Process Technology: An Introduction - Haan A.B. 2015

10 Solid-liquid separation
10.7 Centrifugal filtration

In a centrifugal filter the suspension is directed on the inner surface of a perforated rotating bowl to effect the separation of liquid from the solids. Centrifugal filters essentially consist of a rotating basket equipped with an appropriate filter medium (Fig. 10.34). The driving force for filtration is the centrifugal force acting on the fluid. During cake formation the filtrate passes radially outwards through the filter medium and the bowl. Because the centrifugal forces tend to pull out the liquid from the cake, filtering centrifuges are excellent for dewatering applications. Removal of solids may take place continuously or batchwise. In comparison to other filtration equipment centrifuges show good separation performance at the expense of relatively high costs. This is due to required special foundations to absorb vibrations combined with high manufacturing and maintenance costs associated with the parts rotating at high speed. Centrifuges are generally only applicable to the coarser particles, typically 10 I1m-lO mm.

The simplest fixed-bed centrifuge is the perforated basket centrifuge, which has a vertical axis, a closed bottom, and an overflow restriction at the top end. The slurry is fed through a pipe or rotating feed cone into the basket. The cake is discharged manually or by a scraper that moves into the cake after the basket slows down to a few revolutions per minute. The plough directs the solids towards a discharge opening provided at the bottom of the basket. Some residual cake remains, since the plough cannot be allowed to reach too close to the cloth. The basket centrifuge has a wide range of applications in the filtration of slow draining products that require long feed, rinse, and draining times. It can be applied to the finest suspensions of all filtering centrifuges, because filter cloths may be of pore sizes down to 1 pm. Making the axis horizontal may eliminate the nonuniformity due to gravity with a vertical basket. This is known as the peeler centrifuge, shown in Fig. 10.35, which is designed to operate at constant speed to eliminate nonproductive periods. The cake is also discharged at full speed by means of a sturdy knife which peels off the cake into a screw conveyor or a chute in the center of the basket. The peeler centrifuge is particularly attractive where filtration and dewatering times are short, such as high output duties with nonfragile crystalline materials.

Fig. 10.34: Perforated basket centrifuge.

Fig. 10.35: Peeler centrifuge.

Continuously operating moving-bed centrifuges use either conical or cylindrical screens. The conical centrifuge in Fig. 10.36 has a conical basket rotating either on a vertical or a horizontal axis. The feed suspension is fed into the narrow end of the cone. If the cone angle is sufficiently large for the cake to overcome its friction on the screen, the centrifuge is self discharging. Pusher-type centrifuges have a cylindrical basket with a horizontal axis. The feed is introduced through a distribution cone at the closed end of the basket (Fig. 10.37), and the cake is pushed along the basket by means of a reciprocating piston that rotates with the basket. Pusher centrifuges can be made with multistage screens consisting of several steps of increasing diameter.

Fig.10.36: Conical centrifuge.

Fig.10.37: Pusher centrifuge.