Metal Retention in the Solid Residue after Low-Temperature Pyrolysis of Chromated Copper Arsenate (CCA)-Treated Wood

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Based on previous research work, it is expected that dry recuperation of the metals in chromated copper arsenate (CCA)-treated wood waste can be achieved by low-temperature pyrolysis if a combination of pyrolysis temperature and residence time can be found, for which the two requirements: (1) zero-emission of metals, and (2) metal agglomeration are satisfied. To determine the working conditions for zero metal release, pyrolysis experiments with precisely controlled temperature and residence time and accurately known input metal concentrations of the CCA-treated wood samples, were conducted. Temperatures lower than 327°C are considered, since the release of arsenic is thought to be controlled by the reduction reaction of As2O5 to As2O3, which occurs sharply at 327°C. Metal retentions are calculated based on the measured metal concentrations in the pyrolysis residue and in the dried CCA-treated wood. The results suggest that pyrolysis at 300°C for 20 min gives rise to zero release for the three metals (Cr, Cu, and As), while pyrolysis at 320°C leads to nonnegligible metal releases. Thus, the experimental results suggest that the decomposition temperature of As2O5 is lower than 320°C. However, due to the high experimental uncertainty the observations are inconclusive. For future industrial applications, this study already shows that very careful control of process parameters will be required to ensure a zero arsenic emission. Moreover, this study indicates that an arsenic sampling and analysis method has to be developed and validated to be able to measure the arsenic emissions directly with high accuracy, instead of measuring metal retentions in the solid residue.