Lead-silver anode behavior for zinc electrowinning in sulfuric acid solution

2.1 Basis of zinc extractive metallurgy

Electrowinning is a process of transferring a metal ion in the electrolyte to an elemental metal by the passage of electrical current through a conducting circuit consisting of electrolyte, anode, cathode, and connecting wire. The basic features of an electrowinning cell are shown in Figure 1. The zinc electrolyte is contained in an electrolytic cell. The cell is fitted with electrodes that are immersed in the electrolyte. The external electrical circuit connected to the electrodes causes an electrical current to flow in a controlled manner between the electrodes through the electrolyte (Woollacott & Eric, 1994). These results in electrochemical reactions occur at the interfaces between the electrolyte and the electrodes. Zinc is deposited on the cathode through the transfer of electrons to zinc ions (Nguyen & Atrens, 2009).

Figure 1:

Basic features of an electrowinning cell at 38°C (Nguyen & Atrens, 2009). Permission granted by T. Nguyen.

In zinc electrowinning, zinc is deposited on the cathode, and oxygen evolution is on the anode. Inert Pb-alloys and aluminum are often used as anode and cathode, respectively. The main reaction on the cathode involves the conversion of the zinc ions to a zinc metal by an electron-transfer process. There are also other reactions on the cathode beside the hydrogen evolution during the zinc electrowinning process due to the presence of other ionic species in the zinc electrolyte.

The products during zinc electrowinning are (i) zinc metal at the cathode, (ii) oxygen gas at the anode, and regenerated sulfuric acid in the solution (Nguyen, 2007). In practice, a considerably large amount of energy is required due to overcome factors such as electrical resistance in the circuit, ionic resistance, as well as polarization at the surfaces of the anode and the cathode. These occurred voltage requirements are called overpotentials.

As obvious economic reasons, industrial practice is particularly concerned for the energy efficiency of electrolysis and the quality of the zinc deposit. Other important factors such as design, cleaning anodes and cells, life period, and possibly premature disposal of certain anodes are considered to minimize production costs. Manufacturers have found it appropriate to clean the anodes weekly, then leading to the disruption of the progress of the cathodic process. It is important to indicate that in a similar stop, during the period of anode packaging, the Faraday efficiency of zinc deposition decreases, leading to zinc contamination by lead (Nguyen, 2007).

2.2 Behavior of pure lead in sulfuric acid solution

Figure 2 shows the potential-pH diagram of the Pb-H₂O system; its essential reactions under acidic conditions are (Pourbaix, 1974):

Figure 2:

Potential-pH diagram of the Pb-H₂O system at 25°C (Pourbaix, 1974). Permission granted by M. Pourbaix.

As pure lead anodes have low mechanical behavior and high corrosion rate during zinc electrowinning, it is not applied alone in practice. However, the pure lead reactions in sulfuric acid should be considered.

The following reactions in sulfuric acid electrolytic solution show the formation of PbSO₄, which is only slightly soluble (about 3 mg/l lead at 28°C) and lead oxide. The main reactions to be considered in an acid medium are (Barnes & Mathieson, 1963):

2.3 Potentiokinetic curves of pure lead in sulfuric acid solution

Figure 3 shows a typical polarization curve obtained from a lead electrode in a sulfuric acid solution. The plateau in the solution is clearly observed, in which PbSO₄ is with negative voltages with respect to ENH. Then, the electrode is passive, and the current becomes low, except for a slight re-increase in the current of +50 mV/ENH. The current remains low until voltages reached at above +1900 mV/ENH, and finally, the release of oxygen was observed (Dawson, 1979).

Figure 3:

Potentiokinetic curve showing qualitatively the overall behavior of pure lead anode in a sulfuric acid solution bubbled with air (Dawson, 1979). Permission granted by J.L. Dawson.

The construction of the anode layers in the field at low voltages, corresponding to potentials between 900 and 1100 mV, is independent of the nature of the substrate, regardless of the composition of the electrolyte. A pure lead anode behaves in the same way and shows the same effect as Pb-Ag alloy (0.5%). It can, therefore, conclude that the Ag plays no role in the construction of anodic oxide coatings in the mentioned potential range (Varela et al., 1995).

2.4 Galvanostatic study of pure lead in sulfuric acid solution

Figure 4 shows the evolution of galvanic voltage of anode during a switching constant current density (0.5 mA/cm²) on lead alloy anode electrodes. It seems that the first voltage remains for a time t_p, and the potential level is set by the couple of Pb/PbSO₄/PbO₂ (with a peak of about 0.15 V) and then increases slightly when the oxygen evolution becomes visible. There is first, a removal of absorbed oxygen and, then, decreasing the voltage of the electrode to drop through a series of intermediate levels with more or less marked stages (Pavlov, 1984).

Figure 4:

Curve showing the anodic galvanostatic voltage evolution of a pure lead electrode in a deaerated concentrated sulfuric acid solution at J=0.5 mA/cm² (Pavlov, 1984). Permission granted by D. Pavlov.

3.1 Electrochemical behavior of pure lead

In the electrorefining of metals in sulfuric acid solution, it is recommended that the anodes must have the following characteristics (Msindo, 2010): (a) Inertia, or at least good resistance to protect the anode from corrosion in the electrolyte; (b) good electrical properties, physical and mechanical performances; (c) the cathode of minimum contamination by its decomposition or its reaction products; (d) low prices.

In the zinc industry, lead is usually used as anodes. Lead metal as insoluble anodes are the most common anode material for Zn electrowinning in sulfuric acid solutions. During electrolysis, lead oxidizes to its highest valence, and then, the active surface of the electrode is covered with a deposit of conductor PbO₂, which is adhering and smooth on the surface of lead. However, pure lead is weak material, and it tends to creep and warp during use in electrowinning (Ivanov et al., 2000)). Lead is a good electrical conductor (=20.64·cm) and shows good chemical inertness. It changes instantly to passive state on contact with many corrosive environments (e.g. chromates, sulfates, carbonates, phosphates).

Lead was widely used in some processes, such as electrowinning, operating in sulfuric acid in the type of pure, alloyed with silver (95% Pb-0.5% Ag), which decreases the oxygen evolution overvoltage or with other metals (e.g. Sn, Bi, Sb, Te) to improve its mechanical properties and corrosion resistance. Indeed, the metal corrosion rate at 25°C in contact with sulfuric acid at 50% is 130 mm/year. When subjected to an anodic current density of 1 kA m⁻² in the acid, it was found that its low melting point (327.5°C) is associated with a high coefficient of thermal expansion (30×10⁻⁶·K⁻¹). This low point explains its lack of mechanical strength, which causes problems during operational periods of electrolysis. Lead anodes covered with PbO₂ supplanted the graphite in the production of chlorates and perchlorates in 1950. They were used in hydrometallurgy as an oxygen evolution anode for electroplating of zinc and copper from sulfuric acid solutions (Barnes & Mathieson, 1963).

