Researchers have demonstrated a technique that uses powdered eggshells to produce high-quality magnesium alloys, which have a wide variety of automotive, aerospace and biomedical applications. The eggshells are used as a low-cost, environmentally sustainable alternative to conventional calcium materials, which are manufactured from ore using an energy-intensive process.
Calcium materials, such as calcium carbonate and calcium oxide, are used in a wide variety of applications.
“For example, calcium carbonate and calcium oxide are important materials for manufacturing metal alloys,” says Bharat Gwalani, corresponding author of a paper on the work and an assistant professor of materials science and engineering at North Carolina State University. “But producing those calcium materials relies on a complex process making use of mined materials. We’ve demonstrated a technique that allows us to skip a step. Rather than processing ore to make calcium materials and then using those materials to produce metals or metal alloys, we’ve shown that you can produce a high-quality metal alloy using eggshells.”
Eggshells are 95% calcium carbonate, and the process the researchers use to incorporate the eggshell into alloys converts it into calcium oxide and nascent calcium.
“There are many benefits to this,” Gwalani says. “There are fewer steps. You have a reliable, sustainable supply chain. Eggshells are inexpensive. And you use far less energy, because you do not have to go through the process of creating calcium products from ore.”
For this proof-of-concept work, the researchers used eggshells to produce stronger, harder magnesium alloys, which have a combination of strength and weight that makes them useful for things like electronics and aerospace equipment.
“Calcium is added to magnesium to improve its mechanical properties,” says Gwalani. “We wanted to see if we could use biogenic waste – eggshells – to produce the necessary calcium materials during the manufacturing process.”
For this process, the researchers begin by drilling a series of evenly spaced holes into a cylindrical block of magnesium. Those holes are then filled with finely ground eggshells. This block is then placed into a steel cylinder. A steel mandrel with a hole in the center is then lowered into the cylinder. The mandrel essentially serves as a pestle in a mortar, pressing down on the magnesium block and spinning at 300 rotations per minute.
This process is called friction stir extrusion. As the mandrel presses down and spins, several things happen at about the same time. The eggshell powder is mixed into the surrounding magnesium, creating friction between the particles of eggshell and the metal. This friction converts the calcium carbonate into calcium oxide and calcium – and produces the high-strength alloy Mg2Ca. Lastly, the downward pressure of the mandrel forces the magnesium alloy out through the hole, producing an extruded rod of the finished product.
“This work shows that you can produce high-value, high-quality magnesium alloys using an inexpensive, sustainable, biogenic waste material,” says Gwalani. “This is a scalable, energy-efficient, and environmentally responsible way to produce magnesium-based composites using biogenic waste materials.
“And we’ve already demonstrated that this approach has applications beyond magnesium and eggshells,” Gwalani says. “We demonstrated earlier this year that you can use the same approach – friction stir extrusion – to produce magnetic composites by grinding magnetic samarium-cobalt (SmCo5) powder into scrap aluminum.”
The paper, “Circular Manufacturing of Mg-Eggshell Composites: Transforming Biogenic Waste into Functional Reinforcements,” is published open access in the Journal of Magnesium and Alloys. First author of the paper is Aniruddha Malakar, a former postdoctoral researcher at NC State who is now at the Pacific Northwest National Laboratory. The paper was co-authored by Fu-Yun Tsai, a postdoc at NC State; Md Jasim Uddin, Charles Perkins and Caleb Schenck, Ph.D. students at NC State; M. R. Gaur, a former visiting scholar at NC State; X. Li, Julian Escobar and T. Wang of Pacific Northwest National Laboratory; X. Ma of the City University of Hong Kong; and J. Jain of the Indian Institute of Technology Delhi.
This work was done with support from the Office of Naval Research Global, under grant N00014-23-1-2758; and from the Pacific Northwest Research Laboratory.
