2.2.4 floe which is magnetica lly sati sfied , with little residual attraction for any other wandering beads or flo es . These mu st be captured from the final overflow either by strainers or by magnet ic means. 2.2.3 Flowability A major pract ica l advantage of magnet ised mi~o beads is that a settled bed retai ns its ability to flow through tubes, va lves , pumps, etc . The magneti c floe structure is part ly prese rved in the sett led bed, all owing the bed to retain a few percent more water tha n a set tled bed of co nve nti onal res in. The la tter settles, like sand or grit, into a closepacked lattice stru cture with minimum vo ids. In order for thi s settled bed to flow, thi s lattice mu st ex pa nd to generate shear planes. T he extra water must be supplied from outs id e the bed . T hi s has always bee n a major nui sa nce in start -up of prev ious co ntinu ous io nexchange plants . In contrast, the bed of magnetic resin has suffi cient extra void age to " lubri cate" its fl ow, and it therefore behaves more like a sludge than a sand. Magnetic filters The magnetic properties of the resin enabl e st ray beads to be ca ptured by magnet ic filt ers, preferably of the hi gh-gradi ent type, usin g sharp-edged elements in a strong magnet ic fi eld . Other particul ate matter, however , will pass throu gh such a filter. T hi s enables magnetic resin fluidi sed-bed co ntactors to handle dirty waters or even slurri es. It is eve n possible to treat very thick slurri es , such as fermentat ion brews, wit h magnetic particles, and recover th e particles by means of magnet ic drum se parators such as are used in mineral dressin g. REFERENCES convenhonal BLES ING. N . V . et al. ( 1970). " Ion Exchange in the Process Industries" Soc. of Chem. Ind. London, p . 371. BOLTO. B. A ., DIXON , D.R ., PRIESTLY , A. J ., SWINTON, E. A. (1977). Prog. Wat . Tech 9 p. 833-844, Pergamon 1- u <( LU 0:: Press. CA BLE, P . J .. MURTAG H, R. W.• PILK INGTO , N. Y. (1977). The Chern. Eng. No. 324. CLOETE , F. L. D., STREAT, M. (1963). Bri t. Pat. 1070251. GLUECKHAUF, E. (1955). Trans. Far. Soc. 51 p. 1540. HIM SLEY, A., FARKAS , F . J . (1976). "Theo ry and Practice of Ion Excha nge". Soc. of Chem. Ind. Londo n . NEWMAN, J ., MORLEY , J. B. (1973). "A nu idi sed mov ing bed ... " Proc. 74th Mee ting Am. Ins t. Chem. Eng. LU L:J ~ z w u 0:: w a.. z 0 Vl 5 30 -7/0 ,u I I 4·0 z ct. I I I ;:'; 3·0 0 LLl cD ~ 2·0 t<( Fig. I. Reaction half-time . > X /Tl z "'C 'O < 0 .,u LLl E E cf so,~--------------, -' -!:'.! 50 % "lw Fe 203 16 1-0 -=-- ' - - - ' - - - ' - - - ' - - - L - - L - - ' ----' 0 2 4 6 8 10 12 14 o:: magnet is ed WATER UPFLOW 14 mm/s Fig. S. Fluidisation curves for va rious "Sirotherm " resins. 12 tt- < 10 z"' 0 t- 0 .=10 u w 3 <( 0 -' "' 5 LL.. 8 500 6 o,_ l.l... ::::, 0 er w ::,: w t- f2 <. 3 -' <( 4 2 0 :,: 1 ' - - - - - - - -- ' - - -- - - - - - - ' 10 100 1000 CONCENTRATION meq/ L Fig. 2. Effect of concentration on reaction rate. 0 200 400 600 800 PARTICLE SIZE, micrometres Fig. 6 . Effect of particle size and magnetisation on bed expansion . 1000 500 500 VI "Cl C: 300 3 QJ "' VI 'O C 0 z 0 u 100 ~ t- part i cl e sizes in micrometres u <. LU er LL.. 0 LU 30 l.l... 0 L turbulent ~ flow region t- i LL.. -' <. I '""----- laminar flow region - - - - - - - - - - - - - , E - transitional flow 10L-- - - - - - - -1~.o---- - - ---1~0----- - - -~100 MODIFIED REYNOLDS NUMBER FOR PACKED BED Fig . 4. Shallow-bed kinetics: Effect on particle size and superficial velocity combined, on reaction rate. WATER w ::,: II 50 l.l... -' <( :,: 30 100 200 300 500 900 AVERAGE PARTICLE SIZE micrometres Fig. 3. Effect of particle size on reaction rate. 17