Calcite Scale Formation in the Green Liquor Handling System of the Kraft Chemical Recovery Process

by

Alisha Giglio

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Chemical Engineering and Applied Chemistry

University of Toronto

©Copyright by Alisha Giglio 2018

Calcite Scale Formation in the Green Liquor Handling System of the Kraft Chemical Recovery Process

Alisha Giglio

Master of Applied Science

Department of Engineering and Applied Chemistry

University of Toronto

2018 Abstract

The formation of calcite (CaCO3) scale in green liquor handling systems is a persistent problem

in many kraft pulp mills. CaCO3 precipitates when its concentration in green liquor exceeds its

solubility. In this work, a laboratory study was conducted to determine the effects of green

liquor properties (temperature, TTA, causticity, and sulfidity) on the solubility limits of

CaCO3. This new solubility data was used to develop a database of CaCO3 solubility in green

liquor using the OLI Systems Inc. software. The second part of this work involved field studies

at four kraft pulp mills in Canada and Sweden, to help understand scaling problems affecting

the causticizing plants. The results help explain how CaCO3 scale forms in the green liquor

handling systems. Understanding how CaCO3 scale is formed helps pulp mills develop viable

strategies for mitigating the problem.

ii Acknowledgements

I would first like to thank my supervisors Professor Honghi N. Tran and Professor Vladimiros G. Papangelakis for their motivation, support, and guidance throughout this project. I will never forget the professional and personal skills they have taught me that will be valuable in my future career.

I would like to extend my gratitude to my Reading Committee members, Professor Nikolai DeMartini and Professor Charles Jia for their helpful discussions, feedback, and comments.

Many thanks are due to Sue Mao, whose door was always open whenever I had a question or needed guidance with my research, and Dr. Georgiana Moldoveanu who was always there to assist me with laboratory issues. I am also thankful to Nazin Orang for proof-reading this thesis, and my friends and colleagues whom I have worked with over the past two years.

This work was conducted as part of the research program on “Increasing Energy and Chemical Recovery Efficiency in the Kraft Process - III”, jointly supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and a consortium of the following companies: Andritz, AV Nackawic, Arauco, Celulose Nipo-Brasileira, Canadian Kraft Paper, Clyde- Bergemann, DMI Peace River Pulp, Domtar, Eldorado, ERCO Worldwide, Fibria, FP Innovations, Georgia Pacific, International Paper, Irving Pulp & Paper, Kiln Flame Systems, Klabin, StoraEnso Research, Suzano, Tembec, Valmet, and West Rock. I would like to thank the consortium members for their financial support as well as valuable discussions and information regarding causticizing plant operations, and scaling problems specific to this work. I would like to personally thank Moise Dion of DMI Peace River Pulp, and Blair Rydberg of Canadian Kraft Paper, for their contributions to the field studies. Finally, I would like to express my sincerest gratitude to Maria Björk and Rickard Wadsborn of Stora Enso, as well as the entire Biomaterials group for hosting me and offering their guidance, friendship, and support during my time in Sweden.

Finally, I must express my gratitude to my parents and for providing me with their love and continuous encouragement throughout my years of study and through the process of researching and writing this thesis. This accomplishment would not have been possible without them.

iii Table of Contents

Acknowledgements ...... iii

Table of Contents ...... iv

List of Tables ...... viii

List of Figures ...... ix

List of Abbreviations ...... xvi

Chapter 1 ...... 1

1 Introduction ...... 1

1.1 The Kraft Process ...... 1

1.2 Causticizing Plant ...... 3

1.3 Standard Kraft Pulping Terms ...... 5

1.3.1 Total Titratable Alkali (TTA) ...... 5

1.3.2 Effective Alkali (EA) and Active Alkali (AA) ...... 6

1.3.3 Causticizing Efficiency (CE) ...... 6

1.3.4 Sulfidity ...... 7

1.3.5 Reduction ...... 7

1.4 Scale Formation in Causticizing Plants ...... 7

1.5 Motivation and Objectives ...... 9

Chapter 2 ...... 11

2 Literature Review ...... 11

2.1 Physical Properties of Calcium Carbonate ...... 11

2.2 Calcium Carbonate Scale ...... 11

2.2.1 Scale Deposition Mechanism ...... 12

2.2.2 Calcium Carbonate Phase Transformations ...... 13

iv 2.3 Previous Studies of Calcium Carbonate Solubility ...... 14

2.3.1 Calcium Carbonate Solubility in Water ...... 15

2.3.2 Calcium Carbonate Solubility in Green Liquor ...... 17

2.4 Calcium Carbonate Stability in Green Liquor Systems ...... 18

Chapter 3 ...... 20

3 Current OLI Models of Calcium Carbonate Solubility ...... 20

3.1 OLI Software ...... 20

3.2 Calcium Carbonate Solubility Using OLI’s MSE Model ...... 21

Chapter 4 ...... 24

4 Experimental Investigation of Calcium Carbonate Solubility ...... 24

4.1 Experimental Procedures ...... 24

4.1.1 Apparatus ...... 24

4.1.2 Materials and Methodology ...... 25

4.1.3 Solubility Units ...... 26

4.2 Results and Discussion ...... 26

4.2.1 CaCO3-Na2CO3-H2O System ...... 26

4.2.2 CaCO3-Na2CO3-NaOH-H2O System ...... 35

4.2.3 CaCO3-Na2CO3-Na2S-H2O System ...... 37

4.3 Solid Sample SEM Analysis ...... 39

4.4 Summary ...... 39

Chapter 5 ...... 41

5 Thermodynamic Modelling of Calcium Carbonate Solubility ...... 41

5.1 OLI Software ...... 41

5.2 Calcium Carbonate Database ...... 41

5.3 Modelling of Calcium Carbonate Solubility Using OLI Stream Analyzer ...... 46

v 5.3.1 Effect of Temperature on the Na2CO3-NaOH-Na2S-CaCO3-H2O System ...... 46

5.3.2 Effect of Green Liquor Reduction on CaCO3 Solubility ...... 47

5.3.3 Effect of Chloride on CaCO3 Solubility ...... 48

5.4 Summary ...... 49

Chapter 6 ...... 50

6 Causticizing Plant Analysis of Pulp Mills ...... 50

6.1 Summary of Pulp Mills ...... 50

6.1.1 Mill A ...... 50

6.1.2 Mill B ...... 50

6.1.3 Mill C ...... 51

6.1.4 Mill D ...... 51

6.2 Pulp Mill Liquor Analysis ...... 52

6.3 Causticizing Plant Analysis ...... 54

6.3.1 Mill A ...... 54

6.3.2 Mill B ...... 56

6.3.3 Mill C ...... 56

6.3.4 Mill D ...... 58

6.3.5 Summary ...... 60

6.4 Scale Analysis of Mill A and Mill B ...... 60

6.5 Green Liquor Scale Field Study at Mill A ...... 69

6.5.1 Field Study Details ...... 69

6.5.2 Field Study Methodology ...... 72

6.5.3 Field Study Results ...... 72

6.6 Summary ...... 74

Chapter 7 ...... 75

vi 7 Conclusions ...... 75

Chapter 8 ...... 77

8 Recommendations for Future Work ...... 77

References ...... 78

Appendices ...... 82

Appendix I: TGA/DSC Standard Profiles ...... 82

Appendix II: TGA/DSC and XRD Experimental Sample Profiles ...... 83

Appendix III: Density of Synthetic Green Liquor Solutions ...... 92

Appendix IV: TGA/DSC and XRD Industry Sample Profiles ...... 94

Appendix V: Mill Data ...... 99

vii List of Tables

Table 1. CaCO3 solubility in the CaCO3-Na2CO3-H2O system...... 29

Table 2. CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system (95°C, TTA = 120 g/L

Na2O)...... 35

Table 3. CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system (95°C, TTA= 120 g/L Na2O)...... 37

Table 4. Original standard state properties of CaCO3 found in the MSEPUB database...... 42

Table 5. Original ion interaction parameters found in the MSEPUB database...... 42

Table 6. Regressed standard state properties of CaCO3...... 43

Table 7. Regressed ion interaction parameters (in red) for the green liquor system...... 43

Table 8. Summary of studied pulp mills...... 52

Table 9. Summary of the measured chemical properties of liquors from Mill A, Mill C and Mill D...... 53

Table 10. TGA/DSC Results for mill scale samples...... 66

Table 11. XRF Results for Mill A and Mill B scale samples...... 67

Table 12. Mill A field study results...... 73

Table 13. Comparison of the measured and average chemical properties of Mill A...... 99

Table 14. Comparison of the measured and average chemical properties for Mill C...... 99

Table 15. Comparison of the measured and average chemical properties for Mill D...... 100

viii List of Figures

Figure 1. The kraft process (courtesy of Valmet)...... 2

Figure 2. Schematic of the kraft causticizing plant...... 3

Figure 3. Cross section of a RGL pipeline (8 in. inner diameter) with CaCO3 scale...... 9

Figure 4. Crystal of pure calcite [13]...... 11

Figure 5. XRD patterns of three CaCO3 polymorphs: (a) aragonite, (b) vaterite, (c) calcite [21].14

Figure 6. The solubility of CaCO3 in air (blue) and a CO2 free environment (red). Adapted from OLI Systems Inc. [23]. Conversion to ppm was carried out using the density of water at 25°C (0.996845 kg/L)...... 16

Figure 7. The effect of pH on CaCO3 solubility at 25°C and 1 bar air pressure [29]. Conversion to ppm was carried out using the density of water at 25°C (0.996845 kg/L)...... 17

Figure 8. The solubility of CaCO3 in Na2CO3 at 25°C (adapted from OLI) [22]...... 18

Figure 9. Stability of CaCO3 and pirssonite in the Na2CO3-NaOH-CaCO3-H2O system at 95°C [9]...... 19

Figure 10. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-H2O system using the MSEPUB database...... 21

Figure 11. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system using

the MSEPUB database (TTA= 120 g/L Na2O)...... 22

Figure 12. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system using the

MSEPUB database (TTA= 120 g/L Na2O)...... 23

Figure 13. Experimental setup used for constant temperature reactions...... 24

Figure 14. Experimental setup used for reactions where temperature was varied...... 25

ix Figure 15. Kinetics plot for the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA= 120 g/L

Na2O)...... 27

Figure 16. Kinetics plot for the CaCO3-Na2CO3-H2O system at 25°C...... 28

Figure 17. Kinetics plot for the CaCO3-Na2CO3-H2O system at 95°C...... 28

Figure 18. CaCO3 solubility in the CaCO3-Na2CO3-H2O system...... 30

Figure 19. Thermal profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at

95°C ([Na2CO3] = TTA = 60 g/L Na2O)...... 32

Figure 20. Thermal profile of solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C

([Na2CO3] = TTA = 120 g/L Na2O)...... 32

Figure 21. Thermal profile of solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C

([Na2CO3] = TTA = 180 g/L Na2O)...... 33

Figure 22. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C

([Na2CO3] = TTA = 60 g/L Na2O)...... 33

Figure 23. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C

([Na2CO3] = TTA = 120 g/L Na2O)...... 34

Figure 24. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C

([Na2CO3] = TTA = 180 g/L Na2O)...... 34

Figure 25. Kinetics plot for the CaCO3-Na2CO3-NaOH-H2O system (NaOH = 6 g/L Na2O). .... 36

Figure 26. CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system (95°C, TTA = 120 g/L

Na2O)...... 36

Figure 27. Kinetics plot for the CaCO3-Na2CO3-Na2S-H2O system (Na2S = 48 g/L Na2O)...... 38

Figure 28. CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system (95°C, TTA = 120 g/L

Na2O)...... 38

x Figure 29. SEM images at various magnifications of the solid sample collected from the CaCO3-

Na2CO3-H2O system at 95°C (TTA = 120 g/L Na2O). Left (x850), Middle (x5000), Right (x30000)...... 39

Figure 30. Comparison of experimental data and model performance for the solubility of CaCO3

with temperature in the CaCO3-Na2CO3-H2O system...... 44

Figure 31. Comparison of experimental data and model performance for the solubility of CaCO3

in the CaCO3-Na2CO3-NaOH-H2O system...... 44

Figure 32. Comparison of experimental data and model performance for the solubility of CaCO3

in the CaCO3-Na2CO3-Na2S-H2O system...... 45

Figure 33. The deviation between the experimental data points and OLI model...... 45

Figure 34. OLI generated CaCO3 solubility curve for the CaCO3-Na2CO3-NaOH-Na2S-H2O

system (TTA = 120 g/L Na2O, T=75°C)...... 46

Figure 35. OLI generated CaCO3 solubility curve for the Na2CO3-NaOH-Na2S-CaCO3-H2O

system (TTA = 120 g/L Na2O, T=95°C)...... 46

Figure 36. The effect of green liquor reduction on CaCO3 solubility at 95°C (TTA = 120 g/L Na2O, 10% causticity, 30% sulfidity)...... 47

Figure 37. The effect of chloride on CaCO3 solubility (TTA = 120 g/L Na2O, 10% causticity, 30% sulfidity)...... 49

Figure 38. Calcite scale formation in a green liquor pipeline. (Courtesy of Mill B) ...... 51

Figure 39. Mill A causticizing plant. Locations where scale samples were obtained are indicated in red...... 55

Figure 40. Mill C causticizing plant...... 57

Figure 41. Mill D causticizing plant...... 59

xi Figure 42. Scale samples obtained from the causticizing plant of Mill A (A-F), and Mill B (G). 62

Figure 43. Thermal profile of Mill A, sample A...... 63

Figure 44. Thermal profile of Mill A, sample C...... 64

Figure 45. Thermal profile of Mill A, sample E...... 65

Figure 46. Thermal profile of Mill A, sample F...... 65

Figure 47. Cross section of sample C used for EPMA analysis...... 68

Figure 48. EMPA analysis of Mill A, sample C...... 68

Figure 49. Growth rate of scale sample C...... 69

Figure 50. Schematic (left) and photograph (right) of the scale probe used in this study...... 70

Figure 51. Photograph of two coupons used in this study (left) and coupons fastened to scale probe (right). The top coupon is coated with the hydrophobic coating Durasan® by SilcoTek (left)...... 70

Figure 52. Schematic of Mill A, recausticizing plant with probe locations indicated in red...... 71

Figure 53. Photograph of 1st scale probe location...... 71

Figure 54. Photograph of 2nd scale probe location...... 72

Figure 55. Photograph of coupons after Trial 2...... 73

Figure 56. Photograph of hydrophobic coated coupon after Trial 3. Coupon was removed during WW flow...... 73

