Rhodium Refining Process

— Rhodium Refining Process: From H3RhCl6 to 99.95% Rh Sponge

The rhodium refining process is a hydrometallurgical sequence that dissolves rhodium-bearing feed, separates rhodium from iridium, platinum and base metals by changing the ligand sphere of the rhodium complex rather than its oxidation state, purifies the metal through an intermediate compound, and reduces that compound to rhodium sponge of 99.95% (3N5) purity or higher. YuSai Machinery engineers the complete line — dissolution train, sulfite and ammonia complexation stations, solvent extraction, ion exchange, reduction furnace and off-gas abatement — around your actual feed assay and rhodium head grade.
Final purity: Rh ≥ 99.95% (3N5) as standard; 99.99% (4N) achievable on a repeat purification cycle.
Rhodium recovery: ≥ 95% direct recovery from solution; ≥ 98% overall once barren liquors are scavenged by ion exchange and cementation.

Product Overview

Rhodium is the most difficult platinum group metal to bring to commercial purity, and the difficulty is chemical rather than metallurgical. In a platinum or palladium circuit the separation lever is oxidation state: platinum is dropped as ammonium hexachloroplatinate from one valence state, palladium is oxidised to the tetravalent state and precipitated as ammonium chloropalladate, and base metals are left behind because they cannot form an insoluble ammonium salt under the same conditions. Rhodium offers no such lever. Its useful chemistry in chloride media is dominated by rhodium(III), and the hexachlororhodate complex H₃RhCl₆ is kinetically inert — it does not exchange ligands quickly enough for selective precipitation to work on it directly. Every practical rhodium refining flowsheet therefore does the opposite of what a platinum flowsheet does: instead of exploiting different oxidation states, it deliberately changes the ligand environment, converting rhodium into sulfito, ammine and other complexes whose solubilities and structures differ sharply from those of iridium, platinum, palladium and the base metals.

The second obstacle is getting rhodium into solution at all. Aqua regia dissolves gold and platinum, but it does not meaningfully dissolve rhodium metal, and neither do the strong oxidant–hydrochloric acid mixtures that open platinum and palladium feeds. Four routes are used in practice, and each has a different cost structure: fusion with sodium bisulfate followed by dilute sulfuric acid leaching, which converts rhodium to a soluble rhodium(III) sulfate; a chloridising roast at moderate temperature followed by dilute hydrochloric acid leaching, which converts rhodium to a soluble chlororhodate; pretreatment activation followed by aqua regia dissolution; and electrochemical dissolution in hydrochloric acid. All four require high-temperature equipment, involve operations that are more complex than a platinum dissolution, run over longer cycles, and frequently have to be repeated before dissolution is complete. Route selection is therefore made from the feed assay and from bench-scale testwork, not from preference.

The YuSai line opens with that decision. Incoming material is weighed, riffled, split and assayed by XRF and ICP-OES, and the assay fixes the rhodium head grade, the iridium content, the platinum and palladium load, the base metal inventory and the presence of refractory or oxidic phases. From that point the feed is assigned either to direct dissolution or to a pretreatment — incineration for carbon-supported catalyst, granulation for alloy scrap, fusion or roasting for metal and refractory material. The dissolved liquor is then conditioned: silver chloride, lead chloride and other insoluble residues are crystallised out and filtered off, and the clean chlororhodic acid solution is concentrated to roughly 50 g/L rhodium, because the separation chemistry that follows performs best on concentrated rather than dilute liquors.

Purification is the heart of the line, and it runs on ligand substitution. A dilute alkali adjustment brings the solution to pH 1–1.5, and a 25% sulfurous acid solution is added at a volume ratio of 1:0.75 with respect to the rhodium solution. The batch is boiled with the pH held at 6.4, and rhodium precipitates as milky-white ammonium trisulfitorhodate, (NH₄)₃Rh(SO₃)₃. Iridium does not form an analogous precipitate and stays in the mother liquor, which is exactly the point of the step. The filtered salt is washed with pure water and then boiled with concentrated hydrochloric acid, which converts it to ammonium chlororhodate, (NH₄)₃RhCl₆, and drives sulfur dioxide out of the system. A single sulfite pass removes most of the iridium but recovers only about 95% of the rhodium, so the operation is run as a repeatable cycle with the residues returned, and it is followed by the second and independent branch: ammonia complexation, in which ammonia water is added to precipitate dichloropentaammine rhodium chloride, [Rh(NH₃)₅Cl]Cl₂, while the residual iridium hydrolyses to Ir(OH)₃. The precipitate is washed with sodium chloride solution, the rhodium compound is taken back into solution with sodium hydroxide, and the insoluble Ir(OH)₃ is filtered off. The purified rhodium solution is then acidified with hydrochloric acid, converted with nitric acid to Rh(NH₃)₅Cl₂, boiled and concentrated, and returned to chlororhodic acid by adding hydrochloric acid to destroy and expel the nitrate.

Two polishing technologies back up the precipitation chemistry, and the flowsheet uses them where each is strongest. Solvent extraction handles the bulk separation of base metals and the last of the sister platinum group metals: di-2-ethylhexyl phosphoric acid for copper, iron and lead; controlled oxidation of iridium(III) to iridium(IV) followed by tributylphosphate for iridium and platinum; and a di-n-hexyl sulfide circuit for palladium. Ion exchange resin then works the dilute end of the plant, scavenging rhodium from mother liquors, wash waters and barren solutions where solvent extraction volumes would be uneconomic. What leaves the purification circuit is a clean ammonium chlororhodate solution, which is crystallised with ammonium chloride, dissolved in pure water, and reduced with formic acid to a fine powder known as rhodium black. The rhodium black is washed, dried and reduced in a hydrogen or hydrogen–argon atmosphere to rhodium sponge assaying 99.95% or better. Throughout the line the governing rule is that rhodium is not converted to metallic form until its purity has been confirmed, and that it is produced as a compound whenever the market wants a compound — because a rhodium salt that can be re-purified is worth far more than a metal button that has to be dissolved all over again.

Emissions are captured where they are generated rather than diluted downstream. The chloridising roast, the aqua regia and chloride dissolutions and the nitrate conversion step all evolve nitrogen oxides, chlorine and acid mist; the sulfite route evolves sulfur dioxide; and the ammonia complexation stage evolves free ammonia. Each of these streams is enclosed, vented and routed to a dedicated abatement train — a condenser that recovers hydrochloric acid from the condensate, a nitrogen oxide absorption tower, an alkali washing tower with an acid-mist eliminator, and an ammonia scrubber — so that the plant meets its emission limits without shifting the burden onto the effluent plant. Spent solutions are neutralised, their metal hydroxide sludge is settled and recovered, and the process liquor that finally leaves the site has been through ion exchange and cementation first.

Product Core Selling Points

A flowsheet built on ligand substitution, not on oxidation state. Rhodium(III) in chloride media sits in an inert hexachloro complex that will not respond to the selective-precipitation tricks used for platinum and palladium. The line is therefore designed around deliberately changing the ligand sphere — sulfite first, ammonia second — to move rhodium into a salt that the other metals cannot follow.
Four dissolution routes in one plant. Crude rhodium metal resists aqua regia, so the line is delivered with sodium bisulfate fusion, chloridising roasting, activated aqua regia dissolution and electrochemical dissolution all available, and with the route for your feed fixed in the process design rather than discovered on site.
A coupled sulfite–ammonia purification train. Ammonium sulfite complexation precipitates milky-white ammonium trisulfitorhodate and leaves iridium in solution; ammonia complexation then hydrolyses the remaining iridium to Ir(OH)₃ and drops a pentammine rhodium salt. Run in sequence the two branches clear iridium that neither removes completely alone.
Compound-first process design. Rhodium is purified and held as a compound until its purity is confirmed, never reduced to metal speculatively. That single rule protects the value of a metal whose recovery is difficult and whose intermediates are themselves saleable products.
Recovery engineered to the last part per million. Barren liquors, wash waters and mother liquors are not written off: a three-stage ion-exchange resin train plus cementation returns residual rhodium to the head of the circuit.
Purpose-built for small batches. Reaction in vessels inside a fume hood, bucket-scale reaction with visible endpoints, dedicated filter units and vacuum liquid transfer match the low single-batch tonnages and high value per kilogram that define rhodium refining.

