Metal extraction from ores and waste materials by ultrasound-assisted leaching -an overview

ABSTRACT The traditional leaching process is characterized by extensive reaction time, low efficiency, and considerable leaching reagent consumption. It was well demonstrated that using ultrasound could effectively enhance the leaching reaction rate by removing the passivating layer and increasing the mass transfer rates. This improvement would result from bubble cavitation and other mechanical-chemical mechanisms that ultrasounds can generate during ultrasound-assisted leaching (UAL). Thus, these days using UAL for the recovery and recycling of various valuable metals has markedly received attention as an environmentally friendly process. However, surprisingly no comprehensive overview has been provided to focus on various reaction mechanisms through the applications of UAL and deliberate them for nearly two decades. This work has explored various applications of UAL applied for ore and waste processing by a systematic approach to fill this gap. An overview of different mechanisms (mechanical, thermal, sonochemical) based on main ultrasound operating variables (frequency, power, and time) and their level of leaching effectiveness on the leaching metallurgical responses in varied conditions was discussed in detail. It was indicated that the common approach for conducting UAL investigation mainly focuses on improving the leaching efficiency of metals by using single-frequency ultrasound. While analyzing the correlation between ultrasonic cavitation theory and assisted leaching process and exploring the systematic effect of multi-frequency ultrasonic system need to be further clarified in future research. In general, the present work is going to potentially pave the path for understanding UAL and further its development in the future.


Introduction
Since hydrometallurgical processes (such as leaching) have several advantages over pyrometallurgical techniques, including treatment of low-grade ores with high treatment capacity and low temperature for operation, they have been used widely for extracting and recovering metals from ore and waste materials (Luque-Garcıá and Luque de Castro 2003; Mansur, Guimarães and Petraniková 2022; Mohanty and Devi 2021;Safarzadeh, Moats and Miller 2014;Wei et al. 2015).However, the main disadvantages of conventional leaching processes are large chemical consumption, difficulties in combination with low liberation of metals from ores, and long reaction times (Astuti et al., 2018;Das et al., 2021;Urbańska, 2020).Therefore, various methods have been examined to tackle these drawbacks, and ultrasound-assisted leaching (UAL), as an environmentally friendly method, was introduced to enhance extracting metals from different types of solid samples (Shankar et al. 2011;Swamy et al. 1995;Tyagi et al. 2014;Vyas and Ting 2018;Wang et al. 2014).It was documented that UAL can successfully minimize energy and reagent consumption and optimize reaction conditions (Behera et al. 2019;Jiang et al. 2018;Li et al. 2010aLi et al. , 2012;;Moravvej et al. 2021;Moravvej, Mohebbi, and Daneshpajouh 2018;Swamy and Narayana 2007;Wang et al. 2017).Ultrasounds could initiate the cavitation phenomenon, which can be a source of various mechanochemical reactions for breaking the intermolecular bonds, enhancing leaching agent accessibility (Swamy and Narayana 2007;Swamy et al. 1995), increasing temperature, promoting the mass transfer within the reactions, and finally improving metal dissolution (Meshram, Abhilash, and Pandey 2019;Nguyen and Lee 2021;Tyagi et al. 2014;Vyas and Ting 2018).
In general, forming an inert layer on the surface of particles through the conventional leaching process is the main reason which hinders the further progress of the acid leaching reaction (Figure 1).UAL can enhance the reaction rate between metal elements and acid and improve the leaching efficiency.It is noted that the UAL-type enhancements are limited when the conventional leaching processes are performed at higher temperatures and pressures or longer agitation and retention times (Brunelli and Dabalà 2015;Le and Lee 2021;Meshram and Abhilash 2020).UAL has been proven to have several advantages over other treatment leaching assistant methods based on its functionalities (Meshram and Abhilash 2020;Swamy et al. 1995;Xing, Sohn, and Lee 2020).It was reported that UAL is considered safer and much more expedient than microwaveassisted leaching for the selective leaching of Mo using NaOH (Pinto and Soares 2012).UAL is also more energy-efficient than hydrodynamic cavitation for metal extraction (Shut and Mozzharov 2017).Compared to conventional leaching, the UAL can also significantly increase leaching reaction rates and decrease reagent consumption (Narayana et al. 1997).Thus, Luque-Garcıá and Luque de Castro (2003) concluded that the UAL might be an expeditious, inexpensive, and efficient alternative to other metal extraction methods (Luque-Garcıá and Luque de Castro 2003).
The analyses of the UAL literature showed that the focus of documented investigations was mainly on the improvement of metal leaching recovery and the kinetic enhancement of the process.The experiments were mainly conducted under the laboratory scale's auxiliary conditions of the ultrasonic tank.Therefore, to move toward higher-scale applications, it is necessary to provide a detailed analysis of various UAL applications and suggested mechanisms of reactions to address the role of sonication in leaching processes.Some overall views of UAL applications have been documented (Luque-Garcıá and Luque de Castro 2003; Narayana et al. 1997;Swamy et al. 1995).Since 2000, many published articles on UAL of metals (Figure 2).However, no systematic overview has been conducted concerning UAL for extracting metals from mining and metallurgical resources in recent two decades.The goals of this review are given as follows: • As a strategic approach, this work first introduced the ultrasonic cavitation effects of UAL processes.• After that, the impact of key effective parameters, including ultrasound frequency, power, and time was discussed in detail.In particular, technical factors such as the position of the sample vessel, pulse mode operation, the spreading liquid conditions, the probe tip position, and discrete versus continuous UAL were overviewed.• Finally, potential gaps within the UAL application and possible directions for practical developments were suggested.

