 The oxidized iron is skimmed off, while the sulfur dioxide gas is captured and sent to an on-site acid plant for conversion to valuable sulfuric acid – a million tons of it each year.Left behind is a molten copper sulfide called matte.The 70-percent-copper matte is water-quenched to form a sand-like solid, then injected, with oxygen, into a flash-converting furnace that produces molten, 98.6-percent-pure copper.