use in hazardous and ex-proof
environments when extremely safe operation is required.
MY Nutsche Filters are available in almost any size with the larger
machines for a slurry filling batch of 25 m3 and a cake volume
of 10 m3. Such filters have a filtration area of 15 m2 and are
suitable for fast filtering slurries that produce readily 0.5
m thick cakes.
Operational
Sequence:
The operation of Nutsche Filters requires a complex manipulation
of valves and hydraulics so present day installations are always
semi or fully automated.
Filtration:
The filter is charged with slurry and pressure is applied to displace
the filtrate leaving the cake retained over the filter medium.
For slurries with a wide distribution of coarse fast settling
solids and slow settling fines there is a risk of segregation
with the finer fraction settling over the coarse fraction. When
this happens the fines seal the cake and slow down the cake formation
so keeping the slurry in suspension with rotating arms during
filtration assists in forming a homogeneous cake.
Cake Washing:
In the washing stage a spray ring or connections on top of the
cover introduce the wash liquid over the cake. This displaces
the mother solution with the wash liquid but with such in-situ
washing the efficiency may be quite low if the cake forms with
an uneven thickness.
One of the advantages of the Nutsche Filter is the ability to
smoothen the cake's surface prior to applying spray wash so that
the entire bed is washed evenly.
Washing efficiency may be further improved if air or gas are not
allowed to enter the cake in a multi-washing system so the wash
liquids always displace the solutions in a "piston"
like manner. This is achieved by a special detector that monitors
the surface of the cake for moisture and once air or gas start
entering the bed a signal is transmitted to close the filtrate
valve and reopen it once next washing commences.
Cake Repulping:
Many processes require high washing efficiency to remove the contaminating
liquid from the product and washing the cake by repulping yields
the most efficient product purity. This is done by resuspending
the cake with the paddle arms for thorough mixing with the wash
solution. During resuspension the rotating arms are moving slowly
downwards and are "shaving" the bed gradually layer
after layer until the entire cake enters the slurry.
Pressure Drying:
In the drying stage air or gas purges the cake until the captive
moisture is reduced to an asymptotic level and in practical terms
the cake is considered to be as dry as possible.
To obtain minimum moisture the cake is smoothened by reversing
the rotation of the paddle or auger arms and exerting controlled
pressure on its surface with the hydraulic system. This seals
cracks in the cake so that air or gas will not bypass the bed.
Cake Discharge:
Once all the stages are completed the cake discharge valve opens
and the paddle arms on the smaller machines or the auger arms
on the larger ones are rotated and lowered to convey the dry cake
towards the center. The same procedure also applies to side discharge
machines however it should be noted that in this case the cake
comes out intermittently and not continuously. This may have a
layout impact on the downstream facility such as the conveyor
that handles the product to storage.
On some filters the cloth or woven mesh screen may be backwashed
with water to dislodge and remove any cake residue that adhered
to the medium after cake discharge.
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