CA1110950A - Destabilization of sludge with hydrolyzed starch flocculants - Google Patents

Abstract

DESTABILIZATION OF SLUDGE
WITH HYDROLYZED STARCH FLOCCULANTS

Abstract of the Invention Hydrolyzed corn and potato starches are effective as flocculants in destabilizing dilute as well as thick sludge suspensions. Potato starch flocculants are equal to, or better than, the synthetic polyacrylamide flocculants in destabilizing sludge suspensions, especially when clarity of the suspension is a significant consideration.
Among the potato starch flocculants which were found to be generally better than the corn starch flocculants, those containing A?PO4 were the best. Potato starch flocculants are equally effective on oil-removed and no-oil-removed sludge suspensions.

Description

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

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Athabasca tar sand is a thrcc-component mixture of bitumen, mineral and water. Bitumen is the value for the extraction of which tar sands are mi~ed and proccssed.
The bitumen content is variable, averaging 12 wt.~ of the deposit, but ranging from 0 to 18 wt.~. Water typically runs 3 to 6 wt.~ of the mixture, increasing as bitumen content decreases. The mineral content is relatively constant ranging from 84 to 86 wt.~.
Several basic extraction methods have been known for many years for separating the bitumen from the sands. In the so -called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water. The combined mass is then subjected to a pressure or gravity separation.
The hot water process for primary extraction of bitumen from tar sands consists of three major process steps (a fourth step, final extraction, is used to clean up the recovered bitumen for downstream processing.~ In the first step, called conditioning, tar sand is mix&d with water and heated with open steam to form a pulp of 70 to 85 wt.~ -solids. Sodium hydroxide or other reagents are added as required to maintain pH in the range 8.0 - 8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats (settles upward) to form a cohcrent mass known as froth which is recovered by skimming tlle settling vessel. A third stream may be withdrawn from the settling vessel. This stream, called the middlings drag stream, may be subjected to a third processing step, scavenging. This step provides incremental recovery of suspended bitumen and can be accomplished by conventional froth flotation.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms sand, silt, clay, and fines are used in this specification as particle si~e designations wherein sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh, but is larger than 2 microns, and clay is material smaller than two microns including some siliceous material of that size.
Conditioning tar sands for the recovery of bitumen consists of heating the tar sand/water feed mixture to process temperature (180-200F~, physical mixing of the pulp to uniform composition and consistency, and the consumption (by chemical reaction) of the caustic or other reagents added. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discrete droplets of a partic]e 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 clays which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the natural]y occurringaggregates 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 througllollt the pulp.

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Those skillcd in the art will thercforc understand that the conditioning process, which prcpares the resource (bitumen) for efficient recovery during the, following process steps also prepares the clays to be the most difficul~ to deal with in the tailings disposal operations.
The second process step, called separation, is actually the bitumen recovery step, (the separation having already occurred during conditioning). The conditioned tar sand pulp is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material,"screen oversize", is discarded. The screened pulp is further diluted with water to promote two settling processes: globules of bitumen, essentially mineTal-free, settle (float) upward to form a coherent mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly the sand size mincral, settle down and are removed from the bottom of the separation cell as tailings. The medium through which these two settling processes take place is called the middlings. Middlings consists primarily of water, with suspendcd fine material and bitumen particles.
The particle sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings. Characteristically, as the fines content rises above a certain thresllold (which varies according to the composition of the fines), viscosity rapidly achieves high values with the effect that the settling processes essentially stop. In this operating condition, the scparation cell is said to be "upset". Little or no oil is rccovered, and all streams exiting the cell have abou~ the same composition as the feed.
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As feed fines content increases, more water must he used in the process to maintain middlings viscosity within the operable range.
The third step of the hot water process is scavenging.
The feed fines content sets the process water requirement through the need to control middlings viscosity which, as noted above, is governed by the clay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation cell material balance, and this stream of middlings can be scavenged for recovery of incremental amounts of bitumen. Air flotation is an effective scavenging method for this middlings stream.
Final extraction or froth clean-up is usually accomplished by centrifugation. Froth from primary extraction is diluted with naptha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of an essentially pure (diluted) bitumen. Water and mineral removed from the froth constitute an additional tailing stream which must be disposed of.
In the terminology of extractive processing, tailings is the throwaway material generated in the course of extract-ing the valuable material from an ore. In tar sands processing, tailings consist of the whole tar sand ore body plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be subdivided into three categories; viz: (1) screen oversize,

(2) sand tailings (the fraction that settles rapidly), and

(3) tailings sludge (the fraction that settles slow]y).

