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Extraction of Metals from Ores February 20, 2014

Nick Caggiano !!!!!!!!!!!!!!!!CHEM 113M-003 TA: Michael Coco

Lab Partner: Tyler Boone

Table of Contents !!Introduction 3 ...........................................................................................................................

Procedure 7 ..............................................................................................................................

Results 9 ...................................................................................................................................

Discussion 11 ...........................................................................................................................

Conclusion 15 ..........................................................................................................................

References 16 ...........................................................................................................................

Introduction

The earth’s crust contains an abundance of metals such as aluminum, iron, calcium,

magnesium, and sodium . Copper metal, the focus of this experiment, is the 26th most abundant 1

element found in the earth’s crust, occurring at an an average of 55-75 parts per million . 2

However, the vast majority of metals do not occur naturally in their pure forms. Rather, they are

found in combination with other elements in mineral deposits. Ores are mineral deposits with

high concentrations of a desired metal that enable economic extraction of the metal. Common

ores include bauxite, corundum (aluminum), hematite, magnetite (iron), cuprite, bornite,

malachite (copper)1. Different ores are located in abundance in specific geographical regions of

the earth. For example, the most abundant supplies of bauxite are located in Australia, China, and

Brazil , whereas malachite is mined from locations including North America, Europe, Australia, 3

and southern Africa . 4

The study of metallurgy includes the process of separating and extracting metals from

their respective ores1. The ability to reclaim metals from ore is essential because the pure metals

are more valuable and useful than the ores from which they originate. Different metals have

various properties that make them important to both industry and consumers; for example,

aluminum is used in various applications from beverage containers to aircraft due to its low

weight and corrosion resistance, gold is used for jewelry but also in computers and electronic

devices, and copper is valued for its ability to conduct electricity when used in electrical wires.

Often, a pure metal may be found in several different ores, each of which may require a different

extraction method. Therefore, it is important to select the ore from which the metal may be most

economically and efficiently extracted . 5

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Although metal extraction techniques have become drastically more technically

sophisticated in modern times, evidence indicates that humans have been extracting metals from

the earth using comparatively rudimentary methods as far back as the year 6000BCE. The

earliest metals to be used by ancients, referred to as the Metals of Antiquity, were gold, copper,

silver, lead, tin, iron, and mercury . Some of those metals, such as gold, do exist in isolated form 6

on earth; therefore, gold mining essentially involved the finding and collecting of small gold

particles. However, the discovery that copper tools were used in ancient civilizations carries

more significance. Evidence suggests that ancients were able to smelt malachite ore in pottery

furnaces that were capable of reaching the necessary temperatures of over 1000 degrees celsius.

Additionally, remnants of copper sheets from the time period show that annealing, the process of

heating the copper and then cooling it slowly that strengthens the metal, had been discovered.

Today, the development of more technically sophisticated extraction methods has enabled

more metals to be extracted from their ores using more efficient methods, therefore allowing

society to benefit from higher availability and reduced price of common metals and has lead to

increased availability of rarer, more difficult to extract metals.

Modern extraction techniques may include a variety of methods based on the specific ore

and the intended metal to be recovered. The first step is to remove the ore from the earth. This is

usually carried out by mining (surface or underground based on the depth of the ore to be

mined). However, ores often contain multiple chemical species—some of which contain the

desired metal and some of which are not used in the extraction process. The desired chemical

species that will be used in the extraction of the metal must be isolated. Chemical or physical

processes can be used to carry out this separation, based on the composition of the ore. For

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example, the primary ore of aluminum is bauxite, which contains a mixture of hydrated

aluminum oxide and hydrated iron oxide . Since aluminum is found only in the aluminum oxide 7

part of bauxite, it must be isolated. This is achieved by placing the bauxite in a bath of

concentrated sodium hydroxide. The hydroxide ions combine with the aluminum to form the

soluble species ! , while the iron species remains insoluble and can be filtered out of the

solution. Other ores may be separated by physical means such as froth flotation , in which the 8

ore is treated with a chemical that binds with the desired components to make them hydrophobic.

The ore is then added to a water bath with a foaming agent, which creates bubbles that carry the

hydrophobic particles out of the bath. The undesired components of the ore sink and are left at

the bottom of the bath.

