Note: Descriptions are shown in the official language in which they were submitted.
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Treatment of Fine Tailings
The present invention relates to the treatment of mineral material, especially
waste mineral slur-
ries. The invention is particularly suitable for the disposal of tailings and
other waste material
resulting from mining and mineral processing operations. The invention is
particularly suitable
for the treatment of oil sand tailings and especially mature fine tailings (M
FT) derived from oil
sand tailings.
Processes of treating mineral ores or oil sands in order to extract mineral
values or in the case
of oil sands to extract hydrocarbons will normally result in waste material.
Often the waste mate-
rial consists of an aqueous slurry or sludge comprising particulate mineral
material, for instance
clay, shale, sand, grit, oil sand tailings, metal oxides etc. admixed with
water.
In some cases the waste material such as mine tailings can be conveniently
disposed of in an
underground mine to form backfill. Generally backfill waste comprises a high
proportion of
coarse large sized particles together with other smaller sized particles and
is pumped into the
mine as slurry where it is allowed to dewater leaving the sedimented solids in
place. It is com-
mon practice to use flocculants to assist this process by flocculating the
fine material to increase
the rate of sedimentation. However, in this instance, the coarse material will
normally sediment
at a faster rate than the flocculated fines, resulting in a heterogeneous
deposit of coarse and
fine solids.
For other applications it may not be possible to dispose of the waste in a
mine. In these in-
stances, it is common practice to dispose of this material by pumping the
aqueous slurry to la-
goons, heaps or stacks and allowing it to dewater gradually through the
actions of sedimenta-
tion, drainage and evaporation.
For example in oil sands processing, the ore is processed to recover the
hydrocarbon fraction,
and the remainder, including both process material and the gangue, constitutes
the tailings that
are to be disposed of. In oil sands processing, the main process material is
water, and the
gangue is mostly sand with some silt and clay. Physically, the tailings
consist of a solid part
(sand tailings) and a more or less fluid part (sludge). The most satisfactory
place to dispose of
these tailings is, of course, in the existing excavated hole in the ground. It
turns out, however,
that the sand tailings alone from the one cubic foot of ore occupy just about
one cubic foot. The
amount of sludge is variable, depending on ore quality and process conditions,
but average
about 0.3 cubic feet. The tailings simply will not fit back into the hole in
the ground.
There is a great deal of environmental pressure to minimise the allocation of
new land for dis-
posal purposes and to more effectively use the existing waste areas. One
method is to load
multiple layers of waste onto an area to thus form higher stacks of waste.
However, this pre-
sents a difficulty of ensuring that the waste material can only flow over the
surface of previously
rigidified waste within acceptable boundaries, is allowed to rigidify to form
a stack, and that the
waste is sufficiently consolidated to support multiple layers of rigidified
material, without the risk
of collapse or slip. Thus the requirements for providing a waste material with
the right sort of
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characteristics for stacking is altogether different from those required for
other forms of disposal,
such as back-filling within a relatively enclosed area.
In a typical mineral or oil sands processing operation, waste solids are
separated from solids
that contain mineral values in an aqueous process. The aqueous suspension of
waste solids
often contain clays and other minerals, and are usually referred to as
tailings. These solids are
often concentrated by a flocculation process in a thickener to give a higher
density underflow
and to recover some of the process water. It is usual to pump the underflow to
a surface holding
area, often referred to as a tailings pit or dam or more usually a tailings
pond in the case of oil
sands. Once deposited at this surface holding area, water will continue to be
released from the
aqueous suspension resulting in further concentration of the solids over a
period of time. Once
a sufficient volume of water has been collected this is usually pumped back to
the mineral or oil
sands processing plant.
The tailings pond or dam is often of limited size in order to minimise the
impact on the environ-
ment. In addition, providing larger tailings ponds can be expensive due to the
high costs of
earth moving and the building of containment walls. These ponds tend to have a
gently sloping
bottom which allows any water released from the solids to collect in one area
and which can
then be pumped back to the plant. A problem that frequently occurs is when
fine particles of
solids are carried away with the run-off water, thus contaminating the water
and having a detri-
mental impact on subsequent uses of the water.
In many mineral and oil sands processing operations, for instance a mineral
sands beneficiation
process, it is also common to produce a second waste stream comprising of
mainly coarse (>
0.1 mm) mineral particles. It is particularly desirable to dispose of the
coarse and fine waste
particles as a homogeneous mixture as this improves both the mechanical
properties of the de-
watered solids, greatly reducing the time and the cost eventually required to
rehabilitate the
land. However, this is not usually possible because even if the coarse waste
material is thor-
oughly mixed into the aqueous suspension of fine waste material prior to
deposition in the dis-
posal area, the coarse material will settle much faster than the fine material
resulting in banding
within the dewatered solids. Furthermore, when the quantity of coarse material
to fine material
is relatively high, the rapid sedimentation of the coarse material may produce
excessive beach
angles which promote the run off of aqueous waste containing high proportions
of fine particles,
further contaminating the recovered water. As a result, it is often necessary
to treat the coarse
and fine waste streams separately, and recombine these materials by
mechanically re-working,
once the dewatering process is complete.
Generally oil sands tailings ponds are located within close proximity of the
oil sands mining and
extraction operations in order to facilitate pipeline transportation,
discharging and management
of the tailings. A tailings pond may be contained within a retaining structure
which may be refer-
red to as a dyke structure. A suitable dyke structure may generally be
constructed by placing
the sand fraction of the tailings within cells or on beaches. Tailings streams
initially discharged
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into the ponds may have relatively low densities and solids contents, for
instance around 0.5 to
10% by weight.
In an oil sands tailings pond, the process water, unrecovered hydrocarbons and
minerals settle
naturally to form different strata. The upper stratum can be predominantly
water that maybe
recycled as process water to the extraction process. The lower stratum can
contain settled resi-
dual hydrocarbon and minerals which are predominantly fines. It is usual to
refer to this lower
stratum as "mature fine tailings" (M FT). It is known that mature fine
tailings consolidate extre-
mely slowly and may take many hundreds of years to settle into a consolidated
solid mass.
