2.1 Pulsed power technology

The phenomenon of pulse discharge is widespread in nature, and experts’ research on pulse discharge technology began with the study of the lightning characteristics of the phenomenon. In 1938, American scientists Kingdon and Tanis discovered that high voltage pulsed power discharges could produce microsecond pulse width X-rays [18]. In the same year, Yutkin, a Soviet scientist, conducted a systematic study of pulse discharge technology and proposed the plastic display method to study the mechanical effects of high-voltage pulse discharge impact [19]. Since the 1960s, research in high-voltage electrical pulse technology has seen rapid development, evolving into an independent discipline over time [18, 20, 21]. It is gradually used in the medical field for vivo lithotripsy [22], metal ore crushing [23], solid waste treatment, food sterilization [24], environmental protection [25], etc., because of its high power, high current, and high voltage characteristics.

HVPD equipment consists of pulse generation system, pulse storage system, pulse compression system, pulse transmission system and load system. Pulse switches, as the core of pulse power devices, have a wide variety. According to the classification of media type, they can be divided into gas switch, vacuum switch, liquid switch, and solid switch [26-29]. The components of the pulsed power system are shown in Figure 3. The basic system consists of two parts, one is the slow input of lower power energy into the storage system device over a longer period. The other part is to compress and transform the stored energy to form a high-power pulse. It is efficiently transmitted to the load in a very short time [18, 27].

Figure 3全屏查看

Components of the pulse system (summarize form [28])

In recent years, pulse power technology has seen rapid development. In the field of pulse power device development, universities such as Fudan University, Xi’an Jiaotong University, Zhejiang University, Chongqing University and Southwest Jiaotong University have actively been advancing solid-state Marx generators, Ltd., and pulse forming line technologies based on fully controlled semiconductor devices [29, 30]. Institutes such as the Electrical Engineering Institute, the Institute of High Energy Physics, and the Shanghai Institute of Ceramics have each developed high-voltage nanosecond short pulse technologies including magnetic compression, induction stacking, and silicon carbide photoconductive switch ceramic forming lines [31]. The Chinese Academy of Sciences has achieved a 300 kV/10 ns pulse output using domestically produced gallium arsenide photoconductive switches and ceramic base transmission lines, though further research is required for long-term operation [32, 33].

In terms of the application research of pulse power technology, countries like Russia, Germany, the Netherlands, and Japan have shown prominent applications and research. In recent years, Portugal led the establishment of the A2P, a pulse power technology association involving 18 European countries, dedicated to the promotion and application of pulse power, with products already launched [34]. Japan pays special attention to the application of pulse power technology in areas such as deep ultraviolet light sources, waste gas and water treatment, and food sterilization. The solid-state Ltd being developed by Nagaoka University of Technology, capable of 10 kA/100 ns/10 kHz, has become an alternative for the high-voltage pulse power supply for the new generation of lithography machines by the Japanese lithography giant [35]. In the United States, researchers from Virginia Tech, in collaboration with the R&D team from Angio Dynamics, have developed the Nano Knife, a pulsed electric field device for ablating tumors [36]. Old Dominion University is striving to achieve non-invasive cancer treatment through picosecond pulse focusing technology [37].

China’s research and development in pulse power applications are also advancing rapidly. In the field of advanced manufacturing, universities such as Huazhong University of Science and Technology, China Three Gorges University, and Chongqing University are exploring electromagnetic forming technologies [38]. Significant achievements have been made in the application of pulsed electric fields for tumor treatment, with Chongqing University developing China’s first domestically produced microsecond pulsed electric field tumor treatment device, which has passed the special approval of national class III medical devices, showing significant clinical efficacy in multicenter trials [39]. In material processing technologies, universities such as China University of Mining and Technology, Dalian University of Technology, and Northeastern University have also made breakthroughs in the application of pulse discharge plasma material surface treatment [40-42].

2.2 The fundamental theory of HVPD

Dielectric breakdown occurs when the conductivity of the affected dielectric solid is not high enough to fully accommodate the transfer of electrical charge, whilst its resistivity and ionization potential are too low to fully prevent the flow of this charge [43]. Figure 4 shows the relationship between voltage rise time and breakdown strength for several typical dielectrics. According to the breakdown field strength of the medium, the insulating oil, water, air are ranked in descending order. Theoretically, it is better to use insulating oil as insulating medium, but the demand for insulation applied to industry is extremely high, and it is difficult and costly to obtain insulating oil [44]. In addition, using deionized water as an insulating medium can facilitate the timely rinsing of crushed ore, which not only helps to improve crushing efficiency but also enables the recycling of deionized water [45]. Therefore, deionized water is chosen as the insulating fluid. When the voltage rise time is less than 500 ns, the breakdown field strength of rock is smaller than that of water, and when the ore is crushed by high-voltage electric pulse in aqueous medium, the ore is easier to dielectric breakdown than water. The aqueous medium acts like an insulator, causing the electrical pulse energy to act effectively on the ore [45-47].

Figure 4全屏查看

Relationship between breakdown field strength of mediums and voltage rising time [44]

The application of HVPD on solid materials can be categorized into two distinct methods: electrohydraulic disintegration (EHD) and electrical disintegration (ED) [48]. Their underlying principles are illustrated in Figure 5. EHD relies on compression crushing, where a solid material is immersed in liquid. A plasma channel forms and rapidly expands within the liquid medium, causing the liquid to vaporize instantly. This generates a powerful shock wave, which acts on the solid material's surface, resulting in cracks or even crushing the material [49]. The distinction in ED processes is marked by the electrodes being situated closer to the surface of the solid material. High voltage pulses transmitted through the electrodes induce ionization of gases within the solid material, where the plasma is formed inside the solid materials. This ionization process generates plasma channels that swiftly increase temperature and undergo thermal expansion, creating a destructive force far exceeding the tensile stress of the solid material and causing it to deform and break [50]. The tensile stress required to rupture solid materials is much less than the compressive stress; moreover, fragmentation caused by compressive stress lacks selectivity [51, 52], so the ED method is more efficient and is generally chosen as the main crushing means instead of EHD [53].

