Thickeners mechanically agitated

The thin dispersion rapidly thickens into a gelled matrix and coagulates into a water-repellent agglomeration that doats on the aqueous medium as the mechanical agitation is continued. The agglomeration is dried gentiy shearing must be avoided. 

Mechanically agitated thickeners are usually large, typically 10 to 100 m (30 to 300 ft) in diameter and 2,5 to 3.5 m (8 to 12 ft) deep. In a large thickener the rakes may revolve once every 30 min. These thickeners are especially valuable when large volumes of dilute slurry must be thickened, as in cement manufacture or the production of magnesium from seawater. They are also used extensively in sewage treatment and in water purification. The feed pulp is admitted at the centerline of the unit at a depth of 1 m or so below the surface of the liquid. Above the feed level is a clarification zone in which the liquid is almost free of solids. Below the feed level is a zone of hindered settling and, near the bottom, a... 

Clear-liquid separation is the appearance of a clear supernatant layer above the bulk of opaque paint after shelf-aging. This separation can occur within 1 day and be several inches thick in a 1-gal can. In tinted paints, colorant may float up into this layer and produce a very displeasing in-can appearance. Reincorporation of the supernatant layer by hand stirring or mechanical agitation usually solves the problem only temporarily because the clear-liquid layer returns after several days. HEUR/HEC- and HEUR/hydro-phobically modified HEC-thickened paints are particularly prone to this separation. 

Performance often is improved by appropriate pretreatment of the slurry with flocculants or other means. An operating practice that is receiving increased acceptance is the delaying of cake deposition by some mechanical means such as scraping, brushing, severe agitation, or vibration. In these ways most of the filtrate is expelled before the bulk of the cake is deposited. Moreover, when the cake is finally deposited from a thickened slurry, it does so with an open structure that allows rapid filtration. A similar factor is operative in belt or top feed drum filters in which the coarse particles drop out first and thus form the desirable open structure. A review of such methods of enhancement of filtration rates is by Svarovsky (1981). 

Application of express-flotation is promising for recovery of small particles (Petrovitz, 1981). The process consists of the following operations. Pulp is saturated with microbubbles 100 pm in diameter in a mechanical flotation machine at intensive agitation of a small amount of air (80 to 100 per m pulp). Saturated pulp is released to a thickener where separation into foam and chamber product takes place. Saturation time is 5 min, flotation time is 1 min. Extraction of finely pulverized iron ore of Olenegorskii deposites makes up 98%. In usual flotation such recovery is attained in 10 min. 

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