Destabilization of sludge with hydrolyzed cassava starch flocculants

Cassava starch hydrolyzed in the presence of insoluble metal salts formed in situ is a surprisingly effective flocculant for destabilizing dilute as well as thick sludge suspensions and is particularly effective when used in the treatment of bituminous tar sands tailings and in phosphate slimes.

BACKGROUND OF THE INVENTION 
This invention relates to a novel hydrolyzed cassava starch composition and 
the process for making and using the same. More particularly, this 
invention relates to the discovery that cassava starch which is hydrolyzed 
in the presence of insoluble metal salts formed in situ is a very 
effective flocculant in destabilizing dilute as well as thick sludge 
suspensions. 
In general, the suspensions destabilized by the starch composition of this 
invention are aqueous colloidal suspensions containing either clay 
minerals or metal oxides-hydroxides which are formed in large volume 
during mining operations in the recovery of such materials as coal, 
bitumen from tar sands, and metals and such suspensions are also formed in 
paper making operations. In the case of metal mining operations, 
suspensions known as slimes are formed, typically phosphate slimes or like 
materials produced in the mining of copper, nickel, and titanium. In coal 
and tar sands minings, for example, the mining effluent typically contains 
dilute or thick clay mineral suspensions. In paper making processes 
cellulosic fines, clays and other solids are present in the large volumes 
of liquids obtained. 
In order to properly dispose of these voluminous mining discharges, 
regardless of their source, flocculants have conventionally been employed 
to destablize these suspensions and thus permit the effective separation 
of water from solids. The use of wheat, corn and potato starch which has 
been hydrolyzed in the presence of metal salts formed in situ has been 
disclosed in U.S. Pat. No. 4,289,540 which issued on Sept. 15, 1981 and is 
hereby incorporated by reference. It has now been found that surprisingly 
superior flocculation is obtained if starch from the root of the cassava 
plant is used for preparing the hydrolyzed starch composition to give the 
starch flocculant. 
OBJECTS OF THE INVENTION 
It is, therefore, an object of this invention to provide an effective 
flocculant in destabilizing dilute as well as thick sludge suspensions, 
particularly colloidal suspensions obtained from mining operations. 
It is a further object of our invention to provide an effective 
flocculating agent for treating tar sands tailing streams which carry 
suspended clay particles. 
It is another object of our invention to provide such a flocculating agent 
which is economical to prepare and employ in the treatment of both tar 
sands tailing streams, and phosphate slimes obtained from phosphate mining 
operations. 
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. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, it has been found that the 
foregoing and other objects may be achieved by employing a hydrolyzed 
cassava starch as a flocculant in destabilizing dilute as well as thick 
sludge suspensions. More particularly, it has been found that when these 
starches have been hydrolyzed in the presence of metal salts formed in 
situ there is formed a surprisingly effective composition for 
destabilizing such suspensions. Still more particularly, an especially 
effective flocculant composition may be prepared when calcium alumino 
phosphate cassava starch is used. 
As aforestated, the sludges being treated in accordance with this invention 
are aqueous colloidal suspensions containing either clay minerals or metal 
oxides-hydroxides which are formed during mining operations or other 
operations generating such suspensions. For sake of illustration, and 
simplicity only, the following description will be directed to a colloidal 
clay suspension obtained from bituminous tar sands mining. However, it 
will be understood that this invention is directed generally to the 
destabilization of all such suspensions. 
PREATION OF THE HYDROLYZED STARCH 
As aforementioned, the flocculant is prepared from a cassava starch which 
has been hydrolyzed in the presence of inorganic salts formed in situ. 
The cassava starch itself is readily obtained from the cassava plant which 
is a tropical plant having swollen roots which store the starch. The 
cassava plant is also known as Manioc (Manihut utilissima), family 
Euphorbiaceae and in some areas is referred to as yucca ("Starch--Its 
Sources, Production and Uses", Charles A. Brantlecht, Reinhold Publishing 
Corp., 1953). Separation of the cassava starch from the root is well known 
in the art and need not be repeated in detail here. However, for the sake 
of completeness, the cassava starch used in the examples which follow was 
obtained in the following manner: 
To 15 pounds of the peeled, diced cassava root 1.5 gallons of water was 
added to which 15 gms of potassium metabisulphite had also been added to 
prevent discoloration. The mixture was chopped with water in a blender at 
low speed just long enough to break up the cells (approximately 2 mins.). 
Then, the mass was filtered through a No. 60 mesh (250 micrometers) filter 
to remove the fibrous material from the starch suspension. The fibrous 
material was washed twice with fresh quantities of water and the starch 
suspensions combined and allowed to settle. Finally, the water was 
decanted and the starch was dried in an oven at 60.degree. C. or sundried 
after which it was ready for use. 
The hydrolysis of the cassava starch is achieved by simply heating the 
starch in the form of an aqueous suspension at about 85.degree. to 
95.degree. C. and preferably at about 90.degree. C. in the presence of 
soluble metal salts which will from an insoluble salt in situ. The starch 
should desirably be present in an amount of 1 gm to 10 gm per 100 ml of 
water, and preferably 2 to 6 gms per 100 mls of water. 
Among the salts which may be employed during the hydrolysis are salts of 
metals such as sodium, potassium, ammonium, magnesium, calcium and 
aluminum. The respective anions may be sulfates, acetates, chlorides, 
nitrates, chlorates, bromides, iodides, thiocyanates, phosphates and the 
like. Particularly effective for purposes of this invention, as applied to 
tar sands tailing ponds, is calcium alumino phosphate cassava starch, 
although such other salts containing sodium, aluminum and phosphate ions 
and the like may also be used. 
As indicated, the salts are added as soluble salts in order to form the 
insoluble salt in situ. Thus, for example, the preferred calcium alumino 
phosphate salt is advantageously formed in situ by the addition of 
specified amounts of calcium hydroxide, aluminum sulfate, and sodium 
phosphate to the aqueous cassava starch hydrolysis medium. In any event, 
the resulting salt should desirably be present in amounts of about 10 to 
30 gms per 100 gms of starch, and preferably 15 to 20 gms per 100 gms of 
starch. 
In the case of treatment of known phosphate slimes from phosphate mining 
operations with the flocculant of this invention, it will be understood 
that the presence or addition of a phosphate such as sodium phosphate, is 
not necessary and may be omitted from the salt preparation. 
If desired, the cassava starch hydrolysate may be dried to remove water by 
freeze drying, air drying, spin drying or the like, to provide a solid 
powder composition which is convenient to handle, store and transport and 
which can readily be redispersed in water at the treatment site.