Cavitation
The treatment by ultrasonic equipment in liquids mainly relies on the cavitation mechanism.Cavitation mostly has three stages: formation of cavitation bubbles, growth, and then violent collapse (Simal et al. 1998).Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial (or stable) cavitation.Inertial cavitation is when a liquid's void or bubble rapidly collapses, producing a shock wave.When the intensity of the acoustic field reaches a certain threshold, the growth and rapid collapse of bubbles in a liquid occur within one or limited acoustic cycles.The collapsed bubbles convert into several finer bubbles (Chen et al. 2020b;Leighton 1994;Yasui 2002).These fine bubbles are also collapsed speedily, and the initial bubble annihilation will be completed (referred to as unstable, transient and/or inertial cavitation, which mainly happens at common UAL frequencies  '20-100 kHz') (Adamczyk, Cempa, and Białecka 2022;Crum and Nordling 1972;Leong et al. 2013).Non-inertial cavitation occurs when tiny bubbles in a liquid are driven to oscillate in the presence of an acoustic field when the intensity of the acoustic field is inadequate to induce complete bubble collapse.
As bubble oscillation or repetitive transient mode, non-inertial cavitation could be continued for several hundreds of acoustic cycles (considered a stable system) (Shankar et al. 2011;Yasui 2002).The Blake threshold (P B ) (which can be estimated by the following equation Equation ( 1)), would be the minimum possible acoustic pressure in this cavitation system (Leighton 1994): where P v is the solution vapor pressure, σ is the surface tension, R 0 is the initial nanobubble radius, and P 0 is the system pressure.It would be a way straightforward to create high amplitude acoustic waves (high acoustic pressure) at low frequencies (Kentish and Ashokkumar 2011).The system has to be at high frequencies (>1 MHz) with lower acoustic pressures to have a high dissipation within the liquid.However, cavitation is generally not observed in industrial applications since PB has infrequently exceeded this frequency (Yasui 2002(Yasui , 2018)).
Several thousands of fine bubbles would be generated from liquid vibration when a liquid vessel is exposed to the ultrasonic waves, that is, cavitation bubbles (Avvaru et al. 2008).The generated bubbles spread throughout the negative pressure zone developed by the ultrasonic wave longitudinal propagation and quickly near the positive pressure zone (Singh et al. 2019;Swamy et al. 1995).Bubble implosions associated with acoustic cavitation release high energy and develop a microjet with the speed of around 110 m/s beside a strong impact force, leading to a collision density as strong as 1.5 kg/ cm 2 (Qiang et al. 2021).The instant of rapid bubble collapse creates local high temperature (1000s of Kelvin) and high pressure (100s of atmospheres), and the cooling rate may reach 10 9 K/s (Nagarajan et al. 2006).This ultrasound phenomenon significantly enhances the heterogeneous reaction rate, uniforms the mixture of heterogeneous reactants, accelerates the diffusion of reactants and products, increases forming of new solid phases, and controls the particle size distribution (PSD) (Tyagi et al. 2014;Vyas and Ting 2018).During the UAL process, when the ultrasonic wave propagates through the liquid phase, the environment undergoes physical and chemical changes due to bubble cavitation.The ultrasonic wave continuously produce bubbles, and their collapsing cause mechanical, thermal, and sonochemistry impacts (Figure 3) (Suslick 1990;Thompson and Doraiswamy 1999).These effects have different pluses and minuses (Table 1).

Mechanical
The inertial cavitation will have occurred due to inrushing surrounding fluid after the rapid contraction or gas bubble collapse, which can be symmetrical or asymmetrical (Shankar et al. 2011).Symmetric cavitation and/or micro jets (asymmetric cavitation) generate shock waves that lead to mechanical impacts (Hagenson and Doraiswamy 1998).These mechanical effects enhance the solid particle momentums, and the generated high forces collide between them (Mao et al. 2019;Vyas and Ting 2018).In addition to inertial cavitation, non-inertial cavitation generates no erosion but leads to microstreaming and could be linked to moderate bubble cavity growth that is not over twice the original bubble equilibrium radius (Shankar et al. 2011).Microstreaming specifically refers to the streaming flow of fluid around an oscillating object such as a gas bubble (Elder 1959;Jalal and Leong 2018).The oscillating bubble will generate fluctuations in velocity and pressure in the surrounding fluid (Jalal and Leong 2018;Tho, Manasseh and Ooi 2007).Non-inertial cavitation can result in numerous non-inertial events such as bubble motion and streaming related to the mechanism causing the mass transfer improvement (Birkin et al. 2011).In addition, the mechanical effect induced by ultrasound promotes the development of microcracks and the removal of the inert layer on the solid surface, which is conducive to the further reaction between leachants and reactants (Le and Lee 2021;Margulis and Margulis 2002;Meshram and Abhilash 2020;Paunovic et al. 2019).

Thermal
When ultrasonic waves propagate into the medium, part of their energy converts to heat through friction and conduction processes, called a thermal effect.This phenomenon causes an increase in the local temperature of the medium (Fan et al. 2021).Therefore, the media temperature gradually rises with the ultrasound proportional to its irradiation time.When the temperature reaches a certain level, the rate of temperature enhancement begins to slow down (Abramowicz et al. 2008;Wu et al. 1995).Increasing the temperature affects the thermophysical characteristic of the solution and reaction rate.This phenomenon accelerates acoustic streaming and mass transfer processes (Chen et al. 2021).When the ultrasound propagates into media, bulk movement of the medium occurs.This ultrasonically induced flow is also known as acoustic streaming (Xu, Yasuda and Koda 2013).It is considered that the combination of acoustic streaming and the thermal effect generated by ultrasound impact the mass transfer process (Chen et al. 2021).

Sonochemical
Sonochemistry mostly deals with the chemical reaction and mass transfer rate improvement through different ultrasonic conditions (Kentish and Ashokkumar 2011).In a few special cases, reactions are not beneficial from acoustic cavitation.More OH radicals at 200 kHz could prevent the formation of FeH(C 2 O 4 )+ and oxidize FeH(C 2 O 4 )+ to Fe(C 2 O 4 ), which creates a cover layer on the surface of the particles (Nakamura et al. 2011).Ultrasound-assisted chemical reactions are termed 'sonochemical reactions.'The violent collapse occurs during transient and repetitive transient cavitation generating enormous temperatures at a localized level (>5,000 K) (Ashokkumar and Mason 2007).The simultaneous extensive temperature increases and the violent pressure variations can create chemical changes within the vapor phase in the cavitation bubble and the surrounding immediate fluid.Two basic theories can be explained these impacts; (i) the heat or hot-spot theory of Noltingk and Neppiras (1950) and (ii) the electrical theory of cavitation phenomena proposed by Margulis (1994).Compared to the electrical theory, the heat theory cannot explain many experimental dependences of rates through the sonochemical reactions (Margulis 1994;Margulis and Margulis 2002).Through UAL, the microbubble implosion would generate free radicals with a high tendency to do the reaction (Fischer, Hart and Henglein 1986;Hagenson and Doraiswamy 1998;Hassanzadeh et al. 2021;Mason 1990;Suslick 1990;Vyas and Ting 2018).In aqueous reactions, the formation of free radicals is described as follows (Thompson and Doraiswamy 1999): The temperature inside the collapsing bubbles controls the number of radicals generated.It can be augmented by enhancing sonication power, exceeding external pressure, and reducing external (solution) temperature, leading to more radicals (Kentish and Ashokkumar 2011).In addition, changing from an air-saturated medium to an inert gas saturated such as argon is also effective (Ashokkumar and Mason 2007;Leighton 1994).Variations of ultrasound variables (frequency, power, and time) through the UAL would be essential for enhancing and effectively these three main multi-correlated mechanisms (mechanical, thermal, and sonochemistry).For example, it was documented that chemical effects are more dominant at intermediate frequencies (200 to 500 kHz) as the number of active bubbles generated is higher (Ashokkumar et al. 2008;Kentish and Ashokkumar 2011).In some conditions, the generated free radicals can prevent the reaction between metals and leachants (Nakamura et al. 2011).
A High-frequency system can produce many free radicals, while the enhancement of leaching efficiency should be carefully evaluated.

