ABSTRACT: the sources of raw materials for extracting tellurium are introduced, the recovery and extraction processes are discussed, and the advantages and disadvantages of each method are analyzed. Finally, the key research directions of tellurium recovery and extraction technology are prospected.

Keywords: Tellurium; source; recovery; extraction; progress; classification number: TF 843.5; Document Identification Number: A; article number: 1671-1602(2021)07-0015-01; Author: Wang Jiaxin

Tellurium (Te) is a rare metal with a low abundance in the Earth's crust, usually (1-10)10-9[1] , but it is of great industrial significance and has a wide range of applications, mainly in metallurgy, chemical industry, electronics, aerospace, etc. , known as a "modern industry, defense and cutting-edge technology vitamins" [2,3,4,5]. However, there are few independent tellurium deposits, and only one case of the Dashuigou tellurium deposit in Shimian, Sichuan, has been found so far. Tellurium is mostly associated with copper, gold, lead, zinc and other metal deposits. At present, the main raw material for extracting tellurium in industry comes from anode slime of copper electrolysis refining, and its content is usually 1% ~ 10% , other tellurium-containing lead anode slime, sulfuric acid sludge from sulphuric acid plants, bismuth-tellurium concentrates, tellurium-containing dust from electrostatic precipitators in sulphuric acid plants and smelters, as well as tellurium alloy product residues and refrigeration wastes. With the wide application of tellurium, more and more attention has been paid to the recovery and extraction of tellurium.

1.Progress in traditional recovery and extraction processes

 1. 1Recovery of tellurium from copper anode slime

As the chemical composition of copper anode slime from different smelters is different, the recovery methods of tellurium are also different. The processes mainly include roasting, leaching, separation, extraction and refining. According to the different processes can be roughly divided into fire law, wet law and semi-wet law. The commonly used roasting processes are sulfuric acid method, soda method, chlorination and so on. The sulfuric acid method is suitable for copper anode slime with less than 4% tellurium content, but its treatment efficiency is low. Soda Process can deal with copper anode slime with high content of gold and silver, and its efficiency is high, but the harmful substance produced is difficult to deal with. Chlorination is harmful to machinery. In order to reduce the cost of treatment, some smelters directly Leach anode slime without roasting process. According to the chemical composition of copper anode slime, different leaching methods can be adopted, such as oxygen pressure acid leaching, oxidation acid leaching, oxygen pressure Alkali leaching or chlorination leaching. Although oxygen pressure acid leaching is environmentally friendly, the leaching rate of tellurium is not high. Although the leaching rate of tellurium is improved by oxidizing acid leaching method, the purity is not high. The reagent consumption of oxygen pressure alkaline leaching method is large. Although the leaching rate of tellurium by chlorination leaching is high, it will corrode equipment and is not friendly to environment. The methods of separation and extraction include copper powder reduction, sulfur dioxide reduction and sodium sulfite reduction. The process of copper powder reduction is simple and the recovery of tellurium is high, but the cost is high. The recovery of tellurium in sulfur dioxide reduction is high, but sulfur dioxide is toxic. The sodium sulfite process is simple to operate, low cost and high reduction rate of tellurium, but it is easy to pollute the environment. The main methods for preparation of high purity tellurium from crude tellurium are vacuum distillation, zone melting and chemical purification. Due to the high cost and low recovery rate of pyrometallurgical process, most smelters now adopt hydrometallurgical process.

1.2Recovery of tellurium from lead anode slime

At present, pyrometallurgical process is widely used to treat lead anode slime in large-scale smelters at home and abroad. The tellurium slag is enriched in the oxidation refining process of noble lead, and the tellurium slag is milled in a wet ball mill and leached with water. Sodium tellurite is dissolved into the solution and the insoluble impurities are separated. Then dilute sulfuric acid is added to hydrolyze and precipitate the tellurite slag into tellurium(IV) oxide, after calcining to remove the impurity selenium, the purified tellurium can be obtained by electrodepositing tellurium after adding sodium sodium hydroxide, but the recovery of tellurium is not very high, and further optimization is needed.