Figure 57. Thermal profile for pure calcite used as a standard...... 82

Figure 58. Thermal profile of pure pirssonite used as a standard...... 82

xii Figure 59. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system

at 95°C (TTA = 120 g/L Na2O, NaOH = 6 g/L Na2O)...... 83

Figure 60. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C

(TTA = 120 g/L Na2O, NaOH = 6 g/L Na2O)...... 83

Figure 61. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system

at 95°C (TTA = 120 g/L Na2O, NaOH = 12 g/L Na2O)...... 84

Figure 62. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C

(TTA = 120 g/L Na2O, NaOH = 12 g/L Na2O)...... 84

Figure 63. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system

at 95°C (TTA = 120 g/L Na2O, NaOH = 18 g/L Na2O)...... 85

Figure 64. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C

(TTA = 120 g/L Na2O, NaOH = 18 g/L Na2O)...... 85

Figure 65. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system

at 95°C (TTA = 120 g/L Na2O, NaOH = 30 g/L Na2O)...... 86

Figure 66. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C

(TTA = 120 g/L Na2O, NaOH = 30 g/L Na2O)...... 86

Figure 67. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system

at 95°C (TTA = 120 g/L Na2O, Na2S = 12 g/L Na2O)...... 87

Figure 68. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C

(TTA = 120 g/L Na2O, Na2S = 12 g/L Na2O)...... 87

Figure 69. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system

at 95°C (TTA = 120 g/L Na2O, Na2S = 24 g/L Na2O)...... 88

Figure 70. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C

(TTA = 120 g/L Na2O, Na2S = 24 g/L Na2O)...... 88

xiii Figure 71. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system

at 95°C (TTA = 120 g/L Na2O, Na2S = 36 g/L Na2O)...... 89

Figure 72. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C

(TTA = 120 g/L Na2O, Na2S = 36 g/L Na2O)...... 89

Figure 73. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system

at 95°C (TTA = 120 g/L Na2O, Na2S = 48 g/L Na2O)...... 90

Figure 74. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C

(TTA = 120 g/L Na2O, Na2S = 48 g/L Na2O)...... 90

Figure 75. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system

at 95°C (TTA = 120 g/L Na2O, Na2S = 60 g/L Na2O)...... 91

Figure 76. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C

(TTA = 120 g/L Na2O, Na2S = 60 g/L Na2O)...... 91

Figure 77. Solution density for the CaCO3-Na2CO3-H2O system vs. concentration of Na2CO3 in

g/L Na2O at 25°C...... 92

Figure 78. Solution density for the CaCO3-Na2CO3-NaOH-H2O system vs. concentration of NaOH

in g/L Na2O at 25°C...... 92

Figure 79. Solution density for the CaCO3-Na2CO3-Na2S-H2O system vs. concentration of Na2S

in g/L Na2O at 25°C...... 93

Figure 80. XRD profile of Mill A, sample A...... 94

Figure 81. Thermal profile of Mill A, sample B...... 94

Figure 82. XRD profile of Mill A, sample B...... 95

Figure 83. XRD profile of Mill A, sample C...... 95

Figure 84. Thermal profile of Mill A, sample D...... 96

xiv Figure 85. XRD profile of Mill A, sample D...... 96

Figure 86. XRD profile of Mill A, sample E...... 97

Figure 87. XRD profile of Mill A, sample F...... 97

Figure 88. Thermal profile of Mill B, sample G...... 98

Figure 89. XRD profile of Mill B, sample G...... 98

xv List of Abbreviations

AA Active alkali

ACS American Chemical Society

ADMT Air-dried metric ton

CE Causticizing efficiency

CGL Clarified green liquor

EA Effective alkali

EMPA Electron Probe X-Ray microanalysis

ICP-OES Inductively coupled plasma optical emission spectrometry

MSE Mixed solvent electrolyte model

MSEPUB Mixed solvent electrolyte public database

RGL Raw green liquor

SEM Scanning electron microscope

TGA/DSC Simultaneous Thermogravimetry and Differential Scanning Calorimeter

TTA Total titratable alkali

WW Weakwash

XRD X-ray power diffraction

XRF X-ray florescence

xvi Chapter 1

1 Introduction

The formation of calcite or calcium carbonate (CaCO3) scale is a persistent problem in many kraft pulp mills around the world. CaCO3 scale has been known to form within many areas of the process, particularly within the causticizing plant, however scaling within the green liquor pipelines and pumps is of interest due to the difficulty in removing the scale. This chapter aims to give a background of the kraft process and the calcite scaling problem the pulp and paper industry faces. Standard kraft pulping terms, which will be used throughout this thesis, are also introduced.

1.1 The Kraft Process

The kraft process is the dominant pulping process in the pulp and paper industry. The process of pulping converts wood into wood pulp, which is mainly cellulose fibres, by removing enough lignin so that the cellulose fibres can be easily separated from one another. Wood pulp is used to produce a variety of paper products used every day, such as facial tissues, paper towels, coffee filters, printing and writing papers, diapers, and other absorbent materials. The development of the modern day kraft process, using a mixture of sodium sulfide and sodium hydroxide to pulp wood, has been credited to C.F. Dahl of Danzig, Germany back in 1879, however the use of alkali chemicals for the preparation of papermaking pulp dates to the year A.D. 750 in China [1]. In the present day, approximately 130 million tons of kraft pulp is produced every year, which accounts for over 90% of the world’s chemical pulp production. The kraft process is widely used across the globe due to the ability of the process to handle almost all species of hardwood and softwood, and its chemical recovery efficiency of approximately 97% makes it a favourable economic process [2].

A flow chart of a typical kraft process can be seen in Figure 1. The first step of the process is to dissolve wood chips in the pulping chemicals, aqueous sodium hydroxide (NaOH) and sodium sulfide (Na2S), which together is known as white liquor. This is carried out using a pressurized vessel called a digester, at a high temperature, approximately 170°C, and pressure, between 110 to 150 psi. Within the digester, dissolved lignin produces wood pulp and weak black liquor, which

1 contains any undissolved wood and the spent pulping chemicals. The wood pulp is sent to the bleach plant where it is washed, bleached, and prepared for packaging. The weak black liquor is sent to the chemical recovery cycle for the recovery of white liquor. For every ton of pulp produced, the kraft process produces about 10 tons of weak black liquor that must be processed [2].

Figure 1. The kraft process (courtesy of Valmet).

The chemical recovery cycle is a key component of the kraft process. Its purpose is to regenerate the inorganic pulping chemicals, and to burn the organic materials in a recovery boiler to generate steam and power. To prepare the weak black liquor for burning, the liquor is concentrated in a series of evaporators to produce heavy black liquor which typically contains 65% solids or higher [3]. The concentrated black liquor enters the recovery boiler where the organic component is burned to produce steam, and the inorganic sulfur compounds are reduced in an oxygen deficient environment to reproduce Na2S. The product of black liquor burning is a molten smelt containing mostly Na2S and sodium carbonate (Na2CO3). This step generates a significant amount of energy in the form of superheated steam, which is then harnessed to produce power. Globally, 1.3 billion

2 tons of weak black liquor is processed every year to recover 50 million tons of pulping chemicals

(as Na2O), and to produce 700 million tons of high pressure steam. This makes black liquor the fifth most important fuel in the world [2]. The smelt is further processed in the causticizing plant to regenerate the white liquor.

1.2 Causticizing Plant

Utilizing one simple chemical reaction and a series of liquid-solid separation steps, the causticizing plant regenerates white liquor from the recycled inorganic compounds. A schematic of the causticizing plant can be seen in Figure 2 below.

Smelt (Na2CO3, Na2S) Clarified Green from Liquor (CGL) Recovery Boiler Lime (CaO) Raw Green Grits GLiquor (RGL)reen Liquor GL Clarifier

Dissolving Slaker Tank

Dregs Washer Causticizers Weak Dregs Holding Wash Dregs Tank White Liquor (NaOH, Na2S) WL Clarifier H2O to Digester Mud Washer CO2, H2O Mud

Lime Kiln Mud Slurry Storage Tank

Figure 2. Schematic of the kraft causticizing plant.

The causticizing process begins with molten smelt at approximately 800°C exiting the bottom of the recovery boiler through smelt spouts where it is shattered by steam jets into a spray of droplets. The molten smelt droplets enter the agitated dissolving tank below, where the smelt is dissolved in weakwash (WW), a solution of water and NaOH, to form raw green liquor (RGL). RGL leaves the dissolving tank at approximately 85 – 95°C, and the retention time of the dissolving tank can

3 be anywhere from 1 to 4 hours. Many mills house green liquor equilibration tanks after the dissolving tank which are used to stabilize the temperature and density of the green liquor. These tanks may be agitated and have a retention time between 2 to 4 hours. Any dregs, or suspended solid particles in the green liquor are removed from the RGL in a clarifier. The green liquor clarification system may consist of a sedimentation clarifier, or a series of sock type clarifiers which uses pressure filtration, and some mills may use both filtration units. RGL typically contains about 1000 – 3000 ppm dregs, and the dregs are a light fluffy black material which is mostly composed of unburned carbon from the black liquor as well as iron, silica, alumina, calcium, magnesium, and other trace metals [1]. The dregs from the clarifier are removed to be washed and disposed using a dregs filter which is typically precoated with lime mud. Clarified green liquor (CGL) is sent to the slaker where it is reacted with lime (CaO), initiating the slaking and causticizing reactions. The slaking reaction involves the hydrolysis of lime to produce calcium hydroxide (Ca(OH)2), also known as hydrated or slaked lime (Rxn. 1). The reaction is exothermic, and heaters or coolers are used to ensure the temperature within the slaker is maintained between

101 – 104°C. The causticizing reaction (Rxn. 2) begins as soon as some Ca(OH)2 is produced from the slaking reaction. The causticizing reaction progresses in a series of causticizers where Ca(OH)2 reacts with Na2CO3 to produce NaOH and lime mud (CaCO3). The slaking reaction is faster than the causticizing reaction, and the causticizing reaction is reversible, therefore three or four causticizers are typically used in series to ensure the reaction proceeds as far to completion as possible [4].

Slaking ���() + ��() → ��(��) () (Rxn. 1)

Causticizing ��(��) () + ���� () ⇄ 2����() + ���� () (Rxn. 2)

The causticized green liquor, which is now white liquor, returns to the digester for reuse. The precipitated lime mud is washed and sent to a lime kiln where it is heated to a high temperature to produce CaO for reuse.

4 1.3 Standard Kraft Pulping Terms

A series of standard kraft pulping terms has been adopted by the industry to express the concentrations of all chemicals in the process. The major chemicals within the causticizing plant are:

1. Na2S, sodium sulfide, one of the main constituents of white liquor, it helps reduce damage to the cell walls of the wood in the digester, 2. NaOH, sodium hydroxide, another main constituent of white liquor, it dissolves the lignin bonding wood fibres and is regenerated during the causticizing reaction,

3. Na2CO3, sodium carbonate, the main constituent of smelt from the recovery boiler, dissolving smelt in water produces green liquor,

4. CaCO3, calcite, one of the main constituents of green liquor scale, or also known as lime mud when produced by slaking CaO in green liquor 5. CaO, produced in the lime kiln to be used in the slaking reaction, and,

6. Na2SO4 (salt cake), which is a make-up chemical introduced in the recovery boiler and carried through the kraft cycle as a dead load.

In North America, all chemical concentrations are typically expressed in terms of equivalent amounts of sodium oxide (Na2O), and in Europe, it is more common to express concentration in terms of NaOH. Concentrations are typically expressed as kg/m3, lb/ft3, or g/L [5]. In this study, all concentrations will be expressed as g/L of Na2O. The following section defines the terms commonly used in kraft chemistry, which will be used throughout this thesis.

1.3.1 Total Titratable Alkali (TTA)

Total titratable alkali (TTA) is a term used to describe the sum of the sodium salts, Na2CO3, NaOH, and Na2S, in white and green liquor (Eqn. 1). Theoretically TTA should include ½ Na2SO3 but it is generally ignored. Target values of TTA are set by the digester design [5], and typical values of

TTA in white and green liquor are between 110 – 140 g/L Na2O [6]. It can be noted that as the causticizing reaction progresses most of the carbonate in green liquor is converted into equivalent moles of hydroxide, therefore TTA remains constant throughout the whole process.

5 ��� = ���� + ���� + [���] (Eqn. 1)

Note that concentration, [ ] is an expression of g/L Na2O.

1.3.2 Effective Alkali (EA) and Active Alkali (AA)

The term effective alkali (EA, Eqn. 2), is primarily used in digester operations. This value represents the chemicals that will produce alkali under pulping conditions, and it is used to calculate the amount of cooking liquor needed to process the wood chips in the digester [5].

Typical EA values are between 100 – 110 g/L Na2O [6]. Active alkali (AA, Eqn. 3) as the name suggests, is the active ingredients in the pulping process. The AA required per day determines the amount of white liquor required from the causticizing plant. Target values of AA are set by the digester design [5], and typical values are between 85 – 95 g/L Na2O [6].

�� = ���� + [���] (Eqn. 2)

�� = ���� + [���] (Eqn. 3)

1.3.3 Causticizing Efficiency (CE)

The conversion of Na2CO3 to NaOH in the causticizing reaction is measured by the causticizing efficiency (CE). CE is expressed as a percentage, and the causticizing efficiency of the mixture leaving the last causticizer is typically between 80-90% (Eqn. 4) [4]. It may be noted that much of the causticizing reaction (approximately 75% CE for a total CE of 80 – 82%) occurs within the slaker before even entering the first causticizer due to the speed of the slaking reaction [5]. To obtain an accurate value of CE, NaOH content in white liquor must be corrected for the NaOH content in green liquor.

% ���������� = � 100% (Eqn. 4)

The term causticity (not to be confused with causticizing efficiency) is used to indicate the amount of NaOH present in green liquor before entering the slaker and causticizers. In this thesis, causticity is calculated in the same manner as causticizing efficiency. A typical green liquor has a causticity between 5 – 20% [6].

6 1.3.4 Sulfidity

Sulfidity is a measure of the amount of Na2S in the system, and it can be expressed on an AA or TTA basis. In this study, sulfidity will be expressed on a TTA basis as it remains constant in the system. Typical values of sulfidity in white and green liquor are between 20 – 40% (Eqn. 5) [6].

% ��������� = � 100% (Eqn. 5) [

1.3.5 Reduction

Reduction is the ratio of sodium sulfide to the sum of sodium sulfide and sodium sulfate in green liquor. It is expressed as a percentage, and is a measure of the efficiency of reduction of sulfur species in the recovery boiler. Typical values for the reduction in white and green liquor is between 80 – 90% (Eqn. 6) [6].

[ ] % ��������� = ∙ 100% (Eqn. 6) []

1.4 Scale Formation in Causticizing Plants

A major problem with the kraft causticizing process is the formation of scales in the green liquor handling system. Scale is a term, generally used in industry, that refers to any deposit on an equipment surface. Throughout the kraft process, scale has been known to form on the dissolving tank walls, floor, and agitator blades, green liquor filter cloths, pipelines, pumps, heat exchangers, and slaker walls and classifier screws. The severity of scaling varies from mill to mill, and recurring scaling typically results in unscheduled shutdowns for scale removal which is both time consuming and costly. Scaling was so severe in a kraft pulp mill that in one case a 4-inch diameter pipeline was reduced to 1-inch. This caused the pumps on the system which were originally rated to 260 gal/min to be reduced to 60 gal/min [7].

Green liquor scale was once thought to be primarily composed of pirssonite

(Na2CO3×CaCO3×2H2O), a double salt of sodium and calcium carbonate, however recent analysis of mill scales has found that calcite, the most stable polymorph of CaCO3, is the primary component of green liquor scale [8, 9, 10]. In a study performed by Zakir et al. [8], 12 scale samples

7 from the causticizing plants of 10 kraft mills around the world were analyzed to determine their composition. Eight of the 12 scale samples were found to be composed of CaCO3, and the remaining 4 samples were found to be composed of pirssonite. The majority of the pirssonite scales were collected from the dissolving tank, and 6 of the 8 CaCO3 scales were collected from pipelines following the dissolving tank. Similar observations have been made by other researchers [10] who found that that scales collected from the dissolving tank were primarily composed of pirssonite and sodium carbonate monohydrate (Na2CO3×H2O), and samples collected from the RGL pipeline were mostly composed of CaCO3. In addition to the above, analysis of over 30 scale samples carried out by Honghi Tran’s research group at the University of Toronto show that scales collected from dissolving tanks are mostly pirssonite whereas scales collected downstream of the dissolving tank are primarily CaCO3.

To minimize scale formation in the green liquor pipelines and pumps, many kraft mills run duplicate pipelines between the dissolving tank and the green liquor clarifier. One pipeline carries the RGL from the dissolving tank to the clarifier, while the other pipeline returns the WW to the dissolving tank. This is thought to allow the dilute WW to dissolve any scale that has formed in the RGL line, minimizing scale build up [5]. Mills have been known to switch their WW and RGL lines on a 12 h, daily, or weekly basis. In reality, many mills that follow these practices still endure green liquor lines with intensive scaling.

