- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •1072 1 Introduction
- •Isbn: 3-527-30999-3
- •Inventor of stone groundwood. Right: the second version
- •1074 2 A Short History of Mechanical Pulping
- •In refining, the thinnings (diameter 7–10cm) can also be processed.
- •In mechanical pulping as it causes foam; the situation is especially
- •In mechanical pulping, those fibers that are responsible for strength properties
- •Isbn: 3-527-30999-3
- •In mechanical pulping, the wood should have a high moisture content, and the
- •In the paper and reduced paper quality. The higher the quality of the paper, the
- •1076 3 Raw Materials for Mechanical Pulp
- •1, Transversal resistance; 2, Longitudinal resistance; 3, Tanning limit.
- •3.2 Processing of Wood 1077
- •In the industrial situation in order to avoid problems of pollution and also
- •1078 3 Raw Materials for Mechanical Pulp
- •2, Grinder pit; 3, weir; 4, shower water pipe;
- •5, Wood magazine; 6, finger plate; 7, pulp stone
- •Isbn: 3-527-30999-3
- •4.1.2.1 Softening of the Fibers
- •1080 4 Mechanical Pulping Processes
- •235 °C, whereas according to Styan and Bramshall [4] the softening temperatures
- •Isolated lignin, the softening takes place at 80–90 °c, and additional water
- •4.1 Grinding Processes 1081
- •1082 4 Mechanical Pulping Processes
- •1, Cool wood; 2, strongly heated wood layer; 3, actual grinding
- •4.1.2.2 Defibration (Deliberation) of Single Fibers from the Fiber Compound
- •4 Mechanical Pulping Processes
- •Influence of Parameters on the Properties of Groundwood
- •In the mechanical defibration of wood by grinding, several process parameters
- •Improved by increasing both parameters – grinding pressure and pulp stone
- •In practice, the temperature of the pit pulp is used to control the grinding process,
- •In Fig. 4.8, while the grit material of the pulp stone estimates the microstructure
- •4 Mechanical Pulping Processes
- •4.1 Grinding Processes
- •Is of major importance for process control in grinding.
- •4 Mechanical Pulping Processes
- •4.1.4.2 Chain Grinders
- •Is fed continuously, as shown in Fig. 4.17.
- •Initial thickness of the
- •4 Mechanical Pulping Processes
- •Include:
- •Increases; from the vapor–pressure relationship, the boiling temperature is seen
- •4 Mechanical Pulping Processes
- •In the pgw proves, and to prevent the colder seal waters from bleeding onto the
- •4.1 Grinding Processes
- •In pressure grinding, the grinder shower water temperature and flow are
- •70 °C, a hot loop is no longer used, and the grinding process is
- •4 Mechanical Pulping Processes
- •Very briefly at a high temperature and then refined at high
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •Intensity caused by plate design and rotational speed.
- •4.2 Refiner Processes
- •1. Reduction of the chips sizes to units of matches.
- •2. Reduction of those “matches” to fibers.
- •3. Fibrillation of the deliberated fibers and fiber bundles.
- •1970S as result of the improved tmp technology. Because the key subprocess in
- •4 Mechanical Pulping Processes
- •Impregnation Preheating Cooking Yield
- •30%. Because of their anatomic structure, hardwoods are able to absorb more
- •Is at least 2 mWh t–1 o.D. Pulp for strongly fibrillated tmp and ctmp pulps from
- •4 Mechanical Pulping Processes
- •4.2 Refiner Processes
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa;
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •In hardwoods makes them more favorable than softwoods for this purpose. A
- •4.2 Refiner Processes
- •Isbn: 3-527-30999-3
- •1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning 1115
- •In refiner mechanical pulping, there is virtually no such coarse material in the
- •1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.3 Reject Treatment and Heat Recovery
- •55% Iso and 65% iso. The intensity of the bark removal, the wood species,
- •Isbn: 3-527-30999-3
- •1124 6 Bleaching of Mechanical Pulp
- •Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,
- •2000 Kg of technical-grade product is common. Typically, a small amount of a chelant
- •6.1 Bleaching with Dithionite 1125
- •Vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available
- •1126 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •70 °C, 2 h, amount of NaOh adjusted.
