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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

Such new developments are engineered in the Ekato Technology Center. Here test scales from several liters to 100 m³ are available. For the Combijet+ development work was conducted in the 1 m³ and 30 m³ scale as well as directly in a production plant with original ore suspension in a 30 m³ tank. Additional in-house CFD simulations contributed to a further optimization of the impeller geometry (figure 1). Apart from new impeller solutions novel materials of construction quite often play a vital role enabling the success of new processes. These are likewise tested in the Ekato Technology Center and implemented if assessed to be advantageous. For example abrasion and corrosion are common phenomena in many applications, especially in minerals processing and are challenges in “urban mining” applications as well. An optimized process design and the correct choice of material are of utmost importance to prevent or reduce wear and corrosion of impeller blades and other components. This increases the life-time of the equipment and the time between maintenance works.

Practical Example: Extraction of Metals from Fly Ash

One example of a process being operated at extremely violent conditions is the extraction of metals from fly ash of coal fired power plants, which were previously deposited after separation from the flue gas. These metals, such as aluminum, can be extracted from the fly ash in autoclaves at elevated temperature and thus pressure with hydrochloric acid. From the process point of view, the know-how and experience from conventional autoclaves can be applied. However due to the corrosive properties even typically used titanium is no longer an option as material of construction. In this specific application only zirconium or tantalum product-wetted parts are suitable, which increases the investment and maintenance costs and thus puts the economic viability of the process into question again.

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Besides the high investment costs for these materials, the maintenance costs have to be considered since fly ashes are very abrasive and impeller blades will wear out quite fast. Not only costs for the direct replacement of the agitator blades but also plant downtimes must be taken into account. Here solid ceramic materials offer the great advantage that, due to their chemical and mechanical resistance, the service life and thus the maintenance intervals in the described process can be considerably extended.

Before new materials as ceramics can be adapted and approved for new applications an extensive test program has to be conducted. Here mechanical as well as chemical aspects have to be considered. To determine the abrasion performance of different ceramic materials atmospheric abrasion tests in the Ekato Technology Center were conducted in a model system to compare the performance to standard materials. A flat blade disc turbine with different blade materials was operated in the 1 m³ scale at high tip speeds in an abrasive environment for a period of several weeks. In order to quantify the wear, the chronological weight loss of the impeller blades was determined as shown in figure 2. Based on the measurements, it was then possible to draw conclusions concerning the relative wear resistance of the materials and their differences in service life. A direct comparison of solid ceramic with coated components revealed that the lifetime can be increased by a factor of 10–15 with solid ceramic components. Figure 3 as an example shows an impeller with solid ceramic blades already in use with a diameter of 2.1 m.

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