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Anatomy of a Modern Paper Mill

Bowater Thunder Bay
Bowater Pulp & Paper Canada Inc. - Thunder Bay Mill

Kraft Products for a World of Uses!

Although you might not know it, it's quite possible that many of the items you take for granted in your daily home or office life are made using kraft pulp produced by Bowater Thunder Bay.

Kraft operationThe list starts with all kinds of paper products, such as writing, copying and printing papers, packaging and boxes for consumer products, and high-gloss papers and coated cardboards. But the number of potential uses is astronomical, ranging from facial tissue to masking tapes and photographic papers. The list is limited only by human imagination and is thus expanding daily.

There are two kraft mills at Bowater Thunder Bay. The "A" mill is designed to produce softwood pulp, while "B" mill can produce either softwood or hardwood pulp.

Between them, the two mills produce about 245,000 air-dried metric tonnes (ADMt) of softwood pulp and 240,000 ADMt of hardwood pulp annually. A small percentage of the output is available as furnish for Bowater Thunder Bay's newsprint mill. Most of the balance, representing 90 to 95 percent of the total, is then shipped to convertors (manufacturers who use the pulp as a principal raw material in the production of their finished products) situated throughout North America and abroad. In recent years, for example, Bowater Thunder Bay has annually shipped almost 60,000 ADMt of hardwood pulp to Japan and Korea.

Dumper
Chips arrive by truck and are unloaded by hydraulic dumpers.

Chip pilesBowater's kraft pulp is produced by converting wood chips into pulp through a chemical process. All chips come directly from mobile chipping facilities at the harvest site, or as a by-product from sawmills. Chips are delivered by single or tandem trucks and trailers which are tipped up and unloaded by large hydraulic dumpers. The chips are reclaimed from the huge piles in the mill yard by a massive computerized system of underground and elevated conveyors, augers and hoppers.

The chips are screened to quality and size specifications and transported to the digester, a large pressure vessel, where they are "cooked" (combined with sodium hydroxide and sodium sulphide at high temperature). This process removes some of the lignin (natural "glue" which holds the wood fibres together) to soften the fibres, reduce the discolouration and stabilize the brightness of the pulp. The spent chemicals and lignin, known as black liquor, are recovered through the recovery boiler in order to recover chemicals and generate steam and power, which in turn reduces both effluent and costs. The pulp moves on through a series of washers and screens, in preparation for further processing.

The pulp is then diluted and bleached in a five-stage process. Bowater Thunder Bay is an "Elemental Chlorine Free" (ECF) producer, enabling it to largely eliminate discharges of dioxins, furans and absorbable organic halogens (AOX), all of which are environmental concerns. The pulp is bleached with chlorine dioxide, hydrogen peroxide and oxygen.

The bleached wet pulp, with a brightness of 90 ISO (International Standards Organization) and a consistency similar to wet cotton batting, is ready to be form by the pulp machine.

Here, a moving belt of woven nylon mesh, or "wire", forms the pulp into a thick mat by removing much of the water. Next, a series of presses removes more of the water, until the sheet is about 45 percent dry. Then steam-heated, "air borne" dryers dry the sheet to 94 percent. The sheet, moving at a speed of up to 120 metres per minute, is cooled and cut into smaller sheets of a thick, porous, paper-like card. These sheets are compressed, wrapped in 245-kilogram bales and shipped to customers in 100-tonne lots.

The nearly 600 highly-skilled employees of Bowater's Thunder Bay kraft pulp operation have worked hard to build the high reputation which their products enjoy around the globe. Bowater pulp is noted for its versatility, quality and consistency. Due in part to its great strength characteristics, Bowater Northern Bleached Softwood Kraft is noted as a premium pulp.

Mill Schematic

Bowater Thunder Bay is a producer of both newsprint and kraft pulp.

Newsprint production uses wood fibre obtained from four separate sources, including thermomechanical pulp (TMP), recycled pulp, groundwood pulp and kraft pulp.

Kraft pulp is produced at Bowater's two kraft mills at the Thunder Bay facility. The "A" mill is designed to produce softwood pulp, while "B" mill can produce either softwood or hardwood pulp.

