kraft process

A paper pulp-making process, which uses sodium hydroxide and sodium sulphate instead of sulphite. The pulp is stronger and less crude than that made by the sulphite process. The name derives from Swedish kraft ‘strong’.

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 the wood fiber in large pressure vessels called digesters. The spent pulping liquor, called black liquor, is concentrated by evaporation and burned in the recovery boiler to generate high pressure steam for the mill processes. The inorganic portion of the black liquor is then used to regenerate the sodium hydroxide and sodium sulfide needed for pulping in a process called causticizing.

The high pressure steam is led to turbogenerators, reducing the steam pressure for the mill use and generating electricity.

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. In a modern mill brown pulp is bleached first with oxygen and after that with a combination of acidic (chlorine dioxide) and alkaline stages, reinforced with oxygen and/or hydrogen peroxide.

Various byproducts containing hydrogen sulfide and other volatile sulfur compounds are the cause of the malodorous air emissions characteristic for pulp and paper mills utilizing the kraft process. 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.