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When softwood chips are impregnated with an alkaline solution at a temperature around 100°C the average EA-concentration inside the chips equals that of the surrounding liquor in 45-60 min. However, if the impregnation is continued with a second stage (the old liquor is drained off and new liquor is introduced), the equilibrium in the second stage is reached in 15-30 min. Such a difference between the two stages indicate structural changes in the chips during the first impregnation stage.
Measurements of the diffusion of alkali through an individual piece of wood also indicate that the diffusion is faster if the wood in advance was treated with an alkaline solution.
Moreover, different alkaline solutions for impregnation were used and the amount of organic material varied considerably. Liquors with and without sulphur were also used.
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Impregnation in Alkaline Pulping, 1997 Pulping Conference Proceedings