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Filtration
Definition and Processes
(Chemistry)
What Filtration Is and How It's Done
by Anne Marie Helmenstine, Ph.D.
Filtration is a process used
to separate solids from liquids or gases using a filter medium that
allows the fluid to pass, but not the solid.
The term
"filtration" applies whether the filter is mechanical, biological, or
physical.
The fluid that passes through
the filter is called the filtrate. The filter medium may be a surface filter,
which is a solid that traps solid particles, or a depth filter, which is a bed
of material that traps the solid.
Filtration is typically an
imperfect process. Some fluid remains on the feed side of the filter or
embedded in the filter media and some small solid particulates find their way
through the filter.
As a chemistry and
engineering technique, there is always some lost product, whether it's the
liquid or solid being collected.
Examples of Filtration
While filtration is an
important separation technique in a laboratory, it's also common in everyday
life.
· Brewing coffee involves passing
hot water through the ground coffee and a filter. The liquid coffee is the
filtrate. Steeping tea is much the same, whether you use a tea bag (paper
filter) or tea ball (usually a metal filter).
· The kidneys are an example
of a biological filter. Blood is filtered by the glomerulus. Essential
molecules are reabsorbed back into the blood.
· Air conditioners and many vacuum
cleaners use HEPA filters to remove dust and pollen from the air.
· Many aquariums use filters that
contain fibers that capture particulates.
· Belt filters recover precious
metals during mining.
· Water in an aquifer is
relatively pure because it has been filtered through sand and permeable rock in
the ground.
Filtration Methods
There are different types of
filtration. Which method is used depends largely on whether the solid is a
particulate (suspended) or dissolved in the fluid.
· General
Filtration: The
most basic form of filtration is using gravity to filter a mixture. The mixture
is poured from above onto a filter medium (e.g., filter paper) and gravity
pulls the liquid down. The solid is left on the filter, while the liquid flows
below it.
· Vacuum Filtration: A Büchner flask and hose
are used to pull a vacuum to suck the fluid through the filter (usually with
the aid of gravity). This greatly speeds the separation and can be used to dry
the solid. A related technique uses a pump to form a pressure difference on
both sides of the filter. Pump filters do not need to be vertical because
gravity is not the source of the pressure difference on the sides of the
filter.
· Cold Filtration: Cold filtration is used
to quickly cool a solution, prompting the formation of small crystals. This is
a method used when the solid is initially dissolved. A common method is to
place the container with the solution in an ice bath prior to filtration.
· Hot
Filtration: In hot filtration, the solution, filter, and funnel are heated
to minimize crystal formation during filtration. Stemless funnels are useful
because there is less surface area for crystal growth. This method is used when
crystals would clog the funnel or prevent crystallization of the second
component in a mixture.
Sometimes filter aids are
used to improve flow through a filter. Examples of filter aids are silica,
diatomaceous earth, perlite, and cellulose.
Filter aids may be placed on
the filter prior to filtration or mixed with the liquid.
The aids can help prevent
clogging of the filter and can increase the porosity of the "cake" or
feed into the filter.
Filtration Versus Sieving
A related separation
technique is sieving.
Sieving refers to use of a
single mesh or perforated layer to retain large particles
while allowing the passage of smaller ones.
In
filtration, in contrast, the filter is a lattice or has multiple layers. Fluids
follow channels in the medium to pass through a filter.
Alternatives to Filtration
In some situations, there are
better separation methods than filtration.
For example, for very small
samples where it's important to collect the filtrate, the filter medium may
soak up too much of the fluid.
In other cases, too much of
the solid becomes trapped in the filter medium.
Two other processes that can
be used to separate solids from fluids are decantation and centrifugation.
Centrifugation involves
spinning a sample, forcing the heavier solid to the bottom of a container.
Decantation can be used
following centrifugation or on its own.
In decantation, the fluid is
siphoned or poured off of the solid after it has fallen out of solution.
Anne
Marie Helmenstine, Ph.D.
· Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville - Oak Ridge National Laboratory.
· Science educator with experience teaching chemistry,
biology, astronomy, and physics at the high school, college, and graduate
levels.
· ThoughtCo and About Education chemistry expert since 2001.
· Widely-published graphic artist, responsible for printable
periodic tables and other illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
Anne
Marie Helmenstine, Ph.D.
ThoughtCo
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