Sulfur

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)