An environmentally friendly and high current efficiency acid mist inhibitor for zinc electrowinning

Acid mist inhibitor is an effective additive for suppressing the generation of acid mist during electrowinning process. However, the traditional acid mist inhibitor has a low current efficiency, leading to high energy consumption. Thus, novel acid mist inhibitors are crucial for the development of the electrowinning industry. In this paper, three new acid mist inhibitors (Famigo® FS-101, Famigo® FS-301, and Famigo® FS-401) are used in zinc electrowinning process, while the mechanism of action in suppressing the generation of acid mist is characterized by the volume of bubble attached on the cathode surface and surface tension. The impacts of the concentration of the acid mist inhibitors on the bath voltage, current efficiency, power consumption, and electrochemical properties, as well as the enhancement mechanism on the current efficiency, are studied. The obtained results demonstrate that an adequate concentration of Famigo®FS-101, Famigo®FS-301, and Famigo®FS-401 allows to increase the cathode current efficiency and decrease the energy consumption, from 3219 kWh t−1 in zinc electrolyzer to 2948 kWh t−1, 3090 kWh t−1, and 2884 kWh t−1 using a concentration of 5 mg l−1 of Famigo®FS-101, Famigo®FS-301, and Famigo®FS-401, respectively. The proposed inhibitors have high acid mist inhibition efficiency and current efficiency in the zinc electrowinning process. Furthermore, if engineered, they may contribute to the development of the electroposition industry.


Introduction
Electrodeposition is one of the most widely used methods to extract and refine metal from its salt solution, such as Zn and Cu. During the zinc electrowinning process, zinc is deposited at the cathode surface, hydrogen gas is produced in the form of microbubbles, and a large amount of oxygen is also produced in the form of microbubbles on the anode surface: The number of the oxygen and hydrogen gas microbubbles increases with the progress of the electrolysis process, then increases through the electrolyte and bursts to eject, leading to the generation of acid mist into the air in the form of 'film' [1] and 'jet' droplets [2][3][4]. The acid mist containing sulfuric acid, which is carcinogenic to humans, is classified as group 1 [5,6]. It causes severe corrosion of the cathode, anode, boom, and other equipment in the storage tank, resulting in costly losses [7,8].
Several studies have been conducted to remove sulfuric acid mist, such as the acoustic agglomeration, wet flue gas desulfurization (WFGD) system with double scrubbers, and stacking polymer microspheres matrix [9][10][11][12]. It has been demonstrated that the acid mist inhibitor is an effective additive to suppress the generation of acid mist. Many kinds of acid mist inhibitors have been used to suppress the generation of acid mist during the electroposition process [13][14][15][16][17][18][19]. However, most of these inhibitors result in decreasing the current efficiency by inhibiting the deposition reaction, which further leads to the increase of the tank voltage and energy consumption [20]. For instance, Mackinnon [21] studied the influence of Dowfroth on the current efficiency and polarization during the zinc deposition process. The obtained results demonstrated that Dowfroth can play an active role in suppressing the generation of acid mist at the expense of current efficiency reduction. A similar phenomenon has been reported in other studies. More precisely, Cheng [22] demonstrated that two saponinrich products (Mistop ® and QLZinc ® ) with a commercial licorice product can also play an active role in suppressing the generation of acid mist during the zinc electroposition process while reducing the current efficiency. Dhak [23] studied the influence of saponin, licorice, Dowfroth 250, and Tennafroth 250 on the zinc electrowinning process in industrial waste electrolytes. He deduced that all these surfactants can reduce acid mist. In addition, licorice had larger inhibition efficiency than the other surfactants. However, all these surfactants had a low current efficiency. Dhak [24]studied the influence of Dowfroth 250 (D) and Tennafroth 250 (T) with saponin (S) on the current efficiency and power consumption during the zinc electrodeposition process in industrial waste electrolyte solution. The results showed that the acid mist suppression efficiency can respectively reach 86.1% and 87.0% with the addition of D5+S5 and T5+S5 during zinc electrodeposition in industrial waste electrolyte solutions, at the expense of current efficiency and higher energy consumption. Hosny [25] demonstrated that the nonyl phenoxyethylene surfactant additives have a positive effect in suppressing the generation of acid mist. However, they lead to the increase of the tank voltage and energy consumption.
