Agitation Speed and Gold Recovery in ALS2 Processing

Agitation speed is a key operating variable when ALS2 processes electronic scrap containing thick gold layers. It influences how quickly the working solution reaches the surface, how efficiently dissolved species move away, and how evenly the feed remains exposed during precious-metal recovery. The effect is governed by fluid movement, particle geometry, layer thickness and the chemistry of the acidless separation process.

For recyclers in Australia, this relationship matters across very different operating conditions. A plant receiving concentrated telecommunications boards in Sydney may handle a fairly uniform feed, while a regional facility near Brisbane, Melbourne or Perth may process mixed WEEE arriving in irregular batches. Understanding the hydrodynamics behind mass transfer helps operators set practical agitation ranges rather than simply turning the impeller faster.

Why Thick Gold Layers Change the Process

A thin gold coating can expose a large proportion of its surface quickly once the surrounding liquid is moving. A thick layer behaves differently. As separation progresses, the process may involve transport through surface films, pores, cracks, deposited reaction products or exposed interfaces between gold and its underlying material. The effective path for mass transfer can therefore become longer and less uniform.

The outer liquid boundary layer is especially important. Close to a solid surface, the fluid moves more slowly than the bulk solution. This relatively stagnant zone can limit the delivery of active species to the gold and the removal of dissolved products. Increasing agitation generally reduces the boundary-layer thickness, raising the mass transfer coefficient and improving access to the layer.

However, the process is not controlled by fluid movement alone. If the chemical reaction at the gold surface or within the ALS2 system is slower than diffusion, a higher speed will have a limited effect. The best operating point depends on which resistance dominates at a particular stage of the cycle.

How Speed Influences Mass Transfer

Agitation improves mass transfer by increasing circulation between the bulk liquid and the gold-bearing feed. Turbulence renews the liquid at the surface, distributes dissolved species through the vessel and reduces concentration gradients around individual pieces. With suitable mixing, thick gold layers receive a more consistent chemical environment instead of relying on slow diffusion through a nearly motionless liquid film.

In engineering terms, the mass transfer coefficient often rises with Reynolds number, which increases as impeller speed increases. It also depends on impeller design, liquid viscosity, vessel geometry, solids loading and particle size. The relationship is usually sublinear: doubling the speed does not necessarily double the transfer rate.

At low speed, a small increase can produce a noticeable improvement because poorly wetted surfaces become better exposed. At moderate speed, the process may approach a useful plateau. Beyond that point, extra power can generate heat, air entrainment, splashing, unnecessary shear or particle breakage without a proportional gain in gold recovery.

The Role of Surface Renewal

Surface renewal is valuable when thick layers create local depletion zones. As the ALS2 solution contacts the gold surface, the composition immediately next to the layer can differ from the bulk liquid. Agitation replaces this liquid with fresher solution and carries reaction products away. The result is a stronger driving force for continued transfer.

This effect is often more important for irregular electronic components than for smooth laboratory coupons. Gold may be present on connector edges, plated contacts, cavities and fractured board sections. Faster circulation can improve exposure of these areas, although the impeller must still create movement throughout the working volume rather than simply producing a strong vortex in one zone.

Poorly designed agitation can leave dead spots beneath baffles, around the vessel wall or at the bottom of a solids-rich batch. Operators may then increase rpm to compensate, even though the real problem is circulation pattern. A correctly selected impeller, liquid depth and baffle arrangement can deliver better contact at a lower speed.

Finding the Useful Operating Window

A practical test programme should examine several agitation speeds while holding other variables steady. Feed mass, gold-bearing surface area, liquid-to-solid ratio, temperature, ALS2 dosage and processing time should be controlled as far as possible. Samples can then be assessed for dissolved gold, residual gold on the solids and the condition of the recovered material.

The useful range is identified by comparing recovery improvement with energy demand and equipment effects. If raising speed from 150 to 250 rpm significantly improves gold transfer, that change may be justified. If moving from 350 to 450 rpm produces only a minor increase while causing foaming or heavy wear, the higher setting is unlikely to be the right commercial choice.

Scale-up requires caution. The same rpm does not create the same fluid conditions in vessels of different sizes. Tip speed, power per unit volume, impeller diameter and pumping capacity should be considered together. A pilot vessel in Melbourne and a production vessel in regional New South Wales may require different rpm values to achieve comparable mixing.

