Urban mining with liquid-free recovery of noble metals

Prof. Antti Karttunen (antti.karttunen@aalto.fi)
MSc Jarno Linnera (jarno.linnera@aalto.fi)
Aalto University, Department of Chemistry and Materials Science

One goal of the CloseLoop research consortium is to improve the recovery processes of valuable raw materials from sources that are currently uneconomic. The limited supply of valuable metallic elements requires increasingly sophisticated actions to improve the existing recycling processes towards true circular economy. The scientific as well as the industrial interest in the extraction and separation of valuable components from municipal solid wastes, the so called “urban mining”, is steadily growing. Such components contain valuable metals of lower availability, such as, for example, indium, the noble, or the rare-earth metals. These are usually included in different parts of electronic devices, supplies, or catalysts in quite a low percentage. For example, a cell phone contains about 40 chemical elements, including 300 mg of silver, 30 mg of gold, and also “hazardous” elements such as lead and mercury. Currently, less than 30 of these elements are economically recoverable. The gold concentration in a cell phone is many times times higher than it is in gold ore. Furthermore, it is estimated that electronics in landfills contain large amounts rare-earth metals, while less than 1% of rare-earth metals are currently recycled (Figure 1).

Figure 1. Global postconsumer recycling rates for various metals. Source: United Nations Environment Programme, Recycling Rates of Metals (2011) / C&EN May 30, 2011..

In the past two decades, the most active research area on the recovery of metals from electronic scraps has been the recovery of precious metals by so-called hydrometallurgical techniques. The initial step of any hydrometallurgical recovery method is the process of leaching – extracting a soluble constituent from a solid by means of a solvent. Typical example of a leaching agent used in the recovery of precious metals is cyanide. However, the main problem of the classical hydrometallurgical methods is that they produce large amounts of non-recyclable products dissolved in huge volumes of water. For example, a series of environmental accidents at various gold mines, causing severe contamination of rivers and groundwater, has led to widespread concern over the use of cyanide as a leaching reagent.

One way to improve the sustainability of urban mining is to develop for example cyanide-free leaching methods, a project currently ongoing in the group of Prof. Mari Lundström at Aalto University. Another approach is to develop so-called liquid-free recovery processes of noble metals. In a collaborative project with the group of Prof. Florian Kraus at the University of Marburg, we have recently considered the possibility of using fluorination methods for the recovery of electronic wastes.  The simultaneous treatment of all noble metal compounds in urban solid wastes to convert them into complex fluorides may present interesting possibilities for new sustainable urban mining techniques. In comparison to traditional hydrometallurgical techniques for the homogenization of materials, this method is more convenient mainly because of the fact that there are no liquid wastes formed during the first stage of the process.

During the collaboration we have discovered and characterized several novel inorganic compounds that are potential candidates for the liquid-free recovery of noble metals (Figure 2). In the long term, new scientific breakthroughs in this field will be beneficial for improving the recycling rates of technologically important metals, bringing the society closer to a true circular economy.

Figure 2. Three different fluoridobromate anions. Tetrafluoridobromate (left), heptafluoridodibromate (middle), and decafluoridotribromate (right). Bromine atoms are brown, fluorine atoms blue.

 

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