Some mills have used anti-scaling chemicals to help overcome the scaling problem. These anti- scaling chemicals aim to interrupt the scaling mechanism by blocking crystal growth sites with the chemical anti-scalent, or by causing the scale to form in solution rather than on the surface of equipment where it can easily be swept away by process flows [11]. However, there have been some examples suggesting that anti-scaling chemicals do not appear to be a viable solution to the scaling problem. In October 2017, a pulp mill was forced to shut down due to intensive scaling which completely blocked some sections of the RGL pipeline. This mill had begun to use an anti- scaling chemical in July 2016, and at the same time, discontinued their WW and RGL line switching. Over these 16 months, hard black scale formed within the entire RGL pipeline, as is shown in Figure 3. Scale within RGL pipelines is typically black in colour due to the presence of dregs in the scale. Hydro blasting with high pressure water was attempted to clean the lines,

8 however the scale was so tough that the mill had to use acid cleaning with hydrochloric acid (HCl) to remove all the scale.

Figure 3. Cross section of a RGL pipeline (8 in. inner diameter) with CaCO3 scale.

1.5 Motivation and Objectives

As is apparent, CaCO3 scale is a major problem in the green liquor system that must be mitigated to prevent downtimes and production losses within the kraft pulp mill. Many mills have attempted to remove scale by flushing the green liquor lines with weakwash, or by adding anti-scaling chemicals to their system, however currently there is no viable solution to prevent the formation of CaCO3 scale.

In order to minimize CaCO3 scale formation in the green liquor system, a study must begin with an understanding of CaCO3 solubility in green liquor. To date, there is an extensive database of

CaCO3 solubility in aqueous or low concentrated salt solutions, however the focus has never been placed on CaCO3 formation and solubility within a highly alkaline, supersaturated solution, typical to that of green liquor. There is, therefore, a need to develop fundamental experimental data to understand the solubility of CaCO3 in this system, and how it is affected by the operating conditions of the causticizing plant.

9 The objective of this study was to perform laboratory experiments to examine the effects of green liquor characteristics such as temperature, and concentrations of Na2CO3, NaOH, and Na2S on

CaCO3 solubility, and to use field studies to help understand scaling problems in an industrial setting. Since the kraft process is operated at such a large scale worldwide, understanding and reducing scaling could result in significant time, energy and cost savings. The main objectives of this work are:

1. To determine the effects of green liquor properties (temperature, TTA, causticity, and

sulfidity) on the solubility limits of CaCO3. 2. To incorporate the obtained solubility data into OLI, a commercially available

thermodynamic simulation software, and use it to predict conditions leading to CaCO3 formation. 3. To perform field studies at 4 kraft pulp mills in Canada and Sweden, and to help understand the scaling problems affecting the causticizing plants of kraft pulp mills. Where available, green liquor scales collected from these mills will be analyzed.

10 Chapter 2

2 Literature Review

2.1 Physical Properties of Calcium Carbonate

Calcium carbonate is an important and dominant chemical found on the earth’s surface. It exists in geological deposits of limestone, marble, and chalk, and is used to form the shells of marine organisms, snails, and eggs [12]. The main polymorphs of CaCO3 are calcite, aragonite, and vaterite, listed in increasing solubility and decreasing stability. Most calcium carbonate is found as calcite due to its higher stability. Calcite has a hardness value of 3 on the Mohs hardness scale [13]. It is typically white in colour, but its appearance may vary depending on the impurities in the sample. Figure 4 below shows a crystal of pure calcite.

Figure 4. Crystal of pure calcite [13].

2.2 Calcium Carbonate Scale

In industry, fouling, or scaling, occurs when a dissolved compound precipitates onto the surface of a material, such as the inside of a pipe, or the walls of a vessel. CaCO3 is by far the most common scale in many industrial plans due to its extremely low solubility, and ability to dissolve and reprecipitate in water in the presence of carbon dioxide gas.

11 2.2.1 Scale Deposition Mechanism

For the precipitation of CaCO3 to result in scale, three simultaneous criteria must be met for crystallization to occur on a material’s surface. The three criteria are [14]:

1. Supersaturation 2. Nucleation 3. Crystal Growth

The primary cause of scale formation is supersaturation. A saturated solution is one which is in equilibrium with its solute, whereas a supersaturated solution contains higher concentrations of dissolved solutes than its saturated solution at the same temperature and pressure. A supersaturated solution is not a stable thermodynamic state, and when the solubility of CaCO3 exceeds the metastable limit, crystals will start to form initiating the precipitation process [15]. There are numerous ways to supersaturate a solution including temperature, pressure, or pH changes, addition of solid seeding material, change in flow rates or surface geometry. Supersaturation can be highly localized, and the rate of scale deposition is dependent on the degree of supersaturation [14].

If supersaturation conditions are met, nucleation, the initial form of a precipitate crystal, may begin. Nucleation can occur in two ways, primary or secondary nucleation. Primary nucleation involves the spontaneous formation of new seed crystals within the “mother liquor”. The mechanism of primary nucleation can be homogeneous, where the presence of a foreign substance is not required for nucleation, or heterogeneous, which requires a preexisting foreign crystalline substrate, such as the walls of a pipe or vessel [14, 15]. Secondary nucleation requires the presence of existing crystals, known as “seed sites” within the mother liquor. The mechanism of secondary nucleation is heterogeneous. It is thought that heterogeneous nucleation mainly exists, either by primary or secondary nucleation, as it is nearly impossible to keep a system clean of foreign nucleation sites. Many things can serve as sites for nucleation such as internal stress points in a metal, scratches, microscopic crevices, or rusted surfaces. For nucleation to occur, thousands of molecules must agglomerate and orient themselves in a fixed lattice. Nuclei that are too small will redissolve and redeposit, and once a certain size is reached the system becomes stable. At high degrees of saturation, the nucleation process occurs quite rapidly [14].

12 Once nucleation has begun, crystal growth typically occurs radially from the original nucleation site. Subsequent layers of scale will form on the surface of the crystals, rather than on the metal surface, and in certain cases, the growth rate between the two surfaces can be quite different. The crystals are formed by the aggregation of molecules which continue to grow until they form nuclei. Deposition begins on the nuclei or the nuclei combine to form visible crystals [14]. Many mechanisms have been proposed for crystal growth, with some researchers suggesting that crystallization takes place layer by layer on the crystal surface, while others suggest that the material deposits continuously on the crystal face at a rate proportional to the difference in concentration between the point of deposition and the bulk of the solution [14]. At this point in the crystal growth, the polymorph of CaCO3 is determined, and phase transformations from one polymorph to another may occur.

2.2.2 Calcium Carbonate Phase Transformations

CaCO3 phase transformations are significantly affected by temperature and pH [14]. In aqueous solutions, calcite is typically the preferred polymorph at low temperatures (< 30°C), and aragonite has been found to be the preferred polymorph at high temperatures (³ 40°C) [14]. Recently, much of the published research regarding CaCO3 phase transformations and polymorph determination have been led by the School of Earth and Environment at the University of Leeds, in Leeds, England. Rodriguez-Blanco et al. and Bots et al. of this research group have published a series of articles [16, 17, 18] which have studied the kinetics, mechanisms, and the effects of pH and various ions on CaCO3 crystallization and phase transformations. They have found that the mechanism of formation of crystalline CaCO3 polymorphs often occurs via a nanoparticulate amorphous calcium carbonate (ACC) precursor, which is a monohydrate with the formula CaCO3×H2O [17]. At low temperatures < 30°C, CaCO3 was found to crystallise as calcite via pure ACC and vaterite; while at high temperatures ³ 40°C, CaCO3 was found crystallise as aragonite via pure ACC and vaterite [17]. When studying the effect of pH, it was found that high pH solutions (~ 11.5) promotes the formation of calcite crystals [18]. Similar studies [19, 20] have found that calcite precipitation in water is favoured at high alkalinities.

As mentioned, the only publications which analyze green liquor scale samples has been performed by Zakir et al. and Taylor et al. who determined the phase of CaCO3 scale using X-ray powder

13 diffraction (XRD) spectrometry. Figure 5 demonstrates the differences between the XRD patterns of calcite, aragonite, and vaterite as published by Ni and Ratner, who differentiated CaCO3 polymorphs by various surface analysis techniques [21]. The major peak of calcite (c) can be found at 29.5°, aragonite is typically identified by a peak at 26.2° and vaterite at 27.4°. Both Zakir et al. and Taylor et al. concluded that the CaCO3 scales collected from the green liquor system were in the form of calcite. A detailed study of the crystallization pathway of calcite in green liquor is beyond the scope of this thesis, but it is expected that all CaCO3 green liquor scales will be calcite.

Solid samples will be analyzed using XRD to confirm the CaCO3 phase.

Figure 5. XRD patterns of three CaCO3 polymorphs: (a) aragonite, (b) vaterite, (c) calcite [21].

2.3 Previous Studies of Calcium Carbonate Solubility

Solubility data can be used to describe the solubility limits of products that may cause scale. At the present time, there are no reported studies of CaCO3 solubility in green liquor, however CaCO3 solubility in water is well known. This section reviews the available studies of CaCO3 solubility.

14 2.3.1 Calcium Carbonate Solubility in Water

The solubility of CaCO3 is well known in the CaCO3-H2O-air system, and is described as having an inverse solubility curve (Figure 6, blue), which is common to most scaling materials. This inverse solubility curve is due to the ability of water to absorb CO2 gas, which is found in the atmosphere at concentrations of approximately 300 ppm. When CO2 is absorbed in a solution, is it converted to a weak carbonic acid (H2CO3) (Rxn. 3). This weak acid can dissolve CaCO3, to form soluble calcium bicarbonate (Ca(HCO3)2) (Rxn. 4) [14]. As temperatures increases, the 2+ ability of CO2 to dissolve in water decreases, resulting in the precipitation of soluble Ca as CaCO3 and the reported decrease in CaCO3 solubility. This phenomenon can be used to explain the presence of CaCO3 scale in a kitchen kettle, or on the surface of heat exchanger tubes, which have a higher temperature than the bulk solution.

CO + HO → HCO (Rxn. 3)

CaCO() + HCO → Ca + 2HCO (Rxn. 4)

In a CO2 free system, the solubility of CaCO3 increases with increasing temperature (Figure 6, red). This increase is due to calcium hydrolysis via the formation of CaOH+, which increases with a temperature increase [22]. The increase of CaCO3 solubility in the CaCO3-H2O system can also be explained by simple thermodynamic principles. As the temperature of the solution is increased, the average kinetic energy of the molecules increases. This increase in kinetic energy allows H2O to more effectively break apart solid CaCO3 molecules that are held together by intermolecular interactions, and results in the CaCO3 molecules becoming destabilized in their solid state, thus dissolving more readily in solution. At approximately 100°C, the boiling point of water, it is expected that CO2 is unable to dissolve in H2O, therefore the two systems converge. Figure 6 compares the solubility of CaCO3 in both air and a CO2 free environment. This plot has been adapted from OLI Systems Inc. [23], and the individual data points are a compilation of literature data from 1857 to 2012. Since the green liquor is highly alkaline, any acidity induced by dissolved

CO2 will be immediately neutralized preventing reaction 1, which is an acid attack, from happening. It is therefore likely that the solubility of CaCO3 in green liquor will increase with temperature, similar to the CO2 free system.

15

Figure 6. The solubility of CaCO3 in air (blue) and a CO2 free environment (red). Adapted from OLI Systems Inc. [23]. Conversion to ppm was carried out using the density of water at 25°C (0.996845 kg/L).

The solubility and the kinetics of dissolution and precipitation of CaCO3 and its polymorphs has been experimentally studied in the CaCO3-H2O-air system by numerous researchers [24, 25, 26, 27]. In 1923, Askew was one of the first to study the effect the addition of salts has on carbonate solutions, and found that the presence of NaCl and NaNO3 increases the solubility of calcium carbonate [24]. Plummer et al. studied the solubility of calcite, aragonite and vaterite in the CaCO3-

CO2-H2O system between 0 – 90°C and at 1 atm of total pressure. Their experimental work was 2+ 2- used to determine the equilibrium constants for the reaction CaCO3(s) = Ca + CO3 , and the aqueous model was critically evaluated using previous literature studies [25].

Varying the pH of the CaCO3-H2O-air system is known to have a large effect on the solubility of

CaCO3 [27, 28, 29]. Coto et al. [29], used literature data reported by Nakayama [28] and ASPEN PLUS® 7.1 from Aspen Technology Inc. (Bedford, Massachusetts, USA), a simulation software, to develop a model describing the effects of pH on CaCO3 solubility. As can be seen in Figure 7, as the pH of the system increases, a substantial decrease in CaCO3 solubility occurs.

16

Figure 7. The effect of pH on CaCO3 solubility at 25°C and 1 bar air pressure [29]. Conversion to ppm was carried out using the density of water at 25°C (0.996845 kg/L).

- - Green liquor, with its high concentration of Na2CO3, and presence of OH and HS ions, typically has a pH around 12, and therefore cannot be accurately described by the solubility of CaCO3 in neutral aqueous systems. Literature data for CaCO3 solubility does not exist for systems much above a pH of 9, but following the trend in Figure 7, it can be expected to be extremely low. The lack of such data is most likely due to the difficulty in obtaining accurate measurements of Ca2+ at low concentrations and in the presence of large amounts of sodium in solution. It is therefore important to understand the solubility of CaCO3 in systems with high alkalinities.

2.3.2 Calcium Carbonate Solubility in Green Liquor

Although the solubility of CaCO3 in green liquor has never been studied explicitly, Felmy et al.

[30] looked at the solubility of CaCO3 in the CaCO3-Na2CO3-H2O system. As previously discussed, a typical green liquor has a TTA between 110 – 140 g/L Na2O, which corresponds to approximately 1.7 – 2.3 molal of Na2CO3 in the CaCO3-Na2CO3-H2O system. Felmy et al. analyzed CaCO3 concentrations in solutions up to 2 molal (or approximately a TTA of 124 g/L

Na2O), and found that CaCO3 solubility increases above Na2CO3 concentrations of 0.5 molal (or

17 TTA’s of 31 g/L Na2O), in a CO2 free environment at 25°C. This increase in concentration defies 2- the common ion effect, and can be attributed to the formation of the Ca(CO3)2 complex [30]. OLI

Systems Inc. [22] used this data as well as similar studies which included the presence of CO2 to develop a plot of CaCO3 solubility based on Na2CO3 concentration (Figure 8). This plot predicts rd the solubility of CaCO3 in a CO2 free environment follows a 3 order polynomial shape, as can be seen by the red curve. It should be noted that due to a lack of experimental evidence, it is uncertain that CaCO3 solubility would decrease in this system at Na2CO3 concentrations above 2 molal.

Figure 8. The solubility of CaCO3 in Na2CO3 at 25°C (adapted from OLI) [22].

2.4 Calcium Carbonate Stability in Green Liquor Systems

Past studies within the pulp and paper industry [9, 31, 32, 33, 34], primarily focused on pirssonite solubility, have outlined green liquor phase stability diagrams which may indicate the concentration ranges where CaCO3 is likely to form. The most recent paper published by Zakir et al. [9], suggest that pirssonite solubility and therefore CaCO3 stability regions change with temperature, TTA, causticity and sulfidity. An example of pirssonite solubility in the Na2CO3-

CaCO3-NaOH-H2O system at 95°C is shown in Figure 9. From this plot CaCO3 is likely to precipitate in liquors with concentrations falling below the red pirssonite solubility line. Liquors

18 with a TTA value greater than 150 g/L Na2O can expect pirssonite as the main green liquor precipitant.

Figure 9. Stability of CaCO3 and pirssonite in the Na2CO3-NaOH-CaCO3-H2O system at 95°C [9].

In summary, CaCO3 scale formation and solubility has been investigated by numerous researchers, however a complete study has never been carried out with regards to the green liquor environment.

Thermodynamic simulations exist which may be able to predict the solubility of CaCO3 in a high alkaline environment, but these findings are currently not supported by experimental observations. Therefore, there is a need develop fundamental experimental data to understand the solubility of

CaCO3 in green liquor.

