Chromium speciation in tannery sludge residues after different thermal decomposition processes

Thermal decomposition is a promising technology for tannery sludge disposal, as through thermal decomposition, the energy in tannery sludge could be recovered, and the abundant harmful metal chromium (Cr) could be fixed in the solid residues, which might be recycled as the chromium-rich mineral for further use. In order to provide objective evaluation on the various thermal decomposition technologies for tannery sludge disposal from safety perspective, in this study, different thermal decomposition processes of tannery sludge in typical reactors were simulated on a bench-scale equipment in laboratory, and the transformation characteristics of Cr species in tannery sludge during the processes were systematically investigated. Through characterization of the solid products of decomposition, only Cr (III) species of Cr2O3 and MgCr2O4 were determined in the pyrolysis product, while both Cr (III) species of Cr2O3 and Cr (VI) species of CaCrO4 were detected in the incineration product. For the processes of simulating gasification, when the final stage was combustion, Cr (III) species of Cr2O3 and MgCr2O4 and Cr (VI) species of CaCrO4 were observed, while when the final stage was reduction, only Cr (III) species of Cr2O3, MgCr2O4 and CaCr2O4 were observed. It was found that the Cr (III) species of MgCr2O4 generated in the pyrolysis stage has high stability and is difficult to be oxidized in the combustion stage, while Cr (VI) species of CaCrO4 produced in the combustion stage can be removed in the subsequent reduction stage. Based on the results, among the various thermal decomposition technologies to dispose of tannery sludge, pyrolysis is the safest, while incineration presents a huge risk. The adoption of downdraft gasifier is safer than that of updraft gasifier. Therefore, employing pyrolysis or downdraft gasification technology to dispose of tannery sludge is expected to fix Cr in the form of non-toxic Cr (III) in the solid residue, and then reuse it as tannery agent, so as to achieve the goal of cleaner production.

Automated summary

Thermal decomposition is a promising technology for tannery sludge disposal, where different processes were simulated to investigate the transformation of Cr species. The study found that pyrolysis is the safest method for Cr fixation and reuse.