CA2686831A1 - Process for flocculating, dewatering, depositing and drying oil sand mature fine tailings - Google Patents

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PROCESS FOR FLOCCULATING, DEWATERING, DEPOSITING AND DRYING OIL
SAND MATURE FINE TAILINGS

FIELD OF THE INVENTION
The present invention generally relates to the field of treating oil sand fine tailings.
BACKGROUND
Extraction of bitumen from the Athabasca Oil Sands deposits of north-eastern Alberta, Canada has primarily been through a process known as the Clark Hot Water Extraction process (CHWE). In its purest form, the CHWE involves the addition of hot water to the oil sand ore together with pipeline mixing and floatation in separation cells.
The tailings from this process include slurries of sand, silts, clays and residual bitumen.
When deposited into holding ponds, this slurry forms segregating deposits with the sand settling rapidly to form beaches and a percentage of the fines and clays remaining suspended within the water column. Over time, the fines settle to form a thick material known as Mature Fine Tailings (MFT). This material is characterised as being above 30% solids by weight and typically less than 50%; having 90% of the solid particles less than 44 microns in size; approximately 50% of the particles less than 2 microns; and having very slow settlement rates. Several decades of research into this material have resulted in an excellent fundamental understanding of behaviours, but had not produced a fully implemented, proven technology for dealing with the ever increasing fluid MFT
inventories.

One potential method of dealing with these inventories was to utilise chemical addition to modify the characteristics of the tailings stream sufficiently to allow for thin lift deposition of the material onto shallow slopes, resulting in dewatering through evaporation and freeze/thaw processes. While there are significant advantages to the use of this method, there are also limitations resulting from the climate in northern Alberta.

SUMMARY OF THE INVENTION
The present invention provides a process for flocculating, dewatering, depositing and drying undiluted oil sand mature fine tailings comprising:

a dispersion and floc build-up stage comprising adding a polymer focculant to tailings to cause a rapid increase in yield stress as the polymer flocculant contacts the minerals of the tailings;

a gel state of high shear yield stress stage, which can be a plateau over time depending on the applied shear rate and %solids of the tailings;
a decreasing shear strength and floc breakdown stage, wherein a significant amount of substantially polymer-free water is released; and avoiding an oversheared zone characterized by rapidly decreasing shear strength where the material reverts to a tailings-state and releases very little water.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph of Shear strength progression of flocculated MFT
highlighting four distinct stages.

2 Figure A is a graph of Shear strength progression of flocculated MFT
highlighting four distinct stages.

Figure B is a graph of Maximum water release from polymer-treated MFT during mixing.
Figure 2 is a graph of Variation of polymer dosage with yield stress and water release.
Figure 3 is Scanning electron micrographs of 40 wt% MFT showing the fabric at different shear regimes (a). Untreated MFT, (b) high yield strength and (c) dewatering stage.
Figure 4 is a graph of Shear strength progression for optimally dosed MFT
samples diluted to varying solids concentration.

Figure 5 is a graph of Yield stress progression of MFT optimally dosed with preferred polymer (Poly A) and a high molecular weight anionic polyacrylamide aPAM (Poly B).
Figures 6a-b are Computational fluid dynamic model images of polymer injection systems as a mist (top, a) and as a stream (bottom, b).

Figure 7a is a graph of Shear progression curves of the pilot scale flocculated MFT (35 wt% solid).

Figure 7b is a photograph of Jar samples taken at each sample point in Figure 7a at ideal dosage and low shear.
Figure 8 is a photograph of Flocculated MFT discharged on a slope showing high yield strength material and water channeling around the discharge.

Figure 9 is a graph of Water release rate of flocculated MFT at various distances from the injection point.

Figure 10 is a photograph of Crack formation in an optimally flocculated MFT
after a few days.

DETAILED DESCRPTION
Preliminary advances Regarding the preferred performance requirements for an additive chemical, the focus was put on on strength gain and resistance to shear. Another objective was enhanced dewatering, as several previous attempts to flocculate MFT (Golder 2007, Matthews, J.
2008 and Silawat J. 2008) required dilution of the material prior to mixing with the flocculant, and then only achieved clay to water ratios similar to or slightly less than that found in the source MFT. Commercial application of polymeric flocculation in oil sands is restricted to rapid dewatering of low solids content thin fine tails. In short, flocculants had been unable to collapse the clay matrix any further than that found in the ponds.
During the course of bench scale tests, a new polymer type showed promise in both material strength gain as well as shear resistance. In addition, the polymer appeared to promote initial dewatering of the MFT shortly after mixing by generating a highly permeable floc structure. This means that the process no longer relies on evaporative drying alone, but rather a combination of initial accelerated dewatering and drainage in

