Copper Metal from Malachite circa 4000 B.C.E

In the Laboratory

Copper Metal from Malachite circa 4000 B.C.E.

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Gordon T. Yee* and Jeannine E. Eddleton Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; *[email protected] Cris E. Johnson Department of Chemistry, University of Colorado at Colorado Springs, Colorado Springs, CO, 80933

In an effort to use chemical technology known to ancient civilizations as a focal point for teaching general chemistry, we have examined the feasibility of the laboratory production of copper metal from ore utilizing techniques that are consistent with the time period of around 4000 B.C.E., the beginning of the Copper Age (1). One can imagine that smelting of copper ore might first have occurred accidentally when someone threw green rocks into a fire or pottery kiln on a windy day1 and came back the next morning to find gleaming metal. Inspired by this idea, we have worked out a procedure that starts with an inexpensive and readily available and identifiable mineral, malachite, Cu2CO3(OH)2, which can occur in nature in essentially a pure state. It can be easily roasted in a crucible to the metal oxide, CuO, without the evolution of toxic gases and at modest temperatures attainable with an ordinary Bunsen burner. It can then be reduced with a reagent that was available to ancient civilizations, charcoal, again with a Bunsen burner to give a monolithic product that is unmistakably identifiable as metallic copper and that can be easily tested for malleability and electrical conductivity. The lab can be completed in three hours, requires no specialized equipment, generates no hazardous waste except possibly a small quantity of carbon monoxide, involves no hazardous reagents, and yields a product that the students can take home or utilize in additional experiments. Other authors have recently published procedures in this Journal also aimed at introducing metallurgy into the undergraduate laboratory. For instance, Rodriquez and Vicente have described a complementary procedure for producing copper metal from a mixed copper sulfate and basic copper sulfate ore (2). Their technique, which relies on water solubility differences to separate the former from the latter and uses iron metal (nails) as the reducing agent for copper sulfate, is reminiscent of technology that the Romans were using much later, after the beginning of the Iron Age ca. 1200 B.C.E. Copper sulfate is an unacceptable starting material for the present procedure because roasting would liberate SO2 gas. Similarly, O’Klatner and Rabinovich have recently reported the laboratory preparation of antimony metal from Sb2O3 and potassium cyanide (3).

about 0.8–1 g per student.4 Larger samples can be utilized, but these may take longer to react. More conveniently, malachite is also available in bead form from jewelry stores.5 A one-gram sample of reagent grade basic copper carbonate can be used, but a slightly modified procedure must be employed (see below). The roasting step can also be skipped if reagent grade copper oxide is used as the starting material. Charcoal is made from hardwoods that have been heated in the near absence of oxygen to remove volatile organic compounds. For our purposes, we are treating charcoal as pure carbon and it serves as the reducing agent (reductant) in the second step, which is a redox reaction. The oxidation state of copper changes from +2 to 0. It is unclear whether carbon is oxidized all the way to carbon dioxide (as we have assumed in the balanced reaction below) or only to carbon monoxide. It is likely that a mixture of the two is produced.

Step 1. Roasting Cu2CO3(OH)2(s)

2 CuO(s) + CO2(g) + H2O(g)

The malachite sample, in the form of an 8-mm spherical bead (Figure 1) is weighed and transferred to a 15-mL porcelain crucible. When working with a bead, it is better to use a weighing boat rather than weighing paper to contain the sample because the bead can roll away. The crucible is covered with a lid and placed in a ceramic triangle on a ring stand. A Bunsen burner is used to roast the sample for 15 min. The height of the ring should be adjusted such that the tip of the inner blue cone of the Bunsen burner is touching the bottom of the crucible. Caution: at the beginning of the heating, the sample can sometimes behave like popcorn and jump out of the crucible as the hot gases escape, so it must be covered. After this period, the heat is removed and the crucible is allowed to cool for 15 min, yielding copper(II) oxide (Figure 1). The sample is reweighed and the mass of the CuO is recorded. The percent yield for the roasting step is calculated. Note that the bead is brittle at this point, so extra care should be taken when handling it.