Lead anodes are not stable as they will dissolve in sulfuric acid solution and contribute in lower metal purity due to the corrosion of lead in sulfuric acid solution, and they also exhibit a high oxygen overvoltage, which increases the cell overvoltage and decreases energy efficiency (Marshall & Haverkamp, 2012). During polarization, the lead anode forms a stable protective coating of PbO₂ layer on the surface of the anode. The formation of the stable PbO₂ layer is a slow process on the normally utilized lead-silver anodes. In the process of forming a PbO₂ layer, a competitive oxidation reaction also takes place at the surface of the anode to cause small fine particles of PbO₂, which are not adherent, to detach from the surface of the anode and become suspended in the electrolyte from where they contaminate the cathode with lead (Prengaman & Siegmund, 2000).

However, pure lead is very soft, and the anodes have an effective surface area. It is undergoes significant internal tensions due to corrosion products. Also, it tends to deform because of creep. This can lead, on one hand, to short circuits and, on the other hand, a breakage or cracks in the protective anodic layer, involving an increase in the anodic corrosion of lead. To improve its mechanical strength and corrosion performance, lead is alloyed with silver (Newham, 1992).

3.2 Electrochemical behavior of lead alloy anodes

It can be stated that the first goal of the alloying elements is to improve the mechanical properties of lead, which is known to flow under its own weight at room temperature. Therefore, it is necessary to seek both to increase its elastic limit and breaking load, to obtain a higher hardness and, also, in some cases, to improve its flow ability (Ivanov et al., 2000)).

The first alloys considered for both batteries and electrowinning were the lead-antimony alloy (4–12% Sb), which were already widely known in the printing industry. They remained a very frequent use. For reasons of low ability, tin-lead alloys (0.4% 0.18) were considered but did not seem to have been met with great success. Some works show that lead alloys with alloy elements have interesting properties for corrosion resistance. There is, moreover, a series of patents covering these alloys and others involving copper, silicon, calcium, cerium, and aluminum. Ternary lead alloys are also considered. The results are often disappointing in practice, but show considerable influence of microstructure on the corrosion process (Pavlov, 1984).

Table 1 presents the influence of alloying elements with lead on corrosion rate. Some are commonly used in electrolytic zinc extraction, while other alloys show negative impact on corrosion rate.

Table 2 shows the effect of chloride ions on the lead corrosion in the presence of different alloys, in the case of 100 mg/l of chloride ions, less mass loss for lead-silver alloys was observed for Pb-1% Ag than that of 500 mg/l (Kiryakov & Stender, 1951). Generally, the presence of 500 mg/l shows important corrosion rates of corrosion for the studied alloys.

Anodic dissolution is an important factor to be considered as the dissolved Pb²⁺ ions migrate and diffuse, and can co-deposit with zinc on the cathode. The lead dissolution rate was analyzed according to different parameters, primarily by weight loss measurements. The beneficial role of the presence of silver in a lead anode material is widely demonstrated in numerous studies (Tables 2 and 3).

Pavlov (1984) showed, in particular, that the presence of silver in lead improves corrosion resistance and reduces the anode voltage, based on the fact that using a pure lead electrode in a solution containing silver ions, generates, on the contrary, an increase in the rate of lead dissolution, while the decrease in the overvoltage is continued. The action mode of silver on the anodic process depends differently on whether this element is present in solution or in the substrate (Pavlov, 1984). Table 3 shows the effect of chloride ions on the dissolution of lead alloys in H₂SO₄ solution. It can be concluded that there is no dramatic effect of chloride ions in the zinc electrolyte (Kiryakov & Stender, 1951).

Table 4 shows that the dissolution rate of lead increases with time. It was found that Pb-1% Ag-1% Ca had the least weight loss among the seven samples followed by Pb-Ag1-Ti2, Pb-Ag1-Se1, Pb-Ag1-Se0.1, Pb-Ag1-Ti0.1, Pb-Ag1-Ca0.1, and Pb-Ag1 after 334 and 500 h of polarization.

The commonly used anodes in copper electrowinning are the Pb-6% Sb alloys. This lead-antimony anode is extremely dimensionally stable and is the dominant anode alloy for use in copper electrowinning. It was found that Sb increases the rate of oxidation of Pb at the grain boundaries and at the surface (Prengaman, 2005). Sb promotes the growth of hard, dense, glassy α-PbO₂ (Prengaman, 1984). An increase in the Sb content increases the rate of corrosion (Tshimwanga et al., 2011) and increases the thickness of the oxide layer (Ivanov et al., 2000)). However, antimony lowers the anode oxygen overvoltage (Nguyen & Atrens, 2009). Also, it was reported that the dissolution of Sb in the Pb-Sb alloy is helpful in the formation of PbO and the lead sulfates underneath the PbSO₄ layer in anodic polarization. It also promotes the transformation of PbO→3PbO·PbSO₄·H₂O→PbO·PbSO₄·H₂O→PbSO₄ in the negative potential region, and the large PbSO₄ crystals are easily formed in the transformation (Sun & Guo, 2000).

Studies showed that the addition of Co²⁺ to the electrolyte in a conventional cell leads to an even more pronounced decrease in oxygen overvoltage on the Pb-Sb anode. It was found that the addition of Co²⁺ ions to the electrolyte significantly reduces the corrosion rate of the anode, as well as the cathode contamination with Pb (Eggett & Naden, 1975; Gendron et al., 1975). It is reported that after 96 h of polarization at 45°C, the potential on the Pb-Co₃O₄ anode was about 40 mV lower than that on Pb-Sb. Also, it was found that after polarization, the corrosion rate of the Pb-Co₃O₄ anode was lower by 40% than that of the Pb-Sb anode (Hrussanova et al., 2004).

New alloys of Pb with Ca and/or Sr were developed to reduce the rate of spalling of PbO₂ from the anode compared with the conventional Pb-Sb and, thus, to decrease the Pb level in the cathode. The quantity of Ca and Sr in these alloys must be carefully controlled to produce a fine-grained structure with minimal alloy segregation (Prengaman, 1980). The Pb-Ca and Pb-Sr alloys are relatively weak; for this reason, Sn is added to improve the mechanical properties and to decrease the creep rate. The rolled non-antimony alloys show high corrosion resistance, with more evident effect when Co²⁺ ions are added into the electrolyte. The Pb-Ca-Sn anodes are strong, uniform, dross, and crack free, with a fine-grained structure. Moreover, they show uniform corrosion (Hrussanova et al., 2002). Compared to the conventional lead antimony anode, the lead anode with calcium has a lower rate of spalling of PbO₂, and therefore, there is a lower lead level in the copper cathode (Prengaman, 1984). However, the production of the ternary Pb-Ca-Sn alloy is more complicated than the binary lead alloys.

Other lead alloy with low levels of Sb and Sn were developed for use as an anode (Prengaman, 1980). This alloy shows a uniform fine-grained structure. As the Sb content is sufficiently low, there is no interconnecting network of Sb, and corrosion is reduced. There is sufficient Sb, however, to produce the more adherent β-PbO₂ phase, which prevents passivation. It combines the advantages of the ordinary high Sb alloys and the non-antimonial Pb-Sr and Pb-Ag-Ca alloys (Yang et al., 2013a),b).

