Chemical Engineering October 2011 - 51

Imerys Filtration Minerals USA
Figure 8. Rotary drum filters continuously perform precoating, filtration and
cake washing, drying and discharging for each revolution of the drum. The cutaway
diagram on the left shows the mechanical components. The flowsheet on the right
shows the feeding and recycling of the precoat
The effects of overpressuring and
overloading cause the most permanent
damage to filter elements. This damage
bends and warps the filter plates
causing problems with filter sealing
and performance. Breaks and tears in
the filter media lead to bleed-through
and lack of filtrate clarity. Problems
are compounded when the filter is difficult
to clean.
For the filter station that has a precoat
tank, the general rule-of-thumb
is that the precoat tank should hold
about 1.25 times the holding volume
of the filter. This ratio allows sufficient
volume to fill the filter and the
additional volume needed for the interconnecting
piping. If the resulting
volume of the precoat tank is too large,
it is possible to have a precoat tank of
about one third or one half of the total
volume of the filter. In that case, provisions
are made to fill the filter tank
and the precoat tank prior to circulating
the precoat, so that both tanks are
full of liquid.
Precoating
Clean liquid is used for precoating and
is either the same liquid, or one that is
compatible with the process feed. The
reason for this is that with a clean liquid
only the precoat material is deposited
on the filter media and there is
no contamination from the suspended
solids in the feed that would actually
defeat the purpose of the precoat.
The precoat tank must have internal
baffles to avoid vortexing that
will make the precoat pump cavitate
by sucking air. The tank should have
a properly sized, top-mounted mixer.
Consult the local mixer representative
or distributor to ensure that the mixer
is properly sized, taking into consideration
the specific gravity of the liquid,
the solids concentration and the abrasive
nature of the filter aids. One word
of caution about precoat mixing: avoid
using air or gas lines inserted into the
precoat slurry for mixing, which would
introduce air into the filter cake. The
air or gas bubbles trapped inside the
filter cake, have a tendency to pop,
disturbing the filter cake and affecting
the clarity of the filtrate.
The amount of precoat material required
will depend on the laboratory
findings, but normally the rule is to
use 0.15 to 0.2 lb for each square foot
of filter area. The flowrate required for
feeding the precoat material is related
to the filter area. A rate of about 1.0
gal/min/ft2 generally gives an even
distribution across the plate. The precoat
layer should be thick enough to
produce the desired results without
taking up too much of the cake space
available between the filter elements.
Select the precoat material that allows
the maximum flow in the process
with the minimum resistance to flow
and pressure drop.
The precoat system needs a good
reliable pump, preferably an openimpeller
type centrifugal pump. The
pump is to carry the precoat as quick
as possible with a low pressure drop.
Deposit the precoat layer quickly and
evenly, but do not compact this layer
to cause a high pressure drop and low
process flowrate. When precoating the
filter, fill the precoat tank and the filter
first, thereby allowing the clean liquid
to circulate through the filter plates
establishing the process flow. Fill the
filter with the air vent fully opened to
allow displacement of all the air out of
the filter tank, avoiding air pockets inside
the filter. Avoid any backflow from
getting into the filter tank by having a
check valve on the filter outlet piping.
Also, avoid having any outlet piping
at a higher level than the filter tank
without a check valve to prevent back
flow that will disrupt the filter cake on
the filter elements.
Sometimes, depending on the suspended-solid's
particle sizes, a double
precoat of a fine and then a coarser
grade of filter aid is recommended. The
recommended procedure is to apply
first the finer grade of precoat and then
the coarser grade on top. This technique
provides a depth-filtration type
effect. Your local filter aid representative
will help you with this procedure
and tell you what distribution to use.
Generally the distribution is 60% of
the finer grade and 40% of the coarser,
but this ratio could vary according to
your actual requirements.
When the filter is full and the precoat
liquid is circulating from the precoat
tank to the filter back to the precoat
tank, turn the mixer on and start
adding the precoat filter aid material
to get it on the filter elements quickly
and evenly. Continue this recirculation
until the liquid returning to the
precoat tank is clear, which should
take approximately 10 to 15 minutes.
During this procedure the differential
pressure across the filter will go from
0 psid to not more than 10 psid.
Bodyfeeding
If the nature of the solids being filtered
tends to blind over the precoat, then
once the precoat is formed on the filter
media, a bodyfeed is used to introduce
additional filter aid to the feed to keep
the filter cake porous. While the precoat
rate is according to the filter area
at about 1-1.5 gal/min/ft2, the bodyfeed
rate is according to the percent
by weight of suspended solids. This
bodyfeed ratio is determined in the
laboratory test, and the local filter-aid
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Chemical Engineering October 2011

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