CA1134310A - Method for surcharging with sand the sludge layer of a tar sands tailings pond - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In order to surcharge the sludge layer of industrial process tailings ponds with sand to effect dewatering of the sludge, which sludge has been treated with hydrolyzed starch flocculant, at least two tailings ponds are employed in the system. A first pond receives the process effluent and in-cludes a fines-containing sludge layer. During summer months, sludge is transferred from the first pond to a second pond.
During the winter months, the second pond is allowed to freeze over. Sand is distributed on the ice surface of this pond such that, upon spring thaw, the sand is deposited in a sub-stantially uniform layer over the sludge layer. The process may be repeated annually or as needed to obtain multiple sand surcharge layers interleaved with layers of treated, highly dewatered sludge in the second pond.

Description

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BACKGROUND OF THE INVENTION
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Tar sands twhich are also. known as oil sands and bitu-minous sands) are sand deposits w~ich are impregnated with dense, viscous, petroleum. Tar sands are ~ound throughout the world, o~ten in the sama geographical areas as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca region in the north east o~ the province of Alberta, Canada. This deposit is be-li~ved to contain perhaps 700 billion-one trillion barrels o~
bitumen. For comparison, 700 billion barrels is just about equal to the world-wide reserves of conventional oil, 60% o~
which is found in the Middle East. While much of the Athabasca deposit is not economically recoverable on a commercial scale with current technology, nonetheless, a substantial portion is situated at, or very near, the surface where it may fairly readily be mined and processed into synthetic crude oil, and this procedure is being carried out commercially on a very large scale by Great Canadian Oil Sands (now Suncor Inc~ - Oil Sands Division) and Syncrude near Fort McMurray, Alberta.
Athabasca tar sands is a three-component mixture of bitu-men, rnineral and water. Bitumen is the valuable component for the extraction of which tar sands are mined and processed. The bitumen content is variable, averaging 12wt% of the deposit but ranging from zero to 18wt%. Water typically runs 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 ~or many years J the "hot `~
water" process is the only one o~ present commercial significance

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and is employed by both GCOS and Syncrude. The hot water pro-cess ~or achieving primary extraction o~ b~tumen ~rom tar sand consists o~ three major process steps (a fourth step, ~inal ex-traction, is used to clean up the recovered bitumen ~rom dow~-stream processing). In the ~irst step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85wt% solids. Sodiurn hydroxide or other reagents are added as required to maintain pH in the range o~ 8.0-8.5.
In the second step, called separation, the conditioned pulp i~
diluted ~urther so that settling can take place. The bulk o~
the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most o~ the bitumen rapidly floats (settles upwardly) to forrn a coherent mass known as ~roth which is recovered by skimming the settling vessel. A third stream, called the mid-dlings drag stream, may be withdrawn ~rom the settling vessel ~;
and subjected to a third processing step, scavaging, to provide incremental recovery of suspended bitumen.
The mineral particle size and type distribution is par~ticularly signiflcant to the operation o~ hot water process and to sludge accumulation. The terms "sand" J "silt", "clay" and "~ines" are used in the speciYication as a simplified approxima-tion o~ mineral particle size wher in sand is siliceous material which will not pass 325 mesh screen, silt will pass 325 mesh, but is larger than 2 microns and clay is material smaller than 2 microns, including some siliceous material of that size. Flnes includes both silt and clay, but excludes sand. It should be again noted that these designations are simpliiied approximations.
~or an elegant and in-depth discussion o~ particle size and type .
in tar sands sludgesl re~erence may be taken to the article en- -titled "Mineral Particle Interaction Control o~ Tar Sand Sludge Stability" by Yong and Sethi which appears in The Journal ot Canadian Petroleum Techno`logy, Volume 17, Number 4 ~October-December 1~78), ~L~3~3~
As previously lndicated, conditioning tar sands ~or the recovery o~ bitumen consists o~ heating the tar sands/water ~eed mixture to process temperature (180 - 200 F), physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) o~ the caustic or other reagents added. Under these conditions, bitumen is stripped ~rom the individual sand grains and mixed into the pulp in the form of discreet droplets o~ a size on the s~me order as that of the sand grains. The same process conditions, it turns out, are also ideal for accomplishing deflocculation o~ the ~ines, particularly the clays, which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the naturally occuring aggregates of clay particles to produce a slurry of individual particles. Thus, during conditioning, a ~ -large ~raction o~ the clay particles become well dispersed and mixed throughout the pulp. -~ Those skilled in the art will therefore understand that ~-~
the conditioning proc~ss, which prepares the bitumen resource ~or e~ficient reco~ery during the succeeding process steps, also prepares the clays to be the most difficult to deal with in the tail~ngs 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 ~irst screened to remove rocks and unconditionable lumps of tar sands and clay and the reject materialj "screen oversize", is discarded.
The screened pulp is then ~urther diluted with water to promote `~``
two settling processes: globules o~ bitumen, essentially mineral-free, float upwardly to form a coherent mass of ~roth on the sur~ace of the separation cells; and, at the same time, mineral partlcles, particularly the sand-sized mineral, settle downwardly ~ L~L3~3~0 and are removed ~rom the bottom o~ the separation cell as tail-ings. The medium through which these two settling processes take place is called the middlings. The middlings consi~ts primarily of water with suspended fine material and bitum~n particles.
The particle sizes and densities o~ the sand and o~ the bitumen particles are relatively ~ixed. The parameter which in~luences the settling processes most is the viscosity o~ 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 ab~ut the same composition as the feed. Thus, as ~eed 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 ~eed ~ines content sets the process water requirement through the n~ed to control middlings viscosity which is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream o~ middlings to maintain the separation cell material balance, and this stream of middlings can be soavenged ~or re-covery o~ incremental amounts of bitumen. Air flotation is a~
ef~ective scavenging method for this middlings streamO
Final extraçtion or ~roth clea~-up is typically accomp-lished by centrifugation. Froth from primary extractio~ læ
diluted with ~aphtha, 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 ~L~3~3~C~
the ~roth during this step constitutes an additional tailings stream which must be disposed o~.
In the terminology of extractive processing, tailings is the throw-away material generated in the course o~ extracting the valuable material from an ore. In tar sands processing, tailings consists of the whole tar sand ore body plus net ad-ditions 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 ~raction that settles rapidly), and (3) tailings sludge (the ~raction that settles slow-ly). Screen oversize is typically collected and handled as a separate stream. ;`
Recently, in view o~ the high level of ecological con-sciousness 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 o~ tar sands tailings disposal is straight~orward.
I~ one cubic ~oot of tar sands is mined, a one cubic ~oot hole is le~t in the ground. The ore is processed to recover the bitu-men ~raction, and the remainder, including both process materialand the gangue, constitutes the tailings that are not valuable and are to be disposed o~. III 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 over-size) consist o~ a solid part (sand tailings) and a more or less fluid part (sludge). The most satisfactory place to dispose of these tailings is, o~ cours , in the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings ~ ~-alone ~rom the one cubic foot o~ ore occupy just about one cubic ~oot. The amount o~ sludge is variable, depending ~n ore quallty and process conditions, but averages about 0.3 cubic ~eet. The tailings simply will not ~it back into the hole ln the ground.

