## 5 In-Line Application of LIBS in Industrial Production
### Steel Slags  
Steel slags are multi-component oxide materials that are produced in large quantity in industrial steel production. For the control of the steelmaking processes and for the recycling of the metallurgical slag materials suitable analytical techniques are required. The standard method is XRF. However, LIBS requires less time for the analysis of slags than XRF [466]. For some applications the quantitative analysis of the major components of metallurgical slags is sufficient. This task can be accomplished by calibration-free LIBS (CF-LIBS) where the concentration of major elements is calculated directly from LIBS spectra of the samples without the need to measure reference materials. This approach is of interest if minor and trace elements are not relevant, analysis time is important, and reference materials are not available. The CF-LIBS method has been employed to analyze metals [467; 468; 469; 470], rocks [471; 472; 473], and biological materials [474; 475]. Figure 18a shows a LIBS system installed in the secondary metallurgy of a steel plant of voestalpine Stahl GmbH (Austria). The system measures the concentration of major oxides in solid slag by the CF-LIBS method [375; 476]. The concentration of major oxides in various slag samples determined by CF-LIBS and the nominal concentration determined by reference analysis are shown in Figure 18b. The major oxides in the slag samples were CaO, Al\({}_{2}\)O\({}_{3}\), MgO, SiO\({}_{2}\), FeO, and MnO with concentration values ranging from 5\(\times\)10\({}^{-3}\) wt% to 54 wt%. The concentration values by CF-LIBS (C\({}_{\rm CF}\)) match very closely the nominal concentrations (C\({}_{\rm N}\)). Deviations from the perfect match C\({}_{\rm CF}\) = C\({}_{\rm N}\) (dashed line in Figure 18b) depend on type and concentration of oxide and are up to a few wt%. Figure 17: LIBS at-line measurement of hot steel slabs in the continuous steel casting process. (**a**) Schematic of a slab with steel matrix (a) and various surface layers (b–d). Adapted from [464]. (**b**) LIBS signal of Pb showing the transition between two different steel grades. The inset shows the signal for Ni. Solid lines are 25-point averages. Adapted from [463]. The analysis of slag samples at high temperature is important when short time-to-analysis is required. In order to study the stability of CF-LIBS analysis against sample temperature, ceramic slag samples were heated in a box furnace to high temperature and measured during cool-down in air [374]. The calculated concentration values C\({}_{\rm CF}\) showed only weak variation with sample temperature up to 275 \({}^{\circ}\)C. Larger deviations in concentration were observed at higher sample temperature. The ablation rate, the self-absorption of radiation, the plasma expansion dynamics, and the plasma parameters may depend on the sample temperature [477]. The evaluation of data has led to the conclusion that the CF-LIBS method enables to quantify individual constituents with concentrations \(\geq\)1 wt%. This result agrees with the conclusions from theoretical investigations [371, 478] that the inhomogeneity of plasma is a major limiting factor for the quantitation of smaller concentrations by CF-LIBS. In the crude steel production (Linz-Donawitz process) converter slag is a by-product which can be used as raw material in other industrial branches, e.g., for road construction. The chemical composition of the slag is varying, and chemical analysis is required before further use of the material. An automated LIBS measurement system has been developed to analyze the major oxide components of the liquid slag (T = 600\(-\)1400 \({}^{\circ}\)C) while it is transported in a ladle to slag pits [373]. Figure 19a is a camera view into the slag ladle showing a solidified crust at the slag surface and the laser-induced plasma plume. A measuring probe guides the laser beam onto the slag surface and the plasma radiation from the liquid to the detection unit. The probe is moving across the slag surface during the LIBS measurement (2 min/meas.).