SYNTHESIS OF HYDROTALCITE-LIKE COMPOUNDS FROM ACID WASTE STREAMS FROM THE AI-ANODISING INDUSTRY

摘要

The sulphuric acid process is the most common method for anodisation. Such treatment invariably produces a large volume of waste acid effluents containing mainly sulphuric acid and aluminium. Conventionally these effluents are neutralised with the basic streams from the etching step, producing a large amount of aluminium sludge that causes serious disposal problems. Furthermore, this process implies a continuous loss of resource materials. Alternative ways to deal with the environmental impact of the anodising industry are proposed. These lie in the recovery of materials from the waste streams by means of geochemical engineering. Synthesis of hydrotalcite-like compounds from the acid waste streams can recover almost 100% of the Al lost during the anodising stage. Waste waters of any concentration are suitable for this synthesis, from concentrated ones (about 60 g/l SO_4~(2-) and 6 g/l Al) to the more diluted rinsing effluents. Anionic clays such as hydrotalcite are suitable for catalysis and environmental applications.