Flocculation: reaction, types, applications, examples

During flocculation, due to aggregation, a considerable increase in the size of the particles occurs, becoming macroscopic, visible to the human eye. They also increase their weight , which prevents them from remaining in suspension, causing their sedimentation.

Above is a representation of what is meant by flocculation. Flocs tend to be scaly in shape, and not necessarily round. This process is caused either by external agents, such as the addition of a flocculating agent, or by the abrupt destabilization of the colloidal particles.

In some cases the particles produced by flocculation float to the surface instead of settling. This is due to the low density of some of these particles.

Flocculation reaction

Colloidal state

The colloidal state is formed by small suspended particles that do not form agglomerates. Said particles present electrical charges on their surface, which develop electrostatic repulsions between neighboring particles. For this reason they cannot join in the beginning.

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Therefore, it is these surface electrical charges that support the stability of the colloidal state and its particles, so that there is no sedimentation.


Coagulation occurs by adding chemical compounds that neutralize charges on colloidal particles. The most widely used coagulant is alum, Al 2 (SO 4 ) 3 · 14H 2 O. Coagulants are therefore said to destabilize an electrically charged particle.

The introduction of opposite electrical charges allows the particles to bond together, forming submicron flocs. These flocs are characterized by their stability when kept in suspension. That is, they still do not “break” the colloidal state.

Flocculation requires the mixing to be smooth, that is, it must be carried out at low speeds. In addition, the presence of a high molecular weight polymeric flocculant is required. This flocculant is arranged around a set of submicron flocs, producing their approach.

The flocs bond with each other and increase their interactions governed by the forces of Van der Waals. In this way, they lose energy and the flocculation process continues to advance to give rise to loose flocs.

And then the loose flocs add and strengthen each other until the macroflocs are formed, which are visible in suspension. Once these reach the correct size and weight, their sedimentation finally occurs.

Types of flocculation

There are two types of flocculation: pericinetic flocculation and orthokinetic flocculation.

Perikinetic flocculation

It is the one that occurs thanks to the Brownian motion. This type of flocculation only affects particles with a diameter smaller than one micron. Therefore, perikinetic flocculation occurs only at the beginning of the flocculation process.

Orthokinetic flocculation

This occurs after pericinetic flocculation. It is generated by the movement of particles in all directions and speeds. As there is more movement, the probability that the particles interact with each other increases. These movements are induced by an energy external to the mass of the water or solvent.


Although flocculation is a phenomenon that occurs normally in nature, man has learned to use this process to his advantage. Some of its industrial applications will be mentioned below.

Water treatment

Chemical compounds known as coagulants are added to the water. These coagulants bind to particles and chemical compounds in suspension, producing their aggregation and subsequent sedimentation in the form of masses of solids known as flocs.

The chemical compound most used as a coagulant is aluminum sulfate, although sodium aluminate and ferric sulfate are also used for this purpose. The flocs formed settle, clarifying the water. However, it also requires filtration and a chemical treatment to make the water drinkable.

Precious metal mining

To obtain precious metals it is necessary to separate them from other metals and compounds that contaminate them. For this, selective precipitation of metals is used. This process is usually accompanied by flocculation and sedimentation of the desired metals, as well as contaminants as the case may be.

Beer production

In the fermentation of beer involved cells, such as yeast, forming macroscopic flocs. The yeast present in these particles can settle or float to the top of the fermentation tanks, where it can be collected to be reused in a new fermentation cycle.

Yeast flocculation is dependent on calcium concentration, requiring a concentration of 50-100 ppm. Such flocculation can be reversed with an addition of sulfate, phosphate, or the chelating agent EDTA.

Cheese industry

Flocculation is an indicator of the evolution of curd formation in the initial stages of cheese making, with flocculation indicating the curd establishment time. In it, the micelles that form it can interact with each other and flocculate.

Water quality

There are colloid soils that, when they filter into the water, produce its turbidity and the phenomenon of eutrophication. This consists of the fact that colloids are rich in nutrients that favor the growth and proliferation of some animal and plant species, which hinders the development of other species.

The transformation and flocculation of colloids eliminates their access to rivers and lakes. Therefore, it improves the quality of the water.

Examples of flocculation

Butter making

Whole milk is heated and on cooling it accumulates on its surface lipids that form a cream. This cream is the product of a milk coagulation and flocculation process. Later, the cream is collected and used to make the butter.

Snow production and precipitation

Flocculation is involved during the formation and precipitation of snow in the form of crystals.

Filtration process

In the crops of biological species used in certain industries, there is the generation of particles that could obstruct the filters and the drainage of the crops. Flocculation, by increasing the size of the particles, leaves spaces between them through which the liquid can circulate.

Asphaltene flocculation

Assuming that the asphaltenes in crude oil are in a colloidal state, they are capable of flocculating and sedimenting or precipitating. Many studies maintain that, by varying the natural conditions of the crude oil, the flocculation of asphaltenes is promoted, which sediment as a blackish brown mud. This “mud” or solid represents innumerable problems for the oil industry.

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