濕式冶煉鉻污泥配置廢棄物衍生鋁熱熔融劑回收鉻金屬之研究; Chromium Metal Recovery from Chromium-containing Sludge by Using Thermite Reaction Technology Combined with Hydrometallurgical Approach

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Title:	濕式冶煉鉻污泥配置廢棄物衍生鋁熱熔融劑回收鉻金屬之研究;Chromium Metal Recovery from Chromium-containing Sludge by Using Thermite Reaction Technology Combined with Hydrometallurgical Approach
Authors:	許琇婷;Hsiu-ting Hsu
Contributors:	環境工程研究所
Keywords:	鋁熱反應;濕式冶煉;鉻回收;資源化;Thermite reaction;chromium recycling;hydrometallurgy;resources
Date:	2010-12-22
Issue Date:	2011-06-04 15:05:19 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究使用濕式冶煉法先行萃取電鍍工業設備電解槽底泥中所含之鉻，將其餘重金屬分離，純化成氧化鉻含量較高之產物，再利用鋁熱反應技術回收其中之鉻金屬。研究結果顯示，使用硫酸濃度為6N，鉻電鍍污泥及硫酸之固液比為1：60的條件下，有最佳之鉻溶出率95.34%，還原、鍛燒後可成為氧化鉻含量達82.92wt.%之氧化鉻產物。進行鋁熱反應回收鉻金屬之最佳配比為80克氧化鉻產物混合35.11克鋁屑，即重量百分比2.28：1，有最大放熱溫度2214℃，使氧化鉻產物中之氧化鉻還原為鉻金屬存在於金屬錠中，鉻含量佔金屬錠總重之95.53wt.%，且晶相分析及半定量結果顯示其為金屬鉻，鉻之回收率最高為87.92wt.%，回收後之鉻金屬品位極高，可以投入製程再利用或作為其他用途，減少原物料消耗。添加淨水污泥進行熔渣改質實驗中，最佳配比為氧化鉻產物：鋁屑：淨水污泥=2.28：1：0.16，即混合鉻電鍍污泥80克混合鋁屑35.11克、淨水污泥5.76克進行共同熔融，能夠改善鋁熱反應過於劇烈之特性，幫助金屬錠與熔渣分離，並且在熔融過程中使Si進入熔渣中，形成Si-O-Si鍵結，將仍存在於熔渣中之有害重金屬穩定化，使其不易溶出於環境中，使之具材料化之潛力。研究結果顯示，熔渣改質實驗最佳配比中出現NaAlSiO4及Na6Al4Si4O17矽酸鹽晶相，重金屬鉻溶出濃度最小為0.35mg/L，耐酸鹼試驗在強酸HNO3、HCl及H2SO4與強鹼NAOH中損失最少。綜合上述結果，濕式冶煉法結合鋁熱熔融處理可將鉻污泥中的金屬回收，同時將剩餘熔渣無害化，且所得熔渣具有材料化之潛力。 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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