3 the deposit slope as well as evaporation. No dilution of the MFT was required beyond the polymer make up water and the polymer could be injected in line without the use of a thickener. The polymer was quite effective for MFT up to 40 percent by weight (roughly 0.4 clay-to-water ratio).
Initial field tests produced surprising results, allowing for 20-30cm lifts to reach 80%solids in less than 10 days. Given the weather conditions at the time, the minimum amount of water released as free water was 85% as the potential evaporation rates were too low to account for the dewatering rate. This initial success appeared to be robust and relatively insensitive to changes in fluid density and injection locations.

Subsequent testing began to illustrate, however, that there was a basic understanding of the behaviour of the flocculated material that was not obtained during the initial laboratory or field tests. Deposits were attempted with lower levels of control on the density and flowrates of the source MFT, resulting in a wider variety of deposit dewatering rates. Many of these deposits did not behave as previously observed, and several attempts at enhancing the dewatering performance through additional mixing, changes in the deposition mechanisms, or mechanical manipulation of the deposits met with limited success. It became apparent that more testing was required.
Investigation of Undiluted MFT Flocculation It was attempted to manipulate the MFT floc structure such that initial dewatering is maximized and the MFT gained just enough strength to stack in a thin lift when deposited on a shallow slope. Dewatering occurs as a function of mixing and applied shear during pipeline transport as well as on the deposition slopes.

Bench and pilot scale experiments were conducted to replicate the field observations and to investigate the dewatering potential as a function of polymer dosage, injection type, mixing, total applied shear and clay-to-water ratio of the MFT. The experiments highlight several key factors.

1. Polymer dosage is best determined by clay content, measured as clay activity using methylene blue adsorption method.
2. Mixing of the polymer-treated MFT using laboratory or in-line static mixers can cause less than optimum dewatering potential and stacking in the deposition slopes 3. Shear energy applied to the flocculated materials can greatly affect the dewatering and strength performance. Insufficient shear often create a high strength material with minimal dewatering and excess shear reduces the strength to MFT-like strengths with reduced permeability and dewatering.

4 Polymer Dosage Although it is recognized that the rheology of flocculated systems is governed by the finest particles in a slurry, polymer is often added on a gram per tonne of solids basis.
This is often adequate for a homogeneous slurry. However, fine tailings are deposited in segregating ponds and the mineral size distribution of MFT depends on the sampling depth. Therefore dosing on a solid basis would often result in an underdosed or an overdosed situation affecting maximum water release. This is highlighted in Table 1 for three MFT samples that show large swings in the optimum polymer dosage on solids or fines basis. The MFT samples were sourced from two Suncor ponds at different depths and with similar water chemistries.
Table 1 - Optimum polymer dosage for maximum initial water release.
Optimum polymer dosage Sample ID t% solids t% clay* t% fines(g/tonne o (g/tonne o (g/tonne o on solids on solids solids) fines < 44 pm) clay) MFT A 44.0 8.9 59.8 300 1424 1742 MFT B 32.6 78.9 89.3 1200 1428 1616 MFT C 22.3 99.6 98.8 1700 1707 1693 *Wt% clay is based on the surface area determined from methylene blue adsorption and could be greater than 100 % for high surface area clays (Omotoso and Mikula 2004).
Rheology of Flocculated MFT
A static yield stress progression over time was used to evaluate optimal yield stress for deposition and water release in the laboratory (Spicer P.T., 1996), pilot and field experiments. The shear yield stress was measured by a Brookfield DV-III
rheometer.
The water release was measured by decanting the initial water release and by capillary suction time (CST). The capillary suction time measures the filterability of a slurry and is essentially the time it takes water to percolate through the material and a filter paper medium, and travel between two electrodes placed 1 cm apart. The method is often used as a relative measure of permeability.
Figure 1 shows an optimally dosed MFT mixed in a laboratory jar mixer with the rpm calibrated to the mean velocity gradient. The figure shows the shear yield stress progression curve for a 40 wt% solids MFT. The polymer was injected within a few seconds while stirring the MFT at 220 s-1. Mixing continued at the same mean velocity gradient until the material completely broke down. At each point on the curve, mixing was stopped and the yield stress measured. Water release during mixing is often dramatic and was clearly observed. The extent of water release is given by the capillary suction time. A low suction time correlates to high permeability and a high suction time correlates to low permeability. MFT dosed at ideal rates released the most water and about 20-25% of the initial MFT water was released at the lowest CST.

