Environment
A Novel Process for Recycling of Aluminum Dross Using Alkali Fusion
2020 Volume 61 Issue 11 Pages 2208-2215
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2020 Volume 61 Issue 11 Pages 2208-2215
Aluminum dross discharged from an aluminum production factory can react with water and emit hazardous gases such as hydrogen and ammonia causing serious environmental pollution. Thus, it becomes necessary to recycle the by-products to avoid such problems. In this study, the feasibility of the alkali fusion process to convert the dross into benign and functional material was investigated. The effects of fusion temperature, dross/NaOH ratio, and the heating time on the amount of gas removed from the dross and the soluble contents of Si and Al in the fused dross were examined. Synthesis of zeolite–A from the fused dross was performed by reacting with sodium silicate. The optimum condition to dissolve the minerals Al and Si and maximize the generation of gases was a fusion temperature of 400°C, the ratio of the raw dross to NaOH of 1.0, and the heating time of 3 h. The fused dross can be converted into zeolite-A product with a high cation exchange capacity (3.22 mmol·g−1) by reacting with sodium silicate solution while generating as much gas as that generated in distilled water. These results demonstrate the applicability of the alkali fusion process to recycle the aluminum dross waste generated from an aluminum industry into value-added material, thus contributing to the circular economy while reducing the environmental impact.
To achieve sustainable development in industries with minimal impact on the environment, recycling of industrial waste is essential. Therefore, recycling of waste aluminum dross, which is formed by natural oxidation of molten aluminum surface, is of great importance. Aluminum dross is a hazardous by-product generated primarily in aluminum smelters and aluminum recycling processes. Approximately 5 million tons of aluminum dross is generated annually worldwide.1) Recycling of aluminum dross is challenging and has become a focus of research in recent years. In Japan, aluminum dross is mainly recycled as a deoxidizer in steel-manufacturing processes; however, its demand is limited. Thus, a large amount of aluminum dross is disposed in landfills.2) The main components of dross are metallic aluminum, and its compounds such as oxide, chloride, and nitride, along with other oxides such as SiO2.3–5) The metallic aluminum (Al) and aluminum nitride (AlN) present in the dross can be hazardous when disposed in landfills because of their potential to initiate spontaneous combustion and to emit the hazardous gases including H2, and NH3.6) This is one of the important motivations for the development of effective disposal and recycling technologies for aluminum dross.
The removal of hazardous gases is essential while recycling aluminum dross into valuable products. Several studies focus on utilizing the alkali solution to remove the hazardous gases from the dross. Fukumoto et al. reported that NaOH solution catalyzed the hydrolysis of AlN powder and accelerated the rate of hydrolysis as compared to deionized water.7) Nosaka et al. analyzed the kinetics of the hydrolysis of AlN powder in highly alkaline solutions to generate NH3 and proposed the following equation:8)
| \begin{equation} \text{AlN} + \text{OH$^{-}$} + \text{3H$_{2}$O} \to \text{Al(OH)$_{4}{}^{-}$} + \text{NH$_{3}$} \end{equation} | (1) |
Hiraki et al. reported the evolution of hydrogen gas from waste aluminum powder using NaOH solution by the following equation:9)
| \begin{equation} \text{Al} + \text{OH$^{-}$} + \text{3H$_{2}$O} \to \text{1.5H$_{2}$} + \text{Al(OH)$_{4}{}^{-}$} \end{equation} | (2) |
The reactions (1) and (2) are exothermic.
Moreover, the reaction with alkali solution is also utilized to produce valuable products from aluminum dross. Meshram et al. demonstrated the recovery of hydrogen gas from aluminum dross using NaOH and KOH solutions along with alumina by calcination at 900°C.10) Gil et al. synthesized hydrotalcite-like compounds from the slurries of aluminum saline slag wastes leached with NaOH solution by co-precipitation.11) El-Katatny et al. recovered aluminum in the form of alumina from waste produced from an aluminum factory using atmospheric and high-pressure leaching with caustic soda.12) Mesharm et al. synthesized tamarugite for use in water purification from aluminum dross using NaOH leaching.13) The aluminate ion (Al(OH)4−) which was prepared by dissolving Al or AlN in NaOH solution to produce the value-added materials. However, the residue obtained after filtrating the leachate after leaching of aluminum dross with NaOH contains Al as hardly soluble oxides, such as Al2O3, and MgAl2O4, which become industrial waste.