Figure 5全屏查看

Processes of liquid-electric and ED [48]: (a) Electrohydraulic disintegration; (b) Electrical disintegration

The effect of voltage polarity on material fragmentation is also significant, with the breakdown strength showing notable variation between different polarities. The breakdown strength under positive polarity is lower compared to that under negative polarity. With negative voltage, a substantial number of electrons are injected into the liquid, leading to the formation of a widely dispersed charge shielding layer at the streamer head, thereby weakening the electric field. Simultaneously, the slow-moving positive ions that accumulate at the streamer head give rise to a reversed electrical field, further diminishing the electric field’s strength [54, 55].

3.1 Selectivity of destruction

Despite the prolonged application of HVPD for rock disintegration, the precise mechanism responsible for the failure of ore particles remains a subject of ongoing scientific inquiry. While further research is essential to attain a comprehensive understanding of the HVPD disintegration process, considerable strides have been taken over the years in unraveling various phenomena associated with this technique. It is now broadly acknowledged that two pivotal factors, the radical expansion of the discharge channel and the resultant stress field, play a critical role in particle fragmentation. Nonetheless, the exact mechanisms through which HVPD accomplishes selective destruction, a remarkable and distinguishing feature of this advanced technique, persist as a topic of exploration and debate.

Three mechanisms were proposed by BLUHM et al [16] to explain the genesis of the selective disruption observed during HVPD. A schematic summary of the causes of selectivity of destruction is shown in Figure 6. When an object is embedded in another object with a large difference in its dielectric constant (Figure 6(a)), the embedded object with the larger dielectric constant can attract the discharge channel around it, and the discharge channel continues to extend around it [56]. In this case, the selective separation is directly caused by the discharge channel. Selective liberation is related to the action of compressional waves within the discharge channel [16]. The process of refraction and reflection of the compressional wave inside the object is shown in Figure 6(b). The shrinkage wave is transformed into a stretching wave after several reflections and refractions in the composite material. The greater the wave pressure, the more obvious the separation effect is. And at a sufficiently high wave pressure, the real interface between the inclusions and the matrix is completely separated [57].

Figure 6全屏查看

Mechanism of separation of different composite material in composites by HVPD [16]: (a) Inclusions with high dielectric constants can attract discharge tracks; (b) A compression wave can be transformed into a tensile and shear wave by reflection and refraction at an inclusion and separate it from the matrix; (c) A crack propagating from the discharge channel into the solid can branch around an inclusion if its mechanical properties are different from that of the matrix

The last and most important effect comes from the cracks created around the discharge channels (Figure 6(c)). The results of ANDRES et al [58] showed that the tensile stresses inside the discharge channel exceeded the tensile strength of the material, leading to the formation of cracks; moreover, the number of cracks depended on the total energy released inside the discharge channel.

During the HVPD process, a fascinating phenomenon occurs where radially propagating cracks begin to grow from the initial crack zone. The extent and density of cracks correlate with the rate of energy release [16]. Achieving comminution requires a high-power pulse, while the detachment of fragments is most effectively achieved with high pulse energies deposited over a longer time interval. In the context of selective fragmentation, it is important to consider material inhomogeneities, particularly acoustic inhomogeneities, which can influence crack propagation in composite materials. This is due to the increased mechanical stress concentration at the boundaries of inclusions within the material. These material inhomogeneities can lead to variations in crack propagation paths, resulting in differential fragmentation and selective breakage of mineral phases or components [59]. Furthermore, the dynamic nature of the HVPD process contributes to selective fragmentation. The rapid and intense electrical pulses generate strong shockwaves and stress fields within the material. These stress fields can lead to preferential fracture propagation along planes of weakness, such as pre-existing microcracks or interfaces between different mineral phases (Figure 7). This phenomenon enhances the selective fragmentation of targeted minerals while minimizing damage to other components [60]. It is crucial to highlight that the specific mechanisms of selective fragmentation can vary based on the composition and attributes of the material under treatment. Elements like mineral composition, grain size, and structural integrity all have substantial impacts on how the material selectively responds to HVPD [61].

Figure 7全屏查看

Schematic diagram of HVPD selective crushing (modified from [58])

Researchers have harnessed the property of selective crushing in HVPD and conducted numerous exploratory experiments. They have discovered that selective crushing can effectively weaken materials, with particularly good weakening effects observed in materials with significant differences in dielectric constants [59]. Furthermore, the capacity for selective fragmentation allows for the segregation of materials characterized by substantial variances in their dielectric constants. Notably, materials with higher dielectric constants wield an inducing effect on the configuration of discharge channels. Consequently, a heightened concentration of energy becomes focalized in the proximity of materials boasting larger dielectric constants. This culminates in an amplified degree of fragmentation, ultimately resulting in the advantageous effect of pre-concentration [59, 62].

ZHANG et al [63] compared the HVPD pretreated product with the product without pretreatment after crushing and found that HVPD facilitated the dissociation of monomers from the sample during the crushing process (Figure 8). The preferential breakage of weaker mineral phases during HVPD enables the detachment of valuable minerals from the gangue minerals, enhancing the mineral liberation, leading to higher recovery rates and improved product quality. By selectively breaking down the interlocked mineral particles ensuring that the valuable components are efficiently recovered.

Figure 8全屏查看

HVPD pretreatment-crushed products & mechanical crushed products [63]

3.2 Liberation enhancement of HVPD

The emphasis on advancing valuable mineral extraction with HVPD has been fundamental since its adoption in resource processing. Significant research has focused on refining ore liberation outcomes by experimenting with HVPD on diverse ore materials, meticulously adjusting pulse parameters to optimize liberation processes [6, 63].