3 Shear yield stress 3

5 Capillary suction time `O 8

6

7 5 0 1b S- X

s y Time(s) Figure 1: Shear strength progression of flocculated MFT highlighting four distinct stages In further studies, MFT was mixed with a shear-resistant polymer flocculant in a laboratory jar mixer with the rpm calibrated to total mixing energy input.
Figure A shows the shear yield stress progression curve for a 40 wt% solids MFT dosed at different polymer concentrations. The experiment was conducted in two mixing stages. In the first stage, MFT was mixed at 220 s' during polymer injection. This stage lasts for a few seconds and defines the rate of floc buildup. In the second stage, the material was mixed at 63 s' until the material completely broke down. At each point on the curve, mixing was stopped and the yield stress measured. Water release during mixing is often dramatic and was clearly observed. MFT dosed at 1000 g/tonne of solid released the most water (Figure B). The material released about 20% of the initial MFT
water immediately whereas the under-dosed and over-dosed MFT released very little water through complete floc breakdown.

Figure A: Shear strength progression of flocculated MFT highlighting four distinct stages.

Figure B: Maximum water release from polymer-treated MFT during mixing.

In summary, four distinct stages were identified in the shear progression curve:

= Polymer dispersion or floc build-up stage defined by a rapid increase in yield stress as the polymer contacts the minerals and poor water release.
= A gel state of high shear yield stress which can be a plateau depending on the applied shear rate and %solids of the MFT. The rates of floc build-up and breakdown in this stage appear to be roughly the same.
= A region of decreasing shear strength and floc breakdown where significant amount of polymer-free water is released.
= An oversheared region characterized by rapidly decreasing shear strength where the material quickly reverts to an MFT state and releases very little water.
These stages are used to quantify the behaviour of polymer-dosed MFT and to compare behaviours under different shear regimes and the third stage is the target design basis.
An optimal dose of polymer with a good initial dispersion into MFT will achieves preferred permeability to release water. Without an optimal dose and good dispersion, the MFT remains in the gel state and only dries by evaporation. This is highlighted in Figure 2 where the same MFT in the underdosed or the overdosed state fail to release significant amount of water despite developing significant yield stresses. A
key advantage of preferred polymers is having prolonged resistance to shear which allows operational flexibility when pipelining flocculated MFT to deposition cells.

1000 -#-Underdosed Mix -*--Ideal Dosage a a -.H--Overdosed Mix y 2 y y Time (seconds) E

1000, 2- -+-Underdosed Mix S 100 --*-Ideal Dosage U 6 -H--Overdosed Mix I I

Time (seconds) Figure 2: Variation of polymer dosage with yield stress and water release Shown in Figure 3 are the microstructures corresponding to different shear regimes in the preferred flocculated MFT in Figure 1. The MFT and flocculated slurries were flash dried to preserve the microstructure to some extent. Samples were platinum coated and examined in a scanning electron microscope. The starting MFT showed a more massive

8 microstructure on drying and a greater tendency for the clays to stack along their basal planes in large booklets. This results in a low concentration of interconnected pores and poor dewatering. The middle micrographs in Figure 3 show microstructures exhibited by flocculated MFT in the second stage (383 Pa) at the onset of floc breakdown and water release. The microstructure is dominated by dense aggregates and randomly oriented clay platelets with more interconnected pores. The third set of micrographs (86 Pa) show less massive aggregates and a more open structure most likely responsible for the large water release observed in the third stage. The starting MFT is highly impermeable, whereas the flocculated MFT contains large macropores and significant amounts of micropores not visible in the starting MFT. At higher mixing time, the porosities start to collapse with an attendant reduction in the dewatering rates.

Figure 3: Scanning electron micrographs of 40 wt% MFT showing the fabric at different shear regimes (a). Untreated MFT, (b) high yield strength and (c) dewatering stage Optimally dosed MFT with varying solids content were also investigated (Figure 4). As the solids content decreases polymer dispersion becomes easier. The maximum yield strength of the material also decreases with increasing water content. A
substantial amount of water is released at lower solids content (for example, 10 wt%
settles to 20 wt% immediately - the water release at a lower solids content was much greater at 10 wt% solids (51% of the water in the original MFT) than at 40 wt% solids where 20% of the water in the original MFT was released); however the floc structure is weak and unable to stack in a deposition slope without being washed off.

