CA1123977A - Dike building material comprising sand and treated sludge - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improvement to the basic concept of storing sludge (i.e., fines) in the interstices between the sand grains of a dike associated with a tailings pond is disclosed. The sludge is first treated with hydrolyzed starch flocculant to obtain improved permeability and shear strength characteristics.
Thereafter, the treated sludge is mixed with sand to obtain the fines-containing dike building material which is stronger than that obtained by mixing untreated sludge with sand and thus permits a higher dike in which to store increased quan-tities of fines.

Description

~Z~7~

BACKGROUND OF THE INVENTION

Tar sands (which are also known as oil sands and bitu-minous 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 north-east of the province of Alberta, Canada. This deposit is be lieved 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 sur~ace 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, mineral and water. Bitumen is the valuable component ~or the extraction of which tar sands are mined and processedO The bitumen content is variable, averaging 12wt% o-f the deposit, but ranging from zero to 18wt%. Water typically runs 3 -to 6wt%
o~ the mixture, and generally increases as the bitumen content decreases. The mineral content is relatively constant~ ranging from 84 to 86wt%o While several basic extraction methods -to separate the bitumen ~rom ~he sand have been known ~or many years~ the "hot water" process is the only one o~ present commercial signi~icance 7 q~

~Z39~7 and is employed by both GCOS and Syncrude. The hot water pro-cess ~or achie~ing primary extraction o~ bitumen from tar sand consists o~ three major process steps (a fourth step, ~inal ex-traction, is used to clean up the recovered bitumen from dow~-stream processing~. In the ~irs* step, called conditioning, tar sand is mixed with water and heated with open steam to ~orm 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 ~urther so that settling can take place. The bulk oP
the sand-size mineral rapidly settles and is withdrawn as sa~d tailings. Most o~ the bitumen rapidly floats (settles ~pwardly) to ~orm a coherent mass known as froth 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 o~ suspended bitumen.
The mineral particle size and type distribution is par-ticularly significant to the operation o~ hot water process and to sludge accumulation. The terms "sand", "silt7', "clay" and "~ines" are used in the specification as a simplified approxima-tion o~ mineral particle si~e wherein 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 o~ that sizeO Fines includes both silt and clay, but excludes sand. It should be again noted that these designations are simpli~ied approximationsO
For an elegant and in-depth discussion o~ particle size and type in tar sands sludges, reference 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 Or Canadian Petroleum Technology, Volume 17, Number 4 (October December 19~8). 3 ~z~

As pr~viously indicated, conditioning tar sands for the recovery of bitumen consists of heating the tar sands/water feed mixture to process temperatuxe (180 -20~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 r bitumen is stripped from the individual sand grains and mixed into the puIp 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 occuring 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 unders~and that the conditioning process, which prepares the bi-tumen 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 irst screened to remove rocks and unconditiona~le 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 23~7~

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 middli~gs consistæ
primarily of water ~ith suspended ~ine material and bitume~
part icles .
The lparticle sizes and densities o~ the sand and o:f the bitumen particles are relatively ~ixed. The parameter which influences the settling processes most is the viscosity of the middlings, and viscosity is directly related to fines conten^t.
Characteristically, as the fines content rises abo~e a cer$ai~
threshold, which varies according to the composition o~ the fines, middlings viscosity rapidly reaches high values with the e~fect tha* the settling processes esse~tially stop. ~n this operati~g condition, the separation cell is said to be "upset"~
Little or no oil is recovered, and all ~treams exiting the cell have about the same composition as the ~eed. Thus, as feed fines content increases, more water must be used i~ 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 *he 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 re-covery of incremental amounts of bitumen. Air ~lotation is an effective scavenging method for this middlings stream~
Final extraction or ~roth clean-up is typically accomp lished by centri~ugation~ Froth ~rom primary extractio~ is diluted with naphtha, and the diluted froth is then subjected to a two~stage centrifugatio~. This process yields an essentially pure diluted bitumen oil product. ~Jater and mineral removed ~rom

3~77 the ~roth during this step constitutes an addltional tailings stream which must be disposed o~.
In the terminology o~ extractive processing, tailings is the throw-away material gensrated in the course o~ extracting the valuable material ~rom an ore. In tar sands processing>
tailings consists o~ the whole tar sand ore body plus ~et ad-ditions o~ process water less only the recovered bitumen product~
Tar sa~d tailings can be subdivided into three categories; viz:
(1) screen oversize, (2) sand tailings (the fraction that settles rapidly), and (3) tailings sludge (the fractivn that settles slvw-ly). Screen oversize is typically collected and handled as a separate stream.
Recently, in ~iew of 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 ~ocus on tailings disposal.
The concept of tar sands tailings disposal is straight~orward.
I~ one cubic ~oot of tar sands is mined, a one cubic foot hole is left in the ground. The ore is processed to recover the bitu men fraction, and the remainder, including both process material and the gangue, constitutes the tailings that are not valuable and are to be disposed o~. In tar sands processing, the mai~
process material is water, and the ga~gue is mostly sand with some silt and clay. Physlcally, the tailings ~other than over-size) consist o~ a solid part (sand tailings) and a more or less ~luid part (sludge). The most satisfactory place to dispose o~
these tailings is, of course, in the existing one cubic ~oot hole in the ground. It turns out~ however, that the sand tailings alone ~rom the one cubic ~oot o~ ore occupy just about one cubic ~oot. The amount o~ sludge is variable, depending on ore quality and process conditions, but averages about O . 3 cubic ~eet . The tailings simply will not ~it back into the hole in the ground.