Key Facts

Final purity: Rh ≥ 99.95% (3N5) as standard; 99.99% (4N) achievable on a repeat purification cycle.
Rhodium recovery: ≥ 95% direct recovery from solution; ≥ 98% overall once barren liquors are scavenged by ion exchange and cementation.
The separation principle: rhodium(III) chloro-complexes are kinetically inert, so rhodium is separated by ligand substitution rather than by exploiting differences in oxidation state — the step that makes rhodium the hardest platinum group metal to refine.
Accepted feed: spent three-way catalyst concentrate, Pt–Rh glass-fibre bushing scrap, nitric-acid gauze sludge and equipment scale, Rh–Ir alloy and thermocouple scrap, spent supported rhodium catalysts, plating sludges, anode slimes and PGM black powder.
Product forms: rhodium sponge or powder, ammonium chlororhodate crystals, pentammine chlororhodium dichloride, rhodium chloride and rhodium nitrate solutions, and hydrated rhodium(III) oxide.
Configuration: capacity, footprint, power and utilities are engineered to your feed assay and target throughput rather than taken from fixed catalogue numbers.

Application Scenarios

Secondary precious metal refineries. Stand-alone PGM refineries that buy mixed rhodium-bearing residues and need a repeatable route from assay through to sellable sponge, salt or solution.
Automotive three-way catalyst recyclers. Operations that already produce a PGM concentrate, washcoat or smelter bullion and want to separate rhodium in-house instead of selling a mixed PGM intermediate at a discount to its rhodium content.
Glass fibre and flat glass producers. Manufacture and refurbishment of Pt–Rh bushing systems, where spent bushings, bushing scrap and the platinum–rhodium-bearing refractory brick from furnace relining are processed on site or toll-refined.
Nitric acid and caprolactam plants. Operators of ammonia oxidation gauze systems, where rhodium is lost as volatile oxide to the gas stream and accumulates in the absorption-tower sludge and in the scale on equipment and pipework.
Chemical and petrochemical catalyst users. Spent rhodium catalysts on carbon, alumina and silica supports from hydroformylation, hydrogenation and fine-chemical synthesis, together with the spent reactor residues that come with them.
Copper and nickel electrorefining plants. Anode slime and secondary residue circuits where rhodium reports with the other platinum group metals and is currently exported as a slime for toll refining.
Thermocouple, spark plug and crucible manufacturers. Pt–Rh thermocouple wire scrap and swarf, Rh–Ir and Rh–Pt alloy scrap, and used laboratory ware, where the rhodium content is high and the material is often too valuable to sell as mixed scrap.
Jewellery, plating and surface-treatment operations. Rhodium plating solutions, plating rinse waters, spent electrolytes and sludges, and white-gold and platinum alloy residues from finishing shops.
Electronics and component manufacturers. Rhodium-bearing MLCC and thick-film scrap, hard-disk and sputtering-target residues, and mixed electronic scrap concentrate that carries rhodium with the other precious metals.
Assay, pilot and research lines. Compact installations that validate a rhodium flowsheet on a few kilograms before it is scaled into a production refinery, or that refine the laboratory’s own accumulated residues.

Applicable Raw Materials

Each feed below enters the line at the point its composition requires, and the four dissolution routes are allocated to the feeds they suit. Feeds carrying carbon, organics or a refractory support are first incinerated, mechanically liberated or fused, which is why the route sheets differ even though the dissolution-to-sponge core is common. Output is stated for each feed.

Spent three-way catalyst concentrate and washcoat. PGM-bearing concentrate, smelter bullion or leach residue from automotive catalyst recycling, typically rhodium-lean against platinum and palladium but carrying the highest value per gram. Output: rhodium sponge 99.95%; platinum and palladium intermediates to their own lines.
Pt–Rh glass-fibre bushing scrap and spent refractory brick. Bushing assemblies, bushing scrap and the platinum–rhodium-bearing brick recovered at furnace relining, where contained values run at very high grade. Output: rhodium sponge; platinum sponge on the platinum circuit.
Nitric-acid plant sludges, scale and spent gauze. Absorption-tower sludge, acid sludge and equipment scale from ammonia oxidation gauze systems, plus the spent Pt–Pd–Rh catalytic gauze itself. Output: rhodium sponge; platinum and palladium recovered alongside.
Rh–Ir and Rh–Pt alloy scrap. Crucibles, spinnerets, high-temperature thermocouple elements and spark-plug electrodes, where rhodium is alloyed with iridium or platinum and must be separated from them. Output: rhodium sponge; iridium residue to the iridium circuit.
Pt–Rh thermocouple wire and swarf. Used thermocouple elements and machining swarf, dissolved directly after the platinum–rhodium ratio is established by assay. Output: rhodium sponge; platinum sponge.
Spent supported rhodium catalysts. Rhodium on carbon, alumina, silica and mixed-oxide supports from chemical and petrochemical service, incinerated or support-dissolved before the rhodium circuit. Output: rhodium sponge; support residue for disposal or recovery.
Rhodium-bearing anode slime. Slime from copper and nickel electrorefining, where rhodium reports with gold, silver, platinum, palladium and selenium. Output: rhodium sponge; gold, silver and platinum recovered on separate circuits.
PGM black powder and mixed PGM concentrate. Powder recovered from converters, catalyst manufacture and laboratory residues, with low or negligible base metal content. Output: rhodium sponge; separated salts of the sister platinum group metals.
Rhodium plating solutions and spent electrolytes. Plating baths, colouring baths and their rinse waters, where rhodium is present as a soluble sulfate or phosphate complex and must be concentrated before refining. Output: rhodium concentrate; polished effluent returned to ion exchange.
Plating sludges and filter cakes. Solid residues from plating and surface-treatment effluent plants, treated as a low-grade concentrate and returned to dissolution after enrichment. Output: rhodium-bearing concentrate returned to dissolution.
Glass and ceramic industry residues. Rhodium-bearing crucible fragments, glass-contact refractories and process residues from the optical and speciality glass industries. Output: rhodium sponge; separated platinum where co-present.
Rhodium process residues and spent mother liquors. Catalyst manufacturing residues, spent crystallisation mother liquors, filter cakes and precipitation filtrates generated inside a refinery, all returned to the head of the circuit. Output: rhodium sponge; reclaimed process solution.

Finished Products

The line is configured to deliver the product form the buyer or the downstream catalyst or alloy plant requires, from a saleable intermediate salt through to certified metal.

