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What is Chemical Pulp?
Pulp is wood fiber that is generally used to make paper. Chemical pulp is created by a method that uses chemicals and heat to convert wood into pulp. This process can be, and generally is, used as an alternative to mechanical pulping, which involves obtaining wood fibers by way of a grinding process.
Chemical pulp tends to be more common than mechanically derived pulp. Chemical pulping generally results in the production of paper with greater sheet strength than the paper produced by mechanical pulping. It is also a process that can be regarded as highly efficient due to the possibility of chemical recovery.
Chemical recovery is part of the process in which the chemicals used to treat the wood are reused for another purpose. In some cases, this used liquid, which is often referred to as liquor, is used to produce other chemicals. In other cases, the chemicals are used to create energy.
There are two widely recognized processes for producing chemical pulp. One is known as sulfate pulping. The other is known as sulfite pulping. The major differences between these are the chemicals used, the quality of paper produced, and the economy of chemical recovery.
Sulfate pulping is a process that was developed in Germany in 1879. It is sometimes called kraft pulping because kraft means strength in German, and the paper produced with this chemical pulp is strong. It can be used to make paper bags, writing paper, or diapers. Chemicals are typically always recovered in this process.
Sulfite pulping is a process that was developed in the United States in 1867. It generally results in a light chemical pulp that is easier to bleach and easier to refine. Paper produced by this method can be used for newspaper, writing paper, or cellophane. With this method, however, there is not always chemical recovery.
Both processes are generally executed in a similar manner. Chemical pulping usually begins with debarking trees to create wood chips. Those chips are then cooked in a chemical mixture, or liquor. The purpose of this is to dissolve the wood’s lignin, a natural component that binds the fibers together, and to break down other elements in the wood. When this part of the process is complete the liquor is generally black.
This black liquor is what is used in the chemical recovery process. It can be used in the production of another chemical, such as tall oil. It can also be combusted to create heat or electrical energy.
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Scientist with beakers
Discussion Comments
How is the chemical liquor turned into energy? Do they pump it into a truck and send it to a nearby power plant, or is there some other system? I was also wondering how much energy you could even get out of it.
When they use this system, does the power plant have to be specially equipped to burn the liquor, and does it release any different chemicals into the air compared to coal? The article says the chemicals are sulfur based, and I know that is a big part of acid rain.
I guess the other option would be if the actual paper plant took the liquor and burned it themselves to power part of the plant. I don't know how difficult that would be, but I've heard that's what they do at some of the larger sawmills.
@kentuckycat - I'm not sure about how the whole paper process works. I do know that when you buy recycled paper or get a product that uses it, it always says a certain percentage of it is recycled. That would mean that somewhere along the line, the old paper gets mixed in with new stuff.
As far as I know, once they break the wood apart, they put it in some type of press and squeeze the liquid out then dry it. I would assume that if you put the recycled paper back into the chemicals, it would break down and turn back into pulp that could be used to make new paper. That's just my guess, though.
What is the difference between the sulfate and sulfite chemicals? Where do the chemicals come from?
I was also wondering about how paper gets recycled. I'm sure there would have to be some type of deinking process, but what happens to it after that? Do they put the paper back into the sulfur mixture and create the pulp again, or is there another method?
I knew paper was made from some type of pulp, but I never knew how they got the wood into that form.
What is it about the process that turns the liquid black? Is it some type of reaction between the chemicals and the wood, or are there other things added in that cause the color change?
I've always wondered, too, do different types of trees make different types of paper? Are certain trees harder to use in the paper making process than others?
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