Ultrasound kinetics
Mineral solubility and transport mechanisms are the main factors that dictate the duration of a leaching step.The three crucial steps of a leaching process are as follows (Swamy and Narayana 2007): (i) Contact occurrence between leachants and mineral particles: The leachant contacts with particle surface and penetrates into the mineral matrix, starting the extraction.(ii) Reaction occurrence between leachants and minerals at the liquid/solid interface: The retained analytes are removed by sweeping or displacement from the active sites of the matrix because of a higher affinity and/or extractant concentration.This phenomenon is directly happened by the dissolution (solvation) of minerals in the leachant.(iii) Migration of dissolved minerals from particle surfaces to bulk fluid: The analytes are transported from inside of particle structure to its surface by principally diffusional forces and outside the matrix by initially convective forces, while the leaching continues whether in dynamic or agitated modes.
One of these three stages could be the rate-controlling step, the slowest step in the leaching reaction.Based on which one of them is the rate-controlling, three varied reaction types can obtain, i.e. reaction-controlled leaching, diffusion-controlled leaching, and intermediate-controlled leaching (Asgari et al. 2021;Garg et al. 2020;Kuipa et al. 2014;Li et al. 2017;Romdhane and Gourdon 2002).
The mechanisms of UAL can be attributed to the enhancement of mineral solubility mechanisms and transport phenomena (Swamy and Narayana 2007).Thus, under UAL conditions, the following effects are involved, which affect the process kinetics: (1) Collapse of bubbles produced in the proximity of solid surface leads to high-speed microjets, which can improve transportation rates (into & away from reaction zone), facilitate diffusion of the analytes to the outer zone, and increase the surface area through surface pitting (Hagenson and Doraiswamy 1998;He, Cao, and Duan 2017;Ma et al. 2017;Sharif et al. 2014).( 2) Particle fragmentation during collision enhances the surface area and eases leachant penetration rate into the matrix (fragmentation reduces the size of the matrix) (Avvaru et al. 2006;Le and Lee 2019;Raman and Abbas 2008;Wen et al. 2018).
Phenomena 1 and 2 result from the micro-jetting effect, while phenomenon 3 may happen with the micro-streaming mechanism.A critical issue for selecting the leaching agent is the initial step, namely selective solubilization of target analytes in the former and destruction of the particle-matrix in the latter.All these phenomena are essential for assessing the UAL kinetics.
Based on leaching kinetics, thermodynamic modeling (i.e.estimation of the activation energy) is a useful index to confirm the type of controlled process of the conventional and UAL processes (Thompson and Doraiswamy 1999).The Arrhenius equation is commonly used to characterize the relationship between reaction rate and temperature for various chemical reactions.Equation ( 6) is a common form of the Arrhenius equation (Koerner, Lord and Hsuan 1992) where k is the kinetic rate constant at a given absolute temperature (T), E a is the activation energy, R is the universal gas constant (8.314J•mol −1 •K −1 ), and A is a pre-exponential factor.The parameters (A and E a ) of the Arrhenius equation can be computed by the least-squares method using a nonlinear regression approach (Bu et al. 2016).In addition, the activation energy can also be estimated by plotting ln(k) vs. 1/T (Ashter 2014).The linearized Equation ( 6) is presented in Equation 7(the Arrhenius equation): As seen in Table 2, ultrasound can decrease the activation energy of the reaction compared to conventional conditions.Equation ( 7) indicates both the sizes of the activation energy of the reaction and the pre-exponential factor A influence the reaction rate constant.However, the pre-exponential factor obtained simultaneously is rarely discussed and sometimes not even listed.Some literature reported that the ultrasound The activation energies for the conventional and ultrasound-augmentedleaching process are 13.07 and 6.57 kJ/mol, , respectively.Wang et al., (2014) H 2 SO 4 -CaF 2 leaching of K-feldspar The activation energies with and without ultrasound were found to be 55.67 kJ/mol and 72.33 kJ/mol, respectively.

Ma et al. (2017)
Co-NH 3 -S 2 O 3 2− leaching of gold from gold concentrate Ultrasound decreased the activation energy from 22.65 kJ/mol to 13.86 kJ/mol (conventional conditions.

Gui et al. (2020)
H 2 O 2 -CH 3 COOH leaching of copper from blended copper slag The apparent activation energy was found to be 55.35 kJ/mol, and the kinetics equations of the processes were established.
Turan, Sari, and Demiraslan (2019) Na 2 S 2 O 3 leaching of silver from spent symbiosis lead-zinc mine The apparent activation energy under conventional and ultrasonic conditions is 12.47 kJ/mol and 12.35 kJ/mol, respectively, and it is proved that both are controlled by diffusion.

Li et al. (2018b)
does not influence the activation energy of the reaction, while the effect of ultrasound is solely focused on the pre-exponential factor (Mesci and Sevim 2006;Tekin, Tekin, and Bayramoğlu 2001;Wang, Faraji, and Ghahreman 2020).There is a positive relationship between the pre-exponential factor A and ultrasound power (He, Cao, and Duan 2017;Horst et al. 1996;Ingeç and Tekin 2004;Lahiri et al. 2020;Tekin, Tekin, and Bayramoğlu 2001).Further, Ferrero and Periolatto (2012) found that the apparent activation energy values were decreased from 62.2 to 49.8 kJ/mol in the absence of ultrasound.They also reported that the values of ln A were 18.9 and 13.9, respectively, showing that the strong enhancement of the kinetic constants in the presence of ultrasound is due to the increase of the pre-exponential factor.The pre-exponential factor can be the collision of H+ ions with the radical acid ion in the crystal lattice or the aqueous phase.This effect has been widely reported in many heterogeneous reactions associated with ultrasound and maybe the steric factor (Horst et al. 1996;Lahiri et al. 2020;Tekin, Tekin, and Bayramoğlu 2001).
Since the pre-exponential factor is an important characteristic, a systematic discussion on the pre-exponential factor in UAL applications is necessary in the future.

Ultrasound variables
As mentioned, the effectiveness of UAL mechanisms would completely rely on the level of essential ultrasonic variables (frequency, power, and time).Several investigations have explored the significance of these variables through the UAL of ores and secondary resources, and these processes could be acidic or basic UAL (Tables 1 and 2).Although exploring documented works indicated that few reports examined the frequency variations (Table 1), the focus was mainly on investigating various power and time conditions (Tables 2 and 3).

