Solubility of Sodium Oxalate in Kraft Black Liquors
SOLUBILITY OF SODIUM OXALATE IN KRAFT BLACK LIQUORS Ahmed Khafhafera1 and Nikolai DeMartini2 1 PhD student at the Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada. Email: [email protected] 2 Assistant Professor at the Department of Chemical Engineering and Applied Chemistry and NSERC Industrial Research Chair in the Role of Inorganics in the Industrial Processing of Woody Biomass, University of Toronto, Toronto, ON, Canada. Email: [email protected];
KEYWORDS Kraft pulping, Black liquors, Sodium Oxalate, Scaling, Solubility, Equilibrium, Evaporation.
ABSTRACT Oxalate is both present in wood and formed during pulping. The amount of oxalate present in wood largely depends on the ground the wood is grown on as oxalate is used by plants and trees to bind excess cations such as calcium. The source of oxalate formation in pulping is unknown, but it has been found to depend on wood species and pulping conditions. Sodium oxalate scaling is most prevalent in mills pulping tropical hardwoods, but it is also found in the scales in high solids lines after the concentrators in mills pulping softwoods. There is very limited solubility data for sodium oxalate in black liquor. This study measures the solubility limit in different black liquors through a temperature range below 100 oC of black liquor evaporators at different black liquor dry solids and discusses the implications for kraft pulp mills.
APPLICATION The newly generated solubility data in black liquors could help kraft pulp mills minimize the risk of scale formation. It will also be used in developing a thermodynamic model to provide a quantitative assessment of scale formation under the conditions prevailing in industry.
INTRODUCTION In kraft pulp mills, sodium hydroxide (NaOH) and sodium sulfide (Na2S) are used to pulp the wood. After pulping, the dissolved organics and spent pulping chemicals are washed from the pulp where black liquor is produced. The black liquor is concentrated in the evaporation plant to increase the dry solids content to 65-85% prior to firing in a recovery boiler. The inorganic chemicals are recovered from the recovery boiler, undergo recausticizing and are reused for pulping [1]. During black liquor evaporation, the solubility limit of the inorganic slats may be exceeded which leads to scale formation that fouls the heat transfer surface [2]. The black liquor composition and properties vary from one mill to another depending on the wood species, pulping process and conditions. The following table shows a typical composition of black liquor obtained from North American mills.
Table 1: Typical Composition of Black Liquor [2].
Range Range Range Analyte Analyte Analyte (wt% BLS) (wt% BLS) (mg/kgBLS ) Na2CO3 4.77-14.5 Na 14.0-20.3 C2O4 2000-13400 Na2SO4 1.94-16.1 K 0.82-5.05 Ca 118-1050 Na2S 0.06-2.97 C* 0.54-1.64 Si 367-2080
S2O3 2.40-6.49 C** 30.2-39.7 - - wt% BLS: Weight Percentage on Black Liquor Solids Basis C*: Inorganic Carbon C**: Organic Carbon
Oxalate exists in wood chips primarily as CaC2O4 and K2C2O4 [3]. It is also formed during pulping [4], oxygen delignification [5], and bleaching [6]. The role of oxalate in plants is to balance the charge of excess cations through a formation of crystals; most commonly, calcium oxalate [7]. It has been reported that the content of oxalic acid in North American wood ranges between 0.1 and 0.4 kg/metric ton, depending on the wood species (i.e. softwood, hardwood, and other species). While different, the content in bark can be as high as 15 kg/metric ton [8, 4]. In addition to the oxalate release during pulping, it has been also identified that oxalate is formed instantaneously during pulping, but the mechanisms by which it is formed are still unknown [4]. The concentration of oxalate in black liquor is in the range of 0.20-1.34% dry solids [2], where the lower concentration limit is typical for softwood and the higher limit is typical for tropical hardwood. Of the many organic compounds present in black liquor, sodium oxalate can precipitate when solubility limit is exceeded and thus contributes to scale formation [9]. Since sodium oxalate exhibits normal solubility, it favours precipitation