Experimental Study on Preparation of Nano ZnO by Hydrodynamic Cavitation-Enhanced Carbonization Method and Response Surface Optimization

Abstract

The carbonization method for preparing Nano ZnO is characterized by its simplicity, ease of reaction control, high product purity, environmental friendliness, and potential for CO₂ recycling. However, traditional carbonization processes suffer from poor heat and mass transfer, leading to in situ growth and agglomeration, resulting in low carbonization efficiency, small specific surface area, and inferior product performance. To enhance micro-mixing and mass transfer efficiency, ZnO derived from zinc ash calcination was used as the raw material, and hydrodynamic cavitation technology was employed to intensify the carbonization reaction process. The reaction mechanism of hydrodynamic cavitation was analyzed, and a single-factor experimental study investigated the effects of reaction time, reaction temperature, solid–liquid ratio, calcination temperature, incident angle, cavitation number, and position height on the specific surface area and carbonization rate of Nano ZnO. The response surface method was utilized to explore the significance of the three most influential factors—solid–liquid ratio, cavitation number, and position height—on the carbonization rate and specific surface area. The products were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), laser particle size analysis, and specific surface area analysis. The results showed that the optimal process parameters were a reaction temperature of 80 °C, a reaction time of 120 min, a solid–liquid ratio of 5.011:100, a calcination temperature of 500 °C for 1 h, an incident angle of 60°, a cavitation number of 0.366, and a position height of 301.128 mm. The interaction between solid–liquid ratio and position height significantly influenced the process parameter variations. Under these conditions, the specific surface area and carbonization rate were 63.190 m²/g and 94.623%, respectively. The carbonized product was flaky Nano ZnO with good dispersion and small particle size. Compared to traditional mechanical stirring and bubbling methods, the specific surface area increased by 1.5 times, the carbonization rate improved by 10%, and the particle size decreased by half, significantly enhancing the product performance.

1. Introduction

ZnO nanoparticles (Nano ZnO) have garnered significant attention for their excellent photoelectric properties [1], pyroelectric properties [2], semiconductor characteristics [3], catalytic activity [4], and antibacterial properties [5], and are considered to have broad application potential in various fields. Currently, common methods for preparing Nano ZnO include the sol–gel method [6], hydrothermal method [7], chemical precipitation method [8], and vapor deposition method [9]. The sol–gel and chemical precipitation methods are frequently used due to their simplicity and low cost; however, these methods tend to cause particle agglomeration, making it difficult to control particle size, uniformity, and morphology [10]. In contrast, the hydrothermal and vapor deposition methods can produce high-quality nanofilms or particles with excellent crystal structures, but these methods require high-temperature environments, have high equipment demands, are costly, and involve complex processes [11]. Consequently, most traditional methods struggle to simultaneously meet the requirements of large-scale industrial production and high-quality products [12,13].

The carbothermal method for preparing Nano ZnO features a simple process, easy control over reactions, high purity of products, controllable crystal forms, and an environmentally friendly approach that allows for the recycling of carbon dioxide. It meets the demands of large-scale industrial production and yields products with good performance and high quality [14]. However, the current reactors used for the carbothermal preparation of nanomaterials typically employ traditional mechanical stirring and bubbling techniques. These methods involve little to no external force, leading to issues such as in situ growth, decreased carbonization rates, low specific surface area (<50 m²/g), larger particle sizes, poor heat and mass transfer performance, and low stirring efficiency. These issues collectively impact production efficiency and product performance [13,15].

Cavitation technology offers significant advantages, generating temperatures as high as 4927 °C [16] and pressures up to 51 MPa [17,18] during the formation, growth, and collapse of bubbles, along with high-speed jets of up to 400 km/h and strong shockwaves [19]. Its unique interfacial effects, micro-perturbation effects, turbulence effects, and energy concentration effects increase the mass transfer area, enhance micropore diffusion, weaken boundary layers, and significantly improve mass transfer rates. It can also break large molecular chemical bonds, effectively promoting material separation and reactions [20]. The carbonization reaction process can be modified by techniques such as hydrodynamic cavitation assistance [21], acoustic cavitation assistance [22], and microwave assistance [23] to alter the growth mode of Nano ZnO, thereby improving the particle size distribution and crystal form of Nano ZnO and enhancing its product performance. Hydrodynamic cavitation technology has better advantages compared to acoustic and microwave methods in industrial production. Despite the fine processing and non-contact advantages of acoustic and microwave methods, they suffer from high energy consumption, short effective distances, material limitations, and uneven heating in industrial production. Hydrodynamic cavitation technology, on the other hand, offers strong controllability, low energy consumption in industrial settings, zero pollution, and efficient heat and mass transfer [24,25,26].

Hydrodynamic cavitation, as an emerging process intensification technology, demonstrates significant application potential in the field of nanoparticle preparation [27]. This technology involves a dynamic process where localized pressure drops below the saturated vapor pressure of the liquid, leading to the formation, growth, and collapse of vapor bubbles. This process releases a substantial amount of energy and generates extremely high temperatures, pressures, microflows, microhydraulic cavitation, and shock waves, causing various physicochemical effects. It ensures thorough mixing of particles within the reactor in a short time, thereby enhancing mass transfer, heat transfer, and reaction efficiency [28,29,30]. Xue et al. [31] studied a novel cylindrical rotary hydrodynamic cavitation reactor (CRHCR) with rectangular slots and helical teeth on the rotor surface using hydrodynamic cavitation technology, discovering a phenomenon of simultaneous formation of attached cavitation and shear cavitation. Karacoban et al. [32] employed hydrodynamic cavitation for sludge treatment, demonstrating that it could convert 64% of total Kjeldahl nitrogen and 60% of total phosphorus in waste-activated sludge into soluble substances. Qi et al. [33] applied hydrodynamic cavitation to quantum dot materials and produced cadmium sulfide quantum dots with small particle size, narrow distribution, high concentration, and large Stokes shift. Yu et al. [34] investigated the allergenicity of beer gluten protein using hydrodynamic cavitation and found that it significantly reduced the allergenicity of the product. Zuo et al. [35] used hydrodynamic cavitation technology to address the aggregation issue that occurs during material preparation, successfully producing high-purity and well-crystallized sheet-like Mg(OH)₂ using MgO as the raw material. Guo et al. [36] utilized the powerful local shear generated by the collapse of micro-cavitation bubbles produced by hydrodynamic cavitation technology to prepare FePO₄ with uniform particle size and high specific surface area. Thus, hydrodynamic cavitation, as a novel processing technology, can enhance the dissolution of zinc oxide and mitigate the agglomeration and in situ growth of zinc carbonate intermediate products, thereby improving the carbonization rate, specific surface area, and product performance of Nano ZnO [37]. Currently, there is limited research on using hydrodynamic cavitation for the preparation of advanced inorganic materials, and no studies have yet explored the enhancement of Nano ZnO preparation through carbothermal methods using hydrodynamic cavitation.