Consequently mature fine tailings and the ponds containing them are a major
challenge to tai-
lings management and the mining industry.
The composition of mature fine tailings tends to be highly variable. The upper
part of the stra-
tum may have a mineral content of about 10% by weight but at the bottom of the
stratum the
mineral content may be as high as 50% by weight. The variation in the solids
content is believed
to be as a result of the slow settling of the solids and consolidation
occurring over time. The
average mineral content of the M FT tends to be of about 30% by weight.
The M FT generally comprises a mixture of sand, fines and clay. Generally the
sand may refer-
red to siliceous particles of a size greater than 44 pm and may be present in
the M FT in an
amount of up to 15% by weight. The remainder of the mineral content of the M
FT tends to be
made up of a mixture of clay and fines. Generally the fines refer to mineral
particles no greater
than 44 pm. The clay may be any material traditionally referred to as clays by
virtue of its mine-
ralogy and will generally have a particle size of below 2 pm. Typically the
clays tend to be
water swelling clays, such as montmorillonites. The clay content may be up to
75% of the so-
lids.
Additional variations in the composition of M FT maybe as a result of the
residual hydrocarbon
which may be dispersed in the mineral or may segregate into mat layers of
hydrocarbon. The
M FT in a pond not only has a wide variation of compositions distributed from
top to bottom of
the pond but there may also be pockets of different compositions at random
locations throug-
hout the pond.
In addition, aqueous suspensions waste solids from mining and mineral
processing operations
including mining tailings, such as M FT, held in ponds of holding areas may
also contain coarse
debris. The type and composition of this coarse debris depends on the origin
of the suspension.
In the case of M FT the coarse debris tends to be of different sizes, shapes
and chemical com-
positions. For instance, M FT may include coarse debris such as biomass, such
as wood or
other plant material; petrified matter; solids having a density low enough to
float at or near the
surface of the pond; glass; plastic; metal; bitumen globules; or mats. The
coarse debris found
other mining tailings may include similar debris as in the case of M FT, with
the exception of bi-
tumen materials and may also include other debris materials such as lumps of
ore or other
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masses depending on the geology of the ore mine, the ore extraction processing
technique, or
the location of the tailings pond.
It is known that aqueous suspensions and mining tailings, such as M FT, may be
dewatered and
solidified through the action chemical treatments. A typical chemical
treatment employs the ad-
dition of chemical flocculating agents to bring about flocculation and be so
formed flocculated
suspensions can be subjected to dewatering.
It is well known to concentrate these oil sand tailings in a thickener to give
a higher density un-
derflow and to recover some of the process water as mentioned above.
For example, Xu.Y et al, Mining Engineering, November 2003, p.33-39
describes the addition of anionic flocculants to the oil sand tailings in the
thickener before dis-
posal.
The underflow can be disposed of and/or subjected to further drying for
subsequent disposal in
an oil sand tailings stacking area.
In the Bayer process for recovery of alumina from bauxite, the bauxite is
digested in an aqueous
alkaline liquor to form sodium aluminate which is separated from the insoluble
residue. This
residue consists of both sand, and fine particles of mainly ferric oxide. The
aqueous suspension
of the latter is known as red mud.
After the primary separation of the sodium aluminate solution from the
insoluble residue, the
sand (coarse waste) is separated from the red mud. The supernatant liquor is
further processed
to recover aluminate. The red mud is then washed in a plurality of sequential
washing stages, in
which the red mud is contacted by a wash liquor and is then flocculated by
addition of a floccu-
lating agent. After the final wash stage the red mud slurry is thickened as
much as possible and
then disposed of. Thickening in the context of this specification means that
the solids content of
the red mud is increased. The final thickening stage may comprise settlement
of flocculated
slurry only, or sometimes, includes a filtration step. Alternatively or
additionally, the mud may be
subjected to prolonged settlement in a lagoon. In any case, this final
thickening stage is limited
by the requirement to pump the thickened aqueous suspension to the disposal
area.
The mud can be disposed of and/or subjected to further drying for subsequent
disposal on a
mud stacking area. To be suitable for mud stacking the mud should have a high
solids content
and, when stacked, should not flow but should be relatively rigid in order
that the stacking angle
should be as high as possible so that the stack takes up as little area as
possible for a given
volume. The requirement for high solids content conflicts with the requirement
for the material to
remain pumpable as a fluid, so that even though it may be possible to produce
a mud having
the desired high solids content for stacking, this may render the mud
unpumpable.
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The sand fraction removed from the residue is also washed and transferred to
the disposal area
for separate dewatering and disposal.
EP-A-388108 describes adding a water-absorbent, water-insoluble polymer to a
material com-
5 prising an aqueous liquid with dispersed particulate solids, such as red
mud, prior to pumping
and then pumping the material, allowing the material to stand and then
allowing it to rigidify and
become a stackable solid. The polymer absorbs the aqueous liquid of the slurry
which aids the
binding of the particulate solids and thus solidification of the material.
However this process has
the disadvantage that it requires high doses of absorbent polymer in order to
achieve adequate
solidification. In order to achieve a sufficiently rigidified material it is
often necessary to use dos-
es as high as 10 to 20 kilograms per tonne of mud. Although the use of water
swellable absor-
bent polymer to rigidify the material may appear to give an apparent increase
in solids, the
aqueous liquid is in fact held within the absorbent polymer. This presents the
disadvantage that
as the aqueous liquid has not actually been removed from the rigidified
material and under cer-
tam n conditions the aqueous liquid could be desorbed subsequently and this
could risk re-
fluidisation of the waste material, with the inevitable risk of destabilising
the stack.
