Construction is responsible for 25% of global greenhouse gas emissions, with cement production alone contributing 7-8% of total carbon emissions worldwide. The primary type of cement used, ordinary Portland cement, requires a high-temperature calcination and sintering process at temperatures between 900°C to 1500°C to produce from limestone. This process releases carbon dioxide and consumes a significant amount of energy. To mitigate greenhouse gas emissions, it is crucial to develop cement materials with a lower carbon footprint. Coal fly ash, a by-product of coal combustion in power plant furnaces with an annual global production of up to 750 million tons, is a common additive to cement to lessen the amount of cement needed. However, coal ash contains heavy metals such as lead, arsenic, and cadmium, which leach from the cement/fly ash composite when exposed to rainwater.

Our research group at Rice University has developed a flash Joule heating process, which uses electric pulses to heat materials to 3000°C within milliseconds and then rapidly it cools. We initially demonstrated the conversion of various carbon sources, including coal, coke, waste food, plastics, and rubbers, into flash graphene using this technique (https://doi.org/10.1038/s41586-020-1938-0). The ultrahigh temperatures and ultrafast processing also enable precious metals recovery from electronic waste for urban mining (https://doi.org/10.1038/s41467-021-26038-9), and  recycling of rare earth metals from industrial waste streams (https://doi.org/10.1126/science.289.5488.2295). The electricity cost of the flash Joule heating process is approximately $30 per ton of treated materials.

We have now applied the flash Joule heating process to rapidly and efficiently remove heavy metals from coal fly ash. The heavy metals in coal ash, mostly arsenic, cadmium, cobalt, nickel, and lead, have concentrations ranging from a few to tens of parts per million (Figure 1a-b). The coal fly ash is mixed with carbon conductive additives, and a rapid discharge of capacitors brings the sample temperature to 3000°C, subliming the heavy metals. A single flash heating removes 70-90% of heavy metals within one second (Figure 1c). The heavy metals can be collected using a vacuum trap, while carbon additives are converted to flash graphene, which can be recovered by simple sieving and reused. This process works for different types of coal fly ash, including class C and class F, and coal fly ash from different geological origins. The flash Joule heating purification process can also be extended to decontaminate other large-scale solid wastes like bauxite residue (or red mud), a by-product of the Bayer process for alumina production.

The heavy metals purified coal fly ash was then used as a diluent in ordinary Portland cement up to 30% (Figure 1d), which improved the mechanical performance of cement. Basically, the coal fly ash substituted cement paste exhibits about 50% improvement in compressive strength and about 30% improvement in the modulus of elasticity (Figure 1e). Most significantly, the coal fly ash cement had less heavy metal leakage under mimicked acid rain conditions, outperforming traditional Portland cement (Figure 1f).

Joule heating is a highly efficient technique since almost all the electrical energy directly targets sample heating. This is in striking contrast to a traditional furnace that relies on thermal conduction to heat the sample, leading to reduced energy efficiency. The flash Joule heating purification process has an estimated electricity consumption of $21 per ton of coal ash, based on the industrial electricity rate of Texas, US. A life cycle analysis shows that the reuse of purified coal fly ash in cement reduces greenhouse gas emissions by about 30% and heavy metal emissions by about 40%. Meanwhile, the energy consumption is balanced since the production of coal fly ash is much less energy-consuming than that of cement production.

The flash Joule heating process is scalable. Our lab has already achieved a production rate of 10 kg per day for flash graphene synthesis, using an homemade automation system. We are presently undergoing commercial scale-up for graphene production (https://www.universalmatter.com), with a productivity goal of 1 ton per day by Q2 2023. Although designed for flash graphene synthesis, the equipment and process could be adapted for heavy metal removal purposes since the processing is similar.

Looking ahead, our society is experiencing a paradigm shift from the current linear economy to a circular economy. Waste valorization or upcycling provides a pathway to a low-carbon and sustainable economy. Many waste products contain hazardous materials such as heavy metals and organic pollutants that must be removed before they can be resued. The flash Joule heating process, with its ultrafast processing, no water consumption, and low energy consumption, holds great potential for remediating these hazardous materials and repurposing them to value-added materials and products.