Pulping and Bleaching PSE 476 Lecture #1 Introduction PSE 476: Lecture #1

Introduction to Pulping: History of Papermaking 3000 BC: Papyrus Exterior of reed is laminated and pressed to form sheet Developed by Egyptians Word paper derived from papyrus Making of papyrus http://www.lib.umich.edu/pap/exhibits/papyrus_making/slides.html PSE 476: Lecture #1

Papyrus Making-Harvesting In ancient times, the entire plant was pulled from the root at harvest time. It is unknown at what time of year the ancient Egyptians harvested papyrus, or whether mature papyrus was preferred over young papyrus. PSE 476: Lecture #1

Papyrus Making-Peeling In ancient times, the tough outer layer would have been kept for other uses. Strips of this layer could be woven together to form all manner of useful items, such as baskets or sandals. However, only the inner part of the reed is used to make the writing material. PSE 476: Lecture #1

Papyrus Making-Cutting Once the outer layer is removed, the inner part of the reed is cut into strips. No one is completely sure what method was used in ancient times. Rather than cutting the reed, as shown above, some have suggested that the triangular stalk was peeled into strips. PSE 476: Lecture #1

Papyrus Making-Soaking Soaking the papyrus strips is important for softening the papyrus and activating the plant's natural juices, which act as a glue to hold the strips together. In ancient times, it was thought that the mystical Nile waters were essential to the papyrus-making process, but any water will do. PSE 476: Lecture #1

Papyrus Making-Rolling After they have soaked for a few days in water, a wooden rolling pin is used to drive out the water and flatten the papyrus strips. The strips of flattened, soaked papyrus are laid out in two layers perpendicular to each other. This technique is absolutely essential to papyrus making, and is what gives papyrus its characteristic look and feel. Here, each strip overlaps the next by 1/16 inch. PSE 476: Lecture #1

Papyrus Making-Pressing, Drying When the strips have all been laid out, they are covered with a sheet of linen and felt, and then sandwiched between two boards in a press. The sheet will remain in the press for a few days until it is dry. PSE 476: Lecture #1

Papyrus Making-Final Product Initially, the surface of the papyrus is somewhat rough. It may be burnished slightly with a stone, and then it is ready to receive writing.  PSE 476: Lecture #1

Introduction to Pulping: History of Papermaking 200 BC: Parchment Tanned skin of animals Today parchment refers to high quality paper from vegetable fibers 105 AD: Paper from plant (mulberry) fibers Developed by Chinese Emperor by Ts'ai Lun National secret stolen by Arabs in ~700 AD 1796: First continuous paper machine developed near Paris. Fiber source - rags (cotton) AD 105 is often cited as the year in which papermaking was invented. In that year, historical records show that the invention of paper was reported to the, an official of the Imperial Court. Early Chinese paper appears to have been made by from a suspension of hemp waste in water, washed, soaked, and beaten to a pulp with a wooden mallet. A paper mold, probably a sieve of coarsely woven cloth stretched in a four-sided bamboo frame, was used to dip up the fiber slurry from the vat and hold it for drying. Eventually, tree bark, bamboo, and other plant fibers were used in addition to hemp. The first real advance in papermaking came with the development of a smooth material for the mold covering, which made it possible for the papermaker to free the newly formed sheet and reuse the mold immediately. This covering was made from thin strips of rounded bamboo stitched or laced together with silk, flax, or animal hairs. Other Chinese improvements in papermaking include the use of starch as a sizing material and the use of a yellow dye which doubled as an insect repellent for manuscript paper. PSE 476: Lecture #1

Introduction to Pulping: History of Papermaking 1854: Soda pulping process developed in England 1840: Groundwood pulping developed in Germany 1867: Sulfite (acid) pulping process developed in US 1884: Kraft pulping process developed in Germany PSE 476: Lecture #1

Introduction to Pulping: Two Ways to Generate Fibers Mechanically: “Grind up” raw material (most of the lignin retained). Newsprint Chemically: Dissolve away the lignin Kraft pulping: NaOH/NaSH (dominant process) Sulfite: (Sulfur Dioxide/bisulfite/sulfite) (limited number of mills) Other: Organosolv, steam explosion, etc (very minor) In this class, we are going to discuss the methods and chemistry used in the processes which convert fibrous materials to bleached fibers. PSE 476: Lecture #1