It can be deduced from the literature that the suppression of acid mist is accompanied with the decrease of the current efficiency and the increase of the energy consumption during the electroposition process, which is the main problem in the development of the electrowinning industry. Developing new acid mist inhibitors able to retain high current efficiency and low energy consumption has recently been a key challenge for the electrowinning industry.
In this paper, three new acid mist inhibitors (Famigo ® FS-101, Famigo ® FS-301, and Famigo ® FS-401 (Kopper Chemical Industry Corp., Ltd.)) are proposed, which is stable fluorocarbon organic compound and the main components of these acid mist inhibitors are alkyl acrylate, imidazoline, and utropine. These acid mist inhibitors are dark brown neutral liquid and are completely soluble in water, and it can be directly applied to the electrowinning liquid with densities of 1.20 g cm −3 , 1.61 g cm −3 , and 1.21 g cm −3 , for Famigo ® FS-101, Famigo ® FS-301, and Famigo ® FS-401, respectively. The mechanism of action of these acid mist inhibitors in suppressing the generation of acid mist during the zinc electrowinning process was studied by comparing the numbers and volumes of the bubbles attached on the zinc electrode surface. Furthermore, the enhancement mechanism on the cathode current efficiency was investigated by studying the cathodic polarization degree and nucleation mechanism. The proposed acid mist inhibitors allow to overcome the main problem existing in the electrolysis industry. Moreover, if engineered, they may contribute to the development of the electroposition industry.

Zinc electrowinning experiment
The electrowinning experiments were performed in electrowinning apparatus. The electrolyzer consists of two lead anodes and a central aluminum cathode, and the anode-cathode spacing is fixed at 2.5 cm of the electrolyzer (as shown in figure 1). The anode plate was made of Pb-0.75%Ag (provided by Kunming Polytech Hengda Technology Co., Ltd.), this is due to Pb-0.75%Ag as anode decreases the oxygen overvoltage and increases the corrosion resistance of anodes used in zinc electrowinning [26,27]. The cathode was 1070 pure aluminum, this is due to the current efficiency of the pure aluminum cathode is higher and the deposited zinc plates are flatter and smoother. The edges of the cathode plate were closed with denture base resin to prevent dendrite growth on the side of the plate. During the experiments, the acid mist was collected every half hour and the voltage was measured every hour. The entire equipment was kept in a fume hood with a constant window height throughout the experiment to ensure consistent effects on the electrowinning experiments. The specific parameters are presented in table 1.

Acid mist detection method
The acid mist was collected from the electrolyzer using the Beijing Labor QC-1B atmospheric sampler into a 0.5 l gas sampling bag, as shown in figure 1. The sampling bag was stable in the process of acid mist removal. The MS400HS-H 2 SO 4 acid mist detector (Yunnan Dian Yi Tong Instruments Co., Ltd.) was used with a sampling pump of 500 mg (L·min) −1 speed. In the experiment, the amount of acid mist was obtained when the acid mist detector showed a stable curve. Each additive is repeated more than three times and the results are averaged to remove the errors caused by weather or other objective factors.

Surface tension measurement
The surface tension measurement was performed using the JYW-200B micro-controlled automatic surface tension meter (Shanghai Titan Technology Co., Ltd.) at room temperature and 40°C. During the measurements, the platinum rings were sequentially soaked in pure water, cleaned with petroleum ether, rinsed with acetone, and dried by heating in oxidized fumes from a gas or alcohol lamp.

Electrochemical testing
The linear scanning voltammetric curves is effective in studying the kinetics of electrode processes. They have been widely used in the fields of metal electrolysis, material corrosion, and chemical power sources, as well as the basic electrochemical studies. The relationship between electrode potential and current density was studied by the linear scanning voltammetric curves. The nucleation mechanism of zinc in the acidic zinc sulfate system of 1070 pure aluminum was characterized by constant potential step timing current. The electrochemical workstation (RST-5200, Zhengzhou Shires Instrument Technology Co., Ltd.) was used for testing the LSV. A three-electrode system was used in the electrochemical experiments. For cathodic polarization test, the working electrode, auxiliary electrode, reference electrode is respectively 1070 pure aluminum, platinum electrode, and   For anodic polarization test, the working electrode, auxiliary electrode, reference electrode is respectively Pb-0.75%Ag alloy, platinum electrode, and mercurous sulfate electrode (C(K 2 SO 4 ) −1 , MSE), with a scan rate of 5 mV S −1 at 40°C between the potential from −1 V to −1.7 V.

