Next Article in Journal
Investigation of ASR Models for Low-Resource Kazakh Child Speech: Corpus Development, Model Adaptation, and Evaluation
Previous Article in Journal
Effect of Different Oxygen Atmospheres on Color Stability of Modified Atmosphere Packaged Beef Using Non-Invasive Measurement
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Efficient Recycling Process of Waste Sand with Inorganic Binder via Ultrasonic Treatment

1
Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea
2
Ulsan Technology Application Division, KITECH (Korea Institute of Industrial Technology), 55, Jongga-ro, Jung-gu, Ulsan 44413, Republic of Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(16), 8988; https://doi.org/10.3390/app15168988
Submission received: 22 July 2025 / Revised: 10 August 2025 / Accepted: 12 August 2025 / Published: 14 August 2025

Abstract

The conventional recycling processes for waste sand with inorganic binder (WSIB) in aluminum alloy casting involve washing, heat treatment, and mechanical grinding. However, this process is complex and inefficient for removing the residual binder on the surface of WSIB. This study proposes a simplified and effective recycling process using ultrasound treatment to more efficiently remove residual binder on the surface of WSIB. To evaluate its effectiveness, we characterized ultrasonically recycled sand (URS), conventionally recycled sand (CRS), and virgin sand (VS). The evaluation consisted of the following three steps: (1) characteristics of sand, such as residual binder content, particle size distribution, surface morphology, and specific surface area; (2) measuring the properties of sand cores, including bending strength and the volume of gas evolved during the pouring of A356 Al melts; and (3) measurement of porosity level at the interface between the sand core and A356 Al castings. These results indicate that the ultrasonic recycling process can achieve a technically efficient and simplified recycling process for WSIB.

1. Introduction

The foundry industry contributes significantly to various industries, supplying castings for automotive, construction, shipbuilding, and aerospace [1]. However, it also causes considerable environmental challenges, emphasizing the need for improved sustainability within the industry [2,3]. Among the environmental challenges in the casting industry, waste sand (WS) remains a major concern. Approximately 105 million tons of foundry sand are consumed annually in casting processes [4]. On average, one ton of spent sand is produced per ton of castings, and when the sand becomes unsuitable for reuse, it results in a substantial amount of WS, much of which is disposed of in landfills [5,6]. Disposing of WS without proper treatment not only poses serious environmental concerns but also accelerates the depletion of silica sand resources.
Waste sand with organic binder (WSOB) retains residual resin on its surface that is not fully decomposed during casting. This prevents direct reuse and contaminates the soil environment, including groundwater, with toxic heavy metals such as lead [7,8,9,10]. WSOB is typically recycled via heat treatment. To remove the residual organic binder, WSOB is exposed to high temperatures ranging from 500 °C to 800 °C, where the organic binders are completely combusted. However, this process consumes a large amount of energy, making it economically burdensome [11,12,13]. Although the organic binder content in the core generally ranges from 1 to 3 w t % of the total foundry sand weight, it accounts for up to 70 % of the volatile organic compounds (VOCs) emitted during the casting process [14,15]. In addition, organic binders such as those in the phenol-formaldehyde group can release hazardous air pollutants (HAPs), including formaldehyde, benzyl compounds, phenol, and toluene, during both the casting and recycling process via heat treatment [16,17,18].
To reduce the negative environmental effects caused by using organic binders, research on inorganic binder systems has been developed [19,20]. Inorganic binder systems such as water glass with sodium silicate binder are widely used in mold and core production due to their high dimensional accuracy and low toxic emissions during casting [21,22,23]. However, the major drawback of the inorganic binder system is the difficulty of recycling, which is attributed to the formation of a glass melt in the SiO2-Na2O binary system on the surface of sand particles. At high temperature, a compact monolithic structure with high strength is formed as the binder integrates with the silica sand [24].
WSIB is commonly recycled through dry or wet recycling processes. The dry recycling process removes the residual binder from WSIB through mechanical grinding and interparticle collision. Recent studies aimed to improve the efficiency and cost-effectiveness of WSIB recycling. Wang et al. applied a freeze–mechanical recycling method to used sodium silicate sand and found that freezing at low temperatures caused the residual binder to become brittle and crack, facilitating its removal. At −40 °C, a de-skinning rate of 40.4% was achieved, which refers to the percentage reduction in Na2O content of the reclaimed sand compared to that of the original waste sodium silicate sand [25]. In addition, a method combining heating and dry recycling has been conducted. Due to the hygroscopic nature of sodium silicate, an inorganic binder, the binder film does not adhere tightly to the surface of sand, which hinders its removal through friction and collapse among particles. Preheating the WSIB before dry recycling can induce brittleness in the binder film on the sand surface, thereby facilitating its removal through mechanical impact and friction. This method improves the quality of recycled sand and increases the de-skinning rate. Fan et al. preheated used sodium silicate sand to 320 °C prior to dry recycling. They reported that heating the used sand within this temperature range increased the re-bonding strength and improved recycling efficiency [26]. However, the quality of recycled sand for the dry recycling process is reduced due to particle fracture caused by collision and grinding, and it remains insufficient for complete removal of residual binder from the surface of WSIB [27].
Wet recycling uses chemical solutions to dissolve the residual binder from the surface of WSIB. Kim et al. reported a wet recycling for WSIB involving water washing followed by treatment with a 0.2 M KOH solution to remove residual binder [28]. Hu et al. used acidic chemical agents such as H2C2O4, CaCl2, and MgCl2 solutions to penetrate the WSIB, followed by drying and mechanical grinding, which resulted in the production of high-quality recycled sand [29]. However, wet recycling of WSIB generates alkaline wastewater, and its volume increases with the number of washing cycles, resulting in significant post-treatment and disposal costs [30,31,32].
This study presents a novel approach to the wet recycling of WSIB by using ultrasonic treatment to improve the removal efficiency of residual binder. In contrast to conventional recycling processs that adopt heat treatment or mechanical grinding, the proposed method simplifies the overall procedure, thereby improving operational efficiency. Ultrasonic treatment utilizes high-intensity, high-frequency sound waves in a liquid medium to induce cavitation and acoustic streaming, which effectively detach contaminants from intricate and delicate surfaces without the use of chemical agents. Owing to its superior cleaning efficiency, ultrasonic cleaning has been widely applied in diverse fields such as medical device sterilization, precision component cleaning, marine equipment maintenance, and food processing industries [33,34,35,36,37,38]. URS, produced via ultrasonic recycling, was comprehensively compared with CRS and VS through the following three steps: (1) characteristics of sand, (2) properties of sand core, and (3) porosity in A356 Al castings using sand cores.

