The effect of adsorbed chromium on the pyrolysis behavior of brown coal and the recovery of chromium

Brown coal-based materials are excellent adsorbents for reducing chromium(VI) to chromium(III) and afterward immobilizing these chromium(III) by the binding of oxygenic functional groups in adsorbents. In the study, the approach of pyrolysis is employed for the treatment of Cr-loaded solid waste. The effects of adsorbed chromium on the pyrolysis of Xilingol brown coal were studied, and the solid char residues were collected to characterize with XPS, XRD and SEM/EDX. For the pyrolysis in Ar, the mass loss rates of Cr-loaded samples were much higher than that of unloaded samples above 750 °C, together with the increase in CO and H2 emission. XPS spectra revealed that the increase in CO could be related to formation of [Cr–O–C]. For the pyrolysis in CO2, the presence of chromium was more favorable for the conversion of char, especially demineralized brown coal and kerogen. The maximum decomposition temperatures for the Cr-loaded samples were about 200 °C lower than that of unloaded samples. The char residue yields of Cr-loaded samples were obviously higher than that of corresponding unloaded samples (at 1200 °C). Finally, the chromium in the solid residue was recovered in the form of Cr2O3. The present study exploits an approach method for both brown coal waste treatment and chromium recovery.