Sinter is made from a variety of iron-containing raw materials and is the main raw material for blast-furnace ironmaking in China. Iron-containing raw materials account for nearly 70% of the blast-furnace ironmaking cost, sinter for more than 70% (mass fraction) of the blast-furnace ironmaking materials, and the sintering process for 6%?10% of the total energy consumption of iron and steel enterprises. Therefore, sinter production has a significant impact on blast-furnace ironmaking from the perspectives of cost, burden proportion, and energy savings. Alkalinity is an important parameter of sinter, which is closely related to the quality, output, and energy consumption of blast-furnace smelting. Conventional sinter alkalinity analysis methods have some limitations; therefore, it is necessary to find novel technical methods to measure sinter alkalinity. Laser-induced breakdown spectroscopy (LIBS) is used in many fields, especially in raw material screening and product analysis in the metallurgical industry, owing to its advantages of real-time, rapid, and in-situ detection, simultaneous multi-element analysis without complex sample pretreatment,and remote detection. LIBS is known as the "future chemical analysis superstar" and has attracted the attention of numerous researchers recently.

Ten sinter samples with different alkalinity values are obtained by spiking SiO₂ and CaO standard samples. The experiment is conducted under atmospheric conditions. Continuous spectrum acquisition is performed at 120 different positions on the surface of each sinter sample. To obtain a better signal-to-background ratio (RSD), each spectrum is averaged from 10 consecutive pulsed laser ablation signals, and 12 spectra are obtained for each sample. Six sinter samples are used to establish the calibration model for alkalinity, and the remaining four are used as error analysis samples. The spectral lines of Fe, Si, and Ca with low interference and good stability are selected as reference spectral lines for the experimental parameter study. The effects of pressure, pulse laser energy, and spectrum acquisition delay time on the spectrum signal are studied, and the best experimental parameters are selected. Employing the optimized experimental parameters, the spectral signal stability of the sinter samples is tested. Si and Ca spectral lines with good signal strength and stability are selected as the spectral lines for quantitative analysis of alkalinity. The Fe spectral line, which has a similar spectral line energy level to those of Si and Ca, is selected as the internal standard spectral line. The ratio of the internal standard values of the spectral intensities of Ca, Si, and their corresponding Fe is calculated and used to establish the calibration model of the alkalinity value.

To verify the stability of LIBS-collected spectral signals, 4 spectral lines are selected for each element, totaling 12 reference spectral lines, and the RSD of their spectral intensities is calculated. The results indicate that the RSDs of the four reference spectral lines of Fe, Si, and Ca are below 6% and mostly distributed around 4% (Fig. 6). The RSD of Fe Ⅰ 438.354 nm is 2.5%?1.23% [Fig. 6(a)], and the spectral stabilities of Si and Ca are approximately 4.5% [Fig. 6(b)] and 3% [Fig. 6(c)], respectively. After optimizing the experimental conditions, the fluctuation of the sinter LIBS spectral signal is small, which is conducive to the quantitative analysis of LIBS technology. Fe Ⅱ 248.266 nm and Fe Ⅰ 438.354 nm are selected as the internal standard spectral lines of Si Ⅰ 288.158 nm and Ca Ⅰ 422.673 nm, respectively. The calibration model for alkalinity is determined based on the ratio of their internal standard ratios, and the determination coefficient (R²) approaches 0.951 (Fig. 8). Using the same experimental method to conduct quantitative analysis on the remaining four samples, the deviation between the predicted value and the actual alkalinity value is small, and the relative error of the prediction result is lower than 1.14% (Table 3). The influence of the measurement error caused by the matrix effect and fluctuation of experimental parameters can be reduced by analyzing the sinter alkalinity via the internal standard method to achieve accurate measurement of the sinter alkalinity.

When utilizing LIBS to detect and analyze a sample, the spectral signal fluctuates significantly, owing to the influence of the matrix effect, noise signal, and experimental parameters, which further makes the LIBS measurement result deviate substantially from the actual. To reduce the fluctuation of the spectral signal and ensure the accuracy of alkalinity measurement, first the experimental conditions are optimized, the stability of spectral signal is tested, appropriate analytical spectral lines for internal standard processing are selected, and then a quantitative analysis of alkalinity is conducted. The results demonstrate that after optimizing the sample preparation conditions and experimental parameters, the LIBS spectral signal of the sinter sample fluctuates less and is maintained at approximately 4%, which is conducive to the quantitative analysis of alkalinity. Compared with the non-internal standard method, the R² of the calibration model treated by the internal standard method is increased from 0.468 to 0.951, and the maximum relative error is 1.14%, significantly improving the correlation between the spectral intensity ratios of Ca and Si and alkalinity. This results in the accurate measurement of sinter alkalinity, which has certain reference significance for LIBS detection and analysis of sinter alkalinity.