19 Chapter 3

3 Current OLI Models of Calcium Carbonate Solubility

Although experimental data does not exist for CaCO3 solubility in alkaline green liquor solutions, thermodynamic simulation software can be used to predict the solubility of CaCO3 in this environment. In this chapter, the green liquor system which composed of Na2CO3, NaOH, Na2S, and H2O was divided into three systems: CaCO3-Na2CO3-H2O, CaCO3-Na2CO3-NaOH-H2O, and

CaCO3-Na2CO3-Na2S-H2O, for a solubility analysis using the OLI software. A review of the OLI Systems Inc. software to be used is included, and the existing model predictions are discussed.

3.1 OLI Software

OLI is a thermodynamic simulation software developed by OLI Systems Inc. (Cedar Knolls, New Jersey, USA) which can be used to model conventional, aqueous, and other complex systems. OLI’s software is built in the OLI Engine, which is the basis of OLI’s specialty software. One of OLI’s products is OLI Studio which supports single and multi-point calculations using OLI’s Stream Analyzer software and extensive public databases. The Stream Analyzer software can perform equilibrium calculations for electrolyte chemistry. Single point or multi point survey calculations for trend analysis of temperature, pressure, pH, and composition effects are all possible with this software. The calculations provide vapour, liquid, solid, and 2nd liquid phase separations for a fully speciated model, and properties such as pH, viscosity, density, enthalpy as well as compositions can be determined. OLI’s Stream Analyzer has three available thermodynamic models which are based on experimental literature data. The Aqueous (AQ) model is OLI’s original electrolyte thermodynamic framework. This model is primarily applicable to dilute aqueous solutions up to a mole fraction of 0.3, and uses the Bromley-Zemaitis activity coefficient model [22]. The MSE (Mixed Solvent Electrolyte) model can treat systems of any composition, and is typically more accurate with solutions of high concentration, as they are beyond the AQ model’s capability. The MSE framework is based on the excess Gibbs energy model [22]. The MSE-SRK model is a new model for oil and gas production applications, it is a combination of the MSE model for electrolyte systems, and the SRK (Soave-Redlich-Kwong) for gas phase and the second liquid, or non-electrolyte process [22]. In this work, the MSE model will

20 be used to model the green liquor system. The MSE model houses a public (MSEPUB) database which will be used in this chapter to model calcium carbonate solubility in green liquor and to explore the limitations of the present-day software.

3.2 Calcium Carbonate Solubility Using OLI’s MSE Model

OLI simulations for CaCO3 solubility as a function of temperature for the CaCO3-Na2CO3-H2O system at a TTA of 60 g/L Na2O (0.968 molal Na2CO3), and 120 g/L Na2O (1.935 molal Na2CO3) is shown in Figure 10. The solubility of CaCO3 appears to decrease with increasing temperature, and increases with increasing Na2CO3 concentration as discussed in Chapter 2.

Figure 10. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-H2O system using the MSEPUB database.

OLI simulations of the CaCO3-Na2CO3-NaOH-H2O and CaCO3-Na2CO3-Na2S-H2O systems can - - be used to describe how CaCO3 solubility will be affected by OH and HS ions. Although no experimental studies have been carried out, known thermodynamic properties and ion interaction parameters can be used to estimate solubility using OLI Studio. Figure 11 depicts the OLI generated CaCO3 solubility curves for the CaCO3-Na2CO3-NaOH-H2O system. Solubility appears

21 to decrease with increasing temperature and causticity. According to OLI, the increase in NaOH concentration causes a pH increase from 11.8 to 14.1 at 25°C, thereby causing the decrease in

CaCO3 solubility. The same trend is depicted at high temperatures however the effect is minimal.

Figure 12 depicts the OLI generated solubility curve for the CaCO3-Na2CO3-Na2S-H2O system. A decrease in CaCO3 solubility is depicted, also owing to the decrease in pH. All models appear to suggest the solubility will decrease with increasing temperature.

Figure 11. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system using the MSEPUB database (TTA= 120 g/L Na2O).

22

Figure 12. OLI simulation of CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system using the MSEPUB database (TTA= 120 g/L Na2O).

These OLI simulations all indicate the suggested solubility of CaCO3 in green liquor is extremely low. However, since OLI’s models are based on experimental data, the validity of these simulations should be questioned given the lack of literature CaCO3 solubility data in high alkaline solutions of various sodium salts. To confirm or disprove these models an experimental study of

CaCO3 solubility of green liquor must be carried out, as is discussed in Chapter 4.

23 Chapter 4

4 Experimental Investigation of Calcium Carbonate Solubility

To reliably determine the solubility of CaCO3 in green liquor, laboratory experiments were carried out. As per the OLI model simulations, the green liquor system was divided into three systems for analysis: CaCO3-Na2CO3-H2O, CaCO3-Na2CO3-NaOH-H2O, and CaCO3-Na2CO3-Na2S-H2O. The results of these solubility experiments are summarized in this chapter. In Chapter 5, the results of the solubility experiments are used to develop a predictive model of CaCO3 solubility using the OLI software.

4.1 Experimental Procedures

4.1.1 Apparatus

Two experimental setups were used when analyzing the solubility of CaCO3; a constant temperature and varying temperature setup. Constant temperature experiments were carried out using a silicone oil bath, which held multiple 100 mL Erlenmeyer flasks (Figure 13). Due to the extremely low solubility of CaCO3 and the difficulty in obtaining precise measurements, this setup was used so that identical experiments could be run concurrently. Solubility experiments which involved varying the temperature were carried out in a 1 L jacketed reactor vessel (Figure 14). An automatic stirrer was fastened to the top of the reactor and a circulation bath with silicone oil was used to heat the bath to the desired temperature.

Rubber stopper Silicone oil bath

Erlenmeyer Flasks Stir bar

Stirrer Temperature Control Control

Figure 13. Experimental setup used for constant temperature reactions.

24

Figure 14. Experimental setup used for reactions where temperature was varied.

4.1.2 Materials and Methodology

Sodium carbonate (ACS grade, anhydrous, 99.5%), sodium sulfide (technical grade, mixture of various hydrates), calcium carbonate (calcite, ASC grade, 99.0%), and sodium hydroxide (ACS grade, 97.0%) were obtained from Fischer Scientific and used as received to prepare green liquor solutions for analysis. All solutions were prepared by dissolving chemicals in deionized water (DIW) obtained from a Direct-Q 3 UV System (Millipore Corporation). Liquid samples were taken from the reaction vessel using a 20-gauge needle attached to a plastic syringe which were heated to the reaction temperature, and samples were acquired by puncturing the rubber stopper of the sampling port, ensuring evaporation of the solution never occurred. Immediately following sample acquisition, the liquid was filtered using a 0.22 µm polyethersulfone (PES) membrane sterile filter

(FroggaBio), and the filtrate was diluted with 5% nitric acid (HNO3), or deionized water if the system contained Na2S, to prevent precipitation. Solid samples were collected by filtering the liquid through a 9 cm filter paper with a particle retention of 25 µm (VWR North America). Calcium concentration was determined using an Agilent 700 Series Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), which was calibrated using reference solutions

25 prepared with a standard calcium solution (Fischer Chemical, 1000 ppm ± 1%). Concentrations of 2- - - CO3 , OH , HS ions was determined via titration using a Mettler Toledo G10S Compact Titrator with 1N HCl as the titrant. Thermogravimetric Analysis (TGA) was performed on solid samples using a TA Instruments STD-Q600 Simultaneous Thermogravimetry and Differential Scanning Calorimeter (TGA/DSC). Samples were analyzed using the following procedure: equilibrate the sample in a N2 environment at 20°C then ramp the temperature at 20°C/min to 950°C. TGA/DSC profiles were compared again standard CaCO3 from Fisher Scientific and pirssonite from Mineralogical Research Company shown in Appendix I: TGA/DSC Standard Profiles. Power X- Ray diffraction (XRD) was performed using a Philips XRD system with a PW 1830 HT generator, a PW 1050 goniometer, PW3710 control electronics and X-Pert system software, to identify the phase of the bulk material. Microscopic images were taken of the scale samples using a JEOL JSM-6610LV Scanning Electron Microscope (SEM).

4.1.3 Solubility Units

All expressions of CaCO3 solubility have been presented in parts per million (ppm). In this thesis 1 ppm is equivalent to 1 mg per L of green liquor solution. To convert to standard units of molality, the density of the appropriate synthetic green liquor solution is needed, which has been provided in Appendix III: Density of Synthetic Green Liquor Solutions. Alternatively, if available, OLI simulations provide an accurate estimate of green liquor density.

4.2 Results and Discussion

4.2.1 CaCO3-Na2CO3-H2O System

The CaCO3-Na2CO3-H2O system was investigated at 25°C and between 55°C and 95°C. Na2CO3 concentration was varied to produce synthetic liquors with a TTA of 60, 120, and 180 g/L Na2O, and the effect of temperature and TTA on CaCO3 solubility was studied. It should be noted that at a Na2CO3 concentration, or TTA, of 180 g/L Na2O the precipitant is pirssonite rather than calcite. Kinetics experiments, which required multiple trials, were carried out in the silicone oil bath, and experiments where temperature was varied were carried out using the 1 L jacketed reactor vessel.

Na2CO3 was dissolved in water, and once the solution appeared clear in colour, approximately

26 2000 ppm of CaCO3 was added to the system. Samples were taken at specified time intervals to determine Ca2+ concentration using ICP-OES.

It should be noted that due to the extremely low solubility of CaCO3 in this system, obtaining precise measurements was challenging. Kinetics experiments were carried out at 25°C and 95°C, and Figure 15 shows the kinetics plot for the CaCO3-Na2CO3-H2O system with a Na2CO3 concentration, or TTA of 120 g/L Na2O at 95°C. In this plot, the measured solubility of CaCO3 throughout four identical trials can be seen. At some instances, there is a large deviation in the measured values, therefore to obtain accurate results of CaCO3 solubility, typically 3 or 4 trials were run for each system and two samples were taken at each time interval. Figure 16 and Figure 17 present the results as the mean value of the trials for each system. Data for 25°C is shown in Figure 16 and data for 95°C is shown in Figure 17. The standard error of the mean was calculated for each data point.

Figure 15. Kinetics plot for the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA= 120 g/L Na2O).

27

Figure 16. Kinetics plot for the CaCO3-Na2CO3-H2O system at 25°C.

Figure 17. Kinetics plot for the CaCO3-Na2CO3-H2O system at 95°C.

28 Analyzing Figure 16 and Figure 17 all systems appear to have a high initial CaCO3 solubility which decreases with time, reaching equilibrium after approximately 4 hours. The decrease in

CaCO3 solubility is interesting as experiments were performed by dissolving CaCO3 into the system, so one would expect the soluble calcium concentration to increase over time. The observed high initial concentration of calcium can be explained as follows: the ASC grade Na2CO3 contains approximately 0.01% calcium impurities, when using high concentrations of Na2CO3 up to 2000 ppm of calcium may be introduced into the system resulting in the large initial concentration.

When solid CaCO3 particles are added to the reactor, they act as seeding sites for the dissolved calcium to precipitate, decreasing the calcium concentration until equilibrium is reached.

Table 1 and Figure 18 show the equilibrium solubility of CaCO3 at temperatures between 55°C and 95°C for the three liquors in ppm Ca2+. The solubility was found to increase with respect to 2+ Na2CO3 concentration, or TTA, and the Ca concentration in 60, 120, and 180 g/L Na2O solution at 95°C is about 2.35, 5.46, and 9.76 ppm respectively. An increase in CaCO3 solubility with temperature was also found to be consistent for all liquors. At 25°C, a liquor with a TTA of 60 g/L 2+ Na2O had a Ca concentration of 1.23 ppm compared to 2.35 ppm at 95°C.

Table 1. CaCO3 solubility in the CaCO3-Na2CO3-H2O system. 2+ Temperature Ca (ppm)

(°C) Na2CO3 = 60 g/L Na2O Na2CO3 = 120 g/L Na2O Na2CO3 = 180 g/L Na2O 25 1.23 ± 0.09 4.26 ± 0.14 8.93 ± 0.08 55 1.36 ± 0.01 4.50 ± 0.96 8.97 ± 0.88 65 1.44 ± 0.06 4.62 ± 0.55 9.00 ± 0.55 75 1.67 ± 0.10 4.87 ± 0.31 9.18 ± 0.10 85 2.15 ± 0.04 4.94 ± 0.20 9.44 ± 0.04 95 2.35 ± 0.16 5.46 ± 0.40 9.76 ± 0.50

29

Figure 18. CaCO3 solubility in the CaCO3-Na2CO3-H2O system.

As mentioned in Chapter 2 and Chapter 3, the increase in CaCO3 solubility with temperature is most likely due to the formation of the CaOH+ complex, which increases with a temperature increase. The increase in CaCO3 solubility with increasing Na2CO3 concentration follows the trend first published by Felmy et al., who suggested the increase in solubility at high Na2CO3 molality 2- (> 0.5 molal, or 31 g/L Na2O) is due to the formation of the Ca(CO3)2 complex [30].

Solid samples were collected from the reactor for analysis via TGA/DSC and XRD to determine the composition and phase of the precipitate. The results are shown in Figure 19 - Figure 21 for solid samples collected at equilibrium for the 60, 120, and 180 g/L Na2O systems respectively.

The TGA/DSC figures were compared to the standard profiles of CaCO3 and pirssonite which can be found in Appendix I: TGA/DSC Standard Profiles. A typical thermal profile of CaCO3 depicts a weight loss between 620 – 830°C corresponding to the decomposition of CaCO3 into CO2 and

CaO (Rxn. 5). The theoretical weight loss of CO2 from CaCO3 is 44%.

����() → ���() + ��() (Rxn. 5)

30 The thermal profile of pirssonite is more complicated than that of CaCO3. The first weight loss peak is typically seen between 150 – 250°C and corresponds to pirssonite dehydration (Rxn. 6), resulting in a 15% weight loss. The second weight loss peak is seen between 670 – 770°C and corresponds to the decomposition of CaCO3 within pirssonite (Rxn. 7) resulting in a 18% weight loss. Finally, at approximately 360°C and 430°C solid phase transitions of the anhydrous pirssonite occurs, and at 850°C Na2CO3 begins to melt [31]. Both events result in heat flow fluctuations. It is expected that the solids collected from the 180 g/L Na2O system are pirssonite.

���� ∙ ���� ∙ ��() → ���� ∙ ����() (Rxn. 6)

���� ∙ ����() → ����() + ���() + ��() (Rxn. 7)

The XRD profiles were matched to the figures published by Ni and Ratner [21]. The major peak of calcite (C) can be found at 29.5°, aragonite (A) is typically identified by a peak at 26.2° and vaterite (V) at 27.4°. Pirssonite (P) peaks were identified using a publication by Dickens and Brown found within the American Mineralogist Crystal Structure Database. The most intense peak of pirssonite can be seen at 33.6° [35].

31

Figure 19. Thermal profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 60 g/L Na2O).

Figure 20. Thermal profile of solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 120 g/L Na2O).

32

Figure 21. Thermal profile of solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 180 g/L Na2O).

Figure 22. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 60 g/L Na2O).

33

Figure 23. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 120 g/L Na2O).

Figure 24. XRD profile of the solid sample collected from the CaCO3-Na2CO3-H2O system at 95°C ([Na2CO3] = TTA = 180 g/L Na2O).

The TGA/DSC and XRD profiles for solid samples from the systems with a TTA of 60 and 120 g/L Na2O suggest calcite is the only precipitant in the reaction vessel. The TGA/DSC profile for the solid sample collected from the system with a TTA of 180 g/L Na2O shows the beginning of

34 pirssonite formation. From the TGA profile, a small decrease can be seen between 150 – 200°C from water dehydration, heat flow fluctuations can be seen at 430°C and 850°C. The XRD profile shows small peaks which may be labelled as pirssonite. As the composition of pirssonite in this sample is low, the peaks are much smaller than the calcite peaks indicating a mixture of calcite and pirssonite, most likely due to the excess amount CaCO3.

