Chemical Refining Method
Direct Reduction of AgCl
The widely used hydrazine hydrate reduction method offers strong reducing power, rapid reaction kinetics, and no introduction of new impurities. When the raw material contains low silver content or high levels of impurities such as SiO2, Cu, Ni and Cd, it must first be dissolved in HNO3:
AgCl+HNO3=AgNO3+HCl
After filtering to separate SiO2 residue, the solution is boiled, and HCl or NaCl is added to re-precipitate AgCl. Filtration then removes base metal impurities. The impure silver chloride is slurried in water, and ammonia is added to adjust the solution pH to 10–11, converting AgCl into a soluble ammonia complex:
AgCl+2NH3.H2O=Ag（NH3）2Cl+2H2O
Base metal impurities hydrolyze and precipitate. The filtered silver solution is heated to ~50°C, and hydrazine hydrate (40–80% concentration) is slowly added under stirring at a mass ratio of mAg:mN2H4.H2O=1:(0.3-0.4). After ~30 minutes, grayish-white spongy silver is reduced, with the reaction:
4Ag(NH3)2Cl+N2H4.H2O+3H2O=4Ag+N2+4NH4Cl+4NH3.H2O
The reduction efficiency exceeds 99%, and the silver purity reaches >99%.
Electrolytic Refining Method
Electrolytic refining is applicable to:
- Ag-Au alloys (Ag > 99%) derived from copper/lead anode slimes,
- Doré alloys (Ag 70–75%) from smelting gold chloride slimes,
- Crude silver extracted from secondary silver-bearing resources.
Anode Preparation
Crude silver is melted and cast into anode plates. For silver containing high levels of Cu, Se, and Te, sodium carbonate flux is added during melting, followed by air oxidation blowing to form slag, which is separated before casting. Typical anode composition: Ag > 75%, Au < 20%, with trace Cu and Bi.
Electrolysis Conditions
Electrolysis is performed in a circulating AgNO₃ solution (60–150 g/L Ag). To prevent Bi and other base metals from hydrolyzing and contaminating the silver powder, the electrolyte must contain 10–18 g/L free HNO₃. Excessive nitric acid causes re-dissolution of the cathode silver.
