In my experience cooking large amounts of rice, the heat of the boiling water will not be close to enough to cook it thoroughly.
I have found the key to cooking large batches of rice is to use a lot more water than typical, and heat slowly and stir. I would plan on a few hours at low heat stirring often with plenty of liquid in the pot, any excess liquid merely replaces strike water.
As you already found, high heat for large amounts of rice scorches terribly, just be very patient, may take hours of simmer. Stirring often is a good idea as well.
I agree, found this out the hard way, although I love lite beers made with rice it turns into a sticky paste and also and clogs a recirculating system every time. while trying to clear my false bottom I pushed down too hard and pushed rice through it and it landed on the element and cooked then gave the beer a bad taste."lesson learned"