Ultrasonic Leaching of Precious Metals

Power ultrasound is a effective technique to extract metals such as precious metals and rare earths. This process of ultrasonically assisted solid-liquid extraction is known as sono-leaching, lixiviation or washing. Robust industrial ultrasonicators can be easily installed to leach rare earths from ores, to treat mining slurries for a more complete recovery or to separate high-value metals (e.g. Cu, Zn, Ni) from less valuable metals.

The ultrasonic leaching promotes the reaction by mass transfer and dissolution so that higher yields are obtained in shorter extraction time.
The main benefits of ultrasonic leaching are:

• higher yield
• more complete leaching
• reduced reagent consumption
• milder conditions
• simple feasibility testing
• linear scale-up
• easy installation of full-commercial ultrasound systems
• very robust ultrasonicators for large volume streams

Ultrasonic Leaching of Precious Metals: Faster Extraction Through Cavitation Chemistry

Recovering precious metals such as gold, silver, platinum, palladium, and rhodium is a cornerstone of modern metallurgy and recycling – especially in the processing of ores, concentrates, and secondary resources such as electronic scrap and catalytic converters. While conventional leaching is well established, it is often limited by slow mass transfer, surface passivation, incomplete liberation of valuable phases, and high reagent consumption.
Ultrasonic leaching addresses many of these bottlenecks by introducing high-intensity ultrasound into the leaching slurry, dramatically intensifying reaction kinetics through a phenomenon known as acoustic cavitation.

The Core Mechanism: Acoustic Cavitation

When high-power ultrasound is coupled into a liquid, it creates microscopic cavitation bubbles that rapidly form and collapse. This collapse generates extreme localized conditions, including:

• intense micro-mixing and shear forces
• high-velocity microjets directed at solid surfaces
• localized hot spots (very high temperatures and pressures for microseconds)

Although these effects occur on a microscopic scale, they strongly influence the macroscopic leaching process by continuously renewing the reactive surface and accelerating transport of reagents to and from the solid particles.

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

Why Ultrasound Improves Precious Metal Leaching

In most leaching systems, the rate-limiting step is not the chemical reaction itself, but rather the transport of reactants through boundary layers, pores, or passivating surface films. Ultrasonic cavitation improves leaching efficiency through several synergistic effects:

1. Enhanced Mass Transfer
   Ultrasound reduces the thickness of the stagnant diffusion layer surrounding solid particles. This allows lixiviants (e.g., cyanide, thiosulfate, chloride, iodide, thiourea, or acidic systems) to reach the metal-bearing surface faster, while dissolved metal complexes are removed more efficiently.
2. Particle Surface Activation
   Cavitation microjets and shockwaves continuously erode, clean, and roughen particle surfaces. This exposes fresh mineral phases and increases the effective reactive area – especially important in refractory ores or coated particles.
3. Disruption of Passivation Layers
   Many precious metal-bearing minerals form surface layers during leaching (e.g., oxides, sulfates, elemental sulfur, or silica films). Ultrasound can physically disrupt these barriers, restoring access of the leaching agent to the underlying metal phase.
4. Improved Penetration Into Porous Solids
   For concentrates, catalysts, and e-waste particles, ultrasound helps force liquid into pores and microcracks, improving reagent access to embedded precious metals.

Particle size distributions after ultrasonic and conventional leaching
Image and study: © Canciani et al., 2024

Applications: From Ores to Urban Mining

Ultrasonic leaching is increasingly investigated across both primary and secondary resources:

• Gold and Silver
  Power-ultrasound has been shown to accelerate gold leaching in cyanide and alternative lixiviants by improving transport and removing passivation effects. It is also relevant for silver recovery from ores and industrial residues.
• Platinum Group Metals (PGMs)
  Platinum, palladium, and rhodium recovery – especially from spent catalytic converters – often relies on chloride-based or acidic leaching systems. Ultrasound enhances dissolution kinetics by intensifying surface reactions and improving the breakdown of complex ceramic/metal matrices.
• Electronic Scrap
  Printed circuit boards and electronic components contain valuable precious metals but present strong diffusion barriers due to polymers, oxides, and multi-material structures. Ultrasonic treatment improves leaching uniformity and can reduce required leaching time.

Key Process Advantages

From a process engineering perspective, ultrasonic leaching offers several measurable benefits:

• shorter leaching times through accelerated kinetics
• higher extraction yields due to improved surface access
• lower reagent consumption in many systems (less excess lixiviant needed)
• improved reproducibility through better dispersion and mixing
• potentially lower operating temperature because ultrasound compensates for slower thermal kinetics

Process Considerations and Scale-Up

Successful ultrasonic leaching depends strongly on process design. Critical parameters include:

• ultrasound power density and amplitude
• slurry concentration and particle size distribution
• reactor geometry and flow conditions
• temperature control
• choice of leaching chemistry (acidic, alkaline, chloride, etc.)

Importantly, industrial-scale implementation requires probe-type high-power ultrasonic reactors, as bath sonicators typically do not deliver sufficient energy into dense leaching slurries. Inline ultrasonic flow cells can be integrated into continuous leaching circuits, enabling scalable operation. Hielscher high-performance sonicators are built for processing large volumes under demanding conditions – increasing the yields of leached metals whilst reducing processing time and environmental impact.

Design, Manufacturing and Consulting – Quality Made in Germany

Hielscher ultrasonicators are well-known for their highest quality and design standards. Robustness and easy operation allow the smooth integration of our ultrasonicators into industrial facilities. Rough conditions and demanding environments are easily handled by Hielscher ultrasonicators.

Hielscher Ultrasonics is an ISO certified company and put special emphasis on high-performance ultrasonicators featuring state-of-the-art technology and user-friendliness. Of course, Hielscher ultrasonicators are CE compliant and meet the requirements of UL, CSA and RoHs.

Literature / References