Copper-cobalt cake as a potential source of germanium

Germanium is an important element used in crucial industry sectors like optical fibers for telecommunication or IR optics for night vision systems. However, its global output is limited. It is estimated that its annual global production is ca. 140 tons. Therefore, many world economies, including USA and EU, consider it as a critical raw material. One of the potential sources of germanium is zinc metallurgy. It is assumed that only 3% of germanium present in processed zinc ores is recovered. It was found that during the technological processes of Polish zinc smelters some by-products containing elevated germanium concentrations are produced. In the paper, potential germanium sources, including copper-cobalt cake obtained during the purification of zinc electrolyte and dross from the feeding furnace, are presented. The main components of the cake are cadmium, zinc, copper, nickel, cobalt, and lead, while the dross contains mainly zinc (>65%). Results of leaching tests for both materials using aqueous sulfuric acid solutions are shown. In the case of the cake, wet and dried material was investigated. It was found that the germanium leaching yield for the dried material reached 99%, while for the wet one was 46-86%, depending on leaching conditions (without or with oxidant). On the other hand, the germanium leaching yield for the dross reached 55%. Further processing of the solutions obtained after germanium leaching from the copper-cobalt cake was also analyzed. Two ways were proposed, including precipitation with tannic acid and solvent extraction with addition of complexing agents using a trioctylamine extractant.

Germanium is considered by many world economies, including the EU, USA, Australia, and Japan, as a critical raw material [1].It is associated with its limited output and application in important industry sectors.It is estimated that its annual global production is ca.140 tons.Moreover, germanium production is not uniformly distributed worldwide -ca.2/3 of global output comes from China.Other important germanium producers are Russia, USA, Belgium, Canada, Germany, Japan, and Ukraine.[2].Germanium has several important applications.Almost 1/3 of its supply is used to produce optical fibers, which are the core of fast telecommunication systems.Other critical applications include IR optics (night vision system lenses, contactless thermometers), PET polymerization catalysts, electronics, and solar cells [3].
The typical germanium recovery process may be divided into three steps.Firstly, by-products containing germanium are generated during technological processes associated with zinc metallurgy or coal combustion.They include fly ash from the combustion of Ge-rich lignite, dusts, cakes, or drosses from zinc metallurgy.Some examples of Ge-based sources are presented in Table 1.Secondly, germanium concentrate containing at least 5% Ge is obtained by applying pyro-and/or hydrometallurgical methods.Finally, concentrates are chlorinated and distilled to obtain pure germanium tetrachloride, which may be later processed by water hydrolysis to germanium dioxide and then by reduction to metallic germanium [3][4].Natural sources of germanium in Poland are limited.Especially, Polish coals and zinc ores are not rich in germanium.However, several potential sources, containing 231 ppm to 8.9%, were identified in Polish zinc smelters.They are presented in Table 2.It is estimated that only 3% of germanium present in processed zinc ores is recovered [2].Therefore, it is still huge room for germanium recovery enhancement.This paper is focused on the experimental results of germanium recovery from Polish-based materials.Potential materials, including copper-cobalt cake and liquation-feeding furnace dross, were examined.

Copper-cobalt cake
One of the steps of electrolytic zinc production is the purification of crude zinc electrolyte.The process is performed to remove impurities that may negatively impact zinc electrolysis.One such impurities is germanium.The typical purification process is composed of three steps: cold, hot, and final.In the cold purification step first portion of zinc powder is added to remove the first part of the impurities, mostly cadmium.In the hot step additional zinc powder is added in the presence of activators.The residue from this step is copper-cobalt cake.The final zinc portion is added to remove any remaining impurities in the final step.The composition of investigated copper-cobalt cake is presented in Table 3.Two different cake forms were investigated -wet unconditioned and dried at 70°C for 8 hours.

