Project leader: Harald Brelid
Tel.: +46 31 772 3102
Fax: +46 31 772 2995

Metal in wood

Aim: The object of this study is to increase the knowledge of metal ions in wood and their behavior in the fibre line of the kraft process. 

Background: Wood is a complex cellular composite material. It has been shown that the metal ion content is high in certain morphological regions, such as, cell corners, the middle lamella, ray cells and epithelial cells around resin canals. The modes of attachment vary among different types of metal ions. Divalent metal ions such as Ca, Mg and Mn are to a large extent present as counter ions to carboxylic acids in the wood polymers whereas Fe (probably as Fe3+) has been found to be present as precipitates in the wood structure.

The wood used as raw material in kraft pulping is the main source of several non-process elements, NPEs, (e.g. Ca, Mn, Fe and Cu). NPEs in the kraft process are elements that have a negative impact on the chemical recovery cycle and the fiber line. As the degree of system closure in pulp mills is increased, the negative effects of NPEs are amplified, and thus the problems connected with NPEs have come in focus in the pulping industry. The behavior of the inorganic compounds in the wood chips during pulping is therefore of great importance to the kraft process. In the literature, several studies dealing with the mass balances of different metal ions in the kraft process can be found. The distribution and behavior of different metal ions in the wood material on a morphological level during pulping are, however, for the most part unknown. In the current project the removal and redistribution of metal ions in the wood material during kraft pulping is studied on a morphological level.

Project description: During kraft pulping, organic wood components are fragmented and dissolved in the alkaline pulping liquor. Kraft pulping also leads to a partial redistribution of metal ions originally present in the wood material into the black liquor. It can also be expected that metal ions, to a certain extent, are redistributed in the wood material. Multivalent metal ions, such as Ca and Mn, have a strong tendency to form inorganic precipitates under kraft cooking conditions. Computer simulations have shown that after a completed kraft cook, almost all Ca and Mn should be present as inorganic precipitates. A substantial part of the metal ions is, however, distributed from the wood material into the black liquor during cooking. In the black liquor, these metal ions are probably, at least partly, present in the form of colloidal precipitates. In order to study the redistribution of metal ions, and by that gain a more in depth knowledge of the behavior of different metal ions in kraft cooking, we use analysis methods such as, m-XRF, SEM-EDXA and X-ray diffraction.

Project group:
Harald Brelid, CTH
Anders Rindby
CTH
Geoffrey Daniel, SLU

Industrial mentors:
Jiri Basta, EKA
Ann Marklund, M-real