Rhodium sponge and powder. The primary output, assaying Rh ≥ 99.95% (3N5) with 99.99% (4N) achievable through an additional purification cycle; supplied as free-flowing sponge or powder, or melted and cast to bar, button or shot on request. Output: certified rhodium sponge with certificate of analysis.
Rhodium black. The finely divided metal obtained by formic acid or hydrazine reduction of the purified salt, supplied where a high-surface-area metal powder is wanted or where the buyer operates its own consolidation furnace. Output: washed and dried rhodium black.
Ammonium chlororhodate (NH₄)₃RhCl₆. The crystalline intermediate salt produced by boiling the sulfite salt with concentrated hydrochloric acid, available as a saleable product for buyers who prefer to run their own reduction. Output: washed, dried crystalline salt.
Pentammine chlororhodium dichloride [Rh(NH₃)₅Cl]Cl₂. The soluble rhodium ammine salt produced on the ammonia complexation branch, used as a plating intermediate and catalyst precursor. Output: crystalline rhodium ammine salt.
Chlororhodic acid solution (H₃RhCl₆). The concentrated purified rhodium chloride solution, delivered as make-up electrolyte for plating or as the feedstock for a downstream compounding step. Output: filtered, concentration-adjusted chlororhodic acid.
Rhodium(III) chloride and rhodium(III) nitrate. RhCl₃ and Rh(NO₃)₃ produced where the downstream application calls for a chloride or a nitrate medium rather than a sulfate one. Output: crystallised or solution-form rhodium salts.
Hydrated rhodium(III) oxide Rh₂O₃·xH₂O. Precipitated from purified rhodium solution and dried, supplied either as a precursor for hydrogen reduction or as a saleable compound. Output: washed and dried hydrated oxide.
By-product platinum, palladium and iridium fractions. Sister metals separated during the purification train leave as ammonium chloroplatinate, palladium salt and iridium hydroxide or chloroiridate residues for their own refining lines. Output: (NH₄)₂PtCl₆, palladium salt and Ir(OH)₃ residues.
Optional melted product. Induction melting and casting under protective atmosphere to bar, button, pellet or granulated shot where the buyer requires metallic form rather than powder. Output: cast or granulated rhodium metal.

Rhodium Refining Process Flow

The ten stages below form the standard rhodium refining flowsheet. Pretreatment — incineration of a carbon-supported catalyst, granulation of alloy scrap, or fusion and roasting of metal and refractory feed — is added upstream only where the feed demands it, and the choice between the four dissolution routes is made in process design. Engineering setpoints such as fusion temperature, roasting window, acidity, pH, reagent ratio and reductant dose are fixed against your own assay.

Rhodium refining process flow — feed to certified Rh sponge of 99.95% (3N5) and above.

Steps 1 — Feed Preparation and Assay. Incoming material is weighed, sampled by riffling and splitting, and analysed by XRF and ICP-OES, supported by potentiometric titration where chloride matrices interfere. The assay fixes the rhodium head grade, the iridium content, the platinum and palladium load, the base metal inventory and the presence of refractory or oxidic phases, and it determines both the pretreatment and the dissolution route. Carbon-supported catalyst is incinerated to burn off the support; alloy and metal scrap is granulated or ground; refractory and brick feed is crushed and blended. Sampling matters more here than in most precious-metal circuits, because rhodium feeds arrive in small lots of high unit value and are frequently inhomogeneous, and because the recovery guarantee is calculated from the head grade this step establishes. Each batch is assigned a route sheet and an internal lot number so that the mass balance can be closed at the end of the campaign. Reagent and utility input: riffling and sample splitting equipment, XRF and ICP-OES analysis, crushing and blending. Output: refining route sheet and batch record.

Steps 2 — Crude Rhodium Dissolution. Rhodium metal is not opened by aqua regia, so the dissolution step is selected from four routes rather than fixed by default. In the sodium bisulfate fusion route the prepared feed is mixed with sodium bisulfate and fused at elevated temperature, then the cooled melt is leached in dilute sulfuric acid to give a rhodium(III) sulfate solution. In the chloridising roast route the feed is mixed with a chloridising agent and carbon and roasted in the 750–880 °C band under a chlorine atmosphere, and the calcine is leached in warm water at 50–90 °C to give a sodium chlororhodate liquor; a chlorine atmosphere must be maintained throughout the roast, because rhodium chloride begins to dissociate and volatilise above roughly 550 °C, which both lowers the rhodium yield and loads the off-gas. In the pretreatment activation plus aqua regia route, rhodium that has been activated mechanically or is present as an alloy diluted with platinum is dissolved in aqua regia. In the electrochemical route, rhodium dissolves anodically in hydrochloric acid. Whichever route is used, the objective is the same: a complete, concentrated chlororhodic acid liquor with the minimum quantity of oxidant, because surplus oxidant has to be destroyed later and consumes reagents in the separation stages. Insoluble material — silver chloride, lead chloride, unreacted refractory phases and iridium-rich residue — is retained for separate treatment or returned to the iridium circuit. All four routes need high-temperature equipment, run over long cycles, and may have to be repeated before dissolution is complete, which is why the route is chosen in design rather than discovered on the plant floor. Reagent and utility input: sodium bisulfate, chloridising agent and carbon, chlorine, sulfuric or hydrochloric acid, aqua regia, heating and anodic power supply. Output: chlororhodic acid liquor; Cl₂ and NOx off-gas to scrubber; insoluble residue.

2Rh + 3Cl₂ → 2RhCl₃

2RhCl₃ + 6NaCl → 2Na₃RhCl₆

Steps 3 — Solution Conditioning and Base Metal Removal. The loaded liquor is cooled and held so that silver chloride, lead chloride and other sparingly soluble salts crystallise and settle, then filtered on a vacuum filter plate or candle filter so that the cake can be washed with minimal dilution of the mother liquor. The silver chloride cake is washed free of entrained rhodium and transferred to the silver circuit. The clarified chlororhodic acid solution is then concentrated to roughly 50 g/L rhodium, because both the sulfite and the ammonia complexation stages perform best on concentrated rather than dilute liquors, and dilution increases the volume over which the expensive reagents have to be dosed. Condensate from the evaporator is routed to acid recovery and returned to the process. Cakes are washed until the washings run clean: liquor retained on a filter cake carries dissolved rhodium into the residue and degrades the resolution of the following separation, so washing discipline at this stage is what protects the overall recovery figure. Reagent and utility input: chilled settling, vacuum filtration, low-heat evaporation, acid condensate recovery. Output: AgCl and PbCl₂ cake to the silver circuit; concentrated chlororhodic acid liquor at 50 g/L Rh.

Steps 4 — Ammonium Sulfite Complexation. This is the first of the two ligand-substitution stages and the point at which rhodium and iridium part company. The rhodium solution is adjusted to pH 1–1.5 with dilute alkali, and a 25% sulfurous acid solution is added at a volume ratio of 1:0.75 with respect to the rhodium solution. The batch is boiled with the pH controlled at 6.4, and rhodium precipitates as milky-white ammonium trisulfitorhodate. Iridium does not form an analogous precipitate and remains in the mother liquor, so a well-run sulfite step removes most of the iridium in a single operation — but it also carries a real cost, because the rhodium recovery across one precipitation is only about 95%. That figure is the reason the stage is designed as a repeatable cycle rather than a single pass: the mother liquor and the wash waters go to ion exchange and cementation, the recovered rhodium returns to the head of the circuit, and the iridium-bearing solution is routed to the iridium line. The precipitated salt is filtered and washed with pure water. Reagent and utility input: dilute alkali for pH adjustment, 25% sulfurous acid at 1:0.75 volume ratio, boiling with pH held at 6.4. Output: milky-white (NH₄)₃Rh(SO₃)₃ precipitate; iridium-bearing mother liquor to the iridium circuit.

Na₃RhCl₆ + 3(NH₄)₂SO₃ → (NH₄)₃Rh(SO₃)₃↓ + 3NH₄Cl + 3NaCl

Steps 5 — Conversion to Ammonium Chlororhodate. The washed sulfite salt is slurried in pure water and boiled with concentrated hydrochloric acid, which converts the trisulfito complex into ammonium chlororhodate and releases sulfur dioxide. The salt is filtered, washed and passed forward; the sulfur dioxide leaves with the off-gas and is captured in the alkali washing circuit rather than discharged. This conversion also returns the rhodium to the chloride system, which is the medium the downstream extraction, crystallisation and reduction steps are designed around, and it strips the sulfite ligand that would otherwise interfere with the ammine chemistry of the next stage. The conversion is run to completion, judged by the disappearance of the sulfito salt and by the composition of the wash liquor, not by a fixed time. Reagent and utility input: pure water, concentrated hydrochloric acid, boiling with agitation. Output: (NH₄)₃RhCl₆ precipitate; SO₂ off-gas to the alkali washing circuit.

