CA3101746A1 - Treatment of oil sands whole tailings with lime - Google Patents

Abstract

ABSTRACT
In a slurry-based bitumen extraction process involving the step of passing a flow of oil sands whole tailings from the extraction process through a cyclone, the flow of whole tailings is treated with lime (CaO) or hydrated lime (Ca(OH)2) prior to the cyclone, at a rate in the range from about 600 ppm to about 1,200 ppm by mass. In a bitumen extraction process involving the steps of passing a flow of whole tailings through a cyclone and then passing overflow from the cyclone through a thickener, the whole tailings may be treated with lime before entering the cyclone, or, alternatively, the overflow from the cyclone may be treated with lime before entering the thickener.
Treatment of oil sands tailings with lime in the form of Ca0 or Ca(OH)2) reduces dispersion of silt and clay particles in tailings slurries, promotes settlement of tailings solids, and improves release water chemistry.
Date Recue/Date Received 2020-12-06

Description

TREATMENT OF OIL SANDS WHOLE TAILINGS WITH LIME
FIELD
The present disclosure relates in general to methods for treatment of oil sands whole tailings, and relates in particular to methods that reduce the production of mature fluid fine tailings and improve the chemistry of release water from oil sands slurry-based bitumen extraction processes.
BACKGROUND
The oil sands deposits in the Athabasca region of Alberta, Canada contain about 142 billion cubic meters (or 890 billion barrels) of heavy hydrocarbons, in the form of bitumen, constituting the third largest hydrocarbon deposit in the world.
These oil sands deposits are composed of about 12% bitumen, 82% to 85% mineral matter (solids), and 3% to 6% water (by weight). The fraction of the solids smaller than 45 microns in size (1 micron = 0.001 millimeter) is commonly referred to as "fines", made up of silt and clay particles. The clay fraction of the fines (i.e., smaller than 2 microns in size) is an important factor in both bitumen extraction processes and oil sands tailings disposal processes.
The Clark Hot Water Extraction (CHWE) process, which is based on the use of caustic sodium hydroxide (NaOH) as a process additive, is commonly used in commercial oil sands slurry-based bitumen extraction plants. The use of NaOH
in this process increases the pH of the oil sands ore-water slurry (pH being a measure of acidity defined by the equation pH = -log [H ], where [W] is the molar hydrogen ion concentration) to between 8.1 and 9.0, resulting in the activation of asphaltic acids naturally contained in bitumen to produce surfactant species that reduce water surface tension and bitumen/water interfacial tension in the bitumen extraction process slurry.
The use of NaOH as an extraction process additive promotes liberation of bitumen from the oil sands ore matrix. The water released from the bitumen extraction process Date Recue/Date Received 2020-12-06 (also referred to as release water, released water, or process-affected water) and deposited in tailings ponds is commonly recycled to the extraction plant for further use in the extraction process. Such recycling of release water from the tailings ponds steadily increases the concentration of sodium ions (Nat) in the extraction process water, and causes significant operational challenges for oil sands plants. The NaOH
additive excessively disperses the fines fraction (i.e., silt and clay) of the oil sands ore in the extraction process slurry, producing a tailings material having poor geotechnical characteristics that present tailings management challenges, and promoting the production of toxic mature fluid fine tailings (referred to herein as "mature FI,T").
The use of NaOH as an extraction process additive increases pH of oil sands ore-water slurry systems, reduces water surface tension and bitumen/water interfacial tension, and promotes water wettability (or hydrophilicity) of bitumen droplets in the extraction process slurry. The reduction of bitumen/water interfacial tension promotes liberation of bitumen from the oil sands ore matrix and provides acceptable bitumen extraction efficiency. However, it also suppresses coalescence of bitumen droplets and aeration kinetics, necessitates the use of large process vessels, increases capital costs and operating costs, and reduces bitumen recovery efficiency. Furthermore, the use of NaOH
promotes emulsion stability of the extraction froth, and therefore reduces the efficiency of froth treatment plants. For these reasons, there is a need for novel slurry-based bitumen extraction processes that avoid or mitigate operational challenges experienced in oil sands plants that use the CHWE process.
In typical commercial oil sands operations, the fluid tailings material (i.e., tailings slurry) produced at the bitumen extraction plant is hydraulically transported to and deposited in tailings ponds. The tailings slurry is a mixture of sand particles, dispersed fines, water, and residual bitumen, and the proportions of these slurry constituents can vary. However, typical oil sands tailings comprise about 55% solids (by weight) ¨
consisting of about 82% sand, 17% fines, and 1% residual bitumen. When tailings slurry is discharged into tailings ponds, the coarse sand particles tend to segregate quickly and form a "beach", while the fine tailings (with a solids content of 6% to 10% by weight)

