Flocculation
- Also known as
- flocculant, floc
Flocculation is the process of binding fine suspended particles together into larger clusters, called flocs, so they settle or can be removed more easily. It is usually driven by long-chain polymers called flocculants and is widely used in mineral processing and water treatment.
What is flocculation?
Very fine particles in water settle extremely slowly, and many stay suspended because they carry like electrical charges that keep them apart. Flocculation gathers them into larger, looser clusters, flocs, that settle quickly or can be filtered or floated off. It is central to thickening and dewatering in mineral processing and to clarifying drinking water and wastewater.
How flocculants work
Most flocculants are high-molecular-weight water-soluble polymers, commonly polyacrylamides, available as anionic, nonionic or cationic grades. A long polymer chain adsorbs onto several particles at once, bridging them into a floc. Choosing the right charge and molecular weight for the particles and water chemistry is essential, and the polymer must be mixed gently enough to spread through the slurry without breaking the flocs it forms.
Flocculation vs. coagulation
The two are often used together but work differently:
| Coagulation | Flocculation | |
|---|---|---|
| What it does | Neutralizes the surface charge that keeps particles apart | Bridges particles into larger flocs |
| Typical chemicals | Metal salts such as alum and ferric chloride; low-molecular-weight cationic polymers | High-molecular-weight polymers |
| Result | Small, destabilized particles, microflocs | Large flocs that settle or float |
In water treatment, coagulation usually comes first, followed by flocculation.
What affects flocculation
- Dosage. Too little leaves fines suspended; too much can restabilize particles and waste polymer.
- Particle properties, including size, charge and mineralogy; clays can be difficult.
- Water chemistry, such as pH, dissolved salts and residual reagents.
- Mixing and shear, which help distribute polymer but can break flocs.
Results are judged by settling rate, underflow density and overflow clarity, often measured as turbidity.
Frequently asked questions
Coagulation neutralizes the charges that keep particles apart. Flocculation binds destabilized particles into larger flocs, usually with a polymer. They are often used in sequence.
Dispersion. A dispersant keeps particles separated, while a flocculant draws them together.
Sources
- Wills' Mineral Processing Technology, 8th edition, B. A. Wills and J. A. Finch, Butterworth-Heinemann, 2016
- Water treatment coagulation and flocculation, U.S. Environmental Protection Agency