The most important factors on which filtration depends are:
- The proportion of solids in the slurry.
- Properties of the liquid namely density, viscosity, corrosiveness, etc.
- Properties of the solid such as particle shape, size, size distribution, and compressibility.
- The properties of filter medium especially resistance and initial layers of cake formed.
The following are the factors that affecting the rate of filtration:
(i) Permeability
Coefficient: According to Darcy’s equation
permeability coefficient (K) represents the resistance of both the filter
medium and the filter cake. As the thickness of the cake increase, the rate of
filtration decreases. Also, the surface area of the particles, the porosity of
the cake, and the rigidity or compressibility of the particles could affect the
permeability of the cake.
(ii) Area of Filter
Medium: The total volume of filtrate flowing through the
filter is proportional to the area of the filter. The area can be increased by
using larger filters. For example, in the rotary drum filter, the continuous
removal of the filter cake gives an infinite area for filtration.
(iii) Pressure Drop:
The rate of filtration is proportional to the pressure difference across both
the filter medium and filter cake. The pressure drop can be achieved in several
ways:
- Gravity:
A pressure difference could be obtained by maintaining a sufficient head of
slurry above the filter medium. The pressure developed will depend on the
density of the slurry.
- Vacuum:
The pressure below the filter medium may be reduced below atmospheric pressure
by connecting the filtrate receiver to a vacuum pump and creating a pressure
difference across the filter.
- Pressure:
The simplest method is to pump the slurry onto the filter under pressure.
- Centrifugal force: The
application of higher centrifugal force on slurry increases the rate of
filtration.
(iv) Viscosity of Filtrate: It is considered that an increase in the viscosity of the filtrate increases the resistance of flow so that the rate of filtration is inversely proportional to the viscosity of the fluid. This problem can be overcome by two methods:
- Temperature:
The rate of filtration may be increased by raising the temperature of the
liquid, which lowers its viscosity. However, it is not practicable if
thermolabile materials are involved or if the filtrate is volatile.
- Dilution:
Dilution is another alternative but the rate must be doubled.
(v) Thickness of Filter
Cake: The rate of flow of the filtrate through the filter
cake is inversely proportional to the thickness of the cake. Preliminary
decantation may be useful to decrease the number of solids that ultimately help
to increase the filtration rate.
(vi) Particle Size of
Solids: Generally the larger the particle size the higher the
filtration rate (Kg/m2 /h). Small average particle size and a narrow
distribution range have a high filtration rate.
(vii) Ratio of Slimes to
Coarser Particles: The slimes (or extreme fines) in a slurry
affect filtration rates to a vastly greater extent than their percentage. The particularly,
difficult slurry contains relatively coarse particles and several very fine or
slimy particles with little or no intermediate size.
(viii)
Flocculation/Dispersion of Fine Solids: Flocculation is
generally desirable for slurries of fine solids which are in a dispersed state
and generally filter poorly. The wide variety of polyelectrolyte flocculants
provides space for a substantial improvement in filtration rates. Effective use
of flocculants, especially polyelectrolytes, on moderately high concentration
slurry requires strong agitation to get good solids-flocculant contact. A
minimum of further agitation and minimum aging are important. Some slurry may
be so vicious as to create filtering problems and a dispersant may be a better
way to gain fluidity than dilution.
(ix) Slurry Age:
Sometimes processes involve detention times which provide a conditioning effect
modifying filter performance. Samples stored for a longer time for testing
involve a risk that excessive aging may have some effect on filtration.
(x) Agitation Speed:
Some slurry, particularly with a wide particle size range tends to classify in
the test slurry container or the filter tube. Increasing the agitation speed
(or stirring) to a point that the coarse and fine particles are always
thoroughly mixed may be desirable. A too high speed could limit cake thickness;
prevent coarser particles from forming in the cake or cause delicate flocs to
break down.
(xi) Cycle Time: The
cycle time of the filter drum speed is generally expressed in seconds or
minutes per revolution. Generally the faster the drum speed the higher the
output. However, under these conditions, the cake is thinner and sometimes
wetter, so discharge may deteriorate. At all times, a dischargeable cake must
be produced.
(x) Cake Compression:
Cake compression is normally achieved as an adjunct to the filtration step to
reduce cake moisture of compressible cakes.
(xi) Type of Filter
Medium: Filtering characteristics of fabrics depend mostly on
the type of yarn and weave. Yarns can be mono-filament, multi-filament, spun
from staple fiber, or a combination of the latter two. A high twist can make a
multi-filament perform more like a mono-filament. Permeability and porosity are
prime qualities in cloth selection. The Frazier permeability rating, expressed
as cfm/sq ft is a measure of airflow at one-half inch water pressure through a
dry cloth, and is comparable to percent open area. Porosity and particle
retention may not be accurately indicated by permeability; there is no direct
measure of porosity.
(xii) Filter Cloth
Condition: Cloth conditioning refers to the reduction of pore
size or open area due to the entrapment of fine solids in the interstices.
(xiii) Filter Aids:
Filter aids like diatomaceous earth, perlite, powdered coal, and fly-ash or
paper pulp may be added to the slurry to increase its filtration rate and cake
porosity.
(xiv) Solid Concentration
in Slurry: In general, the greater the percentage of suspended
solids in a given slurry, the higher the cake filter rate in Kg/m2
/h and the lower the filter rate in m3 /m2 /h. Where
maximum solids capacity is desired it is advisable to consider thickening the
slurry by gravity. In some applications involving thickening with sludge recycling,
particle size is increased and both cake and filtrate rates can increase.
(xv) Filter Thickening:
Filter thickening normally occurs in a continuous filter rotating in a tank
containing slurry wherein the solids in the filter tank increase in
concentration and shift to a coarser size distribution. While an equilibrium
concentration and size distribution are usually obtained, it may sometimes be
necessary to dilute the pulp.
(xvi) Slurry pH:
The slurry pH and particle dispersion are closely related. The change in pH can
be one of the most effective methods to achieve flocculation and thus improved
filterability.
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