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Abstract:

After being used as an abrasive agent, copper slag is considered a solid waste in Vietnam because appropriate waste treatment techniques and effective management options are not available. This classification as solid waste is mainly due to the fact that the blasted copper slag (BCS) contain impurities of potentially toxic elements (PTEs), such as As, Cd, Co, Cr, Cu, Mo, Ni, Pb, and Zn that can be released into environment. The present study was conducted with the aim to characterize the solid phase composition of blasted copper slag (BCS) and to investigate the leaching of PTEs from BCS under influence of pH. A wide pH range (1-10) was investigated in order to assess the environmental implications of different management scenarios. Besides amorphous phases, fayalite, magnetite, maghemite, quartz and elemental copper were the main minerals identified by X-ray diffraction (XRD). Field Emission Gun Electron Probe Micro Analysis (FEG-EPMA) results indicated that BCS mainly consisted of Fe, O and Si with minor amounts of S whereas the copper bearing phase was composed of Cu, Pb, and S with small amounts of Si. This indicates that copper in BCS was present not only in metallic phases but also in association with S in the form of sulphides (sulphides of Cu, Cu-Pb, Cu-Pb-Zn). Mapping of selected areas in thin sections with wavelength dispersive spectrometry also confirmed the presence of these sulphide phases which were not detected by XRD. Among the examined PTEs, Cr was found in Fe-O rich phases, while Mo was observed in a glass inclusion. These This might explain the low leachability of these elements in neutral pH conditions. Leaching tests indicated that most of the PTEs showed a low environmental concern in the neutral-alkaline pH range due to the low concentrations in the leachate compared to EU leaching limit value for non-hazardous waste, except As which exceeded this threshold at pH 10. Moreover, in the case of acidification (e.g. as a results of co-disposal with acid waste or as a result of acid rain), special attention should be paid to Cu and Zn which exhibit an important release at pH 3-5. On the other hand, recovery of Cu by acid treatment of the slag could be considered since an extraction with HNO3 1 M during 24 h showed that 39 % of the total Cu content (or 3369 mg/kg) was released from the slag. The feasibility of Cu recovery from the slag should be further investigated.