Continuing with the extraction of aluminum, the pH of the ! solution is lowered

by adding acid, causing aluminum hydroxide ( ! ) to precipitate, which is then heated to

form aluminum oxide ( ! ). The Hall-Héroult process7 is then used to reduce the aluminum

oxide to solid aluminum using electric current. The aluminum oxide is melted at temperatures

over 1000 degrees Celsius in a vat that has an iron cathode and a graphite electrode. Low voltage

but extremely high current electricity flows through the molten solution, and pure aluminum is

produced at the cathode, which sinks to the bottom of the vat and can be recovered. The Hall-

Héroult process is quite energy intensive, but it is necessary because unlike copper and certain

other metals low on the activity series, aluminum cannot be reduced by carbon. Reduction of a

desired metal by electrolysis is common (instead of or in combination with other methods) even

Al(OH )4−

Al(OH )4−

Al(OH )3

Al2O3

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in the extraction of metals that can be reduced by other methods, such as copper, because

electrolysis can produce a purer final product which is more valuable.

In this experiment, copper metal will be extracted from malachite ore. The process will

involve a primary roasting to isolate copper(II) oxide from the malachite and a roasting of the

resulting copper(II) oxide in the presence of solid carbon, which will reduce the copper(II) oxide

to elemental copper solid. The final extraction product will be tested using various methods to

confirm its identity and estimate its purity.

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Procedure

The procedure for this experiment, written by Christopher Ehret and Emily Wilts , was 9

followed exactly and in the specified order with one exception: pre-ground carbon powder was

used instead of being manually-ground by mortar and pestle. Any other deviations from the

prescribed procedure will be noted in this report.

First, two small pieces of malachite (combined mass approximately one gram or less)

were selected. Their combined mass was measured by an electronic balance and recorded, and

the samples were then placed in a ceramic crucible with the lid closed. A ring stand with a metal

triangle was situated above a Bunsen burner, and the crucible was placed in the metal triangle.

The Bunsen burner was lit, and the setup was adjusted such that the inner cone of the flame was

touching the bottom of the crucible. This heating, called the first roasting, was conducted for 15

minutes. After 15 minutes of cooling (5 in the closed crucible with the burner off and 10 with the

lid open) the samples were removed and massed using an electronic balance. This value was

recorded.

Next, a second heating was performed on the malachite samples, this time in the presence

of a reducing agent: carbon powder. In this experiment, commercially available, very finely

ground carbon power was used in place of the manually ground carbon stated in the procedure. A

layer of powder was placed in the bottom of the crucible, on top of which the malachite samples

were placed and then covered with additional carbon powder to fill the crucible to within roughly

a half inch of its top. The lid was then placed back onto the crucible and the sample was heated

for 90 minutes (with the ring/stand/burner setup in the same configuration as previously

described). During this extended heating period, three informational videos were shown in the

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lab describing the extraction processes of lithium, silver, and aluminum from their respective

ores. These videos were intended to provide a reference for the comparison of the extraction

method used for copper in this lab to methods used in large-scale industrial extraction operations.

A short worksheet was completed on the videos.

Returning to the ongoing heat process, the same cooling procedure as previously

described was used after the 90-minute heating period was completed. However, at this point the

small carbon metal pieces were still buried in the fine carbon powder. Physical separation of the

copper metal from the carbon powder was conducted manually on paper towels using tweezers.

The recovered pieces of copper metal were dusted with a paper towel and then massed using an

electronic balance. Finally, the final product was tested to verify that it was in fact copper metal.

The copper pieces were polished with steel wool and then tested with a multi-meter in order to

determine the substance’s resistance in ohms. The samples were additionally hit with a hammer

to verify that they had the same physical properties as copper (e.g. malleability). Percent yield

calculations were carried out using the experimental data.

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Results

Data obtained from the laboratory notebooks of Nick Caggiano and Tyler Boone . 10 11

Percent Yield Calculations

Relevant chemical formulae and reactions:

Malachite (predominantly): Cu2CO3(OH)2(s)

First Roasting: ! (Eqn. 1)

Second Roasting: ! (Eqn. 2)

! (Eqn. 3)

Using Eqn. 1:

!

(Eqn. 4)

Using Eqn. 2:

! (Eqn. 5)

Theoretical Yield of Cu(s) = 0.2364g

!

