Preferential Precipitation and Selective Separation of Rh(III) from Pd(II) and Pt(IV) Using 4‑Alkylanilines as Precipitants

Rhodium
(Rh) is the most expensive platinum group metal (PGM) and
is of great industrial importance. Although the recycling of PGMs
from secondary sources is in high demand, the preferential and selective
separation of Rh from PGM mixtures remains a great challenge. Here,
a selective Rh separation method involving the precipitation of Rh
from an HCl solution containing palladium (Pd), platinum (Pt), and
Rh is reported. 4-Butylaniline and 4-hexylaniline were used as precipitants
for Rh, and selective Rh precipitation was achieved at high HCl concentrations.
We revealed that Rh in HCl selectively forms a unique and highly stable
ion-pair complex comprising [RhCl6]3–/anilinium/chloride ions in a 1:6:3 ratio. The formation and high
stability of the Rh complex were found to play important roles in
the selective recovery of Rh from PGM mixtures.