| Decanter Operation A decanter is an engineered piece of process equipment designed to separate insoluble suspended solids from a mother liquid. The decanter can be used to produce a relatively wet or thin cake (thickening) or a very dry cake (dewatering). The decanter is flexible enough to handle a great fluctuation in the feed material. The decanter can clarify the mother liquid, or classify the solids to obtain specific solids distribution. The decanter will function well if the solids have a higher specific gravity than the liquid. The decanter consists of a solid bowl, which rotates and contains the process. A screw conveyor is contained within the bowl, and it will turn at a slightly different speed from the bowl. 
The feed slurry is introduced into the rotating assembly by a stationary feed tube. The feed is jetted into the conveyor. The feed slurry is accelerated up to speed and will flow through the bowl. The liquid flows from the point where it is introduced on the pond, to the liquid weirs at the large end of the rotating assembly. As the liquid flows through the pond, the g force causes the solids to settle out of the liquid and to sediment against the bowl wall. In effect, there is a blanket of solids building up against the bowl wall. The screw conveyor turns at a slightly different speed than the bowl. The solids building up against the bowl wall are pushed to the small end of the bowl by the screw conveyor. The solids are pushed in the horizontal direction and then up an incline and ultimately out of the liquid prior to being discharged from the bowl. Centrifugal force is constantly being exerted on these solids in an effort to produce the desired solids discharged product. The liquid clarity is controlled by the g force, pond setting, differential speed, flow rate and polymer addition. The liquid clarity will be a function of the hydraulic throughput of the decanter. The higher the flow rate, the poorer the liquid clarity will be. The exact relationship will be dependent on the nature of the feed material. The differential speed will also affect the clarity of the liquid. The lower the differential, typically the less turbulence in the liquid, and the cleaner the liquid being discharged (centrate). However, once the sludge blanked in the bowl is overloaded and the screw conveyor does not remove the solids at the same rate that they enter the unit, the centrate will start to get dirty. The force that removes the solids from the liquid is the relative g force in the bowl. The faster the bowl turns, the greater the g force, and the greater the driving force to remove the solids from the liquid. The greater the g force, the cleaner the centrate will be. The pond setting is an important factor in setting up the centrifuge. The deeper the pond is, the greater the liquid retention time in the bowl, and the cleaner the centrate will be. A deeper pond will also help the screw conveyor convey the solids from the bowl, which again will improve the centrate clarity. Some applications will use polymer to improve the performance of the decanter. The polymer will increase the relative particle size, improving its settling characteristics. This will improve the centrate clarity. Additional polymer will typically improve the liquid clarity up to some point. At this point, overdosing the polymer will cause the polymer to act as an emulsifying agent causing the centrate quality to deteriorate. The solids are also affected by the same parameters as the liquid. As the g force is increased, the force to compact the solids in the sludge blanket increases and the solids become more concentrated. As the solids are conveyed up the beach and are free of the liquid, the increased g force will release more free liquid and produce a drier cake. The differential speed between the bowl and conveyor directly controls the cake solids. The lower the differential, the drier the cake solids will be. This is done by getting the maximum compaction in the sludge blanket, and longer retention time on the beach. The pond depth can affect the cake dryness and the shallower the pond, the longer the retention time of the solids going up the beach and out of the liquid, increasing the cake dryness. The greater the solids loading on the decanter, the wetter the cake will be. This is not a direct relationship, but increasing the solids loading will cause the unit to achieve a wetter cake. The polymer, or chemical condition agent, is used to achieve a better liquid clarity. It also has a secondary effect, and the polymer tends to improve the ability of the decanter to dewater the solids. This again is a variable that must be proven by field-testing. One last facet is the classification effect. As the liquid clarity worsens, more fines are discharged in the liquid and the cake can get drier because of this loss in fines in the effluent. The effect will be dependent on the particle size distribution in the feed material.
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