熔盐氯化法选择性提取废旧荧光粉中稀土元素

Selective Extraction of Rare Earth Elements from Waste Phosphors by Molten Salts Chlorination

废旧稀土荧光粉的回收利用对我国稀土资源的可持续发展和环境保护具有重要的意义。本文以AlCl_(3)-KCl熔盐为氯化剂,对废旧荧光粉中的稀土元素进行氯化提取。在热力学分析的基础上,考察了氯化焙烧条件和碱熔处理对稀土氯化提取率的影响,并采用X射线衍射仪(XRD)和扫描电子显微镜-X射线能谱仪(SEM-EDS)对反应前后废旧荧光粉的物相组成和微观形貌进行了分析。由于稀土元素在荧光粉中的存在形式不同,其氯化提取率也存在着较大差异。在焙烧温度700℃、焙烧时间3 h、熔盐/废旧荧光粉质量比4:1的条件下,Y和Eu的氯化提取率在96%以上,而Ce和Tb的提取率不到30%。碱熔处理可将铝酸盐结构中的稀土元素转化为氧化物,再进行氯化焙烧时Ce和Tb的提取率得到了显著提高。结果表明,采用熔盐氯化-碱熔-二次氯化的方法可对废旧荧光粉中的稀土元素进行高效选择性提取:4种稀土元素的总氯化提取率达97%以上,而Si,Ca,Al,P等主要杂质元素则不被AlCl_(3)-KCl所氯化。

Waste tricolor phosphors were valuable secondary resources of rare earth elements including Y,Eu,Ce and Tb,and thus the recycling of waste phosphors was of great importance both for the sustainable utilization of rare earth elements and environment protection.Currently,the waste phosphors were mainly recovered by hydrometallurgical technology,during which a large quantity of acids,organic reagents and other chemicals were consumed,discharging massive waste of liquids and residues.Moreover,most of the existing researches were focused on recovery of Y and Eu in the red phosphor which was easily dissolved by acids,while the elements Ce and Tb in green and blue phosphors were ignored due to their highly stable aluminate structures.Therefore,the development of efficient and environmentally friendly technology was urgently required for the comprehensive recovery of waste rare earth phosphors.In the present work,the method of chlorination roasting assisted by alkaline fusion was used to selective extract the rare earth elements from waste phosphors,during which molten AlCl_(3)-KCl and Na_(2)CO_(3 )were selected as agents for the chlorination roasting and alkaline fusion,respectively.The waste phosphors were firstly suffered from chlorination roasting to extract the rare earth elements Y and Eu.The roasting products were then suffered from secondary chlorination after alkaline fusion to extract the elements Ce and Tb.From the thermodynamic analysis,the elements of Y,Eu,Ce and Tb in their oxides could be selectively chlorinated to their chlorides,while the impurity elements such as Si,Al and P could not be converted to chlorides by roasting with AlCl_(3).Then,the effects of roasting conditions including roasting temperature,holding time and mass ratio of chlorides/waste phosphors on rare earth element extraction efficiencies were further investigated.Since the rare earth elements existed in different crystal structures in waste phosphors,Y and Eu in the red phosphor were easily chlorinated by molten AlCl_(3)-KCl,but the green phosphor and blue phosphor were hardly attacked by the molten chlorides,and therefore there were great differences in their extraction efficiencies.It was indicated that both the extraction efficiencies of Y and Eu were more than 96%,while those of Ce and Tb were lower than 30%by one-step chlorination roasting with the roasting temperature of 700℃,holding time of 3 h and chlorides/waste phosphors mass ratio of 4∶1.The phase composition and microstructure of the waste phosphors before and after chlorination roasting were examined by using X-ray diffraction(XRD)and scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDS).The results indicated that the rare earth elements Y and Eu in red phosphor were almost completely extracted out,however the green and blue phosphors,as well as the impurities including Ca_(5)(PO_(4))_(3) F_(0.94) Cl_(0.1) and SiO_(2),are still existed in the roasted products.Subsequently,the roasted products were mixed with equivalent mass of Na2 CO3 and roasted at 1000℃for 3 h.The rare earth elements in aluminate green and blue phosphors could be converted into the corresponding oxides by alkaline fusion.Finally,the products after alkaline fusion were processed with chlorination roasting again.The rare earth elements Ce and Tb in the waste phosphors could be readily converted to their chlorides and the extraction efficiencies were significantly improved to 88.51%and 83.06%,respectively,during the secondary chlorination roasting process.Molten salts chlorination assisted by alkaline fusion could be regarded as an effective method for separation and extraction of rare earth elements from waste phosphors.For one-step chlorination,the overall extraction efficiency of Y,Eu,Ce and Tb was 89.15%under the appropriate conditions.However,the extraction efficiency could be enhanced to more than 97%when the waste phosphors were treated via molten salts chlorination followed by alkaline fusion and secondary chlorination,whereas the main impurity elements including Si,Ca,Al and P,could hardly be chlorinated by the molten AlCl_(3)-KCl.The proposed method showed a high selectivity for rare earth elements separation.