CA1085762A - Grinding as a means of reducing flocculant requirements for destabilizing sludge (tailings) - Google Patents

Abstract

GRINDING AS A MEANS OF REDUCING FLOCCULANT
REQUIREMENTS FOR DESTABILIZING SLUDGE (TAILINGS) ABSTRACT OF THE DISCLOSURE
Wet-grinding of the oil-removed sludge suspensions is a better means of producing new reactive surfaces on the constituent minerals in the sludge than blender-grinding.
Prior treatment of the sludge by wet-grinding reduces the requirement of flocculant 573C by several orders of magnitude to achieve similar results of sludge flocculation as esti-mated by sedimentation using centrifugation. This grinding process will work with other applicable flocculants.

Description

BACKGnOUND OF THI~ INVE,NTION

This invention relates to the hot water process for treating bituminous sands, such as Athabasca tar -,ands, and, more particularly, to the treatment of the water and clay-containing effluent discharged from the process.
Tar sands (which are also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous petroleum. Tar sands are found through-out the world, often in the same geographical area as con-ventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca area in the northeast of the Province of Alberta, Canada. This deposit is believed to contain over 700 billion barrels of bitumen. For comparison, this is just about equal to the world-wide reserves of conventional oil, 60% of which is found in tbe middle east.

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Athabasca tar sand is a three-component mixture of bitumen, mineral and water Bitumen is the value for the ex-traction of which tar sands are mined and processed. The bitumen content is variable, averaging 12 wt.% of the deposit, but ranging from 0 to 18 wt.%. Water typically runs 3 to 6 wt.% of the mixture, increasing as bitumen content decreases.
The mineral content is relatively constant ranging from 84 to 86 wt.%.
Several basic extraction methods have been known for many years for separating the bitumen from the sands. In the so-called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water. The combined mass is then subjected to a pressure or gravity separation.
The hot water process for primary extraction of bitu-men from tar sands consists of three major process steps (a fourth step, final extraction, is used to clean up the re-covered bitumen for downstream processing.) In the firststep, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85 wt.% solids.
Sodium hydroxide or other reagents are added as required to maintain pH in the range 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 upward) to form a coherent mass known as froth which is recovered by skimming the settling vessel. A third stream may be withdrawn from ~. ' ~ : ,.

1eD~5~62 the settling vessel. This stream, called the middlings drag stream, may be subjected to a third processing step, scaven-ging. This step provides incremental recovery o~ suspended bitumen and can be accomplished by conventional ~roth flo-tation.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms sand, silt, clay, and fines are used in this speci~ication as particle size designations wherein sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh, but is larger than 2 microns, and clay is material smaller than two microns in-cluding some siliceous material of that size.
Conditioning tar sands ior the recovery o~ bitumen consists of heating the tar sand/water ieed mixture to pro-cess temperature (180-200F), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) oi the caustic or other reagents added.
Under these conditions, bitumen is stripped ~rom the individ-ual sand grains and mixed into the pulp in the ~orm of dis-crete droplets oi a particle size on the same order as that oi the sand grains. The same process conditions, it turns out, are also ideal ior accomplishing deilocculation of the clays which occur naturally in the tar sand feed. Deiloccu-lation, or dispersion, means breaking down the naturally occurring aggregates of clay particles to produce a slurry oi individual particles. Thus, during conditioning, a large ~raction o~ the clay particles become well dispersed and mixed throughout the pulp.

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Those skilled in the art will therefore understand that the conditioning process, which prepares the resource (bitumen) for efficient recovery during the following process steps also prepares the clays to be the most difficult to deal with in the tailings disposal operations.
The second process step, called separation, is actu-ally the bitumen recovery step, (the separation having already occurred during conditioning). The conditioned tar sand pulp is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material, "screen oversize", is discarded. The screened pulp is further diluted with water to promote two settling processes: globules of bitumen, es-sentially mineral-free, settle (float) upward to form a co-herent mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly the -sand size mineral, settle down 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.
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. Characteristically, as the fines content rises above a certain threshold (which varies according to the com-position of the fines), viscosity rapidly achieves high values with theeffect that the settling processes essentially stop.
In this operation 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.

