Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Back~round of the Invention
This invention relates to the hot ~.ater process for
treating bituminous sands, such as Athabasca tar sands,
and, more particularly, to the treatmcnt of the water
and clay-containin~ efflucnt discharlged from the process.
Tar sands (~hich are also known as oil sands and
bituminous sands) are sand deposits which are impregnatcd
with dense, viscous petroleum. Tar sands are found through-
out the world, often in thc same geographical area asconventional ~etroleum. The lar~est deposit, and thc only
one of present commercial importance, is in the Athabasca
arca in the northeast of the Province of Alberta, Canada.
This deposit is bclieved to contain over 700 billion
barrels of bitumen For comparison, this is just about
equal to the ~orld-wide reserves of conventional oil, Sn%
of which is found in the middl~ east.
330~
Athabasca tar sand is a thrce-component mixture of
bitumen, mineral and water. Bitumen is the value for the
extraction of which tar sands are mined and proccssed.
The bitumcn content is variableJ 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 serarating 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 subjectcd to a press~lre
or gravity separation.
The hot water process for primary e~traction of bitumen
from tar sands consists of three major process steps (a
fourth step, final extraction, is used to clean up the
recovered bitumcn for do~nstream processing.) In the first
step, called conditiolling, tar sand is mixed with ~ater and
heate~ with ol~en steam to form a pulp of 70 to 85 ~it.%
solids. Sodium hydroxidc or other reagents are added as
required to maintain pl-l in the range 8.0 - 8.5. In the
second stcp, called sepclration, the con~itioned pulp is
diluted further so that settling can take place. l`he bulk
of the sand-size mineral rapidly settles and is withdrawn
as sand tailings. ~ost of the bitumen rapidly floats
(settles upward) to form a coheTent mass known as froth which
is reco~-ered b~ skimming the scttling vcsscl. A thir~
stream may be withdrawn from the settling vessel. This
stream, callcd the middlin~s drag strcam, may be subjected
to a third processing step, scavcnging. This step l~rovides
incremental rccovery of suspendcd bitumcn and can bc
accomplished by conventional froth flotation.
The mineral particle sizc distribution is particularly
significant to operation of the hot watel process and to
sludge accumulation. The terms sand, silt, clay, and
fines are used in this specification as particle size
designations wherein sand is siliceous material ~hich will
not pass a 325 mesh screen. Silt will pass 325 mcsh, but
is larger than 2 microns, and clay is material smaller than
two microns including some siliceous material o~ that si~e.
Conditioning tar sands for the recovery of bitumen
consists of heating the tar sancl/~.ater ecd mixture to process
ten-perature (lS0-20nF), physical mixinS of the pulp to
uniform composition and consistency, and the consumption
(by cllemical reaction) of the caustic or other reagents
added. Under these conditions, ~itumen is strip~ed from the
individual sand grains and mi~ed into the pulp in the form
oE discretc droplets of a partic]e si~e on the same order
as that o tl~e sand grains. The same proccss conditions,
it turns out, are also ide~l for accomplishing
deflocculation of the cla~-s ~hich occur naturally in the tar
sand feed. Deflocculation, or dispcrsion, mcans brca~ing
down thc natural]y occurringaggregates of clay particlcs
to producc a slurry of individual particlcs. Thus, during
conditioning, a largc fraction of the clay particles bccome
wcll dispcrscd and mixcd througllollt thc pu]p.
309
Those skilled in the art will thercfore understand that
the conditioning process, which prepares the resource
(bitumen) for efficient recovery d~lring the~ following process
steps also prepares the clays to be the most difficult to
deal with in the tailings disposal operations.
The second process step, called separation, is actually
the bitumen recovery step, (the separation having already
occur~ed during conditioning). The conditioned tar sand
pul~ is screened to remove rocks and unconditionable lumps
of tar sands and clay. The reject material,"screen oversize",
is discarded. The screcned pulp is further dilutc~ with
water to promot~ two settling processes: globules of bitumen,
essentially mineral-free, settle ~float) u~ ard 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 mineral, settle down and are removed from the bottom
of the separation cell as tailings. The medium through which
thesc two settling processes ta~e place is called the
middlings. ~liddlings consists primarily of l,~ater, ~ith
suspendcd fine material and bitumen particles.
The particle si es and densitics of the sand and of the
bitumen particlcs are rclatively fi~ed. Thc parameter
whicll in~lucnces the settlillg l)rocesses most is the viscosit~
of thc middlings. Charactcristically, as the fines content
rises above a certain threshold ~ hich varies according
to t11e com~osition of the fines), viscosity rapidly acilieves
high values ~ith the cffcct that tlle settling processes
essentially stop. In this oper~ting condition, the separation
cell is said to be "upset". ~ittle or no oil is rccovered,
and all streams c~iting the ccll havc about the same composition
as the fecd.