2. Experimental Procedure

2.1. Preparation on the WSIB

The inorganic sand core blocks were obtained from a foundry company specializing in aluminum alloy castings. An inorganic sodium silicate binder, supplied by KITECH, was added, with the binder amount being 3 w t % of VS.
The WSIB was prepared as shown in Figure 1. First, VS was mixed with 3 w t % of a sodium silicate binder solution based on the sand weight and cured with hot air at 150 °C to form an inorganic sand core (Figure 1a). The inorganic sand core was then heated at 650 °C for 20 min, followed by crushing and screening to obtain the WSIB (Figure 1b).

2.2. Recycling Process of WSIB

To compare different sand recycling processes, two types of processes were applied to the WSIB, as shown in Figure 2.
A flowchart of the ultrasonic recycling process is shown in Figure 2a. In this process, ultrasonic energy was applied during the washing step to enhance the removal efficiency of the residual binder. The apparatus used for ultrasonic treatment is shown in Figure 3. The system consisted of a 2 k W ultrasonic generator, a piezoelectric transducer, a water-cooled conical booster, and a cylindrical pure titanium sonotrode with a diameter of 40 m m .
The ultrasonic recycling process was performed during the ultrasonic application in 100 m L of distilled water and 100 g of WSIB in an acrylic chamber with a diameter of 70 m m and a height of 100 m m . The sonotrode was immersed 30 m m below the surface of the slurry, which contained WSIB in water. The process was conducted at a frequency of 15 k H z and an amplitude of 40 μ m for 10 min. After ultrasonic recycling, the recycled sand was dried and screened to obtain the URS.
Figure 2b shows a conventional recycling process, including previous studies, which consists of three sequential steps: washing, heat treatment, and mechanical grinding. In each batch process, 1000 g of WSIB was used. The WSIB was washed with 1 L of distilled water for 10 min and dried at 110 °C for 120 min. It was then exposed to a temperature of 500 °C to activate the residual binders. Finally, the mechanical grinding step was performed using a rotating chamber equipped with blades, operating at 500 R P M for 180 min. In the mechanical grinding step, friction and collision among sand particles removed the residual inorganic binder from the surface.
Figure 2. (a) The ultrasonic recycling process for WSIB; (b) The conventional recycling process for WSIB.
Figure 2. (a) The ultrasonic recycling process for WSIB; (b) The conventional recycling process for WSIB.
Applsci 15 08988 g002
Figure 3. Schematic diagram for the ultrasonic treatment for WSIB.
Figure 3. Schematic diagram for the ultrasonic treatment for WSIB.
Applsci 15 08988 g003

2.3. Characteristics of Sand: Residual Binder Content, Particle Size Distribution, Surface Morphology, and Specific Surface Area

The four types of sand—URS, CRS, VS, and WSIB—were comprehensively analyzed for their residual binder content, particle size distribution, surface morphology, and specific surface area. The components of the residual binder on sand were determined using X-ray fluorescence (XRF, XRF-1800, Shimadzu, Kyoto, Japan). Since the inorganic binder consisted of sodium silicate, both Na and Si were present as binder components. However, because SiO2 is also the main component of silica sand, it was not suitable for identifying the residual binder contents [39]. Therefore, the content of Na as a secondary component of the binder was analyzed. The recycling ratio was calculated via Equation (1).
R e c y c l i n g   r a t i o % = ( 1 N a   c o n t e n t s   i n   r e c y c l e d   s a n d N a   c o n t e n t s   i n   w a s t e d   s a n d   w i t h   i n o r g a n i c   b i n d e r ) × 100
For URS, CRS, VS, and WSIB, particle size distribution was expressed as the American Foundry Society (AFS) grain fineness number (GFN). The AFS-GFN was calculated as follows. A total of 100 g of dried sand was screened for 15 min using a sieve shaker (BA-200N, CISA, Barcelona, Spain) equipped with nine sieves (mesh sizes: 20, 30, 40, 50, 70, 100, 140, 200, 270). The percentage retained on each sieve was calculated by dividing the weight of sand on that sieve by the total sample weight and multiplying by 100. Each value was multiplied by the corresponding multiplier based on AFS 1106-12-S [40], and the sum of these products was obtained as the AFS-GFN.
The surface morphology of sand particles was analyzed using a scanning electron microscope (SEM, S-3000H, HITACHI, Tokyo, Japan). Morphological differences among VS, WSIB, CRS, and URS were compared to assess the distribution of residual binder.
The specific surface area of sand, which affects the bending strength of cores, was measured using the Brunauer–Emmett–Teller (BET) method. Measurements were performed with a BELSORP-mini II (MicrotracBEL, Osaka, Japan) using nitrogen adsorption at −196 °C.

