WO2003004831A1

WO2003004831A1 - Oil sands separation process

Info

Publication number: WO2003004831A1
Application number: PCT/EP2002/006959
Authority: WO
Inventors: Donald Allen Luke
Original assignee: Ciba Speciality Chemicals Water Treatments Limited
Priority date: 2001-07-02
Filing date: 2002-06-24
Publication date: 2003-01-16
Also published as: EA200400108A1; AR034656A1; CA2392246A1; PE20030211A1; EA005875B1

Abstract

Oil bearing sands recovery process comprising a main separation stage. The oil bearing sands is slurried in the main separation stage with water to produce an enriched fraction and a dilute aqueous clay waste. The aqueous clay waste is sedimented in a waste sedimentation stage in one or more settling lagoons to provide a substantially solid clay sediment and supernatant. The dilute aqueous clay waste is fed into a well and flocculated by addition of a polymeric flocculant. The flocculated dilute aqueous clay sediments to provide a pumpable thickened clay sediment layer in the base of the well.

Description

OIL SANDS SEPARATION PROCESS

This invention relates to oil recovery processes, which comprises a main separation stage and a waste sedimentation stage. The objective is to treat waste sediment from a tar sand processing facility in a novel and efficient manner.

Tar sands (which are also known as oil sands or bituminous sands) are sand deposits, which are impregnated with dense, viscous, petroleum. Tar sands are found throughout the world, often in the same geographical areas as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca region in the northeast of the province of Alberta, Canada. This deposit is believed to contain perhaps 700 billion to one trillion barrels of bitumen. A substantial portion of the deposits are situated at, or very near, the surface where they may be readily mined and processed into synthetic crude oil. This procedure is being carried out commercially on a very large scale near Fort McMurray, Alberta.

Athabasca tar sands is a three-component mixture of bitumen, mineral and water, bitumen is the valuable component for which the extraction of tar sands are mined and processed. The bitumen content varies, averaging about 12wt% of the deposit, but ranging from zero to 18wt%. Water typically runs from 3 to 6wt% of the mixture, and generally increases as the bitumen content decreases. The mineral content is relatively constant, ranging from 84 to 86wt%.

While several basic extraction methods to separate the bitumen from the sand have been known for many years, the "hot water" process is the one of the most significant. The present invention is not limited to a particular separation process and the following description is for backing. The hot water process for achieving primary extraction of bitumen from tar sand consists of three major process steps (a fourth step, final extraction, is used to clean up the recovered bitumen from downstream processing). In the first step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85wt% solids. Sodium hydroxide or other reagents are added as required to maintain pH in the range of 8.0-8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats (settles upwardly) to form a coherent mass known as froth which is recovered by skimming the settling vessel (sometimes called the primary separation vessel or separation cell). A third stream, called the middlings drag stream, may be withdrawn from the settling vessel and subjected to a third processing step, scavenging, to provide incremental recovery of suspended bitumen.

As previously indicated, conditioning tar sands for the recovery of bitumen consists of heating the tar sands/water feed mixture to process temperature (180-200 F), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic or other reagents added. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discreet droplets of a size on the same order as that of the sand grains. The same process conditions, it turns out, are also ideal for accomplishing deflocculation of the fines, particularly the clays, which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the naturally occurring aggregates of clay particles to produce a slurry of individual particles. Thus, during conditioning, a large fraction of the clay particles become well dispersed and mixed throughout the pulp.

Those skilled in the art will therefore understand that the conditioning process, which prepares the bitumen resource for efficient recovery during the succeeding process steps, also prepares the clays to be the most difficult to deal with in the tailings disposal operation.

The second process step, called separation, is actually the bitumen recovery step since separation occurs during the conditioning step. The conditioned tar sand pulp is first screened to remove rocks and unconditionable lumps of tar sands and clay and the reject material, "screen oversize", is discarded. The screened pulp is then further diluted with water to promote two settling processes: globules of bitumen, essentially mineral-free, float upwardly to form a coherent mass of froth on the surface of the separation cells; and, at the same time, mineral particles, particularly the sand-sized mineral, settle downwardly and are removed from the bottom of the separation cell as tailings. The medium through which these two settling processes take place is called the middlings. The middlings consists primarily of water with suspended fine material and bitumen particles. The particle sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter, which influences the settling processes most is the viscosity of the middlings, and viscosity is directly related to fines content. Characteristically, as the fines content rises above a certain threshold, which varies according to the composition of the fines, middlings viscosity rapidly reaches high values with the effect that the settling processes essentially stop. In this operating condition, the separation cell is said to be "upset". Little or no oil is recovered, and all streams exiting the cell have about the same composition as the feed. Thus, as feed fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.