-- 101~;762 As feed fines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.
The third step o~ the hot water process is scavenging.
The feed fines content sets the process water requirement through the need to control middlings viscosity which, as noted above, 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 of incremental amounts o~ bitumen. Air flotation is an effective scavenging method for this middlings stream.
Final extraction or froth clean-up is usually accom-plished by centrifugation. Froth from primary extraction is diluted with naptha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of an essentially pure (diluted) bitumen. Water and mineral removed from the froth constitute an additional tail-ing stream which must be disposed of.
In the terminology of extractive processing, tailings is the throwaway material generated in the course of extrac-ting the valuable material from an ore. In tar sands pro-cessing, tailings consist 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) sand tailings (the fraction that settles rapidly), and (3) tailings sludge (the fraction that settles slowly). Screen oversize is typ-ically collected and handled as a separate stream.

~0~ilS762 Tailings disposal is all the operations required to place the tailings in a final resting place. One obvious long-range goal of tailings disposal is to replace the tail-ings in the mined out area in a satisfactory form. Thus, there are two main operating modes for tailings disposal: (1) dike building-hydraulic conveying of tailings followed by mechan-ical compaction of the sand tailings fraction; and (2) over-boarding-hydraulic transport with no mechanical compaction.
Recently, in view of the high level oi` ecological consciousness in Canada and the United States, technical in-terest in tar sands operation has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straighti'orward. Visualize mining one cubic foot of tar sands. This leaves a one cubic foot hole in the ground. The ore is processed to recover the resource (bitumen) and the remainder, including both process material and the gangue constitutes the tailings; 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 consists o~ a solid part (sand tailings) and a more or less fluid part (sludge). The most satisfactory place to dispose oi these tailings is, of course, the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings from one cubic foot or ore occupy just above one cubic foot. The amount of sludge is a variable, depending on ore quality and process conditions, but may run up to 0.3 cubic feet. The tailings simply will not iit into the hole in the ground.

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The historical literature covering the hot water pro-cess for the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation of liquid tailings or sludge would occur. Based on analysis of field test unit operations which led to the Great Canadian Oil Sands plant design near Ft. McMurray, Alberta, the existence of sludge accumulation was predicted. This accumulation came to be called the "pond water problem". Observations during start-up and early commercial operations at Ft. McMurray (1967-1969) were of insufficient precision to confirm the prediction. Since 1969, commercial operating data have con-firmed the accumulation in GCOS' tailings disposal area of a layer of fine material and water (sludge) which settles and compacts only very slowly, if at all.
At the GCOS plant, 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 a pool of liquid contained inside. On the dike, sand settles rapidly, and a slurry oi fines, water and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond in-terior commences stratification in settling over a time scale ; of months to years. As a result of this long-term settling, two layers form. The top 5 to 10 feet of the pool are a layer of relatively clear water containing 0 to 5 wt.% solids.
Below this clear water layer is a discontinuity in solids content. Over a matter of a few feet, solids content in-creases to 10 - 15 wt.%, and thereafter, solids content increases regularly toward the pond bottom. In the deepest parts of the pond, solid contents of over 50 wt.% have been recorded. This 10~576Z

second layer is called the sludge layer. The solids content of the sludge layer increases regularly from top to bottom by a factor of 4-5. The clay-water ratio in this layer in-creases also, but by a lower factor of 1.5 - 2.5. The clays, dispersed during processing, apparently have partially re-flocculated into a very fragile gel network. Through this gel, fines of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process 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 point toward the pond in-terior. As the sand settles, fines and water drain into the pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals, ~2) in the hot water extraction process, most of the clays become ~ !
dispersed in the process streams and traverse the circuit, exiting in the tailings, (3) the amount of process water in-put is fixed by the clay content of the feed and the need to control viscosity of the middlings stream, (4) the amount of water required for middlings viscosity control represents a large volume relative to the volume of the ore itself, and (5) upon disposal, clays settle only very, very slowly; thus, the process water component of tailings is only partially ~`
available for reuse via recycle. That which cannot be re-cycled represents a net accumulation of tailings sludge.

