23.6.2020
Due to the Covid-19 situation, I have been working remotely for the last months. Lately I have been working at my summer cottage, totally surrounded by forest. There, burning through 10:s of Gigabytes in meetings and on-line working, it is a perfect place to think about the journey of the growing tree to the final product. Cutting down a tree is today a high tech operation with onboard computers connected to central computers registering the cut trees. I was biking a few days ago and looked at the stacks of logs by the road, sorted according to end use and tree type: softwoods in one stack and hardwoods in another, timber in one stack and pulp wood in another. The final destionation of the logs was marked on each stack. Some of them were going to be transported more than 400 kilometers and this would be just the beginning of the journey of the paper and board we use in our daily life.
At some point, part of these logs will end up in a pulp mill, which is a huge industrial complex. A modern pulp mill in the nordic countries can produce around 1 million tons of pulp annually. This pulp will be further converted into different fiber products and may end up in your toilet paper or in the newspaper/magazine you read. Separating these logs into fibers is not easy and various chemistries are needed. The wood is cooked in an alkaline water solution of NaOH and Na2S at a temperature between 150 and 170°C, which removes most of the lignin that glues the fibers together. After cooking the fibers are as brown as the paper bags you should buy at the supermarket instead of the plastic bags. Brown color is not wanted for most of the paper products, so the fibers need to be bleached. For that, hydrogen peroxide and chlorine dioxide are used in different sequences.
Different chemistries are also needed to prevent scaling of the wood containing inorganics, which reduce heat transfer and clog pipes. Although the amount of inorganics in wood is not high, the total amount of wood going through the mill is so significant, that the possible scaling of these inorganis needs to be controlled by anti-scaling polymers. Extractives, the sticky stuff you get on your hands and clothes when working with wood, cause problems in a pulp mill as well. Alkali in combination with the most common extractives, fatty- and rosin-acids, will create foam which is controlled by defoamers to keep the foam from flooding the floor.
Last but not least, let’s have a look at the water treatment, since pulp mills use lots of water. A modern mill can consume 10-15 m3 of water / ton of pulp, an old mill consumes even more. The water has to be cleaned before usage and before returning the water back to the nature, and for that chemistries are again needed.
As a conclusion, making pulp from the sustainable wood resources is in many ways adding various chemistries to different processes.
Many thanks to my co-worker Saara for checking spelling, grammar and facts.
Writer:
Leif Robertsén, Kemira