Frequency
Ultrasound wavelength (frequency) plays an essential role in bubble formation and in their growth.Short-wavelength ultrasounds which occur at high frequencies, cannot deliver enough time for substantial bubble growth.Thus, their cavitation is not comparable with long wavelengths (Shankar et al. 2011).The frequency level and the power output are inversely proportional.Low-intensity (high-frequency) ultrasound (in the MHz range) does not alter the medium state through which it travels and is commonly used for nondestructive evaluation and medical diagnosis.Nevertheless, high-intensity, low-frequency ultrasound does alter the medium state and is the type of ultrasound typically used for sonochemical applications (Thompson and Doraiswamy 1999).Mainly the frequency examined for UAL was below 100 kHz (Table 3), and mostly as a constant variable, it was kept at 20 kHz (Tables 1 and 2).Leong et al. (2013) have identified three ultrasound frequency regions.Frequency ranging from 20 to 100 kHz belongs to the first region, called power ultrasound, because of the high energy density delivered into the medium.Due to the lowfrequency ultrasound in leaching, enhancements can be attributed to the simultaneous chemical-mechanical impacts, or one of them originating from transient cavitation (Hagenson and Doraiswamy 1998).Particle size reduction revealed particle fractures, which are highly predominant in the low-frequency ultrasound field (Ambedkar, Nagarajan, and Jayanti 2011;Mao et al. 2018;Raman and Abbas 2008).It is experimentally reported that the ultrasound treatment can crush coarse particles into fine particles, and the particle's sphericity also increases with sonication time (Ambedkar, Nagarajan, and Jayanti 2011).Chipping and rounding are mechanisms that lead to preferential mass loss from the corners and edges of particles, causing rounding or conditioning of angular feed particles (i.e. the associated micro-polishing mechanism) (Bu et al. 2019;Cleary and Morrison 2016).The particles disintegrate with a minimum size dependent on the solid characteristics, solvent, and ultrasound intensity.The finest particle size attained is limited by the reduced momentum of smaller particles, which is insufficient to cause further particle breakage (Thompson and Doraiswamy 1999).Sonication produces very fine particles compared to the virgin sample leading to an enhancement in the surface area of particles.In this regard, Wen et al. (2018) reported that the particle size treated by an ultrasound decreased compared to the raw sample.This leads to intimate contact with the strong oxidizing agents generated by the ultrasound in the liquid phase (Ambedkar, Nagarajan, and Jayanti 2011).Comparing the PSD of virgin and sonicated coal samples with two varied ultrasonic frequencies (20 and 25 kHz) at 500 W ultrasound input power revealed the index impact of acoustic waves on the effective mechanical stresses on solids (ultrasonic in tanks).By increasing the frequency to a certain level, the mechanical stresses were more likely to be influenced by the PSD.The particle size of samples decreased after 2 min from an average of 27.51 to 9.02 μm under ultrasonic with a frequency of 25 kHz, while with a frequency of 20 kHz, their sizes had decreased to 11.09 μm (Ambedkar, Nagarajan, and Jayanti 2011).In contrast with the collapse of cavitation bubbles at low frequencies, bubbles generated at high frequencies are much finer and have an affinity to resonate rather than collapse (Mason 2016).However, by further increasing the ultrasonic frequency, the transient cavitation intensity decreases after that certain level, although the stable cavitation effect enhances (Bu and Alheshibri 2021;Chen et al. 2020b;Leong et al. 2013).For instance, considering 40 kHz frequency for cleaning electronic wafers might damage them, while 1 MHz did not (Bulat 1974).In addition, acoustic streaming phenomena are predominant at high ultrasound frequencies.Acoustic streaming macro-and micro-liquid vortex occur in the acoustic chamber space, which does not require cavitation bubble collapse (Lamminen, Walker, and Weavers 2004).Acoustic streaming originates from the acoustic field's inhomogeneous distribution of acoustic pressure.There are three main reasons for the uneven distribution of the pressure field: (1) Attenuation of acoustic energy in the space; (2) Scattering of acoustic waves; and (3) Friction between the vibratory fluid and the chamber wall (Luo et al. 2018).The increasing ultrasound frequency increased acoustic streaming velocity significantly (Shutilov 1988).The excessively faster acoustic streaming is harmful to the mass transfer, which may decrease the leaching efficiency.For such a tendency, Xing et al. (2015) reported that by raising the ultrasound frequency from 45 to 80 kHz, the iron removal by hydrochloric acid UAL at 30°C would improve by 5%; however, enhancing the ultrasound frequency to 100 kHz did not absolutely increase the iron rejection.These variations could be due to the structure of the material, the target element, the way the element is involved, and its crystalline structure in the containing materials, which all are effective in leaching under ultrasonic waves.Nakamura et al. (2011) indicated that the jet flow generated by cavitation bubble collapse and the fracture action of ultrasound irradiation at 28 kHz was stronger than that at 200 kHz through the UAL of iron from green tuff particles at both 20 and 90°C.Therefore, the amount of leached iron at 28 kHz was higher than 200 kHz.As seen in Figure 4, the number of free radicals increases when the ultrasound frequency is increased from 20 kHz to 358 kHz.Increasing the ultrasound frequency can increase the number of active cavitation bubbles (Leighton 1994).In addition, it is known that at a low frequency range the cavitation bubbles are primarily transient, whereas stable bubbles are generated at a higher frequency range (several hundred kHz) (Ashokkumar et al. 2008;Bu and Alheshibri 2021;Chen et al. 2020b;Leong et al. 2013).Thus, the increase of the amount of •OH radicals generated when ultrasound is increased from 20 kHz to 358 kHz is attributed to that both stable cavitation and an increase in the number of active bubbles with an increase in the ultrasound frequency.Thus, in Nakamura et al. (2011)'s study, 200 kHz irradiation could generate more OH radicals compared to 28 kHz.It is concluded that more OH radicals at 200 kHz could prevent the formation of FeH(C 2 O 4 )+ and oxidize FeH(C 2 O 4 )+ to Fe(C 2 O 4 ), which creates a cover layer on the surface of the particles.In addition, Li et al. (2017) reported the lower efficiency at a frequency over 50 kHz could be attributed to the production of more violent cavitation at the lowest ultrasonic frequency, which generated the most localized temperatures, pressures, and more substantial shock waves.Such intensive localized temperatures could significantly enhance some reactions.Furthermore, effective shock waves could increase the particle diffusion into the leach liquor and promote the chemical reaction rate.
The size of the UAL leaching efficiency is related to various effects of acoustic cavitation.The mechanical effects of cavitation and its consequences, such as producing free radicals (sonochemical effect), could be considered the main outcomes of acoustic dissolution-based processes.However, the mechanical influences are damped down when ultrasound frequency increases excessively.And most of the energy impacts and changes in the material's structure lead to dissolution.By applying radio waves and microwaves, the structure of matter undergoes significant changes.These changes in the relatively low spectrum of acoustic energy frequencies are negligible and, in practice, unaffected from this viewpoint (Moravvej et al. 2021;Moravvej, Mohebbi, and Daneshpajouh 2018;Vakylabad 2021).The same pattern was reported in other investigations, where the increase in acoustic wave frequency is either ineffective or even has a negative impact on the recovery and leaching of the target element (Ambedkar, Nagarajan, and Jayanti 2011;Lahiri et al. 2020;Swamy and Narayana 2001).In summary, Compared to the single-frequency system, dual-frequency ultrasound has some advantages in intensifying the leaching process (like lowering process operation time and saving energy).Ambedkar, Nagarajan, and Jayanti (2011) demonstrated that applying two wave sources (58 and 192 kHz; dualfrequency system) would reduce energy consumption by lowering the time of operation process.This could be one of the outcomes of the simultaneous presence of cavitation and streaming impacts.Balakrishnan, Reddy, and Nagarajan (2015) also reported that low (25 kHz) and high-frequency (430 kHz) ultrasound waves had a superior performance for enhancing the removal efficiency of alkali elements in coals using ammonium acetate leaching compared to that of stirring.Swamy and Narayana (2001) observed an intensified phenomenon of ammonia leaching of copper from oxide ores by the simultaneous application at dual frequencies, i.e. 20 and 40 kHz ultrasound.
In summary, low-frequency systems have superior performance in the UAL process.Moreover, the literature review indicates that high frequencies have no essential effect on increasing the leaching metallurgical efficiency (metal recovery).Low-frequency ultrasound (20 to 80 kHz) generates more violent cavitation producing higher localized temperatures and pressures at the cavitation site resulting from transient cavitation (Chen et al. 2020b).In addition, the mechanical impact of the acoustic wave is dominant in low ranges (20-40 kHz) of the frequencies for individual solid materials (Leighton 1994).Those phenomena help enhance mass transfer, decrease the particle size, and remove the covering layer.Sonochemical impacts are mostly dominant at intermediate frequencies, which produces a higher number of radicals (200-500 kHz) (Ashokkumar et al. 2008;Kentish and Ashokkumar 2011).Many radicals can provide more active components (•H and •OH) to react with metals.Thus, it is concluded that mechanical and thermal effects are more significant determining factors for the UAL leaching process of metals than the sonochemical effect.In addition, the attenuation of waves increases with frequency; hence, the penetration depth decreases (Chandrapala et al. 2013;Leighton 1994).It is not economically cost-effective to increase the leaching efficiency by increasing the frequency (especially for large-scale apparatus).The dual-frequency system can be a promising tool for enhancing the leaching efficiency compared to the singlefrequency system (Ambedkar, Nagarajan, and Jayanti 2011).It is reported that the optimum frequency relies on whether intense temperatures and pressures are needed (thus, increased by lower frequencies) or if the rate of single-electron transfer is more central (improved by greater frequencies) (Thompson and Doraiswamy 1999).In general, systematical investigations and detailed discussion on the multi-effects of ultrasound frequency and power on the leaching process and the intercorrelation between them are needed to be performed from the perspective of cavitation theory.

