Retrospective Theses and Dissertations
1980
The simultaneous precipitation of calcium carbonate and magnesium hydroxide in the water softening process Robert William Peters Iowa State University
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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 Split treatment Split treatment with sludge recycle Use of water treatment systems Limitations on Hardness Removal Magnesium removal Calcium removal Ionic strength of solution
25 28 31 35 35 35 37 43
Saturation Index
47
Alkalinity
49
Hydroxide; carbonate- and bicarbonate al]%linity Calculation from alkalinity measurements alone Kvn «4 a nr\'] v Carbonate only Hydroxide-carbonate Carbonate-bicarbonate Bicarbonate only Calculation froa alkalinity pl—s pH measurements
55 56 57 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 Miscellaneous Studies and Observations on .r
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Calr\
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EXPE?.I''IENTAL
132
w wCw I
I II
^ ^ ^A. - -^
^ Feed tanks and pumps Storage tanks In-line filters Puimps Constant temperature baths Rotameters
^ iI 1% 1^9 151 152 153 153
pH meters
15^
iv
Water supply Oven Conductivity meter Filter paper Titration eq^uip'^ent Desiccator Timer Gas dispersion tubes Wet test meter Equilibrium study pump Miscellaneous glassware Pressure filter
155 156 I56 I56 156 157 157 15/ 157 157 158 158
Cheïïiicals
158
Miscellaneous Equipment and Services
léO
Oxygen meters Seaiiiiixig electron microscope Atomic absorption spectrophotometer X-ray diffraction unit
160 I6I I6I I6I
Experimental Design
162
Experimental Procedure
I65
Feed tank preparation Titrations Feed stream pumping to holding tariks Flow rate determination Start up operating conairions Conductivity measurements
I66 I68 170 173 175 i?5 177
Alkalinity measurement
ISO
Sampling for iron analyses Coulver counter sampling SquiHbrium measurements Hun shu'tdown and clean up
181 132 I85 IS5
ANALYSIS OF DATA.
189
Coulter Counter Data Reduction +TnA'nT'
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189 i Cii»
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Alkalinity Data
206
Conductivity Data
209
Activity Coefficients
212
Solubility Product
213
Mass Balances
2l4
Suspension Density Measurement
222
Filtration Material "balance Tiiird moment RESULTS AND DISCUSSION
223 224 225 228
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 Effect of suspended solids on crystal size distribution
284 291
Comparison with Results of Previous Studies
296
Hardness Removal
299
Supersaturation Measurements
314
Mass Balances
3^7
Suspension and deposition aliocations C* C*
3^7
vi
Miscellaneous Experimental Data STATISTICAL ANALYSIS n
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n
33^ 335
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coNcnjsiONS
350
RECOMMENDATIONS
356
BIBLIOGRAPHY
3^1
ACKNOWLEDGMENTS
377
APPENDIX A
379
Calibration Curves APPENDIX B
379 398
Suspension Allocation
398
Reactor Wall Deposition Allocation
400
APPENDIX C Nuclei Density - Growt-h Rate Relationships ATïïspxmxY n Miscellaneous Experimental Data
405 405 LLor\
420
vii
LIST OF TflJBLES Page Table 1. Equations used to describe the experimental c>
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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 bonate
40
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
22?
Densities of the individual cornponftnts
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 32), and calcite calcium "w-CSr
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precipitated in this research %s the aragonite form, with a
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amount of the calcine formed, "rne percentage of calcite crystals in creased by decreasing the
T/? ratio. The brucite form of sagnesium
xxi
hydroxide formed discrete spherical crystals with a flaked structure. The dendritic structure observed for the crystals Indicated a large concentration gradient, due to the supersaturation levels involved.
xxii
DEÎDIGATION
TVixs thesis is dedicated to the memory of Dr. John D, Stevens, who died April 1. I98O.
He served as a constant inspiration on the
project. His insight into the water softening process, crystallization, and water chemistry
proved very helpful. His many thought-provoking
questions were inspirational. His desire for quality data and reports -Tsre reminders of the responsibility of the engineer in society. In his role as major professor, he always had an open door policy of d , V c L X X U X ' W i i a u x uo. u x w i i cLijix the author respected very highly.
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1
INTRODUCTION
The purification and softening of water may "be accomplished by dif ferent methods, depending on the prospective use. Softening is the term applied to processes which remove or reduce the hardness of water. Pur ification, as distinguished from softening, generally refers to the re moval of organic matter and microorganisms from water. Clarification may be important, and may be combined with cold water softening by pre cipitation. More than I3OO municipalities (172) have adopted softening (by precipitation techniques) as part of their water treatment, but ths public as yet does not realize the economic advantages of the process. The savings in soap alone are often more than sufficient to pay for the cost (172), although these savings have been greatly reduced by the re placement of soap by synthetic detergents for washing purposes. Other advantages of soft water, namely, comfort and convenience to the consumer and elimination of the need for the consumer to construct cistern sup plies, are essentially free to tne user.
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chemical treatment to soften water is of prime importance»
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hardened water presents no known health hazard, although.the taste and color may be objectionable to the consumer. Hard water generally con tains objectionable amounts of dissolved salts of calcium and magnesium. TVickso a
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