CALCIUM AND OXALATE IN TEMBEC MAPLE BCTMP LINE

Authors

J. X. Zhang1,3, E. Yu1, Y. Ni1, Y. Zhou2 and D. Joliette2

Organisations and addresses

1Limerick Pulp and Paper Research and Education Centre, University of New Brunswick, Fredericton, NB, E36 6L4  Canada
2High Yield Pulping Division, Tembec, Temiscaming, Quebec, Canada
3Now at: Forestry and Forest Product Research Centre, CSIR, P.O. Box 17001, Congella, Durban, South Africa 4013

email

jxzhang@csir.co.za

Keywords

BCTMP, mechanical pulp, fiber, refining, bleaching, hydrogen peroxide, oxalate, calcium, scale, deposit, CaC2O4 crystal, extraction, chelation, precursor

ABSTRACT

The Tembec maple BCTMP line is an integrated process of refining, washing, and two-stage peroxide bleaching. The analysis of the scale samples located at the Tembec disc-filter shower and reject press showed that circulating process water in the closed water systems caused calcium oxalate deposits. SEM analysis revealed that the deposit at the disc-filter shower imaged polygon facade, which is a characteristic of calcium oxalate monohydrate (COM), while that at reject press was a mixture of COM and COD (calcium oxalate dihydrate).

There are three sources of oxalate in the Tembec BCTMP line, i.e. wood, refining and bleaching. The results of this work show that wood and refining contributed only a small amount of oxalate (about 0.4 and 0.2 kg/T pulp, respectively). However, a significant amount of oxalate, about 2 kg/T pulp, was formed during peroxide bleaching. For unbleached maple pulp, extractions of hot water and benzene alcohol were able to remove most of the oxalate. The oxalate was presumably precipitated and physically adsorbed in the fibres. After the alkaline-hydrogen peroxide bleaching under the same conditions as used in Tembec, the amount of newly formed oxalate from the sample without extraction was found to be about the same as that extracted by hot water and benzene alcohol respectively. This suggests that oxalate precursors (OP) were present in the pulp, and formed oxalate when attacked by hydrogen peroxide during the bleaching. On the other hand, calcium was found to firmly attach onto the fibres because less than 30% of the calcium ions were able to be removed by either benzene-alcohol or hot-water, while about 90% of the calcium ions were able to be removed by super chelation. During hydrogen peroxide bleaching, the oxalate formation occurs due to the attack of OOH- on the end groups of lignin and hemicelluloses. Accordingly, structures and equilibrium were proposed as following:

equation 1

where M is one of metal ions ; Ca, Mg, Mn and Fe, with Ca being the most abundant. OP that is characterized by the position of either -OH or unsaturated groups was likely a functional group of either -CH(OH)-COO-, -CO-COO-, =CH-COO-, or –Ph-O-. The data of the oxalate content after the peroxide bleaching agreed with this hypothesis.

INTRODUCTION

Tembec's Temcell High Yield Pulp mill in Temiscaming, Quebec produces a variety of specialty high yield pulps (i.e. BCTMP) from maple, birch and aspen. A significant amount of its production is bleached maple grade. The maple BCTMP line is an integrated process of refining, washing, and two-stage alkaline-peroxide bleaching, which is simplified and illustrated in Figure 1. The fresh water is pumped into the system for carrying bleaching chemicals into the bleaching stages and for washing the bleached pulp at the last press. A certain amount of the white water is purged from the clear white water tank. Being environmentally friendly, the Tembec BCTMP maple line is operated with three highly closed water loops, as shown in Fig. 1, i.e. refining-washing, latency-washing, and washing -bleaching. The circulation of water in the system causes accumulation of organic and inorganic chemicals, such as oxalate, extractives, lignin, hemicelluloses, cellulose, calcium, magnesium, manganese, iron and sodium. The four major components deposited in pulp mill production lines are calcium oxalate, calcium carbonate, barium sulfate and organic deposit -pitch (Ester (1) and Houdlette (2)).

