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Blinding
Plugging or sealing of any portion of a filter.

Body Feed
Filter aid suspended in the feedstock before and during filtration.

Cake
The accumulation of solids and filter media on the surface of a septum.

Cake Height
Vertical height of the cake.

Calcination
The heating of a solid to a high temperature, below its melting point, to yield the degree of sintering and agglomeration of diatomite particles needed to result in a particular flow rate permeability. Calcining can be effected over a range of temperatures up to about 1300° C.

Capacity
The amount of particulates a filter can remove while maintaining practical filtration and pressure differential.

Celite® Media
A Celite Corporation registered trade name for its line of diatomite filter aids, biocarriers, sorbents, excipients, and fillers.

Cellulose
Filter aids produced by the sulfite or sulfate processing of hard woods. Like perlite, cellulose possesses a less intricate structure than diatomite.

Celpure® Media
An Advanced Minerals Corporation registered trade name for its line of High Purity diatomite filter aids.

Chromosorb® Media
A Celite Corporation registered trade name for its line of chromatography media and sorbents.

Clarity
The degree of insoluble material removed from a feedstock. Clarity is usually measured by optically based methods.

Compressibility
Degree of physical change of the suspended solids or filter cake when subjected to pressure.

Darcy's Law
An empirical law that governs flow through porous media and describes the relationship among flow rate, pressure drop, and resistance. Functional filtration products are usually processed to provide a range of filtration rates that are closely related to their permeability as reported in units of Darcies.

One Darcy corresponds to the permeability through a filter media 1 cm thick which allows 1 cm3 of fluid with a viscosity of 1 centipoise to pass through an area of 1 cm2 in 1 second under a pressure differential of 1 atmosphere.

Darcy Permeability is determined using a specially constructed device designed to form a filter cake on a septum from a suspension of filtration media in water, and then measuring the time required for a specified volume of water to flow through a measured thickness of filter cake of a known cross-sectional area.

DE
A commonly used abbreviation for diatomaceous earth filter aid. (See also Diatomite).

Deformable
Suspended solids that extrude into the interstices of a filter cake and cause rapid filter blinding.

Depth Filtration
A filtration technique that uses depth filters, which are typically characterized by their design to retain particles within a filter matrix. Their capacity is typically defined by the depth of the matrix and thus the holding capacity for solids.

Diatom
See Diatomite.

Diatomite
Diatomite or diatomaceous earth is a sediment greatly enriched in biogenic silica in the form of siliceous skeleta of diatoms, a diverse array of microscopic, single-cell algae. These frustules are sufficiently durable to retain much of their structure through long periods of geologic time and through thermal processing. Diatomite products have an inherently intricate and highly porous structure composed primarily of silica.

Differential Pressure
The difference in pressure between the upstream and downstream sides of a filter or filter media.

Dynamic Depth Filtration
A filtration technique in which filter media is suspended in the feedstock and continually regenerates the filter surface. This differs from dead end filtration, in which the media is fixed within the filter element.

Element
Any structural chamber in a filter on which the septum is supported.

Feedstock
The mixture of solids and liquid that enters the filter.

Filter Aid
Inorganic mineral powders or organic fibrous materials used in combination with filtration hardware to enhance filtration performance. Commonly encountered filter aids include diatomite, perlite, and cellulose. Some of these materials have been in use as filter aids for over seventy-five years.

A process trade-off exists in filter aid technology between the permeability of the porous media and its turbidity removal capabilities. Filter aid products are produced in grades over a wide range of permeability ratings. Each filter aid family has a characteristic curve defining this trade-off in either a pressure or vacuum filtration mode. In general, the tightest marine diatomite can remove submicron size turbidity, but, as in depth filtration, the objective is to select a filter aid that removes only the size of turbidity necessary to achieve the desired clarity without unnecessarily limiting flow rate.

Filter Cake
Retained solids and filter media on the filter element.

Flux
The filtration performance, usually expressed in volume per unit time per area (e.g. L/m2-min)

Flux Calcination
Calcination in the presence of a fluxing agent. In general, the presence of a flux during calcination reduces the temperature at which sintering and agglomeration of diatomite particles occurs, thus permitting larger agglomerates to form and extending the permeability range of filter aids. A common fluxing agent is sodium carbonate (Na2CO3).