Power
Ultrasound intensity can be described as the average power of a wave distributed by the area perpendicular to the direction in which it is spread (Case 1998).It was documented that the UAL process has a direct relation with the dissipated acoustic power per unit volume (kW/m 3 ) rather than the acoustic intensity (W/cm 2 ) (Swamy et al. 1995).However, increasing power in a specific area would be associated with rising ultrasound intensity (Dükkancı and Gündüz 2006;Price, Harris, and Stewart 2010).Such change enhanced the number of cavitation bubbles (Xu, Chu and Graham 2013), and cavitation bubble collapse could be occurred sharply (Zhang et al. 2015a).Furthermore, increasing ultrasonic power increased the energy of cavitation and lowered the cavitation threshold limit (Thompson and Doraiswamy 1999).More cavitation events in a high ultrasound power can lead to various significant influences in the leaching of metals, such as enhancing the mass transfer, removing the passivation layer from the particle surface, and increasing the number of H and OH radicals and the solution temperature.
Several studies have addressed the effect of power variations through various UAL processes (Table 4).It was reported that higher ultrasonic power could enhance the UAL efficiency at a constant time.During UAL, microscopic mass transfer induced by the ultrasound can be combined with proper macroscopic mass transfer at a medium stirring speed (John et al. 2020).Stirring the mixture at an excessively high speed can specifically disrupt the standing ultrasound wave formation, decrease cavitation effectiveness, and reduce leaching efficiency.However, a proper stirring speed must provide sufficient macroscopic mixing as well.Zhang et al. (2015b) reported that during the UAL at a greater power, the solution might be more efficiently agitated with cavitation-generated micro jets.Subsequently, high-power ultrasound would effectively reduce the diffusion layer's thickness and broaden the particle surface area compared to the low-power ones.Generally, increasing the ultrasound power within an accurate range generated sufficient energy to the leaching process in terms of the intensification of the cavitation effect (Thompson and Doraiswamy 1999).As a result, an intense ultrasonic power could rapidly assist in receiving the same leaching recovery or enhance the leaching reaction rate.Similar outcomes have also been stated in the leaching reaction rates of various metals such as Cu (Wang, Faraji, and Ghahreman 2020;Xie et al. 2009;Zhang et al. 2015b), spent cathode carbon (Yuan et al. 2018), iron (Xing et al. 2015), zinc (Wang et al. 2014), vanadium (Chen et al. 2020a), Co and Li (Jiang et al. 2018) and indium (Zhang et al. 2017b).Zhang et al. (2017b) documented that the ultrasonic waves at higher ultrasound power can promote the convective motions among the solution allowing fresh H+ to contact with the indium tin oxide glass particles, transport the dissolved metal ions away and finally promote the diffusion that improved the UAL process recovery (Şayan and Bayramoğlu 2004).Meanwhile, it is notable that an excessive increase in the ultrasound power cannot sufficiently enhance the maximum UAL efficiency, while it might facilitate the leaching reaction rate and reduce the processing time (Jiang et al. 2018;Kim et al. 2006;Li et al. 2015;Wang et al. 2013Wang et al. , 2014;;Yuan et al. 2018;Zhang et al. 2015bZhang et al. , 2017b)).Zhang et al. (2016b) indicated that the recovery of germanium (Ge) through UAL from the waste of zinc extraction processes had been gradually enhanced from 84 to 93% when the ultrasound power was increased from 253 to 701 W.However, by increasing ultrasonic power to 851 W, the Ge recovery dropped to 88%.It was indicated that the ultrasonic power above 700 W could disrupt the GeCl 4 production through the process, enhance the oxidant agent decomposition, and stunt Ge within the crystal structure to react with the leach liquor.Li et al. (2018a) reported that a high ultrasound power (>200 W) leads to a decrease in the leaching efficiency of nickel.They concluded that the leaching efficiency had a certain level of improvement while the ultrasonic power was adjusted at 50-100 W. They also illustrated that the Ag content in residue reduced significantly from 77.34 to 66.8% by increasing the ultrasound power from 100 to 400 W. Zhang et al. (2016b) found that 700 W was the optimum ultrasonic power in the leaching experiment.Further, the leaching efficiency becomes poor when the ultrasonic power exceeds 700 W. They considered that this phenomenon is due to the bubbles produced by cavitation coalescing and then weakening the reaction at high ultrasonic power, deteriorating the leaching process.A similar pattern was observed through the UAL gold leaching (Figure 5), when 180 W ultrasonic power input resulted in better recovery than 120 W.However, the recovery was decreased when 1000 W ultrasonic power input was examined, which is explained by the increased acoustic power resulting in the decomposition of CN− (Zhang et al. 2016).In addition, Daryabor, Ahmadi, and Zilouei (2017) reported that cadmium extraction was decreased especially at the higher level of power (120 W), which is attributed to the decomposition of the oxidant agents (Ca(ClO) 2 ) at higher power.Thus, there is optimum ultrasound power for UAL applications.Meanwhile, they reported that ultrasonic waves could markedly shorten the compression and decompression cycle time.As a result, the liquid molecules would not separate from voids at extremely great powers.As a result of this phenomenon, ultrasonic cavitation could not occur appropriately.Therefore, having the optimum acoustic power would be essential during the UAL process (Yazıcı et al. 2007).
In summary, an increase in the ultrasound power could help enhance the UAL efficiency and improve the extraction reaction rate.The cavitation threshold decreases by increasing ultrasound power, and thus more bubbles collapse, meaning more transient cavitation.This leads to higher mass transfer, excessive powerful oxidants, and deeper removal of coating of the passivation layer.However, it is observed that an excessive increase in the ultrasound power can negatively affect the The Cu leaching efficiency was enhanced when ultrasonic power and ultrasonic treatment time period were increased, while the Fe leaching efficiency was reduced Xie et al. (2009) HCl-NaCl leaching of lead-rich and antimony-rich oxidizing slag 200, 422, 622 Increasing ultrasonic power either enhanced the leaching reaction rate or assisted to quickly reaching the same leaching recovery.

