废SCR脱硝催化剂中砷元素赋存形态与氧化碱浸脱除

Chemical Speciation and Removal Mechanism of Arsenic from Spent SCR Catalysts

以燃煤电厂废SCR脱硝催化剂为研究对象,通过XPS、ICP等检测方法研究废催化剂中杂质As元素的含量及赋存形态,发现废SCR催化剂中As元素存在形态为As_(2)O_(3)与As_(2)O_(5)。采用氧化碱浸技术,将难与NaOH反应的As_(2)O_(3)氧化为更易反应的As_(2)O_(5),实现不同价态As元素的深度脱除。分别研究了氧化剂种类、氧化剂浓度、浸出剂浓度、反应温度、时间、液固比等因素对As元素脱除率的影响规律。结果表明,当反应温度为30℃、反应时间为4 h、H_(2)O_(2)浓度为5%(质量分数)、NaOH浓度为10%(质量分数)、液固比为2 mL/g时脱砷效果最佳,废催化剂中As含量由1334×10^(-6)降低至50×10^(-6),脱除率达到98.5%。所得产物为脱砷后催化剂,主要由锐钛矿型TiO_(2)与少量WO_(3)组成。通过EPR、XPS技术对H_(2)O_(2)强化浸出的反应机理进行研究,发现废SCR催化剂中V元素主要以V^(4+)、V^(5+)两种价态存在,其与NaOH反应生成的HV_(2)O_(5)^(-)、VO4^(3-)和H_(2)O_(2)引发的类Fenton反应,可产生具有强氧化性的羟基自由基,将难以脱除的As^(3+)氧化为更易溶于碱液的As^(5+),提高As元素的脱除率。

The valence state and speciation of As in spent SCR catalysts used in coal-fired power plants was studied by XPS and ICP.The results show that As in spent SCR catalysts exists in the form of As_(2)O_(3)and As_(2)O_(5).As_(2)O_(3)is difficult to be leached directly in weak alkaline solution,so it is difficult to be removed completely.In order to solve this problem,the alkali method with oxidant added was proposed.The effects of oxidizer species,time,temperature,NaOH concentration,H_(2)O_(2)concentration,solid to liquid ratio on the removal efficiency of As were studied.The results show that the removal efficiency of As could reach as high as 98.5%under the optimal leaching conditions of reaction temperature 30℃,reaction time 4 h,NaOH concentration 10wt%,H_(2)O_(2)concentration 5wt%,the ratio of liquid to solid 2 mL/g,and the arsenic oxide content in the solid was reduced from 1334×10^(-6)to 50×10^(-6).The content of As in detoxified SCR catalysts could reach the requirement of fresh SCR catalysts.The removal mechanism of As was studied by EPR and XPS methods,and a new advanced oxidation process was used to treat spent SCR catalysts.The V 2O 5 in spent SCR catalysts reacts with NaOH to form HV_(2)O_(5)^(-)and VO_(4)^(3-).Under the influence of HV_(2)O_(5)^(-)and VO_(4)^(3-)in the leaching solution,H_(2)O_(2)produces hydroxyl radicals(·OH)in alkali solution.Hydroxyl radicals have strong oxidizing properties and oxidize As^(3+),which is not easily soluble in weak alkali,to As^(5+).