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The historical literature covering the hot water process ~or the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation o~ sludge would occur. Based on analysis o~ ~ield test unit operations which led to the Great Canadian Oil Sands plant design near Fort ~cMurray, Alberta, ~he existence o~ sludge accumulation was predicted.
This accumulation came to be called the "pond water problem.lr Observations during start-up and early commercial operations at Fort McMurray (1967-196~) were of insu~icient precision to con-10 ~irm the prediction. Since 1969, commercial operating data havecon~irmed the accumulation in GCOS' tailings disposal area oi a sludge layer o~ ~ines material and water which settles and com-pacts only very slowly, i~ at all, a~ter a ~ew years. For a number o~ reasons, this sludge layer, in co~mon with similar sludge layers observed in tailings ponds associated with mining and extracting processes of many kinds, is particularly important and di~icult to deal with.
At the GCOS plant, for dike building, tailings are con-veyed hydraulically to the disposal area and discharged onto the top o~ a sand dike which is constructed to serve as an impound-ment ~or the pool of ~luid contained inside. On the dike, the sand settles rapidly, and a slurry o~ 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 strati~ication in settling over a time scale of months to years.
Overboarding is the operation in which tailings are dis-charged over the top o~ the sand dike directly into the liquid pool. Rapid and slow settling processes occur, but their dis-tinctio~ is not as sharp as in dike building, and no mechanical , ..
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compaction is carried out. The sand portion o~ the tailings settles rapidly to ~orm a gently sloping beach extending ~rom the discharge position towards the pond interior. As the sand settles, fines and water commence long-term settlin~ in the pond.
The exceedingly complex behavior and characteristics o~
tailings ponds ha~e only recently come -to be understood beyond the simplistic categorization of various zones such as clari~ied water, transition, and sludge/slime. Since a ~ailings pond em-ployed in conjunction with the hot water process ~or processing tar sands is fairly typical, the following characteristics of the layers or zones ln such a taili~gs pond is a good general example~
Tailings ~rom the hot water process containing a dilute suspension o~ ~ine materials in water~ together with sand, are discharged to the tailings pond. The ~ormation o~ sludge by '' settling o~ these tailings is attrlbutable primarily to the presence of dispersed clay minerals. Many of the ~actors which determine the'rate at which the clay minerals settle and the characteristics o~ the sludge formed are set within the tailings discharge. Thesie include'intitial clay concentration (clay/water ratio~, relative proportions o~ various clay mineral species, particle slze, condition o~ clay sur~aces and pore water chemis-try. Exp~rience and laboratory analysis indicate that all these ~actors vary signi~icantly from time to time dependinig 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 settlesO
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. ~ere, additional important determin-ants of settling behavior are 'imposed. These include rate Q~
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in~low and out~low in relation to sur~ace area and clari~ied water volume, pond depth, and degree o~ agitation of pond con~
tents, either through in~lows and outflows or via thermal or by wind e~fects. While initial temperature is inherent in the tailings streams, temperatures in the pond are obviously deteP-mined by numerous other ~actors as well.
Experience and laboratory analyses indicate that when a partly settled sludge remains undisturbed ~or 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 oi more sand and silt particles. These settle either not at all or very slowly because o~ the signi~icant yield strength o~ 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 ~urther change in sludge volume occurs. Consolidation at the bottom o~ the pond is so slow that detection of consolidated material is dif~icult. Slud~e formed in this manner is virtually unchanging over periods of years or decades and ~or practical purposes may be regarded as terminal sludge.
An active pond involving continuous inflow and out~low is more complex. Experience and laboratory tests indicate that, ~ollowing discharge to the pond, clay particles undergo an aging process varying in length ~rom a few days to many weeks. Prior to completion of the aging process, the clay particles do not begi~ to settle. However 9 once they commence to do so, the pro-cess proceeds quite rapidly according to the principles o~ Stokes Law until a clay/water ratio o~ about 0.13/1 is reached at which other ~actors evidently predominate over Stokes Law. In the upper-_g_ most part of a well managed pond, these e~ects result in a moreor less clear water layer at the top underlaid by a layer o~
relatively dilute sludge more or less sharply dif~erentiated from it. This may be termed the sedimentation ~one; its volume is determined by the rate of clay in~low and the average aging time required. If the water layer is permitted to become too small in relation to the ~lay inflow, water out~low and ag~ng time, the upper part o~ the pond becomes overloaded, the clear water layer virtually disappears and the sedimentation zone be-comes much larger since clay is then recycled through the process.GCOS operated under such conditions or on the edge o~ them through much o~ the early yearsO
Sludge in the lower part of a deep active pond which has been in operation for some years is similar to that from an inac-tive pond; i.e., it ma~ be regarded as terminal slud~e. The space helow 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 hy 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 pri-marily a ~unction of the average clay in~low 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 boun-daries within this sludge except on occasion a layer o~ separated bitumen near the interface between water and sludge. However 9 the sludge may be considered as consisting o~ three zones each involving successively larger orders o~ magnitude of time scal~
~or measurable dewatering to occur, and each characterized by the predominance o~ dii~ering dewatering parameters. These three zones may be termed respectively a sedimentation zone, a trans-ition zone and a terminal sludge zone.
Thus~ (1) tar sands contain clay mineral, (2) in the hot water extraction process, most o~ the clays become dispersed i~
the process streams and traverse the circuit~ exiting in the tailings, (3) the amount o~ process water input is ~ixed by the clay content of the feed and the need to control viscosity o~
~ the middlings stream, (4) the amount o~ water required for mid-dlings viscosity control represents a large volume relative to the volume of the ore itseli, and ~5) upon disposal, clays settle only very~ very slowly; thus, the water component o~ tailings is only partially availabl'e for'reuse via recycle. That which can-not be recycled represents a net accumulation o~ -tailings sludge.
The pond water problem, theref'ore, is to devise long-term, economically and ecologically acceptable means to eliminate, minimize, or permanently dispose o~ the accumulation o~ sludge.
Experience has demonstrated that the pro~lem requires a multi-~aceted approach toward its solution, and the present invention is directed at achieving one aspect o~ the solution: a more thoroughly dewatered sludge layer which, as a consequential re-sult, obtains a greater quantity of clarified water for recircu-lation into the proc~ss if necessary in the particular system.
Flocculation o-~ the tailings stream in order to improve the settling characteristics o~ an industrial process tailings pond has been proposed and practiced in the prior art~ In ~loccu-lation, individual particles are united into rather loosely-bound ~' agglomerates or ~locs. The degree o~ ~locculat iOII 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 o~ a ~locculant.

