Rotary and Drum Filter

1. Filter leaf
Rotary Drum filter

2. Filter leaf
 A leaf filter consists of an outer frame containing a drain screen or grooved
plate. There is no restriction on the shape of the frame, be it circular, square
or rectangular. Filter cloth covering the entire unit. Filtrate exits through a
suction connection on the inside of the frame.

3. Principle
 A filter leaf is an apparatus consisting of a longitudinal drainage screen
covered with a cloth. The mechanism is surface filtration and acts as a sieve
or strainer. Vacuum or pressure can be applied to increase the rate of
filtration.
 Solid particles are trapped in the precoat, or filter media while clarified
liquid passes through and into a filter leaf. The liquids pass from leaf to the
shaft’s interior, then out of the filter into a holding tank. Between batches,
liquid cake or sludge is drained from a valve.

4. Construction....
 The general arrangement of a filter leaf is shown in Figure 1.1. It consists of a
narrow frame enclosing a drainage screen or grooved plate. The frame may
be of any shape, circular, square, or rectangular. The whole unit is covered
with a filter cloth. The outlet for the filtrate connects to the interior of the
frame through suction.

5. Working & Uses
 The filter leaf is immersed in the slurry. The vacuum system is connected to
the filtrate outlet. The slurry passes through the filter cloth. Finally, filtrate
enters the drainage canal and goes through the outlet into the receiver. Air is
passed to flow in the reverse direction which facilitates the removal of cake.
 Uses: The filter leaf is satisfactory, if the solid content of the slurry is not too
high, about 5%, i.e., dilute suspensions.

6. Advantages..
 Filter leaf is a versatile piece of equipment. Filter leaf is probably the
simplest form of filter used for batch processes.
 A number of units can be connected in parallel to increase the surface area
for filtration.
 The pressure difference can be obtained either with vacuum or using pressure
up to the order of 800 kilopascals.
 Labor costs for operating the filter leaf are fairly moderate.
 The efficiency of washing is high.
 The slurry can be filtered from any vessel. Simply immersing the filter in a
vessel of water can wash the cake.

7. Rotary drum filter.
 A Rotary Vacuum Filter Drum consists of a cylindrical filter membrane that is
partly sub-merged in a slurry to be filtered. The inside of the drum is held
lower than the ambient pressure.

8. Principle....
 The inside of the drum is held lower than the ambient pressure. As the drum
rotates through the slurry, the liquid is sucked through the membrane, leaving
solids to cake on the membrane surface while the drum is submerged. A knife
or blade is positioned to scrape the product from the surface.
 Vacuum draws liquid through the filter medium (cloth) on the drum surface
which retains the solids. Vacuum is applied using a liquid ring vacuum pump
or other means. The vacuum pulls air (or gas) through the cake and continues
to remove moisture as the drum rotates.

9. Construction.

10. Continue...
 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.

11. Working..
 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.

12. Advantages
 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.

13. Disadvantages
 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

14. 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.

15. Thanks.....