For lead alloys, the electrocatalytical active layer is lead dioxide, which possesses a high overpotential toward oxygen evolution, thus, causing a significant increase in energy use during metal production. Other problems encountered with lead alloys are high cost of some alloying elements such as silver, corrosion and subsequent contamination of cathodes, bending and warping of anodes, uneven power distribution, and necessity to clean anodes, to remove lead and manganese oxide deposits (Ivanov et al., 2000)).

3.3 Influence of fabrication mode on the metallurgy of Pb alloys

When the anodes are chosen to be used in zinc electrowinning industry, it is important to optimize their characteristics. They are manufactured by the industrial method of casting molten metal (550°C). There are two types of molding that give either a smooth or serrated anode (Prengaman & Siegmund, 2000). The advantage of the casting anode is to increase the active surface of the electrode, such as its robustness, and also reduce the amounts of lead and silver dissolved during electrolysis. It is due to better adhesion of the anodic deposit. It has the disadvantage of making it more difficult for anode cleaning.

Thickness of oxides must have a minimum value, typically 8 mm, and corrosion rate should be well controlled. To minimize the anodic dissolution of lead, some industrial methods are used to treat the electrodes to accelerate the formation of a compact anode layer, adherent to the substrate before performing in industrial electrolysis (Prengaman, 1980).

Prengaman and Siegmund (2000) reported that anodes with oriented texture in the rolling direction could produce a grain structure, which is more difficult to corrode and leads to an adherent layer of PbO₂. For example, the rolling surface of anode technologies developed a method to produce a surface on the anode, through controlled rolling and alloying, to which a hard, dense layer of PbO₂ adheres very quickly. So, the different levels of rolling during fabrication can influence the microstructure of these Pb-Ag alloys (Prengaman & Siegmund, 2000). Petrova et al. (1996) reported that the rolled Pb-Ag alloys possess a higher corrosion resistance and lower anodic polarization compared to the cast lead-silver ones due to the structural fineness and homogeneity of the plastic deformed anodes (Petrova et al., 1996). Pb₃Ca precipitation in the solid solution is caused by the plastic deformation of ternary alloys with a calcium content of 0.06%. The hot-rolled alloys form an α-solid solution of Pb₃Ca with fine-grain structure, deformed through the rolling direction. The cold-rolled alloys possess a clearly expressed oriented structure also through the rolling direction. It was stated that the ternary rolled (Pb-0.5% Ag-0.11% Ca) alloys have equal corrosion and electrochemical properties to that of the practically used Pb-Ag anodes. However, the Pb-0.2% Ag-0.2% Sb anode showed good current efficiency with specific energy consumptions similar to traditional anodes in the early days of the run, but presents limited chemical stability, which inhibits its use for a period longer than 14 days (Lupi & Pilone, 1997).

There are two methods to roll the Pb-Ag-Ca anodes: hot rolled and cold rolled. Also, it can be observed that the hot-rolled alloys form an α-solid solution of Pb₃Ca with fine-grained structure, deformed through the rolling direction. The cold-rolled alloys possess clearly expressed oriented structures also through the rolling direction (Petrova et al., 1996). Generally, it can be stated that cold-rolled and hot-rolled Pb-Ag alloys possess higher corrosion resistance and lower anodic polarization than cast lead-silver alloys.

The Pb-Ag-Nd anodes were manufactured by two methods, the cast and rolled fabrications. It was found that the anodic layer formed on the rolled anode is more stable toward the electrolyte than that formed on the cast anode, reducing the corrosion of metallic substrate during current interruption. Hence, the rolled anode has a higher corrosion resistance than the cast anode. However, the anodic potential of the rolled anode was slightly higher than that of the cast anode after 72 h of galvanostatic polarization (Zhong et al., 2015).

3.4 Stability and corrosion rates of lead-silver anodes

Small amounts of Ag (0.7–1.0%) alloyed with lead decrease the oxygen overvoltage and increase the corrosion resistance of the material (McGinnity & Nicol, 2014). Also, the beneficial effect can be achieved by better mechanical properties or by a more fine-grained and more homogeneous microstructure. Pb-Ag alloy anodes were found to be the predominant anode materials for zinc electrowinning in acid sulfate solutions. However, there are also some problems on the lead-silver alloy anode; one is concerned with the incorporation of corrosion products in cathode, which decreases the purity of zinc deposition; the other is that the oxygen overpotential is relatively high, which increases the cell voltage and decreases the energy efficiency (Forsen et al., 1992). In practice, most electrolytic zinc plants are employing Pb-Ag alloys containing 0.7–1.0% Ag as the anode material. The resulting benefits are a longer anode life and a lower Pb content in the cathodic zinc (Tikkanen & Hyvarinen, 1969).

The lead-silver alloy anode in sulfuric acid solution bath has many favorable features, for example, alloying of silver in the alloys resulted in the suppression of the anodic oxidation of the materials, decrease in the anode potential, formation of dense oxide layer closely adhering to the electrode, and appearance of β-PbO₂ composition in the anodic oxide layer. Consequently, the lead-silver alloy anodes are considered in electrolytic production of zinc for these 50 years (Umetsu et al., 1985).

The stability of the anodes is related to the cathode quality – the lead contamination level of the cathode deposits. To improve the performance of the lead anodes, some manufactories of insoluble anodes, such as RSR Technologies (USA), changed the composition and the process of the anodes, for example, adding Ca or Sn in the Pb-Ag alloy and rolling the anode plates to diminish the number of the holes, voids, and laps (Prengaman & Siegmund, 2000). However, there are some difficulties in their use that can create problems: one is concerned with the incorporation of lead corrosion products in the cathode, which decreases the purity. Also, the oxygen overpotential becomes relatively high, which increases the cell voltage and decreases energy efficiency. Because of these problems, there is always an interest in finding ways to improve anode performance and minimize the corrosion and overpotential problems (Yu & O’Keefe, 1999).

Silver meets the main requirements – especially the improvement of lead corrosion resistance and decreasing the oxygen overpotential. An appropriate concentration, widely used in practice, is 0.8% to 1%. It has been reported, generally, that the increase in the silver content in the lead alloy leads to a decrease in corrosion and anodic polarization (Osorio et al., 2013). The corrosion resistance of Pb and Pb-alloy anodes (determined by the decrease of their weight) was studied during electrolysis in 1 m H₂SO₄ at a current density of 400 A/m² and 25°C. The experiments show that the Pb-Ag (1%) alloy is considerably more resistant compared to pure Pb and Pb-Ag (0.5%). The resistance of Pb-Ag alloy containing a large amount of Se (2%) increases. Low concentrations of 0.5% Ca or 0.2% Se improve the resistance, but their action decreases with time (Hine et al., 1988).

