The Kappa number is an assessment of how much lignin is present in pulp, which determines the amount of bleach that needs to be added to it if the goal is a white processed paper product. High Kappa numbers require more bleach, while lower numbers have less lignin and need less bleach. This number is also sometimes known as the pulp brightness measurement, and must be regularly checked at a pulp mill while the facility processes and generates pulp.
A standardized test can be used to determine the Kappa number. Standardization helps to ensure that results are consistent and readily understandable across platforms; a Kappa number of 10%, for example, is an absolute number that provides meaningful information. These numbers can be reported in discussions of the plant's efficiency and systems, and are also important for the logs associated with batch paper production. Plants need consistent papers because consumers rely on a standard brightness in the products they purchase.
To measure the Kappa number, a pulp mill can perform a test with potassium permanganate solution. It determines how much of the solution, in milliliters, is absorbed by a gram of the pulp. This allows the company to calculate and report the Kappa number. A pulp mill may have numerous testing stations to check on pulp as it moves through the processing stages, and can keep continuous logs on its batches of pulp and paper.
Pulp mills can use a variety of techniques to lower the Kappa number and cut down on the amount of bleach they need, including things like washing the pulp to remove excess lignin. These processes may add to processing time, and require careful monitoring to make sure they work as efficiently and effectively as possible. Companies that build machinery and develop processes for the paper industry look at ways to lower Kappa numbers and periodically release new products with increased functionality in this area.
Kappa number logs are typically kept with other records that pertain to production. A pulp mill can use this to determine how different kinds of timbers perform during pulping, and to track quality on its projects. If a company starts to turn out inconsistent pulps and papers, the logs can be checked to find out where the problem is originating, and how to address it. Inadequately maintained machines can be a potential cause, as they may work less effectively and thus remove less lignin during the pulping and processing procedures.