Deposit examination
The crystal structure was investigated using X-ray diffraction (XRD, D8Advance, Bruker, Germany) and the microstructure was studied using scanning electron microscopy (SEM, SU-8010, HITACHI, Japan). The functional group in these three new acid mist inhibitors were analyzed using Fourier transform infrared spectroscopy (FTIR, ALPHA, Bruker, Germany). indicating the different content alkyl acrylate in these three acid mist inhibitors, alkyl acrylate contains oxygen, which forms intermolecular hydrogen bonds with water, which inhibits the electrolysis of water, thus producing less hydrogen and oxygen, which results in less acid fog production. Therefore different levels of alkyl acrylate will have different effects on acid mist inhibitor efficiency. The acid mist suppression efficiency is the most basic indicator for the evaluation of the practical application of the acid mist inhibitor. Thus, the effect of the FS-101, FS-301, and FS-401 acid mist inhibitors and concentration on the suppression of the generation of acid mist during the electrowinning process was evaluated, and the mechanism of action was explored by the volume of bubble attached on the cathode surface. Figure S1 shows the trend variation of acid mist suppression efficiency of FS-101, FS-301 and FS-401 acid mist inhibitors with different concentrations in the zinc electrolyzer, and it can be seen from figure S1 that all the acid mist inhibitors play an active role in suppressing the generation of acid mist during the zinc electrowinning process, and the inhibition efficiency is positively correlated with the concentration of acid mist inhibitors, especially for FS-101. Table 2 lists. The values of acid mist inhibition efficiency, for example, when the concentration is 15 mg l −1 , the acid mist inhibition efficiencies of FS-101, FS-301, and FS-401 are respectively 79.57%, 60.66%, and 58.36%, that are much larger than those obtained in the zinc electrolyzer with 5 mg l −1 of FS-101, FS-301, and FS-401. In addition, FS-101 has higher acid mist inhibition efficiency than the other inhibitors with the same concentration.

Results and discussion
In order to intuitively visualize the action process and illustrate the mechanism of action of these acid mist inhibitors, the YongXian SZM7045-T1 microscope was used to observe the volume changes of the bubble attached on the electrode surface. The obtained results are shown in figure 3. It can be seen from figures 3(b)-(d) that the diameters of the bubbles attached on the electrode surface in the electrolyte using the FS-101, FS-301, and FS-401 acid mist inhibitors are significantly smaller than that for the zinc electrolyzer ( figure 3(a)). The smaller diameters of the bubbles attached on the electrode surface in the electrolyte using the FS acid mist inhibitors are mainly ascribed to the closer packing of the surfactant molecules at the gas/solution interface contributing to the decrease of the surface tension, this reduces force of 'fixing' the bubble to the electrode surface is reduced, resulting in a smaller size separation of the bubble and thus a reduction in bubble size. In addition, a dense foam layer is observed on the anode surface in the zinc electrolytic solution with acid mist inhibitor. The foam is able to absorb the waves, and the acid solution moves down the bubble crater, which is  beneficial for blocking the bubbles from rising and for reducing the bubble rupture rate on the electrolyte surface. This also explains the reasons for the high efficiency in suppressing the generation of acid mist during the zinc electrowinning process.
