CA2582059A1 - A method for producing a non-segregating waste stream - Google Patents

Abstract

A non-segregating waste stream, including tailings obtained from mining, and a method for production of the non--segregating waste stream is provided.

Description

A METHOD FOR PRODUCING A NON-SEGREGATING WASTE STREAM
The present invention relates to a method and system for producing a non-segregating waste stream.

BACKGROUND
Mining operations suffer from significant waste disposal difficulties. One such difficulty is differential settling of the produced waste stream.

In many current open-pit mining operations, for example, waste streams are disposed of by pipelining the waste stream slurry to an external tailings confinement facility, which is essentially a man-made pond enclosed with a dyke system that contains the waste material. Poor settling characteristics of fine inorganic solids create an uppermost solids layer that consists of material that has limited bearing capacity. There is a desire to reclaim mined surfaces, but the low bearing capacity of the top layer of the tailings ponds presents a technical barrier for achieving this.

One example of where this problem is presented is in oil sands mining operations, such as those found in Alberta, Canada. In order to separate valuable bitumen from a surrounding inorganic matrix, large amounts of water and process aids are currently added to mined ore. After separating the bitumen, a large volume of waste slurry remains, which comprises water, sand, dissolved organic and inorganic material, and fine, poorly settling, solids. Once disposed in the tailings confinement, the fine, poorly settling solids do not settle with the other, coarser solids creating a low bearing capacity top layer that is unsuitable for land reclamation.

It has been recognized that one means of generating an in-pit deposit of a waste slurry that is suitable for land reclamation is to generate a waste slurry that does not show differential settling rates between the coarser and finer solids. Several attempts have been made to prevent differential settling. To date, these attempts have shown little success. In the oil sands industry, these attempts have focused on increasing the yield stress of the tailings, but this has failed to generate tailings resistant to differential settling effects under process conditions.
SUMMARY
According to one broad aspect of the present invention, there is provided a substantially non-segregating waste stream.

According to another broad aspect of the present invention, there is provided a method for producing a substantially non-segregating waste stream comprising: (a) determining the apparent viscosity of the waste stream; and (b) modifying the waste stream to increase the apparent viscosity to at least a threshold level over which the waste stream becomes substantially non-segregating.

According to another broad aspect of the present invention, there is provided a method for recovering water from a waste disposal site comprising: (a) determining the apparent viscosity of a waste stream comprising water and solids; (b) modifying the waste stream to increase the apparent viscosity to at least a threshold level over which the waste stream becomes substantially non-segregating; (c) depositing the modified waste stream at the waste disposal site; (d) allowing sufficient time to permit settling of the solids from the water in the modified waste stream; and (e) recovering the water.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a tailings pond exhibiting differential settling.

Figure 2 is a schematic representation of differential settling.

Figure 3 is a schematic representation of non-differential settling.

Figure 4 is a plant layout for generating substantially non-segregating tailings according to one embodiment of the present invention.

Figure 5 is a chart illustrating the effect of apparent viscosity on settling.

Figure 6 shows rheograms of a number of different fluids having different rheological properties.

Figure 7 is the accompanying apparent viscosity profiles for the fluids of Figure 6.

DETAILED DESCRIPTION

A schematic representation of a mine tailings stream produced under current operating conditions is shown in Figure 1. Upon introduction of the mine tailings stream, a poorly settling fine solids layer (100) forms on a settled coarse solids layer (110) in a tailings pond (120), surrounded by a dyke (130). The poorly settling fine solids layer (100) is a result of differential settling, which may be better understood from Figures 2 and 3.

When an homogeneous combined solids slurry is introduced into a containment unit, a heavier fraction will settle to the bottom of the containment unit due to gravitational effects. This is shown in Figure 2. In a typical oil sands operation, the heavier fraction is rich in sand, with the upper fraction comprising a mixture of fines suspended in water. The fine slurry upper layer of the tailings pond will have insufficient bearing capacity to be suited for land reclamation purposes.

ASTM provides a definition for "fines" as particles having a diameter of 74 m or less. In practice, fines are primarily composed of silica and clay. "Coarse"
solids have particles with a larger diameter than the fines (e.g., above 75 m). In practice, these particles are primarily sand.

