Abstract:
A process for flocculating and dewatering oil sands fine tailings in a pipeline is provided, comprising: pumping a tailings feed having a solids content in the range of about 10 wt % to about 45 wt % through a pipeline; injecting an effective amount of a polymeric flocculant into the tailings feed to provide an initial quick dispersion of the polymeric flocculant into the tailings feed; and providing a subsequent conditioning environment to form flocs and release water without overshearing.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process for dewatering oil sands tailings. In particular, tailings are treated with a polymeric flocculant such as a water soluble polymer having a moderate to high molecular weight and an intrinsic viscosity of at least 3 dl/g (measured in 1N NaCl at 25° C.) to form larger structures (flocs) that can be efficiently separated from the water when ultimately deposited in a deposition area. 
       BACKGROUND OF THE INVENTION 
       [0002]    Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. The extraction of bitumen from oil sand using hot water processes yields fine tailings composed of fine silts, clays, residual bitumen and water. Mineral fractions with a particle diameter less than 44 microns are referred to as “fines.” These fines are typically clay mineral suspensions, predominantly kaolinite and illite. 
         [0003]    The fine tailings suspension is typically 85% water and 15% fine particles by mass. Dewatering of fine tailings occurs very slowly. When first discharged in ponds, the very low density material is referred to as thin fine tailings. After a few years when the fine tailings have reached a solids content of about 30-35%, they are referred to as mature fine tailings (MFT), which behave as a fluid-like colloidal material. MFT, which has a low solids to fines ratio (&lt;0.3), is often referred to as a type of fluid fine tailings (FFT). FFT is generally defined a liquid suspension of oil sands fines in water with a solids content greater than 1% and having less than an undrained shear strength of 5 kPa, The fact that fluid fine tailings behave as a fluid and have very slow consolidation rates significantly limits options to reclaim tailings ponds. A challenge facing the industry remains the removal of water from the fluid fine tailings to strengthen the deposits so that they can be reclaimed and no longer require containment. 
         [0004]    Accordingly, there is a need for an improved method to treat fine tailings to reduce their water content to create dry stackable tailings and reclaim the land on which fine tailings are disposed. 
       SUMMARY OF THE INVENTION 
       [0005]    It has been discovered that proper mixing of a flocculant such as a high molecular weight nonionic, anionic, or cationic polymer with oil sands fine tailings such as FFT is critical to creating the right floc structure that will dewater the tailings rapidly. It is contemplated that the present invention can be used in conjunction with centrifugation of the flocculated fine tailings in, for example, decanter centrifuges; thickening of the flocculated fine tailings in thickeners known in the art; accelerated dewatering, or rim ditching, in specially constructed dewatering cells; and “thin lift” operations, where the flocculated fine tailings are spread over an area in a thin layer for rapid dewatering, followed by additional layering and dewatering of flocculated fine tailings. All of these technologies rely on proper mixing of the fluid fine tails with polymer to create a material that can readily dewater. 
         [0006]    The two main methods of mixing polymer with oil sand tailings with polymer are static/in-line mixing and dynamic mixing. Dynamic mixing utilizes a motor driven mixing device such as an impeller to cause fluid mixing while static/in-line mixing uses the energy contained within the flowing fluid stream to mix the polymer and oil sand tailings with polymer. Given that mixing energy is directly coupled with flow rate for a static mixing system, improper design can lead to an unstable mixing system. 
         [0007]    The present invention applies in particular to thixotropic suspensions where the viscosity and yield strength of the slurry can be manipulated and optimized to improve mixing of polymeric flocculants and other process aids. The current application is directed to a process for dewatering oil sands tailings by treating the tailings with flocculant by providing in-line mixing of polymer and raw oil sand tailings such as FFT or MFT to produce a properly flocculated mixture. The introduction of mixing energy to the tailings slurry prior to addition of the flocculant allows one to control the yield point and viscosity of the slurry in order to create a fluid system into which the polymeric flocculant can more easily be mixed. 
