Alkaline kraft process

Pulps have since been made by various chemical processes, covering the full pH range from acid to neutral to alkaline. Each pulping method produces characteristic pulp properties, depending on the type of wood used, and each pulp type lends itself to a different end use. The most widely used chemical cooking process in North America today is the alkaline kraft process . The kraft process was invented in 1884, and the first U.S. kraft mill started production in 1909. As will be detailed later, semi-chemical, chemi-mechanical, and non-suUur cooking processes are also used. These methods are often considered more environmentally friendly than some of the older processes. 

Kraft Process. The dominant chemical wood pulping process is the kraft or sulfate process. The alkaline pulping Hquor or digesting solution contains about a three-to-one ratio of sodium hydroxide, NaOH, and sodium sulfide, Na2S. The name kraft, which means strength in German,... 

PolysuWde Process. One modification to the kraft process being appHed commercially is the polysulfide process (38). Under alkaline conditions and relatively low temperature (100—120°C), polysulfides oxidize the active end group of the polysaccharide polymer to an alkaH-stable aldonic acid. This reaction, known for many years (39), was not produced on a commercial scale until the development of an efficient method for in situ generation of the polysulfide in kraft white Hquor. 

The kraft process evolved from the soda process. The soda process uses an alkaline liquor of only sodium hydroxide (NaOH). The kraft process has virtually replaced the soda process due to the economic benefits of chemical recovery and improved reaction rates (the soda process has a lower yield of pulp per pound of wood furnish than the kraft process). 

The effect of color production from carbohydrate transformation in an alkaline pulping process, such as kraft pulping, is illustrated in Table III. Cotton linters, which originally had a brightness of 90.5%, were treated for 4 h at 180° with kraft liquor, with and without additives. Brightness was measured on paper made from the resulting, washed pulp. 

Kraft pulping involves the cooking of wood chips at 340-350°F and 100-135 psi in liquor that contains sodium hydroxide, sodium sulfide, and sodium carbonate. This process promotes cleavage of the various ether bonds in lignin and the degradative products so formed dissolve in alkaline pulping liquor. The Kraft process normally incorporates several steps to recover chemicals from the spent black liquor [3]. 

The process of pulping, degrading the lignin to a more soluble form so the cellulose fibers can be separated from it, involves some interesting chemistry. The kraft or alkaline sulfate process dominates this part of the industry. Approximately 78% of all pulp is made by the kraft process, 3%... 

Chemical pulps are prepared by the digestion of wood chips with chemicals at high temperature (170-180°C) until much of the lignin has been removed. There is a concomitant loss of hemicelluloses, because of polysaccharide degradation due to chemical attack. Therefore yields are low, from 45-50%. Chemical pulping falls into two main classes, those based on pulping with sulphite liquors at various pH levels, and alkaline pulping of which the kraft process is by far the most important. 

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