By my calculations (assuming his dimensions are inches not feet) that's 17.7 gallons.
If its only 9 inches deep its not really likely suitable for a boil kettle imo. The large surface area to volume ratio would make for some serious boiloff and would make it hard to heat evenly with a basic propane boiler. Its safe to say that if you got the heating element or burner made for that boiler, it could function as a boil kettle, I just don't see it as ideal. Pics might help, it it were 24" deep that would probably be better.
Maybe you could use it as a sanitation bin. Easy to clean and durable. The shallow depth would make it easy to retrieve sanitized items from