Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~Z~39135
329:~-2279
PARTICLE SIZE RANGE SEPAR~TIONS
PRIOR TO DE-WATERING
The invention relates to a method for the dewatering of
an aqueous suspension of slag particles.
When an ash-containing fuel is fully combusted, slag is
obtained as a by-product. The combustion takes place in a reactor
and the resultant slag is generally trapped in a water bath located
underneath the reactor. In slurry form the slag is comparatively
simple to convey.
; 10 Slag is also formed in the partial com~ustion of an ash-
containing fuel such as coal, lignite, peat etc. Here, too, the
resultant slag is usually trapped in a water bath, yielding an
aqueous suspension. The aqueous suspension is than discharged from
the water bath. Because it is preferred to maintain an elevated
pressure in the reactor, a sluice system is used in order to dis-
charge the aqueoussuspension. A~suitable method for this purpose
is described in the US patent specification No. 3,994,702.
It is generally not possible to process or dispose of
the aqueous suspension as such, so that is is very desirable to
~ : : 20 dewater the suspension. The slag particles can be subsequently
reprocessed and empolyed for other~purposes, for example in road
construction. The clhri~ied water can be discharged without
entailing any danger to the environment.
If dewatering o-f the supension is carried ou~ b~ means of
screen units, small slag particles in the suspension are not
caught by the screen units but~ilow along with the water phase.
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On the other hand, if a filter is used, the small slag particles
may cause blockages in the filter. It is possible to cause the
fine particles to agglomerate into larger particles by means of
a flocculant. In that case, however, the larger particles also
react with the flocculant. In order to cause all the small
particles to agglomerate, it is therefore necessary to add a
comparatively large quantity of flocculant to the suspension.
That is uneconomic.
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For that reason, iXL the method according to thLe invention the
slag particles are separated according to diameter si2e. The
present invention therefore relates to a method for the dewatering
of an aqueous suspension of slag particles, characterized in tha
the suspension is separated into different fractions, containiny
coarse, intermediate and fine particles, re~pecti~ely, and that
the fractions are dewatered separately.
By using the methLod accordin~ to the invention it is possible
to choose the most effective dewatering technique for each frac-
tion. For pc~rticle size has an important influence on this choice.In the method according to the invention the separation between
the coaLrse and inLtermediate particles is preferably effected at a
diameter size from 0.5 to 2 mm, and e~en more preferably at 1 mm
The separation between the intermediate and fine particles is
preferably effected at a diameter size from O.Ol to O.l mm, and
even m~re preferably at 5.06 mm. As a result of this the fraction
with coarse particles preferably oontains substantially pa~ticles
with a diameter larger than l mm, the fraction with intermediate
particles preferably contains substantially particles with a i
diclmeter in the range from 0.06 to l mm and ~he fraction with fine
particles preferakly contains substantially particles with a
diameter smaller than 0.06 mm.
e sequence in which tJ~suspension is separated into
~; fractions, hc~ever, is not of material i~portance. It is possible
first to separate ~he fine particles from the coarse and interme-
diate particles and subsequently to separate the remaining suspen-
sion into one fraction~with coarse particles and another fraction
with inte~mediate particles. For preference, however, the coarse
particles are separated fram tJhe suspension first.
In order to sepaxate the coarse particles from the suspen-
sion, use may be made of many types of separators. In de~ermuning
the type of separator, the eroding or abrasive action of the slag
particles is a factor to be considered.
~ydrocyclones, f~x exa~le, are exposed to the risk o
serious damage. For that reason the fraction with coarse particles
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is preferably separated from the suspension and also dewatered by
means of one or more screen units. A preferred ~mbodime~t of the
method according to the invention employs a static screen in order
to separate the greater part of the water from the suspension
whlle separating the coarse particles, followed by a vibrating
screen in order to separate the remaining water, so that the
fraction with coarse particles is sufficiently dewatered. 'L~ese
separated coarse particles are discharged together for further
processing.