3~
The historical literature coveri~g the hot water process for the recovery o~ bitumen from tar sands contains little in the way o~ a recognition that a net accumulation of sludge would occur. Based on analysis o~ ~ield test unit operations which led to the Great Canadian Oil Sands plant design near Fort McMurray, Alberta, the existence of sludge accumulation was predict~d.
This accumulation came to be calIed the "pond water problem."
Observations during start-up and early commercial operations at Fort McMurray (1~67-1969) were o~ insufficient precision to con-firm the prediction. Since 1969, commercial operating data haveconfirmed the accumulation in GCOS' tailings disposal area o~ a sludge layer o~ fines material and water which settles and com- -pacts only very slowly~ if at all, after a ~ew years. For a number o~ 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 import~nt and difficult to deal with.
At the GCOS plant, for dike building, tailings ar~ con-veyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impound-ment ~or the pool o~ fluid contained inside. On the dike, the sand settles rapidly, and a slurry o~ fines, water, and minor amounts of bitumen ~lows 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 ~lows into the pond's interior co~ences stratification in settling over a time scale o~ months to years.
Ove~boarding 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 tinction is not as sharp as in dike building, and no mechanical :~23~77 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 saud settles, fines and water commence long-term settling in the pon~O
The exceedingly complex behavior and characteristics o~
tailings ponds have only recently come to be understood beyond the simplistic categoriza~ion o~ various zones such as clari~ied water, transition, and sludge/slime. Since a tailings pond em-ployed in conJunction with the hot water process for processing -tar sands is fairly typical, the iollowing characteristics o~
the layers or zones in such a tailings pond is a good general example.
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 slud~e by settling o~ these tailings ls attributable primarily to the presence of dispersed clay minerals. Many o~ 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 intitial clay concentration (clay/water ratio), relatlve proportions o~ various clay mineral species, particle size, condition of clay sur~aces and pore water chemi~
try. Experience and laboratory analysis indicate that all these factors vary significantly from time to time depending on the composition o~ the tar sands ~eed and the process conditions.
Typically, tailings are discharged over -the beach (either directly or from dike construction) where most of the sand settlesD
The run-of~ flows continuously into a fluid pool or pond ~rom which water is simultaneously withdrawn as recycle to the tar sands extraction process. Here, additional important determin ants o~ settling behavior are imposed. These include rate oi ~Z3~7~

in~low and outflow in relation to sur~ace area and clarl~ied water volume, pond depth, and degree o~ agitation o~ pond con-tents, either through in~lows and out~lows or via thermal or by wind e~ects. While initial temperature is inherent in the tailings streams, temperatures in the pond are obviously deter mined by numerous other ~actors as well.
Experience and laborator~ analyses indicate that 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 o~ more sand and silt particles. These settle either not a~ all or very slowl~
because o~ the significant yield strength of stagnant sludge.
The clay/water ratio increases only slightly with depth in the upper part o~ 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 di~ficult. Sludge ~ormed .

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 in~low and out~low is more complex. Experience and laboratory tests indicate that 9 ~ollowing discharge to the pond, clay particles undergo an aging process varying in length ~rom a ~ew days to many weeks. Prior to completion o~ the aging process~ the clay particles do not begin to settle. However, once they commence to do so, the pro-cess proceeds quite rapidly according to the principles o~ Stok~s Law until a clay/water ratio oP about 0.13/1 is reached at which other ~actors evidently predominate over Stokes 1aw. In the upper~

_9_ ~3L;23~7~

most part of a well managed pond, these e~ects result in a more or less clear water layer at the top underlaid by ~ layer o~
relatively dllute sludge more or less sharply di~erentiated from it. This may be termed the sedimentation zone; lts volume is determined by the rate of clay in~low and the average aging time required. I~ the water layer is permitted to become too small in relation to the clay in~low, water out~low and aging time, the upper part of 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 of the early years.
Sludge in ~he 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 may be regarded as terminal sludge. I'he ~pace below the sedimentation zone and above th~ 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 prl marily a ~unction of the average clay inflow rate in relatio~ to volume.
In summary, an active pond normally has a well-de~ined clear water layer at the top which can, however, disappear i~
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 of separated bitumen near the inter~ace between water and sludge. However, the sludge may be considered as consisting of three zones each involving successively larger orders o~ magnitude o~ time scale for measurable dewatering to occur, and each characterized by the ~3~7 predominance o~ di~ering dewatering parameters. These three zones may be termed respectively a sedimentation æone, a tran~-ition zone and a terminal sludge zo~e.
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 ~or mid-dlings viscosity control represents a large volume relative to the volume of the ore itsel~, and (5) upon disposal, clays settle only very, very slowly; thus, the water component of tailings is only partially availabIe for reuse via recycle. That which can-not be recycled represents a net accumulation of -tailings sludge~
The pond water problem, therei`ore, is to devise long-term, economically and ecologically acceptable means to eliminate, minimize, or permanently dispose of the accumulation of sludge.
Experience has demonstrated that the problem requires a multi-*aceted approach toward its solution, and the present invention is directed at achieving one aspect oi the solution: a more thoroughly dewatered sludge layer which, as a consequential re sult, obtains a greater quantity o~ clari~ied water ~or recircu-lation into the process i~ neces.sary 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 i~locsO The degree oi ~locculation is controlled by the probability o~ collision between the particles and their tendency toward adhesion a~ter collision. Agitation increase the probability of collision, and adhesion tendency is incr~ased by t~e addltion o~ a i'locculant.

