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(10) International Publication Number
(43) International Publication Date
Process for the purification of a residue
The present invention concerns a process for the purification of a residue,
in particular obtained from cement kiln dust or cement by-pass dusts or from by
products of metal manufacture, and a process for the manufacture of cement or
metal comprising the aforesaid purification process.
The residues of many industrial processes contain soluble salts which make
the disposal of these residues more difficult and expensive, insofar as these
soluble salts run the risk of being leached out and contaminating the subsoil of
the disposal site.
International Patent Application WO-A-201 1/048135 in the name of
Solvay SA the entire contents of which is incorporated by reference into the
present patent application provides processes that make it possible to recycle the
soluble salts contained in some industrial residues, in particular to recycle
potassium chloride or sodium chloride.
It has been found that the disposal of residues which contain valuable raw
materials for aforesaid industrial processes, in particular cement manufacture,
affects the economics of said processes.
The invention now makes available a process which allows for recovering
at least partially the raw materials from residues of industrial processes.
The invention concerns in consequence a process for the purification of a
residue containing solids and mother liquor and having a chloride ion content
greater than 5000 ppm by weight relative to the weight of the residue which
comprises (a) piston washing said residue with a washing fluid and (b)
recovering a purified residue. If desired, e.g. in the case of cement kiln dust, the
purified residue can be recycled in wet form to the cement production process. If
desired, the washing fluid, e.g. water, can be removed partially or completely
from the purified residue. This can be effected by dewatering, e.g. by blowing
air through the residue, or drying, e.g. by applying heat and blowing air through
the residue.
It has been found, surprisingly, that the process according to the invention
allows for recovering a purified residue, having, in particular, adequate residual
chloride content while minimizing use of washing fluid, in particular water.
Short description of the drawing
Figure 1 shows an apparatus useful for performing the process of the
present invention. It is of the vacuum belt filter type, operates at counter current
of washing liquid and residue (filter cake) and comprises a drum roller to
compact the extracted residue, several heat exchangers, and several vacuum
boxes for the extraction fluid ("washing fluid").
The process according to the invention allows for efficient removal of
soluble salts from residues of industrial processes. In particular, it is generally
possible to reduce the chloride ion content to less than 5000 ppm, often to less
than 3000 ppm by weight relative to the weight of the residue. The chloride ion
content is a suitable tracker for the presence of soluble salts in the residue and
can be measured by argentometric titration. Hence it is possible to recover a
purified residue which is generally suitable for re-use in an industrial process.
Usually, said industrial process is the same process from which the residue
containing solids and mother liquor has been recovered.
"Piston washing" is understood to denote in particular a washing operation
wherein washing fluid is transported through a layer of residue by an operation
applying a force to the layer of residue. Such operations include, for example,
applying pressure to the washing fluid or applying vacuum to the residue. In the
process according to the invention, piston washing is generally carried out so as
to substantially or completely displace the mother liquor contained in the residue.
Piston washing can also be applied as a multi staged counter current perfectly
mixed washing carried out so as to substantially or completely displace the
mother liquor contained in the residue.
"Mother liquor" is understood to denote in particular a liquid fraction
containing impurities dissolved therein. Typically, mother liquors in the present
invention include aqueous fractions containing water-soluble salts dissolved
therein.
The residue which is purified in the process according to the invention
wherein the residue can be obtained for example by treating by-products from
metallurgical manufacture, preferably from steel manufacture, or by treating
cement kiln dust or cement by-pass dusts from cement production : Cement kiln
dust (CKD) is preferably the fine-grained, solid, highly alkaline waste removed
from cement kiln exhaust gas by air pollution control devices. A suitable
treatment comprises for example contacting aforesaid by-products or cement kiln
dust with water so as to provide a suspension of solids in a solution of water
soluble impurities. If appropriate, said suspension can be concentrated, for
example filtered to provide a residue containing solids and mother liquor and
having a chloride ion content greater than 5000 ppm by weight relative to the
weight of the residue.
The aforesaid treatment can be suitably carried out in a stirred reactor. If
the residue is containing soluble sulfate ions, a pretreatment can be done with
reagents generating soluble calcium ions. This can be carried out, for example
according to the process described in WO-A-201 1/048135.
