2020 年 61 巻 10 号 p. 1958-1966
To elucidate the effects of polyethylene glycol (PEG) and glue on the deposition of Zn from electrowinning solution and its resulting crystal structure, Zn electrodeposition was performed at a current density of 600 A·m−2 and a charge of 8.64 × 106 C·m−2 in an agitated sulfate solution containing 1.07 and 1.8 mol·dm−3 of ZnSO4 and H2SO4, respectively, at 45°C. With the additions of PEG and glue, the evolution of hydrogen was suppressed at the current density region less than the critical current density for Zn deposition, decreasing the critical current density of Zn. The degree of decrease in the critical current density of Zn was larger with glue than that with PEG. The current efficiency for Zn deposition was higher with PEG and glue than that without at the low current density region because the critical current density of Zn decreased with additives. Since the additives suppressed Zn deposition more than the hydrogen evolution at the high current density region, the current efficiency of Zn decreased by increasing the additive concentration. At the high current density region, little difference was observed in the current efficiency of Zn between PEG and glue. The effect of the molecular weight of PEG on the current efficiency of Zn was rarely observed at the molecular weight above 2000. With the addition of PEG, the deposits became fine platelets with preferred orientation of $\{ 10\bar{1}1\} $ and layered pyramidally, while $\{ 11\bar{2}0\} $ orientation was obtained, and the platelets grew perpendicularly to the substrate with the addition of glue. The surface roughness of deposited Zn decreased with additives, and it decreased further with PEG compared with that with glue.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 58–65.
Fig. 4 Current efficiency for Zn deposition in the solutions containing glue and various molecular weights of PEG. (● additive-free, ▲ Glue 6000, ○ PEG 200, △ PEG 2000, □ PEG 6000, ◇ PEG 35000, Concentration of additives: 10 mg·dm−3)
In Zn electrowinning, a naturally derived glue is added to the electrolytic solution to smooth the cathode surface and prevent impurities from codeposition.1–3) When the glue is added to the electrolytic solution, the cathode potential shifts to the less noble direction, and the crystals of deposits become fine.4–6) However, it is empirically known that the glue degrades gradually during electrolysis, ultimately losing their effectiveness and occasionally exerting a harmful influence on the purity of the deposited metal and on the electric conductivity of the electrolyte. On the other hand, polyethylene glycol (PEG) is reported to be chemically more stable and difficult to degrade than glue.7–9) Therefore, if PEG can be used as a substitute for glue, the lifetime of the additive in the electrolytic solution can be extended. Moreover, PEG addition into the electrolyte solution for Zn deposition is reported to change the morphology of Zn,10,11) but the effect of PEG on the current efficiency for Zn deposition from electrowinning solution and the microstructure of the deposited Zn is little known. Therefore, in this study, it was examined whether PEG can be used in Zn electrowinning as a substitute for glue. The effect of PEG and glue on the deposition behavior of Zn from the electrowinning solution and the crystal texture of Zn was investigated.
Table 1 shows the electrolyte composition and electrolysis conditions used to investigate the Zn deposition behavior and crystal structure of Zn. An electrolytic solution was prepared by dissolving ZnO with a purity of 99.999% (1.07 mol·dm−3) in distilled and ion-exchanged water containing sulfuric acid. The concentration of free sulfuric acid was 1.80 mol·dm−3. In some experiments, the concentration of free sulfuric acid was set to be 0.46 mol·dm−3 to investigate the effect of the ionic strength of the electrolyte solution. Polyethylene glycol (PEG, mean molecular weight of 200 to 35000, standard weight = 6000) and glue with a mean molecular weight of 6000 were respectively added to the electrolyte at concentrations of 1, 5, and 10 mg·dm−3 (standard concentration = 10 mg·dm−3). When the polarization curve was examined, an Al sheet of 1 cm × 2 cm with rear side sealed by epoxy resin and a dimensionally stable electrode (DSE) sheet were used as the cathode and anode, respectively. Prior to electrodeposition, the cathode was carefully polished and buffed with No. 240 emery paper before electrolytic degreasing and acid pickling were performed. The electrodeposition was performed in 0.5 dm3 of a solution agitated at 100 rpm using a magnetic stick-shaped stirrer at 45°C. The partial polarization curves were obtained by electrodeposition under coulostatic (50 kC·m−2) and galvanostatic conditions for a current density range of 10–1,000 A·m−2. The deposits were dissolved from the cathode using nitric acid, and Zn was quantitatively analyzed using inductively coupled plasma spectroscopy to calculate the current efficiency of Zn deposition. The partial current densities for Zn deposition and H2 evolution were calculated in each case by multiplying the total current density by the current efficiency. The current efficiency of the H2 evolution was calculated by subtracting the current efficiency of the Zn deposition from 100%. The cathode potentials were measured using a saturated KCl and Ag/AgCl reference electrode (0.199 V vs. a normal hydrogen electrode, at 25°C). The potentials were plotted with reference to the normal hydrogen electrode.