In 1971, the Rare Elements Department of the Russian Academy of Science in Moscow conducted the first systematic liberation tests on a sample of apatite-nepheline ore from the Kola Peninsula by comparing traditional crushing methods with HVPD processing in both water and transformer oil mediums. The investigation demonstrated that material which had been disintegrated mechanically held 8.77% of compositions where apatite and nepheline were intergrown with other dense minerals. Conversely, material that underwent electrical disintegration in an oil medium showed only a 0.5% presence of these incomplete liberations of apatite and nepheline intergrowths, while in a water medium, this figure was further reduced to 0.48%. Comparatively, the mechanically processed material contained 0.6 times the number of mixed intergrowths with these four minerals than the material processed electrically in water [47].

This study’s results inspired other researchers, who have conducted thorough comparisons of the liberation levels in products from mechanical crushing and HVPD processing across different minerals, with the aim of demonstrating that HVPD offers better liberation effectiveness than mechanical comminution [64-66]. ANDRES et al [67] conducted an analysis of the liberation degree on the comminuted products of complex diamond-bearing kimberlites and emerald-bearing pegmatites. The study revealed that these precious minerals can be non-destructively liberated from the rock matrix, wherein approximately 40% of the diamonds were liberated. Following this, the research team aimed to validate HVPD’s superior liberation capabilities by using magnetite and hematite for further experiments. The outcomes demonstrated technology’s heightened efficiency, notably through the markedly lower inclusion of SiO₂ and P impurities in iron oxide concentrates produced from ores processed electrically [68]. Furthermore, the authors compared the outcomes of processing hematite using mechanical crushing followed by magnetic separation with those using HVPD followed by magnetic separation. It was observed that the concentrate obtained through HVPD-magnetic separation contained 16.98% SiO₂ and 1.07% K, in contrast to the concentrate from mechanical crushing-magnetic separation, which contained 40.26% SiO₂ and 27.72% K. Additionally, the iron loss rate in the tailings from HVPD-magnetic separation was significantly lower at 6.1%, compared to 8.97% in the tailings from mechanical crushing-magnetic separation [66]. GAO et al [6] conducted a systematic study on the effects of HVPD technology on the liberation and separation efficiency of magnetite. The experimental results demonstrated that, compared to mechanically crushed products, HVPD-treated materials exhibited a higher degree of liberation, with an increase of 1.19%. In magnetic separation tests, the iron grade of concentrates from HVPD-magnetic separation improved from 61.06% to 61.86%, and the iron recovery rate increased from 82.60% to 83.26%.

Upon discovering that HVPD technology enhances mineral liberation, thereby significantly aiding in the magnetic separation of minerals, scientists proceeded with an investigation into the impact of HVPD on flotation effectiveness using pyrite and chalcopyrite as test materials. The findings revealed a decrease in recovery rates during flotation for HVPD-treated products compared to those subjected to mechanical crushing, particularly for pyrite, which saw a reduction of 64.1% [69]. This outcome is notably contrary to the theory that higher degrees of liberation improve flotation efficiency [70]. The authors suggest that this phenomenon is due to the formation of both hydrophobic polysulfide species or sulfur elements and hydrophilic oxide species on the surface of chalcopyrite particles. However, these effects on chalcopyrite’s floatability were neutralized, restricting changes in its floatability. In the case of pyrite, the oxidation level of sulfur was significantly heightened by HVPD treatment, resulting in a marked decline in pyrite’s flotation recovery [69].

Scholars have conducted additional liberation tests on a variety of minerals including lead-zinc ore, tin ore, copper-gold ore, wolframite ore, and cobblestone, along with building materials and waste printed circuit boards, as documented in the literature [16, 47, 71-75]. These studies collectively affirm that HVPD markedly betters the liberation of materials over mechanical crushing, leading to an increased yield of single mineral particles while preserving their integrity [71, 75, 76]. Furthermore, HVPD also reduces the production of ultra-fine particles, thus avoiding the occurrence of over-grinding [63].

3.3 Pre-weakening of minerals

ANDRES et al [67] were pioneers in proposing that HVPD had a substantial weakening effect on ore. Their work shed light on the intriguing parallels between HVPD fragmentation and the explosive disintegration techniques employed in the mining industry. While both methods share the fundamental principle of inducing tensile failure to achieve rock fragmentation, the researchers also emphasized a critical factor: the weakening of the solid structure. This weakening arises from the intricate interplay of three-dimensional vibrations within the solids, a consequence of the repetitive reflection and refraction of stress waves occurring at the interfaces between the solids and various phases such as gas, liquid, or solid inclusions. Furthermore, the area of maximum electric field determines the location of the breakdown channel [68].

More comprehensive work with respect to pre-weakening effects using HVPD can be found by Julius Kruttschnitt Mineral Research Centre (JKMRC). JKMRC selected ores from four different regional mines and subjected them to mechanical crushing and HVPD crushing. The test results indicated that the softness indexes (A×b) of all four HVPD products were 9 to 52 times higher than those of the mechanically crushed products at low specific energy levels of 1-3 kW·h/t. [77]. Notably, the authors conducted a comparison of the grindability between the products of the two crushing methods. Surprisingly, they discovered a scenario in which the Bond Work index of the HVPD product closely approximates that of the mechanically crushed product, despite HVPD requiring higher specific energy. This phenomenon was analyzed for the following reasons: the location of the breakdown channel is determined by the area of the maximum electric field. When this maximum electric field is concentrated within solid dielectrics, the path of breakdown traverses this material [68]. This type of streamer propagation on the particle surface prevents disintegration from occurring due to internal electric explosion inside the solid. To overcome this challenge, increasing the output voltage is a viable solution.

