چكيده به لاتين
More than one million tons of secondary aluminum dross (SAD) are produced annually by the aluminum smelter industries. Disposal and recycling of this amount of SAD as hazardous industrial waste is a global issue, so finding appropriate recycling strategies is essential. The purpose of this study was to find a suitable process for extracting aluminum oxide from the SAD. Therefore, in the first step, the dissolution kinetics of SAD in hydrochloric acid was investigated to obtain a better understanding of the acid dissolution behavior of the waste. Fitting different kinetic models to experimental dissolution data showed that the Jander’s model could better predict the data, which means that acid dissolution is under the control of acid diffusion into solid particles. The activation energy of the acidic dissolution reactions was 10.5 kJ/mol. Then, the aluminum oxide was extracted through a five-step hydrometallurgy-based process, including acid leaching, precipitation, re-dissolution, re-precipitation, and calcination. Investigating the effect of different parameters on the aluminum oxide extraction efficiency by adopting the “one-factor-at-a time” approach revealed that the leaching temperature of 85 °C, leaching time of 120 min, the acid concentration of 5 M, the acid-to-SAD ratio of 20 ml/g, and particle size of 38-75 μm resulted in extraction efficiency of 83%. The standard deviation of the measured extraction efficiency data was less than 5.5%. X-ray fluorescence analysis showed that the extracted aluminum oxide had a purity of more than 97%. X-ray diffractometry disclosed that the aluminum oxide contained metastable crystalline phases of γ, κ, and θ with an average crystallite size of 106.7 nm. Field emission scanning electron microscopy revealed a rounded-corner morphology that some particles were more than 600 nm in size. Investigating the effect of pH and aging time as the most influential factors on the structure of aluminum hydroxide gel extracted from the SAD showed that gamma phase of aluminum oxide with a specific surface area of about 137 m2/g and total pore volume of 0.3 cm3/g is possible. In the next step, Mg-Al-CO3 layered double hydroxide (LDH) was synthesized from the SAD. The characterization results showed that the synthesized LDH had an interlayer space of 3.1 Å, a specific surface area of 122 m2/g, and a pore volume of 0.46 cm3/g. In the second phase of the study, the gamma-aluminum oxide and LDH were used to adsorb fluoride and nitrate from aqueous solutions, respectively. The adsorption results revealed that the amount of fluoride adsorbed by gamma-aluminum oxide at room temperature, the adsorbent dosage of 1 g/l, initial fluoride concentration of 20 mg/l, and pH=4, was 16.5 mg/g. The standard deviation of the measured fluoride adsorption data was less than 5.6%. Fluoride adsorption occurs through chemical adsorption, and carbonate and phosphate anions had the most interference with fluoride adsorption. Also, the results of nitrate adsorption by the synthesized LDH showed that nitrate adsorption occurs through chemical adsorption and diffusion mechanisms. The amount of nitrate adsorbed by the LDH at room temperature, adsorbent dosage of 1 g/l, initial nitrate concentration of 100 mg/l and pH=6 was 65.6 mg/g. The standard deviation of measured adsorption data was less than 5.9%. Fluoride and phosphate anions had the most interference with nitrate uptake. Comparing the results with conventional commercial sorbents such as purolite resin showed that the Mg-Al-CO3 LDH had a higher nitrate uptake capacity.