To investigate the effect of organic additives on the current efficiency and crystal texture of Zn deposited by long-term electrolysis, Zn electrodeposition was performed in a solution agitated at 100 rpm under galvanostatic conditions of 600 A·m−2 at 45°C for 4 h using an Al sheet (4 cm × 6.5 cm) and a mesh-like DSE sheet as the cathode and anode, respectively. The current density was fixed at 600 A·m−2 based on a Japanese main zinc electrolytic plant. Table 2 shows the solution composition and conditions for the long-term electrolysis. The size of the electrolytic cell was 13.3 cm (length) × 11 cm (diameter) × 11.7 cm (depth), and the capacity of the electrolyte and the anode–cathode distance were 1.0 dm3 and 2.5 cm, respectively. The current efficiency of Zn deposition was calculated from the mass difference of the cathode before and after electrolysis. The surface morphologies of the deposited Zn were observed using a scanning electron microscope (SEM). The crystal orientation of the deposited Zn was determined using the method introduced by Wilson and Rogers12) with an X-ray diffraction intensity of 0002 reflection to $11\bar{2}2$ reflection. The cross-sectional texture of the deposited Zn was investigated by electron backscatter diffraction (EBSD). Prior to EBSD, the sample was embedded in a conductive resin, and the cross section was polished to a mirror finish using aluminum powders with grain sizes of 1, 0.3 and 0.1 µm, after which etching was performed by Ar ion milling. Using EBSD, the crystallographic orientations of the reference direction (RD, i.e., the direction normal to the surface of the deposited Zn) were examined. The surface roughness of deposited Zn was evaluated by centerline average roughness Ra [Japanese Industrial Standard (JIS) B 0601] using a SURFCOM 1500DX–3DF instrument (Tokyo Seimitsu Co.). The surface roughness was evaluated at a cutoff wavelength of 0.8 mm, measurement length of 5 mm, and measurement rate of 0.15 mm·s−1 based on JIS.
Figure 1 shows the total polarization curves for Zn deposition from the solutions containing 10 mg·dm−3 of organic additives. The total polarization curve rose at the potential region nobler than the equilibrium potential for Zn deposition (−0.76 V), regardless of the presence of an additive, and greatly shifted to a less noble potential direction at the current density range of 50–200 A·m−2. When the potential reached the equilibrium potential for Zn deposition, the total polarization curve rose up again. The current density at the potential region nobler than −0.76 V results from the hydrogen evolution, while the rise of the total polarization at the potential less noble than −0.76 V is attributed to the beginning of Zn deposition. The effect of the additive on the Zn deposition at the potential region less noble than −0.76 V is explained in Fig. 2. On the other hand, the hydrogen evolution at the potential region nobler than −0.76 V is suppressed by the addition of PEG and glue. The suppression effect on the hydrogen evolution was smaller with small-molecular-weight PEG (200) than those obtained with high-molecular-weight PEG. Furthermore, the suppression effect of glue on the hydrogen evolution was larger compared to the PEG of the same molecular weight (6000). The total polarization curve greatly shifted to a less noble potential direction, and the current density at which Zn begins to deposit, that is, the critical current density for Zn deposition was approximately 200 A·m−2 in additive-free solution, while it decreased to 100 A·m−2 with PEG, regardless of its molecular weight, and further decreased to 50 A·m−2 with glue. Since the critical current density for Zn deposition decreases with the suppression of hydrogen evolution,13–15) it decreases due to the suppression of hydrogen evolution with additives.
