Flocculants have been around since the early sixties. As you can imagine, it has led to dramatic changes in operating practice, design and efficiency in most aspects of solid-liquid separation.
SO WHAT IS FLOCCULANT?
In simplest terms, flocculant is a long-chain polymer molecule that is able to attach to fine particles causing them to bridge together to form larger particles known as flocs or agglomerates. In principle, by bridging larger particles together, the flocs (more mass) tend to settle more rapidly. In the case of flotation, as the floc grows in size, the probability of bubble-particle collision efficiency improves.
CHARACTERIZATION OF POLYMER TYPES
Dry polymers are generally recognized as the purest form of polymers commercially available, being considered 100% active. This is really not the case; in reality they contain some residual moisture level of about 5 to 10%.
Diluents (poly blends or blends) can be easily incorporated into solid grade products during manufacture or by simple blending. Some polymer manufacturers call this type of polymer, poly blends. Common diluents include sodium and ammonium salts as well as carbonates or sulphates. The benefits of solid grade polymers are their ability to remain stable for many years if stored correctly in a cool, dry atmosphere.
PHYSICAL FORMS OF SOLID GRADE POLYMERS
- Microbeads have excellent flow characteristics, thus preventing bridging in hoppers, etc. Microbeads have bulk densities 0.8 g per ml. Microbeads are uniform in size, minimizing dusting problems.
- Powders are irregular shaped, granular products with densities of 0.6 to 0.7 g per ml. They are generally more prone to bridging in hoppers and to dusting.
- Flake products have bulk densities of 0.6 to 0.7 g per ml and are particularly susceptible to bridging in hoppers.
Solubility of solid grade products can vary from a few minutes to two hours depending on the ionic character and molecular weight of the product. Solubility rates needs to be known in order to ensure adequate make-up detention time for allowing full dissolution of the polymer.
INVERSE EMULSION POLYMER
These are essentially aqueous polymer gels emulsified in an oil carrier. The chemical nature of emulsions is very complex in order to build in the required properties of low viscosity, emulsion stability (to prevent phase separation) and correct activation on contact with water. Emulsions are metastable (having or characterized by only a slight margin of stability) and product separation is a common problem. Active contents are usually between 25 to 35% with some current development products being offered at higher activities. Shelf life is commonly quoted as one year (note that this may change from manufacture, thus it is recommended that the polymer manufacturer be consulted).
Dissolution of emulsion polymers is rapid and complete under ideal circumstances. However, the characteristics of plant process waters may not be completely compatible with emulsion polymers, thus causing incomplete or slow activation, which can results in a loss of performance.
These may be considered as water-free emulsions. The removal of the water can improve product stability from both a physical and chemical point of view, but the complexity of these products is no less than that of emulsions. Active contents (of commercial products) are 50% with development samples at higher activities.
It should be noted that dispersion polymers are similar to emulsions, although they do tend to perform better when preparation is required in high salinity systems. This is because stronger "activators" can be used in the water free dispersions than in emulsions.
FACTORS THAT AFFECT POLYMER PERFORMANCE
To obtain maximum performance from a selected polymer, a great deal of consideration needs to be given to the following:
- Shelf life
- Water chemistry
- Flocculant type
- Nature of the sludge
- Method(s) of addition
- Rate of solubility
- Polymer solution stability
- Solution viscosity
- Process addition points
- Effects on down-stream operations
Notes: All polymers need to be stored according to the manufacturers' recommendations. Products stored incorrectly or beyond their recommended shelf life may suffer from deterioration by way of hydrolysis, cross linking, depolymerization, or in the case of powders, caking.
Hydrolysis causes a change in the ionicity of the polymer, cross linking renders it less soluble, and depolymerization reduces the molecular weight.
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