Thermodynamics of Sodium Aluminosilicate Formation in Aqueous Alkaline Solutions relevant to ...

235 Pages·2009·9.47 MB·English

Thermodynamics of Sodium Aluminosilicate Formation in Aqueous
Alkaline Solutions relevant to Closed-cycle Kraft Pulp Mills
by
HYEON PARK
B.S., Hanyang University, Korea, 1989
M.S., Hanyang University, Korea, 1993
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUESTMENTS FOR THE DEGREE OF
DOCTOR OF PHTLOSPHY
in
THE FACULTY OF GRADUATE STUDIES
Department of Chemical and Bio-Resource Engineering
We accept this thesis as conforming
to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
August, 1999
©HyeonPark, 1999
in presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, I agree that the Library shall make it
freely available for reference and study. I further agree that permission for extensive
copying of this thesis for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is understood that copying or
publication of this thesis for financial gain shall not be allowed without my written
permission.
Department of 0)CVY\ Iced? W fifa-JxW-^u^ Krg<h ^jzr-l^j
The University of British Columbia
Vancouver, Canada
Date /Uj.J^,
DE-6 (2/88)
11
ABSTRACT
Accumulation of Al and Si ions in the recovery cycle of a kraft pulp mill may
cause sodium aluminosilicate scale formation. This glossy scale forms on process
equipment and is very hard to remove. Thus, formation of the scale can create several
operational problems in mills moving towards progressive system closure and should be
prevented. The purpose of this study is to supply: (a) data on the precipitation conditions
of sodium-aluminosilicates in green and white liquors of the recovery cycle; and (b) a
model to predict such conditions. The data can either be used directly or to test process
models for the design and optimization of progressive system closure strategies.
The precipitation conditions of sodium aluminosilicates in synthetic green and
white liquors at 368.15 K (95 °C) were determined. In the experiments, the effects of
varying the Al/Si ratio and concentrations of OH", C02", SO42', and HS" were studied.
3
The structure of the precipitates was identified by X-ray diffraction, thermogravimetry
and chemical analysis. The precipitates were found to have the structure of
hydroxysodalite dihydrate (Na8(AlSi04)6(OH)-2H20) except in a simulated green liquor
2
system with low OH" and high Cf concentrations where sodalite dihydrate
(Nag(AlSi04)6Cl2-2H20) was formed. The precipitation conditions in mill green and
white liquors at 368.15 K were also measured. The effects of varying the Al/Si ratio,
NaOH, Na2C03, and Na2S concentrations were studied. The precipitates were found to
have the structure of hydroxysodalite dihydrate.
A thermodynamic model for sodium aluminosilicate formation in aqueous
alkaline solutions was developed. Pitzer's method was adopted to calculate the activity of
Ill
water and the activity coefficients of the other species in solution. The system under
consideration contained the ions of Na+, Al(OH)', Si02", OH", C02", S02', Cl', HS"
4 3 3 4
dissolved in water and in equilibrium with two possible solid phases (sodalite dihydrate :
Na8(AlSi04)6Cl2-2H0 and hydroxysodalite dihydrate : Na(AlSi0)(OH)2- 2H0) at
2 8 4 6 2
368.15 K. The equilibrium constants of sodalite dihydrate and hydroxysodalite dihydrate
formation reactions were determined using the thermodynamic properties of the species
involved. Property values that were not available in the literature were estimated by
group contribution methods. The model calculates the molality of all species at
equilibrium including the amount of solid precipitates. The calculations were compared
with published data and were found to be in good agreement.
Meanwhile, since the system contains the Si032" and Al(OH)" ions, knowledge of
4
the relevant Pitzer's model parameters is required. Osmotic coefficient and water activity
data for Na2Si03 and mixed Na2Si03-NaOH aqueous solutions were obtained at 298.15 K
by employing an isopiestic method. The binary Pitzer's parameters, P(0), f3(1), and C*, for
Na2Si03 and the mixing parameters, and were estimated using the
Q - 2- ¥ - i-,
0H Si0 Na+OHsio
osmotic coefficient data. In addition, osmotic coefficient data were obtained for the
aqueous solutions of NaOH-NaCl-NaAl(OH). The solutions were prepared by
4
dissolving AICI36H2O in aqueous NaOH solutions. The osmotic coefficients of the
solutions were measured by the isopiestic method at 298.15 K. The osmotic coefficient
data were used to evaluate the unknown binary and mixing parameters of Pitzer's model
for the aqueous NaOH-NaCl-NaAl(OH) system. The experimental osmotic coefficient
4
data were correlated well with Pitzer's model using the parameters obtained.
