The US incinerates over 2 million tons of municipal solid waste each year that may contain critical materials such as rare earth elements. These materials are often trapped in discarded products such as cell phones or computer batteries. Yet if there was a way to extract these materials, the waste could provide a new source for critical materials, while reducing some of the environmental impacts of disposing them.

With funding from the DOE Advanced Research Projects Agency-Energy, Wencai Zhang, mining and minerals engineering assistant professor at Virginia Tech, is leading a project to characterize ash from Municipal Solid Waste Incineration, or MSWI. Results of his work can lead to the development of downstream processes that can recover critical materials, such as rare earths or precious metals, from MSWI ash.

According to the DOE’s Advanced Research Projects Agency-Energy (ARPA-E), while some research has shown that concentrations of rare earth elements (REEs) are too low within landfills for significant commercial value, the process of incineration results in a higher concentration of the critical materials, allowing them to be reclaimed cost effectively.

“This is a very novel project,” said Zhang. “Studies on the mechanisms and processes for recovering critical materials from municipal waste incinerated ash are rare.”

Aaron Noble, department associate professor, serves as the project's Co-PI. Also working on the project are department graduate student researchers Bin Ji and Qi Li. Dr. Zhang and his research team have extensive experience studying the recovery of critical materials from different feedstocks.

Municipal solid waste makes up almost 30 million tons of the nation’s waste each year. One method for reducing this volume is through incineration. The process leaves behind fly ash, which often contains significant amounts of harmful heavy metals that must be further treated at additional costs.

Diagram of the project's experimental design process
The US incinerates over 2 million tons of municipal solid waste each year that results in residual ash which might contain critical materials such as rare earth elements. The DOE funded research project will characterize the this ash to develop a way to recover these critical materials, while lessening the environmental impacts of disposing it.

“The residual ash from municipal solid waste, under normal circumstances, must be treated by disposal facilities because it contains significant heavy metals, pollutants, and organic proteins,” explained Zhang. “This process is expensive and poses some environmental risks.”

The project has two primary objectives. The first is to characterize municipal solid waste incineration ash. “Understanding the coversion mechanism, or how critical materials change during incineration, can inform us how to develop a recovery process later for recovering them,” said Zhang. Once characterization is completed, the second objective is to design preliminary flow sheets for downstream processing.

In order to study the conversion mechanism, the team will first need to create a synthesized waste ash. This involves combining in-lab created municipal waste, such as food and plastics, combined with electronic waste, such as batteries or cellphones. The e-waste will be provided by one of the project’s industry partners, Covanta, a U.S. Waste-to-Energy company specializing in metals recycling, ash processing, and waste management solutions.

Once combined, the waste sample will be burned in a furnace under controlled laboratory conditions to create a synthetic municipal solid waste ash. “We will be characterizing not only the ash from our industrial partner, but also the synthetic material we prepare in our lab,” said Zhang.

The materials the team eventually hopes to be able to recover from MSWI ash are rare earth elements, such as Cobalt, Manganese, and Niobium. “Rare earths are the raw materials for the production of batteries and permanent magnets, which are critical to many clean energy technologies, such as the manufacture of turbines for wind energy applications,” said Zhang. "In addition, municipal solid waste incinerated ash also may include precious metals, such as gold and platinum, which could be recovered."

With its goal to achieve 100% utilization, the project has the potential to reduce the environmental impact of MSWI ash by characterizing other, non-critical materials in the ash. “Some portions of the ash do not contain critical materials,” said Zhang. “Our project is going to propose additional beneficial uses for those them.”

The MSWI Ash project has significant impacts for both the environment and clean energy resource industries. “Currently, one of the only ways to use the toxic materials remaining in MSWI ash is to sequester them in products like concrete or cinder,” said Zhang. “If we can find a way to recover those materials, it would not only reduce the environmental impact of treating that ash, but it will also bring added value by developing an new domestic supply stream for critical REEs.”