Several authors showed that the lead-silver alloys (usually less than 1% by weight) have excellent corrosion resistance properties. It appears that the oxygen overpotential of these alloys is lowered by approximately 0.15 V relative to pure lead, and this could justify the remarkable stability of the passive film (Kiryakov & Stender, 1951).

The alloyed silver has the advantage of reducing the anode overvoltage and decreasing the corrosion rate (Gui et al., 2015). However, it was further established that the silver migrates through the anode layer. During electrolysis, Pb dissolution from the lead anode is observed.

It is important to examine the corrosion rate as function of current density and content of alloyed silver. Figure 5A shows the corrosion rate as a function of current density and gives the change in corrosion rate of pure lead in sulfuric acid, while Figure 5B illustrates the variation in lead corrosion rate in sulfuric acid in the presence of various Ag quantities of lead (0.5%, 1%, and 2%). For both cases, the corrosion rate is almost zero at zero current and increases with the current density up to 5000 A/m², thereby, indicating that the current density is an important factor. On the other hand, the corrosion rate is almost unchanged by changing the current density in the range above 500A/m² (Hine et al., 1988) Therefore, the electrolysis was conducted at high current densities, mostly at 600 A/m².

Figure 5:

Corrosion rate in 1 m H₂SO₄ at 30°–60°C as function of current density: (A) pure Pb; (B) lead-Ag anodes (Hine et al., 1988). Permission granted by F. Hine.

From the results shown in Figure 5, the positive effect of silver (Ag) on the corrosion rate can be stated as the higher the percentage of silver in the lead alloy, the lower corrosion rate for the lead alloys (Figure 5B) (Hine et al., 1988).

3.5 Development of zinc electrowinning anodes

To save energy and improve the quality of zinc deposits, researchers try, presently, to find new anode materials instead of silver in Pb anodes. Li et al. (2011) found that MnO₂ content has obvious influence on the corrosion rate of Pb/Pb-MnO₂ composite anodes. Higher MnO₂ content in the anode increased effectively the corrosion resistance. Further, the corrosion process of Pb/Pb-MnO₂ anodes will stand a period of heavy intergranular corrosion, which could lead to form a preferable stationary, dense, and fine-grained PbO₂-MnO₂ composite anodic layer compared to that of the Pb and Pb-Ag anodes. Aluminum is employed as a substrate because of its good electrical conductivity and mechanical strength, and then, this is coated by lead as the basic material.

The Pb-Ag anode is widely used as they have good electrochemical behavior in zinc electrowinning. As Ag is an expensive metal, extensive research in its replacement was carried out or decreasing its quantity in the Pb alloy without changing the properties of the standard anodes in industry. To improve the properties of Pb-Ag anodes, cobalt attracted many investigations to examine its influence on the performance of lead-based anodes. For example, coatings of Pb-Ag-Co and Pb-Co were introduced (Yang et al., 2014). Recently, Zhang et al. (2014) found that several other metal elements such as Ag, Ca, Co, Bi, Sr, and Sb could improve its corrosion resistance and decrease the oxygen evolution overpotential.

Most currently proposed systems utilize an oxide-coated anode (OCA) based on a titanium substrate coated with a material that is catalytic toward oxygen evolution. The most common base used for OCAs is titanium (Brungs et al., 1996). Dimensionally stable anodes (DSA) consist of mixed metal-oxide coatings, usually on titanium or nickel substrates, which were attributed to their chemical stability that allows them to maintain nearly constant dimensions during the life of the anode even when operating in an environment with low pH values (Msindo, 2010). These oxide coatings could include tantalum oxide (Ta₂O₅), iridium oxide (IrO₂), ruthenium oxide (RuO₂), and tin oxide (SnO₂) (Morimitsu et al., 2000). Titanium anode coated with Pt, IrO₂, and/or RuO₂ showed the best properties of low-energy consumption and high current efficiency. The modification of titanium electrodes by a metallic oxide may lead to the preparation of the well-known OCAs.

4.1 Formatting and composition of the anodes

4.1.2 Formation of PbO₂

Cyclic voltammetry studies were employed to monitor the appearance of these layers depending on the acidity of the solution and temperature influence on the formation of anodic deposition. The X-ray diffraction is to find an intermediate phase PbO and PbO₂ with two structures α and β (Czerwinski et al., 2000).

The crystallographic structures of these phases and their physical mechanism are described in the literature. The α and β phases of PbO₂ have orthorhombic and tetragonal structures, respectively, and are both very conductive, unlike the lead sulfate, which is an insulator. Certain studies indicate that PbO is orthorhombic, and the others consider it as tetragonal. These studies also indicate that the tetragonal PbO observed during the oxidation of the lead anode is sometimes a semiconductor n-type with a gap equal to 1.9 eV, sometimes a semiconductor p type. According to Burbank (1957), it was found, in fact, that the semiconductor n or p gets the type properties according to the chemical composition of the aqueous medium in which it is formed.

We now recall the different formation mechanisms of these acidic medium layers involved when the field was changed from PbSO₄ to PbO₂. The concern for the formation and growth of PbO₂ essentially resulted from scanning electron microscopy and transmission, as well as X-ray diffraction methods of investigation. Table 5 summarizes the various stages of formation of lead corrosion layers as described by Pavolv and its focus on this section whose potential is greater than 900 mV, where two crystal structures of PbO₂, α and β, were obviously observed (Pavlov, 1984).

Table 5:

The different stages of formation of corrosion layers of lead, according to the potential I/ESS (Pavlov, 1984).

Pb/PbSO4/H2SO4	Pb/PbO/PbSO4/H2SO4	Pb/PbOn/PbSO4/H2SO4	Pb/α&βPbO2/H2SO4	Pb/α PbO2/H2SO4
−0.97<E<−0.4 V/ESS	−0.4<E<0.9 V/ESS	0.9<E<1.2 V/ESS	1.2<E<1.5 V/ESS	E>1.5 V/ESS
Maintenance of electroneutrality of the solution to pore bottom→PbO		Vacancy mechanism→PbOn	(n>1.4)→α PbO2 Dissolution mechanism Oxidation precipitation β PbO2	α PbO2 covers the entire surface of the lead electrode

From 900 mV, α-PbO₂ is formed on the PbO/PbSO₄ interface by a vacancy mechanism. Oxidation of PbO_αPbO₂ involves diffusion through the porosity of PbSO₄ to PbO, O atoms, and/or O²⁻ radicals formed at the conductive interface layer/electrolyte, and migration into the sub-layer of PbO, from the metal to the solution of oxygen vacancies PbO_v²⁺. These processes are illustrated in Figure 6 (Pavlov, 1984).

Figure 6:

Construction under layer of PbO by a vacancy mechanism (Pavlov, 1984). Permission granted by D. Pavlov.