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Reagents act as flocculants through one or a ~ombination o~ three general mechanisms: (1) neutralization of the electrical repulsive ~orces surrounding the small particles which enables the van de Waals cohesive ~orce to hold the particles together once they have collided; t2) precipitation o~ voluminous ~locs, such as metal hydroxides, that entrap ~ine particles; and (3) bridging of particles by natural or synthetic, long-chain9 high-molecular weight polymers. These polyelectrolytes are believed `~
to act by absorption (by ester ~orma$ion or hydrogen bonding~ o~
hydroxyl or ~mide groups on solid sur~aces, each polymer chainbridging between more than one solid particle in~the suspension.
A remarkable number o~ flocculants have been employed in the prior art to obtain precipitation o~ particles in tailings ponds o~ various industrial processes as well as in sewage treat-ment ~acilities. However, a distinct step ~orward in the art has been achieved by the use o~ hydrolyzed corn and potato starch ~locculants as described in co~pending Canadian application S.N. 257,214, ~iled March 31, 19~7, and entitled "Destabilization o~ Sludge with Hydrolyzed Starch Flocculants" and by the use o~
wheat starch flocculants as set ~orth i~ co-pending Canadian ap~
plication S.N. 308,619 ~iled August 2, 1~78, and also entitled "Destabilization o~ Sludge with Hydrolyzed Starch Flocculants".
These speci~ic hydrolyzed starch flocculants, particularly taking into account the economics o~ carrying out flocculation on a large scale, enjoy high per~ormance characteristics ~or their ability to bring about rapid precipitation ~o a substantially terminal settled condition. This characteristic is especially valuable for use in those processes~ such as the hot water process ~or obtaining bitumen from tar sands, in which there is a critical need to recycle clari~ied water from the tailings pond back into . ~
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~L~3~3~0 the process. However, experience has indicated that the simple use o~ these hydrolyzed starch ~locculants, or ~or that matter any other known ~locculant, resultæ in very little t i~ any, im-provement on the ultimate degree o~ dewatering o~ the sludge layer. That is, the terminal status o~ the sludge layer is ~ust about the same as would be obtained over a much longer period o~
time by natural settling proGesses, and this terminal condition is unsatis~actory in that it includes too much water, is to~ vol-uminous, and is too unstable.
Nonetheless, it is not accurate to say that all character-istics o~ a sludge layer obtained as a result of ~locculation by the aforementioned hydrolyzed starch flocculants is the same as that achieved naturally or by~the use o~ other ~locculants. In point o~ ~act, certain very desirable characteristics to the slud~e layer are obtained ~rom.the use o~ the hydrolyzed sta~ch ~locculants 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 characteristics that the pre~ent invention is based. ~ore particularly, it has been ~ound that ~0 the permeability and shear strength characteristics of the sludge layar are both very much i.ncreased; as a result, previously im-possible dewatering techniques may be employed to compact and :
stabilize the sludge layer and to e~tract additional amounts of clari~ied water there~rom. ~-It has been proposed in the pastj as another approach to alleviating pond water problem~, to store the fines in the inter stices between the sand grains iD the material employed ~or dike building. Such a process is disclosed in Canadian p~tent applica~
tion S.N. 244,473, ~iled January 29, 19769 and entitled "Method o~ Sludge Disposal Related to the Hot Water Extra~.tion o~ Tar Sands"

and corresponding ~.S. patent 4,008,146, issued February 15, 1977.

' ~i~3~3~) The experience with the procedure described in that reference i.s -that the height to which the dike can be built is somewhat ].imited; however, it has now been discovered that i~ the sludge mixed with the sand to prepare the dike building material has been treate~ with the aforementioned hyclrolyzed starch floccu-lants~ the strength of the resultant material is notably in-creased such that the dike can be built higher, thereby not only permitting a deeper tailing~ pond, but also storing more s]udge in the inters~ices between the sand grains comprising 1() the dike.

