Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
DESTABILIZATION AND IMPROVEMENT IM
PERMEABILITY AND SYEAR STRRNGTH IN SLUDGE TREATED
WIT~ HYDROLYZED STA2CH FLOCCULANT AND PORTLAND CE~ENT
~AC~GROUND 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 fines-
containing effluent discharged from the process.
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 area in the northeast
of the Province of Alberta, Canada. This deposit is believed
to contain perhaps 700 billion - 1 trillion barrels of bitumen.
For comparison, 700 billion barrels is just about equal to
the world-wide reserves of conventional oil, 60~ of which is
found in the Middle East.
~f~
Athabasca tar sand is a three-component mixture of
bitumen, mineral and water. Bitumen is the value for the
extraction of which tar sands are mined and processed.
The bitumen content is variable, averaging 12 wt.% of
the deposit, but ranging from 0 to 18 wt.%. Water typically
runs 3 to 6 wt.% of the mixture, increasing as bitumen
content decreases. The mineral content is relatively
constant ranging from 84 to 86 wt.%.
Several basic extraction methods have been known for
many years for separating the bitumen from the sands. In
the so-called "cold water" method, the separation is
accomplished by mixing the sands with a solvent capable of
dissolving the bitumen constituent. The mixture is
then introduced into a large volume of water, water with
a surface agent added, or a solution of a neutral salt in
water. The combined mass is then subjected to a pressure
or gravity separation.
The hot water process for primary extraction of 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 mixed with water and
heated with open steam to form a pulp of 70 to 85 wt.%
solids. Sodium hydroxide or other reagents are added as
required to maintain pH in the range 8.0 - 8.5. In the
second step, called separation, the conditioned pulp is
diluted further so that settling can take place. The bulk
of the sand-size mineral rapidly settles and is withdrawn
as sand tailings. Most of the bitumen rapidly floats
(settles upward) to form a coherent mass known as froth which
~ L$~ 3
is recovered by skimming the 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 ac-
complished 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 size designations
wherein sand is siliceous material which will not pass a 325
mesh screen. Silt will pass 325 mesh, but is larger than
2 microns, and clay is material smaller than two microns in-
cluding some siliceious material of that size.
Conditioning tar sands for the recovery of bitumen
; consists of heating the tar sand/water feed mixture to pro-
cess 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 indiv-
idual sand grains and mixed into the pulp in the form of dis-
crete droplets of a particle size on the same order of 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. Defloccu-
lation, or dispersion, means breaking down the naturally
occurring 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 thoroughout the pulp.
v ~
Those skil]ed in the art wlll therefore understand
that the conditioning process, which prepares the resources
(bitumen) for efficient recovery during the following
process steps also prepares the clays to be the most clifficult
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 ancd 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 mineral-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 mineral, settle down and are removed from the
bottom of the separation cell as tailings. The medium through
whieh these two settling processes take place is ealled the
middlings. The middlings eonsists primarily of water, with
suspended fine material and bitumen partieles.
The partiele sizes and densities of the sand and of the
bitumen partieles are relatively fixed. The parameter whieh
influenees the settling proeesses most is the viseosity
of the middlings and viseosity is directly related to fines
eontent. Charae~eristieally, as the fines content rises above
a eertain thresholcl (whieh varies aeeording to the eomposition
of the fines), middlings viscosity rapidly reacnes high values
with the effect that the settling processes essentially stop.
In this operating condition, the separation cell is said to
be "upset". Little or no oil is recovered, and all streams
exiting the eell have about the same composition as the feed.
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 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 in-
cremental amounts of bitumen. Air flotation is an effec-
tive scavenging method for this middlings stream.
Final extraction or froth clean-up is typically ac-
complished by centrifugation. Froth from primary extrac-
tion is diluted with naptha, and the diluted froth is then
subjected to a two stage centrifugation. This process
yields an oil product of an essentailly pure (diluted)
bitumen. Water and mineral removed from the froth, during
this step 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 ex-
tracting 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 sub-
divided into three categories; viz: (1) screen oversize,
(2) sand tailings (the fraction that settles rapidly),
and (3) tailings sludge (the fraction that settles slowly).
