Disc filters are used as save-alls to recuperate costly fibers and fillers, which would otherwise be discarded. In addition, it also contributes towards cleaner effluent discharges. Prior to the introduction of disc filters, open cylinder deckers, sidehill screens, vacuum drum filters, belt filters and air flotation separation cells were used as save-alls.
Disc filters have many applications; thickening of slow-draining stocks, such as, mechanical pulps are the most popular usage. The right disc filtering system, disc segments and filtering media, is key to the recuperation of the fibers and fillers which translates into savings for the mill.
A disc filter unit consists of a series of discs mounted on a hollow center shaft that is equipped with drainage channels terminating at a valve assembly. Each individual disc is made of pie-shaped segments (disc sectors) covered with filtering media, such as polypropylene, kynar and occasionally but not as effective wire mesh.
The discs rotate in a vat filled to about the midpoint of the center shaft with a mixture of white water and sweetener fiber. The center shaft is rotated by variable speed drive. When the disc sector enters the vat, a mat is formed from the fibers in the vat, first by gravity and then by vacuum. This initial fiber mat, not the filtering media, is the actual filtering medium.
As the sector rotates further into the vat, the mat thickens, and the filtrate becomes clearer as fines and inorganic materials are retained to the fiber mat. A splitter mechanism in the valve assembly separates the clear and cloudy filtrates, which are carried to appropriate filtrate tanks via droplegs.
As the sector emerges from the vat, a valve at the end of the center shaft cuts off vacuum, and the mat is removed by knock-off showers. The mat is released into chutes and then to a conveyor that empties into a stock chest. The knock-off position is sometimes located at the top center position of the disc shaft. Alternatively, the knock-off position may be 150* from emergence of the disc from the vat. The showers clean the filter media, as the disc sector continues to rotate and start another cycle.
For the best operation of your disc filter segments, we suggest that the following points are carefully considered. These points refer to a new or replacement segment.
Polypropylene has excellent strength, low moisture absorption and is very resistant to abrasion. It maintains strength after repeated flexing.
Effect of Acids: Excellent resistance to most acids and alkalis with the exception of cholorsulfonic acid and oxidizing agents
Effect of Alkalis: Excellent resistance to alkalis
Effect of Bleaches: Generally good resistance to bleaches and solvents
Maximum Operating Temp: 180°F 82.22°C
Kynar has good mechanical strength, stiffness and creep resistance. It has a high toughness, also at low temperatures. Excellent chemical and hydrolysis resistance.
Effect of Acids: Superior resistance
Effect of Alkalis: Superior resistance
Effect of Solvents: Superior resistance
Maximum Operating Temp: 300°F 150°C
Storage Recommendations: To avoid material shrinkage, Filter Media MUST BE stored in a cool dry environment, temperatures not to exceed 20°C 70°F. (Do not store filter media near Disc Filter System or high humidity places. Also sunlight should be avoided). Maintenance Recommendations: In order to maximize filter media life, it is highly recommended that the sector units and filter media be cleaned with a high-pressure washing at each machine shutdown.
Storage Recommendations: No special storage recommendations required.
Maintenance Recommendations: In order to maximize filter media and disc sector life it is highly recommended that the disc sector be thoroughly cleaned. Thorough cleaning of the showers and nozzles, headers, center shaft and hud with a high pressure shower. (High Pressure Showers are available from P&P Filtration).
Preventive Maintenance: At every shut down, it is recommended that the VAT is emptied, the shaft kept rotating at a slow stable rpm power spray the sector segments as they rotate. Approximate cleaning time 30 minutes.