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The pond water problem is then: to devise long-term economically and ecologically acceptable means to eliminate, minimize, or permanently dispose o~, the accumulation of liquid tailings or sludge.
Flocculation of the drag stream in order to improve the settling characteristics thereto has been proposed and practiced in the prior art. In flocculation, individual par-ticles (in this case clay particles) are united into rather loosely bound agglomerates or flocs. The degree of floccu-lation is controlled by the probability of collisions be-tween the clay particles and their tendency toward adhesion after collision. Agitation increases the probability of collision and adhesion tendency is increased by the addition of flocculants.
Reagents act as flocculants through one or a combina-tion of three general mechanisms: (1) neutralization of the electrical repulsive forces surrounding the small particles which enables the van der Waals cohesive force to hold the particles together once they have collided; (2) precipita-tion of voluminous flocs, such as metal hydroxides, that en-trap fine particles; and (3) bridging of particles by natural or synthetic, long-chain, high-molecular-weight polymers.
These polyelectrolytes are beIieved to act by adsorption (by ester formation or hydrogen bonding) of hydroxyl or amide groups on solid surfaces, each polymer chain bridging be-tween more than one solid particle in the suspension.

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Among the various reagents which have been found useful for flocculating clay are are: aluminum chloride, polyalkylene oxides, such as polyethylene oxide, compounds of calcium such as calcium hydroxide, calcium oxide, calcium chloride, calcium nitrate, calcium acid phosphate, calcium sulfate, calcium tartrate, calcium citrate, calcium sulfonate, calcium lactate, the calcium salt of ethylene diamine tetra-acetate and similar organic sequestering agents. Also useful are quar flour or a high molecular weight acrylamide polymer such as polyacrylamide or a copolymer or acrylamide and a copolymerizable carboxylic acid such as acrylic acid. Ad-ditional flocculants which have been considered include the polymers of acrylic or mephacrylic acid derivitives, for example, acrylic acid, methacrylic acid, the alkali metal and ammonium salts of acrylic acid or methacrylic acid, acrylamide methacrylamide, the aminoaklyl acrylates, the ~;
aminoalkyl acrylamides, the aminoaklyl methacrylamides and the N-alkyl substituted aminoaklyl esters oi' either acrylic or methacrylic acids.
Those skilled in the art will understand that a sat-isfactory solution to the tar sands "pond water problem"
must be economically, as well as ecologically acceptable.
Despite the considerable attention which has been paid to the use of flocculants in the treatment of tailings from the hot water extraction process, the quantities needed to obtain ecologically acceptable results have not been economically acceptable.
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Objects of the Invcntion It is therefore a broad object of our invention to provide mcans for improving the effectiveness of flocculating agents for treating tar sands tailing streams which carry suspended clay particles.
It is another object of our invention to provide such means which is economical to cmploy in the treatment of high volume tar sands tailings streams.
It is a more particular object of our invention to reduce the particle size of the suspended clay particles in tar sands tailings streams by grinding in order to render the sludge more receptive to subsequently applied flocculating agents.
Brief Summary of *he Invention Briefly, these and other objects of the invention are achieved by grinding the sludge prior to its exposure to a flocculating agent in such a manner that the particle size is reduced, preferably through planar cleavage perpendicular to the c-axis of the particle crystals in order to create mostly negatively charged sites. As a result of the grinding, the polymer chains attach themselves better onto the clay colloids and hence reduce the amount of flocculant required to produce the same flocculation obtained without employing the grinding step.
Description of the Drawing The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to the manner in which the grinding step is carried out and its ' .