This study uses ZnO obtained from the calcination of zinc sludge as the raw material and employs a hydrodynamic cavitation-enhanced carbothermal method to prepare Nano ZnO. The research aims to investigate the mechanism of the hydrodynamic cavitation-enhanced carbothermal reaction of zinc oxide. Single-factor experiments and a response surface methodology were used to optimize the hydrodynamic cavitation carbothermal process for Nano ZnO. The reaction mechanism and hydrodynamic cavitation mechanism were analyzed, and the final reaction products were characterized. The large specific surface area of ZnO directly impacts its performance and applications across various fields. A larger specific surface area typically implies higher activity and broader application potential. A high carbonization rate ensures the material’s purity, crystal structure, and reaction stability. This study aims to provide theoretical support for improving the carbonization rate, specific surface area, and quality of industrially produced Nano ZnO products.

2. Experimental Materials and Methods

2.1. Ingredients

The high-purity zinc oxide (99.5% ZnO) used in this experiment comes from Liaoning Boshi Technology Co., Ltd. (Liaoyang, China). It is a byproduct of the wet process for producing sodium formaldehyde sulfoxylate, obtained by high-temperature calcination of zinc sludge. The zinc oxide has a high content of ZnO and low impurity levels; the main components are shown in Table 1. The carbon dioxide is cylinder-packaged, with a volume fraction of 98.5%. Deionized water is self-produced.

2.2. Equipment and Instruments

The total volume of the carbonation reactor is 40 L, and the experimental working volume is set at 50% to 60% of the reactor capacity. The material used is 304 stainless steel. A self-priming flexible pump (MPR-20) is employed, with an adjustable power of 0.55 kW, a head of 30 m, and a flow rate of 50 L/min. The pump speed is regulated by a frequency converter to control the inlet pressure and circulation flow rate of the injector. The inlet pressure is maintained between 0.4 MPa and 0.6 MPa, while the confining pressure of the reactor is controlled at 0.3 MPa. The reaction temperature is regulated by a heat exchange system composed of a heat exchanger, a thermometer, and a control unit, with the maximum temperature reaching 100 °C. The control unit monitors the system in real-time. Throughout the entire reaction process, the ambient temperature remains constant, with a deviation of less than ±1 °C. The remaining equipment includes the following: HCT-4 thermal analyzer; DL-5C filtration machine; DHG-9055A tabletop forced-air drying oven; ZEISS Gemini 300 scanning electron microscope (SEM); Rigaku UltimaIV X-ray diffractometer (XRD); Malvern Mastersizer 2000 laser particle size analyzer; S500-B multi-parameter tester; surface area and pore size analyzer by Beijing Zhongke Huiyu Technology Co., Ltd. (Beijing, China).

Figure 1 shows the equipment setup for the hydrodynamic cavitation-enhanced zinc oxide carbonation experimental apparatus. This setup includes a jacketed carbonation tank, a hydrodynamic cavitation reactor, a power fluid pump, a differential pressure filter, a heater, a temperature controller, a CO₂ cylinder, and various instruments. The schematic diagram of the hydrodynamic cavitation reactor structure is shown in Figure 2.

2.3. Method

2.3.1. Preparation Method

As shown in Figure 3, the process flow diagram for preparing Nano ZnO is described as follows. Using high-purity zinc oxide (ZnO, with a mass content of 99.5%) as the raw material, ZnO and deionized water are added into a jacketed carbonation tank according to a specified ratio to form a slurry. CO₂ is introduced at 25 °C, and a dynamic fluid pump is started. Control the outlet pressure of the dynamic fluid pump and maintain the pressure inside the carbonation tank at 0.3 MPa. During the reaction process, samples are taken at five different times (at 30 min intervals) to analyze the carbonation rate. When the reaction time exceeds 120 min, there is no significant increase in the carbonation rate, and the carbonation rate can reach over 80% in all cases. Open the vent valve to depressurize, then open the bottom valve to discharge. The slurry is subjected to vacuum filtration, and the filtrate can be recycled for slurry preparation or discharged into the environment. The obtained filter cake is washed three times, and the washed filter cake is dried in a constant-temperature drying oven at 110 °C for 4 h. The dried cake is then transferred to a muffle furnace and calcined at 500 °C for 1 h. After cooling, it is pulverized using a circulating tube jet mill and sieved to remove residues, yielding the Nano ZnO product.

The reaction equation is shown in (1) and (2):

5ZnO + 3H₂O + 2CO₂ → Zn₅(CO₃)₂(OH)₆(1)

Zn₅(CO₃)₂(OH)₆ → 5ZnO + 3H₂O + 2CO₂(2)

As shown in Figure 4, the decomposition process of basic zinc carbonate (including the removal of crystallization water, hydroxyl groups, and carbonate groups) has highly overlapping temperature ranges. The separation temperatures of each component are very close, occurring within the same temperature range (200–400 °C). As a result, under different temperature gradients, the entire decomposition process manifests as a single, continuous mass loss area. Hence, a small amount of dried basic zinc carbonate sample from the carbonation reaction is weighed, and the theoretical mass percentages of CO₂ and H₂O are calculated (25.80%). The mass percentages of CO₂ and H₂O evolved during the thermal decomposition of basic zinc carbonate are detected using a thermogravimetric analyzer (TGA). The carbonation rate of ZnO is calculated using formula (3), with three samples taken to compute the average value [38,39]:

(3)$\Phi =\left[{\displaystyle \frac{m_2}{m_1}}\right]\times 100\%$

In the formula, Φ represents the carbonation rate of ZnO, m₁ is the theoretical mass of CO₂ and H₂O(25.8%), and m₂ is the actual mass of CO₂ and H₂O detected by thermogravimetric analysis (TGA) (16.34–24.38%).

Figure 5a,b are the XRD pattern and Raman spectrum of the product after carbonation, respectively. The peak at 319.2 cm⁻¹ in Figure 5b is a characteristic peak of Zn₅(OH)₆(CO₃)₂ [40]. Both figures prove that the precursor of the nano ZnO formed after carbonation is Zn₅(CO₃)₂(OH)₆.

2.3.2. Single-Factor Experimental Analysis

On the premise of not changing other carburizing parameters, sequentially select different reaction times (60, 90, 120, 150, 180 min), reaction temperatures (70, 75, 80, 85, 90 °C), material-to-liquid ratios (2:100, 3:100, 4:100, 5:100, 6:100), calcination temperatures (400, 500, 600, 700, 800 °C), incidence angles (40, 45, 50, 55, 60°), cavitation numbers (0.29, 0.37, 0.44, 0.61, 0.87), and position heights (150, 200, 250, 300, 350) to conduct single-factor experiments for the preparation of Nano ZnO. Use the BET specific surface area and the carbonation rate of zinc oxide as evaluation metrics to determine the influence of each factor on the two dependent variables.