If differential settling were not to occur, the homogeneous slurry would settle under its own weight in a homogeneous fashion. This is shown in Figure 3. In time, a water layer will form over the settled layer, which water can be recycled into, for example, the oil sands mining operation process, reducing the amount of water that must be drawn from external water sources. The consolidated slurry forms a soil layer that will have sufficient bearing capacity to allow for land reclamation.

There have been a number of different approaches taken in an attempt to obtain non-segregating tailings.
According to one theory, it has been suggested that the yield strength of the tailings mix be increased in order to minimize particle settling. On a fundamental level, the terminal settling velocity of a particle is determined by force equilibrium on the particle. Referring to standard textbooks on fluid mechanics (see, for example, Bird, R., W.E. Steward and E.N. Lightfoot, "Transport Phenomena", 1960, John Wiley & Sons), the terminal settling velocity for a single particle in a Newtonian fluid is given by Formula 1.0:

V - gOPdz s 1grI
Formula 1.0 where:

g is the gravitational acceleration constant [m/sZ];
Ap is the difference between particle and fluid density [kg/m3];

d is the particle diameter [m]; and ri is the fluid viscosity [Pa=s].

In Formula 1.0, the density difference for a tailings recipe comprising silica particles and water is typically about 1500 kg/m3. From the functional form of the equation, it is therefore obvious that the settling velocity can only become zero if the fluid viscosity is infinitely high. This suggests that there will always be differential settling (and therefore segregation) when particles settle in a Newtonian fluid.

It is possible to generate tailings mixes that exhibit non-Newtonian behaviour. Unlike with Newtonian fluids, it is theoretically possible to obtain non-segregating tailings with a non-Newtonian fluid. In order to keep a particle suspended in a non-Newtonian fluid (i.e., have zero settling velocity), the fluid must have sufficient strength to counteract the buoyancy force acting on the particle. In rheological terms, this means that the fluid must exhibit yield strength characteristics (see, for example, Hunter, R.J., "Introduction to modern colloid science", Oxford University Press, 1993; and Hiemenz, P.C., "Principles of colloid and surface chemistry", Marcel Dekker, 1997). Physically, the fluid's yield strength required for keeping a particle suspended is given by Formula 2.0:

tio = a0pgd Formula 2.0 where:

tio is the yield strength in Pa;

a is an empirically determined constant with a numerical value between 0.048 and 0.2; and p, d and g are as defined for Formula 1Ø
Applying numbers typical to oil-sands tailings reveals that a yield strength of approximately 0.33 Pa should suffice for generating non-segregating oil sands tailings. But, laboratory testing has revealed that oil sands tailings samples with yield strength higher than 0.33 Pa still show differential settling under simulated process conditions.

Another approach taken in an attempt to obtain a non-segregating tailings has focused on increasing the fines content of the tailings. It has been suggested that if the tested tailings mix is higher in fines content such that a weight ratio of Fines/(Fines+Water) is higher than a certain threshold value, then the tailings should be non-segregating. (See, for example, Fine Tailings Fundamentals Consortium, 1995. "Vol. III, Volume reduction of CHWE fine tailings utilizing non-segregating tailings", in "Advances in oil sands tailings research", Alberta Department of Energy, Oil Sands Research Division.) The present inventors have determined that while the presence of a yield strength of 0.33 Pa may be a factor in generating non-segregating oil sands tailings, this alone is insufficient. It is believed that varying the yield strength alone is insufficient, because this property describes the slurry under stagnant conditions, and not under process conditions. Increasing the Fines/(Fines+Water) ratio has also not resulted in a substantially non-segregating tailings. The present inventors have determined that modifying the apparent viscosity of a tailings product will reduce the undesired effects of differential settling and assist in producing non-segregating tailings.

