Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR TREATING MINERAL SLUDGE AND INSTALLATION FOR
CARRYING OUT SAME
The present invention relates to a method for treating mineral sludge and an
installation for implementing the method. It applies, inter alia, to the
treatment of
effluents from the mining or mineral industry, dredging processes, or public
works
operations generating large quantities of mineral sludge, for their
dehydration and
solidification above the ground.
Due to their substantial volume, these sludges raise a major storage problem
insofar as
they are indestructible.
Conventionally, they were mainly stored by spreading the sludge in closed
areas
where the sludge settled and was then abandoned:
- construction of a dam on entrenched valleys,
- use of the excavations produced by the removal of the materials,
- more or less natural lagoons (basins),
- construction of border strips with embankments.
While certain coarse sludges (sand, gravel, etc.) can dry and reconstitute a
soil usable
in agriculture for example, most effluents, containing large quantities of
suspended
matter in colloidal form (clay, silt, etc.), are particularly stable and
difficult to settle
out and do not dehydrate.
~,
In the context of the present invention, the terms "sand", gravel , "clay"
and "silt"
designate the size of the grain rather than its composition. Sand is a
material passing
through a No. 4 sieve (4.76 mm), but not through a No. 200 sieve (0.074 mm).
Gravel
is a granular material that does not pass a No. 4 sieve and can be as large as
9 cm.
The finest colloidal material passing through the No. 200 sieve iscalled clay
or silt.
Stored in this way, these sludges constitute risk zones in the long term,
particularly
said sludge basins in case of dam break. Moreover, with a more or less dry
surface
crust, covering a soft "colloidal layer", these storage areas cannot withstand
the
weight of animals, pedestrians, or vehicles.
Since conventional storage solutions have proved inadequate or even dangerous,
a
growing body of national regulations had been published prohibiting the
abandonment
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of these zones, accompanied by a remediation obligation, that is treatment and
consolidation.
For this purpose, various techniques have been tested in order to facilitate
the removal
of the trapped water: vibration, porous cores, drains, high-evaporation trees,
but the
layers are so thick (3 to 150 meters) and the quantity of trapped water so
large that
these treatments have all failed.
A first improvement was obtained by thickening the effluents before sending
them to
the basins. The thickening (or conditioning) consists of a flocculation of the
sludge
aimed to promote, with slow mixing, the contact between the colloidal
particles,
which then gather together to form more bulky aggregates (flocs).
The application of this technique has made it possible to double the dry
matter
concentration of the sludges treated, but in most cases (colloidal clays, red
sludge, tar
sand sludge, dredging with high silt, etc.) the results have been inadequate.
In
particular, it has turned out that the transport of these sludges by pipeline
before
storage resulted in the destruction of the flocs, thereby at least partially
returning the
sludge to its original colloidal structure.
In 1979-1980 Alsthom Atlantique and SNF (US 4347140) developed a multistep
flocculation system designed for the special treatment of clay lagoons created
by
phosphate production in Florida.
In a first step, the method applied consisted in using flocculents to
condition the
sludges (originating either from a phosphate washery, or from dredging lagoons
where
they had been stored), in a settler, or a settlerfollowed by a thickener.
The thickened sludge was then pumped to a zone above the ground, where it was
then
deposited after a piped addition at one or more points of an additional
quantity of
flocculent. This overflocculation in the line had the result of enabling the
sludge to
constitute a flow slope, thereby allowing storage above the ground and not in
a border
strip. The water which escaped from the sludge thus overflocculatedwas
recycled via
a peripheral ditch and repumped to the phosphate washery and/or to a
flocculent
dilution unit, in order to increase its effectiveness. It was also
demonstrated that a
ballast layer, located under the storage zone, allowed drainage of the bore
water and
sharply decrease the drying time. Similarly, the addition of the flocculent at
several
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points had the effect of improving its effectiveness and allowing a partial
reduction of
its consumption.
Simultaneously, some tests with flocculent injection from a pressure chamber
incorporated in the piping allowed a significant but very limited iinprovement
of the
system.
The Alsthom method, although innovative, has many drawbacks:
- the need to dilute the flocculent (usually to under 1 g/1) so that it can be
mixed
with the sludge flow within a few seconds, thereby increasing the volume of
sludge by 30 to 40%,
- moreover, due to the pumping speed usually applied in the piping (about 2 to
3 m/sec), a high overbatching of the flocculent is necessary so that the flocs
are not destroyed as they are formed.
The invention overcoines all the drawbacks described above.
Description of the invention
It is therefore the object of the present invention to develop a method and an
installation for treating effluents from the mining or mineral industry for
their
dehydration and subsequent solidification above the ground, so that the
storage
location again becomes usable.