Quality that's Newsworthy!

Bowater Thunder Bay is not only one of the best, it is also one of the biggest of Canadian newsprint mills. Processing wood fibre obtained from four separate sources, including thermomechanical pulp (TMP), recycled pulp, groundwood pulp and kraft pulp, the mill produces more than 540,000 ADMt of newsprint each year. The capability to produce newsprint with a minimum of 25 percent recycled content also makes the mill one of the largest suppliers of recycled content newsprint in North America. About 97 percent of the output is shipped to customers in the United States.

The Bowater goal is to become the preferred supplier to the daily and weekly newspaper and advertising flyer publishing industry across North America. Already the Company counts many of the largest and best known producers of these publications among its customers.

Behind this success lie two main factors. First, the product itself. Bowater newsprint is renowned for its opacity, strength, consistency and resistance to tear, all adding up to exceptional "runnability" and "printability". Second, the commitment of more than 770 individual employees and management to be the best.

This commitment is apparent in the many customer teams, formed from sales, mill and technical representatives, which work closely with customers to achieve superior performance. These teams meet regularly, at both customer plants and the mill, to resolve issues and prevent problems. Mill representatives frequently travel "on call" to customers' sites to help resolve pressroom issues, as well as to report back to production staff at the mill on the performance of their products.

Newsprint Production Processes

Thermomechanical Pulping (TMP) Process

The TMP process converts wood chips into fibers by heat and mechanical forces.

The chips arrive at the mill by truck from local sawmills and mobile chipping sites. Chips are reclaimed from the chip piles as needed, screened to proper size and washed to remove dirt and bark. They are impregnated with water, heated and forced between two circular refining plates, each 1.5 metres in diameter, which "grind" the chips to separate the fibre bundles. These refiners are powered by 30,000-horsepower motors which together draw about 90 megawatts of power (enough electricity to power a city of 80,000 people!). The pulp is then refined in two other stages and screened prior to storage in a latency tank. This improves the flexibility and strength of the fibre before it is pumped to the paper machine blend chest.

Recycle Process

This process redirects magazines, flyers and newspapers that previously went to landfill sites across North America but now go to the paper mill to be repulped, cleaned, de-inked and made into newsprint. The recycled fibres, comprised of 30 percent magazines and 70 percent newsprint, are comparable in strength to those produced using TMP. The yield by weight is about 85 percent of the input.

The used paper is shredded and mixed with water, then repulped and detrashed to remove plastic and other major contaminants. The dense contaminants like staples or fine sand are removed in the high-density cleaners and screens. The pulp is de-inked using soap and air flotation, screened and cleaned. The pulp is washed and pressed, then diluted and stored prior to being pumped to the paper machine blend chest.

Groundwood Process

This process grinds wood fibres from debarked spruce logs. A progression of cleaners and screens removes any remaining bark and contaminants. The pulp is then stored prior to being sent to the blend chest.

The Paper Machines

Total production from the three paper machines is 1,405 ADMt per day from a furnish mix of TMP, recycle and groundwood. Number "5" Paper Machine is one of the most modern in the world and produces nearly half the total production, at a speed of more than 1,300 metres per minute.

Each paper machine has its own blend chest, which combines the different pulps to meet customer specifications. This "furnish" is diluted, screened and pumped through a paper machine headbox onto the forming wire, where 15 percent of the water is removed by natural drainage and vacuum pumps. Another 30 percent is removed by the rolls in the press section.

In the dryer section, heated cylinders evaporate the moisture down to eight percent. The sheet is then calendered, or "ironed", to its final thickness in the calender stack.

The sheet is wound onto huge reels. Each holds a roll of newsprint measuring up to 65 kilometres in length and 30 ADMt in weight. The winder crane removes each reel, and the paper is rewound on a light core, at speeds of 2,400 metres per minute, to the correct width, diameter and length. Rolls meeting customer specifications are then wrapped, labelled, coded and loaded directly into rail cars and trucks for shipping.

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