WO-A-96/05146 describes a process of stacking an aqueous slurry of particulate
solids which
comprises admixing an emulsion of a water-soluble polymer dispersed in a
continuous oil phase
with the slurry. Preference is given to diluting the emulsion polymer with a
diluent, and which is
preferably in a hydrocarbon liquid or gas and which will not invert the
emulsion. Therefore it is a
requirement of the process that the polymer is not added in to the slurry as
an aqueous solution.
WO-A-0192167 describes a process where a material comprising a suspension of
particulate
solids is pumped as a fluid and then allowed to stand and rigidify. The
rigidification is achieved
by introducing into the suspension particles of a water soluble polymer which
has an intrinsic
viscosity of at least 3 dl/g. This treatment enables the material to retain
its fluidity as being
pumped, but upon standing causes the material to rigidify. This process has
the benefit that the
concentrated solids can be easily stacked, which minimises the area of land
required for dis-
posal. The process also has the advantage over the use of cross linked water
absorbent poly-
mers in that water from the suspension is released rather than being absorbed
and retained by
the polymer. The importance of using particles of water soluble polymer is
emphasised and it is
stated that the use of aqueous solutions of the dissolved polymer would be
ineffective. Very
efficient release of water and convenient storage of the waste solids is
achieved by this pro-
cess, especially when applied to a red mud underflow from the Bayer alumina
process.
W02004/060819 describes a process in which material comprising an aqueous
liquid with dis-
persed particulate solids is transferred as a fluid to a deposition area, then
allowed to stand and
rigidify, and in which rigidification is improved whilst retaining the
fluidity of the material during
transfer, by combining with the material an effective rigidifying amount of an
aqueous solution of
a water-soluble polymer. Also described is a process in which dewatering of
the particulate
solids is achieved.
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Canadian patent application 2512324 describes a process for the rigidification
of a suspension
which is or comprises oil sand tailings. The process involves transferring the
suspension as a
fluid to a deposition area in which an effective rigidifying amount of an
aqueous solution of a
water-soluble polymer is combined with the suspension during transfer and then
allowing the so
treated suspension to stand and rigidify. The rigidification is improved
whilst retaining the fluidity
of the material during transfer. The process was particularly suited to the
treatment of tailings as
they are produced from the oil sands processing operation.
However, suspensions which contain a very high proportion of fine solids and
clays, such as
M FT, are particularly difficult to dewater and generally require very high
doses of chemical trea-
tment aids.
It is an objective of the present invention to convert the suspensions that
contain high levels of
fine solids and clays, especially M FT derive from oil sand tailings, into a
form that can be sub-
sequently dewatered. Further, it is an objective by to provide such a
suspension that remains in
a form that can be dewatered after storage or otherwise after a period of
time. Furthermore, it is
an objective of the present invention to achieve a more efficient process for
dewatering a sus-
pension containing high levels of fine solids and clays, especially M FT
derived from oil sand
tailings. In particular it would be desirable if such a process required
reduced levels of chemical
treatment aids. Moreover, it would be desirable for the process of removing
water or dewatering
process is a rigidification process.
According to the present invention which provide a process of preparing a
stabilised suspensi-
on, suitable for dewatering, which suspension comprises particulate solids
dispersed in an
aqueous liquid, said particulate solids comprises clay and mineral particles
of size below 50 pm,
which process comprises the steps of,
(a) subjecting the suspension to a kinetic energy stage to produce a modified
suspension;
(b) addition of a modified suspension stabilising chemical,
in which the kinetic energy stage is a shearing stage and/or application of
ultrasonic energy to
the suspension, wherein the shearing stage comprises subjecting the suspension
to shearing
employing a shearing device and in which the shearing device is selected from
the group con-
sisting of:
a shearing device comprising moving elements which rotate, preferably
impellers, knea-
ding components, or moving plates;
a milling device comprising moving elements;
a static mixer,
in which the operation of the moving elements is at least 5 cycles per second.
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The process enables the suspension to be converted into a form, the stabilised
the suspension,
which is much more readily dewatered, especially in a rigidification process.
By mineral particles of particle size below 50 pm, we mean solid mineral
particles that are not
water swelling clays that may generally be referred to as fines. Often these
mineral particles
may be referred to as silt. Usually these mineral particles have a size of no
greater than 44 pm.
Typically they will have a size between 2 pm and 44 pm, although their size
may be smaller.
The mineral origin of the particles often will be silica and/or quartz and/or
feldspar. The mineral
particles may typically be present in the suspension in an amount of at least
10% by weight of
the mineral content. Often the particles may be present in amount of at least
15% for at least
20% by weight of the solids content. In some cases the solids content of the
suspension may be
made up of up to 50 or 60% by weight.
The clay may be any material traditionally referred to as clays by virtue of
its mineralogy and will
tend to have a particle size of below 2 pm. Generally the claysmay tend to be
a mixture of clays.
Typically the clay component may comprise kaolinite; illite ; chlorite ;
montmorillonites; kaolinite-
smectite mixtures; illite-smectite mixtures. The clay content of the
suspension would usually be
at least 20% of the solids and may be as much as 75% of the solids.
Preferably, the suspension comprises mature fine tailings derived from oil
sands tailings.
Without being limited by theory the inventors believe that suspensions which
contain a high
proportion of very small sized mineral particles and clay particles,
especially where they have
been held in tailings ponds over a considerable time, even many years, such as
oil sands den-
ved M FT, exhibit three-dimensional particle network structures based on these
clays. These
network structures are believed to include clay-clay intra particle networks
and clay-inter-particle
network structures which incorporate the fine mineral particles. The inventors
believe that these
network structures comprise clay particles linked to each other and network
structures where
clay particles and the fine particles are linked together by clay particles.
Further, it is believed
that this network structure is responsible for retaining more water than in
suspensions of equiva-
lent solids. Furthermore, it is considered that the electrostatic forces
within the clay inter-particle
structure may be responsible for the difficulty in achieving adequate water
release with conven-
tional doses of chemical treatment aids.
