This continuous filter is used when the solids in the slurry can be easily suspended in solution and remain there. In this filter a cylindrical drum sets horizontally half submerged in a trough holding the filtration slurry. The drum is coated with a filter media and a vacuum is pulled on the inside of it. The filtrate passes through the filter media leaving the solids on the outside of the drum. The drum is rotated continuously allowing the solids to be washed and dried before they are removed by a knife blade (doctor blade) as the drum moves past it. The cleaned filter media then is rotated through the trough to pick up more solids. An illustration of a rotary drum filter is shown in Figure 12. This machine to the bottom feed group of filtering devices and is one of the oldest filters applied to the chemical process industry. The filter consists of the following subassemblies: • A drum that is supported by a large diameter trunnion on the valve end and a bearing on the drive end and its face is divided into circumferential sectors each forming a separate vacuum cell. The internal piping that is connected to each sector passes through the trunnion and ends up with a wear plate having ports which correspond to the number of sectors. The drum deck piping is arranged so that each sector has a leading pipe to collect the filtrate on the rising side of the drum and a trailing pipe to collect the remaining filtrate from the descending side to ensure complete evacuation prior to cake discharge. However, in some instances, only leading or trailing pipes are provided and this depends on process requirements. The drum is normally driven with a variable speed drive at cycle times of between 1 rpm 10 rom.
• A valve with a bridge setting which controls the sequence of the cycle so that each sector is subjected to vacuum, blow and a dead zone. When a sector enters submergence vacuum commences and continues through washing, if required, to a point that it is cut-off and blow takes place to assist in discharging the cake. The valve has on certain filters adjustable blocks and on others a fixed bridge ring. Adjustable bridge blocks enable the optimization of form to dry ratio within the filtration cycle as well as the effective submergence of the drum when the slurry level in the tank reaches a maximum.
• A cake discharge mechanism that can be either a scraper, belt, roll and in rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge.
The following sketches (Figures 13 through 16) illustrate key features of the operation of this machine.
Selecting the suitable mechanism largely depends on the release characteristics of the cake from the filter media. Scraper discharge mechanisms generally tend to suit cakes that release readily whereas roller discharge mechanisms are best suited for cakes that exhibit thixotropic behavior.
Was this article helpful?