To solve this problem, Singh et al. sintered a mixture of aluminum dross and sodium carbonate at 900°C to break the crystalline phases of quartz and mullite and recovered alumina and ceramics to implement the zero-waste concept.14) Tripathy et al. also applied the calcination of aluminum dross with sodium carbonate at 800°C and dissolved in NaOH solution to recover 90% of Al content in dross.15) Therefore, it would be possible to recover the mostly contents of Al and Si, including hardly soluble oxides, in the dross using alkali fusion on the zero waste concept. However, high temperature over 800°C is needed to fuse the dross using sodium carbonate, and there is no information on the removal of hazardous gases from the dross.
In our previous studies, we successfully converted hardly soluble oxides — such as SiO2 found in crushed stone dust — into highly soluble dust by alkali fusion treatment with NaOH at 400°C to synthesize zeolitic materials.16–18) Additionally, we implemented a large-scale continuous conversion of stone dust into fused materials with high solubility using a rotary kiln from which faujasite zeolite can be synthesized.19,20) Therefore, there is a possibility to convert the dross into mostly soluble fused material at lower than 400°C using NaOH to synthesize high-value added zeolitic material, especially zeolite-A with low Si/Al molar ratio, indicating high cation exchange capacity (CEC). However, little information can be available on the alkali fusion of aluminium dross with NaOH, the removal of hazardous gases, and the synthesis of zeolite from the dross fused with NaOH.
In this study, application of alkali fusion with NaOH to convert the insoluble minerals of the aluminum dross into soluble minerals and synthesize zeolitic material as value-added material was examined. A suitable process has been developed to extract large amounts of aluminum from the dross into distilled water by heating with sodium hydroxide at temperatures of lower than 500°C and utilize the same to synthesize zeolite-A by adding silica sources while removing the hazardous gases generated from aluminum dross during this process.
Aluminum dross used in this study was collected from an aluminum recycling company in Japan. The chemical composition of raw aluminum dross was measured using X-ray fluorescence spectroscopy (XRF) (Epsilon 1, Malvern Panalytical) (Table 1), and the mineralogical composition was analyzed using powder X-ray diffraction (XRD) (MiniFlex600, Rigaku) (Fig. 1). The chemical composition of aluminum dross mainly consisted of Al (46.1%), Si (8.4%), Mg (7.1%), and Cl (6.4%), in the form of metallic Al, Al2O3, AlN, MgAl2O4, SiO2, KCl, and NaCl, and some minor compounds.
XRD patterns of (a) raw dross and fused dross at (b) 200°C, (c) 300°C, (d) 400°C, and (e) 500°C.
Effects of experimental conditions such as fusion temperature, NaOH addition, and heating time, on the properties of fused dross were examined. The dross (10 g) and powdered NaOH (5–20 g) were mixed and added to a nickel crucible, which was heated at 200–400°C in an electric furnace for 0–4 h. After heating, the crucible was cooled to room temperature under natural convection. The resulting fused dross was ground into fused dust (particle size <500 µm) in a ceramic mortar.
The fused dust (5 g) was added to 50 mL of distilled water and stirred at 80°C for 24 h. The amount of gas generated during stirring was measured by collecting in a gas pack. After stirring, the residual solids were filtered and dried overnight in a drying oven. The mineralogical composition of the residue was analyzed using XRD.
The soluble salts of Si and Al present in the fused dust were examined as follows. The fused dust (0.1 g) was added to 10 mL of 1 M HCl and shaken for 24 h at room temperature. After shaking, the solution was centrifuged and the supernatant was analyzed using atomic absorption spectrophotometer (AAS) (AAnalyst200, PerkinElmer) to evaluate the concentrations of Si and Al in the supernatant. To calculate the soluble ratios of Si and Al in the fused dross, the completely fused dross was prepared using sodium carbonate (Wako). The raw dross (1 g) was mixed with sodium carbonate (8 g) to heat at 1000°C for 6 h in a platinum crucible, and dissolve in 10 M HCl solution to analyze the contents of Si and Al in the solution using AAS. These concentrations were used to calculate the soluble ratios of Si and Al in the fused dross to those from the dross fused with sodium carbonate.
2.3 Zeolite synthesisThe fused dross obtained at the fusion temperature of 400°C, mixing ratio of NaOH to raw dross of 1.0 and heating time of 3 h was used for zeolite synthesis. It is noted that the target zeolitic material is zeolite-A with high CEC.
