How do polymers work on hydroxide wastes

In order to understand how polymers improve separation of metal hydroxides, it is first necessary to define certain terms to ensure an understanding of their usage for comparative purposes.

A polymer can be either naturally occurring (starches, gums) or synthetic.  Its high molecular weight results form the large number of regularly repeating groups of atoms in its structure.  It can be either cationic, nonionic or anionic, depending of the charge of its organic functional groups.  Anionic flocculants are used in treating plating wastes because they are more active at the higher pH's where heavy metal precipitation occurs.  A polymer can function either as a coagulant or flocculant.

The term coagulant and the associated process of coagulation implies charge neutralization.  With its high cationic charge density, the coagulant neutralizes negatively charged particles, allowing them to collide, grow in size and subsequently settle.  Coagulants are generally used in treating plating waste when high levels of cleaners or other surfactants requiring some charge neutralization are present.

Flocculants do not rely on charge neutralization.  They bring about liquid/solids separation by another mechanism.  Flocculants are typically of higher molecular weight than coagulants.  This characteristic of a flocculant allows simultaneous contact with many of the small suspended particles.

The phrase "molecular bridging" is used to describe this process.  The long chain organic polymer bridges these particles, compresses them together and thereby produces large flocs that settle rapidly.  As mentioned earlier, it is often necessary to use both a coagulant and flocculant to achieve good settling and a clear effluent, depending on the chemical characteristics of the water as well as the physical aspects of the solids.  In the actual plant treatment of a waste system with polymers, application technology is the all-
important factor in maximizing product activity and thereby lowering operating costs.

Coagulants normally require only a simple dilution with water prior to application.  They can be pumped directly from a drum or bulk storage tank, needing only dilution water to disperse the polymer into the water or wastewater being treated.  Flocculants are of much higher molecular weight than coagulants and require pre-mixing in a make-up tank prior to further dilution and then application.

The point of application of the polymers is also critical in achieving optimum performance.  The coagulant usually is added first (charge neutralization) if needed, and then the flocculant.  To find the exact location for the feed points, it necessary to experiment while checking settling rates and clarities of the treated waste in the clarifier.