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The Kappa number is an indication of the residual lignin content or bleachability of wood pulp by a standardized analysis method.
The Kappa number is determined by ISO 302:2004. ISO 302 is applicable to all kinds of chemical and semi-chemical pulps and gives a Kappa number in the range of 1–100. The Kappa number is a measurement of standard potassium permanganate solution that the pulp will consume. The measurement is inflated by the presence of hexenuronic acids in the pulp. These compounds are formed during the chemical pulping process, from the hemicelluloses.
The Kappa number can be monitored by online Kappa analysers in a pulp mill, but these have to be calibrated with the ISO 302:2004 method.
The Kappa number estimates the amount of chemicals required during bleaching of wood pulp to obtain a pulp with a given degree of whiteness. Since the amount of bleach needed is related to the lignin content of the pulp, the Kappa number can be used to monitor the effectiveness of the lignin-extraction phase of the pulping process. It is approximately proportional to the residual lignin content of the pulp.
K ≈ c*l
K: Kappa number; c: constant ≈ 6,57 (dependent on process and wood); l: lignin content in percent
The Kappa number for bleachable pulps are in the range of 25–30, sack paper pulps in the range 45-55 and pulps for corrugated fiberboard are in the range 60-110.