Several years ago, pilot tests using a prototype of a screen bowl decanter with conical screen discharge were conducted in a coal mine. An analysis of the particle size distribution in the feed slurry’s suspended solids showed that a very large proportion of the fine solids present in the centrate were “generated” in the centrifuge (figure 1).
Reduction of the particle size is often undesirable in solid-liquid separation due to the larger amounts of valuable material lost in the liquid effluent or more difficult dewatering due to the higher capillary forces. Potential areas were thus investigated where unfavourable forces acting on the solids inside decanter centrifuges should be reduced. High shear forces affect the grains present, especially during acceleration and redirection of the feed slurry in the feed ports of a decanter centrifuge. As a result, extensive CFD simulations were performed for different designs of feed port in order to optimise their geometry and thus achieve gentle acceleration and reduce the impact on the rotating liquid pool (figure 2). The model favoured then underwent practical tests on a laboratory-scale decanter and confirmed the theoretical results.
With the current order for two AS9-2 screen bowl decanter centrifuges to be delivered to a Polish coal mine, there was an excellent opportunity available to move up from lab-scale to production-scale testing. One of the centrifuges was supplied with the new “Gentle Feeder” feed port design to allow comparative tests in parallel mode. When the new plant was started up, relevant samples were taken and analysed by an external institution. Although particle fragmentation is limited with the coal from this mine, an impressive result was achieved, with the concentration and, therefore, the loss of solids with the centrate being reduced by about 50% with the new feed port design.
These results will form the basis for launch of the new design, especially at installations where the reduction of particle fragmentation is of particular interest such as those handling crystallized solids like potash, ammonium sulphate or amino acids.
Session: L2 - Decanter Centrifuges
Day: 13 March 2018
Time: 14:45 - 16:00 h