Leaching behaviour of the residue from the thermo-mechanical treatment of Aluminium melting dross

A large amount of currently useless basically oxide residue is produced by the treatment of dross obtained from the melting of alloyed aluminium scrap. Its negligible residual metallic component does not make it feasible for any further metallurgical processing. On the other hand, it may have a high content of chloride salts. Its removal may not only serve the purpose of recycling but it may be mandatory to make this residue suitable for applications in preparing construction and road paving materials, or as a slag forming additive in the steel industry. A hydrometallurgical treatment with pure water – after fine grinding – may be efficient in this respect and it may even reduce the residual metallic content if acidic or alkaline media are also used. In order to develop a suitable method of purification, the composition of the raw residue has been examined by SEM and XRD techniques and carried out experiments to determine the efficiencies of the treatments with water, sulphuric acid and sodium hydroxide. The results show that leaching with water can remove the main salt components of NaCl and KCl in a short time. However, the evolution of some NH3 gas from the side reaction of the AlN compound formed during the preliminary thermos-mechanical treatment cannot be avoided. The application of sulphuric acid can be efficient not only in removing the residual metal content but also in suppressing the evolution of NH3. Although the sodium-hydroxide reagent is capable of aggressively dissolve aluminium not only in the metallic but also in the oxidised states, but it may also enhance the evolution of NH3. According to the phase composition and structure of the treated materials, water leaching perhaps combined with an extra step of sulphuric acid leaching – can be satisfactory for assuring the state of the residual dross to be accepted in non-metallurgical applications.