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Belt Filter Press Processes
Conditioning
Usually the slurry must be conditioned in order to cause the solid particles
to flocculate. Conditioning involves the addition of a polymer, inorganic
salt, etc. to the slurry. The clumping of solids is a result of
flocculation, which leads to the release of free water. The release of the
free water gives the solid matter the necessary consistency to accommodate
pressure filtration. The characteristics of the flocculated slurry vary,
depending upon the type of slurry and the type of conditioning agent used.
These characteristics determine the dewatering ability of the slurry.
Assorted conditioning agents are available. Polymers vary in molecular
weight, functional group, and charge density. Different types of polymers
are tested for effectiveness. The polymer injection ring and mixer is placed
into the feed line upstream of the Klampress. The amount of time the
conditioning agent is in contact with the slurry before thickening (i.e.
retention time) affects the formation of the floc. The amount of mixing
energy applied to the slurry/polymer solution will also affect the formation
of the floc. The polymer injection ring and mixer is adjustable, allowing
mixing energy to be varied for optimum performance.
Gravity Drainage
From the feed line, the conditioned slurry is fed onto the top belt via the
feed chute. This begins the process of liquid/solid separation, where the
solids and the water are physically separated. The liquid/solid mixture is
introduced to the porous belt, allowing the liquid to drain through it. The
liquid is collected in drain pans and routed to a sump or away from the
gravity section. As the thickened slurry moves along the length of the
gravity section, the slurry is gradually turned over by chicanes. The
chicanes greatly increase the gravity thickening process by clearing places
for the water to drain and by turning the solid mass on the porous belt. The
slurry is stopped from running off the sides of the belt by restrainers with
rubber seals.
Wedge Section
The loosely structured cake is directed onto the bottom belt and begins to
be compressed between the bottom belt and the upper belt, forming a wedge.
In the Klampress, this wedge has a horizontal orientation, and the wedge
angle is adjustable while the machine is in operation. The wedge begins to
apply low pressure to the slurry, and prepares the slurry for the high
pressure section.
Shear - Pressure Section
The sandwiched slurry which has most of the water removed by this point,
passes over and around a series of rollers (roller profile is determined at
the factory for each individual slurry). The rollers increase the amount of
pressure applied to the solids. Pressure is applied at a gradual rate to
allow the maximum loading without extruding the slurry out the sides of the
belts. Some rollers are designed to produce a shearing motion on the solids
between the two belts. This shearing motion rearranges the solid mass,
exposing wet inner cake to the belt and allowing more water to be removed.
By the time the solids reach the end of the pressure section, the cake has
been turned over sufficiently to expel nearly all the free water from the
slurry. At the last shearing roller, the belts separate and the cake is
removed from the belts by scraper blades.
Solids that stick to the belt are removed at the discharge end by a scraper
blade. The belts move past the scraper blades to individual belt washboxes.
The washboxes wash the belts of solids that were forced into the pores of
the weave. The washbox cleans the belt of any material that may blind the
belt and cause poor drainage.
The discharge from the machine is the dry solids or cake, and the filtrate.
The filtrate is collected in drainage trays and piped to the base of the
machine for return to the head of the plant for further treatment.
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