- Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid, 0.1–10.0 cm/sec; medium, 0.1–10.0 cm/min; slow, 0.1–10.0 cm/hr.
The selection of a filtration method depends partly on which phase is the valuable one. For liquid phase being the valuable one, filter presses, sand filters, and pressure filters are suitable. If the solid phase is desired, vacuum rotary vacuum filters are
desirable.
- Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min.
- Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges.
- Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges.
- Slow filtering slurries are handled in pressure filters or sedimenting centrifuges.
- Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters.
Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters, when cake washing is critical, when cake drying may be a problem, or
when precoating may be needed.
- For finely ground ores and minerals, rotary drum filtration rates may be 1500 lb/(day)(sqft), at 20 rev/hr and 18–25 in. Hg vacuum.
- Coarse solids and crystals may be filtered by rotary drum filters at rates of 6000 lb/(day)(sqft) at 20 rev/hr, 2–6 in. Hg vacuum.
Cartridge filters are used as final units to clarify a low solid concentration stream. For slurries where excellent cake washing is required, horizontal filters are used. Rotary disk filters are for separations where efficient cake washing is not essential. Rotary drum filters are used in many liquid- solid separations and precoat units capable of producing
clear effluent streams. In applications where flexibility of design and operation are required, plate-and-frame filters are used.
