The hot dip galvanisation is the second way for the protection against corrosion after painting; in consideration of its adaptation to many geometrical forms of objects. Nevertheless, this activity involves a very important pollution, particularly concerning waste acid effluents. This pollution results from several operation included in hot dip galvanisation treatment; for workshops that treats 10 000 tons of sheet metal per a year, we consider the next polluting quantities: * Pickling: operation of elimination of ferrous oxides by hydrochloric acid, this one creates 160 m3 per year of waste effluents containing ferrous chloride (FeC12), acid traces and zinc chloride (ZnC12). * Etching: operation of elimination of zinc by the hydrochloric acid, this task creats 160 m3 per a year of waste effluents containing zinc chloride (ZnC12), acid traces and ferrous chloride (FeC12). In Europe workshops of hot dip galvanisation creat 10 000 m3 per a year of waste acid effluents dedicated to in approved centers. By an other way, the working method in workshops of galvanisation, consisting of suspending the pieces to treat with hooks (tools), conduce to a surconsumption of zinc, and to exhaust more quickly etching baths and pickling baths; because hooks follow the treatment four times a day, in a first task hooks are covered with zinc and the acid eliminate the zinc in a second task.This method involves by one hand, surconsumption of zinc about 9 tons per a year and of acid about 10 tons per a year, for a workshop treating 10 000 tons of sheet metal per a year, and by an other hand the contamination of pickling bath by zinc. The following proposal fits in the framework of research pertaining to innovative solutions allowing the of waste effluents of hot dip galvanisation workshops. The solutions offered for study and development are as follows: - of waste pickling baths through electrochemical transformation of ferrous chloride into ferric chloride, a product commercialised for water treatment. - Valorisation of etching baths through the recovering of ZnC12 by means of crystallisation, and the use of it as raw material in flux baths which serve in the finishing stage of pickling in hot dip galvanisation workshops. Valorisation of pickling baths is only possible subsequent to the elimination of bath contamination by zinc brought into them by handling hooks. This finding defined a direction for research and development namely the formulation of a specific hook coating inert in the diffusion of molten zinc.
Funding SchemeCRS - Cooperative research contracts
20009 San Sebastian