Effect of Oxidation of Zinc Powder on Purification …
677
Results and Discussion
ϕ-pH Diagram of Zn-H 2 O System Concerning Zinc Powder
Replacement
The ϕ-pH diagram of the Zn-H 2 O system is shown in Fig. 2. It is divided into
three stable regions, namely Zn, Zn
2+ , and Zn(OH) 2 , which are involved in leaching, hydrolysis, purification, and electrolysis of zinc hydrometallurgy. Because the
standard electrode potentials of Co and Ge are more positive compared with Zn,
thermodynamically, Zn can replace Co and Ge in an acidic environment, and the
replacement reaction product, Zn
2+ does not cause secondary pollution. Therefore,
almost all zinc hydrometallurgy plants choose zinc powder as a displacer.
In the Zn
2+ ion stable region, when the solution temperature increases from 25
to 80 °C, the Zn
2+ ion stable region decreases. In other words, the starting pH value
of Zn
2+ ion starts to decrease. When the Zn
2+ ion concentration is 150 g/L, or
2.294 mol/L, the activity of zinc α
2+
Zn is 0.0853. The pH of the solution in contact with the zinc powder is higher than that of the bulk of the solution during the
purification process. According to the literature, its value is generally 2–3. It was
mainly attributed to the hydrogen ion consumption by zinc oxide, which changes the
properties of zinc sulfate. Figure 2 shows that Zn
2+ ions can hydrolyze on the surface
Fig. 2 ϕ-pH diagram of Zn-H 2 O system
677
Results and Discussion
ϕ-pH Diagram of Zn-H 2 O System Concerning Zinc Powder
Replacement
The ϕ-pH diagram of the Zn-H 2 O system is shown in Fig. 2. It is divided into
three stable regions, namely Zn, Zn
2+ , and Zn(OH) 2 , which are involved in leaching, hydrolysis, purification, and electrolysis of zinc hydrometallurgy. Because the
standard electrode potentials of Co and Ge are more positive compared with Zn,
thermodynamically, Zn can replace Co and Ge in an acidic environment, and the
replacement reaction product, Zn
2+ does not cause secondary pollution. Therefore,
almost all zinc hydrometallurgy plants choose zinc powder as a displacer.
In the Zn
2+ ion stable region, when the solution temperature increases from 25
to 80 °C, the Zn
2+ ion stable region decreases. In other words, the starting pH value
of Zn
2+ ion starts to decrease. When the Zn
2+ ion concentration is 150 g/L, or
2.294 mol/L, the activity of zinc α
2+
Zn is 0.0853. The pH of the solution in contact with the zinc powder is higher than that of the bulk of the solution during the
purification process. According to the literature, its value is generally 2–3. It was
mainly attributed to the hydrogen ion consumption by zinc oxide, which changes the
properties of zinc sulfate. Figure 2 shows that Zn
2+ ions can hydrolyze on the surface
Fig. 2 ϕ-pH diagram of Zn-H 2 O system