The following examples are provided for purposes of illustrating but not 
limiting the scope of this invention with respect to the preparation and 
use of the starch flocculants. 
SYNTHESIS OF HYDROLYZED CASSAVA 
STARCH FLOCCULANT 
Five grams of cassava starch is weighed into a flask fitted for refluxing. 
Two hundred milliliters of aqueous solution containing 0.617 gm of 
Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O, 0.704 gm Na.sub.3 
PO.sub.4.12H.sub.2 O, and 0.463 gm Ca(OH).sub.2 is added to form calcium 
alumino phosphate in situ in the presence of the starch. 
The suspension is refluxed as 90.degree. C..+-.5.degree. for two hours with 
simultaneous stirring. The hydrolysis is considered complete when the 
insoluble starch is converted into a colloidal solution. The volume is 
then made up to 250 ml with distilled water, thus giving a stock solution 
of 20,000 ppm of calcium aluminum phosphate cassava starch. 
A preferred procedure for making the hydrolyzed cassava starch employs the 
method described in U.S. patent application Ser. No. 327,850, filed Dec. 
7, 1981, which application is also incorporated herein by reference. In 
the preferred method, an aqueous suspension of the starch, alum and lime 
is heated until maximum viscosity of the suspension is obtained and 
immediately thereafter, the salt to be added (e.g. trisodium phosphate) is 
added to the mixture. The general procedure for such preferred method is 
as follows: 
To 2.67 liters of tap water add 109.89 g cassava starch, 10.18 g lime and 
6.78 alum and heat the mixture with stirring at a constant speed of 100 
rpm at a rate of 0.5.degree. C. per minute. Stirring is carried out with a 
Cole-Palmer Master Servodyne torque stirrer calibrated to give viscosity 
value. As heating and stirring are continued the viscosity is observed to 
rise significantly when a temperature of about 60.degree. C. is reached 
and continues to rise until about 70.degree. C. is reached, at which point 
it begins to fall and then 15.46 g of trisodium phosphate (Na.sub.3 
PO.sub.4) is immediately added. Heating and stirring at the same rate is 
continued until a temperature of 85.degree. is reached, and then the heat 
is removed and the material is allowed to cool to room temperature. This 
hydrolyzed cassava starch suspension containing about 4% of the starch 
composition was used in the comparative tests which follow. 
COMATIVE EXAMPLES 
In order to compare and evaluate the efficiency of the cassava starch 
flocculants as settling aids, the test flocculants were used to treat tar 
sand tailings. 
The tar sand tailings used contained 1.93% solids (w/w) which are largely 
silts and clays. The fines are quite dispersed and tend to remain in 
suspension for a long period of time. 
Settling Tests of Tar Sand 
Tailings Treated With Starch Flocculants 
100 ml of the tar sand tailings was poured into a 100 ml cylinder and then 
0.1 ml of alum (0.06 m-mole/1) was added to the tailings sample. The 
cylinder was inverted 5 times to mix the tailings with the alum. Then 0.25 
ml of the 4% hydrolyzed cassava starch flocculant suspension was poured 
into the cylinder. The cylinder was again inverted five times. The dosage 
of treatment is approximatey 100 PPM. The same procedure was used for a 
hydrolyzed wheat starch flocculant used for comparison. The treated 
samples were then allowed to settle in the cylinders without further 
disturbance. The sediment heights were recorded periodically until little 
change was observed. 
4.3 Kynch Analysis of the Settling Tests 
Results of the settling tests were evaluated in terms of Kynch analysis 
which gives a settling time called "ultimate time" (T.sub.u). T.sub.u is 
an indicator of the duration required for a sediment to reach close to the 
ultimate height and this parameter is used to evaluate the efficiency of 
settling aids in suspensions. 
Another parameter for the expression of settling efficiency of sediments is 
called "Unit Area" which is defined as follows: 
##EQU1## 
where C.sub.o =initial solids concentration (ton/cu. ft) H.sub.o =initial 
height (ft) 
T.sub.u =ultimate time (days) 
Unit Area is expressed as sq. ft/ton/day. This is the area required to 
settle one ton of solids per day. Small Unit Area indicates efficient 
settling. The T.sub.u and unit areas of the hydrolyzed cassava starch 
flocculant treated samples, a hydrolyzed wheat starch flocculant treated 
sample and an untreated sample are presented in Table I. 
TABLE I 
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Tar Sand Tailings Treated With Flocculants 
Unit Area 
T.sub.u (Min) 
(sq. ft/ton/day) 
______________________________________ 
Untreated 1440 284.63 
Hydrolyzed Wheat 
98 191.73 
Starch Flocculant* 
Hydrolyzed cassava 
49 95.73 
Starch Flocculant* 
______________________________________ 
*Prepared by the preferred method of S.W. 327,850 
As can be seen from the above table the hydrolyzed cassava starch 
flocculant of the invention is surprisingly superior to the hydrolyzed 
wheat starch flocculant.