JKMRC has done a lot of research on the quantitative assessment of the pre-weakening effect of HVPD and has classified the factors affecting the effect of pre-weakening into two categories, namely, machine-dependent, and ore-dependent [78, 79]. In the study by LI et al [80], additional influencing factors were identified. They found a correlation between the pre-weakening effect and the feed amount. Specifically, as the feed amount increased, the degree of pre-weakening decreased. Moreover, the overall degree of breakage showed a positive correlation with the feeding amount during the HVPD test. Notably, the HVPD product exhibited the highest degree of breakage when the electrode gap was entirely filled with particles. The effectiveness of ore crushing is often affected by the combination of these three factors. As such, there is a growing need to assess ores according to their suitability for the high voltage pulse pre-weakening technique. JKMRC defined the formula for the pre-weakening index for the energy consumption of HVPD crushed ore in order to better quantitatively assess the efficiency of pre-weakening. The formula is as follows [81]:

(1)

where P₀ is the A×b value of the untreated material; P₁ is the A×b value of the pulses-treated material; and E_p (kW·h/t) is the cumulative specific energy used for the pre-weakening characterization. PWI is defined as percentage change in A×b between the pulse-treated and untreated particles per unit of specific energy.

On the basis of evaluating the effect of pre-weakening, the authors proposed three models for crushing probability (D1 model), crushed product fineness (D2 model), and crushed product pre-weakening degree (D3 model) are three models to characterize high-voltage electric pulse crushing, and each model includes particle size, pulse specific energy, and pulse voltage as model input variables. The model parameters were fitted using data from ore crushed by the pilot-scale HVPD plant. Although the ore properties vary greatly, the model agrees well with the measured data, verifying the correctness of the model [82, 83].

To delve deeper into the pre-weakening effects of HVPD on ores, scholars have scrutinized HVPD products through an analysis of micro-morphological and macro-morphological features. HUANG et al [84] conducted an examination of cement specimens before and after subjecting them to HVPD, using X-ray CT (Figure 9). They observed that the porosity of cement specimens filled with single-grained metallic minerals increased by 1.48% after the application of the electric pulse. In contrast, the porosity of cement specimens filled with fine metallic particles increased by 2.67%. This discrepancy suggests that the weakening effect of HVPD is more pronounced in cement specimens loaded with fine-grained metallic minerals compared to those with coarse-grained counterparts [84]. QIN et al [85] employed a scanning electron microscope (SEM) to investigate the micro-morphology of galena samples that underwent both mechanical crushing and HVPD treatment (Figure 10). The results were intriguing: the surface of the mechanically crushed product appeared relatively intact, devoid of visible cracks. In stark contrast, the surface of the HVPD-treated product exhibited a significantly different appearance, with cracks primarily concentrated at the interface between the galena and dolomite. This observation aligns with the concept of selective crushing discussed in Section 3.1, where the cracks predominantly formed at the interface between the galena and dolomite [86]. Additionally, the analysis unveiled the presence of pore-like structures, referred to as melt traces, on the surface of the HVPD product. These structures originated from the substantial heat generated during the formation of the plasma channel, resulting in the melting of minerals due to Joule heat. Concurrently, gases like H₂S, SO₂, and H₂O were released, forming the observed stomatal traces [74].

Figure 9全屏查看

3D visualization of the porosity and minerals distribution after HVPD treatment [84] (blue indicates porosity and yellow indicates heavy minerals; the numbers represent scales with unit of mm)

Figure 10全屏查看

SEM images of HVPD product and mechanically crushed product [85]: (a, b) Mechanically crushed product; (c-f) HVPD product

Having established the pre-weakening effect of HVPD technology on ores and explored their microscopic characteristic morphology, the subsequent critical exploration involves unraveling the multifaceted influence of this phenomenon. QIN et al [87] performed comparative ore crushing experiments using HVPD and mechanical methods (Figure 11). The subsequent analysis of the grinding products revealed noteworthy differences. Specifically, the mass fraction of particles sized <0.074 mm exhibited a significant increase in the samples pre-treated with HVPD in contrast to those processed via mechanical crushing. Furthermore, when the grinding procedure involved 100 pulses and extended from 1 to 3 min, the <0.074 mm content in the HVPD-milled products surpassed that of the mechanically crushed-milled products by a margin ranging from 2.01% to 4.61%, as evidenced by the experimental data. Building on these findings, the authors further explored the impact of HVPD on the leaching efficiency of Carlin-type gold ore. This investigation revealed a substantial 15.65% increase in the leaching rate for the HVPD-treated samples compared to their mechanically crushed counterparts [88]. The findings revealed a substantial 15.65% increase in the leaching rate for the HVPD-treated samples in comparison to those mechanically crushed. This improvement is attributed to the generation of a higher number of microcracks and fissures within the ore structure. These fissures provide entry points for the leaching agent, considerably augmenting the contact surface area between the leaching solution and the gold minerals, thereby enhancing the leaching rate.

Figure 11全屏查看

Effect of HVPD pre-weakening behavior on grinding [87]

ZHANG et al [89] conducted a study to investigate the impact of HVPD on magnetization roasting (Figure 12). They compared the degree of reduction between mechanically crushed-magnetization roasting products and HVPD-magnetization roasting products. The results revealed that after magnetization roasting, the degree of reduction was 37.50% for the mechanically crushed product and 46.45% for the HVPD product. Subsequently, they compared the sorting indexes of these two product types through mechanically crushed-magnetization roasting-magnetic separation and HVPD-magnetization roasting-magnetic separation processes. The findings demonstrated that, under specific roasting conditions (500 ℃ temperature, 20 min duration, and 30% reducing gas concentration), the HVPD product exhibited a 0.09% increase in grade and a 1.6% rise in recovery compared to the mechanically crushed product. The reason behind this phenomenon is akin to the mechanism discussed earlier regarding the effect of HVPD on leaching. The application of HVPD creates intergranular and grain boundary cracks on the ore surface, exposing more hematite to the reducing gas and significantly increasing the contact area with the reducing gas, thus enhancing the reduction efficiency [90, 91].