9 6:
a j: 10$

6 -9-- 10 wt% solids -U-- 20 wt% solids -A- 30 wt% solids 2 -+- 40 wt% solids -1 1 t -7 Time (seconds) Figure 4: Shear strength progression for optimally dosed MFT samples diluted to varying solids concentration.

Further laboratory testing has shown that the strength gain and dewatering effects are possible with many anionic polymers, and are not limited to the particular formulation used in the first successful tests. Figure 5 compares a 40 wt% MFT optimally dosed with the preferred shear-resistant polymer and a high molecular weight anionic polyacrylamide (aPAM) typically used for flocculating oil sands tailings. The optimum dosages for both polymers, in terms of maximum water release, were the same (1000 g/tonne of solids) and were compared at two different shear rates. Polymer dispersion and shear stress response of the polymers differ significantly. Increasing the dispersion rate by increasing the mixer speed increases the yield stress instantaneously, but the traditional aPAM required additional mixing before the onset of flocculation.
This decrease in the dispersion rate means that MFT treated with traditional polymer is more likely to stay in a gel state and not release as much water. In preferred aspects of the process, the polymer flocculant is highly shear-resistant especially during the second and third stages, and is also highly shear-responsive especially in the first stage of dispersing and mixing.

It is generally expected for an aPAM that a higher mixing energy rapidly builds up the yield stress but the floc breakdown also occurs at a faster rate. The lower viscosity of the preferred polymer coupled with a high resistance to shear allow the flocculated MFT to be transported over long distances to deposition cells without significant floc breakdown.

-~- Poly A High Shear 600 -- Poly B High Shear -4- Poly A Low Shear 500 f Poly B Low Shear b Time (seconds) Figure 5: Yield stress progression of MFT optimally dosed with preferred polymer (Poly A) and a high molecular weight anionic polyacrylamide aPAM (Poly B).

5 Various polymers that have been developed with high shear resistance may be used in the process to improve the dewatering. Preferably, such shear-resistant polymers would also be in the general class of high molecular weight 30% anionic polyacrylamide-polyacrylate co-polymer flocculants.

10 In order to optimise the behaviour of the flocculated material, it is preferable to limit the variance in the shear energy applied to the various flocs which are created during mixing. This is achieved with an in-line orifice injector system. The concept here is to inject the polymer as a "mist" through the orifice (Figure 6a) instead of as a stream (Figure 6b). However, it should be understood that the quill-shaped injector device of Figure 6b may be modified by adapting the size of the perforations to approach a mist-like injection into the flow of MFT. When injected into a turbulent back-flow regime as shown in Figure 6a, the polymer is evenly distributed and flocculation is occurring throughout the pipeline cross section within 4 pipe diameters of the injection point. This rapid dispersion allows for precise control of the shear energies from the injection point to the point of deposition, and increases the percentage of the material that falls within the dewatering zone at a design point in the system. This fundamental behavioural understanding is key to the improved application of this technique, and allows results obtained from bench scale testing to be used in CFD modelling and scaled up to field operations.

11 Figure 6: Computational fluid dynamic model of polymer injection systems as a mist (top) and as a stream (bottom).

Pilot Test for the Determination of Mixing Parameters A 20-m long and 0.05-m diameter pipe loop fitted with the in-line orifice injector was used to investigate the shear response and dewatering behaviour of flocculated MFT
(Swift J.D. 2004, Heath A.R. 2003, and D.N. Thomas 1999). Sample ports are fitted to two locations along the length of the pipe. Figure 7a shows that the yield strength

12 progression in the pipe loop is similar to that observed in the laboratory jar mixer although the mixing energies are not directly comparable. MFT flow at 30 Umin corresponds to a mean velocity gradient of 22 s' compared to 63 s' in the bench scale test. Another test conducted at 100 Umin (176 s) showed a more rapid floc buildup and breakdown similar to the the 220 s' test in the jar mixer. Figure 7b shows flocculated MFT sampled at different locations during the test run for the optimally dosed MFT at 30 Umin (1000 g/tonne of solids in this case). Such data from the pilot and field tests may be used to develop a mixing model for process design and monitoring of the commercial scale MFT drying plant.