According to the invention, it has been found surprisingly that the upward
movement
of a sludge issuing from the mining or mineral industry in a vertical tank
serving as a
flocculator, placed at the pipe outlet just before the spreading operation,
and in which
a flocculating agent is introduced via one or more telescopic injection pipes,
served to
significantly improve the settling of the fine solid particles present in the
sludges
during their storage above the ground for the remediation of the areas used.
A primary object of the invention is therefore a method for treating sludge
from the
mining or mineral industry, whereby, before spreading above ground, the sludge
is
subjected to an upward movement during which it is contacted with a
flocculating
agent.
In the rest of the description and in the claims, "sludge" means an effluent
issuing
from the mining or mineral industry, from dredging processes or public works
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operations. The inventive method therefore consists in inserting, before the
spreading
step, a specific flocculation step during which the sludge undergoes an upward
movement.
According to a first feature, the upward speed of the sludge is higher than
its settling
rate. The upward speed is between 0.1 and 0.5 m/s in practice.
Advantageously, the upward movement is duplicated by a centrifugal movement
entraining the mixture of the sludge and the flocculating agent at a speed of
between
0.2 and 0.8 m/s. This produces a uniform mixture. It is also possible to
accelerate the
sludge in the tank through the presence of deflectors obscuring part of the
surface and
changing the direction of the currents, thereby improving the mixing and
flocculation.
According to the invention, the flocculating agent is introduced into the
sludge in a
concentration of between 50 and 500 g per tonne of solid according to the type
and
composition of the sludge to be treated.
In practice, the contact time between the sludge and the flocculating agent is
between.
1 and 10 minutes, advantageously between 2 and 5 minutes.
According to another feature, the flow angle of the sludge at the time of
spreading is
preferably between 2 and 20% (i.e. level difference per 100 meters at the
horizontal).
A further object of the present invention is an installation for implementing
the
method.
This installation is characterized in that it comprises a vertical tank fed at
its base by a
sludge intake line.
According to another feature, the tank is preferably provided in the upper
part with a
30, ramp suitable for removing the sludge.
In practice:
a) the tank:
- has a diameter for maintaining an upward speed higher than the settling rate
of
the flocculated sludge,
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- has an inlet pipe for the sludge to be treated, either via an immersion tube
or a
line, located in the bottom part of the tank and shaped so as to create a
centrifugal effect allowing a mixing at low speed (0.2 to 0.8 m/sec),
- and is dimensioned so that the sludge has a residence time of between 1 and
5 10 minutes, preferably between 2 and 5 minutes,
b) the flocculating agent:
- is introduced via one or more - telescopic - injection pipes or distributed
on
the height of the tank for controlling the flocculation time. For example,
with
a tank and a residence time of 4 minutes, the mixing can take place in I to
4 minutes, depending on the height of the flocculent injection into the
sludge,
- and is preferably injected in dilute form, in a water solution or in a
dispersion
in a solvent medium (or brine) in which the flocculent is insoluble or not
completely soluble,
c) the flocculated sludge:
- is removed via the top of the tank, due to the upward speed, or is deposited
on
its storage area above the ground via a ramp consisting: - of one or more
chutes, optionally perforated (curved grids), preferably made from steel or
wood, or - via a riprap under the flocculation tank or - via large pipes at
low
speed (less than 0.2 m/sec).
This method serves both to very significantly reduce the consumption of
flocculent in
comparison with a direct addition of the same flocculent in the line, and also
to
sharply increase the flow angle of the sludge, which is usually 2 to 7% and
can be
raised to 8 to 20%.
It is also observed that the steeper the slope, the lower the pore water and
the faster its
drainage, despite the pressure exerted by the height of the sludge.
According to the invention, to obtain large storage volumes, it is necessary
to
construct relatively high flocculation tank supports (10, 20 up to 50 meters).
Similarly, once this treated and discharged sludge reaches the bottom of the
tank, the
tank has to be moved or another one reconstructed when it is positioned in a
central
pivot and impossible to recover.
In addition to the above arrangements, the invention also comprises other
alternative
embodiments. Among them, mention can be made of the following in a nonlimiting
manner:
- the addition of sand, lime and/or calcium sulfate to the sludge,
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- the introduction of the flocculent at 2 points in the tank, or at one point
in the
line and the second in the tank according to the invention, or the use of 2
types
of flocculents according to the abovementioned 2 introduction modes,
- the presence of a cleaning opening at the bottom of the tank to remove the
heavy gravel or sand that accumulates therein,
- the use of a tank on wheels or rails, enabling it to move on the embankment,
- the use of the angle created by the embankment as a ramp.
