The precoat filter is used for the filtration of particles being in the single-digit µm range. The actual filter which is used is a pressure or hinged belt filter. The filter belt being used is relatively coarse meshed.
The fineness of the filtration is achieved by applying a precoat medium (e.g. cellulose) on the filter surface. The precoat medium forms a filter cake whose pores are considerably finer than the ones of the filter.
As described above, the precoat medium is “filtered” at first. The filter is fed with effluent as soon as a filter cake has built up. The dirt particles settle on the precoat-filter-cake and form a dirt-filter-cake. The filtrate flows to the bottom. As the filter cake thickens, the counter pressure increases. When a certain limit is exceeded, the feeding is interrupted and a regeneration process is started.
During the regeneration process, the lateral flaps are opened and the filter-belt, together with the filter cake, is ejected. The filter cake is stripped off and the cleaned filter belt is fed in again from the opposite side. The flaps are closed and the filtration process starts again.
The precoat filter is used in situations where very fine particles have to be filtered out of liquids. A common application is, for example, the filtration of oil used in the metal processing industry or the filtration of particles from hone- or grind-processes.
Advantages of the precoat filter:
- outstanding depth filtration due to the three-dimensional organisation of the precoat filter cake
- filter cake is much drier, in comparison to sand filters
- none or only small amounts of backwash water needed
- use of coarse filter belt allows easy cleaning
- no chemical changes of liquids due to the use of inert precoat medium