Application of solvent extraction process for zinc recovery from old silver mine “Davidschachthalde” tailings

The historic mining activities have produced a vast amount of mine tailings covering a huge landscape and containing hazardous substances that are harmful to the environment. In addition, the mine tailings also contain valuable materials that become economical after a certain period depending on the criticality of the commodity. Likewise, the old silver mine tailings “Davidschachthalde” near Freiberg, Germany, bears hazardous substances such as As, Cd and valuable elements such as Zn, In, and Cu. Thus, an innovative flowsheet is developed to recover Zn from mine tailings. Firstly, the Fe and Al are removed using the precipitation method which also removes As. Then, the solution is passed through the cementation steps for Cu and Cd removal. In order to purify and enrich Zn(II) in the aqueous solution before electrowinning the conceptional flowsheet consists of a solvent extraction process.
The filtrate from precipitation steps with the composition of 1120 mg/L Zn(II), 4 mg/L Cu(II), 10 mg/L Al(III), 243 mg/L Ca(II), 21 mg/L Cd(II) and pHini 4.7 is subjected to solvent extraction unit with 3 extraction and 2 stripping stages in MEAB lab scale Mixer Settler. Figure 1 depicts the results for the extraction step which is carried out with 0.5 M Cyanex® 272 in kerosene as the organic phase, A/O ratio of 1:1, a contact time of 10 min with a 1-hour sampling interval. Under the chosen conditions Zn(II) extraction is 89% after reaching equilibrium and shows a high selectivity related to low concentrated impurities Cu(II), Cd(II) and Al(III). However, a Ca(II) co-extraction of up to 22% is observed during the process which would affect the following stripping and electrowinning processes in a negative way. Therefore, a high selectivity between Zn(II) and Ca(II) needs to be achieved in the extraction step.
Here, we report the development of the highly selective solvent extraction process for the Zn(II) containing solutions generated during the previous precipitation and cementation steps. Effects of crucial parameters such as pH control and A/O ratio as well as the composition of the stripping agent on extraction yields, up-concentration and selectivity are discussed in detail.