HCL by Salt and Sulphuric acid method:
NaCl + H2SO4 NaHSO4 + HCl
NaHSO4 + NaCl Na2SO4 + HCl
Both reactions involve the displacement of volatile acid from salt. The equilibrium can be displaced in desired direction by choice of condition i.e. promoting volatilization of HCl
The high temperature process is superior to vacuum for this purpose. To promote reaction rate it is desirable to have temperature sufficiently high to keep at least one of the reacting component in liquid condition.
There is no difficulty in first stage of decomposition but second stage required temperature of about 400 ˚C to liquefy NaHSO4. The higher limit to temperature is the attack of corrosive relative mass on furnace.
The product and unconverted H2SO4 is send to further processing in which there is recovery of H2SO4 by cooling and HCL is recovered as main product from absorber.