Liberating Copper Metal From Stone - The Chemistry of Smelting Copper Ore

Pieces of black glassy slag from the smelter. Note the bubbles in the slag. This was from escaping carbon dioxide bubbles.

NPS - J. Axel

Humanity needs metals to thrive. But how can these metals be freed from rock? This has presented a challenge to people for millennia. Humans began to experiment with mixing different minerals and applying intense heat to ore. Over time, these techniques helped us extract the metals we needed. In fact, copper was the first metal to be smelted, followed by silver, tin, and lead. Other more challenging ores like those that contain iron, mercury, and gold came later. Different techniques were refined to separate these metals from worthless elements like silicon dioxide (quartz) in host rock. Silicon and oxygen are the two most common elements in Earth’s crust. Smelting uses heat to turn rock to liquid and then uses the chemical properties of different types of rock to separate metals from other elements. Prior to the understanding of atoms and the science of chemistry it must have seemed almost magical that mixing different types of rock at high temperatures would create metal!

So how does a smelter refine ore? First, a source of intense heat is needed. Silver melts at 1,763º Fahrenheit (961ºC). Copper melts at 1,984ºF (1,084ºC). Silica melts around 3,000ºF (1,650ºC). However, ‘smelting’ isn’t the same as ‘melting,’ even though everything ends up melted.

Since heating with electricity wasn’t an option in the late 19th century, smelting was accomplished by burning coal that was mined in Southern Utah or with charcoal. Charcoal is made by putting wood in a very hot zero-oxygen kiln for several days until all water and impurities turn to gas and escape the wood. The wood turns black and is now almost pure carbon. The carbon doesn’t burn because there is no available oxygen in the kiln. It is then left to cool slowly over many days. Once cooled the new coal can't spontaneously combust when the kiln door is opened. Both coal and charcoal burn much hotter than firewood. Coal and charcoal can burn over 3,500ºF (1926ºC) in a smelter. Firewood, which is full of impurities and water, only burns around 900-1200º F (480-650º C), which is too cold to melt ore.

 

This closeup shows the liquid patterns from when the slag was molten and freshly poured out of the smelter. It quickly cooled, freezing the fluid patterns in time from when it was liquid about 130 years ago.

NPS - J. Axel

As the ore begins to melt, limestone and iron ore are added. The limestone thermally breaks down because of the heat in the smelter. It is split into quicklime (calcium oxide) and carbon dioxide gas. The quicklime bonds with impurities like silica and oxygen, but not copper or silver. The carbon dioxide bubbles out of the liquid and into the the atmosphere. Iron reduces oxidation of the copper at high temperatures by combining with free oxygen more easily than copper sulfate, preventing copper oxidation. (If you have watched a video of a blacksmith working orange glowing iron and see black scaly flakes form on the surface of the hot metal, that is the same thing where free oxygen in air is bonding to the hot iron and making a thin brittle layer of magnetite.) The burning charcoal also produces a layer of carbon monoxide gas which prevents oxidation. The smelter tender had to carefully keep track of all of this chemistry to produce copper and silver.

Once everything is melted, the higher density liquid copper and silver sink to the bottom of the smelter while the less dense quicklime-silica-oxygen-iron slag floats on top. The smelting process had now separated the valuable copper and silver from the undesirable minerals.

The next step is to skim off the slag. It takes a good eye to pour off slag but not the liquid metal. Liquid slag flows faster and glows brighter than molten metal. The slag was dumped out onto the ground and left to cool. Slag turns into the black glassy rocks you can see below the smelter. They look like obsidian, a glassy volcanic rock (which is also primarily silicon dioxide). The operator would watch for the color change and knew when to stop the slag pour.

Once the brightly glowing slag was poured off, the worker would then tap the hole in the bottom of the smelter and pour out the liquid copper and silver into bar-shaped molds. As the metal cooled, any last slag that came out with the metal floated on top of the bullion in the molds. Once cold, it formed a black glass coating on top of the bullion bars. Workers would hit the bars with a hammer and the glassy slag would shatter and fall off. The bars were shipped by rail for further processing at a Salt Lake City smelter.

It was discovered that the Grand Gulch Mine smelter was not extracting all the copper and silver from the ore. A profitable amount of both metals remained in the slag. It is not known how much slag was produced by the smelter but likely it was a substantial amount. It was put on wagons and sent to a more sophisticated smelter in Salt Lake City to be reprocessed. The small amount of slag that remains at the mine was perhaps from further testing of the smelter before it was finally abandoned.

 

Smelter chimney and black slag.

NPS - J. Axel