~;23977 Reagents act as flacculants through one or a combination of three general mechanisms: (I) nautralization of the electrical repulsive foxces 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 flocs, such as metal hydroxides, that entrap fine particles; and ~3) bridging of particles by natural or synthetic, long-chain, high-molecuIar 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.
A remarka~le number of 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 treat-ment facilities. ~Iowe~er, a distinct step forward in the art has been achieved by the use of hydrolyzed corn and potato starch flocculants as described in co-pending Canadian application S.~. 275,214, filed March 31, 1977 and entitled "Destabilization of Sludge with Hydrolyzed Starch Flocculants" and by the use of wheat starch flocculants as set forth in co-pending Canadian ap-plication S.N. 308,619, filed August 2, 1978, and also entitled "Destabilization of Sludge with Hydrolyzed Starch Flocculants".
These specific hydxolyzed starch flocculants, 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 r in which there is a critical 3n need to recycle clarified water from the tailings pond back into 1J 2397~7 the process. Howeve,r, experience has indicated that the simple use of these hydrolyzed starch 1Occulants, or for that matter any other known flocculant, resul~s in ver~ little, if any, im-provement on the uItimate degree of dewatering of the sludge layer. That is, the terminal s~atus of the sludge layer is ]ust a~out the same as wouId be obtained over a much longer period of tim~ by natural settling processes, and this terminal condition is unsatisfactory in that it includes too much water, is too vol-uminous, and is too unstahle.
Wonetheless, it is not accurats to say that all character-istics of a sludge layer obtained as a result of flocculants 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 flocaulants 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 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 im-possible dewatering techni~ues 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 int~r-stices between the sand grains in the material employed for dike building. Such a process is disclosed in Canadian patent ~,063,956j issued October 9, 1979, and entitled "Method of Sludqe Disposal Related to the Hot ~ater Extraction of Tar Sands"
and corresponding U.S. patent 4,008,146, issued February 15, 1977.

1~;239~7 'I`he expericnce ~v:ith the procedure described in that re~erence i..s -that the height to which the dike can be built i.s somewhat lirnited; however, i t 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 floccu-lants, the strength o~ the resultant material is notably in-creased such that the dlke can be built higher, thereby not ollly permit-tlng a deeper tailings pond, but also .storing more sludge in the interstices ~etween the sand grai.ns comprising 1) the dike.

SUMMARY OF THE INVENTION

It is a broad obJect o~ this invention to minimize the volume of sludge stored in an industrial process tailings pond.
In another aspect, it is an object of this invention to store sludge in the interstices between sand grains o~ a sand dike.
ID a more speci~ic aspect, it is an object of this in-ventlon to provide means for controlling the sludge layer o~
an associated tailings pond by mixing sand with sludge which "!~) has been treated with hydrolyzed starch ilocculant to increase lts strength and permeability characteristics and thereafter employing at least a portion of the sand/sludge mixture as a di.ke building material having improved characteristics such that sludge, particularly the clay component, is stored in the intelstices between sand grains of the dike.

~II',SCRIPTION OF TRE DRAWINGS

The subject matter o~ the invention is particularly pointed out ancl clistl.nctly claimed in the concluding portion of the -14~

~23~7 speci~ication. The invention, however, both as to organizatio~
and method of operation, may best be understood by reference to the ~ollowing description taken in conjunctiQn with the accom-panying drawing of which:
Figure 1 is a somewhat simplified block diagram o~ a hot water process ~or covering bituminous tar sands into bituminous froth for subsequent upgrading to synthetic crude oil;
Figure 2 is a partial cross-sectional view which illus-trates, conceptually and simplistically, the distribution o~
water and sludge in a tailings pond associated with the apparatus illustrated in Figure 1;
Figure 3 is a view similar to Figure 2 and shows the re-sults of prior art attempts to surcharge the sludge layer o~ a tailings pond with sand;
Figure 4 illustrates the ef~ect o-f surcharging the sludge layer of the tailings pond with sand after the sludge layer has been treated with speci~ic hydrolyzed starch ~locculants;
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 e~ect o~ internal surcharging obtained by mixing sand with sludge which has been or is simul-taneously treated with specific hydrolyzed starch flocculants;
Figure 7 illustrates the ef~ect o~ employing a combination of internal and external surcharging techniques with sludge which has been treated with specific hydrolyzed starch ~locculants;
Figure 8 illustrates a general approach ~or increasing the amount of fines stored in the interstices between adjacent sand grains in a dike;
Figure 9 illustrates an exemplary specific method ~or adding hydrolyzed starch flocculant to the tailings o~ a tar sands hot water process;

