Extracting gold encapsulated within a quartz matrix presents a significant challenge because quartz (\(\text{SiO}_2\)) is highly resistant to common chemical leaching methods. The silica structure locks fine gold particles away, preventing contact with traditional solvents like cyanide or mercury. To access this trapped gold, the quartz must be chemically broken down through mineral digestion. This technique uses powerful, corrosive chemicals to dissolve the silicate material, liberating the gold concentrate for purification. This chemical approach is reserved for ores where physical crushing alone is insufficient to expose the microscopic gold.
Physical Preparation of the Ore
Before chemical processing, the ore must be mechanically reduced into a manageable powder. Large pieces of quartz ore are initially processed using jaw crushers, followed by fine grinding in devices like ball mills or hammer mills. The goal is to reach the “liberation size,” where the gold particles are physically separated from the surrounding quartz rock.
Achieving liberation size often requires reducing the material to a fine silt, typically finer than 100 to 200 mesh (about 74 to 149 microns). Grinding the ore to this consistency maximizes the surface area of the quartz particles. A greater surface area ensures the chemical solvent can efficiently penetrate and react with the silica, dissolving the matrix and releasing the encapsulated gold. Proper preparation reduces the necessary reaction time and the amount of corrosive chemical required.
Chemical Dissolution of Quartz
The chemical process to dissolve the quartz matrix requires hydrofluoric acid (HF). This acid is uniquely suited because the fluorine ion readily attacks the silicon-oxygen bonds forming the quartz structure. When powdered quartz (\(\text{SiO}_2\)) is exposed to \(\text{HF}\), the silica converts into a gaseous compound. The reaction is \(\text{SiO}_2(s) + 4\text{HF}(aq) \rightarrow \text{SiF}_4(g) + 2\text{H}_2\text{O}(l)\), producing silicon tetrafluoride gas and water.
This reaction breaks down the solid quartz into a volatile gas that escapes the solution. The gold particles, which are inert to hydrofluoric acid, remain as a fine powder suspended in the liquid. Concentration and temperature control the reaction rate; typically 10% to 40% concentration is used for controlled dissolution. The process is highly exothermic and must be monitored to prevent excessive heat buildup.
The process must be performed in specialized, chemically resistant containers, as HF dissolves most glass and ceramic materials. Specialized plastics, such as polytetrafluoroethylene (PTFE) or polypropylene, are required for all vessels and stirring apparatus. Strict environmental controls are necessary, primarily involving a high-efficiency fume hood, to safely vent the toxic silicon tetrafluoride gas.
Critical Safety Measures for Handling Chemicals
Handling hydrofluoric acid requires extreme caution due to its severe hazards, which surpass those of other strong acids. The fluoride ion in HF penetrates skin and tissue, leaching calcium from the body and causing systemic toxicity. Exposure can lead to severe deep-tissue damage, bone decalcification, and potentially fatal cardiac arrhythmia, even if the initial burn appears minor.
Mandatory Personal Protective Equipment (PPE) includes non-vented chemical splash goggles worn with a full face shield. Technicians must wear specialized, thick neoprene or butyl rubber gloves, often with a secondary pair of nitrile gloves, and a chemically resistant apron over a lab coat. All work must be conducted within a certified, high-flow chemical fume hood to prevent fume inhalation, and working alone with the acid is strictly prohibited.
Immediate emergency procedures must include having a fresh supply of 2.5% Calcium Gluconate gel readily available. This gel is the specific topical antidote for HF exposure; it must be massaged into any affected skin area to bind the fluoride ions and prevent deep tissue damage. Spent acid and waste products must be neutralized with a calcium-containing compound, such as lime or calcium carbonate, before disposal. Proper permitting and disposal protocols must be followed in accordance with environmental protection agency guidelines.
Final Gold Recovery and Refining
Once the quartz is dissolved, the remaining solution contains the liberated gold particles suspended as a fine powder, along with the neutralized acid waste. The first recovery step involves careful washing and decanting to separate the heavy gold powder from the lighter liquid waste. Multiple washes with distilled water are necessary to remove residual acid or dissolved salts that could interfere with subsequent steps.
The remaining gold concentrate, now a heavy sludge, is filtered to isolate the fine metal powder. The collected powder must be thoroughly dried, often in a low-heat environment, to ensure all moisture is removed. This dried gold powder is then prepared for high-temperature smelting by mixing it with a specialized chemical mixture called flux.
Smelting and Purification
The flux, typically a combination of borax, soda ash, and silica, serves to lower the melting point of the gold and chemically bind any remaining non-metallic impurities. When the fluxed powder is heated in a crucible to temperatures exceeding 1,000°C, the gold melts and coalesces into a dense metal droplet. The impurities react with the flux to form slag, a glassy waste product that floats on top of the molten gold. After cooling, the gold “button” can be separated from the slag, providing the final, high-purity metal.