Zhang et al. (2015b) HCI-CaCl 2 leaching of heavy metals from hazardous electroplating sludge waste 100-300
The more powerful the power of ultrasonic agitation, the higher the leaching efficiencies for metals Cu, Ni, and Zn, and the lower the leaching efficiencies for metals Cr and Fe.

Li et al. (2010b)
Ammonia leaching of zinc from lowgrade oxide zinc ore 200, 400, 600 Compared to 200 W power, 600 W power can reduce the leaching duration time from 120 min to 60 min.

Li et al. (2015)
NaOH leaching of chromium from radioactive sludges 6, 13-16, 26-34 As the ultrasound power increases, the Cr removal from the sludge is more efficient.Kim et al. (2006) H 2 SO 4 -H 2 O 2 leaching of nickel sulfate 100-800 The optimum leaching occurred when the ultrasonic power was 200 W.For different element extraction, varied ultrasound would be effective.The highest increase occurred when the ultrasonic power was 50-100 W, and for the Ag content, it increased from 100 to 400 W.

Li et al. (2018b)
NaCN leaching of gold from a refractory ore 120-1,000 There is a negative correlation between ultrasound power and process time.The highest extraction could happen when one of them had its maximum value and another one its minimum (120 W 2 h, or 180 1 h).

Zhang et al. (2016a)
MgSO 4 leaching of rare earth 300, 700 Increasing the ultrasonic power assists in getting the same metallurgical responses in a shorter process time.The highest leaching recovery occurred when the ultrasonic power was between 60-90 W. Li et al. (2014) ultrasound-assisted leaching process.In addition, the optimal ultrasound power varies according to the ultrasound frequency.

Ultrasonication time
Ultrasonication time (UT) is an important variable through the UAL process, which is frequently involved in optimizing leaching tests (Table 5).The UAL can produce a higher leaching recovery of metals than the conventional leaching process with the same leaching time.Moreover, using ultrasound can efficiently shorten the UT and increase the leaching reaction rate.The optimum UT (which can determine the process's kinetics) depends on the particular operating conditions such as the US power, source type, and leaching agent.The sample properties such as PSD are also effective (Swamy and Narayana 2007).Typically, UAL efficiency enhances as UT increases.As mentioned, through the UAL process, the collapsing cavitation bubbles generated by ultrasound assist in removing the saturated cover layer around particles, improving the solution accessibility to the particle surface, quickening the mass transfer into the liquor, and finally improving the process metallurgical responses (Chang et al. 2017;Gamarra Güere et al. 2018;Nakamura et al. 2011;Romdhane and Gourdon 2002;Thompson and Doraiswamy 1999;Wang et al. 2013Wang et al. , 2014)).However, some side reactions through the UAL process, such as the influence of radical dot OH and hydrogen atoms generated by water molecules breakage, would reduce metallurgical responses of manganese extraction when the leaching time was longer than a critical value (Li et al. 2008).After a few seconds to minutes (or even hours), the leaching efficiency reaches a stable level and keeps constant with the further increase in UT (Avvaru et al. 2008;Chang et al. 2017;Chen et al. 2020a;Jiang et al. 2018;Li et al. 2018b;Souada et al. 2018;Wang et al. 2017;Xiao et al. 2018;Yin et al. 2018;Yu et al. 2020;Zhang et al. 2015b).

Other factors
Apart from the major discussed variables, other factors can affect through UAL process, including ultrasound device type, the position of the sample vessel, volume and nature of the transmitting liquid, and discrete and continuous UAL (Swamy and Narayana 2007).

