Cake filtration is a common operation across many sectors of the chemical process industries (CPI). In cake filtration, the solid being filtered acts as a screen so that particles of the suspension are retained by the medium, resulting in the formation of filter cakes.
In some cases, filtration can be difficult if the material being filtered is highly viscous, highly compactible or deformable, or if the filter cake formed in the filtration is very small or highly resistant to flow. For such situations, the addition of filter-aid material can greatly improve the efficiency and effectiveness of the filtration.
Filter aids enable cake filtration to be used for a larger group of suspensions, including those with wider particle-size distributions, larger solid concentrations, and gel-like and highly compactible particles. This one-page reference provides information on the properties of common filter-aid materials and how they are used in industrial filtration.
Filter aid function
Filter aids are inert particulate or fibrous materials that are introduced into a filtration operation in one of two ways — as a filtration pretreatment called “precoat” or by adding filter aid to the liquid to be filtered, referred to as “bodyfeed.”
Precoat. Filter aid precoat is added to form a layer of a second filter medium to protect the main product of the filtration. In a typical precoat process, the filter aid material would be suspended at low concentrations (0.5% is common) in a tank that is agitated to maintain the filter aid in a slurry. Filter aid material is typically added at rates of 500–1,200 g/m2 (10–25 lb/ft2) of filter area. The precoat is formed on the surface of the mesh screen or cloth membrane by recirculating the filter aid slurry through the filter. The coarse particles deposit on the screen (septum) first, followed by the finer particles. This occurs until a thin layer (typically 1.5-3.0 mm) of filter aid forms on the filter septum.
Bodyfeed. Bodyfeed, also called “admix” refers to the addition of filter aid material to the liquid that is to be filtered, either by adding it directly to the tank of liquid to be filtered or by dosing filter aid from a slurry tank into the filter inlet. Bodyfeed filter aid is used to improve the flowrate of difficult-to-filter materials by increasing cake porosity and permeability, and decreasing filter cake compressibility. A basic objective of filtration with bodyfeed filter aid is to achieve the highest possible flowrate that achieves the clarification of the liquid that is required. The type of filter aid and its grade (particle size distribution), as well as the filter aid dosage depends on the solids content and other variables specific to each application. In general, a filter aid dosage of half of the percent solids by weight in the suspension to be filtered is common.
Filter aid materials
Several materials can be used as filter aids, including asbestos, cellulose, agricultural fibers, saw dust, rice-hull ash, paper fibers and others. More common filter aid materials include diatomaceous earth (DE), perlite and cellulose.
Diatomaceous earth. DE (Figure 1) is the silica-based fossilized remains of ancient diatoms (single-celled organisms). DE can be mined from ancient seabeds, processed, and classified to make different grades of filter aids. The smaller the filter-aid particle size, the smaller the process particles can be removed, but the filtration rate is lower. The particle size capture by various filter aids may also vary because of liquid viscosity, surface charge, and so on.
Perlite. Another example of a common mineral filter aid is perlite (Figure 2). The inert perlite is a type of naturally occurring, glassy volcanic rock that forms particles with jagged shapes upon processing. When crushed and heated, perlite expands explosively when water trapped in the mineral structure vaporizes. Perlite can expand to about ten times its original volume. After the expansion, perlite forms jagged particles, which interlock, and create microscopic flow channels that allow filtrate to travel through.
Cellulose. Cellulose is a fibrous, organic material that can be used as a filter aid, and is used in filtration systems where silica-based materials may not be compatible. The fibrous structure of cellulose is designed to adsorb sediment and fine particles. The filterability of cellulose is poorer than DE and perlite, but cellulose can be incinerated, and can lead to filter cakes that hold together more than with other filter aids.
Filter aid selection
Preliminary laboratory tests can help to identify the proper kind of filter aid for a given application. First, the chemical resistance and purity of the filter aid material must be considered for the liquid being filtered. Second, the required grade (particle size) of the filter aid should be determined. Third, the quantity of filter aid needed for the process should be calculated. A first approximation of the quantity of filter aid is one that results in a filter cake volume that is equal to the volume of material to be separated.
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