The third step of the hot water process is scavenging. The feed fines content sets the process water requirement through the need to control middlings viscosity which is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation cell material balance, and this stream of middlings can be scavenged for recovery or incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.

Final extraction or froth clean up is typically accomplished by centrifugation. Froth from primary extraction is diluted with naphtha, and the diluted froth is then subjected to a two- stage centrifugation. This process yields an essentially pure diluted bitumen oil product. Water and mineral removed from the froth during this step constitutes an additional tailings stream, which must be disposed of.

In the terminology of extractive processing, tailings is the throw-away material generated in the course of extracting the valuable material from an ore. In tar sands processing, tailings consists of the whole tar sand ore body plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be subdivided into three categories; viz.: (1 ) screen oversize, (2) coarse or sand tailings (the fraction that settles rapidly), and (3) fine or tailings sludge (the fraction that settles slowly). Screen oversize is typically collected and handled as a separate stream.

Recently, in view of the high level of ecological consciousness in Canada, United States, and elsewhere, technical interests in tar sands operation, as well as other diverse ore handling operations, has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straightforward. If one cubic foot of tar sands is mined, a one cubic foot hole is left in the ground. The ore is processed to recover the bitumen fraction, and the remainder, including both process material and the gangue, constitutes the tailings that are not valuable and are to be disposed of. In tar sands processing, the main process material is water, and the gangue is mostly sand with some silt and clay. Physically, the tailings (other than oversize) consist of a solid part (sand tailings) and a more or less fluid part (sludge). The most satisfactory place to dispose of these tailings is, of course, in the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings alone from the one cubic foot of ore occupy just about one cubic foot. The amount of sludge is variable, depending on ore quality and process conditions, but average about 0.3 cubic feet. The tailings simply will not fit back into the hole in the ground.

Commercial operating data confirms that a sludge layer is accumulating in the tailings disposal area which settles and compacts only very slowly, if at all, after a few years. For a number of reasons, this sludge layer, in common with similar sludge layers observed in tailings ponds associated with mining and extracting processes of many kinds, is particularly important and difficult to deal with. For dike building, tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for the pool of fluid contained inside. On the dike, the sand settles rapidly; and a slurry of fines, water, and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to strengthen the dike as it is built to a higher level. The slurry which flows into the pond's interior commences stratification in settling over a time scale of months to years.

Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. Rapid and slow settling processes occur, but their distinction is not as sharp as in dike building, and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach extending from the discharge position towards the pond interior. As the sand settles, fines and water commence long-term settling in the pond. Tailings from the hot water process containing a dilute suspension of fine materials in water, together with sand, are discharged to the tailings pond. The formation of sludge by settling of these tailings is attributable primarily to the presence of dispersed clay minerals. Many of the factors which determine the rate at which the clay minerals settle and the characteristics of the sludge formed are set within the tailings discharge. These include initial clay concentration (clay/water ratio), relative proportions of various clay mineral species, particle size, condition of clay surfaces and pore water chemistry. Experience and laboratory analysis indicate that all these factors vary significantly from time to time depending on the composition of the tar sands feed and the process conditions.

Typically, tailings are discharged over the beach (either directly or from dike construction) where most of the sand settles. The run-off flows continuously into a fluid pool or pond from which water is simultaneously withdrawn as recycle to the tar sands extraction process. Here, additional important determinants of settling behavior are imposed. These include rate of inflow and outflow in relation to surface area and clarified water volume, pond depth, and degree of agitation of pond contents, either through inflows and outflows or via thermal or by wind effects. While initial temperature is inherent in the tailings streams, temperatures in the pond are obviously determined by numerous other factors as well.

When a partly settled sludge remains undisturbed for between several months and about two years in a deep pond, it separates into two distinct layers, a virtually clear water layer on top and a sludge layer beneath. The density of the sludge layer increases gradually with depth due mainly to the presence of more sand and silt particles. These settle either not at all or very slowly because of the significant yield strength of stagnant sludge. The clay/water ratio increases only slightly with depth in the upper part of the pond and scarcely at all in the lower part. After one or two years, little further change in sludge volume occurs. Consolidation at the bottom of the pond is so slow that detection of consolidated material is difficult. Sludge formed in this manner is virtually unchanging over periods of years or decades and for practical purposes may be regarded as terminal sludge.

