Séparation d'insolubles en phase liquide par électroflottation

Separation of insoluble particles in a liquid phase by electro-flotation

Ingénieurs à Saint-Gobain. Techniques Nouvelles (S.G.N.), Courbevoie.

Résumé

Generally speaking, flotation relies on surface and interface tension properties to get solid particles to "hook" on to gas bubbles produced in a liquid. Conventional flotation methods (e.g. ore beneficiation) result in average enrichment in terms of a given constituent towards the surface, whereas the aim of polluted water treatment is to extract large quantities of fine matter after initial coarse separation. Mechanically-produced gas bubbles are too big for really effective results, but sizes as small as 0.2 mm can be produced by electrolysis, this being the limit for laminar rising flow. The most rational industrial system is one with the polluted liquid and bubble flows in opposite directions which gives the best concentration gradients and highest interception probability. The liquid passes through electrodes arranged in sheets at the bottom of the tank and the sludge is removed from the surface by mechanical means. The liquid flow velocity limit is about 4 m/h ; specific current and voltage are 100 A/m2 and 8-10 V ; the bubbles are swept over an effective depth of 0.5-1.0 m. The liquid can be prepared before treatment if required, for example by flocculation, which sometimes makes treatment more effective. The main advantages of the process are that it is very effective, easy to regulate, requires little room, gives a high sludge concentration, is economical to run and has simple conditioning requirements. For example, paper mill effluent at 1 g/l has been treated sucessfully with an input of 140 Wh/m3 without conditioning and 60 Wh/hm3) after flocculation. Residual solids in the water after treatment were less than 10 mg/l in the first case, and there were none in the second. The process is suitable for any raw or polluted water purification application and thus offers an exceptionally wide scope.