RE: Mill Datapoints for Juniorhttps://www.rusal.com/post_presentation/UBS_Blight_en.pdf
See page 5. Difference between coal and hydro seems to be $150/t refined aluminum. I.e $75/t alumina. But I have let myself understood that zinc smelting business is much less energy intensive (like 50%) than aluminum. Certainly melting points and specific heat capacities support the assumption.
Aluminum, melting point = 660 celcius, specific heat capacity (at 300K) = 900 J/(kg.K) : 0.165 kW/h to melt 1 kg.
Zinc, melting point = 420, c = 388J/(kg.K) : 0.04526 kW/h to melt 1 kg.
This would lead to similar conclusions about opex difference between coal and hydro as in my previous post. But I'll try to find something definite next time.