~239~7~

Figure 10 illustrates a method ~or the addition o~
hydrolyzed starch flocculants to sludge, accompanied by sand inclusion, ~ound in the tailings system v~ a tar sands hot water pr~cess;
Figure 11 illustrates a combination o~ the techniques illustrated in Figures 9 and 10 by which a more rapid recovery of clarified water may be obtained7 and ~ igures 12a, 12b, 12c~ and 12d illustrate a sequen~e of operations by which external sand surcharge to the sludge layer of a tailings pond located in a cold environment can be obtainedO

DETAILED DESCRIPTION OF THE INVENTION

Re~erring now to Figure 1, bituminous tar sands are fed into the system through a line 1 and passed to a conditioning 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 ~orm is a minor amount based on -the weight of the tar sands processed. The tar sands, heated and conditioned with steam and water, pass through a line 3 to a screen 290 The purpose of the screen 29 is to remove from the pulp any debris such as rock or oversized lumps o~ clay as indicated 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 zone 20.
The diluted pulp is continuously flllshed from the feed 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 froth rises to the top and is withdrawn through a line 5 while the bulk of the sand component settles to the bottom as a tailings layer which is withdrawn through line 6. It will be ~L23~

understood, o~ course, that th~ tailings streams ca~ be trans-ferred individually, with or without downstream treatment, as indicated by the alternate lines 23, 24 and optional treatmént 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 VlSCoSityO This middlings material is transferred to a flotation scavenger zone 21 where an air flotation operation is conducted to bring about the f~r mation of additional bituminous froth which passes from the scavenger zone 21 through line 9, in con~unction~with the pri-mary froth ~rom the separatlon zone 20 passi~g through li~e 5, to a fro$h settler zone 22. A bitumen-lean water stream is removed from the bottom of 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 ~rom the flotation scav-enger zone and the sand tailings stream from the separation zon~
20. The bitumen ~rom the set~ler 22 is removed through line ~2 for further treatment, typically final extractio~
Bitumen-lean water from the froth 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 ~
sludge layer 27. The sand included in the ~ailings stream quickly settles in the region 14, and the ~ines-containing water ~lows into the body of the pond 15 where settling takes place. Water from the clarified water layer 26 may be withdrawn by a pump 28 for recycle through a line 17 to be mixed with fresh makeup water and charged into the hot water process.

Re~erring now to Figure 2~ the sludge layer 27 o~ the tallings pond 15 is overlayed with a clarified water layer 26.
(As previously noted, this is a considerable simplification, but is adequate and appropriate ~or an understanding of the present invention.) The sand bottom 23 of the pond defines the lower limit of the sludge layer 27 which, as previously discussed, increases a mineral-to-water ratio from top to bottom, The characteristics o~ the sludge layer 27 so formed is unaccep-tably and insufficiently dewatered and compac-ted 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 s:Ludge layer with a layer o~ sand whereby the sand acts as a permeable piston to compress the sludge and force water out of it. All attempts to carry out this surcharging concept have met with con-plete failure or have been performed under conditions which yield only marginal, if any, benefits under very limi-ted ~onditions.
Seej by way o~ e~ample, U.S. Patent 4,036,752, issued July l9, 1977, and entitled t'Dewatering Clay Slurries."
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 of sand 24 is broadcast over the sludge layer 27, the sand layer is observed to tilt and dump through the sludge layer aæ 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 been 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 settli~g process was accelerated by the use of flocculants, -1~

.4 ~ ~r1~

However, it has been determined that the use of the specific hydrolyzed starch floccuIants described in the above-referenced Canadian patent applications produces a sludge layer with remarkably enhanced shear strength and permeability charac-teristics, and an appreciation of this fact resulted in recon-sideration of the heretoXore substantially impractical and dis-carded sand surcharge concep~. Throughout the remainder of this specification the term "hydrolyzed starch flocculant" means one of the specific starch floccuIants disclosed in ~he above-referenced 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 form.
As shown in Figure 4, a sludge layer 33 which ha been treated with a hydrolyzed starch flocculant is capabl~ of sup-porting a substantial sand surcharge which operates as a porous piston to compact and dewater the sludge layer. In addition, the observed improved permeability of the sludge layer 33 re~ul-ting from treatment with a hydrolyzed starch flocculant af~ords an enhancement to the degree of compaction of dewatering which can be achieved Furthermore, asshown in Figure 5, sludge layer 33, treated with a hydrolyzed starch flocculant, is sufficiently strong that a second layer of sludge 35 may be layed over the sand layer 34 and the second sludge layer, itself, may be sub-jected to a surcharge brought about by another sand layer 36.
For relatively deep tailings ponds, a number of such alternate layers of treated sludge and sand may be employed to obtain a very high degree of compaction and dewatering.
It has also been proposed in the prior art to mix sludge, which has been flocculant-treated with sand to obtain a material which, in effect, i5 "internally surcharged." One may refer, by way of example to U.S. Patent 3,680,693 issued August 1, 1972, A

