Continuous ploughshare mixer with lined mixing vessel
When trying to find the ideal implementation of a mixing application, the mixed product is always the first consideration. It is crucial to have correct and extensive information on this product, and base every detail of the development on this information. However, frequently products have specific properties which push the limits of customary vertical or horizontal mixing systems. Materials with a tendency to cure are one example of this.
Rapidly bonding materials
Examples of such rapidly bonding materials can be found in many fields and industrial sectors: Hardening ashes and ash dusts from combustion plants or power plants containing a high percentage of calcium oxide (CaO) are among them. This is due to the tendency of CaO to react with water. Sludges from production processes may also react with additives, causing them to harden. Similarly, residual sludges have a tendency to harden, for instance if they are mixed with cement to produce a stable product suitable for disposal in a landfill.
What many of these materials have in common is that they are treated in one manner or another, either to make further processing feasible or for efficient disposal. For instance, they are hardened and stabilised through addition of added substances or exhaustive reaction. This conditioning takes place in a mixer. In a continuous process, the materials intended for treatment are fed into the mixer, either from a silo or from filter presses, in the case of sludges. The product to be mixed passes through the mixing vessel in a precisely defined process and is then automatically processed further.
Just one problem remains: When treating or conditioning powder-like or paste-like materials, setting or bonding or both may occur depending on the type of added components. Depending on the degree of hardening, the properties deliberately changed by the process can then cause significant problems in the mixer. A frequently encountered phenomenon is the adherence of hardened materials to the inner wall of the mixer. These adhering materials can lead to oscillation and vibration in connection with severe noise emission, for instance loud “rattling”.
Moreover, the hardened materials increase wear of the mixing tools circulating within the drum. The design of the mixing tool peripherals can only somewhat counteract these negative effects of material adherence.
Non-stick and elastic
lining of the inner wall of the mixing vessel
To solve these problems, Lödige has developed a number of different lining variants especially for the continuous ploughshare mixer. Based on the product properties, a lining made of rubber, polyurethane, polyethylene, teflon or other materials may be used.
Together with the globally renowned Lödige mixing principle and the ideal Froude number, these options provide an ideal combination for difficult product properties of bonding materials. These measures prevent the formation of hardened layers, which may adhere permanently to the inner mixer vessel wall. However, if hardened layers are formed by adhering materials during the mixing process after all, the pressure of the mixing tools against these layers increases. The surface properties of the plastics ensure that the layer is subject to radial and axial movement or is deformed due to a low elasticity (rubber), causing cracks. These cracks in the layer grow with each circulation of the ploughshare shovels, until the layer finally comes away from the wall and enters the mixing process again. This process is repeated continuously. This mixing system permits treatment of materials for which conventional mixers were unsuitable. The design is available in different sizes for throughput rates of up to 1000 t/h.
Lödige puts particular emphasis on easy servicing and the simplest possible replacement of wear-prone and high-maintenance mixer components. For this reason, an optional design of the mixing vessel with divisions across the entire length is available in addition to the design with lateral inspection doors. This solution permits unimpeded access to the entire mixer interior, permitting quick detection of the condition of wear parts and, if necessary, replacement without long downtimes, ensuring the system will soon be ready for operation again. The plastic lining can – if necessary – be replaced directly on the system.
Conclusion
Treatment of materials prone to bonding in conventional mixers is difficult or even impossible due to the adherence of hardened layers on the inner wall of the mixing vessel. Lödige has responded to this problem by designing a system that permits lining of the inner wall of the vessel with a plastic material suitable for the product properties. These linings prevent the formation of hardened layers, as the mixing product can no longer permanently stick to the wall. This permits treatment of materials for which conventional mixers are unsuitable.
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