iv
TABLE OF CONTENTS
ABSTRACT ii
TABLE OF CONTENTS iv
LIST OF TABLES viii
LIST OF FIGURES xi
ACKNOWLEDGEMENTS xiv
CHAPTER 1. INTRODUCTION 1
1.1. Environmental Impact of the Kraft Pulping Industry 3
1.2. Closed-cycle Mill Operation 4
1.3. Non-process Elements in the Kraft Pulping Process 5
1.3.1. Sources and profile of non-process elements 5
1.3.2. Adverse effects of non-process elements 9
1.4. Recent Developments of Closed-cycle Technology 10
1.5. Scale Formation in the Recovery Cycle 13
1.5.1. Sodium Aluminosilicate Scale Formation 15
1.6. Research Objectives 19
CHAPTER 2. THEORETICAL BACKGROUND 21
2.1. Thermodynamic Equilibrium Constant 21
2.2. Equilibrium Constant Calculation at Specific Temperature 23
2.3. Activity of Water, Osmotic Coefficient, and Activity Coefficient 24
2.4. Pitzer's Activity Coefficient Model 28
2.5. Isopiestic Method 32
CHAPTER 3. MATERIALS A ND METHODS 35
3.1. Solubility Experiments using Synthetic Liquors of System A 35
3.1.1. Experimental design 3 5
3.1.2. Experimental procedure 3 7
3.1.3. Identification of the precipitates 39
V
3.2. Solubility Experiments using Synthetic Liquors of System B 40
3.2.1. Experimental design 40
3.2.2. Experimental procedure and analysis 42
3.3. Solubility Experiments using Mill Liquors 42
3.3.1. Experimental design 43
3.3.2. Experimental procedure and analysis 44
3.4. Osmotic Coefficient Measurement for Na2Si03 and Na2Si03-NaOH 45
Systems
3.4.1. Identification of silicic species by a titration method 45
3.4.2. Apparatus and chemicals 46
3.4.3. Experimental procedure 49
3.5. Osmotic Coefficient Measurement for NaOH-NaCl-NaAl(OH) System 50
4
3.5.1. Apparatus and chemicals 50
3.5.2. Experimental procedure 51
CHAPTER 4. THERMODYNAMIC MODELING OF SODIUM 53
ALUMINOSILICATE FORMATION
4.1. Model Equations 53
4.2. Structure of the Thermodynamic Model 55
CHAPTER 5. OSMOTIC COEFFICIENT DATA FOR NaSi0, NaSi0-NaOH, 57
2 3 2 3
AND NaOH-NaCl-NaAl(OH) AQUEOUS SYSTEMS
4
5.1. Identification of Metasilicic Species by Titration 57
5.2. Mole Fraction of Metasilicic Species 63
5.3. Osmotic Coefficient Data for Na2Si0 Aqueous System 63
3
5.4. Osmotic Coefficient Data for NaSi0-NaOH Aqueous System 65
2 3
5.5. Osmotic Coefficient Data for NaOH-NaCl-NaAl(OH) Aqueous System 70
4
CHAPTER 6. DETERMINATION OF PITZER' S PARAMETERS FOR NaSi0, 73
2 3
NaSi0-NaOH, AND NaOH-NaCl-NaAl(OH) AQUEOUS SYSTEMS
2 3 4
6.1. Pitzer's Parameters for Na2Si0 and Na2Si0-NaOH Aqueous Systems 73
3 3
vi
6.2. Pitzer's Parameters for NaOH-NaCl-NaAl(OH) Aqueous System 76
4
6.3. Reliability of the Pitzer's Parameter Determination 81
CHAPTER 7. A PRIORI DETERMINATION OF MODEL PARAMETERS 84
7.1. Pitzer's Parameters 84
7.2. Equilibrium Constants 86
7.3. Estimation of Thermodynamic Properties 88
7.3.1. AHf° and AGf° of sodalite dihydrate and hydroxysodalite dihydrate 88
7.3.2. C° of Sodalite dihydrate and hydroxysodalite dihydrate 91
p
7.3.3. S° of Si02' ) 93
3 (aq
7.3.4. AH° of Si02"(aq) 95
f 3
7.3.5. C° of Si02- 96
p 3 (aq)
7.3.6. Calculation of K at 368.15 K 97
sod
7.3.7. Calculation of K d at 368.15 K 98
hso
7.4. Change of In Khsod with Temperature 99
CHAPTER 8. SOLUBILITY MAPS OF Al A ND Si IN GREEN A ND WHITE 101
LIQUORS
8.1. Synthetic Green and White Liquors of System A 101
8.2. Synthetic Green and White Liquors of System B 106
8.3. Mill Green and White Liquors 112
8.4. Structure of Precipitates from synthetic liquors 118
8.5. Structure of Precipitates from mill liquors 122
8.6. Morphology of Precipitates 122
CHAPTER 9. PREDICTION OF THE PRECIPITATION CONDITIONS OF 125
SODIUM ALUMTNOSILICATES
9.1. Na+- Al(OH)" - Si02- - OFT - C02" - S02- - Cf - H0 (System A) 126
4 3 3 4 2
9.2. Na+ - Al(OH)" - Si02" - OH" - C02_ - Cl' - HS" - H0 (System B) 135
4 3 3 2
CHAPTER 10. CONCLUSIONS A ND RECOMMENDATIONS 139
vii
NOMENCLATURE 142
BIBLOGRAPHY 146
APPENDIX I. Chemical Analysis by Atomic Absorption Spectrophotometer 159
APPENDIX II. Standard Errors and Confidence Intervals for Experimental 161
Solubility Data
APPENDIX III. Tables of Solubility Data 162
APPENDIX IV. Calculation of Titration Curve 165
APPENDIX V. Uncertainty of the Measured Osmotic Coefficient 167
APPENDIX VI. Uncertainty of the Estimated Thermodynamic Properties and 170
Equilibrium Constants
APPENDIX VII. Comparison of Pitzer's Parameters with Published Values 174
APPENDIX VIII. Sensitivity Analysis of Pitzer's Parameters 176
APPENDIX IX. Computational Source Codes in FORTRAN 77 178
Determination of Pitzer's binary parameters for single electrolyte system using 178
osmotic coefficient data
Calculation of osmotic coefficient and activity coefficient using Pitzer's binary 183
parameters for single electrolyte system
Determination of Pitzer's mixing parameters for multi-component electrolyte 186
system using osmotic coefficient data
Calculation of osmotic coefficient and activity coefficient using Pitzer's binary 193
and mixing parameters for multi-component electrolyte system
Computation of equilibrium state for NI^-NI^OH-Ff-HCl-NELiCl-Cr-Na*- 199
NaCl-K+-KCl system at 573.15 K
Computation of equilibrium state for sodium aluminosilicate formation 205
viii
LIST OF TABLES
Table 1.1 Amount of non-process elements from each source. 6
Table 1.2. Metal contents in the bleach effluent. 8
Table 1.3. Concentration range of process and non-process elements in green and 8
white liquors.