~ 3 0 ~ i
As feed fines content increases, more water must be
used in the process to maintain middlings viscosity within
the operable range.
The third step of the hot water process is scavenging.
The feed fines content sets the process water requirement
through the need to control middlings viscosity which, as
noted above, is go~erned by the clay/water ratio. It is
usuall~ necessary to withdraw a drag stream of middlings
to maintain the se~aration cell material balance, and
this stream of middlings can be scaYenged 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 throwa~a) ~aterial 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
~-ody plus net additions of process water less only the
recovered bitumen product. Tar sand tailings can be
subdivided into three categories; viz: (l) screen oversize,
(2) sand tailings ,~the fraction that~ settles rapidly), and
(3) tailings sludge ~the fraction that settles slowly).
.
., ~ . , ! .. .. ..
30g
Screen oversi~e is typically collectcd and handled as a
separate stream.
Tailings disposal is all the operations required to
place the tailings in a final resting place. Onc obvious
long-range goal of tailings disposal is to replace the
tailings in the mined out area in a satisfactory forrn.
Thus, there are two main opcrating modcs for tailings
disposal: (1) dike building-hydraulic conveying of tailings
followed by mechanical compaction of the sand tailings
fractionj and ~2) overboarding-hydraulic ~ransport with no
mechanical compaction.
Recently, in view of the high level of ecological
consciousncss in Canada and the United States, technical
interest in tar sands operation has begun to focus on
tailings disposal. The concept of tar sands tailings disposal
is straightforward. Visuali-e mining one cubic foot of
tar sands. This lea~res a one cubic foot hole in the
ground. The ore is processed to reco~Jer the resource~bitumen)
and the remainder, including both process matcrial and the
gangue constitutes the tailings; tailings that are not
valuable and are to be disposed of. In tar sands processing,
thc main process material is water and the gangue is mostly
s~nd with soole silt and clay. Physically, the tailings
consists o a solid part (sand tailings) and a more or less
fluid part ~sludge). The most satisfactory placc to
dispose of these tailillgs is, of course, the existing one
cubic foot hole in the groulld. It turns out, however, that
the sand tailings from the one cubic foot of ore occupy
just about one cubic foot. The amount of sludgc is a
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33~)9
variable, de~endin~ on ore quality and process con~itions,
but may run up to 0.3 cubic feet. The tailings simply
will not fit into the hole in the ground.
The historical literature coverin~the hot water process
for the recovery ol bitumen from tar sands contains
little in the way of a recognition that a net accumulation
of liquid tailings or slud~e would occur. Based on analysis
of field test unit operations which led to the Great Canadian
Oil Sands plant design near Ft. ~Ic~lurray, 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. ~Ic~lurray (1967-6~) ~ere 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 (slud~e)
which settles and compacts only ~!ery slowly, if at all.
~ t the GCCS plant, for dike building, tailillgs al~e
conveyed hydraulically to the disposal area and discharged
onto the top of a sand dike which is constr~lcted to serve
as an impoundment for a pool oE liquid contained inside.
On the di~e, sand settles rapidly, and a slurr) of fines,
hat~r, and minor amounts of bitumen ~lows into the pond
interior. The settled sand is mecllanically compacted to
build the dike to a higher level. The slurry which drains
into the pond interior commences stratification ill settling
over a time scale of mont}ls to years. As a result of this
long-term settling, two layers form. Thc top 5 to 10 feet
of the pool are a layer of relatively clear wrater containin~
30 0 to 5 wt. r/ sol i~s. Bclow this clcar water layer is a
11;~;3309
discontinuity in solids content. Over a matter of a few
feet, solids content increases to 10-15 wt.~, and thereafter,
solids content increases regularly toward the pond bottom.
In the deepest parts of the pond, solid contents of over
50 wt.% have been recorded. This second layer is
called the sludge layer. The solids content of the sludge
layer increases regularly from top to bottom by a factor
of 4-5. The clay-water ratio in this layer increases also,
but by a lo~er factor of 1.5 - 2.5. The clays, dispersed
during 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 whic]l tailings are
discharged over the top of the sand dike directly illtO 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 carri~d out. The sand portion
of the tailings settles rapidly to form a gently sloping
beach extending from the discharge ~oint to~ard the pond
2G interior. As the sand settles, fines and water drain into
the pool and commence long-term settling.