Unexpectedly, the inventors discovered that applying kinetic energy to the
suspension provides
a modified suspension which is significantly more conducive to releasing water
by chemical tre-
atment. The inventors believe that the action of the kinetic energy on the
suspension directly
interacts with the clay-clay intra-particle network structures and the clay
inter-particle network
structures. In fact it is believed that the shear will at least partially
breakdown these network
structures.
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By kinetic energy we mean that suspension is subjected some energy which is or
induces moti-
on within the suspension. In one form the kinetic energy may be ultrasonic
energy. Generally it
is expected that the application of ultrasonic energy will induce vibrations
which will at least par-
tially break down the network structures. Other forms of kinetic energy may be
alternative me-
ans for inducing vibrations.
One particularly suitable form of kinetic energy is shearing.
However, the inventors realised that unless the sheared suspension is
dewatered once it has
been formed that the clay inter-particle network structure will start to
reform and in doing so will
trap water. By adding a stabilising chemical to the sheared suspension, the
inventors discover-
ed that the clay-clay intra-particle network structures and clay inter-
particle network structure
seem to be prevented or inhibited from reforming.
The modified suspension stabilising chemical may be any suitable substance
which prevents
the clay particles from reattaching to the mineral particles. It has been
found that compounds
traditionally used as clay stabilisers are suitable for this purpose.
Generally suitable stabilising
chemicals include metal salts, hydroxy aluminium, ammonium salts, amine salts,
quaternary
ammonium salts, and polycationic compounds. By polycationic compounds we mean
com-
pounds which contains a plurality of cationic groups. Examples of polycationic
compounds in-
clude polycationic polymers, polycationic oligomers and polycationic
surfactants.
Typical metal salts include alkali metal salts, alkaline earth metals salts,
aluminium salts and
hydroxides, transition metal salts. Suitably the salts are of acid radicals,
such as halides, espe-
cially chlorides; nitrates; carbonates; bicarbonates and carboxylates, for
instance formates, ace-
tates etc.
Examples of metal salts and hydroxides include potassium chloride, calcium
chloride, sodium
chloride, potassium formate, potassium bicarbonate, potassium carbonate and
zirconium salts
such as zirconium chloride.
Examples of quaternary ammonium compounds includes choline chloride, choline
bicarbonate
and choline carbonate.
Examples of polycationic polymers include any of the polymers generally
described as polyeth-
yleneimines, polyamines, polymers of dicyandiamides with formaldehyde or even
cationic vinyl
addition polymers. Typical cationic vinyl addition polymers would include
polymers of water-
soluble cationic ethylenically unsaturated monomers. Typical cationic
ethylenically unsaturated
monomers include dimethyl ammonium halide (e.g. chloride), acid addition or
quaternary am-
monium salts of dialkyl amino alkyl (meth) acrylates and acid addition or
quaternary ammonium
salts of dialkyl amino alkyl (meth) acrylamides. Such polymers may be
homopolymers of one or
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more of the cationic monomers or copolymers of one or more cationic monomers
with non-ionic
ethylenically unsaturated. Other cationic polymers include polymers of vinyl
carboxamides, such
as N-vinyl formamide, followed by partial or complete hydrolysis to yield
vinyl amine units. Pre-
ferred polymers are selected from the group consisting of amino-containing
polymers, in particu-
lar polyethyleneimines, modified polyethyleneimines, polyvinylamines, and
partially hydrolysed
polyvinyl carboxamides.
The polycationic polymers are typically low molecular weight, polymers. For
the purposes of the
invention low molecular weight means an average molecular weight ranging from
about 10,000
to about 1,000,000 g/mol.
It may be preferable in some situations if the stabilising chemical is added
to the suspension
before the kinetic energy, for instance shearing stage. This may provide the
advantage that the
stabilising chemical would be present already to stabilise the modified
suspension, for instance
sheared suspension as it is formed. Alternatively the stabilising chemical may
be added during
the kinetic energy, for instance shearing stage, and this may provide the
advantage that the
kinetic energy, for instance shearing, facilitates the distribution of the
stabilising chemical
throughout the suspension. In some cases it may be preferred that the
stabilising chemical is
added to the suspension after shearing. This may provide the advantage that
the suspension
may be less viscous and it may be easier to mix the stabilising chemical with
the suspension.
Nevertheless it may be desirable that the stabilising chemical is added to the
sheared suspen-
sion within 24 hours of the shearing stage, preferably within 12 hours, more
preferably within 6
hours, and more preferably still within 3 hours, especially within 1 hour and
preferably within 30
min.
The dose of the stabilising chemical may be any amount of the chemical
sufficient to achieve
stabilisation of the modified suspension. Typically the dose will be at least
0.1 g of stabilising
chemical per tonne of modified suspension. Suitably the dose will be at least
0.5 g/tonne and
desirably at least 1 g/tonne, for instance at least 5 g/tonne or even at least
10 g/tonne. The dose
may be even as high as 600 or 700 g/tonne or more for instance up to 1000
g/tonne. Typically
the dose may be up to 500 g/tonne, for instance up to 200 or 300 g/tonne.
The kinetic energy stage, for instance shearing stage, may be carried out in a
kinetic energy
vessel, for instance shearing vessel , before being transferred to the next
step of the process.
Alternatively, the kinetic energy stage, for instance shearing, may be carried
out in line as the
suspension is being transferred.
With regard to shearing any conventional shearing device may be employed as
such devices
are very well known in the industry and also described in the literature.