(NH₄)₃Rh(SO₃)₃ + 6HCl → (NH₄)₃RhCl₆↓ + 3SO₂↑ + 3H₂O

Steps 6 — Ammonia Complexation and Iridium Removal. The second ligand-substitution stage attacks the residual iridium that the sulfite step leaves behind. Ammonia water is added to the rhodium salt and rhodium precipitates as dichloropentaammine rhodium chloride, while iridium present in solution hydrolyses simultaneously to insoluble iridium hydroxide. The precipitate is washed with sodium chloride solution rather than with water, and the rhodium compound is then taken back into solution with sodium hydroxide. Filtration at this point separates the insoluble Ir(OH)₃, which is washed and routed to the iridium circuit, from the purified rhodium solution. Because the two branches work on different principles — sulfito complexation on solubility, ammine complexation on hydrolysis behaviour — running them in sequence clears iridium combinations that neither removes completely on its own, and the number of passes for each branch is set by the impurity profile of your feed. Reagent and utility input: ammonia water, sodium chloride wash solution, sodium hydroxide dissolution, filtration. Output: purified rhodium ammine solution; Ir(OH)₃ residue to the iridium circuit.

(NH₄)₃RhCl₆ + 5NH₃·H₂O → [Rh(NH₃)₅Cl]Cl₂↓ + 3NH₄Cl + 5H₂O

Steps 7 — Nitrate Conversion and Return to Chlororhodic Acid. The purified rhodium solution is acidified with hydrochloric acid and converted with nitric acid to Rh(NH₃)₅Cl₂, which is boiled and concentrated and then returned to chlororhodic acid by adding hydrochloric acid to destroy and expel the nitrate. The purpose of this apparently circular operation is that the ammine route delivers rhodium in a form that is easy to purify but awkward to reduce, while the chloride route delivers a solution that crystallises cleanly as ammonium chlororhodate and reduces predictably. Residual nitrate is the single most common cause of irreproducible separations later in a precious-metal circuit, so the destruction step is run until no further brown fumes evolve when fresh acid is added. Nitrogen oxides evolved here pass to the absorption tower, and hydrochloric acid recovered from the condenser is returned to the process. Reagent and utility input: hydrochloric acid, nitric acid, boiling and concentration, HCl recovery from condensate. Output: clean H₃RhCl₆ solution; NOx to the absorption tower.

Steps 8 — Solvent Extraction and Ion Exchange Polishing. Bulk removal of base metals and the last traces of the sister platinum group metals is carried out by solvent extraction, with ion exchange working the dilute end of the plant. Base metals — copper, iron and lead — are extracted with di-2-ethylhexyl phosphoric acid. Iridium is then oxidised from the trivalent to the tetravalent state with an oxidant such as sodium hypochlorite under heat, the solution is adjusted to a hydrochloric acid medium, and iridium and platinum are extracted with tributylphosphate. Palladium is extracted with di-n-hexyl sulfide and the back-extracted liquid is neutralised with hydrochloric acid before crystallisation. Rhodium itself is deliberately kept in the raffinate, which is where it belongs in this sequence: a high chloride ion concentration suppresses rhodium extraction, so the metal passes through the extraction train while the impurities are removed around it. The neutralised rhodium solution is water-washed to remove sodium and redissolved in hydrochloric acid. Dilute streams — mother liquors, wash waters and barren solutions — are then passed through a three-stage ion-exchange resin train that scavenges residual rhodium before the solution leaves the circuit; a swirl electrowinning module is available where resin capacity is better reserved for other streams. Reagent and utility input: D2EHPA, sodium hypochlorite oxidant, TBP, DHS, hydrochloric acid, sodium hydroxide neutralisation, ion-exchange resin. Output: rhodium raffinate; base metal and Pt/Pd/Ir extract streams to their own circuits.

Steps 9 — Reduction to Rhodium Sponge. The purified rhodium solution is crystallised with ammonium chloride as ammonium chlororhodate, which is dissolved in pure water and reduced with formic acid to a finely divided metal powder known as rhodium black. Hydrazine hydrate is used where formate is not preferred for the product specification, and the alternative precipitation route yields hydrated rhodium(III) oxide, Rh₂O₃·xH₂O, which is washed, dried and then reduced in hydrogen. The rhodium black or the oxide is washed free of chloride, dried, and reduced and annealed in a hydrogen or hydrogen–argon atmosphere to drive off oxygen and residual volatile impurities, producing rhodium sponge assaying 99.95% or better. Because rhodium is the most expensive of the platinum group metals and the reduction stages involve finely divided metal, the vessels, filters and transfer lines in this part of the line are designed for containment and for complete recovery of the powder, and every filter medium, wash and container rinse is returned to the circuit. Melting and casting to bar, button or shot in an induction furnace under protective atmosphere is available where metallic form is required. Reagent and utility input: ammonium chloride, formic acid or hydrazine hydrate, hydrogen or hydrogen–argon reduction gas, induction melting. Output: rhodium sponge ≥ 99.95%; reduction off-gas to scrubber.

2Rh³⁺ + 3HCOOH → 2Rh↓ + 3CO₂↑ + 6H⁺

Rh₂O₃ + 3H₂ → 2Rh + 3H₂O

Steps 10 — Assay Verification and Emission Control. Every filtrate, wash water, residue and finished lot is assayed, and no rhodium solution or compound precipitate is converted to metallic form until its purity has been confirmed. The sponge is sampled and analysed by ICP-OES against a certified reference material, with glow-discharge mass spectrometry available where a full trace-impurity profile including the platinum group metals and the interstitial elements is required, and the batch is released only when the certificate of analysis confirms the specified purity. Barren liquors and wash waters still carry rhodium at the parts-per-million level, so they pass through the three-stage resin train and then iron or zinc cementation; the cementation residue returns to dissolution and the depleted solution goes to neutralisation. Off-gas from the dissolution, sulfite, nitrate and reduction stages passes through a condenser that recovers hydrochloric acid from the condensate, a nitrogen oxide absorption tower, an alkali washing tower with an acid-mist eliminator, and an ammonia scrubber, and the emission log is retained with the batch record. Finished sponge and salts are packed in airtight containers immediately after drying, with tamper-evident sealing and a retained sample held for the warranty period. Reagent and utility input: ICP-OES, XRF and potentiometric titration; resin and cementation reagents; NOx absorption, alkali washing and acid-mist elimination. Output: certificate of analysis; compliance and emission record; polished effluent; retained reference sample.

Product Structure (Main Equipment)

The line is assembled from the equipment groups below. Materials of construction are selected per stream — titanium for the hot chloride and oxidant duty, glass-lined or enamelled steel where acid, oxidant and ammonia are combined, polypropylene and PVDF for atmospheric acid and scrubber service, borosilicate glass where the reaction must be observed directly, and silica or alumina ware for the high-temperature fusion and roasting duty.

Feed preparation, fusion and roasting

Weighing, sampling and splitting station. Riffling and sample division equipment that produces a representative assay sample from each incoming lot before any material enters the process, which is the step the recovery guarantee is calculated from. Output: assay sample and head-grade record.
Incineration and roasting furnace. A controlled-temperature furnace that burns off carbon and organic content from spent supported catalyst and converts the residue to a leachable ash. Output: rhodium-bearing ash for dissolution.
Crushing, granulation and grinding set. Size reduction for alloy scrap, thermocouple and bushing scrap, refractory brick and agglomerated residues, so that dissolution or fusion proceeds at a predictable rate; a reduced and classified particle size is a precondition of an efficient chloridising roast. Output: classified feed of controlled particle size.
Sodium bisulfate fusion unit. A high-temperature fusion station with crucible handling, fume extraction and controlled cooling, used to convert rhodium metal into a water- or acid-soluble rhodium(III) sulfate. Output: fused melt for leaching.
Chloridising roasting furnace. A tube or rotary furnace with chlorine handling, atmosphere control and exhaust capture, operated in the 750–880 °C band with a chlorine atmosphere maintained above 550 °C so that rhodium chloride is not lost to the gas stream. Output: chloridised calcine; Cl₂-bearing off-gas to scrubber.
Warm-water leach tank and agitator. Leaching vessels with heating and agitation that dissolve the chloridised calcine in warm water at 50–90 °C. Output: sodium chlororhodate leach liquor.