Screen oversize is typically collected and handled as a separate stream.
Tailings disposal is all the operations required to place the tailings in a final resting place. One obvious long-range goal of tailings disposal is to replace the tailings in the mined out area in a satisfactory foTm.
Thus, there are two main operating modes for tailings disposal: (1) dike building-hydraulic conveying of tailings followed by mechanical compaction of the sand tailings fract;on; and (2) overboarding-hydraulic transport with no mechanical compaction.

Recently, in view of the high level of ecological consciousness in Canada and the ~nited States, technical interest in tar sands operation has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straightforward. ~isualize mining one cubic foot of tar sands. This leaves a one cubic foot hole in the ground. The ore is processed to recover the resource(bitumen) and the remainder, including both process material and the gangue constitutes the tailings; tailings that are not valuable and are to be dis~osed of. In tar sands processing, the main process material is water and the gangue is mostly sand with some silt and clay. Physically, the tailings consists of a solid part (sand tailings) and a more or less fluid part (sludge). The most satisfactory place to dispose of these tailings is, of course, the existing one c~lbic foot hole in the ground. It turns out, however, that the sand tailings from the one cubic foot of ore occupy just about one cubic foot. The amount of sludge is a s~ -variable, dcl)cnding on ore quality and process conditions, but may run up to 0.3 cubic feet. The tailings simply will not fit into the hole in the ground.
The historical literature coveringthe hot water process for the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation of liquid tailings or sludge would occur. Based on analysis of field test unit operations which led to the Great Canadian Oil Sands plant design near Ft. McMurray, Alberta, the existence of sludge accumulation was predicted. This accumulation came to be called the "pond water problem".
Observations during start-up and early commercial operations at Ft. McMurray (1967-69) were of insufficient precision to confirm the prediction. Since 1969, commerical operating data have confirmed the accumulation in GCOS' tailings disposal area of a layer of fine material and water ~sludge) which settles and compacts only very slowly, if at all.
At the GCOS plant, for dike building, tailings are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for a pool of liquid contained inside.
On the dike, sand settles rapidly, and a slurry of fines, water, and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond interior commences stratification in settling over a time scale of months to years. As a result of this long-term settling, two layers form. The top 5 to 10 feet of the pool are a layer of relatively clear water containing o to 5 wt.% solids. Below this clcar l~ater layer is a discontinuity in solids content. Over a matter of a few - feet, solids content increases to 10-15 wt.%, and thereafter, solids content increases reg~larly toward the pond bottom.
In the deepest parts of the pond, solid contents of over 50 wt.% have been recorded. This second layer is called the sludge layer. ~he solîds content of the sludge layer increases regularly from top to bottom by a factor of 4-5. The clay-water ratio in this layer increases also, but by a lower factor of 1.5 - 2.5. The clays, dispersed dur~ng processing, apparently have partially reflocculated into a very fragile gel network. Through this gel, fines of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process occur but their distinction is not as sharp as in dike building and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to form a gently sloping beach extending from the discharge point toward the pond interior. As the sand settles, fines and water drain into the pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals, (2) in the hot water extraction process, most of the clays become dispersed in the process streams and traverse the circuit, exiting in the tailings, (3) the amount of process water input is fixed by the clay content of the feed and the need to control viscosity of the middlings stream, ~) the amount of water required for middlings viscosity control represents a largc volume re~ative to the volume of the ore 3~ itself, and (5) upon disposal, clays settle only very very .

