Indium and its compounds have a variety of end uses including electrical components,semiconductors,solders and al-loys,mainly due to their attractive physicochemical properties.Particularly,indium tin oxide(ITO)film composed of 90％indium ox-ide and 10％tin oxide is an indispensable part in liquid crystal displays(LCDs)due to the excellent properties such as electrical con-ductivity,optical transparency,work functionality,as well as thermal reflection.The rapid development of electronic information in-dustry has resulted in a significant increase in the production of liquid crystal display electronic devices such as mobile phones,com-puters,and televisions.This has further resulted in a massive increase in global indium demand in the recent years.However,the con-tent of indium in the earth crust is only(50～200)×10-9 parts per billion and is found mostly in heavy metal sulfide ores with sphalerite as the dominant mineral.Due to the limited primary resources of indium,it will be challenging to meet the increasing worldwide de-mand for indium in the future.Therefore,it is crucial to comprehensively recover indium through secondary resources recycling in or-der to resolve the conflict between the rising demand for indium and the scarcity of its primary resources.Waste liquid crystal display is one of the most significant secondary resources of indium,of which the indium content is much higher than that in natural minerals,being as high as about 1000×10-6.The majority of LCD electronic devices have a limited lifespan of approximately 2～5 years,which contributes to the rapid generation of waste LCDs.Thus,efficient indium recovery from waste LCDs will be of great environmental and economic significance from the perspectives of high indium content and large quantity.The hydrometallurgical technology is currently the most important way to recover indium from end-of-life LCDs because of its advantages such as low energy consumption,simplicity in operation,and less environmental burden compared with pyrometallurgical methods.A hydrometallurgical recycling method general-ly includes the procedures of pretreatment,leaching,indium-tin separation,indium extraction,and refinement.The primary purpose of the pretreatment step is to dismantle the discarded LCDs and subsequently crush the obtained ITO glass into fine particles with the size of around 10 μm.Owing to the complicated construction of LCDs,the existing dismantling method is generally laborious and inef-ficient.Therefore,the development of effective automated dismantling technologies is highly needed.Leaching is the use of acid solu-tion treatment to release indium in the form of In3+ions into the aqueous solution.But,during the leaching process,the co-dissolution of some impurities particularly tin will inevitably occur,generating an impurity co-existing indium containing solution.The separation of indium from the impurities in the complex solution is an essential step for indium recovery.Among the impurities,the separation of tin is the most challenging because its chemical similarity to In3+.Several In3+and Sn2+separation technologies have been developed for the recovery of indium from end-of-life LCDs on the basis of hydrometallurgy.The efficiency and selectivity to indium determine the feasibility of the separation process.After the indium-tin separation,indium should be further recovered from the purified indium-con-taining solution in the form of crude indium metal or indium compounds.Currently,the indium recovery methods mainly include alumi-num or zinc cementation for crude indium production,or the hydrolysis recovering indium in the form of In(OH)3.Nowadays,industri-al applications call for indium with a purity of 4N(99.99％)or higher.The three most important purification techniques at the moment are vacuum distillation,zone smelting,and electrolytic refining,and each method has unique refining impact on certain impurities.On the purpose of high purity indium metal production,these refining methods are generally jointly used according to the quality re-quirement of the products.In this paper,the research progress of indium recovery from waste LCD based on hydrometallurgy had been reviewed,and the advantages and drawbacks of the processes such as pretreatment,leaching,indium-tin separation,indium extrac-tion,and refining had been summarized.Moreover,the future directions of research and development for efficient waste LCD recycling for indium recovery had also been discussed.In the future,more scientific and technical efforts should be made to accomplish the sus-tainable development of indium industry in light of the existing worldwide shortage of indium resources and the rising demand.A reli-able secondary resource recycling system was needed to be built to recover indium from a variety of types of indiums bearing wastes,particularly end-of-life LCDs,to supplement the limited primary resources of indium.Second,more scientific researches should be performed to improve and optimize the indium recovery technologies already in use and further develop new energy-saving,low-cost,efficient,and environmental-friendly recycling techniques.These initiatives would be helpful for supporting the sustainable use of indi-um resources and minimizing the environmental impact of the indium bearing wastes,and further promoting the social and economic advancement.
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