Platinum-rhodium crucibles, spinnerets, and filament spinnerets used in the melting of glass and glass fibers are subjected to long-term use at high temperatures of 1200-1300℃. Some platinum and rhodium are oxidized to PtO2 and Rh2O3, which volatilize and permeate into the refractory bricks of the furnace or are embedded in the glass slag. These contain approximately 46-54% SiO2, 39-50% Al2O3, and about 2% CaO+MgO, with approximately 2000 g/t of Pt and 200 g/t of Rh. Direct oxidation and acid leaching of platinum and rhodium are difficult; therefore, methods for separating silicon, aluminum, and aluminum oxides are generally used to enrich platinum and rhodium. The waste residue is mixed with sodium carbonate and sintered at approximately 1000℃, converting silicon and aluminum oxides into sodium silicate and sodium aluminate salts:
Na₂CO₃ + 2SiO₂ = Na₂O·2SiO₂ + CO₂;
Na₂CO₃ + Al₂O₃ = Na₂O·Al₂O₃ + CO₂;
Na₂CO₃ + 6SiO₂ + Al₂O₃ = Al₂O₃·6SiO₂ + CO₂;
The molten material is then ground in a ball mill, and hydrochloric acid is added to the slurry to dissolve the sodium silicate and sodium aluminate:
Na₂O·2SiO₂ + 2HCl + H₂O = 2NaCl + 2H₂SiO₃;
Na₂O·Al₂O₃ + 8HCl = 2AlCl₃ + 2NaCl + 4H₂O;
Na₂O·Al₂O₃·6SiO₂ + 8HCl + 2H₂O = 2AlCl₃ + 2NaCl + 6H₂SiO₃;
Adding a flocculant helps the solid particles settle. After filtration, the filtrate is replaced with Al powder. The resulting filtrate contains Pt < 0.004 g/L and is discarded. The insoluble residue enriched with platinum group metals is treated with hydrofluoric acid solution to remove residual silica, yielding a concentrate containing approximately 50% platinum and rhodium. The concentrate is dissolved in aqua regia or HCl + NaClO₃, and then selectively precipitated or refined into platinum and rhodium products using methods such as solvent extraction.