on cooler surfaces. Such precipitation of sodium oxalate poses numerous problems in the evaporation plant and high solids lines. In the survey of evaporator fouling problem conducted in 1998, only one mill has reported sodium oxalate scaling [10]. A few other cases of sodium oxalate scaling have been reported in the literature. Since sodium oxalate grabbed more attention, a number of mills are suspecting that it could be incorporated with Na2CO3 and Na2SO4 scales. Since the salts of Na-CO3-SO4 system exhibit inverse solubility, sodium oxalate is anticipated to behave as an agglomeration agent for the pre-existed crystals present in the bulk solution, resulting in a formation of layered scales [11]. That said, more work is needed to prove that it is true. To date, there is only one study in the literature about sodium oxalate solubility in black liquor [12]. The experiments were carried out using two softwood black liquors. The experimental matrix included only two solids contents, 18 and 36% dry solids, and a temperature range of 90-150 oC. However, the equilibrium was approached by dissolution and sodium chloride (NaCl) or sodium acetate (C2H3NaO2) were added to the liquor to adjust the total concentration of sodium [12]. The addition of salts to adjust the total sodium can be a limitation since it does not prevail the conditions in the pulp mills. Therefore, the experimental findings do not necessarily represent the sodium oxalate solubility during black liquor evaporation. The solubility of sodium oxalate can be explained by the following chemical reaction:
Na C O s ↔ 2Na C O Eq. 1
The thermodynamic equilibrium constant of sodium oxalate reaction is defined as:
K . Eq. 2 K Na . C O Eq. 3
Ulmgren and Radestrom defined the apparent solubility product [12]:
Eq. 4 L Na C O ,
From a thermodynamic perspective, the true equilibrium constant of any reaction is a function of the activity rather than total concentration, see Eq. 2. The activity by definition is the product of the activity coefficient and concentration, see Eq. 3. Since it is more practical to experimentally determine the total concentration of ions, the solubility product is only expressed as a function of concentration. This imposes implications when dealing with concentrated solution since the activity coefficient of an ion departs further from unity.
log L 0.53 0.024 Eq. 5 log L 1.78 0.044 856/ Eq. 6
According to the experimental findings of Ulmgren and Radestrom, two correlations were developed to describe the apparent solubility product as a function of temperature and sodium ion concentration. However, Eq. 5 is for a temperature equal to 90 oC and Eq. 6 is for a temperature range of 110-150 oC. The findings indicate that at 90 oC the apparent solubility product decreases as the sodium concentration increases. While at 110-150 oC, the apparent solubility increases as the sodium concentration increases. No explanation was provided as to why sodium oxalate exhibits different solubility behaviour at 90 oC compared to at 125 and 150 oC. On the other hand, the OLI simulation in Table 2 shows that the equilibrium concentration of sodium oxalate varies significantly, depending on the salt added. This strongly indicates the sodium activity is different for each solution despite fixing the total sodium concentration, temperature, and pH. This observation is anticipated to be true also for black liquor where the sodium oxalate solubility will depend on the liquor chemistry, process conditions, etc. This study offers solubility data for sodium oxalate in various black liquors at a range of dry solids and temperatures used in the industry. Generating such data will also be helpful in developing a predictive model in OLI for sodium oxalate formation during evaporation.
Table 2: Modelled Sodium Oxalate Solubility in Water at 90 oC and 1 atm Using OLI.
Salt Added Concentration Sodium** pH Soluble Oxalate (g/kg H2O) (g/kg H2O) (g/kg H2O) Sodium Chloride 99.4 46 11.7 9.8 Sodium Formate 115.6 46 11.7 15.3 Sodium Acetate 139.5 46 11.7 16.3 Sodium Sulphate 120.7 46 11.7 22.4 Sodium Carbonate 90.1 46 11.6 22.5 ** The total sodium includes 6.9 g Na/kg H2O coming from sodium hydroxide addition to adjust the pH.
