Title
Pre-impregnation of wood chips for alkaline delignification
Authors
BRUNO LONNBERG

Received This paper is dedicated to the 50^th Anniversary of Cellulose Chemistry and Technology
Published Volume 50 Issue 5-6 May-June
Keywords alkali, chips, diffusion, effective alkali, electrical conductivity, liquid-to-wood ratio, penetration, pre impregnation, residual effective alkali, reversed chlorine number, sodium hydroxide, sodium sulphide, softwood, temperature, time, yield

Abstract
Water-saturated wood pieces were mounted in a diffusion cell between alkaline liquid and pure water. The results indicated that the diffusion of hydroxide ions through a thin wood piece was significantly dependent on its thickness, the alkaline liquid concentration, liquid temperature and, of course, time. The experiments served as a basis for further studies on industrial chips.
In a special pre-impregnation apparatus, controlled thickness industrial chips were treated so as to simulate industrial pre-impregnation processes. The chips were treated under variable initial effective alkali concentration, liquid-to-wood ratio, temperature and pre-impregnation time. The residual effective alkali concentration and the yield were used as tools to evaluate the pre-impregnation process. A reasonable pre-impregnation process would be one where the residual effective alkali is low and the yield is as high as possible.
The main achievements may be described by the residual effective alkali difference between two neighboring initial effective alkali concentrations. The liquid-to-wood ratio of 3:1 was applied in the more detailed evaluations. At the lower end of initial alkali, a difference of, for example, 1 g EA/L was obtained at low temperature after 160 min pre impregnation. Such pre-impregnation equilibrium would guarantee an even distribution of alkali across the wood fiber. However, a high temperature would decrease the time required to 40 min only, but, on the other hand, would include the risk of bad alkali distribution in fibers. At high alkali again, a residual effective alkali difference of the level mentioned would be obtained after 160 min.