Modern Vacuum Distillation Technology for Zinc Recovery in Primary Silver Production

摘要

BBH Stolberg is the biggest primary lead plant in Europe, and its products - lead, sulphuric acid and copper-lead-matte - also include silver obtained from the lead concentrate. In the lead refinery, the silver is separated from the lead bullion by the classic Parkes-Process via the addition of zinc, and the resulting product - the Parkes crust - was until recently processed further by the steps of liquation, distillation in retorts under ambient pressure, and cupellation in a classical hearth furnace. The product of the hearth furnace is a dore containing 99.7 % silver.In the autumn of 2010 it was decided to start planning for an increased production of dore and the project, representing an investment volume of EUR 24 million, was approved by ECOBAT in the spring of 2012. The separation of zinc is a key step from both the economic and technological perspective. The expensive zinc should be recycled as completely as possible in metallic form for use in the Parkes-Process; at the same time, the amount of zinc carried over into the subsequent cupellation process should be minimized as zinc may cause dross formation, silver losses and furnace scaling there. After a brief investigation, vacuum distillation was found to be the best available technology for recovering the zinc and two vacuum induction furnaces supplied by Otto Junker were installed in the new silver plant. Vacuum induction technology delivers the best results in terms of batch size, mass transfer time (or batch time), and optimum product purity. As a genuine first in vacuum distillation technology, a multi-frequency furnace was employed to provide optimum melting and distillation conditions through an adjustable bath movement.