Cadmium process steps and parameters

I. Raw Material Pretreatment and Primary Purification

1. ‌High-Purity Cadmium Feedstock Preparation‌

• ‌Acid Washing‌: Immerse industrial-grade cadmium ingots in 5%-10% nitric acid solution at 40-60°C for 1-2 hours to remove surface oxides and metallic impurities. Rinse with deionized water until neutral pH and vacuum dry.
• ‌Hydrometallurgical Leaching‌: Treat cadmium-containing waste (e.g., copper-cadmium slag) with sulfuric acid (15-20% concentration) at 80-90°C for 4-6 hours, achieving ≥95% cadmium leaching efficiency. Filter and add zinc powder (1.2-1.5 times stoichiometric ratio) for displacement to obtain sponge cadmium‌

1. ‌Melting and Casting‌

• Load sponge cadmium into high-purity graphite crucibles, melt under argon atmosphere at 320-350°C, and pour into graphite molds for slow cooling. Form ingots with density ≥8.65 g/cm³

II. Zone Refining

1. ‌Equipment and Parameters‌

• Use horizontal floating zone melting furnaces with a molten zone width of 5-8 mm, traverse speed of 3-5 mm/h, and 8-12 refining passes. Temperature gradient: 50-80°C/cm; vacuum ≤10⁻³ Pa‌
• ‌Impurity Segregation‌: Repeated zone passes concentrate lead, zinc, and other impurities at the ingot tail. Remove the final 15-20% impurity-rich section, achieving intermediate purity ≥99.999%

1. ‌Key Controls‌

• Molten zone temperature: 400-450°C (slightly above cadmium’s melting point of 321°C);
• Cooling rate: 0.5-1.5°C/min to minimize lattice defects;
• Argon flow rate: 10-15 L/min to prevent oxidation

III. Electrolytic Refining

1. ‌Electrolyte Formulation‌

• Electrolyte composition: Cadmium sulfate (CdSO₄, 80-120 g/L) and sulfuric acid (pH 2-3), with 0.01-0.05 g/L gelatin added to enhance cathode deposit density‌

1. ‌Process Parameters‌

• Anode: Crude cadmium plate; Cathode: Titanium plate;
• Current density: 80-120 A/m²; Cell voltage: 2.0-2.5 V;
• Electrolysis temperature: 30-40°C; Duration: 48-72 hours; Cathode purity ≥99.99%‌

IV. Vacuum Reduction Distillation

1. ‌High-Temperature Reduction and Separation‌

• Place cadmium ingots in a vacuum furnace (pressure ≤10⁻² Pa), introduce hydrogen as a reductant, and heat to 800-1000°C to reduce cadmium oxides to gaseous cadmium. Condenser temperature: 200-250°C; Final purity ≥99.9995%

1. ‌Impurity Removal Efficacy‌

• Residual lead, copper, and other metallic impurities ≤0.1 ppm;
• Oxygen content ≤5 ppm‌

V. Czochralski Single Crystal Growth

1. ‌Melt Control and Seed Crystal Preparation‌

• Load high-purity cadmium ingots into high-purity quartz crucibles, melt under argon at 340-360°C. Use <100>-oriented single-crystal cadmium seeds (diameter 5-8 mm), pre-annealed at 800°C to eliminate internal stress

1. ‌Crystal Pulling Parameters‌

• Pulling speed: 1.0-1.5 mm/min (initial stage), 0.3-0.5 mm/min (steady-state growth);
• Crucible rotation: 5-10 rpm (counter-rotation);
• Temperature gradient: 2-5°C/mm; Solid-liquid interface temperature fluctuation ≤±0.5°C‌

1. ‌Defect Suppression Techniques‌

• ‌Magnetic Field Assistance‌: Apply 0.2-0.5 T axial magnetic field to suppress melt turbulence and reduce impurity striations;
• ‌Controlled Cooling‌: Post-growth cooling rate of 10-20°C/h minimizes dislocation defects caused by thermal stress‌.

VI. Post-Processing and Quality Control

1. ‌Crystal Machining‌

• ‌Cutting‌: Use diamond wire saws to slice into 0.5-1.0 mm wafers at 20-30 m/s wire speed;
• ‌Polishing‌: Chemical mechanical polishing (CMP) with nitric acid-ethanol mixture (1:5 vol. ratio), achieving surface roughness Ra ≤0.5 nm.

1. ‌Quality Standards‌

• ‌Purity‌: GDMS (Glow Discharge Mass Spectrometry) confirms Fe, Cu, Pb ≤0.1 ppm;
• ‌Resistivity‌: ≤5×10⁻⁸ Ω·m (purity ≥99.9999%);
• ‌Crystallographic Orientation‌: Deviation <0.5°; Dislocation density ≤10³/cm²

VII. Process Optimization Directions

1. ‌Targeted Impurity Removal‌

• Use ion-exchange resins for selective adsorption of Cu, Fe, etc., combined with multi-stage zone refining to achieve 6N-grade purity (99.9999%)‌

1. ‌Automation Upgrades‌

• AI algorithms dynamically adjust pulling speed, temperature gradients, etc., increasing yield from 85% to 93%;
• Scale up crucible size to 36 inches, enabling single-batch feedstock of 2800 kg, reducing energy consumption to 80 kWh/kg

1. ‌Sustainability and Resource Recovery‌

• Regenerate acid wash waste via ion exchange (Cd recovery ≥99.5%);
• Treat exhaust gases with activated carbon adsorption + alkaline scrubbing (Cd vapor recovery ≥98%)‌

Summary

The cadmium crystal growth and purification process integrates hydrometallurgy, high-temperature physical refining, and precision crystal growth technologies. Through acid leaching, zone refining, electrolysis, vacuum distillation, and Czochralski growth—coupled with automation and eco-friendly practices—it enables stable production of 6N-grade ultra-high-purity cadmium single crystals. These meet demands for nuclear detectors, photovoltaic materials, and advanced semiconductor devices. Future advancements will focus on large-scale crystal growth, targeted impurity separation, and low-carbon production