Kraft Pulping Process

Logs harvested from our Forest Management Agreement (FMA) forest and other partner suppliers are delivered by truck to the Al-Pac Millsite located 50 km northeast of the Town of Athabasca. The loads are weighed then distributed by crane to our wood storage pile. The storage is over 1.5 km long and holds approximately 1.2 million tonnes – about six months of production. Our cranes feed logs onto conveyor belts to the Woodroom for debarking and chipping. We also receive about 500,000 tonnes of fibre in chip form from local sawmills for our pulp production.

The chips, the size of a large potato chip, are distributed into one of five piles. Piles of bark and other small pieces of fibre are called hog fuel. The hog fuel is burned, generating steam that powers machinery at the Millsite and runs turbines to generate ‘green’ electricity. The sorted wood chips are used in the kraft pulping process. These selected chips are washed then sent to the digester.

At this stage, the wood chips are approximately 50% moisture, and of the remaining solid portion, approximately 40% is cellulose fibre. The fibre is what we use to make our kraft pulp. Most of the remainder of the solids are hemi-celluloses, and a complex organic compound called lignin; it is like a glue that holds the wood fibres together. To access the fibres for our pulp, we remove the lignin. The separation of the material happens in the digesters. The digesters are similar to a pressure cooker – a massive 20-storey one! Our digester uses an optimized process that subjects our pulp to a long, gentle cook, which results in a higher degree of separation from the lignin.

The wood chips are placed in the digester and heated with steam to allow a cooking chemical called white liquor, which dissolves the lignin and breaks the wood chips down. The pulp is then washed and moved to oxygen delignification. The residual lignin left in the pulp after cooking is removed using an oxygen and alkaline solution. This step breaks down the lignin and removes impurities, such as resin.  It also helps reduce the amount of effluent our bleach plant produces. The pulp is washed again and then enters the purification process.

The bleaching or purification process removes any remaining lignin to ensure the highest quality pulp. Unlike a conventional kraft pulping process, Al-Pac was the first mill in the world to eliminate elemental chlorine in its bleaching. We use chlorine dioxide along with other compounds in a four-stage process. Chlorine dioxide is very different from widely known chlorine gas. It has very high water solubility and is a neutral compound. The pulp now looks like white cotton and is ready for the finishing system.

In the finishing system, water is removed from the pulp. The pulp mixture is placed on screens where water is drained off by gravity then suction. The damp sheets pass between rollers where more water drains. Steam-heated dryers further remove water through evaporation. The sheets are chopped into 32″ x 32″ squares, compressed into bales, wrapped, bound and stamped for shipment.

The onsite warehouse holds packaged bales of pulp. A rail line comes directly into our warehouse; bales are loaded for domestic and international transport into about 21-24 boxcars a day or about 7400-7600 railcars loads per year. We also ship some kraft pulp by the truckload.

Pulping Process Waste?

Learn more about Chemical Recovery, Water Treatment and Other Practices we do to Minimize our Environmental Footprint.