Any fluid (Newtonian or non-Newtonian) will show resistance against deformation, e.g. shear. The amount of stress that has to be applied at a certain deformation (or shear) rate depends on the rheological properties of the fluid. For Newtonian fluids, the ratio between shear stress and shear rate is constant (e.g., doubling the shear rate doubles the amount of stress required). This constant ratio is referred to as the fluid viscosity. For non-Newtonian fluids, the ratio between shear stress and shear rate is generally not constant, but is dependent on the shear rate.
For non-Newtonian fluids, an analogous property to viscosity, i.e. the apparent viscosity, can be defined as the ratio between shear stress and shear (i.e., deformation) rate. Apparent viscosity can be measured in a number of ways, as will be known to the person skilled in the art.
It has been observed that the total amount of segregation to be expected is a function of the deformation rate to which the tailings recipe is exposed and the time the tailings mix is exposed to this deformation rate. As described above, the deformation rate is directly related to the apparent viscosity of the mixture.

Tailings slurries are exposed to deformation under process conditions (e.g., during mixing, pipeline transport, in-pit placement). To offset this, the apparent viscosity of the slurry is made sufficiently high during processing so that a substantially non-segregating tailings is obtained.
In practice, this means determining a threshold apparent viscosity above which the tailings become substantially non-segregating. The threshold apparent viscosity can be determined experimentally by observing at what apparent viscosity value the tailings mix becomes substantially non-segregating.

"Substantially non-segregating" as used throughout this application is intended to encompass waste streams that show limited differential settling between coarse and fine particles, which will facilitate generating a sufficient bearing capacity in situ to meet a desired purpose, including, without limitation, land reclamation activities.

It will be readily appreciated that when the waste stream is first introduced into a containment area, it is not immediately suited for land reclamation purposes. A
sufficient consolidation time will have to pass before the material reaches a desired strength.

There are a number of variables that affect the apparent viscosity, including, without limitation, the composition of the tailings mix, the degree of dewatering of the tailings slurry, additives that can modify solution chemistry, and flow devices used during processing.
Increasing the apparent viscosity to a threshold level can therefore be achieved in a number of ways. For example, modifications can be made to mechanical and chemical variables during processing, as well as mechanical factors that may act on the slurry during deposition into a tailings pond. The modifications include, without limitation:

increasing the degree of dewatering of the waste slurry; a regime of conditioning treatments in the process line-up;
and limiting the deformation rate on the slurry during the final stages of deposition.

Apparent viscosity is raised when the water content of a tailings slurry is lowered. There are a number of different dewatering processes that can be used to lower water content, which will be known to the person skilled in the art. Common dewatering options for fine solids include, without limitation, mechanical thickeners and centrifuges.

For coarse tailings, hydrocylones and vibrating screen technology are commonly adopted.

Chemical modification during processing may also be used to increase apparent viscosity. For instance, an apparent viscosity enhancing chemical agent could be added.
Apparent viscosity is raised in oil sands tailings slurries, for example, when the electric double layer around clay material in the slurry is contracted. To achieve this divalent or trivalent cations (e.g., Ca2+, A13+) may be added to the tailings stream. Gypsum (CaSO4), for example, can be used as a source of Ca2+. Other means of chemical modification include, modifying the pH of the tailings stream and/or adding active clays (e.g. Bentonite) to the tailings product. Other suitable chemical modifications will be apparent to the skilled person, or can be determined through routine testing. The appropriate amount of additives can be determined through routine testing.
Mechanical parameters at deposition into a tailings pond may also affect apparent viscosity. For instance, apparent viscosity is adversely affected if the deformation rate applied to the tailings product is too high. For example, a common way of introducing tailings material in the deposition cell is through 'beaching'. In this method, tailings run down the containment wall into the containment. During this process, the deformation to which the material is exposed results in the coarse fraction segregating almost immediately. When the tailings product is introduced in a more controlled way (e.g., by means of feed-wells, diffusers), deformation can be limited and segregation inhibited.

Any of these modifications can be made alone or in combination to minimize deformation conditions in order to meet a defined threshold apparent viscosity suitable for the intended purpose.

A plant layout for generating a substantially non-segregating waste stream is exemplified in Figure 4. This plant layout is described by reference to a tailings obtained from a typical oil sands mining operation.

In Figure 4, a waste stream (400) from an oil sands processing facility is fed into a size classification unit operation (405). The waste stream (400) may be a combination of one or more waste streams, and may comprise one or more of, without limitation, bitumen extraction tailings, Tailings Solvent Recovery Unit (TSRU) tailings, and Mature Fine Tailings (MFT) from a conventional external tailings facility. The size classification unit operation (405) separates the stream into a fines rich stream (410) and a coarse rich stream (420). The size classification unit operation (405) typically consists of (but is not limited to) an appropriate combination of hydrocyclones, gravity separators and equivalents thereof.

