## 5 In-Line Application of LIBS in Industrial Production
### Metal Melts  
The element analysis of metal melts in the production is usually performed off-line by laboratory-based analytical techniques such as XRF and spark OES. A liquid sample is taken and measured after cool-down and solidification. This is a time-consuming process, making the real-time monitoring of the melt composition impossible. With LIBS the melt can be measured directly, i.e., without sampling, and real-time analysis becomes feasible.  
The chemical analysis of aluminum in a primary aluminum smelter has been reported recently [188]. The measurement system was installed at a casting launder system (Figure 8a). The melt temperature was around 730 \({}^{\circ}\)C. Fourteen trace elements (Fe, Si, Cu, Ni, Ti, Cr, Mn, Sn, V, Ga, Zn, Sb, Mg, Na) were measured in the melt and the results were correlated with laboratory measurements on corresponding solid samples. The trace elements Cu, Cr, Mn, and Sn were quantified down to ppm levels and volatile elements, e.g., Na, were measured in real-time down to ppm levels (Figure 8b). It was concluded that the in-line LIBS analysis of many technically important trace elements in the primary  
Figure 7: LIBS analysis of unburned carbon in coal fly ash. (**a**) Schematic of advanced control system using real-time LIBS measurements. (**b**) Unburned carbon content measured by LIBS and by standard chemical analysis method (JIS). Adapted from [106].  
aluminum melt was fully competitive with the off-line laboratory analysis of solid process samples in terms of accuracy and precision. Some earlier reports on the analysis of aluminum melts are [190; 191; 192].  
The LIBS analysis of molten steel has been reported by several groups, e.g., in [194; 197; 198; 199; 200; 201; 202; 203]. In a recent publication, a hollow refractory lancet was immersed into the metal melt to pass the laser light through the surface slag layer onto the liquid steel underneath and to pass the optical radiation of the laser-induced plasma backwards to the optical detection system (using a Cassegrain telescope) and spectrometer (Figure 9a) [197]. The setup was designed to protect all optical and electronic equipment against the high-temperature environment near the steel ladle.  
In laboratory test runs the elements Si, Mn, Cr, Ni, and V in molten steel samples were measured. In the steel plant the elements C (0.21-0.27 wt%), Si (0.52-0.63 wt%), and Mn (1.20-1.38 wt%) were analyzed quantitatively by LIBS and by spark OES as reference. The predicted concentration of test samples was close to the reference concentration with small  
Figure 8: LIBS in-line measurement of molten aluminum. (**a**) LIBS device installed at a casting launder system in a primary aluminum smelter. (**b**) In-situ measurement of sodium concentration in the molten aluminum (dashed line: guide to the eye). Adapted from [188].  
Figure 9: LIBS in-line measurement of molten steel. (**a**) Photograph of the LIBS system refractory lance that is immersed into the liquid steel in the ladle. (**b**) Calibration and test results for C, Si, and Mn of liquid steel in a steel plant (straight lines: linear fits to data). Adapted from [197].  
relative root mean square errors of prediction RMSEP (Figure 9b). The results obtained in the steel plant were not as good as the results in the laboratory. However, the accuracy achieved in the in-line measurements approached the steel plant's requirements. From these results it was concluded that the developed LIBS system is promising for the in situ analysis of melt steel in the steelmaking industry. Immersion probes for LIBS analysis of liquid metals including steel were developed prior by other groups [199; 200; 201].