Summary: An effective separation technology withstanding the test of time, precoat filtration has gone through quite a metamorphosis in the last century. Over time, a few technologies have entered some of the same markets and applications. Still, it has adjusted well and serves as a practical, effective means of reducing solids from liquid.
The use of precoat filtration was first introduced in the late 1890s. At that time, attempts were made to employ diatomaceous earth to clarify sugar syrups prior to granulation. Precoat technology has evolved to the point that our lives are touched daily by a product or service made possible by such filtration. This article describes the evolution of precoat filtration, including development of the medium and equipment, and the commercial and industrial markets served by this technology.
The early years
Precoat filtration involves the placement of a filtration medium on the surface of a filter element and the subsequent passing of a liquid containing solids through the medium to remove the suspended particles from the liquid. The separation of the solids is accomplished by the microporous structure of the filter cake deposited on the filter element. The filter cake “strains” the solid from the liquid, performing a physical separation that doesn’t typically rely on chemical use for charge neutralization or coagulation.
In the beginning, diatomaceous earth—or diatomite—was the only medium used for precoat filtration. The medium was a natural grade of diatomite, meaning it received little processing—typically drying and classifying—after being extracted from the mine site. The earliest record of using diatomite for filtration describes exploratory work performed in Germany in the 1890s and early 1900s to clarify beet sugars. Further work was carried out in California and Colorado with limited success. This was partly due to the fact that diatomite was being misapplied in an attempt to assist the performance of bag filters. The diatomite was added to the sugar liquor as it was fed into the bag filter under gravity. As the diatomite attained a more complete separation, the filter became blinded by the solids and flow ceased prematurely. A better filtration was achieved, but the duration of the filter cycle was costly. In 1914, a successful and economic sugar filtration application was accomplished with use of a pressure filter and, over the years, pressure filtration became the standard production method for obtaining clear sugar liquors.
As the development of precoat filtration technology progressed, modifications to the production process were introduced to alter the permeability and particle separation capability of diatomaceous earth. The natural grades were heated to a softening point to fuse the individual diatomite particles and increase product permeability. Fluxing agents such as table salt and sodium carbonate were developed to further increase the permeability and markets were developed for these faster flow rate filter aids. The addition of filtering media to the filter feed, or body feed, was developed to overcome higher solid concentrations. (Body feeding is the addition of filter medium to the filter feed for the purpose of preventing premature blinding of the filter cake surface and maintaining filter cake porosity and permeability.) This allowed for extension of the filter cycle and maintained economic filter runs. Eventually other media were developed including perlite, cellulose, cellulose filter pads, rice hulls and nutshells; however, diatomaceous earth still maintains the lead role in media used for this technology. Developments in the equipment kept pace with media developments. Plate-and-frame filters were developed for high-pressure filtration (see Defining various filters). Pressure leaf filters enhanced their performance with automated sluicing or washing of the spent filter cake. Vacuum leaf filters evolved, using the pump to pull the liquid from the filter rather than push it through like pressure filters.
Rotary vacuum filters were developed to handle high solids and coarse particulate loading. Tubular element and tube filters were developed, mostly employing outside-in technology (meaning the solids are retained on the outside of the element); however, inside-out filters (where solids are retained inside the element) exist and perform as well. As instrumentation and automation advanced, appropriate adaptations of this equipment have allowed several operations to perform their filtration unattended. Except for the maintenance of media slurries for precoating and body feeding, filtering proceeds without human involvement.
There are many diverse applications for precoat filtration, ranging from wastewater sludge dewatering to the clarification of pharmaceutical fermentation broths. Precoat filtration is used where the solids are considered the contaminant and the filtrate is considered the product having value. It’s also used where the solids are considered the product and the liquid is considered the waste product, as in the Merrill-Crow process used for recovery of gold fines from leachate.
The following applications were developed in the 1930s and many continue to relate to precoat filtration today. The applications include cane sugar refining, plantation white sugars, corn products, beer and wine, pectin, cider, vinegar, citric acid, dyestuffs, soaps, gelatin, glue, vegetable oil, inedible animal fats and oils, varnishes, shellacs and lacquers, lard and tallow, petroleum products and other chemicals. These industries and applications benefit from the flexibility and economy afforded by precoat technology.
