Surface treatment generates an important part of French industrial pollution. Effluents contain a mixture of toxic heavy metals, such as nickel, zinc or chromium. In order to comply with regulation, theses pollutants have to be eliminated before discharge into the environment. The most used process is the hydroxide precipitation with soda or lime. It allows satisfying results in terms of discharge thresholds. Mainly based on craftsmanship, the optimization of the depuration process is then limited. But, facing regulations becoming more and more strict and having to consider the best available technologies, metal finishers must take care of the optimisation of the metal mixture precipitation efficiency Even if there is a lot of research works concerning heavy metals alone in solution, studies on heavy metals mixtures are much limited. Then, second degree mathematical models are built linking the precipitation efficiency to the operating conditions (pH, cation proportion, agitation speed and duration) for bimetallic synthetic solutions of zinc, nickel and chromiumIII. The given results allow to deduce the optimal pH of precipitation for the metal proportion rates. In spite of conclusive validation results, it appears that models concerning the two divalent metals in mixture with chromiumIII are not really satisfying. The quadratic mathematical model seem not to be adapted. Moreover, results shows that the chromiumIII reduces the optimal precipitation pH of zinc and nickel down to 6.4, its own precipitation pH. A pH of 10 allows the zinc and nickel precipitation, in mixture, particularly in a 50/50 rate. In terms of industrial application, this results into the possibility and the interest (reactive economy) to simultaneously precipitate in one hand the chromium with the zinc or the nickel, and on the other hand the zinc and the nickel together. The pilot scale tests confirm the laboratory scale results. The optimisation of the precipitation step in its wholeness includes also the valorisation of the generated hydroxide sludge. Lamellar Double Hydroxides (LDH) are compounds with unique properties permitting among other applications, the absorption of heavy metals. The analysis of industrial sludge and of laboratory sludge generated from industrial effluents showed the presence of structure close to LDH. The possibility of using industrial sludge for absorbing chromates have been exposed by our research work. This exploring phase is promising and opens interesting perspectives for the valorisation of hydroxide sludge.