Effect of PEG and glue on the total polarization curve for Zn electrowinning solutions. (● additive-free, ▲ Glue M.W. 6000, ○ PEG 200, △ PEG 2000, □ PEG 6000, ◇ PEG 35000, Concentration of additives: 10 mg·dm−3)
Partial polarization curves for Zn deposition in the solutions containing glue and various molecular weights of PEG. (● additive-free, ▲ Glue 6000, ○ PEG 200, △ PEG 2000, □ PEG 6000, ◇ PEG 35000, Concentration of additives: 10 mg·dm−3)
Figure 2 shows the partial polarization curves for Zn deposition from the solutions containing 10 mg·dm−3 of additives. The partial polarization curve for Zn deposition shifted to a less noble potential region with the addition of PEG and glue compared to that of the additive-free solution, indicating the presence of polarization with PEG and glue. With the addition of PEG, the polarization effect was small for the molecular weight of 200, while it increased with the molecular weight above 2000, regardless of the magnitude of molecular weight. Comparing PEG and glue with the same molecular weight of 6000, the polarization effect was almost identical.
Figure 3 shows the partial polarization curves for hydrogen evolution during Zn deposition from the solutions containing 10 mg·dm−3 additives. The effect of additives on the hydrogen evolution at the potential region nobler than −0.76 V is mentioned in the total polarization curve shown in Fig. 1. On the contrary, at the potential region less noble than −0.76 V, the effect of additives on the hydrogen evolution was rarely observed. On the other hand, from −0.65 V to −0.78 V, the current density decreased despite the potential shifted to a less noble direction. This decrease in the current density was significant in the additive-free solution. The hydrogen evolution is greatly suppressed by the deposition of intermediate Zn(OH)2 adsorbed on the cathode at the critical current density for Zn deposition.13–15) The decrease in the current density for hydrogen evolution at approximately −0.78 V is attributed to the adsorption of intermediate Zn(OH)2.
Partial polarization curves for H2 evolution in the solutions containing glue and various molecular weights of PEG. (● additive-free, ▲ Glue 6000, ○ PEG 200, △ PEG 2000, □ PEG 6000, ◇ PEG 35000, Concentration of additives: 10 mg·dm−3)
Figure 4 shows the current efficiency for Zn deposition from the solutions containing 10 mg·dm−3 additives. The current efficiency increased with current density, regardless of the presence of an additive, and became almost constant at a current density above 500 A·m−2. Although the current efficiency for Zn deposition at 100 A·m−2 was almost zero from the additive-free solution, it significantly increased with the addition of PEG or glue. With the addition of glue, Zn deposited even at a low current density of 50 A·m−2. As mentioned in Fig. 1, the critical current density for Zn deposition was approximately 200 A·m−2 in additive-free solution, while it decreased to 100 A·m−2 with the addition of PEG and further decreased to 50 A·m−2 with the addition of glue. The current efficiency for Zn deposition from 50 to 200 A·m−2 near the critical current density of Zn corresponded to the critical current density of Zn, and it increased with a decrease in the critical current density of Zn. On the other hand, at the current density region above 200 A·m−2, the current efficiency of Zn was the highest in an additive-free solution, and it decreased with the addition of PEG and glue. Further, focusing on the relationship between the current efficiency of Zn and the molecular weight of PEG, the current efficiency was the highest with molecular weight 200 at a current density region above 200 A·m−2, while it was the smallest with molecular weight 200 at 100 A·m−2. The high current efficiency with molecular weight 200 above 200 A·m−2 is attributed to the small suppression effect on the Zn deposition (Fig. 2), while the low current efficiency with molecular weight 200 at 100 A·m−2 appears to be due to the small suppression effect on the hydrogen evolution near the critical current density of Zn (Fig. 3). Comparing the current efficiency with PEG and that with glue, there was rarely a difference between PEG with a molecular weight above 2000 and glue at the current density region above 100 A·m−2. However, as mentioned above, the current efficiency for Zn deposition was higher with glue than that with PEG at a low current density of 50 A·m−2.
