect to the calculated amount of dry aggregates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The only FIGURE is a diagramatic representation of a plant in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For better explaining the innovatory principles of the present invention and the advantages it offers over the known art, a possible embodiment of the invention applying these principles will be described hereinafter, by way of example, with the aid of the accompanying drawing in which the only FIGURE is a diagrammatic representation of a plant in accordance with the invention. 
     With reference to the FIGURE, a plant, generally identified by  10 , comprises a bitumen deposit generally denoted by  11 , a filler deposit  12  and a supply of aggregates or inert materials, sent from known feeders  13  that can be easily envisaged by a person skilled in the art. 
     Bitumen, filler and aggregates are mixed in a unit  14  consisting of a drier and a mixer from which the bituminous conglomerate exits through a controlled discharge outlet  24 . Known collection and transportation means  15  evacuates the finished product. The filler contained in the aggregates is carried along by the fumes to a recovery plant  31  evacuating the combustion gases and water vapour and recovering the filler by a known filter sending it to container  12  (possibly provided with means for evacuating the excess filler), so that it will be utilised again in the production process. 
     Bitumen is conveyed to the unit by controlled-feed means  29 , the filler being conveyed to the unit by controlled-feed means  30 . 
     Downstream of the controlled discharge outlet  24  of the product from unit  14  there is a hopper  25  having a controlled discharge opening  26  that upon command releases the product into the transportation means  15 . This hopper  25  is provided with weighing means  27 , load cells for example. 
     The described plant is provided with a control system  28  (a suitably programmed computer, for example). 
     The drier-mixer unit can be embodied by a known rotary oven or kiln as shown in the FIGURE, or by a known mixer and a known oven separated from each other, the drying oven discharging into the mixer. In both cases it is advantageous that a mixing region and a drying region should be defined and that to the mixing region at least bitumen and the possible filler should be fed, the aggregates being instead fed to the drying region. 
     During operation of the plant, first a speed is set for aggregate extraction (wet aggregates having an unknown and only presumed real humidity) from the respective feeders  13 , depending on the theoretically required individual percentage. In addition setting of the theoretical bitumen percentage requested in the recipe is carried out. The same is done for the filler (if any) and possible other components (not shown or described, as they can be easily envisaged by a person skilled in the art). 
     The aggregates, through normal transportation means, reach the dryer and travel along it until they reach the discharge region  24 . Bitumen and filler are, for the moment, fed to the foreseen feed regions of the unit in an amount proportional to the theoretical percentage provided by the particular recipe to be applied and to the amount of aggregates extracted from feeders  13 . 
     First the discharge outlet  24  is open and the discharge opening  26  is closed; the finished product coming out of unit  14  is thus accumulated in the weighing hopper  25  and the weighing system begins to register the weight (or best, the weight increase) of the outlet hopper  25 . 
     After a short period of stabilisation, the control system  28  draws the net weight of the aggregates based on the weight increase in time of hopper  25  after deducing the amount of bitumen and of the other additives added to the conglomerate. With a continuous modality or following short-time sequences, the weight value of the (dry) aggregates thus obtained is employed by the control system to adjust at least the infed bitumen amount, so that the desired recipe is respected. Substantially, bitumen is metered in real time depending on the calculated weight of the dry aggregates. 
     As time goes by, the outlet hopper  25  reaches the maximum weight allowed by its volume; the control system therefore closes door  24  and stops the continuous control of the bitumen amount. Admission of the various components goes on depending on the last value calculated. Door  26  is opened and the outlet hopper  25  contents are discharged into the transportation means  15 . 
     Upon a signal that the hopper is empty, system  28  closes door  26  and subsequently opens door  24  again, waits for weight stabilisation and restarts the continuous metering control of the elements as above described. The described cycle can be repeated over the whole desired production time. 
     The bitumen feed means  29  could be accomplished following known systems, such as a mere positive-displacement pump possibly provided with a liter-counter. However due to temperature variations in bitumen, measurement errors could be introduced. 
