What kind of filter aids do exist and how do they “save” the filtration of finely contaminated suspensions?

The filtration of liquids that are contaminated with particularly fine particles is a common filtration task in various industries - but not an easy one. Special filtration technology is required to filter out the small particles. The solution is precoat filtration using so-called filter aids. Diese Mittel, zum Beispiel Kieselgur, Perlit oder Cellulose, sorgen durch ihre speziellen Eigenschaften für einen Filterkuchen, der feinste Partikel zurückhält, dabei aber durchlässig genug ist. Wie diese genau in den Filtrationsprozess integriert werden, welche Arten von Filterhilfsmitteln es gibt und was die Einsatzbereiche sind, klären wir in diesem Artikel.

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Filtration Knowledge

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How are filter aids used?

Filter aids can be introduced into the filtration process in two different ways: Before the actual filtration, in the so-called precoat, and during the filtration process as a body feed.

During precoating, which must always be carried out during precoat filtration, a layer of filter aid is precoated onto the filter cloth or filter fabric before the suspension is introduced. This precoat layer ensures that a permeable filter cake is formed and enables the filtration of fine particles that would otherwise simply pass through the coarse mesh. In this type of precoat filtration, our systems achieve filter finenesses of between 3 and 10 µm, depending on the agent used and the degree of contamination.

In most cases, precoating is sufficient. If, in rare cases, small particles cause the filter layer to block, the filter aid can also be added to the suspension that is to be filtered. The agent is added to the process in order to loosen the filter cake. Such an addition (body feed) may only be carried out until the same amount of filter aid has been added as has already been applied to the filter cartridges in the precoat.

As many different filter aid qualities are now available on the market, especially in terms of the structure and particle size of the powders, the precoat can be completely customised to the requirements of the respective filtration task. In this way, the precoat layer is usually sufficient to achieve a good filtration result.

What kind of filter aids are available?

There are various types of filter aids that are different in terms of their properties and therefore their advantages and disadvantages. Which aid is used should therefore be decided with regard to the specific requirements of the filtration process in question. The choice of the right agent depends, for example, on the type of impurities, the liquid viscosity, the desired filtration result and other process parameters.

In the following, we present three frequently used filter aids as examples: Diatomaceous earth, perlite and cellulose.

Diatomaceous earth

What is diatomaceous earth?

Diatomaceous earth, also known as diatomaceous earth, is a frequently used aid in filtration. It consists of the fossilised remains of diatoms – microscopically small algae. It is particularly suitable as a filter aid due to its porous structure and chemical inactivity.

How is diatomaceous earth produced?

The origin of diatomaceous earth dates back millions of years, when mainly diatoms grew in the world’s oceans. These were deposited on the seabed after they died or were excreted by marine animals. Despite their microscopic size of around 10 µm, deposits several metres high have formed over time. The largest and purest saltwater diatom deposits are found near Lompoc in California, where diatomaceous earth mountains up to 300 metres high can be found in extensive areas. The occurrences of freshwater diatomaceous earth are the result of corresponding deposits in inland lakes. Such deposits are also found in Europe in varying degrees of purity. Even today there are diatoms in all bodies of water and in over 100,000 species.

The diatomite chunks are ground, dried and sorted in air separators to produce diatomaceous earth. To remove organic impurities, the material is annealed (calcined) in rotary kilns at approx. 900 °C, whereby traces of iron oxide produce the familiar pink colour. If the diatomaceous earth is to be white, it is treated with sodium carbonate, which converts iron into a colourless complex. At the same time, the very fine particles are sintered together to form larger particles and some of the microamorphous silicates are converted into the crystalline form known as cristobalite.

Advantages and disadvantages of diatomaceous earth as a filter aid

A major advantage of diatomaceous earth is the internal porosity of the individual particles, which contributes to the formation of a well-permeable filter cake. It is also largely chemically “inert”, meaning that it does not react with the suspension that is to be filtered. Diatomite is also insoluble – a basic requirement for use as a filter aid.

It should be noted that diatomaceous earth should be handled with care. Inhaling diatomaceous earth dust irritates the respiratory tract and can be carcinogenic. Appropriate protective measures should therefore be taken. In addition, the wet cake density is relatively high, which can lead to increased product loss. The abrasiveness of the porous particles must also be considered, which can lead to abrasion of pumps and pipework over time. The high ash content and mineral substances play a role in disposal.