What to Monitor During ALS2 Trials

Visual observation remains useful, especially during early trials. The feed should circulate through the active liquid rather than forming a settled bed or floating raft. Excessive vortexing may draw air into the solution, while violent movement can damage fragile components and create fines that complicate downstream separation.

Process data should include agitation speed, motor load, temperature, pH or other relevant chemical indicators, solids concentration and sampling time. Gold assays should be paired with measurements of other valuable metals and base-metal dissolution where relevant. A faster gold transfer rate is not automatically a better result if selectivity or product quality declines.

For thick layers, time-based sampling is particularly informative. Early samples show whether agitation improves initial surface access; later samples reveal whether the remaining gold becomes reaction-limited or physically sheltered. Microscopic or metallographic inspection of feed before and after treatment can help distinguish chemical passivation from poor hydrodynamic contact.

Managing Australian Feed and Plant Conditions

Australian e-waste streams can be highly variable. A metropolitan processor may receive sorted computer connectors, mobile-phone boards or telecommunications components, whereas a smaller operation may handle mixed material from councils, commercial clear-outs and mining-related equipment. Feed preparation, sorting and size reduction can therefore have as much influence on mass transfer as the chosen agitation speed.

Transport distances also affect plant design. Material moved from remote Western Australia or northern Queensland may arrive in consolidated loads with changing moisture, contamination and composition. A robust agitation strategy should tolerate reasonable variation without requiring constant manual adjustment. Simple operating windows, clear alarm limits and representative sampling are valuable when technical staff are not always beside the reactor.

Australian compliance and market expectations also reward controlled processing. E-waste handling is influenced by state and territory requirements, workplace safety obligations, environmental approvals and restrictions on waste movement. Victoria’s landfill ban on e-waste helped strengthen collection and recovery pathways, while operators elsewhere must account for their own local rules. Acidless processing can support safer resource recovery goals, but the complete plant still needs suitable ventilation, containment, residue management and documented procedures.

Comparing Speed Strategies for Thick Layers

The most suitable strategy depends on whether the priority is rapid throughput, maximum extraction, low energy use or protection of delicate feed. A single constant speed may be convenient, but staged agitation can be more efficient. Lower speed during loading and wetting can reduce splashing, followed by a moderate operating speed once the solids are fully immersed. A later adjustment may be useful as the accessible gold surface changes.

The comparison below describes general process behaviour rather than fixed ALS2 set points. Actual values should be established through controlled trials using the specific vessel, impeller, feed preparation and ALS2 operating conditions.

Agitation approach	Expected mass-transfer behaviour	Main advantages	Main cautions
Low speed	Thicker boundary layer and uneven renewal around complex parts	Lower power use and gentler handling	Settling, dead zones and slow treatment of sheltered gold
Moderate speed	Improved circulation and reduced concentration gradients	Often a practical balance of recovery, energy and control	Requires suitable impeller and solids loading
High speed	Stronger turbulence and surface renewal, with diminishing returns	May assist difficult or compacted feed	Foaming, air entrainment, erosion, heat and higher operating cost
Staged speed	Conditions change during wetting, reaction and final extraction	Can match mixing intensity to process needs	Needs reliable control logic and representative sampling

For an Australian commercial plant, the target should be a repeatable hydrodynamic condition rather than a headline rpm figure. Recording power draw and batch performance makes it easier to compare campaigns, even when feed comes from different suppliers or collection regions. This approach also supports future scale-up for compact precious-metals recovery plants.

The strongest evidence comes from a mass balance. Measure gold entering with the electronic scrap, gold recovered in the product streams and gold remaining in residues. Pair those results with agitation energy, processing time and operating observations. That evidence can show whether a speed increase genuinely improves ALS2 performance on thick gold layers or simply creates a more energetic vessel.

ALS2 development teams, recyclers and equipment integrators can use controlled agitation trials to define a reliable operating window for their own feedstock. Contact Ikoi S.p.A. through the ALS2Project to discuss acidless precious-metals recovery, pilot testing and the engineering data needed to move from laboratory findings to dependable WEEE processing in Australia.