Rotary Drum Filter

The most common type of continuous vacuum filter is the rotary drum filter in which liquor is sucked through a moving septum to deposit a cake of solids. The cake is moved out of the filtering zone, washed, sucked dry and dislodged from the septum.

Construction:

It is consisting on a sheet metal drum. The diameter of drum ranges from 1 to 10 feet. This drum is carried on moving system. A vacuum system is also attached, so that, during filtration whole drum work under vacuum. The surface of the drum is the divided into different section by stripes.

i- Cake formation zoneii- Washing zoneiii- Drying zoneiv- Cake removal zone

These steps are parallel to the axis of drum and welded to the shell. Filter medium is hold by metal strips on drum and covered by filter cloth.

In each section there are many pipes which open inside the cover plates. The side cover plate. There is also a stationary valve plate through which connection are made to the filtrate receiver. For compressed air and wash water connection vale are also given.

After washing, to remove the cake “Doctor knife” is provided. Doctor knife remove only the cake of from septum and does not damage the pre-coated layer of filter aid.

Working:

Before filtration rotary drum filter is dip in tank of filter aid for pre-coating purpose. Commonly is pre-coated up 3” to 5” by diatomaceous earth, perlite or purified wood cellulose.

After that drum is rotated in slurry tank at a speed of 1 to 2 rpm. 1/3 of drum is dip in slurry tank, vacuum is applied through a valve and filtration is started.

A layer solid builds in the form of cake on the surface of panel (filter cloth) in cake formation zone and filtrate pass through pipes and

goes to vacuum receiver and collected in tank. As, the panel leave the cake formation zone it enters to washing zone and in which water is sprayed on cake to extract all filtrate from cake and it sent to wash vacuum receiver. And panel enters to drying zone. In this section high vacuum is applied and all filtrate is sucked and cake is obtained in completely dried form.

Now this panel is moved to cake removal zone in which doctor knife remove dried cake from filter cloth without damaging the filter cloth. After that this panel again enter to cake formation zone for continuous filtration and cycle is repeated.

Applications:

These filter are developed to overcome large problem of filter press and leaf filter. It is very versatile filter.

It is used generally for those operation in which the volume is very large, where process is continuous and where is labor cost must be kept downs. It is used where colloidal particles.

It is used for production of wallboard from gypsum.

Limitations:

It cannot be used for mixture of colloidal and coarse particles.

It must be used continuously.

Filter Auxiliaries:

For rotary drum filter the auxiliaries are shown in figure as,

a- Rotary drum filterb- Vacuum

arrangementc- Filtrate Pumpd- Filtrate receivere- Filtrate separate

Cake Discharge Methods:

The five basic cake discharge types are given bellow,

i- Scraper dischargeii- Belt dischargeiii- Pre-coat discharge iv- Roll dischargev- String discharge

Each is designed to be able to discharge specific types of formed cake solids.

Scraper Discharge: In this method scraper is used to remove cake from septum (filter cloth). For this purpose, scraper having sharp edges is provided in cake removal zone. But filter cloth

selection must provide good wear and air blow back to release cake. Scraper has high resistance against solid blinding. It’s used for wide and hard layer of cake. Due to scraper discharge filtration rate increase. It has simple design.

Belt Discharge: In this method of removing cake endless belt is used made by tensionless material is used. For this belt tracking, cake release and media cleaning are of major importance for efficient operation. The first task is to ensure that a septum (filter cloth) is selected which provides a smooth surface for good release, good wear resistance well dimensional stability.

Cake is removed by high pressure water which is spray on moving belt. The speed of belt is adjusted by the nature of cake and process to maintain the track. Dimensional stability is very important for belt discharge. Best for slurries with moderate solid content. It is most complicated design for cake discharge. In this design cloth continuously cleaned.

Precoat Discharge: This method is designed for those filtration operation in which filter aid is added to slurry. Mean that it is well suited for those slurries which having very low concentration

of solids and difficult to filter. In this design knife can damage the septum so, in this design septum is disposable and renewable. Typical filter medium is diatomaceous earth or perlite. To remove the cake advanced knife is used.

Roll Discharge: Actually it is modified form of scraper discharge. In this method a scraper is adjusted on revolving roll and remove cake from septum. Roll speed must be match to rpm of

drum. This design is used for sticky and mud like cake and where slurry having high content of solids. It having low filtration rate.

String Discharge: In this method a string having high strength is used to discharge cake. But besides the usual media, it requires well maintained track and good alignment. It is used for

slurries having high content of solids. Solids must be fibrous, stringy or pulpy material. In this design of filter cloth is critical.