A Publication on World Pulp, Paper & Allied Industry

October-December' 2001

TECHNOLOGY

   

A Practical Solution to Silica Problem in Straw Pulping (Part II)
Comparison of oxide Added Soda, Soda-oxygen And. Soda-Anthraquinone wheat straw pulping methods with respect to silica contents

 
 

In the first part of this study soda, soda oxygen and soda anthraquinone processes were compared for silica content. Highest silica precipitation on fibers was found in soda oxygen pulping as 80,40 %. In this study, the aim was to enhance silica precipitation by adding 1-3 % oxide such as CaO, MgO and Al2O3

As a result of 1-3 % oxide addition (CaO, MgO and Al2O3) maximum silica precipitation rates were obtained by 3% CaO, MgO and Al2O3 added soda oxygen process as 91.16%; 89,50 and 96,44% respectively. The same values were 45,38; 42,35 and 57,75 for soda anthraquinone pulping and. 40,22; 38,12 and 51,02 for soda pulping. Although oxide addition enhance silica precipitation in soda and soda AQ pulping maximum value of 96,99 % has been obtained by soda oxygen pulping and 3% Al2O3 addition.

Silica was precipitated on fibers as kaolin, talc and calcium silicate depending on the oxide used. Consequently, 5-6 % useful filler has been introduced to improve printing qualities of paper. There will not be an abrasion problem as a result of the transformation of abrasive silica to kaolin, talc and calcium silicate. In fact, this is also visually observed. Hence, this method offers highly practical and efficient solution to silica problem compared to the conventional desilication methods.

Keywords: Wheat straw, desilication methods, metal oxides (Al2O3, CaO, MgO), Soda, Soda oxygen, soda pulping methods.

INTRODUCTION

Existing desilication processes can be divided into three categories.

1. Precipitation of silica as insoluble silicates by using cations like Al+++, Ca++, Mg++ and remove 
    it from black liquor.

2. Lowering black liquor pH by using CO2 or combustion flue gases, thereby, precipitating silica as 
    insoluble silicic acid gel.

3. Other methods such as predesilication. (1).

None of the above mentioned methods can completely solve the silica problem. In addition, they have serious drawbacks such as expensive equipment and installation, cumbersome operations, such as handling of huge amount of water, high chemical, and energy consumption etc.

On the other hand, physical properties of precipitated silica is very important. Because sand like silica presents also serious problems during converting and printing operations. Such as, cutting blades become blunt and abrasion effect on machines.

 

2NaOH + SiO2

Na2SiO3 + H2O  (2)

2Na2SiO3 + Al2O3 + 4H2O

Al2(OH)4Si2O5(Kaolin) + 4NaOH  (3)

Na2SiO3 + CaO + H2O

CaSiO3 (Calcium silicate) + 2NaOH  (2)

4Na2SiO3 + MgO+ 5H2O

Mg3(OH)2Si4O10 (Talc) + 8NaOH  (3)

Whereas, in this method by the addition of metal oxides such as Al2O3, CaO and Mg O the following reactions take place to produce some well known common fillers. On the other hand, in alkaline processes most of the silica exists as sodium silicate in black liquor.

Fillers increase optical and printing properties of papers such as brightness, opacity, surface smoothness, ink absorption, regular burning in cigarette paper etc.

EXPERIMENTAL

9 experiments were performed for every process (soda, soda oxygen and soda anthraquinone) under optimum conditions obtained in the previous study. Oxide quantities (CaO, MgO and Al2O3) were varied at three levels as 1%, 2%, and 3%.

Wheat straw stalks were cut to a length of 6-8 cm. Pulping is performed in an electrically heated, 15 liters volume rotary laboratory digester. All pulps were washed and screened on a 0,15mm slotted screen. Screened pulps beaten in a valley beater to 20,35 and 50 0 SR freeness levels and handsheet were made on a Rapid-Kothen machine.

Physical and optical properties of handsheets were determined according to the TAPPI Standard Methods but viscosity values according to SCAN C 16:88.

Silica contents of black liquor and pulps were determined according to TAPPI T 625 om-85 and TAPPI T 244 om-88 respectively.

 
RESULTS

Screened pulp properties and silica contents of soda, soda oxygen and soda anthraquinone pulps are given in Tables 1,2 and 3 respectively.

On the other hand, physical and optical properties of oxide added soda, soda oxygen and soda anthraquinone wheat straw pulps are given in Table 4,5 and 6. When examining Table 4,5 and 6 highest physical properties were obtained by soda anthraquinone process because of the protective effect of anthraquinone on carbohydrate fraction. On the other hand, the lowest physical properties obtained  by soda oxygen pulping, due to the degradation effect of oxygen. Oxide addition induced a slight decrease in physical properties for all processes. This should be expected when using fillers.

