Phosphorus in the Lime Cycle of Kraft Pulp
Mills
by
Seyedeh Maryam Sadegh Mousavi
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 Seyedeh Maryam Sadegh Mousavi 2019
ii
Phosphorus in the Lime Cycle of Kraft Pulp
Mills
Seyedeh Maryam Sadegh Mousavi
Master of Applied Science
Department of Engineering and Applied Chemistry University of Toronto
2019
Abstract
The accumulation of non-process elements in the recovery cycle of kraft pulp mills
is a current and growing problem. Phosphorus is one of these elements and there is
limited information about its behavior in the recovery cycle. Phosphorous reacts with
lime, reducing the amount of lime available for recausticizing. The first part of this
project, through laboratory work, identified rhenanite (NaCaPO4) as the form of
phosphorous in the lime cycle and showed the negative effect of phosphorus on lime
availability. Now that rhenanite has been identified, it can be used in future work to
build an equilibrium database for equilibrium modeling of phosphorous in the lime
cycle. The second part of this project involved field studies and performing a mass
balance for phosphorus at a Canadian kraft pulp mill to better understand the flows
of phosphorus in the pulp mill and its distribution between the solid and soluble
forms.
iii
Acknowledgments
I would like to thank my enthusiastic and brilliant supervisor, Prof. Nikolai
DeMartini, for the patient guidance, encouragement and advice he has provided
throughout my time as his student. I have been extremely lucky to have a supervisor
who cared so much about my work, and who responded to my questions and queries
so promptly. It was a great privilege and honor to work and study under his guidance.
I would like to extend my gratitude to my committee members, Professor Honghi
Tran and Professor Gisele Azimi for their helpful discussions, feedback, and
comments.
I would like to thank all my friends, research colleagues and members of department
of chemical engineering at University of Toronto who helped me during the past two
years. In particular, I would like to thank Mrs. Sue Mao and Dr. Georgiana
Moldoveanu for their valuable help and guidance during my project. They always
answered all my questions with patience.
I would like to express my sincerest gratitude to Brodie O’Rourke, Blair Rydberg
Steven Reimer and Jody Bertholet of Canadian Kraft Paper for their help, hospitality
and support during my mill visit. I would also like to express my gratitude to Maria
Björk and Rickard Wadsborn of Stora Enso and Peter Hart for their valuable
comments on my experimental plan.
Finally, but by no means least, I must express my deepest and sincerest gratitude to
my mom and dad (Malihe and Ramin) for their unbelievable love and support. They
are the most important people in my world, and I dedicate this thesis to them.
iv
Table of Contents
Acknowledgments ................................................................................................ iii
Table of Contents ...................................................................................................iv
List of Tables ....................................................................................................... vii
List of Figures ..................................................................................................... viii
List of Abbreviations .............................................................................................xi
Chapter 1 ............................................................................................................ 1
Introduction ............................................................................................................ 1
1.1 Kraft Pulping Process .................................................................................... 1
1.2 Recausticizing Process (Lime Cycle) ............................................................ 2
1.3 Non-Process Elements .................................................................................. 7
1.4 Phosphorus in the Lime Cycle ................................................................. 7
1.5 Objectives ..................................................................................................... 8
Chapter 2 ............................................................................................................ 9
Literature Review ............................................................................................... 9
Non-Process Elements in the Pulp and Paper Industry .................................. 9
Non-Process Elements Inputs........................................................................ 9
2.3 Negative Effects of NPEs ............................................................................ 11
2.4 NPE Purge Points ........................................................................................ 11
2.5 NPEs Handling Strategies ........................................................................... 13
2.6 Important Non-Process Elements ................................................................ 15
2.6.1 Aluminum .......................................................................................... 15
2.6.2 Silica .................................................................................................. 16
2.6.3 Magnesium ........................................................................................ 16
2.6.4 Manganese ......................................................................................... 17
2.6.5 Iron .................................................................................................... 17
2.6.6 Phosphorus ......................................................................................... 17
2.7 Phosphorus Compounds in the Lime Cycle ................................................. 18
2.7.1 Calcium Phosphates ........................................................................... 20
2.7.2 Rhenanite (NaCaPO4) ........................................................................ 21
v
Chapter 3 .......................................................................................................... 23
Laboratory Experiments ........................................................................................ 23
3.1 Overview of the Experimental Investigation of P Compounds in the lime
cycle ........................................................................................................... 23
3.2 Experimental Procedure .............................................................................. 23
3.2.1 Identification of Phosphorus Compounds and Their Effects on
Lime Availability .............................................................................. 23
3.2.2 Analytical Method.............................................................................. 25
3.2.3 Results and Discussions ..................................................................... 26
3.3 Effect of Lime Mud Washing on Phase Identification ................................. 41
Chapter 4 .......................................................................................................... 45
Mill Studies .......................................................................................................... 45
4.1 Pulp Mill Background ................................................................................. 45
4.2 Sampling Campaign for P Mass Balance..................................................... 45
4.3 Sample Collection ....................................................................................... 48
4.4 Laboratory Analysis .................................................................................... 54
4.5 Mass Balance Calculations .......................................................................... 55
4.6 Mass Balance Results and Discussions ....................................................... 58
4.6.1 Phosphorus mass balance around different process steps.................... 59
4.7 Kinetic Experiments .................................................................................... 63
4.7.1 Experimental ...................................................................................... 63
4.7.1 Kinetic Experiments Results and Discussions .................................... 64
Chapter 5 .......................................................................................................... 65
5.1 Conclusions ................................................................................................ 65
Chapter 6 .......................................................................................................... 66
6.1 Future Work Recommendations .................................................................. 66
References ........................................................................................................... 67
Appendices ........................................................................................................... 73
Appendix 1 ........................................................................................................... 73
Appendix 2 ........................................................................................................... 83
vi
Appendix3 ............................................................................................................ 86
Appendix4 ............................................................................................................ 87
Appendix 5 ........................................................................................................... 90
vii
List of Tables
Table 1.Standard Pulping Terms (the concentration of each species is expressed in
g/L of Na2O) ......................................................................................................... 5
Table 2.Negative Effects of NPEs [14] [17] ..........................................................11
Table 3.Main inputs and outputs of NPEs [18] [17]. .............................................12
Table 4.Total phosphate concentrations in green and white liquors from eight
Swedish kraft mills [9] ..........................................................................................18
Table 5.Calcium phosphate species based on their Ca/P ratio [36] ........................20
Table 6. Sampling points ......................................................................................45
Table 7.Sampling Points .......................................................................................45
Table 8.P concentration/ (wt.%) in mill samples ...................................................73
Table 9.Density of the mill liquors at 95 ℃ ..........................................................83
Table 10.Dry solid content of mixed streams ........................................................84
Table 11. Black liquor dry solid (%). ....................................................................85
Table 12.Flow through the sampling points ..........................................................87
viii
List of Figures
Figure 1. The kraft pulping process (courtesy of Valmet) ..................................... 1
Figure 2. Schematic of the recausticizing process [48] ........................................... 2
Figure 3, Slaker with cyclone (courtesy of Valmet) ............................................... 3
Figure 4.Lime kiln with lime mud dryer (courtesy of F.L. Smidth) ........................ 6
Figure 5.Distribution of dry mass and non-process elements in different parts of four
investigated trees [12] ...........................................................................................10
Figure 6.Experimental Setup .................................................................................24
Figure 7.P/Ca mole ratio in different cycles ..........................................................26
Figure 8.Effect of P content on lime availability ...................................................27
Figure 9.XRD profile of lime after different cycles, (a): 1st cycle, (b):2nd cycle, (c):
3rd cycle, (d): 4th cycle, (e) :5th cycle, (f):6th cycle .............................................31
Figure 10.XRD profile of lime mud after different cycles, (a): 1st cycle, (b):2nd
cycle, (c): 3rd cycle, (d): 4th cycle, (e) :5th cycle, (f):6th cycle ............................34
Figure 11.SEM image of lime after the first cycle in different areas .....................36
Figure 12.SEM image of lime mud after the first cycle in different areas..............36
Figure 13.Typical thermal profile of lime .............................................................37
Figure 14.Typical thermal profile of lime mud......................................................38
Figure 15.Thermal profile of lime after the first slaking/calcining cycle ...............39
ix
Figure 16.Thermal profile of lime mud after the first slaking/calcining cycle .......39
Figure 17.Thermal profile of lime after the sixth slaking/calcining cycle ..............40
Figure 18.Thermal profile of lime mud after the sixth slaking/calcining cycle ......40
Figure 19.Remaining Na content in the lime mud after multiple washing stages ...42
Figure 20.Na/P mole ratio in the lime mud ...........................................................43
Figure 21.XRD of lime mud after 5th washing stage ............................................44
Figure 22.Schematic of the recovery and fiber line of the mill ..............................47
Figure 23.Wood Chips to digesters sampling point (courtesy of Brodie O'Rourke)
..............................................................................................................................48
Figure 24.Pulp and carryovers from the washers sampling point (courtesy of Brodie
O'Rourke) .............................................................................................................49
Figure 25.Weak black liquor sampling point (courtesy of Brodie O'Rourke) ........49
Figure 26.As-fired black liquor sampling point (courtesy of Brodie O'Rourke) ....50
Figure 27.White Liquor to digesters sampling point (courtesy of Brodie O'Rourke)
..............................................................................................................................50
Figure 28.Hot lime to the slaker sampling point (courtesy of Brodie O'Rourke) ...51
Figure 29.Weak wash (left) and raw green liquor(right)sampling points (courtesy of
Brodie O'Rourke) ..................................................................................................52
Figure 30.The average phosphorus flows in the recovery cycle (g P/ tonnes of air-
dried pulp) ............................................................................................................58
x
Figure 31.P concentration in ash-fired black liquor samples at each sampling period.
