Leaching Process Investigation of Secondary Aluminum Dross: The Effect of CO 2 on Leaching Process of Salt Cake from Alu
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. 2—Molar fractions of dissolved hydrolysis products of mononuclear aluminum hydroxides in equilibrium with amorphous hydroxides.[10]
equilibrium with various amorphous hydroxyl ions in solution. Preliminary experiments (Section IV–C) carried out as part of the current work showed that CO2-saturated water had a pH value of approximately 3.9. From Figure 2, it is observed that at this pH value, the most stable species in solution are Al3+ and Al(OH)2+. Because Al(OH)3 is relatively less stable, it would be expected that Reaction [2] may be hindered when AlN comes into contact with carbonated water. Thus, the use of CO2-saturated water would have two functions: (a) Hinder significantly the ammonia forming reaction, viz. the hydrolysis of AlN. (b) The small amount of NH3 produced can be effectively absorbed by the formation of NH4HCO3 species in aqueous solution. Based on the preceding reasoning, it is expected that leaching with CO2-saturated water would dissolve most of the chlorides and part of the iron present in the salt cake. The residue after leaching would contain Al2O3 and lesser amounts of SiO2, MgO, and AlN, along with metallic Al. VOLUME 43B, OCTOBER 2012—1221
This is likely to form an ideal precursor for the synthesis of high-performance oxynitride ceramics.[11] Experimental work is needed to optimize the conditions for minimizing the hydrolysis of AlN. Another advantage is to utilize the anthropogenic CO2[12]; the fixation of the same as ammonium carbonate can be utilized as a fertilizer. Corresponding reference experiments were conducted with pure synthetic AlN, and the results were compared with those obtained with salt cake leaching. Previous experiments[13] showed an acceleration effect of saline water on the hydrolysis of AlN. In the current work, the leaching of AlN was carried out with both deionized and saline water.
III.
EXPERIMENT
A. Sample Preparation The aluminum dross used in the current work was supplied by Stena Aluminum AB (A¨lmhult, Sweden). The samples consisted of rounded lumps up to approximately 10 mm in size (as well as some smaller metallic fragments) and had a slight smell of ammonia. The samples were crushed using a pulverizer and sieved through a 100-lm aperture screen. A sample before and after crushing is shown in Figure 3. The dry solids were sampled and used for X-ray diffraction, chemical analysis, and subsequent leaching tests. In the case of the leaching treatment, deposits were taken out from the suspensions, filtered, and washed with 2-propanol to remove the adherent water. The residues were dried at 333 K (60 °C) for 8 hours and then stored in plastic, airtight containers before conducting the characterization tests. The leach liquids from each test were also subjected to a chemical analysis. Corresponding experiments were also performed on pure, synthetic AlN samples to understand the impact of CO2 on the hydrolysis behavior. B. The Leaching Experiments with Salt Cake from Al Remelting The experimental apparatus used in the current investigation is shown in Figure 4. Th
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