Current efficiency for Zn deposition in the solutions containing glue and various molecular weights of PEG. (● additive-free, ▲ Glue 6000, ○ PEG 200, △ PEG 2000, □ PEG 6000, ◇ PEG 35000, Concentration of additives: 10 mg·dm−3)
In some experiments, to decrease the ionic strength of the electrolyte solution, the concentration of free H2SO4 was fixed to be 0.46 mol·dm−3 lower than 1.80 mol·dm−3 of the standard concentration. Figure 5 shows the partial polarization curve for Zn deposition from the solution with a lower ionic strength by decreasing the concentration of free H2SO4. The partial polarization curve for Zn deposition was more polarized with the decreased ionic strength of electrolyte solution, irrespective of the presence of an additive. The activity coefficient of the electrolyte depends on the ionic strength of the solution, and it decreases with increasing the concentration of the solution in a dilute solution, while it increases with the concentration of the solution in a concentrated solution.16) In this study, since the concentrated solution was used, the activity coefficient of Zn2+ ion seems to increase with increasing the concentration of free H2SO4 or ionic strength of the solution. Therefore, Zn deposition from the solution with lower ionic strength by decreasing the concentration of free H2SO4 seems to be more polarized than that from the solution of the standard concentration.
Effect of concentration of H2SO4 on the partial polarization curve for Zn deposition in the solutions containing glue and PEG. (●, ▲, ■: H2SO4 1.80 mol·dm−3, ○, △, □: H2SO4 0.46 mol·dm−3, ●, ○: additive-free, ▲, △: with glue, ■, □: with PEG, Concentration and molecular weight of additives: 10 mg·dm−3 and 6000)
The result obtained under the coulostatic condition (50 kC·m−2) is mentioned in the preceding paragraph, while the current efficiency is examined for Zn deposition for 4 h (8.64 × 106 C·m−2) in this section. Figure 6 shows the current efficiency for Zn deposition from the solutions containing 10 mg·dm−3 additives. Except for the PEG molecular weight of 200, the current efficiency of Zn decreased by 4.5–5% with the addition of PEG and glue. The current efficiency of Zn from the solution containing PEG with molecular weight 200 was almost identical to that from the additive-free solution.
Current efficiency for Zn deposition at 600 A·m−2 for 4 h in the solutions containing glue and various molecular weights of PEG. (Concentration of additives: 10 mg·dm−3)
Figure 7 shows the current efficiency for Zn deposition from the solutions containing various amounts of PEG and glue with molecular weights of 6000. The current efficiency of Zn decreased as the concentration of additives increased, regardless of the type of PEG and glue. With the addition of PEG, the current efficiency significantly decreased with the increase at the concentration from 1 to 5 mg·dm−3. There was rarely a difference in the current efficiency between the addition of PEG and glue. Although PEG and glue have the suppression effect on Zn deposition and hydrogen evolution, at the current density region above the critical current density of Zn, the suppression effect of these additives on the Zn deposition was observed (Fig. 2), but the suppression effect on hydrogen evolution was rarely observed (Fig. 3). Therefore, the current efficiency for Zn deposition at 600 A·m−2 decreased with the addition of PEG and glue.
Current efficiency of Zn deposition at 600 A·m−2 for 4 h in solutions containing various amounts of glue and PEG. (■ Additive-free, ▲ Glue, ○ PEG, Molecular weight: 6000)
Figure 8 shows the current efficiency for Zn deposition from the solutions containing free H2SO4 of different concentrations. The current efficiency of Zn greatly increased with the decrease in the concentration of free H2SO4 from 1.80 mol·dm−3 to 0.46 mol·dm3, regardless of the presence of additives. The current efficiency of Zn decreased with the addition of PEG and glue, even in the solution containing 0.46 mol·dm−3 H2SO4, but the degree of decrease was smaller than that in the solution containing 1.80 mol·dm−3 H2SO4. This indicates that PEG and glue suppress hydrogen evolution as well as Zn deposition in the solution containing low concentrations of H2SO4.
