Candle Filters are also used for thickening to produce a concentrated flowable slurry by partial removal of the liquid phase as filtrate. This mode of operation is possible since Candle Filters may operate on very short cycle times taking advantage of the high filtration rates whilst the cakes are still thin. One of the features of Candle Filters when used for thickening is their clear filtrate since gravity Thickeners, in analogy, produce overflows which always contain a certain amount of fine suspended solids.
Another feature of Candle Filters is the cylindrical shape of the elements that retain the formed cake. This shape, opposed to Plate, Leaf or Filterpresses, increases the filtration area as cakes grow thicker and compensate to some extent to the decrease in filtration rates.
Candle Filters are very well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved. These features are not possible on Filterpresses which require the opening of plates to the atmosphere by one to allow cake discharge at the end of each cycle.
Candle Filters are available in all sizes and the larger types may have an area of 200 m2 and contain 250 or more filtering elements installed in a 2.5 to 3.0 meter diameter vessels. The operating pressure range is quite wide but most of the Candle Filters operate at a pressure of 6 bars.
