1. The Vertical Pressure Leaf Filter – Thiết bị lọc lá áp lực
kiểu thẳng đứng
Description
Vertical Pressure Leaf Filters are quite similar to the Horizontal Plate Filters except for
the orientation of the filter elements which are vertical rather than horizontal. They are
applied for the polishing slurries with very low solids content of 1-5% or for cake
filtration with a solids concentration of 20-25%.
2. Thiết bị lọc lá áp lực kiểu thẳng đứng hoàn toàn giống với Thiết bị lọc đĩa kiểu nằm
ngang, chỉ khác việc bố trí các tấm lọc theo chiều thẳng đứng chứ không phải nằm
ngang. Chúng được sử dụng cho việc làm sạch các dung dịch với với hàm lượng chất
rắn rất thấp từ 1 – 5% hay cho bã lọc với nồng độ chất rắn 20 – 25%.
Just like the Horizontal Plate Filter the Vertical Leaf Filters are also very well suited for
handling flammable, toxic and corrosive materials since they are autoclaved and designed
for hazardous environments when high pressure and safe operation are required.
Likewise, they may be readily jacketed for applications whenever hot or cold
temperatures are to be preserved. These features are not possible on Filterpresses which
require the opening of plates to the atmosphere and shifting them one by one to allow
cake discharge at the end of each cycle.
Cũng giống như Thiết bị lọc đĩa kiểu nằm ngang, TB lọc lá rất thích hợp với các loại
nguyên liệu có bản chất dễ bắt cháy, độc hại và ăn mòn khi chúng được đưa vào nồi
hấp và thiết kế cho các môi trường độc hại yêu cầu áp suất cao và an toàn lao động.
Tương tự như vậy, người ta có thể dễ dàng bao bọc thiết bị khi cần giữ ở nhiệt độ
cao hay thấp. Những tính năng này không có trên thiết bị lọc ép vì cần cần mở các
tấm đĩa lọc để lưu thông không khí và tháo rời từng cái một để loại bỏ các cặn rắn ở
cuối mỗi chu kì lọc.
The largest Leaf Filters in horizontal vessels have a filtration area of 300 m2 and vertical
vessels 100 m2 both designed for an operating pressure of 6 bar (see below the section on
The Vessel).
The Leaves
Slurry Feed
Filtrate
Leaf
Screen
Spacers
The slurry is pumped under
pressure into a vessel that is
fitted with a stack of vertical
leaves that serve as filter
elements. Each leaf has a
centrally located neck at its
bottom which is inserted into a
manifold that collects the
filtrate. The leaf is constructed
with ribs on both sides to allow
free flow of filtrate towards the
neck and is covered with coarse
mesh screens that support the
finer woven metal screens or
filter cloth that retain the cake.
The space between the leaves
3. may vary from 30-100 mm
depending on the cake formation
properties and the ability of the
vacuum to hold a thick and
heavy cake to the vertical leaf
surface. The space is set by the
filtrate necks of the leaves at the
bottom end and with spacers at
the top end brackets.
For fast filtering slurries the
space may be doubled by
removing every second plate so
consequently the cake space
doubles but the filtration area is
cut in half.
The Vessel
There are two types of vessel configuration:
• Vertical vessels
• Horizontal vessels
In most of the fine chemicals
processes the leaves are fitted into
vertical vessels whilst horizontal
vessels are used in the heavier
process industries such as the
preparation of sulfur in phosphoric
acid plants. The leaves inside
horizontal tanks may be positioned
either along the tank axis or
perpendicular to the axis.
In order to utilize the tank volume
for maximum filtration area the
width of the leaves is graduated so
they fit to the circular contour of the
tank. This also reduces the slurry
heel volume that surrounds the
leaves.
The vessels are fitted with highly
4. secured cake discharge openings to
ensure safe sealing of the tank under
pressure. The cake that accumulates
on the leaves may be discharged as
a wet thickened sludge or as a dry
cake. For wet cakes the vessel will
normally have a small outlet that is
fitted with a valve whilst for dry
cakes the opening is large and the
closure locks up electrically or
hydraulically with a bayonet wedge.
The head cover of vertical vessels is
often pivoted so that it is swung
away to allow the upwards removal
of the leaves in the stack. It is good
practice to design a special rig that
will support a leaf that is removed
from the vessel.
Special quick opening bolts are
fitted around the cover so that
tightness is secured during
operation but enable easy opening
when access to the stack is required.
Cake Discharge
One of the advantages of the Vertical Leaf
Filter when compared to the Horizontal
Plate Filter is when cakes depart easily
from the filtering medium. In such cases it
is not necessary to incorporate means to
assist discharge since gravity will release
the cake and let it drop towards the
discharge opening. For such cakes that do
not discharge readily a special mechanism
that vibrates the entire stack is
incorporated and this will in most
instances release the cake. However, with
this method care must be taken so that the
cake does not bridge between the two
adjacent plates since this will impair cake
5. discharge.
