| abstract |
Suspensions difficult to filter are filtered on passage through a thick layer of filter aid on the surface of a vacuum filter drum which rotates while partly immersed in a bath of the suspension. The drum interior is divided into separate cells individually connected to suction lines. Outside the drum is a scraper for gradually removing the filter aid surface. Pressure in the drum cell nearest to this scraper is kept constant at a level appreciably below that in the other cells. Should the level of suspension in the vat be lowered, the rate of filter aid removal is automatically reduced, by adjusting the cutter, e.g. to less than 5 mm per hour, while at high suspension levels the removal rate can rise to 10 mm per hour. The appts. is also claimed. Suitable e.g. for 75 wt.% phosphoric acid contg. arsenic sulphide, using calcined Kieselguhr as filter aid. Scraping of the layer, now related to the extent of clogging rather than the number of rotations of the drum, is reduced. In an example, 400 cu. metres of phosphoric acid were filtered before the layer was exhausted, compared with 90 cm metres by the known appts. |