Screen oversize is typically collected and handled as a
separate stream.
, " ~
~$~
Tailings dlsposal is all the operations required to
place the t~.ilings 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 form. Thus,
in tar sands processing, there are two main operating modes
for tailings disposal: (1) "dike building" which is hydraulic
conveying of tailings followed by mechanical compaction of the
sand tailings fraction; and (2) "overboarding" which is
hydraulic transport with no mechanical compaction.
~ecently, in view of the high level of ecological
consciousness 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. Visualize mining one cubic foot of
tar sands. Thisleaves a one cubic foot hole in the ground.
The ore is processed to recover the resource (bitumen)
and the remainder, including both process material and the
gangue, constitutes the tailings; tailings that are not
valuable and are to be disposed of. In tar sands processing,
the main process material is water, and the gangue is mostly
sand with some silt and clay. Physically, the tailings consists
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 cubic foot hole in
the ground. It turns Ollt, however, that the sand tailings from
the one cubic foot of ore occupy just about one cubic foot.
The amount of sludge is a variable, depending on ore quality
and process conditions, but may run up to 0.3 cubic feet.
4~
The tailings simply will no~ fit into the hole in the
ground.
The historical literature covering the 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 (1967-1969) were of insufficient precision to
confirm the prediction. Since 1969, commercial 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 after
a few years.
At the GCOS plant, for dike building, tailings are
conveyed hydraulically to the disposal area and discharged
onto the top of a sand dilce which is constructed to serve
as an impoundment for a pool of liquid contained inside.
On the dike, the 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
0 to 5 wt. % solids. Below this clarified water layer is a
discontinuity in solids content. Over a matter of a few
feet, solids content increases to 10-15 wt.%, and thereaf-
ter, 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 lower factor of 1.5 -2.5. The clays, dispersed during
processing, apparently have partially reflocculated into a
very fragile get network. Through this gel, fines of lar-
ger-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 occurs 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 be-
come dispersed in the process streams and traverse the cir-
cuit, 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, (4) the
amount of water required for middlings viscosity control
represents a large volume relative to the volume of the ore
itself, and (5) upon disposal, clays settle only very very
~' .
slowly; thus, the process water component of tailings is
only partially available for reuse via recycle. That which
can't be recycled represents a net accumulation of tailings
sludge.
The pond water problem is then: to devise long-term
economically and ecologically aeceptable 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 thereto has been proposed and
practiced in the prior art. In flocculation, individual
particles (in this case clay particles) are united into
rather lossely bound agglomerates or flocs. The degree of
flocculation is controlled by the probability of collisions
between the clay particles and their tendeney toward adhesion
after collision. Agitation inereases the probability of
eollision and adhesion tendeney is inereased by the addition
of floeeulants.
Reagents aet as floeeulants through one or more of
three general meehanisms: (1) neutralization of the elee-
trieal repulsive forees surrounding the small partieles
whieh enables the van der Waals eohesive foree to hold the
partieles together onee they have eollided: (2) preeipita-
tion of voluminous floes, sueh as metal hydroxides, that en-
trap fine partieles; and (3) bridging of partieles by natural
or synthetie, long-ehain, high-moleeular-weight polymers.
These polyeleetrolytes are believed to aet by absorption
(by ester formation or hydrogen bonding) of hydroxyl or amide
groups on solid surfaees, eaeh polymer ehain bridging be-
tween more than one solid partiele in the suspension.
Among 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 oxide,
calcium chloride, calcium nitrate, calcium 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
molecular weight acrylamide polymer such as polyacrylamide
or a copolymer of acylamide 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 example, acrylic acid,
methacrylic acid, the alkali metal and ammonium salts of
acrylic acid or methacrylic acid, acrylamide methacrylamide,
the aminoakyly acrylates, the aminoalkyl acrylamides,
the aminoakyl methacrylamides and the N-alkyl substituted
aminoakyly esters of either acrylic or methacrylic acids.