60
Figure 32.P concentration in weak black liquor samples at each sampling period. 61
Figure 33.P concentration variability in liquid samples of the recausticizing plant at
each sampling period ............................................................................................62
Figure 34. Variability of P content in lime mud samples at each sampling period .62
Figure 35. Change of of P concentration in green liquor over time. TTA: 120 g/L
Na2O and sulfidity= 24% .....................................................................................64
Figure 36.P flows in the recovery cycle during the first sampling period ..............78
Figure 37.P flows in the recovery cycle during the second sampling period..........79
Figure 38.P flows in the recovery cycle during the third sampling period .............80
Figure 39.P flows in the recovery cycle during the fourth sampling period ...........81
Figure 40.P flows in the recovery cycle during the fifth sampling period ..............82
Figure 41.Thermal profile of lime mud from white liquor clarifier underflow ......86
Figure 42.Thermal profile of lime after the sixth cycle, a: heating up, b: cooling
down .....................................................................................................................90
Figure 43.Thermal profile of lime mud after the sixth cycle, a: heating up, b: cooling
down .....................................................................................................................91
xi
List of Abbreviations
AA Active alkali
ADMT Air-dried metric ton
BL Black Liquor
CE Causticizing efficiency
CGL Clarified green liquor
CWL Clarified white liquor
EA Effective alkali
GL Green liquor
ICDD International center for diffraction data
ICP-OES Inductively coupled plasma optical emission spectrometry
LM Lime mud
LMC Lime mud after white liquor clarifier
LMC1C2 Lime mud between causticizers #2 and #3
LMS Lime mud after the slaker
LMW Lime mud from the washer
LR Lime rock
NPE Non-Process Element
xii
RGL Raw green liquor
RB Recovery boiler
SEM Scanning electron microscope
TGA/DSC Simultaneous Thermogravimetry and Differential Scanning
Calorimeter
TTA Total titratable alkali
WL White liquor
WLD White liquor to the digester
WLS White liquor from the slaker
WW Weak wash
XRD X-ray power diffraction
1
Chapter 1
Introduction
1.1 Kraft Pulping Process
The Kraft pulping process, also called, the sulfate process is the dominant pulping
process in the world. This method was invented by Karl F. Dahl in 1879 and was
first implemented in 1890 in a Swedish pulp mill [1]. Approximately, 130 million
tons/year of kraft pulp is annually produced which is around 90% of the world’s net
pulp production [2].
A flow chart of the kraft pulping process is shown in Figure 1. Wood chips are
delignified at about 170 ℃ in a digester to free the pulp fibers using white liquor,
which is an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide
(Na2S) that breaks the bonds between lignin, hemicellulose, and cellulose and
produces pulp fibers .The spent solution of pulping chemicals and dissolved organics
washed from the pulp is called the black liquor which is a by-product of the
delignification process [2].
Figure 1. The kraft pulping process (courtesy of Valmet)
2
In the recovery cycle, black liquor is first concentrated in multiple effect evaporators
and then burned in a special boiler called a kraft recovery boiler. The inorganic salts
(predominately Na2CO3 and Na2S) flow out of the recovery boiler as molten smelt
(T ≥800 ℃ ) and are dissolved in water in the smelt dissolving tank forming an
alkaline solution called green liquor. The raw green liquor from the dissolving tank
( at approximately 95 ℃ ) is clarified by settling or filtration and insoluble materials
are mostly removed at this point. The clarified green liquor then goes to the
recausticizing process to regenerate the white liquor [2].
1.2 Recausticizing Process (Lime Cycle)
In the recausticizing plant, clarified green liquor is slaked with lime (CaO) at about
98℃ and white liquor is reproduced. A schematic of the recausticizing process is
shown in Figure 2.
Figure 2. Schematic of the recausticizing process [48]
3
First, quick lime (calcium oxide) is added to the green liquor in the slaker. The lime
reacts with water in the green liquor and produces calcium hydroxide (slaking-Rn1):
Slaking CaO(𝑠) + H2O Ca(OH)2 (𝑠) (Rn1)
Figure 3. shows a schematic of a slaker with cyclone:
Figure 3, Slaker with cyclone (courtesy of Valmet)
4
Following the slaking reaction, calcium hydroxide reacts with carbonate in a series
of three to four agitated tanks called causticizers to form NaOH and
CaCO3 (causticizing-Rn2):
Causticizing Ca(OH)2 (𝑠) + N𝑎2CO3 (𝑎𝑞) ⇌ NaOH(𝑎𝑞) + CaCO3 (𝑠) (Rn2)
The second reaction is slower than the first reaction and the equilibrium dictates a
maximum conversion and causticizing efficiency (CE% (Eq.1)) of approximately
80-90% depending on the process conditions (e.g. Sulfidity (Eq.2), TTA (Eq.3) and
liming ratio (Eq.4)) [3]. Standard pulping terms are shown in table 1.
5
Table 1.Standard Pulping Terms (the concentration of each species is expressed in g/L of Na2O)
Pulping Term Equation
Total Titratable
Alkali (TTA)
(TTA) = [NaOH]+[Na2CO3]+[Na2S] (Eq.1)
Effective Alkali (EA)
EA=[NaOH]+1/2[Na2S] (Eq.2)
Active Alkali (AA) (AA)=[NaOH]+[Na2S] (Eq.3)
Causticizing
Efficiency (CE) (CE) =
[𝑁𝑎𝑂𝐻 (𝑙𝑒𝑠𝑠 𝑁𝑎𝑂𝐻 𝑖𝑛 𝑔𝑟𝑒𝑒𝑛 𝑙𝑖𝑞𝑢𝑜𝑟)]
[N𝑎2CO3 ]+[𝑁𝑎𝑂𝐻(𝑙𝑒𝑠𝑠 𝑁𝑎𝑂𝐻 𝑖𝑛 𝑔𝑟𝑒𝑒𝑛 𝑙𝑖𝑞𝑢𝑜𝑟)]
(Eq.4)
1Causticity Causticity% = [𝑁𝑎𝑂𝐻 ]
[N𝑎2CO3 ]+[𝑁𝑎𝑂𝐻]×100 (Eq.5)
Sulfidity SulfidityTTA = [Na2S]/TTA (Eq. 6)
SulfidityAA = [Na2S]/AA (Eq. 7)
Liming Ratio Liming Ratio=
𝐶𝑎𝑂
𝑁𝑎2𝐶𝑂3 (molar ratio) (Eq.8)
Lime Availability Lime Availability =
CaO available for slaking reaction
Lime (g)
(Eq.9)
Causticizing reaction (Rn-2) can reach higher conversion and proceed to the right
by adding more lime. However, adding too much lime causes over liming which
1 In this thesis, causticity is calculated in the same way as causticizing efficiency
6
means some of the lime remains as calcium hydroxide in the lime mud. This causes
operational problems settling and in the pressure filters [3].
Following the causticizing reaction, the produced white liquor is used in the digester
to be used in pulping. Lime mud is filtered and then washed with mill water or
condensates from the evaporators. The washing solution from the lime mud washer
is called weak wash. The molten salt from the recovery boiler is dissolved in weak
wash in the dissolving tank and makes the green liquor. Lime mud goes to the lime
kiln and it is heated counter- currently to be calcined and be converted to quick lime
[2]:
CaCO3 (𝑠) + Heat CaO(𝑠) + CO2 (𝑔) (Rn3)
Then the produced lime is fed to the slaker again. Figure 4 shows the schematic of a
lime kiln with lime mud dryer and electrostatic precipitator (ESP). The temperature
in the lime kiln can reach up to 1300℃ or higher.
Figure 4.Lime kiln with lime mud dryer (courtesy of F.L. Smidth)
7
1.3 Non-Process Elements
Non-process elements such as magnesium, manganese, silica, aluminum,
phosphorus, barium, copper and iron do not play an active role in the pulping process
[6] [7]. These elements can enter the kraft process with wood, make-up chemicals,
lime rock, biofuels (if burned in lime kiln) and process water [8]. When they exceed
their solubility limit, they precipitate out and cause operational problems such as
scaling of process equipment and blinding of filters, both of which result in reduced
energy efficiency in mill processes and can result in downtime for the mill. Where
these elements build up and precipitate in the recovery cycle depends on where they
are introduced and their solubility limit in the alkaline mill solutions. A brief
description of important NPEs is provided in the next chapter.
1.4 Phosphorus in the Lime Cycle
Phosphorus in one of the NPEs in the recovery cycle. Formation of phosphorus
compounds in lime and lime mud, can become challenging for the pulp mills that
have high phosphorus content in their lime cycle. There is limited information about
the behavior of phosphorous in the recovery cycle. It has been found that
phosphorous in the lime tends to bind with CaO molecules and form Ca-P
compounds that are not soluble in the green liquor[9]. As a result, part of the lime
which is bound to phosphorus becomes unreactive in the slaking reaction[9] [10].
These compounds do not dissolve or decompose in the lime cycle. Therefore, if not
purged, they remain in the cycle and result in dead load.
8
1.5 Objectives
The overall goal of this work is to investigate the fate of phosphorus in the lime
cycle. In this thesis a number of questions are addressed:
• the form of P in lime after it reacts with lime in the recausticizing cycle, and
the affect on lime availability
• the flows of P in the pulp mill
• the distribution of P between the solid and soluble forms, in the recaust
streams and how much this changes between
This thesis consists of the following chapters:
Chapter 2 is an overview of the literature. It consists of a background about non-
process elements more broadly: inputs, purge points and compounds formed. The
information about phosphorus is gathered in more detail while just a general
background is provided for the other non-process elements.
Chapter 3 consists of the methodology and laboratory experiments that were
conducted in this work to identify the phosphorus compounds in the lime and their
effect on the lime availability.
Chapter 4 focuses on the field studies and the mass balance for phosphorus in a
Canadian pulp mill.
Chapter 5 summarizes the main conclusions that can be drawn from the results of
this work.