Dissolution and reaction train

Titanium dissolution reactor. Heated reactor for hydrochloric acid and oxidant dissolution duty, fitted with a condenser, controlled reagent dosing and vapour take-off to the scrubber. Output: chlororhodic acid liquor; acid condensate recovered.
Electrochemical dissolution cell. Anodic dissolution station with rectifier and cell circulation for the electrochemical dissolution route. Output: dissolved rhodium liquor.
Glass-lined and enamelled reaction vessels. Enamelled or glass-lined reactors for acid, alkali, sulfite and ammonia duty, where both oxidising and alkaline conditions occur in the same vessel. Output: conditioned and complexed rhodium solutions.
Glass reaction ware and fume-hood reaction stations. Bucket-scale reaction inside a fume hood with glass vessels, so that operators can follow the endpoint of sulfite precipitation, sulfite conversion and ammonia complexation by colour and behaviour rather than by a timer. Output: batch-wise reaction and endpoint judgement.
Reagent dosing system. Metered pumps with interlocks that add oxidant, sulfurous acid, ammonia and hydrochloric acid in controlled increments and stop on the reaction endpoint rather than on a fixed volume; the pH controller holds the sulfite stage at 6.4 and the pH 1–1.5 adjustment is dosed under the same supervision. Output: reproducible reagent addition; batch log.
Condenser and acid recovery unit. Recovers hydrochloric acid from evaporator and reactor condensate and returns it to the process, reducing reagent make-up and load on the effluent plant. Output: recovered acid returned to circuit.

Solid-liquid separation and liquid transfer

Vacuum filter plate and filter trolley. Mobile filtration units with diaphragm vacuum pumps that separate salts and cakes while keeping wash volumes low, so that the mother liquor is not diluted and entrained rhodium is not carried into the residue. Output: washed salt or cake; clarified filtrate.
Candle filter. Pressure filtration with reverse gas pulse for the cake release duty, used where solids loading makes plate filtration slow and labour-intensive. Output: filtered liquor; dry cake discharge.
Crystallising and settling tanks. Chilled or temperature-controlled vessels that allow silver chloride, lead chloride and salts to crystallise and settle ahead of filtration. Output: settled salts for filtration.
Chloride-resistant transfer and vacuum lift system. Liquid-ring vacuum pumps and vacuum lift stations for liquid transfer, which avoids pumping hot chloride liquors through mechanical seals; the vacuum generator also serves filtration. Output: liquid transfer without mechanical pumping of hot chloride.
Precision and polish filtration. Fine filtration ahead of the extraction, crystallisation and reduction stages, removing suspended solids that would otherwise contaminate the final product. Output: particle-free feed to extraction and crystallisation.

Purification, extraction and recovery

Sulfite complexation reactor. Agitated, heated reactor with pH control and off-gas take-off for the ammonium sulfite stage, arranged so that the milky-white precipitate can be observed, settled and drawn off cleanly. Output: (NH₄)₃Rh(SO₃)₃ precipitate; Ir-bearing mother liquor.
Ammonia complexation and alkaline dissolution station. Reactor with ammonia dosing, agitation and ventilation for the ammine stage, coupled to the sodium hydroxide dissolution vessel and filtration train in which Ir(OH)₃ is separated from the purified rhodium solution. Output: purified rhodium ammine solution; Ir(OH)₃ to the iridium circuit.
Nitrate conversion and denitration unit. Heated reactor with condenser and NOx take-off for the conversion to Rh(NH₃)₅Cl₂ and its return to chlororhodic acid, run until no further brown fumes evolve. Output: clean H₃RhCl₆ solution; NOx to the absorption tower.
Solvent extraction unit. Mixer-settler boxes or centrifugal extractors for the D2EHPA, TBP and DHS circuits, with phase separation and back-extraction, and with the rhodium raffinate taken forward. Output: rhodium raffinate; base metal, Pt/Pd and Ir extract streams.
Ion-exchange resin columns. A three-stage resin train that scavenges residual rhodium from mother liquors, wash waters and barren solutions before the solution reaches neutralisation, with regeneration and elution circuits. Output: recovered rhodium eluate returned to the circuit.
Cementation tank. Iron or zinc cementation vessel for the final polish of the resin effluent. Output: cementation residue returned to dissolution.
Swirl electrowinning unit. Optional module for recovering rhodium and companion metals from very dilute solutions where resin capacity is better reserved for other streams. Output: metal recovered from dilute liquors.

Reduction, melting, control and environmental systems

Reduction reactor. Glass-lined or enamelled reactor with heating, agitation and reagent dosing for formic acid or hydrazine reduction of the purified salt to rhodium black. Output: rhodium black.
Hydrogen reduction furnace. A tube furnace with controlled hydrogen or hydrogen–argon atmosphere, temperature programming and exhaust capture, for reducing rhodium black or Rh₂O₃·xH₂O to sponge and annealing out oxygen and volatile impurities. Output: rhodium sponge ≥ 99.95%.
Induction melting furnace and casting set. Melts rhodium sponge under protective atmosphere into bars, buttons or shot where the buyer requires metallic form rather than powder, and a granulator for shot production. Output: cast or granulated rhodium metal.
PLC control and HMI panel. Supervises dosing, temperature, pH and anodic power, logs every batch, and holds the interlocks that prevent reagent addition out of sequence across the sulfite, ammonia and nitrate stages. Output: batch record and process log.
Nitrogen oxide absorption tower and alkali washing circuit. Two-stage scrubbing for nitrogen oxides plus an alkali wash with an acid-mist eliminator for the chloride and sulfite stages, an ammonia scrubber for the complexation stage, and hydrochloric acid recovery from condensate upstream. Output: compliant stack emission; recovered scrubber liquor.
Effluent neutralisation and settling unit. Neutralises spent acid and alkaline streams, settles the metal hydroxide sludge for recovery and prepares the clarified effluent for discharge or further treatment. Output: settled sludge; treated effluent.
Assay laboratory equipment. XRF, ICP-OES, potentiometric titration, sample preparation and a dedicated fume hood, with glow-discharge mass spectrometry available as a service, so that feed, intermediate and product assays are generated in-house on the same day as production. Output: certificate of analysis per batch.

Product Features

Single-supplier turnkey scope. Assay, flowsheet design, equipment manufacture, installation, commissioning, operator training and handover are delivered as one contract, so the interfaces between fusion, roasting, dissolution, complexation, extraction, reduction and scrubbing are no one else’s problem.
Four dissolution routes built in, not bolted on. Sodium bisulfate fusion, chloridising roasting, activated aqua regia dissolution and electrochemical dissolution are all available in the same line, and the design specifies which route your feed takes instead of leaving the decision to the plant.
Two ligand-substitution branches in one plant. Ammonium sulfite complexation and ammonia complexation are both built into the same line, and the flowsheet specifies how many passes of each branch your feed needs.
Chemistry-led separation, not a generic leach. The precipitation stages are designed around the specific behaviour of rhodium(III) sulfito and ammine complexes, which is what allows iridium to be taken out of the circuit while rhodium is being purified, rather than after the fact.
Compound-first operating discipline. The line is designed to hold rhodium as a purified compound until its purity is confirmed, and to deliver it as a salt or solution whenever the market or the downstream customer prefers a compound to a metal.
Small-batch architecture. Reactions run in vessels inside a fume hood, with bucket-scale reaction, dedicated filter units and vacuum liquid transfer — a layout that suits the low single-batch tonnages of rhodium refining and keeps the working inventory of metal small.
Visible process control. Where the reaction must be watched, glass and transparent polymer vessels are used so that operators can judge the endpoint by colour and behaviour, which on this chemistry is more reliable than a timer.
Recovery designed to the last part per million. Resin scavenging and cementation are part of the standard scope, not an optional extra, and every filter medium, wash and container rinse is returned to the circuit.
Closed-loop reagent and water management. Evaporator condensate returns as recovered acid, sulfite mother liquor and wash waters return to the resin train, and final-stage rinse waters are recycled rather than discharged.
Containment designed for the highest-value metal in the circuit. Enclosed reaction, sealed transfer, wash recovery and tamper-evident packing are specified throughout, because the material being handled is the most expensive of the platinum group metals.
Emissions managed at the source and at the stack. Chlorine, nitrogen oxides, sulfur dioxide, acid mist and ammonia are captured where they are generated and routed through dedicated abatement, with acid recovery upstream.
Assay-linked traceability and feed-matched configuration. Every lot carries a route sheet, a mass balance and the assay results of feed, intermediate and product, and capacity, footprint, power demand and utility consumption are engineered from your assay and target throughput rather than from a standard model number.

