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Views: 0 Author: Site Editor Publish Time: 2021-11-19 Origin: Site
What do carbon black, spices, pigments, tungsten carbide and calcium carbonate have in common? Their production represents 90% of the comminution methods used today to produce powdered materials: air classification, needle, hammer, jet and ball mills.
As products and technologies in today's marketplace become more advanced, the need for application-specific materials with special properties becomes even more important. Due to the wide range of requirements, it is unrealistic to assume that one milling technique can meet all needs. We will describe the air grading approach.
Many process engineers know the material properties they want, but are not familiar with the specific size reduction techniques they may need. Therefore, feed and product specifications need to be determined first.
l How is this determined?
l High Quality Carbon Black Air Classified Mills
l How ACM works?
l ACM Configuration
The most important aspect of selecting a milling technique is knowing some basic information about the product to be processed: feed size material characteristics, desired particle size, and desired size distribution. Depending on the product characteristics, only certain milling techniques may be suitable for a given application. Projects that begin to understand these basic requirements will result in economical and trouble-free milling system solutions.
1. Feed Size
Larger feeds mean longer residence times in the mill and for a given capacity will require larger equipment.
2. Material Properties
It is important to understand the characteristics of the feed material.
Sticky and viscous products can accumulate inside the mill, disrupting the airflow and increasing the pressure drop across the mill.
Low melting point materials are sensitive to high energy input. Heat sensitive products may require cooling to process them efficiently.
Moisture absorbing materials absorb moisture causing buildup and clogging and may require dehumidified air for processing.
Products containing moisture may allow for high energy processing; moisture in the product absorbs heat from grinding and provides cooling during processing.
Material hardness may result in high part wear and increased maintenance.
When processing friable materials, high energy input can result in excessive fines being produced.
Impact forces tend to produce sharp, angular particles.
Compression and shear forces tend to produce more rounded particles.
Certain size reduction techniques can damage materials with high aspect ratios.
Hazardous, toxic or potentially explosive materials also require special system design considerations.
3. Size Reduction Techniques
The four basic techniques used to reduce the size of dry powders are impact, shear, friction and compression. In some cases, a combination of these can be found in a single mill type. Impact and grind comminution methods include air classification mills, pin mills, hammer mills and airflow mills. Shear, impact and compression methods are used in media or ball mills.
For carbon black applications requiring a clean, low particle size product, the Air Classifying Mill (ACM) is the best choice. ACM has a proven track record of successfully replacing other grinding technologies and achieving particle sizes as low as 1 to 2 parts per million (ppm) at -325 mesh or 10 ppm at -500 mesh for many grades of carbon black. ACM rotors are equipped with special plate hammers and hybrid multi-deflection liners.
ACM uses two mechanical stress mechanisms to reduce the particle size. The primary stress mechanism is the mechanical impact caused by direct contact between the hammer/mixer blades and the fluidized powder. The secondary force is particle-to-particle wear caused by vortex currents generated by the plate hammer near the multi-deflector liner through the tip. The combined pressure mechanism causes approximately 80% impact and 20% loss.
The main principle of this mill type is to use air to convey the material to the mill, a rotating disc with hammers to reduce the material, air to separate the product into fine and coarse particles, and air to convey the fine particles out of the mill. The coarse material is returned to the grinding chamber for further size reduction. The process is repeated on a continuous basis.
ACM is a generic term for many types of impact mills that also incorporate dynamic classification technology as part of the mill design. ACMs are the most widely used grinding technology in the powder processing industry today for general size reduction of fine chemicals and food products. The basic advantage of these mills is that they grind and classify in one step.
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