CA1060592A - Clarification of turbid waters - Google Patents

Abstract

A B S T R A C T

A method of treating an aqueous dispersion of negatively charged colloidal particles to flocculate at least a portion of said particles which comprises treating said aqueous dispersion simultaneously with a polyalkylen-imine polymer having a molecular weight from 5,000 to 100,000 and a polyalkylene polyamine polymer having the formula H2N-(CnH2nNH)a-H wherein n ranges from 2 to 4 and "a" ranges from 250 to 5,000, said two polymers being employed in a weight ratio to each other ranging from 1 to 99 to 99 to 1.

Description

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The process of reducing the turbidity of aqueous dispersion (caused by the suspension of colloidal particles therein) by coagulation and flocculation employing organic polymers is a well known art. In one process the charge (zeta potential) on the colloidal particle is first neutra-lizedJ e.g., ~y the use of an oppositely charged water soluble organic polymer which in turn allows the particle, with proper gentle motion of the fluid, to contact other similar particles and agglomerate into floc. Flocculation aids, such as long chain water soluble polymeric materials, may be added to adsorb on the particles after the particle charges have been neutralized. The flocculation aids form bridges between particles, and thus speed the growth of large floc particles which have high settling rates. Poly-acrylamides and activated silica are examples of such flocculation aids. Some cationic water soluble polymers, such as polyamines, because of their charge neutralizing as well as bridging capability, act as both a coagulant and flocculation aid. Polyalkylenimines and polyalkylene polyamines are examples of such cationic water soluble polymers.
A method of treating an aqueous dispersion of ~-negatively charged colloidal particles to flocculate at least a portion of said particles has now been discovered which comprises treating said aqueous dispersion similtan-eously with a polyalkylenimine polymer having a molecular weight from 5,000 to 100,000 and a polyalkylene polyamine polymer having the formula H2N-(CnH2nNH)~-H wherein n ranges from 2 to 4 and "a" ranges from 250 to 5,000, said two polymers being employed in a weight ratio to each other -. ::

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ranging from 1 to g9 to g9 to 1. Polyalkylenimine and polyalkylene polyamine polymers employed in the present invention are those which are soluble in the aqueous dispersion to be treated. Preferably polyaklylenimine polymers employed in the present invention have a molecular weight (as determined by gel permeation chromatography) ranging from 30,000 to 60,000. Examples of such polymers ars polyethylenimine and polypropylenimine.
Polyalkylene polyamine polymers employed in the present invention have a molecular weight (as determined by gel permeation chromatography) ranging from 10,000 to 200,000, preferably at least 40,000. Examples of such polymers are polyethylene polyamine and polypropylene polyamine.
Preferred ranges of the weight ratios of the two polymers depend upon the molecular weight of each polymer, the characteristics of the colloidal dispersion to be treated and other similar parameters.
It has now been discovered that when these two polymers are employed simultaneously, the sedimentation rate (i.e., the amount of floc formed per unit of time) of the colloidal particles is increased over that theoreti-cally expected from the rate of settling caused by treating the aqueous dispersion with the individual polymers. This discovery is improtant since it provides greater process flexibility and a more economical use of those types of polymers. The Figure presenting a portion of the data set forth in Table Il compares the actual sedimentation rate with the expected sedimentation rate based upon the individual polymers.

