In earlier studies the degree of penetration was calculated with a method which gave a penetration degree of over 100%, if mill liquors were used, because the method ignored the diffusion of cooking chemicals and dissolution of wood material. In this paper, a new experimental research method is presented, which allows the penetration of mill liquors to be examined and a new and more accurate calculation method to be derived. The new penetration calculation method takes into account the influence of diffusion and dissolution of wood material in terms of the density of liquor inside wood and the increased void volume caused by yield loss.
In the present research, the new penetration calculation method was used for examining the influence of impregnation conditions on the penetration of cooking liquor into mill chips. Although the amount of alkali penetrating into the chips is much lower than what is needed for cooking, a high degree of penetration is important because it is a prerequisite for fast diffusion. Therefore, proper presteaming, high overpressure, and about 30 minutes’ impregnation time should be used in order to achieve a high penetration degree of over 98 % with softwood mill chips using white and black liquor.