proportioning materials of asphalt batch and continuous plants, there is little difference in principle between batch and continuous plants.
However beyond this point the two types of plant differ considerably and therefore each will be discussed separately.
(a) Batch Plants – Weigh Batching
The fine and coarse aggregates are dropped from the hot storage bins through gates, which are either manually or mechanically operated , onto the weigh batch hopper. The gate of each aggregate bin is closed when the correct weight of material has been discharged into the weigh batch hopper.
The filler should be weighed separately because of the relatively small amounts involved. Weighing may be done manually, but modern plants provide for automatic
The weigh batch hopper empties directly into the mixing chamber or pugmill.
Bitumen may be weighed in a special bucket or it may be measured by a meter for each batch. With either system the bitumen should be distributed uniformly over the full
length of the pugmill.
(b) Continuous Plants – Proportioning Devices
Accurate proportioning materials of the mix components is achieved volumetrically by means of calibrated gates on the bins for aggregate and filler and a metering pump for the binder. The binder is sprayed onto the stream of aggregate as it enters the pugmill.
A positive interlock between the controls of the binder, aggregate and filler feeders is essential for a uniform mix. The positive interlock ensures that aggregate, filler and binder are supplied in constant proportions even if the total output varies.
In both the batch and the continuous plant the mixing unit consists of a pugmill (paddle )The function of the pugmill is to mix the separate constituents, (coarse and fine
aggregate, filler and binder) to achieve complete coating of all particles and homogeneity throughout the mix. This should be achieved as quickly as possible, and hence the mixing unit should be of adequate capacity and mixing efficiency.
The main parts of the pugmill are paddles, paddle tips, liner, shafts, discharge gate and heated jacket.
(a) Pugmills for Batching Plants
Pugmills may be of the single or double shaft type but double shaft pugmills, in which the blades or paddles rotate in opposite directions, are more efficient and will be the only ones followed.
The capacity of the pugmill should be such that the maximum batch does not fill the chamber to a greater depth than the maximum height of the blades. The zone between the maximum height of the blades and the bottom of the pugmill is termed the live zone.
In order to mix efficiently, a pugmill should be neither over-filled nor under-filled.
The mix is discharged from the pugmill by opening a rapid operating, sliding gate in the bottom of the pugmill, This allows the material to fall into a truck (or alternatively into a skip for delivery to a surge bin or hot storage silo). The discharge gate must have a positive action, opening quickly to allow the mixed batch to drop into the truck as a mass. If the batch is discharged as a flow, there is a risk of segregation.
(b) Pugmills for Continuous Plants
The pugmill in a continuous flow plant is similar to that in a batch plant except that it is usually longer and narrower. In order to move the mix forward, most of the paddles are
turned in one direction, except the last few which are usually reversed to prevent the mix from being discharged too rapidly.
The mix is discharged over a level-control gate at one end of the pugmill. Mixing time is controlled by this gate which, when raised or lowered, increases or decreases the amount
of material in the pugmill and consequently the time taken for the mix to pass through. You can see more details of proportioning of materials in batch and continuous plants .