How ALS2 recovers rhodium from catalytic gauze in nitric acid plants
The price of rhodium has done something remarkable in the last decade, climbing past gold and platinum to become one of the most expensive metals traded on the London Bullion Market. That single fact has changed how mining houses, refiners, and industrial recyclers think about every scrap of catalyst they touch. Used catalytic gauze from nitric acid plants, once treated as a low-value consumable, now sits at the centre of a quiet but lucrative global hunt for secondary supply.
Australia has a direct stake in this hunt. The country hosts Incitec Pivot's nitric acid operations near Geelong, alongside Olympic Dam in South Australia, one of the world's few polymetallic deposits that produces rhodium as a by-product of copper and uranium processing. Local refiners in Melbourne and Port Pirie, plus research groups at CSIRO, have long been interested in cleaner ways to recover platinum-group metals (PGMs) from industrial scrap.
That is where the ALS2 project, supported by the European Union's Horizon 2020 programme, comes in. Rather than relying on aqua regia or strong mineral acids, ALS2 uses an acidless, water-based separation route that pulls rhodium, platinum, and palladium out of complex feedstocks, including spent catalytic gauze, with lower emissions and safer handling. The implications for Australian processors, who often sit far from the major European refineries, are considerable.
The role of catalytic gauze in nitric acid manufacturing
Nitric acid is produced through the Ostwald process, where ammonia is oxidised over a platinum-rhodium gauze at temperatures around 900 °C. The gauze, woven from fine wire of typically 90% platinum and 10% rhodium, catalyses the formation of nitrogen oxides that are then absorbed into water. Over months of continuous service, the metal surface becomes brittle, contaminated, and partially volatilised. Operators replace the spent gauze every three to twelve months, depending on pressure and throughput.
Each replacement cycle generates a small but high-value batch of scrap. A single nitric acid train can throw away several kilograms of PGM-bearing wire per shutdown, and globally the figure runs into the tens of thousands of kilograms annually. Rhodium, the smaller fraction of the alloy, is what gives the scrap its real value. Because the metal does not corrode away in service, almost all of it remains locked inside the wire matrix, waiting for an economical recovery route.
Why acid-based recovery has long been the default
The traditional way to liberate rhodium from spent gauze is through acid leaching, typically using hot aqua regia (a mixture of concentrated nitric and hydrochloric acid) or molten lead followed by cupellation. The chemistry is well understood, and most large refiners in Switzerland, Germany, and the United Kingdom have operated variations of it for decades. For Australian operators, the practical problem is that none of the major acid-based refineries sit on Australian soil, so the scrap has historically been containerised and shipped overseas.
Shipping high-value PGM scrap carries its own headaches. Insurance premiums are steep, customs paperwork is heavy, and the time between collection at, say, the Brisbane or Newcastle industrial precincts and final refining in Antwerp can stretch past ten weeks. During that window, metal prices move, and the operator carries the price risk. Acid-based routes also demand skilled handling, fume management, and wastewater treatment infrastructure that smaller recyclers simply cannot afford to maintain.
The acidless approach behind ALS2
ALS2, developed by Ikoi S.p.A. and its Horizon 2020 consortium partners, replaces the acid bath with a sequence of physical and chemical steps carried out in aqueous solution. The technology is built around selective dissolution using benign reagents, followed by targeted precipitation that separates rhodium from platinum and from base-metal contaminants. Because no free mineral acid is required for the main separation stage, off-gas volumes are small, and the liquid effluents are far easier to treat.
For spent catalytic gauze specifically, the system begins with a sizing and cleaning stage that removes dust, ammonia salts, and mechanical debris left over from the nitric acid reactor. The wire is then conditioned, dissolved under controlled potential, and routed through a series of separation cells that exploit subtle differences in the electrochemical behaviour of platinum, rhodium, palladium, and the trace base metals (chiefly iron, nickel, and chromium) picked up during service. The final rhodium fraction reaches purities above 99.9% without the need for high-temperature chlorination.