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slowly; tl-us, the process water componcnt of tailings is only partially availablc for reuse via recycle. That which can't be recyclcd rcpresents a net accumulation of ~ailings sludge.
The pond ~ater problem is then: to devise long-term economically and ecologically acceptable means to eliminate, minimize, or permanently dispose of, the accumulation of liquid tailings or sludge.
Flocculation of the drag stream in order to improve the settling characteristics theretohas been proposed and practiced in the prior art. In flocculation, individual particles (in this case clay particles) are united into rather loosely bound agglomerates or flocs. The degree of flocculation is controlled by the probability of collisions between the clay particles and their tendency toward adhesio after collision. Agitation increases the probability of collision and adhesion tendency is increased by the addition of flocculants.
Reagents act as flocculants through one OT a combination of threegeneral mechanisms: (1) neutralization of the electrical repu~sive forces surrounding the small particles which enables the vander 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-molecular-~eight polymers.
These polyelectrolytes are believed to act by adsorption (by ester formation or hydrogcn bonding) of hydroxyl or amide groups on solid surfaces, each polymcr chain bridging bet~een morc than one solid particle in the suspension.
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g ~mong the various reagents which have been found useful for flocculating clay are: aluminum chloride, polyalkylene oxides, such as polyethylene oxide, compounds of calcium such as calcium hydroxide, calcium o~ide, calcium chloride, calcium nitrate, calc;um acid phosphate, calcium sulfate, calcium tartrate, calcium citrate, calcium sulfonate, calcium lactate, the calcium salt of ethylene diamine tetraacetate and similar organic sequestering agents. Also useful are quar flour or a high 10 molecular weight acrylamide polymer such as polyacrylamide or a copolymer of acrylamide and a copolymerizable carboxylic acid such as acrylic acid. Additional flocculants which have been considered include the polymers of acrylic or methacrylic acid derivitives, for example3 acrylic acid, methacrylic acid, the alkali metal and ammonium salts of acrylic acid or methacrylic acid, acrylamide methacrylamide, the aminoaklyl acrylates, the aminoalkyl acrylamides, the aminoaklyl methacrylamides and the N-alkyl substituted aminoaklyl esters of ei'ther acrylic or methacrylic acids.
Those skilled in the art will understand that a satisfactory solution to the "pond water problem" must be economically, as well as ecologically acceptable. Despite the considerable attention which has been paid to the use of flocculants in the treatment of tailings from the hot water extraction process for tar sands, no flocculant, or flocculant family known in the prior art has been able to meet these fundamental criteria.

Objects of t}lC Invention _ It is therefore a broad objcct of our invention to providc an effective flocculating agent for treating tar sands tailing streams which carry suspended clay particles.
It isanother objcct of our invention to provide such a flocculating agent which is economical to prepare and employ in the treatment of tar sands tailing streams.
In another aspect, it is yet another object of our invention to provide such a flocculant which is safe and easy to handle and which itself offers no ecologically undesirable side effects.
It is a still further object of our invention to provide a flocculant which does not require the prior removal of oil to be effective in flocculating sludge suspensions within the tailing stream from a hot water bitumen extraction process.
Brief SuJn ary of the Invention_ _ ~ riefly, these and other objects of the invention are achieved by employing synthesized flocculants comprising starches. Starches are polysaccharides containing many monosaccharides joined together in long chains. Upon complcte hydrolysis by chemical or enzymatic means, starch yields monosaccharides. Ilydrolyzed corn and potato starches are effective as flocculants in destabilizing dilute as well as thick sludge suspensions. Potato starch flocculants are generally superior to corn starch flocculants, and those potato starch flocculants are equally effective on oil-removed and no-oil removed-sludge suspensions.

Descri~tion of thc Drawi~
Thc sub~ect matter o the invcntion is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to the manner in which the flocculants are prepared and the method of employing them, may best be understood by reference to the following description taken in connection with the drawing of which the single figure is a schematic representation of a hot water extraction process wherein the invention finds particular use.
_etailed Description of the Invention Referring now to the single f;gure, bituminous tar sands are fed into the system through a line 1 and pass to a conditioning drum or muller 18. Water and steam are introduced to the muller through another line 2. The total water so introduced in liquid and vapor form is a minor amount based on the weight of the tar sands processed. The tar sands, conditioned with water, pass through a line 3 to the feed sump 19 which serves as a zone for diluting the pulp with additional water before passage to the separation zone 20.
The pulp tar sands are continuously flushed from the feed sump 19 through a line 4 into a separator 20. The settling zone within the separator 20 is relatively quiescent so that bituminous froth rises to the top and is withdrawn via line 5 while the bulk of the sand settles to the bottom as a tailings layer which is withdrawn through line 6.
A middlings stream is withdrawn through line 7 to be processed as described below. Another middlings stream, h~

