The blending and homogenising of large quantities, say 10 to 500 Tonnes, of bulk materials require all regions of the material to be mixed with all other regions. It is generally impractical to undertake this process by ‘working’ the total mass, but the result can be achieved by continuously mixing and recirculating the bulk in two streams utilising the loading, storage and discharge facilities that are used to hold the product.
A combination of simple techniques accomplishes this by the efficient and economical homogenisation or mixing of large batches of bulk materials utilising conventional storage containers. The process exploits the loading and discharge cycles of the system and recirculating the contents, during which time the mixing takes place. The essential components are: -
-Two Mass Flow storage hoppers that will each hold 40% 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 with pre-set values.
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 process.
When loading the hoppers, the initial 25% of the material is directed to one hopper, then, the transfer screw is reversed to take the next 25% of the batch into the other hopper. Discharge of both hoppers then commences and re-cycled with the continuing feed, equally into the two hoppers. This re-cycling uniformly distributes the two initial sections with a further 25% of the batch quantity. The two hopper contents are discharged together, with new in-feed added to the outgoing material. The two hopper quantities are again re-mixed in this combined discharge and the remaining 25% of the batch quantity is uniformly distributed amongst the bulk. Local mixing takes place at the transfer points and via the diffusion devices in the hoppers.
Key Design requirements.
The simplicity of the concept and of the production operation should not detract from the importance of sensitive design detail that is essential to secure efficient performance. Fundamental to the system is the requirement to guarantee mass flow in the hoppers, by basing their design on measured properties of the bulk material to be stored. 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 sequence of operations. The transfer point from the elevator to the reversing screw must be capable of holding the discharge received during the time taken to reverse the conveyor and the conveyor must be capable of clearing this backlog after reversing. The ‘diffusion devices’ fitted at the inlet to the hoppers spreads the incoming material across the hopper cross section, to diffuse the supply and counter any tendency for the product to segregate.
Efficient Storage Hopper Blending of Large Batches.
Efficient Storage Hopper Blending of Large Batches.
by Lyn Bates339 lyn bates.html
The blending and homogenising of large quantities, say 10 to 500 Tonnes, of bulk materials require all regions of the material to be mixed with all other regions. It is generally impractical to undertake this process by ‘working’ the total mass, but the result can be achieved by continuously mixing and recirculating the bulk in two streams utilising the loading, storage and discharge facilities that are used to hold the product.
A combination of simple techniques accomplishes this by the efficient and economical homogenisation or mixing of large batches of bulk materials utilising conventional storage containers. The process exploits the loading and discharge cycles of the system and recirculating the contents, during which time the mixing takes place. The essential components are: -
-Two Mass Flow storage hoppers that will each hold 40% 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 with pre-set values.
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 process.
When loading the hoppers, the initial 25% of the material is directed to one hopper, then, the transfer screw is reversed to take the next 25% of the batch into the other hopper. Discharge of both hoppers then commences and re-cycled with the continuing feed, equally into the two hoppers. This re-cycling uniformly distributes the two initial sections with a further 25% of the batch quantity. The two hopper contents are discharged together, with new in-feed added to the outgoing material. The two hopper quantities are again re-mixed in this combined discharge and the remaining 25% of the batch quantity is uniformly distributed amongst the bulk. Local mixing takes place at the transfer points and via the diffusion devices in the hoppers.
Key Design requirements.
The simplicity of the concept and of the production operation should not detract from the importance of sensitive design detail that is essential to secure efficient performance. Fundamental to the system is the requirement to guarantee mass flow in the hoppers, by basing their design on measured properties of the bulk material to be stored. 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 sequence of operations. The transfer point from the elevator to the reversing screw must be capable of holding the discharge received during the time taken to reverse the conveyor and the conveyor must be capable of clearing this backlog after reversing. The ‘diffusion devices’ fitted at the inlet to the hoppers spreads the incoming material across the hopper cross section, to diffuse the supply and counter any tendency for the product to segregate.
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