| dc.description.abstract | Previous research has shown that to retrieve chromium metal from electroplating sludge is feasible and beneficial because the electroplating sludge is abundant in oxides of chromium and lead (i.e., PbCrO4, PbO, Cr3O4, and CrO). This study investigated the feasibility of retrieving pure metallic chromium from the
electroplating sludge with or without sewage sludge, by applying a hydrometallurgical method followed by a thermite process.
Sulfuric acid was chosen to leach the chromium in this study. The tested electroplating sludge consisted of 19.62 wt.% Cr and 54.49 wt.% Pb as the main elements in compositions Sulfuric acid was chosen to leach the Cr from the sludge. In this process, chromium was separated as Cr6+ ion in the leachate from Pb that was precipitated as PbSO4. The Cr6+ in the leachate was further reduced by Na2S2O5 and calcined at 1000℃ to yield the calcined product, the solid of chromium oxide. The retrieved calcined product was identified as Cr2O3. To further retrieve pure metallic chromium from Cr2O3, the calcined product was further processed with aluminum powder to activated thermite reaction between Cr2O3 and Al. This thermite process resulted in the reduction of chromium oxide to metallic chromium by the action of aluminum power, thus pure metallic chromium was retrieved. In the thermite process, sewage sludge, with silicon oxide as main composition, was added as a glass former in order to enhance the separation of metallic chromium from the slag mixture, and to modify the mechanical properties of the resultant slag. The results indicate that 95.34 wt.% leaching rate of chromium was achieved by using a 6N sulfuric acid at a L/S ratio of 6. The Cr6+ in the leachate, after being reduced by Na2S2O5, filtered as principate, and calcined at 1000℃ under atmosphere for 1 hour, yielded a product of 82.92 wt.% chromium oxide. Subsequently, in the thermite process, the stoichiometry of aluminum powder was experimentally determined. It was found that a stoichiometry, Cr2O3: Al powder, of 2.28:1 by weight was optimum to yield a maximum purity of metallic chromium (i.e., 95.53wt.%). The retrieval rate of metallic chromium achieved 92.20wt.% by the thermite process from the calcined product (solid of chromium oxide), or 87.90 wt.% based on the chromium in the starting electroplating sludge. Moreover, the purity of retrieved chromium in the ingot achieved 93.88 wt.%.
On the other hand, in experiments with 5-25 wt.% additions of sewage sludge as a glass former, the ingot yielded from the subsequent thermite process, consisted of 95.56 wt.% chromium in purity. In general, the purity of retrieved metallic chromium increased from with the increased addition of 5-25 wt.% sewage sludge, whereas the retrieval rate was lowered to 71-20wt.%, partially due to an increased partition of metallic chromium to the slag, and partially due to the incomplete thermite reaction at lower reaction temperature caused by the addition of sewage sludge. However, the modified slag showed better results for TCLP and other mechanical property tests. These enhanced properties are supposed to be contributed by the crystalline phases of NaAlSiO4 and Na6Al4Si4O17 present in the slag.
The results of this work suggest that to retrieve metallic chromium from chromium-containing electroplating sludge with or without the addition of sewage sludge, by applying a combination of hydrometallurgical method and a thermite process is feasible and recycling-beneficial.
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