Alcan International Limited

Robert Provencher

Gaston Riverin

A process is described for removing all reinforcing particles contained in metal matrix composite scrap materials. This new technique involves providing a treatment vessel for holding a melt of an aluminum matrix composite to be recycled. Heating means are provided for heating the contents of the vessel to a temperature in the range of 700.degree.-750.degree. C. Mixing of the melt is carried out by means of a power driven vaned rotary impeller and this impeller is operated under different conditions for different stages of the process. In the first mixing stage, the impeller is operated under vortex-forming conditions and a fluxing salt is added to the melt while a gas is injected through a gas outlet beneath the surface of the melt and near the rotating impeller. This causes the salt to rapidly mix with the melt and the inlet gas, providing a uniform distribution through the melt of gas bubbles each surrounded by a layer of liquid salt. As a second stage of the treatment process, the impeller is operated at a high differential velocity between the impeller and the melt. These conditions cause collisions between the salt-coated bubbles and wetted reinforcing particles such as to de-wet the reinforcing particles and cause the reinforcing particles and salt flux to rise to the surface of the melt. The surface layer that is formed is essentially a powder which can easily be removed leaving a substantially pure metal with its chemical composition unchanged.

Recovering clean metal and particulates from metal matrix composites

A process is described for recycling scrap aluminum-matrix composites, e.g. gates and risers from a casting operation. A melt of the scrap composite is provided in a treatment vessel by heating at a temperature of 720.degree.-750.degree. C., such as to avoid unwanted stirring and contamination of the melt. While mixing with the impeller, a gas is injected through a gas outlet beneath the surface of the melt and near the rotating impeller. This melt is mixed with a vaned rotor impeller such that no vortex is formed and oxide film is not drawn into the melt. This treatment gas comprises a mixture of a major proportion of an inert gas and a minor proportion of reactive gas. The gas injection is continued only until small gas bubbles have been well distributed through the melt but before the gas causes the reinforcing parties to start rising to the surface. After the gas is stopped, the melt is allowed to rest for a period of time at the reaction temperature of 720.degree.-750.degree. C. with at least periodic gentle mixing to keep the reinforcing particles distributed evenly throughout the melt. During this treatment, hydrogen is removed from the melt by diffusion into the gas bubbles and the other non-metallic impurities are attached to the bubbles and lifted into a dross layer on the surface by flotation, while the reinforcing particles remain dispersed in the melt. The dross layer containing oxide films and other impurities is skimmed from the surface of the melt, leaving a purified aluminum matrix composite. This composite can then be cast in the usual manner.

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Jonquiere, Canada

Robert Provencher