Calibration-free analysis of steel slag by laser-induced breakdown spectroscopy with combined UV and VIS spectra

• Major oxides in steel slag are quantified with improved accuracy by calibration-free laser-induced breakdown spectroscopy.
• For CF-LIBS calculations, LIBS spectra measured in VIS und UV ranges are combined by a mathematical adaption procedure.
• In combined spectra more emission lines are available for calculations with some lines having higher spectral resolution.
• Mean relative errors of calculated concentrations for CaO, Al2O3, MgO, SiO2, and MnO are reduced for combined spectra.

Slag from secondary metallurgy in industrial steel production is analyzed by calibration-free laser-induced breakdown spectroscopy (CF-LIBS). The slag pieces are homogenized by ball milling and sintering. LIBS spectra are measured under ambient conditions using nanosecond Nd:YAG laser pulses (1064 nm) for ablation. A fiber-coupled Echelle spectrometer with two separate detection arms in the UV range (190 to 360 nm) and in the VIS range (298 to 864 nm) and one ICCD camera is used for detection. The UV and VIS spectra are measured sequentially and combined to one spectrum to increase the number of highly resolved emission lines for the calculation of oxide concentrations in slags. The combination of spectra requires an adaption procedure including independent radiometric calibration, the de-convolution of spectrometer functions, baseline corrections, and intensity scaling. More than 60 emission lines of elements Ca, Al, Mg, Si, Fe, Mn, and Ti are evaluated. The electron density is derived from Stark broadening of a Mg line, and the plasma temperature is determined from Saha–Boltzmann plots of Ca, Mn, or Ti lines. With the combined LIBS spectra, the CF-LIBS calculated concentrations CCF are closer to the nominal concentrations CN determined by x-ray fluorescence spectrometry. Significantly reduced relative errors of concentration er = |CCF − CN| / CN are observed for the oxides CaO, Al2O3, MgO, SiO2, and MnO, and no reduction was found for FeO and the minor phase TiO2. The improved accuracy of concentrations is attributed to a better detection of the major elements Ca, Al, and Si in the combined LIBS spectra.