How Does a Pulp Mill Work?

June 20, 2016 9:44 am Published by Stefano Jun 2 Comments

I previously thought all wood products came from a same kind of pulp, but I was wrong. There are several different types of pulp, categorized by their specification and type of tree used. The pulp manufactured by the Weyerhaeuser Grande Prairie Pulp Mill is Northern Bleached Softwood Kraft (NBSK), which is used for tissue, high-quality paper, paper reinforcement, filter, and even medical garments. This mill is well-known for its high quality, consistent grade, and excellent strength.

So, how is it all made? There are many, many steps involved to create the pulp, and that’s not even the final product you see in store shelves. To simplify, the components of wood the process focuses on are cellulose fibers and lignin. Lignin is the “glue” of wood that makes the material hard and durable. Wooden structural elements desire lignin, but not pulp. The kraft process is a chemical way of separating lignin away from cellulose fibers as much as possible without damaging the fiber structures.

Panel 1: Mill Tour

The first step is to make wood easier to process. Just like how shredded cheese melt better than blocks, more surface area makes chemical processes more efficient and even. Logs cut from local forests are sent to our wood processing unit, which cuts them into 1” square chips.

Panel 2: Chip Digester

Now that the chips are ready, they are cooked with chemicals under pressure to start separating lignin. The added chemical is called white liquor. Inside the digester, lignin is dissolved and pumped out. Since lignin is dark, the white liquor becomes black liquor at the end of digesting process. Even though black liquor looks spent, it can be recycled back into white liquor repeatedly.

Panel 3: Pulp Washing and Bleaching

Even with digesting, there is still some lignin left that makes the pulp brown. Since our pulp is used for bright pulp products, they need to be washed and bleached. This “brown stock” is washed to remove any remaining lignin and chemicals. Using various chemicals such as hydrogen peroxide and chlorine, the pulp is bleached to become white stock, simply called stock.

Panel 4: Tour leads to hunger

At this point, pulp has extremely high water content, behaving more like liquid than solid. Headbox lays out the pulp evenly in a large sheet at 99% water content. Gravity and suction is used in fourdriner to reduce the water content to nearly 70%. Hot press rolls squeeze out the water, and then the sheet travels though a large industrial dryer that finally reduces the water content close to 10%.

Once out of the dryer, the pulp is cut, pressed, and packaged. Finally, it is ready for transport using either rail or trucks. I’ve tried to simplify the process in four steps, but in reality there are hundreds of more little processes that involves hundreds of people! Like I showed in the comics, the processes makes you think about food. Maybe it was because I had the tour before lunch, and my stomach was growling. So long for now!

Stefano Kangsan Jun