An active pond involving continuous inflow and outflow is more complex. Following discharge to the pond, clay particles undergo an aging process varying in length from a few days to many weeks. Prior to completion of the aging process, the clay particles do not begin to settle. However, once they commence to do so, the process proceeds quite rapidly according to the principles of Stokes Law until a clay/water ratio of about 0.13/1 is reached at which other factors evidently predominate over Stokes law. In the upper most part of a well managed pond, these effects result in a more or less clear water layer at the top underlaid by a layer of relatively dilute sludge more or less sharply differentiated from it. This layer or diluted sludge may be termed the sedimentation zone; its volume is determined by the rate of clay inflow and the average aging time required. If the water layer is permitted to become too small in relation to the clay inflow, water outflow and aging time, the upper part of the pond becomes overloaded, the clear water layer virtually disappears and the sedimentation zone becomes much larger since clay is then recycled through the process.

Sludge in the lower part of a deep active pond which has been in operation for some years is similar to that from an inactive pond; i.e., it may be regarded as terminal sludge. The space below the sedimentation zone and above the terminal sludge may be regarded as a transition zone lacking clear boundaries at top and bottom. It is characterized by a gradual increase in clay/water ratio with depth and owes its existence to the long time needed to attain the terminal sludge condition. Its thickness is primarily a function of the average clay inflow rate in relation to volume.

In summary, an active pond normally has a well-defined clear water layer at the top which can, however, disappear if overloading occurs. Beneath this is sludge which increases in density with depth. There are generally no clearly defined boundaries within this sludge except on occasion a layer of separated bitumen near the interface between water and sludge. However, the sludge may be considered as consisting of three zones each involving successively larger orders of magnitude of time scale for measurable dewatering to occur, and each characterized by the predominance of differing dewatering parameters. These three zones may be termed respectively a sedimentation zone, a transition zone and a terminal sludge zone.

A continuing challenge remains the development of a long-term economically and ecologically acceptable means to eliminate, minimize, or permanently dispose of the accumulation of sludge. The problem requires a multi-faceted approach toward its solution, and the present invention is directed at achieving one aspect of the solution: a more thoroughly and efficiently dewatered sludge layer which, as a consequence, eliminates the requirement for an active point to function both as a solids disposal and a water clarification site, and thus allows for semi-solid or paste to be disposed of in the primary disposal site.

Flocculation of the tailings stream in order to improve the settling characteristics of an industrial process tailings pond has been proposed and practiced in the prior art. In flocculation, individual particles are united into rather loosely-bound agglomerates or floes. The degree of flocculation is controlled by the probability of collision between the particles and their tendency toward adhesion after collision. Agitation increases the probability of collision, and adhesion tendency is increased by the addition of a flocculant.

Reagents act as flocculants through one or a combination of three general mechanisms: (1) neutralization of the electrical repulsive forces surrounding the small particles which enables the van de Waals cohesive force to hold the particles together once they have collided; (2) precipitation of voluminous floes, such as metal hydroxides, that entrap fine particles; and (3) bridging of particles by natural of synthetic, long-chain, high molecular weight polymers. These polyelectrolytes are believed to act by absorption (by ester formation or hydrogen bonding) of hydroxyl or amide groups on solid surfaces, each polymer chain bridging between more than one solid particle in the suspension.

Flocculants have been employed in the prior art to obtain precipitation of particles in tailings ponds of various industrial processes as well as in sewage treatment facilities. However, a distinct step forward in the art has been achieved by the use of hydrolyzed corn and potato starch flocculants. These specific hydrolyzed starch flocculants (which are characterized by the aqueous hydrolysis of the starch in the presence of one or more metal salts), particularly taking into account the economics of carrying out flocculation on a large scale, enjoy high performance characteristics for their ability to bring about rapid precipitation to a substantially terminal settled condition. This characteristic is especially valuable for use in those processes, such as the hot water process for obtaining bitumen from tar sands, in which there is a critical need to recycle clarified water back into the process. However, experience has indicated that the simple use of these hydrolyzed starch flocculants, or for that matter any other known flocculant results in very little, if any, improvement on the ultimate degree of dewatering of the sludge layer. That is, the terminal status of the sludge layer is just about the same as would be obtained over a much longer period of time by natural settling processes, and this terminal condition is unsatisfactory in that it includes too much water, is too voluminous, and is too unstable.