How the step-by-step recovery cycle works
The actual workflow, when ALS2 is deployed at a compact recovery plant, looks straightforward on paper. First, the spent gauze is logged, weighed, and sampled. Spectroscopy and X-ray fluorescence give a precise assay, which determines how the batch will be routed. Batches with high rhodium content, typical of gauze retired from large ammonia-oxidation units, move to a dedicated processing lane.
The wire is then shredded to increase surface area and loaded into the primary dissolution module. Here, an oxidising aqueous solution selectively attacks the platinum-rhodium matrix while leaving most contaminants untouched. The pregnant liquor is filtered, and platinum is precipitated first through a temperature-controlled step. Rhodium follows in a second precipitation stage, with pH and redox potential finely tuned to lock the metal into a clean salt. The salt is calcined and reduced to a finished rhodium sponge ready for sale to industrial buyers or, in some cases, to a refinery for final pyrometallurgical finishing.
Throughout the cycle, the plant's control system tracks every kilogram of metal in transit, which means there is no in-process metal that auditors cannot reconcile. For a country like Australia, where remote operations need bulletproof traceability, that level of digital bookkeeping is a quiet but important benefit. The project team has also explored how storage time effects influence both the assay and the eventual recovery yield, a subject often overlooked when gauze sits in a yard for months before processing.
How ALS2 stacks up against acid leaching
The differences between the two routes become clearer when laid side by side. The table below summarises the main technical, environmental, and operational contrasts as reported by the ALS2 consortium and benchmarked against conventional aqua-regia refining.
| Parameter | Conventional acid leaching | ALS2 acidless route |
|---|---|---|
| Primary reagent | Aqua regia (HCl + HNO₃) | Mild aqueous oxidising solution |
| Operating temperature | 80–110 °C | 45–70 °C |
| Rhodium recovery yield | 92–96% | 94–98% |
| Platinum-rhodium separation | Multi-stage, complex | Two-stage, selective |
| Off-gas treatment | Scrubbers for NOₓ, Cl₂ | Minimal; mostly water vapour |
| Wastewater treatment burden | High (acidic, metal-laden) | Moderate (near-neutral) |
| Plant footprint | Large, centralised | Compact, modular |
| Capex for a new installation | Reference baseline | 30–40% lower in pilot estimates |
| Suitability for remote sites | Limited | High |
The compactness of the ALS2 footprint is, for Australian conditions, arguably the most attractive feature. A modular ALS2 line could, in principle, be sited near a nitric acid plant in Gladstone or Geelong, processing spent gauze within days of it leaving the reactor. That cuts transport, insurance, and price-risk exposure, while keeping a higher share of the recovered metal's value in the domestic supply chain.
What this means for Australian industry and research
The Australian market for PGM recovery is small by global standards, but it is technically sophisticated. CSIRO's mineral processing teams in Clayton and Waterford have run PGM research programmes for years, and several junior miners in Western Australia have expressed interest in secondary PGM streams as a way to diversify revenue. ALS2 fits neatly into that picture. The Horizon 2020 funding has de-risked the early development work, and the consortium is now looking for industrial partners willing to host demonstration units.
There is also a regulatory angle. Australia's National Television and Computer Recycling Scheme has driven investment in e-waste infrastructure, and several state-based schemes in New South Wales, Victoria, and South Australia are tightening reporting rules around material recovery. While catalytic gauze is not consumer e-waste, the same policy environment rewards technologies that recover value cleanly and document it thoroughly, exactly the niche ALS2 is built for. For an industry that likes to have a red-hot crack at value-adding before shipping concentrates overseas, that is a welcome fit.
Reach out to the ALS2 consortium to explore a demonstration run, a sample shipment for evaluation, or a workshop tailored to Australian operators. The team can share process economics, pilot-plant data, and technical documentation to help your site decide whether the acidless route is fair dinkum for your scrap stream. Get in touch via the contact form on the ALS2Project website and start the conversation.