To extract gold from rich liquid, zinc replacement method and aluminum replacement method are most popular. And the zinc replacement includes 3 methods according to reaction equipment: filter press method, replacement tank method, and Merril Crowe process. The replacement of gold in the rich solution is carried out according to the following chemical reaction: 2Au (CN) 2-+ Zn = 2Au↓ + Zn (CN) 42 -. This reaction is very rapid and complete.
To recover gold from gold-containing solutions, the zinc powder replacement precipitation method began in 1894. Currently, it is the most popular method widely used. Merril Crowe process, also called zinc powder continuous vacuum precipitation method. It is one of typical methods. In the early days, the reaction happens directly in a filter press or a replacement tank. However, the Merrill Crow method adopts an emulgator. The zinc powder for this reaction is produced by distilled zinc. The zinc powder is continuously fed into the emulsifior through the feeder. Then a strong reaction occurs immediately. After that, gold precipitation happens in a vacuum environment. This method can extract more than 99% of gold. While continuous production, every 3-28 days, the gold sediment will be separated by a plate frame filter press or a chamber filter press. Finally, the lean solution is recycled, and gold mud will be sent to manufacture gold ingots.