Figure 1

Figure 1. Simplified Flow Diagram of the Tembec Maple BCTMP Line (Solid lines are wood and pulp streams and dashed lines are fresh water, pressate and white water (WW) streams)

Calcium oxalate deposit is found to be the most common in pulp mills because it is stable and very difficult to remove. Very strong acid and high pressure is required to dissolve the calcium oxalate deposit (3). Millan (4) described that calcium oxalate presents three degrees of hydrates: monoclinic monohydrate (whewelite, CaC2O4·H2O, hereafter referred as COM), tetragonal dihydrate (weddelite, CaC2O4·2H2O, hereafter referred as COD) and triclinic trihydrate (CaC2O4·3H2O, hereafter referred as COT). Since the solubility product of COT (7.88x10-9) is higher than COM (2.12x10-9) (5), according to Ostwald Lussac Rule (6), calcium oxalate precipitate initially forms COT, then readily undergoes transformation to the thermodynamically more stable COM. The results obtained by Ulmgrent et al (3) showed that the formation of COM dominated the deposit process in a chlorine dioxide bleaching filtrate, while COD was also present in the deposit because of the inhibiting effects of dissolved organic compounds on the COM crystal growth. The process of forming calcium oxalate scale includes precipitation and then crystallization. The precipitation dependents on the concentrations of free calcium and oxalate ions in the solution, which is quantitatively described by solubility (or activity) product at equilibrium. The sources that bring oxalate and calcium into the system and the source that forms oxalate inside the system determine the concentrations of free ions of calcium and oxalate. In the study of calcium oxalate in a mechanical pulp of baltic aspen, Terelius et al (7) found that there was a higher oxalate content in the bleached aspen sample compared to the unbleached sample. Elsander et al (8 ) found that a sizable amount of oxalate was produced in peroxide bleaching of a chemical pulp. In the Tembec system, the deposition is observed everywhere and causes detrimental effects on mill production and utility and chemical uses. The strategic intent for this work was to minimize deposition problems. For this reason laboratory work was initially carried out with the objectives of studying the distribution of calcium and oxalate in the Tembec BCTMP line and to study the origin of oxalate in the process.

RESULTS AND DISCUSSION

1. Chemical composition in scale samples

Two scale samples, one at the disc filter shower in the pulp washing unit and the other at the reject press in the refining unit, were analyzed for chemical compositions. The chemical compositions are shown in Table 1. The results show that calcium and oxalate combined, accounted for over 80% (mass) of the scale samples. The other components, such as silicate, sulfate, carbonate, and organic content in those samples contributed (in Table 1) less than 10%. Calcium oxalate therefore dominates the scaling in the Tembec production line. The mole ratios of Ca+2 : C2O4-2 : H2O (Table 1) were about 1 : 0.9 : 0.7 and 1 : 0.9 : 1.2 for the samples respectively at disc filter shower and reject press, where the water in the deposit at disc filter shower is less than that at reject press. This implies that these two deposits have different crystal structures. SEM images are shown in Figure 2, where the scale particles at the disc filter shower are polygonal, which, according to Ulmgrent et al (3), are characteristic of COM. The particles at the reject press (Fig. 2) look bulky and porous, and are likely a mixture of COM and COD.

Table 1. Chemical Composition of Deposits in Tembec BCTMP Line (% as o.d. sample)

Table 1

Figure 2

Figure 2. SEM images of the deposits at disc filter shower and reject press (Courtesy of J. Wesley-Smith at FFP-CSIR, South Africa)

2. Calcium and oxalate from maple wood and refining process

Table 2 shows that the calcium content in the Tembec maple pulp was 1.2 g/kg on pulp, which is in the range of that reported by Krasowski et al (9). Since the bark content in the maple chips is limited to about 0.5%, the calcium brought into the system by the bark is very small (less than 0.03g/kg on wood). In Table 2, the solids in the cloudy-white water contained very high levels of precipitated calcium, while soluble calcium in the water was about 40 ppm, indicating that a large amount of calcium ions were absorbed/adsorbed in fines and colloids that are suspended in the cloudy white water.