2.3.3. Response Surface Optimization Experiment

Based on the results of single-factor experiments, select the material-to-liquid ratio (A), cavitation number (B), and hydrodynamic cavitation height at different positions (C) as the variables to be investigated. The levels of the variables are represented by −1, 0, and 1. Use BET specific surface area (Y₁) and carbonation rate (Y₂) as the response values. Utilize the Design-Expert 13 software to design a three-factor, three-level response surface experiment.

3. Results and Discussion

3.1. Single-Factor Experiment Results

3.1.1. Effect of Process Parameters on Specific Surface Area and Carbonization Rate of Nano ZnO

• (1). Reaction time

Figure 6 illustrates the effect of carbonation reaction time on the BET specific surface area and carbonation rate of Nano ZnO. The results indicate that as the carbonation reaction time (t) increases, both the BET specific surface area and the carbonation rate of Nano ZnO rapidly increase. After a carbonation time of 120 min, the values level off, with the BET specific surface area reaching 54.174 m²/g and the carbonation rate achieving 87.70%. The increase in carbonation reaction time leads to a gradual increase in reaction products, thereby increasing the carbonation rate. When the carbonation time exceeds 120 min, despite the continued increase in reaction time, the reactants and products reach a dynamic equilibrium. As a result, both the carbonation rate and the specific surface area reach their maximum values, and the subsequent reaction tends to stabilize. Therefore, the optimal reaction time is determined to be 120 min.

• (2). Reaction temperature

Figure 7 illustrates the effect of carbonation reaction temperature on the BET specific surface area and carbonation rate of Nano ZnO. The results indicate that as the reaction temperature increases, both the carbonation rate and the specific surface area of ZnO initially rise and then fall. When the reaction temperature increases from 70 °C to 80 °C, the carbonation rate increases from 79.23% to 87.70%, and the specific surface area increases from 31.456 m²/g to 54.174 m²/g. Both the carbonation rate and the specific surface area are significantly influenced by reaction temperature. However, as the temperature continues to rise, the carbonation rate and specific surface area of Nano ZnO decrease. The increase in temperature raises the saturated vapor pressure of the liquid, reducing the cavitation number and enhancing the cavitation effect. Consequently, the carbonation rate and specific surface area increase. Nevertheless, excessively high temperatures may lead to super-cavitation within the hydraulic cavitation reactor, forming large cavitation clouds, which affect the intensity of cavitation bubble collapse and ultimately hinder the reaction between reactants. From a chemical thermodynamics perspective, lower temperatures are favorable for carbonation reactions. The carbonation reaction of zinc oxide slurry is a slightly exothermic reaction, but the solution also contains electrolytes such as H₂CO₃ and Zn(OH)₂, whose ionization equilibrium is influenced by temperature. Elevated temperatures facilitate the diffusion of basic zinc carbonate microcrystals from the ZnO surface into the aqueous solution, increasing the concentration of Zn²⁺ and CO₃²⁻ ions in the solution and promoting the reaction. Considering all these factors, the optimal reaction temperature is determined to be 80 °C, at which the sample achieves the maximum carbonation rate and BET specific surface area.

• (3). Solid–liquid ratio

Figure 8 illustrates the effect of solid–liquid ratio on the BET specific surface area and carbonation rate of Nano ZnO. The results indicate that as the solid–liquid ratio increases from 2:100 to 5:100, both the carbonation rate and BET specific surface area of zinc oxide increase significantly. Compared to the 2:100 ratio, the carbonation rate and BET specific surface area at 5:100 increase by 12.23% and 23.971 m²/g, respectively. At lower solid–liquid ratios, the slurry is relatively dilute, and carbon dioxide gas cannot be uniformly dispersed in the liquid phase. This results in a significant amount of zinc oxide not being converted into basic zinc carbonate. Additionally, the reduced number of collisions between carbon dioxide molecules and zinc oxide particles leads to a decrease in the nucleation rate of the carbonation reaction, resulting in a lower carbonation rate. However, as the solid–liquid ratio continues to increase, both the carbonation rate and BET specific surface area show a decreasing trend. Compared to the 5:100 ratio, the carbonation rate and BET specific surface area at 6:100 decrease by 2.7% and 8.653 m²/g, respectively. This suggests that when the slurry is too thick and viscous, it affects the mass transfer efficiency, which is not conducive to the carbonation reaction. Higher solid content increases the amount of active zinc oxide participating in the reaction, but it also reduces the contact area between active zinc oxide and water, leading to a decrease in reaction speed. Furthermore, excessively high zinc oxide concentration increases the supersaturation of Zn²⁺ in the solution, causing the nucleation rate of basic zinc carbonate to exceed its growth rate. This results in the generation of numerous tiny basic zinc carbonate crystal particles with strong surface polarity, leading to severe agglomeration and in situ growth issues. The agglomerated basic zinc carbonate can encapsulate unreacted zinc oxide, further hindering the progress of the reaction. Considering both the carbonation rate and BET specific surface area, a solid–liquid ratio of 5:100 is selected as the optimal condition for experimental investigation.

• (4). Calcination temperature

Figure 9 illustrates the effect of calcination temperature on the BET specific surface area and carbonation rate of Nano ZnO. The results show that with the increase in calcination temperature, the carbonation rate of ZnO initially rises and then stabilizes, while the specific surface area initially increases and then decreases. As the calcination temperature increases from 300 °C to 500 °C, the carbonation rate of the product increases from 69.23% to 90.41%, and the specific surface area increases from 26.436 m²/g to 59.729 m²/g. The reason for this is that at lower calcination temperatures, basic zinc carbonate is not fully decomposed, resulting in a product that contains some basic zinc carbonate residues. As the temperature continues to rise, basic zinc carbonate completely converts into Nano ZnO, making the carbonation rate stable. It is noteworthy that after reaching 500 °C, the BET specific surface area of Nano ZnO begins to decrease. At lower calcination temperatures, a longer decomposition time is required; otherwise, the decomposition will be incomplete. At higher calcination temperatures, the decomposition time needs to be shortened; otherwise, it will lead to crystal growth, increased particle size unevenness, and severe agglomeration. Therefore, calcination temperature and time significantly influence factors such as particle size, agglomeration state, uniformity, surface coating, and modification of Nano ZnO. When the temperature is low, the calcination time should be extended, and when the temperature is high, the calcination time should be shortened to promote the reaction, increase the specific surface area, and reduce agglomeration. Considering these factors comprehensively, the optimal calcination temperature is determined to be 500 °C with a calcination time of 1 h, at which the sample achieves the maximum specific surface area and carbonation rate.

