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Mining gold with moly

03/04/2018

Modern electronic gadgets contain dozens of metals crammed into the smallest possible space, making them nearly impossible to disassemble for recycling at their end of life. Highly reactive supercritical water can facilitate ‘mining’ precious metals from this electronic waste. Because of the corrosive process environment, molybdenum-containing nickel alloys are used.



It is estimated that there will be nearly 7 billion smartphones in operation by 2022, not including unused and unconnected devices. This mountain of future electronic waste, or e-waste, contains tiny amounts of valuable metals that are part of a giant ‘ore deposit’, distributed around the world. Gold, palladium, silver, and copper can all be ‘mined’ from discarded devices by recycling.

One innovative new process for recycling currently being developed uses supercritical water, which is a powerful oxidizer and a highly-corrosive reactant. It turns carbon-containing matter into gas and dissolves other substances that are insoluble in plain water.

A French research team designed a laboratory-scale supercritical water reactor which could operate at 500°C and 250 bars pressure, but needed a material which could withstand the aggressive environment.

The team, based at Orléans University, chose the nickel-base superalloy Inconel® Alloy 718 containing 3% molybdenum to meet these requirements. The alloy possesses excellent strength, toughness, and stress corrosion-cracking resistance. The team’s laboratory-scale results have been impressive. When electronic circuit boards are immersed in supercritical water for a few hours the water decomposes their polymer resins to a crumbly residue but leaves the metal components intact, allowing their various materials to be separated for further recycling.

The project currently uses experimental vats with small capacities, with a 10-liter vat scheduled to begin operating in 2020. This next development phase will be a semi-continuous closed-cycle processing batches of circuit boards and recovering the gases produced by the reaction to reduce energy consumption and process cost. The commercial process envisioned by the team aims to handle the components of about one million phones annually.

You can read more about this process in MolyReview, in which we showcase some of the most interesting and amazing uses of molybdenum. Jump straight to this article here.

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