A Publication on World Pulp, Paper & Allied Industry

October-December' 2001

TECHNOLOGY

   

A Practical Solution to Silica Problem in Straw Pulping (Part I)
Comparison of Soda, Soda-Oxygen and Soda Anthraquinone Wheat Straw
(Triticum aestivum L.) Pulping Methods With Respect to Silica Contents

by Dr. Ahmet TUTUS* & Dr. Hudaverdi EROGLU**
 

During chemical recovery in straw pulping, two major problems encountered are high silica and high viscosity in black liquor. Desilication methods applied up to now are not technically and eco-nomically feasible.

In this study, soda, soda-oxygen and soda anthraquinone pulping methods were compared from the viewpoints of silica contents, and chemical, physical and optical properties of pulps. After the analysis of the data obtained, it has been concluded that the best method for silica precipitation on fibers and optical properties was soda oxygen process. Whereas, soda anthraquinone method has given the best physical properties. Optimum soda oxygen and soda anthraquinone pulping conditions were as indicated below. Precipitated silica rates on fibers were 27,77 %, 30.079 and 80,40 % for soda, soda anthraquinone and soda oxygen pulping methods respectively. Key - words: Wheat straw, soda, soda-oxygen, soda anthraquinone pulping, and silica precipitation.

Soda oxygen process

Soda anthraquinone process

NaOH %: 16%

NaOH % : 16 %

Temperature : 140 C0

Temperature : 140 C0

Time : 40 minutes

Time : 60 minutes

O2 pressure : 8 kg/cm2

Anthraquinone % : 0,1 %

Liquor / straw ratio : 5/1

Liquor / straw ratio : 5/1

 

INTRODUCTION
Turkey produces about 57 million tons of cereal straw ann-ually of which 30 million tons being wheat straw. Despite the abundance of cereal straw chemical pulping of this raw material has been faced with three important problems.

The first and more important of these is silica problem creating very serious drawbacks in the recovery system. The second one is slow filtration of straw pulp during papermaking. The third one is high viscosity of black liquor due to the presence of high pentosan in wheat straw (30,8 %).

The most important and cumbersome of these problems is the silica presence in black liquor. It continues to harm all the way until the end of recovery sequences. In green liquor silica spoils the settling property of the lime mud during recausticising stage, this in turn affects white liquor clarity. (1).

Ash content of non-wood plants varies between 1% and 20%. Whereas, in softwoods and hardwoods the
same value is generally less than 1%. Consequently, there are no recovery problems originating from silica.
(2). 65-70 % of the cereal straw ash content is SÏO2 and silicates. Silica content in wheat straw varies between 2,5% and 5,5% depending on the soil and the region. (3,4,5)

90% of the silica in the recovery cycle comes from raw material and 6-10% originated from lime used. This
silica creates many problems during recovery operations. With predesilication majority of silica, especially
silica existing on outer surfaces can be removed. However, silica existing in the inner parts of the straw
cannot be removed totally and creates serious problems in the recovery system (5,6,7).

Because of high viscosity of black liquor, it is not possible to obtain sufficiently high concentration by evaporation. High viscosity scale is formed inside evaporator tubes. Evaporator tubes should be cleaned by frequent shutdowns. On the other hand, recovery boiler operation becomes extremely unstable because of fluctuations in black liquor concentration and firing rate. In case of lower concentration huge quantity of additional fuel used to ensure an adequate combustion. Finally, silica in green liquor spoils settling property
of the lime mud in the recausticising stage. Consequently, affects white liquor clarity and results in high soda losses.

Table 1. Soda pulping conditions of wheat straw
Cooking Number

Pulping Conditions
  NaOH (%) Temperature (0C)   Time (min.)
1 16 120 20
2 16 120 40
3 16 120 60
4 16 140 20
5 16 140 40
6 16 140 60
7 16 160 20
8 16 160 40
9 16 160 60
Liquor/Straw ratio was taken constant as 5/1

 

Table 2. Soda oxygen pulping conditions of wheat straw

Cooking Number

Pulping Conditions
NaOH (%) Temperature (0C) Time (Min) Oxygen Pressure  (kg/ cm2)  
1 16 120 20 6
2 16 120 40 6
3 16 120 60 6
4 16 120 20 8
5 16 120 40 8
6 16 120 60 8
7 16 140 20 6
8 16 140 40 6
9 16 140 60 6
10 16 140 20 8
11 16 140 40 8
12 16 140 60 8
13 16 160 20 6
14 16 160 40 6
15 16 160 60 6
16 16 160 20 8
17 16 160 40 8
18 16 160 60 8

Liquor/Straw ratio was taken constant as 5/1

As a result, 25% of the total silica remains in the system (1,7,8). On the other hand, By using, predesilication method maximum silica removal rate was found as 78% with three stages treatment (1).