To examine the effect of adding silica concentration on the gas generation, the fused dross (5 g) was added to 50 mL of sodium silicate solution (0–20 g·L−1) and stirred at 80°C for 24 h. The gas generated during stirring was measured by collecting in a gas pack. After stirring, the solid residue was filtered and dried overnight at 60°C in a drying oven to obtain zeolite-A.
To examine the effect of reaction time on the properties of the product, the fused dross (50 g) was added to 500 mL of distilled water or sodium silicate solution with 20 g·L−1 and stirred at 80°C for 24 h. The gas emitted during stirring was collected and analyzed. Samples (4 mL) of the slurry were collected at regular intervals. The residual solid after the reaction was filtered and dried at 60°C in an oven to obtain zeolite-A. The concentrations of Si and Al in the filtrate were analyzed by AAS.
The mineralogical composition of the product was analyzed using XRD and its CEC was measured by the modified Schörrenberg method,21) and compared with that of target material, commercial pure zeolite-A (MS-4A, Wako). Relative crystallinities of zeolite-A and gibbsite (Al(OH)3) in the product were calculated by the method reported by Machado and Miotto22) using commercial zeolite-A (Wako) and aluminium hydroxide (Wako) as standards, respectively.
The detoxification of the raw aluminum dross was performed by the alkali fusion reaction that promotes the generation of gases such as hydrogen and ammonia.
XRD patterns of raw dross and fused dross obtained at various temperatures are shown in Fig. 1. It should be noted that the ratio of the raw dross to NaOH was kept at 1.0 and the heating time at 3 h. Figure 1 shows that the formation of new compounds (NaAlO2 and Na4Al2Si2O9) in the fused dross along with diminished peaks of SiO2, Al2O3, and MgAl2O4. It can be concluded that the oxide minerals — SiO2, Al2O3, and MgAl2O4 — were converted into sodium salts — NaAlO2 and Na4Al2Si2O9 — by the alkali fusion reaction. The peaks corresponding to unreacted NaOH were observed in the dross fused at 200°C and 300°C indicating that these reaction temperatures are inadequate to convert the NaOH during alkali fusion reaction.
XRD patterns of the residue after distilled water treatment of raw dross as well as that of the fused dross at various temperatures are shown in Fig. 2. In the residue after distilled water treatment of raw dross, the mineral phases of NaCl, KCl, metallic Al and AlN diminished, while that of MgAl2O4, Al2O3, and SiO2 were retained. The XRD of the residues of fused dross after distilled water treatment exhibits broad peaks akin to amorphous phases. It could be concluded that sodium salts of oxides — SiO2, Al2O3, and MgAl2O4 — dissolved and increased the pH of the solution which promoted the dissolution of Al and AlN to create amorphous aluminosilicate or aluminum hydroxide gel. Notably, residue from the fused dross at 200°C and 300°C contained SiO2 due to the incomplete alkali fusion reaction.
XRD patterns of the residue after distilled water treatment of (a) raw dross and fused dross at (b) 200°C, (c) 300°C, (d) 400°C, and (e) 500°C.
The soluble ratios of Al and Si in the fused dross at various temperatures is shown in Fig. 3. The ratios of soluble Al and Si in raw dross were 67.6% and 41.4%, respectively. After the alkali fusion reaction, the soluble aluminum content increases to 100% due to the conversion of aluminum oxides into soluble aluminum salts. The soluble Si content gradually increased to 50% with a rise in the reaction temperature of 200–300°C, and then further increased to 100% when the reaction temperature was 400–500°C. This can be attributed to increased conversion of SiO2, during the fusion reaction as the reaction temperature increases.
Soluble contents of Al and Si in the fused dross obtained at various temperatures.
The amount of gas generated during the distilled water treatment from the fused dross obtained at various reaction temperatures is shown in Fig. 4. It is found that the amount of gas generated from the fused dross increases as the reaction temperature is increased and saturated to 240 mL for temperatures of 300°C and above.
The amount of gas generated during the distilled water treatment from the fused dross obtained at various temperatures.
These results suggest that a fusion temperature of more than 400°C is necessary to solubilize the Al and Si content in the dross and maximize the generation of gases.
XRD patterns of the fused dross obtained at different ratios of raw dross and NaOH are shown in Fig. 5. It should be noted that the temperature was kept at 400°C and the heating time at 3 h. The fused dross has NaAlO2 and Na4Al2Si2O9 as new phases. At the NaOH/raw dross ratio of 0.5, small peaks of SiO2 were observed, these peaks diminished as the ratio increased to 1.0. At ratios of 1.5 and 2.0, peaks of NaOH observed again which could be the excess NaOH.
XRD patterns of the fused dross obtained by the addition of NaOH at the NaOH to raw dross weight ratios of (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0.
XRD patterns of the residue after distilled water treatment of the fused dross obtained with different NaOH/raw dross ratios are shown in Fig. 6. The residues after distilled water treatment of fused dross with NaOH/raw dross ratio of 1.0 or more shows diminished broad peaks akin to amorphous phase, whereas the mineral phases of MgAl2O4, Al2O3, and SiO2 were present when the NaOH/raw dross ratio was 0.5, which could be due to insufficient NaOH for the alkali fusion reaction.
XRD patterns of the residue after distilled water treatment of the fused dross obtained by the addition of NaOH at the NaOH to raw dross weight ratios of (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0.
Soluble ratios of Al and Si in the fused dross obtained at different NaOH/raw dross ratio are shown in Fig. 7. It is found that the soluble Al ratios are about 95% at the ratio of 0.5–1.0, and gradually decrease at the ratio of 1.5–2.0. With an increase in the added NaOH, the soluble Si content increased gradually to 100% due to the alkali fusion of SiO2.
Soluble contents of Al and Si in the fused dross obtained with the addition of NaOH at a weight ratio of NaOH to raw dross of 0.5–2.0.
The amount of gas generated during the distilled water treatment of the fused dross obtained at different NaOH/raw dross ratios is shown in Fig. 8. It is found that the amount of gas generated is maximum (240 mL) at the NaOH/raw dross of 1.
Amount of gas generated during the distilled water treatment from the fused dross with the addition of NaOH at the NaOH to raw dross weight ratio of 0.5–2.0.
These results suggest that the optimum mixing ratio of the raw dross: NaOH is 1:1 to maximize the dissolution of the Al and Si content as well as gas generation.
XRD patterns of the fused dross obtained at various heating times are shown in Fig. 9. It should be noted that the reaction temperature was 400°C and the raw dross to NaOH ratio was 1.0. It was found that sodium salts were formed after 1 h heating.
XRD patterns of fused dross by reacting with NaOH at 400°C for (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.
XRD patterns of the residue after distilled water treatment of the fused dross obtained at various heating times are shown in Fig. 10. It is found that regardless of the heating time, the residues after distilled water treatment show broad peaks akin to amorphous phases.
XRD patterns of the residue after distilled water treatment from the fused dross obtained by reacting with NaOH at 400°C for (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.
The soluble ratios of Al and Si in the fused dross obtained at various heating times is shown in Fig. 11. With an increase in the heating time, the soluble contents of Al and Si increased gradually to 100% due to increased conversion of the Al and Si oxides during the alkali fusion reaction.
Soluble contents of Al and Si in the fused dross obtained by reacting with NaOH at 400°C for various reaction times.
The amount of gas generated from the fused dross during the distilled water treatment obtained by varying the heating time is shown in Fig. 12. It is found that the amount of generated gas saturates at 250 mL for heating times of 3 h or more.
Amount of gas generated during the distilled water treatment from the fused dross obtained by reacting with NaOH at 400°C for various reaction times.
To summarize, the optimum conditions for the alkali fusion reaction to form soluble salts of Al and Si oxides and generate the maximum amount of gases from the fused dross are a fusion temperature of 400°C, the ratio of the raw dross to NaOH of 1.0 and heating time of 3 h.
3.2 Synthesis of zeolite-AZeolite-A was synthesized using the fused dross prepared under optimized conditions by reacting with sodium silicate (0–20 g/L).
XRD patterns of the product synthesized by reacting fused dross with sodium silicate are shown in Fig. 13. It should be noted that the products obtained from the reaction with 0, 5, 10, and 20 g·L−1 of sodium silicate solution are indicated as Product-1, Product-2, Product-3, and Product-4, respectively. It is found that all products contain crystals of zeolite-A. Product-1 is a mixture of zeolite-A and gibbsite (Al(OH)3); however, the amount of gibbsite decreases and that of zeolite-A increases with an increase in the Si concentration during the synthesis.
XRD patterns of (a) Product-1, (b) Product-2, (c) Product-3, and (d) Product-4.
CECs of the products obtained from the reaction are shown in Table 2. It is found that the CEC of the product increases with an increase in the Si concentration during the synthesis. The CEC of Product-1 (without reacting with sodium silicate) is 1.12 mmol·g−1 while that of Product-4 is 3.22 mmol·g−1 which is similar to that of commercial zeolite-A (MS-4A).