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1~6Q59;2 The two polymers can be mixed together in an aqueous liquid to form a premix and the premix then added to the aqueous dispersion of colloidal particles to be treated. Alcohols or other agents may be employed in the -premix to aid in the dispersion of the polymers. As expected when working with high molecular weight polymers ~ -there is a limit on the miscibility of similar polymers in concentrated solutions. It is preferred that such premix be suhstantially free from separated phases and thus desir~
able parameters should be ascertained for the particular polymer~ emplyed. For example, the range in which a speci- -fic mixture of polymers were ~ound to miscible is set forth in the following Table I. The viscosity of the mixture is also set forth. The viscosity was determined with a Brookfield LV model rotational viscosimeter at a room temperature of 23C. Polymer A comprised an aqueous solu-tion containing 33 percent by weight of polyethylenimine having a molecular weight ranging from 20,000 to 30,000.
Polymer B comprised an aqueous solution containing 3.9 percent methanol to increase its fluidity and 37.5 percent by weight of polyethylene polyamine having a molecular , , weight ranging from 40,000 to 70,000.
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Thus when employing a premix containing the a~ove identified mixture of polymers it is preferred to employ less than about 30 percent (volume) of the Polymer A solu-tion to avoid a phase separation in the premix.
In another manner the two polymers may be dis-persed separately into the aqueous fluid to be treated ~nd in that instance the miscibility problems associated with premix are not normally encountered since the quantity employed does not normally approach the solu-bility limits of the polymers.
Parameters well known to those skilled in the art of flocculation are employed to practice the present invention. For example, in the method of treating agueous solutions, e.g., industrial and municipal waste streams and the like wherein the influent turbidities exceed about 50 ppm clarification, flocculation is gener-ally employed ahead of the filtration steps. In the prac-tice of the present invention the two above identified -polymers are added to the influent aqueous stream, gener-ally with rapid agitation, and then passed into a chamber where a more gentle agitation occurs to aid in the forma-tion of floc from the reaction of the two polymers and the turbidity. Following the flocculation chamber the dispersion generally enters a sedimentation step wherein -the floc is concentrated and clarified effluent i8 removed.
Known floculation aids, e.g., polyacrylamides and the like may be employed in conjunction with the two polymers of the present invention~
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Example 1 A clay dispersion was prepared by mixing 200 grams dry Grundite Bond clay (a clay having an approximate chemical analysis of as percent by weight: ignition loss 7.14 percent; silica 61.31 percent; alumina 17.77 percent;
iron oxide 4.97 percent; titania 1.08 percent; lime 0.55 percent; magnesia 1.45 percent; alkalies 4.32 percent and sulphur 1.23 percent from Illinois Clay Products Co.) into 2000 milliliters (ml.) of deionized wa~er while stirring with a high speed air stirrer. The dispersion was then diluted with 2000 ml. of additional deionized water. The solids content of the resulting dispersion was 3.2+ 0.1 percent by weight and had a pH of about 4.4.
Clay sedimentation rates (settling rates) were determined on 100 ml. samples of the clay dispersion described directly herein before in the following manner.
The 100 ml. samples were placed in a closed graduated cylinder and mixed by rotating the cylinder end by end 10 times. The graduated cylinder was then placed in front of a light box. A flocculant solution was added to the cylinder after the cylinder had first remained still for 10 minutes. The sedimentation rate (expressed as percent by volume per minute) was determined by measuring the sediment volume in milliliters at 30 second intervals.
1 25 The above tests were run employing the same two ; polymer dispersions (Polymer A and Polymer B) described hereinbefore just prior to Table I.
The results of the tests are set forth in the following Table II and for the 0.5 ppm concentrations graphically illustrated in the ~igure.

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Claims (3)

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

1. A method of treating an aqueous dispersion of negatively charged colloidal particles to flocculate at least a portion of said particles which comprises treating said aqueous dispersion simultaneously with a polyalkylen-imine polymer having a molecular weight from 5,000 to 100,000 and a polyalkylene polyamine having the formula H2N-(CnH2nNH)a-H wherein n ranges from 2 to 4 and "a"
ranges from 250 to 5,000, said two polymers being employed in a weight ratio to each other ranging from 1 to 99 to 99 to 1.

2. The method of claim 1 wherein the molecular weight of the polyalkylenimine polymer ranges from 30,000 to 60,000.

3. The method of claim 1 wherein the molecular weight of the polyalkylene polyamine polymer is at least 40,000.