There are instances when the cake is
disposed to ponds or repulped for further
treatment and in these cases special
oscillating high impact jet headers sweep
the medium surface and sluice the cake
through the discharge outlet. These
headers also serve to wash the filtering
medium and dislodge particles that clog
the metal screen or cloth.
Selection Criteria
Vertical Leaf Filters are best selected in the following instances:
• When minimum floor space for large filtration areas is required.
• When the liquids are volatile and may not be subjected to vacuum.
• When there is a risk of environmental hazard from toxic, flammable or volatile
cakes specially secured discharge mechanisms may be incorporated.
• When high filtrate clarity is required for polishing applications.
• When handling saturated brines that require elevated temperatures the tank may
be steam jacketed.
• When the cake may be discharged either dry or as a thickened slurry.
They should be selected with care:
• When the cake is thick and heavy and the pressure is not sufficient to hold it on
the leaf.
• When coarse mesh screens are used the filtration step must be preceded with a
precoat to retain cakes with fine particles. Precoating with a thin layer of
diatomite or perlite is not a simple operation and should be avoided whenever
possible.
Advantages
• The cloth or woven mesh screens that cover the leaves of horizontal tanks may be
accessed easily once the stack is pulled out of the vessel. This allows thorough
washing of the medium with high impact jets manually in case that the cake
bridges between the leaves. On vertical tanks the head cover must be unbolted and
removed in order to access the leaf stack.
• Mechanically simple since there are no complex sealing glands or bearings.
6. Disadvantages
• High headroom is required for dismantling the leaves on vertical vessels.
• Large floor space is required for discharging the cake on horizontal vessels.
• The emptying of the vessel in between cake filtration, washing and drying
requires close monitoring of the pressure inside the vessel to ensure that the cake
holds on to the candles.
Operational Sequence
The operation of a Vertical Pressure Leaf Filter is labor intensive and requires a complex
manipulation of valves so present day installations are in most cases fully automated.
7. Precoating
The precoating stage is done only in the
following cases:
• When the contaminants are
gelatinous and sticky the precoat
layer forms a barrier that avoids
cloth blinding. Likewise the
interface between the precoat
and the cloth departs readily so
the cake discharges leaving a
clean cloth.
• When a clear filtrate is required
immediately after the filtration
cycle commences otherwise
recirculation must be employed
until a clear filtrate is obtained.
Filtration
Once the precoating stage is completed
the process slurry is pumped into the
filter, the forming cake is retained on
the leaves and the filtrate flows to
further processing.
When the solids are fine and slow to
filter a body-aid is added to the feed
slurry in order to enhance cake
permeability. However, it should be
kept in mind that the addition of body-
aid increases the solids concentration in
the feed so it occupies additional
volume between the leaves and
increases the amount of cake for
disposal.
Likewise, for all those applications
when the cake is the product, precoat
and filter-aid may not be used since
they mix and discharge together with
the cake.
Please refer to the section on Pressure
8. Maintenance
The Vertical Pressure Leaf Filter requires attention on a regular basis to safety devices
and automation features that accompany modern filters.
The space above the filter should have a hoisting device and sufficient headroom to lift
each leaf and move it horizontally to a location adjacent to the filter tank. It is
recommended to have a special rig that will hold the leaf for maintenance. Space must
also be allocated for the cover which may be either hinged or removed.
The major components that require attention are:
• The filter tank must conform to an Unfired Pressure Vessel code, such as ASME,
and checked periodically as required by the safety regulations.
• The pressure relief valve that is located on the top of the tank must be checked for
emergency functioning.
• The "o"-rings that seal between the leaves necks and the filtrate collecting
manifolds.
• The large diameter caulking gasket of the dished top head cover. The ends must
be cut in an angle to ensure a perfect seal.
• The hinged head cover locking bolts.
• The cleanliness of the filtrate sight glass that monitor on-line or visually enables
inspection of the filtrate clarity.
• The interlock that disables opening the cake discharge when the vessel is still
under pressure.
• The maintenance hoist above the filter must pull out the leaves vertically so that
they will not hit the tank wall.
• The condition of the filter medium, cloth or mesh screen, must be done
periodically to ensure that they are not damaged.
• The vent on top of the head must be checked for free evacuation of air.
• The filter must not be overfilled with cake since this causes the leaves to bend so
they must be checked periodically.