rclationshi~ to the tot~l process, may bcst be undcrstood by refcrence to the following dcscription taken in connection with the drawing of which the single figure is a schematic representation of a hot water extraction process wherein the invention finds particular use.
Detailed Description of the Invention - Referring now to the single figure, bituminous tar sands are fed into the system through a line 1 and pass to a conditioning drum or muller 18. Water and steam are introduced to the muller through another line 2. The total water so introduced in liquid and vapor form is a minor amount based on the weight of the tar sands processed. The tar sands, conditioned with water, pass through a line 3 to the feed sump 19 which serves as a zone for diluting the pulp with additional water before passage to the separation zone 20.
The pulp tar sands are continuously flushed from the feed sump 19 through a line 4 into a separator 20. The settling zone within the se~arator 20 is relatively quiescent so that ~;~
bituminous froth rises to the top and is withdrawn by a line 5 while the bulk of the sand settles to the bottom as a tailings layer which is withdrawn through line 6.
A middlings stream is withdrawn through line 7 to be processed as described below. Another middlings stream, which is relatively oil-rich compared to the stream withdrawn through line 7, is withdrawn from the cell via line 8 to a flotation scavenger zone 21. In this zone, an air flotation ;;
operatlon is conducted to cause the formation of additional `-,'' -12- ~

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oil roth which passcs from the scavcngcr zone through line 9 in mixture with the primary froth from the separator 20 to a froth settler 22. An oil-lean water stream is removed from the bottom of the scavenger zone 21 through line 10 to be further processed as described below. In the settler zone 22, some further oil-lean water is withdrawn from the froth and removed through line 11 to be mixed with the oil-lean water from the flotation scavenger zone, the sand tailings stream from the separation zone and a portion of a lower middlings withdrawn from the separation zone.
The oil-lean water from the froth settler, the scavenger zone, and the separator, and the tailings from the settler, all of which make up an effluent discharge stream, are treated in the sand separation zone 20 by, for example, a single gravity settling process. The sand is withdrawn by a line 13 and discarded and a process water stream is withdrawn by a line 14 to a grinding zone 30.
In the grinding zone 30, the fines-containing process water is ground by ball-grinding, sand-grinding, or any other suitable grinding process by which the fines particle size can be substantially reduced, mostly through planar cleavage perpendicular to the c-axis of the particle crystals. The effluent from the grinding zone is conducted, by line 32, to a flocculation zone 24.
In the flocculation zone 24, a substantial amount of the clay suspended in the effluent is coagulated, and a slurry of coagulated clay and process water is withdrawn in line 15 to a centrifuge zone 25. A portion of the effluent from the flocculation zone 24 may also be passed, via a line 40, to a settling pond 36 wherein the enhanced flocculation ~.
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10~5762 characteristics permit a more rapid and complete settling process to take place over a substantial period of time.
In the ccntrifuge zone ~5, coagulated clay is separated from the process water and discarded via line 16. Water substantially reduced in clay and sand content compared to the effluent discharge is recovered from the centrifuge zone and is recycled by a line 17 to be mixed with fresh water and charged into the hot water extraction process.
Sludge from the subsurface reg;on of the settling zone 36 is withdrawn through line 40 by a pump 38 and is transferred by a line 34 to the grinding zone 30 ~or treatment . .
as hereinabove described. The proportions of the respective feeds through llne 14 and 34 to the grinding zone 30 depend upon the needs of a given system at a given time, and those skilled in the art will appreciate that each source may supply from zero to 100~ of the charge to the grinding zone 30 at a given time. Similarly, the proportions of the effluent from the flocculation zone 24 which are applied to the centrifuge zone 25 or returned to the settling pond 36 will 20 also depend upon the system needs, and each may vary from ;~
zero to 100% of the total.
In order to quantitatively evaluate the results obtained with the method of the present in~ention, grinding experimellts were performed on 4.1 and 11.7 wt.% oil-removed sludge suspensions. Ball-mill grinding was employed although the vast quantities which must be processed in a tar sands ~ ~
operation may be as readily, and more economically, handled -~ ~-by other forms of grinding such as sand-grinding.
Ball-grinding was carried out for twenty-four hours on the sludge suspension, and the ground sludge was thcn exl~osed 108~i762 to various concentrations of a cationic polyacrylamide flocculant 573 C and then centrifuged for predctermincd periods. The results were compared to corresponding unground sludge samples exposed to like flocculant concentrations. The results of the comparative tests for the 4.1 wt.% sludge samples are given in Table 1.
~A~LE 1 Effect of ball-mill grTndlng versus no-grlnding on flocculant requi~e-rents for flocculation of the sludge ~initial concent~ation 4.1~

