Prussian Blue has been found to be a promising candidate in the search to find an efficient method to extract precious metals like gold and platinum-group metals from e-waste, producing up to 10-80 times more gold than the maximum yield from natural ores.
According to the United Nation’s Global E-waste Monitor 2020, in 2019, US$57 billion worth of precious metals and other raw materials were produced. Only 17.4 per cent of this waste was recovered, leaving a large chunk of valuable precious metals in the waste largely untouched.
Increasing the yield of extraction of precious metals for nuclear and e-waste would thus be a goal to work towards in the hopes of having a circular economy. Through a collaboration between researchers at Nagoya University and Tokyo Institute of Technology, an unlikely solution for improving the recycling yield of precious metals from waste was found in a compound more well-known for its use in iconic paintings like Starry Starry Night and The Great Wave off Kanazawa—Prussian Blue.
Though immortalised in such paintings, the pigment also has other useful properties, especially in the field of chemistry. It has a jungle-gym-shaped lattice structure and contains nanometer-sized spaces that can take in platinum-group metals.
The collaboration team, comprising Jun Onoe and Shinta Watanabe from the Graduate School of Engineering, Nagoya University, and Kenji Takeshita from Tokyo Institute of Technology, used UV and X-ray spectroscopy to study the precious metal uptake by Prussian Blue.
“I was surprised to discover that Prussian blue uptakes the platinum-group precious metals by substitution with iron ions in the framework while keeping the jungle-gym structure,” Professor Onoe explained. As a result of this unique substitution effect, Prussian Blue has a higher uptake of platinum-group metals and gold as compared to other regular bio-derived absorbents.
Besides aiding in the extraction of precious metals from e-waste, it also helps to remove platinum-group metals from nuclear waste, reducing contamination in the nuclear waste treatment plant, improving the efficiency and lowering costs of treating nuclear waste before disposal.
This is because nuclear waste is combined with silica, heated, cooled, and then stored away in a designated disposal area, in a process called vitrification. During this process,platinum group metals often deposit on the surface of the melter, making the distribution of heat more uneven. The uneven heat affects the waste treatment process negatively, reducing the stability and quality of the final product. By removing the platinum-group metals from the waste before treatment, money is saved from not having to clean the platinum-group deposits off the wall of the melter before each use, and some valuable raw materials may also be extracted from the waste.
“Our findings demonstrate that Prussian blue or its analogues are a candidate for improving the recycling of precious metals from nuclear and electronic wastes,” said Professor Onoe. “Especially when compared to conventionally used bio-based adsorbents/activated carbons.”
With the ever-increasing amounts of e-waste produced, the wastage of valuable and recyclable precious metals cannot be overlooked, especially since the supply of such precious metals from mining will decline in the long run due to resource depletion. Since Prussian Blue can be produced cheaply and in large quantities in the liquid phase, is non-toxic, and is resistant to temperatures of up to 300°C, it offers a promising avenue to achieving an affordable, efficient, and environmentally friendly precious metal recycling process.
Source: Watanabe et al. (2022). The uptake characteristics of Prussian-blue nanoparticles for rare metal ions for recycling of precious metals from nuclear and electronic wastes. Scientific Reports, 12(1), 1-10.