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.
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.
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).
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.
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.