Dr Zi Li and Dirk Stocker examine the advantages of metallic membranes over standard sintered metals filter media. These include improvements in permeability and backflushing performance.

The most commonly used powders for porous sintered metals are bronze, stainless steel, nickel-based super alloys and titanium. The properties of porous sintered elements can be summarised as follows:
* Stability of shape, for example self-carrying structures even at high pressures.
* Good mechanical strength under permanent loads, impact loading or under pressure peaks.
* High temperature strength and thermal shock resistance. Special high temperature alloys can operate up to 1000°C.
* Chemical stability against acids and (alkaline) detergents.
* Wide spectrum of precisely defined pore size distributions: for filters from metallic powders ranging from 0.1µm to 200µm, and from 3µm to 40µm for sintered metal fibre felt.
* Reverse flow, supersaturated hot steam, chemical agents or high temperatures can be applied for cleansing and rinsing.
* Easy to machine, weldable.
As all technologies and products sinter metal filters have also some limits. Therefore the research is ongoing to eliminate these weak-points. This has led to the development of the metallic membrane the improvements of which can be summarised as follows:
* Improvement of the backflushing performance. Therefore the new filter type had to be designed as a surface filter.
* Improvement of permeability in comparison to standard sintered metals filter media.
* Defined particle retention down to sub micron range.
* Possibility to extend the typical applications for sintered metal filters using the new powder metallurgy approach in the area of the micro filtration.

Because of their properties, metal filter composites' structures are extensively applied in a multitude of different applications. Besides the well-known advantages of stainless steel filter cartridges the wall thickness of 2 - 3mm, which is necessary for the self supporting system, causes a high pressure drop. The wall thickness has to provide sufficient mechanical strength but is not absolutely necessary for particle retention. The separation of particles takes place on the surface of the filter material and in the first tenths of a millimetre of the wall thickness. The other requirements on a filter element are a high flow rate and a good back flushing performance.
To achieve high flow rates without loosing retention performance a lot of different developments of asymmetric composites have been done in the past. These are powder/powder, powder/screen, fibre/screen and powder/fibre composites. All of these different types have their special advantages and limitations. In many cases only the fibre/screen composite has found it's own market for filter applications. The reason why the other types did not really succeeded are as well technical and economical.
The new developed product called SIKA-R ... AS is a powder/powder composite consisting of a coarse metal powder which is the support material and a thin active filter layer (< 200µm) of the same alloy. The thin metallic membrane is applied by a separate process step. The support material and the active filtration layer are diffusion bonded during the sintering process. Fig. 2 shows a micrograph of the asymmetric structure. Metallic membranes can be produced in disk and tube shapes of various sizes.
The behaviour of the function of pressure drop versus wall thickness for conventional and asymmetrically designed filter elements can be demonstrated. Two advantages of the asymmetric design can be easily explained. The first one is the lower pressure drop during the filtration and the second advantage is that this low pressure drop makes it easier to clean the filter element during the back flushing step. Besides the low pressure drop the particle retention behaviours are one of the most important characteristics of filter elements.

In comparison to the conventional sintered metal filters the metallic membranes show tighter pore size distribution. Especially no pore larger than 1µm could be observed in the new material. The conventional sintered metal filters show pore sizes up to 4.5µm. This is one of the important improvements if we are talking about surface filtration. The tighter pore size distribution guarantees, that particles will not move into the filter material and block it over the time, but are kept on the surface of the filter media. The two items, low pressure drop and real surface filtration, give higher operation life time which will reduce the replacement and cleaning costs.
The biggest advantages of SIKA R AS are:
* Coarse support material provides mechanical strength without significantly reducing the flow rate through the filter element.
* Thin filter active layer to achieve a higher flow rate (3 - 4 times) without loosing particle retention performance.
* Improved back flushing performance.
* Narrow pore size distribution.

Applications

The first applications for the newly developed metallic membranes were parts of the traditional market of sintered metal filters. Especially in the field of catalyst recovering, which is important in many chemical processes, the potential of the improved surface filtration properties prolongs the life time on stream. While the conventional sintered metal filters, due to filter clogging, have to be cleaned in an external process after for example 4 weeks the process can continue for 6 - 8 months by using the Metallic Membranes before an external cleaning process is necessary. After external cleaning the filter can be used again.
The industries affected can be summarised as follow:
* Chemical industry (especially petrochemical industry).
* Food and beverage industry.
* Cross flow applications.
* Water filtration.
* General process engineering.
* Laboratory technology.
* Medical technology.
* Environmental technology.
* Filtration of rooling, lubricating and cleaning agents.
* Recovering of catalyst particles.

Enquiry No 71

Dr Zi Li and Dirk Stocker are with GKN Sinter Metals Filters GmbH, Radevormwald, Germany. www.gkn-filters.com