|
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.
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| INTRODUCTION
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|
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.
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|
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.
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.
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 |
| |
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|
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| |
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
(%) |
|
|
|
| |
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|
|
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|
|
|
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 |
| |