Flexibility is provided through selection of a precoat technology that matches the requirements of the process. Rotary vacuum or plate-and-frame filters filter heavy solids concentrations. Sludge de-watering and the production of sodium silicate and hydrogen peroxide are two examples of the “dirty (high solids) end” of the spectrum. Blood fractionation—or separating lipids from human blood—and wine polishing using sterile pad filtration represent the opposite end of the spectrum. Flexibility is provided in the selection of a medium and a specific grade within that medium. Thus, a single technology such as pressure leaf filtration is capable of coarse or fine filtration depending upon the grade of medium selected. Finally, precoat filtration offers flexibility by reacting to changes in the solids concentration of the filter feed.
The amount of body feed is dependent upon and matched to the solids concentration of the filter feed. The nature of the solids also dictates body feed rates. Compressible or deformable solids typically require higher percentages of body-feed. This means that changing concentrations or characteristics may be matched, through experience, to specific body-feeding regimes allowing for the economic continuation of the filter cycle. Economy is provided through the typically lower capital and O&M (operation and maintenance) costs for precoat filtration when compared to technologies offering comparable separation flexibility and efficiency.
Emerging technologies, such as centrifugation and membrane filtration, have been promoted as replacements for precoat filtration. In some industries, they have established a position as standalone technologies; however, integrated system designs also employ precoat filtration as prefiltration before membranes and as polish filters following centrifuges, whereby particles are separated by size through spinning at high speeds. It appears there will always be a market for precoat filtration.
About the author
Jim Larsen is the director of business development for Separmatic™ Fluid Systems, of Milwaukee. He has over 20 years experience in industrial minerals production, sales and marketing. He can be reached at (805) 455-6512, (805) 688-1670 (fax) or email: email@example.com
Defining various filter
Rotary vacuum filters: A precoat filter design utilizing a rotating drum onto which a precoat is applied. As the drum rotates through a vat of liquid, the precoat is immersed in the liquid to be filtered. The liquid is sucked up through the precoat and the solids are retained at the surface. The solids and a very thin layer of precoat are then shaved from the surface of the drum to present a fresh layer of precoat media for the next immersion. The cycle repeats until the precoat is nearly exhausted.
Plate-and-frame filters: A precoat filter design employing several chambers, generally arranged in a horizontal position. The filters’ internal piping allows for the liquid to be filtered through the frame or chamber and withdrawn through the plate. The plate may be precoated with filtering media or fitted with filtration pads. The structural design of the filter allows for pressures as high as 200 pounds per square inch (psi) to build within the plate and frame, thus making this design the choice for de-liquifying heavy sludges and solids.
Pressure leaf filters: A precoat design employing flat filter elements or tubular elements housed within a cylindrical pressure vessel. The liquid to be filtered is pumped into the vessel and pushed through the precoat media—often with the addition of body feed—to a manifold that holds several elements. The manifold sends the filtered liquid out of the filter to the next step in a process.
Vacuum leaf filters: A precoat design that typically employs an open, rectangular tank and flat filter elements. The liquid to be filtered enters the tank and is drawn through the filter elements by a pump pulling the liquid through the elements and out of the filter.
Membrane filtration: A filtration technology that employs a microporous material in sheet or tube form to allow the passage of molecules of a specified size (or smaller) and retention of oversize molecules. Membranes are classified as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, depending upon the size restriction placed on the retained molecules.
Centrifugation: One of the most important and commonly used tools in cell biology, it separates different particle components in a suspension based on differences in their size, shape and density that together define their sedimentation coefficient. Practically, the tube containing a suspension of particles is fixed in the rotor of a centrifuge and allowed to rotate at a high speed. The spinning motion exerts a centrifugal force directed from the center of the rotor toward the bottom of the tube. This force acts on the suspended particles pushing them toward the bottom of the tube at a rate determined by the velocity of the spinning rotor and the particles’ sedimentation coefficients. The different particles of different sizes, shapes, and densities will have different sedimentation coefficients and, therefore, sediment at different rates.