Laser-Induced Breakdown Spectroscopy for 24/7 Automatic Liquid Slag Analysis at a Steel Works

Near-the-Line Steel Slag Analysis Using Laser-Induced Breakdown Spectroscopy: Traditional Univariate Versus Machine Learning Calibration Methods

This work is focused on rapid quantitative analysis of slag in the steel industry for improved process control. The novel approach in this work is a direct comparison of two methods to calibrate and quantify spectral data from the slags. Calibration was first done with the most prevalent method in quantitative optical emission spectroscopy (OES) of solids, the univariate ratio method. The second method is an advanced multivariate analysis (MVA) algorithm termed Elastic Net, allowing to include several lines for each element in the calibration functions. In both methods, the output is mass fraction ratios of the analyte element (or compound) to a matrix element (compound). The actual mass fractions of each compound are calculated by sum normalization assuming the matrix to make up the difference up to 100%. The metric used to evaluate the performance of the methods in terms of accuracy is the parameter σrel calculated as the ratio of the root mean square (RMS) deviation from values obtained by X-ray fluorescence (XRF) divided by the average mass fraction of the compound, expressed in percent. A bit surprising, the main outcome of the comparison is that there is very little difference in the performance of the two methods. One exception is the analysis of MgO, where the elastic net gives significantly better accuracy. Presumably, this is due to the use of multiple lines for Mg to build the calibration function. This is very encouraging, since MgO is a major compound in most slags that needs to be determined accurately. It is suggested to improve accuracy further by means of separate calibrations for a limited number of slag types.

Review of Element Analysis of Industrial Materials by In-Line Laser—Induced Breakdown Spectroscopy (LIBS)

Laser-induced breakdown spectroscopy (LIBS) is a rapidly developing technique for chemical materials analysis. LIBS is applied for fundamental investigations, e.g., the laser plasma matter interaction, for element, molecule, and isotope analysis, and for various technical applications, e.g., minimal destructive materials inspection, the monitoring of production processes, and remote analysis of materials in hostile environment. In this review, we focus on the element analysis of industrial materials and the in-line chemical sensing in industrial production. After a brief introduction we discuss the optical emission of chemical elements in laser-induced plasma and the capability of LIBS for multi-element detection. An overview of the various classes of industrial materials analyzed by LIBS is given. This includes so-called Technology materials that are essential for the functionality of modern high-tech devices (smartphones, computers, cars, etc.). The LIBS technique enables unique applications for rapid element analysis under harsh conditions where other techniques are not available. We present several examples of LIBS-based sensors that are applied in-line and at-line of industrial production processes.

LIBS as a Spectral Sensor for Monitoring Metallic Molten Phase in Metallurgical Applications—A Review

This review article discusses the latest advances on molten phase monitoring in metallurgical processes by using Laser-Induced Breakdown Spectroscopy (LIBS). LIBS is an analytical laser-based technique, where a pulsed laser is focused on a sample to create a plasma. The optical emission from the plasma can be transferred through open-path optical configuration or via an optical fiber to a spectrometer to receive analytical information in the form of elemental composition. Thus, a relatively long-distance analysis can be performed using LIBS. Several modern experimental arrangements, patents and industrial notes are assessed, and the literature is reviewed. The review includes applications of LIBS to analyze steel, iron, aluminum, copper, slags, metal melts, and other materials. Temperature, pressure, and atmospheric composition are crucial parameters of any melting process. Hence, past studies on molten phases describing these parameters have been discussed. Finally, the review addresses the last technological advances for these types of applications. It also points out the need of development in some fields and some limitations to overcome. In addition, the review highlights the use of modern machine learning and data processing techniques to increase the effectiveness of calibration and quantification approaches. These developments are expected to improve the performance of LIBS systems already implemented at an industrial scale and ease the development of new applications in pyrometallurgical processes to address the stringent market and environmental regulations.

In situ laser-induced breakdown spectroscopy measurements during laser welding of superalloy

Laser-induced breakdown spectroscopy (LIBS) has been utilized for in situ diagnostics of the laser welding process. The influence of different weld spot areas (melt pool, solid weld) on LIBS signals and plasma properties has been studied in detail. Liquid metal sampling and high target surface temperature of the melt enhance LIBS plasma intensity and increase plasma temperature. The influence of laser welding process parameters on LIBS measurements has been studied in order to differentiate optimal and defective laser welding. In case of defective laser welding, the melt pool was intensively boiling, so we have observed greater LIBS signals but poor reproducibility. For the first time, the LIBS technique was demonstrated to detect defective laser welding during in situ measurements utilizing atomic and ionic line comparison by paired sample t-test hypotheses testing.

Surface plasma influence on nanosecond laser ablation

The comparison of laser ablation and plasma evolution has been carried out for a molten steel sample in the absence and in the presence of surface plasma. A continuous wave (cw) laser beam was utilized for local melting of a steel (Fe>99  wt.%) sample, but it also induced a surface plasma according to optical emission spectroscopy. The cw laser was switched off for a few milliseconds to dissipate the surface plasma, but the surface temperature did not change according to optical pyrometer measurements. Molten metal was ablated by a nanosecond Nd:YAG laser pulse during cw laser operation and when it was switched off for 5 milliseconds. Comparison of laser ablation and plasma evolution in the presence and in the absence of the near-surface plasma induced by the cw laser beam has been carried out. Time-integrated plasma imaging detected slightly greater emissivity of the plasma induced during cw laser operation. The cw laser operation resulted in a twofold enhancement of the intensity of atomic lines in the spectra as well as slower decay of plasma emission. Plume temperature and electron density were slightly greater at early stages of plume expansion in surface plasma.

