KEY CONCEPTS
• Flash Joule heating is a new technique developed to recycle electronic waste, extract heavy metals and leave a residue that is clean enough to be used in agricultural applications.
• A high voltage discharge of a capacitor bank raises the temperature of the waste by approximately 3,400 K in about 50 milliseconds. 
• Metal recovery yields are improved through the addition of specific halides.

The movement to battery electric vehicles, which contain a higher content of electronics than internal combustion powered vehicles, highlights the current and growing need to deal with electronics waste in general. The higher demand for batteries will eventually further accelerate this issue.

James Tour, T.T. and W.F. Chao professor of chemistry at Rice University in Houston, Texas, noted in a recent article¹ that 40 million tons of electronic waste is produced annually. Only 20% of electronic waste is recycled leaving the majority to be landfilled. This is not a sustainable approach as recycling will afford the opportunity to reuse precious metals such as gold, rhodium, palladium and silver.

Recycling of metals from electronics waste is one example of urban mining. Tour indicates that the traditional approach for recycling electronic is pyrometallurgy. He says, “Pyrometallurgy involves the heating of metals at high temperature. This is an energy intensive process that also has the potential for exposure to heavy metals such as cadmium, lead and mercury that are quite hazardous.”

A second approach using strong aqueous acid media, hydrometallurgy, also is potentially hazardous since acids, bases and cyanide can be used to leach metals. The process is typically slow, difficult to commercialize and has the potential for producing additional waste such as sludge.

With recycling of metals becoming more of a priority, a new technique that can be highly cost effective, not energy intensive, produce good yields, be commercially viable and produce a residue that can be used in agriculture will be well received. Such a technique has now been developed.

Flash Joule heating
Tour and his colleagues used a technique known as flash Joule heating to rapidly heat up electronic waste and extract heavy metals with recovery yields greater than at least 60% for most metals. He says, “In flash Joule heating, a mixture of electronic waste obtained from a printed circuit board was ground into a small powder and mixed with approximately 30% carbon black by weight. The mixture is placed in a quartz tube and squeezed between a porous copper electrode and a graphite electrode. A high voltage discharge of a capacitor bank connected to the two electrodes leads to passing of current through the voltage and raises the temperature of the mixture to approximately 3,400 K in less than 50 milliseconds. The result is that all of the metals present in the printed circuit board are evaporated while the carbon containing materials are carbonized due to their higher sublimation temperature (estimated to be 3,900 K).”

An image of the process is shown in Figure 2.


Figure 2. The experimental set up used to conduct flash Joule heating of electronic waste very effectively recycles metals and saves energy compared to a commercial smelting furnace. Figure courtesy of Rice University.

A cold trap is used to condense the metals. Tour reports recovery yields of greater than 80% for rhodium, platinum and silver and greater than 60% for gold. He says, “The metal power isolated from the printed circuit boards is not very conductive. To facilitate the rapid heating, carbon black is used to increase the conductivity as the current is passed through the reaction mixture.”

The researchers determined that the addition of specific halides helped to improve metal recovery yields. Tour says, “As an example, we found that addition of sodium fluoride will increase the recovery yield for rhodium to over 80%. When sodium chloride was introduced, recovery yields for rhodium, palladium and gold increased with the recovery yield for the latter increasing to over 80%.”

As an alternative to condensation of metals, the researchers developed a leaching process to isolate the volatile components in the flash Joule heating in the quartz tube. Tour says, “We were able to use dilute acids (such as one molar hydrochloric acid) to separate out specific metals. Our technique is more cost effective and environmentally friendly than the strong mineral acids (an example is aqua regia) used in conventional hydrometallurgical recycling.”

Tour states that the residual material left over after the metals are removed is clean enough to be used in agricultural applications. He says, “Our process is set up to recycle every substance that is in the original electronic waste.”

A very attractive feature of the flash Joule heating process is that it uses much less energy than conventional pyrometallurgical recycling. The researchers indicate that their process requires 939 kilowatt-hours per ton of material processed. This is 80 times less energy than is required by a commercial smelting furnace and 500 times less than laboratory tube furnaces.

Currently, the researchers are able to recycle 100 milligrams of electronic waste and are seeking to scale up to recycling one gram. Tour says, “For this process to recycle tons per day, the flash voltage must increase significantly. Our objective is to commercialize this process.”

Tour is optimistic because the original flash Joule process, used to recycle any type of carbon into graphene, is in the process of being scaled up to recycle one ton per day. This work was originally published in 2020.²

Further information on recycling of metals can be found in a recent article¹ or by contacting Tour at tour@rice.edu.

REFERENCES
1. Deng B., Luong, D., Wang, Z., Kittrell, C., McHugh, E. and Tour, J. (2021), “Urban mining by flash Joule heating,” Nature Communications, 12, Article Number: 5794.
2. Luong, D., Bets, K., Algozeeb, A., Stanford, M., Kittrell, C., Chen, W., Salvatierra, R., Ren, M., McHugh, E., Advincula, P., Wang, Z., Bhatt, M., Guo, H., Mancevski, V., Shahsavari, R., Yakobson, B. and Tour, J. (2020), “Gram-scale bottom-up flash graphene synthesis,” Nature, 577 (7792), pp. 647-651.