         [0008]    In one aspect, an end of pipe mixing process for mixing oil sand tailings with polymer is provided which includes in-line mixing at the end of a pipe. There are two key steps when flocculating oil sand tailings and polymer with an end of pipe device. The first step is to very quickly disperse the polymer into the oil sand tailings and then allow a flow conditioning period for floc growth. Quick dispersion of polymer into oil sand tailings can be enhanced if a conventional static mixer is placed directly upstream of the end of pipe flocculating device to pre-shear the raw oil sand tailings. Pre-shear is used to reduce the raw oil sand tailings viscosity plus induce small scale eddies to aid in the dispersion of the polymer in the oil sand tailings. 
         [0009]    It is also important that the piping be sized as to ensure turbulent flow of the raw oil sand tailings. The end of pipe polymer injection device may consist of polymer jets that shoot into the raw oil sand tailings stream to quickly disperse the polymer throughout the oil sand tailings. After the polymer is completely dispersed in the oil sand tailings, a large stilling chamber or flow conditioning section is required to promote floc growth. The flow conditioning section then overflows the flocculated oil sand tailings into a thin lift or other accelerated dewatering deposits. In one embodiment, the flocculated oil sand tailings are treated by centrifugation. 
         [0010]    In one embodiment, the stilling chamber has to be of adequate size as to not cause excessive floc breakup and thereby reducing the ability of the flocculated mixture to release water prior to dewatering by, e.g., centrifugation. 
         [0011]    In another aspect, a mixing system is provided that that will allow for in-line mixing of raw oil sand tailings with polymer within the pipeline itself. There are two important steps when flocculating oil sand tailings with polymer in an in-line mixing system. The first step is to very quickly disperse the polymer into the oil sand tailings and then allow a flow conditioning period for floc growth through the pipeline. Quick dispersion of polymer into oil sand tailings can be enhanced if a conventional static mixer is placed upstream of the polymer injection to pre-shear the raw oil sand tailings. Pre-shear is used to reduce the raw oil sand tailings viscosity plus induce small scale eddies to aid in the dispersion of the polymer in the oil sand tailings. 
         [0012]    It is also important that the piping be sized as to ensure turbulent flow of the raw oil sand tailings. The polymer injection/dispersion section may consist of an injection device that will quickly disperse the polymer throughout the oil sand tailings. After the polymer is completely dispersed in the oil sand tailings, a flow condition section is required to promote floc growth. The conditioning section should not be excessively long otherwise it will result in excessive floc breakup, thereby reducing the ability of the flocculated mixture to release water. 
         [0013]    The present invention is particularly useful with, but not limited to, fluid fine tailings (FFT) such as MFT. Thus, a process is provided for flocculating and dewatering oil sands fine tailings in a pipeline, comprising:
       pumping a tailings feed having a solids content in the range of about 10 wt % to about 45 wt % through a pipeline;   injecting an effective amount of a polymeric flocculant into the tailings feed to provide an initial quick dispersion of the polymeric flocculant into the tailings feed; and   providing a subsequent conditioning environment to form flocs and release water without overshearing.       
 
         [0017]    In one embodiment, the conditioning environment is a lower energy environment that the dispersion environment. In another embodiment, a coagulant such as gypsum is also added to the tailings feed. In another embodiment, polymer is injected using an injector design selected from (1) a plurality of side ports in a spool section of the pipeline, (2) a single side port in a spool section of the pipeline, and (3) a plurality of spargers, each with a plurality of holes, in a spool section of the pipeline. In another embodiment, an in-line mixer or static mixer is used in conjunction with a polymer injector. 
         [0018]    In one embodiment, the oil sands tailings is fluid fine tailings, such as MFT, which fluid may be optionally diluted with water to provide the tailings feed having a solids content in the range of about 10 wt % to about 45 wt %. In another embodiment, the tailings feed has a solids content in the range of about 30 wt % to about 45 wt %. 
         [0019]    In one embodiment, the polymeric flocculant is a water soluble polymer having a moderate to high molecular weight and an intrinsic viscosity of at least 3 dl/g (measured in 1N NaCl at 25° C.). In one embodiment, the polymer dosage is about 1000 g/Tonne of tailings and the polymer jet shear rate is about 200 s −1 . 
         [0020]    In one embodiment, the removed flocculated oil sands fine tailings are added to at least one centrifuge to dewater the oil sands fine tailings and form a high solids cake and a low solids centrate. 
         [0021]    In another embodiment, the removed flocculated oil sands fine tailings are added to a thickener to dewater the oil sands fine tailings and produce thickened oil sands fine tailings and clarified water. 
