Silver foam (also named as silver-zinc crust or silver froth), main content Ag/Zn alloy, is the product of Parkes process when zinc is added to the molten silver-containing lead.

 

Conventionally, the silver foam treatment includes de-zincing by distillation, lead removal by cupellation and silver alloy refined through electro-refining. The zinc distillation process is interrupted and quite energy-consuming as each batch of distillation goes through the steps of temperature rising, distillation and cooling. The lead separation by cupellation firstly oxides lead which has to be reduced to lead bullion before it returns to fire-refining process. Oxidation and reduction of lead, inevitably, requires more energy consumption and results in more metal loss, not to mention the terrible working environment.

 

Taking these challenges, Metalcess developed two solutions after long-term in-depth research and test.

Option 1: using crude zinc as agent for desilvering during lead refining process. The silver-zinc crust is treated by continuous distillation to separate high purity zinc and Ag-Pb alloy. The high purity zinc is casted into ingot for sale. Ag-Pb alloy is further treated by discontinuous distillation to obtain lead bullion and crude silver. The lead bullion returns to lead refining before bismuth removal. The crude silver is electro-refined to produce high purity silver.

Option 2: using refined zinc as agent for desilvering during lead refining process. The silver-zinc crust is treated by liquation and distillation where zinc, lead and silver are separated efficiently at the cost of low energy consumption. The zinc recovered from silver-zinc crust can be recycled as agent to the Parkes process. While the lead alloy can be charged to lead refining pot before bismuth removal.

 

The above two solutions, compared with conventional method, eliminate lead oxidation and reduction steps, thus reducing energy consumption, shortening process flow, requiring less equipment investment, and improving metal recovery. Both solutions are enclosed systems with good working environment.

Metalcess has undertaken two silver foam treatment projects using distillation technology. Its superior technological and economic performance as well as the good working environment is well-received by smelter owners.