Table 3.1. Solubility experiment design for synthetic liquors of system A. 36
Table 3.2. Solubility experiment design for synthetic liquors of system B. 41
Table 3.3. Analysis results of mill liquors. 43
Table 3.4. Solubility experiment design for mill liquors. 44
Table 5.1. Osmotic coefficients and water activities for the Na2Si03 aqueous 66
system at 298.15 K.
Table 5.2. Osmotic coefficients and water activities for the Na2Si0-Na0H 68
3
aqueous system at 298.15 K.
Table 5.3. Osmotic coefficients of NaCl and KC1 as reference and standard 71
solutions at 298.15 K.
Table 5.4. Osmotic coefficients of NaOH-NaCl-NaAl(OH)4 aqueous solutions at 72
298.15 K.
Table 6.1. The Pitzer's parameters of Na2Si0 and NaSi0-NaOH systems at 75
3 2 3
298.15 K.
Table 6.2. The Pitzer's parameters at 298.15 K available in the literature. 79
Table 6.3. The Pitzer's parameters of NaOH-NaCl-NaAl(OH) aqueous system at 80
4
298.15 K obtained in this study.
Table 6.4. The Pitzer's parameters of NaTc0, NaTc0-NaCl, and NaBr-NaC10 82
4 4 4
systems at 298.15 K.
Table 6.5. Comparison of measured osmotic coefficients with calculated those for 82
the NaTc0 system at 298.15 K.
4
ix
Table 7.1. Pitzer's binary parameters for the modeling of sodium aluminosilicate 85
formation.
Table 7.2. Pitzer's mixing parameters for the modeling of sodium aluminosilicate 85
formation.
Table 7.3. Thermodynamic data at 298.15 K and 1 bar published in the literature. 87
Table 7.4. Estimations of AH° and AG° of anhydrous sodalite (Na(AlSi0)Cl). 89
f f 8 4 6 2
Table 7.5. Estimated thermodynamic data at 298.15 K and 1 bar. 90
Table 7.6. Estimation of C° of anhydrous sodalite (Na(AlSi0)6Cl). 92
p 8 4 2
Table 7.7. Contribution by 2H0 for C° estimation. 92
2 p
Table 7.8. Contributions by Cl and (OH) for C° estimation. 93
2 2 p
Table 7.9. Prediction of the entropy of aqueous ions at 298.15 K, cal/mol K. 94
Table 7.10. Thermodynamic data of NaSiC>3(), Na+(aq), and Si032"(aq) for the 95
2 S
calculation of the AG0, AH°, and AS0 of reaction (7.11).
Table 7.11. Heat capacities of aqueous oxy-anions at 298.15 K, cal/mol K. 96
Table 8.1. Repeatability of the experiments in three runs. 102
Table 9.1. Comparison of the calculation results for the NH, - NHjOH - H* - HCl - 126
NH4CI - Cf - Na+ - NaCl - K+ - KC1 system at 573.15 K.
Table 9.2. Example of the modeling calculation results for the Na+ - Al(OH)" - 129
4
Si02" - Off - C02" - S02_ - Cf - H0 system at 368.15 K.
3 3 4 2
Table A. 1.1. Analytical data for the analysis of liquid phase alkaline samples. 159
Table A. 1.2. Analytical data for the analysis of samples from the solid precipitates. 160
Table A.3.1. Solubility data for synthetic liquors of system A. 162
Table A.3.2. Solubility data for synthetic liquors of system B. 163
Table A.3.3. Solubility data for mill liquors. 164
Table A.7.1. Comparison of Pitzer's parameters. 174
Table A.8.1. Sensitivity analysis of Pitzer's parameters for NaSi0 aqueous 176
2 3
system.