The hard part is setting up the formula for the volume of the ice. We want the volume of the ice only, not the ice plus the silo. This ends up being the difference between two hemispheres -- the larger one (silo + ice) minus the smaller one (just the silo), thus leaving us with the ice only. So the equation is:
where is the thickness of the ice. Also, make sure we define because it is decreasing.
At this point you just need to differentiate and start plugging in the appropriate values and solve.