Snorre Ballière Farner

Remelting of Aluminium by Continuous Submersion of Rolled Scrap

Doctor thesis of Snorre Farner, NTNU, 22 Dec 2000. Defended 7 May 2001. My supervisor was Prof. Dr. Thorvald Abel Engh.

The abstract (below), the table of contents (PDF) as well as the full text are available online.

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Abstract

When remelting aluminium scrap, metal losses due to dross generation is a common problem. Reduction of these losses will give substantial economic and environmental benefits. Dross is generated when aluminium metal oxidizes and films of oxide envelope molten metal. When a cold metal object is immersed in a melt, the heat of the melt around this is transferred so rapidly into the object that a shell of melt often solidifies to the surface of the object. When scrap with low bulk density is charged to a melt, solidification of melt on the cold scrap prevents melt from entering the cavities in the bulk of the scrap, and the bulk density remains low. Thus the scrap tends to float on the melt surface. Submersion of this scrap is important to avoid oxidation and subsequent dross generation.

One solution to this is to roll scrap to a strip and feed it into the melt. This system has been examined by studying feeding of a continuous, thin aluminium plate into molten aluminium. Also, the effect of lacquer was considered, as well as feeding the plate into a launder with melt flowing along the surface of the plate.

An analytical, one-dimensional, steady-state model is developed to describe the melting and the melting mechanisms. It is based on a shell solidifying on the plate surface and a gap introducing a thermal resistance 1/h_g between the shell and the plate. The thermal resistance 1/h_l of the boundary layer of the melt is included. Depending on these resistances, the initial temperature of the plate and the melt temperature, a shell will form, and the plate will penetrate a distance P into the melt before it melts away.

An experimental apparatus was designed and constructed to feed aluminium plate from a coil into a melt bath at a specified velocity. The plate could be withdrawn rapidly to "freeze" the situation as it was below the melt surface. The penetration depth P of the plate could be measured and shell formation observed.

More than 200 experiments were performed, and by comparing the penetration depth at different feeding velocities and melt temperatures to model predictions, the two heat-transfer coefficients 1/h_l and 1/h_g could be obtained by curve fitting. They agree reasonably well with values found in the literature and calculated from boundary-layer theory. In a few experiments, the plate feeding was recorded on video tape, and the cross section of some plates was studied in a microscope. Feeding of somewhat thicker plates was also tried. This gave valuable background information for comparing the experiments to the model. Snap-off of the plate due to low mechanical strength around the melting temperature may affect the measurement of the penetration depth of the plate.

Attempts were also made to measure the temperature in the plate by attaching thermocouples to its surface. The obtained temperature profiles in the plate were compared to the model predictions, but the method needs improvement.

A criterion for formation of a shell is formulated and tested against experimental observations. Qualitative agreement is achieved. Even if there is no shell formation, it seems that there will be an air film with thermal resistance 1/h_g. This indicates that the melting rate will be independent of whether a shell is formed or not.

Two additional models with only one heat-transfer coefficient are also developed in order to challenge the main model. From this analysis it is found that the use of two heat-transfer coefficients is necessary to describe the system.

The model should be of direct interest when feeding rolled scrap into molten aluminium.

Improvement of the model can be attained by reconsidering the assumptions made, but then numerical methods must undoubtedly be applied. These new models should include the snap-off mechanism.

Figure 1: (a) experimental apparatus, (b) model illustration, and (c) results of still-melt experiments