Coagulation and electrocoagulation of oil-in-water emulsions

In this work the efficiencies of the chemical and the electrochemical break-up of oil-in-water (O/W) emulsions with hydrolyzing aluminium salts are compared. It has been obtained that the efficiency of the processes does not depend directly on the dosing technology, but on the total concentration of aluminium and pH. This latter parameter changes in a different way in the chemical and the electrochemical processes: the pH increases during the electrochemical experiments since the electrochemical system leads to the formation of aluminum hydroxide as a net final product, but it decreases in the conventional ones due to the acid properties of the aluminum salts added (AlCl3 or Al-2(SO4)(3)). The break-up of the emulsions only takes place in the range of pHs between 5 and 9, and the amount of aluminium necessary to produce the destabilization of the emulsion is proportional to the oil concentration. Electrolytes containing chlorides improve COD removal as compared with those containing sulphate ions. Aluminium hydroxide precipitates were found to be the primary species present in solution in the conditions in which the breaking process is favoured. Consequently, the attachment of more than one droplet of oil at a time to a charged precipitate-particle (bridging flocculation) was proposed as the primary destabilization mechanism. (C) 2007 Elsevier B.V. All rights reserved.