1.3 recovery of tellurium from gold and tellurium bismuth ores

Tellurium is associated with many gold ores. Extraction of tellurium from tellurium-bearing gold ores usually takes a combined process of dressing and metallurgy, then sulfuric acid is added to neutralize the tellurium(IV) oxide precipitates, which are then further processed to obtain tellurium. Although Dashuigou tellurium deposit is the only independent tellurium deposit in the world, the tellurium-bismuth ore reserves are very large, but only a small part of the tellurium ore is rich in ore, most of the tellurium grade is very low, and tellurium mainly occurs in the tellurium-bismuth ore. In order to improve the recovery of tellurium, mineral processing is generally required. It includes separate flotation, combination of magnetic floatation (Foshan) , combination of magnetic floatation and gravity floatation. At present, the recovery of tellurium from Bismuth tellurium ore is mostly by hydrometallurgical process. Jiang Xinyu and Wu Ping leached tellurium bismuth ore with hydrochloric acid and oxidant respectively, so that tellurium and bismuth could enter the leach solution as much as possible, and could be separated from other impurities, and then the leach solution could be reduced. High Purity tellurium powder [10,11] can be obtained by washing the crude tellurium powder with hydrochloric acid slurry. The extraction and separation of tellurium with hydrochloric acid and sulfuric acid were carried out by Wang Ke. After bioleaching with mixed bacteria, te and TBP were used to separate and purify tellurium from leaching simulation solution.

1. 4Recovery of tellurium from various tellurium-containing dust, waste residue of tellurium alloy products and refrigeration wastes

All kinds of tellurium-containing dust, sulfuric acid sludge, cooling waste and so on, these tellurium-containing materials are generally low in tellurium. Therefore, tellurium can only be enriched in alkali slag by alkali melting, so that tellurium can be converted into tellurite or tellurite which is easily soluble in water, the material is then soaked in water and transferred to a solution, where antimony and Bismuth are removed, sodium sulfide is added, dilute sulfuric acid is added after filtration to produce tellurium(IV) oxide, and slurry is then added to sodium hydroxide to produce sodium tellurite for electrodeposition of tellurite. Tellurium can also be recovered from waste residue in HCl-Fe CL3-H2O system.

2.Study on a new type of recovery and extraction process

Now, with the development of technology, liquid membrane separation method and microorganism method have been improved and applied to tellurium recovery process.

2.1 liquid membrane separation

Liquid Membrane separation is a new type of high efficient separation method, which can separate Te4 + coexisting with other ions with liquid membrane system of N503, L113B, liquid paraffin, sulfonated kerosene and HCL solution. Li Yuping and Wang Xianke studied the migration and enrichment of tellurium in different liquid membrane systems. The results showed that tellurium was enriched in inner phase and the common coexisting anions and cations did not penetrate the liquid membrane. This method can be used for the enrichment of tellurium in copper concentrates, lead-zinc ores, fume dust and alloy. But this approach has yet to gain traction in industry.

2.2 microbiological method

Microbial method is a new metallurgical process which is developed in recent years. Through the interaction between microorganisms and target metal compounds in ores, the useful metals in ores can be leached and high purity target metals and their materials can be made, it has the advantages of simple process, short flow, easy operation, low cost, less investment, friendly to environment and less energy consumption, etc. , and it can greatly improve the metal extraction rate [14] . The foreign research on this technology is very early. In the 1980s, this technology was already applied to the treatment of copper ores. In recent years, it has also been applied to the low-grade metal sulfides. At present, it has been commercialized in the extraction of gold, cobalt and other metals, however, the study on leaching of low-grade tellurium ore is still rare. The research on microbial metallurgy in China started late, in the 1980s, but developed rapidly. In 1981, Dexing mine cooperated with the Institute of Microbiology of the Chinese Academy of Sciences to conduct research experiments, in 1995 and 2002, the "microbial heap leaching -- solvent extraction electrodeposition" experimental plant and the electrometallurgy experimental plant of copper were established respectively in Purple Mountain Copper Mine. For the recycling of tellurium, Rajwade et Al. proposed the microbial reduction technology of tellurium-bearing ore solution in 2003. In 2004, Liao Mengxia and others proposed to develop the first independent tellurium deposit in China by using bio-metallurgy technology. The microbial leaching method was used to treat the asbestos ore in Sichuan

In order to improve the leaching rate of tellurium, the strain was activated, identified by molecular and biochemical methods, and mutagenized. Although microbial leaching is suitable for low-grade tellurium-bearing ores, the problems of low bioleaching efficiency, poor adaptability, long period and difficult cultivation of excellent and efficient bacteria have not been effectively solved.

3. Conclusion

Tellurium is widely used in high-tech industry and national defense, which makes it a very important strategic resource. Due to its extensive use, the world's demand for tellurium has doubled, but the production of tellurium has not increased simultaneously, so the research and development of tellurium recovery technology is of great practical significance. The widely used tellurium recovery technologies have their own advantages and disadvantages, so the development of low cost, high tellurium recovery rate, clean and environmental protection recovery process is the focus of future research. 

Source: Chinanews.com, by Wang Jiaxin