How to remove gold from aqua regia solution?
Removing gold from an aqua regia solution primarily involves a chemical process called precipitation, where the dissolved gold ions are converted back into solid gold metal using a suitable reducing agent. A highly effective and commonly used method, as highlighted in refining processes, involves the use of sodium sulfite.
Aqua regia, a highly corrosive mixture of nitric acid and hydrochloric acid, is unique in its ability to dissolve noble metals like gold. This happens because nitric acid acts as an oxidizer, forming gold ions (Au³⁺), while hydrochloric acid provides chloride ions that react with the gold ions to form stable tetrachloroaurate(III) anions ([AuCl₄]⁻), effectively pulling the gold into solution. To recover the gold, these dissolved gold ions must be reduced back to their metallic state (Au⁰).
The Gold Precipitation Process Using Sodium Sulfite
The most direct way to recover gold from an aqua regia solution, specifically by chemical precipitation, involves the precise addition of a reducing agent. Sodium sulfite (Na₂SO₃) is a powerful and widely accepted chemical for this purpose, leading to the precipitation of fine gold particles.
Key Steps for Gold Recovery:
- Preparation of Aqua Regia Solution: Ensure your gold-dissolved aqua regia solution is prepared and settled. For industrial processes, this solution might be a byproduct of other refining steps, such as those involving a "butyl stearate-aqua regia mixture."
- Prepare Sodium Sulfite Solution: According to the reference, you need to prepare a solution of sodium sulfite. A specific example involves preparing a solution by dissolving 875 grams of sodium sulfite in 7.6 liters of water. This concentration ensures an effective reduction reaction.
- Controlled Addition of Reducing Agent: The prepared sodium sulfite solution is then carefully added to the aqua regia mixture (or butyl stearate-aqua regia mixture, as specified in the reference) over a 30-second period. Controlled addition is crucial to manage the reaction and ensure efficient precipitation.
- Gold Precipitation: Upon the addition of sodium sulfite, the gold ions (Au³⁺) in the solution are reduced back to solid gold metal (Au⁰). The gold precipitates from the solution as fine particles. The chemical reaction is generally represented as:
2 AuCl₃ (aq) + 3 Na₂SO₃ (aq) + 3 H₂O (l) → 2 Au (s) + 3 Na₂SO₄ (aq) + 6 HCl (aq)
Alternatively, if considering the tetrachloroaurate ion:
2 [AuCl₄]⁻ (aq) + 3 SO₃²⁻ (aq) + 3 H₂O (l) → 2 Au (s) + 3 SO₄²⁻ (aq) + 8 Cl⁻ (aq) + 6 H⁺ (aq) - Filtration: Once the precipitation is complete and the gold particles have settled, the reaction mixture is filtered. The gold particles collect as the filter cake, effectively separating them from the remaining liquid solution (filtrate).
- Washing and Drying: The collected gold filter cake should be thoroughly washed with distilled water to remove any residual acids or salts, then dried to obtain pure gold metal.
Why Sodium Sulfite is Effective
Sodium sulfite acts as a strong reducing agent. It donates electrons to the gold ions (Au³⁺), causing them to gain electrons and revert to their elemental metallic state (Au⁰). This process is highly efficient and typically yields high-purity gold.
Safety Considerations
Working with aqua regia and chemical precipitants requires strict safety precautions:
- Ventilation: Always work in a well-ventilated area or under a fume hood.
- Personal Protective Equipment (PPE): Wear appropriate PPE, including chemical-resistant gloves, eye protection (safety goggles or face shield), and a lab coat or apron.
- Controlled Addition: Add chemicals slowly and cautiously, especially when dealing with strong acids or exothermic reactions.
- Waste Disposal: Dispose of all chemical waste according to local regulations. Aqua regia and its residues are hazardous.
- Neutralization: Any remaining acidic solutions should be neutralized before disposal.
Other Precipitation Methods (Brief Overview)
While sodium sulfite is highly effective, other reducing agents can also be used, depending on the specific refining context and desired purity:
| Reducing Agent | Chemical Formula | Characteristics of Precipitated Gold |
|---|---|---|
| Sodium Sulfite | Na₂SO₃ | Fine powder, easily filtered. High purity gold. |
| Ferrous Sulfate | FeSO₄ | Commonly used, yields fine gold powder. |
| Hydroquinone | C₆H₄(OH)₂ | Produces very fine gold powder, good for high purity. |
| Oxalic Acid | H₂C₂O₄ | Less common for primary recovery, suitable for very pure gold solutions. |
| Sodium Metabisulfite | Na₂S₂O₅ | Often used interchangeably with sodium sulfite as it forms sulfite in solution. |
For optimal results and safety, it's crucial to follow established protocols and understand the specific chemistry involved.
