How We Make Pulp
See the process details.
Wood fibres, water, chemicals and energy are the essential elements for making pulp. And pulp is the key ingredient in all paper products.
Making pulp entails breaking down the wood structure into individual fibres. At Catalyst, we use a kraft chemical or thermomechanical process to make our pulp — depending on the desired characteristics and type of paper we are producing.
We make kraft pulp by cooking wood chips in digesters with a solution of caustic soda and sodium sulphide to separate the wood fibres from the lignin, a glue-like binding substance. The result is a pulp with a distinctive high strength — "kraft" means strong in German. Our kraft pulp adds strength to printing and writing papers, newsprint, specialty papers and tissue products.
We make thermomechanical pulp by heating the chips under pressure to soften them and then grinding the softened chips between serrated metal plates to separate the fibres from the wood structure. This process does not remove lignin from the fibres, which is why mechanical pulps are less expensive to produce than kraft pulps. We use mechanical pulps to produce newsprint and directory papers.
The common denominator for both pulping processes is the wood chip. Wood chips are residual fibre — waste wood from B.C. sawmills that was once sent to landfills or burned.
We mix a blend of coastal hemlock, fir, cedar and interior whitewood chips to produce pulp with specific characteristics. For example, fully bleached fine-fibred chip pulp is ideal for premium tissue products because of its strength, formation and softness. Pulp with a high fir content provides superior strength for papers requiring high tear strength, such as newsprint; bulk, such as Japanese Washi paper; or porosity, such as filter papers.
Pulp is the building block of many familiar and important products you use daily: writing paper for sending a note, coffee filters for a morning brew or paper towels for cleaning.
Process Details
1. Wood chips
This is where the pulp making begins. A typical wood chip measures 40 x 25 x 10 mm. Each chip comprises primarily water, cellulose wood fibres and the binding agent lignin. To produce pulp, we need to break down the wood structure into individual fibres.
2. Digester
This is our kitchen, where heat, chemicals and pressure dissolve the lignin and convert the chips to pulp. From the digester, the pulp goes into a blow tank, where a rapid change in pressure causes the wood particles to separate into individual fibres.
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3. Screens and washers
Screens remove any uncooked fibre bundles, which are reprocessed. We also wash the pulp thoroughly to remove spent cooking chemicals and dissolved lignin. From here, we screen and store the pulp — now a brown-coloured combination of individual wood fibres and water — before bleaching.
4. Bleaching
Bleaching is a five-stage process of tower soaks and washing. Using a sequence of hydrogen peroxide, chlorine dioxide, oxygen and caustic soda, we dissolve any remaining lignin and brighten the brown pulp fibres, turning them white. Then we put the pulp in another storage tank to await cleaning and drying.
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5. Press and dry
To convert pulp slurry into a sheet, we pass the pulp through a sheet former that drains the water through a combination of gravity and vacuum. The sheet then passes through the press sections — which squeeze out more water — before heading for the dryer, where it runs the gauntlet of steam-heated air jets that reduce water content to less than 10 per cent.
6. Pulp bales
Once the sheet is out of the dryer, we immediately cool it to prevent colour changes. We then cut it into smaller sheets and make bales. The bales pass through a hydraulic press — to make them more compact — before being wrapped and stored until we ship them to customers. Each bale measures 81 x 85 x 38 centimetres and weighs 250 kilograms. Our power and recovery systems recycle much of the water, chemicals and steam used in the pulp-making process.
