Method for producing pre-dried aggregate and method for producing asphalt mixture

Abstract

The present invention provides a method for producing pre-dried aggregate and a method for producing asphalt mixture, which enable improvement in the total energy efficiency of a plant. [Solution] A method for manufacturing pre-dried aggregate in an asphalt plant 1, comprising: an discharge destination switching step in which, immediately after the combustion of the burner 11 of the dryer 2 is stopped, the discharge destination of the aggregate to be processed is switched to outside the system using a discharge destination switching means 29; a wet aggregate supply step in which only wet aggregate of a specific particle size is cut out from aggregate hoppers 7a and 7b and supplied to a drum 10; and a pre-drying step in which, in conjunction with the wet aggregate supply step, the operation of the air supply / exhaust means and the drum 10 is continued, and the wet aggregate is exposed to hot air using the residual heat of the drum 10 as a heat source, and contact heat exchange is performed with the drum 10 to manufacture pre-dried aggregate with a reduced water content.

Description

Technical Field

[0001] The present invention relates to a method for producing pre-dried aggregate in an asphalt plant that is road pavement material production equipment, and a method for producing an asphalt mixture.

Background Art

[0002] An asphalt mixture manufacturing plant that manufactures asphalt mixture as a road pavement material is installed with an asphalt plant that includes a plurality of aggregate hoppers for storing various aggregates such as crushed stone and sand, which are materials of the asphalt mixture, by type, a dryer of rotary kiln structure that heats and dries the plurality of types of aggregates cut out from the aggregate hoppers by exposing them to hot air from a burner, and a conveying means for lifting the heated aggregate discharged from the dryer to the upper part of the adjacent plant body.

[0003] When producing an asphalt mixture in the asphalt plant, various aggregates are cut out from the aggregate hopper, supplied to the kiln body (drum) of the dryer and exposed to hot air from the burner, the aggregates are heated and dried at a predetermined heating temperature corresponding to the mixture shipping temperature (for example, about 160°C to 170°C), and then lifted to the upper part of the plant body via the conveying means.

[0004] In the plant body, the heated aggregate supplied via the conveying means is sieved by particle size with a vibrating sieve provided at the upper part of the plant body, and stored in lower hot bins by particle size. Then, according to the formulation, a predetermined amount of aggregate of each particle size is metered from the hot bins and discharged to the lower mixer, and mixed in the mixer together with a separately metered predetermined amount of filler, molten asphalt, etc., to produce the desired asphalt mixture.

[0005] Incidentally, when raising the aggregate to the predetermined heating temperature, a large amount of burner fuel (for example, about one-third of the total fuel used) is consumed first to evaporate (dry) the moisture contained in the aggregate. In particular, aggregates such as small-grained sand and No. 6 crushed stone have a large specific surface area and are often supplied to the plant as wet aggregate with a relatively high moisture content, so a very large amount of fuel is consumed in the drying process. Therefore, reducing the moisture content of such small-grained wet aggregate can be a promising approach to saving fuel.

[0006] Another approach that contributes to fuel conservation is to improve heat utilization efficiency by recovering and reusing waste heat within the plant system, thereby minimizing ineffective heat that does not contribute to reducing the water content of the aggregate.

[0007] Therefore, if waste heat can be reused and the moisture content of aggregates used in the manufacture of asphalt mixtures, especially small-grained wet aggregates, can be reduced in advance (pre-dried), energy savings can be effectively achieved, and a reduction in CO2 emissions associated with burner combustion can also be expected. From this perspective, several technical proposals have been made regarding the manufacture and utilization of pre-dried aggregates (especially pre-dried sand).

[0008] For example, to address the above problem, the applicant has proposed, as shown in Patent Documents 1 and 2, a separate sand dryer, which is a dedicated dryer for pre-drying high-moisture-content wet sand, to be installed within the facilities of an asphalt mixture manufacturing plant (near the asphalt plant), and to effectively utilize high-temperature hot air (waste heat) exhausted from an existing heat source within the factory facilities, such as a deodorizing furnace, as the heat source for the sand dryer.

[0009] Furthermore, as shown in Patent Documents 3 and 4, a heat exchanger or heat pump unit for recovering exhaust gas heat is provided in the chimney downstream of the dryer's exhaust flue, and outside air heated by the heat exchanger or heat pump unit is supplied to the dryer via a separately provided supply duct, and aggregate such as sand is supplied to the dryer to pre-dry the sand. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2004-36330 [Patent Document 2] Japanese Patent Publication No. 2011-226109 [Patent Document 3] Japanese Patent Publication No. 2011-127270 [Patent Document 4] Japanese Patent Publication No. 2011-149142 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, as described in Patent Documents 1 to 4 above, if sand dryers, heat exchangers, heat pump units, supply ducts, etc. are to be installed separately within the facilities of an asphalt mixture manufacturing plant, the resulting equipment configuration will be relatively large, requiring considerable installation costs and space, and may be difficult to adopt depending on the size of the plant.

[0012] Furthermore, depending on the power consumption of each piece of equipment involved in the manufacturing and utilization of these pre-dried aggregates, the fuel efficiency reduction effect may be offset. Given the current demand for disclosure of CO2 emission reduction effects per unit of production, further technological improvements from the perspective of the overall energy efficiency of the plant (total energy efficiency) are desired.