~L~LZ~3977 and entitled "Process for tha Treatment of Slime and Waste Solids." While this technique has been promising, the amount of sand which can be added to the sludge has been limited by the strength of the slud~e and, as previously noted, no pre-viously known flocculant affords the strength and permeability enhancement to the sludge layer observed to result from use of the hydrolyzed starch floccuIants previously identified.
It has now been found that sand mixed with sludge treated with one of these hydrolyzéd starch floccuIants resuIts în a material which, indeed, exhi~it~ important internal surcharge character-istics resulting in a compacted sand/sludge layer 37 as illus-treated 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 itself overlayed with a sand layer 38. In addition, of course, the multilayering technique illustrated in Figure 5 is equally applicable.
It has been observed at the Suncor-Oil Sands Division plant that on the order of 35~ of the fines (and a larger portion of the clay componentl is discharged into the tailings pond; the remainder is stored in the interstices between adjacent sand grains or is discarded as lumps which are part of the oversize. It has been proposed in the past to increase the quantity of silt, and particularly the quantity of clay, stored in the interstices be-tween adjacent sand grains in the material employed to build a pond-impounding dike. By way of example, one may refer to pre-viously referenced U.S. Patent 4,008~146, issued February 15, 1977, and entitled "Method of Sludge Disposal Related to the Hot Water Extraction of Tar Sands" and also to the corresponding Canadian patent 1,063j956, issued October 9, 1979. As disclosed in that reference, sand and sludge are admixed in a pre-"

~l~23~b77 scribed fashion, and the resultant material is discharged at the dike site to efPect dike building. This is an important concept, but its use in practice has been somewhat limited be-cause the stability o~ the resul~ing dike structure is insu~-iicient to permit building the dike to a height which represents storage of meaning~ul additional quantities of ~ine~.
It has now been determined that, if sand is admixed ~ith sludge which has been treated with a hydrolyzed starch flocculant, an important increase in the strength of the resulting material, 'when employed ~or dike building ~ is observed such that the resul-ting structure is much more stable. Thus, su'bstantially higher dikes can be built, and signi~icantly large quantities o~ silt and, particularly, clay can be stored in the interstices between adjacent sand grains in the material.
An exemplary procedure ior storing silt and clay particles in the interstices between adjacent grains oi sand in a sand dike is illustrated in Figure 8. A tailings pond 41 is enclosed by dike walls 42 and contains a clarified water layer 43 and a sludge layer 44. Sludge is withdrawn ~rom the pond 41 via sludge with-drawal means 51 and is trans~erred to a line 47 by a pump 46 which is supported by flotation means 45 on the surface o~ the pond 41. The sludge material is trans~erred from the line 47 into a line 50 where it is combined with, by way of example, tailings material ~'rom the hot water extraction process for reeovering bitumen ~rom tar sands. This waste water stream ~rom the extrac-tion process is primari'ly water and sand, but inclues minor amounts o~ silt, clay, and biturnen. Thus, the combined streams which are transferred ~rom line 50 into a settling zone 52 contain a sub-stantial amount o~ sand.

~L23~77 In the settling zone 52, an upper layer 53 and a lower layer 54 are ~ormed. The upper layer is withdrawn through line 55 and is trans~erred into a line 56, where it is combined with beach run-o~ water transferred from zone 61 via line 57, and added to the retention pond 41.
The lower layer 54 in settling zone 52 is withdrawn through a line 58 and i.s transferred to an inclined sand pile 59 situated adjacent a dike 60. The lower layer 54 o~ the settling zone 52 typically comprises on the order o~ 2% bitumen, 39% sand, 9% silt,

4% clay, and 46% water. This mixture is dispersed over the sand pile to form additional sand layers whereby a part o~ the clay, silt, and water in the stream is retained in the i.nterstices of the sand layers. The remainder o~ the a~ueous stream percolates down the inclined sand pile zone and settles into the re-tentioD
æone 61. A pump 62 in the retention:zone 61 withdr~ws the aqueous portion o~ that pond and trar.sfers i$ into the line 5? where, as previously noted, it is combined with ~he stream ~rom the upper layer o~ zone 52 in the line 56.
Thus, a part of the sludge from the tailings pond 41 is removed and dispersed with -the sand o~ the wastewater stream over the pond dike wall to carry ou~ dike building. Substantially in creased quantities of the sludge withdrawn ~rom the pond are stored in the interstices o~ 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 o~ the tailings pond 41.
In that manner, only one pond is necessary to conduct the whole process, and there is no need to trans~er clarified water ~rom the zone 61 to the zone 41.