Chapter 6 consists of the recommended future works.
9
Chapter 2
Literature Review
Non-Process Elements in the Pulp and Paper Industry
Pulp and paper mills continue to improve energy efficiency, reduce their CO2
footprint and reduce water use. As mills continue to reduce water consumption,
impurities will tend to accumulate in the system. Accumulation of impurities in the
recovery cycle of pulping facilities is a current and ever increasing problem in kraft
pulp mills around the world. Therefore, a better understanding of NPEs behavior,
can be very helpful to address and prevent the problems associated with their
accumulation.
Non-Process Elements Inputs
The main source of non-process elements in pulp mills is wood [8]. In the
brownstock washing unit, approximately 25-50% of the NPE in wood (K, Mn, Fe,
Mg, Cu, Ba, …) content go to the bleaching plant with pulp, either as a precipitate
trapped among pulp fibers or bound to the pulp, and the rest will end up in the
recovery cycle [11]. The exception is aluminum which does not exit with pulp
because it exists as an anion at basic pH [11].
NPEs concentration varies from mill to mill depending on the location of the mill
and age and type of trees used. Also, different parts of a tree, may have different
NPEs levels (e.g. bark has more NPEs level than wood) [12].
Figure 5 shows the distribution of some NPEs in various parts of four different types
of trees [12]. The concentration of NPE’s in bark is much higher than in the stem
10
wood on a dry mass basis. Thus, poor debarking can result in an increase the input
of NPE’s into digestion.
Besides wood, non-process elements, can also enter into the process through make-
up lime, process water, bio-sludge, petroleum coke in Lime Kiln, corrosion of
process equipment [13], and biofuels if burned in the lime kiln.
Figure 5.Distribution of dry mass and non-process elements in different parts of four investigated trees [12]
11
2.3 Negative Effects of NPEs
As mentioned before, accumulation of NPEs can cause operational problems for the
mills. The main negative effects of NPEs are listed in Table 2.
Table 2.Negative Effects of NPEs [14] [17]
Negative Effects Elements
Scales, deposits Al, Si, Ba
Corrosion K, Cl
Plugging in recovery boiler K, Cl
Inerts in lime cycle P, Mg, Al, Si
Effects on TCF-bleaching Mn, Fe, Cu
Impact on the environment P
Peroxide decomposition [17] Mn, Fe
2.4 NPE Purge Points
There are several purge points (kidneys) for NPE’s in the chemical recovery cycle.
One of the most important kidneys in the chemical recovery process is green liquor
dregs. Elements in smelt that are insoluble when added to weak wash, or that form
insoluble precipitates can be separated from green liquor. These precipitated solids
are called green liquor dergs. Some elements, such as Mn and Mg are less soluble in
green liquor, but others, such as Al and Si are partially soluble in alkaline solutions
and are therefore harder to purge from the lime cycle [11].
12
In addition to the green liquor dregs, ESP2 dust, mill effluents, wastewater, pulp,
and grits from the slaker are important purge points for NPEs as well.
Table 3 summarizes the primary inputs and outputs of different NPEs in the chemical
recovery cycle [18] [17].
Table 3.Main inputs and outputs of NPEs [18] [17].
NPE Input Output
Mn Wood
Process water
Green liquor dregs
Bleach plant filtrates
Mg Wood
Make-up lime
Process water
Green liquor dregs
Lime mud
Pulp
P Wood
Biosludge
Make-up lime
Lime mud
Slaker grits
2 Electrostatic Precipitator.
13
Si Wood
Make-up lime
Process water
Biosludge
Green liquor dregs
Lime mud
Slaker grits
Al Wood
Make-up lime
Process water
Biosludge
Green liquor dregs
Lime mud
Pulp
Fe Wood
Make-up lime
Corrosion
Process water
Green liquor dregs
Slaker grits
2.5 NPEs Handling Strategies
In order to mitigate the effects of non-process elements accumulation on the pulping
process, their concentrations should be controlled and minimized. There are some
ways suggested in the literature to control NPEs content in the kraft pulping process
[6] [14] [17] [19]:
1) Control and reduce the input of NPEs (e.g. by high degree of de-barking and
choosing make-up lime carefully, especially in case of Al and Si)
14
2) Control the efficiency of green liquor clarifier in mills with a green liquor
clarifier
3) Purge lime mud (this will depend on the impurities in the lime mud and lime
rock)
4) Control the NPEs content in the biofuel if it is burned in the lime kiln
5) Analyzing the ESP dust of the lime kiln regularly and purge the ESP dust
instead of the lime mud if the dust has high NPEs concentration
15
2.6 Important Non-Process Elements
In this part, more information about the more important non-process elements is
provided.
2.6.1 Aluminum
Aluminum mainly enters the pulping process with dirt in the wood, make-up lime
and white water from the paper machine (for the mills that bring white water back
to the recovery cycle). The solubility of aluminum in the alkaline mill solutions
depends on pH, temperature and presence of other elements such as magnesium and
silica[20] [21]. Aluminum is more soluble in the white liquor than in the green
liquor, due to an increase in solubility with increasing pH [20]. Therefore, it is
important that dregs are efficiently removed from green liquor. If dregs are not
efficiently removed, there is the potential for aluminum that is insoluble in green
liquor to be solubilized in white liquor.
Aluminum concentration decreases by increasing silica concentration [20] [22].It
can precipitate with silica as aluminosilicate compounds like sodalite (Na8
(Al SiO4)6(OH)2.2H2O) on the black liquor evaporator surface [20] [22].
Aluminosilicate scaling problems on evaporator surfaces has been observed in other
industries as well. For example, during concentrating caustic liquid wastes
containing aluminum and silica for storage; and in the Bayer aluminum production
process where a spent liquor containing sodium hydroxide, aluminum and silica
needs to be concentrated in an evaporator [24].
16
2.6.2 Silica
Silica mainly enters the kraft process with dirt in wood, make-up lime and process
water. It is partially soluble in green and white liquors. Its solubility increases with
alkalinity, so it is more soluble in white liquor than green liquor [20]. As mentioned
in the previous section, silica can precipitate as aluminoslicate compounds by
binding to aluminum on the black liquor evaporator surface [20] [24]. This will have
a negative effect on the temperature profile on the evaporator wall because it reduces
the heat surface area. Besides the black liquor evaporators, silica can cause problems
in the lime cycle as well. If silica accumulates in the lime cycle, it can reduce the
lime availability because it can melt on the surface of lime pallet and decrease
porosity [25].
2.6.3 Magnesium
The main source of Mg is wood. It can also enter the process through MgSO4 added
to oxygen delignification, biosludge, and make-up lime [17]. Magnesium is highly
insoluble in the green liquor, so it can mostly be removed with green liquor dregs. If
it does not precipitate with green liquor dregs due to settling or filtration problems,
it ends up in the recausticizing process and accumulates in the lime mud as Mg(OH)2
which results in a decrease in lime mud filterability [26]. At high concentrations of
magnesium and silicate ions in the black liquor, magnesium silicate hydrate can
precipitate [23]. The magnesium silicate hydrate forms a gel layer on the hot surface
of the tubes and can cause plugging.
17
2.6.4 Manganese
The main source of manganese is wood [27]. The solubility of manganese is very
low in green liquor (approximately 0.6 mg/l), thus if the green liquor is properly
clarified, manganese can be removed easily with green liquor dregs as MnS and it
will not accumulate in the lime cycle [28] [29] [30]. If the green liquor clarifier/filter
does not function well, MnS ends up in the lime mud and causes extra dead load
[27]. It can also affect the lime color and turn it to pale yellow [15].
2.6.5 Iron
Iron is introduced to the pulping system mainly through wood, make-up chemicals
and corrosion of process equipment [17]. Iron has very low solubility in the green
and white liquors (Fe solubility in white liquor is approximately 0.1 mmol/L [17]),
so it can be removed as solid FeS with the green liquor dregs and the grits from the
slaker [17] [15]. If iron is present as FeS in the lime, it can form Ca2Fe2O5 after
calcination. This can lead to an increase in the dust formation in the lime kiln [17].
Iron can also increase bleaching chemical consumption if present in the bleaching
plant [15].
2.6.6 Phosphorus
Phosphorous is a chemical element which is essential for life. Due to its reactive
nature, phosphorus cannot exist as a free element on earth. Most of the phosphorous
containing minerals exist in their oxidized state. Phosphorus (as phosphate ions:
PO43−) is one of the key elements in human DNA, RNA, bones and teeth. The other
high demand application of phosphate is to be used in fertilizers as a replacement
18
for the phosphorous that plants consume from the soil. Phosphate also can be used
as organophosphorus compounds in detergents, nerve agents, and pesticides [31].
In the kraft pulping process, wood chips are the major source of phosphorus. The
phosphorous concentration varies between 40-80 g P/ton dry wood [32]. One study
found that approximately 75% of the phosphorus intake ends up in the recovery cycle
as phosphate ions (PO43−) [9]. Table 4, shows the range of total phosphate ions in
green and white liquors from eight Swedish kraft pulp mills [9]:
Table 4.Total phosphate concentrations in green and white liquors from eight Swedish kraft mills
[9]
[P𝐎𝟒𝟑−] (mmol/L)
Green Liquor 0.7-2.1
White Liquor 0.2-0.5
2.7 Phosphorus Compounds in the Lime Cycle
Phosphorus exists as sodium phosphate in green and white liquors. It is reported that
phosphorus (as phosphate ions) concentration is 3 to 5 times higher in the green
liquor than that of in the white liquor [9]. In the lime cycle, phosphate ions in the
green liquor bind to calcium ions of the lime and precipitate as Ca-PO4 compounds
in the lime mud.