Technical Parameters & Specifications

Engineering setpoints below are the standard design basis and are confirmed or adjusted during process design once the feed assay is known. Capacity, footprint, installed power and utility consumption are engineered to your throughput and site conditions.

Rhodium refining process — technical parameters and specifications.

Parameter	Specification
Feed rhodium grade	Rh ≥ 5% for direct refining; lower-grade material accepted after enrichment, fusion or roasting
Feed forms accepted	Spent three-way catalyst concentrate, Pt–Rh bushing and refractory scrap, nitric-acid sludge and scale, Rh–Ir and Rh–Pt alloy scrap, thermocouple scrap, spent supported Rh catalyst, anode slime, plating solutions and sludges, PGM black powder, process residues
Feed pretreatment	Incineration, granulation, grinding, blending, sodium bisulfate fusion or chloridising roast, selected per feed type
Dissolution routes	Sodium bisulfate fusion and dilute sulfuric acid leach; chloridising roast and dilute hydrochloric acid leach; pretreatment activation and aqua regia dissolution; electrochemical dissolution
Chloridising roast temperature	750–880 °C, with a chlorine atmosphere maintained above 550 °C to prevent rhodium chloride volatilisation
Chloridising roast cycle	Run to completion of the chloridising reaction, typically several hours, followed by controlled cooling
Calcine leaching condition	Warm water at 50–90 °C with agitation, to dissolve sodium chlororhodate
Sulfate route leaching	Dilute sulfuric acid leach of the cooled bisulfate melt, giving rhodium(III) sulfate solution
Solution conditioning	Chilled settling and vacuum filtration to remove AgCl and PbCl₂ ahead of the separation stages
Working Rh concentration at complexation	Approximately 50 g/L Rh in the chlororhodic acid liquor
Sulfite stage pH adjustment	Dilute alkali to pH 1.0–1.5 before reagent addition
Sulfite reagent	25% sulfurous acid (H₂SO₃) solution at a 1:0.75 volume ratio to the rhodium solution
Sulfite stage boil condition	Boiled with pH held at 6.4 through the precipitation
Precipitated rhodium salt	Milky-white ammonium trisulfitorhodate (NH₄)₃Rh(SO₃)₃
Iridium behaviour in the sulfite stage	Iridium does not form an analogous precipitate and remains in the mother liquor
Rhodium recovery per sulfite pass	Approximately 95%, which is why the precipitation is run as a repeat cycle with residues returned
Conversion to chlororhodate	Pure water wash, then boiling with concentrated hydrochloric acid; SO₂ evolved to the alkali washing circuit
Ammonia complexation reagent	Ammonia water (NH₃·H₂O)
Precipitated rhodium ammine	Dichloropentaammine rhodium chloride [Rh(NH₃)₅Cl]Cl₂
Iridium behaviour in the ammine stage	Iridium hydrolyses to Ir(OH)₃, which is dissolved away in sodium hydroxide and filtered off
Ammine wash and dissolution	Sodium chloride solution wash, then sodium hydroxide dissolution of the rhodium compound
Nitrate conversion	Hydrochloric acid acidification, then nitric acid to Rh(NH₃)₅Cl₂, boiled and concentrated
Nitrate destruction	Hydrochloric acid addition to destroy and expel nitrate until no brown fumes evolve on fresh acid addition
Base metal extraction reagent	Di-2-ethylhexyl phosphoric acid (D2EHPA) for copper, iron and lead
Iridium and platinum extraction	Oxidation of Ir(III) to Ir(IV) with sodium hypochlorite under heat, then extraction with tributylphosphate (TBP)
Palladium extraction reagent	Di-n-hexyl sulfide (DHS), with hydrochloric acid neutralisation of the back-extracted liquid
Rhodium extraction behaviour	Rhodium is retained in the raffinate; a high chloride concentration suppresses its extraction, which is used to advantage
Barren liquor recovery	Three-stage ion-exchange resin train followed by iron or zinc cementation, with the residue returned to dissolution
Optional dilute-liquor module	Swirl electrowinning for very dilute streams where resin capacity is reserved for other duties
Crystallisation	Ammonium chloride addition to crystallise clean ammonium chlororhodate for reduction
Reductant	Formic acid (HCOOH) or hydrazine hydrate; precipitation of Rh₂O₃·xH₂O as an alternative route
Hydrogen reduction	Rhodium black or Rh₂O₃·xH₂O reduced and annealed in hydrogen or hydrogen–argon atmosphere
Final rhodium purity	Rh ≥ 99.95% (3N5) as standard; up to 99.99% (4N) with an additional purification cycle
Rhodium recovery	≥ 95% direct recovery from solution; ≥ 98% overall with ion-exchange and cementation scavenging
Residual rhodium in effluent	Design target in the low ppm range after resin polishing, confirmed against your effluent limits
Product forms	Rhodium sponge or powder, rhodium black, ammonium chlororhodate, rhodium ammine salt, chlororhodic acid solution, RhCl₃ and Rh(NO₃)₃, Rh₂O₃·xH₂O, optional cast bar or shot
Product packaging	Airtight containers immediately after drying, tamper-evident sealing, retained reference sample per batch
Materials of construction	Titanium, glass-lined or enamelled steel, polypropylene, PVDF, borosilicate glass, silica and alumina ware for high-temperature duty
Filtration equipment	Vacuum filter plate, filter trolley with diaphragm pump, candle filter with reverse gas pulse, precision polish filtration
Off-gas treatment	Condenser with HCl recovery, NOx absorption tower, alkali washing tower with acid-mist eliminator, ammonia scrubber
Instrumentation and control	PLC with HMI supervision of temperature, pH, dosing, anodic power and batch logging, with sequence interlocks
Quality verification	XRF, ICP-OES and potentiometric titration; glow-discharge mass spectrometry available for full trace-impurity profiles; certificate of analysis per batch
Capacity, footprint, power, utilities	Engineered to feed assay, target throughput and site conditions

