How ALS2 loads gold on activated carbon for downstream electrowinning

ALS2 is a non-acid separation technology developed under the Horizon 2020 programme that targets precious metals locked inside electronic scrap. The process creates a clean pregnant liquor from WEEE feedstocks without the toxic hazards of aqua regia or cyanide leaching. Loading dissolved gold onto activated carbon is the bridge between the ALS2 leach stage and the electrowinning cell that follows.

The chemistry that takes gold out of solution and holds it on carbon granules is centuries old in the goldroom, but ALS2 reshapes the inputs that reach the carbon. Where conventional plants pass a chloride-rich, often acidic stream over carbon, ALS2 delivers a lower-acidity liquor with a controlled oxidation profile. That shift changes the kinetics, the equilibrium, and the way operators tune their carbon columns. The result is a loading step that suits compact urban recyclers as much as remote regional hubs.

Across Australia, electronic waste recovery has become a quietly urgent conversation in cities such as Sydney, Melbourne and Brisbane, where council e-waste drop-offs have multiplied over the past five years. Local refiners also keep close ties with the historic goldfields around Kalgoorlie, where carbon-in-pulp circuits have been refined for decades. Those two traditions are now converging inside modern WEEE plants that want to recover the gram-level gold hidden in circuit boards, connectors and decorative trims.

The chemistry that makes gold stick to carbon

Activated carbon pulls gold out of solution through a combination of adsorption and chemical precipitation. In chloride-bearing liquors, gold typically exists as the aurocyanide, auric chloride or, under ALS2 conditions, a stabilised gold-chloride-hydroxo complex. Each of these species has an affinity for the graphitic surface and the macropore network of the carbon granule.

The surface of activated carbon is not chemically neutral. It carries oxygen functional groups such as carbonyls, phenols and lactones that act as nucleation sites when metal complexes approach. Once a gold complex encounters a favourable site, it loses a ligand, drops to a lower oxidation state, and anchors as metallic gold or as an insoluble gold compound. The granule is then loaded, ready to be stripped in the next stage of the circuit.

Temperature, ionic strength and competing cations all influence how much metal the carbon can hold. Sodium, potassium, copper and zinc ions compete for active sites, which is why ALS2 operators carefully manage the pregnant liquor composition before it reaches the column. Clean liquor in, clean loading out, fewer downstream headaches.

What acidless separation changes about the loading step

Acid-based refining pours strong mineral acids into the pregnant liquor, which keeps metals dissolved but also attacks the carbon surface over time. ALS2 keeps pH in a gentler window, which preserves carbon integrity and lengthens the working life of each batch. The economic impact is significant, because operators do not have to burn through expensive coconut-shell carbon as quickly.

A second change is the absence of nitrate and chloride fumes at high concentration. Workers in Perth, Adelaide and Hobart who handle this kind of plant benefit from a calmer atmosphere and simpler ventilation requirements. A third shift is selectivity. Because ALS2 avoids aggressive oxidisers during leaching, the gold complex that reaches carbon is cleaner, with fewer silver and base-metal competitors. That selectivity translates into higher purity at the electrowinning stage.

The acidless profile also helps with water balance. Closed-loop water treatment is easier when the liquor entering the carbon circuit is not heavily contaminated with sulphate or nitrate salts. For Australian plants that operate under strict state-level environmental discharge licences, this simpler water chemistry is more than a convenience. It is often the difference between a permit that is straightforward and one that drags through years of consultation.

Step by step through the ALS2 carbon loading circuit

Once the leach train produces a clarified pregnant liquor, the stream is filtered and conditioned. pH is adjusted to the narrow window where ALS2 gold complexes are soluble yet carbon-friendly. Redox potential is checked with a probe, and the stream is pushed through a cascade of carbon columns.

Each column holds a fixed bed of granular activated carbon, typically sized between 1 and 3 millimetres. The liquor trickles upward through the bed in a series of contactor vessels, and dissolved gold progressively transfers onto the carbon surface. Operators monitor the difference in gold concentration between the inlet and outlet of each column to track when the carbon becomes loaded.