[0013] In view of the above, the present invention aims to provide a method for producing pre-dried aggregate and a method for producing asphalt mixture that enable improvement of the total energy efficiency of a plant. [Means for solving the problem]

[0014] In order to solve the above problems, the inventors of the present invention conducted thorough research and found that although the dryer (especially the drum) has a suitable heat capacity as a heat storage body, heat originating from the dryer is lost due to dissipation during the down-start operation immediately after the burner combustion stops, which is included in the actual operating process. The inventors considered whether this residual heat could be utilized, and thus arrived at the present invention.

[0015] In other words, the method for manufacturing pre-dried aggregate according to claim 1 of the present invention is a method for manufacturing pre-dried aggregate in an asphalt plant, wherein the asphalt plant comprises: a plurality of aggregate hoppers for storing aggregates containing wet aggregate of a specific particle size according to type; a dryer having the function of heating and drying the plurality of aggregates cut from the aggregate hoppers by exposing them to hot air, and comprising a burner, a supply / exhaust means, and a drum with blades; an aggregate storage bin capable of storing the heated aggregate, which has been made completely dry and discharged from the drum, in a temperature-retaining state; and a system located on the front side of the aggregate storage bin in the aggregate input direction, which determines the discharge destination of the aggregate to be processed discharged from the drum. The asphalt plant is equipped with an outlet switching means that has a function to switch to the outside, and in the production of the pre-dried aggregate in the asphalt plant, it is characterized by including an outlet switching step in which the outlet switching means switches the outlet of the aggregate to be processed to the outside of the system immediately after the combustion of the burner of the dryer stops; a wet aggregate supply step in which only the wet aggregate of a specific particle size is cut out from the aggregate hopper and supplied to the drum; and a pre-drying step in which the operation of the air supply / exhaust means and the drum is continued in conjunction with the wet aggregate supply step, and the wet aggregate is exposed to hot air using the residual heat of the drum as a heat source and contact heat exchange with the drum to produce pre-dried aggregate with a reduced water content.

[0016] Furthermore, the method for producing pre-dried aggregate according to claim 2 of the present invention is characterized by further including a cutting stop step, in which the cutting of the wet aggregate from the aggregate hopper is stopped based on predetermined conditions that correlate with the amount of wet aggregate adhering to the drum.

[0017] Further, the method for producing an asphalt mixture according to claim 3 of the present invention includes the above-described method for producing pre-dried aggregate, and is characterized by further comprising a material mixing step of mixing heated aggregate containing the pre-dried aggregate as a material, molten asphalt, and a filler. Effects of the Invention

[0018] According to the present invention, it is possible to provide a method for producing pre-dried aggregate and a method for producing an asphalt mixture that enable improvement of the total energy efficiency of a plant. Brief Description of the Drawings

[0019] [Figure 1] It is a schematic explanatory diagram of an asphalt plant in the first embodiment. [Figure 2] (a) It is an enlarged view of the main part of the discharge destination switching means adopting a slide gate. (b) It is an enlarged view of the main part of the discharge destination switching means adopting a damper gate. [Figure 3] It is a schematic plan view around the dryer and the pre-dried aggregate yard. [Figure 4] It is a flowchart showing the flow of the pre-drying method for wet aggregate. [Figure 5] It is a schematic diagram showing the internal state of the drum in the pre-drying step. [Figure 6] It is a schematic explanatory diagram of an asphalt plant in the second embodiment. Modes for Carrying Out the Invention

[0020] In the method for producing pre-dried aggregate (aggregate with reduced water content) in the asphalt plant of the present invention, the asphalt plant is provided with a plurality of aggregate hoppers that store aggregates including wet aggregate of a specific particle size with a relatively small particle size, as represented by sand, for each type. The wet aggregate of this specific particle size (according to the required particle size distribution, the representative particle size is, for example, 13 mm or less, preferably 5 mm or less, more preferably 2.5 mm or less) refers to an aggregate that tends to have a particularly high water content due to its relatively large specific surface area.

[0021] Furthermore, the asphalt plant is equipped with a dryer that has the function of heating and drying multiple types of aggregates cut from the aggregate hopper by exposing them to hot air to a predetermined heating temperature (a temperature slightly above the aggregate shipping temperature of approximately 160°C to 170°C). The dryer is composed of a burner, a supply / exhaust means (a blower fan integrated with the burner and / or one or more exhaust fans provided in the exhaust flue downstream of the burner), and a drum with blades, the drum having blades fixed to the inner wall surface of a substantially cylindrical drum, the blades having at least an aggregate scraping function.

[0022] Furthermore, the asphalt plant is equipped with aggregate storage bins in the plant body that allow for the storage of heated aggregate, which has undergone heat drying treatment and is discharged from the dryer, in a state of heat retention maintained at a predetermined heating temperature, particularly in a state of complete dryness (moisture content approximately 0). In the following description, aggregate that has undergone heat drying treatment and whose moisture content has been reduced but has not reached a state of complete dryness will be specifically referred to as pre-dried aggregate.

[0023] Furthermore, the asphalt plant is further equipped with a discharge destination switching means that has the function of switching the discharge destination of the aggregate to be processed to outside the dryer system (any direction on the discharge end side of the drum). This discharge destination switching means corresponds to additional equipment for applying (retrofitting) the present invention to an existing asphalt plant. On the other hand, the aggregate hopper, dryer, and aggregate storage bin described above generally correspond to existing equipment.