SUMMARY OF THE IN~ENTION

It is a broad object of this invention to minimize the volurne of sludge stored in an industrial process tailings pond.
In another ~spect, it is an object of this invention to ~
provide means for surcharging the sludge layer of an industrial ~.
process tailings pond with a layer of sand thereby effecting a p(>rolls piston for compressing, and thus dew~tering, the slud~e 1.ayer.
In a more specific aspect, it is an ob ject of this in-'(~ von1:ion to provide means ~or controlling the sludge layer of an ~;
i.ndllstr:lal proces~ ta.tllngs pond by spreading a layer of sand over ice formed on the tailings pond during a winter season whereby, upon a subsequent thaw, the sand settles to overlay I,ho ~ludg~ layer such that the sand layer functions as a heavy ¦
porous piston which compresses, and thus dewaters, the sludge 1.nyer. .:

DL'SCRIPTION OF T~E DRAWINGS ::
~ ., The subJect matter of the invention is particul~rly pointed ¦ -out and distinctly claimed in the concluding portion of the , :

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specifica-tion. The invention, however, both as -to organization and method of operation, may best be understood by reference ~o the following description taken in conjunction with the accompanying drawing of which:
Figure 1 is a somewhat simplified block diagram of a hot water process for converting bituminous tar sands into bituminous froth for subsequent upgrading to synthetic crude oil;
Figure 2 is a partial cross-sectional view which illustrates, conceptually and simplistically, the distribution of water and sludge in a -tailings pond associated with the apparatus illustrated in ~igure l;
~ igure 3 is a view similar to Figure 2 and shows the results of prior art attempts -to surcharge the sludge layer of a tailings pond with sand;
Figure 4 illustrates the effect of surcharging the sludge layer of the tailings pond with sand after the sludge layer has been treated with specific hydrolyzed starch flocculants;
Figure 5 illustrates the effect obtained by alternating layers of surcharging sand with sludge previously treated with specific hydrolyzed starch flocculants;
Figure 6 illustrates the effect of internal surchargin.g obtained by mix:L.ng sflnd with sludge which has been or is simultaneously treated with speciEic hydrolyzed starch flocculants;
Figure 7 :illustrates the effect of employing a combination of internal and external surcharging techniques with sludge which has been treated with speci~ic hydrolyzed starch flocculants;
Figu.re ~ illustrates a general approach for increasing the amount of fines stored in the interstices between adjacent and grains in a dike;
Figure 9 illustrates an exemplary specific method for adding hydrolyzed starch flocculant to the tailings of a tar sands hot water process;

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Figure 10 illustrates a method ~or the addition o~
hydrolyzed starch ~locculants to sludge, accompanied by sand inclusion, found in the tailings system o~ a tar sands hot water process;
Figure 11 illustra~es a combina~ion o~ the techniques illustrated in Figures 9 and 10 by which a more rapid recovery o~ clari~ied water may be obtained; and Figures 12a, 12b, 12c, and 12d illustrate a sequence o~ :
operations by which external sand surchar~e to the sludge layer ~:
of a tailings pond located in a cold environment can be obtainedO

DETAILED DESCRIPTION OF THE INVENTION

Referring now to ~igure 1, bituminous tar sands are ~ed into the system through a line 1 and passed to a conditionlng drum or muller 18. Water and steam are introduced into the muller through another line 2. The total water so introduced in liquid and vapor form is a minor amount based on the weight o~ the tar sands processed. The tar sands, heated and conditioned with steam and water, pass through a line 3 to a screen 29. The purpose o~ the screen 29 is to remove ~rom the pulp any debris such as rock or oversized lumps o~ clay as indi~ated generally at 30. The oversize material is discarded at a suitable site. ~.
The conditioned pulp passes through a line 31 to a feed sump 19 which serves as a zone ~or diluting the pulp with additional water be~ore it enters a separation æone 20.
The diluted pulp is continuously ~lushed from the ~eed sump 19 through a line 4 into the separation zone 20. The settling zone within the separator 20 is relatively quiescent so that bitu~
minous ~roth rises to the top and is withdrawn through a line 5 while the bulk o~ the sand component settles to the bottom as a tallings layer which is withdrawn through line 6. It will'be ~16-.:

~L34310 understood, o~ course, that the tailings streams can be trans-ferred individually, with ~r without downstream treatment, as indicated by the alternate lines 23, 24 and optional treatment processes 70, 80.
A relatively bitumen-rich middlings stream is withdrawn through line 8 to maintain the middlings layer between the ~roth and the sand layer at a functional ~iscosity. This middlings material is transferred to a ~lotation scav~nger zone 21 where an air ~lotation operation is conducted to bring.about.the ~or mation of additional bituminous ~roth which passes ~rom the scavenger zone 21 through line 9, in conJunction with the pri mary froth ~rom the separation zone 20 passing through line 5, to a froth settler zone 22. ~ bitumen-lean water stream is remov0d ~rom the bottom o~ the .scavenger zone 21 through line 10. ;~
In the froth settler zone 22, some ~urther bitumen-lean water :
is withdrawn ~rom the froth and removed through line 11 to be mixed with the bitumen-lean water stream from the ~lotation scav- :
enger zone and the sand tailings stream from the separation zone 20. The bitumen from the settler 22 is removed through line 12 for ~urther treatment, typically final extraction.
Bitumen-lean water from the ~roth settler 22, the scavenger zone 21, and the separation zone 20, all of which make up an e~-fluent discharge stream carried by line 7, are discharged into a tailings pond 15 which has a clarified water layer 26 and a sludge layer 27. The sand included in the tailings stream quickly settles in the region 14, and the fines-containing water ~lows into the body o~ the pond 15 where settling takes place. Water from the clarified water layer 26 may be withdrawn by a pump 28 ~or recycle through a line 17 to be mixed with fresh makeup water and charged into the hot water process.
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Re~erring now to Figure 2, the sludge layer 27 o~-the tailings pond 15 is overlayed with a clari~ied water layer 260 (As previously noted, this is a considerable simpli~ication9 but is adequate and appropriate ~or an understanding of khe present invention.) The sand bottom 23 o~ the pond de~ines the lower limit of the sludge layer 27 which, as previously discussed, increases a mineral-to-water ratio ~rom top to bottom.
The characteristics of the sludge layer 27 so ~ormed is unaccep-tably and insufficiently dewatered and compacted to minimize the pond volume required to contain the sludge and to obtain a stable sludge structure.
It has been proposed in the past to "surcharge" a sludge layer with a layer of sand whereby the sand acts as a permeable piston to compress the 'sludge and force water out o~ it. All attempts to carry out this surcharging concept have met with con-plete ~ailure or have been performed under conditions which yield only marginal, if any, bene~its under very limited conditions.
Seej by way o~ example, U.~. Patent 4,036,752, issued July 19, 1977, and entltled "Dewatering Clay Slurrles."
What has been observed in practice, when such techniques have been attempted in large, relatively deep tailings ponds, is illustrated in Figure`3. As a layer o~ sand 2 is hroadcast over `
the sludge layer 27, the sand layer is observed to tilt and dump ;~
through the sludge layer as shown generally in the region 32.
The sludge layer is simply incapable of supporting a useful sur-charge o~ sand. Thus, in the prior art, sand surcharging has bee~ ~ ~
theoretically interesting, but totally impractical as a process ' ~or dewatering and compacting sludge, and this has been the case ' whether the sludge was allowed to settle naturally or the settling ~;
process was accelerated by the use o~ ~locculants.

