Removal of metal ions from wood chips during acidic leaching 2: Modeling leaching of calcium ions from softwood chips

The purpose of this work was to model leaching of calcium ions from softwood chips under acidic conditions. The first step was to study the leaching of calcium ions from well-defined sawn samples of spruce wood. Leaching experiments with varying geometries under acidic conditions (pH 2.5 and 60°C) were conducted to determine the diffusion coefficients,giving 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 (in the radial and tangential directions). Results show that leaching of calcium ions from spruce can be described with a model assuming that the wood piece is semi-homogenous, with different diffusion coefficients in different directions.A model describing leaching of industrially cut wood chips,taking into account cracks, rough surfaces and size distribution in the batch of wood chips, was developed using a model of a wood chip with an enlarged surface area. The diffusion coefficient and the chip model were then used to develop models of the leaching process that can be used to simulate and evaluate different leaching designs.