Some factors affecting the brightness and TCF-bleachability of kraft pulp

By varying the cooking conditions, several series of kraft pulps having different brightness levels, i.e. different light absorption coefficients (k), were prepared. A clear correlation between the residual alkali concentration in the black liquor and the k-value was found. The darkening of the pulp (increasing k) was shown to be, in part, due to a higher content of calcium ions that increased lignin redeposition onto the fibers during the cook. Moreover, the production of a bright unbleached pulp (with low k-value) appeared to be beneficial for subsequent TCF-bleaching with hydrogen peroxide.