PRINCIPLES OF OPERATION
The flow of material through both batch and continuous plants is similar up to the point of measuring and mixing. The difference in the types is that in the batch plant, aggregate proportioning is by weight whilst in the continuous plant, aggregate proportioning is by calibrated gate openings which allow a constant flow of hot aggregate and filler into the pugmill mixer.
The essential features of these plants are:-
• Cold aggregate bins fitted with feeders to meter the cold aggregate onto a belt conveyor.
• A dryer which is a rotating steel drum where flames and hot air are forced through the cold aggregate.
• Screens to separate the hot aggregate which is then remixed in the correct proportions.
• Bins to hold hot aggregate and filler.
• Binder storage tanks
• Weight batching or continuous feeding mechanism.
• A pugmill mixer.
• Dust collectors.
• A manual, semi-automatic or automatic control system.
Surge bins may be provided to overcome the disruption due to dump truck delays whilst hot storage silos for the storage of hot mixed material are often provided at batch and continuous plants.
COLD AGGREGATE STORAGE
Aggregate is fed into the plant from one of the following sources:-
(a) Small open-topped bins fed by a front end loader.
(b) Large bins fed by trucks either directly or via a conveyor transfer system.
(c) Large open stockpiles (separated by bulk heads) and situated over a conveyor tunnel. Materials are stockpiled over the tunnel by belt conveyor, truck etc.
The number of bins necessary will depends on the type of mix being produced, and may vary from one, (for a patching premix plant), to four or more (for a plant producing dense graded mixes).
When charging the cold bins, care is required to minimize aggregate segregation and degradation. Enough materials must be maintained in all bins to provide a positive and uniform flow.
When a front-end loader is used to feed the cold bins, the operator should not pick up material from the storage stockpile at ground level. The bucket should be held high enough above the ground to prevent contamination when filling.
If trucks are used to charge the bin, they should deposit their loads directly above the feeder.
COLD AGGREGATE FEEDING
Each size of aggregate must be metered out by the cold feeders in correct proportions and at the correct rate otherwise variation in the final mix, or overflow of the hot bins, may occur.
The feeder units are located beneath the storage bins or stockpiles, and have controls that can be set to produce a uniform flow of aggregate to the cold elevator.
The four types of feeders in common use are:-
• Tilting Chute. These are definitely not suitable for sand and are not particularly good for aggregates as they do not provide a uniform flow. The rate of flow is very much a matter of trail and error as they cannot be calibrated accurately.
• Continuous Belt. These are the most popular for handling sands. They generally have an infinitely variable speed control which is easily adjustable and calibrated. Continuous Belt Feeder
• Reciprocating Plate. These Deliver a uniform flow and are capable of handling most materials. However, flow rates cannot be varied as readily as with the continuous belt and conveyor types.
Reciprocating Plate Feeder
• Electromagnetic Vibrating Chutes. They are generally used for feeding coarse aggregates.
Electromagnetic Vibratory Feeder
Close control over the feeding of cold aggregates into the dryer is necessary in order to avoid variations in grading; temperature and moisture content of dried aggregate; to carryover on screens.
To ensure that the correct quantity of each aggregate is fed to the dryer, the cold bins should be supplied with materials if uniform grading and moisture content, and the levels of the materials in the bins should be kept constant. The feeder gates should be accurately calibrated, securely held in the set position, and kept free from obstructions.