SB Aug16-21 pharma.qxd 8/8/07 10:46 Page 18
Pharmaceutical
apertures of the mesh. When blocking occurs, the useful
screening area is reduced and therefore capacity drops.
The system works on the power by demand (PBD) principle,
which solves the problem of uneven loading. Constant feedback
from the separator screen to the PBD controls monitors the
throughput of material in the system. When there is a heavy
loading on the sieve mesh, PBD increases power, maintaining
the amplitude of the ultrasonics to pass materials through
quickly and efficiently without blinding.
There are several knock on benefits to eliminating blinding via
an ultrasonic deblinding device. The first is that sieving capacities
improve, increasing productivity. The second is that because the
mesh is kept free from blockage for longer, manual cleaning is
more infrequent and therefore the chance of damaging the mesh
is reduced. Finally, ultrasonic deblinding systems enable
powders to be sieved using meshes with smaller apertures. This
enables even finer quality products to be produced than
previously possible or even to screen powders that could not be
sieved before.
The effect of the ATEX directive on sieves
Recent legislation has had a large effect on the design of
ABOVE: Some pharmaceutical powders may be sticky or have irregular
sieving equipment.
shaped particles, which can cause mesh-blinding problems
On March 1, 1996 the European Community adopted a
Directive on equipment and protective systems intended for use
blocking has been used for over 25 years. in potentially explosive atmospheres (94/9/EC). ‘Atmospheres
The ultrasonic frequency is applied to the sieve mesh via an Explosibles’ is more commonly referred to as the ATEX Directive.
acoustically developed transducer. This breaks down the surface The ATEX Directive’s primary function is to eliminate the
tension, effectively making the stainless steel wires friction free possibility of explosions. It applies to electrical and mechanical
and preventing both particles slightly greater and smaller than equipment intended for use in potentially explosive atmospheres.
the mesh from blinding or blocking it. Screen blinding or blocking The Directive affects all industries including food, metal
is a common problem in the sieving of difficult powders on powders, powder paint, pharmaceutical powders, chemicals and
screens of 500 µm and below. It occurs when either one or a any other industry involving powders, dusts and vapours. From
combination of particles sit on or in an aperture of the mesh and July 2003 all new equipment purchased for installation and use,
stay there or particles adhere to the mesh wires, preventing in a potentially explosive atmosphere has to comply to the
other particles from using these openings to pass through. It is requirements of the ATEX Directive.
particularly common with sticky powders or materials which Design changes to sieving units are mainly centred around
contain a large number of particles of a size similar to that of the making sure that the unit is free of any potential sources of
ignition. Therefore it is essential to properly earth all components
and, in the design of the equipment, remove all other
possibilities of a spark or excessive heat generation.
Ultrasonic frequency is applied to the sieve mesh via an acoustically
developed transducer
18 Solids & Bulk Handling • August 2007
www.solidsandbulk.co.uk
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