     In order to further improve precision in carrying out the recipe, still in accordance with the principles of the present invention, more precise and reliable feed means is proposed. This means implements a plant denoted by  29  for weighed feed of bitumen. Said means comprises a hopper  16 , disposed between the bitumen deposit  11  and unit  14 . Bitumen is fed to the intermediate hopper  16  by a filling pump  17  dipping into deposit  11 , and is picked up from hopper  16  by a drawing pump  18  sending bitumen to unit  14 . Hopper  16  is provided with weighing means  19 , load cells for example. Before the plant starting, pump  17  fills the intermediate hopper  16  and then stops. When the plant has been started, pump  18 , a variable delivery pump, dips into hopper  16  and admits bitumen to unit  14  in an amount provided by the recipe. Once reading in hopper  25  has been stabilised, weight decreasing in time in hopper  16  will be proportional to the real amount of dry aggregates as calculated by the control system  28  and this will enable the control system  28  to carry out the feedback adjustment of the picked-up amount of pump  18  to ensure introduction of bitumen into the unit  14  in the desired exact amount based on the calculated weight of dry aggregates. 
     Necessarily, at a certain point hopper  16  reaches a minimum weight value and the filling pump  17  is started again. While pump  17  is, in a short period of time, restoring the weight corresponding to the maximum level in hopper  16  the speed of pump  18  is kept constant, fixed to the last value adjusted, the feedback control being temporarily stopped. When filling of hopper  16  has been completed, the “fedback” bitumen-feeding procedure starts again as above described. For more precision in the system, the filling step of the intermediate hopper  16  can be made to coincide with the discharge step of the outlet hopper  25 . 
     The filler feed means too (as well as the feed means for the other components) may comprise a weighing system, in the same manner as done for bitumen. 
     In order to carry out the above, as shown in the FIGURE, an intermediate hopper  20  is present between the filler deposit  12  and unit  14  and the filler is conveyed to this hopper by a screw conveyor  21  and picked up therefrom by a screw conveyor  22 . In the same manner as the bitumen hopper, the filler hopper  20  is provided with weighing means, denoted by  23 , load cells for example. 
     Operation of plant  30  for weighed feed of filler corresponds to the above description relating to plant  29  for weighed feed of bitumen. First the screw conveyor  21  fills hopper  20  with filler. The filler is then fed to unit  14  by the screw conveyor  22  moved by a controlled-and-variable-speed motor, while system  28  is measuring the weight decrease of the hopper thereby carrying out a feedback adjustment of the operation of the screw conveyor  22 . When hopper  20  reaches its minimum weight, the feedback control of the screw conveyor  22  is momentarily stopped, the screw conveyor  22  goes on carrying out feeding of filler in a constant manner, while the screw conveyor  21  is restarted to fill the hopper again and quickly, until its maximum contents. 
     At this point it is apparent that the intended purposes have been achieved by eliminating the “humidity” variable and, if necessary, the “filler percentage” variable from the production process (without being obliged, on the other hand, to dry and eliminate the filler from the aggregates before the latter are measured) and that a high precision in executing the desired recipe has been reached. 
     It is apparent that possible errors in a plant in accordance with the invention can be only ascribed to the precision in the static weighing instruments and not (as in the plants of the known art) to the correspondence between the contents estimations as regards humidity and filler and the real humidity and filler contents in the aggregates. With the above described system for weighing the incoming bitumen, also eliminated are errors due to variations in the specific gravity of bitumen depending on temperature. By way of example, technical errors and evaluation errors that can be found in a plant of the known art can be summarised as follows (an estimate of the value percent of the errors in the best case is given into brackets): evaluation error in the inert material humidity, therefore in the inert-material loss of weight during the drying step (2%); errors due to a non-correct feed of the individual aggregate fractions and influence of same on calculation of the loss of weight during the drying step, taking into account their different starting humidity content (0.2%); weighing errors for the aggregates introduced into the dryer, due to defects present on the weighing conveyor belt mat (1%), due to the wind push on the weighing conveyor belt (2%), due to materials stuck on the weighing conveyor belt mat (1%); errors due to discarding of the excess filler already weighed on the weighing conveyor belt (2%); errors on the volumetric dosage of bitumen due to varying of the specific gravity as there is a temperature variation (0.5%). 
     All the above errors (that on the whole give error percentages in the order of 10%) are completely eliminated with the plant and method of the present invention. 
     On the other hand, the weighing errors of the finished product, bitumen and filler that may arise in a plant in accordance with the invention can be easily maintained each within a percentage of 0.2%. 
     Obviously, the above description of an embodiment applying the innovatory 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 as claimed in the appended claims. 
     For instance, other weighing means can be adopted depending on specific requirements. In addition, because the lower tolerances in the recipe generally are in the bitumen amount, one can choose to only carry out a feedback adjustment of the amount of bitumen supplied to the unit inlet, depending on the calculated weight of the dry aggregates.