Those skilled in the art will understand that a satis-
factory solution to the "pond water problem" must be economically,
as well as ecologically, acceptable, and the above listed
flocculants fail to meet this fundamental criteria when employed
in the treatment of tailings from the tar sands hot water
extraction process. However, a distinct step forward in
the art was achieved by the use of hydrolyzed starch flocculants
as set forth in copending Canadian patent application
Serial Number 275,214, filed March 31, 1977~ and entitled
Destabilization of Sludge with Eydrolyzed Starch Flocculants.
--10--
As discussed in detail therein, starches are polysaccharides
containing many monosaccharides joined together in long chains.
Upon complete hydrolysis by chemical or enzymatic means, starch
yields monosaccharides. Hydrolyzed corn and potato starches
are effective as folcculants in destabilizin~ dilute as well
as thick sludge suspensions. Potato starch flocculants are
generally superior to corn starch f]occulants, and the potato
starch flocculants are equally effective on oil-removed
and no-oil removed-sludge suspensions.
It has been found that the use of starch flocculants is
very effective in accelerating the rate at which clay and clay-
like silt settles. However, the ultimate extent to which the
sludge layer may be compacted does not appear to be significantly
superior to the compaction observed to take place over a
longer period of time without treatment by a flocculant.
Restated, the solids concentration, shear strength and
permeability of the sludge layer, given sufficient settling
time, is substantially the same whether or not a flocculant,
including starch flocculant, is employed. Thus, those skilled
in the art will appreciate that it would be highly desirable
to provide means by which clays and clay-like silt fines in
- a tailings pond may be rapidly settled to obtain a sludge
layer characterized by increased shear strength and increased
permeability beyond that heretofore obtained.
O~JECTS OF THE INVENTION
It is therefore a broad object of this invention to
provide an effective flocculating agent for treating tar sands
tailings streams which carry suspended clay particles.
It is another object of this invention to provide such
a flocculating agent which is economical to prepare and employ
in the treatment of tar sands tailings streams.
-11 -
In another aspect, it is an object of this invention
to provide such a flocculating agent which is safe and easy to
handle and which itself offers no ecologically undesirable side
effects.
It is a more specific object of this invention to
provide a flocculating agent combination comprising starch
flocculant and cement, the use of which is prescribed portions
results in a rapidly settled sludge layer having improved shear
strength and permeability characteristics.
Briefly, these and other objects of the invention are
achieved by adding Portland cement, preferably in the form of
a dilute slurry, in a concentration on the order of 3.6 pounds
per hundred Imperial gallons of normalized sludge in conjunction
with the addition of starch flocculant within the range of 0.1
lbs. 0.5 lbs. per hundred Imperial gallons.
Now, and in accordance with the present teachings, in
a tar sands tailings pond system for receiving affluent from an
industrial process, a method is provided of obtaining a sludge
layer in the tailings pond which had improved shear strength
and permeability characteristics and which includes the steps
of adding a potato starch flocculant to the effluent and adding
Portland cement to the effluent at a dosage rate of at least
3.0 pounds per lO0 Imperial gallons of normalized sludge dis-
charged into the tailings pond, which normalized sludge is de-
fined as a fluid which has approximately an average of 20%
(w/v) solids content after settling into the sludge layer.