- •6.2 Bleaching with Hydrogen Peroxide
- •Is shown in Fig. 6.5, where silicate addition leads to a higher brightness and a
- •Volume (bulk). For most paper-grade applications, fiber volume should be low in
- •Valid and stiff fibers with a high volume are an advantage; however, this requires
- •1130 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •Very high brightness can be achieved with two-stage peroxide bleaching, although
- •In a first step. This excess must be activated with an addition of caustic soda. The
- •Volume of liquid to be recycled depends on the dilution and dewatering conditions
- •6 Bleaching of Mechanical Pulp
- •6 Bleaching of Mechanical Pulp
- •Is an essential requirement for bleaching effectiveness. Modern twin-wire presses
- •Is discharged to the effluent treatment plant. After the main bleaching stage, the
- •6.3 Technology of Mechanical Pulp Bleaching
- •1136 6 Bleaching of Mechanical Pulp
- •Isbn: 3-527-30999-3
- •7.3 Shows the fractional composition according to the McNett principle versus
- •1138 7 Latency and Properties of Mechanical Pulp
- •7.2 Properties of Mechanical Pulp 1139
1126 6 Bleaching of Mechanical Pulp
6.2 Bleaching with Hydrogen Peroxide
spp.). The wood species, the age of the wood, storage of the logs and the bark content
each have a huge impact on bleachability and the brightness ceiling.
During the bleaching process, a variety of compounds are dissolved, the main ones
being acetic acid (from acetyl groups on carbohydrates) and low molecular-weight
polyoses. Lignin dissolves only to a small extent. However, because the pulping process
solubilizes only a small part of the wood, rather high effluent loads can result
from bleaching. Top brightness requires a high peroxide input and, for its activation,
a similarly large amount of caustic soda. The resultant brightness in bleaching softwood
TMP with increasing input of hydrogen peroxide is shown graphically in
Fig. 6.2, where different amounts of caustic soda were applied to achieve the best
response in brightness. For a given residence time and temperature, there is an optimum
level of activation. The shape of the curves shows, for the ratio of H2O2 to
NaOH, an increasingly wider range of tolerance. Clearly, the more peroxide
applied, the less critical is the correct amount of caustic soda added.
0,5 1,0 1,5 2,0
64
66
68
70
72
74
1% H
2
O
2
2% H
2
O
2
3% H
2
O
2
4% H
2
O
2
Brightness [% ISO]
NaOH-charge [%]
Fig. 6.2 Increase in brightness with optimized charges of NaOH
for different peroxide amounts. Bleaching at 65 °C, 3 h, 20%
consistency, with a constant addition of 2% sodium silicate.
An inadequate activation results in an insufficient consumption of H2O2, but
too-high charges are similarly detrimental, and the alkalinity consumes peroxide
and brightness decreases again. The process cannot be operated with the aim of
consuming all of the hydrogen peroxide applied. An example of the brightness
resulting from a constant input of hydrogen peroxide but a variation of the
amount of caustic soda is shown in Fig. 6.3. The comparison of best brightness
and remaining residual allows the conclusion to be made that the residual must
be higher than about 10–15% of the peroxide input in order to achieve the best
result. As a lower residual results in a poorer gain in brightness, it is therefore
1127
6 Bleaching of Mechanical Pulp
1 1.4 1.8 2.2 2.6 3
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Peroxide residual [%]
NaOH (%)
1,0 1,5 2,0 2,5 3,0
66
68
70
72
74
Brightness [% ISO]
NaOH-charge [%]
Fig. 6.3 Impact of the variation of the caustic soda charge on
brightness. Bleaching of softwood TMP with 4% H2O2 and
2% sodium silicate, constant: 25% consistency, 3 h, 70 °C.
important to control the peroxide residual and to evaluate the ratio between residual
and input.
Bleaching response is improved with consistency. The reason for this is the
higher relative concentration of the chemicals and the lower level of dissolved
compounds. The steep increase in bleaching efficiency with the consistency is
visualized in Fig. 6.4. Because of the importance of high-consistency, modern
bleach plants operate well above a level of 25%.
The addition of sodium silicate in bleaching has several effects. First, it acts as a
buffer, and therefore reduces the peak value of the pH. The instability of diluted
0 5 10 15 20 25 30 35
65
70
75
Brightness [% ISO]
Pulp consistency [%]
Fig. 6.4 Impact of consistency on mechanical pulp bleaching.
Constant application of 1.5% H2O2, 3% sodium silicate at