- •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
1124 6 Bleaching of Mechanical Pulp
with sulfur dioxide generates zinc dithionite which is reacted further with caustic
soda:
Zn + 2SO2 → ZnS2O4
ZnS2O4 + 2NaOH → Zn(OH)2 + NaS2O4
Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,
after concentration of the liquor by evaporation, sodium dithionite is precipitated
by the addition of sodium chloride. The salt, which is dried before shipment, contains
up to 300 p.p.m. zinc.
The dominant process is the reaction of sodium formate with sulfur dioxide
and caustic soda under pressure. The reaction is described by:
HCOONa + 2SO2 + NaOH → NaS2O4 + CO2 + H2O
Commercial grades of sodium dithionite powder typically have a content of
about 88% sodium dithionite, with the sodium salts of bisulfite, sulfite, sulfate
and carbonate as the byproducts. Solutions of dithionite in water must be prepared
on-site. In pulp bleaching, the use of returnable steel containers with up to
2000 Kg of technical-grade product is common. Typically, a small amount of a chelant
(e.g., ethylene diamine tetra-acetic acid; EDTA) is added. This avoids scaling
(precipitation of calcium carbonate) caused by the hardness of the dilution water.
An analysis of the stability of dithionite solutions in the presence of iron and manganese
[33] contradicted the speculation about the metal ion (Fe, Mn) -induced
decomposition of dithionite [34]. The fine powder reacts exothermically with atmospheric
oxygen; the heat of oxidation can lead to ignition. Sodium dithionite is
therefore classified as “spontaneously combustible goods”, and the corresponding
transport and storage regulations must be applied.
Alternatively, some mills operate an on-site process using a solution of sodium
borohydride (~12% by weight) with caustic soda (~40%). This mixture is reacted
with sulfur dioxide to yield dithionite solutions:
NaBH4 + 8NaOH + 8SO2 → 4Na2S2O4 + NaBO2 + 6H2O
Some pulp mills are supplied with an alkaline solution (pH > 12) of sodium
dithionite. Transportation requires cooling to a temperature below 10 °C to maintain
the content at ca. 11–12% dithionite. Cooling of the storage tank is not required
when there is a rapid turnover of the product. Exclusion of air is required
to avoid product losses, as dithionite reacts rapidly with oxygen in the air to yield
sulfite. In the absence of air, decomposition reactions take place, with one of the
products being thiosulfate; this, in turn, may accelerate corrosion reactions.
Bleaching with dithionite typically is conducted at moderately acidic pH, between
pH 4.5 and 6.5. The temperature in tower or tube bleaching is maintained
at 60–80 °C, although a higher temperature produces a faster response to the
chemical addition. The reduction of chromophores occurs very quickly; so the
6.1 Bleaching with Dithionite 1125
reaction time required is short – 15–60 min is sufficient. The amounts of dithionite
Vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available
chromophores for the reduction process is limited, a brightness plateau is typically
reached at an input of between 1.2% and 1.5%. An addition above these levels
triggers decomposition and the formation of thiosulfate. A technological alternative
to tower bleaching is the application of dithionite in the refiner. Although the
brightness gains are slightly inferior, the application is simple and can eliminate
the need for a bleach plant.
As can be imagined, the reduction reactions may be reversible. For example, the
reduction of an o-quinone to the catechol generates the leuco form, which can be
re-oxidized by oxygen in the air. Despite this, the brightness of dithionite-bleached
pulp is relatively stable in heat-induced aging. Stability is affected by light, UV radiation
or transition metal-induced reactions of the phenolic groups within the
lignin, easily generating chromophores [35].
6.2
Bleaching with Hydrogen Peroxide
The application of hydrogen peroxide in bleaching mechanical pulp is by far older
than its use in chemical pulp bleaching. A prerequisite of peroxide bleaching is
the “neutralization” of peroxide-decomposing transition metals (e.g., Mn). Chelation
of the metal ions is achieved with compounds such as diethylene diamine
penta-acetic acid (DTPA) or ethylene diamine tetra-acetic acid (EDTA) (see Section
II-5.4.2.4.5.1, Prerequisites of hydrogen peroxide application). The chelants are
added most beneficially during screening after the refining process. This allows
sufficient residence time and typically has the correct pH level (slightly acidic) for
effective chelation. The demand for chelant is between 0.1% and 0.3% on fiber (of
the commercial product with an active content of ~40%). The addition of chelant,
together with the other chemicals (H2O2, NaOH, silicate) is another option,
though it is slightly less effective because of the higher pH.
The limited aggressivity of hydrogen peroxide might be a disadvantage where
delignification is required, but it may be advantageous if an improvement of
brightness is the only target. In the past therefore, hydrogen peroxide bleaching
was often labeled as being “lignin-conserving”. As described in previous chapters,
this it not the case, and lignin is in fact removed during peroxide bleaching. Lignin
side chains are cleaved and quinones oxidized to more water-soluble carboxylic
acids. However, because hydrogen peroxide will not react easily with the
aromatic systems of lignin, its level of removal is moderate. In mechanical pulp
bleaching, this is an advantage, and the yield and optical properties (opacity) are
only moderately affected by the bleaching process. An example of the response of
mechanical pulp to hydrogen peroxide is shown in Fig. 6.1.
Peroxide addition yields increasing brightness with charges up to about 5%. The
plateau is reached at about 82% ISO for softwood pulp, spruce (Picea spp.) and pine
(Pinus spp.), and 86% ISO for hardwood mechanical pulp, poplar and aspen (Populus