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Cathode Lining

3 min read

The cathode lining is a floor of tightly fitted carbon blocks, sealed at the joints with carbon paste of ground carbon and pitch, laid over refractory brick inside a steel shell and wired through the shell wall to the negative bus bar. It holds molten fluoride bath and molten aluminum at about 960 °C for the life of the cell, so the failure to guard against is a leak through a joint, which would drain the cell and can burn through the steel.

Part of Carbon Electrode Production

Building the permanent carbon floor of an aluminum cell: block fitting, paste joints, insulation and the electrical connection.

The Cathode Is The Floor, And The Metal Pool Is Part Of It

The carrier describes the permanent lining: carbon blocks fitted tightly together on the cell floor, joints sealed with carbon paste (ground carbon plus pitch), and an electrical connection through the cell wall to the external bus bar. Its cell design puts a rectangular steel shell 0.5-1 m long by 0.3-0.5 m wide by 0.3 m deep, lined with refractory brick (alumina or magnesia), with the carbon blocks on the floor connected to DC negative. The liquid aluminum pools on this floor and acts as the real cathode surface.

What You Need

  • Carbon blocks made as in Anode Fabrication; the carrier calls them “the same carbon blocks, but permanent”.
  • Carbon paste (ground carbon and pitch).
  • Refractory brick under and around the carbon (Refractory Materials).
  • A steel shell and copper or steel collector bars through its wall.
  • Bus bars to the supply (Bus Bars).

Method: Building The Floor

Step 1: Lay the insulation. Set the refractory brick in the shell.

Step 2: Lay the carbon blocks tight, with minimal joint width.

Step 3: Fill the joints with carbon paste, packed hard. A joint that gapes lets molten metal run under the blocks.

Step 4: Connect the collector bars through the shell wall to the negative bus.

Step 5: Preheat slowly before the first charge, per the carrier’s pre-heat to 960 °C by electric resistance or gas burners; heating too fast cracks the blocks and the paste (my suggestion; unsourced rate).

Worked Example: The Connection

For 3,000 A (carried over from Operating Parameters), the Bus Bars rule of 1.5-2 A per mm² of copper gives a total cross-section of 3,000 / 2 = 1,500 mm² to 3,000 / 1.5 = 2,000 mm². Split across several collector bars so no single joint carries the whole current.

Common Mistakes

  • Wide joints. Molten bath and metal find them.
  • Wet paste or brick. Moisture flashes to steam on first heating.
  • A shell with no containment beneath. If it leaks, the spill must meet dry sand, not concrete or water.

Safety

Breakout, steam and current

A leak lets bath and aluminum at about 960 °C run onto whatever is below; put a dry sand bed under the cell and never over wet or damp ground, because water under molten metal flashes to steam and throws it. Aluminum attacks steel at these temperatures, as I recall, so a shell can be breached. Stand clear when tapping and keep a disconnect for the DC supply outside the splash zone.

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