"Washing fluid" is understood to denote in particular a liquid fraction
capable of displacing the mother liquor. Usually, the content of impurities
dissolved in the washing fluid is lower than the content of impurities dissolved in
the mother liquor. Typical washing fluids in the present invention include water
or aqueous fractions containing water-soluble salts dissolved therein having a
lower content of water-soluble salts than the mother liquor. In the latter case, the
ratio of the CI- concentration of the washing fluid to the CI- concentration of the
mother liquor is generally from 0.05 to 0.9, often from 0 .1 to 0.8. pH of washing
fluid is generally from 7 to 13, often from 7 to 10. Water is a preferred washing
fluid.
In a particular aspect of the process according to the invention, washing
fluid removed from a first washing step is recovered and used as washing fluid
for a further washing step.
Washing fluid removed from a washing step can also suitably be recovered
and (a) recycled to a treatment to obtain residue or (b) treated to recover soluble
material dissolved therein. If appropriate, KC1 is a preferred soluble material
which can be recovered. This can be carried out, for example according to the
process described in WO-A-201 1/048135.
In the process according to the invention the chloride ion content in the
residue is generally from 3 to 95 % wt., often from 4 to 40 wt. % preferably
from 5 to 20 % wt. relative to the total weight of the residue. In another aspect
the chloride ion content in the residue is at most 30 % wt. In still another aspect
the chloride ion content in the residue is at most 10 % wt.
In the process according to the invention, particular examples of techniques
allowing for piston washing include centrifugation, press filtration and,
preferably, vacuum filtration. In the process according to the invention, the
piston washing is generally carried out by conveying the washing fluid to a layer
of residue disposed in a means for piston washing. Apparatuses are known
which operate according to different methods of applying piston washing.
One alternative of such apparatuses are those applying pressure on the
residue. This pressure can be applied mechanically by a plate or a drum roller
pressed on the filter cake by gravity or by hydraulic forces, and/or by using a
washing fluid applied under pressure. Such apparatus usually contain a filter
cloth which allows fluids, especially the mother liquor and the washing fluid, to
pass, but retains solids. Often, the filter cloth is continuous and moves forward
for regeneration, e.g. when the residue (filter cake) reaches a pre-set pressure
drop. The purified residue may be removed from the filter cloth mechanically,
e.g. by scrapers, optionally after dewatering or drying.
Another alternative are filter apparatuses wherein the pressure of the
atmosphere on the filter side in contact with the residue is higher than the
pressure of the atmosphere on the other side of the filter. Usually, the
atmosphere is air unless inert gases are advantageous due to the nature of the
solids, the mother liquor or the washing fluid. For the treatment of cement kiln
residues, the process is preferably performed in an atmosphere formed from -
potentially moist - air. The lowering of the pressure can be effected by suction
pumps which remove washing fluid having passed the filter. This suction
provides a lowering of the pressure. It is also possible to provide vacuum pumps
which remove a part of the atmosphere on the side of the washing fluid having
passed the filter.
Apparatus are very suitable in which the mother liquor and the washing
fluid are removed mechanically, especially by a roller drum, and by applying a
vacuum.
In existing apparatus, the filter cloth may be supported by belts. Such
apparatus are belt filter apparatus. Belt filter apparatuses are preferred because
of their reliable operation.
In particular the layer of residue may be placed on a vacuum filter.
Suitable apparatuses are available on the market. For example, pressure
belt filters, suction belt filters and vacuum belt filters are available from
Hoffmann Maschinen- und Apparatebau GmbH, Lengede/Germany, as described
in the internet under http://www.hoffmannfilter.de/english/druckbandfilter.htm,
http://www.hoffmannfilter.de/english/saugbandfilter.htm and
http://www.hoffmannfilter.de/english/unterdruck_bandfilter.htm Particular
means for piston washing are selected from a Larox® filter,e.g. the "horizontal
vacuum belt filters", for example, types "RT", "GT", "Pannevis RB/RT" type, or
automatic pressure filters "C series" and "M series", and a centrifuge.
Preferred means for piston washing are vacuum filters, preferably selected
from a drum filter and, most preferably, a vacuum belt filter.
In a particular embodiment, the process according to the invention is a
process for the purification of a residue containing solids and mother liquor and
having a chloride ion content greater than 5000 ppm by weight relative to the
weight of the residue which comprises (a) distributing a layer of said residue on a
belt filter and (b) transporting washing fluid through the layer of residue by an
operation applying a force to the layer of residue and (c) recovering a purified
residue.
As mentioned above, the residue to be purified is often distributed on a
filter, e.g. a filter cloth which preferably is supported by a belt. An even
distribution is preferred. If desired, the mother liquor can be removed by
applying pressure on the residue.