In accordance with a further embodiment, an improve-
ment is provided in an aqueous process for separating oil from
bituminous sands which includes the steps of forming a mixture
of bituminous sand and water; passing the mixture into a separa-
tion zone; settling the mixture in the separation zone to form
an upper oil froth layer, a middlings layer comprising oil,
',~
-12-
t~3
water and clay, and a lower sand tailings layer; withdrawing
separate streams from the oil froth layer, the sand tailings
layer and a middlings layer; collecting an effluent discharge
comprising the effluent from the sand and tailings layer and
the effluent from the middlings layer; and adding a flocculating
agent to the effluent discharge whereby finely divided minerals
including clay settle into a lower sludge layer within the storage
zone for the effluent discharge. The improvement which is pro-
vided in accordance with the present teachings is the flocculating
agent employed comprises a combination of potato starch flocculant
and a Portland cement in which the dosage of Portland cement is
at ]east 3.0 pounds per 100 Imperial gallons of normalized sludge,
which normalized sludge is defined as fluid having approximately
an average solids content of 20~ (w/v) after settIing into the
sludge layer.
DESCRIPTION OF THE DR~WINGS
The subject matter of the invention is particularly
pointed out and distinctly claimed in the concluding portion
of the specification. The invention, however, both as to the
manner in which the flocculating combination is prepared and
the method of employing it, may best be understood by reference
to the following description taken in connection with the
drawing of which:
Figure 1 is a somewhat simplified schematic representa-
tlon of a hot water extraction process wherein the invention finds
particular use; and
Figure 2 is a graph illustrating the results of tests
employing various combinations of starch and cement on normalized
sludge as observed in the laboratory.
-12a-
DETAILED DESCRIPTInN OF TIIE INVENTION
Re-Eerring now to Figure 1, 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 o:E the tar sands processed. The tar sands, heated
and conditioned with steam and 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 relatively bitumen rich middlings stream is withdrawn ;;
through line 8 and transferred to a flotation scavenger zone
21. In this scavenger zone, an air flotation operation is
conducted to cause the formation of additional bituminous froth
which passes from the scavenger zone through line 9 in mixture
with the primary froth from the separation 20 to a froth
settler 22. A bitumen-lean water stream is removed from the
bottom of the scavenger zone 21 through line 10 to be further
processed as described below. In the settler zone 22, some
further bitumen-lean water is withdrawn from the froth and
removed through line 11 to be mixed with the bitumen-lean water
stream from the flotation scavenger zone and the sand tailings
stream from the separation zone. The bitumen from the settler .
is removed through line 12 for further treatment, typically
upgrading to synthetic crude oil.
-13-
Bitumen-lean water from the froth settler 22, the
scavenger zone 21,and the separation zone 20, all of which
make up an effluent discharge stream carried by line 7, are
discharged into a settling pond 15 having a clarified water
layer 26 and a sludge layer 27. The sand included in the
tailings stream quickly settles in the region 14, and
the fines-containing water flows into the body of the pond 15
where settling takes place.
In accordance with the present invention, potato starch
flocculant and cement are mixed with the effluent stream,
preferably as separate or combined slurrys injected, through
individual lines 23 and 24 or combined line 25, into line
7. The quantity of cement injected should be at least 3.0 pounds
(and preferably 3.6 pounds or more) of cement per hundred
Imperial ~allons of sludge which may be expected to accumulate
when the liquid fraction of the tailing stream is discharged
into the pond 15 and settled. The concentration of starch
flocculants injected typically falls within the range 0.1 to
0.5 pounds per hundred Imperial gallons of sludge.
As represented by the dashed lines in Figure 1,
initialization of an existing pond may require broadcas~ing
of the necessary quantities of cement and/or starch flocculant
over the surface of the pond (or by such other means as recirculation
with injection into the recirculates stream) in order to
bring the concentration of cement in the pond to at least 3.0
pounds per hundred Imperial gallons of sludge. As set forth
in more detail below, "sludge", for the purposes only of defining
the concentrations of cement and starch flocculant required,
may be more particularly defined as "normalized" sludge
containing about 20% w/v solids. As previously noted, in an
-14
4~
actual settling pond, the demarcation between a clarified
water layer 26 and a sludge layer 27 is ill-defined and
variable, and the characteristics of the sludge layer 27 change
from top to bottom such that it is necessary to calculate
an "average normalized" sludge from samples from a pond
to determine the minimum dosage of cement and starch flocculant.
