Metallic tin recovery from wave solder dross

Lead-free solders used in electronic components are predominately composed of alloys rich in tin. Significant quantities of partially oxidized tin alloy are generated during typical processing operations. The scrap has generally been considered to be tin oxide and suitable for recovery using electrolysis or reduction with charcoal and fuel oil at temperatures above 1000 °C. This experimental work examines alternative processes for the recovery of metallic tin from this scrap. Microstructural and chemical characterisation of the scrap revealed that metallic tin was the predominant phase and was surrounded by a cover layer of tin oxide ranging in thickness from a few μm (micrometres) to 100 μm. It was shown that even the thin layer of tin oxide prevented the coalescence of the metallic tin and inhibited recovery. It was concluded that removal of the oxide layer is the key step required in recovering the metal. The removal of the tin oxide was attempted using mechanical attrition (grinding in a ball mill) and chemical dissolution of the tin oxide layer by means of hydrochloric acid (HCl). The results of attrition in a ball mill indicated that the tin oxide covering layer was effectively reduced from the material over 53 μm and the removed oxide was concentrated in particles less than this size. Furthermore, the remaining tin oxide layer can be almost completely removed by an acidic treatment with 2 M HCl at room temperature and agitation for 30 min. A flow sheet for the recovery of the metallic tin is presented below based on the obtained experimental results.