Inline Spectroscopy: From Concept to Function

The field of applied spectroscopy is strongly dominated by publications presenting proof-of-concepts, lab set-ups, and demonstrations. In contrast, the corresponding number of commercial successes of inline spectroscopy is surprisingly lower. This article discusses inline spectroscopy from an instrumentation perspective. It is the authors' firm belief that the success of inline spectroscopy lies in the understanding of how the design and implementation of the optical instrumentation affects the data quality, and how this in turn will limit or enhance the performance of the prediction model. This article emphasizes the need for a strong, multidisciplinary design team, whose design process is rooted in first principles, to bridge the technology "valley of death" and convert research in applied spectroscopy into commercially successful solutions.

Combining Raman and laser induced breakdown spectroscopy by double pulse lasing

A new approach combining Raman spectrometry and laser induced breakdown spectrometry (LIBS) within a single laser event was suggested. A pulsed solid state Nd:YAG laser running in double pulse mode (two frequency-doubled sequential nanosecond laser pulses with dozens microseconds delay) was used to combine two spectrometry methods within a single instrument (Raman/LIBS spectrometer). First, a low-energy laser pulse (power density far below ablation threshold) was used for Raman measurements while a second powerful laser pulse created the plasma suitable for LIBS analysis. A short time delay between two successive pulses allows measuring LIBS and Raman spectra at different moments but within a single laser flash-lamp pumping. Principal advantages of the developed instrument include high quality Raman/LIBS spectra acquisition (due to optimal gating for Raman/LIBS independently) and absence of target thermal alteration during Raman measurements. A series of high quality Raman and LIBS spectra were acquired for inorganic salts (gypsum, anhydrite) as well as for pharmaceutical samples (acetylsalicylic acid). To the best of our knowledge, the quantitative analysis feasibility by combined Raman/LIBS instrument was demonstrated for the first time by calibration curves construction for acetylsalicylic acid (Raman) and copper (LIBS) in gypsum matrix. Combining ablation pulses and Raman measurements (LIBS/Raman measurements) within a single instrument makes it an efficient tool for identification of samples hidden by non-transparent covering or performing depth profiling analysis including remote sensing. Graphical abstract Combining Raman and laser induced breakdown spectroscopy by double pulse lasing.

Analysis of charcoal blast furnace slags by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was used for the analysis of charcoal blast furnace slags. Plasma was generated by an application of a 1064 nm wavelength Nd:YAG laser beam to the surface of pellets created from the slags. The presence of Al, Ca, Fe, K, Mg, Mn, and Si was determined by identifying their characteristic spectral signatures. Multivariate analysis was performed for the quantification of these elements. The predicted LIBS results were found in agreement with the inductively coupled plasma optical emission spectrometry analysis. The limit of detection for Al, Ca, Fe, K, Mg, Mn, and Si was calculated to be 0.10%, 0.22%, 0.02%, 0.01%, 0.01%, 0.005%, and 0.18%, respectively.

Indirect measurement of molten steel level in tundish based on laser triangulation

For real-time and precise measurement of molten steel level in tundish during continuous casting, slag level and slag thickness are needed. Among which, the problem of slag thickness measurement has been solved in our previous work. In this paper, a systematic solution for slag level measurement based on laser triangulation is proposed. Being different from traditional laser triangulation, several aspects for measuring precision and robustness have been done. First, laser line is adopted for multi-position measurement to overcome the deficiency of single point laser range finder caused by the uneven surface of the slag. Second, the key parameters, such as installing angle and minimum requirement of the laser power, are analyzed and determined based on the gray-body radiation theory to fulfill the rigorous requirement of measurement accuracy. Third, two kinds of severe noises in the acquired images, which are, respectively, caused by heat radiation and Electro-Magnetic Interference (EMI), are cleaned via morphological characteristic of the liquid slag and color difference between EMI and the laser signals, respectively. Fourth, as false target created by stationary slag usually disorders the measurement, valid signals of the slag are distinguished from the false ones to calculate the slag level. Then, molten steel level is obtained by the slag level minus the slag thickness. The measuring error of this solution is verified by the applications in steel plants, which is ±2.5 mm during steady casting and ±3.2 mm at the end of casting.

Acidity measurement of iron ore powders using laser-induced breakdown spectroscopy with partial least squares regression

Laser-induced breakdown spectroscopy (LIBS) with partial least squares regression (PLSR) has been applied to measuring the acidity of iron ore, which can be defined by the concentrations of oxides: CaO, MgO, Al₂O₃, and SiO₂. With the conventional internal standard calibration, it is difficult to establish the calibration curves of CaO, MgO, Al₂O₃, and SiO₂ in iron ore due to the serious matrix effects. PLSR is effective to address this problem due to its excellent performance in compensating the matrix effects. In this work, fifty samples were used to construct the PLSR calibration models for the above-mentioned oxides. These calibration models were validated by the 10-fold cross-validation method with the minimum root-mean-square errors (RMSE). Another ten samples were used as a test set. The acidities were calculated according to the estimated concentrations of CaO, MgO, Al₂O₃, and SiO₂ using the PLSR models. The average relative error (ARE) and RMSE of the acidity achieved 3.65% and 0.0048, respectively, for the test samples.