Molybdenum Crucibles and Electrodes for Rare Earth Molten Salt Electrolysis Cells

Rare earth metals are electrolysed in fluoride molten salts, with cell temperatures commonly at 950–1050 °C, and fluoride ions in the melt attack metal materials markedly. Molybdenum is relatively stable in this environment, so crucibles, cathodes and some structural parts are often made of molybdenum. But the same material does not mean parts can be combined at will: electrode spacing, cell type and crucible dimensions together determine the current path, and they also affect the actual service life of the crucible. Starting from the molten salt conditions, the following sections set out how crucibles and electrodes are matched.

1. Molten Salt System and Working Conditions

Fluoride molten salt systems are usually made up of rare earth fluorides balanced with lithium fluoride, barium fluoride and similar components, with working temperatures mostly in the 950–1050 °C range. The conductivity and primary crystal temperature of the salt change with the mix, and a change in formulation alters the current distribution and cell voltage; the same electrode parameters used in a differently proportioned salt may behave noticeably differently. The charge is mostly rare earth oxide, and the powder needs time to dissolve and diffuse in the salt after being fed into the cell.

Fluoride ions are aggressive and attack the crucible inner wall, the electrodes and structural parts. Gas evolved at the anode and the crust on the melt surface add to operating difficulty: the crust conducts heat poorly, and covering the melt surface causes local overheating, while crust-breaking tools easily scratch the crucible wall. The temperature gradient is large at the exposed melt zone, and repeated crust formation and detachment make this the location where wear concentrates.

Current passing through the salt generates Joule heat, and electrode spacing together with salt resistance determines cell voltage. Electrode and crucible are on the same loop; if the crucible inner wall is too close to the cathode, current short-circuits along the wall and creates a local hot zone. This must be considered at the cell design stage rather than compensated later by voltage adjustment during operation.

Anode material and the way gas is discharged affect the atmosphere and the convection inside the crucible. If the anode is partly covered by crust, gas discharge is obstructed, the melt surface bubbles and churns, and alternating scouring of the wall increases. Production maintains the gas path by controlling anode immersion depth and breaking crust periodically, while crucible design should leave sufficient gas space at the rim.

2. Cell Type Determines Crucible Opening and Electrode Spacing

Additional note on the basis: electrode products are controlled for composition and tolerance to YS/T 1312-2019 Molybdenum Electrodes, molybdenum rods follow ASTM B387 / GB/T 17792, molybdenum plates and discs follow ASTM B386 / GB/T 3876, and special requirements are executed per the technical agreement between the parties.

The spacing values in the table are typical values and reference ranges; actual service conditions and the contract govern. An open cell is convenient for charging and observation, at the cost of a large exposed melt surface where volatiles easily form crust at the rim; an enclosed cell holds heat better and gives a more uniform wall temperature, but demands higher machining accuracy on the sealing face. The two cell types use different crucible heights and wall thicknesses and cannot share one drawing.

Crucible inner diameter is often derived backwards from electrode spacing. Too small a spacing raises the local current density and increases fluoride salt scouring at a particular height of the inner wall; too large a spacing raises cell voltage and power consumption with it. The drawing should list the distance from the anode outer edge to the wall as a key dimension, and it should be measured again after assembly rather than left at theoretical verification.

The ratio of crucible height to opening must be settled at the same time. If the opening is too large relative to the body, the rim lacks strength and deforms during handling; too small and charging and observation are restricted, with crust-breaking tools unable to reach in. A crucible with a rim should have sufficient thickness there and a permitted flatness deviation marked on the drawing, otherwise the sealing face fits poorly and gas escapes through the gap in operation.

3. Derivation Steps for Crucible Specifications

1. Determine the salt charge from capacity and output per run;
2. Back-calculate the crucible inner diameter from the salt volume and the target melt level;
3. Set the body height from the level as a proportion of height, commonly 60 to 75 percent;
4. Set the wall thickness from inner diameter and depth, balancing thermal shock resistance against creep resistance;
5. Determine the rim form and sealing face dimensions;
6. Lay out the electrode holes and verify spacing and concentricity;
7. Review the general assembly drawing against the handling tooling and charging tools.

Wall thickness is usually 15–30 mm, taking the upper part of the range for larger diameters or greater depth. Too thin a wall creeps and sags during long high-temperature service; too thick a wall stores more heat, and the temperature difference across the wall during heat-up concentrates thermal stress. Specific values must be calculated from the cell type, support method and heating curve, and should not simply be applied by diameter.

Melt level affects electrode spacing and salt convection. A high level increases electrode immersion and lengthens the effective spacing; a low level exposes the upper end of the electrode and intensifies oxidation and crusting. In practice the level is kept within a band, and charge is replenished during feeding and tapping so that the level does not swing widely. The greater the level swing, the more temperature cycles a given height of the wall sees and the more pronounced the thermal fatigue.

4. Loss Paths and Wall Thickness Monitoring

In actual service the melt-line fluctuation zone usually thins first: the temperature gradient there is large, crust forms and detaches repeatedly, and wall thickness falls faster than below the melt line. Concentrating thickness measurements on this band is more effective than taking points evenly over the whole crucible; each measurement should keep the same azimuth and height and be compared run by run.

Salt cleanliness also changes the rate of loss. Moisture and impurities brought in with the raw material form corrosive components at high temperature and loosen the wall surface prematurely. Thorough drying before charging, reasonable control of slag-skimming frequency, and keeping iron tools away from the melt and the inner wall all reduce this kind of loss. These operations do not appear on drawings, yet they often explain why two crucibles of the same model have different lives.