Figure 12全屏查看

Effect of HVPD pre-weakening behavior on magnetization roasting [89]

3.4 Pre-concentration of sources

The property that HVPD can pre-concentration ore was discovered based on the fact that HVPD has a tendency to preferentially crush particles containing metallic minerals. Crushing is not the reduction of large-size ores to small size ores, but the reduction of large size valuable ores to small size ores. The purpose is to utilize the energy generated by the equipment to focus on crushing valuable ores rather than gangue minerals during the crushing process [67]. This targeted approach minimizes energy consumption of gangue minerals, optimizes the use of energy resources, helps to reduce operating costs in the crushing process and Increased product yields.

3.4.1 Pre-concentration of minerals

The High Voltage Pulse Research team at the JKMRC found that when treating single-particle ores with HVPD, there was no uniqueness to the crushing effect, but rather that only some of the ore particles could be crushed with a single pulse, while others could not [81]. JKMRC selected copper-gold ore from Cadia, Australia, and basalt from Mt Marrow quarry with similar mechanical strengths to be processed in a single-particle, single-pulse manner for the two types of ore with particle sizes ranging from 45 to 53 mm, and found a strange phenomenon in that 81% of the copper-gold ore was crushed in one pulse, while only 27% of the basalt particles were crushed under the same conditions [92] (JKMRC define breakage probability, a particle is regarded as broken when lost more than 10% of its initial mass [93]).

Due to the variability in the composition, shape, density and mechanical strength of natural ores (these factors have an impact on the HVPD effect [78].), and in order to reveal why, under the same experimental conditions, there is a 54% difference in the breakage rate of two ores with similar mechanical properties, the JKMRC research team made their own cylindrical artificial ores, which have exactly the same shape, particle size, density and mechanical strength, with the only difference being that some of the artificial ores are purely cement blocks, while others have one pyrite pellet added to them. The results of the study revealed that samples with added pyrite exhibited significantly higher crushing rates than pure cement blocks. Synthetic cement samples containing metallic mineral components were more conducive to pulse discharge currents, leading to the formation of discharge channels and, consequently, preferential crushing when compared to pure cement blocks [59]. The interesting phenomenon attracted the attention of the authors and further work was carried out on it. The authors sieved the pulverized samples and found that the grade of metallic minerals was higher in the finer particles, while the impurities were mainly enriched in the coarse particles (Figure 13(a)) [94].

Figure 13全屏查看

Ore pre-concentration by HVPD: (a) Process of ore pre-concentration by high voltage pulse discharge [93]; (b) Diagram depicting the selective HVPD energy discharge to the mineralized particle [59, 62]

The initial research on ore pre-concentration using HVPD involved the sequential fragmentation of individual particles. This approach, which will be denoted as the single particle (SP), single pulse method in this paper, was initially developed for the purpose of pre-weakening ores with HVPD. In the SP method, pulse energy is evenly distributed across individual particles, maximizing overall weakening. However, it is argued that HVPD pre-concentration may be more efficient when applied to multiple particles (MP). In this mode, pulse energy is expected to be attracted to and preferentially affect particles containing highly conductive or high-permittivity minerals, leaving barren particles intact. This results in better mineral concentration in finer sizes. Moreover, this approach offers additional benefits. In the MP testing approach, a substantial proportion of the generator’s energy is intentionally directed towards the mineralized particles (Figure 13(b)), whereas the barren particles receive a comparatively smaller amount of energy or, in certain cases, are effectively bypassed and receive no energy input at all—particularly when there is a significant difference in electrical conductivity or dielectric properties between the mineralized and barren components, causing the electric discharge to preferentially propagate through the more conductive mineralized phase. This approach provides machinery with the flexibility to distinguish between individual particles, offering the potential for selective fragmentation. While the energy applied to break mineralized particles in the MP test is on par with that in the SP test, the total energy expenditure per unit mass (comprising both mineralized and barren rocks) in a single pulse discharge is markedly reduced, leading to a decreased overall specific energy consumption.

QIN et al [17] selected magnetite quartzite from Liaoning Province, China (Figure 14(a)) as the experimental material and conducted analyses between the particle size <0.074 mm fractions in products from mechanical crushing and HVPD, with a pulse number of 300. The results indicated that the iron contents of the particle <0.074 mm fraction from mechanical crushing and HVPD were 32.93% and 4.07%, respectively. Furthermore, it was found that, compared with mechanical crushing, the iron distribution rate of HVPD in the <0.074 mm fraction improved by 4.08%, 21.96% and 29.04% at pulse numbers of 100, 200 and 300, respectively (Figure 14(b)) [17]. In order to explain this phenomenon, QIN et al [17] employed COMSOL Multiphysics numerical simulation software to construct models for the quartz-quartz, magnetite-magnetite, and magnetite-quartz systems (Figure 14(c)). Their investigation focused on how the high voltage pulse discharge field influenced the distribution of the electric field at the mineral interface. The study’s findings revealed that the electric field strength at the phase interface increased proportionally with voltage. In the quartz-quartz and magnetite-magnetite systems, the electric field at the object-phase interface exhibited symmetric distribution. However, in the magnetite-quartz system, the electric field at the phase interface displayed notable distortion [17]. These observations align with the outcomes of the JKMRC study. Specifically, the electric field intensity was higher on the side of the mineral with a low dielectric constant, indicating selective charge accumulation at the magnetite-quartz boundary. As voltage increased, the distortion of the electric field at the magnetite-quartz interface intensified, along with the strength of the distorted electric field. This distortion was consistent with the evolving characteristics of quartz and magnetite as voltage increased, resulting in enhanced mineral interface distortion and electric field force [7, 78].

Figure 14全屏查看

Preconcentration of magnetite quartzite by HVPD [17]: (a) X-ray diffraction patterns of magnetite quartzite; (b) Iron distribution rates in different pulse numbers and particle fraction; (c) Schematic diagram of mineral composition in COMSOL Multiphysics numerical simulation (M: magnetite; Q: quartz)

3.4.2 Pre-concentration of solid waste

The electronics industry has witnessed significant growth over the past few decades, resulting in a constant influx of various electronic devices. As people’s living standards have improved, the demand for electronic products has steadily risen. Consequently, there has been a corresponding increase in the generation of electronic waste, which has been growing at an annual rate of 3% to 5% [95, 96]. Evidently, electronic waste has evolved into a global environmental challenge. Therefore, the environmentally friendly management and resource utilization of e-waste have become an inevitable trend [97].