Table 3. Soda Anthraquinone pulping conditions of wheat straw

Cooking Number

Pulping Conditions
NaOH (%) Temperature (0C) Time (Min) Anthraquinone  AQ%  
1 16 120 20 0.1
2 16 120 40 0.1
3 16 120 60 0.1
4 16 140 20 0.1
5 16 140 40 0.1
6 16 140 60 0.1
7 16 160 20 0.1
8 16 160 40 0.1
9 16 160 60 0.1

Liquor/Straw ratio was taken constant as 5/1

In this study, soda, soda oxygen and soda anthraquinone methods were compared from the viewpoints of silica contents in black liquor and on pulp fibers, as well as chemical physical and optical properties of pulps obtained.

MATERIAL & METHODS

Wheat straw (Triticum aestivum ssp. vulgare V. graseum, korn) used was taken from Kahramanmaras province near Cilicia region. Chemical analyses of wheat straw were made according to the TAPPI Standard Methods. Fiber properties were measured with a Vizopan light microscope.

Wheat straw cleaned and cut to a length of 6-8 cm. Cooking experiments were made in a laboratory rotary digester of 15 liters capacity and maximum pressure of 25 kg/cm2. Hand sheets were made on Rapid Kothen laboratory handsheet machine. Physical and optical properties of handsheets were measured according to the TAPPI Standard test methods.

Experimental cooking conditions of soda, soda oxygen and soda-anthraquinone processes are given in Tables 1,2 and 3.

Table 4. Chemical composition and fiber properties of wheat straw

Chemical Components

Average %

Holocellulose

77.00

Holocellulose (ash free)

74.43

Cellulose

52.07

Cellulose (ash free)

48.84

Alpha-cellulose

39.82

Alpha-cellulose (ash free)

38.51

Lignin

18.11

Lignin (ash free)

15.59

Pentosans

30.03

Ash

7.04

Silica

5.43

Alcohol-benzene solubility

5.52

% 1 NaOH solubility

40.81

Hot water solubility

12.36

Cold water solubility

7.71

Fiber length (µm)

0.82

Fiber width (µm)

16.07

Lumen width (µm)

5.46

Double wall thickness (µm)

10.81

Table 5. Ash and silica contents of different botanical parts of wheat straw
Botanical Parts Ash Contents

Silica and Silicates

Whole straw 7.04

5.43
Internodes 3.13

1.90
Notes 8.07

5.05
Leaves and blades 13.98

9.92
Rachis 7.07

5.05
Glumes 14.06

12.04
 
CHEMICAL COMPO-SITION AND FIBER PROPERTIES OF WHEAT STRAW

Chemical composition and fiber properties of wheat straw used in this study are given in Table 4. holocellulose rate of wheat straw is approximately equal to those of hardwoods. This is because of high hemicellulose and low lignin content compared to softwoods. High NaOH solubility rate is due to the presence of low molecular weight carbohydrates and other alkali soluble matters.

Ash and silica contents of different botanical parts of wheat straw are given in Table 5. As can be seen from Table 5 wheat straw used contains 7.04 % ash and 5.43% silica based on oven dry straw weight. It should be noticed that leaves and ears contain much more silica than the other parts. On the other hand, internodes have lower silica with1.90%. It can be deduced that dry clearing would noticeably reduce silica content of cereal straw.

Table 9. Overall properties of wheat straw soda, soda oxygen, soda anthraquinone pulps
Processes Screened yield (%) Kappa No. Viscosity (cm3/g) Precipitated Silica on fibers(%) Breaking length (km.) Burst index (kPa.
m2/g)		 Tear index (mN.m
2/g)		 Brightness (%MgO) Opacity (%)
Soda-Oxygen
(Cooking number 11) Soda-AQ		 46.17 18.40 562 80.40 5.65 3.50 6.04 57.8 88.0
(Cooking number 6) Soda 46.96 18.63 698 30.07 8.91 5.38 3.70 53.0 85.8
(Cooking number 6 45.0 20.86 621 27.77 6.75 4.98 3.90 49.1 83.6
 
SOME PHYSICAL, CHEMICAL AND OPTICAL PROPERTIES OF SODA, SODA-OXYGEN AND SODA- ANHRAQUINONE WHEAT STRAW PULPS


Chemical, physical, optical and silica rates remained in black liquor and precipitated on fibers of soda, soda oxygen and soda anthraquinone method pulps are given in Tables 6,7 and 8. All pulps are beaten to 50 ± 3 °SR in a laboratory beater.

As can be observed from the Tables 6,7 and 8 it is clearly seen that higher silica precipitation on fibers is obtained by soda oxygen pulping. Whereas, the same values for soda and soda-anthraquinone pulping vary between 16,57% and. 30,07%. It can be easily concluded that soda oxygen pulping, beside its other advantages such as environmental cleanliness and high brightness, largely contributes to the solution of silica problems.

 

Table 6                        PART-1

  

Chemical, physical, optical and precipitated silica rates on fibers of wheat straw soda pulps
           
  Col. No.1 Col. No.2 Col. No.3 Col. No.4 Col. No.5
           

Cooking No.

Screened yield(%)

 Screen reject(%) Total yield(%) Black liquor pH

Alkali consumption (%)
1. 40.60 10.00 50.60 11.80 73.70
2. 44.74 5.06 49.80 11.48 76.40
3. 46.00 3.10 49.10 10.28 79.80
4. 41.65 8.00 49.65 10.18 80.40
5. 44.50 4.90 49.40 9.98 83.10
6. 45.00 2.00 47.00 9.93 85.00
7. 41.90 7.00 48.90 9.90 88.34

8.

 44.00 4.10 48.10 9.87 90.42

9.

 44.50 1.05 45.05 9.78 93.20
 
 
 

Table 6                        PART-2

  

Chemical, physical, optical and precipitated silica rates on fibers of wheat straw soda pulps
           
  Col. No.6 Col. No.7 Col. No.8 Col. No.9 Col. No.10
           

Cooking No.

Silica precipitated
on fibers %

Silica remained
in black liquor%

% Silica precipitated

Kappa No.

Pulp Visosity
cm3/g
1. 0.90 4.53 16.57 26.60 718
2. 0.98 4.46 18.01 24.08 672
3. 1.00 4.42 18.45 23.16 646
4. 1.20 4.44 23.53 24.10 670
5. 1.31 4.13 24.08 22.80 648
6. 1.50 3.90 27.77 20.86 621
7. 1.43 4.00 26.34 22.64 651

8.

 1.49 3.95 27.39 19.32 623

9.

 1.39 4.06 25.50 18.46 600
 
 
 

Table 6                        PART-3

  

Chemical, physical, optical and precipitated silica rates on fibers of wheat straw soda pulps
           
  Col. No.11 Col. No.12 Col. No.13 Col. No.14 Col. No.15
           

Cooking No.

Breaking Length
km

Burst index
(kPam2/g)

Tear index (mNm2/g)

Brightness 
(% MgO)

Printing opacity (%)
1. 6.91 4.96 3.4 47.6 80.0
2. 7.41 5.40 3.9 49.2 81.8
3. 7.20 5.20 3.7 50.6 82.4
4. 6.54 4.74 3.6 46.0 81.3
5. 6.98 5.16 4.1 48.4 82.0
6. 6.75 4.98 3.9 49.1 83.6
7. 6.01 4.42 3.2 44.2 76.1

8.

 6.54 4.78 3.6 46.4 80.0

9.

 6.27 4.50 3.4 48.0 81.7

On the other hand, brightness of oxygen pulps is about 5 to 10 points higher than those of soda and soda anthraquinone pulps. Finally, soda oxygen pulps give higher tear index values. Contrarily, soda and soda anthraquinone pulps have higher breaking length and bursting strength and higher pulp viscosity. The highest breaking length, burst index and viscosity values obtained by soda anthraquinone method, because of the protective effect of anthraquinone on carbohydrate fraction. A brief summary of the results obtained by three methods is given in Table 9.

 

Table 7                        PART-1

  

Chemical, physical, optical and precipitated silica rates on fibers of
wheat straw soda oxygen pulps
           
  Col. No.1 Col. No.2 Col. No.3 Col. No.4 Col. No.5
           

Cooking No.

Screened yield(%)

 Screen reject(%) Total yield(%) Black liquor pH

Alkali consumption (%)
1. 42.70 4.85 47.55 11.80 72.38
2. 46.07 4.10 50.17 11.48 73.17
3. 46.85 3.80 50.65 10.28 75.54
4. 45.14 4.60 49.74 10.18 81.54
5. 46.04 4.17 50.21 9.98 85.60
6. 46.87 4.03 50.90 9.93 86.40
7. 44.13 4.65 48.78 9.90 88.35

8.

 45.95 4.03 49.98 9.87