Elemental sulfur is a pale yellow, brittle solid, but in large-scale industry, it is typically handled as molten sulfur, a liquid achieved by heating it above its melting point of approximately 115 to 119°C. This liquid form is preferred because it is easier to pump, store, and transport than solid sulfur, minimizing handling losses and dust formation. Molten sulfur, often recovered as a byproduct of oil and natural gas refining, serves as a fundamental feedstock for a vast array of industrial processes. Its efficient handling makes it an important raw material in the global chemical economy.
The Primary Use: Manufacturing Sulfuric Acid
The majority of molten sulfur, estimated to be over 85%, is used to produce sulfuric acid (\(\text{H}_2\text{SO}_4\)). Sulfuric acid is the most-produced industrial chemical globally, and its consumption often indicates a nation’s industrial strength. This conversion occurs primarily through the multi-step Contact Process.
The process begins by burning molten sulfur in air to produce sulfur dioxide (\(\text{SO}_2\)). This gas is then mixed with excess oxygen and passed over a vanadium(V) oxide (\(\text{V}_2\text{O}_5\)) catalyst at approximately 450°C. This exothermic reaction converts the sulfur dioxide into sulfur trioxide (\(\text{SO}_3\)).
The third stage involves absorbing the resulting sulfur trioxide gas into concentrated sulfuric acid, rather than water, which would create a corrosive mist. This reaction forms a viscous liquid known as oleum, or disulfuric acid (\(\text{H}_2\text{S}_2\text{O}_7\)). The oleum is then diluted with water to yield concentrated sulfuric acid.
The resulting sulfuric acid is primarily consumed by the fertilizer industry to manufacture phosphate and ammonium sulfate fertilizers. It is necessary for treating phosphate rock to create soluble phosphate compounds that plants can absorb. Beyond agriculture, sulfuric acid is used in petroleum refining, metal processing, and the synthesis of detergents and pigments.
Direct Uses in Fertilizers and Chemicals
While most molten sulfur is converted into sulfuric acid, it also finds direct application or conversion into other sulfur-containing compounds. Elemental sulfur powder, created by solidifying and milling molten sulfur, is used in agriculture as a soil amendment. This form helps to lower the pH of alkaline soils, making nutrients more accessible to plants.
Elemental sulfur is also a plant nutrient; soil bacteria convert it into soluble sulfate, which aids in protein synthesis and enzyme production in crops. Beyond its role as a nutrient, elemental sulfur is employed as a fungicide and pesticide in agricultural dusting applications.
In the chemical and manufacturing sectors, molten sulfur is reacted to produce compounds like carbon disulfide (\(\text{CS}_2\)), used in the production of rayon and cellophane. It is also necessary for the vulcanization of rubber, a process that creates cross-links between polymer chains to improve elasticity and durability. Furthermore, sulfur is a component in specialized building materials, such as sulfur concrete and sulfur-modified asphalt, where it acts as a binder to improve durability and corrosion resistance.
Handling and Transporting Molten Sulfur
Maintaining sulfur in its molten state is a logistical requirement driven by efficiency, necessitating careful temperature control to avoid handling issues. Elemental sulfur melts between 115°C and 119°C and is typically stored and transported between 135°C and 140°C. This range keeps the sulfur safely above its solidification point while maintaining a low viscosity.
If the temperature rises too high, around 160°C, liquid sulfur undergoes a chemical change: \(\text{S}_8\) rings break open and form long polymer chains, causing a rapid increase in viscosity. This makes the sulfur thick and difficult to pump, leading to process disruptions. To manage this, storage tanks and pipelines are insulated and heated, often using steam tracing or steam coils.
Molten sulfur is transported using specialized equipment, including insulated rail cars, tank trucks, and marine vessels, all designed to maintain the precise temperature. Maintaining this heat during transport and storage is a continuous challenge, but it ensures the sulfur is delivered in a pumpable state. The use of molten sulfur reduces losses and contamination common when handling solid, dusty elemental sulfur.