Heavy metal impact on disposal and reclamation of aluminum‐anodizing residues

Recycling of aluminum and caustic soda solution from waste effluents generated during the cleaning of the extruder matrixes of the aluminum industry

“…Due to this, the final composition of these wastewaters is such (pH ≥ 15.5) that the direct regeneration by caustic process is not feasible [12]. The method developed in this paper for recycling aluminum from the wastewaters of cleaning the extruder matrixes comprises a departure from the prior art methods where water or seeds of Al(OH) 3 are added to the waste solution [6,9,12,13]. In U.S. Patent number 4,372,805 [13], a method for recovering sodium hydroxide and aluminum hydroxide from etching waste is shown; for this purpose, water is added to the solution containing dissolved aluminum to create a supersaturated solution of aluminum hydroxide and crystallize the aluminum hydroxide.…”

“…Conventionally, anodising industries use a small part of this alkaline waste stream by mixing it with part of the acid wastewaters coming from the anodising step, producing the so called anodising mud. This process causes a continuous loss of resource materials (aluminum and sodium hydroxide), as well as it results in generation of very large amounts of solid aluminum hydroxide anodising residues [3]. These residues are typically dewatered to reduce the volume of waste prior to being landfilled.…”

“…In U.S. Patent number 4,372,805 [13], a method for recovering sodium hydroxide and aluminum hydroxide from etching waste is shown; for this purpose, water is added to the solution containing dissolved aluminum to create a supersaturated solution of aluminum hydroxide and crystallize the aluminum hydroxide. On the other hand, Al(OH) 3 was added to the spent aluminum etching as a seed crystal and fine nucleating agent in the agglomeration process [12]. In this work, due to the high pH of the wastewaters produced for cleaning the extruder matrixes (pH ≥ 15.5), the addition of more Al(OH) 3 would be solubilised as Al(OH) 4 − (aq), according to the following equation:…”

“…On the other hand, Al(OH) 3 was added to the spent aluminum etching as a seed crystal and fine nucleating agent in the agglomeration process [12]. In this work, due to the high pH of the wastewaters produced for cleaning the extruder matrixes (pH ≥ 15.5), the addition of more Al(OH) 3 would be solubilised as Al(OH) 4 − (aq), according to the following equation:…”

“…Among them, caustic regeneration is a proven technology and has been in use in the United States and other countries for many years [11]. Such a system, based on the Bayer process, consists of adding fine crystals of aluminum hydroxide, Al(OH) 3 , which act as seeds to enhance crystallization, to the etching solution kept at a proper temperature. After some days, Al(OH) 3 begins to precipitate, resulting in an increase of sodium hydroxide concentration in solution:…”