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Mixing Technology

Urban Mining Made Profitable by Improved Mixing Technology

| Author / Editor: Wolfgang Keller, Annette Rümmele, Dr.-Ing. Wolfgang Last* / Dr. Jörg Kempf

Fig. 3: 2.1 m diameter impeller with solid ceramic blades
Fig. 3: 2.1 m diameter impeller with solid ceramic blades (Source: Ekato)

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On first sight process requirements in urban mining seem to be much less complex as in the classical mining of primary ore bodies since numerous metals are available in high concentrations. Anyhow the presence of many different metals makes the separation to pure end products very demanding and complex.

Besides the direct recovery of metals from waste the recycling from sources as tailings from former ore processing plants gets more and more common. This is mainly due to the fact that technological developments and state-of the-art production processes nowadays allow the reclaiming of these former dumps. Other aspects of the reprocessing of tailings are the extraction of by-products, which were previously not the main focus of the operators. In addition to the expansion of resources such reprocessing of heaps can also serve the protection and preservation of the environment, for example, if the uranium content is reduced or in sulfide ores, the uncontrolled formation of acid is suppressed. However, again the processing methods, especially in the extraction of additional metals, can again become quite complex.

Many of the established and new processes required recovering metals and other raw materials necessitates agitated process steps. Some of the many examples include the recycling of clothing, PET bottles and wood. When it comes to metals examples are the extraction of metals from batteries (e.g. lead), red mud (aluminum), gold tailings and the extraction of zinc or aluminum from fly ashes of waste incineration or coal fired power plants. Many of these recovery processes can be designed using available technologies and know-how, but some require the development of new technologies or process steps. This applies for agitation as for other unit operations as well. Many of the solutions provided rely on the experience and know-how of the equipment suppliers gained over many years for similar or related applications.

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During the development and implementation of new technologies a close cooperation between licensors or end customers with the expertise of equipment suppliers allows developing optimum solutions. This of course applies to the optimization of existing technologies as well and is not limited to the process result itself but also include more economic routes and process steps. Reduced operating and investment costs allow to process low grade or more complex ore bodies with low cost-effectiveness. Mixing as one of the important mechanical unit operations offers diverse opportunities to achieve these objectives. Besides mixing itself other important aspects as materials of construction or mechanical sealing systems have to be included into considerations.

Main Focus on Efficiency and Yield

As already described, many recovery processes are barely cost-efficient, meaning each process step has to be highly efficient and ensure maximum yields. For example in a biological leaching process of gold ores from primary or secondary resources multiple basic mixing tasks have to be fulfilled simultaneously. In a complex biological-chemical reaction sulfur constituents are degraded with the aid of bacteria. These catalyze the breakdown of e.g. pyrite by oxidizing the sulfur and metal using oxygen which is provided by added air. Air feed rates in state-of-the-art reactors with volumes up to 1,500 m³ exceed 10,000 Nm³/h which can only be efficiently dispersed by means of agitators.

Here the efficiency and ability of the impeller to cope with multiply mixing task simultaneously is of utmost importance. One result of a specific new development to accept this challenge is the Combijet impeller. A further technological progress is the implementation of the patented “+”-technology where the added gas is delivered via a central supply line to a rotating distributor integrated into the impeller and transferred into the fluid via hollow agitator arms in the zone of highest turbulence. The Combijet impeller and the “+”-technology already have proven there superiority over other systems in multiple installations. Here power savings of up to 40 % are possible, which for single 1,500 m³ bioleaching reactor resulted in a power saving of approx. € 170,000 per year.

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