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An investigation of gas/liquid mass transfer in mechanically agitated pressure leaching systems DeGraaf, Kenneth Brant 1984

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AN INVESTIGATION OF GAS/LIQUID MASS TRANSFER IN MECHANICALLY AGITATED PRESSURE LEACHING SYSTEMS  by  KENNETH BRANT DEGRAAF •A.Sc. (Chemical Engineering), Queen's University, 1  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  in  THE FACULTY OF GRADUATE STUDIES (Department of M e t a l l u r g i c a l Engineering)  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA September, 1984 ©  Kenneth Brant DeGraaf, 1 9 8 4  In p r e s e n t i n g requirements  this thesis f o r an  of  British  it  freely available  in partial  advanced degree a t  Columbia,  understood that for  Library  s h a l l make  for reference  and  study.  I  f o r extensive copying of  h i s or  be  her  g r a n t e d by  shall  not  be  Metallurgical Engineering  The U n i v e r s i t y o f B r i t i s h 1956 Main Mall V a n c o u v e r , Canada V6T 1Y3 October 1,  1984  of  further this  Columbia  thesis  head o f  this  my  It is thesis  a l l o w e d w i t h o u t my  permission.  Department o f  the  representatives.  copying or p u b l i c a t i o n  f i n a n c i a l gain  University  the  f o r s c h o l a r l y p u r p o s e s may by  the  the  I agree that  agree that permission department o r  f u l f i l m e n t of  written  ii  ABSTRACT Oxygen  pressure  leaching  point  where oxygen  consumption  leach  s o l u t i o n are  obtained  hour.  At  dissolved The  these  high  oxygen a t  purpose of  this  Na S0 2  3  number  of  process  rates,  l i t r e s ) and  variables  and  is  previously  the  terms of the  when  not  accelerated 0  time o f  rates  the  per  2  the  mass  to  the  litre  of  order  of  transfer  1 of  become r a t e - d e t e r m i n g .  examine g a s / l i q u i d mass t r a n s f e r leaching  systems.  U s i n g an  transfer  bubbles  rate  capacity  of  the  in  design  experimentally  are  is  0  2  t r a n s f e r r a t e s and  that  dimensionless for  to  form  transfer  litres  mixing  and  20  gas  correlations  or  scaling-up  system.  It  described  is  and  at  the  be  The  directly practical  the  for  impellor  related  The to  Implications  gas/liquid  potential  for  a  shown  scaled-up  pumping c h a r a c t e r i s t i c s .  theoretically.  great  reducing  gas  studied.  i m p e l l o r v e l o c i t y , which c o r r e s p o n d s to  agitation  have  impellor been  (2  extrapolating  more a p p r o p r i a t e l y  first  impellor.  to  both b e n c h - s c a l e  t i p v e l o c i t i e s and  found  rates,  l i t r e s ) equipment.  discussed  of  transfer  power r e q u i r e m e n t s has  useful  of a c r i t i c a l  and  mass  g a s / l i q u i d mass  impellor  experimentally  the  done u s i n g  (2100  are  agitated  *The e x i s t e n c e  on  volumetric  g a s / l i q u i d systems  the  moles of  consumption  been to  demonstrated  applied  mechanically  point  0.75  residence  a g i t a t e d pressure  pilot-scale  It  in  has  e x p e r i m e n t a l work was  that  a  been  system to measure the oxygen mass t r a n s f e r r a t e s , the e f f e c t of a  pumping The  within  of  have  g a s / l i q u i d i n t e r f a c e may  study  rates i n mechanically  rates  oxygen  the  rates  is  of  power consumption of an  gas  mass  pumping  this principle  systems  improving  confirmed  gas/liquid the  the  are  shown  g a s / l i q u i d mass agitator.  iii  TABLE OF CONTENTS  Abstract iii  Table of Contents  v  L i s t of Tables  vi  L i s t of Figures  x  Acknowledgements Forward  Chapter 1.  2.  *  3.  INTRODUCTION  1  1.1  Pressure Oxygen Leaching Systems  1  1.2  Reaction Kinetics i n Pressure Oxygen Leaching  3  1.3  Mass Transfer i n Pressure Oxygen Leaching  8  1.4  A g i t a t i o n Theory  15  1.5  Summary  24  GAS-LIQUID MASS TRANSFER IN PRESSURE OXYGEN LEACHING  26  2.1  Agitation Theory of Gas Dispersions  -  26  2.2  Previous Work on Gas-Liquid Mass Transfer  32  2.3  Oxygen-Sodium Sulphite System  33  2.4  Purpose and Scope of the Present Investigation  36  EXPERIMENTAL DETAILS  38  3.1  Materials  38  3.2  Apparatus  38  3.3  Experimental Procedures  43  iv  4.  RESULTS AND 4.1  4.2  4.3 5.  DISCUSSION  46  Cominco Mixing Model Experiments  46  4.1.1  Single Impellor Systems  47  4.1.2  Dual Impellor Systems  53  4.1.3  Oxygen Concentration E f f e c t s  57  4.1.4  Small Diameter-High Speed Impellor Tests  59  4.1.5  Special Sparging Mode Experiments  60  4.1.6  Comparison of Results with Theory  61  Bench Scale Experiments  64  4.2.1  Single Impellor Systems  64  4.2.2  Dual Impellor Systems  69  4.2.3  E f f e c t of Oxygen Concentration  70  4.2.4  E f f e c t of Solids  71  4.2.5  Comparison of Results with Theory  71  Summary  CONCLUSIONS,APPLICATIONS AND  73 RECOMMENDATIONS  77  5.1  Conclusions  77  5.2  Applications  79  *  5.3  Recommendations for Further Work  82  6.  REFERENCES  85  FIGURES  1 to 31  88  TABLES  1 to 15  119  APPENDICES  A to J  133  V  LIST OF TABLES  Table 1.  E f f e c t of Impellor Type  119  Table 2.  E f f e c t of Depth of Impellor Immersion  120  Table 3.  C r i t i c a l Tip Velocity Correlation  121  Table 4.  Effect of Impellor Diameter-Mixing Model  122  Table 5.  Effect of Half Baffles on Mass Transfer-Mixing Model  123  Table 6.  E f f e c t of Impellor Immersion with Half Baffles  124  Table 7.  Oxygen Depletion i n Gas Bubbles  125  Table 8.  E f f e c t of Oxygen Concentration  126  Table 9.  Effect of Oxygen Concentration  127  Table 10. Small Diameter-High Speed Impellor Experiments  128  Table 11. Effect of Special Sparging Mode  129  Table 12. Effect of Impellor Diameter-Bench Scale  130  Table 13. Effect of Baffle Length-Bench Scale  131  Table 14. Alternate Dual Impellor Configurations-Bench Scale  132  Table 15. Effect of Solids  133  vi  LIST OF FIGURES Figure 1.  Zinc Pressure Leach Process at Cominco's T r a i l  88  Operations  Figure 2.  Anaconda Arbiter Process  89  Figure 3.  Sherritt-Gordon Ammonia Oxygen Pressure Process  90  for Ni-Cu-Co  Figure 4.  Reaction Steps i n a Typical Leach  91  Figure 5.  Liquid-Side Mass Transfer Models  92  Figure 6.  Liquid-Phase  93  Concentration P r o f i l e for Mass  Transfer with a Chemical Reaction  Figure 7.  L i q u i d - S o l i d Interface  94  Figure 8.  Effect of Agitation on L i q u i d - S o l i d Mass Transfer  95  Coefficient  Figure 9.  Reynolds Number Correlates Dimensionless Parameters 96  Figure 10.  Typical Experimental of Sodium Sulphite  Rate Curve for the Oxidation  97  vii  Figure 11.  Autoclave Mixing Model  98  Figure 12.  E f f e c t of Impellor Type on the Oxygen Mass  99  Transfer Rates Figure 13.  Impellor Positioning  i n Mixing Model  Figure 14.  Effect of Impellor Immersion Depth on Oxygen Mass Transfer Rate for the 4-Bladed Axial  Figure 15.  100  101  Impellor  Effect of Impellor Immersion Depth on Oxygen Mass  102  Transfer Rate for the 4-Bladed Radial Impellor  Figure 16.  E f f e c t of Impellor Immersion Depth on Oxygen Mass Transfer Rate for 6-Bladed Radial Disc  Figure 17.  figure 18.  Impellor  Effect of Impellor Diameter on Oxygen Transfer Rate for the 4-Bladed A x i a l  103  104  Impellor  Effect of Impellor Diameter on Oxygen Transfer  105  Rate f o r the 4-Bladed Radial Impellor  Figure 19.  E f f e c t of Impellor Diameter on Oxygen Transfer  106  Rate for the 6-Bladed Radial Disc Impellor Figure 20.  Effect of Baffle Length on Oxygen Transfer Rate i n the Mixing Model  107  viii  Figure 21.  Standard Dual Impellor Configuration Used i n the Commercial  Figure 22.  108  Autoclave  E f f e c t of Baffle Length on Oxygen Transfer Rate  109  for the Standard Dual Impellor Configuration  Figure 23.  Alternate Dual Impellor Configurations-Unsparged  110  Figure 24.  Alternate Dual Impellor Configurations-Sparged  111  Figure 25.  Effect of Gas Plenum Oxygen Concentration on the  112  Oxygen Mass Transfer Rate  Figure 26.  Effect of Impellor Type on the Oxygen Transfer  113  Rate i n 2 0 - l i t r e s Vessel  Figure 27.  A) Effect of Agitation Rate on Impellor Gas Pumping Capacity and Oxygen Mass Transfer;  *  114  B) E f f e c t of  Surface Aeration of Impellor on Power Consumption (6-Bladed Radial Disc Impellor - 58mm diameter)  Figure 28.  A) E f f e c t of Agitation Rate on Impellor Gas Pumping Capacity and Oxygen Mass Transfer;  B) E f f e c t of  Surface Aeration of Impellor on Power Consumption (6-Bladed Radial Impellor - 58mm diameter)  115  ix  Figure 29.  A) E f f e c t of Agitation  Rate on Impellor Gas Pumping  116  Capacity and Oxygen Mass Transfer; B) E f f e c t of Surface Aeration of Impellor on Power  Consumption  (6-Bladed Axial Impellor - 58mm diameter)  Figure 30.  A) Effect of Agitation  Rate on Impellor Gas Pumping  117  Capacity and Oxygen Mass Transfer; B) E f f e c t of Surface Aeration of Impellor on Power  Consumption  (Standard Dual Impellor - 58mm diameter)  Figure 31.  E f f e c t of High Oxygen P a r t i a l Pressures on Oxygen 118 Mass Transfer Rate (58mm diameter)  X  ACKNOWLEDGEMENTS  I Peters of  would  like  to express my appreciation  f o r h i s assistance, guidance and support  this  interest  to Professor  Ernest  throughout the course  project.  And to Dr. G.M. Swinkels, I give  and support  i n helping to i n i t i a t e this work.  thanks f o r h i s  I would also l i k e to acknowledge the assistance and cooperation of George Parker and Alex Mackie during my stay at the Cominco Technical Research Centre i n T r a i l .  Thanks Horst  Tump,  i s also  Ross  extended  McLeod  to Christine Harrison,  and Ed Klassen  f o r their  Neil  kind  Walker, help and  cooperation with various portions of the project.  The f i n a n c i a l support  of the National Sciences  Research Council of Canada, and the B.C. Science Acknowledged.  Gratitude  i n d u s t r i a l sponsorship  i s also  of this project.  extended  and Engineering  Council i s g r a t e f u l l y to Cominco  f o r their  FORWARD  "Science without A religious  religion  blind  ...  doubt  of the s i g n i f i c a n c e  which  neither  require  person  nor  i s lame, r e l i g i o n without  i s devout  of those are  i n the sense  super-personal  capable  of  because  of  her  essential  loftiness,  but  not  that he has no  objects and goals  rational  Subtle i s the Lord, but malicious he i s not ...  science i s  foundation  ...  Nature hides her secret by  means  of  ruse  "  Albert Einstein  The Lord i s my shepherd;  I s h a l l not want."  Ps. 23:1  'The Science and L i f e of Albert E i n s t e i n " , by Abraham Pais, Oxford Press 1982.  1 1.  1.1  INTRODUCTION  P r e s s u r e Oxygen L e a c h i n g Systems  One  of  hydrometallurgy temperatures  the  most  has  been  to the  involving  oxidative  attention  in  leaching  process at  commercial  plants  years;  which  Trail,  development many  metals  of is  for  of  is  in Even  received  there  remains  competitive  is  are  today,  benefits  of  great  deal  the  zinc  pressure  the  these  and  a  as  compelling.  of  Processes  at yet  direct With  of  Cominco's only  smelting s t i l l  pressure o x i d a t i v e  the  field  pressures  operation  there  practised  the  sulphide o r e s .  and c o n v e n t i o n a l  sulphides  as  though  in  elevated  commercial  routinely  expected  have  example,  existence,  of  fully  leaching  B.C..  in  leaching  application  pressure  main p r o c e s s i n g method pressure  the  developments  l e a c h i n g of complex metal  recent  operations  important  a  few  is  the  oxidative time,  the  leaching plants  for  hydrometallurgical  p l a n t s are r e c o g n i z e d .  The o x i d a t i v e  p r e s s u r e l e a c h i n g of metal  sulphides i s  d i s s o l u t i o n process and can be d e s c r i b e d i n g e n e r a l by the  MS  N  n +  M  + S ° + me~  m +  + e" - N  (  n  _  1  )  +  an anodic  following : 1  [1.1]  [1.2]  2 where MS  = metal  sulphide and  N i s an u n i d e n t i f i e d  oxidizing  agent.  I [|  B e s i d e s oxygen, f e r r i c  i o n (Fe  ) or n i t r i c  acid  can be used as an  oxidizing agent i n the oxidative leaching of metal sulphides. In sulphides  oxidative pressure leaching processes, the leaching of metal l e a d s to the d i s s o l u t i o n of the metal  ions (M )  i n the  m+  aqueous phase and the formation of elemental sulphur accordingly: MS + 2H  +  + j 0  2  M  + S° + H 0  2+  Once formed elemental sulphur may  [ 1 .3]  2  be oxidized further to form  by oxygen or other oxidants present in the leach sytems i . e .  2S°  + 30- + 2H 0 •* 4H 2  2 o  +  4  Since  oxidative pressure  2  + 2S07 4  S° + eFe "^ + 4H„0 •> eFe"^ + 8H  +  leaching produces  sulphate  [1.4]  +  SO?  sulphur  [1.-5]  In  the  elemental  form, one of the major benefits of the process i s the elimination of a i r 'pollution  due  to  the  production  of  sulphur  dioxide.  Furthermore,  sulphur i n the elemental form i s easier to store and handle, and i t i s directly  marketable.  3  1 . 2 Reaction Kinetics i n Pressure Oxygen Leaching  The  reaction mechanisms found In oxidative pressure  complex and varied. phase  reacts  In some cases,  directly  with  the  the dissolved oxygen i n the aqueous  metal  sulphide.  oxygen reacts homogeneously with an intermediate ion,  Fe  metal  + +  );  the  oxidized  sulphides.  pressure  The  leaching are  intermediate  In other species  cases,  the  ( i . e . ferrous  species i n turn reacts with the  c o n t r o l l i n g k i n e t i c mechanism  i n an  oxidative  leaching system has a profound influence on the optimum reactor  design.  Depending reaction mechanism  that  on  is  will  the  percent  acceptable  dictate  backmixed r e a c t o r .  the  in use  extraction a of  and  reactor,  the  either  plug  a  the  average  rate  of  controlling kinetic flow  reactor  or  a  Where the reaction driving force i s controlled by  3  concentration gradients i n the solution a plug flow reactor w i l l perform better than a backmixed reactor. and  In plug flow both percent  average reaction rate are maximized.  For autocatalytic reactions,  *the choice of reactor design i s governed by the percent high  enough conversions  rates, while at low reaction  rates.  the  plug  conversions  The  other  flow reactor gives  is  no  configurations.  difference  extraction.  superior  extreme i s where the  between  selecting  higher  controlling kinetic  kinetics. plug  At  reaction  the backmixed reactor w i l l give  mechanism i s mass transfer limited - zero-order there  extraction  flow  In this case or  backmixed  However, what i s important under these circumstances i s  the means by which mass transfer Is promoted.  4 Examination  of  the  reaction  leaching  kinetics  oxidative  pressure  processes  will  importance  of these design considerations.  pressure leaching processes that i l l u s t r a t e  in  a  few  emphasize  commercial  the  critical  Three commercial oxidative this are as follows ( l i s t e d  in chronological order of development):  1) Sherrltt Gordon Ammonia Pressure Process for Ni-Cu-Co 2) Anaconda Arbiter Process for Cu 3) Cominco's Zinc Pressure Leach  The  Sherritt  Process  Gordon ammonia pressure  leach process  (shown i n  Figure 1) treats a pentlandite-nickel concentrate i n a two-stage counter -current stage.  leach i n which the pregnant  liquor  i s produced  In i t s simplest form the leaching action may  reaction  between  oxygen,  ammonia  the and  sulphide minerals water,  i n which  i n the copper,  i n the  first  be described as a  concentrate, nickel  and  dissolved  cobalt  are  converted to ammines, sulphur i s converted to an oxidized form and iron i s converted to hydrated f e r r i c oxide:  NiS:FeS + 3FeS + 70  -»• N i ( N H ) S 0 3  2(NH ) S 0 4  According  2  2  3  + 40  2  6  + 10NH + 4H 0  2  3  4  + 2Fe 0 'H 0 + 2 ( N H ) S 0 2  + 4NH + H 0  to laboratory and  2  3  2  3  2  3  2  2  [1.6]  3  -> NH »S0 »NH + 3 ( N H ) S 0  pilot-plant  4  3  2  4  2  4  [1.7]  data, under optimum operating  conditions, 95 percent Ni and 60 percent S i s extracted at 70-80 °C and  5 an oxygen p a r t i a l pressure of 0.68 g/1 Ni and 8 g/1 Cu **. g  0 /l*min»atm  The makes use  Anaconda  to give a f i n a l  solution of 45  This gives an average oxygen consumption of  , based  2  atm  on  an  Arbiter  average  process  residence  for copper  time  of  hours.  (shown i n Figure  of the same ammonia chemistry as the S h e r r i t t  goal of the Anaconda Arbiter process was  19  0.18  process.  2) The  to overcome the necessity of  high oxygen p a r t i a l pressures by applying mixing technology, rather than relying  on high pressure reactors.  oxygen rather than a i r .  The  The  process  also  uses  commercial  process treats chalcopyrite concentrates  accordingly:  CuFeS + 4 j o 2  Again,  under  extracted  2  from  gives  an  3  optimum the  pressure of 0.34 *This  + 4NH + 1 ^ 0  atm  + C ^ N H g ) ^ + FeO(OH) + 2S0^ +2H  conditions, 95 concentrate  at  percent 60-90  °C  Cu  and  and  27  an  percent  oxygen  rate of  process equivalent to 0.39  g 0  2  oxygen  consumption  for  the  S is  partial  to give a f i n a l solution containing 43 g/1  average  [1.8]  +  Cu  5  .  Anaconda  /l*min*atm ,based on the residence time  of 5 hours.  Agitation effect  on  reactions.  the  for both  processes  leaching rates due  i s stated to have a pronounced  to the heterogeneous nature  of  the  However, almost a l l the data available i n the l i t e r a t u r e on  6 these processes  i s from  laboratory and  pilot-plant  studies*  From the  oxygen consumption rates i n both of these processes i t i s d i f f i c u l t to determine  i f mass transfer would be  the l i m i t i n g  factor.  As  long as  gas/liquid mass transfer i s r a t e - l i m i t i n g there i s no difference i n the average  rate between a backmixed and a plug flow reactor.  Furthermore,  i n a reactor that i s mass transfer limited there w i l l be an equal rate of heat evolution i n each compartment, therefore equal cooling capacity i s required for each compartment.  In contrast to both of the above processes i s the Cominco zinc pressure  leach  concentrates elemental  process  are  (shown  converted  sulphur.  The  in  Figure  directly  to  3)  zinc  where sulphate  zinc  sulphide  solutions  and  process uses oxygen rather than a i r to treat  sphalerite concentrates as follows:  ZnS + H„SO. + i o . + ZnSO. + S° + H.O 2 4 2 2 4 2  FeS + H S0  4  + |o  2FeS0 + H S0  4  +  2  4  2  Fe (S0 ) 2  4  3  2  -»• FeSO^ + S° + H 0  -> F e ( S 0 ) 2  4  3  [1.91 J  2  [1.10]  + H0  [1-U]  2  + ZnS -• 2FeS0 + ZnS0 + S° 4  1  4  Operating at 145-155 °C and an oxygen p a r t i a l pressure of 7.5 atm  [1.12]  (total  7  pressure yield  =  11.4  atm)  after  1.5  hours  of elemental sulphur are 98% and  much higher temperatures  Information  the extraction of 96%,  zinc  respectively . 6  and  the  These are  and pressures than the previous processes.  available  on  the  commercial  operation shows that  over 80 percent of the reaction takes place i n the f i r s t compartment of the autoclave i n 26 minutes .  Since most of the reaction takes place  7  i n the f i r s t an  initial  compartment, the conditions here are most important. zinc  approximately  concentration of 50 g/1  120  g/1  consumption i s 0.14 of if  g 0  ferrous to f e r r i c the  in 2  the  first  /l»min«atm .  couple  chemical  is  the  controlling  the mass transfer  rate  ferrous ion i s rate c o n t r o l l i n g . support  one  or  compartment  the  average  the  other.  oxygen  At high feed rates the oxidation 6  possibilities;  to  a f i n a l concentration of  iron i s believed to be rate l i m i t i n g .  