Agitation speed and its effect on fine gold separation times
In the rapidly evolving world of electronic waste refining, the relationship between mixing intensity and the time it takes to liberate fine gold from shredded circuit boards is no longer a side note. It is becoming a central design variable. The ALS2 acidless separation technology, developed under the European Union's Horizon 2020 programme, treats agitation speed as a tunable parameter that can compress or extend residence times depending on particle size distribution and pulp density. For operators designing compact recovery plants, understanding that relationship is the difference between a profitable shift and a bottleneck.
Australia sits at an interesting crossroads here. The country has deep roots in gold, stretching back to the 1850s rushes and still visible today in places like the Super Pit at Kalgoorlie and the bullion halls of the Perth Mint. At the same time, Australians are generating more e-waste than ever, with the National Television and Computer Recycling Scheme channeling millions of devices through accredited processors each year. Bridging these two worlds, modern acidless recovery offers a cleaner route for the fine gold locked inside mobile phones, laptops, and the steady stream of retired IT gear that flows through suburban drop-off points.
Why mixing intensity matters in acidless gold recovery
Inside any hydrometallurgical vessel, the speed at which the impeller rotates dictates how quickly reagents reach particle surfaces, how fast reaction products diffuse away, and how evenly suspended solids behave. In acidless systems, where leaching solutions tend to be milder and chemistry is more selective, those hydrodynamic conditions carry even more weight. A faster impeller generally increases mass transfer coefficients, but only up to a point. Beyond that, the gains flatten, energy use climbs, and the very fine fraction ALS2 is designed to capture can start being tossed against the vessel walls rather than staying in productive suspension.
For gold particles below 50 micrometres, the effect is particularly pronounced. These specks are small enough to ride the flow rather than settle, yet large enough to carry significant mass per particle. When agitation is too gentle, they cluster, attach to air bubbles, and effectively hide from the reactive phase. When agitation is too aggressive, the same particles may experience cavitation-induced passivation or simply bypass the reactive zone. The sweet spot usually lies somewhere between 250 and 600 revolutions per minute, depending on tank geometry, baffle configuration, and solids loading.
How fine gold particles behave under shear
Fine gold behaves differently from coarser fractions because its surface-area-to-volume ratio is enormous. A 20-micrometre flake has roughly a thousand times more surface per gram than a 2-millimetre grain. That means the chemical reactions driving dissolution happen quickly, but the transport of reagents to the particle surface becomes the rate-limiting step. Shear introduced by agitation directly governs that transport by thinning the diffusion boundary layer around each particle and increasing the rate at which fresh solution is delivered.
Yet the same shear that helps one particle class can damage another. Fragile flakes can fragment into sub-micron fines that are harder to filter and may be lost downstream. ALS2 modelling treats residence time as a function of three interacting variables: reagent concentration, temperature, and shear rate introduced by the agitator. The shear component is the one operators can adjust most readily without changing chemistry, which is why it receives so much attention in process tuning.
Impeller speed benchmarks across recovery stages
Not every stage of the recovery train benefits from the same rotational speed. In the initial leach tank, where shredded boards are first contacted with the working solution, moderate agitation around 200 to 300 rpm is usually enough to keep solids suspended without shearing the polymer fractions of the feedstock. As the slurry moves toward the precious-metal-selective stage, speeds typically rise into the 350 to 500 rpm band, where the goal shifts from bulk suspension to intimate particle-fluid contact.
The final polishing stage, where fine gold is captured and concentrated, often runs slower again, in the 150 to 250 rpm range, to allow the precious particles to settle or adsorb onto a recovery medium without being swept past it. Operators comparing different reactor configurations have reported that staging agitation in this stepped pattern can reduce total separation time by roughly a fifth compared with a single-vessel approach at constant speed. The trade-off is capital cost, since staged systems require more tanks and more motors.
In Australian pilot work connected to e-waste trials in Melbourne and Brisbane, engineers have begun logging agitation profiles alongside recovery yields to build local datasets. Those records are still maturing, but early indications suggest that the European bench-scale numbers translate well, provided local water chemistry is accounted for, especially the relatively hard water drawn from scheme areas in South Australia and parts of Western Australia.
