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Anode Fabrication

3 min read

Make a carbon anode by grinding charcoal or coke below 1 mm, mixing it with 15-20% coal-tar pitch by weight at 150 °C, pressing the hot mix into blocks, and baking them at 1,000-1,200 °C for 24-48 hours in a sealed kiln that excludes air. The result is a dense, conductive block 50-100 mm thick; heat it slowly, because the pitch gives off gas that cracks a block heated too fast.

Part of Carbon Electrode Production

The four-stage process of making baked carbon anodes: grinding, mixing, pressing and baking.

Pitch Glues The Carbon And Bakes To Carbon

The carrier’s process is: grind the filler below 1 mm; mix with 15-20% pitch by weight at 150 °C; press into blocks with a hydraulic press or heavy weight; bake at 1,000-1,200 °C for 24-48 hours in a reducing atmosphere (sealed kiln); the result is dense carbon blocks with good conductivity, 50-100 mm thick, sized to the cell. During baking the pitch turns into carbon bonding the grains, which is why it works. Pitch comes from Coal Tar Distillation, and a press can be adapted from the lever and screw designs in Press Building, which is a soil-brick press and not rated for this.

What You Need

  • Filler and pitch (Carbon Electrode Production).
  • A sieve or mill to get below 1 mm.
  • A heated mixing pan that holds 150 °C.
  • A steel mould and press.
  • A sealed kiln holding 1,000-1,200 °C, with the blocks buried in carbon or sand to exclude air (my suggestion; the carrier says only “sealed kiln, reducing atmosphere”).

Method

Step 1: Grind and sieve the filler below 1 mm.

Step 2: Mix in 15-20% pitch by weight at 150 °C until every grain is coated. In 100 kg of green mix that is 80-85 kg of filler and 15-20 kg of pitch.

Step 3: Press hot into the mould. A dense block is stronger and conducts better; a loose one crumbles in the cell.

Step 4: Bake slowly up to 1,000-1,200 °C and hold 24-48 hours, then cool slowly. You should see no cracks or blisters; tap the cooled block, and a clear ring suggests it is sound.

Step 5: Check conduction. Pass a known current and measure the voltage drop along the block; blocks with high drop are not dense enough. This is my suggested check; the corpus gives none.

Worked Example

A cell consuming 9.6 kg of anode per day (from Carbon Electrode Production) with blocks 50-100 mm thick: the bake of a batch takes 24-48 hours, so one kiln load per day or every two days supplies the cell if it holds at least 9.6-19.2 kg of baked carbon per load.

Common Mistakes

  • Heating fast. The gas from the pitch splits the block.
  • Letting air in at temperature. The carbon burns instead of baking.
  • Coarse grain. Gaps raise resistance and the block dusts away in the bath.

Safety

Hot pitch, gas and fire

Pitch mixed at 150 °C causes deep burns, and baking drives off flammable gas and smoke that can ignite where it leaves the kiln. Keep the kiln outdoors and away from stores of fuel, ventilate, wear eye and skin protection, and use dry sand and not water on pitch fires. Carbon monoxide forms in a sealed hot kiln: see CO Risk.

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