3.1.2. Effect of Structural Parameters of Carbonization Reactor on Specific Surface Area and Carbonization Rate of Nano ZnO

• (1). Incident angle (α)

Figure 10 illustrates the effect of the incident angle (α) on the BET specific surface area and carbonation rate of Nano ZnO. The results indicate that the incident angle (α) significantly influences the carbonation reaction. As the incident angle (α) increases from 40° to 60°, the carbonation rate of ZnO gradually increases by 11.64%, and the specific surface area increases by 23.069 m²/g. When (α) is 60°, the average carbonation rate reaches 92.95%. The variation in (α) affects the convergence of the water flow. A larger (α) results in faster convergence of the water flow in the contraction section. As the water flow enters the throat diameter, rapid high-speed hydraulic cavitation occurs, creating a low-pressure region that facilitates hydraulic cavitation. In the mixing chamber, gas, liquid, and solid phases mix thoroughly. Due to energy exchange, this mixture is expelled at high speed, maximizing the potential energy of the mixture and enhancing mass transfer efficiency. Additionally, the rapid flow has a stripping effect on the adherent materials on the surface of ZnO solid particles, enhancing the mass transfer process among the gas, liquid, and solid phases. This accelerates the carbonation reaction, reduces agglomeration, and significantly increases the reaction rate. Considering all these factors, the optimal incident angle is determined to be 60°.

• (2). Cavitation number (σ)

The cavitation state can be described by the cavitation number (σ) [41,42]. The cavitation number represents the relationship between the difference in local absolute pressure and the liquid’s vapor pressure, and the specific kinetic energy. It is a dimensionless number used to characterize the likelihood of cavitation occurrence in a flow. The expression is given by Equation (4). A low cavitation number indicates a higher probability of cavitation occurrence, while a high cavitation number indicates that cavitation is unlikely to occur.

(4)$\sigma ={\displaystyle \frac{P-P_{\nu }}{\frac{1}{2}\rho u^2}}$

In the equation, ρ represents the liquid density; P represents the local absolute pressure; P_v represents the vapor pressure of the liquid at the ambient temperature; and u represents the liquid flow velocity.

Figure 11 shows the effect of the cavitation number (σ) on the specific surface area and carbonation rate of Nano ZnO. In this study, only the influence of the liquid phase on the reaction rate was considered, while the flow rate of the gas phase could be regarded as entrainment, making its impact on the overall system negligible and thus excluded from the analysis. During the reaction, the system temperature is maintained constant. By adjusting the pressure at the ejector inlet, the cavitation number is systematically controlled through the circulation flow rate, and the experiments are carried out sequentially. By analyzing the process of reducing the cavitation number σ from 0.87 to 0.37, it can be observed that both the carbonation rate and the specific surface area show an increasing trend. When the cavitation number σ is 0.37, the average carbonation rate reached 93.43%. The hydrodynamic cavitation effect creates cavities in the slurry, and during the formation and collapse of cavitation bubbles, a large number of tiny bubbles and droplets are generated. These tiny bubbles and droplets significantly increase the gas–liquid interfacial area, thereby enhancing the solubility and diffusion rate of gas molecules in both the liquid and solid phases. When the cavities collapse, they generate high temperature and pressure, micro-hydrodynamic cavitation, and strong shock waves [43]. These extreme conditions can effectively destroy the boundary and interfacial layers between the gas, liquid, and solid phases, thinning the boundary layer and thus significantly reducing mass transfer resistance, which increases the diffusion rate of gas molecules in both the liquid and solid phases [44]. In addition, the micro-jets and shock waves induced by cavitation can effectively remove deposits on the solid surface, such as reaction products and passivation layers. This surface cleaning effect exposes more active sites, increasing the opportunities for gas molecules to react with the solid surface, thus inhibiting the formation of large crystal particles and promoting the generation of fine particles, which enhances mass transfer and reaction rates [45]. The stronger turbulence and intense agitation triggered by cavitation increase the turbulence intensity of the fluid. This not only helps to improve the solubility of gas in the liquid phase but also increases the chances of gas molecules coming into contact with the solid surface. A smaller cavitation number means a higher likelihood of cavitation. According to Formula (4), a lower cavitation number corresponds to a higher pressure and liquid flow velocity, indicating higher cavitation performance. Therefore, the reaction materials become more dispersed and uniform, increasing the chances of material contact and reaction, which helps to enhance mass transfer and promote the carbonation reaction. In summary, the application of hydrodynamic cavitation technology can effectively suppress the in situ growth and aggregation of zinc carbonate on the surface of zinc oxide, enhance mass transfer, and increase the specific surface area and carbonation rate. However, when (σ) continued to decrease beyond 0.37, both the carbonation rate and specific surface area declined, possibly due to the ultra-cavitation generated in the hydraulic cavitation reactor, forming a large amount of cavitation cloud that affects the collapse intensity of cavitation bubbles and the reaction between reactants. Considering both the carbonation rate and specific surface area, 0.37 was chosen as the optimal cavitation number for experimental research.

• (3). Height of different positions (H)

Figure 12 shows the effect of different position heights (H) on the specific surface area and carbonation rate of Nano ZnO. Based on Ren et al.’s [46] simulation design, an improved experiment was conducted to study the impact of different position heights on the specific surface area and carbonation rate of Nano ZnO by comparing different lower chamber positions (distance from the top of the reactor to the hydraulic cavitation outlet). The results show that as the position height increased gradually from 350 mm to 300 mm, both the carbonation rate and the specific surface area also gradually increased. At a position height of 350 mm, the carbonation rate and specific surface area were relatively low because the hydraulic cavitation column velocity was too high, preventing the formation of a uniform and stable annular flow field, which did not promote the reaction. At a position height of 300 mm, however, the hydraulic cavitation column velocity was both high and stable. The obstructive effect of the walls on the fluid created bifurcations to both sides, resulting in a relatively stable and uniform flow field. In such a flow field, the materials mixed vigorously, promoting material circulation. The maximum carbonation rate and specific surface area reached 92.46% and 59.223, respectively. However, as the hydraulic cavitation height (H) continued to increase beyond this point, both the carbonation rate and specific surface area rapidly declined, reaching their lowest at 150 mm. This was primarily due to significant velocity loss of the hydraulic cavitation column as it moved downstream, causing turbulent mixing of the slurry and gas at the bottom of the chamber. This led to slurry deposition and poor mixing effectiveness. Considering both the carbonation rate and specific surface area comprehensively, a position height of 300 mm was selected as the optimal height for experimental research.

