EXPERIMENTAL DESIGN OPTIMIZATION OF INTEGRATED ULTRASONIC- AND MICROWAVE-ASSISTED EXTRACTION OF RARE EARTH ELEMENTS FROM COAL ASH

The utilization of coal fly ash as a raw waste material from thermal power plants represents a promising alternative source for recovering rare earth elements (REEs). The significance of these elements lies in their broad application and uneven and limited global distribution. In this study, a method of extraction of REEs from coal ash was developed and optimized through alkaline pretreatment, followed by acid leaching. The optimization process was conducted using the Plackett-Burman and Box-Behnken designs. The identified optimal conditions involved a pretreatment procedure in an ultrasonic bath set at 95°C for 10 minutes with an ash-to-3M NaOH ratio of 0.5:1 (m/V). Acid leaching was a consecutive step in a microwave oven at 174°C for 30 minutes using an HCl/HNO₃ mixture in a 1:1 volumetric ratio. The method was tested on fly and bottom ash samples from the Kostolac thermal power plant in the Republic of Serbia, achieving 76% and 58% recovery efficiencies, respectively.