         [0022]    In another embodiment, the removed flocculated oil sands fine tailings are transported to at least one deposition cell for dewatering. 
         [0023]    In another embodiment, the removed flocculated oil sands fine tailings are spread as a thin layer onto a deposition site. 
         [0024]    In one embodiment, the process further comprises pre-shearing the oil sands fine tailings using at least one in-line or static mixer prior to polymer injection. Without being bound to theory, it is believed that, in certain cases, pre-shearing may increase the maximum dewaterable solids loading. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Referring to the drawings wherein like reference numerals indicate similar parts throughout the several views, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein: 
           [0026]      FIG. 1  is a schematic of one embodiment of the present invention for mixing oil sands fine tailings with polymer. 
           [0027]      FIG. 2  is a schematic of another embodiment of the present invention for mixing oil sands fine tailings with polymer. 
           [0028]      FIGS. 3A to 3F  show schematics of a variety of in-line polymer injectors/mixers useful in the present invention. 
           [0029]      FIG. 4  is a bar graph showing the hydraulic mixing times for a variety of in-line polymer injectors/mixers. 
           [0030]      FIGS. 5A to 5C  shows three examples of polymer injectors useful in the present invention. 
           [0031]      FIG. 6  shows an example of an embodiment of the present invention where a pre-mixer is used to first shear the MFT. 
           [0032]      FIG. 7  is a CoV plot showing a comparison of Computational Fluid Dynamics (CFD) simulations with and without pre-shearing. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention, However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. 
         [0034]    The present invention relates generally to a process for treating tailings derived from oil sands extraction operations and containing a fines fraction, and dewatering the tailings to enable reclamation of tailings disposal areas and to recover water for recycling. As used herein, the term “tailings” means tailings derived from oil sands extraction operations and containing a fines fraction. The term is meant to include fluid fine tailings (FFT) such as mature fine tailings (MFT) from tailings ponds and fine tailings from ongoing extraction operations (for example, thickener underflow or froth treatment tailings) which may bypass a tailings pond. The tailings are treated with a flocculant to aggregate the solids prior to dewatering by thin lift, accelerated dewatering such as rim ditching, centrifugation, etc. 
         [0035]    As used herein, the term “flocculant” refers to a reagent which bridges the neutralized or coagulated particles into larger agglomerates, resulting in more efficient settling. Flocculants useful in the present invention are generally anionic, nonionic, cationic or amphoteric polymers, which may be naturally occurring or synthetic, having relatively high molecular weights. Preferably, the polymeric flocculants are characterized by molecular weights ranging between about 1,000 kD to about 50,000 kD. Suitable natural polymeric flocculants may be polysaccharides such as dextran, starch or guar gum. Suitable synthetic polymeric flocculants include, but are not limited to, charged or uncharged polyacrylamides, for example, a high molecular weight polyacrylamide-sodium polyacrylate co-polymer. 
         [0036]    Other useful polymeric flocculants can be made by the polymerization of (meth)acryamide, N-vinyl pyrrolidone, N-vinyl formamide, N,N dimethylacrylamide, N-vinyl acetamide, N-vinylpyridine, N-vinylimidazole, isopropyl acrylamide and polyethylene glycol methacrylate, and one or more anionic monomer(s) such as acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropane sulphonic acid (ATBS) and salts thereof, or one or more cationic monomer(s) such as dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl methacrylate (MADAME), dimethydiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC) and/or methacrylamido propyltrimethyl ammonium chloride (MAPTAC). 
         [0037]    In one embodiment, the flocculant comprises an aqueous solution of an anionic polyacrylamide. The anionic polyacrylamide preferably has a relatively high molecular weight (about 10,000 kD or higher) and medium charge density (about 20-35% anionicity), for example, a high molecular weight polyacrylamide-sodium polyacrylate co-polymer. The preferred flocculant may be selected according to the oil sand tailings composition and process conditions. 
         [0038]    The flocculant is generally supplied from a flocculant make up system for preparing, hydrating and dosing of the flocculant. Flocculant make-up systems are well known in the art, and typically include a polymer preparation skid, one or more storage tanks, and a dosing pump. The dosage of flocculant may be controlled by a metering pump. In one embodiment, the dosage of flocculant ranges from about 400 grams to about 1,500 grams per tonne of solids in the FFT. In one embodiment, the flocculant is in the form of a 0.4% solution. 