2.4. Evaluation of Bending Strength and the Amount of Gas Evolution of Recycled Sand Cores

Sand cores were prepared with 20 % , 50 % , and 100 % recycled sand, each mixture balancing with VS accordingly. For each condition, 2.5 k g of sand was mixed with 75 g of an inorganic sodium silicate binder, corresponding to 3 w t % of the total sand weight. The mixtures were then cured with hot air at 150 °C to produce cores with dimensions of 20   m m × 180   m m × 20   m m .
Prior to 3-point bending strength testing, sand cores were stored for 1 h under two environmental conditions: 25 °C and 40 % relative humidity (RH), and 38 °C and 65 % RH. The bending strength of each core was measured under both storage conditions to evaluate the effect of exposure to environmental conditions on the bending strength of cores. The cores prepared with mixed URS cores, mixed CRS cores, and VS cores were tested five times each using a digital universal sand strength tester (HJ-0505, Heungjin, Kimpo, Korea), at a crosshead speed of 10 mm/min and a span distance of 100 mm, following the 3-point bending method [41].
The amount of gas evolved from 100 % URS core, 100 % CRS core, and 100 % VS core was measured upon contact with A356 Al melts. Since gas porosity that forms within castings results from gas generated before solidification and subsequently trapped in the melt, gas evolution was analyzed both before and after solidification. Figure 4 shows the schematic of the experimental apparatus used for gas evolution measurement. Molten A356 aluminum was poured onto the sand core placed in a graphite holder. Gas generated from the core during metal contact was released through a gas outlet and measured using a thermal mass flow meter (MF5700, Siargo, Santa Clara, CA, USA). The temperature of A356 Al melts was measured using a data logger (GL240, GRAPHTECH Corporation, Yokohama, Japan) to distinguish gas generation before and after solidification.

2.5. Microstructural Analysis of the Interface Between A356 Al Castings and the Recycled Sand Cores

The interfaces between A356 Al castings and three types of sand cores (100 % URS core, 100 % CRS core, and 100 % VS core) were analyzed and compared. A 1 k g of A356 Al alloy ingot was held in a graphite crucible and melted in an electric furnace at 700 °C. The A356 Al melts were then ultrasonically degassed. A rectangular sand core with a 50 m m × 20 m m × 20 m m was placed inside the stainless-steel crucible with an inner diameter of 100 m m and a height of 150 m m . The A356 Al melts were poured at 690 °C into a crucible and solidified under air. Gas porosity in the approximately 20 m m 2 area at the interface between A356 Al castings and cores was examined using an optical microscope (DM2700M, Leica microsystems GmbH, Wetzlar, Germany).

3. Results and Discussion

3.1. Results of Sand Characterization

As shown in Figure 5, the result of sodium contents of sand samples using XRF analysis. VS contained only 0.15 w t % of sodium content, whereas the WSIB had 4.29 w t % . This significantly higher value reflects a coating of residual binder on grains of WSIB. After the recycling process, the sodium content dropped substantially, with CRS showing 0.85 w t % of sodium content, and URS only 0.33 w t % . The ultrasonic recycling process demonstrated higher recycling ratio efficiency, achieving 92 % recycling compared to 80 % for the conventional recycling process. This enhanced performance is attributed to the effects of cavitation and acoustic streaming during the ultrasonic recycling process. When ultrasonic waves are applied to a liquid medium, the rapid pressure fluctuations induce cavitation, leading to the formation and subsequent collapse of cavitation bubbles. The collapse of these bubbles generates shock waves that impart localized mechanical impacts on particle surfaces, effectively detaching residual binder. Moreover, the explosive growth of bubbles induces strong shear forces and streaming, which remove binder fragments from the surface. As a result, residual binder can be effectively removed from the surface of WSIB [42,43,44].
The particle size distribution of each sand is shown in Table 1 and Figure 6. The AFS-GFN of VS was 31.8, indicating a relatively coarse particle size. In contrast, WSIB had an AFS-GFN of 37.5, reflecting a finer size (under 0.4 m m ) distribution, which is attributed to the sand or binder debris during the crushing of the WSIB core. CRS had an AFS-GFN of approximately 40, suggesting that the conventional recycling process produced additional fine particles due to further breakage of sand. In comparison, URS had an AFS-GFN of 37.9, maintaining a particle size distribution similar to that of WSIB. This indicates that the ultrasonic recycling process effectively removed the residual binder while minimizing damage to the sand particles. These results suggest that while the conventional recycling process increases the amount of fine particles, the ultrasonic recycling process minimizes particle damage during binder removal.
Figure 7 shows the SEM image of VS, WSIB, CRS, and URS. The SEM images corroborated the XRF results, demonstrating that both CRS and URS achieved effective residual binder removal. The surface morphology of WSIB exhibited extensive sodium silicate coating layers, while VS maintained smooth surface morphology. In CRS, most of the binder layer had been removed, leaving only small patches of residual binder, but the particles were fractured. URS showed clean surface morphology and no continuous binder film, with only minimal traces of residual binder observed. Notably, the URS grains appeared less damaged than the CRS grains, with edges and corners remaining more intact, whereas CRS grains occasionally exhibited chipped edges or microfractures. Therefore, the ultrasonic recycling process simplifies the overall process and reduces time by avoiding the need for heat treatment and the mechanical grinding process.
These observations align with the fundamental difference in binder removal mechanisms between the two recycling processes, as presented in Figure 8. When high-frequency ultrasound is applied to water, cavitation bubbles form and collapse near the sonotrode and sand surfaces, generating shock waves that detach the residual binder from the WSIB. The detached binder fragments are then carried upward by acoustic streaming, facilitating their effective separation from the sand [45,46]. By contrast, the mechanical recycling process relies on repeated grinding between WSIB particles under centrifugal force, which is less effective in fully removing binder and may induce surface damage.
Table 2 shows the specific surface area for VS, WSIB, CRS, and URS. VS had the lowest surface area 0.04 m 2 / g due to smooth surface morphology. In contrast, WSIB exhibited a much higher surface area of 0.11 m 2 / g , which is approximately 2.5 times higher than that of VS. This increase is attributed to the rough coating of inorganic binder and the presence of adhered dust on the WSIB. After the recycling process, both recycled sands showed intermediate specific surface areas: 0.07 m 2 / g for CRS and 0.06 m 2 / g for URS. Both CRS and URS showed higher specific surface areas than VS, indicating that residual binder was not completely removed in either recycling process, as also observed in the BET results and SEM images. As a result, the specific surface area of URS was 15% lower than that of CRS, which is attributed to more effective binder removal and reduced generation of fine particles during ultrasonic recycling. The mechanical recycling process induces collapse and grinding between sand particles, leading to microscopic cracks, exposure of internal pores, and retention of fine dust on the grain surfaces, all of which contribute to a higher specific surface area in CRS compared to URS.

