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INCORPORATION OF RECYCLED ASPHALT PRODUCTS IN A BATCH MIX PLANTRecycled asphalt products (RAP) can be incorporated into hot mix asphalt produced in a batching plant up to a rate of approximately 20%. Aggregates are superheated to over 200ºC in the dryer drum to provide the energy required to heat the RAP upon addition.
Our recommendation is to incorporate the RAP at the pugmill mixer and we offer two methods of controlling addition, either by static weigh pod or continuous belt weighing. Whichever method is chosen it is imperative that the system provide sufficient exhaust ducting as the addition of cold wet RAP to superheated aggregate produces an explosion of steam that needs to be evacuated from the pugmill.
Both systems incorporate the feed bin specifically designed to handle materials with poor flow characteristics:
RAP Bin and Feeder
The 12 ton capacity recycle bin has bin top dimensions of 3.66m charging width x 2.4m deep x 4.7m charging height Having a vertical discharge end bin wall with the opposing wall at 15° from vertical and two sloping side walls at 70° which form a self-relieving discharge opening onto the belt feeder the bin is designed to promote clean discharge of the RAP material. Bin spill plates extend the sides of the RAP bin and enclose three sides of the optional inclined grizzly therefore preventing spillage when loading.
The bolt-on type belt feeder has 4.3m drum centres x 800mm belt width and features include a vulcanised endless belt, lagged head drum, spiral tail drum, screw tensioning at the tailend and 100mm diameter intermediate rollers.
Direct drive to head shaft is by a 7.5 kW thermistor protected bevel helical gearbox which provides high torque starting and high efficiency operation.
The outlet of the bin is fitted with an adjustable sliding gate to control the depth of material on to the belt.
From the RAP bin material is conveyed to the batch tower via an inclined conveyor. Various material weighing options are available depending upon customer preference and site layout.
Weighing Conveyor
When a weighing conveyor is chosen the inclined conveyor discharges to a variable speed belt conveyor. The weigh conveyor is suspended on three loadcells within a rigid frame for accurate material weighing. Driven by a bevel helical gear motor under inverter control, the conveyor is driven at slow speed until the desired weight is of material is reached where upon the belt is stopped. The conveyor is started at high speed when the mix calls for RAP which is discharged to the pugmill via the mixer feed chute.
When required the weigh conveyor can be reversed to allow the feed bin and inclined conveyor to be emptied.
In Line Weighing
When an inline weighing system is chosen a double weighbridge with four rubber mounted pivots and central load cell for accurate material weighing is incorporated into the inclined conveyor, which is also fitted with an encoder for accurate belt speed measurement. When material is required the conveyor is started and run until the specified quantity of RAP is supplied to the mix.
Static Weighing
When a static weighing system is chosen the inclined conveyor discharges into a weigh hopper which is charged with the required quantity of material between batches. The weigh hopper is totally enclosed with rubber dust seal, fixed top plate and rubber dust seal to the receiving hopper. The rounded hopper has two vertical walls and two walls at 60° to promote complete discharge and is suspended from three load cells. Double clam shell discharge gates actuated by two pneumatic cylinders ensure rapid discharge to the receiving hopper. From the receiving hopper the RAP is conveyed to the mixer feed chute via a charging conveyor.