The blending and homogenising of large quantities, say of 10 to 500 Te, of bulk materials that requires all regions of the material to be mixed with all other regions, is generally impractical to undertake by ‘working’ the total mass. However, the result can be achieved recirculating the bulk in two streams utilising the loading, storage and discharge facilities that are used to handle the product. This process exploits the loading and discharge equipment of the system to recirculate the contents, during which time the mixing takes place. The essential components are:
- Two Mass Flow storage hoppers that will each hold 50% of the required batch load.
- Two screw feeders that provide a uniform draw from plane flow hopper sections.
- Two ‘diffusion devices’, fitted at the hopper inlets.
- One reversing conveyor to deliver to the hopper inlets.
- One elevator to load the hoppers.
- Variable speed drives on the screw feeders, or variable control of the supply feeder.
The mixing process.
Large scale convection mixing is accomplished by combining feed streams from the supply and the screw feeders on the hopper during the loading and discharge process.
Medium scale convective mixing is effected by progressive extraction from linear storage sections across the width of the hoppers.
Small scale convective mixing is caused by intermingling at transfer points in the process and the diffusion devices at the hopper inlets.
The operational procedure.
When loading the hoppers, the initial third of the batch is directed to one hopper, then, the transfer screw is reversed, to take the next third of the batch into the other hopper. Discharge of both hoppers then commences at a rate similar to the feed and is re-cycled with the continuing feed, equally into the two hoppers in turn. This re-cycling uniformly distributes the two initial sections with a further third of the batch quantity.
The two hopper contents are then discharged together, The two hopper quantities are being re-mixed in this combined discharge. Local mixing during the product handling takes place at the transfer points and via the diffusion devices in the hopper inlets.
Key Design requirements.
The simplicity of the concept and of the production operation should not detract from the importance of sensitive design detail essential to secure efficient performance. Mass flow is fundamental to the system. Similarly, the extraction pattern of the hopper discharge screw feeders is material sensitive and demands experienced attention to design. The feed rates of the discharge screws, or the incoming feed, must be calibrated and changed at the appropriate time in the operating sequence, the transfer point from the elevator to the reversing screw capable of holding the discharge received during the time taken to reverse the conveyor and the conveyor capable of clearing this backlog after reversing. The ‘diffusion devices’ fitted at the inlet to the hoppers diffuses the incoming material across the hopper cross section and counters any tendency for the product to segregate. Contact for details.