3.2. Hydraulic Cavitation Mechanism for the Preparation of Nano ZnO Carbonation Reaction

3.2.1. The Growth Process of Nano ZnO

According to the model by S. V. Dobrydnev [47], the carbonation reaction of ZnO forming basic zinc carbonate and then calcined to form Nano ZnO can be described in the following stages:

CO₂ + H₂O → HCO₃⁻ + H⁺(5)

HCO₃⁻ → CO₃²⁻ + H⁺(6)

ZnO + CO₃²⁻ + 2H⁺ → ZnCO₃ + H₂O(7)

ZnCO₃ + H₂O → Zn²⁺ + 2OH⁻ + CO₂(8)

5Zn²⁺ + 6OH⁻ + 2CO₃²⁻ → Zn₅(CO₃)₂(OH)₆(9)

Zn₅(CO₃)₂(OH)₆ → 5ZnO + 2CO₂ + 3H₂O(10)

The schematic diagram of the carbonation reaction of zinc oxide forming basic zinc carbonate is shown in Figure 13, combining the anion ligand growth unit model and the dissolution–precipitation reaction mechanism. In this reaction process, when CO₂ enters the system, it is first activated to form carbonic acid, which ionizes into HCO₃⁻ and CO₃²⁻. The reactions are shown in Equations (5) and (6). ZnO is an amphoteric oxide that can react with carbonate ions to form zinc carbonate, as shown in Equation (7). Zinc carbonate is the initial product of the reaction, but it is not the final product under the reaction conditions. In an aqueous environment, zinc carbonate easily undergoes hydrolysis with water, releasing a small amount of hydroxide ions (OH⁻) and forming intermediates such as Zn(OH)₂, as shown in Equation (8). During this process, zinc hydroxide is unstable. When the ion product between free Zn²⁺, CO₃²⁻, and OH⁻ exceeds the solubility product of Zn₅(CO₃)₂(OH)₆, Zn₅(CO₃)₂(OH)₆ nuclei form in the system and grow rapidly, leading to the precipitation of Zn₅(CO₃)₂(OH)₆ particles, as shown in Equation (9). As Zn₅(CO₃)₂(OH)₆ precipitates, the concentration of free Zn²⁺ in the system decreases, promoting the dissociation of Zn(OH)₂, which releases more free Zn²⁺ and OH⁻, thereby sustaining the crystallization and precipitation of Zn₅(CO₃)₂(OH)₆. Upon calcination, Zn₅(CO₃)₂(OH)₆ decomposes into Nano ZnO, CO₂, and H₂O, as shown in Equation (10).

3.2.2. Strengthening Mechanism of Hydraulic Cavitation

The hydrodynamic cavitation-enhanced carbonation reaction of zinc oxide mainly utilizes the cavitation effect produced, with the cavity development and collapse process shown in Figure 14. In the liquid, there exist sub-micron-sized small bubbles. Due to the unique structure of the hydrodynamic cavitator, when the liquid flows through the throat of the hydrodynamic cavitator, it experiences a low-pressure, high-velocity state. This induces cavitation within the liquid and allows the bubbles to grow. When these bubbles reach regions where the absolute pressure is greater than the liquid’s vapor pressure (high-pressure zone), the bubbles collapse or “implode”, releasing a tremendous amount of energy, which generates instantaneous localized high temperature and high pressure, along with strong shock waves and micro-hydrodynamic cavitation.

The cavitation phenomenon produced by hydrodynamic cavitation significantly promotes the carbonation efficiency of zinc oxide. The specific effects are illustrated in Figure 15a. At the zinc oxide surface, reactions with water and carbon dioxide occur, synthesizing basic zinc carbonate. Due to the significant surface polarity of the tiny basic zinc carbonate crystals, in situ growth and aggregation of basic zinc carbonate on the zinc oxide surface can occur. Without external force, unreacted zinc oxide becomes coated by the formed basic zinc carbonate, making it difficult to bring water and carbon dioxide into contact, thus inhibiting the carbonation reaction and slowing down the carbonation rate of zinc oxide. This phenomenon is the main reason for the lower carbonation rate and specific surface area of the slurry when preparing basic zinc carbonate from zinc oxide under conventional conditions [48,49]. After utilizing the hydrodynamic cavitation reactor to enhance the reaction, the immense energy generated by the cavitation effect acts on the reactants, stripping off the basic zinc carbonate attached to the surface of zinc oxide. This exposes a new zinc oxide surface, allowing the reaction to continue, thereby accelerating the carbonation reaction rate and improving production efficiency. Additionally, the cavitation effect helps to disperse the agglomerated basic zinc carbonate products, which contributes to enhancing the product’s performance. Figure 15b shows the particle size distribution graph of basic zinc carbonate, and the results prove that the particle size distribution graph of basic zinc carbonate is narrow, with a relatively concentrated particle size distribution range, indicating that the particle size is relatively uniform. The D50 of the sample is 5.7667 μm, which demonstrates that hydrodynamic cavitation can reduce agglomeration and in situ growth.

3.3. Response Surface Method Was Used to Analyze Optimum Conditions of Nano ZnO Preparation

3.3.1. Determination of Factors for Preparation of Nano Zno-Response Surface (RSM) by Carbonization Method

Based on the results of single-factor experiments, response surface design experiments were conducted using the Box–Behnken central composite design principles. Three factors with a significant impact on Nano ZnO preparation were selected for the experiments: solid-to-liquid ratio (%), cavitation number (σ), and height at different positions (H). A response surface analysis was carried out with the specific surface area of Nano ZnO (Y₁) and carbonation rate (Y₂) as the response variables, using a three-factor, three-level design, as shown in Table 2.

3.3.2. Response Surface Optimization Results

Using the Design-Expert 13 software and applying the Box–Behnken experimental design principles, a response surface experimental design was conducted according to the experimental method. A total of 17 experimental points were designed, and the design results are shown in Table 2. The experimental data were then subjected to multiple regression analysis, and the analysis of variance (ANOVA) results for the regression models of Y₁ and Y₂ were obtained (see Table 3). The results were then fitted using multiple linear regression equations, and the outcomes are as follows:

Y₁ = −56.88489 + 12.44171A + 297.97786B + 0.217952C − 9.78571AB + 0.016170AC + 0.060857BC − 1.37685A² − 360.78571B² − 0.000521C²

Y₂ = −81.76911 + 22.99571A + 214.48929B + 0.541928C + 3.46429AB − 0.024950AC + 0.282857BC − 1.66150A² − 438.57143B² − 0.000882C²

Based on the response surface analysis of the data in Table 3 and Table 4 and the analysis of the fitting results, it can be seen that the regression models for BET (Y₁) and carbonation rate (Y₂) are both highly significant, with p < 0.01 (model), indicating that the models are statistically significant and reasonable. The R² values for both models are 0.9634 and 0.9765, respectively, both close to 1, which demonstrates a good consistency between the actual and predicted values. The adjusted R² values are also consistent with the R² values of the responses (difference < 0.2), further confirming the good agreement between the actual and predicted values. The lack-of-fit term is typically used to evaluate the reliability of the equation: if it is not significant, the equation provides a good fit; otherwise, the fit is poor. The lack-of-fit terms for both BET (Y₁) and carbonation rate (Y₂) are greater than 0.05, indicating that they are not significant, meaning the fitted regression equations from this experiment can be used to predict and analyze BET and carbonation rate in the Nano ZnO preparation process instead of relying on actual experimental data. The normal distribution of residuals (Figure 16a,b) shows that all residuals align well with the straight line, with no evident non-normality. Additionally, the scatter plots of residuals versus predicted values (Figure 17a,b) demonstrate that the residuals are randomly distributed around 0, regardless of the predicted value, with a variation range of ±4.82. These results indicate that the model sufficiently describes the relationship between BET, carbonation rate, and the independent variables [50,51].