Water from the clarified water layer 26 may be withdrawn
by a pump 28 for recycle by a line 17 to be mixed with Eresh
water and charged into the hot water process.
In order to substantiate the beneficial effect of
employing cement in conjunction with starch flocculant in the
tailings pond, controlled laboratory tests were undertaken in
which different amounts of Portland Cement, type III, were
added along or in combination with starch flocculant to natural
sludge from the Tar Island Pond at the GCOS mine. The cement
was added in the form of a dilute slurry (5%) and the solids
concentration in the treated sludge samples was adjusted to
14.9% (w/w) prior to centrifugation. The treatment dosages
referred to hereinafter are expressed in pounds per hundred
Imperial gallons of sludge containing 20% (w/v) solids.
A sample of the treated sludge was centrifuged for 320 minutes
for the purpose of determining the amounts of solids present
in the supernatent and sedimented cake. The floc volume
after different periods of centrifugation was also recorded to
provide information on the settling rate.
In the remaining treated sludge samples, the amounts
of soluble Ca and SiO2 in the pore fluid after 1, 7, and 21 days
of aging were determined. In the case of the sludge sample
treated with cement alone, the amounts of amorphous Ca and SiO2
were determined in the sludge solids after 1, 7, and 2 days
of aging.
-15-
In order to test consolidation, permeability, and shear
strength, natural sludge samples with 22.4 to 34.5 (w/w)
initial solids concentration from different depths of the
Tar Island Pond were treated with different dosages of starch
flocculants and cement. In each case, the solids and treated
sludge were concentrated with the aid of a bucket centrifuge.
To accomplish this, sludge was fed into a bucket centrifuge
in batches of about 120 ml, and each sample was centrifuged
for one hour at 3,000 rpm. The resultant concentrated sludge
obtained from bucket centrifugation (with solids content of
50 to 60% (w/w), was consolidated using Farnell consolidometers,
and changes in volume with time at different loads were recorded
manually. The coefficient of consolidation and the permeability
of each sample were calculated from this data at each equilibrium
load. The shear strength of the consolidated sample was
determined by the vane shear method at the conclusion of the
experiment.
As to stabilization, the solids content observed in
both the supernatent and sedimented cake, after centrifuging
the treated sludge for 320 minutes at 1370 RPM, are presented
in Figure 2. The addition of cement with a dosage of less
than 3.6 pounds per hundred Imperial gallons, in the presence
of the starch flocculant, significantly increases the stability
of the sludge as may be seen by the high solids content in
the supernatent. This is, of course, a counterproductive
condition.
-16-
TA9LE I
Influ nc~ of starch flocculant ~t t~o dosages of ccmcnt on th~ solublo Slû2 (pprl) pr~sent In th~ por~ f~uld
_ 5'2 (ppm) concrntratlon In por~ fluld
Ag ing Tlna C men~ dos~ge 1 2 Ib Crmcnt dosag~ I ô Ib
(dly5) tio flocculant 0 12 Ib Starch 0 24 Ib Starch ~Jo Flocculant 0 12 Ib Starch ¦ 0 24 Ib Starch
Flocculan~ Flocculant Flocculant ¦ Flocculant
.,
l 10 99 14 2 16 ~2 12 04 23 31 21 2
2 8 29 21 62 '5 3' lo 65 15 68 16
0 3 ~ 34 21 20 15 68 '~ 65 17 21 18 38
Cement and floccul~nt dos~g~ 3r~ In Ib/lûOgallonso~ sludge contdlnlng 2û~ (W/V) solids
The minimum amount oE cement required to flocculate
the sludge, even in the presence of starch flocculant, is
still obscrved to be 3.6 pounds per hundred Imperial gallons.
