Hot Mix Asphalt Production

1. Hot Mix Asphalt Production Chris Lange PMP, Pr.Tech.(Eng) Director (Technical) Much Asphalt (Pty) Ltd

2. Hot Mix Asphalt Production Plant types and basic components Plant calibration Dust extraction Filler handling and control Aggregate heating Procurement / handling of raw materials Finished product storage Delivery

3. Drum Mix Plant - Conventional This slide shows the material flow in more detail. The gradation is controlled by the proportional flow out of the cold feed bins. The scalping screen removes any oversize material, and the wet aggregate is then weighed on the belt scale. The aggregate is heated and dried in the drum, where asphalt is added to match the dry aggregate flow, based upon the belt scale reading and the moisture compensation factor. The continuous stream of completed mix is then sent to the silos via the slat conveyor.This slide shows the material flow in more detail. The gradation is controlled by the proportional flow out of the cold feed bins. The scalping screen removes any oversize material, and the wet aggregate is then weighed on the belt scale. The aggregate is heated and dried in the drum, where asphalt is added to match the dry aggregate flow, based upon the belt scale reading and the moisture compensation factor. The continuous stream of completed mix is then sent to the silos via the slat conveyor.

5. Old Drum Mix Mix Plant:

6. Twin Drum Plant

7. In this parallel-flow drum mixer, the aggregate enters on the left, or high, end, and flows to the right. Heating and drying are followed by mixing at the low end of the drum, just prior to discharge of the mix.In this parallel-flow drum mixer, the aggregate enters on the left, or high, end, and flows to the right. Heating and drying are followed by mixing at the low end of the drum, just prior to discharge of the mix.

8. This counter-flow dryer also contains a mixing zone. Aggregate flow is still from high to low, or right to left as shown here. Note how burner is inserted farther into the drum to provide a mixing area behind the flame.This counter-flow dryer also contains a mixing zone. Aggregate flow is still from high to low, or right to left as shown here. Note how burner is inserted farther into the drum to provide a mixing area behind the flame.

9. Aggregate enters the uphill end of the dryer, or on the right as shown here. As the drum rotates, the aggregate moves to the lower, or left, end of the dryer. Moisture is driven off, and the aggregate temperature increases. Note direction of aggregate versus gas stream.Aggregate enters the uphill end of the dryer, or on the right as shown here. As the drum rotates, the aggregate moves to the lower, or left, end of the dryer. Moisture is driven off, and the aggregate temperature increases. Note direction of aggregate versus gas stream.

10. The aggregate is introduced via the cold feed bins, but gradation is not controlled here. The aggregate is fed into the dryer, where it is heated and dried, and then transferred into the batch tower. The primary and secondary collectors remove dust from the exhaust stream. When mix is produced, hot aggregates in the batch tower are weighed and asphalt is added. The competed mix is either loaded immediately into trucks, or is stored in silos.The aggregate is introduced via the cold feed bins, but gradation is not controlled here. The aggregate is fed into the dryer, where it is heated and dried, and then transferred into the batch tower. The primary and secondary collectors remove dust from the exhaust stream. When mix is produced, hot aggregates in the batch tower are weighed and asphalt is added. The competed mix is either loaded immediately into trucks, or is stored in silos.

11. This detailed view of the batch tower shows how aggregates are discharged from the dryer into the hot stone elevator, screened, and stored in the hot bins. The weigh hopper is used to add specific amounts from each bin, while the asphalt weigh bucket is filled with the proper quantity of asphalt. The pugmill mixes the aggregates and the asphalt, and the completed mix is discharged.This detailed view of the batch tower shows how aggregates are discharged from the dryer into the hot stone elevator, screened, and stored in the hot bins. The weigh hopper is used to add specific amounts from each bin, while the asphalt weigh bucket is filled with the proper quantity of asphalt. The pugmill mixes the aggregates and the asphalt, and the completed mix is discharged.