According to the significance results of the regression models, for BET (Y₁), the quadratic terms of the solid-to-liquid ratio (A²), cavitation number (B²), and height at different positions (C²) are all highly significant (p < 0.01). The order of influence of the factors on the response value, in terms of their effect on BET, is C > A > B, indicating that the height at different positions has the most significant impact on the specific surface area of BET among the three factors. The interaction term AC is highly significant (p < 0.01), AB is significant (p < 0.05), and BC is not significant (p > 0.05), which suggests that the interaction between A and B, and A and C, is significant. For carbonation rate (Y₂), the quadratic terms of the solid-to-liquid ratio (A²), cavitation number (B²), and height at different positions (C²) are all highly significant (p < 0.01). The order of influence of the factors on the carbonation rate is B > C > A, indicating that the cavitation number has the most significant impact on the carbonation rate of ZnO among the three factors. The interaction terms AC and BC are highly significant (p < 0.01), while AB is not significant (p > 0.05), which shows that the interaction between A and C, and B and C, is significant.

3.3.3. Interaction Analysis of Three Factors on Response Surface

His study systematically investigated the interaction effects between independent variables on the response surface using the Response Surface Methodology (RSM). The 3D response surface plots and contour plots, as shown in Figure 18 and Figure 19, were used to examine the interactions between the three factors and their impact on the carbonation rate of zinc oxide and the BET specific surface area. These findings highlight the complexity of multivariable systems and emphasize the critical role of interaction effects in achieving optimal response outcomes.

The response surface plots visually reflect the magnitude of interaction between two factors. The greater the slope of the surface and the closer the contour plot resembles an elliptical shape, the more significant the interaction between the two factors and the greater their impact on the response value [52]. Conversely, when the surface plot is relatively flat and the corresponding contour plot approximates a circular shape, it indicates that the interaction between the two factors is weaker and has a smaller influence on the response value.

From Figure 18, it can be seen that the interaction terms significantly alter the curvature and overall topological structure of the response surface plot. The contour plot for the interaction term AC is close to an elliptical shape, and the response surface plot exhibits a greater degree of inclination. In contrast, the surface plot for BC has a smaller degree of curvature, and its contour plot is closer to a circular shape.

Combining these observations with the results from the ANOVA table (Table 3), the F-values for the interactions are the following: AB = 9.13, AC = 12.72, BC = 0.8828. In summary, the order of the interaction effects on the specific surface area of Nano ZnO is AC > AB > BC.

As shown in Figure 19, the response surface between A and C exhibits the steepest slope characteristics, and its contour lines approximate an elliptical structure, indicating the significant effect of the A–C interaction on the response variable. Combining these observations with the statistical data in Table 3, the F-values for the interactions between factors are AB = 0.9580, AC = 25.35, and BC = 15.97. From this, it can be inferred that the order of influence of the interactions between factors on the carbonation rate in the preparation of Nano ZnO is AC > BC > AB. In particular, the AC interaction term has the highest F-value, indicating its most significant impact on the carbonation rate, and should be prioritized in process optimization. The AC interaction not only shows a steep change trend on the response surface but also has an F-value far higher than the other interactions, indicating its critical influence on the response variable during the reaction process. Therefore, in process optimization and parameter selection, priority should be given to the combined effect of factors A and C to maximize control over the carbonation rate. In contrast, while the BC interaction is secondary, its F-value still indicates a certain level of influence and should be considered as a secondary regulatory factor. The low F-value of the AB interaction indicates that its contribution to the response variable is relatively weak and can be disregarded. Therefore, in subsequent experimental design and process development, more emphasis should be placed on regulating the AC and BC interactions to further enhance the efficiency and quality of the product preparation. These results underscore the importance of identifying and prioritizing significant interactions in multi-factor optimization to achieve the best reaction outcomes and product performance.

The interactions corresponding to both response values exhibit significant nonlinear deviations, indicating the presence of synergistic or antagonistic relationships between these factors. The A–C interaction has a significant impact on both response values, making it crucial to simultaneously optimize these two variables to improve the specific surface area and carbonation rate of Nano ZnO. It is necessary to carefully consider the combined effects of both variables on the process during the optimization of this procedure.

3.4. Optimal Process Verification and Product Analysis

3.4.1. Verification Experiment

Based on single-factor experiments, the optimal reaction conditions for Nano ZnO preparation were determined as a reaction time of 120 min, a reaction temperature of 80 °C, a calcination temperature of 500 °C for 1 h, and an incident angle of 60°. Through optimization analysis using response surface experiments, the optimal reaction parameters for Nano ZnO preparation were identified as a solid-to-liquid ratio of 5.011:100, a cavitation number of 0.366, and a hydrodynamic cavitation height of 301.128. In actual practice, slight adjustments were made to these parameters, and seven repeated verification experiments were conducted. The experimental results are shown in Table 5. Under optimized reaction conditions, the BET specific surface area predicted by the regression model was 63.190 m²/g, and the carbonation rate was 94.623%. In the actual experimental conditions, the average BET specific surface area and carbonation rate obtained from seven repeated experiments were 62.377 m²/g and 93.937%, respectively. These values are close to the predicted values from the regression equation, indicating that the regression model has a high degree of fit with the actual results and can accurately reflect the influence of various factors on the BET specific surface area and carbonation rate in the preparation of Nano ZnO.

3.4.2. Product Analysis

The microstructural analysis of the prepared Nano ZnO was performed using a scanning electron microscope (SEM), and the results are shown in Figure 20. Figure 20a,b depict the Nano ZnO particles at high and low resolutions, respectively. From Figure 20, it can be observed that the Nano ZnO prepared using hydrodynamic cavitation technology exhibits a multi-layered sheet-like structure, with good dispersion and minimal agglomeration. As seen in Figure 20a, the diameter of the sheet-like Nano ZnO crystals is approximately 500 nm, with a thickness ranging from 25 to 35 nm, reaching the nanoscale.

Figure 21 shows the XRD spectrum of the prepared Nano ZnO. The results indicate that the diffraction peaks at 2θ = 31.74°, 34.38°, 36.24°, 47.54°, 56.60°, 62.86°, and 67.94° correspond to the (100), (002), (101), (102), (110), (103), and (200) crystalline planes of the hexagonal wurtzite structure of ZnO (JCPDS: 36-1451), respectively [53]. There are no other impurity diffraction peaks, indicating that the main phase of the sample is zinc oxide with relatively high purity. The diffraction peaks of Nano ZnO are sharper, suggesting better crystallinity and smaller particle size, implying that the Nano ZnO synthesized in this study exhibits excellent size effect [54]. Using the grain size determination method outlined in GB/T 19589-2004 and the XRD line-broadening method, the average grain size was determined [55]. The half-maximum width of the diffraction peak at 31.7° in ZnO was measured, and the Scherrer formula was used to calculate the average particle size to be 28.679 nm (<100 nm), confirming that the products are at the nanoscale.

