The Pt-Pd-Rh alloy catalytic mesh in the ammonia oxidation tower catalytically oxidizes NH3 to NO2 to produce HNO3 under high temperature (800–950℃) and high pressure (304–912 kPa). Some Pt and Rh are also oxidized to PO2 and Rh2O3, respectively, and volatilized and lost. For every ton of HNO3 produced, an average of 0.1–0.15 g of Pt is lost. There are three loss pathways: approximately 50% is carried away by the high-temperature gas flow and decomposes into Pt and Rh metal particles in the cooler lower part of the tower, depositing in the furnace ash at the bottom of the tower; 25% is carried by the gas flow to the absorption tower and deposited in the acid sludge at the bottom of the nitric acid storage tank; the remainder is deposited in the rust and scale of the equipment. The furnace ash at the bottom of the tower and the acid sludge containing precious metals at the bottom of the nitric acid storage tank are easily collected and recovered.
Cleaning equipment rust is an important measure for recovering precious metals. One method is to soak certain detachable parts in a 10% phosphoric acid solution in a plastic tank for at least 36 hours to remove and recycle the rust. For non-detachable parts or pipes, loosen the rust with a motor-driven rotating nylon brush or wire brush, while simultaneously washing with water.
Fire and hydrometallurgical enrichment – iron trapping smelting: For rust with high iron oxide content, using iron as a trapping agent for smelting and enrichment is an effective method. This involves reduction smelting trapping → acid leaching to remove iron → extraction of platinum-palladium concentrate → separation and refining. Based on its composition, fluxes such as SiO2, CaO, Al2O3, and Ca2F are added according to the requirements for generating silicate slag, along with an appropriate amount of coke powder (C) as a reducing agent. Reduction smelting is then carried out in an electric arc furnace at 1500-1600℃. All three slag types facilitate the smelting process, reducing iron oxide to metallic iron and capturing platinum group metals:
2Fe₂O₃ + 2CO = 4FeO + 2CO₂;
6FeO + 6CO = 6Fe + 6CO₂;
2FeO + C = 2Fe + CO₂;
When ferroalloys containing Pt, Pd, and Rh are poorly settled, some may remain as particulate impurities in the slag. The slag may contain up to 90 g/t of platinum group metals, requiring crushing, grinding, and magnetic separation for recovery. This can reduce the slag content to 5 g/t, achieving a ferroalloy recovery rate >99.3%. After slag separation, FeO is added to the ferroalloy melt for decarburization, breaking down the highly chemically inert platinum group metal carbides, resulting in a C content of <0.5%. Leaching the iron with hydrochloric acid or sulfuric acid yields a crude concentrate containing 10-15% platinum, palladium, and rhodium, with a recovery rate of approximately 95%. The solubility of crude concentrate depends on decarburization, i.e., whether the platinum group metal carbides are completely destroyed. Therefore, decarburization during the smelting stage must be strictly controlled. Crude concentrate with good solubility can be dissolved by aqua regia, HCl + NaClO3, etc., and can be separated and refined from the solution into pure metal products of platinum, palladium, and rhodium using selective precipitation or solvent extraction techniques.