Liquation feeding furnace dross
The pyrometallurgical process of zinc production includes zinc refining using New Jersey process.In this process melted GOB (Good Ordinary Brand) zinc is fed to the rectification column.Zinc vapors volatilize to the top of the column, where they condense and liquid SHG (Super High Grade) zinc is collected.On the other hand, liquid zinc with impurities flows to the bottom of the column and is then directed to the liquation furnace.Due to oxidation, dross is formed on the liquid metal surface.The dross is periodically removed and routinely recycled for the zinc production process.However, it was examined as a potential source for germanium recovery due to elevated germanium content.Due to the presence of a large metallic fraction, the dross was sieved at 0,3 mm mesh prior to investigation.The composition of the dross is presented in Table 4.

Methods
Leaching tests of copper-cobalt cake in wet and dried form as well as liquation feeding furnace dross were performed.All three analyzed materials were leached in aqueous solutions of sulfuric acid (98%, Avantor, Poland).In some tests, the addition of hydrogen peroxide (30%, Avantor, Poland) as an oxidant was also analyzed.All tests were performed using a 500 cm 3 acid solution.A sample of aqueous solution was obtained dissolving concentrated acid in deionized water.Then, the solution was heated to set temperature, and the appropriate amount of investigated material was added depending on the tested solid-to-liquid ratio (S:L).The influence of several parameters like temperature and initial acid concentration was analyzed.The calculation of leaching yields for selected elements was based on the chemical composition of the initial solid material and final solution.Concentrations of elements in solutions were analyzed using ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry; Agilent 5110 SVDV, Agilent Technologies).Solid samples were melted with Na2O2 (p.a.Avantor, Poland) at 600°C and chemically dissolved in 1:1 mixture of concentrated nitric (65%, Avantor, Poland) and hydrofluoric (40%, VWR, UK) acid prior to analysis.

Copper-cobalt cake
The first analyzed material was copper-cobalt cake.Two forms of the cake were investigated -wet and dried.The influence of process temperature and initial acid concentration on the leaching yield of selected elements, i.e., germanium, copper, zinc, and cadmium, was tested.Results of the investigation under non-oxidizing conditions for the dried copper-cobalt cake are presented in Figures 1-2, while for wet cake in Figures 3-4.Results of tests under oxidative conditions are shown in Figures 5-6.It is clear that germanium leaching yield depends on the initial material.Significantly higher leachability, even up to 99%, was achieved for dried cake (Figure 2, 20 wt.% initial H2SO4 concentration).It is associated with oxidation of the initial material during the drying process, even at a moderate temperature of 70°C).It is also supported by a significantly higher leaching yield of copper for dried material.In the case of the wet cake, almost no copper was leached.Leaching yields of zinc and cadmium are not significantly influenced by the kind of initial material.Germanium leaching yield is also significantly influenced by the final pH of suspension -it may be observed especially in tests with different initial acid concentrations (Figures 2 and 4).The general conclusion is that in the case of dried cake, it is possible to achieve almost complete leachability of germanium, while in the case of the wet cake, germanium is only partially leached.
Adding hydrogen peroxide as an oxidant increased the leaching yield of germanium from the wet cake.It may also be noticed that the addition of oxidant had, firstly, an influence on copper leachability, then, an increase in the amount of leached germanium.Oxidant reacts with copper at 0 and I oxidation state present in the cake and oxidize it to soluble Cu(II).Therefore, leachability of germanium, which might have been trapped in the undissolved solid is also increasing.On the other hand, germanium leaching yield was decreased when oxidant was used for leaching of dried cake.The leachabilities of cadmium and zinc were not significantly influenced by H2O2 addition.In this cake decrease of germanium leachability might be explained by formation of sparingly soluble oxides.
Summing up, the highest germanium leaching yield was achieved for the leaching of dried cake at 80°C, 2 hours, and 20 wt.% initial H2SO4 under non-oxidizing conditions.Under these conditions, the selectivity of leaching is moderate, as cadmium, zinc, and copper leaching yields are significant.