Table 2. Calcium Contents in Tembec and Other Maples

Tembec Maple

Other Maple (9)

Pulp
(g/kg on pulp)

Cloudy White Water (ppm on liquor)

Bark
(g/kg on wood)

Wood
(g/kg on wood)

 In Solid

In Liquor

1.2

104

40

3 - 6

1 – 1.6

Table 3 shows that the oxalate content in the maple chips was 0.4 g/kg on pulp. Using the data obtained by Krasowski et al (9), the maximum oxalate brought into the system by both chips and bark is about 0.55 g/kg wood (0.4 g/kg wood + 300.5% g/kg wood). This value is close to those measured in the refined pulp and in the pulp heated at 130oC for 60 minutes (Table 3), indicating that the refining has little effect on the formation of oxalate. Similar conclusions on the refining of aspen and spruce have been made by Terelius et al (7).

Table 3. Oxalate Contents in Tembec and Other Maples

Tembec Maple

Other Maple (9)

Wood
(g/kg wood)

Refined Pulp before latency (g/kg pulp)

Bark
(g/kg wood)

Wood
(g/kg wood)

Control

130oC for 60 min at pH 5

0.4

0.5

0.7

18 – 30

0.2 - 0.6

3. Calcium and oxalate in the Tembec maple BCTMP line

text

Table 4. Total and Free Calcium, Magnesium and Oxalate concentrations

Total

Unit Name

Latency

Cloudy

Clear

P1

P2

Total Solid (% on pulp)

5.3%

2.0%

1.5%

2.3%

2.4%

C2O42- (ppm on liquor)

164

240

219

339

399

Ca2+ (ppm on liquor)

116

202

166

148

104

Mg2+ (ppm on liquor)

60

62

60

84

64

Free

Unit Name

Latency

Cloudy

Clear

P1

P2

C2O42- (ppm on liquor)

6

12

8

66

134

Ca2+ (ppm on liquor)

48

38

38

41

29

Mg2+ (ppm on liquor)

48

48

47

63

69

Bleaching conditions:

P1: 2%H2O2, 2%NaOH, 1.2%NaSiO3, 0.08%MgSO4, 12%consistency, 80oC, 2.5 hrs
P2: 4.2%H2O2, 2.2%NaOH, 1.4%NaSiO3, 0.11%MgSO4, 12%consistency, 80oC, 2.5 hrs

text

Table 4 shows that the concentrations of both total and free oxalates in the bleaching pressates (P1 and P2) were higher than the other streams. Similar observations were also reported by previous researchers (3, 8-10) for hydrogen peroxide bleaching of chemical pulps, and by Terelius (7) for mechanical pulp bleaching.  In the bleaching of a chemical pulp, where most of the extractives and hemicelluloses have been removed during pulping, lignin contributes most to oxalate formation due to the oxidative reactions with bleaching chemicals (13, 14). However, in mechanical pulp bleaching, the extractives and hemicelluloses may play a role in the formation of oxalate. For example, Buchert et al (15) reported that the glucouronic group in xylane contributed to some oxalate formation when the pulp was oxidized by ClO2 or O3. Extractives, chemically similar to lignin, contain unsaturated bonds and may form oxalate when attacked by an oxidant during bleaching.

4Effects of pretreatments on total oxalate and metal ions contents

To remove extractives and metal ions (Ca, Mg, Fe, and Mn) from the pulp, benzene-alcohol extraction, hot-water extraction, and AQAQAQ super chelation were employed. The results in Table 5 show that most of the oxalate ions were removed by these three treatments. This indicates that the oxalate, presumably in the form MC2O4 (where M is Ca, Mg, Fe, and/or Mn), were physically adsorbed inside the fibre wall. The calcium, as well as iron, in the fibres seemed to have chemical bonds with the fibres, because benzene alcohol and hot water only removed about 10 – 30% of calcium ions (Table 5), while the super chelation that was operated under severe conditions removed up to about 90%. It is interesting to note that manganese ions, dramatically affecting brightness gain, were easily removed from the fibres by the above three treatments.