Technical Advantages

The separation problem is solved where it actually exists. Rhodium is not separated by an oxidation-state trick, because its kinetically inert rhodium(III) chloro-complex will not respond to one. The line works on ligand substitution instead, converting rhodium into sulfito and ammine complexes whose solubility and hydrolysis behaviour differ sharply from iridium, platinum, palladium and the base metals. Result: a flowsheet that separates rhodium on the property that actually distinguishes it.
Iridium is removed twice, by two different mechanisms. The sulfite branch removes most of the iridium in a single precipitation. The ammine branch then hydrolyses the remainder to Ir(OH)₃, which is dissolved away and filtered out. Because the two mechanisms are independent, the train clears iridium distributions that neither branch handles alone. Result: applicability across Pt–Rh, Rh–Ir and mixed PGM feeds with one equipment set.
Dissolution is engineered, not assumed. Four routes are available because rhodium metal is not opened by aqua regia, and the choice between fusion, chloridising roast, activated aqua regia and electrochemical dissolution changes both the cycle time and the cost structure. The route is selected in process design from your assay. Result: no campaign lost to a dissolution method that never suited the feed.
Compound-first design protects the value of the metal. Rhodium is purified and held as a compound until its purity is confirmed instead of being reduced to metal speculatively, and it can be delivered as a salt or solution whenever the customer wants a compound. Result: a rhodium salt that can be re-purified instead of a metal button that has to be dissolved again.
The real limitation of the sulfite step is designed around, not hidden. A single sulfite precipitation recovers only about 95% of the rhodium. Rather than treat that as a loss, the flowsheet runs the stage as a repeat cycle, sends the mother liquor and wash waters to ion exchange and cementation, and returns the recovered rhodium to the head of the circuit. Result: overall recovery of 98% and above despite a single-pass figure of 95%.
Concentrated liquors by design. Both complexation stages perform best on concentrated rather than dilute solutions, so the flowsheet deliberately holds the working solution at roughly 50 g/L rhodium and minimises rinse water at every filtration step. Result: higher precipitation efficiency and smaller vessel volumes per kilogram of rhodium.
Washing discipline treated as a recovery control. Liquor retained on a filter cake carries dissolved rhodium into the residue and degrades the resolution of the next separation, so cakes are washed to a defined endpoint rather than for a fixed time. Result: less rhodium reporting to residues and to the iridium circuit.
The law of the system used deliberately in extraction. A high chloride concentration suppresses rhodium extraction, which in most circuits is an inconvenience. Here it is the mechanism: rhodium is kept in the raffinate while base metals, platinum, palladium and iridium are extracted around it. Result: bulk impurity removal without an extractant dedicated to rhodium.
Loss recovery at both ends of the concentration range. Solvent extraction handles the bulk liquors and resin columns work the dilute end, with cementation polishing what the resin leaves. Every stream is accounted for. Result: the parts-per-million fraction is captured rather than written off.
Emissions captured where they form. The dissolution, roasting, sulfite, nitrate and reduction stages each evolve a different gas — chlorine, nitrogen oxides, sulfur dioxide, acid mist, ammonia — and each has its own capture route, with hydrochloric acid recovery upstream of the scrubber. Result: compliance that does not depend on diluting the problem downstream.
Small working inventory of metal. Because the line is built around small batches with vacuum transfer and rapid filtration, the quantity of rhodium held inside the circuit at any moment is small. Result: less capital tied up in in-process metal and shorter campaigns.
Verifiable numbers. Assay-linked mass balance, per-batch logging and a certificate of analysis mean purity and recovery claims can be checked against your own analysis on the same lot, with a retained reference sample held. Result: a refining specification that can be audited by the buyer or by a toll-refining counterparty.
Modular, so a pilot line does not become stranded capital. Additional fusion, roasting, reaction, extraction and resin capacity can be added to a validated pilot installation rather than replacing it. Result: an entry point that scales with the market.
Integration with the wider plant. Feed from an anode slime circuit, a three-way catalyst recycling line or a glass-fibre bushing programme can be routed into this process, and the platinum, palladium and iridium fractions leave as intermediates that downstream refining lines accept. Result: one circuit that closes the rhodium loop without breaking the others.

Service Process

Step 1 — Enquiry and feed data review. You provide the feed type, an assay or typical analysis, expected monthly rhodium volume and any site constraints. We confirm whether the material suits this process or needs an upstream pretreatment or enrichment step first. Output: preliminary process route and a clear scope statement.

Step 2 — Laboratory test and small-batch verification. A representative sample is tested at bench scale through the selected dissolution route, sulfite complexation, conversion, ammine complexation, extraction and reduction. Operating windows, the number of complexation passes and the expected recovery are established on real material, not on assumptions. Output: test report and returned product sample for your own assay.

Step 3 — Process design and flowsheet. We issue the process flow diagram, the reagent balance, the mass balance by metal, the equipment list, vessel sizing and the utility schedule, with all engineering setpoints stated. Output: process design package and mass balance.

Step 4 — Equipment manufacture and factory testing. Fusion and roasting stations, reactors, filtration units, extraction and resin equipment, reduction furnace, scrubbers and the control panel are fabricated, assembled and function-tested before shipment, with material certificates retained. Output: factory acceptance test record.

Step 5 — Layout and site interface design. Equipment layout, civil loading, power supply points, water, compressed air, steam or electric heating, hydrogen supply and ventilation interfaces are defined against your building. Output: layout and utility interface drawings.

Step 6 — Shipment, installation and commissioning. Equipment ships with the documentation set, and our engineers supervise installation, pressure and leak testing, electrical and control checkout, and water commissioning. Output: mechanically complete line, ready for reagent commissioning.

Step 7 — Operator training and SOP handover. Operating procedures, safe handling of chlorine, nitrogen oxides, sulfurous acid, ammonia and hydrogen, endpoint judgement by colour and behaviour, sampling and assay practice, and routine maintenance are taught on site with the actual equipment. Output: trained operators and a signed SOP set.

Step 8 — Production trial and performance acceptance. The line is run on your feed and the agreement is confirmed against measured recovery, product purity and effluent concentration. Output: performance test certificate and first certificate of analysis.

Step 9 — Ongoing technical support. Process troubleshooting, flowsheet refinement as your feed mix changes, spare parts at cost, and reagent or consumable supply on request. Output: continuing production support.

Warranty & After-Sales Service

Warranty period. Twelve months from successful commissioning or eighteen months from shipment, whichever falls earlier, covering design, manufacture and assembly defects of the supplied equipment. Excluded: consumables such as filter cloths, gaskets, membranes, resin and reagents, and damage from misuse or from feed outside the agreed specification.
Performance commitment. Where the feed matches the tested sample, we stand behind the agreed rhodium recovery and product purity figures established in the process design package, with the performance test as the reference point. Output: documented performance acceptance.
Spare parts. Wear and critical spares are quoted at the time of order and held available afterwards, with a recommended on-site stock list issued at handover. Output: spare parts list and supply at cost.
Remote support. Telephone, email and messaging support for operating questions, alarm interpretation and process troubleshooting, whenever your line is running. Output: same-day response on operating issues.
On-site service. Engineer attendance for warranty faults, process problems and periodical check-ups, with response and attendance times agreed in the contract according to your location. Output: on-site intervention and a service report.
Operator retraining and refresher visits. Additional training, refresher sessions and process optimisation visits as your feed mix or product requirement changes. Output: updated training and procedure revisions.
Documentation set. Operation and maintenance manuals, process flow diagram, piping and instrumentation diagram, equipment layout, electrical drawings, spare parts catalogue, safety data for reagents and the emergency response plan. Output: complete documentation handover.
Compliance support. Assistance with emission and effluent parameter declarations, scrubber performance verification, and the handling, storage and disposal requirements for the reagents and gases used in the process. Output: technical data for your permit applications.
Lifetime technical support. The process design remains yours, and our engineering team remains available for modification and expansion advice for the operating life of the plant. Output: long-term engineering continuity.

Frequently Asked Questions

Why is rhodium the hardest platinum group metal to refine?

Because the separation trick that works for platinum and palladium does not exist for rhodium. Platinum is dropped as ammonium hexachloroplatinate and palladium is oxidised to the tetravalent state and precipitated as ammonium chloropalladate, so both can be separated by exploiting differences between oxidation states. Rhodium in chloride media is dominated by rhodium(III), and its hexachloro complex H₃RhCl₆ is kinetically inert — it does not exchange ligands fast enough for selective precipitation to work on it directly. Rhodium refining therefore proceeds by deliberately changing the ligand sphere, converting rhodium into sulfito and ammine complexes whose solubility and hydrolysis behaviour differ from those of iridium, platinum, palladium and the base metals.

How is rhodium dissolved when aqua regia will not dissolve it?

