Nutsche Filter Dryer

Nutsche filter/dryer  construction

1. Rotate joint  2.Driving  3.Lifting  4.Sealing  5.Bellow 

6.paddle  7.Filter mesh   8.Vessel 9.Frame 10. discharge valve

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. 

          Feeding and Filter                   Pulping and Washing                 Pressure Drying                       Cake Discharge