Practices for Solid-Liquid Separation After the ALS2 Reaction

The ALS2 process from Ikoi S.p.A. generates a slurry that carries dissolved precious metals alongside a finely divided solid residue, and the way that mixture is parted shapes every result that follows. Operators who treat the separation step as routine housekeeping routinely leave yield behind, send fines into the filtrate, or wear out equipment faster than the schedule allows. Treating it as a controlled unit operation, with the same respect given to leaching or electrowinning, changes the economics of the whole line.

Australian sites are a useful proving ground because the country runs a steady flow of end-of-life electronics through the National Television and Computer Recycling Scheme, and the gold-room heritage of places such as Ballarat and Bendigo keeps the appetite for cleaner refining strong. Whether a plant sits near the e-waste sorting floors of Sydney or beside the lithium and nickel processing corridors of Western Australia, the same physical rules apply: particle size, slurry rheology, wash temperature, and cake handling decide how much value reaches the recovery tank.

Acidless refining sidesteps the hazards of aqua regia and cyanide, but it does not sidestep the demands of solid-liquid separation. A clean cut between liquor and solids keeps downstream polishing and precipitation stages running smoothly, reduces reagent drag, and produces a cake that can be washed, sampled, and sent to responsible disposal without bleeding metal back into the environment.

Why Solid-Liquid Separation Matters After ALS2

Once the ALS2 reaction reaches completion, the reactor contents need to move on quickly to lock in the dissolved metal. Delays invite re-precipitation, oxidation, or adsorption of metal values onto the very solids that are about to be filtered off. A clear, fast separation protects both yield and the integrity of the downstream recovery train.

Filtrate clarity is the first quality indicator. A hazy liquor arriving at a precipitation stage forces the operator to add polishing steps, burn through more reagents, and accept a higher impurity profile in the final product. Cake quality matters just as much: a wet, channeled cake bleeds dissolved metal into the wash circuit, dragging value into waste and breaking the mass balance that auditors and regulators expect.

For a compact precious-metals plant in Adelaide or Brisbane, footprint is often as decisive as throughput. Vacuum filters need space for cake discharge, pressure filters need room for plate racking, and thickeners need height for settling. Choosing the right separation mode early avoids costly retrofits once the line is already installed and running.

Filtration Choices for ALS2 Slurries

Vacuum filtration remains the workhorse of the ALS2 pilot world. Buchner funnels, rotary drum filters, and belt filters all handle moderate solids loadings with equipment that any Australian metallurgical lab can source from suppliers in Bayswater or Clayton. The key is to tune vacuum level to the particle size: too aggressive and the cloth blinds within minutes; too gentle and the cake stays wet, dragging metal values into the filtrate and onto the floor of the vacuum receiver.

Pressure filtration shines when the slurry contains very fine particles or a high dissolved-metal concentration that justifies a tighter cut. Filter presses and candle filters deliver cake moistures below the 22 percent mark that often signals incomplete recovery, and they do it in shorter cycles. The capital cost is heavier, and plate cleaning adds labour, which is why many Australian sites reserve pressure filtration for premium campaigns or for treating the most concentrated liquor streams.

Crossflow microfiltration is becoming a serious option on smaller modular plants that want a closed loop with minimal operator exposure. Hollow-fibre membranes reject suspended solids while letting the metal-bearing liquor pass, producing a permeate clear enough to drop straight into the next stage. Membrane chemistry has to be matched to the ALS2 liquor, and routine backflushing keeps fouling in check, but the result is a clean, automated separation that fits neatly into a containerised recovery line.

Settling, Decanting, and Cake Washing

Gravity settling in a conical thickener or an Imhoff tank is hard to beat for low-throughput, variable-feed runs. The slurry rests for several hours, clear liquor is decanted from the top, and the underflow drops into a final filter or a wash column. Coagulants and flocculants, dosed in small quantities, can compress settling time from a full shift to a couple of hours. Regional e-waste drives across Victoria and Tasmania often lean on this approach because it forgives feed swings and keeps capital costs modest.