In order to study the mechanism of action of the acid mist inhibitors in reducing the volume of bubbles attached on the electrode surface, the surface tension of zinc sulfate solutions and the zinc sulfate solutions with acid mist inhibitors, which is closely related to the force that 'holds' bubbles to the electrode surface [28][29][30], were investigated at room temperature and 40°C. The obtained results are shown in figure 4. The surface tension curves of zinc sulfate solutions with different concentrations of the FS-101, FS-301, and FS-401 acid mist inhibitors at room temperature, show that the acid mist inhibitors are beneficial for the decrease of the surface tension of zinc sulfate solution, and it is negatively correlated with the concentration of acid mist inhibitors, as shown in figure 4(a). Figure 4(b) shows the surface tension curves of the zinc sulfate solution with different concentrations of FS-101, FS-301, and FS-401 at 40°C. The results are consistent with those obtained at room temperature. The surface tension of zinc sulfate solution is much larger than that of the solution with different concentrations of acid mist inhibitors. In addition, the surface tension of the zinc sulfate solution at 40°C is 13.6 mN m −1 lower than that at room temperature, while the ZnSO 4 solution with 15 mg l −1 FS-101 inhibitor is 2.28 mN m −1 lower than that that at room temperature. This indicates that temperature will affect the surface tension value of electrolyte when acid mist inhibitor is added to some extent. The lower surface tension of the zinc sulfate solution using the FS-101 acid mist inhibitor compared with the other inhibitors having the same concentration, also explains the reasons for the higher mist suppression efficiency and smaller volume of bubble attached on electrode surface.
Based on the previously mentioned results, the mechanism of action of the FS-101, FS-301, and FS-401 acid mist inhibitors in suppressing the generation of acid mist during the zinc electrowinning process relies on the fact that the acid mist inhibitor allows to reduce the surface tension of zinc sulfate solution and the force that 'holds' the bubbles to the electrode surface. Therefore, the bubbles can be separated at a smaller size, which further leads to less acid mist droplets into the air.

Influence of the acid mist inhibitor on the current efficiency and energy consumption
The current efficiency and energy consumption are other important indicators for the commercial applications of acid mist inhibitors. Thus, the influence of acid mist inhibitors on the current efficiencies and polarization degree during the zinc electrowinning process are studied. The obtained results are presented in figures 5-6. The current efficiencies of the zinc electrowinning process with different concentrations of acid mist inhibitor are shown in figure S2(a), and the specific values are presented in table 2. The current efficiency first increased and then decreased with the increase of the concentration of acid mist inhibitor. For example, when the concentration was 5 mg l −1 , the current efficiency was 90.94%, 93.36%, and 91.57% in ZnSO 4 solution using FS-101, FS-301, and FS-401, and the current efficiency decreased to 86.20%, 92.54%, and 89.03% when the concentration increased to 15 mg l −1 , respectively. Table 2 and figure S2(b) shows the impacts of the FS-type acid mist inhibitors on the tank voltage and energy consumption during the zinc electrowinning process. It can be seen that the tank voltage shows an irregular pattern with different kinds of acid mist inhibitor. For example, the tank voltage decrease in the electrolyte with FS-101 and FS-401, while the FS-301 has little effect on tank voltage, which may be ascribed to different types of acid mist inhibitors have different effects on the oxygen evolution overpotential(are shown in figure S3). As shown in table 2 and figure S2(b), all the samples exhibit lower energy consumption in the zinc electrowinning process. When the concentration of the FS-101, FS-301, and FS-401 acid mist inhibitors is 5 mg l −1 , the energy consumption is respectively 2948 kWh t −1 ·Zn, 3090 kWh t −1 ·Zn, and 2884 kWh t −1 ·Zn, and the energy consumption gradually increases with the concentration increase. Since the energy consumption is inversely proportional to the current efficiency, the trend of energy consumption decreasing and then increasing coincides with the trend of current efficiency increasing and then decreasing.
The results of the current efficiency, tank voltage, and energy consumption showed that when the concentration is 5 mg l −1 , the FS-101, FS-301, and FS-401 acid mist inhibitors can effectively suppress the generation of acid mist, improve the current efficiency, and reduce the energy consumption during the zinc electrowinning process. Thus, the optimized concentration of the FS-101, FS-301, and FS-401 acid mist inhibitors is 5 mg l −1 .
