(240g) CFD Modeling of a Molten Slag Jet Free Surface Flow During Mineral Wool Fiberization

Authors: 
Gerogiorgis, D. I., National Technical University of Athens (N.T.U.A.)


Red mud fiberization is a process with remarkable industrial importance, alleviating environmental pressure by ensuring the transformation of this major by-product of aluminium production into useful mineral wool products. The most widespread mineral wool production process is molten rock (in this case, molten red mud) fiberization, either by means of fast rotating spinning wheels (Sirok et al., 2008), or via an impinging air jet; the latter is a promising method which avoids mechanical wear as well as rotating parts, but has not been adequately elucidated. The molten slag which remains after pig iron casting enters via a siphon neck into a heated homogenization reservoir, flows out of a heated ladle orifice at high temperature (1400 °C) and adjustable flowrate, and forms a free-falling vertical jet which visibly radiates its excessive heat: at a given distance, a high-velocity impinging air jet meets the vertical melt jet perpendicularly (or at an angle), inducing intensive droplet generation and breakup. Melt droplets (briefly connected by surface tension and viscous forces to each other) emerge at a high velocity, and nascent fibers are rapidly ejected away from the vertical melt jet, experience rapid cooling along their trajectory and then accumulate in a fiber collection chamber, to undergo further processing towards mineral wool.

Molten red mud fiberization is a free-boundary problem which can be tackled via advanced CFD methods. Typically, a free-boundary problem consists of a set of elliptic partial differential equations (PDE) which must be satisfied within a bounded domain, together with the necessary momentum and heat flux boundary conditions. The actual axisymmetric flow field is not known, but is assumed to be well within a cylindrical bounded domain. A rigorous mathematical formulation has been derived and published by Epikhin et. al. (1981), while other studies focus on multiphase phenomena during jet breakup (Silaev, 1967; Kuan, 2009) and fiber formation (Kulago, 1985). Thermophysical and transport properties of the molten melt have been studied extensively; however, it is unclear how both geometric characteristics (reservoir/ladle/orifice dimensions, melt jet height, impingement angle) as well as operational parameters (reservoir/ladle/ambient temperature, ambient relative humidity, melt flowrate and composition, air jet velocity) affect both the performance and robustness of this novel process, as well as the quality characteristics of the product (fiber size distribution, mass and size distribution of unfiberized droplets).

This paper focuses on high-fidelity CFD modeling of the molten jet free surface flow under external cooling. The CFD model encompasses all interrelated physicochemical phenomena (melt laminar flow, radiative cooling) and considers temperature-dependent transport properties (density, viscosity, surface tension) for the molten slag, in order to understand how the foregoing geometric degrees of freedom (manipulated process variables) affect the shape and temperature of the resulting flow field until the fiberization zone, where it meets the impinging air jet. Sensitivity analyses with respect to the key geometric variables provide further insight and operational guidelines. Experimental validation is also anticipated via data from a novel NTUA pilot plant which is under construction. The validated CFD model will be used for optimization and control studies towards achieving optimal operation.

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