The phosphorous in the lime mud is mostly insoluble during slaking because of the
large amount of calcium added and the low solubility of calcium phosphate
19
compounds [10] [9]. Therefore, phosphorus accumulates in the lime cycle by
calcination of the lime mud. As a result, part of the lime which is bound to
phosphorus becomes unreactive in the slaking reaction and this reduces the lime
availability [9] [10]. Previous studies showed that concentration of 1wt.%
phosphorus in the lime, can decrease the available CaO in lime by 5wt.% [9].
Based on the literature [33] [34] [35] , in alkaline solutions without CO32- ion
present, phosphorus and calcium precipitate as calcium phosphate compounds such
as hydroxyapatite (C𝑎5(PO4)3(OH) ) also known as HAP. However, if other ions
such as Na+ and CO32- are present in the alkaline solution (as in green and white
liquors) ,these ions can be replaced with Ca+ and (PO4)33- in the HAP structure and
form hydroxyapatite compounds containing Na+ and CO32- which are called (CAP)
[33] [34] [35].
Literature Gap
A previous study was done to identify the composition of Ca-P precipitates in green
and white liquors [10]. That study reported that if the total molar ratio of Ca/P in the
solution is less than 0.5 (mol/mol), the precipitates mainly consist of HAP, but when
it is increased to 1.7 (mol/mol), CAP becomes the main component in the
precipitates from green and white liquors [10]:
Ca9±0.5 Na1±0.5 (CO3)1±0.5 (OH)2±0.5 (PO4)5±0.5 if Ca/P (molar ratio) < 0.5
Ca8.5±0.5 Na1.5±0.5 (CO3)2±0.5 (OH)2.5±0.5 (PO4)4±0.5 if Ca/P (molar ratio) ≥1.7
The earlier study also indicated that Na+ ion in the CAP structure is part of the crystal
structure and is not washable [10]. However, this earlier study did not present XRD
spectra and it is unclear how the form of phosphorous was determined. One objective
20
of this work was to clarify the form of calcium phosphate in lime mud. This
information can be used to build a database for the phosphorus compounds that are
formed in the recovery cycle.
2.7.1 Calcium Phosphates
Calcium phosphates are chemical compounds that contain both calcium ions (Ca2+)
and orthophosphates(PO43-) or metaphosphates (PO3
3-) or pyrophosphates (P2O74−)
[31]. Oxide and hydroxide ions can also be found in calcium phosphate compounds
as well. For example, apatite has the formula Ca5(PO4)3X, where X can
be F, Cl, OH, or a mix of all. It is called hydroxyapatite if X is mainly hydroxide
[10]. Most of calcium phosphate compounds can dissolve in neutral or basic
solutions and precipitate as hydroxyapatite. The solubility of calcium phosphate
phases in aqueous solution is an important property and mainly correlated with the
calcium (Ca)/phosphorous (P) ratio] [36]. The solubility of calcium phosphate
species decreases in the order MCPM > DCPD = DCPA > OCP > β-TCP > HA [36].
Different calcium phosphate species are shown in Table 5 based on their Ca/P ratio.
Table 5.Calcium phosphate species based on their Ca/P ratio [36]
Name Abbreviation Formula Ca/P
ratio
Monocalcium phosphate
monohydrate
MCPM Ca(H2PO4)2. H2O 0.5
Dicalcium phosphate
anhydrate (monetite)
DCPA CaHPO4 1.0
Dicalcium phosphate
dihydrate (brushite)
DCPD CaHPO4.2H2O 1.0
Octacalcium phosphate OCP Ca8H2(PO4)6.5H2O 1.33
β-Tricalcium phosphate β-TCP Ca3(PO4)2 1.5
21
Amorphous calcium
phosphate
ACP Ca3(PO4)2. nH2O 1.5
α-Tricalcium phosphate α-TCP α-Ca3(PO4)2 1.5
Hydroxyapatite HA Ca10(PO4)6(OH)2 1.67
Tetracalcium phosphate TetCP Ca4(PO4)2O 2.0
β-Rhenanite3 Na Ca PO4 1
Nowadays, calcium phosphates are widely used in phosphoric acid and fertilizers
production. This group of chemicals are components of biocompatible inorganic
biomaterials found in human teeth and bones, and they are also used in bone grafts
and dental composites production [37].
2.7.2 Rhenanite (NaCaPO4)
Rhenanite is a glass-ceramic bioactive material which means it allows body cells to
grow on it (e.g. bone cells) [38]. Rhenanite is found to have high resorbability and
osteoinductive4 capabilities which are the key factors for a good skeletal repair , so
it is used as a component of bone grafts and self-stetting cements to repair bone
defects [39].
Rhenanite can be produced in the “rhenania process” which is widely used in the
fertilizer industry to produce a soluble phosphate compound that can be used as a
substitute for the phosphates that plants get from the soil [40]. In this process,
hydroxyapatite is mixed with Na2CO3 and SiO2 in which the molar ratio of
Na2CO3/P2O5 is fixed at 1. SiO2 is used to prevent the occurrence of free CaO in the
sintered product [40]. These powder mixtures are then ground together and calcined
3 𝛽-Rhenanite was added to the table by the writer of this thesis.
4 It allows the bony tissue to grow on it.
22
in a rotary kiln at about 1000–1200°C for about few hours which leads to production
of Rhenanite. Above 700℃, hydroxyapatite is not stable and Na+ ions diffuse into
hydroxyapatite structure and form rhenanite [41]. Because rhenanite is treated at a
high temperature, it has a rather low surface area, low surface reactivity, and forms
large powder particles [40]. Rhenanite solubility is 1 g/L of H2O at pH =7 in the
human body [42].
Rhenanite has two structures; 𝛼 and 𝛽. At about 640 ℃, 𝛼-rhenanite transforms to
𝛽-rhenanite [43].𝛽- rhenanite has very similar crystal parameters to that of one of
the apatite species called flouroapatite (Ca5(PO4)3F [43]. As a result, rhenanite has
similar crystal parameters to other apatite species as well [43]. The crystal
parameters of β-rhenanite are: a = 0.523 nm, c = 0.704 nm of the hexagonal system
[44]. Similarly, fluoroapatite has a hexagonal structure with a = 0.9367 nm, and c =
0.6884 nm [45]. The lattice parameter c of both crystals is very similar. Also, given
with the a-axis lattice parameter of β-rhenanite (a = 0.523 nm) by doubling (2 a =
1.046 nm). This value is close to that of fluoroapatite [43].
In this project, the effect of phosphorus on the lime cycle is studied and phosphorus
compounds are identified. Also, a complete phosphorus mass balance is done in a
Canadian pulp mill to find the phosphorus distribution in a kraft pulp mill.
23
Chapter 3
Laboratory Experiments
3.1 Overview of the Experimental Investigation of P Compounds
in the lime cycle
In order to identify the phosphorus compounds in the lime cycle and their effect on
the lime availability, multiple slaking/calcining cycles were carried out with
reburned lime and mill green liquor doped with sodium phosphate. The resulted
products after each cycle were analyzed using different analytical and
characterization methods. The details of these experiments and their results are
summarized in this chapter.
3.2 Experimental Procedure
3.2.1 Identification of Phosphorus Compounds and Their Effects
on Lime Availability
Multiple slaking/calcining cycles were done in mill green liquor doped with 30,000
ppm sodium phosphate which was equivalent to phosphorus concentration of 5,672
ppm. The reason that the sodium phosphate addition was quite high was to ensure
the lime could take up high enough phosphorus in less than 10 slaking cycles to be
analyzed by X-ray diffraction (XRD).
The initial phosphorus concentration in the green liquor was measured by ICP-OES,
and it was (15.8 ppm ± 0.2). The initial reburend lime was digested with 70% nitric
24
acid (HNO3) and was analyzed with ICP-OES to find the initial P content which was
(0.18 wt% ± 0.1). Green liquor TTA and sulfidity were measured using a Mettler
Toledo G10S Compact Titrator and they were (118 g/L Na2O ± 4) and (22.5%±2),
respectively.
The solid calcium compounds after each cycle were analyzed with different
methods: the standard TAPPI lime analysis method (T 617 cm-84) to measure
available CaO for slaking [46], X-Ray Diffraction (XRD), Scanning Electron
Microscopy (SEM), thermogravimetry Analysis (TGA) and Inductively Coupled
Plasma Optical Emission Spectrometer (ICP-OES). The details of all the methods
used are mentioned in this chapter.
All the slaking/causticizing reactions were done at (95 ± 2℃) in a water bath
(ADVANTEC-TBS 181SB) which held a 500 ml high-density polyethylene (HDPE)
bottle with a magnetic stir. The lime mud after each slaking/causticizing cycle was
calcined at (1200 ± 20℃) to form lime. Figure 6 shows a schematic of the
experimental apparatus.
Stirrer
HDPE Bottle
Thermometer
Temperature
Control
Stir Bar
Figure 6.Experimental Setup
25
3.2.2 Analytical Method
Sodium Phosphate (trisodium phosphate, 96%) was used to dope the mill green
liquor that was used in the slaking reactions. After each slaking cycle, the lime mud
was filtered from the white liquor using a 1 L filtration flask with funnel and a 15
cm filter paper with a particle retention of 25 µm (VWR North America). Lime mud
was washed using deionized water and then it was dried at 105 ℃ and was calcined
at (1200 ± 20 ℃) to form lime. Lime availability was measured using the standard
TAPPI lime analysis method (T 617 cm-84) after each cycle [46].
Solid Samples Analysis
To analyze phosphorus , all solid samples were digested with 70% nitric acid
(HNO3) for 2hr at 95 ℃ and then the resulting solution was diluted with 5% nitric
acid (HNO3) and were analyzed by an Agilent 700 Series Inductively Coupled
Plasma Optical Emission Spectrometer (ICP-OES), which was calibrated using
reference solution (Fisher Scientific 100 ppm±1) to obtain the Ca/P ratio.
Furthermore, solid samples were analyzed by Scanning Electron Microscope (SEM)
using a Hitachi VP-SEM SU3500.