The invention has the following main advantages:
- sharp reduction in flocculent consumption,
- significant improvement in dehydration and solidification of the sludge,
- and considerable adaptability to the requirements associated both with the
type
of sludge to be treated (in terms of composition: quantities of fines/sand,
specific surface area, etc.), to the flow rate and to the conditions
associated
with the storage site itself (no electricity, relatively inaccessible, etc.).
Typically and without limitation, the sludgy effluents from the industry and
treated
according to the invention originate from dredging processes, public works
operations,
effluents issuing from ore extraction (coal, alumina, platinum, phosphate,
iron,
diamonds, gold, copper, etc.), tar sands, or any type of aqueous sludge
consisting of
clays or silts.
According to the invention, the flocculating agents used include all types of
organic
polymers soluble in water, including cationic, anionic, nonionic or amphoteric
(co)polymers. The most appropriate flocculating agent for the sludge is first
selected
in the laboratory after routine tests. Preferably, it is selected from the
group
comprising:
- anionic (co)polymers,
- cationic or amphoteric (co)polymers, generally for the most organic
sludges, such as canal dredging sludge,
- polyacrylates, polyhydroxamates, or even natural polymers such as
dextran, generally for red alumina sludge,
- polyamines or polydiallyldimethylamonium chloride (polydadmac),
generally for drilling muds.
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In practice, the polymer used consists preferably of at least one monomer
selected
from:
a) anionic monomers having a carboxylic function (e.g.: acrylic acid,
methacrylic
acid, and salts thereofJ, having a sulfonic acid function (e.g.: 2-acrylamido-
2-
methylpropane sulfonic acid (AMPS) and salts thereof),
b) and/or nonionic monomers: acrylamide, methacrylamide, N-vinyl pyrrolidone,
vinylacetate, vinyl alcohol, acrylate esters, allyl alcohol, N-vinyl
acetamide,
N-vinylformamide,
c) and/or cationic monomers: in particular and in a nonlimiting manner,
mention
can be made of dimethylaminoethyl acrylate (ADAME) and/or
dimethylaminoethyl methacrylate (MADAME) quaternized or salified,
dimethyldiallylammonium chloride (DADMAC), acrylamido propyltrimethyl
ammonium chloride (APTAC) and/or methacrylamido propyltrimethyl
ammonium chloride (MAPTAC).
In combination with these monomers, it is also possible to use monomers
insoluble in
water, such as acrylic, allyl or vinyl monomers, comprising hydrophobic group.
During their use, these monomers are employed in very small quantities, less
than
mol %, preferably less than 10.mo1 %, and they are preferably selected from
the
20 group comprising derivatives of acrylamide such as N-alkylacrylamide for
example,
N-tert-butylacrylamide, octylacrylamide and the N,N-dialkylacrylamides such as
N,N-dihexylacrylamide, derivatives of acrylic acid such as alkyl acrylates and
methacrylates.
The flocculating agents may optionally be branched using a
branching/crosslinking
agent, optionally in the presence of a transfer agent. A nonlimiting list of
branching/crosslinking agents is given below: methylene bisacrylamide (MBA),
ethylene glycol di-acrylate, polyethylene glycol dimethacrylate, diacrylamide,
cyanomethylacrylate, vinyloxyethylacrylate or methacrylate, triallylamine,
formaldehyde, glyoxal, compounds of the glycidylether types such as
ethyleneglycol
diglycidylether, or epoxy or any other means well known to a person skilled in
the art
for crosslinking.
Similarly, the (co)polymers may also:
- issue from the family of polycondensates: epichlorhydrin resin,
dicyandiamide
resin,
- or have undergone one (or more) post-modification reactions such as
hydrolysis, Hoffman degradation, Mannich product, hydroxamation reaction.
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The present invention is aimed particularly at installations for treating
mining or
mineral sludge suitable for the implementation of the invention, but also the
implementation and treatment processes in which these apparatus are included.
Obviously, the following examples are only provided to illustrate the subject
matter of
the invention, but without in any way whatsoever constituting limitation
thereof.
Figure 1 is a schematic representation of the invention of the installation
according to
a first embodiment.
Figure 2 is an alternative of Figure 1.
Figure 3 is a schematic representation of the tank in a first embodiment.
Figure 4 is a schematic representation of the tank in a second embodiment.
Figure 5 is a schematic representation of the installation of the invention
according to
a third embodiment.
Figure6 is a schematic representation of the installation of the invention
applied to
dredging.
