- •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
6.2 Bleaching with Hydrogen Peroxide
When bleaching aspen pulp, the impact on fiber volume of using Mg(OH)2 is
pronounced. Conventional bleaching to very high brightness (>85% ISO) is
accompanied by a decrease in the specific volume from >2 cm3 g–1 to 1.4 cm3 g–1,
or even less. The substitution allows the specific volume (also labeled as “bulk”) to
be maintained. In a set-up using the Mg(OH)2 stage as the main (high consistency)
stage and an additional (medium-consistency) stage with caustic soda as a
brightness adjustment step, it is possible to balance the parameters of bulk and
tensile strength to all levels in between (Fig. 6.8). The more moderate response in
bleaching to the activation with Mg(OH)2 is compensated by a final hydrosulfite
treatment. Consequently, brightness is very high (>85% ISO) and will not effect
any physical properties.
0 1 2 3
NaOH [%]
1.2
1.4
1.6
1.8
2.0
2.2
Bulk [cm./g]
Fig. 6.8 Decrease in specific volume of aspen TMP with input
of caustic soda in a PMg(OH)2-PNaOH-Y sequence
(high consistency–medium consistency–low consistency).
Very high brightness can be achieved with two-stage peroxide bleaching, although
the equipment required is rather complicated (see Section 6.3). A twostage
process can use the high excess of peroxide from the main bleaching stage
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
following the main bleaching step. Typically, volumes are so high that the first
stage must be conducted at medium consistency. The potential savings of this
approach (Fig. 6.9) are reasonable only for a very high brightness target, and this
becomes apparent from the shape of the curves.
Some mills apply in-refiner-bleaching with sodium dithionite. Because of the
high temperature, this reaction is very rapid; sulfonation of the lignin occurs and
typically the residual of sulfite detected in the pulp is extremely small. Therefore,
a peroxide bleaching stage can be added without any fear of activity losses. Y-P
bleaching typically is applied in integrated mills producing paper with different
brightness grades. A part of the reductive bleaching effect is lost by the oxidation
with peroxide, the reason for this being the re-formation of some conjugated
structures with the oxidation. However, the total brightness gain in a Y-P treat-
1131
6 Bleaching of Mechanical Pulp
2,0 2,5 3,0 3,5 4,0
77
78
79
80
81
82
Potential savings
Single stage Two stages
Brightness [% ISO]
H
2
O
2
to 2nd P-stage [%]
Fig. 6.9 Impact of single-stage (high consistency) or
two-stage (medium/high consistency) peroxide bleaching
of a spruce TMP pulp. The second stage saves significant
amounts of peroxide for very high brightness targets.
ment using dithionite in the refiner and peroxide under high-consistency conditions
delivers comparable results with the more conventional P-Y treatment. Post
bleaching with dithionite requires destruction of the excess hydrogen peroxide
with sodium bisulfite or sulfur dioxide gas. These effects are explained graphically
in Fig. 6.10.
2 3 4
0
4
8
12
16
20
Brightness gain [points]
P-Stage
H2O2 [%]
Y-Stage
2 3 4
0
4
8
12
16
20
H2O2 [%]
1st stage
2nd stage 2nd stage
1st stage
Fig. 6.10 Comparison of P-Y and Y-P bleaching. Bleaching
of softwood TMP with constant 1% Na2S2O4 and variable
amounts of peroxide. P-Y with destruction of peroxide
excess by bisulfite, Y-P sequence with refiner application of
dithionite.
1132
6.2 Bleaching with Hydrogen Peroxide
The brightness stability of mechanical pulp is much lower compared with fully
bleached chemical pulp. The reason for this is the high level of lignin remaining
in the fiber. Following bleaching, the structural elements which will re-generate
chromophores easily are mostly eliminated, although the presence of phenols
allows sufficient oxidation processes to yield a low brightness stability [35]. Post
color numbers after heat-or light- induced reversion are significantly higher in
comparison to chemical pulp. This is understandable in light of the large quantities
of phenols and phenol ethers in the remaining lignin. The light-induced
brightness reversion of mechanical pulp can be significant. However, whilst the
reversion of a newspaper in the summer sun might become apparent after only
an hour, this is of limited practical impact. Paper typically is not excessively
exposed to light, and its reversion in the dark is more important. Accelerated
aging in dry or humid tests can describe the stability of a brightness gain. The
response of a TMP to peroxide and P-Y bleaching and to accelerated aging using
hot and humid conditions is compared in Fig. 6.11. Brightness losses described as
points of brightness are relatively constant for the whole range tested. Losses with
humid reversion (100 °C, 100% humidity, 2 h) are around three points, with a
clear tendency for lower losses the higher the brightness. This results in a decreasing
post color number. The results confirm the experience with chemical pulp,
that the greater the removal of chromophores, the better the resulting stability.
The reductive post-treatment has post color numbers that are approximately
equivalent to the results after the P stage; thus, the brightness gained is not easily
lost.
0+1 2+1 3+1 4+1 5+1
62
64
66
68
70
72
74
76
78
ISO-Brightness [%]
P P-Y
H2O2 + Na2S2O4 [%]
Y
Dithionite 2 3 4 5
Peroxide amount [%]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Post color number
Fig. 6.11 Bleaching and aging of spruce TMP with Y, P, or P-Y
bleaching. Initial brightness 55.5% ISO, peroxide bleaching
with silicate stabilization, destruction of the peroxide excess
with bisulfite, constant input of 1% dithionite in Y.
1133