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What Is Pressure Nutsche Filter?
Nutsche filtration is a batch filtration technique that uses vacuum and/or pressure in a closed vessel. The pressure/vacuum is used in combination with a filter plate fitted with the appropriate filter media to force liquid through the media, forming a solid bed often referred to as “cake”.
How Does Pressure Nutsche Filter Work?
When rotated clockwise, it pushes the dry cake towards the vessel wall during each rotation, thus expelling product intermittently out through the side discharge valve. Nutsche filter/dryers can effectively perform the separation of solid matter from a liquid under pressure or vacuum, in a closed system.
Pressure Nutsche Filter
Where Is Pressure Nutsche Filter Used?
Filling And Charging:
The first step involves the transferring of the product to be filtered, usually an aqueous or solvent based solid/liquid slurry, from the primary location (e.g. reactor) to the filter/dryer. The filter should be sized appropriately to handle the solids volume that is being charged. A general rule of thumb to follow is that the maximum solids height should be equal or less than the agitator stroke, typically 12 - 20 inches.
Filtering:
The second step involves the use of pressure and/or vacuum to force liquid through the solid bed and filter media. The process is stopped when solids are visible, or can be continued until all the liquid is pushed out. There are various types of filter media available in nutsche filters – cloth, single layer metal screen, or multi-layer sintered metal. The media should be determined based on the characteristics of the slurry including particle size and shape, cake porosity, and compressibility (which are all factors that, when taken into account with the filter media selected, will determine filtration rate).
Displacement Washing:
Here, fresh wash liquid is sprayed on top of the solid cake taking care not to disturb the surface of the cake. The liquid is forced through with pressure or vacuum. Displacement washing serves several purposes – it removes the liquid and its impurities while keeping the cake intact and it replaces the previous liquid with fresh liquid. There is also the opportunity prior to this step to try and close cracks if the cake has any via smoothing.
Reslurry Washing:
As an additional washing option, a re-slurry washing can be performed if additional extraction or dissolving of impurities is necessary. This is executed by the addition of fresh wash liquid that is mixed with the solids. The agitator is stopped and raised before filtering the wash liquid. The re-slurry process is also used when a long contact time is needed between the wash fluid and the solids or the displacement wash does not provide the required wash quality.
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