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Filtration may be defined as the separation of solid from a fluid by means of a porous medium that retains the solid but allows the fluid to pass.
*The suspension of solid and liquid to be filtered is known as the
slurry.
The porous medium used to retain the solids is described as the
filter medium;
•
The accumulation of solids on the filter
is referred to as the filter cake,
•
while the clear liquid passing through
the filter is the filtrate.
Factors Affecting Filtration
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Filtration
is affected by the characteristics of the slurry, including:
•
1-
The properties of the liquid , such as density, viscosity, and corrosiveness.
•
2-
The properties of the solid, for example, particle shape, particle size,
particle size distribution, and the rigidity or compressibility of the solid.
•
3-
The proportion of solids in the slurry.
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4-
Whether the objective is to collect the solid, the liquid ,or both.
•
5-
Whether the solids have to be washed free from the liquid or a solute.
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The surface upon which solids are deposited
in a filter is called the “Filter medium”.
Filter Medium
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Properties of ideal filter
medium:
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1- It must be capable of delivering a
clear filtrate at a suitable production rate.
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2- It must withstand the mechanical stresses
without rupturing or being compressed.
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3- No chemical or physical interactions
with the components of the filtrate should occur.
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4- It must retain the solids without
plugging at the start of filtration.
•
5-
Sterile filtration imposes a special requirement since the pore size
must not exceed the dimension of bacteria or spores.
Types of Filter Medium
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1- Woven filters: these include a- wire screening . b- fabrics of cotton, wool,
nylon.
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Wire screening e.g. stainless steel is
durable, resistance to plugging and easily cleaned.
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Cotton is a common filter ,however, Nylon is superior for
pharmaceutical use, since it is unaffected by mold, fungus or bacteria and has
negligible absorption properties .
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2- Non- woven filters: Filter paper is a common filter medium since it offers controlled porosity,
limited absorption characteristic, and low cost.
•
3- Membrane filters: These are basic tools for micro-filtration,
useful in the preparation of sterile solutions. These filters are made by
casting of various esters of cellulose, or from nylon,
Teflon, polyvinyl chloride. The filter is a thin membrane with millions of
pores per square centimeter of filter surface.
•
4-Porous plates: These
include perforated metal or rubber plates, natural porous materials such
as stone, porcelain or ceramics, and sintered glass.
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