How are rhodium ruthenium osmium and iridium separated during refining?

The separation of rhodium, ruthenium, osmium, and iridium is a complex process that begins after platinum and palladium have been removed from a mineral concentrate using aqua regia. The remaining residue contains these four metals, which are separated through the following sequential steps:

• Rhodium: The residue is treated with molten sodium bisulfate to create rhodium sulfate, which is then leached with water. The rhodium is precipitated using zinc powder, then purified through a series of chemical treatments involving chlorine, sodium chloride, sodium nitrite, and ammonium chloride to produce pure metal powder.1
• Ruthenium: The remaining residue is fused with alkali nitrate salts to form soluble sodium ruthenate. This is treated with chlorine gas to distill ruthenium as volatile ruthenium tetroxide, which is subsequently reduced to a fine metal powder.1
• Osmium: This metal is recovered similarly to ruthenium, often through distillation, though it can also be distilled from acidic solutions.1
• Iridium: The final residue is treated with sodium peroxide to make the iridium soluble in hydrochloric acid. It is then precipitated using ammonium chloride and calcined into a metal powder.1