which is relatively oil-rich comllared to thc stream withdrawn through line 7, is withdrawn ~rom the cell via line 8 to a flotation scavenger zone 21. In this zone, an air flotation operation is conducted to cause the ~ormation of additional oil froth which passes from the scavenger zone through line 9 in mixture with the primary-froth from the separator 20 to a froth settler 22. An oil-lean water stream is removed from the bottom of the scavenger zone 21 through line 10 to be further processed as described below. In the settler zone Z2, some further oil-lean water is withdrawn from the froth and removed through line 11 to be mixed with the oil lean water stream from the flotation scavenger zone, the sand tailings stream from the separation zone and a portion o-f the lower middlings withdrawn from the separation zone. The bitumen from the settler is removed through line 12 for further treatment.
The oil-lean water from the froth settler, the scavenger zone, and the separator, and the tailings from the settler, all of which make up an effluent discharge stream, are treated in the sand separation zone 20 by, for example, a simple gravity setting process. The sand is withdrawn by a line 13 and discarded, and a process water stream is withdrawn by a line 14 to the flocculation zone 24.
In the flocculation zone 24, a substantial amount of clay suspended in the process water is coagulated~ and a slurry of coagulated clay and process water is withdrawn in line 15 to a centrifuge zone 25. In the centrifuge zone, coagulated clay is separatcd from the process water and discarded via line 16. ~ater substantially rcduced in clay and s~lnd con~ent com~ red to the effluent discharge is recovered from the centrifuge zone and is recycled by a line 17 to be mixed with fresh water and charged into the hot water process.
As previously discussed, a substantial amount of flocculants have been investigated and none are known to have been both effective and economical when used in treating tailings from the hot water process for extracting bitumen from tar sands. I-lowever, according to the present invention, it has been found that hydrolyzed starches synthesized from corn and potato starches can effectively meet these criteria. The major fraction of starch per se is water insoluble. To prepzre the hydrolyzed starch, a 20,000 ppm stock solution was prepared by refluxing a mixture of the starch and an aqueous so~ution containing the requisite amount of electrolyte. The hydrolysis was considered complete when the insoluble starch was converted into a clear colloidal solution. Henceforth in this specification, these hydrolyzed starches will be referred to as starch flocculants. A summary of the prepared starch f]occulants is given in Table 1 on the following page:

TA~LE 1. Sumrn~ry of prepared starct~ floccul~nts from corn an~ potato starches _____ _ ________ __ _ SMRL lype of St~rch Nature ~nd Concentration of Electrolyt~ A~ded L b. Flocculant _ _ _ __ .
1 Na starch 0.05 N NaOH
2 Ca starch 0.05 N C~(OH)2 3 AQ starch 0.10 N AQCQ3

4 Na AQ starch 0.0~ N ~aOH ~ 200 ppm AQ
Ca AQ starch 0.05 ~ Ca(OH2) + 200 ppm A~
6 Na AS,P04 starch 0.05 N ~'aOH + 200 ppm AQ ~- 20~ ~pm P04 7 Ca ~QPOIl starch 0.05 N Ca(OH2) i 200 ppm AQ + 200 ppm P04 8 AQP04 s~arch O.l N AQCQ3 + 200 ppm P04 A~ was a~ded using ~Q2(S04)3.18 H20 P04 was a~ded using 1~a3P011.12 ~2 In order to test the effectiveness of the synthesized starch flocculants, two sludge suspensions containing 5.5 and 17.3 wt.% solids, respect;vely, were employed. In addition, synthetic polyacrylamide f]occulants were used for co~nparative purposes. Test criteria used were:
reflitration rates, self-settling and sedimentation upon centrifugation at a relative centrifugal force of 790g at the bottom of the tube for 30 minutes. The results of reflitration tests and preliminary tests on self-set~ling indicated that the starch flocculants prepared from potato starch were superior to those prepared from corn st~rch; therefore, Table 2 presents only the sedimentation-upon-centrifugation studies done with potato starch floccu]ants.

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LE 2. Solids conccntration tn cakc and s-lpcrnatant upon scdimcntatiol~ by ccntrifugat;on using diffcrcnt flocculants.
__ _ 'Trcatment _ _ Flocculant Initial Solids Final Solids Conc,, %(\~/W) No Type Conccntrat;On Conc-, ~(W/~I) Cake Supcrnatant . _ _ _ _ Polyacrylamide Floccul ants 1 Nonc (untreated sludge) 17.3 1~2.~ 2.4 2 1820~(anionic) 200 ppm 17.3 39 9 1.1 3 573C (cationic) 200 ~ 17~3 37,7 1.7 lû 4 19n6N(non-ionic) 200 " 17.3 42.~ 2.4 Potato Starch Flocculants Na Starch 200 ppm 17.3 36,6' o.o 6 AQ starch 200 " 17.3 35.8 o,o 7 Na AQ starch 200 " 17.3 37.0 o.o Ca ~Q starch 200 " 17.3 36.3 0.0 9 Na AQP01~ starch 200 " 17.3 41.7 Ca ~QP04 starch 200 " 17.3 41.9 0.0 Il AQP04 starch 200 " 17.3 42.~ 0.0 .
, 12 None (untreated sludge) 5.5 35.4 0.4 13 Na AQ starch 200 " 5.5 36.o 0.2 14 Ca AQ starch 200 " 5.5 35.6 0.2 _ .

