- •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
1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
The screening efficiency depends on the following influences [23]:
_ Parameters of the material to be screened (inlet pulp).
– Type of material (mechanical pulp, chemical pulp, recycled pulp)
– Shape of the particles to be separated (cube-like, flake-like,
stretched length directionally)
– Particle size distribution
– Flexibility of the particles
– Drainage resistance and consistency of the inlet slurry
_ Parameters of the screening elements
– Shape of the element (mesh, hole, slit)
– Size of the element (mesh opening, hole diameter, slit width)
– Position and distance of the elements
– Portion of open screening surface
_ Process or design parameters
– Evenness of flow
– Evenness of composition and consistency
– Type of inlet feeding to the screening elements
– Geometry of feeding stream contact with the screening surface
– Speed and length of feeding stream over the screening element
– Transportation force of accept through the screening elements
– Slurry height over the screening surface and specific feed to the
screening surface
– Mechanical measures to prevent plugging of the screening elements
surfing mater
The coarse rejects in the grinding process are separated in the coarse screening
using vibration screens (Fig. 5.3) with holes of 6-mm diameter. The flow rate is
between 40 and 50 t day–1 at a feeding consistency of 1% and a screen area of
1.5 m2.
Fig. 5.3 Bull screen (vibration screen). 1,Groundwood feed
from grinder pit; 2, screen plate; 3,shower water; 4,coarse
reject; 5,accept.
5.2Machines and Aggregates for Screening and Cleaning 1115
The coarse reject is further treated in shredders. In pressure grinding, this
shredder treatment is applied to the whole pulp before pressure relaxation.
In refiner mechanical pulping, there is virtually no such coarse material in the
pulps (TMP, CTMP, and CMP) any longer. These pulps contain more mini-shives,
chops and long fibers than can be separated in fine screening steps, and are
further treated by reject refining.
Pressure screens are the most common equipment in screening and fractionation
of mechanical pulp (Fig. 5.4, left). The pressure screen consists of a cylindrical
screen basket as a screening element, and a concentric positioned rotor to
keep the screen openings unplugged (see also Fig. 5.5). The pulp suspension is
fed from above, either axially or tangentially, with the flow operating either centrifugally,
centripetally, or as a combination of the two. Either the rotor or the screen
basket can rotate, but today the major application is centrifugal flow with a rotor.
Single-stage screens operate with low-consistency or medium-consistency pulps,
whereas in multistage screens several stages are run in one screening apparatus
(Fig. 5.4, right). In future, these screens will increasingly replace existing singlestage
screens. The shives and stiff long fibers are removed as rejects from below,
while the filtrate moves easily through the screen openings, causing a thickening of
the reject flow. Any pad build-up will be repeatedly destroyed by the rotor wings.
Dilution
Feed
Coarse reject
Reject
Accept P2
Accept S2
Accept T2
Pre-screen
stage
3 fine
screening
stages
Fig. 5.4 Left: Single-stage pressure screen (Metso TAP) [24].
Right: Multistage pressure screen (Metso MuST) [25].
The capacity and runnability of a screen, and also the screening efficiency to some
extent, can be controlled bymeans of pulsation. Today, several designs of foil-type pulsation
element are available. The aimis to obtain adequate pulsation by adjusting the
number of foils and their shape, width, clearance, incident angle, and tip speed. A
small difference between the peaks of positive and negative pulses is advantageous
for screening efficiency, but the negative pulses should be sufficiently strong to enable
the suction flow to remix flocks, fibers and contaminants resting on the edge of the
screen opening [26]. The general construction of rotors is shown in Fig. 5.5, with
a cross-sectional view on different pulsation elements of rotors in Fig. 5.6.