A fully automatic specific surface area analyzer was used to conduct nitrogen adsorption–desorption tests on the sample, and the results are shown in Figure 22. The results from Figure 22 indicate that the Nano ZnO exhibits characteristics of a type IV isotherm with an H3-type hysteresis loop. The hysteresis loop of a type IV isotherm is indicative of mesoporous structures with slit-shaped pores, which is consistent with the results shown in the SEM images of the sample. The presence of the H3-type hysteresis loop is generally associated with porous materials formed by the aggregation of well-defined plate-like particles, suggesting that the Nano ZnO prepared in this study is a plate-like particulate material [56].

Nanomaterials tend to undergo secondary agglomeration, which is a common phenomenon. Figure 23 shows the particle size distribution of the product. The results indicate that the particle size distribution of the product obtained through hydrodynamic cavitation is relatively uniform, with a D50 value of 2.8833 μm. The particle size distribution is concentrated, primarily ranging from 1 to 10 μm, demonstrating that the mass transfer in hydrodynamic cavitation is relatively uniform and can reduce the agglomeration of Nano ZnO.

Figure 24 shows the Raman spectrum of the product. In the crystal structure of ZnO, the Raman-active optical phonons are A₁ + E₁ + 2E₂, where A₁ and E₁ split into transverse optical (TO) and longitudinal optical (LO) phonons [57]. For Nano ZnO, Raman characteristic peaks can be observed at 335.9, 395.3, 440.2, 539.1, and 580.8 cm⁻¹, with the vibration mode at 335.9 cm⁻¹ being a second-order multi-phonon mode (E_2h–E_2l), the vibration mode at 395.3 cm⁻¹ being the transverse optical (TO) mode of A₁, and the vibration mode at 580.8 cm⁻¹ being the longitudinal optical (LO) mode of E₁. The strongest Raman peak at 440.2 cm⁻¹ is attributed to the E_2h vibration mode, which is identified as the Raman characteristic peak of hexagonal wurtzite ZnO (B4 phase). This indicates that the synthesized sample is hexagonal wurtzite ZnO, and the sharp and well-defined Raman peaks suggest a high degree of crystallinity. Notably, the peak at 440.2 cm⁻¹ exhibits an asymmetric feature, which is attributed to the Fano asymmetry caused by the phonon confinement effect in nano materials [58].

By using high-purity zinc oxide as a raw material, Nano ZnO products can be prepared through the carbonization method. The main technical specifications of the obtained products are shown in Table 6. As indicated in Table 6, the key technical indicators of the Nano ZnO products prepared from high-purity zinc oxide meet the HG/T2572-2020 standard for active zinc oxide products [59]. Compared to traditional mechanical stirring, the Nano ZnO produced through hydrodynamic cavitation exhibits a larger specific surface area and smaller particle size, indicating that the hydrodynamic cavitation technique significantly enhances the physical and chemical properties of Nano ZnO. The use of high-purity zinc oxide as a raw material for producing Nano ZnO offers the advantage of low raw material costs and also provides an effective solution for the difficult handling of zinc mud byproducts. In summary, the hydrodynamic cavitation-enhanced carbonization method for preparing Nano ZnO has good feasibility.

4. Conclusions

This study proposes a new approach to the preparation of Nano ZnO based on a hydrodynamic cavitation-enhanced carbonization method, using high-purity zinc oxide as the raw material. Through experiments, the effects of various factors on the specific surface area and carbonization rate were investigated, including reaction time, reaction temperature, material concentration (solid–liquid ratio), calcination temperature, hydrodynamic cavitation incident angle, cavitation number, and different positional heights.

• (1). Under conventional conditions, the low carbonization rate is mainly due to the in situ growth of basic magnesium carbonate on the surface of zinc oxide, which inhibits the carbonization reaction. The cavitation effect of hydrodynamic cavitation can generate immense energy to break the in situ growth adsorbed on the zinc oxide surface and improve the agglomeration of basic zinc carbonate. This helps to enhance solid–liquid mass transfer and promote the carbonization reaction, thereby effectively increasing the reaction rate and carbonization rate.

• (2). In the hydrodynamic cavitation-enhanced process, the influence patterns of various process parameters on the carbonization rate and specific surface area are similar to those of the mechanical stirring and bubbling method. Compared to traditional mechanical stirring and bubbling, the specific surface area increased by 1.5 times, and the carbonization rate improved by 10%. Based on single-factor experiments, the effects of seven factors on the carbonization rate and specific surface area in the preparation of Nano ZnO were explored, and four optimal factors were determined: reaction time of 120 min, reaction temperature of 80 °C, an incidence angle of 60°, and calcination temperature of 500 °C for 1 h. These were identified as the best process parameters.

• (3). The process was optimized using response surface methodology (RSM), and the response values were predicted under different combinations of independent variables through a regression equation. The optimal process parameters that maximize the response variables were determined as follows: reaction time of 120 min, reaction temperature of 80 °C, material-to-liquid ratio of 5.011:100, calcination temperature of 500 °C, incidence angle of 60°, cavitation number of 0.366, and position height of 301.128 mm. The interaction between the material-to-liquid ratio and position height was found to have a significant effect on process parameter variations. After conducting seven repeated validation experiments, the measured average values for carbonization rate and specific surface area were 93.937% and 62.377 m²/g, respectively, which are very close to the predicted values from the regression equation—94.623% for the carbonization rate and 63.190 m²/g for the specific surface area. Therefore, the optimized process conditions in this study are reasonable and reliable, providing valuable insights for the preparation of Nano ZnO using the hydrodynamic cavitation-enhanced carbonization method and enabling its comprehensive utilization.

• (4). The experimental verification showed that the product prepared by this process had a high content of Nano ZnO, excellent crystallinity, and a relatively uniform sheet-like morphology. The hydrodynamic cavitation-enhanced carbonization method for preparing Nano ZnO demonstrated good industrial practicability. The properties of the experimentally prepared product met the standards for active zinc oxide, and compared to the products generated by traditional mechanical stirring methods, each performance index was superior. This study focused on the effects of the hydrodynamic cavitation-enhanced carbonization method on the carbonization rate and specific surface area during the preparation of Nano ZnO. Future research needs to further investigate the impact of the cavitation number in hydrodynamic cavitation on the preparation process and the product properties of Nano ZnO. This will help to further optimize the preparation process, improve product performance, and expand its industrial application range.

