Dust extraction of aggregate is the process to remove dust after drying cold aggregate. A certain amount of dust is carried away in the hot gases from the dryer and it is usual for this to be collected in a dust collector.
The type of extractor normally used consists of a set of high energy cyclones and a low energy water scrubbing system. In isolated areas the scrubber may be eliminated.
The fan in the dust extractor creates suction from the dryer, hot bins etc. and delivers the dust to the cyclone type collector, where it is precipitated by centrifugal force. If the dust is found to be suitably graded, it can be returned to the plant for inclusion in the mix
Aggregate from the dryer is fed to screens mounted over the hot bins. The screens separate the aggregate into selected sizes for proportioning, and reject oversize particles. Vibrating screens are preferred. This screening and recombining of aggregate is carried out to correct segregation resulting from passing the combined aggregate through the dryer.
It will also help to correct variations in relative proportions of sand and coarse aggregate. However, as the smallest hot screen size is approximately 4 mm, hot screening will not correct any grading variations in material below this size.
For efficient screening the screens should be:-
• Of Adequate effective area
• Of the correct size opening
• Clean and in good condition
• Property arranged and at the correct slope
• Vibrating in the correct manner.
Hot storage Bins and Screens
For a general purpose plant there should be at least four bins to hold the hot aggregates, but the number in use will depend on the type and size of mix being produced. Bin capacity should be adequate for continuous operation at maximum output. Each bin should be fitted with a contents indicator in full view of the operator, and with an overflow outlet to ensure that should a bin become overfull, the excess will not spill into an adjoining bin. A thermometer should be installed in each hot bin. The hot bins function as small reservoirs to dampen variations in supply rate and to improve batching.
Dust extraction is one step of the asphalt batch and continuous plants. It is recommended to see that post.