Purity Requirements and Performance Comparison of Molybdenum Electrodes for Rare Earth Electrolysis

1. Three Requirements Imposed by Molten Salt Electrolysis Conditions

Rare earth metals are mostly produced by fluoride molten salt electrolysis, and the cells run continuously at high temperature in a strongly corrosive medium. These conditions impose three requirements on molybdenum electrodes: high purity, so that impurities such as Si, Al, Fe, C and O do not enter the molten salt system and interfere with electrolysis; a dense structure that resists salt attack and dissolution loss; and stable conductivity that makes current efficiency and product quality repeatable. Electrodes are usually supplied as rods, plates or threaded connectors.

2. Purity: From the Standard Minimum to High-Purity Grades

The YS/T 1312-2019 industry standard for electrodes sets the minimum molybdenum content at ≥99.93%, which satisfies general high-temperature melting duties. For impurity-sensitive duties such as molten salt electrolysis, high-purity molybdenum with ≥99.95% Mo is normally specified, and still higher purity can be customized. The difference between the two grades is essentially the level of total impurity control, not a trade-off on any single element.

For a detailed discussion of grade logic and application selection, see "High-Purity versus Standard Electrodes"; for a clause-by-clause reading of the standard, see "Key Points of the YS/T 1312-2019 Electrode Industry Standard."

3. Performance Indicators Compared

• Density: ≥10.2 g/cm³; a dense structure reduces the paths along which molten salt penetrates pores, giving more stable resistance to dissolution loss;
• Resistivity: about 0.053 μΩ·m at 20 °C, with conductivity ≥30% IACS, which favors a stable electrolysis current;
• Melting point: 2620 °C, maintaining structural stability in the high-temperature zone of the cell;
• Corrosion-resistant media: fluoride molten salts and other high-temperature electrolysis media, with a service temperature of about 1600 °C.

4. How Impurity Elements Cause Harm

Elements such as Al, Si, K, Ca and O in molybdenum powder exist in the electrode structure as second-phase particles or grain-boundary segregation, and their effects concentrate in three areas: grain-boundary weakening at high temperature, local reaction with the molten salt causing pitting, and dissolution into the melt that contaminates the product. For a continuously running cell, these differences accumulate over the whole service cycle and ultimately show up in electrode consumption rate and melt cleanliness.

5. Selection and Verification

Electrode specifications and purity grades for rare earth molten salt electrolysis are detailed in the process guide.

Higher purity is not automatically better; it must be weighed against the molten salt system, current density and budget. We recommend proceeding in this order: define the cell structure and current regime, determine electrode form and dimensions, select the purity grade, then validate samples on line. Samples can be requested for testing, operating parameters can be submitted through contact us, and the inspection flow is described under quality management.