When the process according to the invention is carried out under pressure,
the filtration pressure is generally from 1 to 30 bar, preferably from 3 to 10 bar
for press filters and from 0.3 to 1 bar, preferably from 0.5 to 0.9 bar for vacuum
filters.
In the process according to the invention, the temperature of the washing
fluid is generally equal to or higher than 10°C. Preferably, it is equal to or lower
than 1°C below the boiling point of the washing fluid. Preferably, the
temperature of the washing fluid is from 10 to 99°C, more preferably from 20
to 80°C ; the preferred and more preferred range applies especially when the
washing fluid is constituted from water which may contain impurities.
When the process according to the invention comprises piston washing a
layer of residue, the thickness of said layer, especially when a belt filter as
described herein before is used, is generally from 0.3 to 10 cm, preferably from 1
to 5 cm, more preferably from 1 to 3 cm. In another aspect, the thickness of said
layer is generally from 2 to 10 cm, preferably from 3 to 7 cm.
In the process according to the invention, especially when a belt filter as
described herein before is used, the ratio of the total volume of washing fluid
supplied to the residue to the volume of mother liquor in the residue supplied to
the washing step is generally equal to or greater than 0.5. Often this ratio is
equal to or greater than 0.8. Preferably this ratio is equal to or greater than 0.9.
This ratio is generally equal to or lower than 5 . Often this ratio is equal to or
lower than 2 . Preferably this ratio is equal to or lower than 1.5.
In a preferred embodiment of the process according to the invention, which
is particularly preferred when the washing is carried out on a belt filter, the
washing fluid is applied to the residue by spraying. More preferably the washing
fluid is applied by flat spraying.
It has been found, surprisingly, that this embodiment allows for
particularly efficient washing, notably by minimizing cracks in the filter cake.
Suitable devices for applying the washing fluid by spraying include
nozzles, in particular nozzles allowing to spray the fluid in the form of a fan, In
that case, the spraying angle I generally from 45° to 110°. When a belt filter is
used, it is preferred to arrange the nozzle such that all the residue moves across
the crest. The feed pressure of the nozzle is suitably adapted to avoid washing
away the residue from the filter.
In a preferred embodiment of the process according to the invention, the
layer is disposed on a transportation device, in particular a moving belt filter and
the piston washing is carried out in counter-current mode, comprising at least 2,
often 2 to 10 washing steps, preferably 2, 3, 4 or 5, more preferably 2 washing
steps. In that case, preferably, a pure washing fluid, for example pure water is
introduced into a first washing step. Washing fluid recovered from said first
washing step is recovered and used as washing fluid for a subsequent washing
step carried out upstream relative to the first washing step.
Figure 1 is illustrating an apparatus designed for counter current washing
in according to a preferred embodiment of the invention. The apparatus used is
of the vacuum belt filter type with drum rollers r l to r5. These drum rollers are
indicated in figure 1 in the form of circles with internal inscription "rl" to "r5"
between the respective vacuum box and the respective heat exchanger, just above
the belt ; the drum rollers serve to compact the residue. Residues R are
distributed on belt filter in the form of a suspension S. The residue is moving
from left to right in the apparatus given in figure 1. Fresh pure washing water
WS is introduced as washing fluid down stream the cake displacement on the
belt at point ©. The washing water WS is heated by means of the heat
exchanger HT1. Filtrate extracted from the cake is collected in vacuum box wl ;
air is blown through the filter cake for effecting air displacement dewatering of
the filter cake. The washing fluid from vacuum box wl is pumped to be heated
by means of the heat exchanger HT2 and used as washing fluid for washing step
at point ©. Drum roller drum r l compacts the residue. The filter cake is once
again dewatered by removing the washing fluid from it by means of air
displacement dewatering. The washing fluid extracted from the filter cake is
collected in vacuum box w2. It is pumped out of the vacuum box w2, heated by
heat exchanger HT3 and used as washing fluid for washing step at point ®.
Drum roller r2 compacts the residue. Filtrate extracted from cake by means of
another air displacement dewatering is collected in vacuum box w3, afterwords
pumped out, heated with the heat exchanger HT4 and used as washing fluid for
washing step at point @. Drum roller r3 compacts the residue. Filtrate being
extracted from the filter cake by means of still another air displacement
dewatering is collected in vacuum box w4,afterwords pumped out , heated with
the heat exchanger HT5 and used as washing solution for washing step at
point (D. Drum roller r4 compacts the residue. Filtrate extracted from the filter
cake during air displacement dewatering is collected in vacuum box w5.
