Recycling Inorganic Residues from Wood-based Industries to the Forest- study of Treatment Techniques and Leaching Characteristics

Doktorsavhandling, 2005

Abstract. In order to minimize the negative impact of loss of nutrients in the forest soil (when harvesting the wood), inorganic solid residues, from wood-based industries, have to be recycled back to the forest soil. However, due to high alkalinity and a high dissolution rate of the elements, the residuals cannot be recycled without being treated into suitable and recyclable products. These products should meet the needs of forest soils without compromising the soil environment. For this purpose, it is necessary to measure and characterize the leaching rate of the product under well-defied conditions. The present study offers theories and methods for characterizing the products prior to recycling. A modified column-pHstat leaching test was developed in which the leaching tests could be performed under well controlled conditions. By using this leaching test, it was possible to make a more accurate evaluation of the leaching characteristics of the products than when using the common column test or batch test. The results of the laboratory leaching tests were then verified by performing field studies. It was found that by thoroughly characterizing the products prior to recycling, it was possible to estimate leaching trends in the field. It was concluded that by using a combination of well controlled experiments and mathematical modeling, it was possible to understand the different phenomena that control the leaching mechanism of both easily limited soluble species. Two different mechanisms were proposed for the leaching rate determining steps. A diffusional mass the transfer model could well describe the leaching rate of easily soluble species, while a heterogeneous reaction mechanism consisting of both surface reaction and mass transfer established the rate determining step for less soluble salts. The influence of treatment techniques on physical structure and leaching properties of the products were also investigated. Depending on the chemical composition of the products, as well as the operational condition during the treatment process, different leaching properties were achieved. It was found that by applying thermal treatment, the physical structure of products becomes harder and more stable than the physical structure of non-treated residues. One of the major findings was that in order to control the leaching of limited soluble species, the chemical structure of the Ca-salts should be modified to a less soluble salt. Moreover, the physical characteristics that correlate with the leaching properties of both easily and limited soluble species need to be adjusted.