The Engineering of Copper Extraction

Added: 2026-05-12

[00:00:02]This rock contains only about a half% copper by weight. That tiny amount is extracted by a process is one of the greatest engineering triumphs of the late 19th century and is still central to our lives. Copper is the backbone of our electrical infrastructure. It conducts electricity better than almost every other metal. Only silver is better, some 6% better. But silver costs 70 times as much. So if you want to wire a building, power an electric motor, or build a circuit board, copper is what you need. And its importance will only increase as we expand beyond fossil fuels because these technologies use more of it. For example, a 3 megawatt wind turbine uses nearly 5 tons. And electric vehicles use three and a half times as much as a gasoline powered car.

[00:00:50]By some estimates, in the next 25 years, demand will increase by 50%.

[00:00:56]We extract copper from rocks with so little copper in them because in the late 1700s and early 1800s, the demand for copper grew fast. This was the age of naval power. Countries with strong navies controlled the seas and their ships needed copper. To prevent shipworms from destroying the wood, workers covered the bottom of ships with copper sheets. This is the remains of a ship off the western coast of Florida.

[00:01:21]The wood is now nearly gone, but the copper is still there. To get it, they mined or rich in copper oxide. This is from a mine active in the United Kingdom in the mid- 19th century. The brownie sections are very likely native copper, elemental metal. And this deep wine red section is coupite, a copper oxide.

[00:01:42]Together, these two regions make this rock perhaps 30% metal by weight, nearly 100 times that of this rock. To extract the copper from this copper rich ore, they heated it in a furnace with charcoal and a flux-like limestone. The intense heat separated the copper from the oxygen and the rocky waste, yielding copper that was largely, though not perfectly pure. The demand for copper increased throughout the 19th century as the world was connected by copper telegraph lines across nations and beneath the ocean. Then demand increased even more as rapid electrification exhausted the world's richest copper deposits, forcing engineers to extract copper from ores far leaner than anything they had processed before. In rocks like this, the copper is contained in these black spots as copper sulfide.

[00:02:32]It has a tiny fraction of the copper contained in the ore used in the 19th century. Yet today's mining companies would love to have a rock with even that much copper. Instead, they have rocks like this. This is from the Bingham Canyon Mine, the world's deepest pit mine. The copper in here is still that black copper sulfide. You can see it here and on the other side of the rock and these tiny black dots. It contains perhaps a half% copper. If you try to extract this rock's copper by tossing it into a furnace, it would take an enormous amount of energy. Instead, to extract it, the rock is pulverized, containing pieces that are about 100 microns in diameter. That level of grinding creates particles that are primarily one of two types, silica or copper sulfide. Next, of course, we need to separate these two. In principle, we could use a pair of tweezers and pick out the black copper sulfide. But on an industrial scale, this would be impossibly inefficient and thus extraordinarily expensive. Instead, we cover the crushed dry solids with water, then a layer of oil, cap the bottle, then mix thoroughly.

[00:03:49]As the mixture sits, the oil rises to the top and the water drains to the bottom, carrying with it the white silica particles. This happens because the surfaces of the silica and the copper sulfide have differing affinities for water. The silica is naturally hydrophilic. It loves water. So, water clings to these particles and they sink happily into the water phase. Copper sulfide, on the other hand, has a sulfide surface that is naturally hydrophobic. It repels water and so stays in the oil phase. So once the water and oil separate, we have a separation of the copper sulfide and silica. Oil and black copper sulfide at the top and water and silica at the bottom. The earliest methods used just this technique. Mix the pulverized rock with a thin layer of oil, mix that with water, then skim the particles from the top. But this used far too much oil to be practical to separate hundreds of tons. Yet this exploitation of differing surface affinities for water led to a way to bring the copper sulfide to the surface on its own. A way that used much less oil. The method was simplicity itself. Pass air bubbles through the liquid. These thousands of bubbles, this chaotic action is extracting the copper sulfide. To see how they do this, let's zoom in and slow down the motion. Look at this set of bubbles. Between them is a black mass, one of the pulverized pieces of copper sulfide. There's a special ingredient that causes the copper sulfide to merge with the bubbles, a molecule called a collector.

