Difference Between Kraft and Sulfite Pulping

The key difference between kraft and sulfite pulping is that kraft pulping involves the treatment of wood chips with a hot mixture of water, sodium hydroxide and sodium sulfide whereas sulfite pulping involves the treatment of wood chips with sulfite or bisulfite salts of sodium, calcium, potassium, magnesium or ammonium.

Wood pulp production is very important in papermaking and other industrial needs. In the process of wood pulping, we separate cellulose fibres from wood either chemically or mechanically. This pulp is useful as a raw material for the production of paper.

CONTENTS

1. Overview and Key Difference
2. What is Kraft Pulping
3. What is Sulfite Pulping 
4. Side by Side Comparison – Kraft vs Sulfite Pulping in Tabular Form
5. Summary

What is Kraft Pulping?

Kraft pulping is a technique used to convert wood into wood pulp using a mixture of water, sodium hydroxide and sodium sulfide. Wood contains almost pure cellulose fibres, which is the main component of paper. Therefore, wood pulp is important in paper production. The kraft process requires a hot mixture of water, NaOH, and Na2S (also named as white liquor) that can break down the bonds between lignin, hemicellulose, and cellulose. This is the major method that is used for the production of wood pulp. It has both chemical and mechanical steps. However, there are some considerations regarding this process since it can release odorous products and sometimes even liquid wastes.

There are several steps in the kraft pulping process: impregnation, cooking, recovery, blowing, screening, washing, and bleaching. The impregnation step involves the pre steaming of wood chips where the wood chips get wetted and preheated with steam. The next step is cooking in which wood chips are cooked in pressurized vessels named as digesters. The next step is the recovery process where excess liquid and solid components are removed using a multiple-effect evaporator in which the mixture is concentrated.

The fourth step of the process is blowing step. In this step, the finished and cooked wood chips are blown into a collection tank (blow tank), which allows the release of a lot of steam and volatiles. Here, the volatiles are condensed and collected. The next step is screening, in which the pulp is separated from large shives, knots, dirt and rest of the debris. Then comes the washing step where cooking liquors are separated from the cellulose fibres in a series of 3-5 washing stages. The last step of this process is bleaching.

What is Sulfite Pulping?

Sulfite pulping is a technique used to produce wood pulp using sulfites or bisulfite salts of sodium, calcium, potassium, magnesium, and ammonium. The solutions of these sulfites or bisulfites are used to treat the wood chips, which leads to the cleavage of chemical bonds between cellulose and lignin. During this process, the lignin is converted into soluble lignosulfonates, and we can easily separate this compound from cellulose fibres. Unlike the kraft process, the sulfite process produces stronger fibres, and this technique is less harmful to the environment.

The first step of the sulfite process is pulping liquor preparation. For most sulfite mills, the pulping liquor is a base (hydroxides of alkali metals and alkaline earth metals) with sulfur dioxide. However, we can use carbonated in place of hydroxide. This step produces sulfites and bisulfites. The next step is the digestion which is conducted in large pressure vessels called digesters. Here, the wood chips are treated with the pulping liquor. This reaction forms lignosulfonates as a byproduct which is useful in other applications. The final step is the chemical recovery using pulp washers, to remove the spent cooking chemicals and degraded lignin and hemicellulose.

What is the Difference Between Kraft and Sulfite Pulping?

Kraft and sulfite process are two methods of producing wood pulp from wood chips. The key difference between kraft and sulfite pulping process is that kraft pulping involves the treatment of wood chips with a hot mixture of water, sodium hydroxide and sodium sulfide whereas sulfite pulping involves the treatment of wood chips with sulfite or bisulfite salts of sodium, calcium, potassium, magnesium or ammonium. Importantly, sulfite pulping is more environmentally friendly than kraft pulping.

Below infographic shows a side by side comparison of the difference between kraft and sulfite pulping.

Summary – Kraft vs Sulfite Pulping

Wood pulp is an important raw material in industries such as paper production industry. Kraft pulping process and sulfite process are used for the production of wood pulp from wood chips. The key difference between kraft and sulfite pulping process is that kraft pulping process includes the treatment of wood chips with a hot mixture of water, sodium hydroxide and sodium sulfide whereas sulfite pulping process includes the treatment of wood chips with sulfite or bisulfite salts of sodium, calcium, potassium, magnesium or ammonium.

Reference:

1. “Sulfite Process.” Wikipedia. Wikimedia Foundation, 17 May 2020. Web. 11 Sept. 2020. Available here.

Image Courtesy:

1. “Woodchips for paper production” By fi:User:MPorciusCato – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “SulfiteProcessSimple” By Smokefoot – Own work (CC BY-SA 4.0) via Commons Wikimedia