The thickness method must match the crucible shape. A conventional ultrasonic probe is hard to couple on curved areas and the readings scatter; a small-diameter probe or a dedicated coupling fixture can be used, with repeated measurement at the same point until a stable value is obtained. Records should state the temperature condition at measurement, since hot and cold readings differ and mixing them misjudges the thinning trend. A single value means little; only a curve of the same point across runs has diagnostic value.

5. Relative Position of Electrode and Crucible

If the cathode is too close to the wall, current short-circuits along the wall surface and the local temperature rises; if the anode is off-centre, the melt forms a stable circulation and one side of the crucible is continuously scoured. Alignment should use the crucible inner circle as the datum, not the outer shell of the cell: the shell changes shape when heated, and aligning to it accumulates error.

Alignment can be done with templates, a taut wire or laser distance measurement, recording the direction and value of the deviation together. Re-measure the spacing after assembly and adjust the electrode position if it is outside the process range; do not compensate by raising voltage, which only raises the current density on the other side and moves the loss elsewhere.

Electrode diameter must match crucible capacity. If the diameter is too small, maintaining the current requires a higher current density and end wear accelerates; too large and it takes up salt space, compresses the spacing and interferes with charging and slag skimming. Diameter, number and spacing should be calculated as one set; changing one item alone usually breaks the balance.

Immersion depth must be checked as well. The working section of the electrode must be fully immersed, but too deep an immersion changes the effective spacing and the convection path; too shallow and the upper end is exposed, with oxidation and crusting starting at the melt line. In practice the distance from the electrode lower end to the crucible base is the controlled dimension, and the drawing should show both the normal level range and the low-level limit so that the site has clear adjustment boundaries.

6. Current Distribution and Cell Voltage

Cell voltage is determined by salt resistance, electrode spacing, the bubble layer and contact resistance. Larger spacing raises cell voltage and power consumption; smaller spacing raises current density and causes local overheating. If cell voltage rises abnormally in operation, check the spacing and the melt level first, then the electrode crust and contact condition; following this order avoids wasted adjustments.

Cells with multiple electrode groups need attention to zonal current balance. Immersion depth and spacing of each group should be as uniform as possible, otherwise the salt temperature distribution is uneven and the composition fluctuates at tapping. Separate measurement can be set up on each group's circuit, recording current over time; when the deviation grows, check that group's contact and crust first.

Contact resistance is easily overlooked. If the connection surface between electrode and busbar or clamp is oxidized or has a gap, contact resistance rises and the joint heats up. Grinding the contact surface before installation, tightening to the specified torque, and re-measuring the temperature rise early in operation can keep this kind of problem out before commissioning.

7. Installation and Start-Up Inspection Checklist

1. Check the crucible inner diameter, height and electrode layout drawing, and confirm that the spacing is within the permitted range;
2. Check whether the crucible rim is flat and whether the sealing face has gaps or deformation;
3. Align the electrodes on the crucible inner circle as datum, and record the direction of concentricity deviation;
4. Check the electrode immersion depth and confirm that the working section is fully below the melt level;
5. Confirm the material of the charging and slag-skimming tools, keeping iron tools away from the salt and the crucible wall;
6. Dry out the furnace according to the heating curve, controlling the rate to avoid thermal shock to the body;
7. Record cell voltage, current and crust condition at the melt line for the first runs as a baseline for later comparison.

The heating rate during drying and start-up melting has a marked effect on crucible life. Too fast and the temperature difference across the body concentrates thermal stress; too slow and commissioning takes longer. Drawing up a curve from body dimensions and wall thickness, with holds at key temperatures, is the more reliable approach. Heating curves for a new cell and after crucible replacement should be prepared separately, because the insulation condition of the old cell has already changed.

8. Operating Records and Scrapping

Recording salt temperature, cell voltage, current, charge weight and metal tapped for each run, together with wall thickness measurement points, builds a life curve for the crucible. Scrapping should not depend on run count alone but should combine wall thinning, local pits and deformation; at the same run count, crucibles of different cell types may differ markedly in remaining wall thickness.

The crucible replacement record should state the reason for scrapping and the service conditions, so that the next crucible can be improved. If several crucibles in a row thin at the same azimuth, the problem is most likely electrode eccentricity or circulation rather than the material itself. Overlaying the loss location on the electrode layout drawing often pinpoints the cause directly.

9. Combined Purchasing and Technical Communication

The crucible and the electrode are two parts on the same current loop, and purchasing them separately easily leads to mismatched dimensions and tolerance interfaces: the crucible inner diameter is machined to one specification while the electrode layout is designed to another, and the shortfall in spacing is only discovered at assembly. It is advisable to list crucible inner diameter, electrode number and spacing on one configuration table and calculate them to a single technical specification.

When requesting a quotation, provide the cell type, salt system, working temperature, capacity, electrode layout and service data for the existing crucible. For a new cell, also provide the general assembly drawing and the heating plan; for a replacement part, the measured dimensions of the old part and the reason for scrapping are more valuable. The more specific the data, the closer the proposal comes to actual service conditions and the lower the cost of trial and error.

Specifications and connection forms of electrodes for rare earth electrolysis can be checked on the electrolysis cell electrode page; dimensional series for crucible products are given in the crucible product series; the behaviour of molybdenum materials in high-temperature and corrosive environments is covered in the remaining entries.