Liquation feeding furnace dross
The second investigated material was liquation feeding furnace dross.The influence of process temperature and initial acid concentration on the germanium and zinc leaching yield was examined.Results of the investigation under non-oxidizing conditions for the dried copper-cobalt cake are shown in Figures 7-8.
In the case of liquation feeding furnace dross, achieving a maximum of 55% germanium leaching yield was possible.On the other hand, almost complete zinc leaching was reached for most of the investigated parameters.The highest germanium leachability was obtained at 80°C, for 2 hours, with an S:L ratio of 1:10 and 30 wt.% H2SO4 initial concentration.It was also clear that temperature and initial acid concentration had some influence on germanium leaching yield.The yield increased as the leaching temperature increased -the highest (53%) for the test performed at 90°C.However, the difference between 80°C and 90°C was not significant.Also, the higher initial acid concentration results in higher germanium leachability.It was noticed that in the range of 5-20 wt.% germanium leaching yield increased significantly -from 1% to 54%.Further increase to 30 wt.% H2SO4 had a minor impact on germanium leachability -it increased to 55%.The highest germanium leachability was obtained at 80°C, for 2 hours, with an S:L ratio of 1:10 and 30 wt.% H2SO4 initial concentration.
The reason why it was not possible to achieve a higher germanium leaching yield may have been associated with the composition of the initial dross.Although the material was significantly oxidated, it might have also contained zinc and elements in metallic elementary form.Therefore, germanium might have been partially reduced and cementated during leaching.Moreover, germanium might have been partially enclosed in the material matrix; therefore, it was impossible to release Ge without complete dissolution of the dross fully.

Further steps
Leaching of the cake resulted in a solution containing germanium along with other elements.The composition of the solution obtained after the leaching of copper-cobalt cake under oxidative conditions is presented in Table 5.The composition of the solution is quite complex, and the germanium concentration is low.Therefore, selective methods allowing the collection of germanium almost completely are highly desirable.It may be done using either precipitation with tannic acid or solvent extraction.Tannic acid is a quite selective agent, forming an insoluble germanium complex used in some industrial applications.Using it is possible to achieve <0.5 mg/dm 3 Ge in the final solution [16][17].Precipitate may be later burned to obtain germanium concentrate.
Solvent extraction was also investigated as a method for germanium separation.Several extractants were examined, including LIX 63, hydroxamic acid, D2EHPA, or Aliquat 336 [18].Especially interesting is the way including complexation of germanium in sulfate solution with an organic acid, like tartaric acid, followed by extraction with an amine extractant, e.g., trioctylamine [19][20].Germanium may be later stripped from organic acid using an aqueous sodium hydroxide solution and precipitated to obtain germanium concentrate.These operations will be further studied as the next steps of the developed germanium recovery process.

Conclusions
Leaching of copper-cobalt cake and liquation feeding furnace dross in aqueous sulfuric acid solutions was investigated.It was found that almost complete germanium leaching was achieved using dried cake at 80°C for 2 hours, S:L 1:5, and initial 20 wt.% H2SO4.Germanium leachability was influenced by temperature and initial H2SO4 concentration -more germanium was leached at 80-90°C and for higher acid concentrations.Also, the cake's form influenced leaching -the dried cake was more oxidized, and therefore more copper was leached.Similarly, adding an oxidant increased the wet cake's copper and germanium leaching yield.However, less germanium was leached under oxidative conditions than for tests with dried cake.
On the other hand, achieving complete germanium leaching from liquation feeding furnace dross was impossible.The highest germanium leaching yield was 55% for 80°C, 2 hours, S:L ratio 1:10 and 30 wt.% H2SO4 initial concentration.

Table 1 .
Composition of selected germanium sources.

Table 2 .
Composition of selected Polish germanium sources.

Table 3 .
Composition of the copper-cobalt cake

Table 4 .
Composition of the liquation feeding furnace dross

Table 5 .
Composition of the solution after oxidative leaching of copper-cobalt cake