- 2 -Date Recue/Date Received 2020-12-06 remain suspended in the tailings ponds. Initially, the fine tailings quickly consolidate to a solids content of about 15% to 20%, but after a few years they consolidate further to become a stable, toxic tailings material having about 30% solids by weight and about 86% water by volume, and this is the material referred to as mature FFT.
The stable structure of the mature FFT is attributable in large part to an excessively high concentration of sodium ions (Nat) and water-soluble asphaltic acids that are naturally present in the bitumen and which reduce the surface tension and bitumen/water interfacial tension and act as clay dispersants. It is also suspected that bi-wetted clay particles (mostly kaolinite type) of submicron size (i.e., less than 0.2 micron) ¨ also called ultra-fines ¨ are at least partially responsible for the formation of the stable gel-like structure of mature FFT.
After more than five decades of commercial experience using the CHWE bitumen extraction process, it has become clear that the production of toxic mature FF1 presents a major environmental challenge. In an effort to reduce mature FFT production, the petroleum industry developed and implemented "composite tailings" (or CT) technology, which uses a cyclone in the bitumen extraction process to separate sand from the whole tailings (as schematically illustrated in the exemplary process flow diagram in Figure 1 herein). This CT technology uses gypsum (i.e., calcium sulfate, or CaSO4) as an additive to prevent segregation of fines from sand particles. However, CT technology has proved to have numerous drawbacks, including the following:
= it does not reduce existing mature FF1' inventory in the tailings ponds, because additional mature FFT are produced from the cyclone overflow;
= it is inefficient in terms of thermal energy conservation, because it discharges the warm cyclone overflow into the tailings ponds where its heat is lost, so that only cold release water can be recycled to the extraction process;
= it results in the emission of hazardous hydrogen sulfide (H2S) due to anaerobic reduction of sulfate ions (S042-) by residual bitumen in tailings;
and

- 3 -Date Recue/Date Received 2020-12-06 = the steady increase of the concentration of sodium ions (Nat) in the process water caused by use of the CHWE process increases the sodium adsorption ratio, giving rise to related challenges in terms of the efficiency of the CT process (e.g., excessive dispersion of clay and silt particles in the extraction process slurry, resulting in reduced ability of thickeners to efficiently thicken the cyclone overflow).
The CT process was subsequently modified to use thickeners to thicken the cyclone overflow, allowing the release water from the extraction process to be recovered by thickening the cyclone overflow and then recycled to the extraction process at as high a temperature as possible. This modified CT process, which reduced thermal energy demand, is schematically illustrated in Figure 2.
BRIEF SUMMARY
In general terms, the present disclosure teaches methods for treatment of oil sands whole tailings with lime (CaO), or, alternatively, hydrated lime (Ca(OH)2), to avoid or reduce the undesirable effects of excessive dispersion of silt-clay size particles in oil sands tailings slurries produced by the CHWE bitumen extraction process.
Benefits of methods in accordance with the present disclosure include improved efficiency of thickeners used to thicken cyclone overflow, and recovery of larger volumes of warm water from the cyclone overflow for recycling to the bitumen extraction process, by treating the cyclone overflow with CaO before it enters the thickeners. This process is schematically illustrated in Figure 3.
Treatment of tailings slurry with CaO has been found to destabilize bitumen-water emulsions, which has the beneficial effect of releasing residual bitumen trapped in the whole tailings. The released residual bitumen can be removed from the tailings effluent by means of a flotation process following CaO treatment.
The present disclosure teaches treatment of oil sand whole tailings with lime (in the form of CaO or Ca(OH)2), which coagulates and flocculates silt-size and clay-size

