Sulfur is the second member of VIA group of the periodic table and has symbol “S” ,it’s a atomic number is 16 while atomic mass is 32 a.m.u. sulfur has been known for its medicinal and germicidal effect from very ancient time before 1000 B.C but its chemical nature remained unknown until 1787 when Lavoisier recognized it is an element.
SULFUR AND ITS ALLOTROPIC FORMS:
The existence of an element in two or more different forms in the same physical state is called allotrope; while different forms are known as allotropic modifications and allotropes.
Sulfur exists in several allotropic forms, out of which the three forms are
(a) Rhombic sulfur (- sulfur)
(b) Mono clinic sulfur ( or prismatic sulfur)
(c) Plastic sulfur (y- sulfur)
It is the most stable crystalline form of sulphur at ordinary condition .free sulfur exits in nature as rhombic sulfur .it is obtained as pale-yellow crystals, giving lemon yellow powder. Its melting point is 113C and its density is 2.08g/cm3 at 20C.it is insoluble in water but dissolves in carbon disulphide (CS2), benzene disulphide dichloride (S2 Cl2) turpentine etc.
MONO CLINIC SULFUR:
Mono clinic sulfur is another crystalline form of sulphur .its stable between 96 to 119C and hence it changes slowly to rhombic sulfur at ordinary temperature.it consist of yellow transparent needle like crystals. On standing the crystals become opaque ,brittle and lemon yellow in color .its melting point is 119C its density is 1.96 g /cm3 .it is soluble in carbon disulphide (CS2) but in soluble in water.
Plastic sulfur is super cooled form of sulfur it is the non crystalline allotrope of sulfur .when ordinary sulfur is heated carefully up to its boiling point about 444.6C and then the molten sulfur is poured cool water. a soft rubber like mass is obtained which looks as if it is made of plastic material .plastic sulfur is generally not considered to be a true allotrope of sulfur because it is unstable reverts to rhombic sulfur on standing.
OCCURRENCE OF EXTRACTION OF SULFUR:
Sulfur is not metal and makes up about 0.1% of the earths crust .it is found in the Free State in the Sicily. Mexico and U.S.A.in U.S.A large deposits of sulfur are found in Louisiana and Texas.it is also found in the Free State in japan and New Zealand .in the combined state.it is largely found as sulphides of iron, zinc, lead, copper and mercury and also as sulphates of magnesium, calcium and barium.
EXTRACTION OF SULFUR:
Most of the world’s sulfur is obtained from underground deposits which may be more then 200 meters below the surface of the earth, above 60-70% native sulfur occur deep below the earth’s surface. For this reason sulfur can not be dug out directly by ordinary mining .for the extractions of sulfur and ingenious method was discovered by Herman Frasch, an American engineer, known as Frasch process.
Sulfuric acid is one of the most important chemical compounds known it is commonly used in the laboratory and in industries in many processes.
INDUSTRIAL PREPARATION OF SULFURIC ACID:
On the large scale sulfuric acid is manufactured by two methods the contact process and the lead chamber process. The contact process is the most recent one and gives sulfuric acid of highest purity.
THE CONTACT PROCESS:
This method was developed in Germany in the early 19th century but came into operation from1912.Now a days sulfuric acid is mostly manufactured by contact process. In Pakistan sulfuric acid is also manufactured by this process and hence contact method is described here. In this method SO2 is produced mainly by burning sulfur in dry air or iron pyrite in pyrite burners.
S(s) + O2 (g) SO2 (g)
4FeS2(s) + 11O2 (g) 2Fe2O3(s) + 8SO2 (g)
It is very important that in contact process SO2.and air must be very pure, free from all sorts of impurities which poison the catalyst; so the mixture of SO2.and air is passed through special dust filters .The solid particles settle down in the dust chamber. Then SO2 is passed through the washing or scrubbing chamber. Here team is injected from the top of the chamber. Solid particles from droplets with steam and settle down. The moist mixture of SO2 and air is passed through the drying towers in which conc.H2SO4 is sprayed from the top.