[0024] Furthermore, when producing pre-dried aggregate at the aforementioned asphalt plant, this is done when the plant is not producing asphalt mixtures and is shut down, specifically immediately after the dryer burner has stopped burning. Here, "immediately after the burner has stopped burning" refers to a predetermined timing that is at least included in the duration of the drum's heat storage state, and as soon as the heated aggregate in the drum is discharged, a series of pre-drying processes begin immediately.

[0025] Specifically, immediately after the dryer burner stops burning, a discharge destination switching process is performed using a discharge destination switching means to switch the discharge destination of the aggregate to be processed from the normal route to the aggregate storage bin to an off-system discharge route located on the opposite side of the drum from the aggregate input side. At this time, the process transitions to pre-dried aggregate production by off-system discharge while residual heat remains in the drum. Preferably, considering the time required from the burner burning stop until the completion of the discharge of heated aggregate from the drum, the discharge destination switching process should be performed within 5 to 10 minutes after the burner burning stops.

[0026] Next, a wet aggregate supply process is performed in which only wet aggregate of a specific particle size is extracted from the aggregate hopper and supplied to the drum. In other words, wet aggregate before heating and drying is supplied into a drum that has sufficient preheating.

[0027] Next, in conjunction with the wet aggregate supply process, the operation of the air supply / exhaust means (driving the air supply / exhaust fan) and the operation of the drum (rotation drive) are continued, and a pre-drying process is carried out in which the wet aggregate is exposed to hot air using the residual heat of the drum as a heat source, and heat exchange occurs in contact with the heat-storing drum (including the blades), thereby producing pre-dried aggregate with a reduced moisture content.

[0028] As a result of this series of processes, the synergistic effects of A) the generation of hot air through heat exchange between the airflow inside the drum generated by the air supply / exhaust means and the heat-storing drum casing and blades, and the contact between the hot air and the wet aggregate, and B) the contact between the heat-storing drum casing and blades and the wet aggregate, enable effective pre-drying of the wet aggregate without consuming fuel and with a reasonable heat exchange efficiency.

[0029] Therefore, according to one aspect of the present invention described above, the residual heat of the drum during the down-cycle operation can be effectively utilized, and the absorption of heat from the drum (including the blades) to the wet aggregate can be promoted, allowing the down-cycle operation for reducing the heat load to be completed relatively early. This can contribute to improving the total energy efficiency, including the driving power of the entire dryer.

[0030] Preferably, a further dispensing stop process is performed to stop the dispensing of wet aggregate from the aggregate hopper based on predetermined conditions (such as the amount of residual heat remaining in the drum over time) that correlate with the amount of wet aggregate adhering to the drum. This suppresses the adhesion of wet aggregate to the inside of the drum (including the blades).

[0031] The pre-dried aggregate produced by the above pre-drying process is temporarily stored in a storage hopper or stockyard. When plant operation resumes, the pre-dried aggregate is heated and dried in a dryer along with other aggregates to create completely dry heated aggregate. Furthermore, the heated aggregate is mixed in a mixer with various predetermined amounts of materials, including molten asphalt and filler, to produce an asphalt mixture of the desired composition. This contributes to improving the total energy efficiency in terms of energy consumption per unit of production during asphalt mixture manufacturing.

[0032] More preferably, the predetermined conditions for executing the cutting stop process (adhesion occurrence conditions) are determined based on any of the following: the elapsed time since the burner stopped burning, the gas temperature and / or humidity at a predetermined location in the asphalt plant, the temperature of the pre-dried aggregate discharged from the drum, or the temperature of the drum shell (which may include the blades). This makes it possible to more reliably suppress the adhesion of wet aggregate into the drum. In particular, if the elapsed time since the burner stopped burning is used as a condition, the control mechanism can be simplified. Alternatively, a separate control unit with calculation functions may be provided to count the elapsed time using a timer, and the applicability of the predetermined conditions (adhesion occurrence conditions) may be determined based on the detected values ​​of various sensors.

[0033] More preferably, the cutting stop process should be performed on the condition that the drum's steel shell temperature falls below a predetermined reference temperature. This makes it possible to more reliably suppress the adhesion of wet aggregate inside the drum. For example, 50°C, 60°C, 70°C, 80°C, and 90°C can be used as reference temperatures. Generally, it is thought that the adhesion suppression effect is greater when the reference temperature is higher, but when the reference temperature is higher, the proportion of heat dissipated from the total heat stored in the drum also increases. Thus, from the viewpoint of balancing the utilization of residual heat and the prevention of adhesion, a reference temperature of around 50°C to 90°C is considered preferable.

[0034] More preferably, the system further includes a lifting means (existing), such as a bucket elevator, for lifting the aggregate to be processed to the upper part of the plant body, and the heated aggregate is loaded by gravity into the existing aggregate storage bin via the lifting means, and the discharge destination switching means is located on the side before the aggregate to be processed is loaded into the lifting means. This makes retrofitting to existing asphalt plants of the so-called tower type / building shape, which are becoming increasingly widespread, even easier, and reduces the power consumption required for lifting pre-dried aggregate.

[0035] More preferably, the wet aggregate of the specified particle size is sand or No. 6 crushed stone. This is advantageous in terms of reducing fuel consumption by focusing the pre-drying treatment on aggregate types that tend to have a relatively high water content due to their small particle size and thus have a large specific surface area. Sand can be obtained in a suitable amount, and because of its small particle size, it has a large specific surface area and therefore tends to have the highest water content, so a high reduction in fuel consumption can be obtained by targeting sand for pre-drying treatment. On the other hand, No. 6 crushed stone is an aggregate type that is characteristically used in the manufacture of asphalt mixtures, and compared to sand, it has a smaller specific surface area due to its particle size and therefore tends to have a relatively low water content, resulting in less adhesion and easier handling.

[0036] Thus, according to the present invention, since the wet aggregate is pre-dried by utilizing the preheating of the drum, the heat is raised to a predetermined heating temperature and heated and dried without expending much thermal energy (burner fuel) on the drying process of the aggregate, and CO2 emissions are reduced. In addition, the power consumption reduction effect due to the shortened down-cooling time obtained by promoting the cooling of the drum is particularly beneficial in terms of energy saving and environmental impact. [Examples]

[0037] <Example 1> Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

[0038] <Overall Structure> Figure 1 shows an asphalt plant for manufacturing asphalt mixtures according to the present invention, which mainly consists of a dryer 2 for heating and drying aggregates of various particle sizes, and a plant body 3 for sieving, storing, weighing, and mixing the heated aggregates that have undergone the heating and drying process in the dryer 2. An aggregate supply unit 4 is located on the material input side of the dryer 2, and the inside of the dryer 2 is in communication with a flue passage 5. The plant also includes a control unit 6 for operating and controlling various devices and equipment within the plant.

[0039] The aggregate supply unit 4, located upstream (material input side) of the dryer 2, includes multiple aggregate hoppers 7a to 7c for storing aggregate, which is the material for the asphalt mixture, according to its particle size, and multiple dispensing belt conveyors 8a to 8c for dispensing aggregate of various particle sizes according to type, corresponding to each aggregate hopper 7a to 7c. The aggregate of various particle sizes dispensed from the aggregate hoppers according to the mix is ​​supplied into the cylindrical drum 10 of the dryer 2 via a connecting belt conveyor 9. The aggregate supplied into the drum 10 is heated and dried to a predetermined heating temperature slightly above the asphalt mixture shipping temperature (the shipping temperature for asphalt mixtures is usually around 160°C to 170°C, but for medium-temperature asphalt mixtures, a lower shipping temperature of around 110°C to 160°C may also be used) by contact with hot air supplied from a burner 11 located at one end of the dryer 2 (opposite the material input side). The heated aggregate, having reached this predetermined heating temperature, is then supplied to the vibrating screen 14 at the top of the plant body 3 via the heated aggregate chute 12 and the vertical conveying device 13.

[0040] The plant body 3 is equipped with a vibrating screen 14 for sieving heated aggregate, aggregate storage bins 15 for storing the heated aggregate sieved by the vibrating screen 14 according to particle size, an aggregate weighing tank 16 for cumulatively weighing heated aggregate of various particle sizes discharged from the aggregate storage bins 15, an asphalt tank 17 for storing molten asphalt, an asphalt weighing tank 19 for weighing the molten asphalt supplied from the asphalt tank 17 by a supply pump 18, a stone powder silo 20 for storing stone powder, a stone powder weighing tank 22 for weighing the stone powder supplied from the stone powder silo via a stone powder screw feeder 21, and a mixer 23 for introducing and agitating the above materials.

[0041] The flue path 5 comprises a first flue 24, a bag filter 25 downstream of the first flue, a second flue 26 downstream of the bag filter 25, and a chimney 27 at the end of the second flue 26. Exhaust containing dust from the dryer 2 is drawn into the flue path 5 by an exhaust fan 28, the dust is removed by the bag filter 25, and then it is discharged through the chimney 27.

[0042] Furthermore, as additional equipment for applying the present invention to an existing asphalt plant, the drum 10 is equipped with a slide gate (discharge destination switching means) 29 and an external discharge chute 30 on the discharge end side, which have the function of switching the discharge destination of the aggregate to be processed to outside the system. The external discharge destination is connected to a pre-dried aggregate yard (described later).

[0043] The operation of the movable parts of the plant equipment, including the slide gate 29 and the aggregate hopper hopper 7a-7c and the cutting belt conveyor 8a-8c, can be controlled automatically or manually via the control unit 6, which is connected to these plant devices via communication.

[0044] <Wet aggregate of specific particle sizes> The aggregates stored in each aggregate hopper 7a to 7c for each type are of a specific particle size, which is relatively small, and are wet aggregates in which moisture adheres to the surface or penetrates the interior, including wet sand and wet No. 6 crushed stone. These specific particle size wet aggregates are stored in the wet sand hopper 7a and the wet No. 6 crushed stone hopper 7b, respectively, and are dispensed by the corresponding dispensing belt conveyors 8a and 8b. The moisture content of the wet sand and wet No. 6 crushed stone depends on various conditions such as the actual particle size of the aggregate, but for example, it is about 7-8% for wet sand.

