Occurrence, isolation & synthesis


In nature, sulfur is found in the Earth's crust, 0.05 %, as both elemental sulfur and as minerals, mainly sulfates and sulfides [6]. The isotopes found in nature are 32S (95.0 %, stable), 33S (0.76 %, stable), 34S (4.22 %, stable) and 36S (0.01 %, stable) [6].

In pure solid form, sulfur exist as:
α-form: orthorhombic crystal of cyclic S8 [6].
β-form: monoclinic crystal of cyclic S8. Stable between 94.8 and 120 °C [6].
γ-form: monoclinic crystal of cyclic S8 [6].
Polymerized form: amorphous sulfur with a molecular weight around 200,000 [6].

When the rhombic sulfur crystals melt at 112.8 °C, it forms a straw colored liquid called λ-sulfur, which is S8 in liquid form. When heated the sulfur ring burst and start to polymerize, At around 230 °C sulfur is a dark red supercooled liquid of S called μ-sulfur or plastic sulfur [5].


Sulfur is extracted from underground deposits using the Frasch process, where superheated water (at 170 °C and a pressure of 100 pounds/square inch) is forced down the outermost of three concentric pipes to the sulfur deposit, located several hundred feet below the surface. When the hot water melts the sulfur, compressed air is forced down the innermost pipe. The air and melted sulfur forms a foam that flows to the surface through the middle pipe, where it is collected and solidifies when cooled. The sulfur obtained by this process is 99.5-99.9 % pure [5].

Sulfur is also obtained by reducing H2S, e.g. from purification of natural gas. This is either done by heating a mixture of H2S and SO2 or burning the H2S in a limited supply of air [5]:

2 H2S(g) + SO2(g) 3 S(s) + 2 H2O(g)
2 H2S(g) + O2(g) 2 S(s) + 2 H2O(g)