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However, it has been determined that the use o~ the specific hydrolyzed starch flocculants described in the above-re~erenced Canadian patent applications produces a sludge layer with remarkably enhanced shear strength and permeability charac~
teristics, and an appreciation of ~his fact resulted in recon-sideration o~ the heretofore substantially impractical and dis-carded sand surcharge concept. Throughout the remainder of this specification the term tThydrolyzed starch flocculant" means one of the speci~ic starch flocculants disclosed in the above-re~erenced Canadian applications or a chemical or fully-functional equivalent comprising, for example, hydrolyzed starch with polyelectrolytes and a low dielectric constant fluid rendered in aqueous ~orm.
As shown in Figure 4, ~a sludge layer 33 which has been treated with a hydrolyzed starch flocculant is capable of sup-porting a substantial sand surcharge which operates as a porous piston to compact and dewa*er the sludge layer. In addition, the observed improved permeability of the sludge layer 33 resul-ting from treatment with a hydrolyzed starch flocculant affords an enhancement to the degree of compaction o~ dewatering which can be achieved. Furthermore, as shown in ~igure 5, sludge layer 339 treated with a hydrolyzed starch ~locculant, is sufficiently strong that a second layer o~ slud~e 35 may be layed over the sand layer 34 and the, itself, may be subjected to a surcharge brought about by another sand layer 36. For relatively deep tail-ings ponds, a number of such alternate layers o~ treated sludge and sand may be employed to obtain a very high degree of compac tion and dewatering.
It has also been proposed in the prior art to mi~ sludge, which has been ~locculant-treated, with sand to obtain a material which, in effect, is "internally surcharged." One may refer, by way of example to U.S. Paten~ 3,680,693 issued August 19 1972, ~3~3~
:
and entitled "Process for the Treatment o~ Slime and Waste Solids." While this technique has been promising, the amount o~ sand which can be added to the.slud~e has been limited by the strength o~ the sludge and, as previously noted, no pre-viously known ~locculant a~ords the strength and permeability enhancement to the sludge layer observed to result from use o~ the hydrolyzed starch ~locculants previously identi~ied.
It has now been found that sand mixed with sludge treated with one of these hydrolyzed starch ~locculants results in a material which, indeed, exhibits important internal surcharge character- ~
istics resulting in a c~mpacted sand/sludge layer 37 as illus- .
trated in Figure 6. Furthermore, as shown in Figure 7, a :`
combination of internal and external sand surcharging techniques may be employed in which the mixed sand/treated-sludge layer 37 ~`~
is itsel~ overlayed with a sand layer 38. In addltion, of course, the multilayering technique illustrated in Figure 5 is equally `' applicable. j"
It has been observed at the Suncor-0l1 Sands Division plant that on the order of.3~% o~ the fines (and a larger portion o~
the clay component) is discharged into the tailings pond; the remalnder is storefl in the interstices between adjacent sand grains or is discarded as lumps which are part o~ the oversize. It has been proposed in the`past to increase the quantity o~ silt, and ;
particularly the quantity o~ clay, stored in the interstices be-tween ad;acent sand grains in the material' employed to build a '~
pond-impounding dike. By way o~ example~ one may re~er to pre-.. ..
viously re~erenced U.S. Patent 4,008~146, issued February 15, 1977, an~ entitled "Method of Sludge Disposal Related to the ~ot ~ater :'~
Extraction of Tar Sands" and also to the corresponding Canadian ' .
~ patent application S.N. 244,477, ~iled January 29, 1976. As dis-closed ln that re~erence, sand and sludge are admixed in a pre- ~ ;

~.

-20~

: ~ . . ,. ., . . : :., - ~3~3~

scribed fashion, and the resultant material is discharged at the dike site to effect dike building. This is an important concept, but its use in practic~ has been somewhat limited be-cause the stability o~ the resulting dike structure is insu~-ficient to permit building the dike to a height which represents storage o~ meaningful additional quantities of ~ines.
It has now been determined tha~, if sand is admixed with sludge which has been treated with a hydrolyzed starch flocculant, an important increase in the strength of the resulting material, when employed for dike building~ is observed such that the resul-ting structure is much more stable. Thus, substantially higher dikes can be built, and signif`icantly large quantities o~ silt and, particularly, clay can bè stored in the interstices between adjacent sand grains in the material.
An exemplary procedure for storing silt and clay particles in the interstices between adjacent grains of sand in a sand dike is illustrated in Figure 8. A tailings pond 41 is enclosed by dike walls 42 and contains a clari~ied water layer 43 and a sludge layer 44. Sludge is withdrawn from the pond 41 via sludge with-drawal means 51 and is transferred to a line 47 by a pump 46 which is supported by flotation means 45 on the surface of the pond 41. The sludge material is trans~erred from the line 47 into a line 50 where it is combined with, by way o~ example, tailings material from the hot water extraction process for recovering bitumen ~rom tar sands. This waste water stream ~rom the extrac tion process is primarily water and sand, but inclues minor i~mounts of silt, clay, and bitumen. Thus, the combined streiams which are transiferred from line 50 into a settling zone 52 contain a sub- ;
stantial amount o~ sand.