Industrial scale shear
devices, for instance shear mixing devices or shear pumps are available from a
variety of manu-
facturers, for instance IKA which manufactures Ultra Turrax high shear
devices, for instance the
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devices in the Ultra Turrax UTL 2000 range; Fluko-high shear mixers; SiIverson
high shear mi-
xers, for instance Ultramix mixers or In-line mixers; Euromixers; Greaves;
Admix Inc which ma-
nufactures Rotosolver high shear devices; Charles Ross and Son Company which
manufac-
tures Ross high shear mixers; Robbins Myers which manufactures Greerco high
shear mixers;
5 Powershear Mixers.
Suitable shearing devices generally have moving elements: such as rotating
components, for
instance impellers; kneeding components; or moving plates. The mixing pumps
may also con-
tain static elements such as baffles or plates, for instance containing
orifices. The moving ele-
10 ments will tend to move quite rapidly in order to generate shear. In
general this will depend up-
on the mode of action within the shearing chamber and the size of the volume
that is being
sheared. This may be for instance at least 5 cycles per second (5 s-1) or at
least 6 s-1, at least 7
s-1, or at least 8 s-1, or at least 9 s-, and usually at least 10 s-1,
suitably at least 20 s-1. Typically
this may be up to 170 s-1, up to 200 s-, or up to 300 s-, or more.
When the suspension, for instance oil sands derived M FT, is subjected to
shearing, the period
of shearing may be referred to as the residence time. The residence time in
the shearing device
may be, for instance at least 1 second. Often it will be at least 5 seconds
and sometimes at
least 10 seconds. It may be up to 30 seconds or more or it may be up to 15
seconds or up to 20
seconds. In some situations it may be at least 20 seconds, for instance at
least 1 min and often
may be several hours, for instance up 10 hours or more. Suitably the residence
time may be at
least 5 min, suitably at least 10 min and often at least 30 min. In many cases
it may be at least
one hour. In some cases the residence time may be up to 8 hours but desirably
less than this.
The shearing device may even be a milling device. Milling devices include
colloid mills, cone
mills and rotor mills etc. In general milling devices tend to have moving
elements, for instance
cones, screens or plates containing gaps, grooves, slots or orifices which
move against other
static elements. The moving elements may move instance by rotation. These
devices tend to
generate a high level of shear stress on liquids and other materials passing
through them. The
moving elements tend to combine high-speed with a very small shear gap which
produces in-
tense friction on the material being processed. The friction and shear that
result is commonly
referred to as wet milling. In one form the milling device may contain a rotor
and a stator, which
are both cone shaped and may have one or more stages of fine grooves, gaps,
slots or orifices.
This stator can be adjusted to obtain the desired gap setting between the
rotor and stator. The
grooves, gaps, slots or orifices may change direction in each stage to
increased turbulence. The
moving elements will tend to move quite rapidly in order to generate
sufficient shear. This may
be for instance at least 5 cycles per second (5 s-1) or at least 6 s-1, at
least 7 s-1, or at least 8 s-1,
or at least 9 s-, and usually at least 10 s-1, suitably at least 20 s-1,
typically up to 170 s-1, up to
200 s-, or up to 300 s-, or more.
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Alternatively the suspension may be passed through a static mixer or other
static elements
which bring about a shearing action, for instance baffles in a pipeline or
alternatively a con-
striction in a pipeline.
The inventors have noted that during the application of kinetic energy, for
instance by shearing
of the suspension, in particular the oil sands derived M FT, a notable
reduction in viscosity of the
suspension can occur. The inventors considered that this may be as a result of
the clay-clay
intra-portable network structures andclay inter-particle network structures
being broken down
and releasing water previously entrained within these networks. It is thought
that this availability
of the water may bring about a reduction in viscosity. Typically viscosity may
be measured by
controlled stress rheometer, such as a Brookfield RS. Viscosity may be
measured at 25 C.
Generally the viscosity of the modified (for instance sheared) suspension
would often be below
90% of the viscosity of the suspension prior to the application of kinetic
energy, such as the
shearing stage. Preferably the viscosity of the modified suspension, for
instance sheared sus-
pension, is no more than 80% of the viscosity of the suspension before the
application of kinetic
energy, such as shearing, and more preferably no more than 70%. More
preferably still the mo-
dified suspension, for instance sheared suspension, viscosity will be up to
60% and in particular
less than 50% of the suspension before the application of kinetic energy, for
instance un-
sheared suspension. In some cases the viscosity of the modified suspension,
for instance
sheared suspension, may be as little as 0.001% of the suspension before the
application of ki-
netic energy, for instance shearing, or even below. Often the modified
suspension, for instance
sheared suspension, will be at least 0.05% or 0.1% of the suspension before
the application of
kinetic energy, for instance un-sheared suspension. In many cases the modified
suspension, for
instance sheared suspension will be at least 1%, at least 5% or at least 10%
of the especially
for the application of kinetic energy, for instance un-sheared suspension.
Generally the change in viscosity from the suspension before the application
of kinetic energy,
for instance without the application of shear, to the modified suspension, for
instance after the
application of shear, tends to increase as the clay content of the suspension
increases.
Following the addition of the modified suspension suspension stabilising
chemical, the so stabi-
lised suspension will remain stable in the sense that the clay-clay intra-
particle network struc-
tures and clay inter-particle network structure is prevented or significantly
inhibited from forming.
Consequently any further treatment of the stabilised suspension may be delayed
after a period
of time if desired. Further, the treated suspension may, if required, be held
in storage. This may,
for instance, be a storage vessel or a storage container or more likely a
storage pond. Desirably
the stabilised suspension should be capable of storage for at least 24 hours
and usually for se-
veral days or several weeks or or even several months or longer. The
stabilised suspension is
particularly suitable for dewatering, especially in a rigidification process.
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Thus the invention also relates to a stabilised suspension, suitable for being
dewatered, which
suspension comprises particulate solids dispersed in an aqueous liquid, said
particulate solids
comprises clay and mineral particles of size below 50 pm,
which suspension is obtainable by a process comprising the steps of,
(a) subjecting the suspension to a kinetic energy stage to produce a modified
suspension;
(b) addition of a modified suspension stabilising chemical.
Specific and preferred embodiments of this stabilised suspension are already
provided in regard
to the process of preparing the stabilised suspension.
The present invention further includes a process of dewatering the stabilised
suspension,
described herein. The process comprises the steps of,
(c) addition of a water-soluble polymer having an intrinsic viscosity of at
least 3 dl/g to the stabilised
suspension; and
(d) dewatering the polymertreated suspension of step (c).