Nonetheless, it is not accurate to say that all characteristics of a sludge layer obtained as a result of flocculation by the aforementioned hydrolyzed starch flocculants is the same as that achieved naturally or by the use of other flocculants. In point of fact, certain very desirable characteristics to the sludge layer are obtained from the use of the hydrolyzed starch flocculants which are not achieved by natural settling or by the use of any other flocculant presently known, and it is on the appreciation and use of these circumstances that the present invention is based. More particularly, it has been found that the permeability and shear strength characteristics of the sludge layer are both very much increased; as a result, previously impossible dewatering techniques may be employed to compact and stabilize the sludge layer and to extract additional amounts of clarified water therefrom.

It has been proposed in the past, as another approach to alleviating pond water problems, to store the fines in the interstices between the sand grains in the material employed for dike building. Such a process is disclosed in Canadian Pat. No. 1 ,063,956, entitled "Method of Sludge Disposal Related to the Hot Water Extraction of Tar Sands" and issued Oct. 9, 1979; and corresponding U.S. Pat. No. 4,008,146, issued Feb. 15, 1 977. The experience with the procedure described in that reference is that the height to which the dike can be built is somewhat limited; however, it has now been discovered that if the sludge mixed with the sand to prepare the dike building material has been treated with the aforementioned hydrolyzed starch flocculants, the strength of the resultant material is notably increased such that the dike can be built higher, thereby not only permitting a deeper tailings pond, but also storing more sludge in the interstices between the sand grains comprising the dike.

There have been numerous proposals in the literature to try to accelerate the sedimentation by flocculating the clay waste, and there have been proposals to improve the structure of the substantially solid clay sediment by adding sands or other materials to the clay waste. Examples of disclosures of such mineral recovery processes utilizing flocculants are U.S. Pat. Nos. 3,418,237, 3,622,087, 3,707,523, 4,194,969, 4,224,149, 4,251 ,363, 4,265,770, 4,342,653, 4,555,346, 4,690,752, 5,688,404, 6,077,441 and 6,039,189 which are each incorporated herein by reference.

Despite the numerous proposals to use flocculants, in practice it has been found that their use frequently is not cost effective. Even when flocculant is used to promote sedimentation and the provision of a supernatant which can be recycled, the quality of the supernatant tends to be rather poor because the supernatant tends to be contaminated with unflocculated clay particles.

In particular, the main separation stage often includes the use of treatment chemicals such as flocculation agents or, especially, flotation agents and the efficiency of their use is decreased (and thus the required dosages are increased) when the water which is used in the flotation or other procedures during the separation stage contains suspended clay particles.

In order to minimize contamination and lack of clarity of the supernatant, it would be desirable to conduct the sedimentation under conditions which provide a considerable depth of sedimenting waste, so as to allow for a deep layer of supernatant above the sediment, thereby permitting supernatant to be drawn off at a height which is as far above the lower sedimented material as is possible. Unfortunately it is difficult to provide for this in lagoons as they tend normally to be relatively shallow. In particular, the problem becomes more acute as the lagoons become filled, over the years, with an increasing depth of substantially solid clay sediment.

A further problem arises from the fact that it is necessary to make optimum use of lagoon areas because of the undesirability of creating new lagoons. Accordingly there is an increasing tendency to need to continue using lagoons until it is impossible to deposit any more solid clay sediment in them, and so there is an increasing tendency to want to use lagoons which are substantially full and are too shallow for useful sedimentation. There is an increasing need to utilize lagoon areas more efficiently.

It is known to utilize sedimentation columns, for instance tubular metal tanks, which are constructed above ground level. Provided such a column has sufficient height it will eventually allow for the formation of a useful depth of supernatant. Unfortunately the volume of aqueous clay wastes which are generated in oil bearing sands recovery processes can be so large that it is impracticable even to contemplate the provision of column separating tanks of this type.

It is also well known to extend the life of a lagoon by digging the solid clay sediment out of it, but this is labor intensive and does not provide any direct solution to the need to conduct the recovery process efficiently and to give a good quality supernatant.