Effect of concentration of H2SO4 on the current efficiency for Zn deposition at 600 A·m−2 for 4 h in the solutions containing glue and PEG. (■ H2SO4 1.80 mol·dm−3, □ H2SO4 0.46 mol·dm−3, Concentration and molecular weight of additives: 10 mg·dm−3 and 6000)
Figure 9 shows the SEM images of the surface of Zn deposited at 600 A·m−2 for 4 h. A lot of traces of evolved hydrogen gas 100–200 µm in size were observed at the surface of Zn (a) deposited from the additive-free solution. The traces of hydrogen gas almost disappeared with the addition of PEG and glue with molecular weight 6000. With the addition of 1 mg·dm−3 glue (b), deposited Zn showed massive crystals, and increasing the concentration of glue to 10 mg·dm−3 (d) resulted in layered Zn platelets perpendicular to the substrate. On the other hand, with the addition of 1 mg·dm−3 PEG (e), the deposited Zn became smooth, and fine particles congregated to form massive crystals, but further increase in PEG concentration (5–10 mg·dm−3) (f, g) led to fine Zn particles.
Surface morphology of Zn deposited at 600 A·m−2 for 4 h in solutions containing various amounts of glue and PEG. [(a) additive-free, (b) Glue 1 mg·dm−3, (c) Glue 5, (d) Glue 10, (e) PEG 1, (f) PEG 5, (g) PEG 10, Molecular weight of additives: 6000]
Figure 10 shows the SEM images of the surface of deposited Zn, which were observed at a higher magnification than those shown in Fig. 9. The Zn deposited from the additive-free solution (a) showed platelets grown parallel to the substrate. The SEM images of the surface of Zn deposited from the solution containing glue shown in Fig. 10(b)–(d) are the enlarged views of the smooth area shown in Fig. 9(b)–(d), which were observed at a higher magnification. The Zn deposited from the solutions containing 1 to 10 mg·dm−3 of glue [Fig. 10(b)–(d)] comprised small platelets with random growth direction. With the addition of 1 mg·dm−3 PEG (e), fine platelet crystals with random orientations were formed, whereas further increasing the PEG concentration (5–10 mg·dm−3) (f, g) resulted in fine platelets that formed pyramidal layers.
Surface morphology except for the concavo-convex area of Zn deposited at 600 A·m−2 for 4 h in solutions containing various amounts of glue and PEG. [(a) additive-free, (b) Glue 1 mg·dm−3, (c) Glue 5, (d) Glue 10, (e) PEG 1, (f) PEG 5, (g) PEG 10, Molecular weight of additives: 6000]
Figure 11 shows the SEM images of Zn deposited from the solutions containing 10 mg·dm−3 PEG with various amounts of molecular weight. For the molecular weight of PEG 200 (a), thin platelet crystals of Zn formed with random orientations. Further increasing the molecular weight of PEG to 4000, 6000 and 35000 (b, c, d) resulted in fine platelets with pyramidal layers. The differences in the surface morphologies of deposited Zn for PEG molecular weights 4000, 6000 and 35000 were negligible.
Surface morphology of Zn deposited at 600 A·m−2 for 4 h in the solutions containing various molecular weights of PEG. (Molecular weight of PEG, (a) 200, (b) 4000, (c) 6000, (d) 35000, Concentration of PEG: 10 mg·dm−3)
Figure 12 shows the crystal orientation of Zn deposited from the solutions containing various amounts of PEG and glue with a molecular weight of 6000. The Zn deposited from the additive-free solution exhibited the orientation of {0001} basal plane of the hexagonal close-packed (hcp) structure. This orientation corresponds to the surface morphology that Zn platelets grow parallel to the substrate, as shown in Fig. 10(a). With the addition of 1 mg·dm−3 glue, the orientation of {0001} plane significantly decreased, while the orientation of $\{ 11\bar{2}2\} $ plane increased. By increasing the concentration of glue to 10 mg·dm−3, the orientation of $\{ 11\bar{2}0\} $ plane further increased. The orientation of $\{ 11\bar{2}0\} $ plane represents that {0001} Zn basal plane is perpendicular to the substrate, indicating that Zn platelets are perpendicular to the substrate, as shown in Fig. 9(d). On the other hand, with the addition of 1 mg·dm−3 PEG, the orientation of {0001} plane significantly decreased, while the orientation of $\{ 11\bar{2}2\} $ plane increased. Further increasing the concentration of PEG to 5 and 10 mg·dm−3 gave Zn with only $\{ 10\bar{1}1\} $ orientation, indicating that the angle between {0001} plane and substrate surface is 65°. The addition of 5 and 10 mg·dm−3 PEG led to the pyramidally layered Zn platelets [Fig. 10(f), (g)], which can be attributed to {0001} Zn basal plane layering at an angle of 65° to the substrate.
