GROUP VI 297
2PbS + 3O 2 -> 2PbO + 2SO 2 t
4FeS 2 + 11O 2 -> 2Fe 2 O 3 -f 8SO 2 T
Since arsenic is often found in nature associated with sulphide
ores, sulphur dioxide obtained by this method may contain some
arsenie(III) oxide as impurity, and in certain processes this is a
distinct disadvantage.
(c) From anhydrite, CaSO 4 (the only sulphur compound found in
large quantities in Great Britain). Anhydrite, shale (SiO 2 ) and coke
are finely powdered, intimately mixed and compressed into pellets
which are fired in a reverberatory furnace at a temperature of about
1700 K.
The carbon reduces a quarter of the anhydrite to the sulphide :
CaSO 4 + 2C -> CaS + 2CO 2
The sulphide then reacts with the remaining anhydrite :
CaS + 3CaSO 4 -» 4CaO 4- 4SO 2
Thus the overall reaction is :
2CaSO 4 + C -> 2CaO + CO 2 + 2SO 2
The gases from the kiln contain about 9% sulphur dioxide. (The
calcium oxide combines with the silica to form a silicate slag which,
when cool, is crushed and mixed with some anhydrite to give
cement, a valuable by-product.)
In all the above methods, the sulphur dioxide obtained is impure.
Dust is removed by first allowing the gases to expand, when some
dust settles, then by passage through electrostatic precipitators
and finally by washing with water. Water is removed by concentrated
sulphuric acid which is kept in use until its concentration falls to
94%.
- The combination of sulphur dioxide and oxygen to form the
trioxide is slow and does not proceed to completion :
2SO 2 + O 2 ^ 2SO 3 : AH = - 94 kJ mol" l
2 vol. 1 vol. 2 vol.
3 vol.
Although the left to right reaction is exothermic, hence giving a
better equilibrium yield of sulphur trioxide at low temperatures,
the reaction is carried out industrially at about 670-720 K. Further-
more, a better yield would be obtained at high pressure, but extra
cost of plant does not apparently justify this. Thus the conditions
are based on economic rather than theoretical grounds (cf. Haber
process).