A new technique enables the use of, for example, cooking oil from fast-food restaurants to dissolve and separate silver. The process requires light and diluted hydrogen peroxide. The technique makes it possible to secure the supply of silver and reduce the burden on the environment.
"Recycling silver from waste materials is becoming increasingly important for securing the supply of this precious metal. It is highly desirable to design new sustainable separation and recycling strategies to replace current processes that strain the environment," says Postdoctoral Researcher Anže Zupanc from the University of Helsinki and the University of Birmingham.
Less than 20% of silver produced annually is recycled, even though the green transition is increasing the amount of waste containing silver. Silver is used, for example, in solar panels. At the same time, mineral resources available for extraction through mining are running out. In the past 25 years, the price of silver has risen sixfold, making its recycling financially viable.
A new recycling concept developed by researchers from the University of Helsinki and the University of Jyväskylä was recently published in Chemical Engineering Journal .
Why does metal dissolve in fats?
To dissolve silver, the most commonly used fatty acids - oleic, linoleic and linolenic - were combined with an aqueous hydrogen peroxide (30%) solution as a green oxidant under mild conditions. This combination effectively dissolved silver into the fatty acids, which acted as both a medium and stabilising ligands for silver ions.
"Computational chemistry enabled us to understand the solubility of metals by investigating the effect of solvents on the thermodynamics of dissolution," says Professor Karoliina Honkala from the University of Jyväskylä.
The results made it possible to explain whether the insolubility of metals is caused by surface passivation or a thermodynamic barrier. Adding ethyl acetate to the silver-fatty acid solution enabled the separation of silver as silver carboxylates from the unreacted fatty acids, which can be recycled. The silver carboxylates were in turn reduced to metallic silver in a light-assisted reduction reactor, an efficient and safe method for separating silver.
Toward urban mining
"The goal of our research is to develop metal recycling techniques from multi-metal substrates using strategies that are inexpensive, sustainable and selective by design," says Professor Timo Repo from the University of Helsinki.
Using fatty acids as solvents has many benefits over using traditional mineral acids and aqueous solutions. In addition to originating in waste material, they are biocompatible, biodegradable, low in acid and non-volatile. This makes them safe and non-corrosive compared with other acids and organic solvents, enabling recycling and reuse.
Since fatty acids are not water-based, metal compounds can be separated from unreacted reaction mixtures by using ethyl acetate and other antisolvents. This allows for both straightforward metal recovery and the recycling of fatty acids. In addition, the possibility of using 30% aqueous hydrogen peroxide as a green oxidant under mild conditions enables urban mining, that is, separating, for example, silver from keyboards with waste silver plating.
