EBSD-XPS法分析磷石膏中杂质物相

Characterization of the Impurity Phases in Phosphogypsum by the EBSD-XPS Method

磷石膏是湿法磷酸过程形成的固体副产物。磷石膏中含有磷、氟、硅等有害杂质组分,极大影响磷石膏制品的质量和性能,巨量磷石膏堆存严重威胁生态环境和生命安全。确定磷石膏中杂质物相的赋存状态,为磷石膏除杂净化和综合利用提供理论指导,非常重要。以低温干燥后的磷石膏为研究对象,利用X射线荧光光谱(XRF)分析确定磷石膏中杂质元素的组成,结果表明,磷石膏中的杂质元素含量较高的有P,Si,F和Al,含量较低的有Ba,Fe和Mg等。因二水硫酸钙物相强峰对杂质物相峰有较强遮蔽作用,X射线衍射光谱(XRD)分析不能确定磷石膏杂质的物相。利用扫描电子显微镜对磷石膏进行电子背散射衍射(EBSD)分析,根据被检样品衬度的区别探明磷石膏的杂质物相,利用X射线能谱分析(EDS)成分确定杂质物相组成;利用X射线光电子能谱(XPS)对硫酸钙晶体表面以及混合杂质物相作进一步分析。EBSD分析结果表明,磷石膏中杂质物相主要包括二氧化硅、氟硅酸钠、氟硅酸钾、氟磷酸钙、氟化钙、硫酸钡、硫化铁、三氧化二铝等,此外还有硅、铝、磷、氟等杂质混合组成的复盐物相,其中二氧化硅、硫酸钡、硫化铁、氟磷酸钙和三氧化二铝为独立赋存物相,氟硅酸钠和氟硅酸钾的物相则混合分布在硫酸钙晶体之间,氟化钙杂质与硅、铝、磷、氟杂质复盐物相结合赋存。XPS分析结果表明,磷石膏中还存在硅酸钙、氟化铝、氟化镁、硫酸铝、磷酸铝、磷酸钙、磷酸氢钙和磷酸二氢钙等物相,其中磷酸钙、磷酸氢钙、磷酸二氢钙和氟磷酸钙四种物相的特征峰位分布极为接近。采用EBSD-XPS组合分析方法,不仅确定了磷石膏中杂质的物相,还阐明了杂质物相与硫酸钙晶体之间的构效关系。该研究为磷石膏杂质物相分析提供新途径,为磷石膏除杂净化及其综合利用提供坚实的理论依据。

Phosphogypsum(PG)is a solid by product generated from the wet phosphoric acid process.A broad spectrum of compositions,including phosphorus,fluorine,silicon,and other hazardous impurities in the PG matrix significantly affect the PG’s quality as the end-products,let alone its persistent ecological and environmental impacts caused by the huge quantity of storage and disposal.Therefore,identification of its impurity phases of PG becomes pivotal.Not only it provides the theoretical guidance for the separation processing but it also sheds insightful lightonits value-added utilization.Different spectral characterization techniques were deployed to unveil its impurity phases in the PG.The X-ray fluorescence spectroscopy(XRF)results show that the elements including P,Si,F and Al remain at a relatively higher level,while the elements such as Ba,Fe and Mg are present at the trace level.The X-ray diffraction spectroscopy(XRD)spectrum presents the dominant crystallite gypsum,whilethe crystal spectrum of other impurities is hardly observed.The electron backscatter diffraction(EBSD)by scanning electron microscope(SEM)results indicate the impurities in the form of silicon dioxide,sodium fluorosilicate,potassium fluorosilicate,calcium fluorophosphate,calcium fluoride,barium sulfate,iron sulfide,and aluminum oxide,etc.These compositions exist as the singular metallic oxides and present in the binary,ternary and multiple metallic oxides complex,making the impurities as a series ofthe composites in the crystallite PG framework.In order to investigate the surface binding energy of the prepared PG,X-ray photoelectron spectroscopy(XPS)was also employed.The results indicate the complexities of the impurities as in the form of calcium silicate,aluminum fluoride,magnesium fluoride,aluminum sulfate,aluminum phosphate,calcium phosphate,calcium hydrogen phosphate and calcium dihydrogen phosphate.In addition,the XPS result also shows the close featured positions,which underpins the characteristic peaks of calcium phosphate,calcium hydrogen phosphate,calcium dihydrogen phosphate and calcium fluorophosphates,indicating their relative similar surface bonding energy.To the best of the author’s knowledge,thecombined characterization techniques using both EBSD and XPS,have been rarely applied in the elucidation of complex phase of the impurities of PG generated from the phosphoric acid process.The inherent advantages of using this proposed hybrid spectrum technique withan accurate establishment of the structure-activity relationship between the impurity phases and calcium sulfate crystal,the comprehensive constructing impurity phases,will pave a new way for the impurity phase characterization,value-added conversion,and integrated utilization of PG.