13. Aggregate Hot Bins

15. Belt Scale Calibration To calibrate the belt scale, aggregates are conveyed from the cold feed system across the belt scale and onto a reference scale. This is usually done by diverting the aggregate flow into a truck after the belt scale, and weighing the truck on a certified platform scale. The readout from the belt scale is compared to the certified weight, and the belt scale is adjusted until the two readings correlate. The larger the calibration sample, the more accurate the calibration can be. (See Appendix K of the Reference Manual).To calibrate the belt scale, aggregates are conveyed from the cold feed system across the belt scale and onto a reference scale. This is usually done by diverting the aggregate flow into a truck after the belt scale, and weighing the truck on a certified platform scale. The readout from the belt scale is compared to the certified weight, and the belt scale is adjusted until the two readings correlate. The larger the calibration sample, the more accurate the calibration can be. (See Appendix K of the Reference Manual).

17. �Graphing the Feeders� Feeders used to be manually calibrated by comparing belt speed (either actual speed or speed control setting) to the tons that are delivered. This can also be done with fixed belt speed and variable gate settings. The results are plotted on a graph. The desired production rate is determined, and the corresponding speed or gate setting is selected. Modern control systems follow the same process, but store the information in the computer.Feeders used to be manually calibrated by comparing belt speed (either actual speed or speed control setting) to the tons that are delivered. This can also be done with fixed belt speed and variable gate settings. The results are plotted on a graph. The desired production rate is determined, and the corresponding speed or gate setting is selected. Modern control systems follow the same process, but store the information in the computer.

18. Tachometer Mounted on Tail Pulley of Cold Feed Bin Belt A tachometer provides both proof the belt is moving, and gives a determination of speed. The tachometer must be mounted on the tail, or non-driven, pulley. Ask: Why is the tachometer mounted on the tail shaft? Expect: The operator needs to know the belt is moving, not just the belt drive. If the belt is slipping, the drive will be turning normally but the belt will be moving erratically, slowly, or not at all.A tachometer provides both proof the belt is moving, and gives a determination of speed. The tachometer must be mounted on the tail, or non-driven, pulley. Ask: Why is the tachometer mounted on the tail shaft? Expect: The operator needs to know the belt is moving, not just the belt drive. If the belt is slipping, the drive will be turning normally but the belt will be moving erratically, slowly, or not at all.

19. �No Flow� Paddle at Feeder Discharge Regardless of the belt scale arrangement, the plant needs to have two safeguards in the cold feed system. The first is the presence of a �no flow� switch which detects the presence of material on each feeder belt. This switch has a limited range of motion.Regardless of the belt scale arrangement, the plant needs to have two safeguards in the cold feed system. The first is the presence of a �no flow� switch which detects the presence of material on each feeder belt. This switch has a limited range of motion.

20. Once the burner has been properly managed to provide efficient combustion and create a hot gas stream, material is �presented� to the gas stream for drying. Steel �lifters� or �flights� are used to accomplish this. The flights need to be properly designed, sized, and located to maximize the interaction between the aggregate and the gas stream for rapid, efficient heating. The amount of material in the dryer, the speed of drum rotation, and the incline of the drum are factored into this design. The flights wear over time due to abrasion by the aggregate, and can deform due to high temperatures, so they must be inspected and maintained. Once the burner has been properly managed to provide efficient combustion and create a hot gas stream, material is �presented� to the gas stream for drying. Steel �lifters� or �flights� are used to accomplish this. The flights need to be properly designed, sized, and located to maximize the interaction between the aggregate and the gas stream for rapid, efficient heating. The amount of material in the dryer, the speed of drum rotation, and the incline of the drum are factored into this design. The flights wear over time due to abrasion by the aggregate, and can deform due to high temperatures, so they must be inspected and maintained.

