Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02936519 2016-07-19
METHOD FOR TREATING SUSPENSIONS OF MINERAL PARTICLES IN
WATER WITH A THICKENER USING A DUAL CHEMICAL TREATMENT
FIELD OF THE INVENTION
The invention relates to a method for treating a suspension of mineral
particles in
water with a thickener using the following sequence: addition an anionic
flocculant/
mixing step/addition of a cationic compound.
Suspensions of mineral particles in water include all types of tailings, or
waste
materials. The suspensions result from mineral ore processes. They are for
instance
industrial tailings and all mine wash and waste products resulting from
exploiting
mines, such as coal mines, diamonds mines, phosphate mines, metal mines
(alumina,
platinum, iron, gold, copper, silver, etc...). Suspensions can also result
from drilling
mud or tailings derived from the extraction of bitumen from oil sand. These
suspensions generally comprise mineral particles such as clays, sediments,
sand, metal
oxides, and may contain oil mixed with water.
The invention is particularly dedicated to the treatment of oil sand tailings
when said
tailings are treated with a thickener.
BACKGROUND OF THE INVENTION
The treatment of tailings has become a technical, environmental and public
policy
issue.
It is common practice to use synthetic or natural polymers such as coagulants
and
flocculants to separate the solids from the liquid.
For a long time, and even nowadays, mineral tailings produced by physical or
chemical
ore treatment methods have been stored above ground in retention lagoons,
ponds, dam
or embankments in semi-liquid form. These large volumes of stored tailings
therefore
create a real hazard, notably if the dikes break.
The improvement of chemical and mechanical treatments of tailings is therefore
a
great challenge that needs to be addressed.
Various attempts were made in the past decades to improve the treatment of
tailings in
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order to efficiently recycle water and reduce the volume of tailings ponds.
Basically
two types of method have been developed to treat tailings and separate solids
from
water: physical treatment and chemical treatment.
The main physical treatments include centrifugation, filtration,
electrophoresis and
electro-coagulation.
On the other hand, chemical methods are emerging. They include processes
involving
the addition of chemicals such as sodium silicate, organic flocculants,
inorganic
coagulants, oxidizing and reducing agents and carbon dioxide.
Thickeners.have been utilized extensively for numerous mining operations.
Thickeners
are large, round tanks used to separate by sedimentation solids from liquids;
clear fluid
overflows from the tank and particles sink to the bottom, the underflow. The
main
objective for a thickener is to obtain a high solid concentration in the
underflow while
maintaining a reasonably low solids concentration in the overflow.
Full scale thickener operations have shown that in general with a single
anionic
polymer treatment, tailings can be well-thickened. However, a single anionic
polymer
treatment cannot deliver satisfactory results when the fine/clay content of
the thickener
feed increases. Dual polymer treatments have been used to improve the
thickener
performance. In these dual polymer treatments, a cationic polymer is added to
neutralize the charges of solid particles prior to the addition of the anionic
polymer.
In all cases of dual polymer treatment for thickeners, the cationic polymer is
always
added before the anionic polymer addition.
Despite great advances in research over the last 10 years, there is still a
need to
develop new solutions to increase quality of overflow by decreasing solids
content, as
well as to improve the compaction of underflow thickener. There is a great
need to
recycle the overflow in order to recycle it immediately back to the process.
SUMMARY OF THE INVENTION
The present invention addresses the above needs by providing a method for
improving
the treatment of suspensions of solid particles in water thanks to the use of
an anionic
flocculant followed by a mixing step and the use of a cationic compound
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According to the invention, it was found that the addition into a suspension,
such as
tailings, of an anionic flocculant, followed by the addition of a cationic
compound
significantly improves the quality of overflow and the compaction of the
underflow of
a thickener when these additions are separated by a mixing step.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to a method for treating an aqueous suspension of
mineral
particles comprising feeding said suspension into a thickener, wherein the
suspension
of mineral particles is first flocculated by addition of an anionic
flocculant, and then
flocculation is enhanced by a subsequent addition of a cationic compound,
wherein the
two addition steps are separated by a mixing step.
In a particular embodiment of the invention, an anionic flocculant is added
into the
pipe transporting the suspension of mineral particles to the thickener, and
the
subsequent addition of a cationic compound is made either into the pipe
transporting
the suspension of mineral particles, and/or either into the thickener.