S~RL ulant Conc Treatment tumulative Centrifugation Time at 280 9 mi ~ o 40 1 60 320 .
Sol Id Conccntrat ion, ~ (W~W) 50 ppm Unground 4.1 4.4 4.6 ' 6.1 81 100 " " 4.1 4.5 5.1 18.7 82 200 " " 4.~ 4.6 19.9 24.5 83 400 " " 4.11 7 . 2 20. 5 23 . 6 ~ 84 50 ~ Cround 4.116;7 21.9 23.6 ; 85 100 " " 4.117.2 21.9 23.1 ~6 200 " " . 4.117.2- 21.9 22.7 87 400 " " 4.11~.0 20.2 21.2 It will be observed that the solids concentration obtained was approximately the same for all prcportions of flocculant added to the ball-ground sludge. That îs, after ball grinding, the lowest proportion ~50 ppm) was as effective as thc highest proportion t400 ppm) of flocculant added to the unground suspensions as may be understood from a comparison of the results obtained from samples 84 and 83, respectively.
The test procedure was also conducted on 11.7 wt.%
oil-removed sludge suspensions to determine if corresponding ,.

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effects could be obtained on somcwhat thic~er suspensions.
The results of the 11.7 wt.~ tests are prescnted in Table 2.
T~ble 2 Effect of ball-mill grlndtng vcrsus no-grtnding on locculant require-ments for flocculation of the sludge (initial conccntration 11.7~,U/~) S~RL 573C Floc~ Treatmcnt Cumulative Ccntrifusation Timc at 2809, m;n.Lab. ulant Conc.
, 0 ~0 160 320 Solid Conccntration, ~ (U/U) 128 0 ppmVnground 11.7 11.7 12.0 12.6 -0 129 100 " " 11.7 12.0 12.1 12.8 130 200 " ^' 11-7 ~ 1l-9 12.4 13.3 131 400 " " 11.7 12;2 16.6 25.6 132 o ppmGround , 11.7 16.0 24.3 ~ 2B.3 133 100 " " 11.7 19.5 24.6 -2B.7 134 200 " " 11.7 20.6 26.3 30.~
135 40û " ", 1~-7 2!.0 26.3 30.6 It will be observed from Table 2 that grinding the sludge suspension before the addition of the flocculant very significantly reduces the amount of flocculant required as indicated by comparison of sample #131 and samples 132 and 135 which produced similar results in terms of solids concentration reached upon centri~ugation. This. it will be understood that grinding sludge prior to flocculation results in a very significant decrease in the amount of ;
flocculant needed to obtain meaningful results.
Plocculants, natural or synthetic, usually are of high ;
molecular weight which may range from 500,000 to 6-8 million.
It is generally believed that the higher the molecular weight, the better the flocculating ability of the polymer.
However, the flocculating ability varies considerably depending upon the nature of the material which is to be :, :

