Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Field of the Invention
The present invention relates to a method of treating the
desulfurization sludge, slurry or suspension (hereinafter referred to simply
as sludge) of A wet-scrubber desulfurization plant and particularly to such a
sludge in which the solids, apart possibly from fly ash, consist predominantly
of calcium sulfate. The invention also relates to an apparatus for carrying
out this process.
Background of the Invention
Typical of known processes and apparatuses is the fact that a flue
gas, especially a flue gas from an electricity-generating power plant, can be
scrubbed in a scrubbing column or tower with a scrubbing liquid containing a
calcium compound or calcium ions so that sulfur oxides are absorbed from the
flue gas in a desulfurization operation.
In general, the scrubbing liquid is collected in a sup of the column
and, with the calcium compounds and/or calcium ions, is recirculated to the
scrubbing zone above the sup which can be traversed by the gas in a
counterfoil to the scrubbing liquid.
An oxidation zone can be provided in the sup to convert calcium
sulfite and calcium bisulfite, formed by the absorption of sulfur dioxide,
into calcium sulfate so that the solids which tend to come out of solution in
the sup to form the sludge, predominantly consist of calcium sulfate.
An important use for this calcium sulfate is as gypsum, i.e. as a
building material.
In the past, the desulfurization sludge drawn from the sup of such a
scrubber and which can contain soluble impurities from the gas stream which
are washed out by the scrubbing liquid was detoured and the resulting solids
washed in one or more washing stages with a washing liquid.
The washing liquid thus resulting or the liquid phase formed on
detouring contains these soluble impurities which are dissolved in the
scrubbing liquid from the flue gas, the scrubbing liquid being generally water.
The liquid containing these impurities is subjected to a
neutralization and as a result of this neutralization the impurities at least
in part can be precipitated out and removed by filtering.
The detouring and the subsequent washing process can be carried out
in a centrifuge or in an assembly of centrifuges or on a vacuum belt filter.
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The impurities generally are silicates, carbonates, hydroxides, copper
compounds, zinc compounds, and the like.
In German patent document-Open Application DEMOS 31 29 878, the
washing liquid is neutralized after it has been recovered by detouring or
from the washing stage. The product is a neutralization sludge which has been
discarded e.g. by landfill deposition. This sometimes poses a disposal
problem but the removal of the neutralization sludge from the liquid allows
disposal of the latter without further specific treatments, e.g. by
discharging into standard waste lines or as runoff.
lo In German patent document-Open Application DEMOS 32 05 636, the
neutralization sludge is returned to the oxidation zone of the scrubbing
column. While this has proved to be a partial answer to the disposal problem,
it does not fully solve this problem because the return of the neutralization
sludge to the scrubbing column and the recirculating of the sup liquid to the
scrubbing zone results in an enrichment in the impurities of the sludge in the
sup and which must be treated.
It should be apparent, therefore, that the very generation of a
neutralization sludge which must be separately handled in various ways, US a
product itself is disadvantageous.
The present invention provides an improved method of processing the
desulfurization sludge of such desulfurization columns or apparatus whereby
the drawbacks enumerated above are obviated.
This invention also provides a process for treating the
desulfurization sludge such that neutralization sludge is not formed as a
byproduct which requires separate handling.
Also the invention provides an improved apparatus for the processing
of desulfurization sludge.
Summary of the Invention
These advantages and others which will become apparent hereinafter
are attained, in accordance with the present invention, in a process in which
a desulfurization sludge is withdrawn from a desulfurization column operating
with a calcium-containing scrubbing liquid and with an oxidation zone so that
the solids in the desulfurization sludge predominantly consist of calcium
sulfate, the sludge is detoured and formed in a thin layer (e.g. may be
detoured in this thin layer through which water is extracted and is then
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washed in at least one but preferably a plurality of stages, and wherein a
liquid is neutralized to precipitate precipitatable impurities scrubbed from
the flue gas by the scrubbing liquid and contained in the neutralized liquid.
According to the invention, the precipitated impurities are filtered
out of the liquid phase in which the precipitate is formed by the thin layer
itself so that the impurities appear in the solids which form the gypsum or
which may be processed subsequently to form the gypsum. This allows these
impurities to be carried off with the gypsum and ultimately to be used in the
building material formed by the gypsum.
This invention is based upon the discovery that when the process is
described above is carried out and the impurities are precipitated, the
precipitated products do not appear to affect the gypsum either as to the
subsequent handling of the gypsum or as to the treatment of the solids in the
thin layer formed, while the thin layer of predominantly calcium sulfate
solids has a high capacity for extracting the precipitated impurities from the
washing liquids.
Naturally, this means that at the conclusion of the process, the
gypsum on which the impurities are deposited should be thoroughly mixed to
spread the impurities uniformly throughout the mass and further that the thin
layer must have a sufficient thickness to retain the precipitated impurities,
i.e. to effect the filtration.
When the gypsum is used as a building material, the impurities do not
adversely affect this use.
According to the a feature of the invention, the washing liquids
containing the impurities are drawn from desulfurization sludge to form the
thin layer and the withdrawn washing liquid is then neutralized. The
neutralized product can then be deposited directly on a previously formed thin
layer and/or can be returned to admixture with the desulfurization sludge and
absorbed on the thin layer of the latter when it is first formed.