3.2. Results of Characteristic of Sand Core

Figure 9 shows the result of the 3-point bending strength test for sand cores with 20 % , 50 % , and 100 % URS and CRS, each mixture balancing with VS accordingly. All specimens were conditioned for 1 h under both standard (25 °C, 40 % RH) and humid (38 °C, 65 % RH) environments prior to testing. Under both conditions, bending strength increased with higher recycled sand content. Cores incorporating 20%, 50%, and 100% URS or CRS consistently exhibited greater strength than those made with 100% VS. These results can be attributed to two main factors. First, the residual binder remaining on the recycled sand provides additional reactive surface sites, promoting more extensive bonding between the fresh binder and sand particles during curing. Although partially inert, these residual films increase the effective surface area, leading to the formation of stronger binder bridges. Second, the finer particle size distribution of CRS and URS compared to VS introduces a wedging effect. When particles of different sizes are blended, they have a spatial influence on each other, described by the wedging effect [47,48]. This phenomenon influences the packing structure. When coarse particles are dominant, fine particles typically fill the voids between them. However, some isolated fine particles may become trapped in narrow gaps between coarse particles rather than fully filling the gaps. This entrapment wedges the coarse particles apart, where smaller particles fill interstitial gaps, resulting in denser packing and improved mechanical interlocking within the core matrix [49,50].
The bending strength of sand cores used in aluminum alloy casting is a critical factor. If the bending strength is too low, the core may crack, deform, or decompose during pouring of molten aluminum, leading to casting defects. Conversely, if the bending strength is too high, it becomes more difficult to remove the core from the castings, potentially increasing post-processing time and labor. Therefore, appropriate strength is essential. In the aluminum casting process, a minimum bending strength of 200 N / c m 2 is generally required to ensure reliable handling, assembly, and casting performance [51].
The distinction between URS and CRS became more evident under humid conditions. While both exhibited increased strength due to enhanced binder interaction and particle packing, URS cores retained a greater proportion of their strength after exposure to high humidity. This is attributed to the cleaner surface of URS grains, achieved through ultrasonic cavitation and acoustic streaming, which more effectively remove residual sodium silicate binder. In contrast, CRS grains tend to retain moisture-sensitive residues that compromise the binder structure under humid conditions. Overall, URS-based cores not only match or exceed the mechanical performance of virgin sand cores under dry conditions but also demonstrate superior durability in humid environments.
Figure 9. Bending strengths of sand cores: weight percentage of recycled sand (VS, CRS, and URS).
Figure 9. Bending strengths of sand cores: weight percentage of recycled sand (VS, CRS, and URS).
Applsci 15 08988 g009
Figure 10 shows the amount of gas volume for sand core samples and the temperature of A356 Al melts during solidification, measured at a position 5 m m above the core. The gray dashed line indicates the time at which the A356 Al alloy reached its solidification temperature of 557 °C. The VS core released about 9 m L of gas. Low gas volume is a known benefit of sodium silicate binders, where the evolved gas consists of water vapor, with negligible CO2 generation unless additives are present. In contrast, the CRS core emitted 14 m L of gas, approximately 1.5 times that of the VS core. This increase is attributed to residual binder remaining on the surface of sand after the conventional recycling process. The URS core released approximately 11 m L of gas, only slightly higher than that of the VS core, indicating that URS removed nearly all residual binder.

3.3. Result of Microstructural Analysis of the Interface Between A356 Al Castings and the Recycled Sand Cores

Figure 11 shows the typical microstructure of the interface area, approximately 20 m m 2 , between the core and A356 Al alloy. Image analysis of the aluminum castings revealed porosity levels of 0.22% by area for the casting made with a VS core, 0.34% for the casting with a CRS core, and only 0.26% for the casting with a URS core. The gas porosity at the interface between the A356 Al castings and the CRS core increased by approximately a factor of 1.5 compared to that at the interface with the VS core. The gas porosity at the interface between the A356 Al casting and the URS core can be explained by residual binder removal and adequate permeability. As discussed, the URS core generated almost no extra gas; furthermore, the URS sand preserved grain size distribution, which likely means the core had higher permeability than the CRS core. Higher permeability allows gases to vent through the core and out of the mold more readily instead of forcing their way into the A356 Al melts. In contrast, they not only produced more gas but also had reduced permeability due to their finer sand clogging the pore spaces.

