Recovery of metal values from zinc solder dross

Zinc solder dross containing 14.8% Sn, 16.3% Pb, 0.41% Al and 64.5% Zn was leached with 3% H₂SO₄ at 45°C for 1 h. Zinc and aluminum went into solution, whereas lead and tin remained with the residue. Aluminum was selectively precipitated as calcium aluminum carbonate by treating the sulphate leachate with limestone at pH 4.8. Zinc sulphate solution was either evaporated to obtain zinc sulphate crystals or precipitated as basic zinc carbonate at pH 6.8. The undissolved lead and tin were leached with 5 M hot hydrochloric acid. The major part of lead chloride ( 73%) was separated by cooling the leached products down to room temperature. From the soluble fraction, tin was recovered as hydrated tin oxide by alkylation with caustic soda at pH 2.4, while the remaining lead was separated at pH 8.5 as lead hydroxide. A process flowsheet had been suggested which involved two-stage hydrometallurgical treatment. Parameters affecting the recovery efficiency of the suggested method such as temperature, time, pH and acid: solid stoichiometric ratio were investigated. Results obtained revealed that the optimum leaching conditions were achieved by using 20 ml of 3% H₂SO₄ acid/g dross for 1 h at 45°C. Recovery efficiency of the metal salts was 99.1, 99.4, 99.6 and 99.5% for Zn, Al, Pb and Sn respectively. Recovery efficiency was related to the solubility of the concerned salts under the given experimental conditions.