Experimental Procedure The starting material is the green mineral malachite, composed of basic copper carbonate, Cu2CO3(OH)2, (4) and closely related to the blue mineral azurite,2 Cu3(CO3)2(OH)2. Because of their attractive colors, both of these compounds are much more important today as semiprecious stones used in jewelry rather than as sources of copper metal. Malachite is readily available at modest expense as polished pebbles from rock collector supply companies.3 If they are too large, the stones can be broken up with a hammer into sample sizes of www.JCE.DivCHED.org

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Figure 1. Three different beads showing the three stages of this reaction (l to r), the green malachite starting material, the black copper(II) oxide intermediate that is produced after roasting, and the final copper metal product after charcoal reduction.

Vol. 81 No. 12 December 2004

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Journal of Chemical Education

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In the Laboratory

Step 2. Reduction 2 CuO(s) + C(s)

Notes for Instructors 2 Cu(s) + CO2(g)

About 1兾4 of a charcoal briquette6 is placed into a folded paper towel and finely crushed with a hammer.4 The roasted CuO sample is returned to the crucible. The crucible is filled with crushed charcoal to within 1兾4 inch of the top and tapped gently to help settle the contents. The crucible is covered and heated on the Bunsen burner for 90 min. Caution: during this period, carbon monoxide gas might be produced. After this period, the heat is removed and the crucible is allowed to cool, covered, for 15 min. The excess charcoal is removed and the copper metal isolated (Figure 1). The product can be brushed to dislodge any charcoal particles adhering to the metal. Wiping the bead on a paper towel also works to remove the weakly adherent carbon layer. The product is weighed and the percent yield for the reduction step is calculated. If the sample appears red and chalky, it has not been reduced fully and should be reacted longer. Alternate instructions if starting with reagent grade powdered Cu2CO3(OH)2: The roasting step does not require modification, but for the reduction step, prior to returning the CuO to the crucible, the bottom of the crucible is lined with a thin layer of fine charcoal powder. This helps to keep the copper metal from sticking to the bottom of the crucible. The heating period is also extended to at least two hours to allow time for the sample to anneal to a monolithic state.

Step 3. Characterization Using the water displacement method, the density of the copper can be determined. Although this can be performed on a single bead, several samples can be combined to get a larger volume change to decrease errors. The literature value for the density of copper is 8.92 g兾cm3. Striking the copper with a hammer demonstrates its malleability. A piece of fine sandpaper (220 grit) or steel wool can be subsequently used to polish the flattened surface if it is not already shiny. Using a simple conductivity meter7 it can be shown that the product is metallic, that is, that it conducts electricity. Hazards The reduction step probably produces a small quantity of carbon monoxide. In trial runs, the carbon monoxide was undetectable with a home-style carbon monoxide detector installed two feet from the experiment. However, bench hoods or other adequate ventilation should be used. Typical Results A sample of malachite, in bead form, was weighed. Its mass was 1.160 g (5.244 mmol). Roasting yielded a CuO bead weighing 0.8292 g (10.42 mmol). This was subsequently reduced to a Cu bead weighing 0.6324 g (9.951 mmol), corresponding to a 95% yield overall. The average density measurement on a collection of six samples was lower than the true value, probably owing to the fact that the hole in the bead is not necessarily wet and the sample might have microscopic voids created as the gases escape. The sample was easily hammered and conductive.

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Occasionally the copper(II) oxide bead will delaminate or shatter, which makes it more difficult to handle in the subsequent step. This can happen spontaneously during the roasting or, because it is spherical, if it rolls away and falls to the floor. A small supply of pre-roasted CuO beads can be kept on hand so that the student can continue the experiment. The broken bead can be saved and its cross-section shown to the students to demonstrate that roasting is not just a surface phenomenon. Although we have not yet attempted this experiment in a guided-inquiry environment, it seems well suited. The students can be supplied with a pile of malachite or basic copper carbonate, a pile of charcoal briquettes, a hammer, paper towels, some crucibles, and Bunsen burners. After showing them an example of metallic copper that is the product of this reaction, the students are given free rein to figure out how it is done. Additional Experiments There are several other experiments that can be attempted in subsequent lab sessions that are related to the one described above.

