Why does production of nickel and copper metals from their ores involve sulfur dioxide emissions?
Well, nickel ores, and copper ores are COMMONLY sulfides...
Please bear in mind that I am a non-specialist, however, nickel is commonly encountered as a MIXED sulfide with iron, i.e.
...the copper oxide is produced, which may be reduced to copper metal, and up the flue we get sulfur dioxide.... i.e. the sulfur in the ore is OXIDIZED...