Bismuth is a rare metal widely used in the production of optical fibers, electronic components, and other advanced materials. The process begins with raw materials containing cerium, which are first refined into crude cerium oxide and further processed into high-purity cerium tetrachloride, cerium oxide, or cerium ingots. These products are then used to manufacture various applications.
The vacuum furnace slag, unique to the Shaoguan smelting plant, is a byproduct from the vacuum dezincification of hard zinc. It contains about 0.5% to 1.5% germanium (Ge) and significant amounts of zinc, lead, silver, and indium. As the zinc distillation process improves, the strontium content in the slag increases, offering better conditions for enriching strontium and producing high-purity cerium oxide. Therefore, recovering germanium from this slag has become an important practical challenge.
The recycling process involves several key steps: ball milling, neutral leaching, oxidative roasting, chlorination distillation, and hydrolysis. Ball-milling reduces the particle size of the slag to below 0.25 mm, making it more reactive. Neutral leaching with sulfuric acid at controlled pH levels allows zinc to dissolve while leaving bismuth and indium in the residue, thus concentrating them for further processing.
Oxidative roasting is performed to partially oxidize metals like zinc, lead, and arsenic, ensuring safe conditions for subsequent chlorination. Chlorination distillation follows, where the material is leached in hydrochloric acid and treated with chlorine gas. This step separates germanium as GeClâ‚„, which is then condensed and hydrolyzed to produce high-purity cerium oxide.
Testing was conducted using specific equipment, including a ball mill, leaching tanks, a filter press, a self-made roaster, distillation units, and an ice machine. The test results showed that the optimal pH during neutral leaching was between 3 and 5, achieving a zinc recovery rate of 80-90%. Oxidative roasting at temperatures between 300 and 450°C improved the efficiency of subsequent processes.
During chlorination distillation, the hydrochloric acid concentration was optimized between 7.5 and 8.5 mol/L, maximizing the evaporation rate of germanium. The condensation temperature of the ice brine was set to -10°C to enhance cooling efficiency. Hydrolysis at low temperatures, followed by drying at 90°C, produced high-quality cerium oxide with a recovery rate of around 96%.
Overall, the entire process achieved a direct yield of 84.7%, demonstrating significant economic benefits. This method provides an efficient way to recover valuable metals from vacuum furnace slag, contributing to sustainable resource utilization.
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