The process of dewatering according to the present invention involves addition
of a water-
soluble polymer which exhibits an intrinsic viscosity of at least 3 di/Q. The
addition of this poly-
mer facilitates the removal of water in the dewatering step. The inventors
believe that the
availability of water released from the clay-clay intra-particle network
structures and clay inter-
particle network structures facilitates the integration of the water-soluble
polymer throughout the
solids of the suspension.
The dewatering of the polymer treated suspension may employ any known
dewatering method.
For instance the dewatering step may involve sedimentation of the polymer
treated suspension
to produce a settled sediment. Such a process may be carried out in a vessel
for example a
gravimetric thickener or in a settlement pond. Alternatively the dewatering
process may involve
pressure dewatering, for example using a filter press, a belt press or a
centrifuge.
Preferably the dewatering process is a process of rigidification of the solids
in the suspension
and the dewatering step is part of the rigidification process. Thus in a
preferred form of the in-
vention the modified, for instance sheared suspension. is transferred as a
fluid to a deposition
area, then allowed to stand and rigidifying, in which the water-soluble
polymer is added to the
sheared suspension during the transfer of the sheared suspension.
Rigidification is a term that refers to a networked structure of particulate
solids. Compared with
settling or sedimentation, rigidification is faster, produces more recovered
water and results in a
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chemically bonded tailings that occupy a smaller surface area, which is more
quickly
rehabilitated. Rigidified tailings are also less likely to spread laterally
after deposition enabling
more efficient land use; and would more rapidly form a solid structure in the
form of a beach or
stack; and have a greater yield stress when deposited, with increased
uniformity or homogenity
of coarse and fine particles. Further by reason of its heaped geometry as a
beach or stack such
rigidified material would result in downward compression forces, driving water
out of the stack
and more rapid release of water, with better clarity.
Desirably the water-soluble polymer may be added to the stabilised suspension,
in the form of
an aqueous solution. The addition of water-soluble polymer, preferably an
aqueous solution of
water-soluble polymer, allows the stabilised suspension to retain sufficient
fluidity during trans-
fer and then once the material is allowed to stand it will form a solid mass
strong enough to
support subsequent layers of rigidified material. We have unexpectedly found
that the addition
of the polymer, preferably as an aqueous solution of the polymer, to the
stabilised suspension,
does not cause instant rigidification or substantially any settling of the
solids prior to standing.
Suitable doses of polymer range from 10 grams to 10,000 grams per tonne of
material solids.
Generally the appropriate dose can vary according to the particular material
and material solids
content. Preferred doses are in the range 30 to 3,000 grams per tonne, while
more preferred
doses are in the range of from 60 to 200 or 400 grams per tonne.
In some instances better results may be obtained when the suspension,
particular the oil sands
derived M FT, is relatively concentrated and homogenous. It may also be
desirable to combine
the addition of the polymer solution with other additives. For instance the
flow properties of the
material through a conduit may be facilitated by including a dispersant.
Typically where a dis-
persant is included it would be included in conventional amounts. However, we
have found that
surprisingly the presence of dispersants or other additives does not impair
the rigidification of
the material on standing. It may also be desirable to pre-treat the material
with either an inor-
ganic or organic coagulant to pre-coagulate the fine material to aid its
retention in the rigidified
solids.
Thus in the present invention the polymer, preferably as an aqueous solution,
is added directly
to the aforementioned sheared suspension. The polymer solution may consist
wholly or partially
of water-soluble polymer. Thus the polymer solution may comprise a blend of
cross-linked p01-
ymer and water soluble polymer, provided sufficient of the polymer is in
solution or behaves as
though it is in solution to bring about rigidification on standing.
This may be a physical blend of swellable polymer and soluble polymer or
alternatively is a
lightly cross-linked polymer for instance as described in EP202780. Although
the polymeric par-
ticles may comprise some cross-linked polymer it is essential to the present
invention that a
significant amount of water soluble polymer is present. When the polymeric
particles comprise
some swellable polymer it is desirable that at least 80% of the polymer is
water-soluble.
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Preferably the polymer comprises polymer which is wholly or at least
substantially water soluble.
The water soluble polymer may be branched by the presence of branching agent,
for instance
as described in WO-A-9829604, for instance in claim 12, or alternatively the
water soluble p01-
ymer is substantially linear.
Preferably the water soluble polymer is of moderate to high molecular weight.
It will have an
intrinsic viscosity of at least 3 dl/g and generally at least 5 or 6 dl/g,
although the polymer may
be of significantly high molecular weight and exhibit an intrinsic viscosity
of 25 dl/g or 30 dl/g or
even higher. Preferably the polymer will have an intrinsic viscosity in the
range of 8dI/g to 25
dl/g, more preferably 11 dl/g or 12 dl/g to 18 dl/g or 20 dl/g.
Intrinsic viscosity of polymers may be determined by preparing an aqueous
solution of the pol-
ymer (0.5-1% w/w) based on the active content of the polymer. 2 g of this 0.5-
1% polymer solu-
tion is diluted to 100 ml in a volumetric flask with 50 ml of 2M sodium
chloride solution that is
buffered to pH 7.0 (using 1.56 g sodium dihydrogen phosphate and 32.26 g
disodium hydrogen
phosphate per litre of deionised water) and the whole is diluted to the 100 ml
mark with deion-
ised water. The intrinsic viscosity of the polymers is measured using a Number
1 suspended
level viscometer at 25 C in 1M buffered salt solution.
The water soluble polymer may be a natural polymer, for instance
polysaccharides such as
starch, guar gum or dextran, or a semi-natural polymer such as carboxymethyl
cellulose or hy-
droxyethyl cellulose. Preferably the polymer is synthetic and preferably it is
formed from an eth-
ylenically unsaturated water-soluble monomer or blend of monomers.
The water soluble polymer may be cationic, non-ionic, amphoteric, or anionic.
