Indium's smelting methods are currently divided into two kinds of primary and renewable. Primary indium refers to the direct extraction of metallic indium from raw ore. In addition, since most of the indium is symbiotic with zinc, lead, copper, and tin, which are similar in nature, we can also recover indium as a by-product from the smelting process of lead, zinc, copper, tin, and other ores. This method becomes recycled indium. Recycled indium accounts for 90% of global production.

At present, indium is mainly based on extraction-electrolysis. The main steps of the extraction-electrolysis method can be simplified as: indium-containing raw materials → enrichment → chemical dissolution → purification → extraction → back extraction → zinc (aluminum) replacement → sponge indium → electrolytic refining → refined indium.

Among these steps, the core step is the extraction of indium metal. Some interesting extraction methods are listed below.

•  Direct solvent extraction of indium from a zinc residue reductive leach solution by D2EHPA^[21]

Figure1. Flow chart about direct solvent extraction of indium from a zinc residue reductive leach solution by D2EHPA

With zinc residue as the indium source, non-zinc electrolyte as the leachate, and zinc concentrate as the reducing agent, at 90℃, the indium source solution reduced the leached zinc residue at a liquid-solid ratio of 190 g / L H₂SO₄. Subsequently, at a certain volume ratio, the organic phase and the aqueous phase are extracted and stripped in an extraction machine. D2EHPA is used for extraction and HCl is used for stripping.^[21] Figure 1. shows the basic process flow sheet about the method. It can be detected that in the process Input high concentration of HCl and low concentration of In solution to obtain a high concentration of In solution and the HCl solution can be reused. Therefore, direct solvent extraction using D2EHPA is an effective method for the concentration and recovery of indium from low concentration solutions, and is suitable to recovering indium from zinc residue reductive leach solutions.

• Solvent extraction of indium by IBZF ^[22]

Preparation of IBZF： ZnO, Fe₂O₃ are mixed in a stoichiometric ratio with zinc ferrite and 0.3 wt. After% indium, the mixture was completely mixed by ball milling for 1 hour. Subsequently, the product was sintered in an oven at 1000 ℃ for 4 hours, washed in 1 M HCl at 40 ℃ for 20 minutes, and finally sintered again at 1000 ℃ for 1 h.

The flask was placed in a constant temperature water bath equipped with a reflux condenser, mechanical stirrer, thermometer and sampling device. When the temperature reaches the appropriate range, use deionized water, sulfuric acid and IBZF to extract indium.

• Ultrasonic-Assisted Acid Leaching of Indium from Blast Furnace Sludge ^[23]

The blast furnace sludge was dried to constant weight under the condition of 80 degrees Celsius, and then, the sample was mixed with 150 g/L sulfuric acid solution and placed in a water bath. At a suitable temperature, the mixed solution is stirred by ultrasound, and the particle distilled water is replenished at regular intervals. Indium is extracted in the supernatant.

In the indium-rich solution extracted, indium is electrolyzed to obtain metallic indium.

Reference：

[21] Xingbin L, Zhigan D, Cunxiong L, Chang W, Minting, LiGang F, Hao R. Direct solvent extraction of indium from a zinc residue reductive leach solution by D2EHPA[J]. Hydrometallurgy, 2015.

https://www-sciencedirect-com.ezproxy.library.qmul.ac.uk/science/article/pii/S0304386X15300098

[22] Zhang Y , Li X , Pan L , et al. Studies on the kinetics of zinc and indium extraction from indium-bearing zinc ferrite[J]. hydrometallurgy, 2010, 100(3-4):172-176.

https://www-sciencedirect-com.ezproxy.library.qmul.ac.uk/science/article/pii/S0304386X09002588

[23] Shen X , Li L , Wu Z , et al. Ultrasonic-Assisted Acid Leaching of Indium from Blast Furnace Sludge[J]. Metallurgical & Materials Transactions B, 2013, 44(6):1324-1328.

https://link-springer-com.ezproxy.library.qmul.ac.uk/article/10.1007/s11663-013-9936-3