EXPERIMENTAL PROCEDURE Softwood and hardwood black liquors were obtained from Canadian kraft mills pulping. As shown in Table 3, the liquor samples were named based on the mill, wood type, and dry solids. The black liquor samples were used to determine the sodium oxalate solubility at a range of temperatures and dry solids. The dry solids of all samples were first measured (TAPPI Standard Test, T650 om-05) to either dilute or concentrate the liquors to adjust the sodium concentration. The dilution is done by adding deionized water to the liquor sample and the concentration is done by an open evaporation under the fume hood. Once the dry solids are set to the target, the liquor sample is transferred to
250 mL Pyrex bottles for the experiment. The experimental matrix includes dry solids of 25-35 wt% and a temperature of 50 oC.
The solubility experiments were performed in the Pyrex bottles immersed in a water bath shaker with a digital temperature control (Memmert, ONE22). However, equilibrium was approached by the dissolution of solid sodium oxalate (Na2C2O4(s)) which was added in excess to the black liquors. The experiment was let to equilibrate at the temperature of interest. Once the equilibrium was reached, liquid samples were withdrawn from the Pyrex bottles using a syringe and dip tube, immediately filtered using 0.22 µm nylon filter, and diluted for analysis. The sampling procedure takes less than 1 min to avoid temperature drop. The solution samples are collected and further diluted for oxalate analysis using ion chromatography (Therom Scientific, DIONEX ICS-2100) and for total soluble sodium analysis using inductively coupled plasma optical emission spectrometry (Agilent Technologies, 700 Series ICP- OES).
Table 3: Nomenclatures of Black Liquor Samples.
Nomenclature Wood Type Sodium (wt% d.s.) Mill SW1 Softwood 19.2 A HW1 Hardwood 18.5 B
PRELIMINARY RESULTS Figure 1 shows the equilibration time of sodium oxalate where the equilibrium concentration is plotted against the experiment time. It should be noted that the oxalate concentration levels off after 1 day of equilibration for both liquors; 3.6 g/kg H2O for softwood and 4.8 g/kg H2O for hardwood. This indicates the time to reach equilibrium is rather short. Based on this observation, the equilibration time for any further experiments is considered to be 1 day. Regarding the analysis, the values of oxalate concentration are an average of two runs. 6
5 O) 2 H
(g/kg 4
Oxalate 3 25.4% SW1 25.1% HW1 2 012345 Time (day)
Figure 1: Equilibration Time of Sodium Oxalate at 50 oC.
Figure 2 shows the oxalate solubility as a function of liquor dry solids. The solubility of sodium oxalate in both liquors decreases as the liquor dry solids increase. This behaviour is expected since the sodium concentration increases as the liquors are concentrated. The oxalate solubility as a function of soluble sodium is plotted, see Figure 3. Although the soluble sodium concentration is about the same in both liquors, oxalate is found to be more soluble in hardwood black liquor than that of softwood. This observation indicates that the oxalate solubility in black liquor can be significantly altered due to the sodium activity which agrees with the modeling results presented in Table 2. One possible explanation that the sodium activity could be different in each liquor is the chemistry complicity. In addition,
it is known that hardwood has less organics compared to softwood liquors. More work with different liquors and dry solids is needed to establish a clear trend. 6 SW1 HW1 5 O) 2 H
(g/kg 4
Oxalate 3
2 15 20 25 30 35 40 45 Dry Solids (%)
Figure 2: The Solubility of Sodium Oxalate vs. Liquor Dry Solids at 50 oC.
6 SW1 HW1 5 O) 2 H
(g/kg 4
Oxalate 3
2 40 60 80 100 120
Soluble Sodium (g/kg H2O)
Figure 3: The Solubility of Sodium Oxalate vs. Soluble Sodium 50 oC.
CONCLUSION Equilibrium of sodium oxalate by dissolution was reached within a day. Preliminary results with one hardwood and one softwood black liquor show that the solubility of sodium oxalate can vary significantly between liquors as similar sodium and dry solids concentrations. The explanation for this difference is likely due to the difference in the activity of the sodium and oxalate ions due to the different compositions. More work is needed to establish a full solubility study of sodium oxalate in black liquor as a function of temperature/solids for various mill liquors to better understand how variable solubility is between different black liquors, followed by solubility modeling.
