Kraft process

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Woodchips for paper production

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Kraft process is a chemical process widely used in the pulp and paper industry to convert wood into wood pulp and then into paper. The process involves the digestion of wood chips at elevated temperature and pressure in a solution of sodium hydroxide and sodium sulfide, known as white liquor, which breaks down the lignin that binds the cellulose fibers together, while preserving the cellulose fibers. The Kraft process, also known as the sulfate process, is the most prevalent method for pulp production worldwide due to its efficiency and ability to handle almost any type of wood or non-wood raw material.

History

The Kraft process was developed by Carl F. Dahl in 1879 in Germany. The name "Kraft" comes from the German word for strength, referring to the high strength of the paper produced by this method. Since its invention, the process has undergone significant modifications and improvements, but the basic principles remain the same.

Process Description

The Kraft process begins with the chipping of wood into small pieces. These wood chips are then cooked in a digester with a mixture of sodium hydroxide and sodium sulfide, known as white liquor. The cooking process removes most of the lignin and hemicellulose, leaving behind a pulp of nearly pure cellulose fibers. After cooking, the solid pulp is separated from the spent cooking liquor, now called black liquor due to the dissolved lignin. The black liquor is concentrated in evaporators and then burned in a recovery boiler, where the heat is used to generate steam and the inorganic chemicals are recovered for reuse in the process. The pulp may then be bleached to produce white paper.

Environmental Impact

The Kraft process is both energy-intensive and a significant source of environmental pollution, primarily due to the emissions from the recovery boiler and the discharge of waste products into water bodies. However, advancements in technology have led to significant improvements in the environmental performance of Kraft mills. Modern mills are equipped with recovery systems that recycle almost all of the inorganic chemicals used in the process. Additionally, the process can be adapted to minimize water usage and reduce the toxicity of effluents.

Advantages

The main advantages of the Kraft process include its ability to process a wide range of wood species, including hardwoods and softwoods, and its superior strength properties of the resulting paper products. The process also allows for the almost complete recovery and reuse of chemicals, making it more sustainable than other pulping methods.

Disadvantages

The primary disadvantages of the Kraft process are related to its environmental impact, including air and water pollution. The process requires significant energy input, and the recovery of chemicals, while efficient, is not complete, leading to the release of sulfur compounds and other pollutants.

Conclusion

The Kraft process remains the dominant method for pulp production worldwide due to its efficiency, adaptability, and the high quality of the paper products it produces. Ongoing research and development efforts aim to further improve the environmental sustainability of the process, ensuring its continued use in the pulp and paper industry.

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