Quantification of slag foaming:

Foaming index = Foam layer thickness/ average gas velocity

Low foaming index means easy escape of gas bubbles which can be obtained either by smaller gas bubbles or higher gas velocities. Foam life is directly proportional to foaming index

Increase in slag viscosity increases foaming index. Presence of solid particles and surface active agents increases the foaming index. Addition of calcium fluoride decreases the foaming index by decreasing the viscosity of slag. Foaming index (FI) can be calculated from the physical properties of slag and size of the gas bubble:

(10)

                                                                        
 
 
 
 

Calculate the foaming index slag of composition  and 5% at 1773 K slag from the following data:

 and  and 0.01m.
Substituting the value of the variables in eq. 10 we get

  for  and 485 for

If the volume fraction of solid particles in slag is 0.1

 s for  and 631 s for

Note that foaming index increases to 1.3 times due to presence of solid particles in slag.

Consider a slag of composition  and  1% Ca F₂ at 1873 K whose 
  and  and  

This slag would have foaming index 9s. Foaming tendency decreases drastically due to production of
Ca F₂  in slag.

Reference to lectures 3 and 4

A.Ghosh and A.Chatterjee:: Ironmaking and steelmaking
Zhang and Fruehan: Metallurgical and Materials Trans. B, 26(8), 1995