Removal of Metal Ions from Wood Chips during Acidic Leaching

Licentiate thesis, 2006

Wood chips are the main source of intake of non-process elements at kraft pulp mills. It would be advantageous to decrease the content of these elements in the chips prior to pulping. In this thesis, leaching of metal ions from wood material has been studied in order to obtain fundamental knowledge regarding a process in which metal ions are leached from wood chips. Experiments with wood powders were conducted to study the influence of pH on the release of metal ions. Results indicate that metal ions are, to a large extent, released when the pH is lowered. It was, however, noted that part of the calcium in the wood material may be present in the form of salts of low solubility. Analyses imply that part of the calcium is present as calcium oxalate. When industrial wood chips were analyzed, the fraction of calcium present as calcium oxalate was found to be: 3% in Scandinavian softwood, 11% in birch and 16% in eucalyptus (E. globulus). Leaching experiments with sawn wood samples under acidic conditions (pH 2.5 and 60°C) have been conducted to determine the diffusion coefficients of calcium in spruce and birch. A diffusion coefficient of 5.9 x 10-10 m2/s in the longitudinal direction and 0.41 x 10-10 m2/s across the grain was determined for spruce. For birch, diffusion coefficients in the longitudinal direction and across the grain could not be determined with good accuracy, due to morphological characteristics. Instead a diffusion coefficient in the wood cells surrounding the vessels was determined to be 2.8 x 10-14 m2/s. Industrial wood chips were leached at different temperatures. Birch and softwood chips were shown to have the same general leaching pattern for calcium, while it was harder to remove calcium from eucalyptus. Potassium was leached from softwood, birch and eucalyptus chips in a similar way. This indicates that there is a difference in accessibility of calcium and potassium ions in eucalyptus chips. A model describing leaching of industrial softwood chips, taking into account cracks, rough surfaces and size distribution within the batch of wood chips, was developed using a model of a wood chip in which an enlarged surface area was applied. This model can be used for simulation and evaluation of different leaching process designs.