(1) Recovery of gold from Au-Sb (Au-Al or Au-As) alloys. First, boil the alloy in dilute aqua regia (acid-to-water ratio of 1:3) to achieve complete dissolution. Evaporate and concentrate the solution until nitrogen dioxide gas ceases to evolve and the volume is reduced to approximately 15% of the original; then, dilute the solution to a gold concentration of 100–150 g/L, allow it to settle, and filter. Reduce the gold in the filtrate using sulfur dioxide, and absorb the nitrogen dioxide in the off-gas using caustic soda.
(2) Recovery of gold from Au-Ag alloys. First, dissolve the silver using dilute nitric acid (acid-to-water ratio of 2:1); add hydrochloric acid to the filtrate to precipitate silver chloride, then dry and cast it into ingots. For the palladium in the residual solution, form an ammine complex using ammonia, acidify with hydrochloric acid, and then add formic acid to precipitate palladium powder. Subsequently, recover the gold from the residue that remained insoluble in the nitric acid.
(3) Recovery of gold from Au-Pt (Au-Pd) alloys. First, dissolve the alloy in aqua regia; add hydrochloric acid and evaporate the solution until it reaches a syrupy consistency to drive off nitrates. Dilute with distilled water and add a saturated ammonium chloride solution to precipitate the platinum as ammonium chloroplatinate. After washing the precipitate with a 5% NH₄Cl solution, recover the gold.
(4) Recovery of gold from Au-Ir alloys. Iridium is a refractory metal; it can first be fused with sodium peroxide (caustic soda may be added simultaneously) at 600–750°C for 60–90 minutes. The melt is poured onto an iron plate to cast thin sheets, which are cooled and then leached with cold water. A small amount of sodium iridate passes into the leachate, while the majority of the iridium remains in the leach residue. Dilute hydrochloric acid is added to the residue to leach the iridium under heating; after filtration, chlorine gas is passed through the solution to oxidize the iridium to the tetravalent state. Saturated ammonium chloride solution is then added to precipitate the iridium as ammonium chloroiridate. Following solid-liquid separation, the precipitate is washed, dried, and calcined to yield crude iridium sponge. Aqua regia is used to dissolve gold from the leach residue, and ferrous salts are used to recover the gold.
(5) Recovery of gold from gold-bearing siliceous alloys. A dilute mixture of hydrofluoric acid and nitric acid (HF:HNO₃ = 6:1; acid:water = 1:3) can be used to leach the alloy; during leaching, the silicon dissolves, causing the gold to detach from the silicon wafers. The gold flakes (or powder) are then boiled in 1:1 dilute hydrochloric acid for 3 hours to remove impurities, washed with water, dried, and cast into ingots. Materials suitable for aqua regia leaching can also be processed via aqueous chlorination leaching. Because the aqueous chlorination process involves a shorter workflow—eliminating the need to drive off nitrates—and offers a better working environment, it is widely applied, particularly for the large-scale treatment of gold-bearing waste, and is gradually replacing aqua regia methods.
