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Investigation of ESEM/EDX to measure liquor penetration and diffusion in Eucalyptus grandis wood chips during kraft pulping

Asheena Hanuman and Jerome Andrew*

CSIR Natural Resources and the Environment – Forestry and Forest Products (FFP) Research Centre

*Corresponding author:


Environmental scanning electron microscopy coupled with energy dispersive x-ray (ESEM/EDX) was optimised to measure the penetration and diffusion of cooking liquor into Eucalyptus grandis wood chips during kraft pulping. The moisture content of the cooked wood chips influenced the ESEM/EDX measurement of sodium and sulphur, whilst the contribution of sodium and sulphur already present in the wood prior to pulping was negligible. Using the optimised ESEM/EDX method, investigations into the use of pulping aids during kraft pulping showed that the penetration and diffusion of sodium and sulphur was enhanced by the use of anthraquinone (AQ) and a combination of AQ and surfactant. As expected, chip thickness was found to influence the penetration and diffusion of cooking liquor into the wood structure during cooking.


The kraft pulping process is one of two predominant chemical pulping processes used in the pulp and paper industry. During this process, delignification occurs when lignin from the wood is degraded and removed with the cooking liquor. The cooking liquor consists of a solution of sodium hydroxide and sodium sulphide. Uniform distribution of cooking liquor throughout the wood chips results in uniform delignification which in turn produces high yield and high quality pulps. The distribution of liquor within the wood structure occurs in two stages. The first stage is penetration of liquor where the cooking liquor flows into the air filled voids of the wood chips under a pressure gradient. This is followed by the second stage where the ions and other soluble matter in the liquor diffuse through the wood chips under a concentration gradient. Liquor penetration and diffusion is affected by chip dimensions (length, width and thickness), with chip thickness being the most critical. It can be enhanced through the use of pulping additives such as anthraquinone (AQ) and surfactants. Anthraquinone increases delignification and protects the carbohydrates from the peeling reaction. Surfactants wet and emulsify the extractives in the wood chips and are able to keep the lignin and extractives in solution. These pulping additives are effective in small quantities and reduce cooking time, kappa number, alkali consumption, cooking temperature and rejects. In addition, they also improve pulp washing, pulp viscosity and yield.

There are currently several methods documented in the literature to measure liquor penetration and diffusion, but no standard method exists. The South African pulp and paper industry expressed their need for a simple and reliable measurement method that, in turn, will facilitate a better understanding of factors affecting liquor penetration and diffusion. A review of the literature has shown that Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) can be used as a method to measure liquor penetration and diffusion in wood chips. The advantage of this method is that it is able to qualitatively show the profile of the ions in the liquor as the ions travel from the edge of the wood chip towards the centre, as well as quantitatively show the amount of ions that have impregnated the wood chip after penetration and diffusion. In this study, Environmental Scanning Electron Microscopy/Energy Dispersive X-ray (ESEM/EDX) was used owing to the added advantage of the technique being non-destructive and simpler to use. A major advantage of ESEM/EDX over SEM/EDX is that it can be used for non-conductive samples without the need for carbon coating the sample to increase its conductivity. This eliminates the interference of carbon from the analysis.

The objective of this study was two-fold. The first objective was to optimise the ESEM/EDX method for measurement of liquor penetration and diffusion in wood chips. This was achieved by investigating factors that affect ESEM/EDX measurement such as moisture content and interference of sodium and sulphur already present in the uncooked wood chips. Subsequent to this, the sensitivity and repeatability of the method was assessed. The second objective was to then use the optimised ESEM/EDX method to illustrate its applicability by measuring the influence of pulping aids and chip dimensions on liquor penetration and diffusion during kraft pulping.

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