Every piece of 24K investment bullion began its life as raw mined ore, old household jewellery, or industrial scrap. Transforming low-karat gold into 99.9% (24K) pure metal requires precise metallurgical and chemical refining. Here is a look inside the three primary methods refiners use to purify gold.
1. Chemical Refining: The Aqua Regia Method
Aqua Regia (a 3:1 mixture of Hydrochloric Acid and Nitric Acid) is the most widely used process for refining old jewellery and shop scrap. It chemically dissolves gold while separating out impurities.
- Inquartation & Granulation: High-karat gold scrap is melted with copper or silver to drop the gold concentration to roughly 25%. It is then poured into cold water to form small metal granules for maximum surface exposure.
- Dissolution: Granules are placed in borosilicate vessels with Hydrochloric and Nitric acid, forming chloroauric acid while releasing undissolved silver chloride.
- Precipitation ("Dropping" the Gold): A reducing agent—typically Sodium Metabisulfite—is added to selectively turn dissolved gold ions back into a fine, heavy brown powder at the bottom of the solution.
- Washing & Melting: The gold powder is thoroughly rinsed with distilled water, dried, and melted in a ceramic crucible to produce 99.9% pure fine gold.
2. Fire Assay & Acid Parting (Inquartation)
A traditional pyrometallurgical method used for high-volume melt testing and alloy separation.
- Silver Ratio Adjustment: Pure silver is added to the alloy so that gold accounts for only one-fourth of the total weight (inquartation).
- Cupellation Furnace: The metal is heated in a porous bone-ash cupel at over 1,000°C. Base metals like copper, lead, and zinc oxidize and absorb into the cupel walls, leaving a pure Gold-Silver bead.
- Nitric Acid Parting: The bead is boiled in dilute Nitric Acid. The acid completely dissolves the silver, leaving behind pure, sponge-like 24K gold that is dried and cast into bars.
3. Electrochemical Refining: The Wohlwill Process
When high-purity investment bars (99.99% or "Four Nines" gold) are needed for official minting, commercial refiners rely on electrolysis.
| Refining Method | Starting Purity | Final Purity | Primary Application |
|---|---|---|---|
| Aqua Regia | 8K to 22K Scrap | 99.95% | Jewellery scrap & e-waste recycling |
| Inquartation & Parting | Mixed Karat Scrap | 99.5% to 99.9% | Scrap jewellers & melt testing |
| Wohlwill Electrolysis | 95%+ Bullion | 99.99%+ | Commercial mints & certified 24K bars |
Rule of thumb: High-karat investment coins are built on rigorous 99.9%+ refinery standards—ensuring that every gram of wealth you hold carries zero unmeasured base metal.
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