Raw Material Fibrous material can come in the form of: Wood Wood (main form in the United States) Other plant material (straw, reeds, etc) Wood Softwoods & hardwoods Logs (chipped directly from logs for pulp production) Chips (residuals from saw mill operations) PSE 476: Lecture #1

Regional Affects on Raw Material For this lecture, we will be looking at raw material use by these regions. PSE 476: Lecture #1

Raw Material Log/Chip Makeup Roundwood: wood high quality, that we can use as to make a lumber Residual: fibre logs (not good enough to make a lumber, not high quality, might have too many branches, rot, tree partially burned, split trees, for pulp and paper they use 85% of fibre logs. Residual staff after harvesting (big branches) PSE 476: Lecture #1

Raw Material Log/Chip Makeup PSE 476: Lecture #1

Raw Material Handling Physical Measurements Physical measurements important for: Determining how much wood is coming into the mill. How much wood is being charged into the digesters. How much actual wood mass (dry) is being charged so to get the correct liquor to wood ratio. Mills use scales and other devices Moisture content meters Laboratory chip screening Wood species determination PSE 476: Lecture #1

Raw Material Handling Physical Measurements Moisture Content - Green (paper industry) Wet basis, amount of water in wood as a fraction of wet weight of wood Typical MCgr of freshly cut wood = 50% (30-60%) Mass of water in wood MC gr = x 100% Wet wood mass Moisture Content - Oven Dry (wood scientists/foresters) Oven dry basis Typical MCOD of freshly cut wood = 100% (45-150%) MCGR amount of water in wood as a fraction of wet weight of wood Mass of water in wood MC OD = x 100% Oven dry wood mass PSE 476: Lecture #1

Raw Material Handling Physical Measurements Solid Wood Density Dry weight of wood Density = Unit volume of green wood Wood contracts 8-15% on volume basis when it dries below 30% moisture. This needs to be taken into account when determining density. Typical units lb/ft3 or kg/m3 PSE 476: Lecture #1

Raw Material Handling Debarking and Chipping There are many different mechanical systems used to debark and chip wood. We will not cover these in this class. There are plenty of references available for you to read. Important points: Get all of the bark of the log. Bark typically used as fuel source. Chip to a very consistent size. PSE 476: Lecture #1

Raw Material Handling Chip Dimensions Uniform Chip Size is very important! Large chips undercook leaving shives (rejects). Small chips clog liquor circulation, use large amount of chemicals, and give a low yield of weak pulp. Chip thickness the primary concern. TOP 1/2 to 1” long Variable Width Side 1/8 to 1/4” thick PSE 476: Lecture #1

Raw Material Handling Wood Deterioration Wood decay requires moisture and oxygen. Moisture content > 20% MCOD. Remove either and slow/stop degradation. Sprinkle solid wood (logs) with water to keep the wood saturated and therefore limit oxygen content. Drying wood to <20% MCOD would slow degradation. Economically unsound. Would slow liquor penetration and therefore pulping. PSE 476: Lecture #1

Raw Material Handling Chip Pile Degradation Conditions that accelerate degradation. Tall chip piles Chip pile compaction Whole tree chips Storage of hardwoods (high starch contents). Method to reduce degradation. FIFO (first in first out) PSE 476: Lecture #1

Raw Material Handling Deterioration in Wood Chip Piles General rule of thumb: 1% loss to decay/month. Respiration of parechyma cells responsible for heat generation. Above 45-55°C, fungal and bacterial degradation stop. Chemical autoxidation takes over above 55°C. This results in severe losses during pulping. Loss of extractives high during storage. PSE 476: Lecture #1

Raw Material Handling Debris Debris is not a good thing! Sources Bark, foliage, plastic, metals, dirt, decayed wood Problems Dirt specks, loss of strength, structural imperfections, wear on equipment. PSE 476: Lecture #1

Raw Material Handling Screening Most mills use a screening system to achieve relative consistency in chip size. Overs: Chips which are oversized or over thick. Accepts: Chips that are in the correct size distribution. Fines: Chips that are too small (includes sawdust). PSE 476: Lecture #1