Synthesis of Nanostructured Alumina from Byproduct Aluminum Filings: Production and Characterization

Abstract

Aluminum oxide production from aluminum filings, which are a byproduct of several industrial machining processes and cannot be recycled to attain bulk aluminum (Al), is vital due to its wide use in scientific research and industry. The goal of this paper is to produce ultrafine and down-to-the-nanoscale alumina powder (Al₂O₃), starting from a waste Al filings. The microstructure and composition of the starting Al used were investigated using scanning electron microscopy (SEM), which was equipped with an attached energy dispersive spectrometer (EDS) unit. The results of this investigation confirmed that the starting Al was mainly Al–Mg alloy. Al₂O₃ was produced using two routes: The first involved the burning of aluminum hydroxide Al(OH)₃ that was precipitated from aluminum chloride solution (AlCl₃) resulting from dissolving the Al filings in 2M HCl. The second route involved direct precipitation as a reaction product of aluminum chloride with sodium carbonate solution. The Al₂O₃ produced using both routes, as well as the intermediate product Al(OH)₃, were studied by SEM. The results demonstrate that the nanoscale range size was reached after milling of the produced Al₂O₃. Following thorough washing with distilled water, the EDS and the XRD techniques confirmed the formation of Al₂O₃, with no residual salt detected. The EDS results showed that the ratios of Al and O in the produced Al₂O₃ were about 96% of the ideal compound ratios. The XRD analysis also revealed the amorphous structure of the standard and the produced Al(OH)₃, whereas the phases of the produced Al₂O₃ were either crystalline or amorphous. In our study, the Al₂O₃ percentage yield was about 77%, and this value obviously depends on the percentage of Al dross in the original Al filings. Overall, this research provides a novel contribution to the production of alumina powder in the nano-range starting from an aluminum filings byproduct, thereby reducing the dependence on known sources of aluminum.

1. Introduction

The recycling of materials generally leads to an extension of their strategic reserves, an environmental improvement, and eventually a reduction in production costs. Aluminum is the most available and the second most consumed metal after steel worldwide [1]. It is usually easy to recycle due to its physical properties of low °C melting point (~660 °C), and light weight, and it suffers little loss upon recycling because of its excellent corrosion resistance [2,3,4,5,6,7]. One of the most common forms of aluminum recycling is to convert it into aluminum oxide, without reaching its melting point, via a chemical reaction followed by a firing route. The aluminum collected for recycling is chemically transformed into white aluminum hydroxide and then burned in normal atmosphere in a temperature range of 700–900 °C to obtain aluminum oxide (alumina, Al₂O₃). The use of aluminum waste such as scraps, food foil, and cans to produce alumina by this chemo-thermal method has been reported by many researchers in this field [8,9,10,11,12,13]. Most authors of previously published articles used waste aluminum foil, due to the large quantities available and its small thickness. This, normally, leads to easier chemical dissolution, and as a result, reduces the production cost and time. Alumina is considered to be a hard and corrosion-resistant ceramic coating and has shown favorable protection properties on aluminum and steel surfaces [14].

In addition, alumina powder can be produced from aluminum dross, a byproduct of aluminum metallurgy. This subject was well investigated in 2022 by Lin et al., who produced several types of alumina powder, such as θ, γ, and α of Al₂O₃, starting from aluminum dross [15]. They compared different routes of calcination to produce Al₂O₃ powder, such as a normal induction furnace and microwave plasma techniques, to produce alumina powder starting from aluminum hydroxide, which had already been produced by a chemical reaction. The advantage of using the microwave plasma technique is that it can produce α-Al₂O₃ within 5 min. at approximately 700 °C, while the same result was achieved at around 1200 °C for 2 h when using the normal induction furnace. The resulting alumina powder is then generally milled to obtain the required particle size range down to the nanoscale for various applications. Therefore, top-down mechanical milling is a simple and adaptable method that yields the required powder when starting from bulk, large-particle-size, powdered raw materials of metals, brittle alloys, and ceramics [16].

