Modern inorganic chemistry

GROUP VI 261

SULPHUR

Large deposits of free sulphur occur in America, Sicily and Japan.

Combined sulphur occurs as sulphides, for example galena, PbS,

zinc blende, ZnS, and iron pyrites, FeS 2 , and as sulphates, notably

as gypsum or anhydrite, CaSO 4.

In America, the sulphur deposits (mostly in Louisiana and Texas)

are dome-shaped layers about 30cm thick, between limestone

above and anhydrite below. From these, the sulphur is extracted

by the Frasch process. A metal tube, about 15 cm diameter and

containing two concentric inner tubes (Figure 10.1) is sunk into

the top of the deposit. Water, superheated to 450 K, is forced

Air

Superheated-

water

Water

* Molten

sulphur

Molten

sulphur

Figure 10.1. The Frasch pump

under pressure down the outer tube, and enters the sulphur layer
through perforations. The sulphur melts (m.p. 388 K) and enters
the inner pipe at the bottom, up which it flows for some distance.
Compressed air is forced down the innermost pipe; this emulsifies
the water and molten sulphur mixture, so lowering its density, and
the emulsion rises to the top of the pipe, where it is run off into vats
to solidify. The purity is usually 99.8 %.
Large quantities of sulphur are recovered from petroleum and
natural gas. Naturally occurring hydrogen sulphide, H 2 S, and that
produced in the cracking and catalytic hydrogenation of petroleum
is first removed by absorption and the regenerated gas is converted
to sulphur by partial combustion with air, the overall reaction being,

6H 2 S + 3O 2 -» 6H 2 O + 6S