Mathematical modeling and computer simulation of the rotating impeller particle flotation process: Part II. Particle agg
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I. INTRODUCTION
THE quality of molten metal can be improved by proper control of “unwanted phases” and impurities. In this context, the term unwanted phases refers to exogenous solid particles and/or liquid phases present above the liquidus temperature of the alloy as well as any gaseous phases dissolved in the melt. Among the various unwanted phases, solid particles and films have the most detrimental effect on a metal’s properties. Consequently, various melt treatment techniques have been developed and are employed to remove solid particles and films from molten alloys.[1] Particle flotation using a rotary degasser, also known as fluxing, is one of the most widely used techniques for removing unwanted phases from molten alloys. In this process, a reactive or inert gas, or a combination of both types of gases is purged through a rotating impeller into the liquid metal. The most commonly used reactive gases are chlorine and fluorine, and the most commonly used inert gases are argon and nitrogen. While the gas, in the form of bubbles, rises to the surface, it encounters the particles and carries them to the top slag.[2] The efficiency of particle removal depends on the interaction between the bubbles and the particles. This interaction largely depends on the flow field inside the melt created by the flow of the bubbles as well as the impeller rotation and the size and number of bubbles. Particle removal also depends on the agglomeration of the particles caused by turbulence in the flow field.[3,4,5] The velocity and turbulence fields govern the transport of particles to the bubbles’ surfaces. The addition of chlorine or other halogens affects the surface tension of the bubbles
M. MANIRUZZAMAN, Postdoctoral Fellow, and M. MAKHLOUF, Associate Professor, are with the Materials Science and Engineering Department, Worcester Polytechnic Institute, Worcester, MA 01609. Manuscript submitted May 11, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS B
in such a way as to make the particles stick to the bubbles’ surfaces more efficiently.[6–9] Figure 1 shows a schematic diagram of a rotary degasser. A useful way of mathematically describing the dynamics of particle agglomeration in a rotary degasser is by means of a particle population balance. Although the mathematical formulation of the population balance is rather simple, it cannot be solved analytically to yield the particle size distribution. Moreover, a straightforward numerical approach to the problem puts practically prohibitive demands on computer time and memory. In this study, a mathematical model is presented to describe the agglomeration and removal of solid particles of varying sizes from an alloy melt during rotary degassing. A particle population balance is used to describe the system mathematically, and a special discretization scheme is employed to reduce the computational complexity and computer time required for solving the population balance equation. The model is used to investigate the effect of the rotary degasser’s operational parameters on particle a
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