The cost of acquiring and disposing of cleaning chemistry is usually a major contributor to the overall cost of an industrial or precision cleaning process. Removing contaminants from cleaning solutions can potentially extend their useful life and reduce overall cleaning costs. Traditional filters including those of the bag and cartridge variety are commonly used to remove solid (particulate) contaminants as small as .1 micron in size. Soluble and emulsified contaminants can also limit the life of cleaning solutions. Some such contaminants can be removed through the use of conventional gravity-type or centrifugal “oil” separators. Gravity separators, however, are not typically effective in removing molecularly divided or emulsified contaminants which can often be removed using micro and ultrafiltration.
In principle, micro and ultrafiltration are not substantially different than any other type of filtration except for the fact that the scale shrinks an order of magnitude or so. Microfiltration encompasses the range from 10 to 0.1 micons while ultrafiltration usually applies to a size range of 0.1 to 0.01 micron. The “filters” used are commonly called “membranes” and vary widely in construction from thin films of flexible material to porous metal. The mechanism of filtration is a combination of restricted passages (mechanical restraint) and “rejection” achieved by coating the membrane with materials that reject certain molecules and other liquid constituents by molecular and chemical means including ionic forces.
Although membranes are made of many different materials and are constructed in many different ways they all share the characteristic of being easily fouled or clogged by retained contaminants if not applied correctly. In order to provide a sufficiently useful life, membranes are configured to have a large amount of surface area and/or in such a way that retained contaminants can be removed by periodic or continuous flushing of the inlet side of the membrane. Flushing is accomplished by mechanical means such as vibration or a “shearing” action which is often provided by a high velocity flow of liquid over the membrane surface. Liquid that successfully passes through the membrane is called the “permeate.” Separated contaminants (whatever did not pass through the membrane), rather than being retained, as in the case of classical filtration, are collected to form a “concentrate” stream. In most cased, the concentrate stream re-circulated and re-presented to the membrane for further extraction of permeate until further extraction is no longer possible or practical at which point the concentrate is collected for re-cycling or disposal.
Fortunately, micro and ultrafiltration are very effective in removing contaminants under the right circumstances but, unfortunately, don’t work under all circumstances. For example, in order to be effective, the membrane must be able to differentiate between those liquid components that need to be saved and those that need to be removed. This doesn’t seem that complex until one realizes that there may not be a lot of difference between the unwanted contaminant and the cleaning chemistry which in many cases has a molecular weight not that much different than oil and other contaminants! If the membrane removes the cleaning chemistry along with the contaminant, the entire purpose of the exercise is defeated. The importance of this is that in order to be effective, cleaning chemistry must be selected that lends itself to this method of recovery. Finding the right chemistry to both clean and be able to pass through a particular membrane for recovery is not a trivial exercise and should not be reliant on parties not familiar with both membrane and chemistry technology. Teaming up with a reputable supplier is highly recommended!