3.3. The enhancement mechanism influence on the current efficiency 3.3.1. Influence of the acid mist inhibitors on the polarization of zinc deposition To further explore the efficiency of the enhancement mechanism of the FS-101, FS-301, and FS-401 acid mist inhibitors on the current efficiency and the reasons for the decrease of energy consumption, the cathodic kinetic parameters and zinc nucleation mechanism during the zinc electrowinning process in zinc sulfate electrolyte were studied with a scan rate of 5 mV s −1 . They were then recorded for a potential ranging between −1.00 V and    Figure 5(a) shows the polarization curves of the electrolyte with FS-101. At the initial stage, the current density is close to zero, and it sharply increases before the scan potential reaches almost −1.6 V, where Zn 2+ starts to reduce to Zn accompanied with hydrogen gas overflow. In contrast to the polarization curves obtained in the Zn 2+ system, the polarization curves obtained in electrolyte with 5 mg l −1 FS-101 shift to positive potential direction, while they shift to negative potential direction in electrolyte with 10 mg l −1 and 15 mg l −1 FS-101. This indicates that zinc grains are more easily precipitated in the electrolyte with 5 mg l −1 FS-101, which is difficult in electrolyte with 10 mg l −1 and 15 mg l −1 FS-101. It also explains the reasons for the higher current efficiency in the electrolyte with 5 mg l −1 FS-101 and the lower current efficiency in the electrolyte with 10 mg l −1 and 15 mg l −1 FS-101.
It can be seen from figure 5(a) that in the zinc sulfate system, the cathodic hydrogen generation competes with zinc precipitation, which is accompanied by hydrogen generation, and therefore it is impractical to measure the hydrogen generation in the zinc sulfate system. Thus, the hydrogen generation process was studied in the 150 g l −1 H 2 SO 4 +124 g l −1 Na + system with different concentrations of FS-101, FS-301, and FS-401, which was obtained by the Debye-Hückel equation to ensure the same ionic strength of the sodium sulfate with the underlying electrolyte. Figure 5(b) shows the linear scanning voltammetric curves of hydrogen generation in the 150 g l −1 H 2 SO 4 +124 g l −1 Na + system with different concentrations of FS-101. It can be seen that the hydrogen evolution potential shifts to the negative direction with the increase of the concentration of FS-101, which indicates that FS-101 can contribute to the suppression of the generation of hydrogen. Figure S4(a) and S5(a) shows the cathodic polarization curves in the ZnSO 4 electrolyte with FS-301 and FS-401, It can be seen from the figure that the polarization curve shifts to positive potential direction, this indicates that FS-301 and FS-401 is beneficial for decreasing zinc deposition overpotential, leading to easier zinc precipitation with the addition of FS-301 and FS-401 acid mist inhibitor. This may also be one of the reasons for the higher current efficiency in the electrolyte with the addition of FS-301 and FS-401. In addition, the shift range of the polarization curve obtained in the electrolyte with 5 mg l −1 is larger compared with others, it means that the electrolyte containing 5 mg l −1 FS-301 or FS-401 promotes zinc precipitation to the greatest extent. Figure S4(b) and S5(b) shows the linear scanning voltammetric curves of hydrogen generation in the sodium sulfate system with different concentrations of FS-301 and FS-401, as can be seen from the figure, when different concentrations of FS-301 and FS-401 were added to zinc electrolyte, hydrogen evolution potential moved in a negative direction, indicating the FS-301 and FS-401 can contribute to suppress the generation of hydrogen, which may be ascribed to the increase in interfacial viscosity due to the adsorption of the additive, which slows down the precipitation of hydrogen.

Influence of the acid mist inhibitors on the zinc nucleation mechanism
The Scharifker and Hills model [31] is widely used for studying the nucleation mechanism. Two cases can occur during the nucleation process. The first one is the instantaneous nucleation process, where nucleation immediately occurs when reaching the desired potential, and the second one is the continuous nucleation process, which indicates the progressive nucleation with an increasing (time-dependent) number of nuclei [32,33]. To further explore the impact of the enhancement mechanism on the current efficiency and the reasons for the decreased energy consumption, the transient characteristics of zinc deposited at a potential of −1.7 V were investigated in zinc electrolyte with and without optimized concentration of FS-101, FS-301, and FS-401 (5 mg l −1 ), as shown in figure 6(a), all curves show that the rise in current for a short time corresponds to the birth and growth of zinc nuclei, after which the diffusion zones of adjacent nuclei overlap and the current reaches a maximum value i m , when the corresponding time is t m .
