Ammonium Chloropalladate Precipitation Method
- Oxidation and Precipitation:
A chloropalladic acid solution containing Pd(Ⅱ)40-50g/L is oxidized with chlorine gas (Cl₂) or hydrogen peroxide (H₂O₂). Excess 10–15% ammonium chloride (NH₄Cl) is added to precipitate dark-red crystalline ammonium chloropalladate （（NH4）2PdCl6):
H2PdCl4+2NH4Cl+Cl2=(NH4)2PdCl6+2HCl
Base metal impurities remain dissolved and do not form ammonium salts. - Washing and Purity Enhancement:
The precipitate is washed with a 10% NH4Cl solution. For initial precipitates with insufficient purity, the ammonium palladium salt is slurried in pure water, boiled until dissolution occurs, converting (NH₄)₂PdCl₆ into soluble dark-red ammonium chloropalladite ((NH₄)₂PdCl₄):
(NH4）2PdCl6+H2O=(NH4)2PdCl4+HCl+HClO After cooling, the (NH4)2PdCl4 solution is re-oxidized and re-precipitated. Filtering and repeating this cycle 2–3 times effectively removes base metal impurities.
Ammonia Complexation Method
- Complex Formation:
A dark-red chloropalladite (H₂PdCl₄) solution containing Pd(II) is heated to 80°C, and ammonia is added to adjust the pH to 8–9, forming soluble tetraammine dichloropalladite:
H2PdCl4+4NH3.H2O=[Pd（NH3）4]Cl2+4H2O+2HCl
Insufficient ammonia may also yield ammonium chloropalladite （NH4）2PdCl. - Impurity Separation:
Soluble impurities are filtered out. The precipitate is slurried in pure water to achieve a Pd concentration of ~80 g/L, followed by re-complexation with NH3.H2O and acidification with HCl. Repeating this cycle 2–3 times effectively separates other precious metals. - Limitations:
Base metals like Cu2+ and Ni2+ form soluble ammonia complexes (e.g., [Me2+(NH3)]Cl2) without hydrolyzing, resulting in poor separation efficiency between precious and base metals.
