SFD responded to a large gas leak at Sherburne Metals.  Arriving and staging at a distance 284 and 280 confirmed the substance to be Nitrogen and determined the possible hazards and no need for evacuation of citizens.  Proceeding into the scene SFD interfaced with staff at Sherburne Metals and contact was made with the Nitrogen supplier and tank maintenance company.  SFD attempted to "warm" the shutoff valve that had frozen shut by the flow of extremely cold Nitrogen with a constant flow of tank water from 284.