I believe what determines the classification of an area is the presence of flammable vapor, dust, etc. If a partition or fire wall can cause an area to no longer contain such vapor, etc., then I don't see why it would need to remain a classified location.
I'm thinking of automotive service shops that have a dividing wall between the service bay and the customer waiting area. The auto bay is classified, the waiting area is not.
Of course, as above, the point of origin of the vapor or dust would still remain classified, up to the limits set by the NEC.
In some cases a fire wall will do the trick, many times it is not the fire rating of the wall, but the physical seperation of the space and the containment of flammable fumes or dust. And then sometimes it is just distance from the source. It really does depend on what the materials involved are,and which categories of atmospheres you are dealing with.
A firewall like the ones around a transformer won't do much for a lighter than air hazard like natural gas. But it might work for gasoline, propane and other heavier than air materials. A door or other opening in the wall would allow the vapors to propagate to the other side and extend the hazardous area.
Surrounding an MCC room with a fire wall in the middle of a classified area doesn't make it safe unless ventilation is provided from a safe lcoation and all hazards are investigated.