3.4. Economic Assessment of the Ultrasonic Recycling Process

To assess the economic feasibility of the proposed ultrasonic recycling process, a comparative analysis was performed against the conventional recycling process, focusing on energy consumption and associated cost for producing 1 k g of recycled sand. In the calculation, the electricity tariff of 142.6 K R W / k W h , based on the Korea Electric Power Corporation (KEPCO) general (A) I industrial electricity tariff as of 1 April 2025 [52], was applied. K R W denotes the Korean currency, and as of 8 August 2025, 1 U S d o l l a r was equivalent to 1390 K R W .
As shown in Figure 2a, the ultrasonic recycling process consists of two steps: ultrasonic treatment and drying. A 2 k W ultrasonic generator was operated at 50 % output power, yielding an effective consumption of 1.0 k W . Treating 0.1 k g of WSIB for 0.17 h consumed 0.17 k W h , and 10 c y c l e s were required to process 1 k g , resulting in energy consumption of ultrasonic treatment as shown in Equation (2).
1.0   k W × 0.17   h × 10   c y c l e s = 1.7   k W h
Drying was performed using an electric furnace with a rated power of 2.2 k W . The furnace was heated at a rate of 10 °C / m i n to reach 110 °C in approximately 0.18 h, consuming of 0.4 k W h . After reaching 110 °C, the temperature was maintained for 2 h under proportional-integral-derivative (PID) temperature control at 50 % output (1.1 k W ), consuming an additional 2.2 k W h . The energy consumption of the drying step is calculated in Equation (3).
2.2   k W × 0.18   h + 1.1   k W × 2.0   h = 2.6   k W h
In addition to the cost of energy consumption, the cost associated with wastewater treatment was evaluated to assess the ultrasonic recycling process. The cost for wastewater treatment facility operation was categorized into chemical usage, energy consumption, sludge disposal, and maintenance. Based on an assumed daily wastewater generation of 500 tons, the annual operating costs were estimated at 95.5 million K R W for chemicals, 40 million K R W for energy consumption, 56 million K R W for sludge disposal, and 25 million K R W for maintenance, totaling 216 million K R W per year [53]. From this estimation, the cost of treating 1 L of wastewater generated during ultrasonic recycling was calculated to be approximately 12 K R W / L , based on the proportion of total annual cost to treated volume. The total energy consumption for producing 1 k g of URS was 4.3 k W h , corresponding to 625 KRW/kg.
The conventional recycling process included washing, heat treatment, and mechanical grinding, as shown in Figure 2b. In the washing step, a three-phase induction motor (380 V , 1.9 A , power factor 0.835) operated for 0.17 h, yielding energy consumption as calculated in Equation (4).
3 × 380   V × 1.9   A × 0.835 ÷ 1000 × 0.17   h = 0.17   k W h
Drying was performed under the same conditions as in the ultrasonic recycling process, consuming 2.6 k W h . In heat treatment, the furnace was ramped to 500 °C in approximately 0.8 h at full output (2.2 k W ), followed by 0.5 h hold at 50 % output (1.1 k W ). Energy consumption of heat treatment as calculated in Equation (5).
2.2   k W × 0.8   h + 1.1   k W × 0.5   h = 2.4   k W h
Mechanical grinding was conducted for 3 h using three-phase induction motor (380 V, 2.0 A, power factor 0.835), resulting in the following energy consumption of mechanical grinding as calculated by Equation (6).
3 × 380   V × 2   A × 0.835 ÷ 1000 × 3   h = 3.3   k W h
The total energy consumption for producing 1 k g of CRS was 8.5 k W h , corresponding to 1212 K R W / k g .
These results indicate that the ultrasonic recycling process reduces total energy consumption and energy costs by approximately 50 % relative to the conventional recycling process. Moreover, the process can be scaled up by increasing the chamber capacity and integrating multiple ultrasonic systems in parallel, enabling the treatment of WSIB on an industrial scale. Thus, the ultrasonic recycling process not only enhances economic feasibility but also confirms its potential as a scalable and energy-efficient alternative for industrial applications.

4. Conclusions

The ultrasonic recycling process was validated as a technically efficient and economically advantageous alternative to the conventional recycling process for WSIB. The comprehensive three-step evaluation validated the effectiveness of the recycling process in terms of sand characteristics, core properties, and porosity at the interface between the sand core and A356 Al castings.
  • The ultrasonic recycling process achieved superior technical performance compared to conventional methods while offering significant operational advantages. Cavitation and acoustic streaming enabled a 92.3% recycling ratio within a simplified process sequence, removing the need for heat treatment and mechanical grinding steps. Therefore, URS had a lower residual binder content (0.33%) than CRS (0.85%), confirming the enhanced binder removal efficiency of ultrasonic treatment.
  • Both the conventional and ultrasonic recycling processes produced recycled sands that substantially improved the bending strength of the core compared to using VS alone. The improvements in bending strength were driven by two factors. Increased specific surface area providing additional binder reaction sites and improved particle packing through wedge effects of optimized size distributions.
  • The ultrasonic recycling process maintained the environmental advantages of inorganic binder systems while achieving superior casting quality. The volume of gas evolved from URS cores (11 m L ) was comparable to that of VS core (9 m L ) and lower than that of the CRS core (14 m L ). Furthermore, the porosity level at the interface between the URS core and A356 aluminum castings was measured at 0.26 % , similar to the 0.22 % that observed with VS cores. The result in porosity level of castings suggests that the mechanical properties of A356 aluminum castings made with URS cores are comparable to those made with VS cores.
The ultrasonic recycling process provides a technically advanced solution for the foundry industry to reduce sand procurement costs, minimize waste disposal requirements, and enhance environmental compliance while achieving superior performance compared to conventional processes.

