Metallurgical reaction behavior of CO₂ as RH lifting gas

Developing new technologies that can utilize CO₂ as a resource or reduce CO₂ emission is an urgent need in the iron and steel industry. The Ruhrstahl-Heraeus (RH) refining process can effectively remove gas and inclusions from molten steel by applying a high vacuum and intense circulation flow of the molten steel. Meanwhile, at the steelmaking temperature, CO₂ can react with carbon in the molten steel to generate CO bubbles, and this enhances the molten bath stirring strength. Therefore, a technology involving the use of CO₂ as the lifting gas in RH refining was proposed. To study the applicability of CO₂ in RH refining, the favorable conditions and limits of CO₂ decarburization under vacuum conditions were analyzed through thermodynamics. Meanwhile, an industrial test platform for CO₂ as RH lifting gas was set up, and the effects of CO₂/Ar as lifting gas on the refining process of molten steel were comparatively studied through industrial tests. The results show that if only the reaction between CO₂ and carbon is considered, CO₂ can still oxidize carbon elements when the carbon content is less than 1.8×10⁻⁶. However, CO₂ selectively oxidizes carbon and aluminum in molten steel. When the aluminum content is below a certain level, CO₂ mainly participates in a decarburization reaction; otherwise, CO₂ will cause certain aluminum loss. Therefore, if the new process is adopted, the timing and amount of aluminum alloy addition should be considered. In addition, CO₂ can be used as RH lifting gas to obtain a dehydrogenation effect equivalent to or even better than that of Ar. Meanwhile, injecting the same amount of CO₂ did not cause a large temperature drop of molten steel; therefore, CO₂ has the potential to be used as RH lifting gas to complete refining.