Types of dewatering equipment covered are Plate and Frame Filters (Pressure)
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Plate and Frame Filter Press
Plate and frame filter presses are dewatering
machines which utilize pressure (60-80 psi, typically)
to remove the liquid from a liquid-solid slurry. They are particularly suited for low solids (<2% solids),
or solids composed of fines (-200 mesh), however they will essentially dewater
many combinations of particle size distribution and percent solid slurries.
The diagram at left, shows the basic operation of a plate and frame filter press.
The feed enters the press at the bottom of the plate, using a pump suitable for pumping up to 80-90 psi.
Then, the feed travels the path of least resistance (up bewteen the filter plates),
which has filter media inserted between the plates, and the void between the plates is
filled with the slurry, as the liquid passes through the filter media, and travels
up to the outlet port at the top of the plate. This liquid is referred to as the
"filtrate", and is discharged from the press. The solids remain in the void
between the plates, until the plates discharge the filtered solids.
Large plate and frame filter presses have mechanical "plate shifters",
to move the plates, allowing the rapid discharge of the solids stuck in between them.
Also, they have the capability of blowing compressed air into the plate, to dry
the cake, and to aid in it's discharge. Typical capacities for a plate and frame filter
will depend upon the solids being dewatered, however, they will range around 1/2
gallon per minute of feed for low solids content slurries (<1% solids) to
over 1 gallon per minute of slurry per square foot of surface area on the plates.
For example, a 50 square foot filter press would dewater 25 GPM of 1/2% solids feed, or
about 50 gpm of 10% solids feed. The volume between the plates will dictate
how often the plates need to be dumped (solids removed). A 10 cubic foot volume press
would hold 10 cubic feet of dewatered solids. Some operations like to have the capacity to
hold a entire work shift of solids, while others prefer to dump on more frequent schedules.
The filter press is "offline" for filtration, while dumping the solids. Solids
dumped from a filter press typically fall into a hopper or directly onto a conveyor
belt for further transportat to the next stage of the operation.
The little press at left, has 8 inch square
plates and is for laboratory use or small scale pilot plant studies, to determine
the feasibility and rates of filtation for various slurries. It is exactly like a full scale press,
and results obtained can be scaled up for plant operations, accurately.
Diatomaceous Earth can be used with plate and frame filters to aid in the
filtration of very dilute solids slurries, or very fine solids, This coating
is first applied to the filter, allowing a even coating of the filter cloth,
then the feed is introduced into the press.