It can be observed from figure 6(a) that the time t m required to reach the peak current in the transient curve of zinc electrodeposition at −1.7 V potential without acid mist inhibitor and with the addition of 5 mg l −1 FS-101, FS-301, and FS-401 are 0.8 s, 0.63 s, 0.3 s, and 0.52 s, respectively. This indicates that FS-101, FS-301, and FS-401 can promote the nucleation rate of zinc on cathode surface, which explains the reason for higher current efficiency in the electrolyte with the addition of 5 mg l −1 FS acid mist inhibitor. Figure 6(b) shows 3D nucleation dimensionless plot of electrodeposited zinc at −1.7 V for optimal FS acid mist inhibitor concentration, this includes theoretical 3D instantaneous nucleation and 3D continuous nucleation curves. It can be seen that the initial nucleation curves of zinc on 1070 pure aluminum after zinc sulfate solution and the addition of 5 mg l −1 FS-101, FS-301, and FS-401 are higher than those of the three-dimensional instantaneous nucleation model curve. Therefore, the initial nucleation of zinc on aluminum with both the zinc sulfate solution and the addition of the optimal concentration of acid mist inhibitor, are consistent with the three-dimensional instantaneous nucleation mechanism, which indicates that the addition of acid mist inhibitor did not change the nucleation mechanism of zinc on aluminum. It can also be seen that the actual curves do not match well with the theoretical three-dimensional instantaneous nucleation curves, because the theoretical model is derived by assuming that nucleation and growth occur on a completely smooth surface. However, in reality, there are dislocations and scratches on the aluminum cathode and the collector, providing additional active nucleation sites for the nucleation and growth of Zn metal. In this case, the initial nucleation curve will gradually deviate from the theoretical curve.
Based on these results, the enhancement of the current efficiency and the reasons for the decrease of the energy consumption can be summarized as follows: 5 mg l −1 of FS acid mist inhibitor is beneficial for reducing the overpotential in Zn sulfate electrolyte, which suppresses the generation of hydrogen and promotes the nucleation rate of zinc on the cathode surface during the zinc electrowinning process.

Influence of the acid mist inhibitor concentration on the deposition morphology and crystal orientation
With the development of modern industry and strict requirements for metal quality, adding suitable additives is an effective measure in order to achieve good economic and technical indicators, reduce labor intensity, improve working environment and obtain high quality zinc with dense structure, smooth surface and low impurity content. For example, the main purpose of adding gums in industrial production is to change the surface morphology of the deposit, reduce dendrite growth, and make the deposit crystallize densely and have a smooth surface.
The previously presented results demonstrate that the addition of acid mist inhibitor allows to decrease the zinc deposition overpotential and promotes the nucleation rate of zinc on the cathode surface, which may affect the microstructure and crystal structure of Zn during the electrowinning process. Therefore, to explore whether this acid mist inhibitor has a positive effect on the morphology of zinc deposits, SEM and XRD were performed to study the microscopic morphology and crystal orientation of zinc plates obtained from the electrolyte with different concentrations of acid mist inhibitor. The obtained results are shown in table 3 and figures 7-10. It can be seen from figure 7(a) that the zinc plating deposited without additives is long strips, and there are many microscopic pores on the surface. The growth characteristic of the (002) preferential orientation of zinc is obtained in the ZnSO 4 electrolyte ( figure 7(b)).   In contrast to the zinc flakes obtained in ZnSO 4 electrolyte, less ordered grain arrangement in zinc sheets obtained from the electrolyte containing FS-101, as shown in figure 8. As shown in figure 8(a), the addition of 5 mg l −1 FS-101 acid mist inhibitor made the grain structure of the deposited zinc plate smaller and more closely aligned, this is similar to previous studies [34] where the addition of perfluorocarboxylic acid to an acidic zinc sulfate electrolyte resulted in smaller and more compactly arranged grain structure and better surface morphology of the deposited zinc sheets. However this is in contrast to the previous Dhak [23] study where the grain size of the microstructure of the zinc plate was larger and less densely arranged when 5 ppm SE/Saponin acid mist inhibitor was added to the electrolyte. Furthermore, no fragments and aggregation phenomenon is observed on the zinc surface, obtained in ZnSO 4 electrolyte with 5 mg l −1 FS-101 ( figure 8(a)). In addition, the fragments and aggregation phenomenon appears again with the increase of the acid mist inhibitor concentration, as shown in figures 8(b) and (c). Moreover, the growth of the preferential orientations of Zn crystal during the electrodeposition process is observed with the addition of acid mist inhibitor. For example, when the concentration is 5 mg l −1 , the growth characteristic of the (101) preferential orientation of Zn is obtained in the electrolyte with 5 mg l −1 FS-101. The disorderly arrangement of zinc flakes, which allows to promote the growth of crystal, and the growth of the preferential orientations of Zn crystal during the electrodeposition process, may be another reasons for the lower energy consumption during the Zn electrodeposition process in electrolyte with FS-101.