ACKNOWLEDGEMENT This work is part of the industrial research consortium project Effective Energy and Chemical Recovery in Pulp and Paper Mills - II at the University of Toronto supported by Andritz, Arauco, Babcock & Wilcox, Canadian Kraft Paper, Cenibra, Clyde Ergemann, CMPC, ERCO Worldwide, FITNIR, FPinnovations, Georgia Pacific, International Paper, IRVING, KFS, Klabin, Mercer, NORAM, Rayonier, Sappi, Södra, Stora Enso, Suzano, Valmet, WestRock. We would also like to acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), [IRCPJ 517715 -16]. Cette recherche a été financée par le Conseil de recherches en sciences naturelles et en génie du Canada (CRSNG), [IRCPJ 517715 -16].
REFERENCES
[1] T. M. Grace and H. Tran, "The Effect of Dead Load Chemicals in the Kraft Pulping and Recovery System," Tappi Journal , pp. 18-24, 2009.
[2] E. Karlsson, "The Formation and Dissolution of Sodium Salt Scales in Black Liquor Evaporators," Doctroal Thesis, Chalmers University of Technology , Gothenburg , 2017.
[3] V. R. Franceschi and F. A. Loewus, "Oxalates in Plants and Fungi: Forms and Distribution," in Calcium Oxalate in Biological Systems; Khan, S.R., Ed, CRC Press: Boca Raton, FL, 1995, pp. Ch. 6, 113-130.
[4] H. Li, X.-S. Chai and N. DeMartini, "Oxalate Release and Formation during Alkaline Pulping," Journal of Wood Chemistry and Technology , vol. 32, pp. 187-197, 2012.
[5] Y. Liu, S. Ge, Y. Li, B. Li and H. Li, "Oxalate Formation during Hydrogen Peroxide-Reinforced Oxygen Delignification," Journal of Industrial and Engineering Chemistry, 2017.
[6] J. A. Krasowski and J. Marton, "The Formation of Oxalic Acid During Bleaching of Kraft Pulp," Journal of Wood Chemistry and Technology , vol. 3(4), pp. 445-458, 1983.
[7] B. Serdar and H. Demiray, "Calcium Oxalate Crystal Types in Three Oak Species (Quercus L.) in Turkey," Turkish Journal of Biology, pp. 386-393, 2012.
[8] M. Haara, "Oxalic Acid and Calcium Oxalate in Production of Wood-Containing Paper," Abo Akademi University , Abo, Finland , 2014.
[9] W. Fu and J. Vaughan, "Morphological Investigation of Sodium Oxalate Crystals Grown in Aqueous Sodium Hydroxide Solution," Light Metals , pp. 191-194, 2013.
[10] W. Schmidl and J. Frederick, "Current Trends in Evaporator Fouling," International Chemical Recovery Conference , pp. 367-377, 1998.
[11] N. DeMartini and C. Verrill, "Evaporator Fouling Mitigation-Case Studies," Tappi Journal , pp. 1-10, 2005.
[12] P. Ulmgren and R. Radestrom, "Deposition of Sodium Oxalate in the Black Liquor Evaporation Plant," Swedish Pulp and Paper Research Institute , vol. 17 no.3, pp. 275-279, 2002.
Gateway to the Future
Solubility of Sodium Oxalate in Kraft Black Liquors
Speaker: Ahmed Khafhafera Supervisor: Nikolai DeMartini
University of Toronto Department of Chemical Engineering and Applied Chemistry Background
2 Oxalate in Black Liquor
• Oxalate concentration in black liquor is 0.2-1.3 wt% d.s.
• Oxalate exists in wood primarily as oxalic acid, CaC2O4 and K2C2O4.
• Oxalate forms during pulping, oxygen delignification, and bleaching.
• It can be concentrated if the wash water of high solids concentrator is rapidly recycled.
