- •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
1. Reduction of the chips sizes to units of matches.
2. Reduction of those “matches” to fibers.
_ Stress by pressure pulsation and shearing of the fiber –
fatigue of the middle lamella
_ Thermal softening of lignin and hemicellulose
_ Rolling effect (= agglomeration of deliberated fibers)
_ Fiber cutting
3. Fibrillation of the deliberated fibers and fiber bundles.
Chemimechanical pulping involves a gentle chemical treatment stage combined
with mechanical defibration such as disc refining. The yields of these pulps are
generally in the range of 80–95%, and their properties are intermediate between
those of high-yield chemical pulps and mechanical pulps. Chemithermomechanical
pulp (CTMP) is produced with pressurized refining. Relatively low chemical
doses are applied, and the yield is typically above 90%. Chemimechanical pulp
(CMP) can be produced with refining at atmospheric pressure; the chemical treatment
stage is more severe than in the CTMP process, and the yield is typically
below 90%. CMP is also the general name for all chemimechanically produced
pulps. CTMP and CMP have been developed for the better use of hardwood and
the improvement of the bonding ability of the stiff long TMP-fibers, with the first
pulping lines beginning operation during the 1950s and 1960s for hardwood applications.
The breakthrough in chemimechanical pulping occurred during the
1970S as result of the improved tmp technology. Because the key subprocess in
chemimechanical pulping is refining, all developments of the TMP process could
also be utilized for CMP production. This caused a rapid growth in the production
of softwood CTMPs during the late 1970s and 1980s.
The difference between chemithermomechanical and chemimechanical pulping
relates mainly to the process conditions utilized (see Tab. 4.2), and is apparent
mainly in terms of the intensity of chemical treatment and pulp yield.
1101
4 Mechanical Pulping Processes
Tab. 4.2 Possible defibration conditions for the production of chemithermomechanical
pulp (CTMP) and chemimechanical pulp (CMP).
Wood type Predamping
[min]
Impregnation Preheating Cooking Yield
[%]
CTMP Softwood 10 1–5% Na2SO3 2–5 min
120–135 °C
91–96
Hardwood 10 1–3% Na2SO3
1–7% NaOH
0–5 min
60–120 °C
88–95
CMP Softwood 10 12–20% Na2SO3 10–60 min
140–175 °C
87–91
Hardwood 10 10–15% Na2SO3 10–60 min
130–160 °C
80–88
Chemimechanical pulps can be produced, in principle, by a variety of combinations
of chemical treatments and mechanical defibration. In practical operation,
sodium sulfite is the dominating chemical in softwood pulping, while sodium hydroxide
and/or sodium sulfite are the common chemicals in hardwood pulping.
Sulfonation opens the wood structure and enables the access of water to the
fiber. Atack and Heitner [20] described this procedure (see Fig. 4.25), and assumed
that the softening of lignin could be led back to an exchange of the aliphatic hydroxyl
groups or ether groups participating in hydrogen bonding between the lignin
molecule chains, by solvated groups that cannot take over any bond between
the molecule chains.
HC
CH
CH2OH
R'O
OR
OH
OMe
HC
CH
CH2OH
R'O
SO3
OH
OMe
+ HSO3
ROH
Fig. 4.25 Exchange of hydroxyl groups by sulfonate groups
(according to Atack et al. [21]).
Atack and Heitner [20] also assumed that the hydrophilicity of wood is caused
by an exchange of the aliphatic hydroxyl or ether groups – acting between the
chains in hydrogen bonding – by solvated groups that cannot build bonding between
the chains, and this results in a softening of lignin. This permanent lignin
softening affects the following defibration in two important ways:
1102
4.2 Refiner Processes
_ The addition of sulfonate groups up to about 1.2% on o.d. wood
leads to an almost complete fiber separation and to diminution of
the disturbing influence of stiff long fibers caused by softening of
the middle lamella [20].
_ The addition of a larger quantity of sulfonate groups (>1.2% up to
2.0% on o.d. wood) makes the stiff long fibers more flexible and
deformable, and this increases the bonding ability, caused by softening
of the fiber walls [20].
In practical CTMP and CMP operations, the chemical treatment of wood is carried
out as pretreatment by the impregnation of wood chips. Besides refining, this
impregnation is one of the most important process steps in chemimechanical
pulping. Generally, impregnation is defined as penetration of the chemicals into
the microstructure of the wood. The physical processes of penetration and diffusion
ensure the distribution of a maximum amount of chemical in a minimum of
time evenly within the wood structure.
A practical solution to this procedure is to dampen and compress the chips on
their way to the impregnator in the feeding screw, and then to expand them in the
cold impregnation liquid. This expansion and condensation effect causes the
chemical to soak into the wood structure. Depending on the chemical concentration
and impregnation time, a chemical consumption of 2% up to 20% on o.d.
wood can be achieved. This means also an increase in wood moisture of 5% to