Sulfur is a pale yellow, crystalline element that is flammable. While pure, solid sulfur is generally odorless, it is widely used in industrial processes, such as fertilizer and sulfuric acid production. Understanding its flammability profile is important, as sulfur poses both a fire and explosion risk.
Ignition and Ease of Combustion
Igniting bulk elemental sulfur requires moderate heat, as the material must transition from a solid to a liquid state before sustaining a flame. Solid sulfur melts at a relatively low temperature, typically between 112°C and 120°C, and the vapor above the molten surface ignites. The spontaneous ignition temperature for pure bulk sulfur is around 232°C to 260°C (450°F to 500°F).
The physical form changes the ignition threshold dramatically; fine sulfur dust presents a much greater hazard. Suspended dust has a significantly lower ignition point, sometimes as low as 190°C (374°F), and can be ignited by weak frictional sparks. The small particle size increases the surface area exposed to oxygen, creating the potential for a dust explosion if a sufficient concentration is present in a confined space. Sulfur is an excellent electrical insulator, and handling operations can easily generate static electricity, which serves as a potent ignition source for airborne dust clouds.
Characteristics of a Sulfur Fire
Once ignited, sulfur burns with a distinctive, low-intensity, pale blue flame that is difficult to spot, especially under bright daylight. The fire is sustained by the combustion of sulfur vapor above the melting solid.
The main hazard of a sulfur fire is the toxic gas it produces: sulfur dioxide (SO2). Sulfur dioxide is a corrosive gas with a pungent, choking odor that is extremely irritating to the eyes and respiratory tract. Unprotected personnel near a fire can quickly suffer from severe irritation, breathing difficulty, and pulmonary edema. Because SO2 is heavier than air, it can settle in low-lying areas or enclosed spaces, posing a suffocation risk and making ventilation a concern.
Safety, Storage, and Extinguishing Fires
Storage and Safety
Safe handling requires isolating sulfur from heat sources and incompatible chemicals. Sulfur should be stored in a cool, well-ventilated area, separated from strong oxidizers such as nitrates or chlorates, which can react violently or explosively. When handling solid sulfur, particularly in powdered form, use non-sparking tools to eliminate ignition sources for dust.
Extinguishing Fires
Extinguishing a sulfur fire requires specialized methods because applying a solid stream of water can be counterproductive and hazardous. A high-pressure stream can scatter the burning material, spreading the fire or triggering a dust explosion. In confined spaces, water hitting hot or molten sulfur can create a sudden, dangerous release of steam, leading to over-pressurization.
For small, contained fires, methods that cut off the oxygen supply are recommended, such as smothering the flames with sand, fine earth, or a dry chemical fire extinguisher. For larger fires, a fine water spray or fog can be used to cool the material and control the blaze without causing dangerous scattering. In industrial settings, introducing steam or inert gas into an enclosed container is effective because it displaces the oxygen and rapidly extinguishes the fire.