0 m e remaining suspension containing the intermediate and fine
particles is subsequently separated into two fractions. To that
end it is conveyed to one or more classifying units. Various
classifying units are suitable. A classifying screw or a classi-
fying rake can be used. It is also possible to employ a hydroclassi-
fying unit or an ordinary classifying cone. Preferably, however,
the separation between the intermediate and fine particles is
effected by means of one or more hydrocyclones. The eroding or
abrasive action of the intermediate and fine particles is consider-
ably smaller than that of the coarse particles. The risk of damage
~` 20 to the hydrocyclones is therefore small. ~oreover, the risk of
damage is reduced even further by providing the interior of the
-~ hydrocyclones with a suitable protective l ming. The hydrocyclones
are so econcmic because they separate very selectively. That is to
say that relatively few fine particles enter the fraction with
25 intermediate slag particles. ~
; The fraction with inter~ediate particles is then dewatered.
For this purpose various types of separators are suitable. Suit-
able types are dewatering screen units (the "Fordertechnik screen"
and the "Elliptexrdewaterizer", see L. Sv~rcvsky, Solid-liquid
0 separation, Butterworths, 1979, pp. 165-167). A number of diffe-
rent filtration units and dewatering screws are also suitable.
~ Preferably, the fraction with the coarse particles is de-; ~latered by means of one or more dewatering rakes. These units are
~ the cheapest and most reliable. Moreover, during the dewatering
;~ 35 treatment a further separation is ef~ected between the entrained
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fine particles and intermediate particles. m e aqueous phase,
which therefore still conta ms a ~ew, substantially f me, slag
particles, is preferably recycled to the classify mg units, in
particular the hydrocyclones. This ensures that no polluted water
enters the environment.
The fraction with fine particles w~ich is discharged frc~ the
classifying units is preferably passed to one or more settling
basins. There concentration of the fine slag particles takes
place. Preferably the concentrate is passed to one or m~re filter
presses where further dewatering takes place.
m e sedi~entation rate of the fine particles in the settling
basins is lcw, because the dImensions of the particles are so
small. Particles of colloidal dimensions will even not settle at
all. It is therefore preferred to add a flocculant to the frac~ion
with fine particles. The addition of flocculant may be made in the
settling basins but also upstream thereof. Because a considerable
proportion of slag particles has already been separated from the
suspension, a ccmparatively small quantity of flocc~lant is
; sufficient. Suitable flocculants are various cationic and/or
anionic polymers.
The wat~r leaving the settling basins, name~y the averflow,
is clear and contains fewer than 50 pFm slag particles in the
water. This overflow can be discharged l:nto the surface water
without any risk of environmEnt pollution. It can also, at least
;~25 partly, be recycled to the process and used as wash mg water
and/or transport medium. The filtrate from the filter presses still
contains a minor quantity of slag particles. Because it is desired
to separate this minor quantity frcm the suspension as well~ the
filtrate is preferably recycled to the settling basms.
The m~ention will now be further elucidated with reference
to the Figure, to ~7hich the invention is hawever by no means
lim~ted. Cc~ressors, valves,~control equip~nt etc. are not shawn
:: in the dia~natic figure.
Via a line 1 an aquecus suspensiQn of slag particles is
35 p ssed to a static screen 2. There a fraction with coarse~pa~
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cles is separated and dewatered. The coarse particles are passed
via a gutter 3 to a vibrating screen 5, where further dewatering
takes place. The dewatered coarse slag particles are discharged
from ~le syste~ by means of a gutter 7 and a con~eyor kelt 8. The
separated water containing the intermediate and ~lne p2rticles is
passed through a line 4 from the static screen 2 and through a
line 6 from the vibrating screen 5 to a line 9~ From there it is
passed via a line 10 to a hydrocyclone 11, where the separation
between the intexmediate and fine particles takes place. A sus-
p~nsion containing substantially intermediate particles leaves thecyclone through a line 12, which carries the suspension to a
dewatering rake 14. In this dewatering unit the intermediate
;~ particles are separated from the suspension and removed from the
system via a gutter 15 and a conveyor belt 16. T.he aqueous sus-
pension with fine particles is passed via a line 17 into the line
9, so that the suspensi~n is recycled via the line lO into the
hydrocyclone ll.