- 4 -Date Recue/Date Received 2020-12-06 particles and improves release water chemistry. Subsequent to introduction of CaO into the tailings slurry before it enters the cyclone, or introduction of CaO into the cyclone overflow before the cyclone overflow enters the thickener, the thickener overflow (typically containing less than about 0.5% solids, plus any excess CaO) can be conditioned for recycling as process water in the bitumen extraction plant, by blending it with the water recycled from the tailings ponds. Such recycling of the thickener overflow containing excess CaO makes it possible to reduce the use of NaOH as a process additive in the bitumen extraction process. This water management practice reduces freshwater demand for the extraction process, and reduces the rate of increase in sodium ion concentration in the process water when using the CHWE process.
The thickener underflow can be blended with cyclone underflow and/or with sand to produce non-segregating tailings (NST) or blended with overburden soils for permanent deposition.
Because of the foregoing advantages, treatment of whole tailings with CaO, in accordance with methods disclosed herein, offers significant operational benefits in terms of tailings management and related environmental and long-term sustainability challenges of oil sands processing plants.
Accordingly, in one aspect the present disclosure teaches treatment of oil sands tailings with CaO (or Ca(OH)2) before the cyclone, or treatment of the cyclone overflow with CaO before the thickener, at a dosage preferably within (but not limited to) the range of 600 to 1,200 parts CaO per million parts whole tailings (by weight), resulting in coagulation and flocculation of silt-size and clay-size particles in the whole tailings or in cyclone overflow, promoting improved efficiency of the thickener to thicken the cyclone overflow to higher solids contents in shorter thickening retention times.
Treatment of oil sands tailings with CaO in accordance with the present disclosure also has the effect of suppressing the activities of surfactant species formed in the bitumen extraction process, and increasing water surface tension and bitumen/water interfacial tension, thereby reducing the stability of bitumen-water emulsions and

- 5 -Date Recue/Date Received 2020-12-06 promoting the release and removal of residual bitumen from the tailings (such as by a flotation process).
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments will now be described with reference to the accompanying Figures, in which:
FIGURE 1 schematically illustrates a prior art composite tailings (CT) treatment process.
FIGURE 2 schematically illustrates a prior art CT treatment process generally as in FIG. 1, but incorporating a thickener to thicken the cyclone overflow.
FIGURE 3 schematically illustrates an exemplary embodiment of a tailings treatment process in accordance with the present disclosure, including treatment of whole tailings with lime (CaO) before the cyclone, or treatment of the cyclone overflow with lime (CaO) before the thickener.
FIGURE 4 photographically illustrates the results of settling tests performed on tailings samples treated with different dosages of lime (CaO) in accordance with the present disclosure.
DESCRIPTION
Examples of the use of lime (CaO) as a process additive for different purposes in the oil sands industry include Canadian Patent Nos. 2,188,064 and 2,522,031 (for NST
production), Canadian Patent No. 2,581,586 and U.S. Patent No. 7,931,800 (for bitumen extraction), Canadian Patent Application No. 2,840,675 (for destabilization of bitumen-water emulsions), and Canadian Patent Application No. 2,977,524 (for dewatering of existing mature FF1 inventories).
Unique chemical properties provided by CaO in aqueous environments include:

