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Introduction To Specialty Polymers

ACT Specialty Polymers consist of both inorganic coagulant aids and high molecular weight anionic, non-ionic, and cationic polymers. These products are used to increase the efficiency of precipitation, clarification, and filtration operations.

The terms coagulation and flocculation have been used interchangeably by uninformed persons. In the water treatment industry the two terms imply two different processes.

COAGULATION

Colloidal particles less than one micron in size undergo Brownian motion and usually will never settle out of suspension.

Coagulation is the process by which the electrical repulsion between individual particles is reduced, inverted, or neutralized. This will cause the individual particles to agglomerate into a pin point floc. Some common coagulants are: aluminum sulfate (ACT WT-9406), aluminum chloride, poly-aluminum chloride (ACT WT- 9413, and ACT WT-9011), ferric chloride, ferrous sulfate, lime, calcium chloride, and magnesium chloride.

FLOCCULATION

Flocculation is used to describe the action of polymeric material which form bridges between individual particles. Bridging occurs when segments of a polymer chain adsorb on different particles and help the particles agglomerate into larger particles. Flocculants have charged groups with a charge that counterbalances the charge of the particles. Flocculants adsorb on the particles and cause destabilization by bridging and/or charge neutralization.

An anionic flocculant is usually used to react against a positively charged suspension. Hydroxide precipitation of heavy metals is an example of this.

The most common polymers are based on polyacrylamide, which is a non-ionic polymer. They bring about agglomeration of the particles by bridging. Polyacrylamides can be given an anionic charge by co-polymerizing acrylamide with acrylic acid. Cationic polymers are prepared by co-polymerizing acrylamide with a cationic monomer.

All available acrylamide based polymers have a specific amount of ionic monomer that gives a certain degree of ionic character. They have a specific molecular weight or chain length and a specific molecular distribution. Usually the flocculating power increases with the molecular weight but it is impossible to select the applicable polymer without bench testing.

FIELD EVALUATION AND SELECTION OF POLYMERS

Flocculant solutions are usually highly viscous and it is almost impossible to prepare highly concentrated solutions. In the field a 0.1% solution is usually used for testing purposes and should be stable for up to 6 days. If a more stable solution is required a 0.5% solution can be used and is normally stable for two weeks. When working with dry polymers, it is advisable to wet the polymer with isopropyl alcohol (IPA) or acetone prior to adding the water. The solution should then be gently stirred with a magnetic or propeller stirrer for two hours to allow the molecule to completely uncoil. Remember that high shear can physically break apart the molecules and decrease the performance of your product.

The table 1 is useful in determining specific amounts of polymer solution to add to a 1,000 ml test solution.

PPM

0.05%

0.1%

0.25%

0.5%

1.0%

0.5

1

0.5

0.2

0.1

.05

1

2

1

0.4

0.2

0.1

1.5

3

1.5

0.6

0.3

0.15

2

4

2

0.8

0.4

0.2

2.5

5

2.5

0.1

0.5

0.25

3

6

3

1.2

0.6

0.3

4

8

4

1.6

0.8

0.4

5

10

5

2

1

0.5

6

12

6

2.4

1.2

0.6

8

16

8

3.2

1.6

0.8

10

20

10

4

2

1

 
Table 1

FIELD BENCH TESTING

Settling Tests


High Solid Suspensions

In this situation, the boundary line between solid phase and liquid phase is clearly evident. The settling speed can be measured in a cylinder by measuring the amount of solid material in a given time frame.

Method

  1. Fill a one liter cylinder with the suspension
  2. Add 1 ml of a 0.1% solution of polymer to the cylinder and invert 4 times.
  3. Measure the height of the solid phase at regular intervals.
  4. Plot the curve of height against time.
  5. Repeat this procedure for all polymers you think will work and select the best one.
  6. Repeat this procedure with different dosages of this polymer until best dosage is arrived at.

This test is very sensitive to stirring so invert each cylinder exactly like the others.

Low Solid Suspensions

In this case, the flocs are usually dispersed and we observe very low settling speeds. Therefore low speed mixing is used (jar test).

Method

  1. Fill all of the beakers with 1,000 ml of the suspension
  2. Add the flocculant at a high revolving speed (100 rpm) during 10 seconds in order to thoroughly disperse the polymer.
  3. Stir slowly for 3 minutes (30-40 rpm) for three minutes.
  4. Compare the different flocculants and the various dosages in terms of floc size, supernate clarity, and settling speeds.

FLOCCULATION TESTS AFTER COAGULATION

All suspensions that have a large proportion of colloidal organic substances must be destabilized through the addition of a di or trivalent metallic salt such as lime, ferrous sulfate, ferric chloride, alum, or PAC. The Polyacrylamides usually make coagulation less pH sensitive and one can use:

  • Calcium salts at pH of 4 -- 14
  • Iron salts at pH of 4 -- 13
  • Aluminum salts at pH 4.5 -- 10

For each suspension there is an optimum pH that has to be found. Evaluations can be done with the jar test.

Method

  1. Add 10, 30, 50, 100, 200 mg/l of coagulant in a 1% solution
  2. If very acidic adjust the pH back to 6 with NaOH.
  3. Mix for 1 minute at 200 rpm.
  4. Add 2 ppm of polymer (usually anionic)
  5. The beaker that gives the first clear supernate contains the optimum amount of coagulant. It may take more than 200 ppm.
  6. Do a jar test by adding the coagulant first and stirring for two minutes at 100 rpm prior to adding the flocculant

FILTRATION TESTS

Method

  1. Add 500 ml solution to be filtered to a 1,000 ml beaker.
  2. Add required amount of polymer.
  3. Pour the solution back and forth between two 1,000 ml beakers exactly four times to mix.
  4. Pour solution into a Buchner Funnel and collect the filtrate in a large graduate for 30 or 60 seconds.
  5. The solution that gives the greatest amount of filtrate after the specified time is dosage of choice.

DOSAGE RATES

In most applications, the amount of flocculant necessary to obtain a good liquid/solid separation is very small. The average range of dosages is:

  • 0.5 -- 3 gm/m3 of diluted suspension
  • 2 --20 gm/m3 of concentrated suspension
  • 25 - 300 gm of flocculant per ton of dry solids for filtration or centrifuge operations.
  • For organic sludge, use 10 - 200 ppm of coagulant and 0.25 - 2.0 ppm of flocculant.

INDUSTRIAL TREATMENT WITH FLOCCULANTS

To ensure maximum efficiency, the flocculant must be used properly. Its use depends on numerous physical and chemical factors that can alter the obtained results. Efficiency usually will be 60 - 120% of the bench test results. Most factors that will influence the final result are:

  • The Location of the injection point has to be in an area that has sufficient turbulence to mix the flocculant but not enough to shear the floc.
  • Multiple point additions usually improve the contact of the flocculant with the system.
  • Secondary dilution will normally enhance the performance of most polymers.

When both coagulants and flocculants are used together, the following equipment improves the efficiency:

  • A coagulating tank with mild stirring and about 5 minutes retention time.
  • A flocculating tank that can be the inner part of the settling tank where the chemical reactions will take place. In the case of vacuum filters or centrifuges, the flocculant is injected into the inlet pipe.

 

 

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