Biekindy,
Without having to go into a detailed investigation, I can tell you some basics about your dryer capacity:
1) Calculate the sensible heat required to warm the air to 212F from the follwoing equation: q=1.08 CFM (212-Ti).
2) The most water you can evaporate from the air is:
M=q/dH = 8,000,000 Btu /970 Btu/lbs = 8,247 lbs. (This is an approximation because you need to subtract the heat required from 1).
3) The actual values will change based on your before and after gravel moisture content and the incoming air properties.
Good luck.