# Evaluation and Current State of Primary and Secondary Zinc Production--A Review
## 4 Zinc Production from Secondary Resources
The following reactions occur in the acid dezincification process:  
* Zinc dissolution: \[\mathrm{Zn}\rightarrow\mathrm{Zn}^{2+}+2\mathrm{e},\] (8)
* Hydrogen release: \[2\mathrm{H}^{+}+2\mathrm{e}\rightarrow\mathrm{H}_{2},\] (9)
* Formation of zinc salts: \[\mathrm{Zn}^{2+}+2\mathrm{NO}_{3}\rightarrow\mathrm{Zn}(\mathrm{NO}_{3})_{2},\] (10) \[\mathrm{Zn}^{2+}+\mathrm{SO}_{4}{}^{2-}\rightarrow\mathrm{ZnO}_{4},\] (11) \[\mathrm{Zn}^{2+}+\mathrm{Cl}^{-}\rightarrow\mathrm{ZnCl}_{2},\] (12) In the alkaline dezincification process, the following reactions take place:
* Oxygen reduction: \[\mathrm{O}_{2}+\mathrm{H}_{2}\mathrm{O}+4\mathrm{e}\rightarrow 4\mathrm{OH}^{-},\] (13)
* Formation of zinc hydroxide: \[\mathrm{Zn}^{2+}+2\mathrm{OH}^{-}\rightarrow\mathrm{Zn}(\mathrm{OH})_{2},\] (14)
* Formation of zinc salts: \[\mathrm{Zn}(\mathrm{OH})_{2}+\mathrm{NaOH}\rightarrow\mathrm{NaHZnO}_{2}+ \mathrm{H}_{2}\mathrm{O},\] (15) \[\mathrm{Zn}(\mathrm{OH})_{2}+2\mathrm{NaOH}\rightarrow\mathrm{Na}_{2}\mathrm{ ZnO}_{2}+\mathrm{H}_{2}\mathrm{O},\] (16)  
In the case of the acid dezincification process, it was found that more concentrated acids (more than 10%) tend to accelerate the dezincification rate but also have an intense effect on the dissolution of the steel substrate. Dilute acids (2% or less) reduce the rate of dissolution but tend to dezincify partially or even ineffectively [84]. To sum up, in hydrometallurgical processes, valuable metals with a high mass fraction can be obtained. The energy consumption of the processes is low due to their low temperature. Unfortunately, these methods require high corrosion resistance of the devices. Slag obtained after leaching is also problematic due to its complex composition, and it cannot meet the storage requirements of environmental protection law. Pyrometallurgical methods are also used for dezincification (e.g., in Japan, there are two plants based on this solution),  
\[\mathrm{Zn}\rightarrow\mathrm{Zn}^{2+}+2\mathrm{e},\]0 Scheme: (**a**) acidic and (**b**) alkaline dezincification of galvanized products. but they are much more expensive. They focus on steel scrap, and the recovery of zinc products is practically low [87]; what is more, these processes are highly energy-intensive and require large equipment. Figure 9 shows the flow of recycled zinc, taking into account the hot dip galvanizing process and finished products withdrawn from circulation [88]. Currently, about 70% of the zinc comes from primary refining of zinc ores (including 10-15% from recycling sources), and about 30% comes directly from recycled zinc (accounting for 80% of the zinc available for recycling). The long life of galvanized steel products in the construction industry makes predicting their appearance in waste streams difficult. In the case of zinc, the following two recycling rates are used: end of life (EOL) and recycled content (RC). The EOL indicator offers a very comprehensive approach by taking into account recycling efficiency, product lifetime and historical production patterns; quantifies the value of the zinc actually recovered at the end of the product's life ('old scrap') and recycled into new metallic zinc. It is estimated that around 45% of the available zinc at the end of the life of various zinc-containing products is recovered and actually recycled (Table 9). In developed regions such as Europe and North America, the EOL rate may be 50% or greater. For developing regions, the rate is 30-40% due to the lack of developed recycling networks and regulatory initiatives aimed at reducing industrial waste [71]. Recycled content RC is a measure of the recycled content in a product. The  
\[\mathrm{Zn}\rightarrow\mathrm{Zn}^{2+}+2\mathrm{e},\]1 Recycling zinc flow, including the hot dip galvanizing process and finished products withdrawn from circulation. recycled content for refined zinc is currently around 8% (scrap combined with concentrate as raw material in the smelter). Table 10 shows the production of zinc, taking into account the method of obtaining it.