- 6 -Date Recue/Date Received 2020-12-06 = provision of hydroxyl ions (OH), which increase the pH of the oil sands ore-water slurry; and = provision of calcium ions (Ca2 ) ions, which simultaneously react with the active species in the slurry, such as clays, bicarbonates (HCO3), bitumen, and water-soluble naphthenates (water-soluble naphthenates being surfactants that reduce water surface tension and bitumen/water interfacial tension in oil sands ore-water slurries involved in bitumen extraction or tailings management operations).
Based on the composition and temperature of the oil sands ore-water slurry, CaO
can be used as an additive at different dosages to improve the efficiency of several oil sands plants processes, including:
= bitumen extraction;
= NST production;
= whole tailings treatment;
= dewatering of existing mature FFT inventory; and = two-stage reclamation of existing mature FFT.
Existing inventories of toxic mature FFT, all produced by using the CHWE
process, currently total more than one billion cubic meters. These mature FFT
inventories are continually growing, and they continue to pose a major environmental challenge.
The present disclosure is directed to treatment of whole tailings with CaO, in order to benefit from advantageous aspects of CaO chemistry in oil sands tailings slurries. In this disclosure, the term "whole tailings" is to be understood as referring to oil sands tailings discharged from slurry-based bitumen extraction plants, either before or after being blended with the tailings discharged from a froth treatment plant.
Methods in according with the present disclosure include treating whole tailings with CaO before they enter the cyclone, or treating the cyclone overflow with CaO before it enters the thickener, as schematically illustrated in Figure 3. Treatment of the whole tailings with CaO by either of these process options coagulates and flocculates silt and clay particles in the tailings slurry, and thereby enhances the thickener's ability to thicken

- 7 -Date Recue/Date Received 2020-12-06 the cyclone overflow, which may be blended with recycled water from tailings ponds and recycled to the bitumen extraction plant for use as process water in the extraction process.
The thickener overflow is typically composed of about 99.5% water and 0.5%
solids, so the solids compositions of the cyclone overflow and the thickener underflow will typically be very similar. Thickener underflow can be blended with sand or cyclone underflow to produce a tailings product having a desired sand-to-fine ratio (SFR), treated with additional CaO and deposited in tailings ponds as a non-segregating tailings (NST) material, or blended with overburden soil for final tailings deposition.
Treatment of the whole tailings or the cyclone overflow with CaO reduces the activity of surfactant species generated from bitumen due to the use of NaOH
as an additive in the CHWE process, and increases water surface tension and bitumen/water interfacial tension. The increase in water surface tension suppresses clay/water attraction, promotes coagulation and flocculation of silt and clay particles, and improves thickener efficiency. As well, ion exchange reactions between Ca2+ ions and clay species promote coagulation and flocculation of clay particles and thus further enhance thickener efficiency.
The increase in bitumen/water interfacial tension reduces bitumen/water attraction, reduces the stability of bitumen-water emulsions, and promotes release of residual bitumen trapped in the whole tailings. The released bitumen can be removed from the tailings slurry using a flotation process subsequent to treatment of either the whole tailings or the cyclone overflow with CaO, as schematically illustrated in Figure 3.
Removal of residual bitumen by flotation will typically be more efficient if the flotation process is implemented after treating the cyclone overflow with CaO, because the solids content of the cyclone overflow will be lower (typically in the range of 10% to 15% by weight), and most of the residual bitumen would be present in cyclone overflow.
Removal of residual bitumen from the tailings is highly desirable, as the residual bitumen is the primary source of toxicity of tailings stored in tailings ponds.

- 8 -Date Recue/Date Received 2020-12-06 In commercial operations, the composition of whole tailings is controlled by the grade of the oil sands ore, the process water chemistry, additives used in the extraction process, the extraction process temperature, and the hydrodynamics of the extraction process slurry. Cyclone overflow composition varies with the composition of the whole tailings and the operating conditions of the cyclone, while the composition of the cyclone underflow tends to be less variable in terms of solids and fines content.
Laboratory Testing Generally speaking, observations made with respect to the treatment of cyclone overflow with CaO can be extrapolated to predict the effects of treating whole tailings with CaO before the cyclone. Such prediction is sound, because the cyclone separates sand grains from the whole tailings, and the chemical reactivity of sand grains with CaO
is much lower than the chemical reactivity of process water and fines with CaO.
Experiments for CaO treatment of oil sands whole tailings were simulated in laboratory conditions by adding CaO to samples of a cyclone overflow material produced at a CHWE extraction plant and containing about 10% solids, approximately 90%
of which solids consisted of fines. The cyclone overflow sample material was prepared by blending process water and mature FF1 comprising about 40% solids, approximately 90% of which solids consisted of fines. The compositions of the mature FFT and process water used in these experiments are presented in Table 1 and Table 2, below:
Bitumen Water Solids Fines SFR
(%) (%) (%) (%) (-) 1.7 58.3 40.0 90.3 0.1 TABLE 1 ¨ Mature FFT sample composition (by weight)