The object of the invention is to provide a oil bearing sands recovery process by which it is possible to obtain supernatant for flotation or other separation steps of improved quality, by which it is possible to utilize sedimentation lagoons more efficiently, and by which it is possible to recover additional hydrocarbon values normally lost to the sedimentation process.

An oil bearing sands recovery process according to the invention comprises a main separation stage in which oil bearing sands is slurried with water and separated into an enriched fraction and a dilute aqueous clay waste, and a waste sedimentation stage in which the dilute aqueous clay waste is sedimented in one or more settling lagoons to provide a substantially solid clay sediment and supernatant and the supernatant is recycled to the main separation stage, and the waste sedimentation stage comprises feeding the dilute aqueous clay waste into a well which has been formed in the ground, flocculating the dilute aqueous waste in the well by mixing polymeric flocculant into the waste, sedimenting the flocculated waste in the well to provide a pumpable thickened clay sediment and a supernatant, recycling the supernatant from the well back to the main separation stage, pumping the thickened clay sediment from beneath the supernatant in the well to one or more final lagoons and allowing the thickened clay sediment to undergo further sedimentation to provide a substantially solid clay sediment in the one or more final lagoons.

In general, the invention is applicable to any process in which the separation of oil bearing sands values from the raw mined rock or other material comprises slurring with water and thereby producing large volumes of a diluted aqueous clay waste which is then subjected to sedimentation in lagoons. Generally the clay waste has the characteristic of a very slowly consolidating clay mineral such as kaolonite, illite, chlorite, smectite, montmorillonite, attapulgite, or other typical naturally occurring clay minerals. The aqueous clay waste may also contain fine particles of other minerals such as sand, quartz or other minerals indigenous to the oil bearing sands source.

The preferred oil bearing sands recovery process to which the invention is applied is the recovery of oil or bitumen values from oil bearing sands , for instance as is practiced in the oil bearing sands recovery processes in Western Canada. Other mineral recovery processes to which the invention may be applied include any of those where the natural settling rate of the sediment is sufficiently slow that lagoon settlement is appropriate and where the settling can be promoted by a flocculating agent. Another particularly preferred example is bauxite mining and refining systems.

When the process is oil bearing sands recovery process, the main separation stage can involve any of the conventional separation procedures in such processes. For instance it may involve cycloning the slurry and may involve subjecting the slurry to flotation. Frequently the material may be recycled one or more times through one or more separation procedures. The advantage of the present invention is the formation of recycle stream with reduced suspended solids. A relatively dirty recycle stream can affect the viscosity of the primary separation vessel and cause excessive losses in the underflow and excessive solids in the recovered oil.

The dilute aqueous clay waste is a slurry consisting primarily of waste clay particles in water. However the clay slurry or waste may contain some useful course mineral values or residual bitumen values. The waste may be primary clay separation waste, typically having a 6 - 20% solids content and containing some useful mineral or bitumen value, or a secondary clay separation waste typically having a lower solids content and less coarse material or bitumen in it. In particular, the clay slurry or waste will often consist of or contain a secondary clay waste, namely a waste obtained from a flotation or other separation process. In some processes, the primary and secondary clay wastes are treated separately while in other processes the primary and secondary clay wastes are mixed together.

In general, the dilute aqueous clay waste generally contains not more than 20% and usually not more than 10% total clay solids by weight, but usually contains at least 0.1 % and usually at least 0.5% by weight total clay solids. The solids generally consist wholly or mainly of clay fines but can include some coarser waste or some coarser mineral values, such that the coarser material can be sedimented from the clay while the fines remain in suspension. The clay fines usually constitute at least 20% and usually at least 10% by weight of the dry matter of the waste. The clay fines, which may constitute the majority of the dry matter, will have physical and chemical characteristics typical of clay waste and, in particular, these characteristics are such that lagoon settlement and evaporation is usually the only practical way for converting the fines into a substantially solid sediment.

If the dilute aqueous clay waste contains coarse mineral values or other coarse settleable material, these materials may be sedimented from the waste as it flows through a ditch towards the well (for instance as described in U.S. Pat. No. 5,688,404) or these values may be sedimented in a lagoon prior to the well treatment of the present invention. Thus the waste containing the mineral values may be directed into an entry area of a settling lagoon, and the resulting reduction in flow velocity which occurs as the waste enters the lagoon causes sedimentation of the mineral values primarily in the entry area. The mineral values can then be recovered from the base of the entry area, or if appropriate from the base of the entire lagoon by excavation.

