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Original copy with light type SrfU 1 » I nt : £ ri*no>* i'vçTSiry / V IK 11 X M: i The simultaneous precipitation of calcium carbonate and magnesium hydroxide in the water softening process by Robert William Peters A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Major: Chemical Engineering A -rs-rrv^rs^ro/^ Signature was redacted for privacy. Signature was redacted for privacy. In C f Major Work Signature was redacted for privacy. For the Major Department Signature was redacted for privacy. Icwa State University Ames, Iowa 1950 Copyright (5) Robert William Peters, 1980. All rights reserved. ii TABLE OF CONTENTS Pàge NOMENdATORE xvi ABSTRACT xix ESEDICATION xxii INTRODUCTION 1 LITERÂÏUSE REVIEW AND THEORETICAL BACKGHOUîîD 7 Lime-Soda Ash Water Softening Reactions 7 Early Water Softening Studies 1? The Water Softening Process 19 Excess lime treatment 25 Split treatment 28 Split treatment with sludge recycle 31 Use of water treatment systems 35 Limitations on Hardness Removal 35 Magnesium removal 35 Calcium removal 37 Ionic strength of solution 43 Saturation Index 47 Alkalinity 49 Hydroxide; carbonate- and bicarbonate al]%lin- ity Calculation from alkalinity measurements alone 55 Kvn «4 a nr\'] v 56 Carbonate only 57 Hydroxide-carbon ate <v Carbonate-bicarbonate 58 Bicarbonate only 58 Calculation froa alkalinity pl—s pH measure- ^ ments iii Hydroxide 59 Carbonate 59 Bicarbonate 60 Calculation from equilibrium equations 60 Carbon dioxide, alkalinity, and pH relation ships in natural waters 62 Saturation and Supersaturation 63 Nucleation 66 Primary nucleation 67 Secondary nucleation 68 Effect of agitation on nucleation 70 The Population Balance 72 Dependence of Kinetics on Supersaturation 80 cTvo-nc;T rsr> Tio-ne'i+'vr "fho Tîl-P'Pan+.c n"P saturation 82 Effects of Impurities on Crystallization Kinetics and Crystal Morphology 87 Crystallization Kinetic Studies for Calcium Carbonate and Magnesium Hydroxide 96 Studies on the Precipitation of Calcium Carbonate in the Presence of Mg"^' Ion 111 Miscellaneous Studies and Observations on Cal- . r "**• - ^ T"..- r\ u-LUiii uc a.xn-4. uuii ju. w wu-t-LC —J.;/ -pTj—• nT3 -1 ?o EXPE?.I''IENTAL 132 w wCw I I II ^ ^ ^A.- - ^ ^ ^ i I Feed tanks and pumps 1% Storage tanks 1^9 In-line filters 151 Puimps 152 Constant temperature baths 153 Rotameters 153 pH meters 15^ iv Water supply 155 Oven 156 Conductivity meter I56 Filter paper I56 Titration eq^uip'^ent 156 Desiccator 157 Timer 157 Gas dispersion tubes 15/ Wet test meter 157 Equilibrium study pump 157 Miscellaneous glassware 158 Pressure filter 158 Cheïïiicals 158 Miscellaneous Equipment and Services léO Oxygen meters 160 Seaiiiiixig electron microscope I6I Atomic absorption spectrophotometer I6I X-ray diffraction unit I6I Experimental Design 162 Experimental Procedure I65 Feed tank preparation I66 Titrations I68 Feed stream pumping to holding tariks 170 Flow rate determination 173 Start up 175 operating conairions i?5 Conductivity measurements 177 Alkalinity measurement ISO Sampling for iron analyses 181 Coulver counter sampling 132 Squi Hbrium measurements I85 Hun shu'tdown and clean up IS5 ANALYSIS OF DATA. 189 Coulter Counter Data Reduction 189 +TnA'nT' rro T)*? c+'i-'î "hiT^T riri T)Q-f-.a i Cii» O - -, r:\jyjyf. # _ w J. J. J» ^\jC^ Alkalinity Data 206 Conductivity Data 209 Activity Coefficients 212 Solubility Product 213 Mass Balances 2l4 Suspension Density Measurement 222 Filtration 223 Material "balance 224 Tiiird moment 225 RESULTS AND DISCUSSION 228 Crystal Morphology of Simultaneously Precipi tated