21. Incorrect veiling allows the heat to bypass the aggregates. This results in poor heating as well as increased dryer shell and exhaust temperatures. Incorrect veiling allows the heat to bypass the aggregates. This results in poor heating as well as increased dryer shell and exhaust temperatures.

22. Photo of induced air burner. Note the visible flame.Photo of induced air burner. Note the visible flame.

23. Fuel Reticulation and Heating System

24. Note how aggregate temperature rises rapidly and then stabilizes due to drum configuration, and how the gas temperature drops rapidly. Ineffective flight arrangement might show slow and insufficient heating of aggregates and dangerously high gas temperatures exiting the drum. A parallel-flow drum would reverse the flame and gas temperature trace, with the gas exit temperature the same as the final aggregate temperature, and the aggregates would not heat significantly until they began to veil after the combustion zone.Note how aggregate temperature rises rapidly and then stabilizes due to drum configuration, and how the gas temperature drops rapidly. Ineffective flight arrangement might show slow and insufficient heating of aggregates and dangerously high gas temperatures exiting the drum. A parallel-flow drum would reverse the flame and gas temperature trace, with the gas exit temperature the same as the final aggregate temperature, and the aggregates would not heat significantly until they began to veil after the combustion zone.

25. The previous slides illustrated a single row of flighting. Successive rows are staggered, so the veiling is staggered. This results in thorough and efficient heat transfer. Mixing flights are also staggered to provide for more thorough mixing. Dams control the depth of material in the dryer. The previous slides illustrated a single row of flighting. Successive rows are staggered, so the veiling is staggered. This results in thorough and efficient heat transfer. Mixing flights are also staggered to provide for more thorough mixing. Dams control the depth of material in the dryer.

26. Primary and Secondary Emission Control

27. Primary Collector:Cyclone Cyclones are effective in removing dust from the gas stream. The size of particle removed is dependent upon the diameter of the cyclone used. If a single cyclone is used, it must be large enough to accommodate the volume of gas created by the plant. This limits the size of the particle it can remove. See page A-10 in the Participant Workbook.Cyclones are effective in removing dust from the gas stream. The size of particle removed is dependent upon the diameter of the cyclone used. If a single cyclone is used, it must be large enough to accommodate the volume of gas created by the plant. This limits the size of the particle it can remove. See page A-10 in the Participant Workbook.

28. Multi-Tube Collector Other types of collectors are also used. Because the moving dust is abrasive, simple devices that can be constructed with heavy-gauge wear-resistant materials are preferred.Other types of collectors are also used. Because the moving dust is abrasive, simple devices that can be constructed with heavy-gauge wear-resistant materials are preferred.

29. Discharge Housing Functions as a Knockout Box The end of the drum is a convenient location for a knockout box, since enlarged ducting is required to transition from the drum to the duct. This arrangement allows the fines to drop into the discharging mix, eliminating the need to convey the fines. This arrangement may or may not utilize a baffle plate. The end of the drum is a convenient location for a knockout box, since enlarged ducting is required to transition from the drum to the duct. This arrangement allows the fines to drop into the discharging mix, eliminating the need to convey the fines. This arrangement may or may not utilize a baffle plate.

30. Cutaway Section of a Typical Baghouse Baghouses have become popular because they eliminate the pond problems associated with wet scrubbers, and they allow reintroduction of the fines from the secondary collector. From a producer standpoint, this saves money in that material that has been fed into the plant, dried, and heated can be sold with the mix, and may result in lower asphalt contents. From a quality standpoint, these fines can be very useful in controlling voids and affecting mix stiffness. They must be carefully controlled for success, however.Baghouses have become popular because they eliminate the pond problems associated with wet scrubbers, and they allow reintroduction of the fines from the secondary collector. From a producer standpoint, this saves money in that material that has been fed into the plant, dried, and heated can be sold with the mix, and may result in lower asphalt contents. From a quality standpoint, these fines can be very useful in controlling voids and affecting mix stiffness. They must be carefully controlled for success, however.

