LU85124A1 - FILTRATION PROCESS IN AN ALLUVIONIZED FILTER, FILTERING MATERIAL THEREFOR AND USE THEREOF - Google Patents

Description

9.

FILTERING PROCESS IN AN ALLUVIONIZED FILTER, FILTERING MATERIAL THEREFOR AND USE THEREOF.

The present invention relates to a method of filtration in an alluvial filter as well as, more specifically, the use of filter materials determined in said type of filter.

We know (cf. RH PERRY and CH HILTON -Chemical Engineers' Handbook, 5th ed., P. 19, 83) that the 10 alluvial filters are generally formed of filter trays, usually made of stainless steel, inside of which a support supports a filter cloth. The filtered liquids are evacuated through the support towards a hollow central axis on which the trays (20 to 15 100 per filter) are stacked. At the time of their commissioning, the filters are alluviated with a thick suspension of filter material so as to create a pre-coating (precoat) on the filter cloth. Filter material can also be added to the liquid to be filtered as a filter aid in order to delay clogging of the filter by formation of the filter cake. At the end of each filtration cycle, the cake and the pre-coating layer of the filter cloth are destroyed by rotating the filter trays.

As filter material, diatomaceous earth is used, in particular kieselguhr. These products have the advantage of being inexpensive and of allowing, within certain limits, to specialize in the different natural deposits ("tripolis") for the various applications. However, they have several major drawbacks: they cannot be incinerated and therefore cause difficulties in getting rid of filter cakes, which are often very polluting; it is difficult to select, for example by sieving, ventilation, etc., particle sizes which are perfectly suitable for the different applications; these / products have a good chemical behavior in medium or 1 slightly acid, but it is impossible to use them in alkaline medium because of the presence of silica; finally, this same silica poses problems from the point of view of industrial hygiene because there is a risk of silicosis.

The present invention aims to provide a means of eliminating the aforementioned drawbacks by proposing the replacement of diatomaceous earth with other substances.

According to the invention, this objective is achieved by applying a filtration process in an alluvial filter in which a metallic powder is used as filter material.

The metal powders, well known per se, are produced according to the principles of powder metallurgy, in particular by sintering small metal particles or also according to the "Coldstream" process.

There are in particular commercially available metal powders based on austenitic stainless steel or based on nickel alloys whose particles have specific shapes and dimensions. Powders with rounded particles have better filtering properties but are difficult to compact. The powders with branched particles have a lower bulk density which makes it possible to adjust the surface / volume ratio. Some powders have mixed and irregular-shaped particles, which makes it possible to find a compromise between the quality of the filtering properties (level of porosity) and obtaining good mechanical resistance. These pou-30 dres, in the context of the invention, must therefore be chosen according to the characteristics of the liquid to be filtered, the treatment conditions (duration of filtration, pressure, temperature, etc.) and the particular filter used. The effective particle size of the powders used must generally be less than or equal to about 1 mm, that is to say the dimensions of the diatomaceous earth materials used up to now. A powder II passing through the sieve 350/500 micrometers of opening of the 3 3 * f j meshes has proved particularly advantageous.

The powders of austenitic steel or nickel alloys discussed above have paramagnetic properties. This has an additional advantage since, given their high cost, the metal powders used in the alluvial filters - in accordance with the invention - must be separated from the filtration residue (filtration cake and pre-coating layer of filter cloth) and recycled.

This separation can be easily done, for example by means of an electromagnet, thanks to said paramagnetic properties of the powders. After separation of the metal powders, the residue can very often be incinerated.

It goes without saying, however, that the invention is not limited to paramagnetic metallic powders but that it also applies to metallic powders which do not have this property. In this case, the powder can be recovered by other known means.

Metallic powders generally have good chemical behavior in a neutral, acidic or alkaline medium. However, certain media to be treated may require the choice of a metal powder having an increased resistance to corrosion. The powders in certain chromium-nickel alloys are known for their good resistance to corrosion and can therefore be advantageously used.

The above shows that the use of metallic powders as filtering material in the alluvial filters, in place of the diatomaceous earths, makes it possible to eliminate all the disadvantages of these earths. The relatively high price of metallic powders is compensated by the possibility of recycling them, an operation which turns out to be particularly easy when metallic powders having paramagnetic properties are used.

.Jlr Although the invention was particularly

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I 4 1 described for the case of application in an alluvial filter with trays, of discontinuous type, it goes without saying that it also finds a valid application in the 1 alluvial filters with sheets or tubes, or 5 in the continuous alluvial filters with drums, discs or belts, operating under vacuum or under pressure.

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Claims (5)

1. Filtration process in an allu-tioning filter characterized in that, as filtering material I, metallic powders. 2. Method according to claim 1 characterized in that the metal powders used have a particle size in a range less than or equal to about 1 millimeter. 3. Method according to claims 1 and 2 • 10 characterized in that one uses metallic powders of austenitic stainless steel or chromium-nickel alloy. 4. Method according to any one of the preceding claims, characterized in that metal powders having ferromagnetic properties are used and in that, after filtration, the metal powders are separated from the filtration residue by means of an electromagnet and we recycle them. 5. Filter material for alluvial filters consisting of metallic powders possibly having paramagnetic properties. / --- yT ~ X 25% n 30 35