Classification is the separation of particle collectives into two products of different particle sizes. Depending on the active principle, it can be divided into SCREEN CLASSIFICATION and FLOW CLASSIFICATION.
During screen classification, particles with different sizes are separated according to their geometrical particle form. In contrast, the particles flow in a fluid during flow classification. Here, different forces for separation are utilized, such as field, flow, and inertia forces.
Both screen and flow classification can be further divided into wet and dry classification. If necessary, the classification units integrated into the grinding circuits can be used as individual units separated from the grinding circuit for making a product. The following separating types or elements are available:
- Vibrating screens for wet and dry screening
- Dry flow classification with air classifiers and aerocyclones
- Wet flow classification with hydrocyclones of different diameters
Screening means that the product is separated by the opening and shape of the screen base. Typically, the screen’s mesh width is indicated to characterize the screen base.
Depending on the application, particle size to be separated, and flow rate, different types of screens are used. These can be divided roughly into the following categories:
- Sieve screens (vibrating screens)
- Flow screens
- Roller screens
- Sieve grates
At CEMTEC’s pilot plant, two vibrating screens with diverse setting options are available for individual sample preparation or semi-industrial tests.
Vibrating screen type IFE:
- Screen area: 1.2 m²
- Flow rate: 150 kg/h–800 kg/h
- Mesh width: 250 µm – 8.0 mm
Air Classifiers, Aerocyclones
Flow classification for dry applications, also called aero-classification or air classification, is based on a dynamic physical effect. In case of the latest generation of air classifiers, this principle is centrifugal acceleration. Depending on the desired product fineness and flow rate, one of the three different air classifiers is preselected. By adapting 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 of the two air classifiers used can be regulated or controlled variably to identify the ideal operating point. In the circuit, they are connected to the ball mill „DORIS“, but they can also be operated autonomously. Depending on the product, the maximum attainable fineness is up to D98 = 1.5 µm (in the case of calcium carbonate, for example).
Another conventional separating element, an aerocyclone, is also available. it is primarily used for dedusting products of vertical mill „STEFANIA“.
Just like air classifiers and aerocyclones, hydrocyclones work with centrifugal acceleration, as well. The suspension with the corresponding solids content is tangentially pumped into the cyclone and forced into a circular path by geometry. The resulting centrifugal force and liquid resistance of the particles in the water causes a separation of the process material suspension into
- fine particle suspension
- coarse particle suspension
The hydrocyclones integrated into the two wet grinding plants are also used in an open circuit to find or produce a given product.
As the fineness of the product is essentially determined by the cyclone’s diameter, cyclones with diameters between ½“ and 6″ are available at CEMTEC.