Author Contributions

Conceptualization, T.H., J.K., Y.L., B.K. and Y.K.; methodology, T.H.; validation, J.K. and B.K.; data curation, T.H., Y.L. and K.K.; writing—original draft preparation, T.H., J.K., Y.L. and B.K.; writing—review and editing, T.H., J.B. and Y.K.; supervision, T.H. and Y.K.; formal analysis, B.K., J.K., K.K. and J.B.; investigation, T.H., Y.L. and J.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Technology Innovation Program (00154970; development of entry-level inorganic binder and exclusive casting process to realize carbon neutral) funded by the Ministry of Trade, Industry & Energy (MOTIE, The Republic of Korea).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed at the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Herfurth, K.; Scharf, S. Casting, Springer Handbook of Mechanical Engineering, 2nd ed.; Springer: Gewerbestrasse, Switzerland, 2021; pp. 325–356. [Google Scholar] [CrossRef]
  2. Sawai, H.; Rahman, I.M.M.; Fujita, M.; Jii, N.; Wakabayashi, T.; Begum, Z.A.; Maki, T.; Mizutani, S.; Hasegawa, H. Decontamination of metal-contaminated waste foundry sands using an EDTA–NaOH–NH3 washing solution. Chem. Eng. J. 2016, 296, 199–208. [Google Scholar] [CrossRef]
  3. Mizuki, T.; Kanno, T. Establishment of casting manufacturing technology by introducing an artificial sand mold with furan resin and realizing a clean foundry. Int. J. Met. 2018, 12, 772–778. [Google Scholar] [CrossRef]
  4. Balulmath, A.B.; Sridhar, G.; Saranya, P. A Critical Review on Potential Use of Waste Foundry Sand in Geotechnical and Pavement Applications. In Proceedings of the Indian Geotechnical Conference, Kochi, India, 15–17 December 2022; Springer: Singapore, 2022; pp. 309–320. [Google Scholar] [CrossRef]
  5. Andrade, R.M.; Cava, S.; Silva, S.N.; Soledade, L.E.B.; Rossi, C.C.; Robertoleite, E.; Paskocimas, C.A.; Varela, J.A.; Longo, E. Foundry Sand Recycling in the Troughs of Blast Furnaces: A Technical Note. J. Mater. Process. Technol. 2005, 159, 125–134. [Google Scholar] [CrossRef]
  6. Ahmad, J.; Zhou, Z.; Martínez-García, R.; Vatin, N.I.; de-Prado-Gil, J.; El-Shorbagy, M.A. Waste Foundry Sand in Concrete Production Instead of Natural River Sand: A Review. Materials 2022, 15, 2365. [Google Scholar] [CrossRef] [PubMed]
  7. Khan, M.M.; Mahajani, S.M. Chemical reclamation of waste green foundry sand and its application in core production. Sustain. Chem. Clim. Action 2024, 4, 100038. [Google Scholar] [CrossRef]
  8. Deng, A.; Tikalsky, P. Metallic characterization of foundry by-products per waste streams and leaching protocols. J. Envrion. Eng. 2006, 132, 586–595. [Google Scholar] [CrossRef]
  9. Deng, A.; Tikalsky, P. Geotechnical and leaching properties of flowable fill incorporating waste foundry sand. Waste Manag. 2008, 28, 2161–2170. [Google Scholar] [CrossRef] [PubMed]
  10. Kmita, A.; Dańko, R.; Holtzer, M.; Dańko, J.; Drożyński, D.; Skrzyński, M.; Tapola, S. Eco-Friendly Inorganic Binders: A Key Alternative for Reducing Harmful Emissions in Molding and Core-Making Technologies. Int. J. Mol. Sci. 2024, 25, 5496. [Google Scholar] [CrossRef] [PubMed]
  11. Rayjadhav, S.B.; Mhamane, D.A.; Shinde, V.D. Assessment of sand reclamation techniques and sand quality in thermal reclamation. Int. J. Product. Qual. Manag. 2020, 30, 343. [Google Scholar] [CrossRef]
  12. Wan, P.; Zhou, J.; Li, Y.; Yin, Y.; Peng, X.; Ji, X.; Shen, X. Kinetic analysis of resin binder for casting in combustion decomposition process. J. Therm. Anal. Calorim. 2022, 147, 6323–6336. [Google Scholar] [CrossRef]
  13. Silva, E.C.; Masiero, I.; Guesser, W.L. Comparing sands from different reclamation processes for use in the core room of cylinder heads and cylinder blocks production. Int. J. Met. 2020, 14, 706–716. [Google Scholar] [CrossRef]
  14. Czerwinski, F.; Mir, M.; Kasprzak, W. Application of cores and binders in metalcasting. Int. J. Cast Met. Res. 2015, 28, 129–139. [Google Scholar] [CrossRef]
  15. Glowacki, S.M.; Crandell, C.R.; Cannon, G.R.; Clobes, F.S.; Voigt, J.K.; Furness, R.C.; McComb, J.C.; Knight, B.A. Emissions studies at a test foundry using an advanced oxidation-clear water system. AFS Trans. 2003, 111, 579–598. [Google Scholar]
  16. Holtzer, M.; Kmita, A. Mold and Core Sands in Metalcasting: Chemistry and Ecology, 1st ed.; Springer: Gewerbestrasse, Switzerland, 2020; pp. 83–107. [Google Scholar] [CrossRef]
  17. Wang, Y.; Zhang, Y.; Su, L.; Li, X.; Duan, L.; Wang, C.; Huang, T. Hazardous air pollutant formation from pyrolysis of typical Chinese casting materials. Environ. Sci. Technol. 2011, 45, 6539–6544. [Google Scholar] [CrossRef]
  18. Wang, Y.; Cannon, F.S.; Salama, M.; Goudzwaard, J.; Furness, J.C. Characterization of hydrocarbon emissions from green sand foundry core binders by analytical pyrolysis. Environ. Sci. Technol. 2007, 41, 7922–7927. [Google Scholar] [CrossRef]
  19. Anwar, N.; Jalava, K.; Orkas, J. Experimental study of inorganic foundry sand binders for mold and cast quality. Int. J. Met. 2023, 17, 1697–1714. [Google Scholar] [CrossRef]
  20. Dańko, R.; Kmita, A.; Holtzer, M.; Dańko, J.; Lehmhus, D.; Tapola, S. Development of inorganic binder systems to minimise emissions in ferrous foundries. Sustain. Mater. Technol. 2023, 37, e00666. [Google Scholar] [CrossRef]