9. The Horizontal Plate Pressure Filter - Thiết bị lọc
lá kiểu đứng
Description
Horizontal Plate Pressure Filters were commonly applied to the fine chemical process
industries such as antibiotics, pesticides or pigments when the load of impure insoluble is
low and polishing is required to obtain high product clarity. However, in recent years
they may be seen more and more in heavier industries such as fertilizers or precious
metals when the product is the cake and efficient washing and low moistures are
required.
10. Slurry Inlet
Filtrate Outlet
Cake Outlet
Air Vent
Cake Scrapers
Filtering Plates
Scavange Plates
Sight Glass
Drive Shaft
Hydraulic Drive
To view the components move
mouse pointer over the menu
Horizontal Plate Filters are very well suited
for handling flammable, toxic and corrosive
materials since they are autoclaved and
designed for hazardous environments when
high pressure and safe operation are
required. Likewise, they may be readily
jacketed for applications whenever hot or
cold temperatures are to be preserved. These
features are not possible on Filterpresses
which require the opening of plates to the
atmosphere and shifting them one by one to
allow cake discharge at the end of each
cycle.
The filter structure consists of a stack of
plates attached to a hollow shaft which are
mounted inside a pressure vessel with each
plate covered with a suitable filter medium.
The slurry is fed under pressure into the vessel and the cake, which is retained by the
filter medium, forms on the top of each plate whilst the filtrate passes through the hollow
shaft further to the process. Please refer to the Operational Sequence below which
describes the filtration cycle in detail.
Filter sizes may vary but generally the maximum is 60 m2 area and designed for a 6 bar
operating pressure.
The Plates
Each circular plate in the stack is constructed with radial ribs that are welded to the
bottom and support a horizontal coarse mesh screen which is covered with a finer woven
metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the
hollow central shaft which lets the filtrate flow freely through circumferential holes and
further down the shaft to the filtrate outlet.
11. The clearance between the plates is maintained by special spacers with "o" rings to
positively seal between the slurry that surrounds the plates and the shaft that collects the
filtrate. The height of the spacers determine the clearance for cake build-up and may be
replaced to meet various process conditions.
One of the obvious differences between polishing and cake filtration is the space between
the plates. For polishing applications the clearances are about 20 mm as opposed to cake
filtration applications where, depending on the percentage of solids and cake build-up
properties, clearances may reach 100 mm. Hence, polishing filters accommodate more
plates than cake filters so for the same vessel size more effective area is available with
polishers.
There are several applications, mainly in the pharmaceutical and paint industries, where
special disposable filter paper is used to cover the plates on both sides so the filtration
area is doubled.
The Vessel
The vessels of Horizontal Plate Filters are, as opposed to Vertical Leaf Filters, always
constructed vertically to accommodate the plates stack.
All have removable dished heads but there are two options for bottom design:
• A conical bottom.
• A dished bottom.
The selection depends largely on the cake discharge arrangement as discussed later.
The head of the larger vertical vessels is often pivoted so that it is swung away to allow
the upwards removal of the plates stack. The layout should provide sufficient headroom
for raising the stack over the vessel and additional floor space next to the filter for stack
maintenance and replacement of damaged plates. It is good practice to design a special
rig that will support the removed stack.
The vessels at their bottoms are fitted with highly secured cake discharge openings to
ensure safe sealing of the tank under pressure.
Cake Discharge
The concept of cake filtration, as opposed to polishing, was enabled by substantial
improvements in the cake discharge mechanisms since such filters are operating on a
short cycle time.
There are two types of cake discharge mechanisms and both use centrifugal force to
throw the cake against the cylindrical wall which then falls to the bottom of the tank:
12. • The rotating disc stack.
• The vibrating disc stack.
The rotating type may be driven from either the top or the bottom whilst
the vibrating type is always driven from the top. The removal of the tank head cover from
top driven filters is generally more complex than those driven from the bottom. On the
other hand bottom driven filters are more susceptible to slurry leaks.
The position of the cake outlet depends on the construction of the tank bottom. There are
two types available:
• With a conical bottom and a central outlet.
• With a dished bottom and a side outlet.
Tanks with conical bottoms discharge cakes by gravity and those with dished bottoms
have a spade that rakes and conveys the cake towards the outlet. Hence, the conical types
require more headroom as compared to the dished type having the same filtration area.
Conical tanks also have often an additional scavenging plate at the lower part of the cone
to filter the residual slurry heel that remains below the main plates. The slurry heel that
remains at the very bottom of the tank is removed through a special dip pipe to avoid
discharging a wet cake.
To facilitate better cake discharge there are designs with sloping plates. With this concept
the cake, owing to the centrifugal force, flies off the plate in a horizontal trajectory
without being dragged and subjected to the frictional radial shear over the surface as with
conventional flat plates.