Ultrasound device
The UAL process could be conducted in the tank-and probetank ultrasound devices (Tables 1 and 2).The indirect ultrasound irradiation of an ultrasound bath is typically used for UAL.As bubbles intensely diminish the ultrasonic wave, the acoustic intensity in a small liquid vessel is affected by the number of bubbles in the surrounding bath (Yasui 2018).
Experimental conditions in the leaching reaction container change with time if bubbles are formed in water in a liquid container.In other words, an ultrasonic bath with the indirect irradiation method should be filled with degassed water to keep the experimental condition constant.However, the probe-type ultrasound device uses the direct irradiation method (Figure 6).Ambedkar, Nagarajan, and Jayanti (2011) compared the size distribution of virgin and sonicated coal samples in two different ultrasonic systems (25 kHz ultrasonic tank and 20 kHz probe).For a constant ultrasonic power input and time, the sample treated by the ultrasonic tank has more particles smaller than 10 μm in comparison with that of the ultrasonic probe (Figure 7).This phenomena can be based on the variations in UAL processes between tank-and probe-type devices due to the difference in the vibration modes (Asakura 2015; Bu and Alheshibri 2021) and the distributions of the acoustic field (Price, Harris, and Stewart 2010).Meanwhile, it is notable that the ultrasonic power of indirect sonication reaches the reaction vessel is moderately less than a probe-type ultrasonic system (Thompson and Doraiswamy 1999).Thus, particular consideration should be made about the type of ultrasound device for UAL.

Vessel position
Since the ultrasonic transducer is mostly installed to a bath sidewall, the generated ultrasonic waves would also be reflected at the other bath sidewall.This phenomenon cause standing wave formation (Yasui 2018).A node would be a point along with a standing wave where the wave has minimum amplitude.
The reverse point of a node is an antinode, a point where the amplitude of the standing wave is a maximum.The cavitation activity is largely concentrated in a 'cone' just under the horn.
As the intensity rises, the field of activity gets larger, representing a huge volume of active bubbles.Thus, the sample vessel's position is also a determining factor for the amount of energy received by the sample (Swamy et al. 1995).

Transmitting media
The presence of bubbles in the leaching media plays an undeniably important role in the cavitation intensity of an ultrasonic bath.Shi, He, and Chang (2004) found that the transmitting water containing 0.2% (w/v) detergent provides the optimum leaching outcomes of Ca, Mg, Mn, and Zn from vegetable samples in an ultrasonic bath.Higher detergent concentration (>0.2%) enhanced the volume of solid particles in the leaching media and hindered the propagation of ultrasonic energy.Thus, the nature of the leaching media should be accurately selected for ultrasound-assisted leaching of metals from ores and solid wastes.The excessively high surfactant concentration helps form a large number of stable bubbles due to the lower surface tension (Bu et al. 2022).However, as bubbles strongly attenuate the ultrasonic wave, the acoustic intensity in a small liquid container is influenced by the presence of bubbles in the surrounding bath.The increase in the number of bubbles in an ultrasonic bath causes a decrease in acoustic intensity in a small liquid container (Yasui 2018).Thus, the decrease in the acoustic intensity weakens the enhancement of the leaching reaction in UAL processes.

Discrete or continuous UAL
The general procedure for developing discrete UAL (DUAL) is easier than that of its dynamic counterpart (continuous USassisted leaching 'CUAL'), as the former involves no precise data to plan and make the continuous manifold and improve interrelated flow variables as in CUAL (Swamy and Narayana 2007).Some benefits of automated analytical methods regarding CUAL over those relying on DUAL are: (1) CUAL saves reagents and samples; (2) CUAL can be coupled to other continuous steps in a very easy and inexpensive manner; (3) CUAL requires no filtration or centrifugation.Detailed comparisons of DUAL and CUAL can be found in the literature by Swamy and Narayana (2007).

Prospects and recommended research directions
Several gaps in the application of UAL can be monitored based on literature, including the role of resonant (stable, gaseous) cavitation in the dual-frequency system, the effect of gas concentration and type, the tip type of the horn-type ultrasound, the synergistic effect of additives/solvents and ultrasound irradiation and interaction between leaching temperature and ultrasound.

Dual-frequency system
As mentioned, the dual-frequency system can provide some superior leaching performance compared to the singlefrequency system.Dual-frequency ultrasound irradiation improves the grinding of particles during a collision by enhancing size reduction, which increases the leaching metallurgical responses (Swamy and Narayana 2007).It is suggested that there is extra transient (vaporous) cavitation happening at 20 kHz and high resonant (stable, gaseous) cavitation at 900 kHz (Entezari and Kruus 1994).In addition, resonant cavitation can increase temperatures to 1800 K (Saksena and Nyborg 1970).This is still insignificant compared with the 'characteristic temperature (bond energy/R) of 60,000 K for the breaking of an O-H bond (498 kJ•mol − 1 ) (Entezari and Kruus 1994).The enhancement mechanisms of transient cavitation under low frequencies in the leaching process are clear.However, the role of stable cavitation in a dual-frequency system needs addressing.Entezari and Kruus (1996) reported that the degassing process could reduce the sonochemical reaction rate of both 20 and 900 kHz ultrasound irradiation.In addition, for 20 kHz ultrasound, the reaction rate using argon is faster than air when employed.However, the use of air provides a faster reaction rate compared to that of argon when 900 kHz ultrasound is employed.The cavitation phenomena are affected by the gas concentration and type (Entezari and Kruus 1996).It is reported that the solution still contains considerable amounts of gases after the degassing procedure (Leighton et al. 1988).Thus, the nature of the effect of gas concentration and type can be attributed to the differences in the number of gas nuclei in solution and the relative times of the cavitation bubble collapse (Entezari and Kruus 1996;Yasui 2018).Thus, further research should be performed to clarify the effect of gas concentration and type on the UAL processes.

Tip type
The ultrasound power is an essential variable during UAL experiments.As shown in Table 6, the different tip types also play an important role in determining the sonochemical reaction rate's magnitude at the same power level.Thus, it is suggested that optimizing the tip shape of horntype ultrasound can be a promising technique to decrease energy consumption.Price, Harris, and Stewart (2010) directly investigated cavitation fields using a 23 kHz horn sonicator and a 515 kHz plate transducer system and reported significant differences in the cavitation fields between these two sonication systems.Li et al. (2008) investigated the impact of various additive concentrations (citric acid and 8-oxyquinoline) on ultrasonic and conventional extraction (using a sulfuric acidhydrochloric acid mixture (4:0.3,v/v) as solvent).It is concluded that the combination of ultrasound and citric acid has a superior ability to increase manganese recovery compared to the combination of 8-oxyquinoline and ultrasound.Rahimi et al. (2020) reported that without H 2 O 2 and ultrasound, the vanadium (V) recovery using lemon juice organic acids reduced greatly, showing that both parameters were critical in the UAL process.With ultrasound and H 2 O 2 , Li et al. (2018a) observed that citric acid was more effective and eco-friendly than H 2 SO 4 and HCl inorganic acids in the leaching process of valuable metals from spent lithium-ion batteries.These phenomena revealed that the synergistic effect of the specific additives/solvents and ultrasound irradiation has contributed to developing a more effective method for extracting valuable metals from ores and solid wastes.Ma et al. (2017) reported that the intensified effect with ultrasound has more obvious at a lower temperature than higher temperature because of the cavitation effect.Yu et al.