X-Ray diffraction (XRD) was done by a benchtop powder X-ray diffraction (XRD)
instrument (Rigaku MiniFlex 600) to identify the phosphorus compound(s) in lime
and lime mud. Thermogravimetric Analysis (TGA) was done on solid samples using
a TA Instruments STD-Q600 Simultaneous Thermogravimetry and Differential
Scanning Calorimeter (TGA/DSC). Samples were equilibrated in air at 20°C then
ramp the temperature at 20°C/min to 1000°C.
26
3.2.3 Results and Discussions
Effect of Phosphorus on Lime Availability
In each slaking/calcining cycle, lime was added to the green liquor at liming ratio of
1±0.05. Then, the resulting lime washed with an amount of water equivalent to the
amount of green liquor in the experiment. This water was used approximately in
thirds so that the lime mud had three rinses. The resulting lime mud was then
calcined at 1200 °C. After the lime mud was calcined, the lime was added to the next
cycle. Series could continue up to 6 cycles. After the 6th cycle, the lime availability
was too low to continue.
Figure 7 shows P (wt.%) and P/Ca molar ratio in lime which were measured using
ICP-OES in all cycles.
Based on Figure 7, as the cycles continued, phosphorus content in lime increased as
expected. This means lime was able to take up more and more phosphorus after each
0 1 2 3 4 5 6
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cycle #
P/C
a m
ole
rati
o
in l
ime
Figure 7.P/Ca mole ratio in different cycles
27
cycle. Also, the fraction of the Ca bound to P increased accordingly. As a result, the
amount of lime available for the slaking reaction decreased.
The availability of the resulting lime after each cycle was analyzed by the standard
TAPPI lime analysis method (T 617 cm-84) [46]. The initial lime availability and
P(wt.%) before the first cycle were (92% ±2) and (0.18 ±0.1), respectively. The
results are shown in Figure 8.
Figure 8.Effect of P content on lime availability
As shown in Figure 8, the lime availability decreases by increasing P (wt.%) in lime.
This is more visible before and after the first cycle (from 92% to 62%) because the
initial lime had low impurities and it was more reactive. In other words, more CaO
was available to react with phosphorus and form Ca-P compounds that are not
reactive in the slaking reaction in the following cycles. So, lime could take up more
phosphorus in cycle 1 than in other cycles. This supports the theory of the negative
effect of phosphorus on the lime availability. Furthermore, Figure 9 shows a linear
y = -5.4562x + 93.677
R² = 0.9937
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14 16
P (wt.%) in lime
Lim
e a
vail
ab
ilit
y %
28
relationship between lime availability and P (wt.%) in the lime; as the P content in
the lime increases by (1wt.%), lime availability decreases by around 5.5 (wt.%),
consistent with earlier findings [9] [10].
X-Ray Diffraction Results
Solid samples after each cycle were analyzed with X-ray diffraction (Rigaku
MiniFlex 600) to identify the Ca-P compounds in the lime and lime mud. Unlike the
previous studies [10] [9], XRD results showed that the Ca-P compound that is
formed in lime and lime mud is rhenanite (NaCaPO4). The XRD profiles of lime and
lime mud samples after each cycle are shown in 5Figures 9 and 10, respectively .
The measured diffraction peaks were analyzed using the PDXL software which is a
full-function powder diffraction analysis software suite. In cycles 1 and 2, it was
difficult to get high intensity diffraction peaks due to lower rhenanite concentration.
The XRD profiles were matched with the figures in the international center for
diffraction data (ICDD) base.
5 Y-axis in the figures changes because intensities are changing
29
P/Ca (mole ratio) =0.14
(a)
P/Ca (mole ratio) =0.25
(b)
0
4000
8000
12000
16000
20000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
R R R R R
0
4000
8000
12000
16000
20000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR RR
RR R R R
R: Rhenanite
R
R: Rhenanite
30
P/Ca (mole ratio) =0.36
(c)
0
1000
2000
3000
4000
5000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
R
R
R
R RR
R RR
0
1000
2000
3000
4000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
R
R
R
R
RRR
R
RR
R
R: Rhenanite
P/Ca (mole ratio) =0.45
(d)
R: Rhenanite
31
P/Ca (mole ratio)=0.55
(e)
P/Ca (mole ratio)=0.66
(f)
Figure 9.XRD profile of lime after different cycles, (a): 1st cycle, (b):2nd cycle, (c): 3rd cycle,
(d): 4th cycle, (e) :5th cycle, (f):6th cycle
0
500
1000
1500
2000
2500
3000
3500
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
RR
R
R
R
R
R
RR
R R R R
0
500
1000
1500
2000
2500
3000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
R RR
R
R
RR
R
R
RR
R R RR R
R: Rhenanite
R: Rhenanite
32
P/Ca (mole ratio) =0.17
(a)
P/Ca (mole ratio) =0.24
(b)
0
1000
2000
3000
4000
15 20 25 30 35 40 45 50 55 60 65 70 75
Inte
nsi
ty
2θ̊
R
RRR
0
2000
4000
6000
8000
10000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
R R
R R
RR
R: Rhenanite
R: Rhenanite
33
P/Ca (mole ratio) = 0.36
(c)
P/Ca (mole ratio) =0.45
(d)
0
2000
4000
6000
8000
10000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
R
R
R
R RR
R: Rhenanite
0
500
1000
1500
2000
2500
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
R
R
R
R
RR
R R
R: Rhenanite
34
P/Ca (mole ratio) =0.52
(e)
P/Ca (mole ratio) =0.59
(f)
0
500
1000
1500
2000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
R
R
R
R
R
R
RR
RR
R
RR R
R
0
500
1000
1500
2000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ̊
RR
R
RR
R
R
RR
R
R
R
R
R
R RR R R RR R R
Figure 10.XRD profile of lime mud after different cycles, (a): 1st cycle, (b):2nd cycle, (c): 3rd
cycle, (d): 4th cycle, (e) :5th cycle, (f):6th cycle
R: Rhenanite
R: Rhenanite
35
Scanning Electron Microscope (SEM) Results
Lime and lime mud after the first cycle were analyzed using SEM (Hitachi VP-
SU3500) to see the rhenanite distribution on lime and lime mud particles. After the
first cycle, the P content of the lime and lime mud was higher than the upper limits
reported in literature. The lime mud sample was a soft white powdery substance and
the lime sample was a light green powder. Particles in both samples had a spherical
shape. SEM-EDS showed phosphorous is fairly homogenously distributed on the
lime and lime mud particles as apposed to being clearly separate crystals. Figure 11
and 12 show the distribution of phosphorous in different areas in lime and lime mud
after the first cycle, respectively.
Element Area#10
(mole%)
Area#11
(mole%)
Area#12
(mole%)
Ca 2.12 2.25 1.98
P 0.27 0.15 0.32
Na 0.26 0.18 0.4
36
Mg 0.023 0.033 0.018
Si 0.013 0.007 0.008
Element Area#4
(mole%)
Area#5
(mole%)
Area#6
(mole%)
Ca 2.34 2.07 1.98
P 0.098 0.208 0.28
Na 0.13 0.43 0.48
Mg 0.017 0.021 0.021
Si 0.004 0.007 0.015
Figure 11.SEM image of lime after the first cycle in different areas
Figure 12.SEM image of lime mud after the first cycle in different areas
37
Thermogravity Analysis Results (TGA)
Besides XRD, lime and lime mud after the first and last cycles were analyzed by TA
Instruments STD-Q600 Simultaneous Thermogravimetry and Differential Scanning
Calorimeter (TGA/DSC) to find the thermal stability of rhenanite formed in lime
and lime mud. The results are shown in Figures 15- 18. The TGA/DSC figures were
compared to the standard profiles of lime and lime mud which are shown in Figures
13 and Figure 14. The standard lime and lime mud thermal profile were obtained by
running TGA/DSC on the original lime and the lime mud resulted from slaking the
original lime with the same green liquor used for all experiments without sodium
phosphate addition.
Figure 13.Typical thermal profile of lime
As shown in Figure 13, a typical thermal profile of lime shows a weight loss between
350- 450 ℃ which corresponds to decomposition of Ca (OH)2 to CaO and H2O
(Rn4). The theoretical weight loss of H2O from Ca (OH)2 is 24%.
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 200 400 600 800 1000
98
98.5
99
99.5
100
Hea
t F
low
(W
/g)
Weig
ht
%
Weight%
Heat Flow (W/g)
38
Ca (OH)2(s) → CaO(s) +H2O (Rn4)
Figure 14 shows the typical thermal profile of lime mud.
Figure 14.Typical thermal profile of lime mud
As shown in Figure 14, a typical thermal profile of lime mud shows two weight
losses; first a weight loss 350- 450 ℃ which was described above and second, a
weight loss between 620 – 830 ℃ which corresponds to decomposition of CaCO3 to
CaO and CO2 (Rn-5) and the theoretical weight loss of CO2 from pure CaCO3 is 44%.
CaCO3(s) → CaO(s) + CO2(g) (Rn5)
The thermal profiles of lime and lime mud samples after the first and the last
slaking/ calcining cycles are shown in Figures 15-18.
