"Maybe these question sound stupid to you guys in the know BUT."
First off, it is not a smelter, but a mill producing concentrate which will be shipped to a smelter. This mill does not produce slag, but does produce a waste in 'sand' form. This waste will be mixed into a cement and returned to mined out areas.
Construction of the ramp and mill area will produce considerable waste (ground up rock), which will be more than enough for all surface aggregate needs.
In a normal building above ground, everything would be housed in one huge structure simply because of economics. In an underground situation, everything can be housed in a series of small areas, with support columns in between to support the ceiling, which will be 100+ meters below ground, in solid bedrock.
Original cost of underground structures may be considerably higher than on surface, but heating and maintenance savings over 20 years will far outweigh those costs.
Environmental hurdles will be minimal for an underground operation, and we could even expect that hard core environmentalists could even come out in support of our applications.
best regards
K