Managing viscosity in ALS2 slurries as solids loading changes

ALS2 acidless separation technology is designed to recover precious metals from electronic waste and WEEE streams without relying on conventional acid-based refining. In this process, slurry behaviour is a practical engineering concern: the concentration of solids affects pumping, mixing, screening, residence time, separation efficiency and the energy required to keep valuable particles in suspension.

Viscosity is not determined by solids loading alone. Particle size distribution, particle shape, moisture, liberated metal content, fines generation and liquid chemistry all influence the apparent viscosity of an ALS2 slurry. Understanding these relationships helps operators specify equipment for compact recovery plants and adapt the process to the varied feedstocks found in Australia, from telecommunications hardware in Sydney to end-of-life electronics moving through Western Australian recycling networks.

Why solids concentration changes flow behaviour

As the proportion of solid particles rises, the liquid has less room to move between particles. At low solids loading, an ALS2 slurry may behave in a relatively fluid manner, with viscosity only modestly above that of the carrier liquid. As concentration increases, particles interact more frequently and form a denser internal structure. The result is a sharp rise in apparent viscosity rather than a simple, linear increase.

The transition becomes especially important when the solids approach a packed or near-packed condition. Small additions of dry feed can then produce a large change in resistance to flow. A slurry that moved easily through a pipe at one concentration may require substantially greater pump torque at a slightly higher concentration. This is why plant design should use a solids-loading curve rather than one nominal viscosity value.

Fine particles have a particularly strong effect. Dust, crushed resin, glass fibre and very small metal fragments fill the spaces between larger particles, increasing surface area and water demand. A mixed electronic-waste feed can therefore become more viscous than a cleaner, coarser fraction at the same mass percentage of solids.

The role of particle characteristics

ALS2 feed preparation influences rheology before the separation stage begins. Shredding and classification determine the distribution of particle sizes, while liberation affects whether metals remain attached to plastics, ceramics or glass. Irregular particles tend to interlock more readily than rounded particles, creating greater resistance to movement and a higher yield stress.

Particle density also matters. Copper, silver-bearing components and other dense fractions can settle quickly if the slurry is too thin or mixing is interrupted. Conversely, a high concentration of lightweight polymer fragments may increase volume and drag without delivering the same settling behaviour. The most useful test data therefore records both mass concentration and volumetric solids concentration.

A feed generated from mobile phones, printed circuit boards and small appliances may contain changing proportions of copper, aluminium, ferrous pieces, polymers and ceramic material. Australian collection streams can be similarly variable because household electronics, business equipment and telecommunications scrap often arrive in separate campaigns. Blending rules and feed characterisation reduce sudden viscosity swings and make the ALS2 process easier to control.

Shear rate, yield stress and pump selection

Many process slurries are non-Newtonian. Their apparent viscosity changes with shear rate, so a value measured in a laboratory beaker may not represent behaviour inside a pipe, impeller or hydrocyclone. ALS2 slurries may show shear-thinning behaviour, where apparent viscosity falls as agitation increases, together with a yield stress that must be exceeded before the material starts to flow.

Yield stress is critical during start-up and shutdown. A pump may circulate a moving slurry successfully but struggle to restart after solids have settled in a line. Short pipe runs, low points and dead legs should be avoided, and tank bottoms should be shaped to prevent stagnant pockets. Agitator selection should consider the torque required to mobilise settled solids, not just the power needed to maintain an already mixed batch.

Positive displacement pumps can handle some high-viscosity duties, while centrifugal pumps may be suitable for lower-viscosity, well-conditioned slurries. The correct choice depends on solids size, abrasiveness, required flow and allowable shear. Oversizing a pump can create excessive shear, wear and energy use; undersizing can cause blockages, unstable flow and poor separation performance.

Measuring ALS2 rheology at useful conditions

A practical test programme should measure viscosity across the expected operating range of solids loading and shear rate. A rotational rheometer or suitable slurry viscometer can identify flow curves, yield stress and thixotropic effects. Testing should include ramp-up and ramp-down cycles because some slurries become temporarily thinner during mixing and gradually rebuild structure when left at rest.

