Characterization of residues from thermal treatment of CCA impregnated wood: Chemical and electrochemical extraction

Thermal treatment of CCA impregnated waste wood is a way to utilize the energy resource of the wood and at the same time to reduce the volume of the waste. An issue of concern in relation to the thermal treatment is As emission to the air. Meanwhile there is still a matter to cope with when methods to avoid As emission are implemented; the residues with increased concentrations of Cu, Cr and As. In the present paper two different residues after thermal treatment are characterized: mixed bottom and fly ash from combustion of CCA impregnated wood and coke from pyrolysis of treated waste wood. By SEM/EDX it was found that the coke still showed wood structure and that Cu, Cr and As was to be found inside this wood structure. Cu was found alone while Cr and As was often found together. By chemical analysis it was found, too, that the coke contained high concentration of Zn, probably from paint. Chemical extraction experiments in acids were conducted with the coke and it was found that the order of extraction (in percentage) was Zn > Cu > Cr (As not measured). A similar investigation of the ash from combustion showed presence of small pieces with wood structure, too, though most particles were without sign of the material combusted. Cr was found build into the structure of some matrix particles, Cu, too, but Cu was also found condensed on the surface of some larger ash particles. As on the other hand was found associated to Ca in particles with an open structure. Chemical extraction with inorganic acids showed the order of percentages mobilized as: As > Cu > Cr. Electrodialytic extraction was tested with the ash. The aim was to test if this method could separate the ash from the mobilized parts of Cu, Cr and As and the method showed promising. As much Cr and Cu as solubilized by chemical extraction at pH below 1 was removed and only 8% As was left in the ash.