Wallace Leung

Dewatering of fine-particle slurries using a compound-beach decanter with cake-flow control

A decanter centrifuge of novel design was developed for the dewatering of fine-solids slurries having micron-range particles that form paste-like, flowable, hard-to-dewater cakes. The design combines a compound beach with a cake-flow control structure that allows expressed liquid from the cake in the dry beach region to be returned back to the liquid pool while maintaining high solids throughput. In order to provide optimal performance with various operating parameters (feed rate, centrifugal gravity, pool, differential speed, concentration of feed solids, and different slurry types), the cake-flow control is adjustable. The second part of the beach is preferably at zero angle. Tests showed that the new design allows increases in both solids dryness and solids throughput.

Torque requirement for high-solids centrifugal sludge dewatering

Abstract In dewatering of compactible sludges to high dryness, the solid-bowl decanter centrifuge is operated with the annular volume between the conveyor hub and the bowl wall packed almost completely with cake solids. The torque required to maintain the conveyor rotating at differential speed with respect to the bowl can be determined for a given type of sludge from the “effective frictional coefficient”, which for municipal and industrial sludges, is typically in the range between 0.5 and 1.5. The back-drive torque, which sustains the conveyor under a small differential speed with respect to the rotating bowl, is used to convey the cake. Concurrently applying a churning action on the cake allows the moisture in the interstitial voids of the cake to be expressed out to the cake surface away from the bowl wall. Higher torque is a consequence of either, or a combination of, higher operating centrifugal gravity, higher solids throughput, thicker cake from lower differential speed, longer transport distance from a large length-to-diameter decanter, and drier cake. The decanter can be operated either under torque control, differential-speed control, or a hybrid algorithm involving both variables. A high-solids decanter mimics a rheometer with continuous inflow and outflow of test fluid material wherein the rheological properties are determined from the torque-speed measurements.

Improved design of conical accelerators for decanter and pusher centrifuges

The simple cone-type feed accelerator commonly used for solid-bowl and screen-bowl decanter and pusher centrifuges delivers to the surface of the decanter pool or the pusher basket a tangenial speed far below the value of Ω R (the rim speed of the cone), with consequences adverse to performance. A novel, more effective cone accelerator is described. This has forward-curved vanes that produce overspeeding and a final unvaned section of cone — called a smoothener — which smears out the jets from the vanes into a nearly uniform conical sheet. Model tests in the laboratory were used for developing the new accelerator technology. Operation in the field has verified the expected improvements in capacity performance.

Efficient double-disc accelerator for continuous-feed centrifuges

Abstract Analysis of the hub-type feed accelerator commonly used in decanter centrifuges reveals serious deficiencies. It delivers to the surface of the pool a tangential speed far below the value of ωR , with a consequent loss of G -level and separation effectiveness. Although the G -level corresponding to the rotation speed is ultimately reached, this occurs by an undesirable ‘churning’ action of the conveyor that causes re-suspension and mechanical wear on the conveyor face. Moreover, the feed exiting the accelerator plunges into the pool with a high radial speed and in a circumferentially non-uniform manner. A novel, more effective accelerator is described here. This has vanes situated between parallel discs whose faces are normal to the axis of the machine. The vanes are forward-curved, and thus produce overspeeding . Downstream of the vanes is a conical smoothener , which smears out the jets from the vanes into a nearly uniform conical sheet. Model tests in the laboratory, using water, were used to develop the improved technology, which is equally adaptable to solid-bowl and screen-bowl decanters and also to pusher centrifuges. Many tests in the field, on both decanters and pushers, have verified that the new technology produces significant improvements in capacity and performance.

In-situ reslurrying and dewatering in screenbowl centrifugues

An innovative patented cake washing technology has been implemented in a continuous-feed screenbowl centrifuge to effectively remove cake impurities. At high wash rates the cake is reslurried and deliquored in sequential stages in-situ in a screenbowl centrifuge to remove residual impurities left in the cake. At low wash rates the new technology increases contact between the cake and the wash liquid promoting cake washing by dissolution and diffusion. The new reslurrying technology was developed in the laboratory with a pilot screenbowl centrifuge on model glass beads and a highly porous polymer. Field tests on sodium carbonate monohydrate crystals using the pilot screenbowl centrifuge with the optimal reslurrying design demonstrated better washing performance with higher cake purity and lower wash liquid.