# Precipitation of Germanium from Sulphate Solutions Containing Tin and Indium Using Tannic Acid
## 1 Introduction  
Germanium is considered as a critical raw material by the European Union [1]. It is important to many different industrial sectors like optic fibers, infrared optics, electronics, photovoltaics or catalysts for polyethylene terephthalate (PET) production [2]. Ge minerals are not present in economically mineable aggregates, but they are scattered in their surroundings. Therefore, Ge is obtained as an accompanying element during the processing of other raw materials--mainly from coal ash and by-products of zinc production. About 30% of Ge production comes from recycling [2].  
One of the ways to process Ge is leaching in aqueous solutions of different acids (usually sulfuric, hydrochloric or oxalic). Dissolved Ge may be later precipitated from sulfuric solutions using hydrogen sulfide [3] or tannic acid, alternatively using ion-exchange resins [4; 5], membranes [6], or solvent extraction [7]. Precipitation of tannin complex is more Ge-selective than precipitation by pH adjustment. It was found that tannic acid gave better results than the application of sodium hydroxide or ammonia [8]. In aqueous solutions, tannic acid forms a water insoluble complex with Ge, firstly described in 1938 by [PERSON] and [PERSON] [9; 10; 11]. The Ge-tannin complex in aqueous solutions is formed according to the following equation [12]:  
\[\mathrm{Ge}^{4+}+\mathrm{H}_{2}\mathrm{T}\rightarrow(\mathrm{Ge}\mathrm{T})^{2 +}+2\mathrm{H}^{+}\]  
where T is tannin. Precipitation with tannic acid allows to recover 99% of the Ge present in the solution and to achieve final Ge concentration below 0.5 mg/dm\({}^{3}\)[12].  
Tannic acid is not a single compound. Generally, its chemical formula is described as C\({}_{76}\)H\({}_{52}\)O\({}_{46}\) and molecular weight 1701.2 g/mol. However, it is a mixture of gallic acid, gallotannins, and theirderivatives [13]. The application of tannins in non-ferrous metallurgy is not limited only to Ge. It is used as a depressant in flotation processes for ore treatment or Au recovery [14].  
The application of tannin for selective precipitation of Ge was investigated during the processing of industrial materials from Yunnan Chihong Zinc and Germanium (China) [12], Cinkur Plant (Kayseri, Turkey) [15], or Puertollano IGCC power plant (Spain) [16]. The composition of the solution used for the precipitation of Ge-tannin complex is different and depends on the source of initial material. Leachates from processing of coal fly ash or zinc refining by-products besides germanium may contain Zn as well as Ni, V, Sb, As, or Fe [8; 12; 16; 17]. However, there is no information concerning processing of industrial solutions containing Ge, Zn, In, and Sn.  
In this study, the precipitation of Ge from sulfate solution containing Sn, In, and Zn using technical and pure tannic acid was analyzed. The detinning operation, which was an essential part of the entire process, was also described. The initial solution was obtained after the leaching of the dorsal coming from thermal oxidation of by-product alloy from New Jersey process, described in the previous study [18; 19].