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The successful removal of sulfur (S) from hot metal is exemplified by the Kanbara Reactor (KR) process. However, a clearer understanding of its operating mechanism is needed. This study investigates the role of lime as the primary desulfurization agent, considering its chemical and mechanical aspects. Lime was found to not only chemically remove S but also facilitate the fragmentation of the desulfurization flux, which is crucial for enhancing the interfacial reaction area during vigorous mechanical stirring in the KR process. Fragmentation depends on the liquid-solid volume fraction in the flux. Traditionally, fluorspar has been used as an additive to aid in lime melting, but due to its environmental toxicity, alternative additives have been proposed. In this study, red mud, an industrial waste from alumina production, and spent MgOC refractory, were used as additives to lime, creating a novel desulfurization flux for hot metal in steelmaking. Laboratory-scale tests were conducted with mechanical stirring. A “DeS Index” was introduced to quantitatively assess desulfurization efficiency, considering extent, rate, and cost. The industrial wastecontaining flux showed comparable efficiency to commercially used fluxes, indicating its potential as an effective alternative. This study contributes to a better understanding of the desulfurization process and offers an environmentally friendly option for desulfurization in steelmaking.