Plugging or sealing of any portion of a filter.
Filter aid suspended in the feedstock before and during
The accumulation of solids and filter media on the surface
of a septum.
Vertical height of the cake.
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.
The amount of particulates a filter can remove while maintaining
practical filtration and pressure differential.
A Celite Corporation registered trade name for its line
of diatomite filter aids, biocarriers, sorbents, excipients,
Filter aids produced by the sulfite or sulfate processing
of hard woods. Like perlite, cellulose possesses a less
intricate structure than diatomite.
An Advanced Minerals Corporation registered trade name for
its line of High Purity diatomite filter aids.
A Celite Corporation registered trade name for its line
of chromatography media and sorbents.
The degree of insoluble material removed from a feedstock.
Clarity is usually measured by optically based methods.
Degree of physical change of the suspended solids or filter
cake when subjected to pressure.
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.
A commonly used abbreviation for diatomaceous earth filter
aid. (See also Diatomite).
Suspended solids that extrude into the interstices of a
filter cake and cause rapid filter blinding.
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
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
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.
Any structural chamber in a filter on which the septum is
The mixture of solids and liquid that enters the filter.
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.
Retained solids and filter media on the filter element.
The filtration performance, usually expressed in volume
per unit time per area (e.g. L/m2-min)
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).
A Harborlite Corporation registered trade name for its line
of perlite filter aids.
A flat filter element that supports the filter septum.
The material that performs the separation of solids from
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.
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.
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.
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
- 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
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
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
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.
The process by which insoluble particles are separated from
a liquid when passing through a permeable filter media.
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).
A liquid containing suspended solids.
An insoluble particle that imparts opacity to a liquid.
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.
Purity Filtration, Sorbents,
Gas-Liquid Chromatography, Sample
Notes, SDS - NAFTA,
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