Temperature
(2020) demonstrated that the UAL metallurgical responses of gold at the low temperature of 10°C were similar to the one in conventional extraction at 25°C, and the unit NaCN consumption was decreased by 16%.Nakamura et al. (2011) showed that ultrasound irradiation effectively leached iron from the green tuff, especially at low temperature (20 °C), and the amount of iron leached by irradiation was twice higher than that obtained with stirring at 20 vs. 90°C.By adequately increasing the temperature, the vapor pressure in the air bubble rises.Thus, the cavitation phenomena are weakened due to the buffer effect enhanced by the closure of the air bubble (Zhang et al. 2015).Moreover, Yang et al. (2012) found an interaction effect between temperature and ultrasound frequency.Thus, developing a low-temperature acidic leaching process assisted by ultrasound is impossible.

Conclusions and recommendations
A critical review of the application of ultrasound as an efficient method to enhance the leaching of metals from mining and metallurgical sources indicated that ultrasound power would be a key factor affecting leaching kinetics.An optimum range of ultrasonic power was recognized as it increased favorable mass transfer, excessive powerful oxidants, and deep removal of passivation layers.However, an excessive increase in the ultrasound power negatively affected the ultrasound-assisted leaching process.In addition, ultrasound-assisted leaching efficiency increased by raising the ultrasonication time.It was concluded that there is a time range in which the leaching efficiency reaches a stable level and remains stationary.It was also pointed out that under appropriate power, time, and frequency, ultrasonic irradiation of aqueous solutions triggered a formation of powerful oxidants such as HO-radicals and H 2 O 2 .
The high cost and low cavitational efficiency of UAL on a large scale are still hurdles for its wide industrial applications.Future efforts should be made to improve the economic availability of UAL from the perspectives of the optimization of ultrasound parameter conditions, the potentials of multifrequency and hydrodynamic-acoustic cavitation (HAC) system, and the disposal optimization of the ultrasonic probe.
(i) The optimal ultrasound parameter range: In applying acoustic waves to increase leaching efficiency, precautions should be taken since the nature of leaching is tied to the low operating costs.Therefore, if the ultrasonic application contains additional costs without a substantial enhancement in the recovery and dissolution of the desired element, its application has no economic justification.However, it is possible that along with the mechanical effects of the acoustic waves, its chemical effects (such as the production of different types of free radicals and its participation in increasing dissolution) participate in the dissolution reaction.Through such a process, ultrasonic waves in the optimal frequency and power ranges can show a significant economic contribution by increasing the dissolution of metals in the leaching environment.Suppose all-acoustic waves' impacts (cavitation, mechanical, thermal, and sonochemical) are effectively optimized in the leaching environment.In that case, the ultrasonic waves in an optimal range of frequencies can significantly increase the dissolution of metals from various sources and increase the process economy.(ii) The potentials of multi-frequency and HAC systems: The review of the current state of UAL revealed that UAL is an efficient method to enhance the leaching recovery of metals.However, it is obvious that the related research is limited at the lab-scale level due to the high cost of ultrasound.It is evident that a multifrequency system has an advantage in energy efficiency and cavitational yield over a single-frequency system (Chen et al. 2018;Matafonova and Batoev 2020;Ye, Zhu, and Liu 2019).The introduction of a second sound wave results in better distribution of the cavitational activity in the reactor resulting in uniform yields, minimizing the formation of standing waves, and more effective utilization of the reactant volume and dissipated sound energy (Tatake and Pandit 2002).The use of multiple transducers with multiple frequencies is recommended for a large system which can provide sufficient cavitational effect and energy efficiency for UAL processes.In addition, acoustic cavitation is usually high intensive but limited to the throughput, while hydrodynamic cavitation, as an alternative method, is easy to be scaled up but limited to the intensity and the uniformity of distribution (Asgharzadehahmadi et al. 2016).It is demonstrated that the range of hydrodynamic-acoustic cavitation (HAC) is evidently widened, and its strength is significantly enhanced compared with hydrodynamic cavitation or acoustic cavitation (Wu et al. 2018).Thus, the combination of ultrasonic and hydrodynamic cavitation can produce a uniform distribution of cavitational activity, which is useful for the industrial application of UAL.(iii) The optimization of the layout of ultrasonic probes: The scale-up design criteria are missing regarding the mass transfer, mixing time, and hydrodynamic characteristics of UAL sonochemical reactors  Table 6.Deviation of the sonochemical reaction rate (μmol h −1 ) at 5 ± 3°C with the intensity (W/cm 2 ) at the same power (20 kHz horn-type ultrasound) (Entezari and Kruus 1996).In particular, more attention should be paid to narrowing the knowledge gap of hydrodynamic characteristics and mixing behavior of UAL reactors with multiple transducers, which is important for selecting the reactor dimensions and position of ultrasonic transducers.

Figure 2 .
Figure 2. Graph showing the number of articles on UAL of metals published by decade.
power, the indium leaching recovery increases significantly.Further increase in the power (>300 W) only produces a slight enhancement.

Figure 5 .
Figure 5.Effect of ultrasonic power on gold's CN ultrasound leaching ratio from refractory ore.Data source: Zhang et al. (2016a).

Figure 6 .
Figure 6.Simplified illustrations of horn-and bath-type ultrasound systems.

Figure 7 .
Figure 7.Comparison of the size distributions of virgin and sonicated coal samples with two different ultrasonic frequencies at 500 W ultrasound input power (a: 25 kHz ultrasonic tank; b: 20 kHz probe).Data source: Ambedkar, Nagarajan, and Jayanti (2011).

Table 1 .
A summary of the pluses and minuses of different effects during UAL.

Table 2 .
Comparisons of the activation energies between UAL and conventional conditions.

Table 3 .
A summary of investigations examined the ultrasonic frequency impacts through UAL.
Xing et al. (2015) from boron carbide waste-scrap 45, 80, 100 The iron leaching ratio is higher in the lower frequency (45 higher than 80-100 kHz).45,80, 100 The iron leaching efficiency increased by increasing the frequency from 45 to 80 kHz; however, no improvement was observed from 80 to 100 kHz.Xing et al. (2015)

Table 4 .
A summary of investigations examined the effect of ultrasonic power through UAL.SO 4 leaching of zinc from zinc residue 80, 160, 240 The leaching reaction of enhanced by increasing the ultrasound power to 160 W, while after that, a negligible improvement was observed

Table 5 .
A summary of investigations examined the effect of ultrasonication time through UAL.Ultrasound shortened the leaching time and increased the initial leaching reaction rate by ∼1.4 times compared to conventional leaching.