One such application is enhancing the thermal conductivity of the base fluids to remove excess heat from photovoltaic collectors, particularly in hot sites, by mixing nanoparticle powders, such as Al₂O₃, CuO, and SiC. Therefore, the desired efficiency of the solar photovoltaic system can be sustained accordingly. This subject has been well investigated by many workers involved in the development of harvesting solar energy for green electricity generation [17,18,19,20]. Al₂O₃ and CuO can also be used to increase the efficiency of solar desalination systems to produce fresh water [21]. Nanoparticles of Al₂O₃ can be successfully used as a protective coating for austenitic stainless steel to reduce its corrosion rate in high-temperature environments [22]. Also, Al₂O₃ is widely used in the production of composite materials and catalysts, which have particular importance in the modern industrial sector. Furthermore, aluminum hydroxide Al(OH)₃, which is an intermediate product in the process of producing aluminum oxide, is widely used in many scientific and industrial applications. The production and characterization of this invaluable inorganic substance have been examined by many researchers [11,23,24,25]. Production of alumina in the nanoscale range starting from aluminum waste was reported by Barakat et al. [26] and Nduni et al. [27]. In addition, an active combustion catalyst can be manufactured with approximately 75% of its composition relying on Al foil waste [28]. This approach is extremely beneficial in automobile applications as it reduces the catalyst production cost. At the same time, it benefits the environment by decreasing greenhouse gases and utilizing Al waste. Researchers used an alumina catalyst support for the application of photocatalytic reforming of glycerol to hydrogen as a proof of concept of the photocatalytic activity and stability of the catalyst [29]. In general, any new work to obtain aluminum oxide and aluminum hydroxide from additional sources that avoids impacting the main sources of aluminum is considered a novel contribution.

However, there is no information about the fabrication of alumina starting from aluminum filings that accumulate as a byproduct in mechanical machining workshops. Also, there is insufficient information about the microscopic structure and composition of recycled aluminum filings or the intermediate product aluminum hydroxide. After producing the aluminum oxide by burning the obtained aluminum hydroxide, the oxide can be milled mechanically until it reaches a nanoscale particle size range.

This research aims to produce nano-aluminum oxide from waste aluminum filings, which is a byproduct of several industrial machining jobs. The microstructure and composition of the used aluminum filings, as well as the intermediate product aluminum hydroxide, are studied using two types of scanning electron microscopy (SEM), one of them with an integrated energy dispersive X-ray spectroscopy (EDS), and an X-ray diffraction system. Overall, this research intends to provide a novel contribution to nanoscale alumina powder production from additional sources without impacting the known sources of aluminum production.

2. Experimental Procedures

2.1. Materials and Equipment

Aluminum filings and bulk starting aluminum were sourced from local aluminum manufacturing workshops. All chemicals, including HCl, HNO₃, acetone, HF, commercial NaOH (96%), and Na₂CO₃, were purchased from chemical companies and used without further purification. De-ionized water produced in our laboratory was used in all experiments. A muffle furnace (Naber, GS Geprüfte Sicherheit, A. STEII 1/78, Lilienthal, Germany) was used for heat treatment. Two scanning electron microscopes were employed for microstructure and chemical analysis, one of which was equipped with an attached energy dispersive X-ray spectroscopy unit (EDS) (Thermo Scientific Phenom Desktop SEM, JU-24112022, Waltham, MA, USA). The other is from Inspect F50-FEI Company, Eindhoven, Netherlands. All masses were measured using a digital micro-balance (Model SEJ-205, Taipei, Taiwan). Emery papers with grade range of 400–2000 were used for grinding, and diamond pastes with various particle sizes (7 µm, 2.5 µm, and 0.25 µm) were used for surface polishing. An AGAR sputter coater instrument (Model AGB 7340, UK) was used to coat the produced powder specimens with platinum before the SEM examination. The ultrasonic bath for cleaning was from JAC ULTRASONIC, 1002. Jeio-Trch Co., Ltd., Seoul, Republic of Korea. A milling machine with variable milling speed manufactured by the Changsha Tiachuang Powder Technology Co., Changsha, China, was used to mill the produced alumina (Al₂O₃) to obtain a nanoscale powder. Zirconia milling balls of 1 cm diameter and a milling pot with a volume of about 50 cm³ were used to run the milling process. The pot is made of stainless steel and has an inner shielding of tungsten carbide to avoid possible wear. A vacuum oven of JEIO TECH, MODEL OV-11, AAH13115K, Seoul, Republic of Korea, with a temperature range from room temperature to 300 °C was used to ensure good final drying of the product. An XRD instrument (Malvern Panalytical, Aeris, Cu k_α1, 0.15406 nm, 0.01 step angle, 2θ ranging from 10° to 90°, Eindhoven, The Netherlands) was employed to characterize the produced products as either amorphous or crystalline.

2.2. Starting Material and Scanning Electron Microscope Examination

Aluminum filings were collected from the mechanical workshop of the School of Science, at the University of Jordan. Figure 1 displays a photograph of a sample of the used aluminum filings. Additionally, a sample from the original aluminum was provided by the workshop for microstructure study using the SEM/EDS unit. The as-received Al was metallographically prepared for the SEM examination. The process started with gradual grinding using emery papers of grades 400 down to 2000. The specimen was polished using the diamond pastes of 7 µm to 2.5 µm and finally with 0.25 µm. Afterward, the polished specimen was etched using Keller’s solution (100 mL distilled water, 2.5 mL 69% HNO₃, 1.5 mL 33% HCl, and 1 mL 48% HF) at about 50 °C for analysis of the microstructure. Metallographic details about aluminum and its alloys are abundant in the literature [30,31].

2.3. Production of Alumina

The collected aluminum filings were manually cut into smaller pieces to increase the surface area, which leads to a much faster chemical reaction. The filings were degreased and cleaned using tap water and soap in the ultrasonic cleaner for about 15 min. This process was repeated more than once as needed. The filings were then washed with distilled water to remove the soap, followed by rinsing with acetone while in the ultrasonic cleaner. Sample amounts of 4.5 g and 9.0 g of the cleaned Al filings were weighed using the digital balance and dissolved in 250 mL and 500 mL of 2M HCl, respectively. Figure 2 shows the production flowchart of Al₂O₃ powder starting from Al filings using two different methods.

Since the reaction of Al with the HCl solution was found to be highly exothermic and to produce hydrogen gas, the filings were added gradually to the acidic solution to avoid any excessive heating and/or explosion. The chemical reaction of aluminum with hydrochloric acid to produce aluminum chloride (AlCl₃) in solution is shown in Equation (1):

2Al + 6HCl → 2AlCl₃ + 3H₂

(1)

After the complete dissolution of aluminum in the diluted hydrochloric acid, resulting in aluminum chloride, the solution was filtered twice to remove any residual solid materials (dross). To precipitate Al(OH)₃ from the acquired AlCl₃ solution, 5M sodium hydroxide (NaOH) solution was slowly added, and the reaction proceeded according to Equation (2) [11]:

AlCl₃ + 3NaOH → Al(OH)₃ + 3NaCl

(2)

As the solution of NaOH was poured slowly into the filtered solution of AlCl₃, a floating gel of semi-transparent Al(OH)₃ compound began to form. The NaOH solution was added until the reaction stopped, i.e., all AlCl₃ was converted into Al(OH)₃ gel. The obtained gel was then separated from its brine liquid, which was produced according to Equation (2), and then washed more than once with room temperature water and then with hot distilled water at approximately 80 °C. The hot water was used for washing to reduce the powder settlement period because, in general, the properties of surface tension and viscosity of liquids decrease with increasing temperature [32,33,34]. Moreover, the hot water accelerates dissolution of salt that may be left over from the production process.

During each washing stage, the gel was left to settle down for a few minutes. The product obtained after the washing step was a wet white powder of Al(OH)₃, which was left to dry in the fume hood of the chemical laboratory overnight. The Al(OH)₃ was then subjected to heat treatment in the vacuum oven at about 105 °C to remove any remaining water. The result of this process was a white powder of Al(OH)₃. The last stage of Al₂O₃ production, starting from the aluminum filings is to heat-treat the produced Al(OH)₃ under an air atmosphere using the muffle furnace at 800 °C for 1 h and 4 h, and then at 1000 °C for 1 h and 4 h with a heating rate of 5 °C/min. More information about converting Al(OH)₃ into Al₂O₃ with several burning temperatures has been previously reported [11]. The thermal reaction equation of Al(OH)₃ forming white Al₂O₃ powder is shown in Equation (3) (H. T. signifies heat treatment):

$$2\mathrm{Al}(\mathrm{OH}{)}_3 \stackrel{H.T., 800 °\mathrm{C}}{\to } {\mathrm{Al}}_2{\mathrm{O}}_3 + 3{\mathrm{H}}_2\mathrm{O}$$

(3)

The solution of AlCl₃ (filtrate) is obtained according to Equation (4) [27]:

2AlCl_{3 (aq)} + 3Na₂CO_{3 (aq)} → 3CO₂ + Al₂O₃ + 6NaCl _(aq)

(4)

The result of the above reaction is a milky gel of alumina in a briny solution. The briny solution was decanted and the produced Al₂O₃ was then washed more than once with hot distilled water and left to dry in the fume hood overnight. After Al₂O₃ powder was produced from the collected Al filings, the milling process was carried out to produce Al₂O₃ powder of nanoscale particle size using the milling facility mentioned in Section 2.1. The milling speed and the milling time were 500 rev/min and 40 h, respectively, with a stop period of approximately 0.5 h every 5 h for the cooling program. The number of zirconia balls was 20, and the mass of the Al₂O₃ powder was about 10 g. Specimens from the produced Al(OH)3 and Al₂O₃ powders were then prepared for the SEM/EDS tests. A small amount of each powder was fixed on an aluminum stud of 12 mm in diameter with a double adhesive carbon sticker and coated with platinum using the sputter coater to be earthed with the SEM specimen stage prior to imaging and chemical analysis.

In general, the mass percentage yield of Al₂O₃ powder was approximately 77% for both production routes. This percentage may be more or less depending on the Al dross that is already discharged during the filtration stage which is subtracted from the starting Al filings mass. The quantity of the dross that is extracted during filtration mainly depends on the Al machining environment and the age of the Al filings.

2.4. X-ray Diffraction (XRD) Test

The XRD system mentioned in the materials and equipment section was used to examine the Al(OH)₃ and Al₂O₃ produced by the calcination route at different temperatures and via direct participation. Fine powders of Al(OH)₃ and Al₂O₃ were hand compacted in the provided sample holder prior to the X-ray examination. After that, the results were analyzed and plotted using the software (X’Pert³ Powder) attached to the XRD system.

3. Results and Discussion

Commonly, the quantity of produced alumina is maximized by adding an excess of HCl solution (a relatively high concentration). This obviously determines the amount of aluminum chloride present after dissolving the Al filings in the HCl solution. Thus, controlling the amount of Al(OH)₃, according to the relevant chemical equations, subsequently governs the obtained Al₂O₃ either by burning the Al(OH)₃ in the air atmosphere or by direct chemical precipitation. It is worth mentioning that the amount of aluminum oxide gained also depends on the purity of the used byproduct Al filings.

3.1. The Microstructure and Chemical Composition Examinations of the As-Received Al

The microstructure and chemical analysis of the as-received Al bulk are displayed in Figure 3 along with the corresponding EDS spectra for the matrix and inclusions of the tested specimen. The EDS spectrum of the matrix in the area of blue rectangle indicates the general chemical composition of the used Al, and the constituent elements are listed in tables within the figure. We can conclude that the aluminum used in this study is an aluminum–magnesium (Al–Mg) alloy with the presence of small amounts of carbon and oxygen. The inclusions that appear in the image mostly consist of Al, Mg, Si, and O elements, which can also be a mixture of aluminum, magnesium oxide, and silicon oxide.

3.2. Inspection of the Produced Al₂O₃

As mentioned in Section 2, Al₂O₃ powder was produced by two routes: heat-treated Al(OH)₃ according to Equation (3), and through direct precipitation according to the reaction of AlCl₃ with Na₂CO₃ as in Equation (4). The final product was a white Al₂O₃ powder which was milled to achieve an ultrafine-nanoscale Al₂O₃ particle size. The washing with hot distilled water was necessary to remove any residual salts and accelerate the sedimentation of either Al(OH)₃ or Al₂O₃ powders. Figure 4 displays photographs of the produced Al(OH)₃ and Al₂O₃ powders after Al(OH)₃ burning as described in Section 2.

Figure 5 shows SEM micrographs of the produced Al(OH)₃ at various magnifications to reveal its micro-textural details. The images show flake-like features with micro- and nanoparticle structures. The figure also shows the general EDS spectrum and elemental analysis (table) of the produced Al(OH)₃ (Figure 5b). The SEM micrographs for the produced Al₂O₃ by the two production routes are displayed in Figure 6. The figure confirms the formation of agglomerated ultrafine particles of Al₂O₃ in the range of 200 nm during the firing stage (Figure 6b). Likewise, the figure reveals that the Al₂O₃ powder produced by chemical precipitation consists of aggregated nanoparticles (Figure 6d).

Moreover, the SEM examination of the milled Al₂O₃, produced as described in Section 2.2, clearly confirmed the presence of particles approaching the nanoscale, the production of which was one of the main objectives of this research. Figure 7 shows SEM images for the milled Al₂O₃ produced by heat treatment of Al(OH)₃ at 1000 °C for 4h in air atmosphere, from which we can observe the nanoparticles at high magnification. Particles in the range of 100 nm and less can be observed in the figure.

Here it must be mentioned that the EDS unit had a prominent role in determining the formation of Al₂O₃ as a result of heat treatment or direct precipitation. Its formation was confirmed by analysis of the chemical composition, specifically the expected percentages of Al and O elements corresponding to this compound. Figure 8 illustrates the SEM micrographs for two specimens of the Al₂O₃ produced by burning Al(OH)₃ at 1000 °C for 1h together with their EDS spectra. The specimen shown in Figure 8a was well washed with hot water to ensure that all the residual salt was removed, and the other (Figure 8c) was washed normally with room temperature water. The results from the EDS unit proved the importance of washing with hot water several times to remove the salt left over from the chemical reaction during the production stages. The molar masses of Al₂O₃, O, and Al are approximately 102 g/mol, 16 g/mol, and 27 g/mol, respectively. Thus, the mass percent of O and Al in Al₂O₃ are 47% and 53%, respectively, which are very near to the values obtained with the EDS, as shown in the chemical analysis table (Figure 8b). These percentages are about 96% of the theoretical values of Al₂O₃. The EDS table in Figure 8d also shows Na and Cl elements, which indicates the presence of sodium chlorine in the tested specimen, and the carbon is attributed to the double adhesive carbon sticker used to prepare the specimen for the SEM test.

3.3. XRD Investigation

The XRD results confirmed that the phase of the produced Al(OH)₃ is amorphous, as shown in Figure 9 (black curve). The figure also shows that the directly precipitated Al₂O₃ was found to be mostly an amorphous phase mixed with other phases of alumina, as shown by the peak at an angle of 2θ of approximately 15° (red curve). This needs more study to identify the XRD pattern of directly precipitated Al₂O₃ with and without calcination. The results illustrate the formation of the crystalline structure of γ-Al₂O₃ when the Al(OH)₃ is calcined at about 1000 °C for 1h. (blue curve). The test was repeated, but for 4h of heat treatment of Al(OH)₃ (green curve), and the results are almost identical. It seems that the optimal firing time for the production of alumina from Al(OH)₃ is 1 h, which is the more time-saving and energy-saving route, if this phase is the required one for a certain application. This XRD finding is in good agreement with published work in this field [35]. Another run of heat treatment was carried out at 1200 °C for 1h to inspect the phase change when the calcination temperature rises. The result of this attempt was a highly crystalline Al₂O₃ (brown top curve), and all the peaks correspond to α-alumina [36,37]. This result confirms that the heat treatment under an air atmosphere is the determining parameter in the crystallinity of Al₂O₃.

In summary, nanoscale Al₂O₃ powder with no salt residues can be produced starting from Al waste filings that exist in most mechanical workshops as a machining byproduct and are unsuitable for Al recycling. However, this work demonstrates that it can be converted into a useful compound for practical applications in several areas. This can be achieved by washing the produced Al(OH)₃ with hot distilled water before the calcination step to produce Al₂O₃, or by washing the directly precipitated Al₂O₃ in the same way after the production stage. An effective method for determining the presence of salt within the product is the SEM/EDS unit. It is worth mentioning that the idea of this research can be extended to other metals and the conversion of various filings into oxides and products with an added value. Our goal in this study was to prove the idea, and perhaps a feasibility study can be considered in the future. The idea is definitely scalable, but again, a detailed study is needed. The main challenges for future research in this field are ensuring the purity of the product and the disposal of the resulting salts during the production steps. In the future, the same research approach could be applied but using other chemicals to dissolve Al filings or other forms of Al waste. This would be followed by burning the solid products of the subsequent processes to produce Al₂O₃ and milling it to reach the appropriate particle size for specific applications.

4. Conclusions

The novel contribution of this research is to successfully producing nanoparticles of Al₂O₃ from waste aluminum filings, a byproduct of machining workshops. The production of Al₂O₃ powder was achieved through two methods:

(i).

Firing precipitated Al(OH)₃ at 800 °C, 1000 °C, and 1200 °C after 1 h and 4 h, and the obtained Al₂O₃ was then well washed with hot distilled water at approximately 80 °C to produce Al₂O₃ with no salt residues.

(ii).

A chemical reaction of an AlCl₃ solution with Na₂CO₃ solution was used to directly precipitate Al₂O₃ powder, which was also well washed with hot distilled water.

The results demonstrate the advantages of producing Al₂O₃ by direct chemical precipitation due to time savings and lower production costs compared with the calcination method. The only drawback of this route is that more washing times are required to remove the residual salt. Furthermore, the SEM and its EDS, and the XRD provided a detailed picture of the microstructure and composition of the used Al filings, the intermediate stage of production represented by Al(OH)₃, and the final Al₂O₃ powders and their phases. Moreover, milling of the Al₂O₃ produced from Al filings resulted in a product with particles in the ultrafine-nanoscale range, which is considered an original work in this field.