Filtration
As particles get
finer the
resistance
against
removing water
increases.
Gravity
dewatering
can no longer be
used,
pressure
has to
be used.
By creating
a differential
pressure
Dp across
a cake
of solids,
liquid
can be
removed
by:
Compression:
“Dewatering
by compression means
removing the
liquid in
a cake
by
compacting the
cake structure”.
Through
blow:
“Dewatering
by
through
blow
means
replacing
the water in
a cake
with air”.
For
tube presses
either compression
or a
combination
of compression
and air
purge
is used.
Similarily
by
the same
method,
cake washing
is possible
on the
Tube
Press.
This
is
known
as displacement
washing where
liquid is
forced
through
the filter
cake
by
hydraulic
pressure
causing the
remaining natural liquid
to
be displaced
by
the incoming
wash liquid.
For
vertical
plate
pressure
filters
either compression
or a
combination of compression
and
air
through
blow is used.
For
vacuum filters
air through blow
is used.
Tube press
The
tube press is a membrane type filter press designed in cylindrical
format and capable of operating at high filtration pressures up to 100 bar.
The use
of
these high
pressures
enables a
higher degree of
separation of the
liquid and
solid
phases.
The tube press is suitable for the
following applications:
Where the process requires very low
cake moisture eg.
smelters
·
Where
cake
handleability
is
critical.
e.g.
underground
slimes
·
Where filtrate
is a
valued product
·
Where cake
transport
costs are
significant
·
Where cake
is
to
be disposed
of
to landfill
Principle of operation
Filtration
takes
place between
two
concentric
cylinders.
The
outer
cylinder
is the casing,
and the inner, the candle.
The
process slurry
is pumped
into
the annular
space between
the filter
media and
the
bladder.
Hydraulic
fluid, water,
is then
pumped between
the bladder
and the
casing putting
the slurry
under pressure
and causing filtration
to
take
place.
When filtration
is complete
the
hydraulic
fluid is
withdrawn
from
the
Tube
unit using
vacuum
until
the
bladder
is
dilated
against
the casing.
The
candle is
then lowered
to
the discharge
position and
a pulse
of air
is blown
between the
candle and
the filter
media.
This
causes
the filter cloth
to
expand,
fracturing
the cake
which is discharged
under
gravity.
When
complete
the candle
closes
to
the slurry
fill
position
to
repeat
the
cycle.
Air purge
and/or cake wash can
be incorporated
into
the
cycle.
At
the completion
of the filtration
element, air
or wash
fluid is
pumped between
the cake
and the
bladder
which is
then
forced
through
the cake
by
a
further
application of
hydraulic
pressure
to be
incorporated
into
the
cycle.
Benefits
·
Driest
possible cake
solids
·
Maximum volume reduction
·
Exceptionally clear
filtrate
Advantages
of on-site tube
press
trials
Tests will
be undertaken
on production
slurry and
will encounter any
variability
of
production
feed
that occurs
during the
trial.
The client
will see
the performance
and in
particular
will be
able
to
handle the
cake
and observe
the filtrate
clarity.
Opportunities
In many
industries a
liquid phase
in the
process
creates
the need
for
liquid/solid separation so that
the solids
and/or filtrates
so produced
can be
handled or
disposed of.
“Traditional”
mechanical dewatering
often
leaves the
cake
too
wet
or
too
sticky
to
handle effectively
and usually
demands further
treatment
such as
thermal drying.
The
Tube
Press,
because of
its unique
design,
will automatically and economically
dewater
most slurries
to
produce a
clear filtrate
and,
without any
form
of thermal
drying,
a hard,
and surface dry
cake
that is
convenient
to
handle
as a product
for
immediate
storage
or
for
disposal.
Vertical
plate
pressure filter
VPA
The
pressure
filter
model VPA
is of
“medium
pressure”
type
operating in
the pressure
range of
6-10
bar.
The
machine mainly
is based
on
the
“air
through
blow”
dewatering
concept, whereby
water
is
displaced
by
air as
it passes
through
a filter
cake.
Simple
mechanical construction
The
pressure
filter is a
heavy
duty
machine,
developed
for
filtration of metallic
minerals,
industrial minerals,
coal and
tailings.
The filter
has simple
mechanical construction
and design,
with
few
moving
parts.
By using a
pulling hydraulic
cylinder
system
for closing and
sealing the filter plate pack,
we
can provide
a compact
layout
and a
light
weight support
frame design.
Polypropylene
filter
plates
are
used
for
good
chemical
resistance and the
light
weight.
Smart fully automatic control system
The
pressure filter
is controlled
by
a system
consisting of a
PLC
connected
to
a PC
screen,
built in
a well-sealed
and well
insulated
cabinet.
All control
logics
are
implemented
in the PLC.
On the
PC monitor
the filter
operator
can
follow
and control
the filter
operation
and information such
as settings,
statistics
and alarms.
High unit capacity
By installing the
pressure filter
on load
cells connected
to
the PLC
a multitude
of
data become
available.
The
weighing system
provides
accurate
production
information,
and can
be used
to
optimise the
filtration and
drying steps
in
the filter
cycle.
Low dewatering cost with the membrane technology
The pressure
filters
use a
rubber
membrane
in each filter
chamber.
The
membrane
technology
eliminates
cracks in
the cakes
and
also compensates
for
the shrinking
in the
cake during the
compressed
air drying.
