| Abstract |
Architectural aluminium production facilities use acid etching and anodizing processes to create their finished products, and these processes produce aluminium-anodizing residue (AAR), which is commonly disposed of in a nearby landfill. There is a risk of aluminium leakage in acidic conditions to the soil and to the ground and surface water. An environmental impact may be caused if the aluminium water concentration rises above the environmental maximum allowable limits. The purpose of the research was to examine the recovery of aluminium, silicon, sodium, potassium, magnesium, and calcium in the AAR through synthesis of zeolites of low molar SiO2/Al2O3 ratio. The aluminium content of the AAR was high, as determined by Scanning Electron Microscopy with Energy Dispersive System (SEM-EDS; average wt% 39.4) and flame atomic absorption spectrometry (FAAS; average wt% 37.2); it also had moderate concentrations of silicon, magnesium, sodium, calcium, iron, and potassium. Because zeolites contain aluminium and all these elements in their structure, we examined upcycling the AAR via zeolite synthesis, rather than discharging it in a landfill as a way to prevent the environmental impact and create an economical benefit to the waste. The synthesis was carried out by a hydrothermal process under alkaline conditions, which has been used to synthetize laboratory and commercial zeolites. A series of gel samples were prepared, combining moist commercial-grade sodium silicate (CSS) with wet and dried AAR to enrich the silicon content of the mixture. It process took advantage of the moisture in the AAR to avoid the need to add water. Proportions of AAR:CSS (w/w) used were 2.6, 0.83, and 3.8. The mixtures were kept at constant temperature (85 degrees C) in a closed system at reaction times of 4 and 8 h. Sodium hydroxide (1.5M) solution was tested as mineralizing agent. Blends 1A and replicate led to the synthesis of a Linde Type A (LTA) dehydrated zeolite with an empirical formula of Na-91.7[Si96Al96O384], a crystalline phase material identified using X-ray diffraction (XRD). The final dried material shows a SiO2/Al2O3 molar ratio 2.1 (samples 1A, 1B); 2.1; 1.8 (samples 2A, 2B). SEM image 8, blends 2A, 2B, indicates an improvement of crystals morphology with a reaction time of 8 h. The remainder samples show a molar ratio SiO2/Al2O3 lower than 1.0 and in this case the SEM-EDS images do not show a well-defined geometrical shape. The observed XRD peaks were consistent with characteristic peaks of synthetic dehydrated LTA zeolites. The XRD results demonstrate that a pure LTA crystalline zeolite was created. A complex, high magnification image of the zeolite surface topography was obtained using SEM-EDS. Well-defined spherical particles were observed with a blend of 2.6 moist AAR:1.0CSS (w/w), 85 degrees C reaction temperature, and 8 h reaction time. These results demonstrate that it is possible to recover aluminium, silicon, potassium, calcium, and sodium from dewatered gelatinous AAR, which can contribute to sustainable development and promotion of the circular economy. |