Chelating extraction and recovery of copper from hazardous heavy metals sludge
|Keywords:||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||Issue Date:||2007||Abstract:||
在螯合萃取方面，以0.1 M EDTA、DTPA或EDDS即可有效萃取B與C污泥中之重金屬，對A污泥則需提高EDTA、DTPA或EDDS之濃度至0.25 M才能達到較佳之萃取率；這與MINEQL+之模擬結果相似；且不論螯合劑的種類，萃取效率隨著液固比的增加而上升。A污泥連續萃取實驗，以使用EDTA連續萃取效果較佳；B與C污泥，則不論連續萃取的組合，均可獲得不錯的效果。0.1 M EDTA、DTPA或EDDS萃取後，污泥中重金屬結合型態均偏向生物不可利用，即萃取後會趨於穩定，這與萃取後污泥之TCLP試驗結果相符。比較EDTA、DTPA或EDDS對A、B與C污泥中Cu之萃取效率，發現EDDS對Cu的萃取效率與EDTA或DTPA相當且大於NTA，因此，EDDS的生物易分解性將使螯合萃取技術更具環境競爭力。
螯合後重金屬污泥資源化研究方面，混合40%之螯合後重金屬污泥與60%之石材污泥，在燒結時間15 min、溫度1150oC時，可資源化成密度0.74 g/cm3、抗壓強度4.41 MPa之輕質骨材。序列萃取分析重金屬污泥為基礎的輕質骨材則發現，污泥中重金屬多鍵結在鐵錳氧化態、有機態與矽酸鹽態；隨燒結溫度愈高，燒結輕質骨材之溶出濃度愈低，在燒結溫度達1150oC時，序列萃取之溶出總量即可符合TCLP之標準。
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.
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