Samples need careful preparation. Solids should be representative of the actual ALS2 feed, including the expected fines fraction and moisture content. Results can be distorted if coarse metal pieces settle during sampling or if a sample is diluted unintentionally. Recording temperature is also important because liquid viscosity generally falls as temperature rises, although process safety and material compatibility should determine whether temperature adjustment is appropriate.

Plant trials should connect rheology with operating indicators such as motor current, pump discharge pressure, flow stability, mixing time and solids recovery. A case study involving telecom scrap recovery is relevant when considering how telecommunications equipment can create a distinct blend of metals, plastics and fine fractions. Such feed-specific evidence is more useful than transferring a viscosity number from an unrelated mineral slurry.

Controlling concentration in an Australian plant

Solids loading can be managed through controlled feed metering, dilution water, screening and recirculation. Online density measurement, tank level monitoring and pump power trends can provide early warning of a thickening slurry. Operators should establish an acceptable operating window rather than treating maximum solids concentration as the sole objective. A slightly lower concentration may improve throughput if it prevents stoppages and reduces mixing demand.

Australian conditions add practical considerations. A compact plant near Melbourne or Sydney may receive irregular batches from electronics collection and commercial dismantling, making feed blending essential. In Queensland or Western Australia, longer transport distances can increase the importance of dewatering and storage logistics. Hot conditions around Perth or inland sites can alter evaporation and moisture content, so water balance checks should be part of routine process control.

The regulatory setting also supports careful material tracking. Australia’s Recycling and Waste Reduction Act 2020 and product stewardship framework encourage better recovery of valuable materials, while state and territory environmental regulators set requirements for waste handling, transport, water management and discharge. An acidless process can reduce dependence on hazardous reagents, but operators still need documented controls for contaminated dust, residues, stormwater and recovered fractions.

Designing for stable operation and resource efficiency

A sensible design basis uses several rheological scenarios: normal feed, high-fines feed, maximum planned solids loading and a restart condition after settling. Each scenario should be checked against pipe velocity, pump curve, impeller torque, tank geometry and cleaning procedures. Wear-resistant materials may be necessary where dense metal particles or glass-rich fines create abrasion.

Water use should be considered alongside viscosity. Adding liquid can restore pumpability, but excessive dilution increases tank volume, downstream separation duty and dewatering requirements. The best operating point balances flowability with recovery efficiency and water consumption. This is particularly relevant for Australian facilities seeking to minimise freshwater demand and manage discharge obligations in regions affected by seasonal scarcity.

Digital process monitoring can improve this balance. A plant may combine density, flow, pressure and motor-load data with periodic laboratory rheology measurements. Specialist optical and analytical capabilities, such as those available through GenOptics measurement tools, can complement conventional sampling when operators need better visibility of particle or process conditions. The objective is a repeatable operating envelope that keeps valuable solids moving without unnecessary energy or dilution.

Solids loading condition	Typical flow behaviour	Main operational risk	Useful control response
Low	Relatively mobile slurry with limited particle interaction	Dense particles may settle	Maintain adequate velocity and agitation
Moderate	Rising apparent viscosity; possible shear thinning	Increased pump and mixer demand	Monitor density, motor load and pressure
High	Strong particle interaction and higher yield stress	Blockages, poor restart and unstable flow	Adjust feed rate, add controlled dilution and improve mixing
High fines or irregular particles	Viscosity may exceed that predicted from concentration alone	Excessive torque and abrasive wear	Characterise fines, use suitable impellers and inspect wear points
Settled or aged slurry	Structure rebuilds during rest and requires extra force to mobilise	Start-up failure or line plugging	Design for restart torque, recirculation and drainability

For an ALS2 recovery plant, viscosity management is therefore part of metallurgical performance, not a separate utility issue. Reliable solids handling protects precious-metal recovery, reduces downtime and helps the acidless process deliver its environmental advantages at commercial scale.

Teams assessing an Australian installation should build a feed-specific rheology dataset early, test several concentration levels, and translate the results into pump, tank and control specifications. This evidence-based approach gives Ikoi S.p.A.’s technology a stronger foundation for recovering value from local e-waste streams while supporting safer and cleaner resource recovery.