~L3~3~

In the sektling zone 52, an upper layer 53 and a lower layer 54 are formed. The upper layer is withdr~wn through line 55 and is trans~erred into a line 56, where it is combined with beach run-o~ water trans~erred from zone 61 via line 57, and added to the retention pond 410 The lower layer 54 in settling zone 52 is withdrawn through : ;
a line 58 and is transferred to an inclined sand pile 59 situated adjacent a di~e 60. The lower layer 54 of the settling zone 52 typically comprises on the order of 2% bitumen, 39% sand, 9% silt,

4% clay, and 46% water. This mixture is dispersed over the sand pile to ~orm additional sand layers whereby a part o~ the c~ay, silt, and water in the'stream is retained în the interstices of the sand layers. The remainder o~ the aqueous stream percolates down the inclined sand pile zone and settles into the retention zone 61. A pump 62 in the retention:zone 61 withdraws the aqueous portion of that pond and transfers it into the line 57 where, as previously noted, it is combined with the stre'am from the upper layer of zone 52 in the 'line '56. ' ~:
Thus, a part of the sludge'from the tailings pond 41 is removed and dlspersed with the sand of the wastewater stream over the pond dike wall to carry out dike building. Substantially in~
creased qualltiti0s of the sludge withdrawn from the pond are stored ""
in the interstices of the sand pile zone 59 thereby providing a ' ;.
means ~or reducing the'solids content and, more importantly, clay :~ :
content of the tailings pond 41. It may be noted that tailings ;~
pond 41 and retention zone 61 can be unitary wherein the sand : ;
pile 59 is located on the dike walls 42 of the tailings pond 41. -In that manner, only one pond is necessary ~o conduct the who~e process, and there is no need to transfer clari~ied water ~rom the zone 61 to the'zone'41. ~.

~L~3~
I~, as previously discussed, the sludge layer 44 in the retention pond 41 has been treated with the hydrolyzed starch ~locculant, the strength o~ the resultant sand/sludge mixture discharged onto the sand pile 59 to increase the height o~ the dike will be very much stronger, thereby permitting the dike to be built to a substantially greater height without compromising its integrity~
An exemplary system ~or a~ding hydrolyzed starch floccu-lant to the tailings from the separati~n zone 20 discharged through the line 6 and alternative line 23 (~igure 1) is illus-trated in ~igure 9. Tailings from the separation cell are trans~
~erred, via line 23, to a sand separation æone 71 in which the :~
sand component rapidly settles to the bottom for discharge as wet sand through a line 72 to a tailings sump 73. Tailings water is withdrawn ~rom the sand separation zone 71 at a higher point via line 74 into which the hydrolyzed starch flocculant is intro-duced through a line 75. The flocculated tailings water is then discharged into a thickening pond 76 which ~unctions as a holding zone during the several days residence period required ~or the ~locculant to settle the ~ines ~principally clay) well below the sur~ace. Opti~nallyJ the hydrolyzed starch flocculant may be broadcast on the sur~ace of the thickening pond as indicated in the region 77, or a combination o~ ~locculant dosing techniques : ;
may be applied to the tailings water. Virtually clear water may be withdrawn ~rom the upper layer o~ the thickening pond 76 via line 78 for recycle into the hot water process.
Thickened tailings water is drawn ~rom the lower regions -of the thickening pond 76 and is trans~erred, via line 79, to the tailings sump 73. The content of the tailings sump 73, which ; ;
will be a sand and ~locculated thickened tailings water mixture, is withdrawn via line 81 and trans~erred to a sand pond 82. I~

~ 9L3~3~0 the sand po~d 82, ~urther settling takes place and, because o~
the use o~ the hydrolyzed starch ~locculant, an e~ect takes place corresponding to that illustrated in Figure 6; i.e., a higher degree o~ dewatering and compaction results than would be obtained i~ another type of ~locculant were used. As a re~
sult, a clari~ied water layer 160 is also present o~ the sur~ace of the sand pond 82, and this clari~ied water layer may be with-drawn by pump 83 for trans~er via line 84 to a primary tailings pond 85.
Tailings from downstream incremental bitumen recovery pro- ' ;
cesses, which essentially comprise fines laden water, may also be ~, conducted via line 24 for discharge into the primary tailings pond 85. Eydrolyzed starch ~locculant may also be added to this tail-ings stream as indicated at'87 in order to maintain the floccu lant dosage in the primary tailings pond 85 at an optimum level, Clari~ied water is withdrawn ky pump 88 from the upper level o~
the primary tailings pond'85 for recycle via line 89 to the hot '~
water process. ,;
Figure 10 illustrates an examplary system ~or accomplish-~ ing addition o~ hydrolyzed starch,flocculant to sludge accompanied by sand inclusion to obtain the ef~ect illustrated in Figure 6 and dlscus~ed above. Tailings from the separation cell are con~
veyed via line 23 to a sand separation zone 90 wherein the sand component rapidly settles to the bottom ~or discharge through '' line 91 to a tailings sump 92.' Fines-containing tailings water ~, is withdrawn from an upper region o~ the sand separation zone 90 ,`,' through line 93 for discharge into a primary tailings pond 94.' The primary tailings pond ~4 also receives, via line 24, the tail- '' ings from the downstream processes ior extraotin~ incremental ~' ,' amounts o~ bitumen. As indicated at 95, hydrolyzed starch ~loccu- ;
lant may be added to this stream tQ maintain the ~locculant dosage ~3~3~1~

