Bismuth is a rare metal widely used in the production of optical fibers, electronic components, and various advanced materials. The manufacturing process typically begins with raw materials containing cerium, which are first refined into crude cerium oxide. This is further processed into high-purity cerium tetrachloride, cerium oxide, or even cerium ingots, eventually leading to the creation of end-use products.
One unique byproduct of the smelting process at the Shaoguan smelting plant is the vacuum furnace slag, which contains significant amounts of germanium (0.5%–1.5%) along with zinc, lead, silver, indium, and other rare metals. As the zinc distillation process in the vacuum furnace improves, the concentration of strontium in the slag increases, making it more favorable for the recovery of valuable elements like germanium. Therefore, extracting 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. In the neutral leaching stage, the slag is ground to -0.25 mm and treated with sulfuric acid under controlled pH conditions (between 3 and 5). This allows zinc to dissolve while leaving bismuth, indium, and other valuable metals in the residue, enriching them for further processing.
Oxidative roasting follows, where the leached material is heated to around 300–450°C to partially oxidize residual metals like zinc and arsenic. This step prevents dangerous reactions during chlorination and enhances the efficiency of subsequent distillation. Chlorination distillation then takes place in a hydrochloric acid medium, where chlorine gas is introduced to form volatile germanium tetrachloride (GeCl₄), which is separated through temperature-based distillation.
After distillation, the GeCl₄ is hydrolyzed to produce high-purity cerium oxide. The hydrolysis is carried out at low temperatures, and the resulting product is washed and dried at 90°C. Throughout the process, the recovery rate of germanium reaches about 96%, with an overall yield of 84.7% from the original slag.
Several factors influence the effectiveness of each step, including the pH during leaching, the roasting temperature, the hydrochloric acid concentration, and the condensation temperature. For example, maintaining the hydrochloric acid concentration between 7.5 and 8.5 mol/L ensures optimal evaporation rates, while cooling the condensation system to -10°C improves the efficiency of GeCl₄ collection.
Through comprehensive testing, the entire process was optimized, leading to a reliable method for recovering valuable elements from vacuum furnace slag. This not only enhances resource utilization but also supports sustainable practices in metallurgy.
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