Chelating extraction and recovery of copper from hazardous heavy metals sludge
Date Issued
2007
Date
2007
Author(s)
Chang, Fang-Chih
DOI
zh-TW
Abstract
Sludge containing heavy metals is a widespread and complicated headache for many related industries. The TCLP leaching concentration of sludge is higher than the standards for defining hazardous waste. Thus, the resource recovery of heavy metal from sludge is an emergent environmental issue. In this study, we evaluate the performances of novel copper removal processes for printed circuit board and electroplating wastewater sludge applying chelant extraction (Biodegradable chelate and Persistent chelate) and powdered iron cementation, followed with the reuse of chelating agents to chelate supplementary fresh copper-containing sludge. The contents of this study are: (1) to extract the heavy metals from sludge; (2) to recover the heavy metal and to recycle the chelating solution; (3) to recycle the hazardous heavy metal sludge that is pretreated by chelating extraction; (4) to evaluate the leaching behavior of heavy metals from green materials that is produced from the heavy metal sludge after chelating extraction.
The results of this study indicated that sludges A, C, and D were slightly alkaline, but sludge B was very slightly acidic. The study showed that the sludges contained copper of high total concentrations (about 7.2-28.2 wt. %), with small total concentrations of nickel and zinc. The leaching concentrations of copper in all sludges were extremely high, especially in sludge B. Based on this data, the recovery of copper from sludges appears to be of practical, as well as environmental, value. The results of sequential extraction indicated that heavy metals in sludge A and C existed as the forms of Fe/Mn-oxide bound and organically bound mostly, but the forms of exchangeable bound and carbonate bound mostly for sludge B. Thus, the metal mobility and potential bioavailability was lower for sludge A and C, but contrary to sludge B.
For the extraction experiments, the results indicated that the best extraction efficiency of heavy metals was 0.25 M EDTA or DTPA or EDDS for sludge A, 0.1 M chelating agents for sludge B and sludge C. The experimental results were similar to the simulated results using MINEQL+. The extraction efficiency of heavy metals increased when the ratio of liquid to solid increased, irrespective of the kind of chelating agent. The successive extraction using EDTA would achieve the better extraction efficiency for sludge A. The distribution of the metal fractions in the sludge would become stable after chelating extraction. For Cu, the order of extraction efficiency was EDDS ≥ EDTA ≥ DTPA > NTA. The easily biodegradable chelating agent EDDS has been proposed as a safe and environmentally benign replacement for EDTA in sludge extraction.
Results of the cementation experiments showed that precipitation efficiencies of Cu of were higher than 80% when the Fe:Cu molar ratio was as high as 6:1 at pH 3 for each sludge sample. The deposit of zinc and nickel results from the coprecipitation on copper precipitated by cementation processes. The XRD analysis results of recovered copper from the chelated cupric wastewater indicated that copper deposits on the iron surface almost entirely in the form of the copper molecules.
The more powdered iron used, the higher the recovered efficiency of EDTA and DTPA. The efficiency of re-extraction using reused EDTA reached the original level of chelating extraction only for sludge B. However, the copper extraction efficiency for each sludge is quite approximate when using DTPA recovered at various iron concentrations. This is because the leachability of sludge B was superior to that of sludge A or C.
The removal of Cu, Zn, and Ni from clelated wastewater by sulfide precipitation was well, irrespective of the kind of chelating agent or sludge. This may be related to the fact that CuS has a higher pKa value than CuEDTA or CuDTPA. The supernatant could be recovered and reused again as chelants for sludge extracting solutions. However, the extraction efficiency of the supernatant after being recycled over three cycles was lower than that of fresh chelating agents. Reduction ratios of copper from supplementary sludge using the extract from the metal-sulfide precipitation were 36-54% for sludge A, 6-10% for sludge B, and 16-24% for sludge C in comparison with previous extraction.
The heavy metal sludge after chelating extraction and mining residues were evenly mixed at a weight ratio of 40% : 60% into raw aggregate pellets of 3-5 mm diameter. The lowest density of 0.74 g/cm3 and low compressive strength of 4.41 MPa could be obtained at sintering temperature of 1150°C for 15 min. The concentrations of heavy metals leached tend to decrease with increasing sintering temperature. Results obtained by sequential extraction show that concentrations of Cd, Cr, Cu, and Pb in LWA sintered at 1150°C for 15 min dropped significantly to the regulatory threshold.
Subjects
含銅重金屬污泥
螯合萃取
化學置換反應
螯合再萃取
輕質骨材
序列萃取
Copper contained sludge
Chelant extraction
Cementation
Re-extraction
Lightweight aggregate
Sequential extraction
SDGs
Type
thesis
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