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The slurry is filtered on a rotating drum surface by applying a vacuum.
Construction of Rotary Drum Filter
It consists of a hollow horizontal metal drum that is 1 to 20 ft long. The drum rotates at a speed of less than one rpm. The face of the drum is divided into circumferential sectors each forming a separate vacuum cell. The internal piping is used to attach each sector to the center of the drum through a rotating valve. The valve has certain filter adjustable blocks. A valve with a bridge setting controls the sequence of the cycle so that each sector is subjected to vacuum, blow, and a dead zone. This helps in discharging of cake. The slurry is placed into the tank. The agitator is used to maintain the uniformity of the slurry. Filter cloth is attached to the face of the drum by inserting special caulking ropes into the grooved strips. The cloth ends are attached to clippers. The filter cloth is used to retain the cake. The multifilament strings are threaded across the entire cloth width. Sprays are attached to wash the cake.
Working of Rotary Drum Filter
The working of rotary drum filter is divided into various zones such as pick-up zone, drainage zone, washing zone, drying zone, and cake removal zone. The drum is immersed to the required depth in the slurry, which is agitated with help of an agitator to prevent settling of the solids. A drum rotates, vacuum is applied to those sectors of the drum which is submerged. A cake of the desired thickness is produced by adjusting the speed of rotation of the drum. The filtrate is passed out to the receiver. The drum leaves the drainage zone and enters into the water wash zone. The cake is then washed with sprays. The cake is partially dried through a current of air and the cake enters into the drying zone. Finally, pressure is applied under the cloth to aid the removal of the cake. The washed and partially dried cake is removed using a doctor’s knife. The cake is also discharged by string discharge and belt discharge filters. In the string discharge filter, numbers of endless strings are placed over the width of the drum. String discharge filters are used when the cake is sticky. The wear of filter cloth is less in this case. In belt discharge filters short endless belts are used. Higher filtration rates may be achieved using belt discharge. All these steps are completed in one cycle of a drum. Then drum again receive a fresh lot of slurry. When the solids of the slurry are too much that the filter cloth becomes blocked with the particles, a pre-coat filter may be used.
Advantages of Rotary Drum Filter
- The rotary filter is continuous in operation
- This filter is suitable for filter slurries containing a high proportion of solids.
- Labor costs are very low due to automatic operation.
- A rotary filter is suitable for filtration of highly concentrated solutions or thick slurries containing 15 30% of solids.
- Variation of the speed of rotation enables the cake thickness to be controlled.
- Very high capacity.
Disadvantages of Rotary Drum Filter
- Complex design with many moving parts.
- Very expensive because a lot of accessories are connected such as vacuum pumps, vacuum receivers. slurry pumps and agitators.
- The cake tends to crack under vacuum, so washing and drying are not efficient. 4. As a vacuum is applied, it is unsuitable for liquids near boiling point.
- Gelatinous or slimy precipitates forming impenetrable cake will not separate cleanly from cloth
Uses of Rotary Drum Filter
- A rotary filter is used for the separation of the mycelium from the fermentation liquor in the manufacture of antibiotics.
- It is suitable for slurry containing considerable amounts of solids in the range of 15-30%.
- These are used for the collection of calcium carbonate, magnesium carbonate, and starch.
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