The Oxygen Flash Furnace
At the Asarco copper smelter in Hayden, Arizona, the oxygen flash furnace is initially heated up with natural gas. It becomes so hot that the copper concentrate immediately ignites when it is blown into the furnace along with oxygen and a silica-bearing flux.  The silica flux binds with the iron and other impurities and keeps them separated from the copper.Liquid copper and iron fall to the bottom of the furnace like a molten metal rain, while the sulfur combines with oxygen to form sulfur dioxide gas. This combustion releases a tremendous amount of heat that sustains the high temperature in the furnace.The copper, iron, and silica form two layers at the bottom of the furnace. The lighter, top layer is a slag of mostly iron, silica, and other impurities. The bottom layer of mostly copper and some iron is called matte. 
 furnace_diagram
 acid_plantThe Acid Plant
Sulfur dioxide gas from the flash furnace is captured and combined with water in an adjacent acid plant to produce sulfuric acid.The acid is sold as a by-product to chemical manufacturers in the United States.   The Hayden smelter produces 2,500 tons of high-purity sulfuric acid every day.

Tapping the Flash Furnace
flash_furnace_tapThe matte is tapped from a hole near the bottom of the furnace and poured into a huge ladle.   An overhead crane picks up the ladle and pours the molten matte into another type of furnace called a  converter.Even though it is more than 50 percent copper, if cooled now the metal would be silvery in color because of the relatively high iron content.  For this reason, it is sometimes called ‘white metal’ at this stage.

 
Slag
slag_pourThe slag is a dense, glassy material of mostly iron and silica. It is tapped from a hole in the upper part of the furnace wall and carried outside to the slag dump to cool.A slag pour resembles a man-made lava flow and can be quite spectacular at night. The slag cools quickly and tends to break into pieces by itself.  It will be recovered and sent back through the mill and concentrator to recover any copper that might have been trapped in the slag.
 
The Converter Furnace
converter_furnaceIn the converter, air is blown into the matte to burn away the iron and any remaining sulfur.   This burning releases more heat which keeps the molten metal hot. The iron and sulfur gases are trapped and cleaned to keep them out of the air.After blowing air through the converter for a few hours, the entire furnace rotates on its side to pour the molten metal, now called blister copper, into another large ladle which carries it to the anode furnace.  
 
The Anode Furnaceanode_furnace
The blister copper is more than 98 percent pure, but it contains too much oxygen. The anode furnace blows natural gas into the melt to burn off the oxygen.At first, a tall yellow flame billows from the top of the anode furnace, but after a few hours it becomes a beautiful blue-green when most of the oxygen in the copper has been burned away .
 
The Anode Casting Wheels
By rotating on its side, the anode furnace pours the molten metal, now called anode copper, into a heated trough. Natural gas flames are blown over the river of copper to prevent oxygen from being absorbed into the metal.  The copper flows into a rocking device that alternately pours the liquid metal into one or the other of two scales that measure out the copper into specially shaped molds. The finished castings,  called anodes, are 98 to 99 percent pure copper.anode_castingTwo anode casting wheels, each with sixteen molds, continually rotate under the scales to receive between 750 and 850 pounds of copper per mold. The molds themselves are made of about 2,000 pounds of solid copper that gets coated with a barite (barium sulfate) powder to keep the casting from sticking to it.While still glowing red-hot in the mold, the solidified anode is quickly cooled with a water spray, and in just a few minutes it can be lifted out of the mold. The two casting wheels each complete one rotation every fifteen minutes, together producing more than 120 anodes per hour.

anodesEven though they are made of copper, the molds never melt. Copper has a very high heat capacity, meaning it must absorb a lot of heat before reaching the melting point.  The molds are so much more massive than the castings, and both are cooled with a water spray, so the molds never get close to being hot enough to melt.

Anodes are basically two-inch thick slabs of copper about three feet wide and three-and-a-half feet tall including the two handles (or ‘lugs’) molded into the top.

They are loaded into special rail cars or bundled onto flatbed trucks and shipped to the Asarco Copper Refinery in Amarillo, Texas.

 TOUR NOTE:
Special clothing and safety equipment are required to visit a smelter, so we can’t take you there on a tour.  But the exhibits about smelting at the Mineral Discovery Center are the next best thing.