Stream (410) is fed into a fine tailings conditioning unit operation (415). In conditioning unit operation (415), one or more tailings process aids (430) may be added to the fines rich stream (410). Processing aids may comprise (but are not limited to) one or more anionic and/or cationic flocculants, as well as dilution water. The conditioned fine tailings stream (440) is fed to a fine tailings dewatering unit operation (425). This unit operation (425) may comprise a combination of mechanical thickeners and/or centrifuges or equivalent unit operations.
The unit operation (425) achieves a degree of dewatering of the conditioned fine tailings stream (440) for generating a fine paste having a sufficient apparent viscosity to produce a substantially non-segregating tailings product. Unit operation (425) produces an overflow stream (450) that is lean in particles and an underflow stream (460) that is rich in particles. Stream (450) may be fed to water treatment unit operation (435), which separates the stream into an internal recycle stream (470) and a product stream (480).

Unit operation (435) is aimed at removing sufficient ionic material from the aqueous phase to make the water acceptable for recycling into the oil sands processing plant as stream (480). Unit operation (435) will typically comprise a final solids removal step and a combination of Reverse-Osmosis/Nanofiltration or equivalent. Stream (450) may be treated in whole or in part, and unit operation (435) may comprise a means for bypassing a part of stream (435) directly to stream (480). Stream (470) is comprised of a concentrated 'brine' that can be used as dilution water for unit operation (405).

The coarse rich stream (420) may not be of high enough solids content for producing a substantially non-segregating tailings product. Stream (420) may therefore be further treated in a coarse tailings dewatering unit operation (445). This unit operation (445) may comprise vibrating screen technology, for example, or an equivalent thereof. A water stream (490) from this unit operation may contain fine material that is recycled into the size classification unit (405). Unit operation (445) dewaters the coarse material to sufficiently low water content for substantially non-segregating tailings production. A
dewatered coarse materials stream (500) is obtained from unit operation (445). Both the dewatered fines stream (460) and the dewatered coarse stream (500) are fed at correct proportions to product mixing unit operation (455). After the streams are homogenized sufficiently in unit operation (455), the homogenized material stream obtained (510) is fed to deposition unit operation (465), which pumps out and distributes stream (520) in-pit (not shown). Unit operation (465) may comprise equipment equivalent to feed-wells in commercially available mechanical thickeners.

Example 1 In Figure 5, laboratory data on a combined coarse-fine tailings mix is presented. Physical characteristics of the tailings mix are:

Remoulded yield strengthl: 2 Pa.
Unmoulded yield strength2: > 10 Pa.
Fines/(Fines+Water): 38 wt%

Solids content: 75.5 wt%
1"Remoulded yield strength" is the residual yield strength after thorough homogenisation.

2"Unmoulded yield strength" is the yield strength if the tailings mix is left undisturbed.

The yield strength of the sample is higher than 0.33 Pa, and should therefore be non-segregating according to Formula 2Ø Also, the tested tailings mix is higher in total solids content and a ratio of Fines/(Fines+Water) than the segregation boundary as defined in "Advances in Oil Sands Tailings Research" (see above). This also suggests that the tailings should be non-segregating.

The tailings recipe was tested by monitoring the degree of sand drop-out over time under simulated process conditions. For this purpose, laboratory equipment that monitors sand fraction in a slurry as a function of time was used. Process conditions are mimicked by applying shear rates representative of pipeline transport and deposition to the sample while measuring the change in sand fraction in the sample. A decreasing sand fraction in the sample indicates that the sand has settled past the detection point, which means that sand has segregated from the slurry mix. The lines shown in Figure 5 show the amount of sand present at a certain height in the test cell. For the condition indicated as "low apparent viscosity" (typically around 100 mPa s), it can be seen that after around 6000 seconds, the sand content starts decreasing. The solids content continues to decrease over time. When the same sample was tested under higher apparent viscosity conditions, as shown by lines labelled "intermediate apparent viscosity" and "high apparent viscosity" (typically around 500 mPa s), both the time at which segregation commences and the extent to which segregation occurs decreases. The apparent viscosity was changed by altering the shear rate applied to the sample.