in the primary tailings pond at a desired level. Clarified water is withdrawn, by pump 96, ~or recycle via line 97 back into the hot water process. .
Sludge is withdrawn ~rom the sludge layer o~ primary tailings pond 94 by a pump 98 and is trans~erred via line 99 to an auxiliary pond 100 which ~unctions essentially as a sludge holding areaO Sludge is wi.thdrawn from the auxiliary pond 100 by a pump 101 and is trans~erred via line 102 to the tailings sump 92. It will be understood that, i~ the sludge withdrawal rates ~rom the primary tailings pond 94 is commensurate with the .
capacity of the tailings sump 92, the transfer of sludge to the auxiliary pond 100 need not necessarily be carried out. As a practical matter, such nice adjustments cannot always be achieved, and it is therefore often desirable to provide the auxiliary pond 100. ,, Hydrolyzed starch ~locculant is added to the wet sand/sludge mi~ture by injecting it into the sludge stream ~rom the auxiliary pond 100 (as indicated at 103), by adding the ~locculant to the tailings sump 92 (as indica$ed at 104), and/or by adding the flocculant to the mixture discharged from the tailings sump 9 through line 105 ~or discharge into a third pond 106. In the third pond 106, a hlgh degree o~ dewatering and compaction of the sand/hydrolyzed starch flocculated slud~e mixture, generally ~:
as depicted in Figure 6, takes place. As a result J clari~ied water ~rom a layer 161 may be wi~hdrawn, by pump 107 ~rom the upper layer o~ the pond 106 and trans~erred via line 108 to the ~;;
primary tailings pond 94 ~rom which it is available as recycle ~ :
water to the hot water process. - .;
It may be noted that the system sludge has a bitumen con- -tent which may be su~ficient ~or economic recovery as the price o~ crude oil continues to increase. For that reason, provision . ' .

3 ~3~3~
may be made to bypass section 102a of line 102 by a circuit ~.~
which includes line lO9, optional tertiary bitumen recovery ~ . .
process 100, and line 111.
Figure 11 illustrates a system which combines the tech~
ni-ques illustrated in Figures 9 and 10 in order to obtain a higher rate of recovery o~ recycle water and, particularly9 to minimize the containment volume required to hold the sludge.
Such a higher water recovery rate may be dictated by the fresh water requirements o~ the en$ire hot water process system or, in a given installation, may only be necessary during periods when relatively poor (i.e., high in clay content) tar sands feed ~`
is being worked. The containment volume problem is critical at ~;~
sites o~ limited area and is, ~or example, more important at the Suncor-Oil Sands Division lease site than the fresh water .
aspect.
Tailings from the separation cell are transferredJ via ;~
line 23, to a sand separation zone 140 in which the sand compon-ent rapidly settles to the bottom for discharge as wet sand through a line 141 to a tailings sump 142. Tailings water is withdrawn ~rom the sand separation zone at a higher point via line 143 into which hydrolyzed starch ~locculant is introduced through a line 151. The :elocculated -tailings water~is then discharged into a thickening pond 152 which functions as a holding zone dur- ~ ^
ing the resi.dence period ton the order o~ up to one day) required .
~or the ~locculant to set-tle the:~ines (principally clay) well : ~
be~ow the sur~ace. Optionally, the hydrolyzed starch ~locculant ~:
may be broadcast on the sur~ace o~ the thickening pond as indica~
. .
ted in the.region 157, or a combination of ~locculant dosing techniques may be administered to the tailings water. Virtually clear recycle water may be withdrawn from the upper level of the thickening pond 152 via line 157 ~or recycle into the hot water ~3~3 IL~

process. Thickened tailings water is withdrawn from the lower region of the thickening pond 152 and is trans~erred, via line 153, to the tailings sump 142O
Because clay particles u~dergo an aging process varying in length from a few days to many weeks before they begin to settle, an individual practical installation may require the addition of a holding pond 170 which receives the taili~gs water via a line 171. Aged tailings water is withdrawn through line 172 and transferred to the thickening pond 152.
A first tailings pond 144 receives, via line 24, the tail-ings ~rom downstream processes for ~xtracting incremental amounts o~ bitumen. As indicated at I45, ~ydrolyzed starch flocculant may be added to this stream to maintain the flocculant dosage in the first tailings pond at a desired le~el. Clarified water is withdrawn, by pump 146, for recycle, via line 147, back into the hot water process along with the rerycle water obtained from the thickening pond 152. ~ ;`
Sludge is withdrawn from the sludge layer o~ the first tailings pond 144 by pump 148 and is transferred via line 149 to a second tailings pond 150 which ~unctions essentially as a sludge holding area. Sludge is withdrawn -Erom the lower region o~ the ;~
second tailings pond 150 by a pump 131 and is transferred via line 132 to the tailings sump 142. It will be understood that ~:
if the sludge withdrawal rate from the first tailings pond 144 is commensurate with the capacity of the tailings s~mp 142, the transfer of sludge to the second tailings pond 150 need not neces~
sarily be carried out, ~ydrolyzed starch flocculant is added to the wet sand/sludge mixture by injecting it into the sludge stream ~rom the second ::
tailings pond 150 as i~dicated at 133, by adding the flocculant .

