Process engineers struggle to clarify process liquids. Particle fine removal is part of the ongoing battle. But there are ways to automate the clarification processes to improve filtration and minimize operator exposure. The cake solid’s structure and the nature of the process will determine which types of pressure-filtration automated clarification technologies are best for you.
Before we discuss removal of particle fines, let’s discuss how they are generated. Tom Blackwood, in his Chemical Processing article, Fend Off Fine Particle Frustrations, talks about two main areas for fines generation. First, the problem can start in the particle formation step (crystallization, reaction or extraction). Excess supersaturation or the lack of nucleation control can allow fine particles to persist through the process.
Next, upstream processing can lead to particle fines through attrition. Attrition can occur in centrifugation, drying, conveying and storage. Tom has even seen attrition in liquid/liquid separation processes where crystals have formed due to the immiscibility of the chemical in a solvent. Centrifuges are a common source of attrition due to their filtration and discharge mechanism via the solids hitting the cloth or solid surface due to a poorly designed inlet.
Particle Fine Removal Approaches
How can pesky particle fines be removed? Throughout my career in the solid-liquid separation market space, I have seen some interesting solutions. At one melamine resin facility, the slurry was in a formaldehyde process. The operators were wearing masks and opening up a manual plate filter in a room with residential floor fans to dig out the cake from the paper filter media.
In another case for zeolites, the client had multiple bag filters to clarify the filtrates following a vacuum belt filter. When the filtrates, the final product, remained cloudy, to my surprise, the client decided to add another set of bag filters!
There are better ways! Engineers have many choices to automate the particle fine removal process. This might involve candle filters, pressure plate filters, or sintered metal filters. Each type has its place. With testing and careful evaluation, engineers can make the correct choice.
Candle filters, for instance, are best suited for filter cakes that are vertically stable. Pressure plate filters are used for filtration of cakes up to 75 mm thick. Sintered metal cartridges are used for high temperature applications greater than 200 degrees Celsius where the solids are well-defined hard crystalline shaped. But there’s so much more to know about these options, check out what I’ve written about these in the past.
In the meantime, I’ll tell you filter aids are generally the last resort. Filter aid improves filtration, but there’s more work involved.
Finding the right approach to particle fine removal is going to take into consideration cake structure and thickness, filtration pressure, filter media and more. At least with automated clarification technologies the process can be a little easier once you find the best choice for your needs.