Targeted recovery of precious metals from catalyst waste using ionic media and ion exchange

Precious metals (PMs) are rare, naturally occurring metallic chemical elements of high economic value. This group consists of gold, silver and platinum group metals (PGM), palladium, platinum, iridium, osmium, rhodium and ruthenium. The production, use and price of PMs has grown considerably in the last century due to rapid growth in demand. The process of metal production from their ores to produce pure metal predominantly takes place through open-pit mining creating a lot of unwanted gases to be released into the environment; high energy consumption and water which cannot be reused due to the presence of hazardous chemicals. Additionally, the growing social concern for the way that metals are mined has created a growing number of activists that are demanding countries and companies no longer obtain their precious metals from countries where metals are produced via eco-unfriendly, immoral and socially damaging practices. An exploration into using more environmentally and economically sustainable methods of recovering PMs from ores and recycling them from waste would significantly improve the supply and demand discrepancy that is currently being seen especially with the metal's iridium, osmium, rhodium and ruthenium. More environmentally benign and sustainable practices will be a greatly sort after methodology due to the increased use of precious metals in the electronics and automobile industries.

Current methods that are being used for PM recovery are usually economically costly, and require the consumption of large amounts of energy and/or hazardous chemicals. Development of more environmentally and economically sustainable recovery methods that will conform to increasingly stringent environmental regulations is highly needed. The growing demand for PMs and the rarity of them makes their recovering from waste highly sort after; current methods do currently have high recovery rates, but are impacted by high cost and high energy usage. Currently activated carbon (AC) and ion exchange (IX) based processes have high rates of recovery but are hampered by slow kinetics and the usage of cyanide for both AC and IX. Additionally, the cost of resins has caused a high cost of the process and high toxicity of the waste is a major drawback of those methods.

Furthermore, the use of Deep Eutectic Solvents (DES) has only recently been used for PM recovery specifically gold and silver. There is no literature on how DES would work with recovery of PGMs. The same applies to metal on metal oxide catalysts (MoMOs), as the PMs are sparingly soluble in concentrated acids. Moreover, the PGMs that are used with metals such as cobalt in catalysts, separating the PGMs from transition metals is currently extremely hard to do in acidic media. There is currently insufficient research regarding the application of DES in metal recovery to be able to develop an environmentally green leaching process. Additionally, being able to use IX and DES combined, with IX having an affinity towards low-grade metals and DES potentially having affinity to separate PMs from transition metals. This combination of unit operation has the potential to create both an economically and environmentally sustainable recovery methodology.