-5
-4
-3
-2
-1
0
1
2
0
10
20
30
40
50
60
70
80
90
100
0 200 400 600 800 1000
Hea
t F
low
(W
/g)
Weig
ht
%
Weight%
Heat Flow (W/g)
39
Figure 15.Thermal profile of lime after the first slaking/calcining cycle
Figure 16.Thermal profile of lime mud after the first slaking/calcining cycle
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0 200 400 600 800 1000
96
96.5
97
97.5
98
98.5
99
99.5
100
Hea
t F
low
(W
/g)
Temperature (℃)
Weig
ht
%
Weight %
Heat Flow (W/g)
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
0 200 400 600 800 1000
50
60
70
80
90
100
Hea
t F
low
(W
/g)
Weig
ht
%
Weight %
Heat Flow (W/g)
Temperature (℃)
40
Figure 17.Thermal profile of lime after the sixth slaking/calcining cycle
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 200 400 600 800 1000
96
96.5
97
97.5
98
98.5
99
99.5
100
Hea
t F
low
(W
/g)
Wei
gh
t %
Weight %
Heat Flow (W/g)
Temperature (℃)
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0 200 400 600 800 1000
86
88
90
92
94
96
98
100
Hea
t F
low
(W
/g)
Weig
ht
%
Weight %
Heat Flow (W/g)
Temperature (℃)
Figure 18.Thermal profile of lime mud after the sixth slaking/calcining cycle
41
As shown in Figures 15-18, DSC results for the lime and lime mud samples with
high phosphorus contents, in addition to the peaks seen in typical lime and lime mud
samples, show an endothermic reaction (with no weight loss) between 600 -640 ℃
which can be due to a crystal structure change of the 𝛼 -rhenanite to 𝛽-rhenanite
[43]. This was confirmed by running TGA on the lime and lime mud after the sixth
cycle which had the highest P content in the way that the samples were first heated
up to 950 ℃ and then they were cooled down. The results were in good agreement
with the literature [43]. Thermal profiles are shown in appendix 5.
3.3 Effect of Lime Mud Washing on Phase Identification
In order to be sure the sodium in the rhenanite structure is not washable and not
present only because of the sodium phosphate addition or insufficient lime mud
washing, a series of washing experiments were done on a lime mud sample. In this
experiment, the first slaking/calcining cycle was repeated. Lime mud was washed
with deionized water in multiple stages as before, but this time with more stages
(five instead of three) and more water in each washing stage (in total, more than the
equivalent to the amount of green liquor used in the slaking/causticizing cycle: 250
ml GL for slaking and 350 ml wash water). The lime mud and washed solutions were
analyzed by ICP-OES to find sodium levels after each washing stage. Figure 19
shows the effect of washing on sodium concentration in the lime mud.
42
As can be seen in Figure 19, sodium level in the lime mud decreased from stage one
to four and after the fourth stage, it became constant. Also, after the fourth cycle
little to no Na was washed from the lime mud.
Besides sodium, the phosphorus concentration in the washed solution was analyzed
as well. The results showed no phosphorus was washed out in any of the washing
stages. This suggests that the washed sodium was not from the rhenanite structure.
It could be the sodium in residual green liquor. Thus, all the remaining sodium in
the lime mud after washing is bound to phosphorus in the rhenanite structure.
Figure 20 shows the effect of washing on Na/P molar ratio in the lime mud.
0
1
2
3
4
5
6
1 2 3 4 5
Na
wt%
in
lim
e m
ud
Washing stage
Figure 19.Remaining Na content in the lime mud after multiple washing stages
P(wt.%) =4.88
43
As can be seen in Figure 20, the molar ratio of Na and P in the lime mud after the
4rd washing stage becomes constant and is very close to 1 which is the molar ratio of
Na and P in the rhenanite (Na Ca PO4). This molar ratio is close to the molar ratio
found in an earlier study [35]. To confirm this theory, the lime mud after the 5th
washing stage was analyzed by X-ray diffraction. The results are shown in Figure
21. The washing experiments were done on lime mud after the first slaking/calcining
cycle, so the intensity of the rhenanite peaks in the XRD profile are low. However,
rhenanite’s major peaks are still visible.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1 2 3 4 5
Na
/P m
ole
ra
tio i
n l
ime m
ud
Washing Stage
Figure 20.Na/P mole ratio in the lime mud which are all in the rhenanite structure
44
Figure 21 confirms rhenanite is still present in the lime mud after the 5th washing
stage.
Figure 21.XRD of lime mud after 5th washing stage
0
500
1000
1500
2000
2500
3000
3500
4000
15 25 35 45 55 65 75
Inte
nsi
ty
2θ°
R
R R
C
C C CC
C
CC C CR
C
R: Rhenanite
C: Calcite
45
Chapter 4
Mill Studies
4.1 Pulp Mill Background
This mill has a conventional single line recovery cycle. The mill produces
unbleached kraft paper from softwood (equal mix of spruce and pine). The annual
production rate is 182,500 admt/year.
4.2 Sampling Campaign for P Mass Balance
The sampling campaign contained 17 sampling points which are shown in table
Table 6. Sampling points
Sample # Description
1 Wood chips
2 Pulp+carryover
3 Weak BL
4 Ash-fired black liquor to the recovery boiler
5 White liquor to the digesters
6 Raw Green liquor before green liquor clarifier
46
7 Green liquor dregs
8 Clarified green liquor
9 Slaker grits
10 WL after the slaker
11 WL between causticizer 2 and 3
12 Clarified WL to lime mud washer
13 Lime mud off the lime mud washer
14 Weak wash
15 Scrubber water
16 Hot lime from the lime kiln
17 Lime rock added to the lime kiln
47
All of the samples were collected during a mill visit in April 2019 and they were
analyzed at the University of Toronto. The sampling campaign was carried out over
three days. On the first day, only one round of samples was collected while on the
second and third days two rounds were collected. A simplified schematic of the
mill’s general recovery cycle is shown in Figure 22. Sample points (1-17) are shown
in Figure 22 as well.
Figure 22.Schematic of the recovery and fiber line of the mill
48
4.3 Sample Collection
Wood chips samples to the digesters were obtained by the mill operators once a day
with a shovel. Weak black liquor and ash fired black liquor samples were collected
using a wide-mouthed high-density polyethylene (HDPE) bottle. As-fired black
liquor samples were collected by the mill operators into wide-mouthed HDPE bottles
containing a weighed amount of deionized water and the weight of the bottle with
water and black liquor was used to determine the amount of as-fired black liquor
sampled. Pulp samples were collected from the last washing stage using a shovel
and the pulp samples were pressed to get carryover samples. The carryover was
stored in wide-mouthed HDPE bottles and pulp samples were stored in the sealed
3L plastic bags. Wood chips, pulp and black liquor samples were stored at 4 ℃. All
bottles were immediately sealed with Parafilm™. Sampling points (1-5) are shown
in Figures 23-27.
Figure 23.Wood Chips to digesters sampling point (courtesy of Brodie O'Rourke)
49
Figure 24.Pulp and carryovers from the washers sampling point (courtesy of Brodie O'Rourke)
Figure 25.Weak black liquor sampling point (courtesy of Brodie O'Rourke)
50
Figure 26.As-fired black liquor sampling point (courtesy of Brodie O'Rourke)
Figure 27.White Liquor to digesters sampling point (courtesy of Brodie O'Rourke)
51
Hot lime samples were obtained directly from the lime kiln and were cooled down
before storing in wide-mouthed HDPE bottles. Figure 28 shows hot lime sampling
point.
Figure 28.Hot lime to the slaker sampling point (courtesy of Brodie O'Rourke)
Lime rock samples were collected from where they entered the lime kiln and were
stored in a sealed 3L plastic bag.
52
In order to get dregs samples, raw green liquor with known volume was filtered
while hot using a 24 cm cellulose filter paper, medium grain, (from Fisher Scientific)
at the mill. Dregs were then washed using deionized water (obtained from the main
lab at the mill) and were dried in the oven at 105 ℃. Figure 29 shows raw green
liquor and weak wash sampling point.
Figure 29.Weak wash (left) and raw green liquor(right)sampling points (courtesy of
Brodie O'Rourke)
Grits samples were taken using a shovel from the grits storage and were stored in
sealed 3L plastic bags. The flow used for calculation of the P mass flow with the
grits was the dumping rate.
53
All other liquid samples from the recausticizing plant were taken using a 1L thermos.
The thermos was rinsed with the sample solution at least twice before sample
collection. Then, the samples were hot filtered immediately using a 1L filtration
flask with a 24 cm cellulose filter paper (medium grain) bought from Fisher
Scientific. In some cases, they were filtered using a 0.2 µm polyethersulfone (PES)
membrane sterile filter (FroggaBio) attached to a plastic syringe. Samples were
stored in 50 ml plastic bottles bought from Fisher Scientific and were sealed
immediately with Parafilm™.
Solid samples were washed with deionized water and were dried in oven at 105℃ at
the mill after separation from the mill solutions and were stored in 3L plastic bags.
Dry Solid Measurements
The dry solid content of the dregs and white liquor samples were measured at the
mill. A given volume of the mixed solution was filtered using a 1L filtration flask
with a 24 cm cellulose filter paper (medium grain) bought from Fisher Scientific.
Then, the solid part was washed with deionized water and dried at 105 ℃ and the
dry weight was measured. Results are shown in Appendix 2.
To measure the dry solids of the black liquor samples, an aliquot of black liquor was
spread over sand and the sample before and after drying at 105°C for 24 hours.
Results are shown in Appendix 2 as well.
54
4.4 Laboratory Analysis
All samples were analyzed at the University of Toronto. The elemental composition
was analyzed with ICP-OES (at least five times repetitions for each sample).
All liquid and solid samples (except dregs and grits) were digested with 70% nitric
acid (HNO3) at 95℃ for 2hr and were analyzed by ICP-OES.
Wood samples were completely air dried at 40 ℃ and were ashed at 540 ℃ prior to
digestion. Pulp samples were ashed at 540 ℃ as well.
Dregs and grits samples were prepared using fusion with lithium tetra boride and
were analyzed with ICP-OES. They were ashed at 540 ℃ prior to fusion.
Density Measurements
The density of the liquors obtained from the recausticizing plant was measured at
room temperature at University of Toronto and was corrected to 95℃ by using a
correction factor obtained by assuming that mill liquors behave like water in terms
of density at different temperatures. In order to confirm this assumption, some
solutions were heated up to 95℃ and density was compared with that of calculated
based on the water behavior. Results were in very good agreement and the error was
less than 2%. Density measurement results are shown in the appendix 2.
