The kraft process is a process for creating wood pulp out of wood, for use in paper production. Unlike many other chemical engineering processes, the kraft process is not named after its inventor, but instead derived from the German word kraft, meaning “strong.”
This name was chosen by the inventor of the process himself, Carl Ferdinand Dahl, who intended to market the superior strength of the paper created from this process.
A resident of Danzig, Kingdom of Prussia (present-day Germany), Dahl invented the process in 1879, and had himself awarded a U.S. patent for the invention on April 15, 1884. His invention was first put into action when a pulp mill in Sweden first began using it in 1890.
The kraft process has undergone significant improvements throughout the century, especially since the invention of the recovery boiler during the early 1930s by G.H. Tomlinson. The innovation helped it surpass the sulfite process, another pulp-making process, in usage and catapulted it to the widespread popularity that it enjoys today.
The kraft process begins with presteaming common wood chips. This involves collecting wood chips that are 12–25 millimeters (0.47–0.98 inches) in length and 2–10 mm (0.079–0.39 in) in width, and wetting them before heating with steam. This causes cavities within the wood chips to be filled with both air and moisture.
After this, the wood chips are impregnated with white and weak black liquor by heating up to 100 °C (212 °F). During this process, liquor penetrates the capillary structure of the wood chips, and saturates them homogeneously throughout.
White liquor, so-named because of its white opaque color, is a strongly alkaline, aqueous solution of sodium sulfide (Na2S), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sodium thiosulfate (Na2S2O3), sodium chloride (NaCl), calcium carbonate (CaCO3) and water. However, only the first two (and to a lesser extent, the third) compounds actually contribute to the breakage of extractives–cellulose fiber bonds; the other components of white liquor are considered to be chemically inert.
Black liquor, on the other hand, is simply the residue created from the consumption of white liquor during the previous batches of the kraft process. Black liquor is thus a mixture of woodchip residues in white liquor. Aside from being used as a digesting agent during the early stages of the kraft process, black liquor is also combusted in the recovery burner in order to recover useful compounds from the black liquor and generate extra power for the pulp mill.
The rationale for recycling spent white liquor is pretty simple: economy. Not all of the active components of white liquor are spent up during digestion, and disposing them right after just one use is fiscally imprudent and environmentally irresponsible as well. Black liquor is, as its name suggests, a viscous, aqueous, black liquid that turns water to dark caramel upon contamination, and is very toxic to aquatic life. About 7 tons of black liquor is produced for every 1 ton of pulp manufactured under the kraft process. Recycling black liquor (i.e. spent-up white liquor) greatly reduces the amount of it that goes into our ecosystem.
During digestion, the wood chip–liquor mixture is placed into a highly pressurized vat for several hours at temperatures ranging from 170 to 176 °C (338 to 349 °F). The liquor mixture act to digest the pulpwood into paper pulp by removing lignin (a complex chemical compound found in the wood’s secondary cell wall), hemicellulose (a polymer also found in the cell wall) and other extractives. This is done in order the pulpwood cellulose fibers that are used as ingredient in making paper. Reactions between nucleophilic bisulfide (HS-) or sulfide (S2-) and the woodchip components underpin this step of the kraft process.
Digestion produces a solid pulp known as a “brown stock.” This product is then collected and washed to rid it off the inorganic compounds that came from liquor impregnation. Atmospheric pressure is reduced in the containers in order to let steam arise from the brown stock, and cool them down. Efficiently designed pulp mills recycle this steam to turbines in order to generate electrical power.
Afterwards, the pulp is passed through sieves in order to remove dirt and other unwanted contaminants; and then washed again for several times in order to produce a final product that is clean pulp. Finally, the pulp is bleached to give it paper’s familiar white color. Several chemicals may be added after this process in order to improve the quality of the pulp.
The kraft process produces a lot of by-products, the most notable of them being crude sulfate turpentine and tall oil soap. Both of which can be used as ingredients of a wide range of retail and industrial products, including lubricants, soaps, solvents, inks, binders and many more. Effluent produced by kraft-process pulp mills are extremely detrimental to the environment and should be recycled whenever possible.