As can be observed from Table 1,2 and 3. Highest silica precipitation was in oxide added soda oxygen pulping with 96,49% when using 3% Al2O3. Whereas, maximum silica precipitation values were 51.02% and 57,75% for soda and soda anthraquinone processes respectively. Generally, it has been observed that silica precipitation rates were increased by the increase of oxide rate for all processes. But, precipitation rates are not sufficient for soda and soda anthraquinone processes. On the other hand, maximum precipitation rate has been obtained by 3% Al2O3 addition for all methods. However, the best result has been obtained by soda oxygen process and 3% Al2O3 addition.

Table 1                         PART-1

Pulp properties and silica contents of oxide added soda wheat straw pulps

             
 

Col. No.1

Col. No.2

Col. No.3

Col. No.4

Cooking No.

Oxide Added

Screened Yield (%) Black Liquor PH Silica  Precipitated on fibres (%)
  A12O3 (%) CaO (%) MgO (%)      
             

0*

0

0

0

45.00 9.93 1.50
1. 1     44.83 10.78 1.86
2. 2     43.25 10.57 2.10
3. 3     41.84 10.40 2.76
4.   1   43.76 10.17 1.79
5.   2   43.01 10.01 2.09
6.   3   41.15 9.98 2.18
7.     1 41.88 10.40 1.67
8.     2 40.13 10.27 1.86
9.     3 39.67 10.05 2.07

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1 are constant, soda cooking No.6

 
 

Table 1                         PART-2

Pulp properties and silica contents of oxide added soda wheat straw pulps

 
 

Col. No.5

Col. No.6 Col. No.7 Col. No.8
Cooking No. Silica in Black Liquor (%) % Silica Precipitated Kappa No Pulp Viscosity (cm3/gr)
         
0* 3.90 27.77 20.9 621
1. 3.55 34.38 20.0 614
2. 3.31 38.81 19.4 605
3. 2.65 51.02 18.7 594
4. 3.63 32.90 19.3 602
5. 3.33 38.56 18.7 580
6. 3.24 40.22 17.9 574
7. 3.74 30.87 19.4 585
8. 3.55 34.38 18.9 560
9. 3.36 38.12 18.2 545

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1 are constant, soda cooking No.6

 
 

Table 2                         PART-1

Pulp properties and silica contents of oxide added soda oxygen wheat straw pulps

             
 

Col. No.1

Col. No.2

Col. No.3

Col. No.4

Cooking No.

Oxide Added

Screened Yield (%) Black Liquor PH Silica  Precipitated on fibres (%)
  A12O3 (%) CaO (%) MgO (%)      
             

0*

0

0

0

46.17 9.90 4.35
1. 1     45.68 10.80 4.75
2. 2     43.94 10.61 4.96
3. 3     42.86 10.45 5.23
4.   1   44.51 10.23 4.46
5.   2   42.85 10.11 4.79
6.   3   41.16 10.03 4.95
7.     1 43.13 10.60 4.41
8.     2 42.66 10.50 4.64
9.     3 40.52 10.41 4.86

0* NaOH 16%, Temperature: 1400C, Oxygen pressure: 8kg/cm2, Time: 40min., Liquor to straw ratio: 5/1 are constant, soda-oxygen cooking No.11

 
 

Table 2                         PART-2

Pulp properties and silica contents of oxide added soda oxygen wheat straw pulps

 
 

Col. No.5

Col. No.6 Col. No.7 Col. No.8
Cooking No. Silica in Black Liquor (%) % Silica Precipitated Kappa No Pulp Viscosity (cm3/gr)
         
0* 1.06 80.40 18.4 562
1. 0.68 87.47 17.8 558
2. 0.47 91.34 17.0 540
3. 0.19 96.49 16.6 517
4. 0.97 82.14 18.10 546
5. 0.64 88.21 17.6 528
6. 0.48 91.16 17.0 514
7. 1.01 81.36 17.9 540
8. 0.79 85.45 17.1 515
9. 0.57 89.50 16.8 506

0* NaOH 16%, Temperature: 1400C, Oxygen pressure: 8kg/cm2, Time: 40min., Liquor to straw ratio: 5/1 are constant, soda-oxygen cooking No.11

 
 

Table 3                         PART-1

Pulp properties and silica contents of oxide added soda anthraquinone wheat straw pulps

             
 

Col. No.1

Col. No.2

Col. No.3

Col. No.4

Cooking No.

