special case of emulsions

The conditions governing the flocculation of hydrocarbon or oil emulsions will depend on the nature of these emulsions :

• mechanical emulsions are relatively unstable and characterised, after a one-hour preliminary static sedimentation, by micelle measuring from ten to one hundred microns;

• chemical emulsions are relatively stable either because of the nature of the hydrocarbons concerned (asphaltenes, naphtenates), or because of the co-presence of dispersants (alkaline salts, detergents…). After a one-hour preliminary static sedimentation stage, these emulsions can be characterised by micelle measuring between 0.1 micron (micro-emulsion) and some tens of microns and a very variable hydrocarbon concentration ranging from 100 mg · L^–1 (complex petrochemical effluent) to 50 g · L^–1 (water-based cutting fluids).

As in the case of colloids, the coagulation process applicable to these emulsions require the zeta potential to be neutralised; however, afterwards, droplet coalescence mechanisms may predominate.

Treatment of mechanical emulsions may include destabilisation or partial coagulation using an organic coagulant direct followed by coalescent filtration or flotation.

Chemical emulsion processing must include comprehensive coagulation followed by flocculation and a separation step using dissolved-air flotation.

Ultrafiltration membranes are increasingly being used to achieve an immediate droplet/water separation without the need to destabilise the emulsion; on the contrary, the more stable the emulsion, the more it can be concentrated by preventing it from coating the membrane with a film. This method produces a concentrate containing up to 30-40% of oil and a hydrocarbon-free aqueous phase.