Calcium Carbonate and Magnesium Hydroxide 228 Dissolved Oxygen Concentration 24-9 Reactor Vail Deposition 250 Kinetic Measurements 252 Suspension Density 281 Effects and Applications of the Kinetic Order 284 Effects of supersaturation on crystal size distribution 284 Effect of suspended solids on crystal size distribution 291 Comparison with Results of Previous Studies 296 Hardness Removal 299 Supersaturation Measurements 314 Mass Balances 3^7 Suspension and deposition aliocations 3^7 C* C* vi Miscellaneous Experimental Data 33^ STATISTICAL ANALYSIS 335 nVJJL V M LVUii i-lCL. J—UC n -» _ J. • OOOf "L"^ ^ /% rVw^ o"** ^ ^ 4» 4 yw^ Ç» coNcnjsiONS 350 RECOMMENDATIONS 356 BIBLIOGRAPHY 3^1 ACKNOWLEDGMENTS 377 APPENDIX A 379 Calibration Curves 379 APPENDIX B 398 Suspension Allocation 398 Reactor Wall Deposition Allocation 400 APPENDIX C 405 Nuclei Density - Growt-h Rate Relationships 405 ATïïspxmxY n LLor\ Miscellaneous Experimental Data 420 vii LIST OF TflJBLES Page Table 1. Equations used to describe the experimental c> cTirc+o Tnc; 13 Table 2. Equations used to describe the experimental system in this study 15 Table 3. Principal cations causing hardness in water and the major anions associated with them 21 Table 4. Degree of hardness classifications 22 Table 5. Reported solubility products for magnesium hydroxide (brucite) 38 Table 6. Reported solubility products for calcium car 40 bonate Table 7. Relative surface areas of various water soften ing reactors 141 Table 8. Crystal size distribution in run 33= Dilution = 10:1, sample size = 2.0 ml. Aperture = 280 pim 192 Table 9. Average cr^'stal size distribution in run 33 193 Table 10. Void fraction estimates 226 Table 4 -i Densities of the individual cornponftnts 22? Table 12. Kinetic data for individual runs 255 Table 13. Kinetic order values 257 Table 14. Comparison of crystallization kinetics before and after aragonite to calcite transformation 2? 3 Table 15. Comparison of suspension density measurements 282 Table 16. Comparison cf results for <0 percent increase in residence time for various kinetic distributions 268 Table 17. Comparison of actual increase in n°; G,- and L_ 290 Table 18. Percent increase in n°. G. and for a 10-fold viii Table 19. Comparison of suspension density effects on the crystallization kinetics 295 Table 20. Summary of experimental kinetic expressions for the calcium carbonate and magnesium hydroxide sys-cems 297 Table 21. Hardness data of feed conditions for individual experimental runs 300 Table 22. Residual hardness at "pseudo" steady state con dition Initial magnesium hardness ~ 55 ppm Initial calcium hardness ~ 230-310 ppm (de pendent upon lime dosage) 302 Table 23- Ionic strength data 3^4 Table 24. Summary of activity coefficient data 305 Table 2,5. Summary of iron concentration levels and iron removal efficiencies 307 Table 26. Softening efficiencies at "pseudo" steady state conditions ^08 Table 2?. Overall residual hardness and softening efficiencies 311 Table 28. Supersaturation data 315 Table 29. Relative supersaturation results JlS Table 30. Summary of deposition and suspension density (by filtration) 320 Table 31» Effect of iron addition on the wall deposition 323 Table 32. Percent error for nass b-alance clos''jre 325 Table 33- Summary of alkalinity distribution results 330 Table 34. O-zerall alkalinity distribut'" in results 332 Table 35. TeSTs for kineTic order Null hypothesis - ^2 Alternative hypothesis r.^9 336 kinetic rate constant Null hypothesis ix Table 37. Summary of growth rate correlation with separate growth rates of CaCO^ and Mg(OH)g 33? Table 38.