The influence of the pre-dewatering time and the feed-rate of the wash-liquid on the filter-cake washing in the centrifugal field

摘要

The processing of suspensions with filtering centrifuges is divided into severalrnsteps: filling of the centrifuge, cake-building, pre-dewatering, washing, dewatering and unloadingrnof the filter-cake. On the one hand the dewatering of the filter-cake itself improves the purity ofrnthe product by reducing the amount of mother-liquid which contains the dissolved impurities. Onrnthe other hand the washing-step is used to even increase the purity of the particulate product. Inrngeneral both, the dewatering and the washing, occur parallel and very often, especially inrncontinuous centrifuges, the filter-cake is already partly dewatered before the washing-medium isrnadded. Therefore the interaction between the pre-dewatering- and the washing-cycle has to bernunderstood in order to optimize the purification process.rnDepending on the distribution of the mother liquid inside the filter-cake before the washing steprnstarts the purification can be achieved by ideal displacement or by convective mass transferrnbetween a mother-liquid- and a wash-liquid-film. The requirement for the mentioned idealrndisplacement is the existence of a capillary height of mother-liquid inside the filter cake. But notrnonly the pre-dewatering and therefore the distribution of the mother-liquid but also the flowrnpattern of the penetrating wash-liquid affects the washing procedure. A high flow-rate causes arnplug-flow of wash-liquid inside the pores and a liquid layer on top of the cake whereas lowerrnflow-rates result in an instantaneous penetration and a film-flow over the surface of the particlesrnor the remaining mother-liquid. If the remaining mother-liquid inside the filter cake is only at therncontact points or on the surface of the particles a plug-flow of wash-liquid could be a waste sincernonly a very small ratio of the wash-fluid has contact to the impure mother-liquid. The most partrnpenetrates the filter-cake without contributing to the purification process.rnThe results show that the distribution of the remaining mother-liquid inside the filter-cake at thernbeginning of the washing step affects the chance of ideal displacement of the contaminatedrnmother-liquid and the convective mass transfer between the mother- and the pure wash-liquid.rnThey also show that for the investigated product a film-flow of the wash–liquid seems to be morernefficient compared to piston-flow concerning the purity of the particulate product.