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Designing a Polymer or Poly-Electrolyte Storage Day Tank Mixing System:

Advantages 4,000 gallon Polymer Poly-Electrolyte Storage Day Tank:

Polymer, Polyelectrolyte, or flocculants consist of various molecular weight anionic, nonionic and cationic polymers.  They are used to increase the efficiency of settling, clarification, filtration and centrifuge dewatering operations. 

Coagulation and flocculation are used to separate suspended solids from water.  Although the terms are used interchangeably, they are in fact two distinct processes.  

Finely dispersed solids (colloids, less than 1 micron in size) are stabilized by their negative electric charges, which act to suspend themselves in wastewater.  The colloids negative charges act to repel each other, preventing them from forming larger masses which hinder the small particles from settling.  To remove the colloidal particles both flocculation and coagulation are required.  

Coagulation is the process of destabilization, by introducing common mineral salts such as aluminum sulfate, ferric chloride, lime, calcium and magnesium chloride which either reduce, neutralize or invert the electrical repulsion between the colloidal particles, which give them the opportunity to grow in size or aggregate, to form a floc.  

Flocculation is used to describe the action of polymeric materials, which carry active charge groups, which counterbalance the charge of the colloidal particles, which acts to bridge into long polymer chains.  The floc then settles which is then easily separated.  Anionic flocculant will usually react against positively charge suspension such as salts and metallic hydroxides.  Cationic flocculant will react against negatively charged suspensions like silica or organic substances.  However, some anionic flocculants agglomerate clays which are electronegative.

Although there are both mineral (silica, bentonite, etc.) and natural (starch, alginates, etc.) flocculants, the most commonly used is synthetic polyacrylamide flocculation polymer, which is a nonionic polymer.  Polymers can be given anionic or cationic character by copolymerizing them with acrylic acid or cationic monomer, respectively.  Every polyacrylamide has a specific amount of ionic monomer to give a certain degree of ionic character.

Since it is difficult to prepare highly concentrated polyacrylamide flocculants, they are commonly dissolved into 0.5% to 0.1% solutions in cold water.  High shear devices can degrade (deteriorate) the polymer chains so avoid using high speed (agitator shaft output RPM of 3600, 1800, 1200 RPM's) mixers, dispersers, homogenizers, disintegrators or centrifugal pumps.  All polymer solutions will degrade over time, where degradation time is dependent upon the solutions concentration.    

Polymer Mix or Day Storage Tanks:
For polymer day tanks from 50 to approximately 5,000 gallons, mixers with agitator shaft speeds of 350 RPM's are quite common.  Typical solution viscosity will range from 500 to 5,000 centipoise.  Although a 100 RPM mixer will have a lower shear rate than a 350 RPM mixer, which is beneficial to the efficiency of the very sensitive polymer solution, economics generally don't consider this option to be feasible.  Since 350 RPM mixers are common, 400 RPM mixers are not recommended.  Above 1,500 gallons, consideration may be given to a slower speed mixer, as the initial cost of the mixer may prove beneficial versus the continual chemical usage cost efficiencies.  Since the polymer solution is sensitive to shear (over mixing), it is recommended that the customer run the mixer intermittently after the slurry is made.  Hand, Off, Auto Switches, with dual timers, are generally available upon request, which will automatically mix the tank contents at specified intervals.      

Vertical tanks, with height to diameter ratios of 1.0 to1,2 are preferred over horizontal tanks since they take up less space, are easily supported on a concrete slab and are simpler to mix.  Tanks having mixers installed vertical-on-tank-centerline generally do not require anti-swirl baffles due to the process viscosity of the application.  If you are using a lower viscosity solution, three (3) anti-swirl baffles installed 120 degrees apart, maybe required to prevent foaming and other considerations.  Contact your mixer supplier for their recommendation.  

As for the mixer design, the use of an upper impeller maybe recommended, especially if a solid feeding system such as a funnel eductor is used.  In the event of dewatering, the upper impellers purpose will be to capture and entrain the dewatered top phase and pump it downward quickly into to lower regions of the storage tank to achieve a solution.  Without the upper impeller, the lower viscosity water phase may stratify on the surface and take time to reincorporate into the batch exposing the solution to over mixing.        

04.12.11 

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