Method and apparatus for clarifying liquids

Claim 2

Method of operating a filter containing a layer of particulate material sufficiently fine to become fluidized during purging but not during filtering, and a layer of particulate material too coarse to become fluidized during purging, which method comprises the steps of introducing into a liquid requiring said clarification a polyelectrolyte adapted to cause particles suspended in said liquid to adhere to other particles, passing said liquid upwardly through said filter, proceeding from the coarse layer to the fine layer, preventing substantial migration of said coarse material into said fine material by maintaining between said fine and coarse material a lower screen having openings too small to freely pass the individual particles of relatively coarse material, and preventing breakup of said layer of fine material, while unfluidized, by imposing thereon an upper screen having openings so small as to prevent such breakup under a liquid pressure less than 50 percent greater than that required to advance the liquid to be clarified through said filter when clean.

Claim 3

Method as claimed in claim 3 in which each of said screens has a total open area less than 70 percent of the total cross-sectional area of a layer but at least as great as the total cross-sectional area through which fluid is afforded access to said coarse layer.

Claim 4

Method as claimed in claim 4 comprising the step of interrupting the filtering liquid to be clarified when the pressure required to force it through said layers reaches a predetermined point more than 50 percent in excess of that required to force it through said layers when clean, forcing a purging fluid through said layers in the same direction as that of said liquid to be clarified at a rate of flow sufficient to fluidize only said layer of fine particulate material until said layers are substantially cleansed of filtrate, and then reintroducing said fluid to be clarified.

Claim 5

Method as claimed in claim 3 in which said liquid is passed through said filter until the pressure across said filter reaches at least said 50 percent greater pressure but is still insufficient to fluidize said layer of fine particulate material and said filter is then purged by forcing both a gas and a liquid through said bed in the same direction as the liquid to be clarified at a pressure which is sufficient to fluidize said layer of fine particulate material.

Claim 6

Clarifier including a filter bed comprising a series of layers of particulate material substantially chemically inert to the liquiD to be clarified and to its contaminants, at least one layer consisting of particulate material so fine that it becomes fluidized during purging and at least one layer consisting of particulate material too coarse to become fluidized during purging and positioned beneath said layer of fine material, a lower screen between said layers of fine and coarse materials having openings too small to permit upward migration of said coarse material, an upper screen atop said layer of fine material having openings sufficiently larger than any of said fine particles to permit breakup of said layer of fine material and subsequent passage of fine material therethrough when said layer of fine material is fluidized during purging but small enough to prevent breakup of said layer of fine material, while unfluidized, at any liquid pressure less than 50 percent greater than that required to advance a liquid to be clarified through said filter bed when said bed is clean, means for introducing a liquid to be clarified beneath said layer of coarse material, and means for withdrawing said liquid from above said layer of fine material.

Claim 7

Clarifier as claimed in claim 7 comprising a perforated plate supporting said layer of coarse material and a plurality of nozzles extending through said plate through which a liquid to be clarified and a purging fluid may be introduced during clarification and purging respectively.

Claim 8

Clarifier as claimed in claim 8 in which each nozzle is provided with a series of axially spaced radial openings.

Claim 9

Clarifier as claimed in claim 7 in which said each of said screens has a total open area between 9 and 70 percent of the cross-sectional area of said filter bed.