In summary: ~1) tar sands cont~in clay minerals, ~2)
in tl~e hot water e~traction ~rocess, most o the clays
become dispersed in the process streams and tra-erse the
circuit, exiting in the tailings, ~3) the amoullt of process
water input is fixed by the clay content of the feed and
the need to control viscosit~ of the middlings stream,
~4) the amount of l~ater required for middlings viscosity control
represents a large volunle re~ative to the volume of the ore
itself, and (5) u})on dis~osal, clays settle onl~ very very
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3309
slowly; thus, the process water component of tailings is only
partially available for reuse via recycle. That which
can't be recycl~d re~resents a net accumulation of tailings
sludge.
The pond ~ater problem is then: to devise long-term
economically and ecolo~ically 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
bett~een the clay particles and tlleir tendency to-~ard adhesion
after collision. Agitation increases the probability of
collision and adhesion tendency is increased by the additio
of floccu]ants.
Reagents act as flocculants through one or a combination
of threegeneral mechanis~ls: ~1) neutralization o the
electrical repulsi~e forces surrounding the small particles
whicll ena~les tl~e ~ander ~'aals collesive force to hold the
particles togetiler once the~ have collided; (2) precipitation
of voluminous flocs, such as metal hydroxides, that entrap
fine particles; and t3) bridgin~ of particles by natural
or synthetic, long-chain, high-molecular-~eight polymers.
T}lese polyelectrolytes are believed to act by adsorption
(by ester formatioll or hydroRen bonding) of hydroxyl or amide
groups on solid surfaces, each poly]l~er chain bridging
hetl~een more tl~an one solid particle in the suspension.
3 3~9
Amon~ the various reagents which have been found
useful for flocculating clay are: aluminum chloride,
polyalk~lcne o~idcs, sucll as polyethylenc oxide, compounds
of calcium such as calcium hydroxide, calcium oxidc,
calcium chloride, calcium nitrate, calcium acid phosphate,
calcium sul~ate, calcium tartrate, calcium citrate,
calcium sulfonate, calcium ]actate, the calcium salt of
ethylene diamine tetraacetate and similar or~anic
sequestering agents. Also useful are quar flour or a high
molecular weight acrylamide polymer such as polyacrylamide
or a copolymcr of acrylamide and a copolymerizable
carbo~ylic acid such as acrylic acid. Additional flocculants
which ha~e been considered include the polymers of acrylic
or methacrylic acid derivitives, for e~ample, acrylic acid,
mcthacrylic acid, the alkali metal and ammonium salts of
acrylic acid or metllaclylic acid, acTylamide methacrylamide,
the aminoaklyl acrylates, the aminoal~yl acrylamides,
the aminozklyl methacrylamides and the N-al~yl substituted
aminoaklyl estcrs of either acrylic or methacrylic acids.
Those s~illed in the art ~ill understand that a satisfactory
solution to the "pond ~ater problem" must be economically,
as ~e].l as ecologically acce~tablc. Desp;te the considerable
attclltion ~ ich has becn paicl to the llse of flocculants
in the treatmcnt of tailillgs from the IIOt water e~traction
process for tar sands, no flocculant, or flocculant family
kno-m in the ~rior art has been able to mcet thcse
fundamental criteria.
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3309
Objects of the Invcntion
.
It is thereforc a broad object of our invention to
provide an effectivc flocculating agcnt for treatin~ tar
sands tailing streams w}lich carly suspended clay particles.
I~ isanother object of our invention to provide such
a flocculatin~, agent which is economical to prepare and
employ in the treatnlent of tar sands tailin~ streams.
In another aspect, it is yet another object o~ our
invention to provide such a flocculant ;hich is safe and
easy to handle and which its~lf offers no ecologically
undesirable side e~fects.
It is a still further object of our invention to
pro~ide a flocculant ~-hich dces not require the prior removal
of oil to be effective in flocculating sludge suspensions
within the tailing stream from a hot l~atcr bitumen
extraction process.
~rief Sur"mar~ of the ~n~ention
Briefly, these and other objects of the invention are
achieved by elnplo~ing synthesi~.ed flocculants comprising
starches. Starches are polysaccllarides containing many
mono.saccllarides joincd together in long chains, ~pon
cor,~l)lcte h)~rolysis b~ chemical or en~ymatic mcans, starch
~-ie]ds monosaccharides. Ilydroly~ed COI`II and potato
starc~cs arc e~fectivc ~s flocculants in destabili,ing
d]lute as l~ell as thicX sllldge suspensions. Potato starch
flocculants are gencrally superior to corn starch flocculants,
and those potato st:arch ~locculants are equally effective
on oil-removc~ and no-oil removed-sludge suspension~s .
