Alkali wood extraction

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Alkali wood extraction is a method of producing oxalic acid, formic acid and methanol from sawdust or wood chips by dry-distilling them in the presence of a strong alkali (usually sodium or potassium hydroxide), then treating the "melt" of wood with acid to produce a soluble alkaline oxalate.[1]

  1. Combine a concentrated solution of potassium hydroxide or sodium hydroxide with sawdust in a metal (iron) pan
  2. Mix thoroughly over heat
  3. Evaporate to dryness by heating to 110°C
  4. Raise temperature to the range 180-240°C for 20-40 minutes
  5. Leach with water
  6. Boil until density approaches 1.355 g/cm
  7. Allow to cool
  8. Filter
  9. Discard filtrate (toxic)
  10. Redissolve residue in hot water
  11. Add calcium hydroxide
    Ca(OH)2 + Na2C2O4 → CaC2O4(s) + 2 NaOH(aq)
  12. Filter
  13. Evaporate filtrate (hydroxides) and recycle
  14. Add water to the residue to create a slurry
  15. Add sulfuric acid to the slurry
  16. Heat gently and stir for 2h
    CaC2O4 + H2SO4 → H2C2O4(aq) + CaSO4(s)
  17. Filter.
  18. Residue is calcium sulfate
  19. Evaporate to density of 1.116
  20. Filter
  21. Residue is calcium sulfate
  22. Dry filtrate
  23. Powder is oxalic acid

References

  1. ↑ Othmer, Donald F.; Gamer, Carl H.; Jacobs, Joseph J. (1942) "Oxalic Acid from Sawdust - Optimum Conditions for Manufacture"
    Ind. Eng. Chem. 34(3); pp262–267. 
    link courtesy ACS.
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