The dilute aqueous clay waste, optionally after preliminary sedimentation of coarse materials, then flows into a well that has been formed in the ground. The well can be located within or in the vicinity of a primary lagoon, existing mine cuts, a channel, an emergency spillway or secondary containment area or virgin territory. In a preferred embodiment, the well is located in any convenient location, such as a primary lagoon, more particularly at the base or waste entrance of a primary lagoon. The well may be made by excavation of, for example, a square, rectangle, circle or oval area in the ground to a sufficient depth. If desired the well can be lined in order to prevent erosion of the walls, but this is usually unnecessary.

As a result of utilizing a well, instead of a settlement column, it is possible to generate a very large volume and deep settlement zone very cost effectively. A supernatant layer and a thickened sediment layer are formed within the well from the dilute aqueous waste layer fed into the well. The supernatant can be removed, pumped or otherwise drawn from the top of the well provided this removal does not disturb the thickened sediment layer at the bottom of the well. Generally the supernatant is taken from the well by overflowing the well during substantially continuous feed into the well. The removal of the supernatant can be in any convenient manner, for instance through a channel and piped back to the separation stage, in which event the well can be dug in any suitable location.

As described above, a preferred embodiment provides that the well is dug in an area that facilitates easy introduction of clay waste and easy return of clarified water and may be at the base of a primary lagoon and the supernatant overflows from the top of the well and may flow across the exposed base of the primary lagoon. Generally the primary lagoon would have been used already for collecting solid clay sediment from the mineral recovery process and so the supernatant flows over the substantially solid clay sediment in the primary lagoon.

The flow of the supernatant from the well over the sediment before recycling of the supernatant to the main separation stage has the effect of polishing the supernatant and thereby increasing the clarity and reducing the suspended solids of the supernatant which is returned to the flotation or other main separation step.

Generally, in the preferred embodiment, the well is formed in a lagoon that is already substantially filled with substantially solid clay sediment. Thus, by practicing the invention, a lagoon that has already been substantially filled with sediment can be given a new and very important purpose by excavating a treatment well and then relying on the existing sediment in the lagoon to provide a polishing of the supernatant. The rate of increase of sediment in the lagoon as a result of this polishing process is extremely slow and so the lagoon can be given an almost indefinite extension in its useful life.

The phrase "substantially full" means that the lagoon is too shallow to be useful for separation of clear supernatant from sediment, for instance as a result of the horizontal component of the flow velocity exceeding the vertical component of the settling rate.

Suitable dimensions for the well in the inventive process comprise a depth of from about 6 to about 60 or deeper as convenient, preferably about 25 to about 60, feet and an upper surface area (generally approximating a square or round area) giving a flow rate of 0.01 to 1 , preferably 0.1 to 0.5 U.S. gallons per minute per square foot. The required surface area depends on the flow rate of the clay waste.

The size of a conventional primary lagoon is from about 50 to about 2000, generally 250 to 1 000, acres. The distance of travel of the supernatant over the substantially solid clay sediment in the primary lagoon is generally at least 300 feet and usually at least 700 feet, e.g., 1000 to 5000 feet. The flow conditions of the supernatant as it travels from the well overflow to the primary lagoon exit are preferably such as to cause minimum disturbance of clay which is sedimenting down onto the substantially solid clay sediment in the base of the lagoon. Preferably the depth of the supernatant above the settling clay and the solid clay sediment is such that there is a layer of at least about 6 inches, generally at least about 1 foot and preferably at least about 3 feet deep which appears, to the naked eye, to be clear. The speed of flow of the supernatant is generally in the range of about 0.000001 to 0.1 gallons per minute per square foot, so as to promote the opportunity of sedimentation and to minimize the risk of disturbing the sedimenting clay.

The dilute aqueous clay waste is flocculated in the well by mixing a polymeric flocculant into the waste in such a way as to promote optimum flocculation with minimum polymer usage. The polymeric flocculant can be added in solid form but more usually is added as a preformed solution in conventional manner, typically having a polymer concentration of about 0.1 -2% by weight. The polymeric flocculant can be added to the waste after the waste has entered the well but may be added to the waste before the waste enters the well or may be added to the recycled supernatant prior to mixing in the well. The addition point can be just prior to the entry to the well or it can be at a substantially earlier position, for instance as described in U.S. Pat. No. 5,688,404.