Comparing agitation regimes for fine gold capture
The table below summarises how four common agitation regimes affect separation time, energy draw, and gold recovery in acidless circuits. The values reflect typical ranges reported across ALS2 trials and related hydrometallurgical literature, rather than a single fixed recipe.
| Agitation regime | Typical impeller speed | Approx. separation time | Energy draw | Fine gold recovery |
|---|---|---|---|---|
| Gentle suspension | 100–200 rpm | 75–110 min | Low | 70–80% |
| Moderate mixing | 250–400 rpm | 40–70 min | Moderate | 85–92% |
| High shear | 450–650 rpm | 25–45 min | High | 88–94% |
| Pulsed/intermittent | 200–500 rpm cycled | 35–60 min | Moderate | 90–95% |
The pulsed regime, which alternates between higher and lower speeds during a single batch, has attracted attention because it appears to combine the recovery benefits of high shear with the energy economy of moderate mixing. It is also gentler on mechanical seals, a point that matters in remote sites where maintenance trips cost real money and spare parts sometimes take days to arrive.
Australian market realities for compact recovery plants
Australia's e-waste stream is unusual in a few ways that affect how ALS2-style plants can be deployed. The country is geographically vast, with population concentrated in coastal cities, while much of the inland produces relatively little material per square kilometre. That favours modular, containerised recovery units that can be sited near regional collection hubs rather than centralised mega-plants. The model fits well with the ALS2 footprint, which has been promoted as scalable from a single shipping container up to small industrial halls.
Aussie operators also have to navigate the country's gold heritage, both as an opportunity and a sensitivity. Many regional communities still remember the smelters and battery operations of the late twentieth century and remain cautious about any new chemical process. Pitching acidless technology as a clean alternative, with no nitric acid or aqua regia in the loop, has generally landed well in community consultations held in places like Ballarat and Charters Towers. Operators using the local vernacular, dropping in the occasional "no worries" or "she'll be right" when explaining timelines, tend to find meetings run more smoothly.
Regulatory realities matter too. The National Measurement Institute oversees hallmarking and assay standards, while state-level environment authorities set discharge limits that vary noticeably between New South Wales, Victoria, and Queensland. Compact ALS2 plants have been promoted as easier to permit precisely because their closed-loop design keeps the working solution on site for far longer than acid-based systems, reducing the volume of effluent requiring treatment.
Practical considerations for plant designers and operators
When specifying agitators for an ALS2 line, designers should weigh three trade-offs in roughly this order: residence time versus recovery, recovery versus reagent consumption, and reagent consumption versus energy use. Agitation speed sits at the centre of all three. A common mistake in early designs is to oversize the motor to chase marginal time savings, then discover that the additional heat load accelerates reagent degradation and shortens campaign life.
Materials of construction also deserve attention. Even in an acidless system, chloride-bearing working solutions can be aggressive on standard stainless steels above certain temperatures. Many pilot plants have moved to higher-nickel alloys or rubber-lined tanks for the gold-selective stages, accepting the higher upfront cost in exchange for longer service intervals. Operators in humid coastal zones, from the Gold Coast down to Geelong, have reported that lined tanks also reduce condensation-related scaling on instrument probes.
Control philosophy matters as well. Fixed-speed drives are still common and cheap, but variable-frequency drives allow operators to ramp agitation through the cycle, holding the high-shear phase only as long as the kinetics demand. The ALS2 team has published refinements in this area, and the recent project news covers benchmark figures from new installations.
Scaling the technology for regional sites
For a compact plant processing around one tonne of e-waste per day, shaving twenty minutes off each batch cycle is a real economic gain. Across three shifts it adds up to roughly an extra batch per day, which at current gold prices represents a meaningful bump in recovered value. That is why agitation speed, unglamorous as it sounds, has become a focus of optimisation work in the ALS2 programme and across the wider WEEE recovery community.
The technology continues to attract interest from Australian researchers, recyclers, and regional councils looking for cleaner ways to recover value from retired electronics. Whether the plant is destined for a suburban industrial estate in Western Sydney or a regional hub near the resources corridor of central Queensland, the same principle holds: tune the impeller to the particle, not the other way round, and the separation times will look after themselves.
Teams interested in deploying ALS2 in Australia are encouraged to get in touch with the consortium, share local feedstock samples, and contribute operational data to the growing knowledge base. The next phase of optimisation will benefit from Australian input, and partners across the recycling, mining, and research sectors are invited to join the conversation.