The proof-of-concept study, conducted by researchers at North Carolina State University, uses finely ground eggshells directly in the production of magnesium alloys through a process known as friction stir extrusion.
The approach allows calcium-containing materials needed to improve magnesium’s mechanical properties to be produced during the manufacturing process itself, rather than being separately processed from mined ore before being added to the alloy.
The researchers say the method could offer a cheaper and more sustainable supply of calcium while putting a common form of food waste to productive use.
Eggshells are composed of about 95 percent calcium carbonate, making them a potentially useful source of calcium for alloy production.
During friction stir extrusion, the calcium carbonate in the eggshell powder is converted into calcium oxide and calcium, contributing to the formation of Mg2Ca, a compound that strengthens magnesium.
“For example, calcium carbonate and calcium oxide are important materials for manufacturing metal alloys,” said Bharat Gwalani, corresponding author of the paper 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,” he added.
The researchers say eliminating the separate processing of mined materials could reduce both the number of manufacturing stages and the energy required to produce calcium-based alloying materials.
“There are many benefits to this. 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,” Gwalani said.
The study focused on magnesium because of the metal’s combination of low weight and strength, which has made magnesium alloys useful in sectors including automotive manufacturing, aerospace equipment and biomedical applications.
To produce the alloy, the researchers began with a cylindrical block of magnesium and drilled evenly spaced holes into it before filling the openings with finely ground eggshell powder.
The magnesium block was then placed inside a steel cylinder, where a steel mandrel with a central opening was lowered onto it.
Rotating at 300 revolutions per minute, the mandrel simultaneously pressed down on the magnesium and generated friction that mixed the eggshell particles into the metal.
The resulting pressure and heat drove the transformation of calcium carbonate into calcium-containing materials and promoted the formation of Mg2Ca within the magnesium.
At the same time, the downward force pushed the processed material through the central opening of the mandrel, producing an extruded rod of the finished magnesium alloy.
“Calcium is added to magnesium to improve its mechanical properties,” Gwalani said.
“We wanted to see if we could use biogenic waste—eggshells—to produce the necessary calcium materials during the manufacturing process,” he added.
The researchers said the process could have applications beyond eggshells and magnesium, describing friction stir extrusion as a potentially scalable and energy-efficient manufacturing technique.
Earlier this year, the same team demonstrated another application of the process by combining magnetic samarium-cobalt powder with scrap aluminum to produce magnetic composites.
The latest study, published in the Journal of Magnesium and Alloys, demonstrates how a widely discarded biological waste product can be incorporated directly into metal processing to supply useful alloying materials while reducing dependence on conventional mined sources.