• Ultrasound applied at different acoustic frequencies produced distinct gypsum scale distributions within the mixing tank. • A clear non‑linear relationship was observed between acoustic frequency and scale deposition under constant acoustic power. • Low‑frequency ultrasound (22 kHz) delivered the strongest scale suppression, yielding ≈ 72 % mass inhibition compared with the highest frequency (142 kHz) tested. • Cavitation effects - confirmed through visual evidence of bubble formation and collapse - played a key role in disrupting gypsum scale, especially in dead‑zone regions of the tank. • Up to 92% scale inhibition performance was achieved at the lowest frequency (22 kHz) using only 15 W acoustic power at a 2.5% duty cycle. Gypsum scale formation presents a persistent challenge across various unit operations in the minerals processing industry. This study explores the use of ultrasound as a non-invasive technique to mitigate gypsum scaling under laboratory conditions. A 2-litre cylindrical, unbaffled mixing tank equipped with a SWIRLFLOW® impeller and an ultrasonic transducer was used to investigate the impact of different acoustic frequencies (22, 44, 98, and 142 kHz) on scale formation at fixed acoustic power of 15 W and a duty cycle of 2.5 %. Results indicated that 22 kHz provided the most effective scale inhibition, yielding a total scale mass of 2.36 g and a maximum scale thickness of 1.76 mm in comparison to 142 kHz with 8.57 g and 4.12 mm, respectively. A non-linear trend was observed between ultrasonic frequency and both scale mass and thickness, with the most significant mitigation (≥ 87 %) observed between 22 and 44 kHz. The formation of gypsum-coated shells along the tank’s base and walls points to the active role of bubble formation and collapse in the dead zones of the flow during mixing. Analytical modelling of bubble dynamics revealed that lower frequencies produce relatively larger bubbles and higher bubble-wall velocities, which decrease with increasing frequency. These larger bubbles collapse more violently, disrupting existing scale and impeding fresh scale nucleation. Additionally, higher bubble-wall velocities contribute to enhanced suspension by propelling scale-laden liquid away from the bubble interface. These findings suggest that optimising ultrasonic frequency at moderate acoustic power is critical for improving scale control and mixing efficiency in industrial processes involving gypsum precipitation.
Mondal et al. (Fri,) studied this question.