Sulfur is a nonmetallic chemical element, recognized in its elemental form as a bright, pale yellow, brittle solid. Known since ancient times as “brimstone,” the element is a poor conductor of electricity and is highly reactive, forming compounds with nearly all other elements except gold and platinum. Today, the vast majority of sulfur is used in the production of sulfuric acid. Sulfuric acid is the most widely produced industrial chemical, primarily used for manufacturing phosphate fertilizers.
Primary Sources of Sulfur
Commercial sulfur availability is linked to two primary sources: natural deposits and sulfur compounds contained within fossil fuels. Over 90% of the world’s sulfur supply comes from the latter, recovered during the purification of natural gas and crude oil. These hydrocarbon sources contain sulfur as hydrogen sulfide (H2S), known as “sour gas,” or as complex organic sulfur compounds within “sour crude” oil.
These sulfur compounds must be removed before the fuels are used or transported, primarily to prevent the release of the pollutant sulfur dioxide (SO2) upon combustion. Natural elemental deposits, once the main source, occur in subsurface formations like the caprock of salt domes, volcanic regions, and sedimentary beds. A smaller volume of sulfur is also obtained from mineral ores, such as pyrite (iron sulfide). These natural deposits are now secondary to the quantities recovered from the petroleum and natural gas industries.
Modern Recovery: The Claus Process
The Claus process is the dominant method for converting gaseous hydrogen sulfide (H2S) into elemental sulfur. Driven by environmental regulations, this technology is applied in refineries and natural gas processing plants to “sweeten” sour gas streams by removing H2S. The process is divided into two main stages: a high-temperature thermal stage and a lower-temperature catalytic stage, followed by condensation.
The process begins in the thermal stage, where approximately one-third of the H2S is combusted with air in a reaction furnace at temperatures exceeding 1000°C to produce sulfur dioxide (SO2). This partial oxidation sets up the necessary stoichiometric ratio for subsequent reactions. The hot gas stream then passes through a waste heat boiler, where the first significant portion of elemental sulfur is formed and condensed as a liquid.
The remaining gas mixture, containing H2S and SO2 in the ideal 2:1 ratio, moves to the catalytic stage to maximize conversion. This gas is reheated and flowed over a catalyst bed, typically composed of activated alumina or titanium dioxide, which facilitates the main Claus reaction. Here, the H2S reacts with the SO2 to form elemental sulfur and water vapor. Because the reaction is limited by chemical equilibrium, the process often employs two or three catalytic stages in series to achieve a higher overall recovery rate.
After each pass through a catalyst bed, the stream is cooled in a condenser to collect the newly formed liquid sulfur. This liquid must be continuously drained to prevent catalyst poisoning. A two-stage Claus unit achieves 95% to 97% recovery, but modern plants often incorporate a third catalytic stage or a downstream tail-gas treatment unit to push recovery to over 99.9%. The recovered elemental sulfur is highly pure and is stored and transported in its molten state.
Historical Extraction: The Frasch Process
The Frasch process, developed in the late 19th century by Herman Frasch, enabled the commercial extraction of elemental sulfur from deep underground deposits. This technique was historically applied to pure sulfur beds found in the caprock of salt domes, primarily along the U.S. Gulf Coast. Unlike traditional mining, this method bypasses excavation by capitalizing on sulfur’s low melting point, approximately 115°C.
The process uses three concentric pipes drilled into the sulfur deposit, often hundreds of meters below the surface. Superheated water, pressurized to prevent boiling and heated to about 165°C, is injected through the outermost pipe into the porous rock formation. This hot water melts the surrounding solid sulfur, creating a pool of molten material at the bottom of the well.
Compressed air is forced down the innermost pipe, mixing with the molten sulfur in the middle pipe. This air lightens the density of the sulfur, creating a froth that the pressure of the superheated water column forces upward to the surface. The sulfur is collected in large vats where it cools and solidifies. While the Frasch process once dominated global supply, it is now largely historical; its high energy cost and the abundance of recovered sulfur from the Claus process have made it obsolete in most regions.