Generally the polymeric flocculant is added to the waste as it flows through a mixing device which discharges into the well. The mixing device can be a duct or other suitable device, such as a tank, or small well formed in the groun, through which the waste flows with sufficient turbulence to promote good mixing of the flocculant into the waste. The turbulence may be generated solely by the rate of flow through the duct or by baffles or other turbulence inducers by the injection of water within the duct. If desired, mechanical rotors or other mechanical mixing apparatus can be provided to achieve suitable mixing of the polymeric flocculant into the waste, sufficient to give substantially uniform flocculation.

The polymeric flocculant can be any water soluble polymeric flocculant which is capable of promoting flocculation and therefore separation of the aqueous waste into a supernatant and a thickened clay sediment The polymer is generally a water soluble polymer formed from one or more ethylenically unsaturated monomers. The monomers may be non-ionic, anionic or cationic. Similarly, the polymer may be non-ionic, anionic or cationic, or it may be amphoteric.

Suitable anionic monomers include ethylenically unsaturated carboxylic or sulphonic monomers such as aryclic acid, methacrylic acid and 2-acrylamido-2-methyl propanesulfonic acid (AMPS) (a US trademark of the Lubrizol Corporation). Acrylamide is a suitable non-ionic monomer. Suitable cationic monomers are dialkylaminoalkyl (meth) -acrylates and - acrylamides, usually as their quaternary ammonium or acid addition salts, or diallyl dimethyl ammonium chloride.

Preferred anionic polymers are copolymers of 5-70% by weight generally 10-50% by weight anionic monomers such as acrylic acid (usually as sodium acrylate) and/or AMPS with other monomers generally acrylamide. Particularly preferred anionic copolymers are Percol 336, Percol 727, Percol 358 all from Ciba Specialty Chemicals, Water Treatments Inc. Suitable cationic polymers are formed of 1 -50% by weight, generally 2-15% by weight cationic monomer such as dimethyl aminoethyl-acrylate or -methacrylate acid additions or quaternary salts together with other monomers, generally acrylamide. Particularly preferred cationic copolymers are Percol 455, Percol 352, also from Ciba Specialty Chemicals, Water Treatments Inc.

The molecular weight of the polymer is generally such that the polymer has an intrinsic viscosity ("IV") (measured using a suspended level viscometer, 1 N sodium chloride buffered to pH 7 at 20 DEG C.) of at least 4 dl/g and usually at least 8 dl/g. When the polymer is anionic, the IV is typically 10-30 dl/g and when it is cationic the IV is typically 8-15 dl/g. The polymer can made by gel polymerization, reverse phase bead polymerization or reverse phase emulsion polymerization or by any other suitable technique in known manner.

The effective dosage of the polymer is selected in conventional manner for sedimentation applications and is usually 0.01 to 1 , preferably about 0.0125 to about 0.75 pounds polymer per ton solids in the waste which is being flocculated.

The selection of the polymer and dosage amount can be conducted by conventional selection procedures so as to obtain the optimum combination of clarity and depth of supernatant and the rate of settling on the one hand and pumpable thickened clay sediment on the other.

The theoretical residence time of the dilute aqueous clay waste in the well is usually from 5 minutes to 5 hours, preferably 10 minutes to 3 hours, e.g., 45 to 120 minutes.

Promotion of the flocculation process can be achieved by mixing dilution water with the flocculant solution into the dilute aqueous clay waste because the waste entering the well often has a solids content above the value which gives optimum settling rate. The optimum amount of dilution water can be determined by routine testing. An added benefit of this process, when operated at the optimum dilution rate, such that polymer usage is optimized, is that a film of oil is observed floating on the surface of the supernatant. This oil film is substantial enough that in time it will accumulate to an amount which can be recovered by any convenient manner, thus adding to the total bitumen recovery, which is the object of the present invention.

The thickened clay sediment is removed from the well at a position significantly below the supernatant and/or at a time such that removal does not undesirably impair the quality of the supernatant. The removal may be continuous or discontinuous. The solids -content of sediment will generally increase towards the bottom of the well and, in order to minimize the risk of the well gradually filling up with sediment, it is therefore desirable to remove the thickened clay sediment from as close to the bottom of the well as possible. Th e sediment that is removed from the well generally has a solids content at least 2 or 3 times and often up to 10 times the solids content of the original dilute aqueous clay waste stream which is being flocculated. Often the solids content of the thickened sediment is from about 10 to about 30% dry weight solids. Measured by taking a sample of the thickened sediment of known weight and evaporating the liquid component or moisture at a known temperature (typically 105 degrees Celsius) in a standard laboratory drying. The solids content should preferably be as high as is practicable but must not be so high that the sediment is not conveniently pumpable.