[0045] As will be understood by those skilled in the art, in various aggregates containing wet aggregate of a specific particle size, aggregates of various particle sizes are mixed within a predetermined size range, and as a whole, they constitute one group / category of aggregate types that have a specified particle size distribution. The wet aggregate may be natural aggregate, crushed stone / crushed sand, artificial lightweight aggregate such as Mesalite (registered trademark), recycled aggregate including old asphalt, etc. Furthermore, as will be understood by those skilled in the art, the aggregate inside the aggregate hoppers 7a to 7c is in a wet state with a corresponding moisture content due to the classification process during manufacturing, exposure to rainwater during transportation and storage, etc.

[0046] According to the regulations, the sand is fine aggregate as defined in JIS A 0203:2019 (Concrete Terminology), which passes completely through a 10mm sieve and at least 85% by mass through a 5mm sieve. The No. 6 crushed stone is single-sized crushed stone as defined in JIS A 5001:1995 (Crushed Stone for Roads), which passes at least 85% by mass through a 13mm sieve.

[0047] Furthermore, in this specification, among the treated aggregates that have undergone heat drying treatment in the dryer 2, aggregates that are in a completely dry state (moisture content of approximately 0%) and stored in the aggregate storage bin 15 in a warm state maintained at a predetermined heating temperature corresponding to the aggregate shipping temperature are specifically referred to as heated aggregates. On the other hand, aggregates whose moisture content has been reduced but which have not reached a completely dry state are specifically referred to as pre-dried aggregates.

[0048] <Hair dryer> The rotary kiln structure dryer 2 includes a burner 11 integrated with a blower 31 and a drum 10 with blades 32. Multiple types of aggregate cut from aggregate hoppers 7a to 7c are exposed to the hot air from the burner 11, allowing the aggregate to be heated and dried to a predetermined heating temperature corresponding to the predetermined shipping temperature. The burner 11 is a combustion device for industrial furnaces that can burn heavy oil, city gas, solid biomass, hydrogen, and various other fuels individually or in combination. As previously described, an exhaust fan 28 is provided on the flue path 5 that communicates with the inside of the drum 10 of the dryer 2, allowing exhaust gas from the dryer 2 to be drawn in. The exhaust fan 28, together with the blower 31 of the burner 11, has the function of generating airflow inside the drum 10 of the dryer 2, and in this specification, it may be treated in parallel with the components of the dryer 2 as a blower / exhaust fan.

[0049] The roughly cylindrical drum 10 is rotatably supported on the machine base 33 by rollers 34 and is positioned at a slight incline in the aggregate transport direction. Various shapes and functions of blades 32, such as scraping and holding, are fixed to the inner wall surface of the drum 10. The drum 10 and blades 32 (rotary kiln with blades) function as heat storage bodies with a corresponding heat capacity and are manufactured from iron-based materials such as various types of carbon steel or alloy steel.

[0050] The air supply / exhaust means includes both a blower 31 integrated with the burner 11 as described above, and an exhaust fan 28 located on the flue path 5 (second flue 26 in this embodiment) that communicates with the inside of the drum 10. These blowers 31 and exhaust fans 28 can each be operated independently or simultaneously, and can generate airflow inside the drum 10. In particular, during combustion of the burner 11 or during the heat storage period of the drum 10, hot air can be generated inside the drum 10 by operating the air supply / exhaust means. The direction of hot air generation can be either counterflow or parallel flow relative to the aggregate transport direction, but in this embodiment, a counterflow type is adopted, which is generally adopted for dryers 2 for new aggregates in terms of thermal efficiency.

[0051] <Discharge destination switching means> The asphalt plant 1 in this embodiment has a slide gate 29, which is a discharge destination switching means, which allows the aggregate to be processed to be discharged from the aggregate discharge end side of the dryer 2 to the vertical conveying device 13 via the heated aggregate chute 12, which is an aggregate discharge means, as well as a switching path for discharging the aggregate outside the system via the off-system discharge chute 30, and both paths can be switched and selected at the desired timing.

[0052] Figure 2 shows an enlarged view of the main part of the discharge destination switching means. In this embodiment, as a specific form of the discharge destination switching means, as shown in Figure 2(a), a configuration is adopted in which a slide gate 29 is provided in a part of the heated aggregate chute 12 to guide the aggregate to be processed to the system discharge chute 30. The slide gate 29 constitutes a part of the bottom plate of the heated aggregate chute 12, and by the operation of the slide gate 29, the aggregate is pulled downward near the entrance of the heated aggregate chute 12 and guided to the system discharge chute 30.

[0053] Another embodiment of the discharge destination switching mechanism is shown in Figure 2(b), which includes a branching mechanism equipped with a switching damper 29' for switching the discharge path of the aggregate to be processed, located along the discharge path of the external discharge chute 30. However, the mechanism is not limited to this, and any other appropriate mechanism may be adopted.

[0054] Figure 3 shows a schematic plan view of the area surrounding the dryer 2 and the pre-dried aggregate yard. As a discharge route for the aggregate to be processed, an aggregate screw conveyor 35, which is a transport means, is provided below the discharge chute 30, with its transport end facing the pre-dried aggregate yard 36. In the pre-dried aggregate yard 36, the pre-dried aggregate can be received by receiving vehicles such as wheel loaders waiting in the dashed area X.

[0055] Furthermore, pre-dried aggregate may be temporarily stored in the pre-dried aggregate yard 36, but in this case, it is desirable to provide a roof over the pre-dried aggregate yard 36 to prevent rainwater from flowing in. Alternatively, storage means such as a storage hopper may be provided instead of the pre-dried aggregate yard 36. In addition, a belt conveyor or other appropriate conveying mechanism may be used instead of the screw conveyor 35 as a conveying means. However, the screw conveyor 35 has the advantage of being less subject to layout constraints, such as having a high degree of freedom in placement and being easy to place while avoiding inspection doors of existing plants.