Crystal orientation of Zn deposition at 600 A·m−2 for 4 h in solutions containing various amounts of glue and PEG. (● 0002, ▲ $10\bar{1}3$, ■ $11\bar{2}2$, ◆ $10\bar{1}1$, ○ $11\bar{2}0$, △ $10\bar{1}0$, Molecular weight of additives: 6000)
Figure 13 shows the crystal orientation of Zn deposited from the solutions containing 10 mg·dm−3 PEG with various molecular weights. Adding PEG with molecular weight 200 significantly decreased the orientation of {0001} plane, while the orientation of $\{ 11\bar{2}2\} $ plane increased. Increasing the molecular weight of PEG to 4000, 6000 and 35000 led to Zn with only $\{ 10\bar{1}1\} $ plane orientation. The difference in the crystal orientation of deposited Zn was rarely observed among molecular weights of PEG above 4000. This corresponds to the surface morphology shown in Fig. 10.
Crystal orientation of Zn deposition at 600 A·m−2 for 4 h in solutions containing glue and various molecular weights of PEG. (● 0002, ▲ $10\bar{1}3$, ■ $11\bar{2}2$, ◆ $10\bar{1}1$, ○ $11\bar{2}0$, △ $10\bar{1}0$, Concentration of additives: 10 mg·dm−3)
Figure 14 shows the crystal orientation mapping images by EBSD of cross section of Zn deposited from the solutions containing 10 mg·dm−3 PEG and glue with molecular weight 6000. For the EBSD, the crystallographic orientations of the RD (i.e., the direction normal to the surface of the deposited Zn) are shown. In the solution without an additive (a), the deposited Zn exhibited a non-oriented dispersed type of fine crystals at the initial stage of deposition, and at a thickness above 50 µm, the deposited Zn shifted to fibrous-type large crystals with the ⟨0001⟩ orientation. With the addition of 10 mg·dm−3 glue (b), the fine fibrous-type crystals with the $\langle 10\bar{1}1\rangle $ orientation were observed in the first-half stage of deposition, then shifted to the inclined fibrous-type crystals with $\langle 11\bar{2}0\rangle $ and $\langle 10\bar{1}0\rangle $ orientations from the middle of the deposition onward and largely grew to the surface. This orientation of inclined fibrous-type crystals represents the orientation of platelets crystals perpendicular to the substrate shown in Fig. 9(d). With the addition of 10 mg·dm−3 PEG (c), the deposited Zn comprised both fine fibrous-type crystals with the $\langle 10\bar{1}1\rangle $ orientation and granular crystals. The crystal orientation of the cross section of deposited Zn evaluated by EBSD corresponded to the crystal orientation of the surface of deposited Zn shown in Figs. 12 and 13.
