Mechanical Pulping

Objective: obtain a general understanding of the process of mechanical defiberizing of woody biomass for the formation of pulp products.

Important Concepts
Groundwood processes
Refiner-based processes
Canadian Standard Freeness
Mechanical Pulping Overview---Before 1960, all mechanical pulps were prepared by the stone groundwood (SGW) process. This involves the "brute force" approach of forcing a log (bolt) against a spinning stone. The mechanical action breaks apart the fibers which can be used to make things such as newsprint. The use of refiners has slowly replaced SGW processes, and in the manufacture of refiner mechanical pulps (RMP), chips are passed into a piece of equipment which has either one or two spinning plates (see image above). These plates break up the chips into a pulp to prepare a pulp similar to an SGW pulp. By 1990, more than half of all mechanical pulps were prepared by by this method. Refiners allow more control over the pulping process providing a stronger pulp, avoids the need for log storage and allows the use of sawmill residue.
Canadian Standard Freeness
One of the key measurements used by pulp mills is Canadian Standard Freeness (CSF). This is a crude measure of pulp particle size and the ability of a pulp to drain water. This is a key issue for the manufacture of a sheet of paper, where water drainage is crucial for obtaining good sheet formation and a consistent product. By definition the Canadian Standard Freeness (CSF) is the number of milliliters of water collected from the side orifice of the standard freeness tester when a pulp suspension drains through the screen plate at 0.30% consistency and 20 °C.
A pulp that drains rapidly is considered to be "free." A pulp that drains slowly is said to be "slow." More water will come through the orifice with a free pulp and this number rarely gets above 700. The lower the freeness the slower the drainage characteristics. Low range CSF values are between 50 - 100. A class handout shows the freeness tester (there is one in the Brooks pilot plant of you want to see a real one).
Mechanical Pulping
Stone groundwood (SGW) and Pressurized groundwood (PGW).
Refiner Mechanical pulp (RMP).
Thermomechanical Pulp (TMP).
Mechanical pulps are typically prepared to a specified CSF.
 
A. Stone Groundwood (SGW).
1. Several grinders typically feed one paper machine.
2. Grinder: a round concrete slab with "stones" bolted on. The stones have a gritty material as well as an overall design.
3. The abrasive action causes the stones to wear down and occasional "sharpening" is required.
4. Pressure of the wood on the stone causes heat. The amount of heating is controlled by water showers. The heating causes the lignin of the middle lamella to loosen somewhat allowing the fibers to break apart. Screening is required (shives).
5. Desired wood is spruce (most mills in Canada and Northeast U.S.).
6. High yield, poor strength, good printing properties.
7. Newsprint, book, tissue, and board grades are produced.
 
B. Pressure Groundwood (PGW).
1. Newer process-first commercial system installed in 1980.
2. Differs from SGW in that it uses 30 psi pressure during grinding (air). Shower temperatures maintained at 95 °C.
3. Higher operating temperatures allows for better lignin softening for less tearing and more loosening.
4. Higher strength properties than SGW.
5. Closed system allows for addition of chemicals (chemigroundwood). Alkaline peroxide for a whiter pulp (1990).
 
C. Refiner Mechanical Pulp.
1. Production initiated in 1960.
2. Mechanical reduction of chips and/or sawdust in a disc refiner.
3. Process is typically two-stages in series.
4. Longer fibers than GW, are also freer and bulkier, but somewhat darker. Less tearing than either GW processes; the mechanism is considered more of an unravelling.
5. Plate clearance is critical. Chips are fed in at the center and move radially outward, breaking up by passage through smaller and smaller plate bars.
6. Can add heat and/or chemicals (TMP and CTMP processes).
 
D. Thermomechanical Pulping (TMP).
1. First major modification of the RMP process.
2. Employed for making high-tear newsprint and board grades.
3. Steaming prior and during refining softens the chip providing long fibers and less shives.
4. Steam pretreatment should not be too long - darkening occurs.
5. Refining must be done below 140 °C or else lignin becomes too soft and forms a glass on the fiber upon cooling. Typical temperature range is 120-130 °C.
6. Strongest of all mechanical pulps.
 
E. Chemithermomechanical Pulping (CTMP).
1. Introduction of chemicals prior and/or during refining.
2. Saves energy and also provides a pulp of different qualities (typically "softer).
3. Usual treatments are with sodium sulfite - sulfonation occurs to the lignin which is a "permanent" softening as compared to the temporary softening of TMP pulps.
4. The most common approach is chemical pretreatment during steaming in combination with refining and subsequent alkaline peroxide bleaching. This is the most cost-effective way to produce high brightness pulps (however they are not permanently bright).
5. Chemical sulfite treatments range from 1-5% of the weight of the wood.
a. Low levels: can be used as the sole furnish for newsprint.
b. Medium levels: middle layer of multi-ply boards (adds bulk and rigidity at a lower cost than kraft).
c. High levels: tissue and fluff pulp.
6. Fastest growing segment of the pulping industry.
 
F. Chemimechanical Pulping (CMP).
1. Relatively "severe" chemical treatment followed by atmospheric refining.
2. Yields in 85-90% range.
3. Good bonding and strength properties but ill-suited for printing (poor opacity). Typically a reinforcement pulp for newsprint.
 
G. Chip Impregnation for CTMP/CMP.
1. Chips are usually compressed and then allowed to expand in aqueous solution of impregnation liquor-air is objectionable.
2. Treatment of partially refined pulps provides a larger surface area for sulfonation. This improves the printing properties.
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