4.2.2 CaCO3-Na2CO3-NaOH-H2O System

The CaCO3-Na2CO3-NaOH-H2O system was investigated at 95°C for a liquor with a constant TTA of 120 g/L Na2O to determine the effect of varying NaOH concentration, or causticity on CaCO3 solubility. As mentioned, a typical green liquor has a causticity between 5-20%, which corresponds to a NaOH concentration between 6 to 24 g/L Na2O when the TTA is to 120 g/L Na2O. For these experiments, the effect of NaOH concentration was studied up to 30 g/L Na2O. Concentrations of 2- - 2+ CO3 , and OH , ions was determined via titration, and Ca concentration was determined using ICP-OES.

Figure 25 shows the kinetics results for a liquor with a NaOH concentration of 6 g/L Na2O. As with the CaCO3-Na2CO3-H2O system, equilibrium is reached after approximately 4 hours. The standard error of the mean was calculated for each data point. Table 2 and Figure 26 show the equilibrium CaCO3 concentration at NaOH concentrations up to 30 g/L Na2O at 95°C. A slight decrease in CaCO3 solubility with increasing NaOH concentration was found, which can be attributed to the increase in the alkalinity as explained earlier. Solid samples were collected from the reaction vessel for analysis via TGA/DSC and XRD. The results can be found in Appendix II: TGA/DSC and XRD Experimental Sample Profiles. All solid samples were found to be pure calcite.

Table 2. CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system (95°C, TTA = 120 g/L Na2O).

2+ NaOH (g/L Na2O) Ca (ppm) 0 5.49 ± 0.40 6 5.34 ± 0.29 12 5.17 ± 0.30 18 5.07 ± 0.12 30 4.98 ± 0.36

35

Figure 25. Kinetics plot for the CaCO3-Na2CO3-NaOH-H2O system (NaOH = 6 g/L Na2O).

Figure 26. CaCO3 solubility in the CaCO3-Na2CO3-NaOH-H2O system (95°C, TTA = 120 g/L Na2O).

36 4.2.3 CaCO3-Na2CO3-Na2S-H2O System

The CaCO3-Na2CO3-Na2S-H2O system was investigated at 95°C for a liquor with a TTA of 120 g/L Na2O to determine the effect of Na2S concentration on CaCO3 solubility. A typical green liquor has a sulfidity between 30-40%, and for a liquor with a TTA of 120 g/L Na2O, corresponds to a

Na2S concentration between 24 to 48 g/L Na2O. For these experiments, the solubility of CaCO3 was investigated in liquors with Na2S concentrations up to 60 g/L Na2O. The physical appearance of this system was yellow/green which differed from the previous two systems which appeared white in colour. It is likely the Na2S gives green liquor its characteristic colour. Concentrations of 2- - 2+ CO3 , and HS , ions was determined via titration, and Ca concentration was determined using ICP-OES.

Figure 27 shows the kinetics results for a liquor with a Na2S concentration of 48 g/L Na2O. Equilibrium is reached after approximately 4 hours. The standard error of the mean was calculated for each data point. Table 3 and Figure 28 shows the equilibrium CaCO3 concentration at Na2S concentrations up to 60 g/L Na2O. A decrease in CaCO3 solubility with increasing Na2S concentration was found, and this can be attributed to an increase in the pH of the system as well as an effect of Na2S addition. The solubility of CaCO3 decreases more dramatically when compared to the CaCO3-Na2CO3-NaOH-H2O system, therefore it is likely that Na2S has an additional effect on CaCO3 solubility. Solid samples were collected from the reaction vessel for analysis via TGA/DSC and XRD. The results can be found in Appendix II: TGA/DSC and XRD Experimental Sample Profiles. All solid samples were found to be pure calcite.

Table 3. CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system (95°C, TTA= 120 g/L Na2O).

2+ Na2S (g/L Na2O) Ca (ppm) 0 5.49 ± 0.40 12 4.67 ± 0.11 24 3.93 ± 0.01 36 3.49 ± 0.23 48 2.27 ± 0.17 60 1.80 ± 0.06

37

Figure 27. Kinetics plot for the CaCO3-Na2CO3-Na2S-H2O system (Na2S = 48 g/L Na2O).

Figure 28. CaCO3 solubility in the CaCO3-Na2CO3-Na2S-H2O system (95°C, TTA = 120 g/L Na2O).

38 4.3 Solid Sample SEM Analysis

In addition to TGA/DSC and XRD analysis of the solid precipitate, SEM imaging was used to study the surface of the crystals. All precipitates collected from the reaction vessels were a soft white powdery substance. Analyzing the SEM images of the precipitate collected from the CaCO3-

Na2CO3-H2O system with a TTA of 120 g/L Na2O at 95°C in Figure 29, the CaCO3 particles have a spherical shape with a grain size of less than 0.5 µm, and tend to agglomerate in groups with a diameter of approximately 5 µm. The physical appearance of the precipitate, although still calcite, is significantly different from the hard scale found in industry. This topic will be further discussed in Chapter 6.

Figure 29. SEM images at various magnifications of the solid sample collected from the CaCO3-Na2CO3- H2O system at 95°C (TTA = 120 g/L Na2O). Left (x850), Middle (x5000), Right (x30000).

4.4 Summary

In this chapter, the solubility of CaCO3 in synthetic green liquor solutions was found to increase with temperature, which contradict what OLI’s MSEPUB database predicts as presented in

Chapter 3. Experimental evidence found that CaCO3 solubility increases with increasing Na2CO3 2- concentration, most likely due to the formation of the Ca(CO3)2 complex as suggested by Felmy et al, and CaCO3 solubility was found to decrease with NaOH and Na2S concentration due to an increase in alkalinity and an effect of sulfide addition. Using this new solubility data, a predictive model for CaCO3 solubility in green liquor will be developed using the OLI software in Chapter 5.

39 The solid calcite precipitate collected during the experiments was significantly different from the calcite scale seen in industry. The solid collected throughout this study was soft and powdery, however green liquor scale is typically extremely hard. Due to this difference, a study of calcite scale formation in industry is necessary to help understand the hardness and the mechanism behind calcite scale formation in mills.

40 Chapter 5

5 Thermodynamic Modelling of Calcium Carbonate Solubility

This chapter focuses on the creation of a new database in OLI which will be able to predict the solubility of CaCO3 within green liquor. To develop this database, the experimental data discussed in Chapter 4 will be used.

5.1 OLI Software

As discussed in Chapter 3, OLI is a thermodynamic simulation software developed by OLI Systems Inc. which can be used to simulate electrolyte systems. Introduced in Chapter 3 was the program, OLI Studio, which supports the Stream Analyzer software to perform equilibrium calculations for electrolyte chemistry. It was shown that despite its ability to run simulations using the MSEPUB database to determine the solubility of CaCO3 in green liquor, there is no experimental evidence to validate these simulations. In this chapter, the Environmental Simulation

Program, ESP, will be used to develop a prediction of CaCO3 solubility using MSE model and the new solubility data discussed in Chapter 4. The database will be built by regressing standard state thermodynamic properties of CaCO3, as well as ion-interaction parameters for the green liquor system. Once the database has been created, it can be uploaded to the Stream Analyzer software to perform accurate equilibrium calculations for CaCO3 solubility in green liquor.

5.2 Calcium Carbonate Database

OLI/ESP has a public database called MSEPUB which contains thermodynamic data for most compounds, including CaCO3. The original standard state properties for CaCO3 found in the MSEPUB database are shown in Table 4. To create a private database, the original standard state properties and new ion interaction parameters were regressed using the new CaCO3 solubility data.

It should be noted that the database was developed for green liquor solutions below Na2CO3 saturation (i.e. below the pirssonite solubility curves presented by Zakir [31]), therefore solubility data for the CaCO3-Na2CO3-H2O system with a TTA of 180 g/L Na2O was excluded for this regression. Numerous iterations found that regressing the ion interaction parameters between the

41 Ca2+ and Na+, Ca2+ and OH-, Ca2+ and HS-, and HS- and Na+ ions, provided the most accurate representation of the CaCO3 solubility data in green liquor. As a best practice, ion interaction parameters that were already in the MSEPUB database were not changed, however new parameters were added. The original ion interaction parameters found in the MSEPUB database are shown in Table 5.

When running regressions and simulations in the OLI software, all concentrations must be expressed in units of molality. All CaCO3 solubility concentrations have been converted between ppm and molality using the solution density at 25°C provided by the OLI software. To ensure the density values within OLI were accurate, the density of the CaCO3-Na2CO3-H2O, CaCO3-Na2CO3-

NaOH-H2O, and CaCO3-Na2CO3-Na2S-H2O systems were verified experimentally. A comparison between the density provided by OLI and the experimentally determined density can be found in Appendix III: Density of Synthetic Green Liquor Solutions.

Table 4. Original standard state properties of CaCO3 found in the MSEPUB database.

Property Aqueous Phase Solid Phase

° DfG 298 (kJ/mol) -1099.61 -1128.87

° DfH 298 (kJ/mol) -1193.11 -1207.86

° S 298 (J/mol/K) 41.24 89.90

Table 5. Original ion interaction parameters found in the MSEPUB database.

Ion Parameter Interaction BMD0 BMD1 BMD2 BMD3 CMD0 CMD1 CMD2 Ca2+ and Na+ 11.27 -2.64E-2 2905.50 - - - -6685.21 Ca2+ and OH------Ca2+ and HS------HS- and Na+ 50.25 -0.12 - - -56.67 0.12 -

Table 6 and Table 7 summarize the regressed standard state properties of CaCO3 and the ion interaction parameters that were used to create a new database in OLI/ESP. The new ion interaction parameters are highlighted in red and underlined, and the original ion interaction parameters (in black) are the same as the parameters summarized in Table 5.

42 Table 6. Regressed standard state properties of CaCO3.

Property Aqueous Phase Solid Phase

° DfG 298 (kJ/mol) -1103.91 -1136.53

° DfH 298 (kJ/mol) -1157.94 -1198.69

° S 298 (J/mol/K) 173.63 146.34

Table 7. Regressed ion interaction parameters (in red) for the green liquor system.

Ion Parameter Interaction BMD0 BMD1 BMD2 BMD3 CMD0 CMD1 CMD2 Ca2+ and Na+ 11.27 -2.64E-2 2905.50 -1.51E-3 -318.31 1.79 -6685.21 Ca2+ and OH- 6.68 - - - 9.37 - - Ca2+ and HS- 2616.17 - - - -3729.06 - - HS- and Na+ 50.25 -0.12 377043.7 - -56.67 0.12 -248412.4

Plots of the experimental values and OLI’s prediction can be seen in Figure 30, Figure 31, and Figure 32. The absolute average relative deviation for the experimental data point and the OLI prediction was determined for each data point, and the results are shown in Figure 33. The largest deviation between the experimental data and the OLI model is 8.26%, and the smallest deviation is 0.31%. On average, the experimental data and OLI model has a 2.20% deviation.

43

Figure 30. Comparison of experimental data and model performance for the solubility of CaCO3 with temperature in the CaCO3-Na2CO3-H2O system.

Figure 31. Comparison of experimental data and model performance for the solubility of CaCO3 in the CaCO3-Na2CO3-NaOH-H2O system.

44

Figure 32. Comparison of experimental data and model performance for the solubility of CaCO3 in the CaCO3-Na2CO3-Na2S-H2O system.

Figure 33. The deviation between the experimental data points and OLI model.

45 5.3 Modelling of Calcium Carbonate Solubility Using OLI Stream Analyzer

The newly created CaCO3 database can be uploaded to OLI Stream Analyzer to perform CaCO3 solubility calculations outside of the range of the experimental systems. Results of OLI simulations using the developed database can be seen in Figure 34 to Figure 37. Note that the OLI software

operates in units of molality. All CaCO3 solubility concentrations have been converted to ppm using the solution density provided by the OLI software.

5.3.1 Effect of Temperature on the Na2CO3-NaOH-Na2S-CaCO3-H2O System

The OLI model was used to predict the solubility of CaCO3 in the complete green liquor system

(CaCO3-Na2CO3-NaOH-Na2S-H2O) at a TTA of 120 g/L Na2O. Two temperatures were compared, 75 °C and 95 °C, and the results are shown in Figure 34 and Figure 35. As expected, the solubility

of CaCO3 decreases with increasing causticity and sulfidity, whereas it increases with increasing

temperature. The sulfidity of the liquor appears to have a greater effect on CaCO3 solubility than the causticity, and the effect of temperature appears to be more significant at lower sulfidity and causticity.

Figure 34. OLI generated CaCO3 solubility curve Figure 35. OLI generated CaCO3 solubility curve for the CaCO3-Na2CO3-NaOH-Na2S-H2O system for the Na2CO3-NaOH-Na2S-CaCO3-H2O system (TTA = 120 g/L Na2O, T=75°C). (TTA = 120 g/L Na2O, T=95°C).

These results suggest that mills operating with green liquors at lower temperatures, higher sulfidity and higher causticity will most likely see increased calcite scaling. These results can be used to

46 predict the amount of calcium that will precipitate from the green liquor solution as a result of temperature change. For example, if a mill has a green liquor with 120 g/L Na2O TTA, 10% causticity and 30% sulfidity, a decrease in green liquor temperature from 95 °C to 75 °C will lower the amount of dissolved calcium from 2.8 to 2.6 ppm.

5.3.2 Effect of Green Liquor Reduction on CaCO3 Solubility

Introduced in Chapter 1 was green liquor reduction is commonly expressed as percentage of

Na2S/(Na2S + Na2SO4), where Na2S and Na2SO4 are concentrations in g/L Na2O. Typical values for the reduction in green liquor are between 80 – 90%. To determine the effect of Na2SO4, the concentration of Na2S was kept constant and the Na2SO4 concentration was varied. Calculations using the newly created OLI database show that increasing the green liquor reduction will lower the solubility of CaCO3 (Figure 36). This implies that operating at a low green liquor reduction efficiency should not be the cause of the calcite scale formation problem.

Figure 36. The effect of green liquor reduction on CaCO3 solubility at 95°C (TTA = 120 g/L Na2O, 10% causticity, 30% sulfidity).

47 5.3.3 Effect of Chloride on CaCO3 Solubility

Depending on the mill, liquor cycles may have problems with non-process element accumulation, most notably chloride (Cl) and potassium (K) which can enter the process from the wood [2]. If there are sufficient levels of chloride in black liquor, HCl may be emitted from the recovery boiler which can be a problem due to increasing stringent environmental regulations. The amount of Cl and K in the recovery cycle is typically expressed as fractional units such as Cl/(Na+K) and K/(Na+K) [36]. The typical concentration of Cl in the recovery cycle is 1.5 mol % Cl/(Na+K) [36].

Figure 37 summarizes the effect of Cl in green liquor on CaCO3 solubility by adding NaCl to the system. The potassium concentration in this simulation is zero. Increasing Cl concentration results in an increase in CaCO3 solubility. From Figure 37, at 0 mol % Cl/(Na+K) and 95 °C the solubility of CaCO3 is 2.87 ppm, which increases to 3.95 ppm at 10 mol % Cl/(Na+K). At 1.5 mol % Cl/(Na+K), which is a more realistic chloride concentration in the green liquor system, the solubility of CaCO3 has increased to 3 ppm. The results suggest that although the presence of a large amount of Cl in the recovery cycle may cause problems in the recovery boiler, it does not appear to have a negative effect on CaCO3 scale formation. As expected, the 95°C liquor has a higher solubility than the 75°C liquor.

48

Figure 37. The effect of chloride on CaCO3 solubility (TTA = 120 g/L Na2O, 10% causticity, 30% sulfidity).

5.4 Summary

In this chapter, a model was developed using OLI software to accurately predict CaCO3 solubility in the green liquor system based on the experimental results obtained in Chapter 4. The OLI model simulates the experimental results with approximately a 2% error. Through running simulations in

OLI Studio using the newly created database it was found that the presence of Na2SO4 and Cl in green liquor increases the solubility of CaCO3, demonstrating that their presence should not contribute to increased calcite scale formation.