[0056] <Department Head> The control panel PC terminal constituting the control unit 6 is a computer having a processor such as a CPU or GPU, memory such as RAM, and an auxiliary storage device such as a hard disk drive or SSD. Furthermore, the power sources for the movable parts of the plant, including the drive means (not shown) such as an air cylinder that drives the slide gate 29 (discharge destination switching means), the drive motor (not shown) for the cutting belt conveyor of the aggregate hopper 7a to 7c, and the drive motor (not shown) for the blower / exhaust fan (burner fan 31 and exhaust fan 28) of the dryer 2, are electrically connected to a PLC (Programmable Logic Controller, not shown), and the PLC is connected to the control panel PC terminal for communication. Hereinafter, the control unit 6 refers collectively to the PLC and the control panel PC terminal.

[0057] <Pre-drying process> The specific steps of the pre-drying process will be explained below using the flowchart shown in Figure 4. In the diagram, S represents a step.

[0058] When producing pre-dried aggregate in asphalt plant 1, this is done when the plant is not producing asphalt mixture and is shut down, specifically immediately after the burner 11 of dryer 2 stops burning. Here, "immediately after the burner stops burning" refers to a predetermined timing that is included in the period during which the drum 10 of dryer 2 maintains a heat storage state, and as soon as the heated aggregate in the drum 10 is discharged, a series of pre-drying processes shall be started immediately.

[0059] First, the combustion of the burner 11 of the dryer 2 is stopped (S1), and once the discharge of the heated aggregate from the drum 10 is complete (5 minutes have elapsed in this embodiment) (S2:YES), the slide gate 29, which is a means for switching the discharge destination, is slid to execute a discharge destination switching process that switches the discharge destination of the aggregate to be processed to the outside of the system via the outside discharge chute 30 (S3). Immediately after the combustion of the burner 11 is stopped, the inside of the drum 10 of the dryer 2 still has enough residual heat to reduce the moisture content of the wet aggregate and turn it into pre-dried aggregate, even if it is not yet heated to a completely dry state.

[0060] Next, a wet aggregate supply process is performed in which, in this embodiment, only wet sand of a specific particle size is cut out from the aggregate hopper 7a and supplied to the dryer 2 by a belt conveyor 8a (S4). In this way, wet aggregate before heating and drying is supplied into the drum 10 of the dryer 2, which has sufficient preheating.

[0061] Next, in conjunction with the wet aggregate supply process, the operation of the blowing / exhausting means and the rotational drive of the drum 10 are continued, and the wet aggregate is exposed to hot air using the residual heat of the dryer 2 as the heat source, and heat exchange occurs with the drum 10 and the blades 32, thereby performing a pre-drying process to produce pre-dried aggregate with a reduced moisture content. In this embodiment, the blower 31 of the burner 11 is stopped when the heated aggregate discharge is completed (S2), and only the exhaust fan 28 of the second flue 26 is operated, but the blower 31 of the burner 11 may be operated as appropriate during the pre-drying process.

[0062] Figure 5 shows a schematic diagram of the inside of the drum 10 during the pre-drying process. In the dryer 2, with the combustion of the burner 11 stopped, the operation of the exhaust fan 28 of the second flue 26 and the rotational drive of the drum 10 are continued, and wet aggregate WS is supplied into the drum 10 and scraped up by multiple blades 32. In order to increase the contact area between the wet aggregate WS and the hot air, it is dropped in layers at predetermined intervals to form a so-called aggregate bale, and this scraping and dropping is repeated, causing it to move (rolling down) inside the drum 10 and be sent out in the discharge direction. At this time, inside the drum 10 of the dryer 2, by utilizing the aforementioned residual heat, the pre-drying treatment of the wet aggregate can be effectively performed with a reasonable heat exchange efficiency without consuming burner fuel, through the synergistic effects of A) in particular in the bale-forming region α, the generation of hot air by heat exchange between the airflow inside the drum 10 and the heat-storing drum 10 casing and blades 32, and the contact between the hot air and the wet aggregate WS, and B) in particular in the holding region β, the contact between the heat-storing drum 10 casing and blades 32 and the wet aggregate WS.

[0063] Next, a determination is made as to whether the amount of wet aggregate adhering to the drum 10 meets the predetermined conditions (adhesion occurrence conditions) (S5: YES), and a cutting stop process is executed to stop the cutting of wet aggregate from the aggregate hopper 7a (S6).