Removal can preferably be accomplished by pumping, such as by using a fixed pump positioned on the ground near the side of the well and connected by a pipe to near the base of the well for drawing off thickened sediment from the well, or a floating pump which floats on the supernatant and has a pipe extending down to near the base of the well.

The thickened sediment is removed from the well to one or more final lagoons where it is spread over the lagoon and allowed to sediment and evaporate to form the desired final substantially solid clay sediment. Because the thickened sediment removed from the well has a much higher solids content than conventional clay waste, the amount of sedimentation and evaporation which is required to provide the final solid sediment is much less than in conventional processes, and there may be no incentive to try to recycle any supernatant (because of the large amount of supernatant which has been recycled from the well). Accordingly the final one or more lagoons do not have to have as deep a settling volume as is normally considered necessary. As a result, the thickened sediment can be pumped into lagoons which are partially or almost entirely full of sediment.

The invention thus the advantage that it can simultaneously give good recovery of supernatant (often of very high clarity) while using lagoons which would normally be considered to be too full and to shallow for many purposes.

The following are examples of the invention.

Example 1 : A oil bearing sands recovery process is conducted by slurring oil bearing sands with water and separating the desired bitumen in the primary separation vessel, from which generally bitumen, coarse sand tailings and fine clay tailings are removed. These fractions are subjected to further separations (including flotation) to give oil bearing fractions and aqueous clay waste having a solids content which varies between about 0.2% solids and 20% solids and which contains a residual amount of bitumen values.

Claims

Claims:

1. An oil bearing sands recovery process comprising a main separation stage of which oil bearing sands is slurried with water and separated into an enriched fraction and a dilute aqueous clay waste, and a waste sedimentation stage in which the dilute aqueous clay waste is sedimented in one or more settling lagoons to provide a substantially solid clay sediment and supernatant, and in which the waste sedimentation stage comprises feeding the dilute aqueous clay waste into a well, flocculating the dilute aqueous waste by mixing a polymeric flocculant into the dilute aqueous clay waste, sedimenting the flocculated dilute aqueous clay waste to provide a pumpable thickened clay sediment layer in the well.

2. A process according to claim 1 further comprising the step of removing at least a portion of the thickened clay sediment layer from the well, and allowing the removed thicken clay sediment to undergo further sedimentation and evaporation to provide a substantially solid clay sediment in one or more final lagoons.

3. A process according to claim 1 in which the well has been dug in a primary lagoon which contains substantially solid clay sediment from the mineral recovery process.

4. A process according to claim 3 in which the supernatant flows over the substantially solid clay sediment in the primary lagoon before being recycled to the main separation stage.

5. A process according to claim 1 in which the main separation stage includes at least one flotation stage and the recycling of the supernatant includes recycling to at least one flotation stage.

6. A process according to claim 1 in which the thickened clay sediment layer has a solids content 5 to 1 00 times the solids content of the dilute aqueous clay waste.

7. A process according to claim 1 in which the dilute aqueous clay waste has a clay solids content of 0.1 -15% and the thickened clay sediment layer has a solids content of 10-60%.

8. A process according to claim 4 in which the primary lagoon is substantially filled with a substantially solid clay sediment from the mineral recovery process.

9. A process according to claim 2 in which the one or more settling lagoons and the one or more final lagoons are substantially filled with a substantially solid clay sediment from the mineral recovery process.

10. A process according to claim 1 in which the dilute aqueous clay waste contains particulate mineral values that are sedimented from the dilute aqueous clay waste before the dilute aqueous clay waste is fed into the well.

11. A process according to claim 1 in which the recovery process is a oil bearing sands recovery process.

12. A process according to claim 1 1 in which the flocculant is a water soluble cationic polymer having an intrinsic viscosity of at least 4 dl/g.

13. A process according to claim 11 in which the main separation stage comprises a flotation stage utilizing an amine flotation agent and the dilute aqueous clay waste is treated with mineral acid, and the flocculant is a water soluble anionic polymer.