In an alternative form of the invention, an anionic flocculant is added into
the thickener
and the subsequent addition of a cationic compound is made into the thickener.
In another alternative form of the invention, an anionic flocculant and a
portion of the
cationic compound are added into the pipe transporting the suspension of
mineral
particles to the thickener, and the remaining of the cationic compound is
added into the
thickener.
Within the thickener, the flocculated solids settle to the bottom of the
thickener and are
preferably mechanically raked to increase compaction of the solids and push
the solids
to the discharge at the bottom of the thickener, referred to as the underflow.
Recovered
water, called overflow, flows over a weir at the top of the thickener.
According to a preferred embodiment, the overflow of the thickener can be used
to
prepare a solution' of the anionic flocculant or the cationic compound, said
solution
being used into the method of the invention.
According to the invention, the underflow of the thickener may be pumped to a
deposition area for dewatering. In a preferred embodiment; the underflow of
the
thickener is flocculated during transport from the thickener to the deposition
area. This
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secondary flocculation is made with the addition of at least an anionic
flocculant.
In general, aqueous suspensions of mineral particles are concentrated and
contains
preferably between 2% and 30% solids, more preferably between 5% and 30%
solids
by weight, and even more preferably between 10 and 15% solids by weight of the
suspension .of mineral particles. They are also called mineral tailings. But
suspensions
having lower solids content may be efficiently treated with the method of the
invention.
It has been discovered that the method according to the invention is
especially efficient
when the suspension of mineral particles is a tailings resulting from oil sand
extraction.
The treatment of oil sand tailings has recently become an increasing issue in
Canada.
The oil sands tailings are alkaline aqueous suspensions which contain un-
recovered
residual bitumen, salts, soluble organic compounds, sands and clays. The
tailings are
discharged to tailings ponds for storage.
The dual chemical treatment of the invention outperforms the known processes.
It
increases underflow solids content which leads to significant reduction of
tailings
volume and faster reclamation of the tailings. It increases recovered water
quality.
Recovered 'water can be recycled immediately back to a process, for example
for the
bitumen extraction process or for making down polymer solution. This
considerably
reduces operation cost.
It also increases robustness of the process (widens operation windows). The
dual
chemical treatment of the invention is less sensitive to thickener feed
variation,
maintaining good performance with wide dosage windows, mixing.
Anionic flocculant
In the present invention, the anionic flocculant is preferably a synthetic
water soluble
polymer and is advantageously obtained by polymerization of at least one non-
ionic
monomer and at least one anionic monomer.
The anionic flocculant is preferably a synthetic water soluble polymer having
an
anionic,ity ranging from between 10 to 70 mol%, preferably from 10 to 90 mol%,
more
preferably from 20 to 40 mol %. The molecular weight of the anionic flocculant
is
preferably comprised between 5 and 40 million daltons, and more preferably
between
10 and 20 million.
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Cationic Compound
The cationic compound may be selected from the group consisting of a synthetic
cationic flocculant, a synthetic cationic coagulant, a cationic inorganic
coagulant, a
5 cationic natural polymer and semi-natural polymer.
In a preferred embodiment, the cationic compound is a synthetic cationic
flocculant or
a synthetic cationic coagulant.
Cationic Flocculant
In the present invention, the cationic flocculant is preferably a synthetic
water soluble
polymer and is advantageously obtained by polymerization of at least one non-
ionic
monomer and at least one cationic monomer. The molecular weight of the
cationic
flocculant is preferably more than 2 million and less than 40 million dal-
tons.
The cationic flocculant has a cationicity preferably ranging from between 20
to 100
mol%, more preferably from 40 to 80 mot %. The molecular weight of the
cationic
flocculant is preferably comprised between 5 and 40 million daltons, and more
preferably between 7 and 20 million.
Cationic coagulant
In the present invention, the cationic coagulant is preferably a synthetic
water soluble
polymer and is obtained advantageously by polymerization of at least one
cationic
monomer. The molecular weight of the cationic coagulant is preferably 1
million
daltons or less.
The cationic coagulant has preferably a 100 mol% cationicity. The molecular
weight of
the cationic coagulant is preferably comprised between 50.000 and 1 million
daltons,
and more preferably between 200.000 and 1 million.