[0064] In this embodiment 1, the cutting stop process (S6) is performed on the condition that the shell temperature of the drum 10 falls below 70°C (S5: YES). According to the inventor's experiments, under specific conditions, when the shell temperature of the drum 10 measured by a radiation thermometer falls below the standard value of 70°C, adhesion of wet aggregate to the inside of the drum 10 becomes significant, while when the shell temperature is 70°C or higher, adhesion of wet aggregate is suppressed. Note that the value of 70°C is for when the wet aggregate is sand, and a lower standard value may be adopted for crushed stone No. 6 (adhesion is considered less likely). Conversely, depending on the moisture content and various properties of the wet aggregate, it may be possible to adopt a standard value higher than this. In this way, a predetermined period until the shell temperature reaches the lower limit value at which adhesion occurs is registered in the time timer of the control unit 6, and the determination of whether the conditions for adhesion occurrence are met is made when the timer goes up. This ensures the certainty of suppressing the adhesion of wet aggregate to the drum 10 and blades 32, while simplifying the control mechanism by omitting linked control with temperature sensors, etc. Alternatively, depending on the properties of the supplied wet aggregate, such as its water content and water absorption rate, as well as the ambient temperature and supply amount, a predetermined period may be calculated in advance for which the drum 10 steel shell temperature drop meets the conditions for adhesion to occur, and a predetermined period until the timer goes up may be determined.

[0065] When stopping the supply of wet aggregate (S6), the cutting belt conveyor 8a of the aggregate hopper 7a is controlled to stop, and when all of the wet aggregate supplied into the drum 10 has been discharged outside the drum 10 as pre-dried aggregate (after 5 minutes in this embodiment) (S7:YES), the rotation drive of the drum 10 is stopped (S8) and the pre-drying process is completed.

[0066] The pre-dried aggregate produced by the above pre-drying process is then collected by receiving vehicles such as wheel loaders waiting in the dashed area X of the pre-dried aggregate yard 36 via the system discharge chute 30 and the aggregate screw conveyor 35. When plant operation is resumed, various aggregates containing the pre-dried aggregate are heated and dried in the dryer 2 to produce completely dry heated aggregate. Furthermore, a material mixing process is carried out in the mixer 23 to mix the heated aggregate with various predetermined amounts of materials including molten asphalt and filler to produce an asphalt mixture of the desired composition. This can contribute to improving the total energy efficiency in terms of energy consumption per unit of production during asphalt mixture manufacturing. In this embodiment 1, the recovered pre-dried aggregate was directly fed into the input side of the drum 10 of the dryer 2 from the receiving vehicle. However, for example, the aggregate hopper 7c in the figure could be used for pre-dried aggregate, and the pre-dried aggregate could be temporarily stored in this aggregate hopper 7c using the receiving vehicle, and then cut again by the cutting belt conveyor 8c when the plant operation is restarted.

[0067] Thus, according to the pre-drying process of the present invention, the residual heat of the dryer 2 during the down-cycle operation can be effectively utilized, and the absorption of heat from the drum 10 (and blades 32) to the wet aggregate can be promoted, allowing the down-cycle operation for reducing the heat load to be completed relatively early. This can contribute to improving the total energy efficiency, including the power consumption of the dryer 2.

[0068] Furthermore, while the thermal efficiency within the plant system has already been increased to near its limit through various innovative measures, the residual heat from dryer 2 during down-start operation had to be dissipated. However, in the pre-drying process of this embodiment, this residual heat can be newly utilized.

[0069] In other words, in conventional asphalt plants, when there is material inside the dryer drum, energy is used to raise its temperature, so much of the heat stored in the drum does not become wasted heat, and therefore a heat recovery mechanism is generally not provided. On the other hand, when no material is being fed in during down-pressure operation, the heat capacity of the drum cannot be fully utilized. In the asphalt plant of the present invention, in addition to being able to utilize the residual heat of the drum 10, the cooling time of the drum 10 can be shortened by heat exchange with the wet aggregate, so a reduction in the heat load on the equipment can also be expected.

[0070] Thus, in the dryer 2 of the asphalt plant 1, which is supplied with pre-dried aggregate, the aggregate can be heated to a predetermined temperature without expending much thermal energy (burner fuel) and while suppressing CO2 emissions, making it particularly beneficial in terms of energy saving and environmental impact. Furthermore, considering that in conventional down-cooling operations, the drum 10 was driven empty for at least the required period to protect the dryer 2, shortening the down-cooling operation time by promoting cooling is also effective in reducing electricity consumption, and together with fuel consumption reduction, can contribute to improving the overall energy efficiency of the plant. For example, consider a scenario where 3.3 tons of wet sand are pre-dried using the residual heat from dryer 2 during the shipment of a standard asphalt mixture (around 160°C). By starting pre-drying 5 minutes after burner 11 stops burning, and performing 10-minute pre-drying cycles twice a day, a reduction in moisture content of approximately 4% (equivalent to about 130 kg of water) can be achieved. Consequently, a fuel consumption reduction of approximately 9 liters in heavy oil equivalent can be expected, and a corresponding reduction in electricity costs can be anticipated due to the shortened cooldown time. These effects will vary depending on plant operating conditions such as ambient temperature, humidity, and the number of dryer start-ups / cooldowns per day.

[0071] <Example 2 - Temperature Sensor> Figure 6 is an overall diagram of the asphalt plant according to Example 2. The only difference from Example 1 is that multiple temperature sensors T1 to T4 are provided at predetermined locations within the plant, and the detected values ​​can be sent to the control unit 6. Specifically, the installation locations are the aggregate temperature sensor T1 in the pre-dried aggregate chute 12, the steel shell temperature sensor T2 in the drum 10, the exhaust gas temperature sensor T3 in the first flue, and the exhaust gas temperature sensor T4 in the second flue.

