Matte

matte, crude mixture of molten sulfides formed as an intermediate product of the smelting of sulfide ores of metals, especially copper, nickel, and lead. Instead of being smelted directly to metal, copper ores are usually smelted to matte, preferably containing 40–45 percent copper along with iron and sulfur, which is then treated by converting in a Bessemer-type converter. Air is blown into the molten matte, oxidizing the sulfur to sulfur dioxide and the iron to oxide that combines with a silica flux to form slag, leaving the copper in the metallic state. Smelting of nickel sulfide ores yields a matte in which nickel and copper make up about 15 percent, iron about 50 percent, and sulfur the rest; the iron is removed in a converting furnace, and the sulfides of copper and nickel are separated before being reduced to the metals. Smelting of lead sulfide ores produces a liquid layer of copper sulfide matte that can be decanted, along with slag and speiss, from the lead bullion.