The Frasch Process

The Frasch Process is a method in which sulfur is extracted from naturally occurring elemental deposits of sulfur. It has been used since the 1890s to extract vast deposits of sulfur, as it has solved the problems of poisonous gas penetrating into mine shafts.

  1. Concentric pipes are bored down to an elemental deposits.
  2. Superheated water (170oC, 1.5MPa) is injected into the sulfur deposit through the outer pipe.
  3. Sulfur's melting point is low so it is melted (approx. 115.21oC)
  4. Both liquids form an emulsion (foaming dispersion).
  5. Compressed air is injected down the central pipe, forcing the emulsion to the surface through the middle pipe.
  6. At the surface, this emulsion is collected in large vats and cooled.
  7. As this dispersion cools, it separates into water and solid sulfur (99.5% pure).
  8. It should be cooled quickly in order to avoid oxidation, which produces toxic SO2.

Properties of Sulfur allowing Extraction

  • Low melting point of roughly 115oC due to weak dispersion forces (super heated water melts this easily).
  • Insoluble in water and does not react with it chemically (allows it to be collected on the surface)
  • Low density (2.07g/cm3) forms a sulfur-water emulsion that is light and can be readily transported to the surface with compressed air.
  • Inert, non-toxic and non-volatile, meaning no special precautions are required to protect health of miners.

Environmental Issues

Recovered water contains some dissolved minerals. The effluent need to be cooled to avoid thermal pollution and should be recycled to avoid contamination of local ecosystems. Also once sulfur is extracted the caverns left over are prone to subsidence (collapse). They are difficult to backfill and can become filled with ground water over time.