Particle shape and ALS2 gold flotation in Australian e-waste
Electronic waste is not a uniform ore. A batch of retired phones, laptops and televisions can contain gold-plated connectors, copper alloys, solder, engineered plastics and laminated housings, all mixed at the millimetre scale. For ALS2Project, the target is not simply gold-bearing material, but a reliable route to separate valuable metal particles from complex WEEE without relying on aggressive acid refining. Particle shape is a variable that can change how those particles behave during flotation.
Shape also records the route a piece of scrap has taken. Shredding creates flat and elongated fragments, impact can round sharp edges, and manual disassembly can preserve intact connectors. The same electronics recycler in Melbourne, Sydney or Adelaide may therefore deliver feeds with different size-by-shape populations. Australian plant designers need to understand those differences because a concentrate that performs well in one batch can lose recovery when the geometry of the feed changes.
Flotation responds to both surface chemistry and mechanics. A particle must collide with a bubble, attach, travel with the froth, and avoid being swept into the product without attaching. Its size, shape, surface roughness and conditioning all affect that sequence. ALS2 changes the surrounding separation chemistry; it does not remove the need to control hydrodynamics and particle-level behaviour during a froth-flotation stage.
These issues have particular value in Australia, where gold recovery expertise around Kalgoorlie-Boulder is well established, yet urban-mining feed differs greatly from run-of-mine ore. Scale, contamination and product mix vary more widely in WEEE. Under Australia’s National Waste Policy and state product-stewardship arrangements, cleaner recovery can support better waste outcomes, while compact plants near population centres may reduce transport of low-value fractions.
Why shape changes the bubble interaction
A particle’s geometry affects the contact area presented to a rising bubble and the drag acting on it after attachment. Flat fragments expose a broad face and often make strong initial contact, while needles and rods meet bubbles at a small angle and may tumble. Compact grains can be more stable in the pulp but offer less face area for collector attachment. Angular particles also create greater local pressure and shear around a contact point.
| Particle class | Typical geometry | Likely flotation behaviour | Principal operating risk |
|---|---|---|---|
| Equant or blocky | Roughly equal dimensions | Moderate bubble contact and relatively stable movement | Detachment when bubbles are small or attachment is weak |
| Plate-like or tabular | Wide, thin, flattened | High initial contact and potential rapid flotation | Excessive entrainment or loss of fine plates to tailings |
| Elongated or needle-like | High aspect ratio | Directional movement, rotation and variable attachment | Tumbling, detachment and entanglement |
| Rounded or spherical | Smooth, equant outline | Predictable movement and lower drag | Less surface area for conditioning and lower attachment probability |
The ranking changes with particle size and surface condition. A thin plate can have excellent contact yet be too large to remain in the froth; a rounded particle may float efficiently after effective conditioning. Needle-like particles can rotate, detach or become entangled, while very fine plates may report to tailings through entrainment or poor bubble capture. ALS2 flotation trials should compare shape classes at matched size and chemistry, then check whether the observed trend persists in mixed feed.
Useful shape descriptors include aspect ratio, circularity, projected area and perimeter. No single measurement captures all three-dimensional behaviour, so imaging data should be linked to mass, thickness and density. Shape alone cannot determine ALS2 performance, but it provides a valuable part of the feedstock fingerprint.
Where shape enters the ALS2 separation route
The European Union’s Horizon 2020 research and innovation programme has supported ALS2Project, led by Ikoi S.p.A., to develop safer and cleaner separation technology for precious metals from electronic waste. Its acidless approach seeks to avoid the corrosive reagents and liquid effluents associated with conventional acid-based refining. Froth flotation can provide a physical concentration step within this broader recovery strategy.
Feed preparation determines the geometry reaching that step. A coarse crusher may release intact plated contacts, while a fine shredder can generate paper-like fragments, irregular powder and particles with newly exposed surfaces. Attrition rounds edges and can detach fine gold from larger carriers. Each unit operation therefore changes not only particle size but also the proportion of plates, rods and equant grains available for flotation.
Conditioning then acts on those different surfaces. Where collectors are used, their distribution can depend on exposed area, roughness and residual coatings. A plate may present more apparent surface to a reagent solution than a compact particle of equal mass, yet its behaviour can remain dominated by detachment once the particle enters the froth. Particle-level testing helps distinguish chemical attachment problems from mechanical losses.
From electronics feed to liberated gold particles
Gold in WEEE is commonly associated with connector plating, electronic contacts, die-bonding materials and printed circuit boards. It may occur as extremely thin coatings, small fragments or particles still locked within resin, glass or another metal. A high bulk grade does not necessarily indicate a high concentration of readily recoverable gold, because liberation can be incomplete and uneven across the feed.
