## 3 Results  
### Subsection Samples Characterization  
According to the X-ray diffraction (XRD) spectra (Figure 2), in all thermally treated samples, the peaks related to the presence of chromium were attributed to the hexavalent form CaCrO\({}_{4}\) (Joint Center for Powder Diffraction Studies--JCPDS card No. 08-0458), except for the samples treated at 750 \({}^{\circ}\)C, where the observed peaks were attributed to the trivalent form MgCr\({}_{2}\)O\({}_{4}\) (JCPDS card No. 10-0351). It should be clarified that the identification of chromium's form in the initial tannery sludge was not possible, due to the amorphous structure of these samples. Instead, the crystallinity increased after the thermal treatment, resulting in the appearance of sharp peaks. The remaining peaks of these spectra present mainly the CaCO\({}_{3}\)-calcite (JCPDS card No. 47-1743) and the anhydrous CaSO\({}_{4}\) (JCPDS card No. 37-1496). The intensity of calcite peaks decreased as the treatment temperature increased, due to its decomposition, while in the case of anhydrite CaSO\({}_{4}\), they remain almost unaffected in the examined temperature range.  
Figure 1: Flow diagram of the batch-recirculation prototype reactor experimental set-up.  
As Table 1 shows, a significant weight loss was observed, slightly higher than 50% and almost stable, for all the examined treatment temperatures. These results can be attributed to dehydration/dehydroxylation processes, organic carbon combustion and calcium carbonate decomposition, which mainly took place at temperatures below 500 \({}^{\circ}\)C. Such a weight loss led to almost doubling of metal content in the corresponding residues/ashes. Also, it was verified that the initial tannery sludge contained only Cr(III), whereas after the application of thermal treatment, the dominant species were Cr(VI), results which are in good agreement with the respective XRD spectra. The importance of pH and electrical conductivity measurements lies in the determination of appropriate equilibrium conditions during the following hydrometallurgical process, i.e., the dominant species of eluted/leached ions and their corresponding quantity, respectively. In fact, all samples (i.e., the primary waste and the resulting ashes) have been shown to be highly alkaline materials, due to the dissolution of calcium compounds. In contrast, the quantity of leached ions after combustion was substantially higher than that in the initial sludge, according also to the corresponding electrical conductivity measurements. These results can be attributed to the decomposition of organic matter, leading to the production of soluble ash constituents and, therefore, the release of more dissolved ions.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Treatment Temperature (\({}^{\circ}\)C)**} \\ \hline  & **Initial** & **500** & **550** & **600** & **650** & **700** & **750** \\ \cline{2-7}  & & & & **\% w/w** & & & \\ \hline Weight loss & - & 52.6 & 52.8 & 53 & 54.5 & 55.4 & 55.5 \\ Organic matter & 22 & 1.4 & 0.2 & 0 & 0 & 0 & 0 \\ Organic carbon & 12.2 & 0.8 & 0.1 & 0 & 0 & 0 & 0 \\ Inorganic carbon & 3.1 & 1.7 & 1.3 & 0.8 & 0.4 & 0.1 & 0.1 \\ Crtotal: & 14.1 & 28.3 & 28.5 & 28.5 & 28.6 & 29 & 29 \\ -Cr(III) & 14.1 & 6 & 5.7 & 4.5 & 3.5 & 3 & 3.2 \\ -Cr(VI) & - & 22.3 & 22.8 & 24 & 25.1 & 26 & 25.8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physico-chemical characterization of raw tannery sludge and the corresponding ashes after the application of thermal treatment.  
Figure 2: X-ray diffraction (XRD) diagrams of raw tannery sludge and the corresponding ashes after the application of thermal treatment.