Author Contributions

Conceptualization, J.G.; methodology, J.G.; software, H.Y. (Hanshuo Yang); validation, J.G. and H.Y. (Hanshuo Yang); formal analysis, J.G.; Investigation, J.G.; resources, J.G.; writing—original draft, J.G.; writing—review and editing, H.Y. (Honglei Yu); supervision, L.F.; funding acquisition, D.W. and G.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Hydrodynamic cavitation-enhanced ZnO carbonation reaction experimental apparatus.

Figure 2. Schematic diagram of hydrodynamic cavitation reactor structure (α is the inlet angle, β (40°) is the outlet cone angle, L (10 mm) is the throat length, d0 is the throat diameter, d1 (20 mm) is the inlet diameter, d2 (20 mm) is the outlet diameter, d3 (15 mm) is the carbon dioxide gas inlet diameter).

Figure 3. Process flow chart for preparing Nano ZnO.

Figure 4. Thermogravimetric curve.

Figure 5. (a) XRD of Zn5(CO3)2(OH)6; (b) Raman spectrum of Zn5(CO3)2(OH)6.

Figure 5. (a) XRD of Zn5(CO3)2(OH)6; (b) Raman spectrum of Zn5(CO3)2(OH)6.

Figure 6. Effect of reaction time on specific surface area and carbonization ratio of Nano ZnO.

Figure 7. Effect of reaction temperature on specific surface area and carbonization ratio of Nano ZnO.

Figure 8. Effects of solid–liquid ratio on specific surface area and carbonization ratio of Nano ZnO.

Figure 9. Effect of calcination temperature on specific surface area and carbonization ratio of Nano ZnO.

Figure 10. Effect of incident angle on specific surface area and carbonization ratio of Nano ZnO.

Figure 11. Effect of cavitation number on specific surface area and carbonization ratio of Nano ZnO.

Figure 12. Effects of different position heights on specific surface area and carbonization ratio of Nano ZnO.

Figure 13. Schematic diagram of nucleation and growth of basic zinc carbonate.

Figure 14. Hydraulic cavitation strengthens carbonization mechanism.

Figure 15. (a) Formation of basic zinc carbonate by hydro-cavitation-enhanced carbonization; (b) particle size distribution graph of basic zinc carbonate.

Figure 16. Residual normal distribution diagram. (a) BET; (b) Φ.

Figure 17. Scatter plot between residuals and predicted values. (a) BET; (b) Φ.

Figure 18. Influence of interaction of three factors on specific surface area of BET. (a) AB; (b) AC; (c) BC.

Figure 19. Influence of interaction of three factors on specific surface area of carbonization rate. (a) AB; (b) AC; (c) BC.

Figure 19. Influence of interaction of three factors on specific surface area of carbonization rate. (a) AB; (b) AC; (c) BC.

Figure 20. Product SEM image. (a) High resolution; (b) Low resolution.

Figure 21. Product XRD pattern.

Figure 22. Product absorption and desorption curve.

Figure 23. Product particle size distribution map.

Figure 24. Product Raman spectrum.

Table 1

Material composition.

Chemical Constituents	ZnO	PbO	CdO	MnO	CuO	Hydrotrope
Content (%)	99.5	0.0005	0.0005	0.001	0.002	0.10

Table 2

Experimental factors and levels of response surface.

Factor	Standard
	−1	0	1
Solid–liquid ratio (%)	4	5	6
Cavitation number (σ)	0.30	0.37	0.44
Different exit positions (H)	250	300	350

Table 3

Response surface experiment results.

Run	Solid–Liquid Ratio(%)	Cavitation Number(σ)	Different Exit Positions(mm)	BET(m2/g)	Φ(%)
1	5	0.3	250	59.917	92.25
2	6	0.44	300	59.248	90.34
3	5	0.44	250	59.426	89.36
4	4	0.37	350	60.146	91.42
5	5	0.37	300	63.621	94.87
6	5	0.3	350	60.385	89.17
7	5	0.37	300	63.579	94.62
8	5	0.37	300	62.812	95.12
9	5	0.44	350	60.746	90.24
10	4	0.3	300	59.469	91.74
11	5	0.37	300	62.346	94.59
12	4	0.37	250	60.882	89.25
13	4	0.44	300	60.995	89.96
14	6	0.37	350	61.754	89.74
15	5	0.37	300	63.583	93.84
16	6	0.3	300	60.462	91.15
17	6	0.37	250	59.256	92.56

Table 4

Regression model and variance analysis.

Source	df	BET			Φ
		Sum of Squares	F	p	Sum of Squares	F	p
Model	9	37.84	20.45	0.0003	71.54	32.37	<0.0001
A—Solid–liquid ratio	1	0.0745	0.3624	0.5662	0.2521	1.03	0.3447
B—Cavitation number	1	0.0041	0.0201	0.8911	2.43	9.90	0.0162
C—Different jet positions	1	1.58	7.66	0.0278	1.02	4.14	0.0815
AB	1	1.88	9.13	0.0193	0.2352	0.9580	0.3603
AC	1	2.61	12.72	0.0091	6.23	25.35	0.0015
BC	1	0.1815	0.8828	0.3787	3.92	15.97	0.0052
A2	1	7.98	38.83	0.0004	11.62	47.34	0.0002
B2	1	13.16	64.01	<0.0001	19.45	79.20	<0.0001
C2	1	7.14	34.71	0.0006	20.45	83.30	<0.0001
Resiudal	7	1.44			1.72
Lack of Fit	3	0.0922	0.0913	0.9610	0.7976	1.15	0.4297
R2	0.9634				0.9765
Adjusted R2	0.9163				0.9464

Data in bold indicates a significant correlation with experimental variables.

Table 5

BET specific surface area and carbonization rate after parameter optimization.

Actual Test Condition	Repeated Trials	Bet (m2/g)	Ratio of Coalification (%)
Response time (120 min)	1	62.684	94.52
Reaction temperature (80 °C)	2	61.851	93.68
Solid–liquid ratio (5.011:100)	3	62.732	93.5
Calcination temperature (500 °C)	4	62.556	93.79
Incident angle/Diameter (60°)	5	62.342	94.3
Cavitation number (0.366 ± 0.03)	6	61.742	94.11
Different jet positions (301.13)	7	62.733	93.66
	Average value	62.377	93.937

Table 6

Product main technical indicators.

Parameters	Product	Traditional Mechanical Mixing Products [60]		HG/T2572-2020
				Top Quality Goods	Qualified Product
ZnO percentage content, %	97~98	95~98	-	95~98	95~98
H2O, %	0.2	≤0.7	-	≤0.7	≤0.7
Grain size, nm	25~35	50~80	≤50 nm	≤100	≤100
Product appearance	White	White or Faint yellow	White	White	White
Bulk density, g/mL	0.28~0.30	0.36~0.40	-	≤0.35	≤0.40
Specific surface area, m2/g	62.377	38.46~47.75	49.89	≥45	≥35
Loss on ignition, %	1.6	-	-	1~4	1~4

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