In the latter case, the neutralized washing liquid is combined with
the desulfurization sludge before it is spread to form the thin layer. When
mixture with the desulfurization sludge is effected thorough mixing should be
effected by the use of an agitator, although direct addition of the
neutralization agent to the sludge as it is deposited on the filter may also
be used.
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The preferred or best mode embodiment of the invention, however,
provides that the neutralization sludge which is formed by neutralizing and
washing liquid obtained from washing the sludge in the form of a thin layer,
is then mixed with further quantities of the desulfurization sludge before
they are spread to form the thin layer for detouring and filtration so that
the precipitation of precipitatable impurities in the sludge is effected at
the same time as the desulfurization sludge is filtered.
In a modification, the neutralization agent is added directly to the
desulfurization sludge so that the precipitation of impurities is effected
directly in the desulfurization sludge, in which case, although a recycling of
washing liquid can still be provided, all of the precipitated impurities may
have been collected at the outset in the thin layer filtered product which is
then detoured and washed.
It will be appreciated that especially in the latter case, no
neutralization sludge per so requires collection and separate handling and
thus the entire problem of disposal of a neutralization sludge is eliminated.
The washing liquid which is obtained, free from the neutralization sludge and
the precipitatable impurities which can be separated outright by
! neutralization, could be recycled directly to the scrubber as makeup scrubbing
liquid or can be discharged as an innocuous waste water.
Brief Description of the Drawing
The above and other features and advantages of the present invention
will become more readily apparent from the following description, reference
being made to the accompanying drawing in which:
FIG. 1 is a flow diagram illustrating one approach to the method of
the present invention;
FIG. 2 is a flow diagram showing an alternative; and
FIG. 3 is a flow diagram illustrating a method utilizing features of
both approaches.
Specific Description
In all of the FIGURES, there is shown an inlet line 1 which derives
via a pump from the sup of a flue was desulfurization scrubber, such as one
of the scrubbers previously described. The desulfurization sludge in which
the solids component consists predominantly of calcium sulfate, is delivered
to a hydrocyclone 2 which performs as initial separation or detouring, the
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recycled liquid being returned at pa to the scrubber while the desulfurization
sludge is recovered at 2b.
The desulfurization sludge is deposited continuously in a thin layer
on a vacuum belt filter 3 carrying the sludge over four zones 4, 5, 6, 7 which
may be suction boxes, whose suction pumps have not been shown and which apply
suction to the underside of the belt. The suction pumps deliver respective
liquids to the collecting vessels 8.
The first zone 4 is a detouring zone and this zone is followed by
washing zones 5, 6 and 7.
The detouring liquid drawn from the sludge in zone 4 is delivered
via line 9 to a tank 13 provided with a motor-driven stirrer 14. A
neutralizing solution inlet 15 opens into this tank to introduce, for example,
calcium hydroxide as a neutralizing spent or sodium hydroxide, e.g., in an
aqueous solution. The amount of neutralizing agent thus added should be
sufficient to precipitate the precipitatable impurities mentioned previously
and which are entrained in a solution from a desulfurization column.
The precipitate of the impurities, in suspension in the liquid, is
displaced by a pump 20 to the first washing stage 5 where it is deposited on
the thin detoured layer of desulfurization sludge so that the gypsum here
acts as a filter mass absorbing the fine precipitate particles.
The liquid which is thus recovered at 10 can be disposed of as an
innocuous effluent or recycled as has been shown at 24, as a makeup addition
to the scrubbing liquid scrubber.
Fresh water as a washing liquid is introduced at stage 7 via line 23
and the washing liquid from this stage is recovered at line 12 and delivered
by a pump 24 and a line 25 to the stage 6.
The gypsum product is collected in the mixing bin 26 in which it is
thoroughly blended for subsequent use as a precursor in the manufacture of
plastic building materials. The liquid collected to the washing stage 6 is
combined with recycled effluent from stage 5 in line 21 as makeup water for
the scrubber or scrubbers.
The method illustrated in FIG. 2 differs from that shown in FIG. 1 in
that the impurities precipitate is not formed in the liquid phase recovered by
detouring but rather the entire desulfurization sludge is introduced into the
basin 13' where it is agitated by the motor-driven stirrer 14' and thoroughly
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mixed with the neutralization solution added at 15'. As a consequence, the
slurry deposited at 27 on the vacuum belt filter in the detouring stave 4
already contains the precipitated impurities which collect upon the thin layer
of detoured calcium sulfite residue.
The liquid returned at 9 as the first scrubbing solution, therefore,
is already free from these impurities.
The remainder of the process in FIG. 2 is similar to that of FIG. 1.
FIG. 3 shows a modification of both of these processes wherein the
basin 13 is provided in addition to the basin 13' and has its agitator 14 and
neutralization solution inlet lo.
Part of the neutralization is thus effected in basin 13' while any
necessary remaining impurities which can be separated out by further addition
of calcium hydroxide or sodium hydroxide solutions can be removed at the basin
13 and deposited on the thin layer at zone 5.
Line 15" seen in FIG. 3 also indicates that the neutralization
solution can be supplied to the desulfurization sludge as it is applied to the
vacuum belt factor to deposit the twin layer therein.
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