Table 5. Effect of Pretreatment on Oxalate (g/kg on pulp) and Metal Ions (ppm)

Sample

Control

Benzene Alcohol

Hot Water

Super Chelation

Total Oxalate

1.6

0.1

0.1

0.1

Ca

1007

934

700

98

Mg

507

260

35

22

Mn

70

17

13

5

Fe

24

/

10

15

5. Mechanism of oxalate formation during hydrogen peroxide bleaching

Table 6 shows the effect of caustic charge on the oxalate formation during hydrogen peroxide bleaching. A remarkable amount of oxalate, about 2g/kg pulp (in Table 6) was formed in the laboratory bleaching trials, which is in agreement with the oxalate concentrations in the mill bleaching filtrates in Table 4. As shown in Table 6, an increase in caustic charge to its optimum level increased the number of perhydroxyl anions (OOH-), which resulted in greater pulp brightness gain. Extracted by benzene alcohol, the bleached sample that contained lower levels of extractives, manganese and iron, was found to have a higher final brightness with a higher caustic charge. However, for a given caustic charge, the oxalate in the sample extracted by benzene-alcohol was found to be about the same as that in the controlled sample. This suggests that extractives do not produce significant amounts of oxalate during peroxide bleaching. The majority of oxalate formed during the peroxide bleaching must occur in the reactions between OOH- and the pulp components (i.e. lignin and hemicelluloses). However, the mechanism for the oxalate formation is different to that of brightening during the peroxide bleaching, This is demonstrated by the fact that a plot (Figure 3) of the newly formed oxalate versus brightness gain, shows that the controlled sample does not overlap with the benzene alcohol extracted sample. Even so, at a certain level of brightness gain, the formation of oxalate was substantially increased. Since the formation of oxalate happens due to the peroxide oxidation of reactive end groups of lignin and hemicelluloses in hydrogen peroxide bleaching, the pulp most likely contains oxalate precursors (OP). Consequently, structures of Fiber-OP-M+1 and Fiber-OP-H were present in the pulp and equilibrated between each other through the ion exchange of H+ and M2+:

equation 2

text3

equation 3

Peroxide bleaching is a heterogeneous process, where OOH- attacked Fibre-OP-H and Fiber-OP-M+ to form free and precipitated oxalates respectively. These newly formed oxalates are dissolved into the liquid phase. On the fibre side, a carboxylic acid is formed and then is neutralized by caustic during the peroxide bleaching. The following mechanisms (3) and (4) were proposed:

equation 4

The above equations (3) and (4) are supported by the results shown in Table 6. The content of the precipitated oxalate in the filtrate is only slightly affected by caustic charge, while the free oxalate content in the filtrate increases with an increase in caustic charge.

Table 6. Effect of Caustic Charge on Oxalate Contents (g/kg on pulp) During Peroxide Bleaching

Sample

Controlled

Benzene Alcohol

Sample

Unbleached

Bleached

Caustic Charge

/

4.2

2.8

1.4

4.2

2.8

1.4

Brightness (% ISO)

57

79

78

75

82

80

77

Newly Formed Oxalate

0

2.2

1.9

1.5

2.1

1.6

1.3

Oxalate in Pulp

1.2

0.9

0.9

1.1

(-0.2)

0.1

0.3

Oxalate in Filtrate

 

Total

0.4

2.9

2.5

1.9

2.2

1.6

1.2

Free

0.1

1.4

1.0

0.8

1.4

0.8

0.5

Cal. ppt*

0.3

1.5

1.5

1.1

0.8

0.8

0.7

*  Cal. ppt is calculated precipitated oxalate. Peroxide Bleaching Conditions: 6.2% H2O2, 2.6% NaSiO3, 0.13% MgSO4, 80oC, 3 hours

Figure 3

Figure 3.  Newly formed oxalate versus brightness gain (Data from Table 6)

CONCLUSIONS

Three sources of oxalate are present in the Tembec maple BCTMP line: wood, refining and bleaching process. The results in this work show that the bleaching process contributed most to oxalate formation, which was due to the reactions between oxalate precursors and hydrogen peroxide, while only small amounts of oxalate were formed by the reactions between peroxide and extractives.

The extractions of hot-water and benzene-alcohol, and super chelation removed most of oxalate in the maple pulp. The super chelation removed about 90% of the calcium in the pulp, while the hot-water and the benzene-alcohol extractions removed only about 10 - 30%.

text 4

FUTURE WORK

To minimize calcium oxalate deposit in Tembec BCTMP production line.

EXPERIMENTAL

1) Pulp sample:

The pulp at about 22% consistency was collected from disc filter at Tembec line. They were used as is.