3- -I-- 50% Ideal Dosage Low Shear 2 -U- 80% Ideal Dosage Low Shear --A- Ideal Dosage Low Shear -- Ideal Dosage High Shear =A

Time (seconds) Figure 7a: Shear progression curves of the pilot scale flocculated MFT (35 wt%
solid)

13 Figure 7b: Jar samples taken at each sample point in Figure 7a at ideal dosage and low shear Field observations The rapid polymer dispersion by the orifice mixer caused the yield strength of flocculated material to increase very rapidly and results in the deposition of a two-phase fluid. This is shown in Figure 8 where the flocculated MFT and a separate water stream are observed at the discharge in one of the pilot tests.

A scaled up version of the orifice mixer was investigated in the field with optimally dosed 35 - 40 wt% MFT flowing at - 7500 Umin (32 s) in 0.3 m pipe diameter, and deposited in cells at various distances from the injection point. Figure 9 shows the extent of water release for each cell, both from actual sampling after 24 h and a capillary suction test conducted on the as-deposited flocculated MFT. The dewatering trend is analogous to the shear progression profile for the laboratory and pilot tests. Over 25% of the MFT
water was released immediately after injection up to 175 m. Beyond this length, the water release rate decreased rapidly and the flocculated material properties resemble MFT.

Further dewatering occurs in the deposition slopes through drainage enhanced by the slope and by evaporation. The under-mixed material deposited at roughly 7 m from the discharge was further dewatered by mechanically working the material to reach the floc breakdown stage where more water is released from the flocculated material.
Aggressive mechanical working however could break the deposit structure resulting in lower permeability and a restricted water release. Once the permeable structure is broken, dewatering is only by evaporation.

Evaporation results in crack formation as shown in Figure 10. Deepening cracks through dewatering allow for side drainage of release water into cracks and down the slope.
Typical deposits up to 20 cm thick was found to dry beyond 80 wt% solids in 6-10 days after which a subsequent lift could be placed.

14 Figure 8: Flocculated MFT discharged on a slope showing high yield strength material and water channeling around the discharge Time in pipe (s) J Mechanical working --W- Capillary suction during discharge -i -- % Water loss after 24 h 1:0 .2 9-5 "

n en Inn Icn inn V )V Iv/ IJV tA!
Distance from polymer injection (m) 30 Figure 9: Water release rate of flocculated MFT at various distances from the injection point.

35 Figure 10: Crack formation in an optimally flocculated MFT after a few days Conclusions Industry based research efforts into oil sands tailings and reclamation represent the final, crucial step in developing successful technologies to deal with the legacy and future fluid MFT inventories. Fundamental research, while a first critical step, can only identify potential solutions and directions for development. Scale issues are major determining factors in successful oil sands technology implementations. Use of the present techniques can reduce fluid inventories, minimise the need for tailings pond storage, and allow for accelerated reclamation of mining areas. Further focus on determining the technical controls of the process will result in ever more effective systems, reducing costs, land disturbance, and lower water requirements.

References 1. Wells, P. S and Riley, D. MFT Drying - case Study for the use of Rheological Modification and Dewatering of Fine Tailings Through Thin Lift Deposition in Oil Sands Tails. In Paste 2007 - A.B Fourie and R. J. Jewell (eds). 2007. Austalian Centre for Geomechanics, Perth.
2. Matthew J. Past Present and Future Tailings Experience at Albian Sands.
International Oil Sands Tailings Conference. December 7 - 10, 2008.
3. Silawat Jeeravipoolvarn. Deposition of inline thickened fine tailings.
International Oil Sands Tailings Conference. December 7 - 10, 2008.
4. Golder Paste Technology Ltd. Conceptual study on applying paste technology to oil sands tailings management at Suncor, Fort McMurray. 2007.
5. Spicer P. T. Pratsinis S. E. (1996): Shear Induced flocculation: The evolution of floc structure and the shape and the size distribution at steady state. Water Research 30 (5):
1049-1056.
6. Omotoso, O. and Mikula, R. J. (2004): High surface area caused by smectitic interstratification of kaolinite and illite in Athabasca oil sands. Applied Clay Science 25 (1-2): 37-47.
7. Swift J.D. , Simic K., Johnston R.R.M. , Fawell P.D. , Farrow J.B. (2004):
A study of the polymer flocculation reaction in a linear pipe with a focused beam reflectance measurement probe. International journal of mineral processing 73: 103-118.
8. Heath A.R., Koh P.T.L (2003): Combined population balance and CFD modeling of particle aggregation by polymeric flocculant. Third International Conference on CFD in the Minerals and Process Industries. December 10-12 2003.
9. Thomas D.N., Judd S.J., Fawcett N. (1999): Flocculation Modeling: A review.
Water Research 33 (7): 1579-1592.

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