ferrous/ferric  or  and  With  rate  constant  limiting for  the  there reaction  However, are  two  i s rate  of oxygen f o r the oxidation of the There i s no evidence i n the l i t e r a t u r e  However, the  reactor design would  be  affected d i f f e r e n t l y for each case.  Although appears  to be  the  oxygen  consumption  considerably lower  leach processes  i t i s important  only from the p i l o t  plant.  than  of  the  zinc  pressure  leach  the ammonia oxidative pressure  to remember that the Anaconda data i s  I f commercial data were available i t would  probably be found that the residence times are much higher than the zinc pressure leach process; rates  comparable  this would make the average oxygen consumption  to the zinc  pressure  leach rates.  I f they are not  8  comparable to the zinc pressure the  result  It leaching  of  can rates  consumption obtained  chemical  be  have  rates  within  seen  that  been  of  0.75  a residence  leach rate then the descrepencies may  be  differences  between  the  systems.  for  pressure  leach  processes  oxidative  accelerated mole  0  to  the  per  litre  time of the  order  2  point  of  where  leach  of one  oxygen  solution  hour.  At  are  these  high oxygen consumption rates the mass transfer of dissolved oxygen at the  gas/liquid or  liquid/solid  interface may  become  rate-determining.  C l e a r l y , an understanding of the mass transfer processes i n an oxidative pressure  leach process are important.  1.3 Mass Transfer i n Pressure Oxygen Leaching  Before pressure  considering  the  mass  transfer  processes  in  oxidative  leaching i t i s useful to be reminded of the reaction steps that  take place i n the leach solution (see Figure A). oxidative * solution,  leach  i s the  requiring  interface.  d i s s o l u t i o n of  oxygen  Next, the  to  be  dissolved  reactants  leached,  liquid/solid  The  interfacial  film.  gaseous oxygen .Into  transported  interface of the p a r t i c l e being  The f i r s t step i n any  across  must  the  d i f f u s e to  the  leach  gas/liquid the  solid  involving transport across  reactants  are  then adsorbed on  the to  the s o l i d surface which i s then followed by the chemical leach reaction. After  the  reaction takes place any  soluble products are  desorbed from  the i n t e r f a c e which i s then followed  by d i f f u s i o n of the products from  the interface into the bulk s o l u t i o n .  In an oxidative leach process any  9  s i n g l e  s t e p  or  In  c o m b i n a t i o n  most  d e s o r p t i o n  of  systems  steps  at  the  s i n g l e  r e l a t e d  p a r t i c l e  c h e m i c a l  r e a c t i o n  o r  T h e r e f o r e  the  s i g n i f i c a n t  the  most  c o n s i d e r  l e a c h i n g .  The  whereby  gas  d i s s o l v e s  c o n c e p t  of  r a t e  of  where t h e  a  a b s o r p t i o n  =  i s  k  the  s y s t e m ,  p a r t i a l  are  r a t e  to  a d s o r p t i o n  compared  a c r o s s  r e s i s t a n c e  l i m i t i n g .  the  f a s t  t r a n s f e r  e q u i l i b r i u m the  G  a (  i n  the o f  i s  -  P  i s  l i q u i d  a  to  e i t h e r  phase  mass  and the  i n t e r f a c e s .  t r a n s p o r t  g a s - p h a s e  o c c u r s  at  o f  r e a c t i n g ,  r e s i s t a n c e  r e s i s t a n c e  can  a  i s  and be  i n  p r e s s u r e  s o l u t e  based a  g a s ,  upon  the  l i q u i d - p h a s e  n e g l e c t e d .  The  t h e n ,  P  ±  )  =  t h e o f  k^C* - C  a r e a a v e r a g e the  s o l u b l e  w i t h  p^,  and  i s F i g u r e  )  [1.13]  r a t e  c o n c e n t r a t i o n  (see  A Q  between  t h e  l i q u i d  t r a n s f e r  t r a n s p o r t  w i t h o u t  i s  b u l k  mass  f o r  i n t e r f a c i a l  i n t e r f a c i a l r  g a s / l i q u i d  r e p r e s e n t a t i o n  the  p r e s s u r e s  i n t e r f a c e ,  i n  b a s i c  assuming  r  gas  be  l e a c h i n g  s u r f a c e  mass  the  a d d i t i v i t y  r e s i s t a n c e ,  the  the  to  c o u l d  i n t e r f a c e s .  F i r s t  * o f  s t e p s  the 5 ) .  gas of  l i q u i d  t r a n s f e r  gas o f  and  i n  the  d i s s o l v e d  average  o f  g a s ,  b u l k gas  per  u n i t p  gas  and and  volume p^ at  c o r r e s p o n d i n g  c o n c e n t r a t i o n  of  are the to  d i s s o l v e d  10  The  coefficient  k^ i s the gas-side mass transfer c o e f f i c i e n t ,  which refers to the gas  f i l m resistance, implying there i s a stagnant  f i l m across which the soluble gas i s transferred by molecular alone  (while the bulk of the gas has  been demonstrated by Westerterp et a l  diffusion  a uniform composition). 8  f o r oxygen t r a n s p o r t i s n e g l i g i b l e  I t has  that gas-side transfer resistance (ie. kg»  k^), even i n a i r . This  does not account for gas-side resistance due to oxygen p a r t i a l pressures reduced  below  the  oxygen  partial  pressures  found  i n a i r , which  can  become s i g n i f i c a n t .  The  c o e f f i c i e n t k^ i s the p h y s i c a l l i q u i d - s i d e mass transfer  c o e f f i c i e n t i n the absence of a chemical reaction.  There are two main  types of models that represent the physical significance of Ic^: the f i l m models  and  the  surface-renewal  models.  The  stagnant  f i l m at the surface of the l i q u i d next  rest  the  of  composition  liquid by  away from  turbulent  the  film  agitation .  p r e d i c t that k^ i s proportional to D^, *gas i n the l i q u i d .  Although  models  assume  to the gas while  boundary The  9  film  i s kept  film  models  a  the  uniform  in  consequently  the d i f f u s i v i t y of the dissolved  t h i s i s a simple model i t does not  agree 1/2  w e l l with the experimental evidence that suggests The of  k^ varies as  surface renewal models assume the periodic replacement liquid  at  the  interface  by  liquid  from  the  (D^)  of elements  interior  of  bulk  composition, where the rate of absorption i s a function of the time of exposure of the e l e m e n t ' . 10  11  The surface renewal models r e a l i s t i c a l l y 1/2  predict  that k  v a r i e s as ( D )  .  However, both lead to the same  11  predictions  c o n c e r n i n g the e f f e c t of the driving force ( C. - C. ) on A Ao  average rate of gas t r a n s f e r .  Except f o r very slow chemical reactions,  the basic effect of a chemical reaction on the mass transport i s to hold C^  Q  in 6).  equal to zero i n the bulk l i q u i d ; that i s , as soon as A i s dissolved the bulk l i q u i d Therefore,  i t i s consumed by a chemical reaction ( see Figure  the process i s e s s e n t i a l l y one of physical  absorption  followed by reaction i n the bulk l i q u i d .  It earlier  was  that  there  transfer rates. therefore  found  f o r the three  commercial  are s i g n i f i c a n t differences  processes  discussed  i n the oxygen mass  These processes each employ s i m i l a r agitator designs;  the differences i n the oxygen mass transfer rates must exist  i n the d e t a i l s of the chemical and mass transfer processes. possible to discuss information examine how  It i s not  these d e t a i l s for any of the processes because the  i n the l i t e r a t u r e i s incomplete. the oxygen mass transfer  However, i t i s possible to  could  be affected  by d i f f e r e n t  chemical conditions.  The  oxygen  consuming  reaction  i n a pressure  must take place i n one of three possible zones * . 1  on  8  oxidative  leach  One possible zone i s  a solute monolayer at the gas/liquid interface which i s a reducing  agent reactive to oxygen.  Secondly, the oxygen could  be dissolved i n  the bulk l i q u i d  and after dispersion by d i f f u s i o n and convection  with a reducing  agent dissolved i n the solution or suspended as a s o l i d  such as the mineral being leached.  The third  react  zone i s intermediate,  and  oxygen i s consumed somewhere i n the  agent dissolved essential forming  that  i n the there be  dissolved  oxygen.  For  solution.  boundary layer by  In the  first  chemical reactions  reducing  that  agents without  and  last  a reducing cases i t i s  decompose the  simultaneous  mineral  consumption  of  example i n the ammine processes cuprous ammine i s such a  reducing agent being formed by the reaction of the mineral with cupric ammines, while in acid systems i t i s ferrous  The the  ion.  zone where the consumption of oxygen takes place depends on  concentration  profile  boundary layer and  of  the  dissolved  reducing  the chemical reaction rate.  agent across  the  Therefore, i n d i f f e r e n t  chemical systems there are l i k e l y to be differences i n the  concentration  p r o f i l e s , as well as in the chemical rates, even though both the systems may  be g a s / l i q u i d mass transfer l i m i t e d .  comparing some preliminary against the  the  oxygen consumption rate  i n the  0 l*min»atm as compared to 0.27 /  2  possible  that  and  the  mass  2  rate-determining. reacting  rate  of  found that to  1.02  g  sulphite system  In both systems gas/liquid mass but  i n the  cuprous complex i s oxidized transfer  It was  g 0 /l»min»atm i n the  r a t e - l i m i t i n g step,  the  experiments.  ammine system i s equal  under similar a g i t a t i o n conditions. i s the  by  experiments using a cuprous ammine oxidation  sodium sulphite oxidation  transfer  This point i s i l l u s t r a t e d  this  ammine system, i t i s  at the bubble i n t e r f a c e , species  would  then  be  In the second case, i t i s probable that most of the  oxygen dissolves, passes through the boundary layer, and  does  not react with sulphite u n t i l a catalyst ion i s available to mediate the reaction.  In  t h i s case  i t i s the  mass transfer  rate  of  dissolved  oxygen that  i s rate-limiting.  In both cases, however, the g a s / l i q u i d  interface area determines the o v e r a l l rate, even though the same amount of  area  cannot  The pressure  lead  to exactly  leaching  the surface  transport  resistances  R = k  of a dissolved  solid  ( C - C ) = L s L  i n the l i q u i d phase  upon the concept of I n t e r f a c i a l resistance  T  r  D ^ - ( C - C ) = r s c  [1-14]  v  l  J  i s the l i q u i d - s i d e mass transfer c o e f f i c i e n t ; C, C and C are s c  T  species i n the bulk solution, at the  l i q u i d / s o l i d interface and at the receding chemical reaction rate; D  g  chemical reaction front; r i s  i s the e f f e c t i v e d i f f u s i v i t y through the  product layer and L i s the product layer thickness  The liquid/solid transfer. significant product  i n oxidative  The rate of transfer i s then:  the concentrations of the transport  the  that  rate.  As with g a s / l i q u i d  species  p a r t i c l e i s based  i n s e r i e s , assuming  consumption  takes place  i s l i q u i d / s o l i d mass transfer.  of a  can be neglected.  where k  oxygen  other mass transfer process that  mass transfer, the transport to  the same  (see Figure 7 ) . 3  l i q u i d - s i d e mass t r a n s f e r c o e f f i c i e n t (k^) i s treated i n mass transfer  the same way as i t i s i n g a s / l i q u i d mass  In leach reactions where a product layers forms there may be resistance  layers  to mass transport  the reactants  across the layer.  In porous  d i f f u s e by means of molecular d i f f u s i o n  through cracks and channels i n the product layer.  14  Since fluid  leaching  systems are generally  hydrodynamics on the mass transfer  agitated  coefficients  the e f f e c t of i s an important  consideration.  In the case of s o l i d s , the agitation usually does not e f f e c t the particle off  size  dramatically.  Furthermore, once p a r t i c l e s are suspended  the bottom the mass transfer  rates increase l i t t l e as the p a r t i c l e s  approach uniform suspension ( see Figure 8 )  .  Once the p a r t i c l e s are  i n a uniform suspension, the p a r t i c l e s are e s s e n t i a l l y i n free f a l l most of can  the time. be based  given  Consequently, the l i q u i d - s i d e mass transfer on correlations  particle  sizes.  using  coefficients  the terminal s l i p v e l o c i t y of the  In cases where i n t e r n a l  diffusion  through a  product layer i s mass transfer c o n t r o l l i n g the l i q u i d hydrodynamics have no  effect.  Generally, transfer 'leaching.  coefficient  controlled  transfer,  i s the l i m i t i n g  transfer factor  the l i q u i d - s i d e i n oxidative  mass  pressure  the product of the mass transfer  and the i n t e r f a c i a l area per unit volume of system (a) d i r e c t l y by the hydrodynamics.  As i n l i q u i d / s o l i d mass  the l i q u i d - s i d e c o e f f i c i e n t i s r e l a t i v e l y i n s e n s i t i v e  degree of a g i t a t i o n .  However, the i n t e r f a c i a l area i s greatly  by the hydrodynamic conditions. gas/liquid  mass  In gas/liquid mass transfer  coefficient is  i n gas/liquid  affected  Therefore, the purpose of a g i t a t i o n i n  dispersions i s primarily  per unit volume.  to the  one of increasing  the surface area  15  Clearly, gas/liquid  systems  understood different is  that  design than  the  of  in  agitators  liquid/solid  process  of  is  far  to  properties,  reduce  dispersing  or  nonuniformities  temperature  i n gas  more  processes. a  gas  from the process of mixing a l i q u i d .  meant  agitation  the  or  critical  in  It  be  must  in liquid  i s very  The mixing of a l i q u i d  gradients  of material i n bulk  .  dispersion i s to create gas/liquid  in  compostion,  The  function of  interfacial  area.  This d i s t i n c t i o n i s important, and w i l l be emphasized i n the discussion on a g i t a t i o n theory.  1.4 A g i t a t i o n Theory  In general, agitation results i n f l u i d motion. any  agitated system must  momentum.  For  a  obey  the  laws of  F l u i d motion i n  conservation of mass  constant-density, Newtonian  liquid,  the  mass  and and  momemtum balance i n terms of l o c a l pressure and v e l o c i t y i s represented by the Navier-Stokes equation  :  «  = ~g  c  P +  v +  where Dv/Dt i s the substantial t  g  time  [1.15]  derivative of v e l o c i t y ,  d e n s i t y , g i s the a c c e l e r a t i o n due to gravity, g conversion  factor,  vector d i f f e r e n t i a l  p i s the  pressure,  operator, and  c  i s the  i s the gravitaional  i s the v i s c o s i t y ,  i s the Laplacian operator.  i s the It i s  not  intended  details The  i n this  are applied  specifically, will  to give an exhaustive treatment  of the Navier-Stokes equation, or for that matter,  primary purpose  they  thesis  of the  agitation.  i s to give an appreciation of the fundamentals as to  the engineering  the l i m i t a t i o n s  be addressed.  Other  of  agitation  of the concepts  systems.  More  behind a g i t a t i o n theory  sources are available  f o r a more detailed  „. 13,14,15 treatise  As large  can be seen  number  operational,  of  from  independant  and physical  the Equation variables  variables.  1.15, a g i t a t i o n which  include  involves a  geometrical,  It i s common practise  i n the  engineering d i s c i p l i n e s to simplify the Navier-Stokes equation by means of the  dimensional a n a l y s i s .  The method of dimensional analysis rewrites  1 6  Navier-Stokes  characteristic  equation  quantities  in a  dimensionless  to represent  form  the p r i n c i p a l  by  selecting  dimensions  of  length, time and mass.  The 'diameter, D.  characteristic  in agitation  i s the impellor  The c h a r a c t e r i s t i c time i s the r e c i p r o c a l of the agitator  r o t a t i o n speed, 1/N. density,  length used  The c h a r a c t e r i s t i c mass i s the product of l i q u i d  p and the cube of the impellor diameter, D . 3  1 7  Accordingly,  dimensionless lengths and times are defined as:  x' = x/D  [1.16]  y' = y/D  [1.17]  z' = z/D  [1.18]  t' = t/D  [1.19]  The dimensionless v e l o c i t y , V ,  i s the r a t i o of the actual v e l o c i t y , V ,  to the c h a r a c t e r i s t i c v e l o c i t y ,  ND:  V  The  dimensionless  = V/ND  [1'20]  p r e s s u r e i s d e f i n e d as f o l l o w s ,  g r a v i t a t i o n a l conversion factor, and p selected  to simplify the boundary  Q  where g  £  i s the  i s a reference pressure which i s  conditions:  (P " P j  g  c  pN D  If  these dimensionless variables  rearranged  17  and  g  u  = -V'p' + ( Dt'  )V*V  + (  pD N  ) DN  2  In Equation 1.22,  two dimensionless groups appear as  Reynolds Number for agitation,  Fr  = DN /g  forces to g r a v i t a t i o n a l  2  parameters.  Re = pD N/u , which represents the  c o e f f i c i e n t for the viscous d i s s i p a t i o n agitation,  [1.22]  2  2  r a t i o n of i n e r t i a l to viscous forces, appears  for  into equation 1.15  a dimensionless form of the Navier-Stokes equation r e s u l t s ,  DV  The  are substituted  i n r e c i p r o c a l form as the  term.  , which represents  forces, appears.  Also, the Froude number the  ratio  From Equation 1.22  of  inertial  i t follows  18 that  f o r a given  geometric  set of i n i t i a l  similarity,  and boundary conditions that require  the v e l o c i t y  and pressure  distributions  can be  expressed as functions of the Reynolds and Froude numbers:  It  V ' C x ' . y ' . z ' . t ' ) - f(Re,Fr)  [1.23]  p ' ( x ' , y ' , z \ t ' ) = f(Re,Fr)  [1.24]  i s common  practice  where the l i q u i d  to consider  gravitational  surface i s e s s e n t i a l l y f l a t ,  effects  unimportant  such as i n f u l l y  baffled  tanks, therefore the v e l o c i t y and pressure d i s t r i b u t i o n s are determined only by the Reynolds number.  Impellor distribution  power  along  consumption  i s related  the face of the impellor blade.  to  the  pressure  Power, P, i s the  product of r o t a t i o n a l speed and applied torque, where the applied torque i s determined of  by integrating  a flat-blade  turbine.  the pressure d i s t r i b u t i o n over the surface  Thus, power i s related  to f l u i d  pressure at  the blade surface:  f " o P  P  }  blade  [1.25]  It follows from Equation 1.21 that dimensionless pressure and power are related as follows:  19  From Equation 1.24 the Power number, N^, i s found to be only a function of  the Reynolds number  when g r a v i t a t i o n a l  effects  are not a  factor.  Therefore, the equation usually takes the following form for c o r r e l a t i n g a g i t a t i o n power data:  P g  c  — = f(Re) pN D 3  [1.27]  5  In most applications of an agitator the i n e r t i a l forces dominate the viscous forces;  these are considered turbulent conditions and are  associated with large Reynolds numbers (Re>10 ). Under these conditions 1+  the terms i n the Navier-Stokes equation which represent the viscous and g r a v i t a t i o n a l forces can be neglected, and this yields an equation f o r f l u i d motion known as Euler's equation  11+  :  DV* = -V'p'  [1.28]  Dt'  * In Equation  1.28, the Reynolds number i s not included as a parameter,  therefore the v e l o c i t y and pressure d i s t r i b u t i o n s are constant for t h i s limiting  case.  As shown i n Figure 9, the Power number i s constant at  high Reynolds number therefore we find the power draw of an impellor i s proportional  to the agitation speed  for a given geometric arrangement:  and impellor diameter  as follows,  20 P « pN D 3  If  a constant  [1-29]  5  t i p speed, ND, i s maintained  1.29 the power consumption w i l l  then according  vary with D .  to Equation  An important point i n  2  this power c o r r e l a t i o n i s that i t applies only to single phase systems, therefore power consumption i n gas/liquid systems cannot be described by this  equation.  As  important  understanding  as  understanding  the behaviour  of  fluid  power  i n agitated  v e l o c i t y , since  systems i s  by  definition  a g i t a t i o n i s the f l u i d motion produced by impellor r o t a t i o n .  