Cationic inorganic coagulant
Cationic inorganic coagulant is preferably chosen , in the following group:
(poly)aluminium chloride, (poly)aluminium sulfate, (poly)aluminium
chlorohydrtate,
ferric chloride, ferric sulfate, ferrous chloride, ferrous sulfate, ferrous
chlorosulfate,
bentonite, gypsum, and calcium chloride, etc.
Cationic natural or semi-natural polymer
The cationic natural polymer is preferably chosen in the following group:
cationic
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cellulose, cationic dextrin, cationic dextran, chitosan, starch and starch
derivates,
polysaccharides, and alginates.
Monomers
The monomers useful for the preparation of the water soluble polymer (anionic
flocculent and cationic compound) according to the invention, are described
hereafter.
Non-ionic monomers are preferably selected from the group comprising
acrylamide;
methacrylamide; N-mono derivatives of acrylamide; N-mono derivatives of
methacrylamide; N,N derivatives of acrylamide; N,N derivatives of
methacrylamide;
acrylic 'esters; and methacrylic esters.
The most preferred non-ionic monomer is acrylamide.
Anionic monomers are preferably selected from the group comprising monomers
having a carboxylic function and salts thereof; monomers having a sulfonic
acid
function and salts thereof; monomers having a phosphonic acid function and
salts
thereof, They include for instance acrylic acid, acrylamide tertio butyl
sulfonic acid,
methacryliC acid, maleic acid, itaconic acid; and hemi esters thereof.
The most preferred anionic monomers are acrylic acid, acrylamide tertio butyl
sulfonic
acid (ATBS), and salts thereof. Generally, salts are alkaline salts, alkaline
earth salts or
ammonium salts.
Cationic monomers are preferably selected from the group comprising
dimethYlaminoethyl acrylate (DMAEA) quatemized or salified; dimethylaminoethyl
methacrylate (DMAEMA) quaternized or salified; diallyldimethyl ammonium
chloride
(DADMAC); acrylamidopropyltrimethylammonium chloride (APTAC); and
methacrylamidopropyltrimethylammonium chloride (MAPTAC).
Monomers having a hydrophobic character may also be used as comonomer for the
preparation of the water soluble polymer (anionic or cationic) but at a
concentration in
weight. based on the total monomer content of less than 5%. They are
preferably
selected from the group comprising (meth)acrylic acid esters having an alkyl,
arylalkyl
or ethoxylated chain; derivatives of (meth)acrylamide having an alkyl,
arylalkyl or
dialkyl chain; cationic allyl derivatives; anionic or cationic hydrophobic
(meth)acryloyl derivatives; and anionic or cationic monomers derivatives of
(meth)acrylamide bearing a hydrophobic chain.
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Polymers of the invention are linear or structured. As is known, a structured
polymer is
a polymer that can have the form of a star, a comb, or has pending groups of
pending
chains on the side of the main chain. The polymers of the invention, when
structured,
remain water soluble.
The polymerization can be carried out according to any polymerization
techniques well
known to= a person skilled in the art: solution polymerization, suspension
polymerization, gel polymerization, precipitation polymerization, emulsion
polymerization (aqueous or reverse) followed by an isolation step in order to
obtain a
powder, for example a spray drying step, or micellar polymerization followed
by an
isolation step for example a precipitation step in order to obtain a powder.
The polymerization is generally a free radical polymerization, preferably by
inverse
emulsion polymerization or gel polymerization. By free radical polymerization,
we
include free radical polymerization by means of U.V. azoic, redox or thermal
initiators
and also Controlled Radical Polymerization (CRP) techniques or template
polymerization techniques.
According to particular embodiment of the invention, the anionic flocculant is
a
copolymer of acrylamide and acrylate, preferably a copolymer of 70mol%
acrylamide
and 30mol% of sodium acrylate.
According to another particular embodiment of the invention, the cationic
compound is
a synthetic cationic flocculant of quaternized dimethylaminoethyl acrylate.
According to another particular embodiment of the invention, the cationic
compound is
a synthetic cationic coagulant of diallyldimethyl ammonium chloride.
Dosage
In the method of the present invention, the anionic flocculant is added into
the aqueous
suspension of mineral particles to treat ill an amount preferably comprised
between 25
and 500 g/ ton of solid mineral particles of the suspension of mineral
particles,
preferably between 75 and 150 g/ton.
The synthetic cationic flocculant is added into the aqueous suspension of
mineral
particles to treat in an amount comprised between 25 and 250 g/ ton of solid
mineral
particles of-the suspension of mineral particles, preferably between 25 and 75
g/ton.
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The synthetic cationic coagulant is added into the aqueous suspension of
mineral
particles, to treat in an amount comprised between 25 and 250 g/ ton of solid
mineral
particles of the suspension of mineral particles, preferably between 25 and 75
g/ton.
The cationic inorganic coagulant is added into the aqueous suspension of
mineral
particles to treat in an amount preferably comprised between 50 and 2500 g/
ton of
solids, preferably between 100 and 2000 g/ton of solids.
The cationic natural polymer is added into the aqueous suspension of mineral
particles
to treat in an amount comprised between 25 and 2500 g/ ton of solid mineral
particles
of the suspension of mineral particles, preferably between 100 and 2000 g/ton.
Anionic flocculant and cationic compound addition
According to the invention, the water soluble polymers are preferably added in
the
form of an aqueous solution. Polymers are often available in powder form,
sometimes
in emulsion form. Prior to their addition, they are preferably prepared in
concentrated
solution, called "mother solution" at a concentration comprised between 0.1
and 2
weight ,%. A solution is prepared for each of the anionic and cationic water
soluble
polymer, generally by diluting the "mother solution". Then, said diluted
solutions are
added into the suspension of mineral particles in the appropriate amount and
according
the specific sequence of the method of the invention.
Mixing Step
The mixing step may occur in the pipeline transporting the treated tailings to
the
thickener, in the thickener, or may be caused by inline static mixer or inline
dynamic
mixer.
Obviously, the following examples and figures are only given to illustrate the
subject
matter of the invention, which is in no way restricted to them.
Figure 1 is a diagram showing the underflow solids content (in weight %) and
the
overflow clarity (recovered water quality) (in clarity wedge) of thickener
feed from an
oil sand extraction site 1, treated by anionic and/or cationic polymer in
using different
addition sequences.
Figure 2 shows flocculation results for a thickener feed obtained from a
different Oil
Sands operator treated using varying addition sequences of anionic and
cationic
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polymer.
EXAMPLE
Laboratory tests show that the dual polymer process according to the invention
increases the overflow quality (decreased solids content) and compaction of
the
thickener underflow.
Methodology
Undiluted thickener feed and process water samples are obtained from two Oil
Sands
operators. The solids content of the thickener feed tailings is10%.
Three polymers are used:
- anionic flocculant, AF: copolymer of acrylamide and sodium acrylate (7 /3
molar ratio) ¨ Molecular weight of 13-14 Million daltons;
- cationic flocculant, CF: polymer of DMAEA MeCI - Molecular weight of
7-8
Millions daltons, and
- cationic coagulant, CC: PolyDADMAC ¨ Molecular weight of 600-800
Thousand daltons.
Polymer solutions are prepared at a concentration of 0.1% by weight using
process
water. 500 mL samples of thickener feed are placed in graduated cylinders and
are
treated either with anionic polymer alone (single AF), with anionic flocculant
and
cationic flocculant or coagulant in different sequences. Each polymer solution
(anionic
and cationic) was added in stages and the cylinder was inverted a number of
times
between different stages.
The flocculated samples were allowed to settle for 10 min after which, the
quality of
the overflow, as well as solid content of the resulting underflow, were
determined.
Results
The properties of the thickener feed tested are shown in Table 1.
Figure 1 shows the effect of polymer addition sequence on the underflow solids
content and overflow clarity of thickener feed obtained from Site 1. It can be
seen that
the addition of cationic flocculant (CF) before anionic flocculant (AF)
improves clarity
and underflow solids content slightly. However, if CF is added after AF, the
clarity
and solids content increase significantly.
=
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Figure 2 shows similar results for thickener feed obtained from Site 2. As
seen
previously, adding cationic coagulant (CC) after AF increases both clarity and
underflow compaction.
5
Sample Solids content SFR Clay content pH
[wt%] [ /0]
Site 1 10.0 0.8 34 8.2
Site 2 10.0 1.0 27 8.5
Table 1: Thickener feed properties
SFR is the Sand / Fine Ratio: ratio of mass of sand (coarse solids, >44 urn)
to fines
(fine solids, <44um).
=
