The present invention relates to a method of and an apparatus for
thickening lime sludge with a drum filter according to which lime sludge is thickened
on a filtering layer formed of the lime sludge on the surface of a filter drum
partly immersed in a vat containing lime sludge.
Conventionally, a lime sludge having a dry-solids content of approx.
25 % is thickened to a dry-solids content of 70 - 85 % with a drum vacuum filter,
prior to supplying the lime sludge to a lime sludge reburning kiln. At the beginning
of the filtering process, a so-called precoat filter layer of the lime sludge is
formed on the surface of the filter drum, the thickness of the layer being normally
10 - 20 mm. A scraper is used for scraping off the dried lime sludge from the surface
of the precoat. The dried lime sludge falls onto a conveyor and further onto a
feed screw which introduces the dried lime sludge into a lime sludge reburning
The surface layer of the precoat becomes clogged while the filtering
proceeds and it must be removed from time to time. The method normally used is
to move an automatic scraper gradually inwards according to a certain program whereby
the clogged surface layer is removed. After the scraper has moved in several steps
closer to the drum the whole precoat is removed and a new precoat formed.
All the movements of the scraper described above and the removal
of the precoat from the surface of the drum result in disturbances in the operation
of the lime sludge reburning kiln. After the scraper has moved, the filtering of
the lime sludge is more efficient and the lime sludge volume bigger for a time.
Usually also the dry-solids content of the lime sludge is higher after a movement
of the scraper, which result in a change in the operation of the lime sludge reburning
kiln. When the whole precoat is renewed the supply of lime sludge to the kiln is
stopped for a while which causes a major disturbance in the kiln. Typically, the
precoat is replaced once a shift or three times during 24 hours. During a longer
period of time, the filter cloth becomes so badly clogged that it must be washed
either with an acid or with a high-pressure washer. This operation also causes
a longer disturbance or a cut in the production of the lime sludge reburning kiln.
It is an object of the present invention to provide an improved method
and an apparatus for thickening lime sludge. The main characteristic feature of
the method of the present invention resides in the removal of a strip at a time
of the lime sludge from the filtering layer by means of a liquid jet reciprocating
continuously in the longitudinal direction of the drum.
The characteristic feature of the apparatus of the invention is that
at least one nozzle is provided under the scraper and continuously reciprocable
in the longitudinal direction of the drum.
High-pressure nozzles, two for example, are provided under the scraper
in such a way that one of the nozzles breaks the precoat and drops it into the
vat and the other nozzle washes the filter cloth clean. The pressure of the water
in the nozzles is approx. 50 - 100 bar which gives a very good cleaning effect.
The nozzle spreads the jet in such a way that the precoat is removed from a strip
of 10 - 50 mm in width and the same area of filter cloth is washed at a time.
The washing nozzles are installed in a rack designed for displacing
the nozzles in the lateral direction. The nozzles are continuously reciprocated
in the longitudinal direction of extension of the drum which continuously renews
the precoat and washes the filter cloth.
Advantages of continuous high-pressure water scraping are:-
- the dry-solids content of the lime sludge from the lime sludge filter to the
lime sludge reburning kiln is maintained continuously uniform which allows constant
operation of the kiln;
- change of precoat is not necessary as the precoat is renewed continuously a
strip at a time. Disturbances in the reburning kiln due to a change of precoat
- there is no need to interrupt filtering for washing of the filter cloth.
The invention is described more closely, by way of example, with
reference to the accompanying drawing which illustrates schematically an embodiment
of the invention and in which:-
- Fig. 1 is a cross-sectional view of the apparatus; and
- Fig. 2 is a side view of the apparatus partly in section.
The Figures illustrate a drum vacuum filter comprising a drum 2 mounted
on a shaft 4. The shell of the drum is composed of a perforated plate 6 covered
with a filter cloth 8. The drum is partly immersed in a vat 12 containing lime
sludge 10, the vat being provided with an inlet 13 via which lime sludge is introduced
into the vat with a dry-solids content of 20 - 25 % for filtering. The drum is
connected to a vacuum system via an end of the shaft, i.e. the hollow end 14, by
a method known per se.
A scraper 16 is provided adjacent to the drum, extending from one
end of the drum to the other, for removing dried lime sludge from the surface of
the drum and for guiding it onto an adjacent belt conveyor 18 which transports
it to a lime sludge reburning kiln.
There are two high-pressure washer nozzles 20 and 22 disposed one
on top of the other and under the scraper and both mounted to be reciprocably displaceable
along a bar 24 extending in the longitudinal direction of the drum. The nozzles
are connected by a method known per se to a drive means (not illustrated) which
continuously reciprocate such in the longitudinal direction of the drum from its
one end to the other.
When the drum rotates down into the lime sludge in the vat the lime
sludge adheres in a layer onto the filter cloth. The partial vacuum in the drum
provides the pressure difference which extracts liquid from the lime sludge through
the filter cloth. When the immersed surface of the drum is rotated up from the
lime sludge the partial vacuum results in liquid extraction from the lime sludge
layer whereby the dry-solids content of the lime sludge increases remarkably. The
liquid separated from the lime sludge, i.e. white liquor, is removed via the hollow
shaft of the drum.
The thickening of lime sludge is preferably performed by means of
a precoat layer 26 formed on the surface of a filter drum, i.e. the thickening
takes place through both the filter cloth and a filtering lime sludge layer formed
on the filter cloth. The scraper 16 scrapes lime sludge of a dry-solids content
of 70 - 85 % from the surface of the precoat and drops it onto a belt conveyor
18. The tip of the scraper 16 is kept at a certain constant distance from the surface
of the drum so as to maintain a precoat thickness of approx. 10 - 15 mm.
During each rotation of the drum, the displaceable nozzles 20 and
22 move under the scraper and remove a part of the precoat layer. The water jet
from the upper nozzle 20 breaks the precoat and causes it to drop into the vat
12 and the lower nozzle 22 washes the filter cloth clean. The nozzles are provided
with such a high pressure, for example 100 bar, that a good cleaning effect is
achieved. The nozzles spread the water jet in such a way that the precoat is removed
from a strip of 50 mm in width at a time and the same area of filter cloth is washed
at a time.
The invention is not limited by the embodiment described as an example
only but it can be modified and applied within the scope of protection defined
by the appended patent claims.