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Introduction Hard-rock mining has been a considerable force in the West, influencing the history, politics, environment, and economy of the region. As I have included mining topics in my freshman Earth Science course, I have discovered that students have many misconceptions about how metals are removed from rock. For one, few realize that the metal is usually "locked up" in mineral compounds that are very different from the pure metal, just as sodium is very different from sodium chloride. Secondly, few understand the extent to which chemistry is involved in removing the metal from the compound. Purpose This is a convincing demonstration that will show students one way that chemistry is used to extract a metal from an ore. How to do the demonstration 1. You will need a small piece of malachite (about 1 cubic inch), a greenish mineral whose formula can be written as Cu2CO3(OH)2 or CuCO3 . Cu(OH)2 . This can be purchased from science supply companies (10 pieces for under $10). As with most copper ores, the malachite does not contain pure copper (native copper), and will contain other minerals besides the malachite. Malachite, one of several minerals mined to obtain copper, is mined primarily in Arizona, the leading copper-producing state. 2. As with most real mines, the first thing you will need to do is crush the ore. One way to accomplish this is to take the lid cut from a coffee can, bend it in half like a taco shell, put the malachite inside, and then place it into a sandwich baggie. Lay the baggie onto some pavement, and gently pound with a hammer until the ore is crushed. Be sure to wear goggles. At an actual mine site the ore may be put into huge rotating cylinders containing steel balls that roll around , crushing the ore (called "ball mills"). 3. At your demonstration site set up a ringstand with a funnel, containing a coffee filter, and then empty the crushed ore into the filter/funnel. 3. Next you will need two 250 ml. beakers, one containing 75-125 ml. of 1 molar H2SO4 . Position the empty beaker beneath the funnel, and then carefully pour the acid over the crushed ore. Be sure to wear goggles. Once it has leached through, switch the beakers and repeat the process one or two more times until the solution becomes sky-blue. The blue color is due to the presence of Cu2+ ions which were put into the solution as the acid reacted with the malachite (a neutralization reaction). Part of what happened is shown in this equation: 4. Next, remove the solution from beneath the funnel and put the empty beaker in its place to catch the remaining drops. Finally, take several iron nails that have been cleaned with steel wool and place them into the leachate solution. You will want your students to be close by as you do this so that they can observe the nails as they become coated with copper. Use forceps to lift them out of the solution and show the brilliant copper color. The mining industry uses scrap iron, such as discarded cars, for this process. Chemically, what is happening is that the copper ions are "stealing" electrons from the iron atoms as shown in this ionic equation: 5. The demonstration leaves you with two waste products. . . the acidic copper sulfate solution and the crushed ore, or tailings. Ask your students how these mine wastes might be dealt with so that this does not become a future Superfund Site. Disposal Set the beaker of leachate beneath the funnel and pour 50 ml. of tapwater over the tailings to rinse the acid off somewhat. Then take .5 square foot of aluminum foil, wad it up, place it in the leachate solution, and let it set overnight. The copper ions will be reduced, forming copper metal, and the aluminum ions will go into the solution in their place. The next day, remove any undissolved foil. Now what you have is a combination of aluminum sulfate and sulfuric acid, with a copper sludge. Your science department should have a copy of the Flinn Chemical Catalog/Reference Manual. Follow Flinn's guidelines for disposal of these materials. Remove the filter cone, containing the tailings, from the funnel, put it in a baggie, and then place it in the garbage. Etc. In Montana, most of the copper ores are "sulfide ores" containing compounds such as Cu2S (chalcocite) or CuFeS2 (chalcopyrite). Processing these ores requires additional steps to deal with SO2 emissions and concerns related to acid mine drainage. |
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