How is gold and silver refined

Gold Refining Methods
Aqua Regia Process
Purpose: Refines gold from scrap, electronics, or jewelry.

Steps:
Dissolve gold in a mixture of hydrochloric acid (HCl) and nitric acid (HNO₃) (3:1 ratio) to form gold chloride.
Filter out insoluble impurities.
Precipitate pure gold using a reducing agent (e.g., sodium metabisulfite or oxalic acid).
Wash and melt the gold powder into a bar.

Purity: Up to 99.95% pure.
Risks: Toxic fumes (NOₓ, Cl₂), corrosive acids. Requires a fume hood and PPE.
Miller Process (Chlorination)
Purpose: Industrial refining of large quantities.

Steps:
Melt impure gold and bubble chlorine gas through it.
Chlorine reacts with impurities (e.g., silver, base metals) to form chlorides, which float to the surface as slag.
Pour off the slag, leaving behind high-purity gold.

Purity: 99.5–99.7% pure.
Electrolytic Refining (Wohlwill Process)
Purpose: Produces ultra-pure gold (99.99%).

Steps:
Dissolve gold in aqua regia and cast it into an anode.
Use a pure gold cathode and a gold chloride electrolyte solution.
Apply electric current to deposit pure gold on the cathode.
Used by: Mints and high-end refineries.

Silver Refining Methods
Cupellation
Purpose: Refine silver from lead-containing ores or alloys.

Steps:
Melt the impure silver with lead in a cupel (magnesia or bone ash crucible).
Blow air over the molten metal to oxidize impurities (lead, base metals).
The oxidized impurities absorb into the cupel, leaving pure silver.
Purity: ~99% pure.
Electrolytic Refining

Purpose: High-purity silver (99.99%).
Steps:
Cast impure silver into an anode.
Use a pure silver cathode and a silver nitrate electrolyte.
Apply current to dissolve the anode and deposit pure silver on the cathode.
Chemical Precipitation (Nitric Acid Method)
Purpose: Refine silver from scrap or solutions.

Steps:
Dissolve silver in nitric acid to form silver nitrate.
Precipitate pure silver by adding copper or salt (NaCl) to form silver chloride, then reduce it to metallic silver.
Melt the silver powder into a bar.

Risks: Toxic NOₓ fumes, corrosive acids.
Key Considerations
Safety:
Always work in a well-ventilated area (fume hood) with PPE (gloves, goggles, respirator).
Neutralize and dispose of acids/chemicals according to local regulations.

Equipment:
Heat-resistant crucibles, borax (flux), furnaces, electrolytic cells.

Legality:
Check local laws; refining precious metals often requires permits.

When to Use Professional Services
For significant quantities or high-purity requirements (e.g., investment-grade bullion), professional refiners are recommended. They use advanced techniques like induction furnaces and spectrometers to ensure purity and safety.