Pulping and Bleaching PSE 476 Lecture #2 Mechanical Pulping

Agenda Mechanical pulping Grinding-logs Refining-chips Mechanical pulp properties Process conditions (chemicals, steam, etc.) Canadian Standard Freeness Test

Mechanical Pulping Mechanical pulping is the process by which fibers are produced through mechanical methods: Grinding-Stone Groundwood (SGW) Logs (very occasionally chips) are pressed into a turning stone thus releasing fibers. Refining-Refiner Pulp Chips are fed between 2 disks. One disk is always turning while the other can be fixed or turning. Often heat or a chemical pretreatment is utilized.

Mechanical Pulping General Info Raw Material Quality Pulps cannot be brightened very much; therefore good quality material must be used Chips used should be less than 2 weeks old (oxidation and biological decay darken chips after this point). Low bark and dirt tolerance (color and machinery wear issues). Species Dependence Different wood species work better in different processes.

Properties of Mechanical Pulp Relatively cheap High yield of product (85-95%) Low capital costs relative to Kraft mill High opacity product Large amount of fines in product scatter light Allows printing on both sides of thin sheet Good printing surface Broad fiber size distribution gives smooth surface Good bulk

Properties of Mechanical Pulp Relatively weak product Not strong enough to get through printing press Need to add chemical fibers to product Can use recycle fibers Limited brightness Bleached with lignin retaining bleaching agents Photoyellowing Formation of chromophores from lignin compounds through the reaction with light and oxygen

Stone Groundwood: Equipment Not used at all in the Pacific Northwest—all our wood leftovers from saw mills as chips. Different designs. Logs are pressed into a rotating stone. The fibers are washed off the stone with water (cools the stone). Running this system under a slight pressure improves the process.

Mechanical Pulping Overview Stone Groundwood: Equipment The surface of the stones are cut with patterns (burrs) using a metal burr. Patterns are 1.6 mm deep. The stones must be sharpened every 50-150 hours.

Pulpstone “sharpening” Sharpening= fracturing the softer bond posts. This causes the worn grains to be removed, uncovering new sharp abrasive grains. Why sharpen? To expose fresh abrasive grit. To control the compression / decompression frequency on the wood fibres. To control fibre length. To clean stone pores. To carry water into the grinding zone and pulp out of the grinding zone. A pulpstone, manufactured from either Ceramic bonded abrasive or Cement bonded abrasive, consists of abrasive grains held together by a bonding material. Pulpstone "sharpening" is actually a misnomer, it does not sharpen the individual abrasive grains as such since these grains are much harder than the sharpening burr. Pulpstone sharpening is, in effect crush dressing and is accomplished by fracturing the softer bond material called bond posts, which surrounds the abrasive grains. This causes the worn grains to be removed, uncovering new sharp abrasive grains.

Mechanical Pulping Overview Stone Groundwood: Mechanism In the grinding process the lands and grooves of the stone, as well as the individual grits, pass rapidly over the wood surface. Each time a land passes over the wood fibre it compresses the surface of the wood and each time a groove passes over the wood fibre it relaxes the surface of the wood thereby causing localized heating of the wood. This heat softens the lignin, which binds the wood fibres together. When the lignin has been adequately softened, the fibres will separate from the surface of the wood. A pattern having a narrow land area will provide increased unit or specific grinding pressure. This higher pressure increases the deforming pressure on the wood surface causing increased localized heating of the wood. Increased softening of lignin will cause longer length fibres and fibre bundles to be separated from the wood. This results in a pulp of higher freeness. This is a poor reproduction of a drawing showing the action of the stone on the surface of the wood. Source: Handbook of Pulping and Papermaking , Biermann, C, 1996, page 65

Mechanical Pulping Overview Stone Groundwood: Mechanism The burrs on the stone alternately compress and decompress the fibers. This loosens the fibers. Additionally, the heat developed softens the lignin in the middle lamella which helps the process. The application of heat or chemical will also soften the lignin and improve this process.