The polymers
may be formed from any suitable water-soluble monomers. Typically the water
soluble mono-
mers have a solubility in water of at least 5g/100cc at 25 C. Particularly
preferred anionic poly-
mers are formed from monomers selected from ethylenically unsaturated
carboxylic acid and
sulphonic acid monomers, preferably selected from (meth) acrylic acid, allyl
sulphonic acid and
2-acrylamido-2-methyl propane sulphonic acid (AMPS), and their salts,
optionally in combina-
tion with non-ionic co-monomers, preferably selected from (meth) acrylamide,
hydroxy alkyl es-
ters of (meth) acrylic acid and N-vinyl pyrrolidone.
Preferred non-ionic polymers are formed from ethylenically unsaturated
monomers selected
from (meth) acrylamide, hydroxy alkyl esters of (meth) acrylic acid and N-
vinyl pyrrolidone.
Preferred cationic polymers are formed from ethylenically unsaturated monomers
selected from
dimethyl amino ethyl (meth) acrylate - methyl chloride, (DMAEA.MeCI) quat,
diallyl dimethyl
ammonium chloride (DADMAC), trimethyl amino propyl (meth) acrylamide chloride
(ATPAC)
optionally in combination with non-ionic co-monomers, preferably selected from
(meth) acryla-
mide, hydroxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.
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In some instances, it has been found advantageous to separately add
combinations of polymer
types. Thus an aqueous solution of an anionic, cationic or non-ionic polymer
may be added to
the above mentioned material first, followed by a second dose of either a
similar or different
5 water soluble polymer of any type.
In the invention, the water soluble polymer may be formed by any suitable
polymerisation pro-
cess. The polymers may be prepared for instance as gel polymers by solution
polymerisation,
water-in-oil suspension polymerisation or by water-in-oil emulsion
polymerisation. When prepar-
10 ing gel polymers by solution polymerisation the initiators are generally
introduced into the mon-
omer solution.
Optionally a thermal initiator system may be included. Typically a thermal
initiator would include
any suitable initiator compound that releases radicals at an elevated
temperature, for instance
15 azo compounds, such as azo-bis-isobutyronitrile. The temperature during
polymerisation should
rise to at least 70 C but preferably below 95 C. Alternatively polymerisation
may be effected by
irradiation (ultra violet light, microwave energy, heat etc.) optionally also
using suitable radiation
initiators. Once the polymerisation is complete and the polymer gel has been
allowed to cool
sufficiently the gel can be processed in a standard way by first comminuting
the gel into smaller
pieces, drying to the substantially dehydrated polymer followed by grinding to
a powder. Alter-
natively polymer gels may be supplied in the form of polymer gels, for
instance as neutron type
gel polymer logs.
Such polymer gels may be prepared by suitable polymerisation techniques as
described above,
for instance by irradiation. The gels may be chopped to an appropriate size as
required and
then on application mixed with the material as partially hydrated water
soluble polymer particles.
The polymers may be produced as beads by suspension polymerisation or as a
water-in-oil
emulsion or dispersion by water-in-oil emulsion polymerisation, for example
according to a pro-
cess defined by EP-A-150933, EP-A-102760 or EP-A126528.
Alternatively the water soluble polymer may be provided as a dispersion in an
aqueous medium.
This may for instance be a dispersion of polymer particles of at least 20
microns in an aqueous
medium containing an equilibrating agent as given in EP-A-170394. This may for
example also
include aqueous dispersions of polymer particles prepared by the
polymerisation of aqueous
monomers in the presence of an aqueous medium containing dissolved low IV
polymers such
as poly diallyl dimethyl ammonium chloride and optionally other dissolved
materials for instance
electrolyte and/or multi-hydroxy compounds e. g. polyalkylene glycols, as
given in WO-A-
9831749 or WO-A-9831748.
The aqueous solution of water-soluble polymer is typically obtained by
dissolving the polymer in
water or by diluting a more concentrated solution of the polymer. Generally
solid particulate
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polymer, for instance in the form of powder or beads, is dispersed in water
and allowed to dis-
solve with agitation. This may be achieved using conventional make up
equipment. Desirably,
the polymer solution can be prepared using the Auto Jet Wet (trademark)
supplied by BASF.
Alternatively, the polymer may be supplied in the form of a reverse phase
emulsion or disper-
sion which can then be inverted into water.
When the polymer is added as an aqueous solution the aqueous polymer solution
may be add-
ed in any suitable concentration. It may be desirable to employ a relatively
concentrated solu-
tion, for instance up to 10 % or more based on weight of polymer in order to
minimise the
amount of water introduced into the material. Usually though it will be
desirable to add the pol-
ymer solution at a lower concentration to minimise problems resulting from the
high viscosity of
the polymer solution and to facilitate distribution of the polymer throughout
the material. The
polymer solution can be added at a relatively dilute concentration, for
instance as low as 0.01%
by weight of polymer. Typically the polymer solution will normally be used at
a concentration
between 0.05 and 5% by weight of polymer. Preferably the polymer concentration
will be the
range 0.1% to 2 or 3%. More preferably the concentration will range from 0.25%
to about 1 or
1.5%.
A suitable and effective rigidifying amount of the water-soluble polymer,
preferably as an ague-
ous solution, can be mixed with the stabilised suspension prior to a pumping
stage. In this way
the polymer solution can be distributed throughout the stabilised suspension.
Alternatively, the
polymer solution can be introduced and mixed with the stabilised suspension
during a pumping
stage or subsequently. The most effective point of addition will depend upon
the substrate and
the distance from the stabilised stage to the deposition area. If the conduit
is relatively short it
may be advantageous to dose the polymer solution close to where the stabilised
suspension
flows from the stabilisation stage or kinetic energy stage, for instance
shearing stage. On the
other hand, where the deposition area is significantly remote from the
stabilisation stage or ki-
netic energy stage, for instance shearing stage, in may be desirable to
introduce the polymer
solution closer to the outlet. In some instances in may be convenient to
introduce the polymer
solution into the stabilised suspension as it exits the outlet.
