Frequently asked questions regarding membrane filtration, process bath treatment and Atec filtration systems

Which substances a membrane retains depends on the membrane technology used. With membrane processes ranging from microfiltration (MF) to reverse osmosis (RO), theoretically almost the entire spectrum of impurities – from coarse particles to dissolved salts – can be removed.

However, in the field of industrial process bath maintenance, Atec primarily uses microfiltration and ultrafiltration, or in some applications, coarse nanofiltration.

Microfiltration (MF)

Microfiltration is particularly suitable for removing:

• Oils and fat droplets
• Solid particles
• Abrasion and dirt
• Metal hydroxides and other precipitates
• Bacteria

Dissolved bath chemicals such as cleaners, alkalinity, and surfactants pass through the membrane and remain in the process bath. This allows impurities to be removed without significantly altering the bath chemistry.

Ultrafiltration (UF) and coarse Nanofiltration (NF)

In rinse bath maintenance, tighter membranes are often used. In addition to the impurities mentioned above (oil, particles,…), these can additionally retain:

• Macromolecular organic compounds
• Surfactants
• Other larger dissolved molecules

This often allows rinse baths to be used significantly longer and, if necessary, combined with further treatment processes, for example, to reduce conductivity.

The selection of the appropriate membrane is always based on the composition of the process medium and the desired treatment goal.

Ceramic membranes are characterized by an exceptionally long service life. Unlike polymeric membranes, they are extremely robust against chemical, mechanical, and thermal stresses. The ceramic membrane material is inert and therefore does not react with most chemicals used in industrial processes. High temperatures and intensive cleaning cycles also pose no problem for the membranes.

In standard applications (e.g., degreasing baths and rinsing baths), ceramic membranes are therefore usually not considered wear parts. Provided that the membranes are operated within their design limits and no mechanical damage occurs, they can be reliably used for many years. The actual achievable service life depends on the specific application, operating conditions, and type of cleaning. However, in many industrial plants, ceramic membranes are in use many times longer than comparable membranes made of polymeric materials. The cost savings in plant maintenance due to the elimination of regular membrane changes are enormous.

Membrane filtration processes such as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) have established themselves in numerous industrial processes as efficient solutions for bath maintenance, service life extension, and, in some cases, the recovery of valuable materials. Whether membrane filtration is suitable for a specific process bath depends primarily on the composition of the bath and the contaminants to be removed.

Basic principle of membrane filtration

In membrane filtration, undesirable substances are separated from the process medium based on their size. The membrane has a fixed pore size that clearly defines the separation limit. This allows contaminants to be continuously removed while desired bath components remain in the process (e.g., cleaners, surfactants, etc.).

Degreasing baths / cleaning baths

Degreasing baths are among the most common applications for membrane filtration at Atec.

During the cleaning process, oils, fats, and particles continuously enter the cleaning bath. These contaminants can be reliably removed from the bath using microfiltration or ultrafiltration. The cleaning solution remains largely in the process. The service life of the bath is often significantly extended, and the quality of the washing process improves.

Rinsing baths after degreasing

Rinse baths following degreasing baths are contaminated by pollutants and bath chemicals carried over from the degreasing stage via components, workpiece carriers, conveyor belts, or similar. This makes it clear: the best rinse bath maintenance often begins with the maintenance of the degreasing bath. A clean and well-maintained degreasing bath enormously reduces the carry-over of contamination into the rinse.

If the actual rinsing bath is to be treated, tighter membranes can often be used here. These can then separate carried-over surfactants in addition to the usual contaminants. In some cases, membrane filtration can be combined with further processes (e.g., to reduce conductivity).

Pickling baths

In acidic or alkaline pickling processes, membrane filtration can remove typical contaminants:

• Metal hydroxides
• Other reaction products
• Oils and other contaminants

Through the continuous removal of these undissolved substances, the activity of the pickle can be improved and its service life extended.

Pickling / etching of aluminum: Chemical milling and cleaning of extrusion dies

An application that is often technically and economically interesting is the treatment of highly contaminated alkaline aluminum pickling or etching baths. The most heavily contaminated pickling baths are typically chemical milling baths and pickling baths for the cleaning of extrusion dies / tools.

Rinsing baths after pickling / etching

The rinse after pickling is contaminated by the carried-over pickling medium (acidic or alkaline) and carried-over impurities. Therefore, a pickling baths well-maintained via membrane filtration reduces carry-over into the rinse bath.

Depending on the requirements of the specific application, the rinses can be treated purely by membrane filtration or using combined processes.

Sealing baths

After the anodizing of aluminum, the open pores of the material are closed in hot sealing baths. These baths often have a short service life (approx. 1 week), as precipitates lead to streaks and spots on the components. Occasional cooling of the bath (e.g., overnight) further exacerbates the problem.

Membrane filtration offers the ideal solution here. Precipitates are continuously and reliably removed from the medium. Occasional cooling even benefits the membrane filtration, because dissolved solids precipitate as the bath cools and can then be filtered out. With sealing baths, an enormous extension of service life is possible through membrane filtration (e.g., 6 weeks instead of 1 week).

Other baths

There are a variety of other process baths and process media that can also be effectively treated using membrane filtration. Examples include electrolytes from ECM processes, certain phosphating baths, and the process medium from vibratory finishing systems.

Conventional oil separators are based on separating oil and water by gravity. The less dense oil floats on top of the denser water. For this process to work properly, the medium must have sufficient residence time in the oil separator. It therefore has to have a certain minimum size. As a rule, the better the oil separator is expected to work, the larger it has to be. However, space is often very limited in industrial plants.

Another limiting factor is the bath chemistry, i.e. the cleaner. If a demulsifying cleaner is used, which is the case in most degreasing baths, it is almost impossible for an oil separator to remove the oil from the bath. The cleaner keeps the oil emulsified, i.e. it remains very stable as small droplets suspended in the water.

The advantage of membrane filtration is that separation takes place via the membrane pore size. This pore size is clearly defined and does not change. The water and the cleaner can pass through the pores, while the oil and particles are retained. This process is independent of the cleaner, temperature, and also the oil and dirt concentration. The resulting filtrate is always free of oil and turbidity.

This makes membrane filtration the more robust and reliable process compared to oil separators for maintaining cleaning baths. These advantages have increased further in recent years through the use of robust ceramic membranes.