Common wood chips are first "pre-steamed" (wetted and preheated with steam) to fill cavities with either liquid and air. The steam treatment causes the air to expand and about 25% of the air to be expelled from the chips. The chips are then saturated with black and white liquor (trapping any remaining air) and cooked in a mixture of white liquor, water in chips, condensed steam and weak black liquor.
The cooking liquor penetrates into the capillary structure of the chips and low temperature chemical reactions with the wood begin in pressurized vessels called digesters. In a continuous digester, the materials are fed at a rate that allows the pulping reaction to complete by the time the materials exit the reactor.
The solid pulp, known as brown stock (due to its color), represents about 50% (by weight) of the dry wood chips and is collected and washed. The combined liquids, known as black liquor (because of its color), contain lignin fragments, carbohydrates (from the breakdown of hemicellulose), sodium carbonate, sodium sulfate and other inorganic salts.
The excess black liquor contains about 15% solids and is concentrated in a multiple effect evaporator. This multi-stage process frees rosin soap to rise to the surface, which is then collected and processed to tall oil. The remaining "weak" black liquor is further evaporated until it is 65-80% solid, before finally burned in a recovery boiler to recover the inorganic chemicals for reuse in the pulping process.
During combustion, sodium sulfate is reduced to sodium sulfide by the organic carbon in the mixture. The molten salts ("smelt") from the recovery boiler are dissolved in a process water ("weak white liquor") composed of all liquors used to wash lime mud and green liquor precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as "green liquor".
This liquid is mixed with calcium oxide, which becomes calcium hydroxide in solution, to regenerate the white liquor used in the pulping process. Calcium carbonate precipitates from the white liquor is recovered and heated in a lime kiln where it is converted to calcium oxide (lime).
Finally, the calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used in recovering the white liquor.
The combination of these reactions form a closed cycle with respect to sodium, sulfur and calcium and is the main concept of the so-called recausticizing process where sodium carbonate is reacted to regenerate sodium hydroxide.
The recovery boiler also generates high pressure steam which is fed 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 which can be used by an associated paper mill or sold to neighboring industries or communities through to the local electrical grid. Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.
The finished cooked wood chips are blown to a collection tank ("blow tank") that operates at atmospheric pressure. This releases a lot of steam and volatiles, chiefly raw turpentine. These volatiles are condensed and collected.
After pulping, the pulp ("accept") is separated from large shives, knots, dirt and other debris ("reject") by a combination of different types of sieves (screens) and centrifugal cleaning. The sieves are normally set up in a multistage cascade operation because considerable amounts of good fibers 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 solid pulp ("brown stock") from the blowing process goes to the washing stages, where the used cooking liquors are separated from the cellulose fibers. Pulp washers use counter-current flow between the stages such that the pulp moves in the opposite direction to the flow of washing waters.
After washing the cellulose fibers ("brown stock") is further delignified by a variety of bleaching stages. Bleaching decreases the mass of pulp produced by about 5%, decreases the strength of the fibers and adds to the cost of manufacture.
For plants designed to produce pulp for brown sack paper or linerboard (boxes and packaging), the pulp does not need to be bleached to a high brightness.
1. Kraft Process. (2020, February 13). In Wikipedia.