  21. Polzin, H. Inorganic Binders for Mould and Core Production in the Foundry, 1st ed.; Fachverlag Schiele und Schön GmbH: Berlin, Germany, 2014; pp. 105–120. [Google Scholar]
  22. Fortini, A.; Merlin, M.; Raminella, G. A comparative analysis on organic and inorganic core binders for a gravity diecasting Al alloy component. Int. J. Met. 2022, 16, 674–688. [Google Scholar] [CrossRef]
  23. Anwar, N.; Major-Gabryś, K.; Jalava, K.; Orkas, J. Effect of additives on heat hardened inorganic solid foundry binder. Int. J. Met. 2025, 19, 129–144. [Google Scholar] [CrossRef]
  24. Jelinek, P. Pojivové Soustavy Slévárenských Formovacích Směsí, 1st ed.; OFTIS: Ostrava, Czech Republic, 2004; pp. 156–158. [Google Scholar]
  25. Wang, J.N.; Fan, Z.T. Freezing-mechanical reclamation of used sodium silicate sands. Int. J. Cast. Metal. Res. 2010, 23, 257–263. [Google Scholar] [CrossRef]
  26. Fan, Z.T.; Wang, H.F. Research and new advances in application of sodium silicate sand casting technology. MW Met. Form. 2011, 19, 23–26. [Google Scholar]
  27. Gong, X.L.; Hu, S.L.; Fan, Z.T. Research, application and development of inorganic binder for casting process. China Foundry 2024, 21, 461–475. [Google Scholar] [CrossRef]
  28. Kim, K.H.; Bae, M.A.; Lee, M.S.; Park, H.; Baek, J.H. Regeneration of used sand with sodium silicate binder by wet method and their core manufacturing. J. Mater.Cycles Waste 2021, 23, 121–129. [Google Scholar] [CrossRef]
  29. Hu, S.; Gong, X.; Wu, W.; Cai, G.; Ren, W.; Fan, Z. A Novel Reclamation Method of Chemical–Mechanical Grinding for Inorganic Binder Waste Sand in Aluminum Alloy Casting Process. Int. J. Met. 2024, 19, 1569–1578. [Google Scholar] [CrossRef]
  30. Wen, J.; Dong, H.; Zeng, G. Application of zeolite in removing salinity/sodicity from wastewater: A review of mechanisms, challenges and opportunities. J. Clean. Prod. 2018, 197, 1435–1446. [Google Scholar] [CrossRef]
  31. Xue, A.; Tang, Y.; Li, Y.; Dai, W.; Liu, J.; Wang, H. Reclaiming sodium silicate into diatom. J. Clean. Prod. 2025, 486, 144575. [Google Scholar] [CrossRef]
  32. Wang, L.; Jiang, W.; Gong, X.; Liu, F.; Fan, Z. Recycling water glass from wet reclamation sewage of waste sodium silicate-bonded sand. China Foundry 2019, 16, 198–203. [Google Scholar] [CrossRef]
  33. Nguyen, D.D.; Ngo, H.H.; Yoon, Y.S.; Chang, S.W.; Bui, H.H. A new approach involving a multi-transducer ultrasonic system for cleaning turbine engines’ oil filters under practical conditions. Ultrasonics 2016, 71, 256–263. [Google Scholar] [CrossRef] [PubMed]
  34. Choi, J.; Kim, T.H.; Kim, H.Y.; Kim, W. Ultrasonic washing of textiles. Ultrason. Sonochem. 2016, 29, 563–567. [Google Scholar] [CrossRef] [PubMed]
  35. Lais, H.; Lowe, P.S.; Gan, T.H.; Wrobel, L.C. Numerical modelling of acoustic pressure fields to optimize the ultrasonic cleaning technique for cylinders. Ultrason. Sonochem. 2018, 45, 7–16. [Google Scholar] [CrossRef] [PubMed]
  36. Yu, C.; Huang, X.; Fan, Y.; Deng, Z. A new household ultrasonic cleaning method for pyrethroids in cabbage. Food Sci. Hum. Wellness 2020, 9, 304–312. [Google Scholar] [CrossRef]
  37. Huang, X.; Niu, G.; Xie, Y.; Chen, X.; Hu, H.; Pan, G. Application of ultrasonic cavitation in ship and marine engineering. J. Mar. Sci. Appl. 2024, 23, 23–38. [Google Scholar] [CrossRef]
  38. Fulford, M.R.; Stankiewicz, N.R. Cleaning methods for dental instruments. Br. Dent. J. 2023, 235, 105–111. [Google Scholar] [CrossRef]
  39. Ko, E.Y.; Kim, K.H.; Baek, J.H.; Hwang, I.; Lee, M.S. Wet regeneration of waste artificial sand used in sand casting using chemical solutions. Environ. Eng. Res. 2021, 26, 200421. [Google Scholar] [CrossRef]
  40. Thomas, S. Mold&Core Test Handbook, 5th ed.; American Foundry Society: Schaumburg, IL, USA, 2019; pp. 21–22. [Google Scholar]
  41. KS L 3314:2017; Testing Method of Bending Strength for Insulating Fire Bricks. Korean Agency for Technology and Standards: Seoul, Republic of Korea, 2017.
  42. Yamashita, T.; Ando, K. Low-intensity ultrasound induced cavitation and streaming in oxygen-supersaturated water: Role of cavitation bubbles as physical cleaning agents. Ultrason. Sonochem. 2019, 52, 268–279. [Google Scholar] [CrossRef]
  43. Uemura, Y.; Sasaki, K.; Minami, K.; Sato, T.; Choi, P.K.; Takeuchi, S. Observation of cavitation bubbles and acoustic streaming in high intensity ultrasound fields. Jpn. J. Appl. Phys. 2015, 54, 07HB05. [Google Scholar] [CrossRef]
  44. Park, R.; Choi, M.; Park, E.H.; Shon, W.J.; Kim, H.Y.; Kim, W. Comparing cleaning effects of gas and vapor bubbles in ultrasonic fields. Ultrason. Sonochem. 2021, 76, 105618. [Google Scholar] [CrossRef]
  45. Chahine, G.L.; Kapahi, A.; Choi, J.K.; Hsiao, C.T. Modeling of surface cleaning by cavitation bubble dynamics and collapse. Ultrason. Sonochem. 2016, 29, 528–549. [Google Scholar] [CrossRef]
  46. Lee, Y.; Kim, J.; Ha, T.; Kang, B.; Kim, Y. Effect of Stable and Transient Cavitation on Ultrasonic Degassing of Al Alloy. Metals 2024, 14, 1372. [Google Scholar] [CrossRef]
  47. Cheng, Y.H.; Zhu, B.L.; Yang, S.H.; Tong, B.Q. Design of concrete mix proportion based on particle packing voidage and test research on compressive strength and elastic modulus of concrete. Materials 2021, 14, 623. [Google Scholar] [CrossRef]