         [0039]    As used herein, “fluid fine tailings” or “FFT” is a liquid suspension of oil sand fines in water with a solids content greater than 2%. “Fines” are mineral solids with a particle size equal to or less than 44μ. “Mature fine tailings” or “MFT” are FFT with a low solids to fines ratio (SFR), i.e., less than about 0.3, and a solids content greater than about 30%. 
         [0040]    As used herein, the term “in-line flow” means a flow contained within a continuous fluid transportation line such as a pipe or another fluid transport structure which preferably has an enclosed tubular construction. 
         [0041]      FIG. 1  is a flow diagram of one embodiment of the process of the present invention. In this embodiment, oil sands fine tailings are mature fine tailings (MFT) obtained from a settling basin  110 . However, it should be understood that the fine tailings treated according the process of the present invention are not necessarily obtained from a settling pond and may also be obtained from ongoing oil sands extraction operations. 
         [0042]    The tailings stream from bitumen extraction is typically transferred to a settling basin  10  where the tailings stream separates into an upper water layer, a middle MFT layer, and a bottom layer of settled solids. The MFT layer is removed from between the water layer and solids layer via a dredge or floating barge having a submersible pump. In one embodiment, the MFT has a solids content ranging from about 10 wt % to about 45 wt %. In another embodiment, the 
         [0043]    MFT has a solids content ranging from about 30 wt % to about 45 wt %. In one embodiment, the MFT has a solids content ranging from about 37 wt % to about 40 wt %. The MFT is preferably undiluted. 
         [0044]    The MFT is then pumped through pipeline  112  (in-line flow). In this embodiment, the MFT first passes through an in-line static mixer  114  for pre-shearing the MFT. Suitable static mixers for use in the present invention for pre-shearing oil sands fine tailings are known in the art. A static mixer is a motionless mixer which is inserted into a housing or pipeline with the objective of manipulating fluid streams, in this instance, to significantly accelerate the in-line reaction of flocculation. Typical designs of static mixers comprise plates, baffles, helical elements or geometric grids positioned at precise angles to direct flow and increase turbulence. 
         [0045]    In the embodiment of  FIG. 1 , the pre-sheared MFT is then introduced into an end of pipe chamber  116  comprising a mixing chamber  118  where polymer  122  is injected as a jet using a jet mixer as shown in  FIG. 3B . Other in-line polymer injection and mixing concepts can also be used. End of pipe chamber  116  further comprises a stilling chamber  120  where complete flocculation of the FFT can occur without overshearing of the flocs taking place. The flocculated FFT can then be further treated by centrifugation or directly deposited in thin sloping layers (thin-lift), subjected to accelerated dewatering (rim ditching) or deposited into other tailings deposition cells. 
         [0046]      FIG. 2  is a flow diagram of another embodiment of the process of the present invention. In this embodiment, fluid fine tailings (FFT) are primarily mature fine tailings (MFT) obtained from a settling basin  210 . However, it should be understood that the fine tailings treated according the process of the present invention are not necessarily obtained from a settling pond and may also be obtained from ongoing oil sands extraction operations. 
         [0047]    The MFT is pumped through pipeline  212  into an in-line first static mixer  214  for pre-shearing the MFT. Suitable static mixers for use in pre-shearing the MFT are known in the art. The pre-sheared MFT is then introduced into a polymer injection/dispersion zone  230  where polymer  232  is injected using a polymer injector, for example, a 3″ Tee  80  on an 8″ pipe  312 , as shown in  FIG. 5A , or a 1.5″ Tee  84  on an 8″ pipe  312 , as shown in FIG,  5 B. Tee injectors were investigated due to the simplicity of their design. In one embodiment, polymer injection/dispersion zone  230  comprises a venturi, as shown in FIG,  5 C. With reference to  FIG. 5C , venturi  86  can be inserted between two pieces of pipe  312 ′ forming pipeline  312 . Polymer is injected into the venturi  86  via a plurality of polymer lines (not shown) connected to a plurality of polymer inlets  88 . 