Figure 7 shows β-PbO₂ schematically for the region between 1200 mV and 1500 mV and the infiltration of the electrolyte through the porosity of the deposits. Diagram (A) represents the electrode covered by the layers of PbO and PbSO₄. It can be stated that some conclusions from Figure 7B to F on the PbO₂ form process give rise to a model that describes the formation mechanism of the α phase and β-PbO₂. It is clear that silver promotes the growth of the β phase of PbO₂, but the exact role of this element in the layers of the construction process is unclear. According to the investigation from X-ray diffraction, the composition of the surface film changes as follows: From −306 to 350 mV/SHE: PbSO₄ is only visible, from 350 to 1550 mV/SHE includes tetragonal PbO and PbSO₄ mainly with a light PbO·PbSO₄, 3PbO·PbSO₄·H₂O, 5PbO·H₂O, PbO, and PbO₂ orthorhombic α (Pavlov & Rogachev, 1986). Above 1550 mV/ENH, the amount of α-PbO₂ increases at the extent of the other constituents; above 1850 mV/ENH, β-PbO₂ appears and becomes predominant. In addition, Valeriote et al. (1991) recently showed that the potential of the PbO₂ area is formed, the current density is controlled sequentially by at least three scattering mechanisms followed by at least two stages of crystallization and yet other diffusion steps. These authors believe that precipitation begins with PbO₂, α-PbO₂ below +1.7 V/ENH. When the electrode voltage is lower than this value, the reaction stops. Above this value, β-PbO₂ can be formed, and the reaction continues with β-PbO₂ outside the electrode in zinc electrolyte (Valeriote et al., 1991).

Figure 7:

Construction of the anode layers of corrosion: (A) Pb/PbSO₄/H₂SO₄; (B–Ð) Pb/α-PbO₂/PbSO₄; (E, F) formation of β-PbO₂ (Pavlov, 1984). Permission granted by D. Pavlov.

A lead fluoride coating for passivating 0.8% Ag alloy of the lead anodes is formed through a preconditioning process. The anodic polarization in the sulfuric acid solution with F⁻ ion produces a thin F-PbF₂ layer, which affects the catalytic conversion of the PbSO₄ isolation layer in the protective β-PbO₂ layer and hinders the α-PbO₂ crystallization, which causes much less corrosion of the Pb base possibilities (Jaksic et al., 1987).

4.2 The effect of Ag on the formation of β-PbO₂ for lead-silver alloys in acid solutions

It was stated that Ag promotes the occurrence of β-PbO₂ in the oxide film (Pavlov, 1984). The curves in Figure 8A and B show the intensity values for Pb and Pb-2.5% Ag as a function of the anode potential, which were measured after 24 h of exposure. It is evident that on pure Pb, α-PbO₂ is the main constituent of a fine anodic layer at potentials higher than +1800 mV. Silver increases the relative amount of β-PbO₂ and broadens its stability region (Tikkanen & Hyvarinen, 1969).

Figure 8:

X-ray diffraction intensity vs. electrode potential curves for (A) Pb-2.5% Ag alloy and (B) Pb in 2 N H₂SO₄ at 25°C and 24 h (Tikkanen & Hyvarinen, 1969). Permission granted by M.H. Tikkanen.

Tikkanen and Hyvarinen (1969) stated that there is evidence that alloying Ag into the lead base promotes β-PbO₂ formation. This can increase its relative proportion in the oxide phase and broaden its stability region toward higher anodic potentials. Furthermore, silver helps to transform the morphology from inter-crystalline of the typical pure lead into a more planar one of the lead-silver alloy.

Considering the practical consequence of the results, it is clearly shown that alloying lead with Ag gives rise to a converted material surface with β-PbO₂ more easily than that of pure Pb during the electrolytic process. The stability of the β-PbO₂ film is also increased; it is a factor of utmost importance in the electrolysis of zinc where the anodic current density is a regulating factor (Tikkanen & Hyvarinen, 1969). Ruetschi and Cahan (1958) reported that electrodepositing β-PbO₂ exhibited a stable potential about 7 mV below that of α-PbO₂ in 4.4 m H₂SO_4. β-PbO₂ electrodes, prepared by pressing β-PbO₂ power into a perforated lead sheet, also exhibited potentials lower than those of α-PbO₂ prepared in the same manner. However, these electrodes exhibited limited stability due to the self-discharge effects.

It was reported also that the formation free energy of α-PbO₂ would be higher than that of β-PbO₂ (Bone et al., 1961). Hence, a higher electrode potential would be expected for α-PbO₂ than that for β-PbO_2. The manufacture of rolled samples can influence the α and β PbO₂ layer of Pb-0.8% Ag anodes. Yang et al. (2013a,b) stated that for Pb-0.8% Ag anode, the β-PbO₂ layer can be increased, and α-PbO₂ layer decreased with increasing electrolysis time.

5.1 Effect of pH on lead alloy passivation

Takehara and Kanumura (1987) studied the effect of H₂SO₄ concentration on the processes of PbSO₄ oxidized to PbO₂. They found that the oxidation rate of PbSO₄ increased with ascending H₂SO₄ concentration. The fitted result revealed that the Pb (II) resistance increases with increasing H₂SO₄ concentration, and the growth of the high-resistance PbO film was encouraged with a descending pH value, according to Takehara and Kanumura (1987).

It was reported that Li et al. (2011) employed the electrochemical impedance spectroscopy to investigate the Pb0.8Sn electrode in 4.5 m H₂SO₄ solution at different temperatures. It was found that the semicircle increased with the decrease in pH value, implying the increased film protection. By analysis of the equivalent electron circuit, it can be noticed that the resistance of the PbSO₄ film, PbO film and the transfer resistance increased with an increment of H₂SO₄ concentration.

The influence of H₂SO₄ on the Pb0.8Sn alloy in H₂SO₄ was studied by potentiodynamic measurements. It was revealed that the Pb0.8Sn electrode was in the passive state in all kinds of solutions with a passive potential region from about 0.94 V–1.7 V, the passive potential regions slightly enlarged with increasing H₂SO₄ concentration. While for the steady passive currents of Pb0.8Sn electrode in 0.5 m, 1.5 m, 2.5 m, 3.5 m, and 4.5 m H₂SO₄, the solutions decreased from 4.75E-4, 3.6E-4, 3.08E-4, 2.42 E-4, and 1.77E-4 A cm⁻², respectively. It means that the protection of the passive oxide film on Pb0.8Sn alloy can be increased with H₂SO₄ concentration from 0.5 to 4.5 m (Li et al., 2011).

5.2 Effect of temperature on lead alloy in sulfuric acid solution

By the study of the Pb0.8Sn electrode in 4.5 m H₂SO₄ solution at different temperatures via the potentiodynamic technique, it was found that the Pb0.8Sn alloy was in the passive state from about 0.013–1.5 V. The passive state was related to the formation of the protective film on the PbSn alloy, the passive potential regions significantly decreased with increasing temperature, and the steady passive currents at 25°C, 40°C, 55°C, 70°C, and 80°C were 1.77E-4, 2.02E-4, 2.79E-4, 3.41E-4, and 4.71E-4 A/cm², respectively. It means that they increased with increasing temperature; this indicated also that the protective effect of the anodic film on the Pb0.8Sn alloy decreased with temperature. Also, the electrochemical impedance spectroscopy method was employed to investigate the Pb0.8Sn electrode in 4.5 m H₂SO₄ solution at different temperatures, and it can be observed that the semicircle decreased with an increase in the temperature. Obviously, the corrosion rate of PbSn increased by 166% with an increase in the temperature from 25°C to 80°C (Li et al., 2011).

