A Flocculation Test Procedure is used to determine how much Flocculants need be added to solids in suspension (slurry or pulp) to cause the individual particles to collect in the form of flocs. The formation of flocs aids in improved settling rates, better overhead clarity and/or faster filtration rates. There are three main methods for achieving aggregation of very small or colloidal particles in suspension:
- Coagulation, i.e. aggregation caused by the compression of the electrical double layer of the solids by addition of simple electrolytes.
- Aggregation caused by the action of hydrolysing electrolytes such as ferric or aluminium sulphate, the mechanism involved being a composite of both coagulation by the polyvalent ions and bridging flocculation by the precipitating hydroxides.
- Flocculation, i.e. aggregation through the use of long-chain organic polymers that bridge between adjoining particles. Flocculation is, by far, most import of aggregation methods used in the mineral industry.
There are basically three types of organic polymers (or flocculants) that can be used, nonionics, anionics or cationics. The nonionics which are a neutral polymer molecule, have a wide application and can be used as flocculants for most solids in acid, basic or neutral conditions. In some systems where flocculation is dependent upon the “ionic” charge of the particles a polymer with either a positive or negative charge will aid in the attachment to the solid particles and hence improve efficiency of flocculation. The charge of the particles is dependent on a number of factors and is usually pH dependent. If the charge on the particle is overall positive, then an anionic polymer will be most suitable and vice versa.
The solubility of polymers varies considerably, but in general is very low and therefore they should be prepared as a very dilute solution, usually less than 1% by weight. For this Flocculation Test, the addition of the polymer aqueous solution should be very slow, with slow agitation.
The method of application is also important. With most flocculants adsorption is rapid and irreversible, so thorough and complete dispersion of the flocculant throughout the slurry is a necessity. The recommended test techniques of application for maximum efficiency are as follows:
- Application as a very dilute solution
- Multiple point addition to the suspension
- Gentle agitation or mixing, that is agitation sufficient to ensure distribution throughout the pulp but not violent enough to destroy rapidly formed flocs.
Consumption of flocculants usually range from 0.005 to 0.2 kg/tonne of dry solids.