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Why
use filter powder?
Filtration is accomplished by forcing liquid under pressure
through a cloth or screen (septum). The solids to be filtered
may be non-rigid, slimy or colloidal in size and occur in
most organic and food products.
Theoretically the liquid should pass through the opening of
the filter cloth and the impurities remain on the cloth. In
practice, the finer, suspended solids pass through the coarse
openings in the cloth and larger particles remain behind to
clog the openings, smear the cloth and slow down or, most
likely, stop the flow.
In such systems where the filtration resistance is high, unwanted
solids can only be removed efficiently and economically by
use of filter powder. The filter powder forms a porous layer
on the filter septum, which acts principally as a support
for this cake / bed. The filter aid is now the filtering medium
that traps the solids to be removed and prevents them from
blinding the septum.
Use of filter aid also allows quick and easy cake removal
without damage to the cloth.
What
is Diatomite? (Also known as Kieselguhr and diatomaceous
earth)
Diatomite is a fossilised mineral of vegetable origin. It
is the accumulation of an enormous number of fossil diatoms.
Diatoms are single cell plants of microscopic size. There
are many varieties that live in both freshwater lakes and
saltwater oceans, staying close to the surface where they
form part of the abundant plankton. Alive, they are at the
base of the food chain. Dead, they leave a tiny skeleton known
as a frustule.
Whereas most other plants use solar energy to synthesise cellulose
for their cell walls, the diatom extracts amorphous silica
from the water, building for itself what amounts to an elegant,
but strong shell with highly symmetrical perforations.
Diatoms reproduce themselves in bi-cell divisions every few
hours and it has been estimated that a single diatom could
produce up to 1,000,000,000 diatoms
In its natural (milled) state, the individual diatoms are
extremely small and would form a very fine (low permeability)
filter-cake, which would be too ‘slow’ for most
filtration applications. To create larger particles, the diatoms
are fused together at high temperatures. These larger particles
are then classified, according to their specification.
Large deposits are now open-cast mined in the USA, Northern
Europe and Iceland.
Two types of Diatomite are processed:
- Freshwater origin, which are tubular and
extremely effective and efficient in the removal of very
fine solids and applications where fast flow-rates are required.
All types are processed at temperatures ranging from 8000
to 11000°C. Both calcined and process-calcined Diatomite
filter aids are free of organic matter and are non-absorptive.
Celatom is a fresh water brand of diatomite.
- Seawater origin, which are composed of
rods, flowers, boats and snowflake shapes. These have the
same use as freshwater daitomites, they are just a different
shape.
What
is perlite?
Perlite, in the natural state, is a dense glassy rock formed
by volcanic action. When crushed and heated, it ‘pops’
like popcorn, expanding to 20 or more times its original volume.
The explosion creates a porous structure. By special milling
techniques, the structure necessary for a good filter aid
can be obtained.
Perlite is an amorphous mineral consisting of fused sodium
potassium aluminium silicate. It is processed in the same
temperature range as Diatomite, in order to eliminate organic
matter.
Perlite does not possess the very fine interstices or internal
area of Diatomite and, therefore, in general does not have
the same very fine solids removal capability. However, because
perlite is bulkier, it can be more economical to use in many
applications.
What is
cellulose?
Pure Alpha cellulose is extracted from beech wood. The fibres
produced are snow white, chemically inert and contain practically
no ash. In general terms, the different permeability’s
/ clarifying characteristics of each cellulose grade are determined
by the length of the fibres.
What products
can be filtered utilising filter powder:
Typical applications may be found in the Celatom® Filtration
Spectrum Performance Scanner. (160k PDF)
What is
a pre-coat?
A filter aid pre-coat protects the filter septum (the support
for the filter cake) and ensures clarity by retaining solids
at the surface. Filter aid slurry is made from filtered liquid
or, sometimes, water and filter aid. The concentration should
be a slow as possible, with 0.5% being typical. Agitation
in the pre-coat tank should be sufficient to keep the filter
aid in suspension. Filter aids should be added at 500 to 1200g/m2
of filter area.
The pre-coat is formed by re-circulating the filter aid slurry
through the filter. The coarser particles deposit themselves
first on the screen, followed by smaller ones. A pre-coating
rate of 40 litres / m2 / min is standard. Much lower rates
are used with higher viscosity liquids. Differential pressure
should be at least 0.07 kg /m2 (1psi) during the pre-coat
process.
Pre-coat may be thick or thin:
- Thin layer pre-coats
for low viscosity liquids with less than 50ppm impurities
on pressure filters
- Thin layer pre-coats plus body-feed addition,
as used in most pressure filtration systems.
- Thick layer pre-coats where high solids
of up to 30% are present and large volumes are involved.
Rotary vacuum filters are used for this process.
What
is a body-feed?
Body-feed is the addition of filter aid to the liquid being
filtered. Body-feed filter aid is continuously added to the
unfiltered liquid to keep the dirt solids apart, thus maintaining
a porous cake. Body-feed maintains clarity and flow-rate
throughout the filter run. The solids content will determine
the filter aid dosage levels. Selection of the optimum type
and grade of filter aid is crucial and our technical engineers
would be pleased to assist in this analysis.
Why choose
Celatom® freshwater Diatomite ahead of other brands?
Being hollow and tubular, Celatom® freshwater diatomite
has a low bulk density and greater efficiency versus seawater
diatomite. In filtration, it is usually possible to use one
20 kg bag of Celatom® fresh water diatomite, where you
might currently be using a 23 kg bag of a competitor’s
sea water diatomite – thus saving 3 kgs per bag, in use.
In addition, by reducing body-feed doses, further savings
are often possible. Filter run-lengths remain the same, whilst
clarities and micro-counts, as in the production of beer for
example, are improved.
How
is Celatom® packaged?
Product can be supplied in 20kg Kraft multi-ply sacks, 400kg
to 600kg bulk bags, or in bulk tanker loads.
How should
I select the best filter powder system for my process?
Please contact sarah@flexibulk.co.uk
our sales team at Flexibulk to discuss which filter powder
management system is most suitable for you. Our technical
managers are pleased to receive your questions regarding solids
/ liquid separation. Where appropriate, Flexibulk’s technical
team is available to make on-site
visits and conduct laboratory-scale filtration trials using
our small pressure and vacuum filters.
Dose
rates: how much should I use?
By running lab-scale trials, the optimum grades and
dose rates can be selected, which avoids costly ‘trial
and error’ full-scale trials.
As a general indication, a pre-coating rate of 40 litres /
m2 / min is standard. Much lower rates are used with higher
viscosity liquids. Filter aids should be added at 500 to 1200g/m2
of filter area. For bodyfeed a dosage of 0.5 of the percent
solids by weight is a good starting point.
What
makes beer hazy?
Many factors contribute to clarity of beer post-filter, and
usually the choice of filter powder is a major factor contributing
to the quality of packaged beer. In general terms, reducing
the permeability of the filter powder (cake) will improve
filtrate clarity, however, reducing the permeability can often
increase the pressure rise per hour during filtration.
It is very important to work with your filter powder supplier
to optimise the filter powder regime: i.e. to choose the optimum
blend of perlite, Diatomite or cellulose for the beer qualities
produced.
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