Application Of Laser-induced Breakdown Spectroscopy In The Analysis Of Trace Elements And Industrial Indicators In Petroleum Coke

Petroleum coke is a by-product of crude oil refining process and an important industrial material in electrolytic aluminum,steel-making and cement industries.According to its quality,petroleum coke can be used to produce pre-baked anodes for aluminum smelting,graphite electrode for electric furnace steel-making and raw material for other chemical industries.The content of trace elements(vanadium,nickel,iron,sodium,calcium and other elements)and industrial indicators(sulfur,ash and volatile content)of petroleum coke are important factors to evaluate the quality of petroleum coke and determine its use.Rapid and accurate detection of trace elements and industrial indicators in petroleum coke is of great significance to petroleum coke industry.Laser induced breakdown spectroscopy(LIBS)has the advantages of rapid detection,no complex sample preparation,and simultaneous analysis of multiple elements.In this paper,LIBS technology is applied to the detection of trace elements and industrial indicators of petroleum coke.The methods to improve the detection accuracy,repeatability and sensitivity of LIBS analysis for trace elements and industrial indicators of petroleum coke are systematically studied.The specific research contents are as follows:(1)In view of the matrix effects caused by the volatile content of petroleum coke,two different methods,establishing calibration model based on classification and sample preparation with binder,are proposed to reduce the influence of matrix effects on the quantitative analysis of LIBS.For the first method,a two-step approach is implemented.Due to the large difference of volatile content in petroleum coke,petroleum coke samples are divided into two different types of samples according to volatile content.Next,calibration models of V,Na and Ca elements were established for each two types of samples.The results show that the calibration model based on classification can overcome the difficulty of insufficient prediction ability of the single model alone for samples with large matrix differences.The two-step approach could significantly improve the detection accuracy of LIBS for V,Na and Ca elements in petroleum coke.For the second method,the method of reducing the matrix effects using sample preparation with binder is studied.Through the analysis of the repeatability of spectral signal and plasma morphology,the percentage of the stearic acid binder was optimized.It is found that the percentage of the binder has a significant effect on the repeatability of spectral signal and plasma morphology.The pellet sample with 30 wt%stearic acid binder has the best signal repeatability,which significantly reduced the fluctuation of the LIBS signal.Finally,based on the optimal percentage of the binder(30 wt%),the content of V,Na and Ca in petroleum coke was quantitatively analyzed.The results show that the two different methods,establishing calibration model based on classification and sample preparation with binder,both can reduce the matrix effects and improve the detection accuracy.In comparison,the method of sample preparation with binder can not only improve the detection accuracy,but also improve the measurement repeatability.(2)Spatial cavity confinement combined with normalization using acoustic energy(Cavity&NAE)is proposed to improve the measurement repeatability and sensitivity of LIBS.Firstly,the laser energy and the diameter of cavity are optimized.By comparing the Pearson correlation coefficient between the first peak of acoustic signal(acoustic amplitude,acoustic area,acoustic energy)generated by laser-induced plasma and the spectral lines,the results show that the acoustic energy has the highest correlation with the studied spectral lines of V,Na and Ca element.Therefore,acoustic energy is selected as the reference signal for spectral normalization.Then,the results of improving spectral repeatability by three methods,namely,non-spatial cavity confinement,spatial cavity confinement and Cavity&NAE,are compared.The results show that Cavity&NAE has the best effect on improving spectral repeatability.Finally,the limit of detection(LOD)and measurement repeatability of LIBS for the quantitative analysis of trace elements V,Na and Ca in petroleum coke were studied based on the Cavity&NAE method.The results show that compared with method without spatial confinement,the Cavity&NAE method can significantly improve the detection accuracy,repeatability and LOD of trace elements in petroleum coke.Moreover,the detection accuracy and measurement repeatability of Cavity&NAE are better than those of using spatial cavity confinement method alone.(3)The optimization of the secondary breakdown of DP-LIBS is proposed to improve the detection sensitivity of trace elements in petroleum coke.The effects of focusing geometry,focusing angle and axial focusing position on orthogonal re-heating DP-LIBS have been investigated.The plasma morphology,spectral intensity,signal repeatability,plasma temporal evolution and plasma properties were systematically studied.The results indicated that the focusing geometry,focusing angle and focusing position of the second laser pulse should be considered as critical parameters which could significantly influence the signal repeatability and intensity of DP-LIBS.The double lens focusing geometry and appropriate focusing angle can effectively reduce the spherical aberration,avoid the multiple breakdown phenomenon,and improve the energy coupling efficiency of the second laser pulse and the first laser formed plasma(Laser-formed plasma,LFP),which is beneficial to enhance the plasma emission intensity.It is found that the secondary breakdown almost occurs on the boundary of the LFP.When the secondary breakdown occurs on both sides of the LFP boundary,a confinement effect and emission enhancement can be realized,which is beneficial to enhance the signal intensity and stabilizing the plasma morphology.Based on the optimal condition of the secondary breakdown,the trace elements V,Ni and Fe in petroleum coke were quantitatively studied by DP-LIBS.This method further improves the sensitivity of LIBS to trace elements in petroleum coke and solves the problem of weak spectral intensity of Ni and Fe elements using traditional SP-LIBS methods.(4)The method of LIBS combined with XRF technology(LIBS&XRF)to detect the industrial indicators of petroleum coke:sulfur,ash and volatile content is proposed.Based on the complementarity of the detection range of LIBS and XRF technology,LIBS spectrum and XRF spectrum data are selected to establish calibration models for different industrial indicators.The results show that the prediction accuracy of LIBS&XRF method for sulfur content in petroleum coke is better than that using XRF method alone.For the analysis of ash content,the prediction accuracy of LIBS&XRF method is better than that of XRF and LIBSmethod alone,and the prediction repeatability is significantly better than that of LIBS method.For the analysis of volatile content,the prediction accuracy and repeatability of LIBS&XRF method are better than those of LIBS method alone.LIBS&XRF method has advantages of rapid multiple elements detection of LIBS and high repeatability of XRF,which can meet the requirements of online detection of industrial indicators of petroleum coke.Summarily,this paper systematically studies the application of LIBS technology in thedetection of trace elements and industrial indicators of petroleum coke,and proposes methods to reduce matrix effects and improve detection accuracy,sensitivity and repeatability.The LIBS technology and XRF technology are combined to realize the accurate and rapid detection of industrial indicators of petroleum coke.The results of this paper provide theoretical and technical support for the application of LIBS technology in petroleum coke detection.