3 Rationale
• Sodium oxalate (Na2C2O4) scale has been reported in several Kraft pulp mills.
•Na2C2O4 scale is found in some HW mills below 50% BL d.s.
•Na2C2O4 is found in ring-header scaling in some SW mills.
• Lack of solubility data of sodium oxalate in BL.
4 Sodium Oxalate Scales
Second Effect High Solids Storage Tank HW black liquor SW black liquor Dry solids: 45% Dry solids: 70-80%
Na2C2O4 ~ 50% d.s. Na2C2O4 ~ 10% d.s.
DeMartini & Verrill (2005) 5 Literature
6 Sodium Oxalate Solubility
• Only one study published (Ulmgren & Radestrom, 2002)
• One softwood liquor • Two solid contents ( 18 and 36% d.s.) • Three temperatures (90, 125, & 150 C)
• Sodium concentration (2 – 6 mol/LBL)
• Sodium concentration adjusted by the addition of sodium chloride and sodium acetate • Key limitation
7 Sodium Oxalate Solubility
0
‐0.1 150 oC ‐0.2 L = [Na]2 [C O ] 125 oC s 2 4 ‐0.3 Log L 1.78 0.044 Na 856/T ) s [Na] & [C2O4]are in mol/L (L ‐0.4 Log ‐0.5 Log L 0.53 0.024 Na 90 oC ‐0.6
‐0.7
‐0.8 123456 Ulmgren & Radestrom Sodium (mol/L) 8 (2002) This Work
9 Black Liquors
1.5 20
1.25 16 Mill Wood Type Location 1 d.s.) d.s.)
A Softwood North America 12 B Hardwood North America (wt% (wt% 0.75
8 C Softwood South America 0.5 Oxalate Sodium 4 0.25
0 0 Mill AMill BMill C Mill A Mill B Mill C 10 Experiment Matrix
Temperature (oC) Liquor 50 60 70 80 90 Mill A X X X X X Mill B X X X Mill C X X X
Liquor Dry Solids (%) (23, 28, 33, 39, 43, and 47)
11 Experimental
Dry solids: 23‐47% Concentration Temp.: 50‐90 oC Black Liquor as received
Na2C2O4 added
Dilution
Sampling
Dilution IC Analysis 12 Equilibration Time – Mill A
10 o 9 60 C 8 O) 2 H 7 Solids 6 (g/kg
5 23% 4 Oxalate 28%
3 33% 39%
Soluble 2 43% 1 47% 0 0 5 10 15 20 25 30 Sampling Time (hrs) 13 Solubility Behaviour
10
9 Solids 8 23% O) 2 H 7 6 28% (g/kg
5 33%
Oxalate 4 39% 3 43%
Soluble 2 47% 1
0 40 50 60 70 80 90 100 Temperature (oC) 14 Solubility – Mill A
10 9
8 o O) 90 C 2 H 7 6 (g/kg 70 oC 5 50 oC
Oxalate 4
3
Soluble 2
1
0 15 20 25 30 35 40 45 50 Liquor Dry Solids (%) 15 Solubility – Mill A-B-C at 70 oC
10 9 8 O) 2 H 7 6 (g/kg
5
Oxalate 4
3
Soluble 2 1
0 15 20 25 30 35 40 45 50 Liquor Dry Solids (%) 16 Solubility – Mill A-B-C at 90 oC
10 9 8 O) 2 H 7 6 (g/kg
5
Oxalate 4
3
Soluble 2
1
0 15 20 25 30 35 40 45 50 Liquor Dry Solids (%) 17 Solubility – Mill A-B-C at 90 oC
5
4.8 3 O) 2 H
4.6 (g/kg
4.4 (Ls)
Log 4.2
4 0 25 50 75 100 125 150 175 200 Sodium (g/kg H2O) 18 Conclusion & Future Work
• Solubility of oxalate in three mill black liquors showed similar behaviour as a function of solids and temperature • A positive correlation was found between apparent solubility of oxalate and the sodium concentration • Future work will include measuring the solubility at higher temperatures
19 Thank you Questions?
20