By one of four routes, selected from the feed assay. Sodium bisulfate fusion breaks the metal down into a melt that is then leached in dilute sulfuric acid to give a rhodium(III) sulfate solution. A chloridising roast at 750–880 °C under a chlorine atmosphere converts rhodium to a chloride that is leached in warm water at 50–90 °C to give a sodium chlororhodate liquor; the chlorine atmosphere must be maintained above about 550 °C, because rhodium chloride begins to dissociate and volatilise above that point. Pretreatment activation followed by aqua regia works where the rhodium has been activated mechanically or is present as an alloy diluted with platinum, and electrochemical dissolution in hydrochloric acid provides an anodic route. All four require high-temperature equipment, run over long cycles, and may need to be repeated before dissolution is complete.

Which rhodium-bearing feed materials can the line accept?

Spent three-way catalyst concentrate, washcoat and smelter bullion; Pt–Rh glass-fibre bushing scrap and the platinum–rhodium-bearing refractory brick recovered at furnace relining; nitric-acid plant absorption-tower sludge, acid sludge, equipment scale and spent Pt–Pd–Rh gauze; Rh–Ir and Rh–Pt alloy scrap including crucibles, spinnerets, spark-plug electrodes and thermocouple elements; pt–Rh thermocouple wire and machining swarf; spent supported rhodium catalysts on carbon, alumina and silica; rhodium-bearing anode slime from copper and nickel electrorefining; PGM black powder and mixed PGM concentrate; rhodium plating solutions, spent electrolytes, rinse waters and sludges; glass and ceramic industry residues; and the refinery’s own spent mother liquors, filter cakes and process residues. Feeds carrying carbon, organics or refractory supports are incinerated, granulated or fused first, and that pretreatment is designed to match the specific feed.

How are rhodium and iridium separated from each other?

By two independent mechanisms run in sequence. In the ammonium sulfite stage the solution is adjusted to pH 1–1.5 and a 25% sulfurous acid solution is added at a 1:0.75 volume ratio; the batch is boiled with the pH held at 6.4 and rhodium precipitates as milky-white ammonium trisulfitorhodate, while iridium does not form an analogous precipitate and stays in the mother liquor. In the ammonia complexation stage that follows, rhodium precipitates as dichloropentaammine rhodium chloride while residual iridium hydrolyses to Ir(OH)₃; the rhodium compound is dissolved in sodium hydroxide and the insoluble Ir(OH)₃ is filtered off. Because the two branches work on different principles, running them in sequence clears iridium distributions that neither handles completely on its own.

What is the difference between the ammonium sulfite route and the ammonia complexation route?

The ammonium sulfite route works on solubility. Rhodium is complexed with sulfite and precipitated as (NH₄)₃Rh(SO₃)₃, a milky-white salt, while iridium remains dissolved; the salt is then boiled with concentrated hydrochloric acid to convert it to ammonium chlororhodate and release sulfur dioxide, which returns the rhodium to the chloride system. The ammonia complexation route works on hydrolysis behaviour. Ammonia water precipitates [Rh(NH₃)₅Cl]Cl₂ while iridium hydrolyses to Ir(OH)₃, which sodium hydroxide dissolves away from the rhodium. The sulfite branch removes the bulk of the iridium in one operation but recovers only about 95% of the rhodium; the ammine branch removes residual iridium and delivers a rhodium solution that is easy to purify. Both branches are built into the same line, and the flowsheet specifies how many passes of each your feed needs.

Why is rhodium held as a purified compound instead of being reduced to metal immediately?

Because on this metal a purity mistake cannot be corrected cheaply. Rhodium dissolves with difficulty, so any metal that fails specification has to go back through fusion or roasting and the whole dissolution train again. Keeping rhodium as a compound until its purity is confirmed — and the process is designed so that it can be delivered as a salt or a solution rather than always as metal — means a batch that needs another purification pass simply gets one, instead of being melted, re-dissolved and re-refined from the beginning. The rule applies to every intermediate in the line, and it is also why the intermediate salts are offered as products in their own right.

What rhodium purity does the line deliver?

Rh ≥ 99.95% (3N5) is the standard specification, delivered as sponge or powder and released against a certificate of analysis. An additional purification cycle — further sulfite and ammine complexation passes before crystallisation — brings the product to 99.99% (4N), and the achievable figure depends on the impurity profile of your particular feed, and in particular on how much iridium, platinum and palladium it carries into the head of the circuit. The exact figure for your material is established in the laboratory test before the plant is built, and the full trace-impurity profile can be verified by glow-discharge mass spectrometry where that level of documentation is required.

What rhodium recovery can be guaranteed?

Direct recovery from solution is 95% or higher, rising to 98% or higher overall once the barren liquors, mother liquors and wash waters are treated. The gap between the two figures is not inefficiency that can be designed away: a single ammonium sulfite precipitation recovers only about 95% of the rhodium, which is a property of that chemistry rather than a defect of the equipment. It is closed at the other end of the line, where a three-stage ion-exchange resin train adsorbs residual rhodium from the process liquors and the polish filtrate is then cemented with iron or zinc. The cementation residue returns to dissolution rather than leaving the plant.

How long does a rhodium campaign run?

Longer than a palladium or platinum campaign, for two structural reasons rather than one. Crude rhodium has to be broken down by fusion, chloridising roast, activated aqua regia or electrochemical dissolution before the separation chemistry can begin, and that step may have to be repeated; and purification depends on repeated complexation passes, with a single sulfite pass recovering only about 95% of the rhodium, so several passes are normal for most feeds. What can be stated is the structure: dissolution and conditioning come first, the sulfite and ammine branches deliver the purification, extraction and resin polishing follow, and reduction and drying are comparatively short. Individual stages are governed by endpoints rather than fixed times — the chloridising roast runs until the reaction is complete, the nitrate destruction runs until no brown fumes evolve, and each precipitation runs until no further salt forms. The campaign length for your material is estimated from the laboratory test.

How are NOx, SO₂ and chlorine emissions controlled?

Each gas is captured where it is generated and routed to the abatement stage that suits it. Gas from the dissolution, roasting and nitrate conversion stages passes through a condenser that recovers hydrochloric acid from the condensate, then a nitrogen oxide absorption tower, then an alkali washing tower with an acid-mist eliminator for the chlorine and acid mist. Sulfur dioxide from the sulfite conversion stage goes to the alkali washing circuit. Free ammonia from the complexation stage goes to a dedicated ammonia scrubber that doubles as the site’s chlorine emergency absorber, and the absorbent is dosed with oxidant so nitrogen oxides are converted rather than carried through. Emission parameters are logged per batch and retained with the batch record, and scrubber performance data are supplied for your permit applications.

Is my feed tested before a plant is quoted?

Yes, and it is the step that decides the flowsheet. A representative sample is run at bench scale through the selected dissolution route and then through sulfite complexation, conversion, ammine complexation, extraction and reduction, which establishes the operating windows, the number of complexation passes your feed needs, the expected recovery and the final purity. Because rhodium feeds are frequently inhomogeneous and arrive in small high-value lots, sampling and splitting are treated as part of the test rather than as a preliminary to it, and part of the recovered product is returned to you so that you can verify it with your own assay. The equipment quotation and the process design package are based on that result, not on a generic assumption about your material.

What throughput can be engineered?

Throughput is engineered to your feed assay and your target rhodium input rather than taken from a catalogue of fixed sizes. What matters in this process is the rhodium content of the feed, the dissolution route it needs, and the number of complexation passes required, so two plants of the same nominal vessel size can have very different daily metal output depending on grade and on how much iridium has to be separated. We size the fusion and roasting stations, reaction vessels, filtration, extraction and resin equipment and the scrubber train from your assay and your monthly rhodium volume, and we state the resulting figures in the process design package. The layout is modular, so a pilot line that validates the flowsheet on a few kilograms can be extended rather than replaced.