31. Pulse Jet Baghouse In the pulse jet baghouse, each row of bags is plumbed with a manifold carrying compressed air. When that row is to be cleaned, a solenoid valve opens briefly, causing a pressure wave to knock the dust off the bag.In the pulse jet baghouse, each row of bags is plumbed with a manifold carrying compressed air. When that row is to be cleaned, a solenoid valve opens briefly, causing a pressure wave to knock the dust off the bag.

32. The exhaust fan runs at constant speed, and a damper is used to provide the proper draft. The draft necessary will depend on the type of mix, the production rate, the moisture content, and a variety of other factors. Automatic damper controls measure the pressure in the plant and adjust the damper accordingly. The idea is to provide just enough draft to balance the burner, so operators who adjust manually look for dust open the damper until dust stops puffing around the dryer seals. See page A-6 in the Participant Workbook. The exhaust fan runs at constant speed, and a damper is used to provide the proper draft. The draft necessary will depend on the type of mix, the production rate, the moisture content, and a variety of other factors. Automatic damper controls measure the pressure in the plant and adjust the damper accordingly. The idea is to provide just enough draft to balance the burner, so operators who adjust manually look for dust open the damper until dust stops puffing around the dryer seals. See page A-6 in the Participant Workbook.

33. Cyclone, Screw Collector, and Hot Elevator The dust collected by the primary collector is normally returned to the mix without control or metering. In this illustration, the dust is returned to the hot stone elevator and carried up to the number one bin.The dust collected by the primary collector is normally returned to the mix without control or metering. In this illustration, the dust is returned to the hot stone elevator and carried up to the number one bin.

34. Filler Weigh-Pod After Surge Silo Surge silos make metering easier. In this case, a weigh depletion hopper is suspended on load cells below the surge silo. The hopper is filled, and the valve feeding it is closed. The system then monitors the change in mass, or depletion of weight, as the vane feeder discharges the fines into the mix. When the hopper is nearly empty, the system assumes the discharge rate remains constant while it fills the hopper again, and so forth. See page A-12 in the Participant Workbook. Surge silos make metering easier. In this case, a weigh depletion hopper is suspended on load cells below the surge silo. The hopper is filled, and the valve feeding it is closed. The system then monitors the change in mass, or depletion of weight, as the vane feeder discharges the fines into the mix. When the hopper is nearly empty, the system assumes the discharge rate remains constant while it fills the hopper again, and so forth. See page A-12 in the Participant Workbook.

35. Emission Control Summary Remove particulate matter Primary Secondary Return fines to the mix Affects void content Affects mix stiffness Difficult to control Emission control systems are designed to remove particulate matter from plant exhaust. Primary collectors are usually static devices using the inertia of the dust particles to remove some of the dust. Secondary collectors are active filtering devices, usually baghouses. The collected fines can be returned to the mix, but this must be done carefully as the fines affect void content and binder stiffness. The fines are also difficult to control.Emission control systems are designed to remove particulate matter from plant exhaust. Primary collectors are usually static devices using the inertia of the dust particles to remove some of the dust. Secondary collectors are active filtering devices, usually baghouses. The collected fines can be returned to the mix, but this must be done carefully as the fines affect void content and binder stiffness. The fines are also difficult to control.

36. Batch plant filtration

37. Aggregate storage

38. Aggregate Storage

39. Bitumen Storage

40. Old Style Operating Cabin

41. Plant automation: Modern control cabin:

42. Plant automation

43. Roodepoort Op Screen

44. Batch Plant Schematic

45. Batch Plant Op Screen

46. Laboratory from the past

47. Modern Asphalt Laboratory

48. Quality Control Trends Monitored

50. Slat Conveyor Feeding Storage Silos The other method is with a slat conveyor.The other method is with a slat conveyor.

51. Hot Storage � Skip and Winch System

52. Storage Silo: 165 t Capacity

53. Correct Loading Sequence

54. The good old days