This point  i s frequently missed, consequently the volumetric vessel i s often,used to describe  power d i s s i p a t i o n i n a  the degree of a g i t a t i o n .  However, i f  we examine the dimensionless f l u i d v e l o c i t y i n the turbulent region (see figure 9) i t i s found to be fixed just as the Power number i s , therefore the  same degree of f l u i d  motion can be achieved  with d i f f e r e n t power  l e v e l s i n the same vessel by varying the impellor diameter and agitator speed.  Another important feature of an agitator i s the impellor pumping capacity. directly  The conclusions to pumping  v e l o c i t y across  capacity.  an area  stated as follows:  related to v e l o c i t y behaviour can be applied The r e l a t i o n s h i p between the average  and the pumping capacity  through that area i s  [1.30]  where  Q Q - « A D  Therefore,  if  [1-31] 2  Q/D  is  2  substituted  into  the  dimensionless  v e l o c i t y relationship the the following r e s u l t s :  v (Q/D i -^s. = = ND ND 2  average  1 7  Q = f(Re) ND  [1.32]  3  Since the behaviour of the pumping number i s similar to the behaviour of velocity,  the  pumping  number  region as shown i n Figure and  9.  is  found  to  be  fixed  in  the  A detailed examination of the t h e o r e t i c a l  empirical relationships between discharge v e l o c i t i e s and  impellor  geometry and  Volume I, Chapter  As was  4.  turbulent  rotational  speed  i s given  by  Uhl  flow rates, and  Gray i n  13  the case for power consumption, i t i s not  fundamentally  correct to apply the c o r r e l a t i o n for pumping capacity (Equation 1.32) g a s / l i q u i d systems. density  liquid.  literature  that  to  These correlations are only correct for a constant A  correlation  successfully  has  predicts  not  yet  been  published  power consumption  pumping c h a r a c t e r i s t i c s i n gas/liquid.systems.  and  in  the  impellor  The present correlations  22  do  not  account  for the effect of sparging  or the impellor  pumping  gas  into the l i q u i d from the gas plenum.  Dimensional analysis has been applied to gas dispersion only i n limited a p p l i c a t i o n s .  Remembering that the primary e f f e c t of a g i t a t i o n  i n gas dispersion i s to create i n t e r f a c i a l area, i t i s found that under some conditions  properties  such as  bubble  size can  be  correlated  by  dimensionless parameters.  In gas of  gas  liquid  dispersions there are two  sparging  predominates and  from  gas  the  the  plenum exerts  regimes; one  other  control.  where the e f f e c t  where gas  pumped into  Dispersions  are  the  brought  about by f l u i d dynamical forces which have to overcome the s t a t i c forces of  surface  attempting  tension. to  Surface  r e t a i n bubble  leading to breakup.  forces  s p h e r i c i t y and  prevent  dispersion  by  gross d i s t o r t i o n  be due  to induced f l u i d flow or buoyancy.  In systems where gas sparging i s present, and bubbles are formed by means other  than surface aeration, dimensionless analysis of bubble  break-up y i e l d s a dimensionless parameter c a l l e d The t  resist  These dynamic forces which induce a shear stress to  produce a dispersion may  *  tension  the Weber number,  Weber number i s a r a t i o of i n e r t i a l forces to the surface  forces, which i s formulated We  = "  •  p  We.  tension  for agitator systems as follows: fl-331  23 In most p r a c t i c a l liquid  from  systems  the impellor  above, therefore  this  i s l i k e l y to pump gas into the  type  of c o r r e l a t i o n  i s of limited  value. The power consumption by  correlating  the power  power consumption  under  i n gas sparged systems has been predicted  consumption  ungassed  under  gassed  conditions  to the  conditions by means of the impellor  pumping number : 20  ^ = f ( ^ ) where P and P  8  [1-34]  a r e the power consumptions  conditions, respectively. correlate  a l l data  taken  under ungassed and gassed  However, this method has been shown to not over  a  wide  range  of  conditions . 21  A  c o r r e l a t i o n soundly based on physical p r i n c i p l e s has not been published.  In systems where the gas pumped into the l i q u i d by the impellor from the gas plenum predominates, dimensional analysis has been shown to seriously underestimate the i n t e r f a c i a l area at high rates of a g i t a t i o n . Calderbank  demonstrated  22  the deficiency  of such  an  analysis  where  surface aeration exists at the impellor.  It  i s seen that dimensional analysis does not account  formation of bubbles •  Calderbank  22  due to surface  for the  aeration of the impellor.  As  has shown the i n t e r f a c i a l areas where surface aeration i s  present are much higher than where bubbles are formed only by sparging. Therefore,  dimensional analysis  cannot  be  applied  to the design of  24  practical usually  gas/liquid mass transfer  systems where surface  aeration i s  present.  1.5 Summary  Examination of the development of pressure  oxidative  leaching  processes indicates that mass transfer aspects are becoming increasingly important.  In pressure  oxidative leaching gas/liquid mass transfer i s  p a r t i c u l a r l y important. In gas/liquid mass transfer i t has been demonstrated  that the  l i q u i d - s i d e mass t r a n s f e r , k^a , i s r a t e c o n t r o l l i n g , i n g e n e r a l . Furthermore, the l i q u i d - s i d e mass t r a n s f e r c o e f f i c i e n t , k^, has been shown to be r e l a t i v e l y i n s e n s i t i v e to a g i t a t i o n ; whereas, the e f f e c t of a g i t a t i o n on the i n t e r f a c i a l area, a , i s quite s u b s t a n t i a l .  Therefore,  the primary  purpose  of a g i t a t i o n i n a g a s / l i q u i d  system i s to generate i n t e r f a c i a l area.  The i n t e r f a c i a l areas capable  of being produced by surface aeration are considerably larger than those 'produced  solely  classical  by gas sparging  dimensional  analysis  i n an agitated  i n the theory  system.  However,  of a g i t a t i o n does not  account f o r the gas pumped into the l i q u i d by the impellor from the gas plenum, and f o r this  reason  i t cannot  be applied  to the design of  i n d u s t r i a l gas/liquid operations where obtaining large i n t e r f a c i a l areas i s the aim.  This leads to the conclusion, that f o r the successful  operation  of pressure oxidative effect  of  agitation  leaching on  gas  pumping and  area i n gas/liquid dispersions to  this  oxidative  topic  in  leaching.  gas/liquid  processes a clearer understanding of the  generation of  i s necessary. mass transfer  This as  the  interfacial  thesis i s addressed  i t applies  to  pressure  26 2. Gas-Liquid Mass Transfer i n Pressure Oxidative Leaching  2.1 Agitation Theory of Gas Dispersions  2.1.1  C r i t i c a l Agitation Speed  As Calderbank is  a critical  gas/liquid  and others**' 3,24,25,26  22  z  agitation  interfacial  speed, N .  Q  much  agitation  on  corresponds impellor  the gas hold-up and speed.  to the hydrodynamic  due  effects.  the  Q  demonstrated»  the re  the gas hold-up and the  area are e s s e n t i a l l y dependent on the gas sparge  rate, while above N more  Below N ,  Q  n a v e  the i n t e r f a c i a l  This  critical  area depend  agitation  speed  conditions where bubbles form at the  to surface entrainment of gas, as opposed  to c a v i t a t i o n  However, the c r i t i c a l agitation speed has only been correlated  by empirical equations with no discussion of the underlying fundamental theories.  Westerterp  8  and  others  23  '  24  '  25  have used  the  following  * r e l a t i o n s h i p , or a modified version, to correlate the c r i t i c a l a g i t a t i o n speed to different mechanically agitated vessels:  ND  T  Q  (og/p)  where T  = A + B (  1/lt  i s the vessel  )  [2.1]  D  diameter,  p i s the l i q u i d  density,  a i s the  27 surface  tension,  constants  and  g i s the  acceleration of g r a v i t y .  A and  B are  which are a function of the agitator type.  Pandit  et  al  Tiave  2 6  correlated  the  critical  a g i t a t i o n speed  based on l i q u i d c i r c u l a t i o n v e l o c i t i e s to give the following:  N  0  D  T  = 0.865 (  (V /e ) G  )( D  G  1/3  T  ) '  1  v e l o c i t y of the sparged gas, and  under  the  i s the s u p e r f i c i a l  gas  i s the f r a c t i o n a l gas hold-up.  these relaionships have been shown to correlate well  conditions  questionable.  J  1.0  where T i s the v e s s e l diameter i n metres,  Although  r2.21  they  Firstly,  were  applied,  i t is difficult  depth of immersion.  their  to believe Neither  universality is that  there  i s no  effect  of impellor  equation includes  the  effect  of impellor depth of immersion; surely, for s i m i l a r conditions,  an impellor at a depth of 1 metre i s not going to give the same c r i t i c a l • a g i t a t i o n speed as an for  Equation 2.2  on the c r i t i c a l as Vg  •* 0 (no  impellor at a depth of 10 metres.  Furthermore,  the e f f e c t of s u p e r f i c i a l gas v e l o c i t y and a g i t a t i o n speed i s d i f f i c u l t  gas  s p a r g i n g ) equation 2.2  to accept.  gas hold-up  In the  implies that N  Q  limit,  -*• 0; t h i s i s  contrary to experimental evidence.  A more appropriate speed  has  been postulated  approach to predicting the c r i t i c a l a g i t a t i o n by  Peters  2 7  which  i s based  on  fundamental  28 considerations The  viscous  of momentum and viscous  e f f e c t s i n a g a s / l i q u i d system.  effects are a function of the complex shear stresses around  the impellor, these shear stresses are induced by f l u i d dynamical forces which are influenced Neglecting  by the v e l o c i t y d i s t r i b u t i o n around an impellor.  the viscous  e f f e c t s , the c r i t i c a l  a g i t a t i o n speed  can be  approximated by c a l c u l a t i n g i t on the basis of momentum e f f e c t s only. This assumes that the k i n e t i c energy needed to generate a bubble at the blade  t i p of an impellor  equivalent  immersed  at a depth, h, i n the l i q u i d i s  to the potential energy a gas bubble positioned at a height,  h, below the free l i q u i d  surface loses i n r i s i n g to the l i q u i d  surface  in the absence of drag:  mgh = 1/2 mV  [2.3]  2  This  i s also equivalent  to the potential energy of a drop of l i q u i d  positioned at a height, h, above the free l i q u i d surface.  Therefore,  on the basis of momentum e f f e c t s only, the c r i t i c a l t i p v e l o c i t y i s :  «  * V  1/2 = (2gh)  [2.4]  i / Z  where h i s the depth of impellor immersion, g i s the acceleration due to g r a v i t y , and V that  there  i s the c r i t i c a l t i p v e l o c i t y .  i s no density  cancel each other.  I t i s important to note  e f f e c t since the mass terms i n Equation 2.3  Therefore,  as i t i s written this equation would be  equally applicable i n l i q u i d mercury.  On immersion.  this  basis  we  expect  Momentum effects  standing  waves  due to wall  possibly  affect  the c r i t i c a l  to observe  an e f f e c t  of depth of  can be complicated by the existence of effects,  therefore  t i p velocity.  vessel  geometry  Furthermore,  can  the viscous  e f f e c t s are also expected to change with depth of impellor immersion due to changes i n the v e l o c i t y d i s t r i b u t i o n around the impellor.  The dispersion of a gas by the impellor pumping gas into the l i q quid from the gas plenum above the l i q u i d two  step process.  The f i r s t  t i p of an impellor blade; critical  t i p velocity  surface i s considered to be a  step i s the formation of a bubble at the  the formation of a bubble i s governed by the  as discussed  above.  The second  break-up of the bubble that has been formed. brought static the  about  by f l u i d  dynamical  forces  The breakup of a bubble i s  which  have  forces of surface tension which were discussed  equilibrium  bubble size dispersion  state  process i s the  to overcome the i n section 1.4;  between these two forces determines the maximum  that can e x i s t .  The concept of a two step process i n gas  i s important because i t demonstrates that the prediction of  'bubble s i z e , gas hold-up or i n t e r f a c i a l area i s meaningless unless the conditions for bubble formation have been s a t i s f i e d  2.1.2  Gas/Liquid I n t e r f a c i a l Area  It  was  established  a g i t a t i o n i n gas/liquid the  first.  earlier  systems  that  the primary  i s to create i n t e r f a c i a l  function area.  of  Above  c r i t i c a l t i p v e l o c i t y , the gas/liquid i n t e r f a c i a l area, a, increases  30 linearly  with impellor  t i p velocity  and  i s affected  less  by  the  gas  8 24 25 sparge r a t e  '  '  .  The l i n e a r r e l a t i o n s h i p between i n t e r f a c i a l area  and impellor t i p v e l o c i t y i s a compound e f f e c t of gas hold-up and bubble diameter as demonstrated by W e s t e r t e r p . 28  The critical  average  bubble  t i p velocity.  size  i s seen to decrease rapidly  This i s mainly due  to the fact  near  the  that near the  c r i t i c a l t i p v e l o c i t y the bubbles are not yet homogeneously dispersed In the  liquid.  than  above  Below the impellor the bubble density i s s t i l l much lower the  impellor.  constant value. fractional  gas  The  average  bubble  size  rapidly  reaches a  Only at extremely high a g i t a t i o n rates, and above a hold-up  of  approximately  0.4,  does  the  bubble  size  decrease.  According to Westerterp , the gas hold-up i s shown to have a 28  non-zero value below the c r i t i c a l  t i p v e l o c i t y , and this i s due to the  presence of gas sparging i n h i s experiments. shows  how  the  fractional  * a g i t a t i o n rate. limit the  of 0.4;  agitation  gives  a  Finally  hold-up  the f r a c t i o n a l  increases  linearly  gas hold-up  with  reaches an  the upper  thereafter, i t increases very slowly with increases i n rate.  theoretical  approximately 0.6. agitation  gas  Nevertheless, Westerterp  rates  The most dense spherical packing of gas upper  limit  where the  fractional  gas  bubbles  hold-up i s  The gas hold-up ceases to be l i n e a r with respect to  only  at  t i p velocities  encountered in i n d u s t r i a l processes.  much  higher  than  typically  In result  of  summary, increases increased  i n the  fractional  size i s e s s e n t i a l l y constant  gas  interfacial  area  hold-up, while  the  are mainly  the  average bubble  over the range of a g i t a t i o n rates t y p i c a l l y  used i n industry.  It i s interesting to r e c a l l that the impellor pumping  capacity  linear  was  also  Equation 1.32. gas  hold-up  gas.  with  to  a g i t a t i o n rate  This suggests that a r e l a t i o n s h i p may  (or i n t e r f a c i a l  In the  respect  area) and  an  acording  to  exist between the  impeller's  capacity  to pump  experimental results section evidence w i l l be presented  that confirms this r e l a t i o n s h i p .  2.1.3  Impellor  Power Consumption  As discussed gas  previously, the prediction of power consumption i n  dispersions must account for the  presence of gas i n the l i q u i d .  The  reduced e f f e c t i v e density  to  unloading  at the impellor caused by bubble  the  r a p i d l y decrease above the  rate.  critical  t i p v e l o c i t y due  pumping number  been correlated to  (Q/ND ), where Q i s the 3  gas  sparge  C o r r e l a t i o n of the power consumption has also been related to the  s u p e r f i c i a l gas v e l o c i t y . However, these correlations only consider effects is  to  formation.  power consumption i n gassed systems has  dimensionless  to  power consumption of an impellor i s  expected  The  due  of  gas  pumping  gas  surface.  sparging, into the  and liquid  cannot  be  from the  applied gas  where  the  the  impellor  plenum above the  liquid  32 In surface aerated  systems the e f f e c t i v e l i q u i d density i s going  to be related to the gas hold-up. the  impellor  tip velocity  in  surface  density w i l l likewise be r e l a t e d . is  probably  c l o s e l y related  Since the gas hold-up i s related to aerated  systems  the  effective  In p a r t i c u l a r , the e f f e c t i v e density  to  the  gas  pumping  capacity  of  a  given  impellor type as follows:  Q  a ND^  [2.5]  g where w i s the impellor blade width, and Q  i s the impellor gas pumping  capacity.  Therefore,  any  c o r r e l a t i o n for predicting the power consumption  of a surface  aerated  unloading  to bubble formation  2.2  due  impellor must account for the e f f e c t of  Previous Work on Gas/Liquid  The *gas/liquid  at the impellor.  Mass Transfer i n Agitated Systems  following i s a b r i e f review of the l i t e r a t u r e available on mass  Comprehensive  transfer  reviews  in  have  3 0  31  most systematic  mechanically  been  V a l e n t i n , Nagata , C h a r p e n t i e r  The  32  published and  8  2 6  These  31  authors  agitated by  confirm  Sideman  vessels. et  al  2 9  ,  Shah . 33  experimental work has  Westerterp et- a l , Reith *, Mehta and Joshi .  impellor  been c a r r i e d out  Sharma , M i l l e r * and Pandit  the  23  existence  21  of  a  critical  v e l o c i t y , however, only empirical correlations are reported.  by and tip  There i s  33 no  discussion of  the  underlying  fundamentals related to  the  critical  t i p v e l o c i t y i n the l i t e r a t u r e .  The  gas/liquid  interfacial  area  has  been  reported  to  be  increased by increases in ionic strength, v i s c o s i t y , by the presence of s o l i d s or Immiscible l i q u i d , or by a decrease in l i q u i d surface tension.  There velocities,  have  flow  been  rates,  some and  exhaustive  flow  patterns  studies  done  to  examine  for rotating impellors  by  35 36 37 Nagata et a l . is  expected  '  '  that  However, v e l o c i t y measuring tubes were used and i t the  measuring devices.  velocity profiles  Nevertheless,  would  be  tangential v e l o c i t i e s  are  greatly decreased  b a f f l e s and both axial and r a d i a l v e l o c i t i e s are Charpentier chemical  32  by  the  they show some Interesting e f f e c t s of  impellor depth of immersion on the v e l o c i t y p r o f i l e s . that  affected  by  Aiba the  has shown  3 8  i n s e r t i o n of  increased.  has given an excellent review of both physical and  methods for measuring g a s / l i q u i d i n t e r f a c i a l  areas and  mass  'transfer c o e f f i c i e n t s .  2.3  Oxygen-Sodium Sulphite  System  for Measuring  Interfacial  Area  and  i n t e r f a c i a l area  and  Gas/Liquid Mass Transfer Rates  The  methods used to measure the  specific  mass transfer rates i n gas/liquid contacting devices can be into two  classified  categories: l o c a l measurement with physical techniques  such as  34  light  scattering  methods;  and  or r e f l e c t i o n ,  secondly,  Experimental  work  in  global this  photography, or e l e c t r i c measurements  project  has  only  measurement of oxygen mass transfer rates. reaction  of oxygen  measure  the  with  oxygen  mass  with  conductivity  chemical concerned  techniques. the  global  To be s p e c i f i c , the chemical  dissolved  sodium  transfer  rates.  