PROCESS AND APPARATUS FOR MANUFACTURE OF ASPHALT

Abstract: In the manufacture of asphalt containing at least some reclaimed asphalt material, the reclaimed asphalt is heat-ex­ changed with hot, dried and screened aggregate prior to mixing the reclaimed asphalt with the aggregate and with fresh bitu­ men.

Description: PROCESS AND APPARATUS FOR MANUFACTURE OF ASPHALT This invention relates to the manufacture of asphalt and more particularly to a process and apparatus for manufacturing asphalt mixes in which at least a portion of the starting material comprises salvaged or reclaimed asphalt. The use of reclaimed asphalt provides economic advantages not only in that it reduces the quantity of virgin or new material required to produce a given batch of asphalt but also because it reduces the problem of disposing of old asphalt which, for example, may have been removed from the surface of a roadway. Salvaged or reclaimed asphalt which contains bitumen is not suitable for incorporation into conventional asphalt making processes. When recycling in these processes the materials are raised to a relatively high temperature to produce an adequate transfer of heat. At these temperatures bitumen may degrade and it may create a pollution problem. Also, at raised temperatures the reclaimed material which contains bitumen may become tacky so that it is particularly difficult to handle and to assimilate in the asphalt making process particularly in relation to batch plant operations. Moreover, higher operating temperatures inevitably lead to increased energy consumption and maintenance costs. The present invention seeks to provide a process for producing asphalt which utilizes at least a proportion of reclaimed asphalt as a starting material and which alleviates or minimises these difficulties, and an apparatus for carrying out such a process. According to the invention there is provided a process for producing asphalt mixes in which at least a portion of the starting material is reclaimed asphalt, the process comprising the steps of: providing at least one grade of aggregate material; transferring a metered quantity of the aggregate to a dryer for drying and heating the aggregate; screening the dried and heated aggregate into separate holding compartment according to grade; providing a supply of reclaimed asphalt of suitable consistency; transferring the reclaimed asphalt to a reclaim holding compartment located in heat transfer relationship with at least one of the aggregate holding compartments whilst being physically separated therefrom, to permit heat transfer to take place between the dried and heated aggregate and the reclaimed asphalt; ; discharging the aggregate and the reclaimed asphalt from the holding compartments to effect at least partial blending of the aggregate and the reclaimed asphalt; and adding bitumen to the at least partially blended aggregate and reclaimed asphalt. Filler material may be added as required. A proportion of the screened aggregate may be diverted into the reclaim holding compartment to effect a degree of preblending and preferably heat transfer of the aggregate and asphalt in the reclaim holding compartment. The aggregate preferably is diverted in the same proportions in which it is fed to the screening step. According to the invention there is also provided apparatus for use in such a process, the apparatus comprising a batch tower including a plurality of aggregate holding compartments for holding preheated aggregate and at least one reclaim holding compartment for holding reclaimed asphalt, the reclaim holding compartment being in heat-exchange relationship with at least one of the aggregate holding compartments so that, in use, heat may pass from the preheated aggregate to heat the reclaimed asphalt. The apparatus may also include at least one cold feed hopper for storing starting material aggregate. Preferably the apparatus includes a plurality of cold feed hoppers for storing aggregate of various sizes to be used in producing the final mix. In one form the apparatus in accordance with the invention may include six cold feed hoppers. The apparatus preferably includes a dryer for aggregate. A belt conveyor or the like may be provided to transfer aggregate from the cold feed hoppers to the dryer to preheat and dry the aggrregate. The belt feeding conveyor may be controlled by means of an automatic control system to ensure that the correct proportion of aggregate is fed from each cold feed hopper. The hot and dried aggregate may be transported to the batch tower by means of a transfer belt, elevator or other suitable means. The apparatus may include a sizing screen arrangement for separating the preheated and dried aggregate into various grades and a holding system for holding the graded material. The sizing screen arrangement may be provided in any suitable form to separate the aggregate into any suitable number of size ranges as may be desired. The holding system may include one or more separate compartments for holding sized and graded aggregate. Each holding compartment is preferably provided by an aggregate holding bin. The aggregate holding bins may be arranged in any convenient manner and preferably are arranged in line beneath the sizing screen arrangement. In an alternative, the aggregate holding compartments may be provided as separate aggregate compartments within a holding bin. Preferably the aggregate holding bins or compartments are arranged in mutual heat transfer relationship thereby to provide heat transfer therebetween and to minimise heat loss and temperature variation. The reclaim holding compartment may be provided by means of a reclaim holding bin or a reclaim holding compartment of an aggregate holding bin. The reclaim holding compartment is located in heat transfer relationship with at least one-aggregate holding bin, preferably such that it receives optimum heat transfer from the aggregate holding bin or bins. This arrangement permits transfer of heat between the dried and heated aggregate held in the holding system and the reclaimed material. Each holding bin may include one or more outlet gates for the discharge of held material. Preferably the outlet gate of the preferred reclaim holding bin is arranged such that it enhances mixing of the aggregate and reclaimed material. In one form the outlet gate of the reclaim holding bin may be located between the outlet gates of the aggregate holding bins. The apparatus of the invention preferably includes also a separate recycle feed hopper for storing reclaimed asphalt material to be recycled prior to it passing to the reclaim holding bin. The reclaimed material which is stored in the recycle feed hopper may be metered to the reclaim holding bin through a separate recycle belt feeder as desired. The recycle belt feeder preferably is controlled by the same automatic control system as that preferably used for controlling the preferred belt feeding arrangement associated with the aggregate feed hoppers. This permits the proportions of aggregate and reclaimed material to be relatively adjusted as required. The reclaimed material may be transported to the batch tower by means of a transfer belt or elevator. Preferably a conveyor belt is used to transfer the reclaimed material from the recycle hopper to the appropriate holding compartment. The reclaimed material preferably is arranged to enter the reclaim holding compartment or bin approximately, at the centre thereof. The apparatus may include a weighing hopper for receiving aggregate and/or reclaimed asphalt from the batch tower. The proportion of aggregate and reclaimed asphalt discharged from the holding bins into the weighing hopper may be controlled according to the grade of mix required in the final batch. Bitumen may be added to the mix from a suitable bitumen storage tank or other facility. The bitumen preferably is added to the mix in a suitable mixing chamber. Filler materials, such as fine material or powder may also be added to the mixing chamber or elsewhere. An associated filler tank or silo may be provided for this purpose. In a preferred arrangement of the invention fines may be collected by extraction from exhaust of the preferred aggregate dryer and used to provide at least part of the filler. Extraction may be by means of a bag filter for example. According to a further aspect of the invention, a diverter gate may be associated with the reclaim holding compartment or bin. In one form the diverter gate may be arranged to run along a dividing wall between the aggregate holding bins and the reclaim holding bin. Preferably the diverter gate runs the full length along the dividing wall. In one form the diverter gate may be actuated to draw at least a proportion of aggregate into the reclaim holding bin. One advantage of allowing aggregate to enter the reclaim holding bin is that it permits preblending and heat transfer between the reclaimed and aggregate materials as the blend travels towards the discharge section of the reclaim holding bin. The degree of blend and heat transfer may be controlled to some extent by adjusting the speed of the belt conveyor supplying the reclaimed material and/or the position of the diverter gate. In one form the diverter gate may be located on the underside of the sizing screen. Preferably the diverter gate is positioned across the full length of the sizing screen. The diverter gate may be adapted to draw aggregate into the reclaim holding bin preferably in the proportions in which they are fed to the screen by the hot elevator. The quantity of aggregate material which is diverted into the reclaim holding bin preferably is a function of the position of the diverter gate. In one form the diverter gate may be arranged to divert for example from 0 to 30% or more of the incoming aggregate into the reclaim holding bin and the amount will depend upon the position of the gate. The position of the diverter gate may be set by adjusting a manual stop. The diverter gate preferably is actuated pneumatically from a control station. The proportion of aggregate which enters the reclaim holding bin thus may be adjusted by selecting the corresponding gate stop position. The proportion of reclaimed material which enters the reclaim holding bin preferably is selected by the belt feeder speed and discharge gate opening of the recycle feed hopper. Typically in a recycling process, the aggregate material feeders are driven in the correct proportions to the reclaimed material feeder so that the blend in the reclaim holding bin is as designed to suit parameters of the mix and the materials. The belt feeders preferably operate on a pre-calibrated drive volumetric basis which does not fluctuate with moisture. A load cell preferably is provided on a recycle transfer belt to indicate presence of reclaimed material. This may provide an operator with information as to when the diverter gate should be operated to commence the simultaneous flow of reclaimed and aggregate materials into the reclaim holding compartment. When recycling is not required a diverter chute may be provided at the end of the sizing screen to permit material passing over the top deck to be diverted into the reclaim holding bin so that for coarse mixes using all new material the reclaim bin can be used as the largest stone bin. The process and apparatus described above provides versatility in the proportion of reclaimed material able to be fed through the asphalt making system. The process allows time for heat transfer between reclaimed material and raw aggregate and may be readily set and adjusted in accordance with the appropriate mix design. Because the reclaimed material is introduced into the asphalt making process subsequent to grading taking place, clogging of the grading screen with tacky reclaimed material is effectively avoided. The temperature of the blend is