Bands of Aluminum Monoxide for Alumina Determination in Cryolites by Means of Laser-Induced Breakdown Spectroscopy

Abstract

Alumina (Al₂O₃), added to melt of the cryolite (Na₃AlF₆) in an electrolysis bath to produce metallic aluminum, remains often in the melt and leads to an increase in energy inputs. Therefore, monitoring the aluminum oxide content in cryolite is an important analytical task in aluminum production. In this paper, it is proposed to use the bands of the green system of aluminum monoxide AlO to estimate the alumina content in cryolites using laser-induced breakdown spectroscopy. For this purpose, a series of samples of NaF-Na₃AlF₆-Al₂O₃ system with a constant cryolite ratio (1.7) were used. It was found that focusing the radiation below the sample surface to a depth of 3–6 mm provides the minimum RSD values (4-8% for the 0-0 band and 6–10% for the 1–1 band) and the maximum signal-to-background ratio (80–120 for the 0-0 band and 40–75 for the 1–1 band) in the time window of 4–16 μs after the laser pulse. The selected focusing and time window conditions made it possible to detect a dependence between the intensity of the 0–0 and 1–1 bands of AlO and the alumina content, which is characterized by a high background and relatively low sensitivity. Normalization to the background makes it possible to use this dependence for qualitative separation of cryolite systems with high and low alumina content.