sulphite We  has  will  been  used to  briefly  examine  the reaction of oxygen with sodium s u l p h i t e . The  absorption of oxygen i n sulphite  solutions  i s very  often  used to measure mass transfer rates and i n t e r f a c i a l areas as reviewed by Reith and Beek.  39  The chemical reaction i s :  2Na S0 + 0 2  3  2  CoSO^ — ---*•  2Na S0 2  [2.6]  lt  with the rate equation represented by:  r  ^The  reaction  catalyzed 0  k  2  Na S0 , 2  3  C  C 2  Na S0 2  3  ^oSO^  i s considered  by the presence  and N a S 0  2  c 0  3  a  rapid  of cobalt  [ 2  pseudo-mth-order ions.  2  7 ]  which i s  When the reaction  i n the l i q u i d phase i s mth-order i n 0  under conditions  type  '  between  and nth-order i n  where the concentration of N a S 0 2  3  i s the same  everywhere i n the bulk solution (k C„ " i s constant) the reaction i s mn N a S 0 ' 3  2  said to be pseudo mth-order i n 0 > 2  3  35  The  reaction i s reported  to be second  order  with  respect to  oxygen (m=2) for partial, pressures of oxygen below 1 atm and f i r s t with respect to oxygen above 1 atm. * 4  with  a i r at atmospheric  oxygen.  The reaction  pressures rate  Therefore, reactions carried out  0  are second  i s independent  order  with  respect to  of the sodium  concentration i n the concentration range 0.4 - 0.8 M, this zero  order  first  kinetics.  order  influence  with  respect  of catalyst  to  sodium  concentration  unity f o r cobalt concentrations M.  40  Any cobalt  sulphite represents  Below 0.4 M, the reaction i s reported  4 0  sulphite  concentration.  to be The  (Co" ") gives a reaction order of 1-1  i n the range of 1 X 1 0  concentrations  order  higher  than these  will  - 4  to 2 X 10"  3  form a s o l i d  p r e c i p i t a t e with the sulphite.  The oxidation of sodium sulphite i s inhibited by the presence of trace amounts of copper i n s o l u t i o n . ((NH^)280^) determined complex  concentration  the ammonium  concentration  The effect of ammonium sulphate  the rate  i n the l i t e r a t u r e .  with  constant  on  4 1  of oxidation  It i s posssible that cobalt ion w i l l  species  of ammonium  under  c e r t a i n conditions.  sulphate  i s used  presence of ammonium sulphate should not be important. e f f e c t of ammonium sulphate  has not been  If a  throughout the The most obvious  i s the decrease i n bubble diameter related  to the increased ionic strength of the s o l u t i o n .  The average volumetric rate of absorption of oxygen per unit of interfacial volumetric  area rate  i s independent varies  of the gas hold-up ;  proportionally with  32  however, the  the i n t e r f a c i a l  area . 4 0  36  Therefore, the  the reaction of oxygen with sodium sulphite i s applicable f o r  measurement of gas/liquid mass transfer rates and the i n t e r f a c i a l  areas  i n the mechanically agitated  systems being  studied.  A typical  experimental rate curve from this study demonstrates zero order k i n e t i c s i n d i c a t i n g the reaction i s i n the mass transfer controlled regime (see 3  Figure  10).  above  this  interfacial  Furthermore, under reaction area.  system  the appropriate  can  be  used  to  conditions measure  discussed  the  average  4+2  2.4 Purpose and Scope of the Present Investigation  This thesis i s an examination of the gas/liquid mass transfer i n pressure leach processes. interfacial  area  The emphasis i s on the creation of gas/liquid  i n mechanically  agitated  vessels.  It w i l l  be  demonstrated that the mechanisms by which gas dispersion take place are different  from  the mechanisms by which f l u i d  Furthermore, the conditions systems w i l l  be discussed.  that  mixing  i s accomplished.  optimize gas dispersion i n gas/liquid  The optimum conditions  f o r mixing are not  *the same as f o r gas dispersion; therefore, a clear d i s t i n c t i o n between mixing  a fluid  and dispersing  a gas i s necessary  to achieve optimum  process conditions i n a gas/liquid system.  Furthermore, i t w i l l be demonstrated that because of the above d i s t i n c t i o n dimensionless correlations are not useful f o r extrapolating or  s c a l i n g up a gas/liquid mass transfer system.  Instead  i t w i l l be  37  shown that such gas/liquid systems are more appropriately described scaled-up  by  means  characteristics.  of  the  impellor  t i p velocity  and  gas  and  pumping  This view of gas/liquid systems i s shown to follow  from fundamental considerations of the momentum transfer and the viscous e f f e c t s around an impellor.  The e f f e c t of a number of variables on the oxygen mass transfer rates,  and  variables  volumetric include  power  the  type  requirements of  impellor,  sparging modes, oxygen p a r t i a l pressure, use  of  full  variables  were  arrangements. equipment reactor  length  The  and  half  studied  ( 2 1 0 0 litres).  the  baffle  both  experimental work was  (a 2 l i t r e autoclave  been  impellor  dispersion  positioning,  single  and  done using  dual  These impellor  both bench  scale  and a 2 0 l i t r e vessel) and a p i l o t scale  The p i l o t  i s demonstrated  These  configurations.  scale experiments were performed at  Cominco's Technical Research centre i n T r a i l , B.C.. gas  studied.  the presence of s o l i d s , and the  length  using  has  A novel approach to  to have p o t e n t i a l for a much reduced  power consumption over the conventional  systems presently  employed.  38 3.  Experimental Details o  3.1  Materials  Technical grade anhydrous sodium sulphite (Na S0 ) was used f o r 2  the oxidation tests. was  provided  3  The 100 pound bags i n which the sodium  did not provide any vapour  barrier,  sulphite  therefore  it  is  possible that atmospheric moisture was absorbed.  As a catalyst aqueous  solution.  i n the oxidation tests, cobalt was dissolved i n  This  was added  as reagent grade  cobalt  chloride  ( C o C l '6H 0). 2  2  To  simulate  the mixing environment  of an e l e c t r o l y t e ammonium  sulphate with surface active agents removed was dissolved i n the aqueous solution.  Reagent grade ammonium sulphate was used f o r the bench scale  experiments.  The only  distinguishable  difference  between  this and  commercial grade ammonium sulphate was the amount of froth produced.  3.2 Apparatus  3.2.1  Cominco Mixing Model Tests  The  oxidation  tests  performed  at Cominco's Technical Research  Centre were performed i n a scaled-down model of the f i r s t compartment of  39 the  commercial autoclave.  diameter  cylindrical  constructed  of  tank were two from  the  fiber  The  section of  lucite  reinforced  plastic  b a f f l e s 0.18  agitator  model was  shaft  constructed with (see  one  of a 1.22  bulged  Figure  end  either  b a f f l e s were used interchangeably:  end one  from top to bottom, the other set was  of  the  set was  0.61  which i s  11).  Inside  metres wide which were located  at  tank.  the  equidistant  Two  48 inches  metres  lengths  of  long and went  metres long and was  placed i n  the lower half of the tank.  It and  use  was  various  possible  to  remove  impellors  impellor combinations.  from  the  Three types of  agitator  shaft  impellors were  tested:  i ) four-bladed i i ) four-bladed  45° pitched blade ( a x i a l flow) impellor 90° f l a t blade ( r a d i a l flow) impellor  i i i ) six-bladed 90° f l a t blade ( r a d i a l flow) impellor  The  diameter of these impellors could be varied to be either 46 cm  (18  * inches) or 53 cm (21 inches)(see Appendix A for dimensional d e t a i l s ) .  The motor.  By  agitator shaft was changing sheaves on  agitator shaft i t was rates (50-300 rpm). tachometer.  driven by a constant  speed 3-phase 11  the motor shaft, the  jack shaft or  kW the  possible to study an extended range of a g i t a t i o n These a g i t a t i o n rates were measured by a hand held  40 The  agitator  drive  motor  was  instrumented  so  that  power  measurements could be made during operation. A voltmeter, wattmeter primary and secondary ammeters were provided. the  current  components), by 13.65  was  2.73:1  (ratio  these  with t h i s  There  of  Appendix  B).  15 mm  to  was  used  5:1 and  secondary  reading must be m u l t i p l i e d i s some question as to  size motor the e f f i c i e n c y  i s low  power  how  input  converting energy  The power draw when i n a i r i s 0.982 kW.  Gas could be sparged into the tank. diameter  primary  power measurements represent the actual  form e l e c t r i c a l to mechanical.  The sparger with an o r i f i c e  to sparge under  the bottom  impellor  (see  Another sparger was designed to sparge gas just above the  bottom impellor. of 15 mm  of  therefore the actual wattmeter  to obtain the power draw.  accurately because  stepdown  The voltage was  and  This sparger has two o r i f i c e s , each having a diameter  (see Appendix B).  Gas flow to the sparger was measured with a  Fischer and Porter rotameter using B6-35-10 tube and BSVT-64 f l o a t .  A  second F and P rotameter FP-1-35-G10/27 was used to measure oxygen flow rates when oxygen enriched a i r was sparged.  Both rotameters were f i t t e d  with a pressure gauge so pressure corrections could be made.  I t was  possible to introduce a i r or oxygen into the vapour space of the mixing model through PVC tubing; flow through the tubing was measured using the rotameters used for the sparger (see Appendix C).  Measurements of dissolved oxygen were made with a YSI model 51B dissolved  oxygen meter.  The  sensing probe  l u c i t e port i n the bulged end of the tank.  was  inserted  through  the  41 For the purposes of a special experimental arrangement to test the effect speed  of a small diameter high speed  impellor  a 1/2 Hp v a r i a b l e  (0-1200 rpm) motor was mounted above the opening of the mixing  model.  A 6-inch six-bladed disc impellor was mounted on the shaft and  was provided a depth of immersion up to 5.5 inches.  3.2.2 Bench-Scale Experiments  The pressure  bench-scale tests  at U.B.C. were performed  autoclave and a 2 0 - l i t r e  atmospheric  vessel.  in a  2-litre  The  2-litre  pressure autoclave was constructed of titanium with a zirconium l i n i n g ; the vessel diameter was 10 cm. thick-walled  pyrex  cross-section.  glass  Inside  symmetrically around  The 2 0 - l i t r e vessel was constructed from with  each  the walls.  a  28  reactor  diameter  four  baffles  cylindrical were  placed  The b a f f l e widths were equivalent to  one-tenth the diameter of the v e s s e l . Interchangeably:  cm  one set of b a f f l e s  Two lengths of b a f f l e were used went  from  top to bottom of the  v e s s e l , the other set of b a f f l e s went from the bottom of the vessel to * halfway up the vessel w a l l . Both reactors were f i t t e d posssible  to sample the solution during an experiment.  so that  i t was  the pressure  autoclave was f i t t e d with a 600 p s i pressure gauge.  As with the p i l o t - s c a l e  tests,  i t was possible  to remove the  impellors from the agitator shaft and use various impellor combinations. The  impellors used i n the bench-scale experiments were the same three  types used i n the p i l o t - s c a l e tests (see Appendix D).  The diameter of  42 these impellors could be varied to be either 58 mm or 40  The  agitator  shaft  for the two-litre  mm.  pressure  driven by a constant speed 1-phase 44 Watt motor.  autoclave  By changing pulleys  i t was possible to use a range of a g i t a t i o n rates (150-1450 rpm). agitator shaft  was  The  for the 2 0 - l i t r e vessel was driven by a variable speed  (0-1400 rpm) 75 Watt motor.  The a g i t a t i o n speeds were measured by a  hand-heId tachometer.  It  was  not possible  2 - l i t r e autoclave. constructed motor.  to make power  draw measurements  on the  However, the motor mount for the 2 0 - l i t r e vessel was  with a mechanism  for measuring the torque delivered by the  This was done by mounting the motor v e r t i c a l l y along the shaft  axis on a thrust bearing  so that the motor casing rotates f r e e l y .  The  motor was connected by means of a r a d i a l arm to a spring balance (500g) which measured the force exerted  by the rotating motor (see Appendix E  for schematic).  For  the 2 0 - l i t r e vessel, a portion of the free l i q u i d  area was enclosed  so that gas bubbles r i s i n g out of s o l u t i o n could be  trapped, and the volumetric Appendix  F).  surface  The  gas  flow rate of bubbles could be measured (see flow  rate  was  measured  using  Gilmont  flowmeters ( a 2000 ml/min capacity and a 8000 ml/min capacity).  gas  43 3.3 E x p e r i m e n t a l , Procedure  3.3.1 Cominco M i x i n g Model T e s t s  The  r a t e o f oxygen mass t r a n s f e r to the s o l u t i o n was measured by  sampling  the  analyzing  the c o n c e n t r a t i o n  Na S0 2  by  3  solution  was e s t a b l i s h e d ,  stoichiometry  2  analysis  3  0  +  2  (usually  of Na S0 . 2  Once  3  every the r a t e  and  of depletion  of  determined  -• 2Na SO^  [3.1]  2  3  was done by t i t r a t i n g  with  0.1N  iodine-iodide  indicator.  concentration  o f 5 ppm Co  was used  i n the ammonium  only  a  few e x c e p t i o n s ,  the t e s t  work was  performed  solution  o f ammonium s u l p h a t e  catalyst  and ammonium s u l p h a t e a measured amount The N a S 0 2  repeatedly, oxygen  minutes)  solution.  With  added.  3  the r a t e o f oxygen mass t r a n s f e r was  an a c e t i c - s t a r c h  A catalyst sulphate  2  for Na S0  s o l u t i o n using  intervals  a c c o r d i n g to the r e a c t i o n :  2Na S0  This  at  meter  sparger  was  samples  were  3  (14kg/m ).  was d i s s o l v e d  3  Following  first taken  to less started,  than then  d i s s o l u t i o n o f the  (1-4 kg) o f N a S 0 2  3  was  by s t a r t i n g and s t o p p i n g the a g i t a t o r  so as not t o cause any gas d i s p e r s i o n , fell  in a  0.3  ppm.  I f sparging  the a g i t a t o r  at i n t e r v a l s u n t i l  until  was  the d i s s o l v e d  the d i s s o l v e d was  used, the  started.  Solution  oxygen meter  read  44 greater than 0.3 ppm.  The volume of the mixing model tank i s 2116 l i t r e s at 100% f u l l . The testwork was performed at 82% f u l l which i s a l i q u i d volume of 1735 litres.  For the  the oxygen enriched tests no additional a i r was admitted to  vapour space and the open top of the tank was covered.  were taken from the headspace  Gas samples  and analyzed f o r percent oxygen using an  Orsat analyzer.  For  the purpose of mixing the solution homogeneously during the  small diameter high speed impellor tests a single 46cm four-bladed a x i a l impellor was placed  a half  impellor diameter above the bottom  of the  tank and rotated at 104 rpm. _ At this a g i t a t i o n rate the single impellor at  the tank bottom  did not cause any disturbance of the free  liquid  surface.  Throughout  the experimental study  unstable mixing  conditions  were prevented by maintaining agitation rates that did not cause loss of solution through the top of the mixing model due to splashing.  3.3.2 Bench-Scale Experiments  For 0.1g/l  the bench-scale experiments  CoCl »6H 0 2  2  was  used  a catalyst  i n a l l experiments.  concentration of  A l l test  work was  45 performed  i n 14kg/m  3  ammonium sulphate solutions.  For experiments i n  the 2 - l i t r e pressure autoclave 8-15g of Na SG" was added at the start of 2  the experiment. in  In the 20 l i t r e vessel  solution was i n i t i a l l y  20-60 g.  3  the amount of Na S0 2  dissolved  3  The solutions were s t i r r e d  gently  with a s t i r r i n g rod u n t i l a l l components were completely dissolved. sample of the solution was taken and analyzed started.  A  before the agitator was  After the agitator was started samples were taken at i n t e r v a l s  (usually 3-5 minutes) and analyzed immediately; t h i s continued u n t i l the Na S0 2  3  concentration f e l l below 0.05 kg  During  experiments  consumption of the impellor  with  the  20-litre  vessel,  the  and the gas flowrate due to r i s i n g  power bubbles  were recorded.  Tests done i n the 2 - l i t r e autoclave were performed with a l i q u i d volume of 1.5 l i t r e s , and the work i n the 2 0 - l i t r e vessel used a l i q u i d volume of 16 l i t r e s .  For  the oxygen enriched  tests  done  i n the 2 - l i t r e  pressure  autoclave the autoclave was f i r s t purged with pure oxygen to remove any residual a i r .  After the purge was complete,  the autoclave was  to the operating pressure then the agitator was started.  brought  46 4. Results and Discussion  4.1 Cominco Mixing Model Tests  The  impellor  commercial autoclave impellor blade  for  flow)  i s presently  45° pitched  on the bottom  and curves  i n the  blade ( a x i a l  (see figure  i s used as the standard  purposes,  used  of the shaft, and a four-bladed  impellor  configuration  comparison  that  has a four-bladed  on the upper part  (radial  impellor  configuration  impellor  on any graph  flow)  90° f l a t  11).  This  configuration  representing  this  autoclave  are  configuration are labelled 'STD'.  Although geometrically the  the  similar,  model  and  the  commercial  the agitator speed i n the autoclave  that  same process results as the model w i l l not be the same speed.  gives the  scale-up  formula used for v e r t i c a l c y l i n d r i c a l tanks to give equal mass  transfer  is:  4 3  , U2/3 l D~  [4.1]  D  n  where  2  =  n  (  n^ and n  respectively,  2  )  are the agitator  and D^  and D  2  rpm f o r the model and  are s i m i l a r l y  the impellor  autoclave, diameters.  Although this scale-up c o r r e l a t i o n applies only to a single phase system it  i s used here only  to give  an idea of the order  of magnitude that  scale-up  represents.  Applying  this  equation  to  the horizontal  c y l i n d r i c a l vessel, the test results f o r the model at 198 rpm should be approximately equivalent 100 rpm.  to the results f o r the commercial autoclave at  I t i s not correct to apply  experiments Nevertheless,  because  the  systems  this equation to the bench scale are  not  geometrically  similar.  f o r the purpose of rough comparisons the test r e s u l t s for  the model at 198 rpm would be equivalent  to the results f o r the bench  scale impellors at 785 rpm.  4.1.1 Single Impellor  Systems  4.1.1.1 E f f e c t of Impellor Type  A series of oxidation tests were done to determine the e f f e c t of impellor  type on the rate of oxygen mass transfer (g 0 /l*min). 2  results  are shown i n Figure  C l e a r l y , a single six-bladed the  standard  autoclave impellor  dual  impellor  12 for a range of impellor  more  t i p speeds.  f l a t disc impellor i s more e f f e c t i v e than configuration  used  f o r promoting gas/liquid mass transfer. was  The  e f f e c t i v e than  the other  two  in  the  commercial  The six-bladed disc types  of  impellors  probably f o r two reasons: i t provides greater shear force than the a x i a l impellor,  and due to the disc i t forces more gas to flow  high shear zone at the t i p of the impellor  through the  blades.  S i m i l a r l y , the six-bladed f l a t disc impellor made more e f f e c t i v e use  of the energy  i t consumed to produce  oxygen mass  transfer.  A  48 measure of this  e f f e c t i s the r a t i o of the oxygen mass transfer rate  (g0 /l*min) to the volumetric power consumption of the agitator (kW/m ), 3  2  and  i s termed the r e l a t i v e mass transfer e f f i c i e n c y (g0 /l*min)/(kW/m ). 3  2  A comparison of the r e l a t i v e mass transfer e f f i c i e n c y of each impellor type  at a constant  values  impellor  t i p v e l o c i t y i s given  i n Table  1.  The  f o r the power draw (kW) have had the power draw of the impellor  rotating  i n a i r (P  = 0.98 kW) subtracted from them.  Although, i n a  3- X X"  non-gassed l i q u i d , the  other  six-bladed flat  a six-bladed  two impellors, disc  impellor.  impellor This  disc impellor consumes more power than  i t was found that  i n g a s / l i q u i d systems the  consumed no more power than the  four-bladed  lower power consumption i s due to the extensive  gassing of the impellor.  