Prior teachings suggest that this tailings recipe should be non-segregating, but segregation is observed under process conditions. However, it is observed in this example that increasing apparent viscosity reduces the degree of segregation, and that the onset of segregation can be delayed.

Example 2 The rheological behaviour of two tailings mixes is shown in Figure 6. Figure 6 shows the stress-deformation rate behaviour of a Newtonian fluid (600), an oil sands tailings slurry having a low yield stress (610) of about 0.4 Pa, and an oil sands tailings slurry having a high yield stress (620) of greater than 3 Pa. The Newtonian fluid (600) has a viscosity that is 10 times the viscosity of water. The Newtonian fluid (600) was not actually measured, but is a theoretical construct included for reference purposes only.

In Figure 7 the data plotted in Figure 6 is converted into apparent viscosity values. The horizontal line (700) illustrates an approximate threshold value for the apparent viscosity over which segregation was observed to be negligible. The reference Newtonian fluid from Figure 6 is plotted as line (710). The low yield strength fluid is plotted as line (720) and also fails to exceed the threshold value for deformation rates over about 10 s-1: for deformation rates below 10 s-1, the material acts as non-segregating, while for values over 10 s-1 the material segregates. Finally, the higher yield strength fluid is plotted as line (730). The constraint on deformation rate is alleviated: the material stays non-segregating throughout the deformation rate regime shown.

In this example, it is observed that a material with a yield stress of 0.33 Pa will segregate under process conditions when the apparent viscosity is not carefully controlled.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. By way of example, specific reference to tailings from a mining operation as the waste stream has been made, but the invention is not intended to be so limited. The method of the invention may be extended to any suitable manufacturing waste stream, including one comprising organic matter.

It must be noted that as used in the specification and the appended claims, the singular forms of "a", "and"
"the" include plural reference unless the context clearly indicates otherwise.

Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill and the art to which this invention belongs.

Claims (19)

1. A method for producing a substantially non-segregating waste stream comprising:

(a) determining the apparent viscosity of the waste stream; and (b) modifying the waste stream to increase the apparent viscosity to at least a threshold level over which the waste stream becomes substantially non-segregating.

2. The method according to claim 1, wherein the waste stream is tailings obtained from a mining operation.

3. The method according to claim 2, wherein the tailings is obtained from an oil sands mining operation.

4. The method according to claim 3, wherein the tailings comprise tailings from bitumen extraction, tailings from a tailings solvent recovery unit, or mature fine tailings, or a combination thereof.

5. The method according to claim 1, 2, 3 or 4, wherein the apparent viscosity is increased to at least a threshold level sufficient to permit land reclamation following disposal of the waste stream.

6. The method according to claim 1, 2, 3, 4 or 5, wherein the waste stream is modified by dewatering, addition of an apparent viscosity enhancing chemical agent, pH
modification, addition of active clays, or minimizing mechanical deformation forces acting on the waste stream, or a combination thereof.

7. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by dewatering.

8. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by addition of an apparent viscosity enhancing chemical agent.

9. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by addition of a divalent or trivalent cation.

10. The method according to claim 9, wherein the cation is Ca2+ or Al3+.

11. The method according to claim 10, wherein the Ca2+
is added as gypsum.

12. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by modifying the pH.

13. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by adding an active clay.

14. The method according to claim 13, wherein bentonite is added.

15. The method according to claim 2, 3, 4 or 5, wherein the waste stream is modified by minimizing mechanical deformation forces acting on the tailings composition during disposal.

16. A substantially non-segregating waste stream obtained from the method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15.

17. A method for recovering water from a waste disposal site comprising:

(a) determining the apparent viscosity of a waste stream comprising water and solids;

(b) modifying the waste stream to increase the apparent viscosity to at least a threshold level over which the waste stream becomes substantially non-segregating;

(c) depositing the modified waste stream at the waste disposal site;

(d) allowing sufficient time to permit settling of the solids from the water in the modified waste stream; and (e) recovering the water.

18. The method according to claim 17, wherein the waste stream is tailings obtained from a mining operation.

19. The method according to claim 18, wherein the tailings is obtained from an oil sands mining operation.