The seventh lecture in the module Particle Technology, delivered to second year students who have already studied basic fluid mechanics. Centrifugal Separation covers both sedimenting and filtering centrifuges as well as hydrocyclones. Adaptation of the gravity settling and conventional filtration models, to account for the conceptual centrifugal acceleration, is included. Examples of industrial equipment for centrifugal separation are included.

1. Centrifugal Separation Chapter 8 in Fundamentals Watch this lecture at http://www.vimeo.com/10203052 Visit http://www.midlandit.co.uk/particletechnology.htm for further resources. Course details: Particle Technology, module code: CGB019 and CGB919, 2nd year of study. Professor Richard Holdich R.G.Holdich@Lboro.ac.uk

3. Particle motion in a centrifugal field

4. Sigma theory

5. Hydrocyclones

6. Grade efficiency & cut size

7. Filtering centrifuges

8. Adaptation of filtration equations

10. Scroll Discharge Decanter Screw rotates at only slight differential speed to the centrifuge - solids leave at one end, centrate at the other. Image courtesy of Siebtechnik GmbH

11. Tubular bowl centrifuge This one is vertical axis - simple design with no internals for clarification or liquid/liquid separation - a more complicated design is the chamber bowl. Image removed for copyright reasons. For an example product please see http://www.sharpenntechnologies.com/pcat-gifs/products-large2/high-speed-centrifuge1111-2.jpg.

12. Disc stack centrifuge Like a lamella clarifier: internal surfaces to encourage settling - usually used in oil/water separation and cream

13. Sedimenting Centrifuges – Let’s confine our analysis to a simple geometry - ignoring the complicated internal structures required to remove deposited solids and oil concentrates. Liquid flow out Inner radius Air core Outer radius

18. Giving:i.e. the radial residence time in the machine

19. Horizontal/axial residence time where

20. Sedimenting Centrifuges

23. Flue gas desulphurisation Feed:CaSO4 - 35water - 65 100% Cake:CaSO4 - 70water - 30 100% Centrate:CaSO4 - 2.7water - 97.3 100% All concentrations as mass percent

24. Hydrocyclone Single unit and array: Defined by diameterof cylindrical section Image showing "Krebs gMAX® Hydrocyclones" courtesy of FLSmidth Krebs Inc.

25. Means of separation Centrifugal: 800 g in 300 mm hydrocyclone 50000 g in 10 mm hydrocyclone Type of separator: a classifier (i.e. splits into sizes) a thickener (i.e. concentrates suspensions)

26. Operating data Diameters: 0.01 to 1 metre Solid (cut) sizes: 2 to 250 microns Flow rates (single unit): 0.1 - 5000 m3 h-1 Pressure drop: 6 to 0.4 bar U/F solid content: up to 50% v/v (claimed)

27. Principal features Note: primary & secondary vortex, air core, U/F, O/F, tangential feed

28. Tangential velocity

29. Radial velocity

30. Axial velocity

37. Normalised reduced grade efficiency:100%

38. Equilibrium Orbit Theory A particle orbiting on the LZVV has no net tendency to move into the primary vortex (then O/F) or secondary vortex (then U/F). It must be equal to the cut size x50%.

40. Liquid drag:FD FC

41. Hydrocyclones - types and configurations Oil/water separation - often offshore

42. Filtering Centrifuges A perforated bowl - similar to a spin dryer See box on page 83 for descriptions

43. Filtering Centrifuge – Section 8.3 Pusher generally coarse solids > 50 microns (semi)-continuous solids output careful balance of slurry in Image courtesy of Siebtechnik GmbH

44. Filtering Centrifuge Peeler generally solids > 5 microns usually intermittent solids output - slow to 50 rpm Image removed for copyright reasons. Please search online for an image of a peeler centrifuge.

45. Filtering Centrifuge Inverting Bag generally solids > 5 microns intermittent solids output Image removed for copyright reasons. Please search online for an image of an inverting bag centrifuge.

46. Filtering centrifuge - full cycle Function Time(s) Time(%) Accelerate from 50 to 500 rpm 40 5 Load/Filter at 500 rpm 277 32 Accelerate to 1050 rpm 90 10 Spin dry at 1050 rpm 119 14 Wash at 1050 rpm 10 1 Spin dry at 1050 rpm 236 27 Slow down to 50 rpm 90 10 Unload at 50 rpm 15 2 Total cycle time 877 100 Basket load per cycle of solids 140 kg Productivity 575 kg/hour

47. Centrifuge - simple analysis – Fig 8.9 Ptotal = Pcake + Pmedium Definitions:

48. Centrifuge - simple analysis - same as for conventional filtration However, the radius at which the cake forms is continually moving inwards and the geometry is not planar. where:

49. Centrifuge - simple analysis Centrifugal head - the driving pressure: where omega is in seconds-1 = (2 pi/60)RPM Density is that of the slurry or liquid depending upon the operation: filtering or washing

50. Centrifuge - washing but rc remains constant during the washing stage. The time to wash with Vw m3 of solvent is:

51. Centrifuge - washing Typical washing performance: 1 Solute concn. Initial concn. 0.5 Flooded cake Dewatered cake 0 1 2 3 Wash volumes

52. Centrifuge - drainage 1 Relative saturation 0.5 S* = S Irreducible saturation Sinitial 0 0.2 0.4 0.6 Time or dimensionless drainage time

53. This resource was created by Loughborough University and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. Slide 3 (Left). Image of a decanter centrifuge provided courtesy of Thomas Broadbent and Sons Ltd. See http://www.broadbent.co.uk/en/about for details. Slides 3 (right), 4, and 42. Images courtesy of Siebtechnik GmbH. See http://www.tema.co.uk/images/products/7_1.jpg for details. Slide 24. Image of"Krebs gMAX® Hydrocyclones" photo courtesy of FLSmidth Krebs Inc. See http://www.flsmidthminerals.com/Products/Classification/Hydrocyclones/Hydrocyclones.htm for details. © 2009 Loughborough University This work is licensed under a Creative Commons Attribution 2.0 License. The name of Loughborough University, and the Loughborough University logo are the name and registered marks of Loughborough University. To the fullest extent permitted by law Loughborough University reserves all its rights in its name and marks which may not be used except with its written permission. The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher.