Decanter Centrifuge Separation is using the principle of separation of two phases (liquid/liquid or solid/liquid), based on the difference of density of the two phases: the two phases of different density and not mutually soluble in the mixture are separated under the action of the gravity force or centrifugal force field applied in the decanter centrifuge. The principle is called gravity separation by centrifugal force.
Due to the different density, the mixture in the container is gradually separated after some time into 3 layers, with the heavy solid phase sinking to the bottom, the light phase floating to the surface and the heavy liquid phase staying between the light liquid phase and the solid phase. The separation speed is a result from the density difference of the phases. The separation speed is a result of the difference in density of the different phases.
2 phases centrifugal separation
When centrifugal force is applied to a mixture containing 2 phases of different density in a mixture, the heavy phase separates from the light phase and settles on the inner wall of the bowl. The higher speed the bowl rotates, and the higher is the centrifugal force applied and the faster the mixture separates. A decanter is equipped with a screw conveyor or “scroll” feeding the mixture of liquid and solids, carrying and discharging the solids, to provide a high efficiency, high speed, continuous gravity separation device.
The process liquid enters the decanter through a central feed tube and flows into the inlet chamber of the scroll from where it flows into the separation space of the bowl where it is accelerated to operating speed. The solid particles settle against the bowl wall due to the action of centrifugal force. The scroll, rotating at a slightly higher speed than the bowl, conveys the separated solids to the narrow bowl end of the conical bowl. Due to the conical shape of the bowl, the solids are lifted out of the liquid in the dewatering zone where remaining liquid is removed due to the action of the centrifugal force. At the end of the bowl, the solids are discharged into the receiving chamber of the housing. The liquid flows between the scroll flights in the opposite direction of the solids to the other end of the decanter where it is discharged under gravity through adjustable regulating plates.
Main Motor and Back drive
The main motor is driving the bowl through a V-belt. The pulleys of motor shaft and bowl shaft determine the speed of the bowl.
The back drive (auxiliary motor) is used to control the speed of the central gear shaft to adjust the differential speed between the basket and the screw conveyor.
There are two systems controlling the back drive (differential speed): the first is to control the differential speed with the auxiliary motor shaft through the coupling with main motor; the second is to control the differential speed with the motor driving the differential belt pulley through the V-belt.
The decanter can be adapted to meet a wide range of conditions and requirements and the effective throughput capacity of the decanter depends on:
• the separability or clarifiability of the product (size and density of the solid particles versus the density liquid phase),
• the flow rate of the mixture and the concentration of the solids in the product,
• the required residual moisture content in the discharged solids,
The decanter can be adjusted to suit individual requirements by varying the following control parameters:
- Bowl Speed: By varying the rotational speed of the bowl, the G-force can be adjusted to suit the application. The higher speed, the better separation, so usually the decanter runs at maximum speed for optimal performance.
- Feed rate: The lower the feed rate, the better the separation.
- Liquid Level: Adjust the liquid level (pond depth) to give the optimal balance between liquid clarity and solids dryness by selecting different plate dams. In general terms, the effluent liquid becomes more clear and the solid cake more wet when diminishing the liquid radius (increasing plate size) and vice versa.
• Differential Speed (∆n or ∆RPM): The dryness of the solids cake can be increased when operating with a lower differential speed, but the effluent liquid will be less clear and vice versa. The torque increases with the lower Differential Speed as the solids becomes drier. This property can be used to regulate the dryness of the solid cake: the adjustment of differential speed based on the torque can be used to compensate for varying content of solids in the feed and automatically regulate the dryness of the solid cake.