SEM image of raw dross and Product-4 is compared in Fig. 14. Cubic shaped crystals were observed in Product-4 while a mixture of different morphologies such as rod-like and spherical, was observed in raw dross.
SEM images of (a) raw dross and (b) Product-4.
The concentrations of Si and Al and the relative crystallinities of the product phases during the reaction of fused dross with 0 g·L−1 and 20 g·L−1 sodium silicate are shown in Fig. 15. In the case of the reaction with 0 g·L−1 sodium silicate solution, the concentration of soluble Al always exceeded that of Si during the reaction. Initially, the concentrations of Si and Al increased upon addition of the fused dross but as the reaction progressed, the Si concentration reduced to zero by forming aluminosilicate gel. However, the Al concentration starts to decrease gradually after 6-h of reaction time thus accelerating the formation of zeolite-A and gibbsite. The relative crystallinities of zeolite-A and gibbsite were almost constant until 8 h and were found to increase afterward. In the case of reaction with 20 g·L−1 sodium silicate solution, the concentration of Al increases initially, reaching a steady-state after 2 h. This residual Al concentration is lower than in that in the former case indicating that a large amount of aluminosilicate precipitates out. The relative crystallinity of zeolite-A increases in the early stage and saturates after 4 h.
Concentrations of Si and Al in the filtrate and intensities of the product phases obtained during the reaction in (a) distilled water and (b) 20 g/L sodium silicate solution.
The CEC of the product during the reaction is shown in Fig. 16. CEC of Product-4 increased with increasing the reaction time and saturates after 6 h of reaction. In contrast, the CEC of Product-1 saturates almost immediately from the beginning of the reaction. These results are consistent with the appearance of zeolite-A peaks (Fig. 15). The CEC of Product-4 (4 mmol·g−1) is approximately four times higher than that of Product-1 (1 mmol·g−1) and can be attributed to the higher crystallinity of zeolite-A in Product-4.
CEC of the samples during the synthesis of Product-1 and Product-4.
The amount of gas generated during the reaction of fused dross with distilled water (0 g·L−1 sodium silicate solution) and that with 20 g·L−1 sodium silicate solution is shown in Fig. 17. In both cases, the amount of gas generated increases sharply at first and saturates to a final value of 800 mL. The saturation time for reaction with distilled water was 0.5 h while that with 20 g·L−1 sodium silicate solution was 2 h. Therefore, the amount of gas generated in both cases was the same but the gas generation rate was higher in distilled water than in 20 g·L−1 sodium silicate solution. Thus, 2 h of reaction time is sufficient during zeolite synthesis from the fused dross to generate gas in the same amount as that in distilled water.
The amount of gas generated from the reaction of fused dross with distilled water and with 20 g/L of sodium silicate solution.
This suggests that the fused dross can be converted into zeolite-A with high CEC by reacting with 20 g·L−1 sodium silicate solution while removing the hazardous gases.
Waste aluminum dross obtained from a recycling facility consisted mainly of aluminum and silica in the form of Al, AlN, Al2O3, MgAl2O4, and SiO2, which are insoluble in distilled water. The alkali fusion treatment of the raw dross to convert the oxide minerals — Al2O3, MgAl2O4, and SiO2 — into soluble sodium salts — NaAlO2 and Na4Al2Si2O9 — while Al and AlN remained intact. When the fused dross was added to distilled water sodium salts dissolved to form an alkaline solution which promoted the dissolution of Al and AlN to generate gases. The optimum fusion conditions to solubilize the Al and Si content of the dross and generate the maximum amount of gases are a fusion temperature of 400°C, a ratio of raw dross to NaOH of 1.0, and heating time of 3 h.
The fused dross obtained under the optimum fusion conditions was converted into zeolite-A crystals using a sodium silicate solution. The CEC of the resulting product (zeolite) increased upon increasing the concentration of sodium silicate in the solution. The highest CEC of the product is 3.22 mmol·g−1, which is almost the same as that of commercial zeolite-A. Also, the reaction with sodium silicate did not affect the amount of gas generated from the fused dross.
In this study, the alkali fusion treatment on aluminum dross waste to detoxify the dross by removing the hazardous gas and converting it into a value-added product, zeolite-A was succeeded.
The financial support for this work was provided by the Aluminum Research Grant Program of the Japan Aluminum Association.
