Indium, a silvery-white, soft post-transition metal with atomic number 49, is one of the softest elements. Its physical and chemical characteristics place it between gallium and thallium in the periodic table. Indium is relatively rare, with an estimated abundance in Earth’s crust ranging from 0.05 to 0.1 parts per million, a concentration comparable to that of silver or mercury. This scarcity, combined with its distinct properties like electrical conductivity and transparency in oxide form, makes it a valuable component in various advanced technologies. Understanding its sources is important for its continued availability in modern applications.
Natural Occurrence and Extraction
Indium rarely exists as a standalone element or in high concentrations within its own minerals, making direct extraction uneconomical. Instead, it is found as a trace impurity within the ores of other, more abundant metals. The primary natural source is zinc ore, particularly the sulfide mineral sphalerite, which accounts for approximately 95% of refined indium derived from natural deposits. Smaller quantities of indium are also present in the ores of lead, tin, and copper.
Extraction Process
The extraction of indium occurs as a byproduct during the smelting and refining processes of these host metals. During zinc smelting, indium accumulates in the iron-rich residues, from which it can then be separated. Indium-bearing primary ores are found in geological formations such as Mississippi Valley-type deposits, skarns, and polymetallic veins. This byproduct status means that indium production volumes are tied to the global output of zinc and other base metals.
Global Production and Reserves
China has consistently been the leading global producer of refined indium, historically supplying well over half, and at times up to 70%, of the world’s primary indium. Other significant producers include South Korea, Japan, and Canada. For example, Korea Zinc in South Korea is a substantial producer of refined indium, contributing notably to the global supply chain. Canada has considerable mining operations, particularly in British Columbia and Ontario, extracting zinc and lead, and yielding indium as a coproduct. Russia maintains notable deposits, concentrated in regions like the Ural Mountains and Siberia.
Precisely estimating global indium reserves is challenging due to its trace nature within other ores and its status as a byproduct. Theoretical reserves are estimated at approximately 16,000 tons globally, with about 11,000 tons deemed economically recoverable. Zinc deposits alone are thought to contain around 95,000 tons of indium.
Recycling and Urban Mining
Beyond natural geological occurrences, recycled materials represent an increasingly important source of indium. A substantial amount of indium is recovered from end-of-life electronic devices, particularly those containing indium tin oxide (ITO). ITO is a transparent and electrically conductive coating widely used in LCD screens, touchscreens, and solar panels. The practice of urban mining, which involves extracting valuable materials from discarded consumer products, is expanding to meet demand for elements like indium.
Recycling processes involve disassembling electronic devices, followed by various methods like chemical leaching to dissolve indium from the components. After leaching, purification steps are used to obtain high-purity indium suitable for reuse. Recovering indium through recycling reduces the need for primary mining, conserves natural resources, and lessens the environmental impact associated with traditional extraction and processing. This approach also helps mitigate supply chain risks, especially considering that primary indium production is concentrated in a limited number of countries.