[00:05:28]The collector binds to the copper sulfide surface through a polar end containing sulfur and oxygen. The other end is a long hydrocarbon tail that repels water, making the carbon sulfide particle strongly water repelling.

[00:05:44]That's the key. Water repelling particles naturally attach to air bubbles. They prefer the air water interface to being surrounded by water.

[00:05:53]This process is done on an industrial scale. At some mines, as much as 150,000 tons a day. Each of these is a froth flotation unit. In each tank, a slurry of pulverized ore containing bits of copper sulfide flows into the tank. At the same time, air is blown in from the bottom. The air passes through a spinning impeller with small holes so that the air enters the tank as bubbles.

[00:06:22]These bubbles collect the copper sulfide and rise to the top where they gather as foam. The slurry stripped of copper sulfide exits his waist while the foam flows over the edge of the tank where it's collected. Once you have this copper rich foam, you can put it through a furnace much as you did with a copper rich rock to create copper that is about 99% pure. Now, this isn't pure enough.

[00:06:46]Most copper is used in electrical applications. Copper's conductivity drops dramatically with impurities. So, you need copper at least 99.9% pure. So, there's one final step needed to refine the copper. The impure molten copper is cast into a flat plate which is used as an electrode. These electrodes are about 1 m by 1 meter and about 50 mm in thickness. They weigh about 500 kg. To show you how they're used, I've created a miniature version of that electrode.

[00:07:19]That electrode of impure copper is paired with a stainless steel electrode of the same size. The key idea is to transfer the pure copper to the steel electrode and to leave behind the impurities. To do that, these electrodes are covered with a solution of copper sulfide dissolved in water. This creates a sea of copper ions around the electrodes. Next, we create a voltage between these two plates by connecting them outside of the solution with a battery. This causes electrons to flow from the copper electrode to the stainless steel electrode. The electrons leaving the electrode on the right come from the pure copper. The removal of the electrons causes the solid copper to dissolve into solution as an ion. At the same time, the electrons that enter the steel electrode combine with a copper ion in solution to create solid copper.

[00:08:13]As you see in this time lapse, the copper and the impure electrode dissolves.

[00:08:24]When we drain the water solution, we see the remains of the electrode, the impurities. Recall that the other electrode began as bare stainless steel, but is now covered with a layer of very pure copper. The copper is astonishingly easy to remove from the stainless steel electrode. On this electrode, I'm going to open up a gap right here. When I flex the copper covered electrode slightly, notice the gap here. It's enough that with a blade, I can cleanly peel the copper away.

[00:08:54]And that's no accident. Stainless steel was chosen precisely because this separation is easy. The surface of the stainless steel is covered with a thin, stable layer of chromium oxide. The copper is sitting on the surface rather than fused to it. Industrially, robotic arms bend the plate slightly, then drive hydraulic knives between the copper and the electrode surface to peel off copper sheets. These stacks of copper are sold to fabricators who roll or draw them into wire, sheet, or plate. The bare stainless steel cathode goes straight back into the tank house and is reused hundreds of times. Now, the extraction of copper from rocks is only one source of the copper we need for the future.

[00:09:37]The other is the copper we've discarded.

[00:09:40]The largest copper bearing waste is discarded electronics, followed by copper from the demolition of buildings.

[00:09:46]industrial waste, motors, generators, switch boxes, and junked vehicles. For electronic waste, the copper is recovered by dissolving the metallic components and acid. For high-grade scrap like wire and cable, which is over 99% pure copper, it's mostly physical separation.

[00:10:05]Every year, over 10 million tons of this copper bearing scrap is created. Right now, a bit under 50% of the copper in that waste is recovered. But for our energy future, we'll need to increase that. Thanks for listening. I'm Bill Hammock, the engineer guy.