- 9 -Date Recue/Date Received 2020-12-06 Total Cations Anions Water Conduc. Alkalinity Ca2+
Mg2+
Na K+ cr so;
Origin pH mS/cm (mg-CaCO3/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Mature FFT 8.4 1.9 429 50 20 280 18 145 PW-1 8.4 1.7 401 45 20 276 15 147 PW-2 8.4 1.7 402 44 20 274 15 145 PW: Process Water TABLE 2¨ Process water sample composition In these experiments, samples of cyclone overflow material were treated with CaO
at dosages of 600, 800, 1,000, and 1,200 milligrams of CaO per kilogram of cyclone overflow (i.e., parts per million). A control sample of untreated cyclone overflow material was also prepared. Static standpipe settling tests were conducted on the treated and untreated cyclone overflow samples, and the results of these static standpipe settling tests after two hours of settling time are photographically presented in Figure 4.
The static standpipe settling tests indicated that the untreated sample of cyclone overflow recovered from the CHWE process would not settle in the thickener sufficiently to produce thickener underflow having a desired solids content (which would be about 40%). However, the settling tests indicated improvement of the settling characteristics of the cyclone overflow samples treated with CaO. Visual inspection of the tested samples, as presented in Figure 4, indicates that treatment of cyclone overflow with CaO at dosages of 1,000 to 1,200 ppm improves settling characteristics, which would enhance a thickener's ability to thicken the cyclone overflow.
Test results for the cyclone overflow samples treated with CaO at dosages of 600 ppm and 800 ppm also indicated improved settling characteristics, although less so than for the samples treated at CaO dosages of 1,000 ppm and 1,200 ppm. It is reasonable to predict that CaO dosages up to 2,000 ppm or more would produce beneficial results as well, as the presence of some residual CaO (i.e., above a dosage that is maximally effective to improve thickener efficiency) should be benign.
Accordingly,

- 10 -Date Recue/Date Received 2020-12-06 CaO dosages in accordance with the present disclosure do not have a specific maximum limit, so "erring on the high side" with CaO dosages may be reasonable in practice.
The settled fractions of the four CaO-treated cyclone overflow samples were blended with sand to produce tailings materials with having sand-to-fines ratios of 2:1;
3:1, 4:1, and 5:1 (by mass). These tests simulate commercial operations of CT
or NST
production processes by blending thickener underflow with cyclone underflow materials.
In commercial operations, additional CaO could also be used to improve geotechnical characteristics of the CT or NST materials for permanent deposition, as described in CA 2,188,064 and CA 2,522,031 with respect to NST production using CaO as a process additive. Results obtained from these experiments suggest that these tailings materials are suitable for permanent deposition.
The water chemistry of the thickener overflow (i.e., the release water recovered from the CaO-treated cyclone overflow) was analyzed, along with the chemistry of the blend of recovered water (i.e., thickener overflow) and process water, and the results of these analyses are set out in Table 3, below:
RW/PW Total CaO Volume Ratio EC Alkalinity Na+ K+ Ca2+ Mg2+
Cl- SO4 (PPm) (1 pH (nnS/cnn) (mg-CaCO3/L) (nng/L) (nng/L) (nng/L) (nng/L) (nng/L) (nng/L) PW 9.0 1.54 382 337 17.7 16.6 15.1 - RW 8.6 1.60 424 355 17.0 37.0 14.5 600 RW 10.6 1.40 112 298 8.5 7.8 0.8 800 RW 11.6 2.11 111 359 13.0 4.8 0.1 1,000 RW 11.9 2.61 115 382 16.0 13.8 0.1 1,200 RW 12.1 3.41 116 393 18.6 86.1 0.1 800 1:1 10.4 1.55 221 347 15.1 4.8 6.5 800 1:2 10.2 1.52 224 342 15.5 5.6 8.0 800 1:3 9.7 1.50 282 345 18.8 6.7 10.8 1,000 1:1 10.8 1.61 198 373 17.1 3.6 5.6 1,000 1:2 10.3 1.53 246 355 17.1 4.5 9.1 1,000 1:3 9.9 1.51 266 352 17.1 5.6 10.6 1,200 1:1 11.5 1.89 141 365 18.0 1.8 0.5 1,200 1:2 10.8 1.60 216 359 18.0 4.0 8.0 1,200 1:3 10.3 1.56 243 355 17.8 3.6 10.1 PW: Process Water; RW: Released Water; EC: Electric Conductivity TABLE 3- Water chemistry

