**Alberto N. Conejo**

School of Metallurgical and Ecological Engineering, University of Science and Technology, 30 Xueyuan Road, Haidian District, Beijing 100083, China; aconejonava@hotmail.com or aconejo@ustb.edu.cn

Received: 9 April 2020; Accepted: 8 May 2020; Published: 27 June 2020

**Abstract:** Steelmaking involves high-temperature processing. At high temperatures mass transport is usually the rate limiting step. In steelmaking there are several mass transport phenomena occurring simultaneously such as melting and dissolution of additions, decarburization, refining (De-P and De-S), etc. In ladle metallurgy, refining is one of the most important operations. To improve the rate of mass transfer bottom gas injection is applied. In the past, most relationships between the mass transfer coefficient (*mtc*) and gas injection have been associated with stirring energy as the dominant variable. The current review analyzes a broad range of physical and mathematical modeling investigations to expose that a large number of variables contribute to define the final value of the *mtc*. Since bottom gas injection attempts to improve mixing phenomena in the whole slag/steel system, our current knowledge shows limitations to improve mixing conditions in both phases simultaneously. Nevertheless, some variables can be optimized to reach a better performance in metallurgical ladles. In addition to this, the review also provides a state of the art on liquid–liquid mass transfer and suggests the current challenges in this field.

**Keywords:** mass transfer coefficient; mixing time; physical modeling; mathematical modeling; kinetic models
