The simultaneous precipitation of calcium carbonate and magnesium hydroxide in the water softening process

Retrospective Theses and Dissertations 1980 The simultaneous precipitation of calcium carbonate and magnesium hydroxide in the water softening proce...
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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

Follow this and additional works at: http://lib.dr.iastate.edu/rtd Part of the Chemical Engineering Commons Recommended Citation Peters, Robert William, "The simultaneous precipitation of calcium carbonate and magnesium hydroxide in the water softening process " (1980). Retrospective Theses and Dissertations. Paper 6800.

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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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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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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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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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