The effect of temperature on the corrosion rate of the Pb-0.7% Ag anode in sulfuric acid solution was reported. It was obtained that the corrosion rates of the Pb-0.7% Ag anode increased with an increase in temperature. The corrosion rate of the Pb-0.7% Ag anode at 25°C, 38°C, and 55°C were 0.89, 1.04, and 1.51 μA cm^-2 after 5 h of potential decay following 5 h of electrolysis at 50 mA/cm², respectively (Zhang & Houlachi, 2010).

5.3 Influence of cobalt ions on the anodic oxidation of lead alloy

The presence of cobalt ions (500 ppm) in electrolyte changed the structure, morphology, and chemical composition of the surface film from a loose porous film with a substantial PbO₂ content, to a thin dense film. This change in surface film is interpreted as the cause for the decreased rate of oxidation of the lead anode in the presence of cobalt ions. Also, the addition of cobalt to electrolyte facilitates oxygen evolution and improves the anode life service. However, it was found that the presence of cobalt ions (500 ppm) increases the imperviousness of the α-PbO₂ layer in hindering the oxidation of metallic lead to α-PbO₂ and also in hindering the oxidation of PbSO₄ to β-PbO₂ (Nguyen & Atrens, 2009).

6.1 In the presence of manganese in sulfuric acid solution

Zinc electrowinning was performed in a H₂SO₄-ZnSO₄ electrolyte with a significant amount of Mn²⁺ ions. In industrial conditions, the main anodic reaction of oxygen evolution is accompanied by several other reactions, which permit, on one hand, to form the lead oxidation layers (PbSO₄, PbO₂) and, on the other hand, to oxidize the Mn²⁺ ions in the solution (Huang et al., 2010).

The oxidation of Mn²⁺ ions involves the formation of an anodic layer of MnO₂ that influences the morphology of the lead electrode. It also causes the formation of various other ions, complex or simple, in the electrolyte (Ivanov et al., 2000)).

Mn²⁺ ions, which diffuse into solution and arrive to the interface, cause the current dependence of the hydrodynamic condition of the environment. This diffusion partially controls the current condition, as it is also exercised through the layer of MnO₂, and adds it to that of the SO₄²⁻ ions in the layer of PbSO₄ (Yu & O’Keefe, 2002).

The oxidation reaction of Mn²⁺ into the MnO₂ causes transfer resistance through the sub-layer of PbO. Moreover, this charge transfer, which probably involves two successive stages, can also generate an additional capacitive effect (Yu & O’Keefe, 2002; Tunnicliffe et al., 2012).

During polarization, Mn²⁺ ions in the electrolyte could be oxidized to MnO₄⁻ (Vereecken & Winand, 1972). Newham, 1992 established that the corrosion rate of Pb-Ag anodes in 1.8 m H₂SO₄ decreases with the presence of Mn²⁺. It was found that the presence of Mn²⁺ in the electrolyte causes the formation of a thin layer of MnO₂ on the anode. Ivanov & Stefanov (2002) found that addition of 1.5–3 Mn²⁺ g dm⁻³ to the zinc electrolyte can minimize the corrosion of lead anodes and reduce the contamination of the cathodic zinc with lead (Zhang & Hua, 2009). Zhang & Houlachi (2010) found that the addition of manganese sulfate from 4 to 8 g dm⁻³ Mn²⁺ to the zinc electrolyte gave almost the same overpotentials and then increased for 10 and 12 g dm⁻³ for the lead-silver anode. Also, the corrosion rate of the Pb-Ag alloy anode decreased with the MnSO₄ addition. In addition, adding MnSO₄ to the zinc electrolyte could improve the structure of the zinc deposit at a low concentration but decreased the current efficiency by a few percent when the concentration exceeded 10 g dm⁻³ (Zhang & Cheng, 2007). However, higher concentrations of manganese in the electrolyte can cause a significant decrease in current efficiency. Also, with longer operation times, the formation of MnO₂ may increase the corrosion of the lead anode (Yu & O’Keefe, 2002; Schmachtel et al., 2009).

6.2 Effect of chloride ion on lead dissolution

Lead dissolution increased using an industrial electrolyte without manganese, which contains various impurities, including chloride ions that are probably the most dangerous. We can assume that chloride ions attack the anodic layer of MnO₂. This could weaken it until the loss of its protective role (Ivanov et al., 2000)).

An anode with low silver content is much more exposed for dissolution phenomenon in sulfuric acid solution without Mn²⁺ in the presence of chloride ions, which was proved by Pb dissolution of the Pb-Ag (0.52%) anode. The decrease in the percentage of silver in the material engendered an increase in lead dissolution. This increase is more pronounced in zinc electrolysis (Umetsu et al., 1985). The silver content of the material is, therefore, a key factor against anodic dissolution of lead (Ivanov et al., 2000)).

For each type of lead anodes, the presence of Mn²⁺ ions in the electrolyte reduces lead dissolution, even for the lead silver anode with low silver content. These results imply that the anodic oxide layer of MnO₂ is a protective barrier against the leakage of Pb²⁺ ions in solution. However, it was demonstrated that the impurities contained in industrial electrolyte, and perhaps more specifically chloride ions, annihilate the protective layer by the MnO₂ (Ivanov et al., 2000)). Newham, 1992 investigated the effect of chloride ions on the anodic dissolution of lead in sulfuric acid solution. It was shown that in the absence of manganese in solution, these ions do not have much effect, but on the other hand, they cause an increase in the rate of lead dissolution in the presence of manganese in sulfuric acid solution.

Finally, industrial electrolyte showed a similar behavior of the electrode and giving the same results as in the zinc electrolyte with MnSO₄. It can then be concluded that either chloride ions present in the industrial electrolyte or other elements play a role in the construction of the electrode layers at low anodic polarization (Ivanov et al., 2000)). It was reported also that after 15 days of galvanostatic electrolysis in the zinc electrolyte containing 600 mg l⁻¹ of ions, the presence of the chloride ion had significantly an effect on the formation and stabilization of the Pb-0.8%Ag anodic oxide layer; the α-PbO₂ was more stable than β-PbO₂ (Yang et al., 2014).

6.3 The formation of MnO₂ on lead electrode

In the supporting electrolyte containing MnSO₄, the presence of Mn²⁺ ions in solution generates a different behavior of the electrode. Above 940 mV, the oxidation of these ions was corresponding to a type of electrode Pb/PbO/MnO₂. The overall behavior of the electrode will then result in areas of Pb/PbO/PbSO₄ and Pb/PbO/MnO₂, which share the active surface of the anode (Yu & O’Keefe, 2002).