Afterwords, filtrate F is pumped out from vacuum box w5 and send to a further
chemical treatment if needed. Drum roller r5 compacts the residue. Purified
residue C is recycled to production.
In a further preferred embodiment of the process according to the invention
the piston washing is carried out under conditions minimizing, preferably
substantially avoiding formation of cracks in the residue. A particular means for
minimizing, preferably substantially avoiding formation of cracks in the residue
comprises carrying out the washing on a filter, in particular a belt filter further
equipped with a means for applying mechanical pressure to the residue.
In that case, the mechanical pressure is generally from 0.5 to 20ba,
preferably from 1 to 5 bar.
A drum roller is a preferred means for applying mechanical pressure to the
residue. In a particular aspect, the means for applying mechanical pressure is
movable relative to the residue surface. The term "movable" means especially
that the means for applying pressure, especially a drum roller, is arranged in the
apparatus in such a way than the linear speed of the drum roller surface can be
higher than the linear speed of the cake. The relative speed of the roller versus
the residue (filter cake) linear speed can be the same as the residue. It may be
higher, preferably equal to or lower than 30 % higher. A preferred range is
from 0 (0 being included) to 30 % higher, more preferably, the range is from 5
to 10 %.
The process according to the invention often further comprises expelling
washing fluid, preferably dewatering the residue after primary removal of the
washing fluid. Such expelling is preferably carried out by contacting purified
residue with a gas stream, in particular an air stream. In that case, the chloride
content of the residue obtained after expelling washing fluid is generally less
than 5000 ppm w , preferably less than 3000 ppm.
In that case, the expelling of the washing fluid from the residue is generally
carried under conditions allowing to obtain a washing fluid content in the residue
of at most 40 % weight relative to the weight of the residue. Preferably, this
content is equal to or less than 35 % weight relative to the weight of the residue.
Generally this content is at least 28 % weight relative to the weight of the
residue. Preferably, this content is equal to or more than 30 % weight relative to
the weight of the residue.
It has been found, surprisingly, that the residues treated according to the
invention may display thixotropic behaviour. Controlling the washing fluid
content in the residue in the ranges cited here before allow to obtain a non
thixotropic residue. This residue is particularly interesting for processing in
further processes such as cement manufacture or metal manufacture. The
invention also concerns such non-thixotropic residue obtainable by the process
according to the invention.
The invention concerns also a manufacturing process for the manufacture
of cement or a metal which comprises
a) the process according to the invention
b) using purified residue obtained in step a . as a raw material for said
manufacture.
The examples hereafter are intended to illustrate the invention without
however limiting it.
Example 1 - Piston washing of a cement kiln dust (CKD) residue on a belt filter
with a 3-step washing
A flow rate of 150 kg/h of CKD is agitated in a stirred reactor with 70 L/h
of water and 120 L/h of a recycled brine from the filtration of the produced
suspension. The suspension is filtered over a belt filter.
The residue containing a mother liquor having a CI content of 15,6 % wt.
relative to the total liquid weight and a solid content of 27 % wt. relative to the
suspension weight is continuously disposed as an, on the average, 15 mm thick
layer on a belt filter to which a vacuum of 600 mbar is applied.
The filter cake produced without washing has a CI content of 11% wt.
relative to the cake weight after drying.
Countercurrent piston washing is carried out by carrying out a first
washing with a flow rate of 150 L/h of pure water as washing fluid, a second
upstream washing with washing fluid recovered from the first washing and a
third upstream washing with washing fluid recovered from the second washing.
The washing fluid from the third washing is used as raw material for the process
described in WO-A-201 1/048135. Downstream from the first washing, an air
stream is applied to the purified residue to expel water. The chloride ion content
of the dewatered residue is lower than 5000 ppm. The dewatered residue is
recycled to a cement manufacture.
Example 2 - Piston washing of a CKD residue on a belt filter equipped with a
drum roller
The process according to the invention is carried out as in example 1, but
in a belt filter equipped with a drum roller, applying a mechanical pressure
of 2 bar to the layer of residue at the end of the last washing. The chloride ion
content of the dewatered residue is lower than 3000 ppm.
Example 3 - Piston washing of a cement kiln dust (CKD) residue on a belt filter
with a 2-step washing
42 kg/h of CKD by-pass residue was agitated in a stirred reactor with
70 L/h of water. The suspension is filtered over a belt filter.
The residue containing a mother liquor having a CI content of 12 % wt.
relative to the total liquid weight and a solid content of 30 % wt. relative to the
suspension weight is continuously disposed through a goose-foot shaped weir
system on a zone of the belt fabric filter which is not under vacuum as an, on the
average, 15 mm thick layer. The layer was moved along the belt filter while a
vacuum of 500 mbar was applied.
