Filling and Filtration
- Filling and Homogenisation
After filling the vessel, the unfiltered product or precoat
is circulated from the batch or precoat tank, through the vessel and
out through the overflow nozzle to ensure an even suspension.
- Precoating and Prefiltration
Unfiltered product or precoat is circulated through the filter elements and back to the feed tank to produce a porous primary layer (precoat layer or cake by direct filtration).
For solids with a fast sedimentation speed, part of the feed stream is recirculated through the overflow nozzle.
The filtration route first passes through the cloudy product, filter cake, filter plate, spacer ring area, the hollow shaft and filtrate outlet.
The same route is taken by the wash liquid during cake washing. Cascade washing by the principle of heel volume filtration (see step 4) is used to reduce the wash liquid volume.
- Heel Volume Filtration
The heel volume filtration starts with a full filter at the end of the filtration or wash cycle.
the product is pumped by either the filtration pump or a separate pump from the heel volume nozzle to the nozzle in the vessel head, back to the filter.
The medium passes from the distribution plate and the overflow plate forming a curtain of liquid over the filter nest and fills the dished area above the filter cake.
Part of the liquid passes through the hollow shaft. The surplus recycled material falls into a pool of remaining liquid whose level drops according to the filtrate volume.
The filtration pressure is created by compressed air or nitrogen. If there is no longer any liquid falling from above, the liquid on the cake is pushed through by the gas pressure. After this, a gas breakthrough occurs as all of the liquid is filtered.