Crystal orientation mapping images by EBSD of Zn deposited at 600 A·m−2 for 4 h in solutions containing glue and PEG. [(a) Additive-free, (b) Glue, (c) PEG, Concentration and molecular weight of additives: 10 mg·dm−3 and 6000]
The crystal orientation of deposited Zn depends on the overpotential for deposition and changes from the {0001} plane to the $\{ 10\bar{1}1\} $, $\{ 11\bar{2}0\} $, and $\{ 10\bar{1}0\} $ planes (in that order) with increasing deposition overpotential.17–19) This represents that Zn platelets grow parallel to the substrate under low deposition overpotential, but they are inclined to the substrate with increasing deposition overpotential. In this study, the overpotential for long-term electrolysis at 600 A·m−2 was 48 mV for the additive-free solution, while it was 64 mV for the solutions containing 10 mg·dm−3 glue or PEG with molecular weight 6000. The preferred orientation of {0001} plane of Zn deposited from the additive-free solution is attributed to the low overpotential for deposition. On the contrary, the preferred orientations of $\{ 11\bar{2}0\} $ and $\{ 10\bar{1}1\} $ planes in the solutions containing glue and PEG, respectively, appear to be due to the increase in the overpotential for deposition because of these additives (Fig. 2). However, although the overpotential for deposition from the solution containing PEG was almost identical with that from the glue-containing solution (Fig. 2), their crystal orientations were different, which cannot be explained by deposition overpotential alone. Two-dimensional nucleation on various crystal planes is reported to change regardless of the overpotential when additives are adsorbed on the cathode.20–23) The PEG and glue in this study affect the nucleation and growth rate of specific crystal planes. When the overpotential for metal deposition increases, the nucleation rate of the deposited metal becomes larger than the growth rate of the metal, thereby decreasing the crystallite size.24–26) The PEG and glue decrease the particle size of Zn. Since these additives increase the overpotential for deposition, the decrease in the particle size of deposited Zn can be explained by the overpotential theory.
Figure 15 shows the surface roughness of Zn deposited from the solutions containing various amounts of PEG and glue with molecular weight 6000. The surface roughness of deposited Zn significantly decreased with the addition of PEG and glue. This is attributed to the decrease in trace of hydrogen gas and Zn platelet size. Comparing the addition of PEG and glue, the surface roughness was smaller with PEG. With the addition of PEG, the surface roughness decreased by increasing the concentration to 5 mg·dm−3. This decrease in surface roughness is due to Zn platelets becoming fine and growing pyramidally. With the addition of glue, the surface roughness was larger with the concentration of 10 mg·dm−3 than that with additive-free solution, which is attributed to some Zn platelet crystals being perpendicular to the substrate [Fig. 9(d)].
Surface roughness of Zn deposited at 600 A·m−2 for 4 h in solutions containing various amounts of glue and PEG. (■ Additive-free, ▲ Glue, ○ PEG, Molecular weight: 6000)
From the perspectives of current efficiency for Zn deposition and surface roughness of deposited Zn, PEG can be substituted for glue at a current density above 100 A·m−2 in Zn electrowinning. However, at a low current density (50 A·m−2) near the critical current density for Zn deposition, the current efficiency for Zn deposition from the solution containing PEG was lower than that from glue-containing solution. (Fig. 4) In Japan, the commercial production of Zn electrowinning is operated at approximately 600 A·m−2 at night since the cost of electricity is low, whereas it is operated at approximately 50 A·m−2 where the dissolution of deposited Zn does not occur during the day. The effect of glue and PEG on the critical current density for Zn deposition on the Zn electrode and the difference in the current efficiency of Zn at a low current density near the critical current density of Zn between glue and PEG are required to be investigated further in the future.
The effects of polyethylene glycol (PEG) and glue on the deposition behavior of Zn from the electrowinning solutions and its crystal structure were investigated. With the additions of PEG and glue, the evolution of hydrogen was suppressed at the current density region less than the critical current density for Zn deposition, decreasing the critical current density of Zn. The degree of decrease in the critical current density of Zn was larger with glue than that with PEG. The current efficiency for Zn deposition increased with the addition of PEG and glue at the low current density region because the critical current density of Zn decreased with additives. Since the additives suppressed Zn deposition more than the hydrogen evolution at the high current density region, the current efficiency of Zn decreased with increasing concentration of additives in solution. At the high current density region, there was a minor difference in the current efficiency of Zn between PEG and glue. The effect of the molecular weight of PEG on the current efficiency of Zn was rarely observed for molecular weights above 2000. With the addition of PEG, the deposits became fine platelet crystals with $\{ 10\bar{1}1\} $ as the preferred orientation and pyramidal layers, while the deposits had preferential $\{ 11\bar{2}0\} $ orientation, and the platelet crystals grew perpendicularly to the substrate with the addition of glue. The surface roughness of deposited Zn decreased with additives, and it was smaller with PEG than that with glue.