When the leading column reaches a target loading, it is taken offline and the next column in the cascade becomes the lead vessel. The loaded carbon is then sent to a stripping circuit where gold is desorbed using a warm eluent. The carbon itself is regenerated, reactivated in a kiln if needed, and returned to the duty rotation. The whole sequence is designed for continuous operation rather than batch interruptions.

Preparing carbon for the electrowinning cell

Downstream of the strip circuit, the gold-rich eluate feeds an electrowinning cell. Steel wool cathodes inside the cell capture gold as a sludge that is later smelted into doré bars or granules. The quality of the eluate directly determines the purity of that cathodic deposit, which is why the upstream carbon-loading step matters so much.

Carbon that has been loaded cleanly delivers a more concentrated eluate. That higher concentration drives faster plating kinetics in the electrowinning cell and lowers power consumption per ounce recovered. Operators on leaner margins, such as community e-waste hubs and small recyclers across regional Victoria and Western Australia, depend on this efficiency to keep unit economics workable.

Regeneration of carbon is also a quiet contributor to plant sustainability. Spent carbon that has lost capacity can be returned to a reactivation furnace, where controlled heat removes organic foulants and restores pore structure. ALS2's lower-acid feed prolongs the interval between reactivation cycles, reducing both fuel use and the carbon dust that operators must manage.

Real-world applications and the Australian recovery picture

In practical terms, ALS2 carbon loading makes sense wherever the feed has low to moderate gold grades. Decorative plating on consumer goods is a good example. Watches, costume jewellery and even small appliance trim contain micron-thin gold layers that ALS2 can dissolve efficiently and load onto carbon. A practical case study on recovering gold from decorative plating on eyeglass frames walks through the kind of low-mass feedstock that compact Australian recyclers handle every week.

Larger feedstocks also fit the model. End-of-life mobile phones harvested through the Mobile Muster programme, decommissioned routers and printed circuit board offcuts from Brisbane electronics manufacturers all carry recoverable gold. When these materials enter the ALS2 circuit, the carbon-loading step handles the resulting liquor without the corrosion risk of older acid-based flowsheets.

State-level regulations matter too. Victoria's e-waste landfill ban, which took full effect in 2019, has pushed councils to set up dedicated collection points and partner with private recyclers. Those recyclers increasingly look for technologies that match their environmental commitments, and acidless gold recovery fits comfortably with their public-facing sustainability messaging.

Tuning loading parameters for stronger gold yields

Operators who want to push loading higher focus on three levers: contact time, carbon dosage and liquor purity. Longer contact time means more opportunities for the gold complex to find an active site, but it also means a larger carbon inventory on site. Balancing the two is a daily judgement.

Carbon dosage is the second lever. Adding more carbon to the cascade raises the total surface area available for gold complexes. The trade-off is that each individual granule loads to a lower final grade, which can lengthen stripping times. Plants that know their feed mix can optimise this balance and keep their electrowinning cell fed with consistent eluate strength.

Liquor purity is the third lever, and arguably the most rewarding. Pre-treatment steps that precipitate base metals, scrub copper and remove fines all help the carbon do its job more efficiently. ALS2 already produces a relatively clean pregnant liquor, but adding polishing filtration or selective ion exchange before the carbon contactors can lift gold loading by several percentage points. For Australian operators watching both recovery rate and operating cost, those percentage points quickly add up across a year of throughput.

ALS2's loading stage is the quiet heart of an acidless gold recovery circuit. It takes a clean pregnant liquor, captures dissolved gold onto activated carbon with minimal environmental burden, and feeds a downstream electrowinning cell that delivers saleable metal. Operators across Sydney, Melbourne and the goldfields of Western Australia are already showing how this approach can serve both industrial recyclers and community e-waste programmes.

Explore the project resources, request a technical briefing, or connect with the ALS2 team to discuss pilot-scale trials on your specific feedstock. The next step toward a cleaner, safer precious-metals circuit starts with the carbon column.