An aqueous suspension with fine slag particles lea~es the
; hydrocyclone 11 through a line 13. In the line 13 an aqueous
: 20 solution of flocculant îs added to the suspension fru~ a vessel 25
.~: via a line 26. The aqueous suspension with the flocculant is
passed via a line 18 into a settling basin l9. m ere thi ~ of
the:suspension takes place. The ~hickened suspension is passed
through the bottom of the settl mg basin 19 via a line 20 to a
~ 25 filter press 22. m e slag particles dewatered there are disch3rged
: frcm the sysbem via a line 23. The filtrate from the filter press
22 is recycled via a line 24~to:the suspension m the line 13.
~: ~ The over low from the settling basin 19 is discharged via a
line 21. A portion of the clarified water is passed via a line 27
: 30 t~ the vibrat mg screen 5 where it is used as washing water. The
remainLng portion of the clarified water is discharged from the
system via a l me 28.
lthcugh:they are not shown ln the Figure, it is possible to
: incorpDrate a number of buf~er:vessels in the ~ethod according to
the invention m order to co~nteract:fluctuations in the fe~d to
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continuously oFerating installations, or in order to serve as
storage vessels for units operating batchwise. Suit-~ble locations
for a buffer vessel are upstream of the hydrocyclone, between the
h~drocyclone and the settling basin, and between the settling
5 basin and the filter press.
EX~LE
In a plant as diagrammatically shown in the Figure the
following experiment was carried out.
Via the line 1, 34.886 kg/s of water with 1.438 kg/s of slag
; 1~ particles is supplied. After separation on the static screen 2,
the c se slag particles are washed on the vibrating screen with
5.94 kg/s of water from the line 27. ~his produces a stream of
coarse slag particles of 0.708 kg/s with 0.236 kg/s of water which
is discharged via the gutter 7 and conveyor belt 8 for further
15 treatment.
The suspensions frcm the lines 4 and 6 are ccmbined in the
line 9, through which 40.59 kg of water and 0.73 kg of slag
particles flow per second. Via the line 17, 3.76 kg/s of wat~r
with O.I88 kg/s of slag particles is added t~ the stream in the
20 line 9. After separation in the hydrocyclone 11, via~the;line 12 a
quantity of 3.87 kg/s of water and 0.430 kg/s of slag is passed to
the dewatering rake 14, where 0.242 kg/s of coarse sl~g particles
with 0.110 kg/s of water is separated.
Via the stream in the line 13 of 40.48 kg/s of water and
25 0.488 kg/s of fine slag particles, to which via the line 24 an
; amount of 4.06 kg/s of water and 0.027 kg/s of slag is added, in
addition to O.Ol kg/s ~f water and 0.0002 kg/s of flocculant via
the l m e 26, via the line 18 a quantity of 44.55 kg/s of water and
0.515 kg/s of slag and 0.0002 kg/s of flocculant is supplied to
30 the settling basin 19. Via the line 20 a quantity~of 4.39 kg/s
with 0.515~kg/s of sl g and 0.0002 kg/ of flocculant is supplied
~ to the filter press 22. ~ ~
-~ ~ There 0.4882 kg/s of solid matter with 0.330 kg/s of water is
~ separated. The filtrate is~recycled via the line 24.
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From the settliny basin 19 a quantity of 40.16 kg/s of clearwater is discharged via the line 21. Of that quantity, 5.94 kg/s
is passed to the vibrating screen 5 via the line 27, so that a
quantit~ of 34.22 kg/s of water is discharged frcm the system via
5 the line 28.
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