Flocculation
As flocculation or flocculation ( latin flocculus , small tufts wool ') refers to processes in which finest suspended or colloidal solid particles (<1 micron) in water to form larger flakes are combined, to then by sedimentation or filtration deposit better from the water and to be able to remove. This technology is used in particular for water and wastewater treatment . These are often purely physical processes, but chemical complex formations can also play a role. After a previous precipitation , it is also possible to remove dissolved substances such as phosphates or parts of the dissolved organic carbon from the water by means of flocculation.
When it comes to the chemicals used for flocculation, a distinction is made between flocculants and flocculants . Flocculants (also known as coagulants ) are mainly used to destabilize the colloidal state and the associated approach of the particles. This process is also known as coagulation . In particular, compounds are used which, as counter ions or complexing agents, neutralize the charge on the particles and thus reduce their repulsive potential. In particular, iron and aluminum salts, but also other hydroxide formers, are used. The setting of the optimal pH value is of decisive importance.
Flocculation aids, on the other hand, serve to accelerate the further agglomeration of the resulting microflakes to form larger macroflocks. For this purpose, high molecular weight polyelectrolytes (in common parlance “polymeric flocculants”) are used, mostly copolymers based on acrylamide , which can be anionic or cationic.
Coagulation and agglomeration usually take place in parallel. In the practical application of flocculation, mixing conditions (agitation conditions) and maturation time can play an important role in the efficiency of the separation, especially in systems with a low solid content (e.g. surface water). Flocculation is also used in sludge dewatering or in sewage and dewatering processes in industry (paper manufacture, mining, chemical processes). However, it can also be used to remove phosphate from surface waters, such as in the OWA (surface water treatment) in Berlin-Tegel, in order to avoid the eutrophication of natural waters.
Occurrence in metallurgy
Larger workpieces made of alloyed steel tend to lead to undesired material separation due to the precipitation of hydrogen from solid solution with reduced diffusibility. This flocculation can be avoided by preheating the aggregates , adding alloys and using special forming processes with delayed cooling.
See also
Individual evidence
- ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 424.