Ultrasonic Leaching of Rare Earth Elements from Ashes

Ultrasonic leaching can increase extraction yields and shorten processing times when recovering rare earth elements from ash. By improving contact between ash particles and leaching agents, power ultrasound accelerates dissolution and helps make valuable elements more accessible. Studies on coal fly ash demonstrate the potential: researchers reported 97% yttrium extraction, while a separate study on gallium and vanadium reduced the acid leaching time from 180 to 20 minutes.

Ash as a Secondary Resource for Rare Earth Elements

Ashes represent a promising secondary resource for rare earth elements (REEs). Coal combustion residues can contain yttrium and other rare earths used in electronics, energy technologies and advanced materials. Recovering these elements from existing ash streams and deposits could diversify raw material supplies and turn a disposal material into a useful feedstock.
The extraction potential depends on both elemental concentration and accessibility. An ash may contain valuable elements, yet release only a limited fraction during leaching if the surrounding mineral matrix restricts reagent access. Consequently, ash composition, pretreatment and leaching chemistry must be considered together. Recovering several valuable elements from the same feedstock may also improve the prospects of a process.
Ultrasonic leaching fragments ash particles making rare earths available to leaching reagents. By improving mass transfer between particles and reagent, sonication increases recovery whilst shortening processing time!

How Ultrasound Improves Ash Leaching

Power ultrasound intensifies solid–liquid extraction through acoustic cavitation. Microscopic bubbles form and collapse in the liquid, generating local shear forces, microjets and intense mixing. These effects can disperse particle agglomerates, clean exposed surfaces and improve transport of leaching agents to reactive sites. Dissolved species are transported away more rapidly, helping sustain dissolution.

For ash processing, these effects offer several practical advantages:

• Faster leaching: Enhanced mass transfer can shorten the time needed to extract accessible metals.
• Higher extraction yields: Improved surface contact can help release material that remains insufficiently leached under conventional mixing.
• Combination with pretreatment: Ultrasound can intensify the leaching stage after activation has made resistant phases more accessible.
• Flexible process development: Sonication can be developed adapted to varying acid concentration, solids loading, temperature and treatment time.

Research Examples: High Yields and Shorter Leaching Times

The following studies illustrate these advantages under specific experimental conditions. Yttrium is a rare earth element; gallium and vanadium are other valuable metals. Their extraction provides complementary evidence for ultrasonic processing of ash.

Yttrium and Vanadium: Up to 97% and 100% Extraction

Masoum et al. (2021) investigated the extraction of vanadium and yttrium from coal fly ash using sulfuric acid, hydrogen peroxide and ultrasound. The researchers applied response surface methodology to optimize the solid-to-liquid ratio and the proportions of sulfuric acid and hydrogen peroxide. Under the optimized conditions, reported extraction reached 100% for vanadium and 97% for yttrium.
A modified shrinking core model indicated that mass diffusion controlled the reaction rate. This finding highlights the importance of transport processes in the investigated system and supports targeting mass transfer during process optimization. The results demonstrate that high yttrium extraction is achievable with the combined treatment, although the reported yields reflect the complete reagent and ultrasound system.

FESEM images of raw coal ash (a) left before leaching coal ash, (b) after ultrasonic leaching.
Study and images: ©Masoum et al., 2021

Gallium and Vanadium: 88.89% Shorter Leaching Time

Zhang et al. (2026) investigated ultrasonic acid leaching following sodium carbonate activation roasting. The optimized pretreatment consisted of roasting at 850°C for 60 minutes, using a fly ash-to-Na₂CO₃ mass ratio of 2:1. Subsequent ultrasonic acid leaching achieved high gallium and vanadium extraction within 20 minutes.

The reduction from 180 to 20 minutes corresponds to an 88.89% shorter leaching stage. Kinetic analysis indicated increased rate constants and enhancement of both mass transfer and interfacial reaction processes. These results demonstrate the potential of combining activation roasting with ultrasonic leaching. The time saving applies to leaching; roasting remains an additional processing step.

Ultrasonically enhanced acid leaching operates at a rate twelve times faster than conventional acid leaching, due to the beneficial mechanical action of cavitation bubbles bursting near the surface. This phenomenon improves acid solution mixing, thereby enhancing transport properties.
Image and study: © Canciani et al., 2024

Develop your Ultrasonic Leaching Process for High Rare Earth Recovery

For practical rare earth extraction, leaching is followed by separation and purification of the dissolved elements. High leaching efficiency therefore represents an important intermediate result, rather than the yield of a finished rare earth product. Process evaluation should include downstream recovery, reagent use and the energy required for both pretreatment and sonication.
Hielscher offers industrial-grade bench-top sonicators and heavy-duty ultrasonic flow reactors for developing and implementing intensified leaching processes. Equipment selection and trials can establish suitable amplitude, solids loading, residence time and temperature control for a particular ash.
Simple scale-up to large volumes: Once established parameters for optimal recovery can be then linearly scaled-up to any commercial leaching volume.

SEM images for particles at different times of conventional (a–c) and ultrasonic (d–f) leaching.
Image and study: © Canciani et al., 2024

Literature / References