9L~a 39~3~

to the tailings sump 142 as indicated at 134, and/or by adding the ~locculant to the sand/sludge mixture discharged irom the tailings sump 142 through line 135 into a third tailings pond 136, as generally indicated at 139. In the third tailings po~d 13~ a high degree o~ dewatering and compaction of the sand/hydro lyzed starch ~locculated sludge mixture, in the manner depicted :~
in Figure 6, i.s obtained. As a result, clari~ied water may be withdrawn, by pump 137, ~rom the upper layer 162 of the third ~ :~
tailings pond 13S for trans~er via line 138 to the first tailings pond 144 ~rom which it is available as recycle water to the hot water process. .
As previously noted, the sludge has a significant bitumen content. Hence, optional tertiary bitumen recovery may be sought in the bypass loop comprising line 154, process 1.55, and line 156 disposed around the line section 132a between the pump 131 and the tailings sump 142.
It may be notedf with respect to the discussions relevant ~ :.
to Figures 8, 9, 10, and 11, that, in many instances9 the plur-ality o~ ponds illustrated ~or clarity in explaining the processes may often be, in practicel a single pond. In that instance, cer-tain o~ the process steps, such as pumping clarified water and/or sludge between the ponds, takes place naturally so that no special provisions need be made for carrying out these steps~ ;~
It will be appreciated by those skilled in the art, o~
course, that the systems illustrated in Figures 9, 10, and 11 are merely exemplary of ~pproaches toward practical installation which will vary with the process material, type o~ process, cli-mateJ and according to many other factors. The approaches in-volved are basically to emply the thickening pond f sludge recycled from the field, or a combination o~ both. The ways in which these approaches can be applied together or separately are quite "
-2~-.-. .

~ ~39~3~

numerous. Merely by way of example, (1) either one or both sludges may be added to the tailings be~ore sand separation; t2) either one or both sludges may be added to the tailings a~ter sand separation (such as into a tailings sump); (3) extra stages involving repeated sand separation and remixing with fresh sludge may be added with recycle o~ surplus sludge back to the thicken-ing pond or out to the field; or (4) a settling vessel or cyclo~e may be used for sand separation or the displacement technique disclosed within previously referenced U.S. Patent 4,0889146, may be used.
Figures 12a, 12bJ 12c, a~d 12d illustrate sequential steps in a process by which an external sand surcharge achieving the result illustrated in ~igures 4, 5, and 7 can be obtained in re- ;
gions (such as northwest Alberta) having harsh winters. Consider, as shown in Figure 12a, a first summer perlod in which a first auxiliary pond 110 contains sludge received, by way of example from a primary tailings pond, not shown in ~igures 12a, 12b, 12c, o~ 12d. The sludge is withdrawn by pump 111, for transfer, via line 112 to a second auxiliary pond 113. Hydrolyzed starch flocculant may be added, as indicated at 114, if the sludge has not previously been -treated with the starch ~locculant or if the dosage needs to be renewed or increased. The sludge transfer ~rom pond llO to pond 113 is carried out throughout the summer.
Subsequently, as illustrated in Figure 12b, during the first winter, sludge from the primary tailings pond is transferred into the ~irst auxiliary pond 110 via line 115. Because of the harshly cold environmen* at the site o~ the Athabasca tar sands, a thick ice layer 116 forms on top the sludge 117. After the ice has be-~ome su~ficiently thick to bear the weight of heavy machinery~ a layer 118 of sand is spread on *op of the ice layer 116.

~L3~L3~i Upon spring thaw, the ice layer 116 melts to permit the sand layer 118 to settle on top the hydrolyzed starch ~locculant treated sludge layer 117 to be supported thereby and to fu~ction as a porous piston to e~fect further dewatering and c~mpa~tion ~;
of the sludge layer 117. During the second summer, Figure 12c, sludge is again withdrawn from the ~irst auxiliary pond 110 by the pump 111 and is transferred Yia line 112 to the second aux~
iliary pond 113 for deposit as another sludge layer 119 over the sand layer 118. Hydrolyzed starch flocculant is added as in~
dicated at 114 if the transferred sludge has not been previously treated to the desired dosage.
During a second winter, Figure 12d, sludge ~rom a primary tailings pond is again received into the first auxiliary pond 110 via line 115. In the second auxiliary pond, a new ice layer 120 ~orms on top the second sludge layer 119, and when the ice ~
layer 120 reaches sufficient thickness, a second layer of sand ~:
121 is spread over it such that, upon spring thaw, the sand l~yer .
121 settles atop the sludge layer 119 to obtain additional ex~
ternal surcharging of the entire system below it.
20 . The foregoing yearly cycle may be repeated until the cap-acity of the second au~iliary pond is reached whereupon another auxiliary pond can begin to receive sludge from the ~irst aux-iliary pond 110.
It will be readily apparent that many diverse techniques may be employed to emplace a sand surcharge over a sludge layer in a tailings pond. For example, the sand may simply be broad-cast over the pond sur~ace as illustrated ln previously re~erenced U.S. Patent 4,036,752, or any other workable technique may be used to obtain the effect illustrated in Figure 4~ etcO, so long as the sludge 7ayer is first treated with hydrolyzed starch floccu-lant to improve its shear strength and permeability characteristics.

~L~3~3~
While the principles o~ the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art, many modi~ications of struc- :
ture, arrangement, proportions, the elements, materials, and ;~
components used in the practice o~ the invention which are par-ticularly adapted for specific environments and operation re-quirements without departing from those principles.

Claims (6)

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

1. In a tailings pond system for receiving fines-containing effluent from an industrial process in a region having sufficiently harsh winter weather to reach a hard, prolonged freeze, the method for obtaining highly dewatered sludge comprising the steps of:
A) during a first relatively warm period, pumping sludge from a first pond to a second pond;
B) subsequently, during a cold period sufficient to freeze over the second pond, distributing a layer of sand over the frozen surface of the second pond;
whereby, during a subsequent period sufficiently warm to melt the ice layer on the second pond, the sand layer settles over the sludge to apply a porous surcharge thereto which forces water in the sludge to migrate upwardly through the sand layer.

2. The method of Claim 1 in which the sludge is treated with hydrolyzed starch flocculant to increase the shear strength and permeability of the sludge layer prior to settling the sand layer thereover.

3. The method of Claim 1 in which the steps are subsequently repeated employing the same second pond whereby alternating layers of sludge and sand are deposited in the second pond.

4. The method of Claim 2 in which the steps are subsequently repeated employing the same second pond whereby alternating layers of sludge and sand are deposited in the second pond.

5. The method of Claim 3 in which the same first pond is employed for repetition of the steps comprising the method.

6. The method of Claim 4 in which the steps are subsequently repeated employing the same second pond whereby alternating layers of sludge and sand are deposited in the second pond.

PL:dd PL196