2397~

I~, as previously discussed, the sludge layer 44 in the retention pond 41 has been treated with the hydrolyzed starch -~locculant, the strength of the resultant sand/sludge mixture discharged onto the sand pile 5~ 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 for adding hydrolyzed starch flocc~-lant to the tailings from the separation zone 20 discharged through the line 6 and alternative line 23 (Figure 1) is illus-trated in Figure 9. Tailings ~rom the separation cell are trans-ferred, via line 23, to a sand separation zone 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 from the sand separation zone 71 at a higher point via line 74 into which the hydrolyzed starch flocculant is intro-duced throug~ a line 75. The flocculated tailings water is the~
discharged into a thickening pond 76 which functions as a holding zone during the several days residence period required for the flocculant to settle the fines ~principally clay) well below the surface. Optionally, the hydrolyzed starch flocculant may be broadcast on the surface oP the thickening pond as indi~ated in the region 77, or a combination o~ floccularlt dosing techniques may be applied to the tailings water. Virtually clear water may be withdrawn from the upper layer o~ the thickening pond 76 via line 78 for recycle into the hot water processO
Thickened tailings water is drawn from the lower regions o~ the thickening pond 76 and is transferred, 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 transferred to a sand pond ~2. In ~23~

the sand pond 82, ~urther settling takes place and, because oi the use of the hydrolyzed starch ~locculant, an e~ect takes place corresponding to that illus~rated in Fi~ure 6; i.e., a higher degree of dewatering and compaction results than would bè obtained i~ another type o~ ~locculant were used. As a re-sult, a clari~ied water layer 160 is also present on the sur~ace of the sand pond 82, and this clari~ied water layer may be wit~-drawn by pump 83 for transfer 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 ~or discharge into the primary tailings pond 85. Hydrolyzed starch ~locculant may also be added to this tail ings stream as indicated at 87 in order to maintain the iloccu-lant dosage in the primary tailings pond 85 at an optimum level.
Cla~ified water is withdrawn by pump ~8 ~rom the upper level oi the primary tailings pond 85 for recycle via line 89 to the hot water process.
Figure 10 illustrates an examplary system for accomplish-ing addition of hydrolyzed starchflocculant to sludge accompanied by sand inclusion to obtain the e~fect illustrated in Figure 6 and discussed 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 for 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 ~or discharge into a primary tailings pond 94.
The primary tailings pond 94 also receives, via line 24, the tail-ings ~rom the downstream processes ~or extracting incremental amounts o~ bitumen. As indicated at 95, hydrolyzed starch ~loccu~

lant may be added to this stream to maintain the ~looculant dosage ~L12;3~

in the primary tailings pond at a deslred level. Clari~ied water is withdrawn, by pump 96, ~or recycle via line 97 back into the hot water processO
Sludge is withdrawn ~rvm the sludge layer o~ primary tailings pond 94 by a pump 9B and is transferred via line 99 to an auxiliary pond 100 which functions essentially as a sludge holding area. Sludge is withdrawn ~rom the auxiliary pond 100 by a pump 101 and is trans~erred via li~e 102 to the tailings sump 92. It will be understood that, if the sludge withdrawal rates from the primary tailings pond 94 is commensurate with the capacity o~ the tailings sump 92, the transfer of sludge to the auxiliary pond 100 need not necessarily be carried out. hs a practical matter, such nice adjustments cannot always be achieved9 and it is therefore o~ten desirable to provide the auxiliary pond 10~.
Hydrolyzed starch ~locculant is added to the wet sand/sludge mixture by injecting it into the sludge stream from the auxiliary pond 100 ~as indicated at 103), by adding the ~locculant to the tailings sump 92 ~as indicated at 104), and/or by adding ~he ~locculant to the mixture discharged ~rom the tailings sump 92 through line 105 for discharge into a third pond 106. In the third pond 106, a high degree o~ dewatering and compaction of the sand/hydrolyzed starch ~locculated sludge mixture, generally as depicted in Figure 6~ takes place. As a result, clarified water ~rom a layer 161 may be withdrawn, by pump 107 from the upper layer of the pond 106 and trans~erred ~ia line 108 to the primary tailings pond 94 from 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 sufficient for economic recovery as the priceo~ crude oil continues to increase. For that reason, provision ~Z39~77 may ~e made to bypass section 102a o~ line 102 by a circuit which includes line 109, optional terti~ry bitumen recovery process 100, and line 111~
~ igure 11 illustrates a system which combines the tech-niques illustrated in Figures 9 and 10 in order to obtain a higher rate of recovery o~ recycle water and, par-ticularly9 to minimize the containment volume required to hold the sludge.
Such a higher water recovery rate may be dictated by the fresh water requirements of the entire 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 ~.ontainment volume problem is critical at sites of limited area and is, ~or example, more important at the Suncor-Oil Sands Division lease site than the fresh water aspect.
Tailings ~rom the separation cell are transferred, via line 23, to a sand separation zone 140 in which the sand compon-ent rapidly settles -to the bott~m ~or discharge as wet sand through a line 141 to a tailings sump 1420 Tailings water is withdrawn from the sand separation zone at a higher point via line 143 into which hydrolyzed starch flocculant is introduoed through a line 1510 The ~locculated tailings water is then discharged into a thickening pond 152 which functions as a holding zone dur-ing the residence period (on the order of up to one day) required for the flocculant to settle the ~ines (principally clay) well below the sur.~ace. Optionally, the hydrolyzed starch ~loccula~t may be broadcast on the surface of the thickening pond as indica-ted in the region 167, or a combination o~ ~locculant dosing techniques may be administered to the tailings water. Virtually clear recycle water may be withdrawn ~rom the upper level of the thickening pond l52 via line 157 for recycle into the hot water -2~-1~35~77 process. Thickened tailin~s water is withdrawn from the lower region o~ the thlckening pond 152 and is trans~erred, via line 153, to the tailings sump 142.
Because clay particles undergo an aging process varyi~g in length from a ~ew days to many weeks before they begin to settle, an individual practical installation may require the addition o~ ~ holding pond 170 which receives the tailings water via a line 1710 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 ~or extracting incremental amounts o~ bitumen. As indicated at 145, ~ydrolyzed starch :~locculant may be added to this stream to maintain the flocculant dosage in - the first tailings pond at a desired level. Clarified water is withdrawn, by pump 146, for recycle, via line 147, back i~to the hot water process along with the recycle water obtained ~rom the thickening pond 152.
Sludge is withdrawn from the sludge layer o~ the ~irst tailings pond 144 by pump 148 and is transferred via line 149 to a second tailings pond 150 which ~unctions essen~ially as a sludge holding area. Sludge is withdrawn from the lower region of 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 sump 142" the trans~er o~ sludge to the second tailings pond 150 need not neces-sarily be carried out, Hydrolyzed starch ~locculant is added to the wet sand/sludge mixture by injecting it into the sludge stream from the second tailings pond 150 as indicated at 133, by adding the flocculant -27- .

