Kraft process

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The Kraft process (also known as Kraft pulping or sulfate process) is used in production of paper pulp and involves the use of caustic sodium hydroxide and sodium sulfide to extract the lignin from wood chips in large pressure vessels called digesters. Some digesters operate in a batch manner and some in a continuous process such as the Kamyr digester.

The spent, extracted pulping liquor, called black liquor, is concentrated by evaporation and burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process. The recovery boiler also generates high pressure steam for the mill processes. The inorganic portion of the black liquor is used to regenerate the sodium hydroxide and sodium sulfide needed for pulping in a process called causticizing.

In the case of softwood (conifer) pulping, a soaplike substance is collected from the liquor during evaporation. The soap is acidified to produce tall oil, a source of resin acids, fatty acids and other chemicals. Also turpentine originates from softwood.

The high pressure steam, from the recovery boiler, is led to turbogenerators, reducing the steam pressure for the mill use and generating electricity. A modern kraft pulp mill is more than self-sufficient in its electrical generation and normally will provide a net flow of energy to the local electrical grid. Additionally: bark and wood residues are often fired in a separate power boiler to generate steam.

The process name is derived from German kraft, meaning strong; both capitalized and lowercase names (Kraft process and kraft process) appear in the literature. It was developed by Carl Dahl in 1884 and now is used for about 80% of production volume of paper.

Kraft process differs from the sulfite process by using alkaline solution, which is less corrosive to the equipment. Sulfite process cannot process pulp from all wood species, for example from pine. Kraft process is also more efficient than the sulfite process. It produces stronger fiber, however the fiber is also rougher and darker that makes it somewhat more challenging to bleach.

In a modern mill, brown pulp, (cellulose fibers containing residual lignin), following the cooking process is first washed to remove some of the dissolved organic material and then further delignified by an oxygen/alkali reaction and subsequently bleached with a combination of acidic (chlorine dioxide) and alkaline (sodium hydroxide) stages, reinforced with oxygen and/or hydrogen peroxide.

In the case of a plant designed to produce pulp to make brown sack paper or linerboard for boxes and packaging, the pulp does not always need to be bleached to a high brightness. In these cases, a higher yield of fiber from wood can be achieved.

Various byproducts containing hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and other volatile sulfur compounds are the cause of the malodorous air emissions characteristic for pulp and paper mills utilizing the kraft process. Outside the modern mills the odour is perceivable only during disturbance situations, for example when shutting the mill down for maintenance break. The sulphur dioxide emissions of the kraft pulp mills are much lower than sulphur dioxide emissions from sulfite mills.

The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.