As shown in Figure 15, waste printed circuit board (WPCB) constitutes the predominant component of electronic products [98], comprising approximately 4% to 7% of the total e-waste mass. Within WPCBs, roughly 40% is comprised of metals and alloys, 30% consists of oxides, and the remaining 30% encompasses various other compounds [99-108]. While various researchers have explored metal recovery through mechanical [109, 110] and heating treatment methods [111] due to the high recycling value of metals, it is imperative to acknowledge the inherent limitations in these approaches. While various researchers have explored metal recovery through mechanical and hot treatment methods due to the high recycling value of metals, it is imperative to acknowledge the inherent limitations in these approaches. Therefore, there is a pressing need to develop recycling technologies capable of efficiently and cost-effectively segregating these materials.

Figure 15全屏查看

Composition of WPCB: (a) An example of WPCB from the optical mouse, marking the presence of the different classes of materials (metals, oxides, and polymers) used in the board, depicting the complexity of the materials problem [98]; (b) A pie diagram of the fraction of constituent materials in a representative WPCB (data are summarized from Refs. [99-108])

An intriguing observation highlighted in Section 3.1 pertains to the formation of discharge channels at phase interfaces exhibiting significant variations in dielectric constants. This phenomenon has prompted scholars to explore the potential of employing HVPD techniques for the recovery of valuable metals from WPCB.

ZHAO et al [112] discovered that copper primarily concentrated within the particle size range from 0.5 to 1.5 mm in HVPD products when processing WPCB using this technology. They also developed a program grounded in fractal theory and particle morphology to compute the fractal dimensions of WPCBs components. This program increases, so does the pressure, gas content, and gas conditions for high pressure pulses to achieve the best copper content distribution ratio (0.5-1.5 mm). DUAN et al [72] completed liberation of WPCBs using Selfrag Lab, and the surface morphology and chemical composition of suspended particles were analyzed by SEM. The results show that 97.92% of copper was accumulated in the products <2 mm using HVPD. ZHAO et al [74] applied HVPD technology for the disintegration and enrichment of waste photovoltaic panels. The experimental findings demonstrated differences in the selectivity of various components during high-voltage pulse crushing, with the ranking of selectivity being Ag>Si>glass. This property renders high-voltage pulse crushing particularly effective for enhancing photovoltaic panels. Subsequently, a sorting test was conducted on the product with a particle size <0.5 mm, leading to the recovery of a pure glass product with a purity rate of 98.99%. Moreover, when the crushed product <0.5 mm underwent processing by a falcon sorter, it successfully recovered 9.78% of silver and 94.22% of copper.

Figure 16 illustrates the process of waste circuit board crushing facilitated by HVPD [7, 113]. When subjected to an electric field, copper’s free electrons gain kinetic energy, enabling them to move towards the empty band and creating directed electron movement [114]. If the energy acquired by the electrons is insufficient to compensate for the energy lost due to mutual electron and lattice vibration, a new state of electron and lattice vibration is reached, resulting in the generation of new electrons [115]. Under the influence of an electric field, electron collisions lead to the production of additional electrons and holes, initiating breakdown [116].

Figure 16全屏查看

Schematic plot of the formation and expansion of the discharge channel [7, 113]

Crushing of WPCBs under HVPD can be categorized into four stages. Initially, when the discharge electrode rapidly forms high voltage due to the electric field, electrons accumulate at the copper foil-glass fiber cloth (GFC) interface. Subsequently, as the potential increases, electrons, propelled by the tunnel effect, begin to cross the barrier, and undergo injection. Many of these free electrons reach the GFC-copper foil interface, initiating the breakdown process [117]. Additionally, collision ionization generates a large number of carriers. When the resulting increase in current reaches a certain threshold, numerous conducting channels are established at the copper foil-GFC and GFC-GFC interfaces. These channels contain high-temperature, high-pressure plasma, which efficiently breaks down the epoxy resin, producing small gaseous molecules [118]. This, in turn, rapidly elevates the pressure within the conducting channels. The high-resistance plasma within the discharge channel generates shock waves [119], causing expansion cracks along the copper foil-GFC interface. These cracks separate or entirely disassociate the various components of WPCBs. Furthermore, the original cracks within WPCBs continue to expand under the influence of tensile waves, either piercing the WPCB section or merging with other cracks. As the conductive channel expands outward, it assumes the appearance of a branching tree. Electrical tree growth occurs between the layers and at the copper foil-GFC and GFC-GFC interfaces. Additionally, the GFC, influenced by an external electric field and the copper foil’s dielectric, experiences material strains proportionate to the square of the electric field’s strength [72]. This phenomenon, arising from dielectric polarization, represents an electrostrictive effect and may further aid the dissociation process between the copper foil and GFC.

Furthermore, while HVPD demonstrates remarkable effectiveness in enriching valuable metals during the crushing of WPCBs, it incurs higher energy costs compared to mechanical crushing when applied to PCBs. Even under nearly optimal conditions (with a similarity criterion K=0., where K depends on the capacitance, inductance, and output voltage of the discharge circuit of the generator), the minimum energy cost remains at 11-12 kJ/g (3000 kW·h/t). In contrast, mechanical crushing maintains a significantly lower energy cost, approximately 100 kW·h/t. Consequently, this method has not yet seen widespread industrial implementation [120]. AKIMOTO et al [121] offer an alternative perspective, firmly believing in the commercial potential of HVPD technology. Their view is based on a meticulous evaluation of the cost-effectiveness of HVPD technology. Their analysis, conducted under optimal conditions (The sum of discharge energies under the optimal conditions was 21.2 kJ) and factoring in an electricity cost of 0.123 USD/(kW·h), was diligently adjusted to match the proportions of experimental samples and real panels. The outcome of this thorough assessment indicated a remarkably low processing cost of 0.0019 USD/W for high-voltage pulse crushing. This cost estimate not only demonstrates its economic viability but also aligns with the targeted process cost set by the New Energy and Industrial Technology Development Organization (NEDO), which stands at approximately 0.045 USD/W.