Preferably the polymer treated suspension will be pumped as a fluid to an
outlet at the deposi-
tion area and the so treated suspension allowed to flow over the surface of
rigidified material.
The suspension is allowed to stand and rigidify and therefore forming a stack
of rigidified mate-
rial. This process may be repeated several times to form a stack that
comprises several layers
of rigidified solids of the suspension. The formation of stacks of rigidified
material has the ad-
vantage that less area is required for disposal.
The rheological characteristics of the polymer treated suspension as it flows
through the conduit
to the deposition area is important, since any significant reduction in flow
characteristics could
seriously impair the efficiency of the process. It is important that there is
no significant settling of
the solids as this could result in a blockage, which may mean that the plant
has to be closed to
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allow the blockage to be cleared. In addition it is important that there is no
significant reduction
in flow characteristics, since this could drastically impair the pumpability
on the suspension.
Such a deleterious effect could result in significantly increased energy costs
as pumping be-
comes harder and the likelihood of increased wear on the pumping equipment.
The rheological characteristics of the suspension as it rigidifies is
important, since once the pol-
ymer treated suspension is allowed to stand it is important that flow is
minimised and that solidi-
fication of the polymer treated suspension proceeds rapidly. If the polymer
treated suspension
is too fluid then it will not form an effective stack and there is also a risk
that it will contaminate
water released from the suspension. It is also necessary that the rigidified
material is sufficient-
ly strong to remain intact and withstand the weight of subsequent layers of
rigidified suspension
being applied to it.
Preferably the process of the invention will achieve a heaped disposal
geometry and will co-
immobilise the fine and any coarse fractions of the solids in the suspension
and also allowing
any released water to have a higher driving force to separate it from the
suspension by virtue of
hydraulic gravity drainage. The heaped geometry appears to give a higher
downward compac-
tion pressure on underlying solids which seems to be responsible for enhancing
the rate of de-
watering. We find that this geometry results in a higher volume of waste per
surface area, which
is both environmentally and economically beneficial.
A preferred feature of the present invention is the release of aqueous liquor
that often occurs
during the rigidification step. Thus in a preferred form of the invention the
suspension is de-
watered during rigidification to release liquor containing significantly less
solids. The liquor can
then be returned to the process thus reducing the volume of imported water
required and there-
fore it is important that the liquor is clear and substantially free of
contaminants, especially mi-
grating particulate fines. Suitably the liquor may for instance be recycled to
the mining opera-
tion, for instance oil sands operation, from which the suspension originates.
Alternatively, the
liquor can be recycled to the spirals or other processes within the same
plant.
Furthermore clarifying polymers may optionally be added after the thickener to
the underflow
but before disposal by rigidification. This may enhance the clarity of the
water released from the
rigidifying stack.
The clarifying polymers are typically low molecular weight, polymers. For the
purposes of the
invention low molecular weight means an average molecular weight ranging from
about 10,000
to about 1,000,000 g/mol. For example, anionic polymers in the range of about
10,000 to about
500,000 g/mol may be used.
These can be anionic, non-ionic or cationic. They can be synthetic or
naturally derived, e.g.
from starch, gums or cellulose, e.g. carboxymethyl cellulose. Preferably they
are anionic, e.g. a
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homopolymer of sodium acrylate, or as a copolymer with acrylamide, or
hydrolysed polyacrylo-
nitrile or hydrolysed acrylamide.
The amount of clarifying polymer will be determined by the composition of the
oil sands tailings
but generally about 5 to about 500 g/tonne of dry solids. For example the
amount of clarifying
polymer may be about 5 g to about 100 g/tonne of dry solids.
The clarifying polymer may be added as a solution and may be added in any
suitable concen-
tration. It may be desirable to employ a relatively concentrated solution, for
instance up to 10%
or more based on weight of polymer in order to minimise the amount of water
introduced into
the material. The clarifying polymer solution can be added at a relatively
dilute concentration, for
instance as low as 0.01% by weight of polymer. Typically the clarifying
polymer solution will
normally be used at a concentration between 0.05 and 5% by weight of polymer.
Preferably the
polymer concentration will be the range 0.1% to 2 or 3%. More preferably the
concentration will
range from 0.25% to about 1 or 1.5%.
The clarifying polymer may be added before, simultaneously, or after the
rigidifying amount of
the water-soluble polymer added according to the present invention.
The following is an illustration of the invention.
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Examples
Example 1
One tonne of aqueous suspension comprising mature fines tailings (MFT) derived
from oil
sands is fed in to a shearing device (Ultra Turrax MK 2000). The aqueous
suspension compris-
ing M FT is sheared at 9 cycles per second for a duration of 30 seconds to
produce a sheared
suspension.
A 0.5% aqueous solution of cationic homopolymer of diallyl dimethyl ammonium
chloride of mo-
lecular weight 300,000 Da at a dose of 2000 gitonne is mixed with the sheared
suspension to
produce a stabilised sheared suspension of MFT. The stabilised suspension is
back to a holding
vessel where it is stored for 24 hours.
Example 2
The stabilised sheared suspension of Example 1 that has been stored for 24
hours is then
passed along a conduit and an anionic polyacrylamide, consisting of an aqueous
solution of a
copolymer of acrylamide with sodium acrylate (30/70 on a weight basis) having
an intrinsic viscosity
of 19 dl/g at a concentration of 0.5%, is introduced into the suspension at a
dose of 2100
g/tonne (based on active polymer per dry aqueous sheared suspension).
The polymer treated suspension continues to flow along the conduit to an
outlet where the pol-
ymer treated sheared suspension is allowed to flow on to an inclinded
deposition site. The so
treated sheared suspension very quickly dewaters and forms a heap of
rigidified, dewatered
MFT solids. As the MFT dewaters substantially clear aqueous fluid flows from
the heap.
30