Harborlite® Media
A Harborlite Corporation registered trade name for its line of perlite filter aids.

Leaf
A flat filter element that supports the filter septum.

Media
The material that performs the separation of solids from liquids.

Mesh
Grading of particle size according to passage through a standard sieve with a known number of filaments per inch. Substances graded at 80/100 mesh are those which pass through a screen with 80 filaments per inch but are retained by a screen with 100 filaments per inch.

Perlite
A naturally occurring volcanic glass, that thermally expands upon processing. Perlite is chemically a sodium potassium aluminum silicate. After milling, a three-dimensional multihedral structure is present, but because its structure is not as intricate (or tortuous) as that of diatomite, perlite is better suited to the separation of coarse microparticulates from liquids having high solids loading. Perlite and diatomite are common functional filtration components of depth filter sheets and pads.

Permeability
A measure of resistance to flow reported in units of Darcies, for filter aid products. For use in filtration, filter aid products are usually processed to provide a range of filtration rates that are closely related to their permeability. Diatomite filter aids are available in a wide range of permeability, from about 0.001 to about 15 Darcies. The selection of a filter aid with a particular permeability suitable for a specific filtration process depends on the flow rate characteristics of the fluid to be filtered and the degree of fluid clarification desired for the particular application.

Precoat
A thin layer, 1.5 to 3.0 mm, of filter aid that is applied to the septum before the actual filtration process. A precoat is typically used in filter aid filtration when the following is needed:

  • A high degree of clarity from the beginning of the filtration cycle
  • The prevention of penetration and blinding of the septum by the solids
  • Ease of septum cleaning (with reusable septums only)

Some applications do not require a precoat; this is especially true where the septum is a disposable filter pad or sheet that can achieve the above functions without the use of a precoat.

Pressure Precoat Filtration
A method of solid-liquid separation using a precoat with diatomite filter aids that has been practiced for over 75 years. One premise is that the retained solids have minimal permeability with this method of filtration; otherwise, conventional dead end filtration would suffice. The filter aid is the essential feature that serves two functions: 1) as a precoat, it protects the filter septum and provides immediate filtrate clarity; 2) as body feed, it increases permeability in the accumulating filter cake and improves liquid drainage.

Filter aids act as dynamic depth filters, because their rigid nature resists cake compression, thereby retaining permeability and significantly increasing solids retention capacity versus standard depth devices. This extended permeability slows the rise in differential pressure for constant flow filtration, and retards the loss in flow for constant pressure processes. Each filtration cycle starts as the feedstock slurry enters the filter, and it ends when either the flow is minimal (<15% of initial rate) at peak differential pressure, or the available cake space within the filter is depleted. The filter cake stays permeable even at the end of the cycle, offering added benefits for product recovery.

After the filtration cycle, and depending on the filtrate value and equipment design, the cake can first be rinsed in place with one to two bed (cake) volumes of a process-compatible buffer to displace any retained filtrate. Pressurized gas can then be used to void the cake of this post-wash buffer.

Solid-Liquid Separations
The process by which insoluble particles are separated from a liquid when passing through a permeable filter media.

Septum
A permeable material used to support the filter aid or other filter media. A filter septum can be disposable (e.g. pad, paper, fabric, or plastic woven wire screen) or reusable (e.g. metallic woven wire screen).

Slurry
A liquid containing suspended solids.

Turbidity
An insoluble particle that imparts opacity to a liquid.

Wet Density
An indicator of the degree to which a diatomite filter aid retains the porous and intricate structure. Wet density reflects the void volume available to entrain particulate matter in a filtration process; it is one of the most important criteria used to determine the filtration efficiency. Diatomite products with lower wet densities have greater void volumes, and thus greater filtration efficiency.

 

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Yeast and bacteria cells trapped in a diatomite filter cake.

 


Scanning electron micrograph of Celpure® 65 (2500x)

 


Depth filter elements

 


 

 
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