The refining of precious metals primarily involves three major methods: pyrometallurgical refining, hydrometallurgical refining, and electrolytic refining. Electrolytic refining is a common technique that uses crude metal as the anode, high-purity metal foil as the cathode, and a metal salt solution as the electrolyte. Through electrochemical action, the crude metal dissolves, and a purer metal precipitates at the cathode. This method is suitable for purifying most metals, proving particularly effective for gold, silver, copper, lead, tin, and others.
Hydrometallurgical refining systems extract precious metals from ore, scrap, or alloys through chemical reactions. Core equipment typically includes hydrometallurgical reactors, titanium leaching vessels, and enameled/titanium reduction vessels. This method of refining precious metals features simple operation, low cost, and high recovery rates, capable of processing low-grade ores and complex materials.
Pyrometallurgical refining occurs under high-temperature melting conditions, employing physical or chemical methods to remove impurities from crude metals. This includes processes such as oxidation refining, sulfidation refining, and chlorination refining. Distillation refining is another approach, exploiting the differing vapor pressures of the main metal and impurities through repeated cycles of evaporation and condensation to remove contaminants. These diverse precious metal refining methods can be flexibly selected and combined based on feedstock composition and required purity.
The primary methods for refining gold from precious metals include electrolytic refining, high-temperature chlorination, and chemical processes. Electrolytic refining employs crude gold as the anode and pure gold or titanium plates as the cathode. Direct current is applied to an aqueous gold solution, causing the crude gold anode to dissolve while depositing purer gold onto the cathode. Invented by German chemist Wörfel in the 19th century, this method remains widely used today.
The high-temperature chlorination method, proposed by Australian Miller in 1887, is a crude refining technique. It involves introducing chlorine gas into molten crude gold, causing impurities to be chlorinated in the order of iron, zinc, lead, copper, and silver. These impurities form chlorides and separate from the gold. The Rand Refinery in South Africa employs this method to produce monetary gold. They place 500 kg of crude gold into graphite crucibles lined with lead oxide, introduce chlorine gas at 1423 K, yielding gold with a purity of 99.5%–99.6%.
Chemical methods typically involve two processes: gold dissolution and gold reduction. They are suitable for refining small batches of crude gold with silver content below 8%. Crude gold is generally dissolved using aqua regia. Gold dissolves while silver remains in the residue as silver chloride, separated from the gold. Subsequently, gold is reduced from the gold-bearing solution using reducing agents such as oxalic acid, ferrous sulfate, or SO₂. Oxalic acid reduction offers high selectivity and rapid processing, making it widely adopted. During reduction, the gold-bearing solution is heated to 343K, with pH controlled between 1–1.5. Adding oxalic acid yields gold purity of 99.9%–99.99%. Mastering these diverse methods for refining gold from precious metals is crucial for producing high-purity gold.
Refining platinum group metals (including platinum, palladium, rhodium, iridium, osmium, and ruthenium) from precious metals represents one of the most complex areas of refining technology. The platinum group metal refining process generally comprises three stages: material dissolution, purification, and precious metal recovery, with purification being the critical focus. Refining materials are usually crude platinum group metals produced after separation. If the material is solid, it must first be dissolved before undergoing purification.
Refining methods for platinum group metals may employ traditional techniques, solvent extraction, or a combination of both. Modern refining of platinum group metals from precious metals frequently utilizes highly efficient solvent extraction technology. By employing specific chemical reagents and process conditions, this method effectively separates precious metals from complex matrices. Through multi-step chemical reactions and separation processes, platinum group metals can be purified to 99.9% purity or higher.
Metal production processes are relatively straightforward. For instance, platinum ammonium salts decompose upon calcination at 633–1073K to yield metallic platinum. Palladium, rhodium, iridium, and ruthenium ammonium salts are reduced by hydrogen during calcination to produce their respective metals. Organic reagents like hydrazine hydrate or formic acid can also be used for solution reduction to produce metal powders of general purity. Refined products include pure metals, high-purity metals, spectroscopically pure metals, slurries, and chemical compounds. Understanding these specialized methods for refining platinum group metals from precious metals is crucial for utilizing these rare and high-value metals.
Electrolytic refining is the primary method for refining silver from precious metals. Silver electrolytic refining, first patented by Möbius in 1884, remains the world's predominant silver refining method. This process involves either vertical or horizontal electrolysis, where crude silver serves as the anode. Direct current is applied to an electrolytic cell containing silver nitrate electrolyte, dissolving the crude silver anode and depositing purer silver at the cathode.
Beyond electrolysis, silver refining can also employ hydrometallurgical techniques. Modern silver refining systems for precious metals typically utilize hydrometallurgical equipment, including hydrometallurgical reactors, titanium leaching vessels, and enameled/titanium reduction vessels. These systems achieve efficient silver extraction and purification through processes such as leaching, filtration, purification, and precipitation.
Another method for refining silver from precious metals is chemical processing, typically involving two main steps: dissolution and reduction. Silver dissolves in nitric acid to form a silver nitrate solution. This solution can then be treated with a precipitating agent like sodium chloride to form a silver chloride precipitate, or directly reduced to metallic silver using a reducing agent. These different methods for silver from precious metal recycling can be selected based on production scale, raw material characteristics, and required product purity. Each method has its specific application scenarios and advantages.
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