The
result means
lower
compressed
air consumption and lower
total
dewatering
cost
Low maintenance cost
Because of
the
simple
mechanical design
with
few
moving
parts
as
well
as the
filter
cloth design
and system,
with extremely
short
downtime
for
cloth changing,
the
pressure filters
have
very
low maintenance costs.
VPA = Vertical Pressure filter Air through blow
·
Lightweight polypropylene
filter
plates
are
mounted
on a
bolted
steel frame and
are
moved
by
hydraulic
cylinders
·
Adjacent “filter
and
compression”
plates
form
a filtration
chamber.
The
filter
cloths hang
between
each pair
of plates.
Rubber
membranes are
protected
by
the filter
cloth
thereby
reducing
wear.
·
By mounting the
filter
on a load
cell system
the filtration
cycle is
monitored
and
controlled.
·
Chambers are
top
fed
for
optimum
filling.
Two
sided filtration speeds up
the “filling”
cycle.
·
Openings
for
pulp,
water
and air
are
generously
dimensioned to
reduce
energy
losses and
wear
·
Service and maintenance
requirements
are
low.
The
VPA
design
facilitates
easy
cloth changing.
·
Air blow
pressure
5-8 bar (73-116 psi). Membrane
pressure
6-9 bar
(87-131 psi).
Drum
vacuum filters
Vacuum filtration is a well-established technique used
in industrial dewatering.
All
rotary
vacuum filters
operate
on a
similar principle.
A pressure
differential
between
the surface
and
inside of
the drum
is applied
by
means of vacuum.
This pressure
differential
causes
transport of
liquid through
the filtration surface
while the
filter
medium arrests
solid particles
and
a cake
is
formed.
As
the drum
rotates,
the cake
rises
above
the slurry
level
in the
filter
tank and
air
is drawn
through
the cake,
forcing
out liquid.
The
liquid (filtrate)
exits the
filter
through
the internal
piping and
the vacuum
head.
Principal components
of the
drum
filter
model
BTF
1.
Cake
discharge
system
counter
weight
and lever arms
2.
Pneumatic
diaphragm
cylinder
3.
Cylinder
tensioning
spring
4.
Optical or
pneumatic
cloth edge sensor
5.
Guide
pulley
Drum
filter
model
BTF
1.
Drum
2.
Drum
drive
3.
Support
Frame
4.
Tank
5.
Vacuum
Head
6.
Agitator
The
range of rotary vacuum filters incorporates two basic design principles:
Metsos filters
are
specially designed
for
high
vacuum
operation allowing
very
high air
flowrates.
This results
in:
–
Higher
capacities
–
Lower
cake
moisture
–
Extended
application range
Modularized components and equipment results in:
–
Flexibility in
machine configuration
–
Features may be added
or altered
to
accommodate
process
changes
–
Construction from
proven,
standard
components
–
Simplified maintenance
and parts
replacement
–
Lower
equipment and
operating costs
Belt drum filter model BTF
Belt drum filters,
model BTF, are
identical in
all
respects
to the TF
range with one
fundamental exception:
cake
discharge.
With
the belt
drum filter
the cloth
is
removed
from
the drum
after
the dewatering
zone
and passes
over the
cake
discharge
system
before
washing and
return
to
the
drum
prior
to
the cake
pick-up
zone.
The
three key
system
features
of the
belt
drum
filter
are
thus:
•
Cake
discharge
•
Cloth
washing
•
Cloth
tracking
Cloth washing system
Cloth washing increases
the capacity
of the filter
and extends
the cloth
service
life
by
preventing
blinding.
After
passing the
cake
discharge system
the cloth
is washed
over
its
full
width
by two
high pressure
sprays,
one
above
and one
below
the cloth.
Wash
water
is
confined
by a soft
rubber curtain
and collected
for
separate
discharge
through
a launder.
Cloth tracking system
Cloth tracking
is achieved
by
means of
a single
hinged guide
pulley mounted
in spherical
roller bearings
after
the cloth
passes from
the
washing system.
The
guide pulley
is
positioned
by
pneumatic diaphragm
cylinders
activated
by
optical or
pneumatic sensors
at each
side of
the cloth.
Correct
cloth
tension is maintained
by
adjustable counter-
weights acting via
levers
to the
cake dis-
charge
system.
The cloth tracking
system
–
Maintains
cloth position
within a
few mm
–
Eliminates need
for
special cloth
edgings
–
Provides
self-adjusting cloth
tensioning
Top
feed drum filters model TFF
The
top
feed
drum filter,
model TFF,
is especially
suited
for
dewatering
slurries
containing coarser
solid fractions.
The
top
feed
principle promotes
segregation
of the coarser
particles
in the
feed
box
forming
a
“precoat”
which
results
in a
high filtration
rate
cake.
Our
capability
to incorporate
very
high
capacity
vacuum heads
and internal
drum
drainage
systems
into
the TFF
design
results
in
high machine
capacities and
lower
moisture
cake.
Standard features include:
Special seal between
feed
box
and drum
prevents
leakage
of
feed
and
reduces
mechanical
wear
of filter
cloth
Snap-blow
cake
discharge
Optional features:
High pressure
water,
air or water/air mixture
cloth cleaning
between cake
discharge
and
feed
point.