In  the unsparged  cases,  the top impellor  i n a dual  impellor  system i s mainly doing the work to create gas/liquid mass t r a n s f e r . four-bladed the  six-bladed disc  45° pitched blade impellor i s used as the upper impellor i n  standard  four-bladed  The  autoclave  configuration.  45° pitched blade impellor  I t i s shown  here  that  the  i s much less e f f e c t i v e than the  disc impellor; therefore, i t i s expected that the six-bladed  impellor  alone  would  be more  e f f e c t i v e than  the standard  dual  impellor configuration.  4.1.1.2 E f f e c t of Depth of Impellor  The  depth  of  the impellor  Immersion  immersion  below  the free  surface had a d i f f e r e n t e f f e c t for each type of impellor.  liquid  Each impellor  49 was  studied at three depths - 22.9  (0.75  diameter),  and 45.7  cm  cm  (0.5  impellor diameter),  (1.0 diameter).  Figure flat  for each impellor 16.  The  type are  cm  A cross-sectional view of  the impellor positions i n the mixing model i s shown i n Figure results  34.3  shown In Figure  14,  13.  Figure  oxygen mass transfer rates produced by the  15  The and  six-bladed  disc impellor were found to be the most sensitive to the depth of  impellor immersion. sensitive only While  the  The  four-bladed  to placements of the  four-bladed  45°  f l a t blade impellor appeared to be impellor below a depth of 34  pitched  blade  impellor  was  affected  s i g n i f i c a n t l y only by impellor placements above a depth of 34  If  the  concept  of  a  critical  section 2.1.1, i s representative  t i p v e l o c i t y , as  of a surface aerated  due  explained  The  difference  (this represents  due  to the p a r t i t i o n i n g e f f e c t the disc has  on f l u i d flow (there i s no flow a x i a l l y through the impellor). disc  a  The s e n s i t i v i t y of the six-bladed disc impellor to the  depth of immersion i s l i k e l y  six-bladed  in  to the difference i n  the v e l o c i t y p r o f i l e s around each type of impellor viscous e f f e c t ) .  cm.  impellor then an  e f f e c t of depth of immersion i s expected to be observed. between the three types of impellors i s l i k e l y  cm.  impellor  from the other two  will  have a very  Thus the  different velocity profile  impellors.  The r e l a t i v e mass transfer e f f i c i e n c y for each impellor displays trends  similar to the  oxygen mass transfer rates.  Table 2 shows the  e f f e c t of impellor immersion depth on the impellor power draw (kW/m ). 3  Only  the  power  draw  for  the  six-bladed  disc  impellor  was  greatly  50 affected by the depth of impellor immersion; the power increased r a p i d l y with depth, l i k e l y due to reductions  i n the gas pumping capacity of the  impellor.  * As expected from Equation 2.4, the c r i t i c a l t i p v e l o c i t i e s , V , for  the six-bladed  impellor were found to be proportional to the square 1/2  root of the immersion 0.997 (see Table 3).  depth, h  , w i t h a c o r r e l a t i o n c o e f f i c i e n t of  The e f f e c t of depth of immersion did not correlate  1/2 as w e l l to h  f o r the other two  viscous e f f e c t s . 4.1.1.3 E f f e c t of Impellor  impellors;  t h i s i s l i k e l y due to  Diameter  For each of the impellor types a comparison of the oxygen mass transfer rate was made for two impellor diameters - 46cm and 53 cm. impellors were placed The  at the same depth of 23 cm for each  r e s u l t s for each impellor  shown i n Figure types  an  17, Figure  increase  in  The  experiment.  type over a range of agitation rates i s  18 and Figure 19.  impellor  diameter  For each of the impellor  increases  the  oxygen mass  transfer rate once the agitation rate i s above the c r i t i c a l impellor t i p velocity.  The c r i t i c a l t i p v e l o c t i y corresponds to the point where the  extrapolated  l i n e a r portion of the curve intersects the x-axis.  the  i t appears  graphs  significantly  affected  that  the  critical  t i p velocity  by a change i n the impellor  diameter.  From  is  not  Again,  considering Equation 2.4 a change i n the impellor diameter should have no e f f e c t on the c r i t i c a l t i p v e l o c i t y as shown by the graphs.  51 Although, with  an  for  the  due  to  i n general,  i n c r e a s e i n the  the  increased  increased  circular  increased  gas  The  effect  swept  c r o s s - s e c t i o n a l area,  which  rates  s i x - b l a d e d d i s c i m p e l l o r was due  was  increased  only  marginal  i n c r e a s e d r a t e s a r e thought to  c i r c u m f e r e n t i a l area  pumping  diameter i s l i k e l y  transfer rate  i m p e l l o r d i a m e t e r , the  six-bladed disc impellor. the  oxygen mass  through  the  out  by  both  t i p and  combine  impellor.  only marginally  the  The  to  reason  be the  give the  a f f e c t e d by the i n c r e a s e i n  to the presence of the d i s c which r e s u l t s i n o n l y  a s m a l l i n c r e a s e i n the c i r c u m f e r e n t i a l a r e a i s e s s e n t i a l l y only r a d i a l f l u i d  swept out by the t i p ( t h e r e  f l o w ) ; t h e r e i s e f f e c t i v e l y no  increase  i m p e l l o r d i a m e t e r a f f e c t e d the  impellor  i n the c r o s s - s e c t i o n a l a r e a .  4  Table power  consumption  cases,  the  impellor. relative impellor;  shear  and  power Only  mass  the  i n the  case of  transfer  The  relative  consumption  a l l others  efficiency. radial  summarizes how  relative  mass  i m p e l l o r i n c r e a s e d as forces  increase reaction.  in  increasing shear  a  the  with  four-bladed increase  decrease  in  the  with  the  a  means  power  more  of  surface  larger  area  the  the  diameter  four-bladed due  to  increased. is  the  mass t r a n s f e r  probably  consumption  of  impellor did  relative  transfer efficiency  than  In a l l  diameter  radial  the diameter i n c r e a s e d  more  forces  transfer efficiency.  increased  efficiency  showed  mass  produced  the An for  52 A.1.1.4 Effect of B a f f l e Length  Conventionally,  the design of b a f f l e s i n a chemical  reactor has  the length of the b a f f l e run from the bottom of the tank to just above the l i q u i d l e v e l of the s o l u t i o n . examine the effect  A series of experiments were done to  of using  b a f f l e s which are shorter i n length.  The  b a f f l e s used were 58cm long  (half the mixing model diameter) and  19cm  wide,  and  were  experiments short  placed  using  length  the  at  the  four-bladed  b a f f l e s i n Figure  resulted i n a more uniform mass transfer rate. pronounced  bottom  of  axial  20.  The  the  tank.  impellor  use of short  appears short  of  length b a f f l e s  increase of 0.0014 kg0 /m *min i n the oxygen 3  2  The use of short length b a f f l e s resulted i n a more  vortex.  efficiency  were used  r e s u l t s of  show the e f f e c t  Improvements i n the impellor power draw and transfer  The  were also observed when the short  instead of the f u l l  that  as  baffles  the  the r e l a t i v e mass  length b a f f l e s as shown i n Table  a g i t a t i o n rate  instead  of  However, the improvement  length b a f f l e s  long  increases,  baffles  the power saved  becomes  more  5.  It  using  substantial.  i n the r e l a t i v e mass transfer e f f i c i e n c y i s  larger at the lower a g i t a t i o n rates than i t i s at the higher a g i t a t i o n rates, when short b a f f l e s are used instead of long b a f f l e s . power draw when short  b a f f l e s are employed  i s probably  viscous d i s s i p a t i o n of energy i n the solution. the less  more pronounced beneficial  vortex.  at higher  The reduced due  to lower  This would also explain  Furthermore, half length  a g i t a t i o n rates probably  b a f f l e s become  because  at  higher  53 a g i t a t i o n rates that  removal  of  the  viscous  baffles in  d i s s i p a t i o n of the  top  impellor  portion of  the  power i s so tank has  high  only  a  marginal e f f e c t .  A similar diminishing improvement i s observed when the e f f e c t of short  length  immersion  baffles i s  as  considered  shown i n Table  6.  at  The  different  depths  benefits of using  of  impellor  short b a f f l e s  instead of long b a f f l e s at a shallow depth are about a factor of greater  i n each case  than  the  benefits  at  a deep depth of  3-3.5  impellor  immersion.  4.1.2  Dual Impellor Systems  4.1.2.1 The Standard Dual Impellor  The pitched  standard  dual  blade-upper, and  Configuration  Impellor  a  four-bladed  used i n the commercial autoclave was range (125-290 rpm).  The  arrangement 90°  flat  (a  four-bladed  blade-lower) that i s  studied over an extended a g i t a t i o n  r e s u l t s for both unsparged and  sparged  normal m /min or 10 normal ft /min) are shown i n Figure 21. 3  45°  3  (0.28  For both  the sparged and the unsparged conditions there appears to be two  linear  regions.  In the unsparged case, the slopes of the lines on the two curve  can  separately.  be  explained The  by  considering  the  effect  of  each  region  impellor  slope of the l i n e i n the f i r s t region i s approximately  54 equal  to the slope  impellor.  expected  The c r i t i c a l  f o r a single 4-bladed 45° pitched  blade  t i p v e l o c i t y of the f i r s t region i s much lower  than that measured f o r a single four-bladed 45° pitched blade impellor, and  i s l i k e l y due to the assistance provided by the a d d i t i o n a l momentum  of the bottom impellor. The d i s c o n t i n u i t y i n the curve has been attributed to effect of the lower impellor becoming dramatically important. second  linear  four-bladed  region  i s approximately  45° pitched  blade  equal  impellor  immersion) plus the slope f o r a four-bladed diameters immersion). the  critical  approximately  to  (at 0.5  The slope of the the slope impellor  for a diameter  f l a t - b l a d e impellor (at 1.0  I f the second l i n e a r region i s extrapolated to  t i p velocity  i t i s found  to  be  equal to 3.85 m/s for a four-bladed of 46 cm (see Figure  4.5  m/s  which i s  r a d i a l blade impellor  immersed  at a depth  observed  for the sparged case, however, because oxygen gas i s already  present i n the solution without first  linear  15).  The same trends are  any a g i t a t i o n due to the sparge gas, the  region intersects the y-axis at a s l i g h t  positive oxygen  mass transfer rate.  4.1.2.2. Effect of Baffle Length on Standard Configuration  The  results  from  the single impellor  oxygen mass transfer rates could instead that  of long  same effect  baffles; would  be improved  studies by using  therefore, i t was reasonable be observed  i n a dual  suggested short  that  baffles  to assume that  impellor  system.  The  r e s u l t s of tests done using the standard dual impellor configuration are  55 shown i n Figure 22. was  As i t was  i n the single impellor experiments, there  an increase i n the oxygen mass transfer rate when short b a f f l e s were  used  instead  of  long  baffles.  However, this was  impellor t i p speed of approximately 4.5 m/s.  true  a  plateau  value  of  up  to  an  Beyond this a g i t a t i o n rate  the oxygen mass transfer rate increases only marginally, reach  only  approximately  0.011  and  appears to  kg0 /m min.  This  3  2  agitation rate corresponds approximately to the c r i t i c a l t i p v e l o c i t y of the  second  linear  region  found  in  the  unsparged  experiments.  This  suggests there i s some negative e f f e c t of the lower impellor, however an adequate explanation  for this phenomena cannot be  4.1.2.3 Alternate Dual Impellor  Considering impellor were  System - Unsparged  results  of  experiments  done  using  single  systems a series of impellor configurations were tested which  believed  standard  to  give  higher  impellor configuration.  shown to be choice  the  provided.  the  most  disc  The  mass  transfer  impellor  impellor or a four-bladed  rates  than  the  six-bladed disc impellor had been  e f f e c t i v e impellor, therefore  for the upper impellor.  six-bladed  oxygen  Unfortunately,  available,  so  only  i t was  there was a  the  first  only a single  four-bladed  radial  a x i a l impellor could be selected for the lower  impellor.  For different  the  case  impellor  shown i n Figure  23.  where  no  sparging  arrangements against The  was the  used, standard  a  comparison  of  configuration i s  combination of six-bladed disc  impellor-upper  56 and  four-bladed  more  r a d i a l impellor-lower  effective  four-bladed  than  radial  was found to be nearly four times  the standard  impellor  impellor  configuration.  as the lower impellor  mass transfer rate that i s 27 percent impellor as the lower impellor.  higher  produced  The  an oxygen  than the four-bladed  axial  This was expected based on the single  impellor studies.  Because the top impellor i s the one mainly responsible gas  dispersion i n the unsparged case there was expected  f o r the  to be only a  small difference between the 21 inch diameter and the 18 inch diameter upper six-bladed  disc impellor.  impellor system.  This was found to be true i n the dual  This follows from the single impellor studies of the  oxygen mass transfer rates.  A.1.2.4 Alternate Dual Impellor  The  Systems - Sparged  same series of experiments that were performed above were  run with a sparger under the lower impellor.  The effect of sparging on  the oxygen mass transfer rate f o r this series of impellor i s shown i n Figure  24.  The use of sparging  showed improvements ranging transfer disc  rates.  impellor  Again,  as the upper  as compared to no sparging  from 60 to 100 percent  the impellor impellor,  configurations  higher  combination with the six-bladed and the four-bladed  Impellor  as the lower impellor  proved to have the highest  transfer  rates.  from  dispersed  It i s clear  by the bottom  impellor  oxygen mass  the results that  i s very  important;  flat  blade  oxygen mass  the sparged gas the oxygen mass  57 transfer rates were almost doubled.  The  surprising result i n t h i s comparison i s that  arrangement with the upper 53 cm six-bladed  the impellor  disc impellor had a lower  oxygen mass transfer rate than the 46 cm impellor.  However, because the  comparison i s based on the impellor t i p v e l o c i t y of the upper impellor only, and because the bottom impellor has a very s i g n i f i c a n t e f f e c t , the lower oxygen mass transfer rate i n the 53 cm arrangement i s the result of  the  lower  impellor  i n the 53  cm  arrangement  having  a lower t i p  v e l o c i t y than the lower impellor i n the 46 cm arrangement.  4.1.3  Oxygen Concentration  Effects  4.1.3.1 Oxygen Depletion in Reacted Gas Bubbles  A set of experiments were done to examine the e f f e c t of oxygen concentration only  i n the headspace on the oxygen mass transfer rate when  surface incorporation of the gas i s used ( i . e . no sparging).  results  showed  that  the oxygen mass transfer rate  increases  The  linearly  with oxygen concentration i n the headspace, as shown i n Figure 25.  This  l i n e a r relationship agrees with the mass transfer equation discussed i n section 1.3 for conditions where C^ =0: o  r  • v < 2- AO> c  l ' 1  c  The dissolved oxygen concentration  i n the bulk l i q u i d  i s considered  1 3  !  to  58 be zero i n the oxidation tests, and i n the pressure leach.  Furthermore, an experiment was designed  so that the gas bubbles  r i s i n g from the solution could be trapped and analyzed f o r their oxygen concentration. inverted  This  was  beakers before  achieved  by  collecting  the gas could mix with  rising  bubbles  in  the fresh a i r i n the  headspace.  The respective oxygen concentrations i n the headspace and  the  gas bubbles  reacted  are given  i n Table  7.  The results  show a  s i g n i f i c a n t depletion of oxygen i n the bubbles.  The  depletion of oxygen can present serious problems f o r oxygen  mass transfer i f the gas bubbles experience long residence times i n the solution  before  oxygen. appeared  they  Although  rise  only  to the headspace  a i r was used  to be no e f f e c t  to be replenished  i n the experimental  of oxygen depletion over  experiment; a l l experimental  with  work there  the course  of an  rate curves displayed zero order kinetics  ( i . e . l i n e a r with time).  4.1.3.2 Oxygen Enrichment for the Dual Impellor Systems  The disc-upper/ sparge  oxygen mass transfer rates were measured f o r the six-bladed four-bladed  under  headspace.  radial-lower arrangement  the bottom The improved  impellor  and a  pure  oxygen mass transfer  using oxygen  a pure oxygen purge  i n the  rates are compared i n  Tables 8 and 9 against a i r sparge for the same impellor arrangement.  59 4.1.4  Small Diameter-High Speed Impellor Experiments  A  series of  experiments were designed  to demonstrate a  novel  approach to gas dispersion, and to i l l u s t r a t e that the dispersion of gas (production of i n t e r f a c i a l areas) and different aqueous  conditions systems  well-mixed,  to  very  be  optimally  little  homogeneous  the mixing of f l u i d s each require effective.  agitation  solution;  is  this  In  necessary  can  be  low impellor power consumption.  we  far  depth of does  thus  immersion are  not  seem  to  consumption and considered, serve  the  impellor  important any  type,  operating  correlation  cm  diameter six-bladed  dispersion  function  with  the  low  t i p v e l o c i t y and  impellor  there power  With these assumptions  disc impellor was  which was  a  dispersion  variables; moreover,  between  for  provide  For gas  impellor  the oxygen mass transfer rate.  a 15 gas  be  that  to  achieved  a g i t a t i o n rates and have seen  general,  run  at  selected  agitation  to  rates  between 500-1000 rpm so as to provide impellor t i p speeds s i m i l a r to the standard  configuration.  The  small impellor was  placed at a depth of 14  cm i n the solution.  To provide solution mixing a 46 cm four-blade  impellor was  a half impellor  tank, and the  free  placed  agitated at 104 liquid  surface).  against the standard  The  rpm A  diameter above the bottom of  (at this rate there comparison  of  the  r e s u l t s indicate that with the  experimental results  small impellor  the oxygen mass transfer rates are 39 percent  the  i s no disruption of  impellor configuration i s given i n Table  impellor configuration.  axial  higher  10.  at 1000  than the  rpm  standard  Furthermore, although the actual power draw of  60 the  small  impellor  equivalent (0.5  could  not  be measured, the maximum i t could  to the motor's power capacity,  Hp).  The t o t a l  volumetric  assumed maximum, i s 0.359 kW/m .  to  The emphasis  3  demonstrate  the  distinction  by the  i n this  between the  mechanisms of gas dispersion.  the  power of two impellors,  impellor configuration, at an a g i t a t i o n rate of 169  0.832 kW/m .  provided  which i s rated at 0.375 kW  small impellor  experimental  rpm,  this  equal  series was  mechanisms of mixing  C l e a r l y , the  to  and the  oxygen mass transfer was  and the mixing of the solution done by  large impellor at the bottom.  be achieved  under  This compares to a power draw for the  3  standard  be i s  Therefore,  oxygen mass transfer can  with much lower power consumption.  According  to t h e o r e t i c a l c a l c u l a t i o n s done using  power  curves,  i n an ungassed aqueous system the small impellor would consume 1.90 kW (2.55  Hp),  and  assuming that the e f f i c i e n c y of the agitator drive t r a i n  i s 90 percent a motor of at least 2.11 operate  the Impellor  Nevertheless, (0.5  impellor  at 1000 rpm (see  the small impellor was  Hp) motor.  Therefore,  the  i s less than 20 percent  ungassed impellor.  kW (2.83  Appendix  E  for details).  