3309
Dcsc_ ~tion of thc nrawin~
Thc sub~cct matter of the invcntion is particularly
pointed out and distinctly claimed in the concluding
portion of the spccification. The invention, however,
both as to the mallner in which the flocculants are prepared
and the method of employing them, may best bc understood
by reference to thc follo~in~ dcscriptiorl taken in
connection ~ith the dra~ing of which the single figure is
a schematic representation of a hot water extraction process
whcrein the invention finds particular use.
Detailed Description of the In~ention
Referring now to thc single figure, bituminous tar sands
are fed into the system through a line 1 and pass to a
conditioning drum or muller 18. Water and steam are introduced
to the mullcr through another line 2. The total ~ater so
introduced in liquid and vapor form is a minor amount based
on the weight of the tar sand~ proccssed. The tar sands,
conditioncd ~ith water, pass through a line 3 to the feed
sump 1~ which servcs as a ZOIIC for diluting the pulp Wit]l
additional ~ater bcrore ~assage to the separation zone 20.
The pulp tar sands are continuously flushed from the
~ced sump 19 tlllollgll a line 4 into a separator 20. The
settling zone within thc separator 20 is relatively
quiescent so that bituminous froth rises to the top and
;s witlldra~n via line 5 ~hile thc bulk of the sand settles
to t11e bottom as a tailings layer which is withdrawn through
line 6.
A middlings strealn is .~ithclrawn through linc 7 to be
processed as describcd belo~.~. Anothcr middlings strcam,
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33Qg
which is relatively oil-rich compared to thc stream
withdrawn throu~h line 7, is withdrawn from the cell
via line 8 to a flot~tion scavenger zone 21. In this
zone, an air flotation operation is conducted to cause
the formation of additional oil froth which passes from the
scaYenger 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 remo~ed from the bottom of the
scavenger zone 21 through line 10 to be further processed
as described below. In the settler zone 22, some further
oil-lean water is withdral.n 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 of 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
-one, and the scparator, and the tailings from the settler,
all of ~hich ]na~e up an effluent discharge stream, are
treated in the sand separation zone 20 by, for e~ample,
a simple gravit~ s~ttil~g process. The sand is withdrawn
by a line 13 and discarclcd, and a process wAter stream is
withdrawn by a line 14 to thc 1Occulation 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 ~a~er and
discarded via line 16. Watcr substantially rcduced in clay
330~
and sand content comp;lrcd to the efflucnt dischar~e is
recovered from the centrifuge 7One ancl is recycled hy a
line 17 to be mixecl with fresh water ancl charged into the
hot water process.
As previously cliscussed, a substantia] amount of
flocculants havc been invcstigated and none are known to
have been both effecti~~e and economical when used in
treating tailings from the hot water process Eor ex~racting
bitumen from tar sands. Ho~cver, according to the present
invention, it has been found that h~drolyzed starches
synthesized from corn and potato starches can effectively
meet these criteria. The major fraction of starch per se
is ~ater insoluble. To prep?.re the h~droly~ed starch,
a 20,000 ~pm stock solution ~as prepared by reflu~ing
a mi~ture of the starch and an aqueous solution containing
the requisite amount of electrolyte. The hydrolysis was
considered complete ~hcn the insolublc starch was con~erted
into a clear colloidal solution. Henceforth in this
specification~ these l-)drol~ed starches will be referrecl
to as starcll flocculallts. A sumTnaly of the preparecl starch
flocculants is gi~en in T~ble 1 on the following page:
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11~3309
TABLE 1. Sul[rnary of prepared starch flocculants from corn and
potato starches
SMRL Type of Starch Nature and Concentration of
Lab. Flocculant Electrolyte Added
Na starch 0.05 N NaOH
2 Ca starch 0.05 N CatOH)2
3 AQ starch 0.10 N A~C~3
4 Na AQ starch 0.05 N NaOH ~ 200 ppm AQ
Ca AQ starch 0.05 N Ca(OH2) + 200 ppm AQ
Na AQPO4 starch 0.05 N NaOH + 200 ppm AQ + 200 ppm PO4
7 Ca AQPO4 starch 0.05 N Ca(OH2) ~ 200 ppm AQ +200 ppm PO4
8 AQPO4 starch 0.1 AQCQ3 ~ 200 ppm PO4
* AQ was added using AQ2(SO4)3.1~ H2O
** PO~ was added using Na3PO4.12 H20
Thus, in accordance with the present teachings, a
composition is provided which comprises a hydrolyzed corn or
potato starch which is obtained by aqueous hydrolysis of starch
in the presence of insoluble metal salts formed in situ. The
salt which is employed may contain calcium, aluminum and
phosphate ions with the salt being AlPO4 being preferred.
In order to test the effectiveness of synthesized
starch flocculants, two sludge suspensions containing 5.5
and 17.3 wt. % solids, respectively, were employed. In
addition, synthetic polyacrylamide flocculants were used
for comparative 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-settling
indicated that the starch flocculants prepared from potato
--15--
33~9
starch were superior to those prepared from corn starch;
therefore~ Table 2 presents only the sedimentation-upon-
centrifugation studies done with potato starch flocculants.
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3309
'eLE 2. Sollds conccntration In cakc and supcrnatant upon scdimcntation by
ccntrifugation using diffcrcnt flocc-llants.
.
Trcatment Flocculant Initlal Solids Final Solids Conc., ~(W/W)
Typc Conccntrat;~n Conc., ~(W/~ Cake Su~crnatant
Po!yacrylamldc_Flocculants
1 Nonc (untrcatcd sludgc~ 17.3 1I2.1 2.4
2 1820A(anionic) 200 ppm 17.3 39.9 1.1
3 573C ~cationic) 200 " 17~3 37.7 1.7
4 19n6N(non-ionic) 200 " 17.3 42.9 2.4
Pota~o Starch Floccu!ants
Na Starch 200 ppm 17,3 36,6 o.o
6 AQ starch 200 " 17.3 35.8 0,o
7 Na A~ starch 200 " 17.3 37.o o.o
~ Ca ~ starch 200 " 17.3 36.3 o~o
9 Na A~P04 starch 200 " 17.3 41.7 o,o
Ca ~QP04 starch 200 " 17.3 41.~ o,o
11 AQP04 starch 200 " 17.3 42.9 o.o
12 None (untreated sludge) 5.5 35,4 o.4
13 Na A~ starch 200 " 5.5 36.o 0.2
14 Ca A~ starch 200 " 5.5 35.6 0.2
,
From thc data sct forth in table 2, it is evident that the
starch Elocculants are decide~]y superior to the polyacrylamide
flocculants vis-a-vis the quality of the resultant
supernatant. ~or thosc in which no flocculants ~erc used
or in which synthetic polyacrylamide flocculants were used,
the su~ernatant had up to 2.4 wt.~ solids in it, wl~ereas
the runs in which the starch flocculants were emp]oyed with
a 17.3 wt.~ sludge concentration had no suspended solids
in the supcTnatant at all. Among the starch flocculants,
-16-
30g
it appcars that those starches containing A2P04 were thc
best. Further, it was found that thc starch flocculants
are c~lally cffective on no-oil-remov~d sludge as in treating
oil-removed sludge whereas the polyacrylamide flocculants
were more effective on oil-removed than on no-oil-removed
slud~e 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.
Primary minerals, ~hich 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 ~aolinite and illite with some montmorillonite
and intergrade mi~ed-layer minerals, have high specific
surface areas and a substantial amount of negative charge.
There is also some positive charge, usually disnosed at
the edges of the crystals of solids~
As previousl~ noted, starches are polysaccharides
containing many monosaccharides joined together in long
chains. Upon complete hydrolysis, b~ chemical or en7ymatic
means, starch )ields monosacc}larides. Starch consists
primarily of two components: amylose and amylol)c~ctin. The
am)~lose flaction m~kes up from ]0 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 weig]lt of the starches varies from 10,000
to 1,000,000. Thc mechanism by which starch functions as
a destabilizin~ agent for sludgc appears to be one where
the free hydroxyl groups of the starch attach themselvcs
onto the surfaces of solid particles, probably throu~h
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~ 330~
hydrogen bonding. The clay particles with adsorbed
starch polymers are then no longer able to attract water
molecules as before, and hence, attrac~ each other and
are flocculat~d~ 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 thè 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 ~han those starches containing no phosphate.
Further, starches ~ith branched chains appear to be more
effective than straight chain varieties.
While the principles of the in~ention have now been
made clear in an illustrative embodiment, there ~ill be
immedia~ely obvious to those skilled in the art many
modifications of structure, arrangement, proportions, the
elements, materials and componen~s used in the practice
of the in~ention which are particularly adapted for
specific environments and operat;ng retluirements Wit]lOut
departing from those principles.
