Kappa Number → Lignin
Estimate the residual lignin content of a cellulose pulp from the Kappa number, Lignin% ≈ Kappa · 0.15. The Kappa number measures the pulp's permanganate consumption, proportional to the lignin remaining after cooking — the higher the Kappa, the more lignin (dark color, stiffer fibers) remained. It is the main control of delignification, defining the bleaching load needed. Enter the Kappa number.
Result
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Kappa number → lignin
After cooking, pulp still holds residual lignin - the less of it, the brighter the pulp and the easier the bleaching. Measuring lignin directly is laborious, so the industry runs a quick and clever test: the Kappa number, which gauges how much potassium permanganate (an oxidant) the pulp consumes under standardised conditions. Since permanganate reacts with lignin, consumption stays proportional to the lignin present - and the practical relation is Lignin% ≈ Kappa · 0.15. A Kappa of 20 points to ~3% lignin. The Kappa number works as the thermometer of delignification: it tells the mill when to stop the cook (a very low Kappa means overcooking, which degrades the fibres; a very high one leaves more lignin than the bleach plant can handle) and it sizes the charge of bleaching chemicals (chlorine, chlorine dioxide, peroxide, ozone) needed to reach the target brightness. Pulps for kraft packaging paper (sacks, cardboard) stay at high Kappa, as they need no brightness; pulps for printing grades are cooked and bleached down to a very low Kappa. Enter the Kappa number.
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