Higher metal recovery by Outotec Pure Jarosite process

Björn Saxén, Marko Lahtinen, Juho Savikangas

Jarosite precipitation is a widely utilized iron removal process in the zinc industry. However, in its standard realization the process has a significant drawback of causing losses of valuable metals like zinc, silver and lead to the jarosite. This comes mainly from the content of these metals in the calcine used for neutralization. Outotec has developed an improved version of the jarosite precipitation process, which minimizes the need for neutralization, minimizing also the metal losses to the jarosite. Thus, the pure jarosite also contains less amount of environmentally hazardous metals. The main feature of this process is to start the precipitation from ferrous iron after having reduced the iron by zinc concentrate in a separate step before jarosite precipitation. Iron is then oxidized back to the ferric form simultaneously with jarosite precipitation. By this method, the formation of sulphuric acid is only half of that compared to the conventional method and neutralization with calcine is avoided. Outotec has now verified this pure jarosite process by extensive laboratory and continuous bench-scale tests. The change to pure jarosite is not only a change of chemistry and flowsheet, as it also emphasizes the requirements for reactor performance. High performing reactors ensure the precipitation efficiency and maximize oxygen utilization; i.e., minimize operation costs. The required efficient mixing and oxygen dispersion capabilities are found in the Outotec OKTOP® reactor series. Optimal solution to guarantee the best technical and operation result is the OKTOP® plant product including not only the reactors, but also all the related equipment for the reactor series. In addition to the jarosite precipitation itself, the change to pure jarosite implies new process steps for reduction and neutralization. However, our case studies show that the increased metal recoveries achieved with the pure jarosite process easily make the investment highly attractive, with a payback time of only a few years. Outotec can design all needed process and equipment changes, and typically delivers at least the new key equipment. One responsible technology supplier ensures efficient and fast delivery and can provide both mechanical and process performance guarantees for the entire delivery.