3.5 Coal crushing and penetration enhancement by HVPD

Coalbed methane (CBM) stands as a noteworthy clean energy source, receiving prominence in the energy strategies of several nations, including China, the United States, Canada, and Australia [122-124]. China boasts substantial CBM reserves, estimated at approximately 37 trillion m², positioning the country as the third-largest holder of these reserves globally [125]. Currently, China’s ongoing coal mining operations have progressively ventured into greater depths, advancing at a rate of 10 to 25 m annually. In certain instances, mines have reached staggering depths exceeding 1 km [126-128]. This deep mining has engendered a series of challenges associated with CBM extraction. As mining depth increases, so does the pressure, gas content, and gas emissions from CBM. Moreover, the inherently low permeability of China’s CBM reservoirs presents a risk of drilling blockages [129, 130], jeopardizing both the safety of miners [131] and the efficiency of extraction [132]. Hence, the efficient extraction of CBM plays a pivotal role in sustaining resource availability and ensuring environmental conservation. To address these challenges, conventional techniques like hydraulic fracturing [133], hydraulic cutting [40], and deep-hole loose blasting [134] have been employed. Additionally, novel methods such as liquid nitrogen (N₂) and liquid carbon dioxide (CO₂) fracturing, as well as microwave technology, have been proposed [135-137]. However, these methods have inherent limitations. For instance, hydraulic methods can result in water lock issues, deep-hole blasting techniques fence for liquid N₂, liquid CO₂, and microwave technologies remain restricted [138-140].

As illustrated in Figures 17 and 18, Chongqing University has independently developed a novel multifield coupled high-voltage electric pulse fracturing coal rock permeability enhancement experimental system [53]. This system chiefly consists of a servo loading and control system, high voltage charging and energy storage system, and data acquisition system. It applies confining pressure and axial pressure to the samples through an axial compression loading system and a confining pressure loading system, effectively simulating the actual conditions of extraction operations. This setup stands apart from ore crushing devices lacking a pressure loading system. Since ore crushing is involved in the ore processing phase, it does not necessitate simulating mining environments. Conversely, enhancing coal seam permeability is an essential aspect of the coalbed methane extraction process, thus necessitating equipment capable of simulating the three-dimensional stress state of coal-rock masses and the occurrence state of CBM.

Figure 17全屏查看

Schematic diagram of servo loading and control system [53]: (a) Overall structure; (b) Negative electrode and inlet channel; (c) Specimen loading and pulse impact through the cavity; (d) Positive electrode; (e) Outlet channel (1-Epoxy resin insulated pipe, 2-Coal-rock specimen, 3-Electrode, 4-Conductive air bolt, 5-Epoxy resin insulation board, 6-Insulated column, 7-Axial pressure cylinder, 8-Reaction frame, 9-Spring, 10-PEEK column, 11-Insulating rubber sleeves, 12-Nylon base, 13-Sealing ring, 14-Stainless steel reinforcing sleeves)

Figure 18全屏查看

Experimental setup including servo loading and control system [53]

The fundamental mechanism underlying HVPD ability to enhance coal permeability shares similarities with its ore-crushing capability [141]. Electrical breakdown of coal can be delineated into three distinct stages. In the initial phase (Figure 19(a)), when the voltage applied between the anode and the cathode surpasses the breakdown voltage of the coal sample situated between the two electrodes, discharge develops within the coal sample near the anode. The discharge develops in a dendritic fashion, extending from the anode to the cathode [142]. In the second stage (Figure 19(b)), a discharge channel emerges between the anode and cathode within one of the many branches. Once this discharge channel forms, the development of the other branches ceases. In the third stage (Figure 19(c)), the electrical energy stored in the capacitor is rapidly discharged into the plasma channel, leading to elevated temperatures within the channel. Consequently, the plasma channel disintegrates due to the thermal expansion stress, thereby disrupting the original structure of the coal body and giving rise to numerous cracks and pores [143, 144].

Figure 19全屏查看

The coal rupture process by HVPD [144]

It has been demonstrated that coal is characterized as an anisotropic and porous medium, and water saturation within coal seams is a common occurrence [145]. The intrusion of water into the coal seam displaces the gas present in the coal body [146]. Within the three-phase system comprised of water, gas, and coal, the formation of the discharge channel is influenced by the electrical conductivity of each phase. Consequently, the water content plays a pivotal role in shaping the development of the discharge channel. YAN et al found that water content was positively correlated with cracks and pores in the coal body under HVPD, and that there were more pores in coal bodies with 1% and 1.5% water content than in those with 0% and 5% water content, respectively [140]. The increase of water content can also reduce the energy cost of mining by decreasing the maximum breakdown voltage and the minimum breakdown voltage of the coal body. In addition, the surface of the plasma channel is oxidized in the high-temperature, high-pressure environment, and the increase of water content in the coal facilitates the provision of additional oxygen-containing functional groups, which promote the uptake of methane (CH₄) [147]. In addition, the formation water contains NaCl, KCl, CaCl₂, and NaSO₄, of which 70%-95% is NaCl. The NaCl solution infiltrates the fractures within the coal body, bridging the non-conductive or poorly conductive zones with the highly conductive regions, thereby establishing a continuous conductive pathway. Furthermore, the Na⁺ and Cl^- ions within the coal migrated directionally due to the electric field’s influence, causing a shift in the primary mode of conductivity from electronic to ionic. The presence of NaCl ions further bolstered the development of cracks under HVPD’s influence, contributing to increased permeability [148].