2) Sample Preparation:

-Benzene-Alcohol Sample:

The pulp was thoroughly washed with benzene-alcohol (50% v/v) to displace all the water, extracted with the benzene-alcohol mixture in a soxhlet extractor for 6 hours. The sample was then thoroughly washed with 95% alcohol to displace all benzene-alcohol and then extracted with 95% alcohol in the soxhlet extractor for 6 hours. The sample was then further subjected to distilled water washing to remove all the alcohol, followed by extraction in the soxhlet extractor with distilled water for 6 hours.

-Hot Water Sample:

The pulp was subjected to distilled water extraction in a soxhlet extractor for 12 hours.

-Super-Chelation:

The pulp was treated by AQAQAQ washing sequence. The conditions for A were at a pH of 1 .5 (using HCl), 70oC and 30 minutes, then well washed with distilled water. The conditions for Q were 0.5% DTPA, 70oC, 30 minutes, followed by washing with distilled water.

-Total Oxalate:

2N HCl at 65oC for 4 hrs, filtrated with 0.45 µm membrane, 10 times diluted. The 40ml sample was extracted by 1gram Zeolyst to further remove some polymers, then filtered with 0.1 µm membrane, followed by ion chromatographic (IC) analysis. It was found that the presence of the residual peroxide in the sample did not affect the oxalate content.

-Free Oxalate:

40 ml of ten time diluted pressate mixed with 1g Zeolyst for about 5 minutes, filtered with 0 .1 µm membrane, then acidified the filtrate to 0.1N HCl at 20oC for about 5 minutes, and then IC analysis. The use of the Zeolyst did not affect the oxalate content.

-IC conditions:

Column: AS4A-SC, Elluent: 1.8mM Na2CO3/1.7mMNaHCO3 with a flow rate of 2.3 ml/min, Regenerate: 25mN H2SO4 with a flow rate of 3 – 5 ml/min, Backpressure: <2000psi.

3) Chemical composition of scaling samples

Moisture content was determined by drying the sample at 105 oC for 12 hours. Metal and silicate contents were determined by ashing the samples at 900 oC for 12 hours, where carbonate and oxalate were converted into CO2. The balance remained as ash. This ash was further treated with 6N HCl to dissolve metals (Ca, Mg, Fe, Mn, Cu, and Na) and analysis with an atomic absorption (AA) spectrometer. The portion of the ash not dissolved by 6N HCl was counted as silicate (SiO2). Oxalate and sulfate were determined with an IC. The samples for IC analysis were the filtrates prepared by dissolving the scaling samples in 3N HCl solution at 65 oC for 5 hours. The substances that were not dissolved by 3N HCl were considered to be organic compounds and silicate in the scaling samples. 

REFERENCES

1. D. R. Ester, Pulp & Paper, September, 135(1994).

2. G. R. Houdlette, Pulp & Paper, June, 154(1985).

3. P. Ulmgren and R. Radestrom, J. Pulp and Paper Science, 27(11), 391(2001).

4. A. Millan, Crystal Growth & Design, 1(3), 245(2001).

5. B. Tomazic and GH Nancollas, ibid, 46, 355-361(1979).

6. D. Skrtic, J. of Crystal Growth, 66, 431-440(1984).

7. H. Terelius, M. Nilsson, and T. Blomberg, International Mechanical Pulping Conference 2001 , p125-p132.

8. A. Elsander, M. Ek, and G. Gellerstedt, Tappi J. 83(2), 73(2000).

9. J. A. Krasowski and J. Marton, J. Wood Chemistry and Technology, 3(4), 445-458(1983).

10. R. J. Dexter, and X. H. Wang, Proceedings of 1998 Tappi Pulping Conference, p1341 -p1347.

11. B. Hultman, C. Nilsson, and S. Sjoberg, Svesk Paperstidning, 84(18): R163-R168(1981).

12. B. Bernard-Michel, M.N. Pnns, H. Vivier, and S. Rohani, Chemical Engineering Journal, 75 (1999) 93-103.

13. C. W. Bailey and C. W. Dence, Tappi J. 52(3), 491(1969).

14. K. Kratzi, P. K. Claus, A. Hruschka, and F. W. Vierhapper, Zcellulose Chem. Technol., 12, 445-462(1978).

15. J. Burchert, and A. Harjunpaa, Tappi J. 78(11): 125(1995).

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