Construction is imminent for a carbon-capture project aimed at reducing CO2 emissions from steel production while producing carbon-neutral calcium carbonate. At the U.S. Steel (Pittsburgh, Pa.; www.ussteel.com) manufacturing facility in Gary, Ind., a carbon-capture system designed by CarbonFree (San Antonio, Tex.; www.carbonfree.cc) will capture CO2 emissions from the fluegas of the steel plant and convert it into powdered calcium carbonate for a wide range of applications, including as a component in paints, coatings and adhesives, as well as food, plastics and others.
CarbonFree has developed a process, known as SkyCycle™, to convert exhaust CO2 into high-purity CaCO3. The process works by contacting the flue gas with a magnesium hydroxide solution that captures the CO2 as magnesium bicarbonate (diagram). The magnesium bicarbonate is reacted with CaCl2 derived from calcium-containing slag onsite, allowing the product CaCO3 to precipitate out of solution. Slag is a byproduct of the blast-furnace steel process that contains a mix of metal oxides. The process also involves recovering MgCl2 from the reaction that produces the CaCO3 product and heating it to decompose it into MgOH and HCl, which are then used in another cycle of the process. Treating the slag with HCl can produce the required CaCl2.
“We use LeChatelier’s principle of chemical equilibrium to drive the reactions in the cycle,” explains Bill Bryant, marketing director at CarbonFree. “Instead of talking about the cost of carbon required for carbon capture, this approach allows us to talk about lowering the carbon footprint of our customers’ products, while making a sustainable and profitable business.”
The carbon-neutral CaCO3 product, known as endurocal®, was launched in November 2024, and Bryant says pilot-scale samples of the product are available for applications testing from the company’s demonstration plant at Southwest Research Institute (SwRI; San Antonio, Tex.; www.swri.org).
CarbonFree says endurocal has the same properties as conventionally produced precipitated calcium carbonate (PCC), in terms of particle shape, particle size distribution and purity, but with a lower cost and zero carbon footprint.