CA2892983C - Treatment of fine tailings - Google Patents

Abstract

The invention relates to process of preparing a stabilized suspension, 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 µm, which process comprises the steps of, (a) subjecting the suspension to a kinetic energy stage to produce a modified suspension; (b) adding a modified suspension stabilizing chemical, in which the kinetic energy stage is a shearing stage and/or application of ultrasonic energy to the suspension. The process also relates to a so stabilized suspension and a process of dewatering said stabilized suspension by the steps of, (c) addition of a water-soluble polymer intrinsic viscosity of at least 3 dl/g to the stabilized suspension of step (a); (d) dewatering the polymer treated suspension of step (c). The invention is particularly suited to dewatering of mature fine tailings derived from oil sands tailings. The process of the invention preferably involves rigidification of the suspension.

Description

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

2 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

3 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

4 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.

5 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.

6 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.

7 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.

8 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

9 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 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.

11 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.

12 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

13 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.

14 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.

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

16 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

17 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

18 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.

WO 211141111885 PC111132(114111583411

19 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.

Claims (18)

Claims

1. A process of preparing a stabilised suspension and then dewatering the stabilised suspension, which suspension comprises particulate solids dispersed in an aqueous liquid, said particulate solids comprises clay and mineral particles of size below 50 µm, 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 thereby forming the stabilised suspension, (c) addition of a water-soluble polymer of intrinsic viscosity of at least 3 dl/g to the stabilised suspension; and (d) dewatering the polymer treated suspension of step (c), in which the kinetic energy stage is a shearing stage and/or application of ultrasonic energy to the suspension.

2. A process according to claim 1 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 consisting of:
a shearing device comprising moving elements which rotate;
a milling device comprising moving elements;
a static mixer, in which the operation of the moving elements is at least 5 cycles per second.

3. A process according to claim 2 wherein the shearing device comprising moving elements which rotate are impellers, kneading components, or moving plates.

4. A process according to any one of claims 1 to 3 in which the suspension comprises mature fine tailings derived from oil sands tailings.

5. A process according to any one of claims 1 to 4 in which the modified suspension stabilising chemical is selected from the group consisting of metal salts, hydroxy aluminium, ammonium salts, amine salts, quaternary ammonium salts, and polycationic compounds.

6. A process according to any one of claims 1 to 5 in which the modified suspension stabilising chemical is added to the suspension before subjecting the suspension to a shearing stage in step (a).

7. A process according to any one of claims 1 to 6 in which the modified suspension stabilising chemical is added to the suspension simultaneously with subjecting the suspension to a shearing stage in step (a).

8. A process according to any one of claims 1 to 7 in which the modified suspension stabilising chemical is added to the suspension after subjecting the suspension to a shearing stage in step (a).

9. A process according to any one of claims 1 to 8 in which the modified suspension has a viscosity which is less than 90% of the viscosity of the suspension prior to the kinetic energy stage.

10. A process according to any one of claims 1 to 9 in which the modified suspension as a viscosity which is less than 50% of the viscosity of the suspension prior to the kinetic energy stage.

11. A process according to any one of claims 1 to 10 in which the stabilised suspension is transferred as a fluid to a deposition area, then allowed to stand and rigidify, in which the water-soluble polymer is added to the stabilised suspension in step (c) during transfer of the stabilised suspension.

12. A process according to any one of claims 1 to 11 in which the water soluble poly-mer is formed from ethylenically unsaturated water-soluble monomer or blend of mono-mers.

13. A process according to any one of claims 1 to 12 in which the water soluble polymer is anionic.

14. A process according to any one of claims 1 to 13 in which the polymer is formed from monomer(s) selected from the group consisting of (meth)acrylic acid, allyl sulphonic acid and 2-acrylamido-2-methyl propane sulphonic acid as the free acids or salts thereof, optionally in combination with non-ionic co-monomers, selected from the group consisting of (meth)acrylamide, hydroxy alkyl esters of (meth)acrylic acid and N-vinyl pyrrolidone.

15. A process according to any one of claims 1 to 12 in which the water soluble polymer is non-ionic.

16. A process according to any one of claims 1 to 12 or claim 12 in which the polymer is formed from monomer(s) selected from the group consisting of (meth) acrylamide, hy-droxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.

17. A process according to any one of claims 1 to 12 in which the water soluble polymer is cationic.

18. A process according to any one of claims 1 to 12 or claim 17 in which the polymer is formed from monomer(s) selected from the group consisting of dimethyl amino ethyl (meth) acrylate - methyl chloride, (DMAEA.MeCl) quat, diallyl dimethyl ammonium chlo-ride (DADMAC), trimethyl amino propyl (meth) acrylamide chloride (ATPAC) optionally in combination with non-ionic co-monomers, selected from the group consisting of (meth) acrylamide, hydroxy alkyl esters of (meth) acrylic acid and N-vinyl pyrrolidone.