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Filters made of metal

Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. : 09 September, 2006  (Technical Article)
Microfilters soon clog up and require elaborate cleaning. A new filter made of a metal fabric simply shakes off the offending deposits. It has an evenly sized pore structure of a defined granularity and is more durable than existing products.
Not everyone appreciates yeast particles in the beer, like those in Bavaria’s famous wheat beer. Many prefer a sparkling clear lager. Brewers therefore remove the suspended particles before the beverage is bottled, using filters with a pore diameter of one micrometer. Microfiltration, as the technique is called, is common industrial practice. It is used, for example, in the sterilization of milk and drinking water. But ordinary filters have certain drawbacks: the particles collect on their surface, impeding the flow of liquid. This makes regular cleaning necessary, during which time production comes to a standstill. A further problem is that the pores are rarely of exactly the same size. The materials of which they are made, typically plastic-coated non-woven fabric, ceramics, or the fine-pored natural mineral diatomite, contain tiny holes of varying diameters. This makes it impossible to extract particles of a specific size from the liquid.

Researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen are developing a new type of filter, which can be cleaned continuously while in use and can be manufactured with pores of precisely the same size. The throughput of liquid, or its so-called permeability coefficient, is also ten times higher than that of conventional soil barriers. This is because the new filter is made of a very fine metallic fabric with very smooth surfaces, which offers little resistance to the liquid flowing through it. To manufacture the perfectly aligned metal grid, the researchers use micro embossing plates onto which they deposit a microscopically fine film of metal. Different hole patterns can be embossed into the layer of metal by modifying the structure of the plates. Standard filter materials have a significantly less even structure.

Because the metal filter is also far more robust and flexible than conventional filter materials, it can be cleaned by mechanical vibration. The collected deposit is shaken off the surface of the filter well before its permeability is affected. “Ceramic filters would break apart under this kind of mechanical load”, says UMSICHT project manager Josef Robert. And non-woven fabrics are usually not robust enough to stand up to this kind of treatment, not to mention the fact that their permeability lessens over the course of time, as more and more particles adhere to the fabric. Robert and his colleagues are now testing various substrate materials for the metal fabric, and adapting the process to different applications and production facilities of different scales.
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