49 Chapter 6

6 Causticizing Plant Analysis of Pulp Mills

The results of the laboratory studies demonstrate the solubility of CaCO3 in green liquor, however the scale collected throughout the experiments was found to be a soft powdery substance unlike the extremely hard scale found in industry. Therefore, CaCO3 scale formation needs to be looked at from a wider perspective.

An analysis of pulp mills in Canada and Sweden was carried out to understand each individual causticizing plant and compare the scaling problems within their green liquor systems. A summary of each pulp mill has been included and the chemical properties of each pulp mill have been compared. An onsite trial was implemented at Mill A which will be used to monitor scale growth within the green liquor pipelines.

6.1 Summary of Pulp Mills

6.1.1 Mill A

Mill A is a single line kraft mill located in Alberta, Canada. It was designed in 1989 produce to 1110 air dried metric tons/day (admt/d) of bleached hardwood. Upgrades have resulted in an increase of production targets to 1650 admt/d of bleached hardwood pulp and 1200 admt/d of bleached softwood pulp, and the mill currently runs in campaigns of hardwood and softwood. The flow rate of the clarified green liquor lines must be above 70 L/s to reach the increased production targets [37]. The mill uses an anti-scaling agent in their green liquor system to prevent calcite scale build up, however scaling within the green liquor lines is still reported. Two visits were made to Mill A in July 2017, and February 2018.

6.1.2 Mill B

Mill B is located in Manitoba, Canada, and was first constructed in 1970. The mill produces 500 admt/d of High Performance Unbleached Sack Kraft Paper using an equal mixture of softwood spruce and pine feed. The mill is a single line with a conventional recovery cycle. Intensive scaling

50 was reported at Mill B in October 2017 which caused the mill to shut down due to complete blocking of the RGL lines (Figure 38). This mill was not visited, however scale samples from the blockage in 2017 were collected and sent to UofT for analysis.

Figure 38. Calcite scale formation in a green liquor pipeline. (Courtesy of Mill B)

6.1.3 Mill C

Mill C is located in central Sweden, and was built in 1917. The mill currently has a production capacity of 775 000 admt/y (approximately 2120 admt/d). The wood feed is mainly pine (softwood) with up to 10% birch (hardwood). Mill C produces bleached and unbleached kraft pulp as well as chemithermomechanical pulp (CTMP), and the pulp products are used to make liquid packaging board. Green liquor scaling is rarely reported as a problem at Mill C.

6.1.4 Mill D

Mill D is located on Sweden’s eastern coastline, and is a non-integrated kraft pulp mill which began its first production in 1895, and has grown to have a current capacity of 540 000 admt/y (1480 admt/d). The mill has three production lines, or digesters, producing three products: supreme which is a stand quality pulp used for folding boxboard, newsprint, and coated fine paper and is produced from softwood (pine and spruce), select which has a high purity and low brightness reduction and is used for folding boxboard and is produced from hardwood (birch), and care which

51 is a fluff grade used for hygienic purposes and is produced from softwood (pine and spruce). The mill is the largest fluff producer in Europe with a capacity of 260 000 t/a. Green liquor scaling rarely causes problems Mill D.

A summary of each pulp mill can be found in Table 8.

Table 8. Summary of studied pulp mills.

Property Mill A Mill B Mill C Mill D

Location Canada Canada Sweden Sweden Year Production Began 1989 1970 1917 1895 Annual Production 520 000 182 500 775 000 540 000 (admt/y) Hardwood / Softwood, 2/3 softwood, Wood Type softwood Softwood 10% 1/3 hardwood campaigns hardwood Anti Scaling Chemical Yes Yes No No Pipeline Material SS 304 SS 304 SS 304 SS 304 Pipeline Insulation Mostly Fully Fully Fully Pipeline ID (cm) 20 15/20 20 20

6.2 Pulp Mill Liquor Analysis

During visits to Mill A in February 2018, and Mill C and D in June 2018 the RGL, CGL, and WW was analyzed using laboratory techniques for TTA, causticity, sulfidity, density, and suspended solids. The purpose of this analysis is to identify any abnormalities in the mill liquor, which could be used to explain the reported scaling at Mill A, and the lack of scaling at Mill C and Mill D. Hot liquor samples from Mill A and Mill C were collected and filtered on site and sent back to UofT for soluble calcium and total calcium analysis using ICP-OES. The total calcium was determined by digesting the liquors using nitric acid (HNO3) at 95°C for two hours. The soluble calcium calculated by OLI is indicated in brackets.

Table 9 summarizes the measured chemical properties for each mill. Two measured values are included for the RGL of Mill C as there are two RGL lines (one from each dissolving tank). The

52 measured data was compared to the average hourly data from the past 6 months, when available, and this comparison can be found in Appendix V: Mill Data. The agreement between the laboratory data and the online mill data is good in all cases.

Table 9. Summary of the measured chemical properties of liquors from Mill A, Mill C and Mill D.

Property Mill A Mill C Mill D

RGL TTA (g/L Na2O) 137 140 125/131 RGL Causticity (%) 14 10 3/4 RGL Sulfidity (%) 24 31 25/29 RGL Density (kg/L) 1.20 1.20 1.18/1.19 RGL Suspended Solids (ppm) 2612 963 1389/1392 RGL Soluble Ca2+ (ppm) 14.7 (4.55)1 3.4 (4.21) *2 RGL Total Ca2+ (ppm) 101 106 *

CGL TTA (g/L Na2O) 115 136 130 CGL Causticity (%) 17 11 3 CGL Sulfidity (%) 25 32 29 CGL Density (kg/L) 1.16 1.19 1.19 CGL Suspended Solids (ppm) 35 80 22 CGL Soluble Ca2+ (ppm) 8.1 (2.75) 5.1 (3.88) * CGL Total Ca2+ (ppm) 7.6 115 *

WW TTA (g/L Na2O) 17 11 14 WW Causticity (%) 74 66 19 WW Sulfidity (%) 15 33 28 WW Density (kg/L) 1.02 1.01 1.02 WW Suspended Solids (ppm) 4 392 579 WW Soluble Ca2+ (ppm) 0.2 (0.99) 1.0 (0.57) * WW Total Ca2+ (ppm) 5.5 103 *

1 Calculated soluble calcium (OLI), 2 Property was not analyzed

53 All measured data appears to fall within the normal ranges of a kraft pulp mill. There are minimal differences between the TTA, causticity and sulfidity of the 3 mills, suggesting a slight change in liquor concentration does not play a large role in scale formation. It should be noted that the measured RGL TTA (137 g/L Na2O) for Mill A is significantly higher than the average value (120 g/L Na2O, Table 13). The operator mentioned it was very difficult to control the TTA in the dissolving tank of Mill A, and TTA swings from 115 g/L Na2O to 135 g/L Na2O in a 12 hour period were not uncommon. These TTA swings most likely lead to increased scaling occurrences in Mill A’s RGL line.

Looking at the Ca2+ concentrations in liquor, it can be seen that the measured concentration is higher than OLI’s calculated concentration in almost all cases. In Mill C, it is apparent that the total Ca2+ concentration in the WW is much higher compared to Mill A. The higher total Ca2+ in

Mill C is likely due to the high number of suspended CaCO3 solids present in the WW. It is hypothesized that an increased solid CaCO3 concentration in the WW may help reduce scale formation in the green liquor system, as the solid particles can act as seeding sites for the calcium in the green liquor to precipitate in the bulk. This was demonstrated when a mill (not summarized in this report) had large amounts of scaling on an agitator within the dissolving tank. The mill added lime mud particles (CaCO3) to the WW, and fed the line into the dissolving tank near the agitator blade. A decrease in scale formation on the agitator was seen.

6.3 Causticizing Plant Analysis

Since the composition of the mill liquors is relatively the same and the total calcium concentration does not appear to play a role in the reported scale formation, analyzing the causticizing plant of each mill may give insight as to why Mill A and Mill B reported more scaling problems. A summary of each causticizing plant is provided below.

6.3.1 Mill A

The causticizing plant of Mill A is equipped with a dissolving tank, a green liquor sedimentation clarifier, a heat exchanger and slaker (Figure 39). The mill does not have any green liquor storage tanks. The pipelines are made of 304L stainless steel, and all major lines have an inner diameter of 8 inches (20.3 cm). The last major line cleaning of the entire green liquor system was performed

54 in 2008, and since then only scaled sections have been acid cleaned. Green liquor composition is controlled in the RGL and CGL lines using density control, and the temperature of the system is controlled in the dissolving tank, at approximately 96°C, and in the slaker, at 103°C. The heat exchanger can act as a heat exchanger and a cooling system for the CGL. During start up, which occurs about 6 times a year, the heat exchanger will heat the CGL to process temperature. Once the system has equilibrated (the flow rate is greater than 60 L/s), it will act as a cooling system for CGL, as the temperature exiting the heat exchanger must be 88°C to ensure the temperature does not exceed 103°C in the slaker (due to the exothermic slaking reaction). The pipelines between the dissolving tank and clarifier are switched between RGL and WW every 12 hours. There is no line switchingDMI Causticizing in the CGLPlant lines.– Scale Locations

Heat Exchanger C T = 88ºC Weak Wash T = 75ºC T = 93ºC A D 1 B 1 2 Slaker T = 93ºC T = 103ºC E F 2 T = 96ºC TTA = 113 g/L Na2O Dissolving Tank T = 96ºC Clarifier TTA = 118 g/L Na2O T = 93ºC TTA = 113 g/L Na2O

20 ft 300 ft 50 ft 50 ft 50 ft

Figure 39. Mill A causticizing plant. Locations where scale samples were obtained are indicated in red.

Scale formation is a problem within the RGL lines between the dissolving tank and the clarifier, and the CGL line out of the heat exchanger. As mentioned the mill used an anti-scaling chemical8 which enters the system in the dissolving tank, however it is not uncommon for the process to be stopped every few months to acid clean the scaled sections of the pipeline. Major vessels, including the dissolving tank and slaker are cleaned out once a year during shutdown.

55 6.3.2 Mill B

The causticizing plant of Mill B is equipped with a dissolving tank, sedimentation clarifier (which also acts as a storage tank), a slaker, and a single line of causticizers (similar to Mill A). In June 2016, an anti-scaling chemical was introduced to the green liquor system to prevent scale buildup, and line switching between the RGL and WW lines was stopped. All lines were cleaned prior to this implementation, and in October 2017 the mill had to be shut down due to complete blocking of the green liquor lines. The scale was attempted to be pressure cleaned for 120 hours using 20,000 psi water, which was relatively unsuccessful, and eventually the lines were cleaned using HCl. Scale formation appeared to be more intensive on the inside elbows of the pipes compared to the outside, and much of the scaled section was horizontal, however some vertical sections were also scaled to roughly the same extent. The scale was only found in the RGL lines, beginning immediately out of the dissolving tank and running to the green liquor clarifier. The scaled section of piping was approximately 250-300 ft.

6.3.3 Mill C

Although Mill C was constructed in 1917, much of the causticizing plant was upgraded between 1960 - 1993. The causticizing plant is single line from the recovery boiler to the green liquor filtering system, dual from the slaker to the lime mud cleaners, and then converges once again to feed a single lime kiln. A schematic of the causticizing plant can be seen in Figure 40.

56 Skoghall Causticizing Plant Overview

Weak Wash T = 61ºC To green liquor filtering

785 m3 1000 m3

325 m3 400 m3 GL Clarifier GL Storage Tank 20 m Dissolving GL Stabilizing Tank T = 91ºC Tank GL ”Sock” TTA = 140 g/L Na O 2 200 m 20 m 20 m T = 99ºC Clarifiers T = 88ºC Heat Exchanger

To white liquor clarifier, lime mud Causticizers Line 1 Slaker 1 washer (x2), lime 50 m kiln

Causticizers Line 2 Slaker 2

Figure 40. Mill C causticizing plant.

The RGL and WW lines between the dissolving tank and GL stabilizing tank are switched every 24 hours to prevent scale buildup. The four sock clarifiers can be found in parallel, and the residence time is approximately 15 minutes. As was seen at Mill A, the heat exchanger is primarily used to cool the CGL from the clarification unit, and the average cooling temperature is approximately 3°C. The slaking and causticizing reactions are carried out in two lines, both of which are equipped with a slaker, causticizers and a lime mud filter. Line 1 has 2 large causticizers, and Line 2 has 5 small causticizers. To successfully carry out the slaking and causticing reactions, approximately 1/3 of the lime added to the slakers must be fresh lime. Additionally, the lime kiln does not currently have the capacity to regenerate all the CaCO3 as CaO, therefore the CaCO3 from one of the lime mud filters is sent to the lime kiln, and CaCO3 from the second lime mud filter is sent to landfill. The pipelines in the causticizing plant are stainless steel (AISI304), insulated, and generally have an inner diameter of 20 cm.

Mill C rarely reported process shutdowns affecting production targets due to scaling issues, however discussions with mill personnel led to an understanding of where scale commonly forms and how it is dealt with. Scale was found to form on the agitator blades of the dissolving tank and

57 within the pipelines that run between the causticizers. During the annual shutdown the dissolving tank, slakers, and causticizers are emptied and hydroblasted to remove any scale, and all pipelines within the causticizing plant are cleaned using pigging, which mechanically removes any debris. Due to frequent buildup throughout the pipelines between the causticizers, these lines are typically hydroblasted up to 3 times a year. The dual line of causticizers allows one of the lines to undergo cleaning while the second line continues to operate, and since the lime kiln cannot handle all the

CaCO3 product, NaOH production is not affected. The cloths of each sock clarifiers are changed approximately 3 times a year. To prevent disruptions, only one clarifier is offline at any given time. The green liquor stabilizing tank, sedimentation clarifier, and storage tank are only emptied during mandatory safety and maintenance renewal, once every 6 years. It is obvious that the causticizing plant of Mill C does experience a similar amount of scale formation to Mill A, however considerable cleaning procedures are in place to ensure the buildup does not become problematic.

6.3.4 Mill D

The causticizing plant of Mill D is equipped with two recovery boilers and two dissolving tanks, which merge into a single line throughout the green liquor filtering system, and return to a dual line system at the slakers (Figure 41).

58 Skutskär Causticizing Plant Overview Weak Wash To green liquor filtering

2000m3

T = 94ºC Dissolving GL Storage 3 Tank 1 3 2500m 2000m Tank

20 m T = 88ºC GL Clarifier GL Storage GL ”Sock” Tank Clarifiers Dissolving 250 m Tank 2 100 m T = 85ºC Heat TTA = 125 g/L Na2O Exchanger

To white liquor clarifier, lime mud Causticizers Line 2 Slaker 1 washer (x2), and lime 40 m kiln (x2)

Slaker 2 Causticizers Line 1

Figure 41. Mill D causticizing plant.

The black liquor from the three digesters is mixed and sent to the two recovery boilers which have a capacity of 2000 dry weight solids (dws)/day and 600 dws/d. The smelt from the two recovery boilers enters their own dissolving tanks where it is mixed with WW to form RGL. The retention time of the dissolving tanks is between 2 – 4 hours. To prevent scale buildup, the WW and RGL lines between the dissolving tanks and green liquor clarifier are switched once every 24 hours. Before entering a sedimentation clarifier, which has a residence time between 6 – 9 hours, the two RGL are mixed, and the semi-clarified green liquor from sedimentation clarifier is stored in a green liquor storage tank prior to entering the sock clarifiers. Five sock clarifiers are found in parallel, and finally the CGL enters a second storage tank before it is sent to a heat exchanger to be cooled before entering the slakers. The slaking reaction is carried out in two causticizing lines which both contain a slaker and 3 causticizers. The white liquor from both lines is combined and filtered to be sent back to the digesters, and the lime mud is washed and sent to one of the two lime kilns to regenerate CaO for reuse. The pipelines are stainless steel, insulated, and generally have an inner diameter of 20 or 25 cm.