Crushing and screening can improve liberation, but they may also create different shape populations. Delamination can release plate-like gold-bearing material, whereas abrasion may produce rounded particles coated with plastic dust. Under a microscope, apparently similar particles can therefore have very different attachment characteristics. This is where shape characterisation complements conventional assays and size analysis.
These differences matter to Australian recyclers dealing with mixed collections from households and businesses. A feed containing office equipment may have different connector and coating profiles from consumer phones collected during a mobile-device campaign. Rather than assigning one flotation setting to an entire product category, operators can identify the shapes associated with each valuable stream and design separate collection, preparation or recovery routes.
Measuring particle shape in mixed WEEE
Representative sampling is the first challenge. A scoop taken from a conveyor belt may over-represent one particle size, while a composite sample can obscure valuable rare shapes unless it is split carefully. Secure sampling, dust control and separate handling of hazardous fractions are essential because WEEE can contain lead, brominated flame retardants and other contaminants.
Sieves and mass measurements establish the coarse size profile, while microscopy or automated imaging adds shape information. Equipment suppliers such as Cavendish Instruments can help laboratories compare particle-characterisation options suitable for mixed electronic scrap. Image analysis should record more than photographs: particles need to be assigned to defined shape classes, measured consistently and linked to their flotation results.
A robust study should preserve information about both weight and number. A few large plates can dominate a mass-based sample while representing only a small proportion of individual particles. Results based on mass recovery, particle counts and chemical assays can reveal whether a shape is truly absent from the concentrate or merely lost during sample preparation.
Reading flotation performance without false conclusions
Flotation performance should be reported through several connected measures. Recovery indicates how much target gold reaches the concentrate, while grade and selectivity show whether the separation is commercially useful. Tailings losses, gangue entrainment and concentrate mass provide the mechanical context. A high recovery achieved by floating large quantities of plastic is not a successful gold-separation result.
Shape comparisons need a controlled test matrix. Researchers can hold particle size and surface chemistry constant while changing geometry, or sort a real feed into shape fractions before flotation. Testing only hand-picked particles can exaggerate performance and will not reveal losses caused by mixed feeds. Replicate batches, consistent froth depth, bubble size and reagent addition are needed to separate meaningful effects from normal plant variation.
The expected trends should also be interpreted through hydrodynamics. Collision efficiency depends on particle trajectory and orientation, attachment depends on contact conditions, and detachment depends on the balance between bubble forces, particle weight and drag. Plate-like particles may show strong attachment but poor retention; equant particles may show weaker contact but greater stability. These are mechanisms to test, not universal rules to assume.
Planning a compact Australian recovery plant
Compact precious-metals recovery plants can sit near collection depots, regional transfer stations or industrial precincts instead of concentrating every fraction at one distant refinery. This model suits Australia’s large geographic distances and the freight costs involved in moving mixed WEEE. Local treatment can remove dense or saleable fractions early, leaving lower-mass streams for established downstream processors.
Economics still demand careful planning. Australian operators must “run the numbers” on gate fees, metal values, reagent consumption, water treatment and residue disposal. Particle shape influences several of those costs through recovery, entrainment and concentrate quality. Under tightening state stewardship requirements, a plant that produces a measurable improvement in gold recovery can also document the diversion of electronics from disposal pathways.
Experience from Kalgoorlie-Boulder and other Australian mining regions is relevant, but conventional mining flotation cannot be transferred unchanged to WEEE. Ore feeders handle mineral particles with relatively consistent liberation; electronics contain coatings, laminates and mixed composites. The transferable skills are disciplined sampling, metallurgical accounting and operator training, while the process development must reflect urban-mining feed.
Turn particle findings into plant decisions
Start with a material and shape audit across representative WEEE batches. The audit should connect particle classes to assay values, liberation, flotation response and final residue quality. This creates a practical database for choosing crushers, screens, conditioners and collection points without assuming that one preparation method suits every product stream.
Pilot testing should then use those results to define a plant-specific operating window. Particle-size controls, froth conditions and reagent additions can be adjusted around the observed attachment and detachment behaviour. Training should teach operators to recognise changes in feed geometry, because a rising proportion of fine plates or elongated fragments can signal a shift in recovery performance.
The immediate action is to review the ALS2Project technical findings, request a particle-characterisation test plan from Ikoi S.p.A., and require each Australian pilot plant to maintain a verified mass balance before scale-up.