It can be seen from figures 9(a) and (c) that FS-301 has a clear effect on the microstructure of Zn during the electrodeposition process. In contrast to the morphology of Zn obtained in ZnSO 4 electrolyte, the grain arrangement of zinc plates obtained in the electrolyte containing low concentration of FS-301 was less ordered, and the grain arrangement of zinc plates obtained in the electrolyte with high concentration of FS-301 was more ordered, which is consistent with the previously mentioned result. In addition, there are many small pieces of mesh structure observed on the surface of zinc flakes, the flakes become thicker, and the aggregation phenomenon becomes clear with the concentration increase. Moreover, the growth of the preferential orientations of Zn crystal during the electrodeposition process is observed with the addition of FS-301. For example, when the concentration is 5 mg l −1 , the growth characteristic of the (101), (102), and (103) preferential orientations of Zn is obtained in the electrolyte with 5 mg l −1 FS-301. The disorderly arrangement of zinc flakes and the growth of the preferential orientations of Zn crystal during the electrodeposition process may be another reason for the lower energy consumption during the Zn electrodeposition process in electrolyte with FS-301.
The addition of lower concentration of FS-401 acid mist inhibitor has no clear effect on the morphology during the electrodeposition process, except that the flakes become thicker and the aggregation phenomenon becomes clear with the concentration increase, as shown in figures 10(a) and (b). On the contrary, the zinc plating obtained in the electrolyte with 15 mg l −1 FS-401 showed irregular long strips with inconsistent crystal development, and many pores are observed on its surface [35]. In addition, the growth of the preferential orientations of Zn crystal during the electrodeposition process is also observed with the addition of FS-401. For example, the growth characteristic of the (101) and (102) preferential orientations of Zn is obtained in the electrolyte with 5 mg l −1 FS-401. The disorderly arrangement of zinc flakes and the growth of the preferential orientations of Zn crystal during the electrodeposition process may be another reason for the lower energy consumption in the electrolyte with the addition of FS-401.

Conclusion
In this paper, it is demonstrated that Famigo ® FS-101, Famigo ® FS-301, and Famigo ® FS-401 are an environmentally friendly acid mist inhibitors in the zinc electrowinning process. for example, the acid mist inhibition efficiency of 5 mg l −1 Famigo ® FS-101, Famigo ® FS-301 and Famigo ® FS-401 are 58.03%, 55.74% and 42.95%, enabling approximately 50% of the acid mist in the electrolyzer to be eliminated, thereby reducing the risk to humans and the environment as well as other equipment. In addition, Famigo ® FS acid mist inhibitor is a neutral liquid, non-toxic and harmless, and does not pollute water.
The action mechanism of the acid mist inhibitors is consistent with the theoretical hypotheses presented in previous studies. The acid mist inhibitor allows to reduce the surface tension of the zinc sulfate solution, thus allowing the bubbles to be separated with a smaller size, which further leads to less acid mist droplets into the air.
The adequate concentration of Famigo ® FS-101, Famigo ® FS-301, and Famigo ® FS-401 can also contribute to the increase of the cathode current efficiency and the decrease of the energy consumption, from 3219 kWh t −1 in zinc electrolyzer to 2948 kWh t −1 , 3090 kWh t −1 , and 2884 kWh t −1 with a concentration of 5 mg l −1 of Famigo ® FS-101, Famigo ® FS-301, and Famigo ® FS-401, respectively. This is due to the fact that the acid mist inhibitor can decrease the zinc deposition overpotential, suppressing the generation of hydrogen, and promoting the nucleation rate of zinc on the cathode surface at the same potential. In this paper, three new acid mist inhibitors with high inhibition efficiency and current efficiency are proposed for the zinc electrowinning process. If engineered, they may contribute to the development of the electroposition industry.