  48. Kwan, A.K.H.; Chan, K.W.; Wong, V. A 3-parameter particle packing model incorporating the wedging effect. Powder Technol. 2013, 237, 172–179. [Google Scholar] [CrossRef]
  49. Gyarmati, G.; Budavári, I.; Fegyverneki, G.; Varga, L. The effect of sand quality on the bending strength and thermal distortion of chemically bonded sand cores. Heliyon 2021, 7, e07624. [Google Scholar] [CrossRef]
  50. Vasková, I.; Varga, L.; Prass, I.; Dargai, V.; Conev, M.; Hrubovčáková, M.; Demeter, P. Examination of behavior from selected foundry sands with alkali silicate-based inorganic binders. Metals 2020, 10, 235. [Google Scholar] [CrossRef]
  51. Li, J.; Zhang, H.; Hu, S.; Du, M.; Xiang, T.; Chen, J.; Cheng, Y. Wettability, flowability and core bending strength of wet silica sand particles based on the interfacial properties of liquid silicate binders. Powder Technol. 2024, 433, 119195. [Google Scholar] [CrossRef]
  52. Korea Electric Power Corporation, Electricity Tariff Structure. 2025. Available online: https://online.kepco.co.kr/PRM015D00 (accessed on 5 August 2025).
  53. Kim, K.H. Wet Regeneration and Optimization for Circular Use of Foundry Sand in Casting Process. Ph.D. Thesis, University of Ulsan, Ulsan, Republic of Korea, 2021. Available online: https://oak.ulsan.ac.kr/handle/2021.oak/5645 (accessed on 5 August 2025).
Figure 1. (a) shows a virgin sand core with inorganic binder; (b) shows WSIB after heating at 650 °C for 20 min.
Figure 1. (a) shows a virgin sand core with inorganic binder; (b) shows WSIB after heating at 650 °C for 20 min.
Applsci 15 08988 g001
Figure 4. Schematic diagram of apparatus used to measure gas evolution from sand cores in contact with A356 Al melts.
Figure 4. Schematic diagram of apparatus used to measure gas evolution from sand cores in contact with A356 Al melts.
Applsci 15 08988 g004
Figure 5. Result of XRF analysis of sand samples: VS, WSIB, CRS, and URS.
Figure 5. Result of XRF analysis of sand samples: VS, WSIB, CRS, and URS.
Applsci 15 08988 g005
Figure 6. Result of particle size distribution of sand samples: VS, WSIB, CRS, and URS.
Figure 6. Result of particle size distribution of sand samples: VS, WSIB, CRS, and URS.
Applsci 15 08988 g006
Figure 7. Surface morphology of sand samples: (a) VS, (b) WSIB, (c) CRS, and (d) URS.
Figure 7. Surface morphology of sand samples: (a) VS, (b) WSIB, (c) CRS, and (d) URS.
Applsci 15 08988 g007
Figure 8. Schematic diagram of binder removal process: (a) ultrasonic recycling removes binder via cavitation and acoustic streaming; (b) mechanical recycling removes binder through collapse and grinding between particles by centrifugal force.
Figure 8. Schematic diagram of binder removal process: (a) ultrasonic recycling removes binder via cavitation and acoustic streaming; (b) mechanical recycling removes binder through collapse and grinding between particles by centrifugal force.
Applsci 15 08988 g008
Figure 10. Gas volume evolved from 100 % VS, CRS, and URS core upon contact with A356 Al aluminum melts.
Figure 10. Gas volume evolved from 100 % VS, CRS, and URS core upon contact with A356 Al aluminum melts.
Applsci 15 08988 g010
Figure 11. The typical microstructure at the interface between the sand core and A356 Al castings: (a) VS core and A356 Al castings, (b) CRS core and A356 Al castings, and (c) URS core and A356 Al castings.
Figure 11. The typical microstructure at the interface between the sand core and A356 Al castings: (a) VS core and A356 Al castings, (b) CRS core and A356 Al castings, and (c) URS core and A356 Al castings.
Applsci 15 08988 g011
Table 1. AFS-GFN for VS, WSIB, CRS, and URS.
Table 1. AFS-GFN for VS, WSIB, CRS, and URS.
Sieve Size
(Mesh)
MultiplierProducts ( %   S a n d   R e t a i n e d × M u l t i p l i e r )
VSWSIBCRSURS
200.10.30.10.10.1
300.23.53.12.73.1
400.316.913.712.813.5
500.45.98.48.18.0
700.52.84.76.65.5
1000.71.32.52.92.6
14010.73.03.83.0
2001.40.20.81.80.8
27020.21.00.80.8
PAN300.30.30.3
AFS-GFN31.837.539.937.7
Table 2. Specific surface area of VS, WSIB, CRS, and URS.
Table 2. Specific surface area of VS, WSIB, CRS, and URS.
Sand Specific   Surface   Area   ( m 2 / g )
VS0.04
WSIB0.11
CRS0.07
URS0.06
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Ha, T.; Kim, J.; Lee, Y.; Kang, B.; Baek, J.; Kim, K.; Kim, Y. Efficient Recycling Process of Waste Sand with Inorganic Binder via Ultrasonic Treatment. Appl. Sci. 2025, 15, 8988. https://doi.org/10.3390/app15168988

AMA Style

Ha T, Kim J, Lee Y, Kang B, Baek J, Kim K, Kim Y. Efficient Recycling Process of Waste Sand with Inorganic Binder via Ultrasonic Treatment. Applied Sciences. 2025; 15(16):8988. https://doi.org/10.3390/app15168988

Chicago/Turabian Style

Ha, Taekyu, Jongmin Kim, Youngki Lee, Byungil Kang, Jaeho Baek, Kyungho Kim, and Youngjig Kim. 2025. "Efficient Recycling Process of Waste Sand with Inorganic Binder via Ultrasonic Treatment" Applied Sciences 15, no. 16: 8988. https://doi.org/10.3390/app15168988

APA Style

Ha, T., Kim, J., Lee, Y., Kang, B., Baek, J., Kim, K., & Kim, Y. (2025). Efficient Recycling Process of Waste Sand with Inorganic Binder via Ultrasonic Treatment. Applied Sciences, 15(16), 8988. https://doi.org/10.3390/app15168988

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop