1. Introduction
Hard disk drives (HDDs), the most recognizable electronic storage device [
1], have recently been replaced by solid state drives in personal computers [
2]. There has been a renewed demand for HDDs in recent years due to their relatively low price that makes them ideal for large cloud data centers. As shown in
Table 1, hard disk drives contain NdFeB magnets, aluminum, stainless steel, and printed circuit boards (PCBs) [
3], which also contain copper and precious metals, such as gold and silver [
4,
5,
6,
7,
8,
9,
10]. The PCB accounts for 82.5% of the economic values of HDD, and rare earth elements (REE) such as Nd have been recognized as a strategic resource [
11]. Therefore, recycling has been suggested to recover valuable metals from HDD PCBs and NdFeB magnets.
A new process for recycling and destroying data in spent HDD was proposed as follows. Firstly, PCB is disassembled from spent HDDs, a relatively straightforward task since it is located outside of these devises. Secondly, HDDs without PCB is scrapped for data security. Finally, scrapped NdFeB magnets are recovered with low-intensity magnetic separator. The aluminum (Al) and stainless steel (STS) scraps generated by the scrapping of HDDs should be separated for recycling because they accounted for about 90% of the weight of HDD scrap, as shown in
Table 1 [
3]. Gravity separation processes are promising methods to separate Al and STS because the former is lighter than the latter. Generally, gravity separation techniques such as jigging use water as a medium [
12], which requires the drying of products after separation. Because this operation consumes huge amounts of energy, dry gravity separation techniques are a more environmentally friendly and sustainable alternative.
The behaviors of particles in vibrating particulate bed have been investigated for predicting size separation of particulates in vibrating vessels [
13,
14,
15,
16]. The vibrating particulate bed method was applied recently to recycling processes as follows [
17,
18,
19]. A recycling process using the vibrating ball (1.8–2.6 g·cm
–3) with different ratio of soda glass beads (2.44 g·cm
–3) and alumina beads (3.57 g·cm
–3) was investigated to separate Al and Mg scraps [
17,
18]. In these tests, the pure Al block (2.69 g·cm
−3) sunk, while the pure Mg block (1.72 g·cm
−3) floated on the ball bed, resulting in the successful separation of pure Al and Mg blocks [
17,
18]. Even though the separation efficiency of Al and Mg increased to 100% under certain conditions, however, high efficiency was not maintained during the separation tests.
For spent HDD recycling, the present study is aimed at developing the dry-gravity separation of Al and STS scraps using vibrating ball beds, which has not been studied before. Although the ratio of vessel and ball sizes, the ratio of ball volume and vessel size, the ratio of scrap sample and ball sizes are important factors in separation efficiency [
6], in this study, the effects of ball size and vibrating power (i.e., vibration amplitude) on the separation efficiency of Al and STS scraps was investigated using zirconia balls as vibrating media. The behavior of Al and STS scraps was also examined by using a ball bed medium of mixed sizes to maintain high separation efficiency.
3. Results and Discussion
Convection and the ‘Brazil nut effect’ are phenomena observed when vibrating a particle-filled vessel as shown in
Figure 1 [
13]. Convection represents the circulation of particles within the vessel (
Figure 1a), which occurs by frictional interaction of balls and side wall of vessel [
13,
14,
15,
16]. The convection could enhance the mixing of balls and size separation [
13]. The ‘Brazil nut effect’ is a phenomenon where the largest pieces rise to the top of a particle bed (
Figure 1b) [
13]. A previous study has also observed that a large particle with higher density tends to sink to the bottom of the particle bed consisting of lower density particles, a phenomenon called the ‘reverse Brazil nut effect’ [
20]. Therefore, zirconia balls (5.6 g·cm
–3) were used as a medium to separate Al (2.65 g·cm
−3) and STS (7.3 g·cm
–3) scraps in this study.
Figure 2 shows the behaviors of Al and STS scraps during the vibrating test with a 2–3-mm vibration amplitude using 1-mm zirconia balls as the medium. When vibrating the vessel at a vibration amplitude of 2 mm as shown in
Figure 2a, the floating ratio of Al scrap was 100% over the entire vibrating test duration while the floating ratio of STS scrap decreased from 100% to 25% (i.e., only one out of four STS scraps floated on the zirconia ball bed). The floating ratio of scrap remains at 25% after 4 min but the floated scrap changed in turn with time by convection during the vibrating test. This result suggests that the heavier scrap (STS) could go downward more easily than the lighter scrap (Al) and indicates that the scraps of STS and Al could be separated by a specific gravity difference. In
Figure 2b, both floating ratios fluctuated with time, and it was observed that convection speed increases with increasing vibration amplitude from 2 to 3 mm.
Figure 2c shows lower floating ratios of Al and STS scraps with 3 mm than with either 2 or 2.5 mm vibration amplitude. In the test shown in
Figure 2c, some scraps appeared on the top of the ball medium but the scraps soon sunk by convection.
Figure 3 shows the separation efficiencies of results shown in
Figure 2. The separation efficiency with a vibration amplitude of 2 mm increased rapidly to 86.6% after 4 min while the efficiencies with 2.5-mm and 3-mm vibration amplitudes increased and then decreased to 0% and 50%, respectively. This low separation efficiency resulted from the ball convection followed by scraps movements.
Figure 4 shows the behaviors of Al and STS scraps during the vibrating test with vibration amplitudes of 2.5 mm (
Figure 4a) and 3 mm (
Figure 4b) using 2-mm zirconia balls as the medium. Conventional studies reported that the ratio of ball diameter to vessel size is one of the key factors of size segregation during the vibrating motion [
13]. The 2-mm vibration amplitude was insufficient to vibrate the 2-mm ball bed in this study, and only the balls at the top of the ball bed bounced during the test, so all scraps remained on the top of the ball bed after the test. This kind of phenomena was rarely reported, and the ratio of ball bed volume and vibrating power is also an important factor for the separation.
Figure 4a shows the floating ratio of Al and STS scraps with a 2.5-mm vibration amplitude using a 2-mm ball as a medium, and the result indicates that at least one of the scraps remained on the top of the ball bed (no 0% floating ratio).
Figure 4b shows the result performed under the condition of a 3-mm vibration amplitude using 2-mm balls as a medium, and the floating ratio fluctuated more than the other tests.
Different behaviors of convection were observed with ball sizes during the tests, and, as shown in
Figure 5, all 1-mm balls moved via convection in the vessel (
Figure 5a) but only the upper half of the ball bed moved convectively when 2-mm balls were used as the medium (
Figure 5b). Therefore, the scraps move up and down rapidly in the results shown in
Figure 4b. When 3-mm balls were used as the medium, convection was not observed and all scraps remained above the ball bed, so the separation of Al and STS scraps was not achieved at all. The ratio of vessel and ball sizes and vibrating power are important factors on the movement of balls [
13], so the vessel size and vibrating power would not be enough for 3-mm balls in this study. This fact indicates that the balance of ball bed volume and vibrating power is an important factor to separate the scraps. The separation efficiencies of the tests with a 2-mm ball medium shown in
Figure 4 are summarized in
Figure 6. As discussed above, the separation efficiency was 0% at the vibration amplitude of 2 mm because the ball bed was not vibrated sufficiently. The separation efficiency increased to 86.6% at 10 min with a 2.5-mm vibration amplitude, and it fluctuated during the test with a vibration amplitude of 3 mm.
Although the separation efficiency increased to 86.6% as discussed above, where more Al scraps floated than STS scraps, the efficiency was not maintained due to convection. It is also interesting to note that more power (vibration amplitude) was required to vibrate the larger ball bed. When the ratio of ball diameter to vessel size is larger, no convection was observed but some STS scrap intruded within the 3-mm balls. Therefore, a ball bed with mixed sizes of balls was prepared by mixing 1-mm and 3-mm zirconia balls. The 3-mm balls were placed on top of the 1-mm ball bed by the ‘Brazil nut’ effect. It was expected that this 3-mm ball bed could act as a barrier, preventing the sunken STS scrap from floating again.
Figure 7 shows the behaviors of Al and STS scraps with vibration amplitude of 3 mm using a mixed size ball bed with a 2:1 ratio of 1-mm and 3 mm-balls. All Al scraps remained on top of the mixed-size ball bed during the test because Al scrap cannot sink through the 3-mm ball bed. The floating ratio of STS scrap decreased rapidly to 0%, which remained for 2 min. The STS scraps sank in turn through the 3-mm ball bed, and disappeared before 2 min. However, some of the 1-mm balls began to climb up along the wall after 4 min, so an STS scrap showed up on top of the ball bed at 5 min. As shown in
Figure 8, the STS scraps intruded into the 3-mm ball bed and then sank to the bottom of the 1-mm ball bed. Since no convection occurred in the 3-mm ball bed, the STS scraps also remained under the 3-mm ball bed for more than 2 min although the scraps moved up by convection through the 1-mm ball bed. The same behavior was observed five times under the same conditions, so this result indicates that the separation of Al and STS scraps was achieved successfully. Furthermore, 100% of separation efficiency could be maintained longer.
Figure 9 shows the floating ratios of Al and STS scraps at 4 min by vibrating with amplitudes of 2–3 mm using a ball bed with various ratios of 1-mm and 3-mm balls. When vibrating the vessel with a vibration amplitude of 2 mm (see
Figure 9a), most of the scraps remained on top of the ball bed because the vibrating power was insufficient.
Figure 9b,c illustrate the results with vibration amplitudes of 2.5 and 3 mm, respectively. With the 2:1 ratio of 1-mm and 3-mm balls, all STS scraps sank but all Al scraps remained on top of the mixed size ball bed, which indicates that Al and STS scraps were separated perfectly. With the 1:1 ratio of 1-mm and 3-mm balls, more 3-mm balls prevented the STS scraps from sinking but the floating ratio of STS scraps decreased from 75 to 50% with increasing vibration amplitude from 2.5 mm to 3 mm. When using the 1:2 ratio of 1-mm and 3-mm balls, the same behavior of STS scraps was not observed in five repeated tests (data not shown) because the intrusion of STS scraps through a 3-mm ball bed occurs randomly while the Al scraps were not sunk during the test. The results shown in
Figure 9 are summarized in
Figure 10 as separation efficiency. The separation efficiencies at 4 min show 100% with the 2:1 ratio of 1-mm and 3-mm balls using 2.5-mm and 3-mm vibration amplitudes while the efficiency was 0% when the ball bed consisted of only 3-mm balls regardless of the vibration amplitude.