The potential range corresponding to the high polarization begins at 1160 mV vs. SHE, and is characterized by a strong evolution of oxygen evolution reaction from water at the anode. It becomes the major reaction on the lead electrode, covered with a porous conductive deposit of PbO₂ with MnO₂ if the electrolyte contains manganese. By increasing the anodic potential, it was found consistently that a pink coloration of the electrolyte occurred at the more anodic potential for the small silver content in lead: the solution becomes pink at E=1320 mV for silver grades 0.5% (Varela et al., 1995).

In the presence of Mn²⁺ ions in solution, permanganate ions are formed on the electrode [reaction (16)], and then diffuse into the solution. Manganese dioxide is also formed, both electrochemically on the lead electrode [reaction (17)] and chemically within the solution [reaction (18)]:

The MnO₂ precipitates in the solution, allows the co-precipitation of impurities, and forms the anodic slime that settles at the bottom of the cells. These, then, require regular cleaning. This MnO₂ chemically formed can also precipitate on the cathode (Ivanov et al., 2000)).

The anodic deposition is, itself, comprised of a mixed film of PbO₂/MnO₂ having an adhesive inner layer to the anode and a poorly adhering outer layer. Palvov et al. (1998) studied the formation rate of the anodic deposition of MnO₂, depending on the nature of the electrode. It was shown that the MnO₂ layers are formed on the Pb-Ag anode more quickly than on a pure lead electrode. However, no difference in MnO₂ structure was observed for both anodes, despite their difference in morphology (coarse for pure lead, and small grains for Pb-Ag) (Pavlov et al., 1998).

The manganese dioxide that was formed electrochemically on the anode produces conditions of operation of the lead electrode. The mixed film of PbO₂/MnO₂ helps to slow the diffusion of Pb²⁺ ions to the electrolyte. The part of MnO₂ provides the adherent deposit. A critical amount of electrodeposited MnO₂ (the anode crust) dissociates from the electrode, increasing the anode slimes and the amount of lead co-deposited zinc. Other studies analyzed the value of the oxygen overvoltage on the β-PbO₂ and MnO₂ on the surface of lead alloy anodes. It was proven that the oxygen overvoltage is lower on the MnO₂ than that on the β- PbO₂. In fact, the presence of Mn²⁺ ions in solution is beneficial for the anodic process, with the formation of a MnO₂, which is electrochemically deposited on the electrode, but dangerous to the cathodic process, as the MnO₂ precipitates chemically throughout the cell (Yu & O’Keefe, 2002).

At the potential of oxygen evolution, it is reported that there was an anodic formation of various species other than PbO₂ and MnO₂, forming the oxidation layer of the lead anode in the sulfuric acid medium containing manganese (Nijjer, 2000). Further, the anodic polarization in the cells of zinc electrolysis in the presence of Mn²⁺ ions created glassy deposits of mixed oxides, MnO₂, β-PbO₂, which presented the advanced protective film and extends the life of the allied electrodes Pb for 2, 6, or 10 years, respectively (Rajkovic et al., 1998).

The presence of permanganate ions is to induce a coloration in the solution, and Mn³⁺ ion is not precluded. The formation of these species depends on the initial E⁰=0.851 V at the concentration of Mn²⁺ ions in the electrolyte. If there is an excess of Mn²⁺ ions in solution, reaction (19) is followed by reactions (20) and (21), involving the formation of the Mn³⁺ and MnO₂ species in the electrolyte (Mohammadi & Alfantazi, 2015):

If [Mn²⁺] ≤200 mg/l, the oxidized MnO₄⁻ is the most stable form, while for [Mn²⁺] ≥1 g/l, the oxidized Mn³⁺ state is the most stable. The other secondary species proved to be capable of generating a decrease in the yield of electro crystallization of zinc on the cathode, and it is even more clear when the initial concentration of Mn²⁺ is high (Nijjer, 2000).

7.1 Lead contamination in sulfuric acid solution

The electro crystallization of zinc in sulfuric acid solution has already been the subject of numerous publications. It is important to determine the mechanism of formation and growth of the deposit, as to analyze the effects of impurities contained in the electrolyte on the cathodic deposit process (Joao et al., 1995).

Anodic dissolution is an important factor to be considered because the Pb²⁺ ions that go into solution can diffuse and intervene in the cathodic process. The lead dissolution rate was analyzed according to different parameters, primarily by weight loss measurements. It seems important to detect the deposits such as PbO₂/MnO₂, which protect the electrode. The preforming of anode treatment is, in this sense, a remedy against the dissolution of lead (Ivanov et al., 2000)).

Under industrial conditions of polarization, the value of the anode potential enables the formation of hydrogen oxide that can then diffuse into solution to reach the cathode where the harmful effect of Pb ion was demonstrated (Bozhkov et al., 1990).

The dissolution of lead to form a Pb²⁺ ion implies their reduction at the cathode. Mansfeld & Gilman (1970) showed that traces of lead on the cathode were beneficial for inhibiting the dendritic growth process of the deposit. Mackinnon et al. (1979) claimed that the lead co-deposited with zinc causes a change in orientation of the zinc crystals and increases cathodic polarization.

There are other secondary species anodically formed, as the original electrolyte containing manganese. These species may influence the cathodic process. Figures 9 and 10, respectively, show the Pb-Ca-Ag and Pb-Ag anodes. The two figures show the amount of Pb in zinc with time. It notes that the anode contains silver as an additive, and without pretreatment (Figure 10), the amount of lead present in the electrolyte is higher, if compared to other cases (Umetsu et al., 1985).

Figure 9:

Zinc contamination from the Pb-Ca-Ag anodes during zinc electrolysis for 24 h at a current density of 660 A/m² (anode), 580 A/m² (cathode), and 40°C. (A) Untreated: no precon, (B) untreated: precon 2 h, (C) sandblasted: no precon, (D) sandblasted: precon 12 h (Umetsu et al., 1985) (Here: precon means preconditioning). Permission granted by Y. Umetsu.

Figure 10:

Zinc contamination from the Pb-Ag anodes during zinc electrolysis for 24 h at current density of 660 A/m² (anode), 580 A/m² (cathode), and 40°C. (A) Untreated: no precon, (B) untreated: precon 2 h, (c) sandblasted: no precon, (D) sandblated: 8 h (Umetsu et al., 1985) (Here: precon means preconditioning). Permission granted by Y. Umetsu.

The best results obtained with the anodes Pb-Ag-Ca were performed with the preconditioning (cleaning), particularly for short periods from 2 to 4 h (Table 6). The average lead content in zinc is 27–25 ppm, respectively. Results of the sandblasted surface (41 ppm) competed favorably with longer periods of preconditioning (8, 12, and 24 h), where the average lead content in zinc ranged from 32 to 40 ppm. Also, it was not beneficial for the combination of preconditioning and sandblasting (Umetsu et al., 1985).