59 Through discussions with engineers and operators on site, a summary of the scaling problem within the causticizing plant has been compiled. An operator who has been working within the causticizing plant for approximately 30 years stated that pirssonite scaling in the green liquor system was a major problem 20 years ago when the TTA of the system was typically hovering above 140 g/L Na2O. Since then, the TTA has been lowered to a target of 125 g/L Na2O and the mill does not experience green liquor scaling which causes unscheduled process disruptions.

The mill shuts down once a year, and during this time major vessels such as the dissolving tanks and causticizers are hydroblasted to remove any buildup. The most severe scaling is typically found within the dissolving tanks, as scale can be seen built up on the walls and impeller blades. There are rarely any problems with the green liquor sedimentation clarifier, or storage tanks, and most years these vessels are not emptied or cleaned during shut down. The green liquor sock filters must be acid cleaned regularly, and one sock filter is typically offline for cleaning every month, resulting in cleaning to each sock filter approximately 2 times a year. The classifier screws of the slaker occasionally experience scale build up, and mill personnel believe this is a result of over liming (adding excess CaO). In the past operators have increased the TTA within the slaker to prevent over liming, however the increase in TTA resulted in an increase in scale.

6.3.5 Summary

Although Mill A and Mill B reported more problems due to scale formation which led to a loss in production, scaling appears to be present at all mills to roughly the same extent. Mill C and Mill D have considerable procedures in place to overcome scale buildup and the design of the causticizing plants allows for commonly scaled sections to be cleaned without causing disruptions. The smaller sock clarifiers at Mill C and D can be taken offline individually and the dual line of causticizers at both mills help keep buildup manageable, particularly when cleaning procedures are introduced into the daily operations of the mill.

6.4 Scale Analysis of Mill A and Mill B

A total of 6 scale samples were collected from Mill A between the dissolving tank and green liquor slaker, and one sample was collected from the blockage at Mill B for analysis. TGA/DSC, XRD, and SEM imaging was carried out as outlined in Section 4.1 Experimental Procedures. X-Ray

60 Fluorescence (XRF) was performed using a Bruker S2 Ranger system to identify the bulk material composition. Electron Probe X-Ray microanalysis (EPMA) was performed using a JEOL JXA8230 instrument. Samples were encased in epoxy and the surface was prepared by a polishing sequence with 9 µm, 3 µm, and 1 µm oil based diamond suspensions. The approximate location of Mill A scale samples was shown in Figure 39.

Photographs of the scale samples from Mill A can be seen in Figure 42. The first scale sample (sample A) was obtained from the RGL pipeline directly after the dissolving tank in February 2018. The sample is very smooth and light grey in colour. This location has RGL and WW line switching every 12 hours. Sample B was obtained from the RGL “switching tree” in June 2017. The scale is black in colour and can break apart with minimal force. Sample C was obtained from the heat exchanger outlet in June 2017. This sample is very hard and takes exceptional force to break. The remaining three samples were collected during a dissolving tank shut down in September 2017, and they are of interest due to their unusual morphologies. Sample D is a scale encasing the temperature probe within the dissolving tank. Although it is similar to Sample C it can break apart quite easy. Sample E is very fine sand like particles, and sample F is small black spherical beads which range in size from a diameter of 3 to 7 mm. Both sample E and F were found on the bottom of the dissolving tank. Sample G (Mill B) was recovered from the blockage in the RGL line. An analysis of each scale sample is outlined in the following sections.

61

Sample A: RGL Dissolving Tank Exit Sample B: RGL Switching Tree

Sample C: CGL H-X Outlet Sample D: DT Temperature Probe

Sample E: DT Sand Sample F: DT Beads Sample G: RGL Pipe

Figure 42. Scale samples obtained from the causticizing plant of Mill A (A-F), and Mill B (G).

TGA/DSC Analysis

Similar characteristics can be seen between the thermal profiles of the RGL scales (sample A, B, G) the CGL scale (sample C) and sample D, and the scales found on the floor of the dissolving tank (sample E and F). The thermal profile of sample A is shown in Figure 43 and the thermal profile of sample B and G can be found in Figure 81 and Figure 88 respectively (Appendix IV).

62

Figure 43. Thermal profile of Mill A, sample A.

For samples A, B, and G which are RGL scale samples, a weight loss between 395°C and 520°C can be seen and this is speculated to be due to the anti-scaling chemical Mill A and B uses, or carbon entrapped in the scale from the presence of dregs in the RGL. Heat loss is not seen between these temperatures, suggesting the weight % is likely due to the anti-scaling chemical. The weight percent of anti-scaling chemical is approximately 10% in sample A, 25% in sample B, and 30% in sample G. The endothermic peak between 620°C and 830°C is characteristic of CaCO3 decomposition to CaO and CO2. Pure CaCO3 has a weight loss of 44%, and the weight loss from

CaCO3 decomposition in sample A was found to be 37%, sample B was found to be 28%, and sample G was found to be 30%. Therefore, according to TGA/DSC analysis it can be concluded that the samples contained approximately 85%, 64% and 68% pure CaCO3 for sample A, B and G respectively.

63

Figure 44. Thermal profile of Mill A, sample C.

Sample C and sample D follow similar trends to A and B, however there is no weight loss between 395°C and 520°C indicating there is little to none of the anti-scaling chemicals trapped within these samples, as can be seen by the TGA/DSC profiles in Figure 44 and Figure 84 (Appendix IV). Since sample C was recovered from the CGL line after the green liquor filtration it is hypothesized that the anti-scaling chemical was removed in the clarification system. Sample D was extracted from the temperature probe in the dissolving tank, and the anti-scaling chemical enters the RGL line exiting the dissolving tank. TGA/DSC analysis concludes these samples to have 93% and 85% pure CaCO3 for sample C and D respectively.

64

Figure 45. Thermal profile of Mill A, sample E.

Figure 46. Thermal profile of Mill A, sample F.

The thermal profiles of sample E (Figure 45) and sample F (Figure 46) are quite unique. Analyzing the profile of sample E, a weight loss between 0 – 120°C is due to water loss and is approximately 10%. Between 150 – 250°C is water loss from pirssonite dehydration, and for a sample of pure

65 pirssonite this loss is 15%. Sample E only shows about 3% water loss, and assuming the water dehydration is only due to pirssonite, this indicates the sample is 20% pure pirssonite. The weight loss between 620°C and 830°C is typically 18% for pure pirssonite, however sample E shows a weight loss of 31%, suggesting the sample is a mixture of 20% pirssonite and 62% CaCO3. The thermal profile of sample F is much closer to that of pure pirssonite. Between 150 – 250°C and a weight loss of 7.6% is seen. Again, assuming the water dehydration is only due to pirssonite, this suggests the sample is 50% pure pirssonite. The weight loss from the decomposition of CaCO3, was found to be 17% in sample F. Since we assume the sample is 50% pirssonite 9% of the 17% weight loss is due to CaCO3 decomposition within pirssonite. The remaining 8% weight loss corresponds to 18% of pure CaCO3 within the sample. A summary of the TGA/DSC results can be seen in Table 10.

Table 10. TGA/DSC Results for mill scale samples. Sample Sample Sample Sample Sample Sample Sample Parameter A B C D E F G Scale Chemical wt% 10 25 0 0 0 0 30

CaCO3 wt% 85 64 93 85 62 18 68 Pirssonite wt% 0 0 0 0 20 50 0

XRD Analysis

The XRD profiles for all samples can be found in Appendix IV. The profiles from sample A – D show only calcite peaks. Sample E and F show small peaks which can be attributed to pirssonite, in addition to calcite peaks.

XRF Analysis

Using the results from the XRF and the loss on ignition (LOI) from TGA/DSC the weight percent of each element, displayed as oxides, can be found in Table 11. The weight percent of CaCO3 for each sample was found to be consistent with TGA/DSC results for all samples.

66 Table 11. XRF Results for Mill A and Mill B scale samples.

Weight % Element Sample Sample Sample Sample Sample Sample Sample A B C D E F G CaO 50.98 49.84 50.84 51.54 48.9 41.91 50.69

Na2O 0.42 0.63 0.6324 0.51 10.54 7.24 0.77 MgO 1.76 2.41 3.21 2.09 2.06 0.5 0.73

Al2O3 0.33 0.37 0.34 0.29 0.59 0.25 0.22

SiO2 0.56 0.91 1.24 0.75 0.51 0.6 0.27

P2O5 1.22 0.86 0.83 0.85 0.79 0.4 0.85

SO3 3.69 6.76 7.99 3.07 3.14 4.25 2.00 Cl 0.37 0.31 0.34 0.31 0.28 0.21 0.49

K2O 0.31 0.49 0.45 0.31 0.35 0.45 0.24

V2O5 0.14 0.11 0.06 0.12 0.02 0.05 0.09 MnO 0.32 1.15 0.57 0.23 0.36 0.19 0.49

Fe2O3 0.34 0.58 1.21 0.16 0.59 0.33 0.13 ZnO 0.48 0.7 0.16 0.08 0.19 0.2 0.15 BaO 1.62 1.33 0.77 1.38 0.22 0.52 1.06 LOI* 48 57 44 41 52 38 63 Total 110 123 112 102 120 95 121 *LOI = loss on ignition (determined from TGA/DSC)

EPMA Analysis of Sample C

Due to the thickness of sample C (approximately 3.5 cm), EPMA was carried out to determine if the composition changes across a cross section of scale. A photograph of the piece of scale used for this analysis can be seen in Figure 47. The sample was analyzed in a line from the pipe side to the liquor side. Results of the EPMA analysis are shown in Figure 48. The calcium concentration, indicated on the left axis, is significantly higher than all other elemental concentrations, indicated on the right axis. The concentration of elements is relatively even across the cross section, and it is interesting to note that in the areas where calcium concentration decreases, the concentration of sulfur appears to increase. This is most likely due to the presence of CaSO4 in the sample.

67

Figure 47. Cross section of sample C used for EPMA analysis.

Figure 48. EMPA analysis of Mill A, sample C.

The thickness of sample C can also be used to estimate the growth rate of scale in this section of the green liquor pipeline. As mentioned, the inner diameter of this pipeline is 8 inches, or 20 cm. The width of the scale collected was 3.5 cm, and this particular piece of scale formed over a period of 6 months. With this information, it can be determined that the scale covered approximately 50% of the cross sectional area, and the growth rate of the scale is approximately 0.2 mm per day.

68

Figure 49. Growth rate of scale sample C.

6.5 Green Liquor Scale Field Study at Mill A

A field study was conducted at Mill A to obtain immediate results regarding calcite scale formation, and long term results which will help minimize downtime cleaning and cost savings in the causticizing plant. Two main hypothesis were tested in this study:

1. Temperature gradients within the pipe drive the precipitation of CaCO3. 2. Hydrophobic materials, which have low surface energy, may prevent calcite nucleation.

6.5.1 Field Study Details

Two scaling probes were inserted into the green liquor pipeline at two locations to monitor scale growth over time. A schematic of the scale probe used in this study can be seen in Figure 50.

69

Figure 50. Schematic (left) and photograph (right) of the scale probe used in this study.

Coupons of different materials were used to monitor the scale buildup. One coupon was stainless steel, and the other was of the same material with a hydrophobic coating known as Durasan® provided by SilcoTek. Durasan® has a low surface energy which inhibits the rate of nucleation. It has a wear resistance of 6.13 which is less than that of 304 stainless steel, and can be used at temperatures below 450°C. A photograph of the coupons used in this study can be seen in Figure 51. Two coupons, one of each material will be fastened to the end of each scale probe as shown.

Figure 51. Photograph of two coupons used in this study (left) and coupons fastened to scale probe (right). The top coupon is coated with the hydrophobic coating Durasan® by SilcoTek (left).

There are two insertion locations for the two probes. The first location is between the raw green liquor transfer pump #1 and flow control valve B. This is the same location where scale sample A was collected (see previous section). The second probe location is after the heat exchanger on the

70 clarified green liquor line. This is the same location where scale sample C was collected. Note line switching occurs at location 1, but not location 2. Due to the proximity of location 1 to the dissolving tank, the temperature will be assumed to be the same. The temperature at location 2 is monitored. A schematic of the causticizing system with the probe locations indicated by red stars is depicted in Figure 52. Photographs of the probe locations can be seen in Figure 53 and Figure 54. DMI Causticizing Plant – Probe Locations

2 Heat Exchanger T = 88ºC Weak Wash T = 75ºC T = 93ºC

1 1 1 2 Slaker T = 93ºC T = 103ºC 2 T = 96ºC TTA = 113 g/L Na2O Dissolving Tank T = 96ºC Clarifier TTA = 118 g/L Na2O T = 93ºC TTA = 113 g/L Na2O

5 m 100 m 15 m 15 m 15 m

Figure 52. Schematic of Mill A, recausticizing plant with probe locations indicated in red.

10

Figure 53. Photograph of 1st scale probe location.

71

Figure 54. Photograph of 2nd scale probe location.

6.5.2 Field Study Methodology

Various trials were run, making use of both probe locations. During each trial, the initial weight of the coupons was measured before they were secured to the probe and inserted into the pipeline. Once the trial was complete, the coupons were removed from the pipeline and dried. The coupons were weighed, then loose char and debris was washed away from the coupon surface and collected, and the coupons were weighed once again. The difference in the initial weight and the weight immediately after the trial was used to determine the percent change of the coupons.

6.5.3 Field Study Results

A summary of the trials can be seen in Table 12. A photograph of the same coupons shown in Figure 51 can be seen after Trial 2 in Figure 55. The coupons appeared to turn a dark black colour when immersed in both the RGL and CGL pipelines. When the coupons were removed while WW was flowing through location 1 they appeared bright and shiny in colour (Figure 56).

72 Table 12. Mill A field study results. Parameters Trial 1 Trial 2 Trial 3 Trial 4 Probe Location 1 1 1 2 Trial Length 18 h 48 h 10 days 10 days Liquor RGL WW/RGL WW/RGL CGL Yes Yes Line Switching No No (12 h) (12 h)

Stainless Steel Coupon 0.82 % - 3.25 % 0.25 % 0.91 % Weight Change

Hydrophobic Coupon 0.44 % - 3.31 % 0.43 % 0.75 % Weight Change

Figure 55. Photograph of coupons after Trial 2.

Figure 56. Photograph of hydrophobic coated coupon after Trial 3. Coupon was removed during WW flow.

Unfortunately, significant scale did not appear to build up on the coupons making conclusions rather difficult. One reason for this may be because the coupons have been placed in the center of the cross section of the pipe, and any scale formation may have been mechanically removed due to the force of the green liquor flow. To improve this trial, it is recommended a method for monitoring scale formation on the walls of the pipe is used. As it is nearly impossible to place a coupon on the wall of the pipeline without disrupting the system, a simpler method for monitoring

73 could involve taking borescope images of the pipe daily to observe the scale growth over time. The small amount of scale formation may be because the longest trial lasted 10 days. As was shown in the previous section, the scale formed in 6 months, therefore scale formation can be an extremely slow process, and even a 10 day trial may not have been significant time for scaling to begin.

The trial results suggest that WW/RGL line switching aids in preventing scale build up, as expected. When line switching was stopped in Trial 1, a slight increase in accumulation was seen compared to Trial 2. Comparing Trial 3 and 4, Trial 4, which only saw CGL flow, also saw an increase in accumulation. An additional explanation for the increased accumulation in Trial 4 is that there is a temperature drop of approximately 5°C which may induce scaling. However, it is impossible to conclude whether the increased accumulation is due to the lack of line switching, temperature fluctuations, or a combination of both effects.

Visual inspection of the coupons shows that the hydrophobic coating may have been removed by the liquor, suggesting these coatings are not a viable solution to mitigate scaling. Further tests should be completed to verify these results.

6.6 Summary

In this chapter, 4 pulp mills were analyzed to compare their scaling problems and determine why some mills experience more intense scaling than others. Mill A and B reported more scaling problems, however analysis of the causticizing plant concluded that all mills experience scaling to the same extent. Mill C and Mill D can remove scale build up more effectively without causing process disruptions.

