Crude sulfur is produced from the Frasch process or recovered from "sour" natural gas or petroleum. Although termed "crude", this sulfur possesses a minimum purity of 99.5 percent and is suitable for a majority of uses. The impurities consist primarily of trapped organic matter.
Until recently, a significant amount of the world's sulfur supply came from sulfur-bearing limestone deposits found in the gulf coast region of the North American continent. The sulfur occurs at depths of 500 to 3,000 feet in domes subterraneously up-thrust by columns of salt. The Frasch process utilizes a steel tube made up of three concentric pipes that are driven underground to reach the sulfur deposit. Superheated water is then pumped down under great pressure in the outermost pipe to melt the sulfur. Air pressure from the innermost pipe forces the sulfur up the third pipe to the surface where it cools and solidifies.
Pyrites is the term given to a variety of sulfide ores. In the United States, iron pyrites are used in the production of only a limited amount of sulfur. The salt dome deposits and sulfur recovered from petroleum and natural gas are much more economical sources. On a worldwide basis, however, sulfur tonnages from pyrite ore are substantial. Pyrites are obtained either as run-of-mine or beneficiated ore from straight pyrite deposits or recovered, as it usually is in the United States, as by-product flotation concentrates during the refining of iron ore. Depending upon the process used, pyrites can be made to yield elemental sulfur or sulfuric acid.
Recovered sulfur from petroleum refining and from "sour" natural gas has surpassed Frasch process sulfur as the world's most important source of supply. Recovered sulfur tonnages are expected to increase as the demand for clean-emission fuel continues.
In petroleum refining, sulfur is reacted with hydrogen to form hydrogen sulfide. In "sour" natural gas, the sulfur occurs as hydrogen sulfide. In either case, the hydrogen sulfide is converted to elemental sulfur. Recovered sulfur is produced by counter-current absorption to collect the hydrogen sulfide in a solution. The solution is normally an alkanolamine from which the gas is distilled, then burned to produce a flue gas consisting of hydrogen sulfide and sulfur dioxide. This is cooled and catalyzed in a converter to produce sulfur vapor and water. The sulfur vapor is removed from the mixture by scrubbing with makeup liquid sulfur.