Preparation of Bronze Bronze is an alloy of copper with a small quantity of tin that is harder than either pure element. An explanation for this observation may be found in an article by Geselbracht et al. (5). Adapting the present procedure, the charcoal reduction reaction can be performed with roughly 10% by mass of reagent grade tin(IV) oxide added to the CuO. SnO2 is the principal tin-containing mineral in nature and is known as cassiterite. The copper and tin oxides can be ground together with mortar and pestle, but then should be heated for at least two hours. For added complexity, the hardness and color of the bronze can be examined as a function of added tin. Preparation of Tin The smelting of tin can also be carried out using the procedure described here starting with reagent grade tin(IV) oxide. However, longer times are required for the reduction (∼5 h) and under these conditions, the tin melts, so the product consists of tiny beads of metallic tin. Electrolytic Purification The final step in the industrial purification of copper metal is an electrolytic purification. This process has been modeled in an experiment described by Osella et al. (6) using a copper alloy as starting material, but the crude copper synthesized in the present experiment can probably be used instead. Conclusion We have presented a convenient procedure for making copper metal from a readily available, naturally occurring mineral, malachite. The starting materials are inexpensive and convenient and more importantly, the procedure involves no


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In the Laboratory

hazardous reagents and produces no hazardous waste. It thus represents a safe and compelling introduction to metallurgy in the laboratory. Acknowledgment We thank Susan Eriksson from the geology department at Virginia Tech for the initial samples of malachite. W

Supplemental Material

Instructions for the students, data worksheet, and notes for the instructor, including answers to the pre- and postlab questions, are available in this issue of JCE Online.

Fine Facets, 888 Brannan Street, Suite 2172, San Francisco, CA 94103, (415) 255-0268. It should be possible to obtain them for less than $0.25 per bead, strung on fishing line. Comparably sized ellipsoid beads that we obtained from a local jewelry store worked equally well. A keyword search on “malachite beads” on the Internet yields several other possible suppliers. 6. We used Kingsford charcoal briquettes. “Match-light” type charcoal, which has been treated with a volatile and flammable material, should not be used. 7. This can either be a commercial digital voltmeter or one constructed from a battery and a few LEDs and resistors. See Katz, D. A.; Willis, C. J. Chem. Educ. 1994, 71, 330–332 and references therein for a design.

Literature Cited

Notes 1. Forced air, such as that supplied by a bellows, makes the fire burn hotter. Wind could have accidentally served this role. 2. We have found that this experiment does not work well with azurite, perhaps because our samples of azurite tended to be mixed with other minerals. 3. One can perform a keyword search “tumbled malachite” on the Internet. 4. A sheet of plywood can be used to protect the laboratory bench top. We have observed that materials broken up with a hammer sometimes flake apart upon roasting. 5. We have used 8-mm spherical beads from the company

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1. Craddock, P. T. Early Metal Mining and Production; Smithsonian Institution: Washington, DC, 1995; pp 122–155. 2. Rodríguez, E.; Vicente, M. A. J. Chem. Educ. 2002, 79, 486– 488. 3. O’Klatner, B. L.; Rabinovich, D. J. Chem. Educ. 2000, 77, 251–252. 4. Malachite Home Page. http://mineral.galleries.com/minerals/ carbonat/malachit/malachit.htm (accessed Aug 2004). 5. Gesselbracht, M. J.; Ellis, A. B.; Penn, R. L.; Lisensky, G. C.; Stone, D. S. J. Chem. Educ. 1994, 71, 254–261. 6. Osella, D.; Ravera, M.; Soave, C.; Scorza, S. J. Chem. Educ. 2002, 79, 343–344.


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Copper Metal from Malachite circa 4000 B.C.E

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