Overview of Rhodium Refining Process
The process consists of two stages: preparation of pure rhodium compounds and production of pure rhodium metal. While numerous methods exist for preparing pure rhodium compounds, none are highly efficient, with relatively low refining recovery rates and no standardized procedures. Consequently, research into the properties of rhodium compounds and complexes, as well as the refinement of refining technologies, remains a focal point in PGM metallurgy.
Rhodium refining is conducted entirely in solution. Crude rhodium metal must first be dissolved using methods such as:
- Sodium bisulfate fusion followed by dilute sulfuric acid leaching
- Medium-temperature chlorination roasting followed by dilute hydrochloric acid leaching
- Pretreatment activation followed by aqua regia dissolution
- Electrochemical dissolution
These methods yield impure H3RhCl6 solutions. All dissolution techniques require high-temperature equipment, involve complex operations, and have long processing cycles, often necessitating repeated steps for complete dissolution. A key principle in rhodium refining is that rhodium solutions or compound precipitates should not be converted to metallic form until their purity is confirmed. Additionally, efforts should be made to produce rhodium compounds as market-ready products at the earliest opportunity.
Preparation of Pure Chlororhodic Acid (Salt)
Producing high-purity chlororhodic acid (H₃RhCl₆) or its salt compounds is a fundamental step in rhodium refining. Key methods include sodium sulfite complexation, ammonia complexation, ammonium sulfite complexation, solvent extraction, and ion exchange, often requiring combined approaches for optimal results.
Ammonium Sulfite Complexation-Ammonia Complexation Method
Initial Precipitation:
A sodium chlororhodate solution containing ~50 g/L Rh is adjusted to pH 1–1.5 using dilute alkali. A 25% sulfurous acid (H₂SO₃) solution is added at a 1:0.75 volume ratio, followed by boiling and maintaining pH 6.4 to precipitate milky-white ammonium trisulfito rhodate ((NH₄)₃Rh(SO₃)₃):
Na3RhCl6+3(NH4)3SO3=(NH4)3Rh(SO3)3↓ +3NH4Cl +3NaCI
Iridium (Ir) does not form a similar precipitate and remains in solution.
Washing and Conversion:
The rhodium precipitate is washed with pure water, then dissolved in concentrated HCl under boiling conditions to form ammonium chlororhodate ((NH₄)₃RhCl₆):
(NH4)3Rh(SO3)3+6HCl=(NH4)3RhCl6↓+3SO2 +3H2O
A single sulfite precipitation step removes most iridium but achieves only ~95% rhodium recovery.
Ammonia Complexation Purification:
The ammonium chlororhodate is further purified via ammonia complexation to precipitate pentammine chlororhodium dichloride ([Rh(NH₃)₅Cl]Cl₂):
(NH4)3RhCl6+5NH3·H2O=[Rh(NH3)5CI]Cl2↓ +3NH4Cl +5H2O
During ammonia complexation, iridium hydrolyzes to Ir(OH)₃. The precipitate is washed with NaCl solution, dissolved in NaOH, and filtered to remove Ir(OH)₃. The rhodium solution is then acidified with HCl and treated with HNO₃ to form[Rh(NH3)5CI](NO3)2, which is boiled, concentrated, and converted back to chlororhodic acid (H₃RhCl₆) by adding HCl to eliminate nitrate ions.
