Femtosecond laser processing for blast-hole analysis: Laser removal of slurry and effect on rocks

This study explores the use of femtosecond laser pulses to remove iron ore slurry used by mining industries to stabilise blast-hole structures, aiming to preserve wall stability and the chemical composition of the underlying rock. In blast holes, minerals are often coated with deposits such as dust or slurry, which must be removed to analyse the rock beneath. The ablation depth per pulse and ablation efficiency of the slurry were assessed, followed by studies on the ablation thresholds and rates of economically significant Australian rocks, including banded iron, limonite, goethite, shale, and hematite. Surface damage was evaluated using optical microscopy, profilometry, colourimetry, VIS/NIR spectroscopy, and Fourier Transform Infrared spectroscopy (FTIR) spectroscopy. Changes in the Fe³⁺ electronic transition band at 0.60 µm suggest alterations in iron's oxidation state, while shifts in OH bands at 3400 cm⁻¹ indicate water or molecular OH loss. Surface discoloration was observed, and safe laser fluence thresholds were established to minimise this effect. Despite these changes, the rocks remained identifiable, highlighting the potential of femtosecond laser cleaning for rock analysis with minimal alteration.