Example 1 (counter-example)
The effluents from a phosphate washery, after separation of the sand, are
flocculated
in a settler with an anionic flocculent (acrylamide/sodium acrylate 70/30,
molecular
weight 18 million) enabling the sludge thus treated to reach a concentration
of
150 g/liter of dry matter.
The sludge is then pumped at a rate of 300 m3/h via a line to an embankment
20 meters high constructed on a bed ""of drained sand.
The same flocculent as the one used in the settler is introduced into the line
after
dilution to a concentration of 0.05 gLliter:
- at a first point 100 meters from the end of the line,
- and at a second point 15 meters from the end of the line.
The quantities of flocculent required are then varied in order to create the
maximum
slope of the sludge thus flocculated (value of 5.5%).
The quantities of flocculating agent required are:
- about 200 g/tonne of sludge at the first injection point,
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- and 300 g/tonne of sludge at the second point,
making a total of 500 g/tonne.
A sample of spread sludge is taken two days after the end of pumping. It
contains
480 g/liter of solid matter. After 12 months of storage, the surface of the
spreading
site is cracked and sufficiently consolidated to permit the passage of alight
vehicle.
Example 2
Example 1 above is reproduced. The effluents from a phosphate washery, after
separation of the sand, are flocculated in a settler (1) with an anionic
flocculent
(acrylamide/sodium acrylate 70/30, molecular weight 18 million) enabling the
sludge
thus treated to reach a concentration of 150 g/liter of dry matter.
The sludge is then pumped at a rate of 300 m3/h via a line (3) to an
embankment (2)
20 meters high constructed on a bed of drained sand.
A vertical cylindrical tank (4) with the useful volume of 6 m3 and a diameter
of
1.20 meters, is installed after the line (3). A flocculation pipe (5.1)
(Figure 2) is
lowered into the tank in order to visually obtain the best flocculation both
at the
overflow and at the bottom of the ramp (6). The flocculation pipe must be
immersed
by about 4 meters. In an alternative shown in Figure 3, three side
flocculation
pipes (5.2) are used. The optimal quantity of flocculating agent required is
320
g/tonne of sludge. The ramp has an angle of 20 to the ground. The flow slope
(7)
obtained is 9%.
After two days, the concentration of the spread sludge is 640 g/liter of solid
matter.
The surface tracking is considerable after one morith of storage with very
deep cracks.
After seven months, the sludge supports a light vehicle.
In the embodiment shown in Figure 4, the slope (2) is used as a ramp (6).
Example 3
A suction dredge (8) is used for the construction of a port in low level dunes
consisting of about:
- 85 % of very fine sand with a maximum grain size of 1 mm.
- and silts consisting of very fine clays and a small quantity of organic
matter.
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Pumping is carried out on a 12 meter high dam via a 60 cm diameter pipe at an
average flow rate of 1500 m3/h.
The tank, which is placed on an embankment (2), has a diameter of 2 meters and
a
5 height of 6 meters. The ramp (6) has an angle of 20 to the ground.
The flocculent pipe (5) is introduced at the bottom of the tank (4) and the
flocculent
rate is determined visually (ditto example 1) to obtain good flocculation and
a
perfectly clear pore water.
The quantity of flocculent required is about 90 g/tonne of dry matter. Due to
the
variations in flow rate and composition, the average steady state consumption
is about
110 g/tonne of treated sludge.
The slope obtained is very steep, on average 16%. The tank must therefore
often be
moved. The sand-silt mixture obtained is completely uniform. After one month,
it is
possible to level the treated surfaces by bulldozer and construct the storage
buildings
of the port within 6 months.
By way of comparison, when the same flocculent is introduced into a line 10
meters
before the exit, about 180 g/tonne is needed to obtain a flocculation that is
partially
broken by the height of fall. Similarly,with an introduction at 2 points (at
100 meters
and 10 meters from the exit) the consumption remains high at about 150
g/tonne. In
this case, due to the partial breakage of the flocs, part of the silts is
dispersed in the
recovery water, therefore requiring passage through a settling lagoon before
being
discharged in the sea.
Example 4
In this embodiment, the tank.(4) is mounted on a support (9) at a height of
about
20 meters. The tank is provided on its whole periphery with a ramp (6). It
follows
that the treated sludge is dumped on either side of the tank, above the
ground. When
the level of the treated sludge reaches the base of the tank, it is replaced
or buried.
The invention and the advantages thereof clearly appear from the above
description.
Particularly noteworthy is the significant improvement in the settling of the
fine solid
particles present in the sludge during its storage above the ground, to permit
the
remediation of the surfaces used.
