Optimization of water leaching of chlorides from aluminum salt slag

Abstract

Aluminum recycling generates large amounts of hazardous wastes, known as salt slags, consisting mainly of oxides, metallic aluminum, and salt fluxes. Water leaching is a common technique used for salt removal, being a decisive operation due to water usage and the need to achieve sufficient salt recovery. In this study, water leaching tests under varied operational conditions (water type, slag particle size, solid content, and leaching time) were carried out in salt slag samples obtained from a Brazilian aluminum scrap melting company. Leaching efficiency was assessed by the % chlorine leached. The optimal leaching condition, defined as the one that resulted in the highest chloride removal from the slag together with appropriate operational conditions (larger viable slag size, lower leaching time, etc.), was identified for a slag size below 2.8 mm, 30 wt% of solids in pulp, and a leaching time of 90 min. The results showed that it was possible to recover more than 92% of the salts initially contained in the raw slag, resulting in a leached material with less than 2.5% salts. The recovered salt characteristics showed potential for recycling and could be re-mixed to the salt flux used for aluminum melting. The possibility of achieving higher efficiencies and lower water consumption during leaching was also discussed, as was a preliminary flowsheet for salt slag treatment.