Cake washing is where recovered value is either saved or surrendered. A single displacement wash with warm demineralised water can push more than 90 percent of the residual liquor out of the cake; a second wash recovers most of what remains, with diminishing returns beyond that point. Wash ratios need to balance water cost, especially in the Murray-Darling Basin where processors feel the pinch of tightening allocations, against the value of the metal still trapped in the moisture.

Reslurrying the cake in a small repulp tank before a final filtration pass is a robust trick that experienced ALS2 operators swear by. It re-suspends any channeled solids, evens out the bed, and lets the second filter cycle produce a cleaner cake with less metal loss. The extra tank adds a modest footprint cost, but it pays back quickly in tighter mass balance and cleaner discharge streams.

Process Controls and Practical Operating Tips

Inline density meters, conductivity probes, and turbidity sensors turn the separation stage from a black box into a controllable unit operation. Watching filtrate clarity in real time tells the operator when a filter cloth is blinding, often long before lab assays catch the drift. In a country where senior metallurgists are concentrated in Perth and Kalgoorlie, remote sites in Townsville, Cairns, or Hobart benefit enormously from this kind of instrumentation and remote monitoring.

Maintenance discipline matters as much as the technology choice. Filter media should be inspected at every shift change, with cloths rotated or replaced on a schedule rather than after a failure. A small inventory of spare cloths kept on the workshop shelf keeps unplanned downtime to a single shift rather than a full day, and operators on Australian smelter sites have learned that fair-dinkum preparation beats heroic troubleshooting every time.

Documentation closes the loop. Recording vacuum pressure, cycle time, cake moisture, wash ratio, and filtrate turbidity for every batch creates a dataset that, after a few months, exposes trends and bottlenecks that no amount of intuition can match. Teams that share this data across the ALS2 network build a collective knowledge base that benefits new adopters in Newcastle, Geelong, or Cairns, where comparing notes makes new builds come together quicker than going it alone.

Comparing the Main Separation Options

No two ALS2 sites face the same constraints, so the separation choice that suits a small regional e-waste hub will look very different from the one that fits a high-throughput central refinery. The options that follow capture the main trade-offs that operators weigh during flowsheet design. Each method carries a different balance of capital cost, footprint, and skill requirement, and the right choice depends on the feed and the site's priorities.

Method	Best For	Cake Moisture	Footprint	Operating Cost	Typical ALS2 Use
Vacuum filtration (Buchner, drum, belt)	Moderate solids, general campaigns	25–35 percent	Medium	Low to moderate	Routine batch work, pilot scale
Pressure filtration (filter press, candle)	Fine particles, high-value liquor	15–22 percent	Medium-large	Moderate to high	Premium campaigns, tight mass balance
Crossflow microfiltration	Closed-loop, low exposure	Liquid only (clean permeate)	Small to medium	Moderate	Small modular plants, automated lines
Gravity settling plus decant	Variable feed, batch operation	40–60 percent after final filter	Large	Low	Regional collections, intermittent runs

Vacuum filtration tends to win where budgets are tight and operators have solid in-house mechanical know-how. Pressure filtration pays back when the value of the trapped liquor justifies the capital, and crossflow microfiltration earns its place when a closed, automated loop is the goal. Gravity settling sits at the budget end of the spectrum and is often the first step in a phased build-out that adds filtration later as volumes grow.

Stay Connected with the ALS2 Project

The team behind ALS2 continues to publish pilot results, technical notes, and event updates that help operators refine every step of the recovery train. Engineers interested in applying these practices to their own flowsheet can follow the latest developments on the ALS2 project news page, where new case studies, workshop announcements, and partner updates appear regularly. Reaching out through the project channels is a straightforward way to compare notes with other adopters, and there are no worries about getting started because the team is happy to walk newcomers through the basics.