What Does Kraft Process Mean?
The kraft process is a chemical method used to convert wood into wood pulp of pure cellulose fibers. The process is used to separate the fibers from the natural glue-like substance (lignin) that binds them together. The pulp produced is mainly used in the paper-making industry.
The cellulose fibers produced are very strong and sometimes referred to as kraft fiber.
Corrosionpedia Explains Kraft Process
In the kraft process, wood chips are cooked in an aqueous solution of sodium hydroxide and sodium sulfide (white liquor) under pressure in the digester system. This causes the lignin to dissolve and the chips are converted into pulp. The pulp is then separated and taken into the blow tank where a change in pressure causes the wood to separate into individual fibers. The separation is followed by screening, washing, bleaching, pressing and drying.
Corrosion and pollution are some of the challenges associated with the kraft process. The continuous digesters constructed from carbon steel suffer from stress corrosion cracking that can lead to fatal cracks.
The carbon steel surfaces exposed to the alkaline sulfide liquors become corroded, largely due to the chemical environment in the processes, which has the ability of causing corrosion and corrosion-assisted cracking.
The most affected sections are typically liquid level lines in storage tanks and clarifiers as well as carbon steel pressure vessels in the digestion process.
The inner surface of the upper cone of the shell in the vapor zone continuous digester is also one of the most affected sections, especially where the white liquor and the wood chips enter into the digester through the rotating valves. This can be minimized by applying shot peening at the inner surfaces of the upper cone sector.
Stainless steel is also sometimes used in this equipment to minimize corrosion.