Numerical investigation of breakup process of molten blast furnace slag through air quenching dry granulation technique

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ORIGINAL PAPER

Numerical investigation of breakup process of molten blast furnace slag through air quenching dry granulation technique Li-li Wang1 • Yu-zhu Zhang1 • Yue Long1 Received: 31 October 2019 / Revised: 10 January 2020 / Accepted: 22 March 2020  China Iron and Steel Research Institute Group 2020

Abstract Molten slag is broken up by supersonic air into droplets through the air quenching dry slag granulation technique. The breakup process of blast furnace slag directly determines the droplet diameter and the waste heat recovery. In order to gain deep insight into the granulation mechanism and visualize the breakup process, three-dimensional unsteady numerical simulation based on the k-x based shear stress transport turbulence model was conducted to simulate the transient breakup process of molten slag (k is the turbulent kinetic energy, and x is the specific dissipation rate). The coupled level-set and volume-of-fluid method was utilized to capture the sharp air–liquid interface. The results show that a flat film is formed firstly under the effects of air impingement, recirculation zone and pressure gradients. Then, the axial wave and the spanwise wave appear simultaneously and the film is broken up into ligaments owing to the generation of vortex and hole structure at the intersection of axial trough and spanwise trough. Finally, the ligaments are broken up into droplets owing to Rayleigh–Taylor instability at the air–liquid interface. The droplets smaller than 3.00 mm account for 80%, with the average diameter of 1.95 mm. Keywords Breakup process  Granulation mechanism  Molten slag  Air quenching  Computational fluid dynamics

1 Introduction Blast furnace slag (BFS) is the main by-product in ironmaking process. As its discharge temperature is between 1450 and 1550 C, each ton of slag contains a large amount of high-quality thermal energy which is equivalent to 60 kg standard coal [1–3]. Thus, there is a great potential for waste heat recovery from high-temperature molten slag. In addition, blast furnace slag is rich in SiO2, CaO, Al2O3 and MgO. Accordingly, vitreous phase can be obtained when the slag is cooled rapidly and the cooled slag is normally used as building materials [4]. In order to achieve high cooling rate, the water quenching method has been widely & Yu-zhu Zhang [email protected] Li-li Wang [email protected] Yue Long [email protected] 1

College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210, Hebei, China

utilized. Although the vitreous phase content of produced solid slag particles can reach up to 95% [5], this kind of water treatment method brings about several serious problems like the large consumption of water and waste of sensible heat as well as emissions of polluted gases [6, 7]. Therefore, for the purpose of energy saving and emission reduction, alternative techniques need to be developed, and the dry slag granulation concept has received close attention [8]. The air quench