Kraft pulp is a chemical wood pulp that forms the basis for a vast majority of the world’s paper and paperboard products. It is the dominant technology for producing wood pulp, accounting for over 80% of all chemical pulping processes worldwide. The name is derived from the German word “Kraft,” which translates to “strength,” referencing the superior durability of the resulting fiber. This method converts wood into nearly pure cellulose fibers, the fundamental building blocks of paper.
Wood Fiber Components
Wood is primarily made up of three polymeric components: cellulose, hemicellulose, and lignin. Cellulose, a long-chain carbohydrate polymer, forms the structural foundation of the wood fibers and provides the strength. Hemicellulose is a shorter, branched carbohydrate that interacts closely with the cellulose fibers. Lignin, a complex organic polymer, acts as a rigid glue that binds the individual cellulose fibers together, giving wood its stiffness. The main objective of the Kraft pulping method is to isolate the desired cellulose fibers by chemically dissolving this lignin binder.
The Alkaline Pulping Process
The transformation of wood chips into pulp, known as delignification, occurs in a controlled chemical environment. Wood chips are first loaded into large, high-pressure vessels called digesters. They are immersed in a strong alkaline solution referred to as “white liquor,” principally a mixture of sodium hydroxide and sodium sulfide.
The chips are then “cooked” under elevated temperatures, typically ranging between 130 and 180 degrees Celsius, for several hours. This combination of heat, pressure, and white liquor chemically attacks the lignin and, to a lesser extent, the hemicellulose. The sulfide ions in the liquor accelerate the breakdown of lignin’s complex chemical bonds without severely degrading the cellulose molecules.
This chemical action causes the lignin to dissolve into soluble fragments, releasing the individual cellulose fibers. Once cooking is complete, the resulting mixture is discharged, and the solid material, called brown stock, is separated from the spent cooking liquid. The pulp then undergoes washing and screening steps to remove remaining chemical residue and undigested wood fragments.
Resulting Material Strength and Uses
The Kraft process selectively removes lignin while preserving the length and integrity of the cellulose fibers, which is the source of the pulp’s strength. Minimal damage to the cellulose results in fibers that form strong bonds during the papermaking process. This leads to a material characterized by high tear resistance and excellent tensile strength. The unbleached Kraft pulp retains residual lignin, giving it a distinctive dark brown color. It is widely used in the production of corrugated boxes, multi-wall sacks, and heavy-duty paper bags.
Chemical Recovery and Mill Operations
A defining characteristic of the Kraft process is its sophisticated chemical recovery cycle, which is fundamental to economic and environmental viability. The spent cooking liquid, dark brown from dissolved lignin and organic wood materials, is termed “black liquor.” This liquor contains both the unusable organic material and the valuable inorganic pulping chemicals.
The black liquor is concentrated and then directed to a recovery boiler, where it is burned. This combustion serves two purposes: the organic compounds are consumed to generate high-pressure steam and electricity, making modern mills largely energy self-sufficient. Simultaneously, the inorganic sodium and sulfur chemicals are recovered from the molten ash at the bottom of the boiler.
This recovered inorganic material is dissolved to form “green liquor.” It is then treated through recausticizing to regenerate the original sodium hydroxide and sodium sulfide mixture, the “white liquor.” This closed-loop system allows the mill to recover and reuse approximately 90 to 97% of the inorganic pulping chemicals. The cycle’s efficiency offsets the high cost of initial chemicals and minimizes environmental waste, although sulfur compounds can release volatile gases that produce a characteristic odor.