Clarifiers/thickeners overflow systems

The separation of solids is a purely mechanical process involving thickening of the reactor effluent slurry from 2-5 wt. % to 20-30 wt. % solids, followed by filtration to produce a waste filter cake. While the cake is being formed, it is washed with fresh water to recover sodium salts that would otherwise be lost in the process liquor that is entrained in the moist cake. The filter cake represents the only waste discharged from the process. There are no other purges from the system. The clarified and regenerated thickener overflow liquor is fed forward to the absorber thus completing the liquor loop. 

As can be seen, the two-stage system with a clarifier thickens and classifies a little better but the two stage with overflow recycle reduces the cut size to a little lower value. 

Sedimentation systems consist of a collection of components, each of which can be supplied in a number of variations. The basic components are the same, whether the system is for thickening or clarifying tank, drive-support structure, drive unit and lifting device, rake structure, feedwell, overflow arrangement, underflow arrangement, instrumentation, and flocculation facilities. 

Slurries exhibiting Type II behavior ate sufficiently thick sed flocculent that the solids tend to settle as a mass, giving a rather sharp line of demarcation between the clear liquid overflow and the settling solids. For such systems the design is normally controlled by the thickening capability of the besin, although the basin design must be adequate to provide sufficient overflow ares to clarify the liquid overflow. 

Overflow from the clarifier is combined with a similar stream from leaching circuit B and passed through six 600-ft (58-m ) U.S. pressure filters in parallel, each operated at a feed rate of 300 to 500 gal/min (1.1 to 1.9 m /min). Filters are precoated with about 0.1 lb (0.05 kg) of precoat per short ton of ore, and 0.35 lb (0.17 kg) of filter aid per ton is added to the filter feed. The filter loading cycle lasts from 4 to 24 h, depending on the solids contents of clarifier overflow. Filter cake is returned to 2 or 3 thickener. Filtered leach liquor containing about 1 g UaOg/liter is the product of the leaching system and the feed to the solvent extraction system. 

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