Rotary furnace.
But how clean is it?
What is your heat transfer efficiency?
Why do think that efficiency is "low" - compared to what other processes, and how did you measure those processes?
How do you measure your own process - equally important!
Lead is notorious for the health effects of breathing the vapors, so a trap and vent system is essential = heat loss. So, What are you doing to trap the hot vapors, filter or recycle to heated gases and fumes around the furnace?