Notably, in China, approximately 33% of the total coal resources exhibit sulfur content surpassing 1%. Among these resources, 15.5% have sulfur levels exceeding 2%, and 8% have sulfur content exceeding 3% [149]. Coal rich in sulfur and ash not only emits sulfur oxides during combustion but also results in low thermal efficiency, elevated disposal costs, and significant slagging [150]. HVPD technology has been shown to be effective in enhancing coal permeability as well as significantly reducing sulfur and ash content. Following HVPD treatment, the desulfurization rate was 36.6% and the deashing rate was 21.3% [151]. The findings underscore the potential of HVPD technology for coal treatment, presenting a promising avenue for permeability enhancement and coal quality improvement.

4.1 Energy consumption

One of the most significant obstacles impeding the industrialization of HVPD is the energy consumption. Throughout most of the literature, the recorded values for energy consumption during these tests vary significantly due to differences in equipment and material properties.

ANDRES [46] concluded that the energy consumption of HVPD is 25%-40% higher than that of mechanical compression and impact, and in some cases, even higher. However, USOV and TSUKERMAN [152] argued that while the energy consumption of HVPD technology exceeds that of mechanical comminution, the issue of energy consumption can be addressed. WANG et al [73] compared the energy consumption of HVPD and mechanical comminution technologies using chalcopyrite as the experimental raw material. The experimental results show that at low specific energy input levels (8.9 kW·h/t), electrical comminution requires 46% less energy than mechanical comminution to produce a similar degree of liberated chalcopyrite.

BRU et al [153] conducted pilot experiments utilizing HVPD technology for recycling ultra-high-performance fibre-reinforced concrete. The results yielded a significant insight: the highest fibre recovery rate, approximately 60%, was achieved at an energy consumption level of approximately 13 kW·h/t. However, as energy consumption increased to 30.5 kW·h/t and 60.13 kW·h/t, fibre recovery rates decreased substantially to just 0% and 4%, respectively. This underscores that a blind escalation in applied energy doesn’t necessarily correlate with improved recovery rates. It highlights the importance of precise parameter selection, which not only conserves energy but also enhances test results. In addition, an interesting phenomenon was found in the pilot test: when compared with the laboratory equipment, the continuous pilot-scale equipment used in the test process showed a significant reduction in specific energy consumption.

Research carried out at JKMRC has shown that reducing the size of the material leads to a reduction in crushing efficiency when using Selfrag Lab equipment [5, 77]. When applying the HVPD technique to process materials with smaller particle sizes, increased energy consumption is necessary to attain the desired outcomes, which results in a significant loss of energy [153]. Hence, selecting the appropriate particle size becomes a fundamental consideration in the experimental setup. The authors conducted mechanical crushing and HVPD experiments to pulverize the porphyry copper ore to an appropriate and consistent size. The results demonstrated a substantial contrast in energy consumption: HVPD crushing required 21.8 kW·h/t, while mechanical crushing consumed a mere 1.5 kW·h/t [154]. When solely considering the ore-crushing aspect, HVPD technology may appear disadvantaged, as it typically requires energy inputs ranging from 72 to 95 kW·h/t to induce surface and internal particle cracking [5, 77]. However, this process notably reduces the ore’s mechanical properties. WANG et al [77] selected four ore types-Bornite and a copper-gold deposit from New South Wales, and copper-gold deposit and sphalerite from Queensland as raw materials. Their study demonstrated that across all four ores, the high-voltage pulse products generated within a 1-3 kW·h/t energy range had significantly higher A×b values, with an increase of 9%-52% compared to conventional crusher products at similar specific energies. This decrease in mechanical properties can facilitate energy savings of up to 24% during subsequent ore milling.

When dealing with low-grade samples, the demand for specific energy input remains low, typically under 5 kW·h/t [94]. This is due to the fact that the feed particles often contain a significant proportion of barren material, and HVPD breakage is primarily needed for size-based separation of mineralized particles. Conversely, in the case of high-grade ore samples, energy consumption can surge to as much as 10.8 kW·h/t. This is attributed to the fact that mineralization is frequently distributed throughout most of the feed particles. To achieve pre-concentration in these scenarios, a process of progressive crushing is required, entailing size reduction for a majority of the feed particles [62].

4.2 Equipment wear

The assessment of machine wear and tear remains a crucial factor for a comprehensive economic evaluation of the technology. It has been reported that, unlike mechanical crushers used for crushing, the crushing chamber of HVPD machines is reported to be made primarily of insulating plastic material and metal electrodes, which need to be adjusted and replaced periodically, but this part of the machine is extremely low maintenance [16, 51]. There are no other moving parts in HVPD machines, and the machine body is made of ordinary structural steel, which is virtually immune to wear and tear when in operation. In addition, the relatively simple structure of HVPD equipment makes it easy to automate the crushing process, and maintenance of the equipment does not require a large number of highly skilled workers [155].

The life of the equipment mainly depends on the service life of the capacitor [156]. HUANG et al [157] analyzed the distribution of the electric field in the dielectric during the capacitor pulse discharge process through electrical field simulation. The results indicate that the treatment of electrode ends, the presence of air gaps in the dielectric, and the distribution of impurities in the oil affect the electric field distribution, leading to electric field distortion and breakdown phenomena. This causes overheating near the breakdown point, resulting in ablation. The authors suggest that insulation breakdown caused by electrothermal ageing in the insulating medium of pulse capacitors is a primary factor in reducing the lifespan of pulse capacitors.

In order to extend the service life of the capacitor, scholars have put forward some technical solutions for the improvement of the equipment [158, 159]. LI et al [160] believed that adopting a rational electrode structure design can reduce the probability of thermal breakdown occurring within components. At varying operational field strengths of the device, it is necessary to select different electrode structures based on the proportion of capacity loss generated by local discharge at the pulse capacitor electrode edges and self-healing in the film. This approach allows pulse capacitors to achieve optimal self-healing performance at an ideal working temperature, thereby extending the device’ s lifespan.