Mechanical Pulping Groundwood Flowsheet Logs Drum Washer There is a latency tank before bleaching which serves to “uncurl” the fibers. Grinder Bull Screen Refiner Refiner Fine Screen Press Centrifugal Cleaner Screen Thickener Bleaching Storage Paper Machine

Refiner Pulping Refiner Mechanical Pulping (RMP) Thermo-mechanical Pulping (TMP) Chemi-mechanical Pulping (CMP) Chemi-thermomechanical Pulping (CTMP) The differences between these systems are discussed in the notes section and on the next slide. Refiner Mechanical Pulping: Chips are fed through the refiner with no treatments. Thermo-mechanical Pulping: Chips are heated to 110-140°C prior to the first refining stage. The increase in temperature softens the lignin thus making it easier to remove fibers. Chemi-mechanical Pulping: There are a variety of CMP processes. The commonality of these processes is that a chemical treatment is applied to the chips prior to refining. The purpose of these treatments is to soften the lignin. Chemi-thermomenchanical Pulping: Is a CMP process which is similar to TMP. The chips are treated with a relatively small charge of sodium sulfite (or NaOH/hydrogen peroxide) prior to refining. Small laboratory refiner Notes

Mechanical Pulping Process Conditions

Refiner Plates The refiners contain opposing plates that shred the chips. These plate are separated into different sections 1st section (inlet) rips chips into wood slivers. Each subsequent step reduces the size of the bundles. Water carries the fibers through the disk.

Refiner Plates (2) Directional Variable pitch Chicane 

Refiner Pulping Typical Refiner Flowsheet Chip Screen Rechipper Chip Washer Oversize Fines First Stage Refiner Second Stage Screens Thickener Reject Centrifugal Cleaners Thickener Bleaching Storage Paper Machine Rejects

Refiner Pulping CMP (CTMP) Process Chemicals Hot Sulfite (NaOH + Na2SO3) NaOH swells wood for easier refining. Sulfite prevents the wood from darkening upon this treatment. Cold Soda (NaOH) NaOH swells the wood. Some hemicelluloses and all resins lost. Softwoods resist this treatment so used only for hardwoods; a very very minor process.

Refiner Pulping CMP Process Chemicals Alkaline Peroxide (NaOH + H2O2) Pulping and bleaching together. Difficult with Softwoods. Need to stabilize peroxides against: Thermal degradation and degradation by metals Silicates used to stabilize peroxides can deposit on refining equipment. CTMP (2-5% Na2SO3) Most common of the chemical processes. The lignin is sulfonated to only a very slight extent. This softens the lignin making fiber removal easier.

Mechanical Pulping Determination of Endpoint In chemical pulping, the pulping reaction is allowed to go until a certain level of lignin is reached as determined through the determination of Kappa number. In mechanical pulping, the material is refined until a specific freeness is reached. Freeness is the ease with which water leaves the fiber mat formed on a wire mesh from a dilute slurry. Here the drainage time of pulp is discussed in reference to market pulp and/or unrefined pulp. The drainage time of pulp or freeness or slowness of pulp is modified to have some desired properties in the paper, here that is not discussed.   Drainage of unrefined pulp which is measured as freeness can give an indication on : 1) Fiber Length of pulp, as long fiber pulps have more freeness compared to short fiber pulps, 2) Damage to fiber during pulping, bleaching or drying as short fibers or fines produced during pulping operation, reduces pulp freeness, 3) Refining energy required to achieve certain slowness during stock preparation.  

Mechanical Pulping Overview Endpoint Determination Canadian Standard Freeness Measurement of ease at which water leaves a fiber mat Simple, quick, reproducible but not always meaningful High Freeness 750 ml water: Unrefined -chemical pulp Low Freeness 20 ml water: Heavily refined - mechanical pulp