The additional benefits of flocculant addition to this dosage
of cement is that it further reduces the interparticle distances
and increases the solids concentration in the sediment. For
example, when starch flocculant is added at the rate of 0.48
pounds per hundred gallons along with 3.6 pounds of cement
per hundred gallons of sludge, the final solids content in
the sedimented cake obtained is about the same as with five
pounds of cemellt alone per hundred gallons of sludge. If the
amount of soluble SiO2 present in the pore fluid is any
indication of a higher degree of hydration reaction oE cement,
and if there is a minimum amount of cement required to
destabilize the sludge, it is then apparent from Table 1
that the addition of staTch flocculant does increase the
hydration reaction of cement. lhis decreases the amount of
cement re~uired to destabilize the sludge as seen by the
increase in the amount of soluble silica present in the pore
fluid.
~,'3~2~
It is clear that, ir' one on]y considers the pond water
ol~lelll in tcrms Or soli(ls contcnt:, the beneficial effect
of f] occu1ant when used in combination with cement is marginal .
It is essent;al to recognize that the final solids content
rcflects only tl-e compressabi1ity of- the sedimented mass and
not rates o-f compression, nor such important properties as
permeability and shear strength.
The influence o-E starch flocculant in combination
with different dosa~es of cement on physico-mechanica1
prol,ertics Or s1ud~e is summarizecl in Table 2. The two sets of
consolidation data obtained on sludge Wit}l different initial
solids content (34 . 5 and 22 . 4% w/w) are also given in Table
2. It is eviclent from a study of the results that the use of
cement in combination with starch flocculant is beneficial in
increasing consolidation, permeability and shear strength
significantly only when the amount of cement added is greater
than about 3 . 6 pounds per hundred Imperial gallons of
"normalized" sludge as best shown in Fi~ure 2.
TA9LC 2
.tm~nt ~nd con,ol~datlon dat~
¦ Natur~ and Dosage of Coeff~clent oF Permeabillty Shear
No. conc. I per ioo ra i ions ( l~p ) Consoi idat i ion. cv K (10 5 cm s~c ' ) Strength
tW/W) ¦ with 2û~6 (W/V) Solids c~ sec (p.s.i.)
__
I . 34.5 No treatment 0.06 0.22 3.0
2 34.5 0.18 starch fir~ccuiant 0.36 1.0 3.8
3 34.5 0,18 s~Drch flocculAnt 1.3 1.7 16.4
3 0 i 52 alcohoi
4 22.4 No ~reatmen~ o os o.os 6
22.4 o.lB starch flocculant o.o8 0.04 5.2
~ 1.54 ~Icohol
6 22.4 0.16 starch flocculant 0.21 0.37 6.5
t 1.6 alcohol
+ 3.1 Portland cem~nt
7 2 2 4 + i . 5 4 a i coho i 0 . 2 5 0 . 4 2 8 . 2
~ 3.RS Portland Cem~nt
4 0 8 2 2, 4 t i . 5 4 a i c oho I o . I s o . 6 7 7 . 5
~ 4.63 Portl~nd cr~nt
Th~ v~lu~s f Cv and K glven h~re ar~ at ~ vold rat~o of 1.25.
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~.
4 ~
The observed and demonstrated increase in consolidation,
permeability, and shear strength of the sludge obtained by
practicing the present invention is believed to occur for the
following reasons: The fines contained in the sludge
suspension consist of primary, as well as secondary, minerals.
Primary minerals, which are most quartz and some feldspars,
have very low specific sur-face area 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
area and a substantial amount of negative charge. There are
some positive charges, usually at the edges of the crystal of
some clay minerals. The addition of starch flocculant
destabilizes the sludge through the formation of tactoids
and the additional treatment of flocculated sludge with Portlan~
Cement causes an increase in shear strength and permeability
of the flocculated mass through creation of inter and intra floc
bonding.
While the principles of the invention have now been
made clear in an illustrative embodiment, there will be
immediately obvious to those skilled in the art many
modifications of structure, arrangements, proportions, the
elements, materials, and components, used in the practice of the
invention which are particularly adapted for specific
environments, and operating requirements without departing from
those principles.
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