         [0048]    It is understood, however, that other polymer injectors/mixers can be used in the polymer injection/dispersion zone  230 .  FIG. 3  shows a number of different polymer injectors/mixers that could be used in the present invention.  FIG. 3A  illustrates sparger-like injectors with spatial distribution;  FIG. 3B  shows various nozzle-type injectors;  FIG. 3C  shows a number of venturi-type injectors/mixers;  FIG. 3D  shows polymer injection coupled with static mixers,  FIG. 3E  show wake mixers/injectors and  FIG. 3F  shows vortex generators. 
         [0049]    The MFT/polymer mixture continues through the pipeline  212  so that flow conditioning can occur, i.e., complete flocculation of the MFT without over-shearing of the flocs taking place. The flocculated MFT can then be further treated by centrifugation, directly deposited in thin sloping layers (thin-lift), subjected to accelerated dewatering (rim ditching) or deposited into other tailings deposition cells. 
         [0050]    Thus, in both embodiments described above, it important to have a first step comprising quick dispersion of the polymer into the FFT followed by a second step of providing a subsequent lower energy region to promote floc growth. A well-dispersed polymer/MFT product flowing through the conditioning pipe or stilling chamber develops increasingly large flocs, builds rheological strength, and then begins to dewater, either within the pipe or stilling chamber. 
         [0051]    In one embodiment, a coagulant is also introduced into the in-line flow of FFT. As used herein, the term “coagulant” refers to a reagent which neutralizes repulsive electrical charges surrounding particles to destabilize suspended solids and to cause the solids to agglomerate. Suitable coagulants include, but are not limited to, gypsum, lime, alum, polyacrylamide, or any combination thereof. In one embodiment, the coagulant comprises gypsum or lime, In one embodiment, the dosage of the coagulant ranges from about 300 grams to about 1,500 grams per tonne of solids in the FFT. 
         [0052]    Exemplary embodiments of the present invention are described in the following Examples, which are set forth to aid in the understanding of the invention, and should not be construed to limit in any way the scope of the invention as defined in the claims which follow thereafter. 
       EXAMPLE 1 
       [0053]    The measure of uniformity or “mixedness” that is most often used is the radical variation coefficient (CoV). A low CoV number indicates better uniformity of flocculant and tailings (good mixedness).  FIG. 4  is a bar graph which illustrates the hydraulic mixing time necessary to achieve a CoV reduction using a variety of polymer injection devices and 32% MFT. It was observed that, with the simpler designs such as the quill, tee and sparger, it took longer for CoV reduction to occur with MFT having a relatively high solids content (32%). However, use of Computational Fluid Dynamics (CFD) suggested that mixing of 32% MFT and polymer flocculant could be improved by using a pre-mixer.  FIG. 7  is a CoV plot versus distance from the injector point (m) showing a comparison of Computational Fluid Dynamics simulations of polymer injectors comprising a sparger, a 2″ quill in a 12″ pipe and a 3″ Tee in an 8″ pipe without and with pre-shearing. It can be seen from  FIG. 7  that with pre-shearing (pre-mixing) a lower CoV number could be obtained for all injectors, which indicates better uniformity of flocculant and tailings (good mixedness) when using 32% MFT. 
       EXAMPLE 2 
       [0054]    In one test, a plurality of Komax™ flanged carbon steel static mixers were placed in an 8″ pipeline upstream from the polymer injection site for pre-mixing MFT. Following the static mixers, a venturi (4″ contraction) having eight (8) port openings was used for injecting polymer. Following the venturi was a length of 8″ pipe, as shown in  FIG. 6 . The flocculated MFT was deposited in a ditch and observed for dewatering properties. It was observed that the addition of the Komax mixers promoted flocculation and allowed excellent dewatering results over a wide range of conditions, including a different number of injectors, polymer dosages and MFT solids wt %. A Tee injector was also tested and provided good dewatering conditions at discharge. It was further observed that aggressive post mixing after polymer injection was actually detrimental to instant dewatering observed at the end of pipe and at the discharge. 
         [0055]    One of the more surprising results when using a pre-mixer for pre-shearing the MFT was that much higher solids content MFT could be used. For example, MFT having a wt % solids of 31% and the MFT density (kg/m3) of 1.24 still showed good dewatering properties with good water runoff in the ditch when using a MFT flow rate (m 3 /hr) of 250 and a polymer dosage (kg/Tonne MFT) of 1050. 
         [0056]    The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.