Countercurrent washing was carried out by applying a first washing with a
flow rate of 35 L/h of demineralized water as washing fluid. The washing fluid
was sprayed onto the layer through a DELAVAN AC2 nozzle. A second
upstream washing was carried out with washing fluid recovered from the first
washing Downstream from the first washing, an air stream was applied to the
purified residue to expel water. The water content of the treated residue was
37 % wt. and the chloride-ion content was 0.6 g/kg. The residue was not
thixotropic and can be recycled for cement production. A sample of the residue
was completely dried. The chloride ion content of that residue was 0.9 g/kg.
C L A I M S
1. Process for the purification of a residue containing solids and mother
liquor and having a chloride ion content greater than 5000 ppm by weight
relative to the weight of the residue which comprises (a) piston washing said
residue with a washing fluid and (b) recovering a purified residue.
2 . Process according to claim 1, wherein the piston washing is carried out
by conveying the washing fluid to the residue placed on a vacuum filter.
3 . Process according claim 2, wherein the vacuum filter is selected from
a drum filter and, preferably, a belt filter.
4 . Process according to anyone of claims 1 to 3, wherein the piston
washing comprises at least 2, often 2 to 5 preferably 2 or 3 washing steps and is
carried out in a counter-current mode.
5 . Process according to anyone of claims 1 to 4 wherein said washing
fluid is water.
6 . Process according to anyone of claims 1 to 5, wherein washing fluid
removed from a first washing step is recovered and used as washing fluid for a
further washing step.
7 . Process according to anyone of claims 1 to 6, wherein the piston
washing is carried out under conditions minimizing, preferably substantially
avoiding formation of cracks in the residue.
8 . Process according to anyone of claims 1 to 7, wherein the washing is
carried out on a belt filter.
9 . Process for the purification of a residue containing solids and mother
liquor and having a chloride ion content greater than 5000 ppm by weight
relative to the weight of the residue which comprises (a) distributing a layer of
said residue on a belt filter and (b) transporting washing fluid through the layer
of residue by an operation applying a force to the layer of residue and (c)
recovering a purified residue.
10. Process according to claim 9, wherein the washing fluid is transported
through the layer of residue by applying vacuum to the residue.
11. Process according to claim 9 or 10, wherein the thickness of the layer
is from 0.8 to 3 cm.
12. Process according to anyone of claims 1 to 11, wherein the ratio of the
total volume of washing fluid supplied to the residue to the volume of mother
liquor in the residue supplied to the washing is from 0.5 to 5, preferably from 0.9
to 2 .
13. Process according to anyone of claims 1 to 12, wherein the washing
fluid is applied to the residue by spraying, preferably by flat spraying.
14. Process according to anyone of claims 1 to 13, wherein the chloride
content of the residue is from 3 to 95 % w , preferably from 5 to 20 % wt.
15. Process according to anyone of claims 1 to 14 further comprising
expelling washing fluid, preferably dewatering the residue after removal of the
washing fluid, preferably by expelling water from said residue with a gas stream,
in particular an air stream.
16. Process according to anyone of claims 1 to 15 wherein the expelling of
the washing fluid from the residue is carried under conditions allowing to obtain
a washing fluid content in the residue of at most 40 % weight relative to the
weight of the residue.
17. Process according to anyone of claims 1 to 16, wherein the chloride
content of the purified residue obtained after expelling washing fluid is less
than 5000 ppm wt., preferably less than 3000 ppm.
18. Process according to anyone of claims 1 to 17, wherein washing fluid
removed from a washing step is recovered and (a) recycled to a manufacture of
slurry or (b) treated to recover soluble material dissolved therein.
19. Process according to anyone of claims 1 to 18 wherein the residue is
obtained by dissolving the water-soluble fraction of cement kiln dust from
cement production with water, preferably in a stirred reactor.
20. Process according to anyone of claims 1 to 19, wherein the slurry is
obtained by treating by-products from metallurgical manufacture, preferably
steel manufacture.
2 1. Process for the manufacture of cement or a metal which comprises
a . The process according to anyone of claims 1 to 20 and
b. using purified residue as a raw material for said manufacture
22. A non-thixotropic residue obtainable by the process according to
anyone of claims 1 to 20, comprising 28 to 40 wt. % washing fluid, preferably
water, relative to the weight of the residue and less than 5000 ppm of chloride
ions relative to the weight of the residue.