The classical fire assay method remains the definitive standard for precious metal evaluation in gold exploration and grade control. However, complex mineral matrices containing high sulfides, copper, or arsenic require tailored flux chemistry to ensure total precious metal collection into the lead button.
1. Fundamental Flux Components and Functions
- Lead Monoxide (Litharge – PbO): Serves as both an oxidizing/desulphurizing agent and the primary source of lead collector metal when reduced by flour or carbon.
- Sodium Carbonate (Soda Ash – Na2CO3): Powerful basic flux that reacts with silica (SiO2) to form fusible sodium silicate slags.
- Sodium Tetraborate (Borax Glass – Na2B4O7): Acidic flux that lowers slag melting temperature, decreases viscosity, and dissolves refractory metal oxides.
- Silica (SiO2): Modulates slag acidity and protects clay crucibles from excessive corrosion by basic oxides.
2. Adjusting for High-Sulfide / Refractory Ore Matrices
When assaying pyrite-rich or arsenopyrite ore samples (>5% S), standard flux formulations risk producing brittle speiss or undersized lead buttons. The assay chemist must:
- Increase Potassium Nitrate (KNO3) oxidant additions to selectively oxidize excess sulfides before fusion.
- Elevate the Litharge-to-ore ratio to guarantee a 28g–35g lead button for complete gold and silver collection.
- Maintain a slightly acidic slag (Silica to base ratio ≈ 1.0–1.2) to prevent heavy metal oxide entrapment.
Nebrix Limited supplies ultra-pure certified Lead Monoxide (Gold Assay Flux grade), Borax Anhydrous, Soda Ash, and assay crucibles tested for zero background gold contamination.
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