Out of Whose Hills You Can Dig Copper - Pt I

The mines in Timna are reported to be King Solomon's Mines. The area is full of beautiful blue and green copper ore, which was known even to ancients as a valuable asset for making bronze and so forth. The title of this blog article is from Deuteronomy, where God promises this to the Israelites. Copper is still very valuable. Valuable enough that pennies are mostly zinc now, people steal copper wiring from walls, and vast operations extract ore from the earth. As a collector, the intense blues and greens are irresistible. And as a rockhound, they are easy to find and identify given the colors!  Malachite, the primary ore of copper, is known for taking a beautiful polish to expose its bubbly bands of light and dark greens. If you have the time, look up the Malachite Room in the Winter Palace of St. Petersburg, Russia.

Malachite is a copper carbonate. Its molecular formula is Cu2CO3(OH)2. In this experiment we will try to extract pure copper from malachite ore. I was too proud of my own found specimens to destroy them (yet), so I bought some bags of polished pebbles on Ebay. For starters, I dumped out 24g of this gem - and then crushed it with a hammer into small pieces. I didn't want to pulverize it completely as that is unnecessary and inhaling copper ore dust can be dangerous. This is what it looked like before and after:

Now, we could smelt the copper out with a furnace, like people have done for thousands of years, but I want 99.x% pure copper and a high yield using a chemical process.

Cu2CO3(OH)2 has a molecular weight of around ~225g/mol, of which ~127g is copper (or ~56% of the ore's mass). With 24g or ore being processed, the best we could hope for for perfectly pure malachite with no impurities (wrong, I can see bits of quartz and manganese oxides already!) is a yield of a little more than 13g. Even if I process it all perfectly, I'll probably lose microscopic bits in my container and filters. So my goal is a relatively reasonable 10g or more.

This process is not hard at all, but I made some mistakes the first time and failed miserably. My "copper" was mostly iron filings that I added later. The "copper" tested successfully with a propane torch - it gave off the green flame I wanted. However, something wasn't quite right and when I put a magnet near it, I knew my copper (which does look grey/black after being oxidized by flame) was almost all iron. I had to start over, so it was smart not to use up all of my ore at once.

(If you want to see some of my beautiful mistakes, look at the pictures from my Hello World post)

The first reaction we want (which created all sorts of odd internet arguments about the balanced equation, reaction type, and by-products) is as follows:

Cu2CO3(OH)2 + 4HCl -> CO2 + 3H20 + 2CuCl2

2 coppers, one carbon, 5 oxygens, 6 hydrogens, and 4 chlorines on each side

Solving for the oxidation numbers, I can calculate that the copper is 3+ on the left and reduced to 2+ on the right.

We will now make carbon dioxide gas, water, and something called Copper(ii) Chloride. This is fairly easy, dump about 50ml of 20 baume (31.5%) hydrochloric acid (HCl) into the container. Instantly it fizzes like crazy and turns to a very dark green liquid in almost no time. 

This almost black liquid is water from diluted acid, water created by the reaction, and a lot of copper(ii) chloride. The latter is some nasty stuff, and I would not get it on too much if I were you. The turquoise-colored crystals on the side of my tub above are copper(ii) chloride dihydrate. By the way, turquoise is another pretty copper mineral, a phosphate instead of carbonate. Here is another view of such residual crystals from my erstwhile experiment:

Back to the dark green liquid. Note that if I had added 100ml or so water to make it a ~10% solution of HCl, it would be a lighter color. This time I just forwent caution and skipped the water. The down side of that is that I could not see well into the dark liquid. That would be a problem later. In the mean time, I discovered a fringe benefit of keeping the water content down. Without any heat or other stimulus, I was shocked to see a dense cap of crystallization quickly form over the solution. These are hydrated copper (ii) chloride crystals that have a higher concentration of chlorine. The first photo shows the crust, and the second zooms in on a bit that I broke and flipped over to see crystals that were growing down into the solution.   

Ok, we are now ready for the second reaction that yields copper. Please read Pt II for that.

Thanks for reading,

Paul

p.s. It has come to my attention that some people would like to repeat these experiments without the hassle of finding the chemicals and/or having to buy too much, etc. I will try to keep an Ebay store alive (seller: cinnabarminerals) that offers low cost and related material for the purpose of enabling education. I will title offerings "Brave New Chemist Science Pack - (related Blog post name)." You can always comment and ask where to buy certain equipment too.