[0072] The predetermined conditions (adhesion conditions) for executing the wet aggregate cutting and supply stop process (S6) are determined based on the values ​​detected by sensors T1 to T4 at these predetermined locations within the asphalt plant 1, namely the gas temperature and / or humidity, the temperature of the pre-dried aggregate discharged from the drum 10, and the steel shell temperature of the drum 10 (which may include the blades 32), or a combination thereof (S5). The other pre-drying processes are the same as in Example 1.

[0073] To determine whether the conditions for adhesion are met, various sensors may be used individually or in combination. These include contact or non-contact gas temperature sensors if installed inside the drum 10 or in the flue path 5, aggregate contact thermometer sensors if installed at the drum 10 outlet, and radiation thermometers installed near the surface of the drum 10 when measuring the drum's shell temperature. Temperature sensors may also be replaced with humidity sensors. Various sensor types may be used, including contact, non-contact, image-based, and others.

[0074] Then, the dispensing and supply stop process (S6) is executed on the condition that the temperature values ​​detected by each temperature sensor T1 to T4 exceed or fall below a predetermined threshold that causes adhesion inside the drum 10.

[0075] <Variation> In addition, in order to suppress the adhesion of wet aggregate into the drum 10, various conditions may be determined by experimentation or simulation, taking into account the fluctuations in the moisture content of the wet aggregate in accordance with the amount of residual heat remaining in the drum 10 over time. For example, when determining the timing of the end of the pre-drying process, it is advisable to set the time so that the amount of residual heat in the drum 10 becomes unsuitable for pre-drying the wet aggregate, calculated from the start of the pre-drying process. Specifically, for example, pre-drying may be performed on a specific type of wet aggregate, such as sand, and the time until the moisture content no longer decreases may be measured. Based on this, the parameters for adhesion conditions may be fine-tuned, taking into account the moisture content of various wet aggregates in the aggregate hoppers 7a and 7b, as well as the influence of the ambient temperature.

[0076] Alternatively, pre-dried aggregate may be produced by pre-drying recycled material RC13-0 as wet aggregate. In this case, the pre-drying process may be carried out using the heat stored in the drum of a recycling dryer (not shown).

[0077] Furthermore, the installation location of the discharge destination switching means is not limited to the description in each embodiment, as long as it is on the upstream side of the aggregate storage bin 15. Also, the number of aggregate hoppers is not limited to the description in each embodiment. Regardless of the description in each embodiment, the components described in each embodiment may be combined or replaced with mechanisms and processes of equivalent function, without departing from the spirit of the present invention. [Industrial applicability]

[0078] The present invention can be widely used in the pre-drying process of various wet aggregates in asphalt plants, and in the production of various asphalt mixtures using the pre-dried aggregate produced thereby as a material. [Explanation of symbols]

[0079] 1...Asphalt plant 2...Dryer 3...Plant main body 4...Aggregate supply unit 5… Flue path 6… Control unit 7a-7c... Aggregate hopper 8a-8c... Cutting belt conveyor 10...Drums 11...Burner 28... Exhaust fan 29... Slide gate (discharge destination switching mechanism) 30...External discharge chute 31...Blower 32... Feathers 36... Pre-drying yard

Claims

1. A method for producing pre-dried aggregate in an asphalt plant, The aforementioned asphalt plant, Multiple aggregate hoppers for storing aggregates containing wet aggregate of specific particle sizes, separated by type, The function involves exposing multiple types of aggregate cut from the aggregate hopper to hot air for heat drying. A dryer comprising a burner, a supply / exhaust means, and a drum with blades, The processed aggregate discharged from the drum, which has been completely dried, is in a heat-retaining state. A storage bin for aggregates that can be stored in a certain state, Located on the front side of the aggregate storage bin in the direction of aggregate input, and discharged from the drum The system includes a discharge destination switching means that has the function of switching the discharge destination of the aggregate to be processed to outside the system, In the production of the pre-dried aggregate at the asphalt plant, Immediately after the burner of the dryer stops burning, the discharge destination switching means controls the bone to be processed. A discharge destination switching process that switches the discharge destination of the material to outside the aforementioned system, Only the wet aggregate of the specified particle size is cut from the aggregate hopper and supplied to the drum. The wet aggregate supply process, In conjunction with the wet aggregate supply process, the operation of the air supply / exhaust means and the drum is continued. The wet aggregate is exposed to hot air using the residual heat of the drum as the heat source, and the contact heat of the drum is also exposed to hot air. A pre-drying process that involves replacing the material to produce pre-dried aggregate with a reduced moisture content, The process includes a cutting stop step, in which the cutting of the wet aggregate from the aggregate hopper is stopped based on predetermined conditions that correlate with the amount of wet aggregate adhering to the drum, The aforementioned wet aggregate of a specific particle size is fine aggregate as specified in JIS A 2023:2019, The cutting stop process is performed on the condition that the temperature of the drum's metal shell falls below 70°C. A method for producing pre-dried aggregate.

2. The predetermined condition is the elapsed time from the burner's combustion stop, and the predetermined period until the metal shell temperature reaches 70°C, which is the lower limit for the adhesion of the fine aggregate, is registered in the time timer of the control unit of the asphalt plant, and the cutting stop process is performed when the timer goes up. A method for producing pre-dried aggregate according to claim 1.

3. The method for producing pre-dried aggregate according to claim 1 or 2, comprising the pre-dried aggregate as a material. The process further includes a material mixing step of mixing heated aggregate, molten asphalt, and filler. A method for producing a fart mixture.

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