Synthesis of dawsonite: a method to treat the etching waste streams of the aluminium anodising industry

Synthesis of dawsonite was studied as a way to deal with the etching waste streams of the aluminium anodising industry in order to reduce the emissions to the environment and also to recover useful and marketable mineral resource materials. The process of synthesis was carried out using two different waste streams arising from the etching section of an anodising process when a cascade rinsing system is employed, the spent etching bath solution (132 g/l of Al and 151 g/l of Na), and the first stage effluent from the cascade rinsing system (67 g/l of Al and 71 g/l of Na). The synthesis of dawsonite was studied as a function of NaHCO3/Al molar ratio (1-10), crystallization temperature (30-150 degrees C), and reaction time (2-48 h) using supersaturated NaHCO3 solutions. A NaHCO3/Al molar ratio of 3 was optimal to obtain dawsonite as a single phase, and a reaction time of 24 h and high crystallization temperature (150 degrees C) to improve its crystallinity. The mineral characterisation was performed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential thermal analysis (DTA), all of which indicated characteristics typical of the desired compound. Almost 100% of the aluminium initially present in the etching waste streams was recovered in the form of dawsonite when the appropriate conditions for its synthesis were used.