able to operate with only a 0.37 kW  power required  to operate the gassed  of the power required to operate  the  However, i t would be necessary to provide a 2.11 kW  (2.83  Hp) motor for start up conditions.  4.1.5  Special Sparging Mode Experiments  Two  Hp) would be required to  experiments  were done  to examine  the difference  between  61 sparging above an a x i a l flow impellor and impellor  (the a x i a l  experiments using  flow of  sparging below an a x i a l flow  solution was  a six-bladed disc  downward).  impellor-upper  a x i a l impellor-lower are shown i n Table 11.  The  and  There was  results  a  of  four-bladed  only a marginal  improvement i n the oxygen mass transfer rate when sparging i s done from above  as  opposed  to  sparging  from  below.  There  i s no  apparent  difference i n power consumption.  4.1.6  Comparison of Results with Theoretical Predictions  It was  proposed i n section 2.1.1  that the c r i t i c a l t i p v e l o c i t y  should be related to momentum e f f e c t s , and according to the r e l a t i o n s h i p developed  to  describe  these  effects  (Equation  2.4)  the  critical  tip  v e l o c i t y , V , should be d i r e c t l y proportional to the square root of the 1/2 depth of i m p e l l o r immersion, h  .  For the six-bladed disc impellor,  when a l i n e a r regression analysis was  performed on a straight l i n e plot  * of  V  This  1/2 versus h experimental  Equation  evidence  gives  excellent support  *  idea that  [2.4]  i/  a fundamentally  other  the  1/2 = (2gh) ^  correct concept.  The  other two  not correlate as well to the above equation. to  to  obtained.  2.4:  V  is  a good correlation c o e f f i c i e n t of 0.997 was  effects  such  as  non-radial  impellor that are not accounted  impellor types did  This i s believed to be due  momentum  vectors  through  for by this obviously simple  the  equation.  62  The proposed  above equation represents  to be  liquid.  The  a  two  first  step step  process  i s the  governed by the c r i t i c a l  only  the  for the  first  step  i n what i s  dispersion of a gas  formation  of  the  in a  bubbles, which i s  t i p v e l o c i t y , V , should not be affected by  changes i n the impellor diameter according  to Equation 2.4.  For each of  the impellor types the experimental curves showed e s s e n t i a l l y no change i n the c r i t i c a l second  step  t i p v e l o c i t y as the impellor diameter was  after the  formation  of  the  changed.  bubbles i s establishment  The of a  stable bubble diameter which i s determined by the hydrodynamics at  the  impellor.  the  gas  The bubble diameter i s rapidly established and  hold-up i s s i g n i f i c a n t  area.  It was  i n determining  oxygen mass transfer the  gas/liquid  interfacial  suggested that the gas hold-up i s d i r e c t l y related to gas  pumping capacity of the impellor.  diameter;  the  then only  gas  rates  This i s supported by the increase i n  which  pumping  followed  capacity  of  increases  an  i n the  impellor  is  impellor  expected  to  increase as impellor diameter becomes l a r g e r .  The  r e s u l t s obtained  well explained  for  the  dual  impellor  systems were also  by the t h e o r e t i c a l concepts put forward.  Examination of  the dual impellor system revealed a compound e f f e c t of the two  impellors  which resulted in a discontinuity i n the rate of oxygen mass transfer as the impellor t i p v e l o c i t y increased. explained  i n terms of the  the  arrangement.  dual  linear  section of  the  The  The  discontinuity i n the curve i s  i n d i v i d u a l contribution of each impellor i n slope  curve was  and  critical  t i p v e l o c i t y f o r each  found to relate well to the  additive  63  e f f e c t of the predictions for each single impellor.  It  has  been stressed  that f l u i d mixing and gas dispersion are  optimized under d i f f e r e n t process conditions. uses  baffles  to  improve  c y l i n d r i c a l reactors. decrease v e l o c i t i e s gas  dispersions,  bubbles  are  solution. provide  fluid  in a  reactor,  especially  in fluid  flow which r e s u l t s  high  shear  are desirable  stresses  to minimize  i n energy losses. in  the  the size  shear  only  in  the  immediate  vacinity  In  region where of bubbles i n  High shear stresses are enhanced by high v e l o c i t i e s .  high  in  By their nature, b a f f l e s change the d i r e c t i o n and  however,  formed  mixing  C l a s s i c a l mixing practice  of  Baffles  the  baffles;  therefore, the removal of baffles from the top h a l f of the mixing model resulted  i n higher oxygen mass tansfer rates and  consumption. effect  lower impellor power  Normally, the removal of b a f f l e s would have a detrimental  on f l u i d  mixing, but i n the case of gas dispersion the optimum  conditions are d i f f e r e n t , consequently the removal of baffles i s found to Improve the dispersion of gas.  The  small  discriminated experiments  diameter-high  between f l u i d  were found  draw of  an  impellor  diameter of the impellor (Power  tt  lowering  D ). 2  mixing and  to further  process for gas dispersion. power  speed  impellor  experiments  similarly  the dispersion of gas.  confirm the concept  of a two  These step  According to c l a s s i c a l mixing theory the can be  greatly  (Power <* D ),or 5  reduced  by  decreasing the  at constant t i p v e l o c i t y ,  ND  If oxygen mass transfer rates can be maintained while  the power consumption,  there i s potential for many benefits.  64 Bubble formation is  mainly  i s only dependent on the c r i t i c a l  a function of  the  impellor  t i p v e l o c i t y , which  immersion depth.  If a  smaller  diameter Is desired then a higher agitation rate i s required to maintain t i p v e l o c i t y ; power i s not as strong a function of a g i t a t i o n rate as i t i s of impellor diameter (Power <* N ) .  Furthermore, i f the impellor i s  3  immersed at a shallower required  increase  diameter-high  of  arrangement was the the  i n a g i t a t i o n rate w i l l  speed  implications  impellor  the able  two  experiments  step  with  process  standard  diameter-high  approximately  not  be  so  large.  The  demonstrated  for  gas  well  the  small these  dispesion.  This  to improve the oxygen mass transfer rates beyond  rates obtainable small  depth the c r i t i c a l t i p v e l o c i t y reduces, and  half  the  speed power  dual  impellor  impellor of  the  configuration, while  system standard  only  consumed  dual  Impellor  configuration.  4.2  Bench-Scale Experiments  4.2.1  Single Impellor  Systems  4.2.1.1 Effect of Impellor  Type  A series of oxidation tests were done i n a 2 0 - l i t r e vessel  to  further examine the  oxygen mass t r a n s f e r . impellor between  t i p speeds. each  impellor  effect of impellor  type on  cylindrical the rate of  The results are shown i n Figure 26 for a range of In these small-scale experiments the type  i s not  as  pronounced  as  difference  i t was  in  the  65  experiments six-bladed  using  the  mixing  disc impellor  other two impellor  model  is s t i l l  (2100  litres).  However,  the  found to be more e f f e c t i v e than the  types.  The diminished  difference between the impellor types i s l i k e l y  due to two reasons: geometric s i m i l a r i t y between mixing model and the 2 0 - l i t r e vessel has not been maintained, and secondly, the scale i s a hundred  times smaller  the most c r i t i c a l  than the mixing vessel.  The former i s probably  criterion.  4.2.1.2 Impellor Gas Pumping Rates  For  was  done to  measure the gas pumping rate as a function of a g i t a t i o n rate.  The gas  pumping  rate  each  impellor  i s based  type  on a measurement  region amounting to 80 percent The  measurements  impellor  cannot  gas pumping  a series of experiments  be  of gas flow  the annular  of the t o t a l surface area of the l i q u i d . considered  rate, therefore  an  absolute  the volumetric  have been termed the r e l a t i v e gas pumping rate. type of impellor  from  are shown i n Figure  27a, Figure  measure  of the  flow measurements  The r e s u l t s f o r each 28a and Figure 29a.  The r e s u l t s c l e a r l y show that the r e l a t i v e impellor gas pumping rate i s a l i n e a r function of the a g i t a t i o n rate.  As discussed  earlier,  the oxygen mass transfer rate i s mainly  dependent on the gas hold-up, the gas hold-up has i n turn been shown to be a l i n e a r function of the a g i t a t i o n rate.  I t i s believed that the gas  66 hold-up i s d i r e c t l y therefore  gas  pumping  on  the  agitation r a t e .  impellor gas the  rate  gas  is  pumping rate of the  also  expected  mass  transfer  t i p v e l o c i t y that  to  Furthermore,  pumping rate linear with respect  oxygen  critical  the  the  dependence  is  related to  rate, closely  but  the  the gas hold-up i n a surface aerated  have  not  a  only  linear is  the  to the a g i t a t i o n rate as gas  coincides  v e l o c i t y for the oxygen mass transfer rate.  impellor,  pumping  with  the  rate  has  critical  a  tip  This supports the idea that  vessel i s d i r e c t l y related to the  gas pumping rate of the impellor.  4.2.1.3 Impellor  It impellor  was  of  interest to  examine the  type  as  a  of  consumption using  Power Consumption  for each  function impellor  the  agitation  type was  where no  gas  was  entrained  rate.  The  measured under two  a torque measuring device: f i r s t l y ,  conditions  power consumption of  the power was  i n the  each power  conditions  measured under  solution; secondly,  the  power consumption for each impellor was  measured under conditions where  surface  Impellor  aeration  takes  place  and  the  becomes gassed.  r e s u l t s for each impellor type are shown i n Figure 27b, Figure  29b.  The  ungassed  curve  for each impellor  The  Figure 28b  and  type displays  the  c l a s s i c a l relationship between power and a g i t a t i o n rate, where the power increases with respect to the cube of the a g i t a t i o n rate, N . 3  the gassed curve has  a much reduced power consumption beginning at the  a g i t a t i o n rate that corresponds to the c r i t i c a l the  point  where the  However,  impellor  tip velocity.  begins to pump gas.  After the  This i s gassing  begins,  the  power  curve  increases monotonically  flattens  a much reduced power consumption.  at  least p a r t i a l l y due as  initially,  then  eventually  with the same shape as the ungassed curve, but  at  viscosity  out  The  reduced power consumption i s  to the much lower e f f e c t i v e s o l u t i o n density  a r e s u l t of  the  impellor  pumping gas  and  i n t o the solution.  If Figures 27, 28 and 29 are each examined there appears to be a c o r r e l a t i o n between the oxygen mass transfer rate, the gas pumping rate and  the  mass  gassed power curve.  transfer  rate  are  approximately  coincident  gassed  curve  power  agitation  rate  Both the gas  linear  which  functions  critical  deviates is  pumping rate and of  agitation  tip velocities.  from  the  approximately  ungassed equal  the oxygen rate  with  Furthermore, power  to  curve  the  at  critical  the an tip  velocity.  4.2.1.5 E f f e c t of Impellor  The  e f f e c t of  Diameter  changes i n the  impellor  mass transfer rate were done using a 40 mm disc impellor.  The  diameter on  and a 58 mm  results are shown i n Table 12.  the  oxygen  six-bladed  radial  As i t was  found i n  the mixing model r e s u l t s , the oxygen mass tranfer rates increase with an increase  i n impellor  diameter.  The  rate correlated well to the increase from the rate  and  diameter  increase the was  consumption  increase i n gas  i n impellor diameter.  impellor increased  increased  gas  pumping  from as  40  mm  expected  pumping rate that resulted  Both the oxygen mass transfer  rates to when  i n oxygen mass transfer  double  58  mm.  the  when The  impellor  the  impellor  impellor diameter  power was  68 increased. Whereas,  However, the power consumption increased by only 77 percent. i f the c l a s s i c a l  r e l a t i o n s h i p between  power  and Impellor  diameter i s governing (P <*• D^), then the power consumption f o r the 58 mm impellor should have been 2.1 times higher than the power for  the 40 mm impellor.  consumption  This much lower increase i n the impellor power  consumption i s l i k e l y due to the increased gas pumping that r e s u l t s from a larger impellor diameter.  The increased gas pumping rate reduces the  effective  and  solution  density  i n part  counteracts the e f f e c t  of  increased power consumption that normally results from larger impellors.  4.2.1.6 Effect of B a f f l e Length  The effect of b a f f l e length was examined using a 58 mm diameter six-bladed r a d i a l disc impellor. shown i n Table 13.  The r e s u l t s of these experiments are  The oxygen mass transfer rates were found to be  lower when half length b a f f l e s are used instead of f u l l length b a f f l e s . in  This i s contrary to the effect short length b a f f l e s had  the mixing model experiments.  the  top-to-bottom  The difference i s most l i k e l y due to  difference i n the geometry of the two vessels, and possibly the  vessel  scale has an e f f e c t .  The impellor  gas pumping  rate was also  lower when half length b a f f l e s are used instead of f u l l length b a f f l e s . Furthermore, the impellor power consumption was found to be lower when half  length  b a f f l e s are used.  This  result  obtained i n the mixing model experiments.  agrees with the r e s u l t s  69  A.2.2 Dual Impellor  Systems  4.2.2.1 The Standard Dual Impellor  The pitched  standard  dual  Configuration  impellor  configuration  (a six-bladed  45°  blade-upper and a six-bladed 90° f l a t blade-lower) was examined  to measure the e f f e c t of a g i t a t i o n on the impellor power consumption and the impellor gas pumping rate.  The r e s u l t s are shown i n Figure 30. The  oxygen mass transfer rate and the impellor  gas pumping rate are both  linear  The slope  functions  of the a g i t a t i o n r a t e .  of oxygen mass  transfer rate curve i s the same as the slope f o r the single six-bladed a x i a l impellor; this i s expected since the upper blade of the standard dual  configuration i s also a six-bladed  noted that the standard experiments  used  only  axial  impellor.  I t should be  dual impellor configuration i n the mixing model four-bladed  impellors,  whereas  the  configuration i n the bench-scale experiments used six-bladed Although,  there  was  found  to be l i t t l e  transfer rates f o r a four-bladed in  difference  and a six-bladed  between  standard  impellors. the mass  r a d i a l disc impellor  the mixing model experiment, i t appears to be important f o r r a d i a l  impellors transfer  and a x i a l rate  impellors.  For example, the slope of oxygen mass  curve for the six-bladed  bench-scale impellor  g 0 *s/m»1»min, while the slope the four_bladed 2  i s 0.0082  mixing model impellors  i s 0.0042 g 0 »s/m*l«min. 2  As with the single impellor experiments, the power consumption for  the dual  impellor  configuration  i s lower when the impellors are  70  gassed  due  to  surface  aeration.  However,  the  reduction  in  power  consumption i s not as large as that found for single impellor studies. The  reason for this i s because the lower impellor i n the configuration  is s t i l l  s i g n i f i c a n t l y ungassed by surface aeration at the impellor t i p  speeds.  Furthermore, there was  scale  experiments  because  the  no d i s c o n t i n u i t y observed i n the bencha g i t a t i o n rates  were not  sufficiently  high.  4.2.2.2 Alternate Dual Impellor  The  most  effective  against the standard i n Table 14.  The  configuration had standard rates  dual  i s not  experiments. i n the  experiments  Impellor  configuration  dual impellor configuration. six-bladed  was  radial-lower  an oxygen mass transfer rate s l i g h t l y higher  large as was  compared  The r e s u l t s are shown  r a d i a l disc-upper/six-bladed  than the  This difference i n oxygen mass transfer found  Furthermore, there was  values  consumption.  dual  configuration. as  Configurations  i n the  case of  the  mixing model  found to be only a small difference  of the r e l a t i v e gas pumping rate and  the impellor power  These differences between the r e s u l t s from the bench-scale and  the  results  from  the  mixing  model  experiments  are  attributed to changes i n geometry and scale.  4.2.3  E f f e c t of Oxygen  The  effect  of  Concentration  oxygen  concentration  extended range of oxygen p a r t i a l pressures  was  studied  than was  over  a more  possible with  the  mixing model apparatus. was  0.21  atma  (3.1  plotted in Figure  The respect  to  according  The  psia)  range of oxygen p a r t i a l pressures examined  to  6.8  atma  (85.3  psia).  The  results  are  31.  rate of oxygen mass transfer i s found to vary l i n e a r l y with the  oxygen  partial  to Equation 1.13,  up  to  6.8  atma as  would  be  expected  where the dissolved oxygen concentration  is  e s s e n t i a l l y zero, C^ =0, i n the bulk l i q u i d . q  4.2.4  Effect of Solids  Experiments were done to examine the e f f e c t on gas mass transfer rates  when s o l i d s  experiments  silica  concentration Table 15.  are  present  particles  of 46 kg/m . 3  The  The  i n the (240  gas/liquid  mesh)  were  system.  For  used  a  in  these solids  r e s u l t s of the experiment are shown i n  oxygen mass transfer rate and  the  impellor  gas  pumping  rate are lower when s o l i d s are present, however the e f f e c t i s marginal. The  solids  also  had  only  a  small  effect  on  the  impellor  power  consumption.  4.2.5  Comparison of Results with Theoretical Predictions  The gas hold-up has been reported a g i t a t i o n rate as discussed rate has  been predicted  the gas hold-up and  earlier  to be a l i n e a r function of the  i n section 2.1.2.  The  to be d i r e c t l y related to the gas  the impellor  gas  gas  pumping  hold-up.  if  pumping rate are d i r e c t l y related  to each other then the impellor pumping rate should be a l i n e a r function of  the agitation rate, just as the gas hold-up  results  obtained  from  measurements of the impellor  firmly support these predictions. found  to vary  is.  linearly  with  The experimental gas pumping rates  The impellor gas pumping rates were  the a g i t a t i o n , as did the oxygen mass  transfer rates.  According consumption  to  for an  the  classical  impellor  dimensional  the power  draw  analysis  of an  of  impellor  power should  increase proportionally with the cube of the a g i t a t i o n (P « N ) for the 3  case of ungassed correlate well  impellors.  The measured power curves were found to  to this r e l a t i o n s h i p f o r ungassed  impellors.  However,  the gassed impellors were found to have a lower power consumption than  the ungassed  impellors.  For impellors  rate  which were affected  by  surface aeration the power curve was coincidental with the curve for an ungassed  impellor u n t i l the a g i t a t i o n rate was approximately  to the impellor's c r i t i c a l  t i p velocity.  rate no gas Is incorporated so  the impellors  agitation rate  Below the c r i t i c a l a g i t a t i o n  into the solution due to surface aeration,  are expected  the impellor  to behave s i m i l a r l y .  At the c r i t i c a l  becomes gassed, and the two power curves  diverge with the gassed impellor curve having a lower power This was found  to be true  equivalent  consumption.  f o r both dual and single impellor  systems.  