IV.

Extraction

Several methods exist for the extraction of free sulfur from the ground. In Sicily the sulfur-containing rock is placed in large piles on sloping ground and ignited. The liquid sulfur resulting from this heating is allowed to run into a series of wooden molds, in which it solidifies; in this form it is known as roll sulfur. The roll sulfur may be further purified through distillation, the vapor being passed into a large brick chamber in which it condenses on the walls as a fine powder called flowers of sulfur. In areas where natural sulfur deposits may lie some 275 m (about 900 ft) or more below the surface of the earth, as in Louisiana and Texas, the method most commonly used for extraction is the Frasch process, invented in 1891 by the American chemist Herman Frasch. In this method four concentric pipes, the largest being 20 cm (8 in) in diameter, are driven down into the sulfur-containing deposits. Water, heated under pressure to 170°C (338°F), is forced through the two outer pipes into the deposit, melting the sulfur. When a sufficient quantity of sulfur has been melted, hot air is forced down the inmost pipe to form a froth with the molten sulfur, and the mixture is forced up to the surface through the remaining pipe. The sulfur is run into wooden bins and solidified, yielding a product that is about 99.5 percent pure. Sulfur is also obtained from pyrites by distillation in iron or fireclay retorts, but it usually contains traces of arsenic when produced in this manner.