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

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.

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.

Cellulose

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.