Indium Refining & ITO Semiconductor Lab

Sphalerite zinc concentrates contain 50 to 1,200 ppm indium. In roast-leach-electrowin (RLE) circuits, indium concentrates in the neutral leach residue (lead-silver jarosite cake). High-temperature concentrated sulfuric acid leaching at 85°C to 95°C solubilizes indium along with iron and zinc: In2O3 + 3H2SO4 -> In2(SO4)3 + 3H2O. Crucially, because ferric iron (Fe3+) co-extracts with indium into di-(2-ethylhexyl) phosphoric acid (D2EHPA / P204), the pregnant leach solution is pre-reduced with zinc dust or SO2 gas to convert Fe3+ to non-extractable ferrous iron (Fe2+). D2EHPA in kerosene then selectively extracts In3+ with separation factors over 1,000 relative to Zn2+, Cd2+, and Fe2+. Stripping with 4 to 6 M hydrochloric acid produces an ultra-concentrated indium chloride eluate.

Indium Tin Oxide (ITO, typically 90 wt% In2O3 and 10 wt% SnO2) is the industry standard transparent conductive oxide (TCO). In2O3 is a wide direct bandgap semiconductor (~3.6 eV), rendering thin films (>85%) transparent to visible photons. Doping with Sn4+ substitutes for In3+ cations, contributing free electron carriers that yield low electrical resistivity (10^-4 ohm-cm) and low sheet resistance (<10 ohms/sq). Sputtered ITO films serve as transparent pixel electrodes in smartphones, OLED/microLED displays, capacitive touchscreens, smart architectural electrochromic windows, and heterojunction (HJT) silicon solar cells.

Planar magnetron sputtering utilizes crossed electric and magnetic fields to confine argon plasma, creating a localized high-density erosion groove known as the 'racetrack'. As a consequence, only 25% to 35% of the sintered ITO target mass is eroded onto display glass before the target reaches end-of-life and must be replaced to prevent backing-plate burn-through. Closed-loop hydrometallurgical recycling facilities dissolve spent target remnants in hydrochloric acid, recover over 98% of the high-purity indium via solvent extraction or ion exchange, and resynthesize fresh ITO sputtering powder, reducing primary indium demand and cutting fabrication costs by 40%.

Crude indium sponge produced by zinc cementation is cast into anodes and electrorefined in indium sulfamate (In(SO3NH2)3) or chloride electrolytes at 80 to 150 A/m2, yielding 4N (99.99%) commercial metal. For advanced optoelectronic applications like Indium Phosphide (InP) semiconductor laser diodes, photodetectors for datacom transceivers, and high-electron-mobility transistors (HEMTs), purity must exceed 6N (99.9999%) or 7N (99.99999%). This is achieved via multi-stage quartz high-vacuum distillation at 800°C to 950°C and 10^-3 Pa to volatilize high vapor-pressure impurities (cadmium, zinc, thallium), followed by multi-pass horizontal RF induction zone refining to segregate trace transition metals to the ingot ends.

The Levelized Cost of Indium (LCOIn) is determined by zinc residue feed conditioning and freight ($15 to $25/ton), concentrated sulfuric acid consumption (1.2 to 2.0 tons H2SO4 per ton residue), D2EHPA solvent replenishment, zinc dust reductant, electrowinning electric power (~8.0 kWh/kg In), and high-density ITO sintering plant capital amortization. For an integrated 60-tonne/year refinery, net levelized costs range between $160 and $280 per kg In, allowing healthy EBITDA margins against historical market prices ranging from $240 to over $450 per kg.