permitted to stabilize and become uniform due to progressive heat transfer or "heat soaking" which occurs at the holding stage and to a further extent at the preblending and mixing stages. As a result the temperature of the final mix is such as to avoid undesirable degradation of the bitumen in the final mixing stage. Because the temperature in the final mix stabilizes progressively in the process it is possible to run the dryer at a lower temperature and obtain acceptable heat transfer between the aggregate and reclaimed materials. This results in significantly lower energy consumption as well as reduced maintenance costs. A preferred form of asphalt recycling system according to the invention will now be described in greater detail by way of example only with reference to the accompanying drawing which is a schematic representation of the system. Referring to the drawing, there are shown six cold feed hoppers 10 for storing virgin aggregate material. The aggregate material is metered accurately by six corresponding belt feeders 11 having variable speed drive adjustable by means of an automatic feedback control system (not shown). A transfer belt 12 transfers the aggregate material to a dryer 13. The dried and hot aggregate material is transported to a sizing screen 15 of a oatch tower by means of a hot elevator 14. The batch tower includes four holding bins 16 for holding graded aggregate after the latter passes through the sizing screen 15. A fifth holding bin 18 is provided alongside holding bin 16 for accepting reclaimed material. The reclaimed material is supplied from a separate feed hopper 19 and is metered to the system via a belt feeder 20. Recycle feeder 20 is controlled automatically in correct proportion to the aggregate feeders. The reclaimed material is elevated to the fifth holding bin 18 by means of a conveyor belt 21. A diverter gate 22 is provided along a dividing wall 23 between holding bins 16 and 18. The diverter gate 22 is selectively positioned by means of a pneumatic control system (not shown) to divert from 0 to 30% or more of incoming aggregate into holding bin 18 as may be required. When not recycling, the sizing screen 15 may be fitted with a diverter chute (not shown) to divert coarse material into holding bin 18. Holding bin 18 then acts as the largest stone bin. Holding bins 16 and 18 are provided with a plurality of outlet gates shown generally at 24. Material is discharged via the outlet gates to a weighing hopper 25. Finally, bitumen, and filler if used, is added to the mix prior to it being blended and discharged for transport.

Claims: CLAIMS: 1. A process for producing asphalt mixes in which at least a portion of the starting material is reclaimed asphalt, the process comprising the steps of: providing at least one grade of aggregate material; transferring a metered quantity of the aggregate to a dryer for drying and heating the aggregate; screening the dried and heated aggregate into separate holding compartment according to grade; providing a supply of reclaimed asphalt of suitable consistency; transferring the reclaimed asphalt to a reclaim holding compartment located in heat transfer relationship with at least one of the aggregate holding compartments whilst being physically separated therefrom, to permit heat transfer to take place between the dried and heated aggregate and the reclaimed asphalt; ; discharging the aggregate and the reclaimed asphalt from the holding compartments to effect at least partial blending of the aggregate and the reclaimed asphalt; and adding bitumen to the at least partially blended aggregate and reclaimed asphalt. 2. A process according to claim 1, including the further step of finally mixing the aggregate, reclaimed asphalt and bitumen. 3. A process according to claim 1 or claim 2, including the step of diverting a portion of the screened aggregate to the recycle holding compartment to effect a degree of preblending and heat transfer. 4. Apparatus for use in the production of asphalt mixes wherein at least a portion of the starting material is reclaimed asphalt, the apparatus comprising a batch tower including a plurality of aggregate holding compartments for holding preheated aggregate and at least one reclaim holding compartment for holding reclaimed asphalt, the reclaim holding compartment being in heat-exchange relationship with at least one of the aggregate holding compartments so that, in use, heat may pass from the preheated aggregate to heat the reclaimed asphalt. 5. Apparatus according to claim 4, wherein the batch tower also includes screening means to separate pre-heated aggregate into selected aggregate holding compartments according to the size of the aggregate. 6. Apparatus according to claim 5, wherein the screening means comprises a sizing screen located above the aggregate holding compartments. 7. Apparatus according to any one of claims 4 to 6, wherein each holding compartment includes at least one outlet gate for discharging held material. 8. Apparatus according to any one of claims 4 to 7, wherein the reclaim holding compartment incorporates a diverter gate actuable to draw at least a portion of aggregate into the reclaim holding compartment. 9. Apparatus according to any one of claims 4 to 8, which also includes a weighing hopper for receiving aggregate and reclaimed asphalt from the holding compartments. 10. Apparatus according to any one of claims 4 to 8 or claim 9 including a mixing chamber for receiving aggregate and reclaimed asphalt from the holding compartments or from the weighing hopper, respectively. 11. Apparatus according to any one of claims 4 to 10, further including means for adjusting the supply of reclaimed asphalt as a proportion of the aggregate material.

Inventors: LEONARD, DAVID JOHN, MANNES, JOHN PETER WINFIELD, WENBAN, BERTRAM KEITH
Applicants: MOBIL OIL CORPORATION
Application year: 1984
Main classification: E01C19
IPC codes: E01C19, C08L95