Monthly Archives: July 2019

The mystery of decanter centrifuge

We are very familiar with the process of settlement, where solids in liquid fall down to the bottom of a container, leaving liquid phase crystal clear. This is the case when we sit a cup on top a desk, fill it with water including some solids. After some time the solids will settle at the bottom. Equipment like this is called clarifier.

However, this process happens in natural gravity and it takes too long for the solids to settle down. Then people thought centrifugal force may help to speed up this process. So they tried to tie a bottle on one end of a rope and rotated the rope at certain speed. They found that the settlement happened much quicker. This initiated the decanter centrifuge, which works on the same principle as the rope except that decanter is more complex, withstand more load, and make the settlement process much faster.

With decanter centrifuge the separation of solids from liquid is greatly expedited. For instance it can take hours for solids to separate from liquid while it takes only a few second to achieve the same result. This is because G force generated at decanter centrifuge is a few thousand time that of regular gravity.

There are many factors controlling the performance of decanter centrifuge, but among them the following four factors are most influential.

  1. Centrifugal force generated. This determines separation efficiency. When the gravity difference between solids and liquid become very small, exceptional strong G force is need to separate solid from liquid. This also applies to small particles, in the magnitude of microns, as buoyance is in the same magnitude of gravity force an the particles will float in the liquid;
  2. Differential speed required to send settled solids from decanter centrifuge
  3. Design of screw conveyors
  4. Settlement section for particular type of applications.

There are much more considerations in practical applications.

Consideration of the application of decanter centrifuge

Many people know that decanter centrifuge works on the principle of centrifugal force, which makes heavier solids to settle down on bowl wall and lighter liquid phase to “float” on top of the deposited solids. However, there is much more to take into consideration when decanter centrifuge is to be applied to different applications.

The first consideration is the capability required to in the production process. This can differ tremendously, from a few million liter per minute to thousands of cubic meters per hour. This is mainly due to economic consideration.

The other consideration is the length and diameter of rotating bow. The longer the rotating bowl, the longer slurry takes to travel the whole length, and therefore better settlement of the solids. This is particularly important when lighter solids phase is to be separated from the liquid. If the solids is much heavier, such as barite in drilling process, bowl length does not need to be that long. It is common for the length of rotating bowl to be 2, 3 or even 4 times the diameter of rotating bow.

Another consideration is the conical section of decanter centrifuge bowl, this concerns the texture of the solids to be separated. If solids is easily compactable, a bigger angle can be applied so as to increase throughput speed. However if the solids to be separated is quite lose, and difficult to get compacted, a gentle angle is usually required to have them compressed properly before being discharged from the rotation bowl.

One important factor is the centrifugal force. There is much to consider when this factor is to be considered. In a slurry there is always liquid in between solids particles. The finer the solids particle is, the more liquid content, therefore a much larger centrifugal force will be needed to move liquid content out from between solids particles.