55
4.5 Mass Balance Calculations
The daily online data log of the mill was obtained for the three days of the sampling
campaign and it was used to calculate the mass flow of phosphorus through the
sampling points.
The liming ratio was calculated by (Eq 10) based on the white liquor causticity and
the Ca (OH)2 and CaCO3 content of lime mud from the clarifier underflow obtained
from its thermal profile [47].
Causticity(W.L.)−Causticity(G.L.)
1−Causticity(G.L.)∗ (1 +
𝐶𝑎(𝑂𝐻)2
𝐶𝑎𝐶𝑂3∗
100
74)
* Where the Ca (OH)2 and CaCO3 are the (wt.%) in lime mud from white liquor
clarifier underflow
Lime availability was measured using TAPPI standard lime analysis method [46]
and it was 92%. Based on the (Eq 10) liming ratio was 0.82. The thermal profile of
lime mud from the white liquor underflow is provided in appendix 3.
The mill uses the slaker temperature to adjust the lime feed screw drive speed (lime
addition) to the slaker. Since these variables remained fairly constant in three days
of sampling campaign (based on the mill data log), the liming ratio was calculated
based on the first day of samples and assumed to be the same for the other two days.
Table 7 shows a summary of the mill’s online data log in that three days (daily
average).
Liming Ratio = (Eq 10)
56
Table 7. Mill’s data log (daily average)
6 Measured at University of Toronto by titration with HCl (1N) and was used to calculate liming ratio and lime
addition. This value is the average of three days.
Property Value
RGL TTA (g/L Na2O) 120-122
RGL Sulfidity (%) 22-26
CGL TTA (g/L Na2O) 120-122
CGL Sulfidity (%) 22-26
6CGL (Na2 CO3) (mol/L) 1.354
WL in #2 Causticizer TTA (g/L Na2O) 120-122
WL in #2 Causticizer Causticity (%) 80
WL in #2 Causticizer Sulfidity (%) 22-26
WL in #1 Clarifier TTA (g/L Na2O) 120-122
WL in #1 Clarifier Causticity (%) 80
WL in #1 Clarifier Sulfidity (%) 23.5
WW TTA (g/L Na2O) 18
Slaker Temperature (℃) 102.2
Lime Feed Screw Speed (%) 33-36
57
Besides the liquor properties, the daily average flow rates of the liquors through the
sampling points were obtained from the mill for the three days of sampling campaign
(see appendix 4). The flow that was used for dregs and grits was their dumping rate.
58
4.6 Mass Balance Results and Discussions
By using the P concentrations in the samples and flow through the sampling points,
P mass flows in the sampling points were calculated. The result of the mass balance
(average) for the three days of sampling is shown in Figure 30. The figures of the
phosphorus flows for the separate 5 times of sampling are shown in appendix 1. The
concentration of P in all samples (measured by ICP-OES) can be found in appendix
1 as well.
Figure 30 shows the P flows when the mill is on 100% lime mud. If the mill goes on
lime rock, the lime rock is added to the kiln instead of lime mud. Based on the mill
Figure 30.The average phosphorus flows in the recovery cycle (g P/ tonnes of air-dried pulp)
59
data for the 6 hours that they were on 100 % lime rock one night during the sampling
campaign, P flow from the addition of lime rock to lime kiln is 590 g P/MT pulp.
As shown in Figure 30, the main input of phosphorous to the recovery cycle is the
wood used in pulping. Almost all of the phosphorous in the wood is solubilized
during pulping and enters the recovery cycle with black liquor. The phosphorous
remains in the black liquor in evaporation. It appears that perhaps as much as a third
of the phosphorous in black liquor is released during combustion in the recovery
boiler. Unfortunately, ESP ash sample was not collected to experimentally prove this
hypothesis. About two-thirds of the phosphorous in the black liquor exits the
recovery boiler with the smelt and is dissolved to form green liquor. About 80% of
the phosphorous in the raw green liquor exiting the dissolving tank is soluble while
about 20% is insoluble. The insoluble fraction is removed with the dregs and the
remaining phosphorous goes with the green liquor to the slaker. The mill balance
showed no phosphorous pick-up by the lime during slaking and recausticizing. This
is likely because the concentration of phosphorous in the green liquor at this mill
was on the low end for mill liquors. More work is needed in the future to better
understand under what conditions P is removed from the green liquor by the lime.
4.6.1 Phosphorus mass balance around different process steps
Digestion and Evaporation
In the digestion unit less than 4 % of the P in wood goes with the pulp and its
carryovers. The remaining 96% of the P ends up in the weak black liquor. A fraction
of the weak black liquor produced from the digestion unit goes back to the digester
to obtain the desired liquid to wood ratio. In this way, around 43% of the total P of
60
the weak black liquor goes back to the digester and the rest is sent to evaporation.
Figure 31 and Figure 32 show P concentrations in as-fired black liquor and weak
black liquor samples during the sampling period, respectively. The P flows around
the evaporation unit are fairly balanced.
Figure 31.P concentration in ash-fired black liquor samples at each sampling period. Error bars
represent 1 standard deviation of 6 replicate analysis of the samples.
0
0.002
0.004
0.006
0.008
0.01
1 2 3 4 5
P (
wt.
%)
dry s
oli
ds
Sampling Period
61
Figure 32.P concentration in weak black liquor samples at each sampling period. Error bars
represent 1 standard deviation of 6 replicate analysis of the samples.
Causticizing Plant
The fate of phosphorus in the recusticizing plant is of special interest. Figures 33-34
show the variability of P concentration in the 7liquid and 8solid samples of
recausticizing process at each sampling period, respectively. Error bars represent 1
standard deviation of 6 replicate analysis of the samples.
7 WLS and CWL samples were not collected on the third round of sample collection.
8 LMS, LMC1C2, LMC and LMW samples were not collected on the third round of sample collection.
0
0.004
0.008
0.012
0.016
0.02
1 2 3 4 5
P(w
t.%
)dry s
oli
ds
Sampling Period
62
Lime rock sample was only collected once on the first day of sampling period.
0
5
10
15
20
25
30
35
40
RGL CGL WW WLS CWL WLD
P(g
/L)
1
2
3
4
5
0
500
1000
1500
2000
2500
LR LMS LMC1C2 LMC LMW
P (
mg/K
g)
1
2
3
4
5
Figure 33.P concentration variability in liquid samples of the recausticizing
plant at each sampling period
Figure 34. Variability of P content in lime mud samples at each sampling period
63
Based on Figures 33-34, very little phosphorus is picked up by lime mud after the
slaker, between causticizers #2 and # 3, and from the white liquor clarifier
underflow. Therefore, approximately all of the total phosphorus entering the slaker
with clarified green liquor continued with white liquor to pulping. This is in contrast
to other studies which showed concentration of P in the green liquor is three to five
times higher than that of in white liquor [9].This may be due to the fact that the initial
P concentration of the green liquor at this mill is low, being only 20-30 ppm based
on the mass balance results. Based on the literature data, P concentration in the green
liquor is between 20-65 ppm [9]. The other reason would be that the lime muds were
already at equilibrium, so they did not pick up any phosphorus from the liquors of
the causticizing plant.
4.7 Kinetic Experiments
4.7.1 Experimental
Sample of raw green liquor from the dissolving tank was collected using a 1L
thermos. About 200 ml of the hot, unfiltered samples were put into pyrex glass
bottles and held at 95 °C using a water bath with shaker (Memmert GmbH+CoK G).
This temperature is about the same as the temperature in the dissolving tank (~95
°C) and slaker (~102 °C). One unfiltered sample was taken before putting the
samples in the water bath to measure total P concentration at time = 0 min. Samples
were pulled after 15 min, 30 min, 45 min, 60 min, 120 min and 240 min using a
needle attached to a plastic syringe and were filtered using a 0.2 µm polyethersulfone
(PES) membrane sterile filter (FroggaBio). It was about 10 minutes between the time
when the samples were pulled when the sample was transferred to the pyrex bottles.
64
The filtered samples were put directly into digestion tubes and sent to University of
Toronto where they were digested with 70% nitric acid (HNO3) at 95℃ for 2hr and
then analyzed by ICP-OES.
4.7.1 Kinetic Experiments Results and Discussions
Figure 35 shows the change in P concentration as a function of time in RGL from
the dissolving tank. These experiments were carried out to determine if residence
time plays a significant role in the distribution of P between the soluble and insoluble
phases. At time =0, both total and soluble P in the green liquor were measured.
Figure 35.Change of P concentration in the raw green liquor over time. TTA: 120 g/L Na2O and
sulfidity= 24%
Based on the results shown in Figure 35, there was little change in the concentration
of soluble phosphorus in the green liquor, indicating that equilibrium is
approximately reached in the dissolving tank. Also, there is a slight increase in
concentration after 1 hr, which might be due to dissolution of the P from the glass.
65
Chapter 5
5.1 Conclusions
The primary source of phosphorous entering the pulp mill comes from wood, and
almost all of this phosphorous is solubilized during digestion and it ends up in the
black liquor. Mass balance results suggest that after digestion, most of the black
liquor phosphorous is retained in the smelt and the remainder is believed to be in the
ESP ash. Unfortunately, this assumption was not experimentally verified because an
ESP ash sample was not collected. The phosphorous in smelt is nearly completely
soluble in green liquor. It partially reacts with calcium that comes with the lime
during slaking and causticizing to form rhenanite which reduces the lime availability
by 5.4% for every 1 wt% of P. However, the mill balance indicates that the
distribution between soluble phosphorous and rhenanite during slaking and
recausticizing is a function of phosphorus concentration in the green liquor. This
needs to be studied further. Phosphorous that does not react with calcium in slaking
and recausticizing, enters the digester with the white liquor and ultimately ends up
back in the black liquor. Now that the form of phosphorus in the lime cycle is known,
a solubility database for rhenanite can be made.