Despite these points, diatomaceous earth is a very popular filter aid thanks to its excellent filtration properties. A wide range of qualities is available worldwide, meaning that this filter aid can be used for many different applications.


What are perlites?

Perlite is a dense, glass-like rock structure of volcanic origin. It is widely used in industry. This is mainly due to its unique property: when heated at high temperatures, it expands and forms a light, foamy structure. The foamed perlite can be used in a variety of ways – in addition to its use in filtration, it can also be used as insulation material in the construction industry or as a planting substrate.

How are perlites produced?

Volcanic glass such as perlite is formed by the rapid cooling of lava, for example through contact with water or glacial ice. The rapid cooling causes the lava to solidify without crystallising. The resulting amorphous rock glass is extracted in open-cast mines as perlite ore.

For its industrial use, the ore is heated to approx. 1300 °C in a rotary kiln. This vaporises the water it contains and expands the perlite to around 20 times its original volume. This also causes perlite to change colour – from grey-black glass to the characteristic pure white of industrial perlite. After cooling, the “stone foam” is ground and sorted according to grain size. The particle size distribution is between 1 µm and 100 µm, but mainly between 40 µm and 60 µm.

Advantages and disadvantages of perlite as a filter aid

The perlite particles produced by the manufacturing process described correspond in shape and type to the irregular particles of a crushed glass bead. Overall, the siliceous volcanic rock is similar to diatomaceous earth in terms of its filter properties, both in terms of its favourable and unfavourable characteristics.

In contrast to diatomaceous earth, perlite has no internal porosity, but is just as chemically inactive and insoluble, which makes it a good filter aid. In terms of disadvantages, the wet cake density is also similar, as is the abrasiveness. Air inclusions (“floaters”) caused by the manufacturing process, which cannot be floated, are also to be expected. On the other hand, perlite is often cheaper than diatomaceous earth and is available worldwide, including from local producers.

However, a special property of perlite is its low weight. Compared to diatomaceous earth and cellulose, perlite has the lowest density, up to 50 % less than that of “standard” diatomaceous earth.


What is cellulose?

Cellulose is one of the most abundant biopolymers on earth and the main component of plant cell walls. It is a polysaccharide, which means that it consists of a long chain of linked glucose molecules. It is used as a filter aid in modern precoat filter systems (pressure and vacuum filters). It is a particularly versatile filter aid with a wide range of fibre lengths and fibre structures that can be adapted to almost any filtration task.

Advantages and disadvantages of cellulose as a filter aid

The organic filter aid has many advantages, starting with the fact that it is made from natural, renewable raw materials. This makes cellulose particularly environmentally friendly to dispose of and also harmless from an occupational health perspective.

With regard to filtration, however, its excellent separation effect is particularly noteworthy. Cellulose forms an open, porous filter cake and therefore has a low product loss. It also has a low dead weight (although perlite is even lighter) and is non-abrasive due to its soft structure.

The only points worth mentioning when using organic filter aids are a possible fire hazard due to dust formation, which must be prevented, and possible bridging with certain products. The latter means that the filter aid can “get stuck” in the dosing devices so that no filter aid slips into the dosing screws. This can be remedied by skilful design, for example with FAUDI dosing units.

Where is precoat filtration used?

As mentioned at the beginning, filtration with filter aids is used especially when particularly small turbid matter is to be filtered out of a suspension. Precoat filtration therefore achieves particularly pure liquids as a result.

A frequent task of our precoat filters is the treatment of cooling lubricants in all metalworking industries. Cooling lubricants that are used in grinding and honing processes, rolling, lapping, eroding, pressing or finishing, for example, can be returned to the production process with the desired degree of purity using precoat filtration. Ultra-fine filtration is also possible.

But precoat filter systems are also used in the food production sector, for example in water treatment, the purification of food and vegetable oil and beverage production. Our filter systems have also been used for the filtration of brine in cheese production.

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Filter aids are the central component of precoat filtration, which is used in particular to filter out very fine particles from liquids. As filtration usually only takes place through the use of these substances, they are actually more of a “filter medium” than an “aid”.

Filter aids are used to facilitate the filtration process mechanically by influencing the filter cake.

Thanks to the wide range of filter aids with different grain sizes and structures, there is a suitable solution for every filtration task. We will be happy to advise you on precoat filtration in your production process.