Utilization of secondary zinc

Utilisation of zinc ashes from galvanizing plants and cathod sheet melting shops of electro-winning zinc plants, and of zinc hydroxide from sodium hydrosulphite industries, has been examined with a view to production of either electrolytic zinc or pure zinc oxide, meeting the specifications of rubber grade. Zinc ash from cathode sheet melting shops could be dechlorinated to the tolerable level by calcining at 900°C for 45 min and could be taken directly to the leaching stage in the conventional electrolytic zinc circuit. In some cases, with galvanizer's ash, washing with 1% NaOH solution is also found to remove chlorine to the desired level. Metallic zinc in the galvanizer's ash could be separated by a selective leach in ammoniacal ammonium carbonate media and pure zinc oxide could be produced from the leached zinc content by precipitation as zinc carbonate. Grinding and sieving could also separate out the heavier metallic zinc portion, and the chlorine from the lighter fraction could be removed either by calcination or by washing with sodium hydroxide solution. Direct calcination of the washed zinc hydroxide sludge from Na2S2O4 industry gives zinc oxide with a higher lead content. Calcination of zinc carbonate obtained from zinc hydroxide by ammoniacal ammonium carbonate leaching gives pure zinc oxide. (Sarvashri G. Basu and P.K. Sinha, Senior Scientific Assistants; Shri S.C. Aush and Dr. N. Dhananjayan, Scientists; and Prof. V.A. Altekar, Director, National Metallurgical Laboratory)