Oxide Added

Screened Yield (%) Black Liquor PH Silica  Precipitated on fibres (%)
  A12O3 (%) CaO (%) MgO (%)      
             

0*

0

0

0

46.96 12.05 1.63
1. 1     46.18 11.20 2.10
2. 2     44.38 10.98 2.45
3. 3     43.65 10.90 3.13
4.   1   44.98 11.10 1.95
5.   2   42.17 11.30 2.17
6.   3   40.72 11.48 2.46
7.     1 43.68 12.40 1.81
8.     2 42.94 11.90 2.06
9.     3 41.73 11.45 2.30

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, anthraquinone: 0.1% are constant, soda-anthraquinone cooking No.6.

 
 

Table 3                         PART-2

Pulp properties and silica contents of oxide added soda anthraquinone wheat straw pulps

 
 

Col. No.5

Col. No.6 Col. No.7 Col. No.8
Cooking No. Silica in Black Liquor (%) % Silica Precipitated Kappa No Pulp Viscosity (cm3/gr)
         
0* 3.79 30.07 18.6 698
1. 3.32 38.67 18.1 686
2. 2.97 45.20 16.1 670
3. 2.29 57.75 14.8 620
4. 3.48 35.91 17.8 680
5. 3.26 40.07 16.3 663
6. 2.96 45.38 15.8 612
7. 3.62 33.33 18.0 665
8. 3.36 38.01 17.8 650
9. 3.13 42.35 16.9 604

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, anthraquinone: 0.1% are constant, soda-anthraquinone cooking No.6.

 
 

Table 4                         PART-1

Pulp properties and silica contents of oxide added soda wheat straw pulps

           
 

Col. No.1

Col. No.2

Col. No.3

Cooking No.

Oxide Added

Breaking Length (km) Burst Index (kPa.m2/g)
  A12O3 (%) CaO (%) MgO (%)    
           

0*

0

0

0

6.75 4.98
1. 1     6.54 4.84
2. 2     6.32 4.68
3. 3     6.18 4.50
4.   1   6.42 4.70
5.   2   6.18 4.52
6.   3   6.02 4.36
7.     1 6.49 4.78
8.     2 6.26 4.60
9.     3 6.10 4.44

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1 are constant, soda cooking No.6.

 
 

Table 4                         PART-2

Pulp properties and silica contents of oxide added soda wheat straw pulps

 
 

Col. No.4

Col. No.5 Col. No.6
Cooking No. Tear Index (mN.m2/g) Brightness (% MgO) Printing Opacity (%)
       
0* 3.9 49.1 83.6
1. 3.80 51.0 81.0
2. 3.68 52.8 82.2
3. 3.56 54.0 83.7
4. 3.64 50.2 82.3
5. 3.52 51.6 84.0
6. 3.40 53.2 79.4
7. 3.70 50.0 79.4
8. 3.62 51.2 81.5
9. 3.48 52.4 82.9

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1 are constant, soda cooking No.6.

It is interesting to note that brightness values of 3% Al2O3 added soda oxygen pulps increased from 57,8% to 63 %. This can be easily attributed to the filling action of silica precipitation as kaolin on fibers. In the same manner, opacity value increased from 88,0% to 92,7% by oxide addition. In fact, when visually observing, raw material ash has sand like structure, whereas oxide added pulp ash has pure and white powder structure. According to the laboratory particle size measurement, soda oxygen 3% Al2O3 added straw pulp ash particle size has been found 95 % lower than 10 µ.

The same observation can be made for soda and soda anthraquinone methods, but to a lesser extent because of lower precipitation rate. For example, brightness and opacity of soda anthraquinone pulps increased from 54,1 and 85,8 to 55,2 and 88,5 respectively by the addition of 3% Al2O3. On the other hand, abrasivity was found between 20-25 mg which considered as lower value and under the maximum value described as 25 mg in coating application. Overall properties of soda, soda oxygen and soda anthraquinone pulps. It is clearly seen that highest screened yield and the lowest Kappa number was obtained by soda AQ method. The above-mentioned observations are clearly seen concerning silica precipitation rates and physical and optical properties.

CONCLUSIONS

1. 96,49% of the silica existing in raw material precipitated on fibers as kaolin by using %3 
     A12O3
added soda oxygen pulping. In this manner, the entire problem encountered in straw 
    pulping related to silica presence in black liquor will be totally eliminated. In addition, 
    resulting paper has been 5-6% filled by kaolin. Particle size of precipitated ash was 95% 
    lower than 10µ of Hence, some improvements in optical properties and printing properties 
    have been obtained.

2. Due to the presence of oxygen somewhat higher physical strength loss was observed such 
    as breaking length and burst index. However, by using oxide added soda oxygen AQ 
    process certain strength losses would be regained (5). Additional experiments need to be 
    done in this respect.

        3. Finally, it should be stated that oxide added soda oxygen pulping of wheat straw presents a 
            practical and beneficial solution to silica problem, which is the most important tiresome problem 
            in non-wood pulping. Silica remained in black liquor is about 0,19%. This amount is nearly equal 
            to the amount encountered in wood pulping.

Table 5                         PART-1

Physical and optical properties of oxide added wheat straw soda-oxygen pulps at 50 ± 3 0SR

           
 

Col. No.1

Col. No.2

Col. No.3

Cooking No.

Oxide Added

Breaking Length (km) Burst Index (kPa.m2/g)
  A12O3 (%) CaO (%) MgO (%)    
           

0*

0 0 0 5.65 3.50
1. 1     5.54 3.30
2. 2     5.37 3.18
3. 3     5.18 3.10
4.   1   5.48 3.14
5.   2   5.29 3.08
6.   3   5.01 3.00
7.     1 5.55 3.24
8.     2 5.34 3.12
9.     3 5.20 3.06

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, Anthraquinone: 0.1% are constant, Soda-Anthraquinone cooking No.6.

 
 

Table 5                         PART-2

Physical and optical properties of oxide added wheat straw soda-oxygen pulps at 50 ± 3 0SR

 
 

Col. No.4

Col. No.5 Col. No.6
Cooking No. Tear Index (mN.m2/g) Brightness (% MgO) Printing Opacity (%)
       
0* 6.04 57.8 88.0
1. 5.90 59.2 90.8
2. 5.76 61.4 91.4
3. 5.58 63.0 92.7
4. 5.64 58.6 89.2
5. 5.52 60.2 90.3
6. 5.40 61.4 91.1
7. 5.76 58.0 90.0
8. 5.60 58.6 91.2
9. 5.50 59.8  

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, Anthraquinone: 0.1% are constant, Soda-Anthraquinone cooking No.6.

 
 

Table 6                         PART-1

Physical and optical properties of oxide added wheat straw soda-oxygen pulps at 50 ± 3 0SR

           
 

Col. No.1

Col. No.2

Col. No.3

Cooking No.

Oxide Added

Breaking Length (km) Burst Index (kPa.m2/g)
  A12O3 (%) CaO (%) MgO (%)    
           

0*

0

0

0

8.91 5.38
1. 1     8.70 5.10
2. 2     8.56 4.86
3. 3     8.32 4.64
4.   1   8.64 4.96
5.   2   8.32 4.70
6.   3   8.06 4.46
7.     1 8.67 5.04
8.     2 8.45 4.78
9.     3 8.28 4.52

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, Anthraquinone: 0.1% are constant, Soda-Anthraquinone cooking No.6.

 
 

Table 6                         PART-2

Physical and optical properties of oxide added wheat straw soda-oxygen pulps at 50 ± 3 0SR

 
 

Col. No.4

Col. No.5 Col. No.6
Cooking No. Tear Index (mN.m2/g) Brightness (% MgO) Printing Opacity (%)
       
0* 3.70 53.0 85.8
1. 3.62 54.1 86.0
2. 3.50 54.8 87.4
3. 3.36 55.2 88.5
4. 3.48 53.6 85.2
5. 3.32 54.0 86.3
6. 3.20 54.6 87.5
7. 3.54 53.2 84.1
8. 3.40 53.8 85.0
9. 3.32 54.2 86.4

0* NaOH 16%, Temperature: 1400C, Time: 60min., Liquor to straw ratio: 5/1, Anthraquinone: 0.1% are constant, Soda-Anthraquinone cooking No.6.

 

REFERENCES

  • Deniz, I. Predesilication of Wheat (Triticum aestivum L.) Straw and The Action of This 
    Pretreatment on O2-NaOH Pulping Conditions, Ph.D. Thesis, Karadeniz Technical University, Pulp 
    and Paper Technology Dept., Trabzon (1994). 

  • Hurter, A. M. Utilization of Annual Plants and Agricultural Residues for The Production of 
    Pulp and Paper, Pulping Conference, TAPPI Proceeding, New Orleans. p.139-160 (1988).

  • Huheey, E. J. Inorganic Chemistry, Principles of Structure and Reactivity, Harper & Row Publishers, New York. p. 508-509 (1972).

  • Tutuŗ, A. Comparison Of Soda-Oxygen, Soda-Anthraquinone And Soda Pulping Wheat (Triticum Aestivum L.) Straw With Respect To Silica Problem And Other Aspects, Ph.D. Thesis, Zonguldak Karaelmas University, Forest Product Engineering Dept., Zonguldak, s. 6 (2000).

  • Erošlu, H. (1987) Soda-Oxygen Anthraquinone Pulping of Wheat Straw, Non-wood Plant Fiber Pulping Progress Report, No. 17, Houston.

  • Erošlu, H., Tutuŗ, A., 2001. Patent Pending, 20/04/2001, Turkish Patent Institute.

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