Leaching Behavior of the Anode Slime Generated by the Electrorefining of Sn-Ag-Cu Alloys

Abstract

The waste material produced in lead-free wave soldering is melted, skimmed and cast into anodes. Anode slime, the by-product of electrorefining in hydrochloric solutions is collected and the basically undissolved silver and copper constituents are removed by selective leaching. The first step is the elimination of metallic tin using HCl of high concentration and temperature, followed by the leaching of the nobler metallic components in HNO 3 under similar conditions. Kinetic study on the leaching behavior of the raw anode slime in both media has shown the optimum conditions. The metallic tin content of the anode slime can be dissolved virtually completely in 10 M HCl at the highest possible temperature of 85 o C, however it takes about 3 hours. On the other hand, silver can be o quickly dissolved in about 15 minutes by applying approx. 50% HNO 3 at 90 C. Combination of the two steps in this order can result in an efficient recycling of the slime by producing relatively pure SnCl 2 and AgNO 3 solutions, respectively. The tin content can be recycled conveniently to the tin electrolysis and the silver containing solution can be used for recovering silver by electrowinning or precipitation.