Kraft process
The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose.
[6] The invention of the recovery boiler by G. H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.
During impregnation, cooking liquors penetrate into the capillary structure of the chips and low temperature chemical reactions with the wood begin.
Under digesting conditions, lignin and hemicellulose degrade to give fragments that are soluble in the strongly basic liquid.
One of the main chemical reactions that underpin the kraft process is the scission of ether bonds by the nucleophilic sulfide (S2−) or bisulfide (HS−) ions.
The molten salts ("smelt") from the recovery boiler are dissolved in a process water known as "weak wash".
[14] Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.
Tomlinson's invention have been in general use since the early 1930s, attempts have been made to find a more efficient process for the recovery of cooking chemicals.
The volatiles are condensed and collected; in the case of northern softwoods this consists mainly of raw turpentine.
The sieves are normally set up in a multistage cascade operation because considerable amounts of good fibres can go to the reject stream when trying to achieve maximum purity in the accept flow.
The fiber containing shives and knots are separated from the rest of the reject and reprocessed either in a refiner or is sent back to the digester.
The brownstock from the blowing goes to the washing stages where the used cooking liquors are separated from the cellulose fibers.
Several types of washing equipment are in use: In a modern mill, brownstock (cellulose fibers containing approximately 5% residual lignin) produced by the pulping is first washed to remove some of the dissolved organic material and then further delignified by a variety of bleaching stages.
Fully bleached kraft pulp is used to make high-quality paper where strength, whiteness, and resistance to yellowing are important.
The main byproducts of kraft pulping are crude sulfate turpentine and tall oil soap.
The availability of these is strongly dependent on wood species, growth conditions, storage time of logs and chips, and the mill's process.
Control of odours is achieved through the collection and burning of these odorous gases in the recovery boiler alongside the black liquor.
Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes.
The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents.
