# Evaluation and Current State of Primary and Secondary Zinc Production--A Review
## 4 Zinc Production from Secondary Resources
After the zinc bath reaches the state of saturation with iron (0.03 wt.% at 450 \({}^{\circ}\)C), excess iron reacts with liquid zinc and is released in the form of fine particles of intermetallic phases \(\delta_{1}\) (FeZn\({}_{7}\)) or \(\zeta\) (FeZn\({}_{13}\)) forming the so-called hard zinc. When the galvanizing bath contains an Al additive above 0.14 wt.% (continuous galvanizing), the reaction of Al with dissolved iron leads to the formation of particles of the Fe\({}_{2}\)Al\({}_{5}\)Zn\({}_{\rm x}\) phase. Hard zinc, which can be identified as surface drops (mainly Fe\({}_{2}\)Al\({}_{5}\)Zn\({}_{\rm x}\)) and bottom drops (mainly \(\delta_{1}\) (FeZn\({}_{7}\)) or \(\zeta\) (FeZn\({}_{13}\)) phase), hinders the galvanizing process and requires periodic removal--the surface drops is skimmed off by labor or machine at intervals, while the bottom drops can be removed when the operation is shut down. One novel efficient method called the supergravity method for recovering zinc from galvanizing dress has been proposed by the University of Science and Technology Beijing in China [70]. The hot dip galvanizing process also provides zinc-rich waste as a residue from the purification of finished products. Zinc thickening and drips formed on the surface are removed by mechanical grinding, and the removed zinc is re-melted. Zinc-rich wastes are also zinc sludges formed when wet fluxing is used. However, wet galvanizing is now much less frequently used. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Waste** & **Source of Formation** & **Chemical Composition** \\ \hline Zinc ashes & Oxide surface on galvanizing bath & 60–80\% ZnO, 2–12\% CI, 0.2–1.5\% Fe, \textless{}0.3\% Al, \\  & Oxide surface on galvanizing bath & small amounts of alloy additives for the bath \\  & & Pb, Bi, Sn \\ \hline Hard zinc & Particles of the intermetallic phases of the Fe-Zn system that fall to the bottom of the galvanizing bath & 96\% Zn, 4\% Fe \\ \hline Zinc slimes & Wet flux on the surface of the galvanizing bath & 5.6\% Zn, 48.1\% ZnCl\({}_{2}\), 27.4\% ZnO, 3.1 AlCl\({}_{3}\), 15.8\% NH\({}_{4}\)Cl and Fe and other oxides \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Characteristics of various zinc wastes generated in the industry and during the production process, worked out based on [57; 71; 72]. Another important source of zinc is electric arc furnace (EAF) dust. It is estimated that the EAF furnace generates 15-25 kg of dust per ton of raw steel, the zinc content varies from 10 to 40%. The minimum zinc content necessary for the process of zinc recovery from steelmaking dust to be profitable depends on various factors, such as the proximity of the steelworks, energy costs, slag cost; however, it should be at the level of 15% [73]. Zinc is also contained in steelmaking dust from the converting process (BOF). However, the content of zinc in it varies between only 0.5 and 6%. This value is too small for the profitable recovery of zinc from these dusts. Zinc is not recovered from the BOF process dusts; the suggested solution, in this case, is to collect galvanized sheet scrap and send it to the EAF process or to dezcincify steel scrap [74; 75; 76]. The hydrometallurgical production of zinc produces deposits and leaching and purification residues. In the production of rayon fiber yarn, zinc is used as a modifier for cellulose regeneration. After the rayon is made, it is washed and the solution contains 30-50 ppm of zinc, which precipitates in the form of zinc hydroxide [71]. Pyrometallurgical methods used to treat secondary zinc waste include the following: electrothermic retort processes (ERP), Imperial Smelting Process (ISP) and Waelz furnace. Hydrometallurgical processes include the following: molecular recognition technology (MRT), EZINEX process, Zincex process, CASHMAN process, sulfuric acid leaching process, chloride leaching process, chloride-sulfate leaching process, alkaline leaching process, crystallization and solvent extraction [77]. Table 8 presents the basic characteristics of the mentioned processes.