- 11 -Date Recue/Date Received 2020-12-06 These analyses suggest that, in commercial operations, thickener overflow contains enough alkalinity to soften large volumes of recycle water taken from existing tailings ponds containing tailings produced by CHWE processes. Recycling the thickener overflow, followed by CaO treatment of the whole tailings, would reduce the use of NaOH as an additive in the bitumen extraction process. This practice would mitigate the steady increase of the sodium ion concentration in the extraction process slurry, thereby providing significant benefits and advantages in terms of bitumen extraction plant operation, tailings management, and long-term sustainability for oil sands plants.
It will be readily appreciated by those skilled in the art that various modifications to embodiments in accordance with the present disclosure may be devised without departing from the present teachings, including modifications which may use structures or materials later conceived or developed. It is to be especially understood that the scope of the present disclosure should not be limited by or to any particular embodiments described, illustrated, and/or claimed herein, but should be given the broadest interpretation consistent with the disclosure as a whole. It is also to be understood that the substitution of a variant of a claimed element or feature, without any substantial resultant change in functionality, will not constitute a departure from the scope of the disclosure or claims.
In this patent document, any form of the word "comprise" is intended to be understood in a non-limiting sense, meaning that any element or feature following such word is included, but elements or features not specifically mentioned are not excluded. A
reference to an element or feature by the indefinite article "a" does not exclude the possibility that more than one such element or feature is present, unless the context clearly requires that there be one and only one such element. Wherever used herein, any form of the word "typical" is to be understood in the sense of representative or common usage or practice, and not as implying invariability or essentiality.

- 12 -Date Recue/Date Received 2020-12-06

Claims (7)

EMBODIMENTS IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS
CLAIMED ARE DEFINED AS FOLLOWS:

1. In a slurry-based bitumen extraction process involving the step of passing a flow of oil sands whole tailings through a cyclone, the method of treating the flow of whole tailings with Ca0 at a rate of about 600 ppm to about 1,200 ppm by weight, before the flow of whole tailings enters the cyclone.

2. In a slurry-based bitumen extraction process involving the steps of passing a flow of oil sands whole tailings through a cyclone and then passing overflow from the cyclone through a thickener, the method of treating the overflow from the cyclone with Ca0 at a rate of about 600 ppm to about 1,200 ppm by weight, before the cyclone overflow enters the thickener.

3. A method as in Claim 2, comprising the further steps of blending overflow from the thickener with recycled water from tailings produced by a CHWE bitumen extraction process, and using the blended water in the bitumen extraction process.

4. A method as in Claim 2, comprising the further step of blending underflow from the thickener with sand to produce non-segregating tailings.

5. A method as in Claim 2, comprising the further step of blending underflow from the thickener with underflow from the cyclone to produce non-segregating tailings.

6. A method as in Claim 2, comprising the step of blending underflow from the thickener with sand and underflow from the cyclone to produce non-segregating tailings.

7. A method as in Claim 2, comprising the step of blending underflow from the thickener with overburden soils for permanent tailings deposition.

Date Recue/Date Received 2020-12-06