Analyzing the collected scale from Mill A and B found that 5 of the 7 scale samples were CaCO3. Scale samples collected from the RGL pipelines appeared to have increased amounts of unburned carbon, whereas scale from the CGL pipelines was mostly pure CaCO3. An industrial field study at Mill A attempted to gather results regarding calcite scale formation, however the main conclusion from this study was that scale formation is an extremely slow process

74 Chapter 7

7 Conclusions

A fundamental study was performed to investigate calcite scale formation in the green liquor system, first by determining the effects of green liquor properties (temperature, TTA, causticity, and sulfidity) on the solubility limits of CaCO3, and second by conducting field studies to analyze scale formation within the causticizing plant. The major conclusions from this work are summarized below.

1. The solubility of CaCO3 increases with increasing temperature and TTA (Na2CO3

concentration), and decreases with increasing causticity and sulfidity (NaOH and Na2S concentration). Increasing solubility with temperature is opposite to the behavior of the

CaCO3-H2O-air system, however this trend is expected due to the high alkalinity of the

green liquor system, which neutralizes any dissolved CO2 preventing the inverse solubility curve.

2. The solubility of CaCO3 in green liquor was modelled in this work using OLI software. The OLI model simulated the experimental results with approximately a 2% error. The effect of green liquor reduction and Cl concentration was analyzed using the OLI model,

and it was found that the presence of Na2SO4 and Cl both increases the solubility of CaCO3. 3. The field study involving scaling probes at Mill A discussed in Chapter 6 was inconclusive during the 2 weeks of trials, suggesting scale formation is an extremely slow process. 4. Green liquor scales collected from various areas of the causticizing plant were characterized from Mill A and Mill B. Five of the 7 samples were found to be composed

of CaCO3. Calcium carbonate scales from the raw green liquor lines were found to be

approximately 65 – 85% pure CaCO3, and the remaining weight percent of the sample is thought to be anti-scaling chemicals and dregs. The calcium carbonate scale from the

clarified green liquor line was found to be 93% pure CaCO3. From this analysis, it can be

concluded that CaCO3 scales in the clarified green liquor line of Mill A, and the raw green

liquor line of Mill B formed due to the direct precipitation of CaCO3 as there was no line switching between weakwash and green liquor in these pipelines. It is unclear whether the

75 CaCO3 scales in the raw green liquor line of Mill A formed due to selective dissolution of

pirssonite, leaving behind CaCO3, or if they formed from the direct precipitation of CaCO3. 5. Mill A and Mill B reported more scaling problems with their causticizing plants compared to Mill C and Mill D. Analysis of the causticizing plants show that scale formation may be present to the same extent in all mills, however scale removal has been introduced into the daily operations of Mill C and Mill D resulting in decreased scaling problems and process downtimes. Increased suspended solids in the weakwash of Mill C and D may act as

seeding sites in the green liquor system causing CaCO3 to precipitate in the bulk rather than on equipment surfaces.

6. The solubility of CaCO3 in the synthetic green liquor experiments in Chapter 3, and in the mill green liquor in Chapter 5 was found to be less than 15 ppm in all cases. An analysis of the liquor from Mill A and Mill C found that the total calcium concentration was higher

than the soluble calcium concentration. Therefore, a driving force for CaCO3 precipitation always exists within the causticizing plant.

76 Chapter 8

8 Recommendations for Future Work

The solubility of CaCO3 in green liquor was found to be extremely low. Since CaCO3 is always present in the green liquor system from the lime mud particles in the weakwash and the wood, the total calcium concentration is always higher than the soluble calcium concentration. This means there is always a driving force for CaCO3 precipitation. To mitigate CaCO3 scaling in the causticizing plant it is recommended to further extend this work by investigating ways to control

CaCO3 precipitation. This could include:

• Determining if lime mud particles in the weakwash can provide seed sites for CaCO3 precipitation in the green liquor bulk compared to equipment surfaces. • A study of anti-scaling chemicals. Many mills spend millions of dollars adding these chemicals to their green liquor system, improving their effectiveness or even disproving their usage could result in significant cost savings

The mechanism of CaCO3 formation in green liquor was not experimentally studied in this work. Understanding this mechanism could play an important role in mitigating scale formation.

77 References

[1] D. W. Reeve, Pulp and Paper Maufacture Volume 5 Alkaline Pulping, Atlanta, GA: The Joint Textbook Committee of the Paper Inustry, 1989.

[2] H. N. Tran and E. K. Vakkilainnen, "The Kraft Chemical Recovery Process," in TAPPI Kraft Recovery Short Course, St. Petersburg, FL, 2012.

[3] D. T. Clay, "Evaporation Principles & Black Liquor Properties," in TAPPI Kraft Recovery Short Course, St. Petersburg, FL, 2012.

[4] B. E. Dotson and A. Krishnagopalan, "Causticizing Reaction Kinetics," in 1990 Pulping Conference Proceedings, 1990.

[5] D. R. Sanchez, "Recausticizing Principles and Practice," in TAPPI Kraft Recovery Course, 2009.

[6] P. Bajpai, Pulp and Paper Industry: Chemical Recovery, Oxford: Elsevier, 2016.

[7] B. Sithole, "Scale Deposit Problems in Pulp and Paper Mills," in African Pulp and Paper Week, Durban, ZA, 2002.

[8] T. Zakir, H. Tran and V. Papangelakis, "Characterization of hard scale formed in the kraft mill green liquor processing equipment," TAPPI Journal, vol. 12, no. 11, pp. 55-61, 2013.

[9] T. Zakir, H. Tran and V. Papangelakis, "Formation of pirssonite in green liquor handling systems," TAPPI Journal, vol. 12, no. 7, pp. 33-41, 2013.

[10] K. Taylor and B. McGuffie, "Investigation of non-process element chemistry at Elk Falls mill - green liquor clarifier and lime cycle," Pulp & Paper Canada, vol. 108, no. 2, pp. 27- 32, 2007.

78 [11] C. S. Greer, Inorganic scale control in today's pulp mills, Naperville, Il: Nalco, an Ecolab Company , 2015.

[12] J. W. Morse, R. S. Arvidson and A. Luttge, "Calcium Carbonate Formation and Dissolution," Chemical Reviews, vol. 107, no. 2, pp. 342-381, 2007.

[13] Hudson Institute of Minerology, "Calcite," 2018. [Online]. Available: https://www.mindat.org/min-859.html. [Accessed 2018].

[14] J. C. Cowan and D. J. Weintritt, Water-Formed Scale Deposits, Houston, TX: Gulf Publishing Company, 1976.

[15] M. C. Area and F. E. Felissia, "Scaling in Alkaline Spent Pulping Liquor Evaporators," in Mineral Scales and Deposits, Elsevier, 2015, pp. 557-568.

[16] P. Bots, L. G. Benning, J.-D. Rodriguez-Blanco, T. Roncal-Herrero and S. Shaw, "Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)," Crystal Growth and Design, vol. 12, pp. 3806-3814, 2012.

[17] J. D. Rodriguez-Blanco, S. Shaw and L. G. Benning, "The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite," Nanoscale, vol. 3, pp. 265-271, 2011.

[18] J.-D. Rodriguez-Blanco, S. Shaw, L. G. Benning, P. Bots and T. Roncal-Herrero, "The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate," Journal of Alloys and Compounds, vol. 536S, pp. S477-S479, 2012.

[19] Y. F. Ma, Y. H. Gao and Q. L. Feng, "Effects of pH and temperature on CaCO3 crystallization in aqueous solution with water soluble matrix of pearls," Journal of Crystal Growth, vol. 312, pp. 3165-3170, 2010.

79 [20] Y. S. Han, G. Hadiko, M. Fuji and M. Takahashi, " Influence of initial CaCl2 concentration on the phase and morphology of CaCO3 prepared by carbonation," Journal of Materials Science, vol. 41, pp. 4663-4667, 2006.

[21] M. Ni and B. D. Ratner, "Differentiation of Calcium Carbonate Polymorphs by Surface Analysis Techniques – An XPS and TOF-SIMS study," Surface Interface Analysis, vol. 40, no. 10, pp. 1356-1361, October 2008.

[22] OLI Systems Inc., "OLI Systems," 2018. [Online]. Available: https://www.olisystems.com/. [Accessed March 2018].

[23] J. Berthold, "Calcium Carbonate Solubility," January 2016. [Online]. Available: http://wiki.olisystems.com/wiki/Calcium_Carbonate_Solubility. [Accessed March 2018].

[24] H. O. Askew, "The solubility and hydrolysis of calcium carbonate," Transactions, vol. 49, pp. 791-796, 1923.

[25] N. L. Plummer and E. Busenberg, "The solubilities of calcite, aragonite and vaterite in COrHzO solutions between 0 and 90C, and an evaluation of the aqueous model for the system CaCO3-CO-H2O," Geochimica et Cosmochimica Acta, vol. 46, pp. 1011-1040, 1982.

[26] W. P. Inskeep and P. R. Bloom, "An evaluation of rate equations for calcite precipitation kinetics at pCO2 less than 0.1 atm and pH greater than 8," Geochimica at Cosmochimica Acta, vol. 49, pp. 2165-2180, 1985.

[27] J.-Y. Gal, J.-C. Bollinger, H. Tolosa and N. Gache, " Calcium carbonate solubility: a reappraisal of scale formation and inhibition," Talanta, vol. 43, pp. 1497-1509, 1996.

[28] F. S. Nakayama, "Calcium activity, complex and ion-pair in saturated CaCO3 solutions," Soil Science, vol. 106, pp. 429-434, 1968.

80 [29] B. Coto, C. Martos, J. Pena, R. Rodriguez and G. Pastor, "Effects in the solubility of CaCO3: Experimental study and model description," Fluid Phase Equilibria, vol. 324, pp. 1-7, 2012.

[30] A. R. Felmy, D. A. Dixon, J. R. Rustad, M. J. Mason and L. M. Onishi, "The hydrolysis and carbonate complexation of strontium and calcium in aqueous solution. Use of molecular modeling calculations in the development of aqueous thermodynamic models," J. Chem. Thermodynamics, vol. 30, pp. 1103-1120, 1998.

[31] T. Zakir, Evaluation and Control of Pirssonite Scale Formation in Green Liquor Systems of the Kraft Process, Toronto: University of Toronto, 2011.

[32] C. R. Bury and R. Redd, "The system sodium carbonate - calcium carbonate - water," Journal of the Chemical Society (Resumed), no. 0, pp. 1160-1162, 1933.

[33] F. E. Littman and H. J. Gaspari, "Causticization of carbonate solutions," Industrial and Engineering Chemistry, vol. 48, no. 3, pp. 408-410, 1956.

[34] W. J. Frederick and R. Krishnan, "Pirssonite deposits in green liquor processing," TAPPI Journal, pp. 135-140, 1990.

[35] B. Dickens and W. Brown, "The crystal structures of CaNa2(CO3)2*5(H2O), synthetic gaylussite, and CaNa2(CO3)2*2(H2O), synthetic pirssonite," Inorganic Chemistry, vol. 8, pp. 2093-2103, 1969.

[36] H. Tran, D. Barham and D. Reeve, "Chloride and potassium in the kraft chemical recovery cycle," Pulp and Paper Canada, vol. 91, no. 5, pp. 185-190, 1990.

[37] M. Dion, J. Fawcett and N. Lorincz, "CFD Review of Flow Dispersion in Green Liquor Clarifiers," DMI Peace River Pulp Division.

81 Appendices

Appendix I: TGA/DSC Standard Profiles

Figure 57. Thermal profile for pure calcite used as a standard.

Figure 58. Thermal profile of pure pirssonite used as a standard.

82 Appendix II: TGA/DSC and XRD Experimental Sample Profiles

Figure 59. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 6 g/L Na2O).

Figure 60. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 6 g/L Na2O).

83

Figure 61. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 12 g/L Na2O).

Figure 62. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 12 g/L Na2O).

84

Figure 63. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 18 g/L Na2O).

Figure 64. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 18 g/L Na2O).

85

Figure 65. Thermal profile of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 30 g/L Na2O).

Figure 66. XRD of solid sample collected from the CaCO3-Na2CO3-NaOH-H2O system at 95°C (TTA = 120 g/L Na2O, NaOH = 30 g/L Na2O).

86

Figure 67. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 12 g/L Na2O).

Figure 68. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 12 g/L Na2O).

87

Figure 69. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 24 g/L Na2O).

Figure 70. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 24 g/L Na2O).

88

Figure 71. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 36 g/L Na2O).

Figure 72. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 36 g/L Na2O).

89

Figure 73. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 48 g/L Na2O).

Figure 74. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 48 g/L Na2O).

90

Figure 75. Thermal profile of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 60 g/L Na2O).

Figure 76. XRD of solid sample collected from the CaCO3-Na2CO3-Na2S-H2O system at 95°C (TTA = 120 g/L Na2O, Na2S = 60 g/L Na2O).

91 Appendix III: Density of Synthetic Green Liquor Solutions

Figure 77. Solution density for the CaCO3-Na2CO3-H2O system vs. concentration of Na2CO3 in g/L Na2O at 25°C.

Figure 78. Solution density for the CaCO3-Na2CO3-NaOH-H2O system vs. concentration of NaOH in g/L Na2O at 25°C.

92

Figure 79. Solution density for the CaCO3-Na2CO3-Na2S-H2O system vs. concentration of Na2S in g/L Na2O at 25°C.

93 Appendix IV: TGA/DSC and XRD Industry Sample Profiles

Figure 80. XRD profile of Mill A, sample A.

Figure 81. Thermal profile of Mill A, sample B.

94

Figure 82. XRD profile of Mill A, sample B.

Figure 83. XRD profile of Mill A, sample C.

95

Figure 84. Thermal profile of Mill A, sample D.

Figure 85. XRD profile of Mill A, sample D.

96

Figure 86. XRD profile of Mill A, sample E.

Figure 87. XRD profile of Mill A, sample F.

97

Figure 88. Thermal profile of Mill B, sample G.

Figure 89. XRD profile of Mill B, sample G.

98 Appendix V: Mill Data

Note: the average values are based off 6 months of online data.

Table 13. Comparison of the measured and average chemical properties of Mill A. Clarified Green Raw Green Liquor Weakwash Property Liquor Measured Average Measured Average Measured Average TTA 137 120 115 114 17 - (g/L Na2O) Causticity (%) 14 17 17 17 74 - Sulfidity (%) 24 30 25 27 15 - Density (kg/L) 1.20 1.20 1.16 1.14 1.02 - Suspended 2612 1000 35 < 100 4 1 Solids (ppm) Soluble Ca2+ 14.7 - 8.1 - 0.24 - (ppm) Total Ca2+ 101 - 7.6 - 5.5 - (ppm)

Table 14. Comparison of the measured and average chemical properties for Mill C. Clarified Green Raw Green Liquor Weakwash Property Liquor Measured Average Measured Average Measured Average TTA 140 150 136 130 11 13 (g/L Na2O) Causticity (%) 10 6 11 6 66 57 Sulfidity (%) 31 35 32 35 33 29 Density (kg/L) 1.20 - 1.19 - 1.01 - Suspended 963 855 80 177 392 176 Solids (ppm) Soluble Ca2+ 3.4 - 5.1 - 1.0 - (ppm) Total Ca2+ 106 - 115 - 103 - (ppm)

99 Table 15. Comparison of the measured and average chemical properties for Mill D. Property Raw Green Liquor Clarified Green Weakwash Liquor Measured Average Measured Average Measured Average TTA 125/131 - 130 - 14 - (g/L Na2O) Causticity (%) 3/4 - 3 5 19 - Sulfidity (%) 25/19 - 29 28 28 - Density (kg/L) 1.18/1.19 1.14/1.17 1.19 1.31 1.02 - Suspended 1389/1392 2837/2917 22 106 597 - Solids (ppm)

100