How Salt Concentration Shapes ALS2 Separation Rates

ALS2Project presents Ikoi S.p.A.’s acidless separation technology for recovering precious metals from electronic waste and WEEE recycling streams. The Horizon 2020-supported project is developing a safer alternative to conventional acid refining and a compact recovery model suited to smaller regional facilities. At the centre of that work is a practical operating question: how much sodium chloride should be present when an ALS2 separation stage begins, and where does additional salt stop producing a useful gain?

Sodium chloride concentration describes the amount of NaCl in the process liquid, usually reported in grams per litre or as a percentage of the liquid mass. It is different from total salinity, which may include other dissolved compounds, and from the salt-to-feed ratio. In ALS2 operations, concentration can affect solution density, solid-liquid contact, reaction activity and phase separation. It can therefore change precious-metal recovery, product purity and processing throughput at the same time.

For Australian deployment, this operating variable carries additional operational weight. Sydney and Melbourne generate dense and diverse e-waste streams, while many householders simply take obsolete electronics to a council tip or approved e-waste drop-off. State and territory stewardship arrangements within Australia’s broader National Waste Policy make traceability important. Long freight routes to metropolitan refineries also make compact recovery plants more attractive closer to the source material.

The central finding is a response curve rather than a universal recipe. Moderate salt addition may improve wetting, transport or settling enough to raise recovery, while excessive concentration can increase viscosity, residual solids, corrosion or metal carry-over. The best operating point depends on component composition, particle size, water chemistry and downstream washing. ALS2Project trials should therefore treat concentration as a measured, repeatable control rather than assuming that more salt is always better.

Why sodium chloride changes ALS2 performance

Sodium chloride changes more than one property of the separation system. It raises ionic strength, which can improve wetting and alter reaction activity, while also increasing liquid density and, at stronger concentrations, viscosity. Where ALS2 separates suspended solids through contact, settling or phase partitioning, these changes can move target metal from reject streams into the recovered product. Recovery, purity and cycle time may consequently shift together.

Chemistry can also make the response less predictable. Chloride may promote dissolution of selected metal surfaces, but silver can form poorly soluble silver chloride, while gold behaviour changes in strongly chlorinated conditions. An acidless process is therefore chemically active rather than chemically passive. Feed mineralogy, prior treatment and competing elements determine whether a higher NaCl concentration improves selectivity or merely increases the dissolved-solids load.

Operations need to define the separation rate before comparing trials. Precious-metal recovery into the selected product stream, product purity, losses to rejects and tonnes processed per hour are separate performance indicators. A short test showing excellent recovery but slow throughput may be less useful for a compact plant than a slightly lower rate with steady output and a cleaner residue.

Reading the salt-dose response

ALS2 test results can be interpreted through three broad operating zones. The useful zone is normally identified by balancing improved separation against rising viscosity, salt carry-over and downstream handling demands. It is rarely the highest concentration the equipment can physically tolerate.

A sound comparison holds pH, temperature, agitation, liquid-to-solid ratio and residence time constant while changing only the NaCl dose. Representative feed samples should then be assayed before and after separation, with duplicate trials to separate real effects from sampling variation. Product and reject streams must both be measured so that apparent recovery can be checked against a mass balance.

Relative NaCl condition	Main process effect	Expected ALS2 response	Primary control
Below the effective range	Weak wetting and ionic activity; uneven dispersion	Target material may remain with rejects, reducing recovery and purity	Establish a salt-free baseline and confirm feed composition
Controlled working range	Better solid contact and useful density-driven separation	Strongest combined recovery, selectivity and throughput	Keep pH, temperature and liquid-to-solid ratio constant
Above the working range	Higher viscosity, residual salt and possible entrainment	Separation rates may plateau or decline despite additional salt	Check washing, stage retention and downstream brine handling
Feed-specific excess	Complexation or precipitation changes metal distribution	May benefit one metal while reducing the recovery of another	Confirm metal speciation and downstream treatment capacity

The preferred concentration is the one that repeatedly produces the best process result across realistic feed batches. Conductivity can help track salt make-up, but it should not be the only control because temperature and other dissolved ions also affect the reading. Trial records should connect each concentration with reagent consumption, water use and residue quality so the apparent separation gain remains commercially meaningful.

Optimising NaCl and process controls

A staged test programme can turn sodium chloride concentration into a dependable operating variable. Initial trials should bracket low, intermediate and high doses around the current ALS2 reference condition. Once the useful response zone is visible, repeated tests should examine the boundaries and confirm whether temperature or residence time shifts the optimum. A small factorial trial can then reveal interactions that one-variable-at-a-time testing may miss.

Process water needs equal attention. Hardness, alkalinity and background chloride can change the response to added NaCl, particularly if calcium or magnesium contributes to scale or residue formation. Australian plants should meter purified or consistently treated water rather than assuming every source is equivalent. In hot operating environments, evaporation from tanks can raise concentration between shifts, so sealed storage, level control and regular calibration are essential.

Salt dosing should be logged as mass per batch or make-up volume, supported by conductivity and equipment readings. Operators also need to record pH, temperature, pump output and residence time because a concentration change cannot be assessed in isolation. Automatic dosing can help a compact installation, but it should include limits for excessive addition and clear instructions for responding to an upset. Concentrated brine needs suitable handling equipment even though it avoids the chemical hazards associated with acid refining.

Scaling ALS2 for Australian e-waste

Compact ALS2 systems could reduce the distance between waste generation and metal recovery. Dense urban collections around Melbourne and Sydney may justify larger installations, while regional facilities near Canberra, Adelaide, Perth or smaller collection hubs may benefit from modular preprocessing. Shredding and concentration near the source can lower transport volumes before valuable metal-bearing fractions reach the ALS2 separation unit.

Deployment must align with Australian waste traceability requirements. Operators need clear records for incoming WEEE, hazardous components removed before processing, final residues and recovered metal. Local council approvals, fire safety provisions and industrial waste controls may apply even when the core separation chemistry is acidless. Chloride-bearing residues also require an identified disposal route because extracting a valuable metal does not automatically make every remaining fraction harmless.

Commercial performance depends on more than laboratory separation rates. Gold, silver and other precious metals are priced in global markets, often in US dollars, even when Australian contracts are settled in Australian dollars. Buyers will assess fineness, contamination and assay consistency, while plant operators face currency exposure, changing commodity prices and transport costs. Local partnerships between councils, licensed e-waste processors and refiners can therefore improve feed consistency and make a compact plant commercially more resilient.

Building an evidence-led deployment path

Evidence should connect laboratory chemistry with full-scale operating conditions. ALS2Project results should report the feed source, NaCl concentration, liquid-to-solid ratio, residence time, recovery, purity, energy use and replicate variation. Acidless separation can reduce exposure to corrosive reagents and acid fumes, but concentrated salt remains a controlled industrial input. Appropriate personal protection, electrical safety, corrosion management and containment of process liquids remain essential.

Public communication is part of that evidence. ALS2Project’s salt-response charts and operating explanations should remain readable on the phones and tablets commonly used in Australian workshops. A browser-based external test environment can support acceptance testing of stakeholder-facing pages and dashboard mock-ups across screen sizes, while separate commissioning tests verify readings from the actual plant controller.

Australian e-waste organisations should nominate a representative feed, run matched ALS2 trials at several NaCl concentrations and publish the resulting mass balance. Recyclers, technology partners and councils can then select the concentration that balances recovery, residue quality, water consumption and cost before committing to a regional installation. This evidence-led sequence turns sodium chloride from a fixed recipe ingredient into a controlled variable with a credible path toward compact precious-metals recovery.