The c l a s s i c a l analysis of power curves relates the power consumption to the Reynolds number; these power functions do not account f o r the e f f e c t of  gassing at the impellor  and are inadequate  for describing  dispersion system which i s affected by surface aeration.  a gas  73  Examination of the effect the  impellor  increases.  diameter  of impellor  i s increased  The oxygen mass  diameter showed that as  the oxygen  transfer  rate  mass  transfer  i s a function  rate  of the gas  hold-up, and the gas hold-up i s a function of the impellor gas pumping rate.  Based on this analysis the oxygen mass transfer rate could be  expected to increase with an increase  i n the impellor diameter because  an Increase i n the impellor diameter w i l l result i n a higher gas pumping rate.  The c l a s s i c a l power curves state that the power consumption of an  impellor  varies according  (P * D ) .  to the f i f t h  power of the impellor  This was not found to be true i n the case where an impellor  5  experiences surface aeration and becomes gassed. increased pumping  at a much lower rate. rate  effective  diameter  with  an  increase  s o l u t i o n density  This  The power consumption  i s due to the increase  i n impellor  diameter,  i n gas  therefore  the  i s reduced, thus the power consumption i s  reduced.  4.3 Summary  4.3.1 Mechanisms of Gas/Liquid  It liquid step  has been demonstrated  that  s o l u t i o n i s well represented  blades.  The second step  the dispersion of a gas into a  by a two step process.  i n the process i s the formation  impellor size.  Dispersions  The f i r s t  of gas bubbles at the t i p of the  i s the establishment  of the bubble  Once the bubble size has been set the gas/liquid i n t e r f a c i a l area  74 i s determined almost exclusively  The  by the gas hold-up.  formation of bubbles depends on the c r i t i c a l  t i p velocity,  * V  , which i s a f u n c t i o n  of both momentum and viscous e f f e c t s .  Based  only on the momentum e f f e c t s , the c r i t i c a l t i p v e l o c i t y i s shown to vary with the square root of the impellor depth of immersion according to the fundamental equation: * V  This  1/2 = (2gh)  i s a simplified  fundamental  [2.4]  i / Z  representation  viewpoint  of a complex  which  serves  situation.  only  to provide a  A more  comprehensive  model would be expanded from this equation to include both momentum and viscous fluid  effects.  Momentum  flow deflections  effects  are altered  from walls and b a f f l e s ;  by the presence of these could add to or  subtract from momentum vectors a r i s i n g from the agitator to  geometrical  system.  distortions  of the c r i t i c a l  Viscous effects are related  gases i n a solution  and physical  velocity  blade, leading  c r i t e r i o n by the  to the d i s t r i b u t i o n of s o l i d s and  properties of the f l u i d .  A l l these  aspects must be incorported to improve the accuracy of the c r i t i c a l t i p v e l o c i t y model.  The equilibrium  size  of bubbles i n a gas dispersion  between  shear  tension forces present.  stresses  i s determined  i n the solution  by an  and the surface  The bubble size i s r e l a t i v e l y i n s e n s i t i v e to  the a g i t a t i o n rate, except at high agitation rates which are not t y p i c a l  75 in industrial applications.  The gas hold-up i s d i r e c t l y related to the  i n t e r f a c i a l area which ultimately determines the gas mass transfer rate. The gas hold-up has been shown to be l i n e a r l y related to the gas pumping rate  of an  impellor,  as  long  as the a g i t a t i o n  rate  i s above the  c r i t i c a l tip velocity.  The surface  power  aerated  dimensionless  curves impellor  power  that  describe  are v a s t l y  curves.  The  the power different  power  for an ungassed  impellor.  from  consumption  gassed by surface aeration i s as much as 75 percent consumption  consumption  The  the  of a  classical  of an  impellor  lower than the power  power  consumption  is a  complex function of not only impellor diameter and a g i t a t i o n rate, but also  a  function  of  the impellor  type,  impellor  positioning,  fluid  properties and vessel geometry.  The  dispersion  demonstrated making  up  to be  of  gas  using  the compound  the configuration.  dual  effect  impellor  systems  of the i n d i v i d u a l  Therefore,  based  on  was  impellors  single  impellor  studies i t i s possible to predict the composite effect of two impellors placed i n a dual configuration.  4.3.2 Scale-Up  The this  Considerations  problem of scale-up  study,  nevertheless  has not been s p e c i f i c a l l y addressed i n some  conclusions  can  be  made.  76  The  scale-up  of  gas/liquid dispersions  for  equal  interfacial  area, which corresponds i n most cases to equal gas mass transfer rates, cannot  be  based  on  the  criteria  dimensionless correlations) or on  of the  Reynolds basis  number  (or  similar  of  power inputs  per  unit  i s maintained  i n both the  impellors  volume of l i q u i d . When geometric and  the  vessel  similarity  shape, the  impellor  independent parameter of scale-up  t i p v e l o c i t y remains l i k e l y as  criteria.  In applications to oxygen pressure  leaching, i t seems compelling  that agitator size, type, depth of immersion, and need  to  be  selected  on  the  an  basis  of  gas  v e l o c i t y c r i t e r i a rather than mixing theory,  b a f f l e configuration  pumping  and  i f i t has  critical  tip  been determined  that gas/liquid mass transfer i s a substantial resistance i n the rate of the autoclave  process.  77  5. Conclusions,  5.1  Conclusions  This in  Applications and Recommendations  study has  mechanically  particular liquid  agitated  emphasis on  phase.  transfer  rate  been an examination of g a s / l i q u i d mass transfer  The and  oxidative  pressure  leaching  systems  with  the effect of a g i t a t i o n on gas  dispersion i n a  e f f e c t of a number of variables on  the oxygen mass  the  impellor  power consumption  has  been examined.  These variables include the type of impellor, the impellor positioning, oxygen concentration, sparging modes, the presence of s o l i d s i n solution and  the use of f u l l length over half length b a f f l e configurations.  following conclusions  1) It has  can be made:  been shown that the dispersion of a gas i n a l i q u i d  can be described by a two  step  process.  2) The mechanisms by which a gas i s dispersed different  The  from  accomplished.  the  mechanisms  by  which  i n a l i q u i d are  fluid  mixing  Consequently, the optimum conditions  is  for each  process are not the same.  3) The  critical  corresponding the  t i p v e l o c i t y , which  to the  i s the  point when bubbles are  a g i t a t i o n rate first  formed at  impellor, can be modelled by a t h e o r e t i c a l equation based  78  on the fundamental momentum e f f e c t s , V  The  min "  experimental  (  2  «  h  I '**] 2  r e s u l t s support  the basic  nature  of t h i s  model, however i t i s necessary to further develop t h i s model to include both viscous and momentum e f f e c t s .  4) C l a s s i c a l  dimensionless correlations are not adequate f o r  extrapolating  gas/liquid mass transfer systems where surface  aeration of the impellor i s present.  5) The scale-up of a gas/liquid mass transfer system cannot be done on the basis  of dimensionless correlations nor on the  basis of power inputs per-unit volume of l i q u i d .  6 ) I t i s more appropriate  to describe and scale-up by means of  the impellor t i p v e l o c i t y and gas pumping c h a r a c t e r i s t i c s .  Some of the more p r a c t i c a l  implications  of these  conclusions  are as  follows:  1) The most e f f e c t i v e type of impellor f o r the dispersion of gas  was found  followed  to be the flat-bladed r a d i a l  by the flat-bladed r a d i a l  impellor  45°-pitched flat-bladed a x i a l impellor. speeds, increased  disc  impellor,  and l a s t l y the  At equal impellor t i p  impellor diameter was found to increase the  79  oxygen mass transfer  2) Gas  sparging  rates.  can s i g n i f i c a n t l y  transfer i n dual impellor  the oxygen mass  systems.  3) Under certain conditions found  enhance  the use of half length b a f f l e s was  to improve the gas mass transfer  rates  and reduce the  power consumption. 4) The oxygen mass transfer  rate  was found  to be a l i n e a r  function of the oxygen p a r t i a l pressure over the range studied ( 0.21 - 6.8 atma).  5) The presence  of s o l i d s i n a gas dispersion  marginally lower the oxygen mass transfer  5.2  were found to  rates.  Applications  The application of these concepts could enhance the operation of a v a r i e t y of hydrometallurgical is  rate  limiting.  processes where gas/liquid mass transfer  In processes  where the rate  limiting  process i s  undefined, but where gas/liquid mass transfer i s p o t e n t i a l l y important, the use of these concepts will  eliminate  can provide a guide to process changes that  any potential  gas/liquid  mass transfer  problems In an  e f f e c t i v e and economical way.  The zinc pressure leach process i s an example of a process where  80 the rate l i m i t i n g process i s undefined.  The process i s believed to be  limited i n the f i r s t compartment by either the g a s / l i q u i d mass transfer of  oxygen  to the solution, or by the reactions  ferrous-ferric  (Fe^/Fe  l i t e r a t u r e at present limiting.  Although  1 11  couple.  There  i s no  with the  evidence  i n the  to indicate that one or the other process i s rate the p r i n c i p l e s presented  dispersion of gas should the  )  associated  here  f o r improving the  have no e f f e c t on the f e r r o u s / f e r r i c couple,  rate of oxygen mass transfer to the solution could be improved by  these methods u n t i l i t i s clear that g a s / l i q u i d i s not i n the zinc pressure  leach process.  rate-determining  The r e s u l t s obtained  i n the mixing  model indicate that i t i s possible to increase the oxygen transfer rates much  more  than  current  industrially  experienced  rates  are i n the  autoclave.  Another  oxidative  pressure  leaching  process  where gas/liquid  mass transfer i s l i k e l y to play a v i t a l role i s the Equity S i l v e r Mines leach gold  process  for removing antimony and arsenic  bearing  operating  copper  concentrates.  leach  plant,  which  began  i n 1981 at Houston, B.C., makes use of sodium hydroxide and  sodium sulphide  (fed as sodium hydrosulphide) to leach soluble antimony  and  arsenic from the concentrate.  to  sequentially  sulphate. mass  The  from h i g h - s i l v e r and  45  solution : 4 9  antimony,  arsenic  and  excess  sodium  It i s i n the antimony p r e c i p i t a t i o n step that gas/liquid  transfer  autoclaves  crystallize  The leach liquors are then processed  i s likely  to play  a  the o x i d a t i o n of S~~, SbS  vital 3  role.  and A s S  3  In the antimony  ions takes place i n  81  Na S + 20 2  Na AsS + 0 3  Na SbS + 13/2 3  is  very  rapid  limited  0  3  At the operating  3  2  2  3  + 4NaOH + H 0  the  reactions  i n view of  the  large  U  C,  [5.1]  1+  1+  + 3H S0 2  6  2  of  [5.3]  1+  the oxidation of these species  are  probably  oxygen  quantities of oxygen that  dispersion  [5.2]  1+  -• NaSb(OH) + 3Na S0  2  temperature of 150  improved  2  + 3H 0 + Na As0  2  and  Therefore,  •*• Na S0  2  the  oxidizing gas  mass  transfer  are consumed.  i n the  solution  would l i k e l y enhance the reaction rates.  The  accelerated  microbiological  leaching  systems  has  of gold and  been  s i l v e r concentrates  successfully  demonstrated  using in  the  laboratory, along with processes for the b i o l o g i c a l leaching of copper, zinc  and  uranium. * * 1  1  In  microbiological  leaching,  oxygen  and  carbon  dioxide are e s s e n t i a l constituents which must be dissolved i n the solution by means of gas/liquid contacting. gold  and  silver  recovery  using  biological  In one  laboratory study on  leaching * 1  * that considerably higher oxidation rates were obtained as  compared  stirred  to  a  pachuca  tank i s l i k e l y due  tank.  The  improved  to the increased  leach  6  i t was  reported  i n a s t i r r e d tank  oxidation  rate  in  the  oxygen mass transfer rate.  C l e a r l y , g a s / l i q u i d mass transfer, and more s p e c i f i c a l l y gas dispersion, i s l i k e l y to be important i n the commercial use of b i o l o g i c a l leaching.  wherever i t i s necessary to disperse a gas i n a l i q u i d i n Order to successfully operate a process the p r i n c i p l e s discussed i n this paper  82 can  be  applied  to  aid  in  the  design  of  the  gas/liquid  contacting  equipment.  5.3 Recommendations for Further Work  This mechanisms considered  study has  of  gas  emphasized  dispersion.  the In  Importance of understanding  this  regard  the  study  the  cannot  be  complete, but serves to demonstrate that there i s s t i l l much  research yet to be done i n this area.  There i s s t i l l no u n i f i e d  theory  of mixing which incorporates a l l the e f f o r t s related to f l u i d mixing and gas  dispersion.  As  an extension  of t h i s study i t i s recommended that  the following be undertaken:  1) The  effect  determine  of  fluid  their  effect  properties on  gas  consumption i n gas  dispersion.  fluid  fluid  viscosity,  strength  of  the  should mass  examined  transfer  and  to  power  Of p a r t i c u l a r importance are  density,  solution, and  be  the  surface effect  of  tension,  ionic  surface  active  agents.  2) The liquid  positioning of surface  should  impellors  in  solution r e l a t i v e  to  the  be examined more extensively.  In dual  impellor configurations the r e l a t i v e p o s i t i o n of one  impellor  to the other should be investigated.  3) In dual  impellor systems, the methods of sparging  gas  into  83  the  solution should  be studied to determine whether  sparging  i s b e n e f i c i a l only at the lower impellor.  4) Although v e l o c i t y p r o f i l e s of impellors have been studied i n the  past,  measurements have been limited  hot-wire  anemometer instruments.  to pitot-tube and  By measuring  with  these  devices the v e l o c i t y p r o f i l e s are changed due to the physical presence of the instrument means  of  studying  laser-Doppler  i n the system.  velocity  velocimetry.  profiles  A more  would  accurate  make  use of  V e l o c i t y measurements have been  made i n both single phase and two phase g a s / l i q u i d s y s t e m s . 47  A  careful  impellor  study  of  how  the impellor  diameter,  depth of  immersion, a g i t a t i o n rate, and b a f f l e length a f f e c t  the v e l o c i t y d i s t r i b u t i o n would give a greater  understanding  of viscous e f f e c t s .  5 ) The effect of vessel geometry should be given some attention, this  i s often  evidence  dismissed  to support  as  unimportant  but  there  i s no  t h i s conclusion i n the case of g a s / l i q u i d  systems.  F i n a l l y , i n a more general way, the effect of a g i t a t i o n on other mass transfer processes picture  of the mass  should  be investigated to give a more complete  transfer i n a hydrometallurgical  process,  either gas/liquid or l i q u i d / s o l i d transfer can be rate l i m i t i n g :  since  84  1) The e f f e c t of a g i t a i o n on l i q u i d / s o l i d mass t r a n s f e r should be studied. the  A possible system f o r i n v e s t i g a t i n g t h i s would be  oxidation  of  solid  cuprous  chloride  (CuCl) to  c h l o r i d e ( C u C l ) which i s soluble i n aequeous systems. 2  cupric  85 REFERENCES 1.  Hiskey, J.E. ; Wadsworth, M.E.; "Electrochemical Processes in the Leacing of Metal Sulphides and Oxides", Process and Fundamental Considerations of Selected Hydrometallurgical Systems, AIME (1981); M.C. Kuhn, Ed., p303  2.  McKay, D.R.;  3.  Levenspiel, 0.; Chemical Reaction Engineering, 2ed ; John Wiley & Sons, N.Y. (1972)  4.  Forward, F.A.;  5.  Kuhn, M.C.;  6.  Parker, E.G.;  7.  Parker, E.G.; McKay, D.R.; Salmon-De-Friedberg, H.; Proc. of 3rd Int. Symp. on Hydrometallurgy, AIME; Atlanta, Georgia (March 1983) p927-940  8.  Westerterp, K.R.; Van Dierendouck, (1963) v o l 18 157-196 .  Halpern, J . ; Trans AIME, v o l 212; 301  Mackiw, V.N.;  (1958)  Trans AIME, v o l 203, 457-463 (1955)  A r b i t e r , N.; Kling, H.; CIM B u l l e t i n , (Feb 1974) p62-74 CIM B u l l e t i n , (may  1981) pl45-150  L.L.; de Kraa, J.A.;  Chem Eng Sci  P  9.  Whitman, W.G.;  Chem. M e t a l l . Eng., 29, 147 (1983)  10. Higbie, R.; Trans. Am. 11. Danckwerts, P.V.;  Inst. Chem. Eng., 35, 365 (1935)  Ind. Eng. Chem.,43,1460 (1951)  12. Oldshue, J.Y.; Chem. Eng. (Junel3, 1984) p82-108 13. Uhl, V.W.; Gray, J.B.; Mixing; Theory and Practice , v o l I and I I , (1966), Academic Press, N.Y. * 14. Bird, R.B.; Wiley, N.Y.  Stewart, W.E.; (1960)  Lightfoot, E.N.;  Tranport Phenomena,  15. Nagata, S.; Mixing, P r i n c i p l e s and Applications, Halsted Press, Wiley,N.Y. (1975) 16. Langhaar, H.W.; Dimensional Analysis and Theory of Models, John Wiley & Sons, N.Y. (1951) 17. Dickey, D.S.; 18. Hinze, J.O.; 19. Dickey, D.S.;  Fenic, J.C.; Chem Eng (January 5, 1976) pl39-145 AIChE J . , 5, 289 (1955) Hicks, R.W.;  Chem Eng (February 2, 1976) p93-100  86 20. Calderbank, P.H.; Trans Inst Chem Engrs (London), 36 443 (1958) 21. Michel, B.J.; M i l l e r , S.A.; AIChE J . , 8,262 (1962) 22. Calderbank, P.H.; Trans Inst Chem Engrs (London), 37,175 (1959) 23. Mehta, V.D.; Sharm, M.M.; Chem Eng S c i , 26, 461 (1971) 24. M i l l e r , D.N.; AIChE J . , 20, 445 (1974) 25. Boerma, H.; Lankester, J.H.; Chem Eng S c i , 23, 799 (1968) 26. Pandit, A.B.; Joshi, J.B.; Chem Eng S c i , 38, 1189 (1983) 27. Peters, E.; Private Communication (1982) 28. Westerterp, K.R.; Chem Eng S c i , 18, 495 (1963) 29. Sideman, S.; Horatacsu, 0.; Fulton, J.W.; Ind Eng Chem, 58, 32(1966) 30. Valentin, F.H.H.; "Absorption i n Gas-Liquid Dispersions", Spon, London (1967) 31. Nagata, S.; "Mixing", Wiley, N.Y. (1975) 32. Charpentier, J - C ; "Mass Transfer Rates i n Gas-Liquid Absorbers and Reactors", Advances i n Chemical Eng., v o l I I , Academic Press, Inc (London), (1981) 33. Shah, Y.T.; "Gas-Liquid-Solid Reactor Design", McGraw-Hill,N.Y. (1979) 34. Reith, T.; Ph.D. Thesis, Delft University (1968) 35. Nagata, S.; Yamamoto, K.; Hashimoto, K.; Naruse, Y.; Mem. Fac. Eng. Kyoto Univ., 20, 336 (1958) * 36. Ibid; 21,260 (1959) 37. Ibid; 22,68 (1960) 38. Aiba, S.; AIChE J . , 4, 435 (1958) 39. Reith, T.; Beek, W.J.; Chem Eng S c i , 28, 1331 (1973) t  40. Laurent, A.; Doctoral Thesis, University of Nancy, France (1975) 41. Wesselingh, J.A.; Van't Hoog, A.C; Trans. Inst. Chem. Engrs., 48, T69 (1970) 42. DeWaal, K.J.A.; Okeson, J.C.; Chem Eng S c i , 21, 559 (1966)  87 43. Rautzen, R.R. et a l . ; Chem Eng (October 25, 1976) p21 44. Bruynesteyn, A.; Working Paper on M i c r o b i o l o g i c a l Leaching as a Means f o r Metal Recovery, B.C. Research, Vancouver, B.C. (1983) 45. Dayton, S.; Eng. and Mining J . (January, 1982) p78-83 46. Lawrence, R.W.; pl07-110  Bruynesteyn, A.; CIM B u l l e t i n (September, 1983)  47. Mahallngham, R.; Llmaye, R.S.; Brink, J r . , J.A.; AIChE J . (1976) volume 22, No. 6 48. Gollakota, S.V.; Guin, J.A.; Ind Eng Chem Process Pes Dev (1984) volume 23, p52-59 49. Jones, D.; Private Communication (1984)  88  Nickel Concentrate  First Stage Leach  Gases-  Water  Gases  1  NH Scrubber 3  Nitrogen  Solution  Solids AirNH-  Second Stage Leach  NH (Aaua) 3  Steam  NH (Aqua) 3  Weak Liquor  Solids Counter Current Wash  Water  Wash  H S2  Water  Solids  Final Tailings  Copper Sulphide Copper-Free Solution To Nickel-Cobalt Precipitation And Ammonium Sulphate Recovery  F i g u r e 1.  