The use of ultrasound technology for the inhibition of mineral scale formation in mixing tanks

Gypsum scale formation is a major challenge in many unit operations within the minerals processing industry. Conventional techniques for removing such scales in mixing tanks are time-consuming, labour-intensive and potentially hazardous, highlighting the need for new inhibition strategies. An experimental study on the effect of ultrasound in inhibiting gypsum scale in mixing tanks at laboratory scale was conducted. A 2 l cylindrical tank, employing an unbaffled agitator system and ultrasonic transducer, was used to test the effect of varying sonication times (1 to 6 h per day), and power levels (3 to 4.25 W). Scale-mitigation parameters based on the area fraction of descaled surface, the total scale mass and scale thickness were proposed and assessed. Results revealed that intermittent application of ultrasound significantly reduces gypsum scale deposition, while the scale inhibition improved with an increase in sonication time and power. The most effective setup was found to be 6 h/day of ultrasound at 4.25 W, which achieved almost 97 % reduction in scale mass and thickness at the end of five days of scaling compared to cases without ultrasound. Acoustic analysis indicated that ultrasonic waves with multiple frequencies between 20 and 400 kHz, alongside cavitation and streaming flows, help inhibit scale formation on the tank surfaces. • Ultrasound applied intermittently can reduce gypsum scale deposition on the internal surfaces of mixing tanks. • The power level and duration of sonication of the applied ultrasound strongly affects the nature of scale inhibition. • Different parameters are proposed and calculated to compare the scale inhibition performance under various operating conditions of ultrasound. • 97 % reduction in gypsum scale deposition was achieved at high power for 6 h of ultrasound daily over a 5-day test campaign in 2 l mixing tanks.