From the data set forth in table 2, it is evi.dent that the starch flocculants are decided].y superior to the polyacrylamide flocculants vis-a-vis the quality of the resultant supernatant. For those in which no flocculants were used or in which synthetic polyacrylamide flocculants were used, the supernatant had up to 2.4 wt.% solids in it, whercas the runs in which the starch flocculants were employed with a 17.3 wt.% slud~e concentration had no suspended solids in the supernatant at all. Among the starch flocculants, it appcars that those starchcs containing AQP04 were the best. ~urther, it was found that the starch flocculants are e~ually effective on no-oil-removed sludge as in treating oil-removed sludge whereas the polyacrylamide flocculants were more effective on oil-removed than on no-oil-removed sludge suspensions.
The fines contained in the sludge suspension associated with the hot water process for extracting bitumen from tar sands consists of primary, as well as secondary minerals.
. 10 Primary minerals, which are mostly quartz and some feldspars, have very low specific surface areas and little of any kind of charge. In contrast, the secondary minerals, which are mostly kaolinite and illite with some montmorillonite and intergrade mixed-layer minerals, have high specific surface areas and a substantial amount of negative charge.
There is also some positive charge, usually disposed at the edges of the crystals of solids.
As previously noted, starches are polysaccharides containing many monosaccharides joined together in long chains. Upon complete hydrolysis, by chemical or enzymatic means, starch yields monosaccharides. Starch consists primarily of two components: amylose and amylopectin. The amylose fraction makes up from 10 to 20% of the starch and is water soluble. The other portion, amylopectic, constitutes 80 to 90% of the starch and is water insoluble.
; The molecular weight of the starches varies from 10,000 to 1,000,000. The mechanism by which starch functions as a destabilizing agent for sludge appears to be one where the free hydroxyl groups of the starch attach themselves onto the surfaces of solid particles, probably through hydrogcn bonding. The clay particles with adsorbed starch polymcrs are then no longer able to attract water molecules as before, and hence, attract each other and are flocculated. The presence of electrolyte in the system enhances the effectiveness of the starch flocculants by reducing the repulsive forces between the electric double layers of the solid particles, thereby ma~ing it easier for the starch polymer to adsord and form a floc. The presence of phosphate is notably helpful, because the starch polymers can readily interlink through this radical.
It may be noted that potato starch contains 0.07 to 0.13%
phosphate and has generally been considered as a better flocculant than those starches containing no phosphate.
Further, starches with branched chains appear to be more effective than straight chain varieties.
While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately ohvious to those skilled in the art many modifications of structure, arrangement, proportions, the elements, materials and components used in the practice of the invention which are particularly adapted for speciflc environments and operating requirements without departing from those principles.

Claims (3)

WHAT IS CLAIMED IS:

1. In an aqueous process for separating oil from bituminous sands comprising the steps of:

(a) forming a mixture of bituminous sand and water;

(b) passing the mixture into a separation zone;

(c) settling the mixture in the separation zone to form an upper oil froth layer; a middlings layer comprising oil, water, and clay; and a lower sand tailings layer;

(d) withdrawing separate streams from the oil froth layer; the sand tailings layer; and the middlings layer;

(e) collecting an effluent discharge comprising the effluent from the sand tailings layer and the effluent from the middlings layer; and f) adding a flocculating reagent to the effluent discharge; whereby finely divided minerals including clay settle into a lower sludge layer within a storage zone for the effluent discharge;

the improvement wherein the flocculating reagent employed is selected from the group consisting of hydrolyzed corn starches and hydrolyzed potato starches.

2. The process of Claim 1 in which the improvement comprises selecting the flocculating reagent from the group of hydrolyzed corn and potato starches comprising: Na starch, Ca starch, Al starch, Na Al starch, Ca Al starch, Na AlP04 starch, Ca AlP04 starch, and AlP04 starch.

3. The process of Claim 2 in which the concentration of the flocculating reagent in the effluent discharge is controlled to not exceed 200 parts per million.