S h e r r i t t - G o r d o n Ammonia Oxygen P r e s s u r e P r o c e s s f o r Ni-Cu-Co  Ammonia Recycle Ammonia Ment Recovery  Condensate Return Process, Water Lime Slurry Boiler r Feed Water Steam Steam Lime Boil Plant Sysjem Gypsum Residue Raffinate Recycle Ammonia Vent Wash Water Moke - U P  Moke Up Ammonia Oxygen Concentrate Feed From Smelter  I  Cooling Wote^  Scrubber Water Ammonia Froctionator  I  Leach Reactors  CCDj : Thickeners  Dilution and Launder Spooy Water Leach Reside Flotation  —  C  Z  I  al  H.sa  Solvent Extraction  Pregnant Liquor FilItration " .Filter r Cake  miuen In? t  Make Up Water  Flotation Tails  Electrowinning  •Return to Smelter  F i g u r e  2.  Anaconda  A r b i t e r  P r o c e s s  LZ=  Cathode. Copper  CO 1X>  Fortified Spent Electrolyte  Zinc Concentrate L  Flash Steam  tPh  Conditioning Tank  Zinc Sulphate Slurry (To Zinc Recovery)  Oxygen i i  [Molten S ° .Sffjotatjon ~\ ! Cone i  _  l  p"p"ty"*-fiWfl- ~* Clean  Steam  Figure 3 .  . j  Sulpher  Unreacted Sulphides (To Roasters)  o  °2  Gas Phase  0 (ln Solution) o  F e  2°3  X H  F i g u r e  2°  4.  R e a c t i o n  S t e p s  i n  a  T y p i c a l  L e a c h  Cos Film 1  ! Bulk of 1 Gos '  Liquid Film L  6  C  A  V  \  !•I  1  [  !  1  1 1 Bulk of  ! j  j Liquid  |  \ « So  Liquid Layer of  1  Infinite Depth  •  «  p  i  1 Higbie Model in the Liquid  1  Film Model in the Liquid  Liquid Layers of Infinite Oeplhs  _.i  t, » Donckwerts Model in the Liquid  no  F i g u r e  5.  L i q u i d - S i d e  Mass  T r a n s f e r  M o d e l s  92  Figure 6 .  Liquid-Phase Concentration P r o f i l e f o r Mass Transfer with a Chemical Reaction  94  — Receding Reaction Front Reaction Rate = kC„  Liquid Film Diffusion Rote=k AC l  Unreacted / Solid  Liquid Solid Interface  F i g u r e  7.  L i q u i d - S o l i d  I n t e r f a c e  56  96  it  w  \ \ \ \  Q.  E 3  Viscous  1 1  Transition  Reynolds (a) Power number  Number  Reynolds Number (b) Dimensionless velocity Q Z  , 1 1  Turbulent  D  I  £ 3  Z o> c  "a.  E 3  a.  Viscous  I  Transition  Reynolds (c) Pumping number  F i g u r e  9.  R e y n o l d s  Number  Number  I  Turbulent  D  C o r r e l a t e s  D i m e n s i o n l e s s  P a r a m e t e r s  97  10 Time Figure 10.  15 (min)  25  T y p i c a l Experimental Rate Curve f o r the Oxidation of Sodium Sulphite  98  99  Figure 1 2 .  E f f e c t of Impellor Type on the Oxygen Mass Transfer Rates  100  Figure 13.  Impellor Positioning i n Mixing Model  101  F i g u r e 14.  E f f e c t of Impellor Immersion Depth on Oxygen Mass Transfer Rate f o r the 4-Bladed A x i a l Impellor  102  1  P„ '21  I  kPo  Impellor Tip Speed  Figure 15.  1  (m/s)  E f f e c t of Impellor Immersion Depth on Oxygen Mass Transfer Rate f o r the 4-Bladed R a d i a l I m p e l l o r  103  1  r  l4ko/rn(NH)S0 3  4 2  4  Impellor Tip Speed  F i g u r e 16.  (m/s)  E f f e c t of Impellor Immersion Depth on Oxygen Mass Transfer Rate f o r 6-Bladed R a d i a l Disc Impellor  104  F i g u r e 17.  E f f e c t of Impellor Diameter on Oxygen T r a n s f e r Rate f o r the 4-Bladed A x i a l Impellor  105  Impellor Tip Speed  Figure 18.  (m/s)  E f f e c t of Impellor Diameter on Oxygen T r a n s f e r Rate f o r the 4-Bladed R a d i a l Impellor  106  Impellor Tip Speed  Figure 19.  (m/s)  E f f e c t of Impellor Diameter on Oxygen T r a n s f e r Rate f o r the 6-Bladed R a d i a l Disc I m p e l l o r  107  14 kg/m* (NH^S0  8 x  30  4  P. «2l:'kPa.t °2  1  22 9 cm  4B-A-I8  3|  cf STD  a) 2-0 o or <*lO c o 10 to  .£  10  c a)  x O  2-5 Impellor Tip Speed  Figure 20.  50 (m/s)  7-5  E f f e c t of B a f f l e Length on Oxygen Transfer Rate i n the Mixing Model  108  Impellor  F i g u r e 21.  Tip Speed  (m/s)  Standard Dual Impellor C o n f i g u r a t i o n Used I n the Commercial Autoclave  109  l4kg/m 21 (Nr^S0 kPo 8  4  1  229cm  4B-A-I8 4B-R-I8  22-9cm  E f f e c t of B a f f l e Length on Oxygen T r a n s f e r Rate f o r the Standard Dual Impellor C o n f i g u r a t i o n  110  Sparge Rate =0 Normal mVmin Upper Impellor Tip Velocity = 4 0 5 m/s l4kg/m (NH ) S0 3  per  CM  CP 0)  B  O  cm  Q. 3  cm  O  wer  O 20O  to IT) (J  CO  10  c o  5a 3. ?a > a . o_ l 3  S§  CO CO  Q  Q  15  *5  •5  o CD  a. 5 CL  3  6 D  o —I  E o  co u>  "D O  o or  o to  cc —  o  4  u>  •o  CO CO  2  u  to  w  4  "O  o _  <D  TJ _  _o aci co  •6 .9 o x or  •a TJ o  <  =  CD  c cn  co  • X  "S  a co  O  TO  CO  F i g u r e 23.  A l t e r n a t e Dual  Impellor  Configurations-Unsparged  Ill  Sparge Rate = 0-28 Normal mvmin Upper Impellor Tip Velocity = 405m/s 14 kg/m (NH ) S0 3  4  2  4  O  o X  '£  0 ~ 3  3t  w.  aj Q. a. o 3  O  E u  a  ro 2  c o  in u  '6 "5 T5  o  or  o  a> o  "O  GQ 10  c CP  £  u Q0  a> a. Q.  3  £  CuD co u  ST  -  o cm  w  tn  SI S3  Si tn  Q  "5  o  or  •a T3  O CQ CO  g _ '•o o o x  or < "O  a>  oCD CO ^J"  X  O  O T3 C O (?)  0 Figure 24.  Alternate Dual Impellor  Configurations-Sparged  112  c  1  1  1  Tip Speed =4-75 m/s I4kg/m (NH ) S0 ?  4  o  or  co  ^  O06  • /  4  1  -  2 2-9 cm t  4B-AH8 6B-RD-I8  -  i  1  008 • 0)  2  1—  \  i—i  2 2-9 cm t  V  /  /  0O4  o 002  c >» X  o  F i g u r e 25.  —  y  s 1  20  i  i  40  60  Oxygen Concentration  Effect  80 (%Volume)  of Gas Plenum Oxygen C o n c e n t r a t i o n  Oxygen Mass T r a n s f e r  Rate  100  on  113  Impellor  Figure  26.  Tip Velocity (m/£)  E f f e c t of I m p e l l o r Type on the Oxygen T r a n s f e r Rate i n 2 0 - l i t r e s  Vessel  114  •8  2 5  x  1  68058  •g  "E HO *  £ o  14 kg/hf (N^) S0  or  2  s  « o  4  Full Baffles  or  a.  in e  m  .e  8  12  2  E >>  O  or  1  2 3 Impellor Tip Velocity (m/s)  4  50mm _  1 A  a  Ungassed /  / / •  l4KgV(NVVzS0 Full Baffles  4  15  A  e  .9 a.  I  .0 A  /  % o  a.  I  /  5  0  Figure 27.  -0-0Gassed  •  1 2 3 Impellor Tip Velocity (m/s)  <  4  A) E f f e c t of A g i t a t i o n Rate on Impellor Gas Pumping Capacity and Oxygen Mass Transfer;  B) E f f e c t of  Surface Aeration of Impellor on Power Consumption (6-Bladed Radial Disc Impellor - 58mm diameter)  8 x o5  1  £  X  £ o az  Wkg/hT ( N ^ ) S 0 2  1-0  4  Full Baffles  * o  a:  6 3 3  c o  0.  |2 &  0-5  e $ 11  c  0)  o> >> K  O  01  1  I4kg/ri  3  2 3 Impellor Tip Velocity  (NH ) S0 4  Full Baffles  2  4  (m/s)  4  6BR58  I  15  Ungassed A  c  o  3 O  u tt  * I  5  tt CL  E  w  Figure 28.  0  I  2 3 Impellor Tip Velocity (m/s)  4  A) E f f e c t of A g i t a t i o n Rate on Impellor Gas Pumping Capacity and Oxygen Mass Transfer;  B) E f f e c t of  Surface Aeration of Impellor on Power Consumption (6-Bladed  Radial Impellor - 58mm diameter)  116  X  2 5 I4kg/n  3  (NH ) S0 4  2  50mm 4  Full Bottles  6BA58  o 10  o  or  o  or  E  01  3  Q. in o  I  §  0-5  2  &  2  e a> a> >»  «E >  I  in in a  1  O  IT  1  2 3 4 Impellor Tip Speed (m/s)  Wkg/ta' ( N ^ j S C ,  Full Baffles  * 10  50mm 68A58  Ungassed A  o. E  3  in c o O v  I 0)  Q.  E  1  Figure 29.  2 3 4 Impellor Tip Speed (m/s)  A) E f f e c t of A g i t a t i o n  Rate on Impellor Gas Pumping  Capacity and Oxygen Mass Transfer; B) E f f e c t of Surface Aeration of Impellor on Power Consumption (6-Bladed  A x i a l Impellor - 58mm diameter)  o  T  £ 5  o  -68A58« 6BR58c  4-  or  ? "5. £  5  O  117  g  l4kg/m (N^LS0 s  &  50 mm 60 mm  E  o  4  Full Baffles  IO o. 4) O  3-  or tn  §  &2-  0-5 »i in o  2  c a>  « or  -lAl~ 2 3 Impellor Tip Velocity Kkg/m {NH ) S0 3  4 2  4  Full Baffles  50mm 60mm  (m/s)  4  6BA58 6BR58  15 Ungassed / e  .2 Q.  E 3 in e o O  10  a>  s. a  a.  E  0  I 2 3 Impellor Tip Velocity  A) E f f e c t of Agitation  (m/s)  4  Rate on Impellor Gas Pumping  Capacity and Oxygen Mass Transfer; B) E f f e c t of Surface Aeration of Impellor on Power Consumption (Standard Dual Impellor - 58mm diameter)  118  Figure 31.  E f f e c t of High Oxygen P a r t i a l Pressures on Oxygen Mass Transfer Rate (58mm diameter)  119  TABLE 1 EFFECT OF IMPELLOR TYPE  IMPELLOR TYPE  AGITATION RATE  VOLUMETRIC POWER  RELATIVE MASS TRANSFER EFFICIENCY  m/s  kW/m  Single 6-Bladed Radial Disc - 46 cm  4.05  0.558  0.0341  Single 4-Bladed Radial - 46 cm  4.05  0.558  0.0054  4.05  0.224  0.0027  4.05  0.832  0.0059  Single 4-Bladed 45°Pitched Blade A x i a l - 46 cm Standard Autoclave Configuration - 46 cm  VOLUMETRIC POWER:  (kg0 /m »min)/(kW/m )  3  3  3  2  The power draw of the impellor i n a i r has been subtracted = 0.566 kW/m 3  SINGLE IMPELLOR IMMERSION DEPTH = 23.9 cm = 9.0 inches  120  TABLE 2 EFFECT OF IMPELLOR IMMERSION  DEPTH OF IMMERSION  IMPELLOR TYPE 23cm kW/m  RMTE  kW/m  RMTE  kW/m  RMTE  0.558  0.0341  0.929  0.0078  1.067  0.0007  0.558  0.0054  0.477  0.0050  0.495  0.0012  0.224  0.0027  0.268  0.0015  0.260  0.0008  0.832  0.0059  3  6-Bladed Radial Disc - 46 cm Single 4-Bladed Radial - 46 cm Single 4-Bladed 45°Pitched Blade A x i a l - 46 cm  STD Autoclave Configuration - 46 cm  46cm  34 cm 3  ^RMTE: Relative Mass Transfer E f f i c i e n c y AGITATION RATE = 4.05 m/s = 169 rpm  3  (kg0 /m •min)/(kW/m ) 3  2  3  121  TABLE 3  CRITICAL TIP VELOCITY CORRELATION  IMPELLOR DEPTH (m)  EXPERIMENTAL  PREDICTED  VALUE  VALUE  1  (m/s)  V critical  (m^)  V critical  0.229  2.42  2.12  0.343  2.93  2.59  0.457  3.27  2.99  1.  Refer to Figure 16.  2.  Equation 2.4  2  122  TABLE 4  IMPELLOR TYPE  EFFECT OF IMPELLOR DIAMETER  VOL. POWER (kW/m ) 3  RMTE kg0 /m 'min J  2  (  kW/m  3  '  AGITATION (m/s)  6-Bladed Disc - 53 cm  1.200  0.0188  4.05  6-Bladed Disc - 46 cm  0.558  0.0341  4.05  4-Bladed Radial - 53 cm  0.811  0.0107  4.05  4-Bladed Radial - 46 cm  0.558  0.0054  4.05  4-Bladed 45°Pitched Blade -53cm  0.403  0.0012  4.05  4-Bladed 45°Pitched Blade -46cm  0.224  0.0027  4.05  0.832  0.0059  4.05  STD Autoclave Configuration  *  RMTE: Relative Mass Transfer E f f i c i e n c y .  123  TABLE 5  BAFFLE  EFFECT OF HALF BAFFLES ON MASS TRANSFER  AGITATION  POWER  VOL. POWER  RMTE kg0 /m •min  LENGTH  3  2  (m/s)  (kW)  (kW/m ) 3  (  — >  kW/m  3  HALF  4.05  0.388  0.224  0.0080  FULL  4.05  0.388  0.224  0.0027  HALF  6.30  1.268  0.731  0.0115  FULL  6.30  1.748  1.008  0.0071  46 cm-FOUR-BLADED IMPELLOR 45°-PITCHED IMPELLOR  RMTE: Relative Mass Transfer E f f i c i e n c y  124  TABLE 6 EFFECT OF IMPELLOR IMMERSION ON HALF BAFFLES  DEPTH OF IMMERSION  BAFFLE LENGTH 23cm  34cm  46cm  kW/m  RMTE  kW/m  RMTE  kW/m  RMTE  HALF  0.224  0.0080  0.221  0.0050  0.260  0.0027  FULL  0.224  0.0027  0.268  0.0015  0.260  0.0008  3  3  RMTE: Relative Mass Transfer E f f i c i e n c y  3  (kg0 /m •min)/(kW/m )  AGITATION RATE = 4.05 m/s = 169 rpm 46 cm-Four-Bladed 45°-Pitched Blade Impellor  3  2  3  125  TABLE 7  OXYGEN DEPLETION IN GAS BUBBLES  OXYGEN CONCENTRATION (% VOLUME)  HEADSPACE GAS  20.6  REACTED BUBBLES  13.0  53cm- 6-BLADED RADIAL DISC IMPELLOR-UPPER 46cm- 4-BLADED RADIAL IMPELLOR-LOWER IMPELLOR TIP SPEED =2.90 m/s - UPPER 2.49 m/s - LOWER OXYGEN MASS TRANSFER RATE = 0.0061 kg0 /m »min 3  2  126  TABLE 8  EFFECT OF OXYGEN CONCENTRATION  OXYGEN MASS TRANSFER RATE (kgO/m •min) 3  2  AIR SPARGE  0.0327  PURE OXYGEN SPARGE  0.1137  46cm- 6-BLADED RADIAL DISC IMPELLOR-UPPER 46cm- 4-BLADED RADIAL IMPELLOR-LOWER IMPELLOR TIP SPEED =4.05 m/s  TABLE 9  EFFECT OF OXYGEN CONCENTRATION  OXYGEN MASS TRANSFER RATE(kg0 /m min) 3  2  AIR SPARGE  0.0409  PURE OXYGEN SPARGE  0.1277  53cm- 6-BLADED RADIAL DISC IMPELLOR-UPPER 46cm- 4-BLADED RADIAL IMPELLOR-LOWER IMPELLOR TIP SPEED = 4.72, m/s - UPPER 4.05 m/s - LOWER  128  TABLE 10.  SMALL DIAMETER-HIGH SPEED IMPELLOR EXPERIMENTS  Small High Speed Impellor Standard  Oxygen Mass Transfer Rate (g/1 0 /min)  Agitator Speed (m/s)  2  3-93  00029  7-98  0-0068  405  00049  _]  140 cm  e-  ~~f  6B-RD-6 1 22-9 cm  T  • 4B-A-I8 4B-A-I8:  Tip Speed = 2 4 9  6B-RD-6  6Bladed radial disc impellor  4B-A-I8  4Bladed  axial  impellor  m/s 15-2 cm diameter  4 5 8 cm diameter  129  TABLE 11 EFFECT OF SPECIAL SPARGER MODE  SPARGER MODE  kW/m  RMTE  SPARGER ABOVE  0.850  0.0369  BOTTOM IMPELLOR  0.929  0.0290  AVERAGE  0.890  0.0330  SPARGER BELOW  0.890  0.0327  3  BOTTOM IMPELLOR  RMTE: Relative Mass Transfer (kgO /m 'min)/(kW/m ) 3  2  3  Efficiency  130  TABLE  EFFECT  OXYGEN  DIAMETER  (mm)  OF  MASS  IMPELLOR  RELATIVE  TRANSFER  RATE  PUMPING  ( k g 0  m i n )  ( m  2  / m  3  3  DIAMETER  GAS  POWER  RATE  CONSUMPTION  / m i n )  (W)  40  0 . 0 0 4 2  0.0011  2 . 5 5  58  0 . 0 0 7 9  0.0022  4 . 5 0  6-BLADE  TIP  12  RADIAL  SPEED  =  DISC  2.82  m/s  IMPELLOR,  ,  DEPTH  FULL  OF  BAFFLES  IMMERSION  =  50  mm  131  TABLE 13  EFFECT OF BAFFLE LENGTH  BAFFLE  OXYGEN MASS  RELATIVE GAS  POWER  LENGTH  TRANSFER RATE  PUMPING RATE  CONSUMPTION  (kg0 /m min)  (m /min)  (W)  3  2  3  HALF  0.0100  0.0026  FULL  0.0134  0.0053  2.75  1  4.60  6-BLADE RADIAL DISC IMPELLOR, 58 nm DIAMETER TIP SPEED = 3.26 m/s , DEPTH OF IMMERSION = 50 mm  1.  Extrapolated Value  132  TABLE 14  ALTERNATE DUAL IMPELLOR CONFIGURATION  IMPELLOR  OXYGEN MASS  RELATIVE GAS  POWER  CONFIGURATION  TRANSFER RATE  PUMPING RATE  CONSUMPTION  (kg0 /m m i n )  (m /min)  (W)  3  2  Standard  0.0118  3  10.4  0.0040  Configuration  *  6-Bladed Radial Disc-Upper/  0.0123  0.0032  10.3  6-Bladed Radial -Lower  FULL BAFFLES , 58 mm DIAMETER TIP SPEED *> 3.28 m/s , DEPTH OF IMMERSION = 50 mm  -UPPER  = 110 mm -LOWER * Extrapolated Value  133  TABLE 15  EFFECT OF SOLIDS  SOLID  OXYGEN MASS  RELATIVE GAS  POWER  CONCENTRATION  TRANSFER RATE  PUMPING RATE  CONSUMPTION  (kg/m )  (kg0 /m •min)  (m /min)  (W)  3  3  2  3  0  0.0113  0.0038  9.50  46  0.0104  0.0036  9.33  STANDARD DUAL IMPELLOR CONFIGURATION, 58 mm DIAMETER, FULL BAFFLES TIP SPEED - 3.23 m/s , DEPTH OF IMMERSION - 50 mm  -UPPER  = 110 mm -LOWER  134  APPENDIX A MIXING MODEL EXPERIMENTS Below a r e shown t h e t h r e e t y p e s o f i m p e l l o r s used d u r i n g t h e p i l o t - s c a l e work a t C o m i n c o * s T e c h n i c a l R e s e a r c h c e n t r e ( t h e d i a m e t e r s v a r i e d between 46cm and 53cm):  4-Bloded Axial Impellor  m  TJ  -23cm-  4-Bladed Radiol Impellor  -265cm—  •19cm15 I cm - 2 3 ccm-J 6- Bladed  Radiol  Disc Impellor  —26-5cm—  75em  135  APPENDIX B SPARGING MODES F o r c e r t a i n e x p e r i m e n t s i n t h e m i x i n g model t h e r e was s p a r g i n g used.  One s p a r g e r was d e s i g n e d t o r e s t on t h e bottom o f t h e v e s s e l and  s p a r g e f r o m u n d e r n e a t h t h e bottom i m p e l l o r .  The o t h e r s p a r g e r was made  so t h a t gas c o u l d be s p a r g e d f r o m j u s t a b o v e t h e l o w e r i m p e l l o r .  Mode-. Below Lower Impellor  Mode: Above Lower Impellor  136  APPENDIX C AIR AND OXYGEN FLOW DIAGRAM D i f f e r e n t m i x t u r e s o f oxygen and n i t r o g e n were used i n t h e m i x i n g model e x p e r i m e n t s schematic  t o v a r y t h e oxygen c o n c e n t r a t i o n .  Relow i s a p i p i n g  showing t h e placement o f v a l v e s and gauges.  Valve Pressure Gauge  To Headspace Purge  Xo  Rotameters •  (X) Valve  o  Oxygen Cylinder  ®  Pressure Gouge  Valve  6  Compressed Air or Oxygen Cylinder  To Sparger  137  APPENDIX D BENCH-SCALE EXPERIMENTS Below a r e shown t h e t h r e e t y p e s o f i m p e l l o r s u s e d d u r i n g t h e experiments  a t U.B.C. ( t h e d i a m e t e r s v a r i e d between 40mm and 58mm):  6-Bloded .Axial Impellor  'J2 7-5 —20mm—  6-Bloded Radial Impellor  6-Bladed Radial Oisc Impellor  • 29mm —  Tim  138  APPENDIX E POWER MEASUREMENTS In t h e b e n c h - s c a l e e x p e r i m e n t s power measurements were made f r o m t h e t o r q u e m e a s u r i n g d e v i c e shown s c h e m a t i c a l l y b e l o w .  Knowing t h e  r o t a t i o n a l speed o f t h e i m p e l l o r a n d t h e a p p l i e d t o r q u e t h e power was c a l c u l a t e d as f o l l o w s : s p r i n g b a l a n c e r e a d i n g : lOOg o r 0.1kg arm l e n g t h : 0.1m r o t a t i o n a l s p e e d : 900 rpm Power:  0.1kg x 0.1m x 9.81m/s x 2 9.25  Watts  x 900rpm x 1/60  139  APPENDIX F VESSEL DESIGN FOR MEASURING IMPELLOR GAS PUMPING RATES B e l o w i s shown a s e c t i o n a l d r a w i n g o f t h e 2 0 l i t r e v e s s e l u s e d t o m e a s u r e t h e r e l a t i v e i m p e l l o r gas pumping r a t e i n t h e bench experiments.  scale  A s e a l e d gas plenum e x i s t e d above t h e l i q u i d , e x c e p t f o r  t h e g a s s a m p l i n g p o r t w h i c h c o u l d be u s e d t o draw gas samples f r o m t h e plenum.  By d r a w i n g t h e gas o u t o f t h e plenum t h r o u g h a r o t a m e t e r a t a  r a t e t h a t k e p t t h e l i q u i d l e v e l c o n s t a n t t h e s t e a d y s t a t e i m p e l l o r gas pumping r a t e c o u l d be r e a d from t h e r o t a m e t e r :  Plenum Gas Sampling Port "150mm"  100  mm  Gas Seal Around Perimeter  •o w u a a. cn  •130  mm"  'Liquid Sampling Port O  00  c  01 u  8  el i  •f Each Set of Four Baffles is Removable  300  mm  140 APPENDIX 6 POWER DRAW CALCULATION FOR SMALL DIAMETER HIGH SPEED IMPELLOR  Impellor r o t a t i o n a l speed = N Impellor blade width = w Density = ^ P Newton's law conversion f a c t o r = g Power = P Impellor Diameter = D Power number = N„  P = 4.63 x 10 -16 N  PC  For a small i m p e l l o r (15.2 cm) 6-bladed d i s c i m p e l l o r (w/D=l/5)  N  P  = 5.0  f  D  N = lOOOrpm P = 4.63 x 1 0 "  16  1.0 15.2 cm (6 inches)  (5.0)(1.0)(1000) (15.2) 3  5  = 1.90 kW = 2.55 Hp  Assuming the motor has a 90 % e f f i c i e n c y , a 2.11 kW motor i s required.  141  APPENDIX H C a l c u l a t i o n o f Oxygen Consumption Rate in Anaconda A r b i t e r Process  Reference:  Kuhn, M.C., e t a l ; CIM B u l l e t i n ( F e b . 1974), p62-74  Temperature = 60-90 °C Residence Time = 5 hours 0 P a r t i a l Pressure = 34kPa = 0.34 atm 2  Weed Concentrate Composition: wt % Cu Fe % Extraction:  26.6 . 95.0  S  21.4 25.0  The i r o n i s e x t r a c t e d as  33.2 26.7  Fe(0H)g. 3  The f i n a l s o l u t i o n has a copper c o n c e n t r a t i o n o f 45.25 kg/m . 45.25 kg/mn  kg concentrate/m = 3  0.95 S o l u t i o n Composition (kg/m ):  x  3  Cu Fe S  ^  3  0.266  Cu  FeCFeCOH)^)  45.25 Oxygen Demand:  ^ ^  9.58  45.25 x 16/63.5 = 9.58 x 24/55.8 = 15.87 x 48/32.0 =  S 15.87  11.40 4.12 23.81 3  Total Oxygen Demand  =  39.33  =  0.39  3  Total Oxygen Demand (kg O^/m min atm)  kg/m  142 APPENDIX I Calculation of Oxygen Consumption Rate in Sherritt Gordon Ammonia Leach Reference:  Forward, F.A., et a l ; Trans AIME, vol 203 (1955) p457 Nashner, S.; CIM Bulletin, vol 58, (1955) p212  Temperature = 70-80°C Residence Time = 19.2 hours 0 Partial Pressure = 68kPa = 0.68 atm 2  Concentrate Composition: Ni 12.0 Extraction % 95.0  wt % Fe  S  30.0 60.0  28.0 60.0  The iron is assumed to extract as FeCOrl)^ and the S as S0^~.  3  The final solution  has a nickel concentration of 45.0 kg/m .  kg concentrate/m  3  45.0 kg/m  3  =  = 395 kg/m  3  0.95 x 0.12 Solution Composition: (kg/m ) 3  Oxygen Demand:  Ni Fe S  Ni  Fe(Fe(0H) )  45.0  71.1  45.0 x 16/58.7 = 71.1 x 24/55.8 = 66.4 x 48/32.0 =  Total Oxygen Demand = 3  Total Oxygen Demand (kg O^/m min atm) =  3  S(S0 ) 4  66.4  12.3 30.6 99.6 142.5 0.18  3  kg/m  143 APPENDIX J Calculation of Oxygen Mass Transfer Rate in Cominco Zinc Pressure Leach References:  Parker, E.G.; CIM B u l l e t i n (May 1981), p 145 Parker, E.G., et a l ; Proc 3rd Inter. Symp. on Hydrometallurgy, AIME (March 1983), p927  Temperature = 145-155°C Residence Time (1st Compartment) = 26min O2 Partial Pressure 750kPa =7.5 atm Concentrate Composition:  wt %  Fe % Extraction  11.5 82!o(Fe )  S 30.5 8o!3(S°)  3+  Pb  Zn  6.5 82.0  49.0 82!o  It i s assumed that 2.4% of the sulphur reacts to form S0^~. 3  The f i n a l solution has a zinc concentration of 120 kg/m in the 1st compartment and an i n i t i a l concentration of 50 kg/m in the feed. 3  1 2 0  kg concentrate/m - =  Oxygen Demand:  "  5 0  0.82 x 0.49  3  = 174 kg/m  Zn (120-50) x 16/65.4 Fe (0.115x174x0.82) x 24/55.8 Pb: {0.065x174x0.820 x 16/207 S(S0 ) (2.4/100x174x0.305) x 64/32.0 Total Oxygen Demand 4  3  Total Oxygen Demand (kg 02/m min atm)  = 0.14  = 17.13 = 7.06 = 0.72 = 2.55 = 27.46  kg/m 3  

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