~Z3~

to the tailings sump 142 as indicated at 134, and/or by a~ding the flocculant to the æand/sludge mix~ure discharyed from the tailings sump 142 through line 135 into a third tailings pond 136, as generally indicated at 139. In the third tailinqs pond 136 a high degree of dewateriny and compaction of the sand/hydro-lyzed starch floccuIated sludge mixture, in the manner depicted in Figure 6, is obtained. As a result, clax:ified water may be withdrawn, by pump 137, from the upper layer 162 o the third tailings pond 136 for transfer via line 138 to the first tailings pond 144 from which it i5 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 155, and line 156, disposed around the line sectlon 132a between the pump 131 and the tailings sump 142.
It may be noted, with respect to the discussions relevant to Figures 8, 9, 10, and ll, that, in many instances, the plur-ality of ponds illustrated for clarity in explaining the processes may often be, in practice, a single pond. In that instance, cer-tain of the process steps, such as pumping clarified water and/or sludge hetween 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, of course, that the systems illustrated in Figures 9, lO, and 11 are merely exemplary of approaches toward practical installation which will vary with the process material, type of process, cli-mate, and according to many other factors. The approaches in-volved are basically to employ the thickeniny pond, sludge recycled from the field, or a combination of both. The ways in which these approaches can be applie~ together or separately are quite -2~-~L~23~

numerous. Merely by way o~ example, (1) either one or both sludges may be added to the taillngs be~ore sand separation; (2) 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 of surplus sludge back to the thicken ing pond or out to the field; or (4) a settling vessel or cyclon~
may be used for sand separation or the displacement technique disclosed within previously referenced U.S. Patent 4,088~146, may be used.
Figures 12a~ 12b, 12c, and 12d illustrate sequential steps in a process by which an external sand surGharge achieving the result illustrated in Figures 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 Figures 12a, 12b~
12c, or 12d. The sludge is withdrawn by pump 111, for transfer9 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 flocculant or if the dosage needs to be renewed or increased. The sludge trans~er from pond 110 to pond 113 is carried out throughout the summer.
Subsequently, as illustrated in ~igure 12b, during the first winter, sludge ~rom the primary tailings pond is transfer:red into the first auxiliary pond ~10 via line 115. Because of the harshly cold environment at the site of the Athabasca tar sands, a thick ice layer 116 forms on top the sludge 117. After the ice has be-come su~ficiently thick to bear the weight of heavy machinery~ a layer 118 of sand is spread on top of the ice layer 116.

~3~7 Upon spring thaw, the ice layer 116 melts to permit the sand layer 118 to settle on top the hydrolyzed starch ~loccula~t treated sludge layer 117 to be supported thereby and to iu~ction as a porous piston to effect further dewatering and compaction of the sludge layer 117. During the second summer, Figure 12c, sludge is again withdrawn from th~ ~irst auxiliary pond 110 by the pump 111 and is trans~erred via line 112 to the second aux-iliary pond 113 for deposit as another sludge layer 119 over the sand layer 118. Hydrolyzed starch ~locculant is added as i~
dicated at 114 i~ the transferred sludge h~s not been previously treated to the desired dosageO
During a second winter, Figure 12d~ sludge from a primary tailingæ pond is again received into the first auxiliary po~d 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 layer 121 settles atop the sludge layer 119 to o~tain additional ex-ternal surcharging of the entire system below it.
The foregoing yearly cycle may be repeated until the cap-acity of the second auxiliary pond is reached whereupon another au~iliary pond can begin to receive sludge ~rom the first 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 in previous~y referenced U.S. Patent 4,036,752, or any other workable technique may be used to obtain the e~ect illustrated in Figure 4, etc., so long as the sludge layer is ~irst treated with hydrolyzed starch ~loccu-lant to improve its shear strength and permeahility characteristic~

13 ~3977 While the principles of the invention have now been made clear in an illustrative embodlment, there will be immediately obvious to those skilled in the art, many modifications of struc-ture, arrangement, proportions, the elements, materials, and components used in the practice o~ the invention whieh are par-ticularly adapted for speci~ic environments and operation re-quirements without departing from those principle~s.

Claims (14)

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

1. In the method of reducing the sludge content of an industrial process tailings pond which includes the steps of:
A) withdrawing a sludge stream from the tailings pond sludge layer;
B) mixing the sludge stream with sand;
C) settling the sand/sludge mixture in a settling zone to form a substantially aqueous upper layer and a lower layer comprised of an aqueous/sand/sludge mixture; and D) dispensing the aqueous/sand/sludge mixture over a sand pile zone to provide additional sand layers thereon having at least a part of the sludge in the mixture entrapped within the interstices between the said grains;
the improvement comprising treating the sludge with hydrolyzed starch flocculant to improve the shear strength and permeability of the aqueous/sand/sludge mixture whereby the sand pile can be built to greater height than without such treatment.

2. The method of Claim 1 wherein the sand pile is em-ployed as a pond wall whereby the portion of the aqueous/sand/
sludge mixture which is neither entrapped in the interstices be-tween the sand grains nor evaporates is collected in a pond.

3. The method of Claim 2 wherein the pond of which the sand pile is employed as a wall is separate from the tailings pond.

4. The method of Claim 3 which includes the additional step of transferring water from the separate pond to the tailings pond.

5. The method of Claim 1 which includes the additional step of pumping fluid from the substantially aqueous upper layer in the settling zone to the tailings pond.

6. The method of Claim 2 wherein the pond of which the sand pile is employed as a wall is the tailings pond.

7. In an industrial process from which a fines and sand-containing aqueous effluent is discharged into a tailings pond, the method for reducing the sludge content of the tailings pond comprising:
A) treating the tailings pond sludge with hydrolyzed starch flocculant;
B) withdrawing a sludge stream from the tailings pond sludge layer;
C) mixing the sludge stream with the fines and sand-containing aqueous effluent;
D) settling the sludge/effluent mixture in a settling zone to form a substantially aqueous upper layer and a lower layer comprised of an aqueous/sand/sludge mix-ture; and E) dispensing the aqueous/sand/sludge mixture over a sand pile zone to provide additional sand layers thereon having at least a part of the sludge in the mixture en-trapped within the interstices between the sand grains.

8. In an industrial process from which a fines and sand-containing effluent is discharged into a tailings pond, the method for reducing the sludge content of the tailings pond com-prising:

A) treating the fines and sand-containing effluent with hydrolyzed starch flocculant;
B) withdrawing a sludge stream from the tailings pond sludge layer;

C) mixing the sludge stream with the treated fines and sand-containing aqueous effluent;
D) settling the sludge/effluent mixture in a settling zone to form a substantially aqueous upper layer and a lower layer comprised of an aqueous/sand/sludge mixture; and E) dispensing the aqueous/sand/sludge mixture over a sand pile zone to provide additional sand layers thereon having at least a part of the sludge in the mixture entrapped within the inter-stices between the sand grains.

9. In an industrial process from which a fines and sand-containing effluent is discharged into a tailings pond, the method for reducing the sludge content of the tailings pond comprising:
A) withdrawing a sludge stream from the tailings pond sludge layer;
B) mixing the sludge stream with the fines and sand-containing effluent;
C) treating the sludge/effluent mixture with hydro-lyzed starch flocculant;
D) settling the treated sludge/effluent mixture in a settling zone to form a substantially aqueous upper layer and a lower layer comprised of an aqueous/sand/
sludge mixture; and (E) dispensing the aqueous/sand/sludge mixture over a sand pile zone to provide additional sand layers thereon having at least a part of the sludge in the mixture entrapped within the interstices between the sand grains.

10. A method according to Claims 7, 8, or 9 wherein the sand pile is employed as a pond wall whereby the portion of the aqueous/sand/sludge mixture which is neither entrapped in the interstices between the sand grains nor evaporates is collected in a pond.

11. The method of Claim 10 wherein the pond of which the sand pile is employed as a wall is separate from the tail-ings pond.

12. The method of Claim 11 which includes the additional step of transferring water from the separate pond to the tail-ings pond.

13. The method of Claims 7, 8, or 9 which includes the additional step of pumping fluid from the substantially aqueous upper layer in the settling zone to the tailings pond.

14. The method of Claim 10 wherein the pond of which the sand pile is employed as a wall is the tailings pond.