66
Chapter 6
6.1 Future Work Recommendations
One question that arose from the results of this work is the effect of phosphate and
carbonate concentrations in green liquor on the reaction of phosphate with calcium
during slaking and recausticizing. There might be a competition between carbonate
and phosphate ions to bind with calcium. It is recommended to study how P is picked
up by the calcium as the result of this ion competition. These experiments can be
done as kinetic and equilibrium experiments to determine which reaction is faster
and what the equilibrium distribution is for phosphorous between the solid and
soluble form. . The equilibrium studies can be used to build an equilibrium database
for modeling the reaction of phosphate in green liquor with lime.
The other question that arose from the mass balance results was the fact that there
might be some phosphorus being volatilized during the combustion of black liquor
in the recovery boiler. This phosphorous appears to end up in the ESP ash. However,
the ESP ash was not sampled. Phosphorus release during combustion of black liquor
in the recovery boiler is highly recommended to be studied in the future to
understand how much phosphorous is released during combustion of black liquor.
67
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73
Appendices
Appendix 1:
A: Phosphorus concentration or wt.% measured by ICP-OES
Table 8.P concentration/ (wt.%) in mill samples
Sample Day 1 Day 2-AM Day 2-PM Day 3-AM Day 3-PM
Ash-fired
black liquor
(g/L)
0.064
0.088
0.070
0.077
0.054
Weak black
liquor (g/L)
0.022
0.024
0.024
0.017
0.018
Pulp
carryover(g/L)
0.00099
0.00077
0.00052
0.00030
0.00064
Pulp (wt.%) 0.0011 0.0011 0.0013 0.0012 0.0013
Wood (wt.%) 0.0040 0.0050 0.0050 0.0045 0.0045
White liquor
to digester
(g/L)
0.020
0.021
0.017
0.016
0.019
74
Weak black
liquor to the
digester (g/L)
0.023
0.024
0.024
0.016
0.018
Weak wash
(g/L)
0.0034 0.0023
0.0017
0.0011
0.0018
Raw green
liquor(g/L)
0.021
0.033
0.021
0.027
0.025
Clarified
green liquor
(g/L)
0.02
0.029
0.020
0.019
0.027
Dregs(wt.%) 0.043 0.022 0.092 0.024 0.026
Hot lime to
the slaker
(wt.%)
0.32
0.26
0.17
0.23
0.25
White Liquor
from the
Slaker (wt.%)
0.023
0.035
- 0.016
0.030
75
Lime mud
from the
slaker (wt.%)
0.19
0.19
- 0.14
0.17
White Liquor
between
causticizers #2
and #3
(g/L)
0.021
0.040
0.040 0.022
0.031
Lime Mud
between
causticizers #2
and #3 (wt.%)
0.16
0.17
-
0.17
0.17
Clarified
White Liquor
(g/L)
0.018
0.023
-
0.019
0.020
76
Lime Mud
from Clarifier
Underflow
(wt.%)
0.18
0.20
-
0.17
0.13
Lime mud
after Lime
Mud
washer(wt.%)
0.19
0.18
-
0.18
0.16
White Liquor
after LM
washer (g/L)
0.0014
0.0011
-
0.00036
0.00094
Grits (wt.%) 0.14
0.075
-
0.11
0.11
77
Scrubber
Water (g/L)
90.0095
100.0095
0.0065
0.015
0.0066
Lime Mud
from
Scrubber
Water (wt.%)
0.18
0.18
0.19
0.19
0.18
Lime Rock
(wt.%)
0.17
- - - -
9 Average of the second (pm) and third day (am and pm) samples.
10 Average of the second (pm) and third day (am and pm) samples.
78
B: Phosphorus mass flows in five separate sampling periods
Figure 36.P flows in the recovery cycle during the first sampling period
(g P/tonnes of air-dried pulp)
79
Figure 37.P flows in the recovery cycle during the second sampling period
(g P/tonnes of air- dried pulp)
80
Figure 38.P flows in the recovery cycle during the third sampling period
(g P/tonnes of air-dried pulp)
81
Figure 39.P flows in the recovery cycle during the fourth sampling period
(g P/tonnes of air-dried pulp)
82
Figure 40.P flows in the recovery cycle during the fifth sampling period
(g P / tonnes of air-dried pulp)
83
Appendix 2: Density and Dry Solid Results
A: Density
Table 9.Density of the mill liquors at 95 ℃
Sample Density-Day 1
(g/L)
Density-Day 2
(g/L)
Density-Day 3
(g/L)
Carryover with pulp 977 952 950
Weak Black Liquor 1070 1058 1073
Raw Green liquor before
green liquor clarifier
1121 1151 1104
Clarified green liquor 1127 1125 1108
White liquor after the
slaker
1118 1116 1100
White liquor between
causticizers #2 and #3
1107 1107 1083
White liquor after the
white liquor clarifier
1099 1099 1073
White liquor to digester 1106 1085 1088
Weak wash 995 979 963
Lime mud washer 972 936 901
84
Scrubber water - 940 950
B: Dry Solid
Table 10.Dry solid content of mixed streams
Sample Dry Solid (g/L)
Green liquor dregs 2.5
Lime mud after slaker 90.2
LM (washed) between #2 and #3
causticizers
93
Lime mud from white liquor clarifier
underflow
591
Lime mud from scrubber water 8.8
85
Table 11. Black liquor dry solid (%).
sample Name Dry solid
Ash-fired BL, first day 60%
WBL, first day 15.30%
Ash-fired BL, second day (AM) 60.50%
WBL, second day (AM) 13.34%
Ash-fired BL, second day (PM) 61.01%
WBL, second day (PM) 14.80%
Ash-fired, third day (AM) 61%
WBL, third day (AM) 15.60%
Ash-fired, third day (PM) 63.97%
WBL, third day (PM) 14.61%
86
Appendix3: Liming Ratio
Figure 41.Thermal profile of lime mud from white liquor clarifier underflow
By using the thermal profile of lime mud from white liquor clarifier underflow
(Figure 41), (Rn4) and (Rn5), Ca (OH)2 and CaCO3 content of lime mud were
determined. Then, by using (Eq 10) and liquors properties, liming ratio was
calculated [47].
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
200 400 600 800 1000
0
10
20
30
40
50
60
70
80
90
100
Hea
t F
low
(W
/g)
Temperature(℃)
Wei
gh
t%
weight%
heat flow w/g
87
Appendix4: Flow Through the Sampling Points
Table 12.Flow through the sampling points
Location (Flow) Value – 1st day Value – 2nd day Value – 3rd day
Wood to Digesters
(g/min)
1,768,000
1,695,666 1,768,683
WL to Digesters (L/min)
1,628
1,610 1,678
BL to Digesters
(L/min)
1,282 1,275 1,328
Pulp Production
(g/min)
350,666 313,500
342,833
Weak BL to Evaporators
(L/min)
3,124
2,810 2,919
As-fired BL to RB
(L/min)
662 619
662
88
11WW to dissolving tank
(L/min)
and
RGL (L/min)
1,243 1,144 1,279
12CGL to slaker (L/min)
and
WL from slaker (L/min)
and
WL between
causticizer#2 #3 (L/min)
1,316 1,325 1,338
CWL to lime Mud
washer (L/min)
175 206 203
13Lime rock to
kiln(g/min)
117,187 -
-
WL from lime mud
washer underflow
(L/min)
341 318 315
Lime mud from the
washer to lime kiln
(g/min)
119,792 120.933 120,933
Scrubber water
(L/min)
364 288 380
11 In order to maintain a certain level in the dissolving tank, RGL flow and WW flow should be the same.
12 Tanks are in series and their levels is constant therefore input and output flows are assumed to be the same.
13 Mill was on lime rock for 6 hours during the first night of sampling campaign.
89
14Hot lime to slaker
(g/min)
87,546 90,224 91,359
Slaker grits (L/min)
20% solid
3.15 3.15 3.15
Lime kiln production
(g/min)
72,917 73,612 73,612
The mass flow of lime mud from the lime mud washer to the lime kiln was
calculated based on the kiln hot lime production and (Rn 3):
CaCO3 (𝑠) + Heat → CaO(𝑠) + CO2 (𝑔) (Rn3)
Lime availability was measured using TAPPI standard lime analysis method [46]
and it was 92%
14 Calculated based on the liming ratio and Na2CO3 (mol/L) in clarified green liquors
90
Appendix 5: Rhenanite Crystal Structure Change
(a)
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
86
88
90
92
94
96
98
100
0 200 400 600 800 1000
Hea
t F
low
(W
/g)
Weig
ht
%
Temperature (℃)
Wt.% Heat Flow (W/g)
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
88
88.3
88.6
88.9
0 200 400 600 800 1000
Hea
t F
low
(W
/g)
Weig
ht%
Temperature(℃)
Wt.% Heat Flow (W/g)
(b)
Figure 42.Thermal profile of lime after the sixth cycle, a: heating up, b: cooling down
91
(a)
(b)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
82
84
86
88
90
92
94
96
98
100
0 200 400 600 800 1000
Heat
Flo
w (
W/g
)
Weig
ht%
Temperrature(℃ )
Wt.%
Heat Flow (W/g)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
84.1
84.6
85.1
85.6
0 200 400 600 800 1000
Hea
t F
low
(W/g
)
Weig
ht
%
Temperature (℃)
Wt.%
Heat Flow (W/g)
Figure 43.Thermal profile of lime mud after the sixth cycle, a: heating up, b: cooling down
![NORTHERN BLEACHED HARDWOOD KRAFT PULP FROM · PDF fileUNITED STATES TARIFF COMMISSION Washington [AA1921-105] September ·23, 1974 NORTHERN BLEACHED HARDWOOD KRAFT PULP FROM CANADA](https://static.documents.pub/doc/80x56/5a6ffe347f8b9ab1538b8911/northern-bleached-hardwood-kraft-pulp-from-nbsppdf-fileunited-states.jpg)