The cake that accumulates on the plates may be discharged as a wet thickened sludge or
as a dry cake. For wet cakes the vessel will normally have a small outlet that is fitted with
a valve whilst for dry cakes the opening is large and the closure locks up electrically or
hydraulically with a bayonet wedge.
Selection Criteria
Horizontal Plate Filters are best selected in the following
instances:
13. • When minimum floor space for large filtration areas is required.
• When the liquids are volatile and may not be subjected to vacuum.
• When there is a risk of environmental hazard from toxic, flammable or volatile
cakes specially secured discharge mechanisms may be incorporated.
• When high filtrate clarity is required for polishing applications.
• When handling saturated brines that require elevated temperatures the tank may
be steam jacketed.
• When efficient washing is required.
• When the cake is heavy and must be supported as opposed to a Vertical Leaf
Filter where the cake forms on a vertical surface and may fall-off once the
pressure drops.
• When the cake may be discharged either dry or as a thickened slurry.
They should be selected with care:
• When the cake does not discharge readily accessing the filter medium between the
plates for washing is difficult.
• When coarse mesh screens are used the filtration step must be preceded with a
precoat to retain cakes with fine particles. Precoating with a thin layer of
diatomite or perlite is not a simple operation and should be avoided whenever
possible.
Advantages
• The removal of the plate stack on bottom driven filters is simpler than on top
driven machines since on the later the entire drive has to be removed to allow
access to the stack.
• Plates with the screens mounted on the topside, as opposed to two sided plates,
provide good support for the forming cake and therefore are always used on
applications with thick and heavy cakes.
Disadvantages
• High headroom is required for dismantling the entire plate stack.
• The bearing of top and bottom driven filters, that supports the rotating plate stack
and its sealing, is complex since it has to withstand the internal pressure and the
side forces imposed by the mechanical drive. However, side loads on some
machines are eliminated by the use of hydraulic motors.
• The emptying of the vessel in between cake filtration, washing and drying
requires close monitoring of the pressure inside the vessel to ensure that the cake
holds on to the candles.
Operational Sequence
14. The operation of a Horizontal Plates Filter is labor intensive and requires a complex
manipulation of valves so present day installations are in most cases fully automated.
15. Precoating
The precoating stage is done
only in the following cases:
• When the contaminants
are gelatinous and
sticky it forms a barrier
that avoids cloth
blinding. Likewise the
interface between the
precoat and the cloth
departs readily so the
cake discharges leaving
a clean cloth.
• When a clear filtrate is
required immediately
after the filtration cycle
commences otherwise
recirculation must be
employed until a clear
filtrate is obtained.
Filtration
Once the precoating stage is
completed the process slurry is
pumped into the filter, the
forming cake is retained on the
plates and the filtrate flows to
further processing.
When the solids are fine and
slow to filter a body-aid is
added to the feed slurry in
order to enhance cake
permeability. However, it
should be kept in mind that the
addition of body-aid increases
the solids concentration in the
feed so it occupies additional
volume between the plates and
increases the amount of cake
for disposal.
Likewise, for all those
16. Maintenance
The Horizontal Plate Filter requires attention on a regular basis to safety devices and
automation features that accompany modern filters.
The space above the filter should have a hoisting device and sufficient headroom to lift
the entire disc stack and move it horizontally to a location adjacent to the filter tank. It is
recommended to have a special rig that will hold the plate stack for maintenance since the
bigger ones may reach a length 3 meters or more. Space must also be allocated for the
cover which may be either if it is hinged or removed.
The major components that require attention are:
• The filter tank must conform to an Unfired Pressure
Vessel code, such as ASME, and checked as required by
the safety regulations.
• The pressure relief valve that is located on the top of the
tank.
• The bearings, retainers, o-rings, gaskets and seals where
the center shaft enters the vessel. This applies to both top
and bottom driven stacks with the former being
susceptible to air or gas leaks and the later to slurry leaks.
• The large diameter caulking gasket of the dished top head
cover. The ends must be cut in an angle to ensure a perfect
seal.
• The hinged head cover locking bolts.
• The spacers' ring seals that press the entire plate stack
together.
• The cleanliness of the filtrate sight glass that monitor on-
line or visually enables inspection of the filtrate clarity.
• The interlock that disables stack spinning with a closed
cake discharge outlet.
• The interlock that disables opening the cake discharge
when the vessel is still under pressure.
• The maintenance hoist above the filter must pull out the
entire stack vertically so that the plates will not hit the
tank wall. Its exact positioning is also essential for bottom
driven stacks since on many types there is no access to the
bearings and gland seals from the bottom and the entire
insertion is done from the top.
• The condition of the filter medium, cloth or mesh screen,
must be done periodically to ensure that they are not
damaged.
• The vent on top of the head must be checked for free
evacuation of air.
• The filter must not be overfilled with cake since this