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flocculated. In order for the flocculant to be effective, it is necessary forlthe polymer chain to be attached to several particles and subsequently to engulf or entrap other particles into a floc. Cationic polymers are bonded to the negatively-charged sites and anionic on the positively-charged sites, ~usually edges) on the clay surfaces.
Non-ionic polymers a~tach themselves through hydrogen bonding and non-specific van der Waals bonds. Grinding according to the method of this invention apparently increases the electronic charge through the creation of new surfaces and also increascs the specific surface area, this enhancing the reactivity of the ground material. In addition, the physical hindrances, such as coating of the exchange sites with bitumen or amorphous materials, are likely to be removed by the process. This treatment of wet grinding apparently makes the polymer chains adapt themselves better onto the clay colloids and hence reduces-the amount of flocculant required to produce the same results as are achieved without grinding.
While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications of structure, arrangement, proportions, the elements, materials, and components used in the practice of the invention which are particulalry adapted for specific environments and operating requirements without departing from those principles.

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Claims (10)

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

1. In an aqueous process for separating oil from bituminous sands comprising the steps of:
(a) forming a mixture of bituminous sand and water;
(b) passing the mixture into a separation zone;
(c) settling the mixture in the separation zone to form an upper oil froth layer; a middlings layer comprising oil, water and clay; and a lower sand tailings layer;
(d) withdrawing separate effluent streams from the oil froth layer; the sand tailings layer; and the middlings layer;
(e) collecting an effluent discharge including effluent from the middlings layer; and (f) adding a flocculating reagent to the effluent discharge; whereby finely divided minerals including clay settle into a lower sludge layer within a storage zone adapted to receive the effluent discharge;
the improvement wherein the effect of the flocculating reagent on the finely divided minerals in the effluent discharge is enhanced by grinding at least a fraction of the effluent discharge to reduce the particle size of the finely divided minerals therein.

2. The process of Claim 1 in which the grinding step is carried out prior to the step of adding a flocculating reagent to the effluent discharge.

3. The process of Claim 2 in which the grinding step is carried out by ball grinding.

4. The process of Claim 2 in which the grinding is carried out by sand grinding.

5. The process of Claim 1 in which sludge is withdrawn from the storage zone and ground during the grinding step.

6. An aqueous process for separating oil from bituminous sands comprising the steps of:
(a) forming a mixture of bituminous sand and water;
(b) passing the mixture into a separation zone;
(c) settling the mixture in the separation zone to form an upper oil froth layer; a middlings layer comprising oil, water and clay; and a lower sand tailings layer;
(d) withdrawing separate effluent streams from the oil froth layer; the sand tailings layer and the middlings layer;
(e) collecting an effluent discharge including the effluent from the middlings layer;
(f) grinding at least a fraction of the effluent discharge to reduce the particle size of the finely divided minerals therein; and (g) adding a flocculating reagent to the ground effluent discharge.

7. An aqueous process for separating oil from bituminous sands comprising the steps of:
(a) forming a mixture of bituminous sand and water;
(b) passing the mixture into a separation zone;
(c) settling the mixture in the separation zone to form an upper oil froth layer; a middlings layer comprising oil, water, and clay; and a lower sand tailings layer;
(d) withdrawing separate effluent streams from the oil froth layer; the sand tailings layer; and the middlings layer;
(e) discharging the effluent streams from the middlings layer into a settling zone to form an upper clarified water layer and a lower sludge layer;
(f) withdrawing sludge from the sludge layer;
(g) grinding the withdrawn sludge to reduce the particle size of the mineral fines therein; and (h) discharging the ground sludge into the settling zone.

8. The process of Claim 7 in which the ground sludge is mixed with a flocculating reagent prior to its discharge into the settling zone.

9. The process of Claim 9 in which the ground sludge and flocculated reagent mixture is centrifuged to increase the solids content thereof prior to discharge into the settling zone.

10. The process of Claim 9 in which at least a portion of the supernatent from the centrifuging step is employed in forming the mixture of bituminous sand and water.