For the Pb-Ag anodes, it expected that the blasting preconditioning resulted in lower contamination for only the sandblasted sample; the trend is similar to that obtained with the Pb-Ag-Ca anodes. Preconditioning of Pb-Ag anodes (Table 7) seems also to be more effective by sandblasting in minimizing lead contamination in the initial period.

7.2 Nickel and cobalt impurities in sulfuric acid solution

The effects of other impurities present in solution, and the insoluble additive behavior of the cathode during electrolysis, were studied. Many authors analyzed the effects of the impurities present in solution on the cathode behavior during electrolysis (Mackinnon et al., 1979). In acid electrolytes, it contains certain metal impurities, such as Ni and Co. During a certain period, zinc deposits are uniform, fully adherent to the cathode, and the current efficiency of zinc is around 95%. Subsequently, a “self-dissolution” spontaneous zinc dissolution occurs, accompanied by a strong release of hydrogen. When the zinc deposit is completely dissolved, the filling process restarts. The deposition cycle/dissolution can occur several times in 24 h (Muresan et al., 1996).

It is often mentioned in the literature that an induction period is observed under acidic electrolytes containing certain metal impurities, such as Ni or Co. During this period, zinc deposits are uniform, fully adhered to the cathode, and the yield of zinc current efficiency is 95%. Subsequently, a “self-dissolution” spontaneous zinc deposition occurs, accompanied by a strong release of hydrogen. When the zinc cathode is relatively dissolved, the filling process restarts. The deposition cycle/dissolution can occur several times in 24 h (Mackinnon et al., 1979).

Nickel impurity in the zinc electrolyte causes the process of dissolving zinc in activating the hydrogen release. The bubbles generated by this reaction create the electrolytes confined to the areas where the micro cells are located between the couples Zn²⁺/Zn and Ni²⁺/Ni. The self-dissolution of zinc occurs when the potential of the confined environment of the two couples reached the corrosion potential (Mackinnon et al., 1979). Nickel causes the process of dissolving zinc in activating the hydrogen release as bubbles by this reaction in the zinc electrolyte. This is confined by areas where the microcells are located between the couples Zn²⁺/Zn and Ni²⁺/Ni (Bozhkov et al., 1990). The self-dissolution of zinc occurs when the potential of the confined environment of the two couples reaches the corrosion potential (Bozhkov et al., 1990).

Many impurities present in solution, other than nickel, have a negative effect on the cathodic process. There are 15 impurities influencing the electrometallurgy of zinc. They showed, in particular, that Ge and Sb instantly ease the reduction of the hydrogen ions at the cathode, while inhibiting the zinc deposition process. Under industrial conditions of polarization, the value of the anode potential enables the formation of hydrogen peroxide, which can then diffuse in solution to reach the cathode where the harmful effect of this compound was demonstrated (Mackinnon et al., 1979).

7.3 Lead removal from zinc electrolyte

Lead can be detrimental to the properties of zinc deposit, so it is important to control its concentration in various deposition processes. In the electrolytic extraction of zinc, it is desirable to minimize the lead to insure an acceptable purity for high-grade applications. One specific example of the adverse influence of lead in zinc is the poor adhesion, which gives electrogalvanized steel. When the lead content is too high, the zinc coating can be easily removed from the steel substrate after heating in the temperature range of 215°C–280°C. Insoluble lead-silver anodes and the source of lead in the zinc electrolyte, whether in the form of metal or compound, are possible sources of lead contamination (Zhang et al., 2009).

For instance, good quality zinc oxide or special high-grade zinc contains enough lead to supply 2 mg/l of lead in an electrolyte containing 100 mg l⁻¹ of zinc (Srinivasan et al., 1990). As the theoretical reduction potential of the lead ions at 10 mg l⁻¹ present in the electrolyte is −0.223 V (SHE), it leads to co-deposits with zinc. The deposition rate of lead may be quite low, as it will be depositing at its limiting current density at such low concentrations (Ault & Frazer, 1988).

It is a common practice to add strontium carbonate or similar chemicals to the electrolyte to control the lead content in sulfuric acid solution during zinc electrowinning, where lead anodes are used. Therefore, the solutions, which contain 80 g l⁻¹ of zinc and 10 mg l⁻¹ of lead at pH 1.5, were treated with strontium carbonate at 55°C, and the solution was analyzed for lead. It is important to note that in all cases, filtering the electrolyte is necessary in order to achieve a reduction in the level of lead in solution. When 20 mg l⁻¹ of strontium carbonate was added into the zinc electrolyte; no decrease in the lead content was noted until the solution was filtered, giving a final concentration of 4 mg l⁻¹ of lead. A 100 mg l⁻¹ of strontium carbonate addition, followed by stirring for 5 min and filtering, reduced the lead content below the detection limit (Bratt & Smith, 1965).

Deposits were also prepared for adhesion testing in electrolytes initially containing 10 mg l⁻¹ of added lead but which had also been treated by strontium carbonate. A 20-mg/l of strontium addition with 5 min of agitation, whether followed by filtration or not, did not remove lead to the threshold lever needed (i.e. approximately 4–5 mg/l to prevent failure). A 100 mg/l of strontium carbonate addition in the absence of filtration did not prevent poor hot adhesion; however, the deposit made from the electrolyte after filtration exhibited good adhesion after heating. The actual chemical form of the lead present in the electrolyte was not determined, but at the levels used, it was assumed to be ionic, rather than as a particulate compound (Bratt & Smith, 1965).

Depending on the electrolysis conditions, one or several organic additives may be added to the electrolyte in order to counteract the detrimental effects caused by impurities (Gonzalez & Lew, 1995). The effect of organic additives in the electrolyte on the nature of the crystallization presents one of the important aspects. Additives could be adsorbed preferentially on the cathode to completely alter the growth of the deposit. Additives also reduce the grain size by creation of more nucleation sites during the electrodeposition (Sato, 1959). However, they are also susceptible to decomposition by the presence of a large amount of Mn²⁺, which is gradually oxidized to MnO⁴⁻ or MnO₂; this also can cause further alterations in the electro-crystallization (Sorour et al., 2015).

Glues and gelatin are the most commonly used additives in the industry to prompt the deposit growth and minimize the negative effect of metallic impurities (Robinson & O’Keefe, 1976). The addition of gelatin into the electrolyte led to very good results in increasing current efficiency, reducing overpotentials, producing better smooth and compact deposits in presence of antimony traces (Lafront et al., 2009). Sodium lauryl sulfate (SLS), as additive at low concentrations in the zinc electrolyte containing Sb (III), was examined (Tripathy et al., 1997). It showed good results in increasing current efficiency, reducing power consumption, and improving surface morphology. In addition, triethyl benzylammonium (TEBA) (Ivanov, 2004), 1-butyl-3-methylimidazolium hydrogen sulfate [BMIM]HSO₄ (Zhang & Hua, 2009), and perfluorinate surfactant were employed as additives to study their effects on the electrodeposition characteristics of zinc from the zinc electrolytes (Cachet & Wiart, 1990).