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Classification

Classification is the separation of particle collectives into two products of different particle sizes. Depending on the active principle, these can be divided into two categories

• Screen classification
• Flow classification

In screen classification, the different particle sizes are separated according to the geometrical particle form. In contrast to this, the particles flow in a fluid during flow classification. Here different forces for separation such as field forces, flow forces and inertia forces are utilized.

A distinction can be made between wet and dry classification in both screen classification and flow classification. The classification units integrated in the grinding circuits can be used as an individual unit, if necessary, separated from the grinding circuit for the production of a product. The following separating types or elements are available for this:

• Vibrating screens
for wet and dry screening

• Dry flow classification
with air classifiers and aero-cyclones

• Wet flow classification
with hydro-cyclones of different diameter

 
Screening

During screening, the product is separated by the opening and shape of the screen base. The mesh width of the screen is typically indicated for characterization of the screen base.

Depending on the application, particle size to be separated and throughput, different types of screen are used. These can be divided roughly into the following categories
• Sieve screens (vibrating screens)
• Flow screens
• Roller screens
• Sieve grates

Two vibrating screens with diverse setting options are available in the CEMTEC pilot plant for individual sample preparation or for semi-industrial tests.

 

Vibrating screen type IFE

The essential parameters are as follows:

Vibrating screen type IFE
Screen area: 1.2 m²
Throughput: 150 kg/h – 800 kg/h
Mesh widths: 250 µm – 8.0 mm

Vibrating screen type Fluid-System for wet screening
Screen area: 2.0 m²
Throughput: 50 kg/h – 200 kg/h
Mesh widths: 180 µm – 1000 µm

 

Air classifiers, aero-cyclones

Flow classification in the dry area, also designated as aero-classification or air classification, is based on a dynamic physical effect; in the case of the latest generation of air classifiers on the principle of centrifugal acceleration. Depending on the product fineness and throughput rate aimed for, one of the three different air classifiers is preselected. By adapting the speed, air volume and angle of incidence of the guide vane ring, the basis of the fine adjustment for efficient production of the product and design of the separating device is determined.

All main process parameters can be regulated or controlled variably for optimum operating point identification in the case of the two air classifiers in use. These are connected to the ball mill "DORIS" in the circuit, but can also be operated autonomously. Depending on the product, the maximum attainable fineness is up to D50 = 1.5 µm (example of calcium carbonate).

An aero-cyclone is also available as a further conventional separating element. This is primarily used for removing dust from products of the vertical mill "STEFANIA".

Air classifier
Aero-cyclone

 

Hydro-cyclones

Hydro-cyclones are based on centrifugal acceleration in exactly the same way as air classifiers and aero-cyclones. The suspension with corresponding solid content is pumped tangentially to the cyclone and forced into a circular path by the geometry. The resultant centrifugal force and liquid resistance of the particles in the water causes a separation of the process material suspension into
• fine particle suspension and
• coarse particle suspension

As the fineness of the product is essentially determined by the cyclone diameter, cyclones with diameters of ½" to 6" are available at CEMTEC.

 

2" hydro-cyclone in circuit with ball mill
2" hydro-cyclone; limestone suspension after APEX nozzle