Overall reaction (Eqn. 1 + Eqn. 2)

! (Eqn. 6)

Table 1. Mass of Sample

Mass (grams) ∆ (grams)

Original (Malachite) 0.4113

After First Roasting (CuO(s)) 0.2975 -0.1138

After Second Roasting (Cu(s)) 0.2346 -0.0629

Cu2CO3(OH )2 (s)→ 2CuO(s)+CO2 (g)+ H2O(g)

2CuO(s)+C(s)→ 2Cu(s)+CO2 (g)

% Yield = 100 Experimental YieldTheoretical Yield

⎡⎣⎢

⎤⎦⎥

0.4113g Cu2CO3(OH )2 (s) 1 mol Cu2CO3(OH )2 (s)221.11 g

⎡⎣⎢

⎤⎦⎥

2 mol CuO(s)1 mol Cu2CO3(OH )2 (s)⎡

⎣⎢

⎤

⎦⎥ = 0.003720 mol CuO(s)

0.003720 mol CuO(s) 2 mol Cu(s)2 mol CuO(s)

⎡⎣⎢

⎤⎦⎥

63.546g Cu(s)1 mol Cu(s)

⎡⎣⎢

⎤⎦⎥= 0.2364g Cu(s)

% Yield = 100 0.2346g0.2364g

⎡⎣⎢

⎤⎦⎥= 99.24%

Cu2CO3(OH )2 (s)+C(s)→ 2Cu(s)+ 2CO2 (g)+ H2O(g)

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Observations

• The malachite was observed to be dark green in color and opaque.

• The intermediate (copper(II) oxide) was observed to be a very dark black color.

• The final product, copper solid (Cu(s)), was observed to be generally copper-colored, but it

was dull gray in some spots.

Verification of final product:

• The sample was also hit with a hammer. It was observed that the metal did deform slightly as a

result.

Table 2. Resistance

Resistance Value (Ω)

Trial 1 0.2

Trial 2 0.1

Trial 3 0.1

Average 0.1

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Discussion

From the results, it was apparent that the mass of the sample decreased after each

roasting. This is supported by Eqn. 1 and Eqn. 2. In Eqn. 1, which describes the reaction

occurring during the first roasting, malachite is decomposed into copper(II) oxide, carbon

dioxide gas, and water vapor. The mass of the sample decreased by 0.1138g (Table 1) because

the carbon dioxide and water vapor gases produced during the decomposition dissipated and

therefore were not taken into account in the measured mass. The mass measured primarily

copper(II) oxide (and potentially any unreacted malachite). During the second roasting (Eqn. 2),

the mass decreased yet again because carbon dioxide gas is evolved as a result of the redox

reaction that reduces copper(II) oxide to Cu(s) with copper powder as the reducing agent.

The experiment supports that these two reactions are endothermic. The malachite would

not decompose at room temperatures, and likewise the redox reaction that produced solid

elemental copper would similarly be prevented at room temperature by an energy barrier.

Therefore, the Bunsen burner was needed in order to provide the necessary energy for the

reactions to occur.

The resulting product after both roastings was tested to verify that is was copper. The

resistance values (Table 2), as measured by a multimeter, were quite low, with an average of 0.1

ohms. This suggests that the final product was primarily copper; however, although resistance

depends on the size and cross-sectional area of the sample, the actual resistance of the sample—

if it were 100% pure copper—could reasonably be expected to be less than 0.1 ohms based on a

published resistivity value of 1.68 x 10-8 Ω•m . However, the accuracy of the resistance 12

measurement was limited by the precision of the multi-meter, which only reported resistance

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values to the nearest 0.1 ohm. The duller color of the sample also appears to suggest that the final

product is not pure copper. Pure copper would be shiny and copper-colored instead of the tinge

of gray observed.

The percent yield for this experiment was calculated to be 99.24%. However, the

observational evidence that suggests the final product is not pure copper also suggests that this

percent yield calculation has been skewed. The percent yield could have been affect by a variety

of causes. A first hypothesis might be that excess carbon powder clung to the copper sample

pieces, therefore increasing their mass and artificially inflating the experimental yield

measurement. However, the copper pieces were dusted and cleaned until they were visibly clean.

There could (and very likely was) some amount of carbon on the surface of the copper, but it was

only a trace amount. With such a small amount of carbon, and considering the fact that the

weight of carbon is less than that of copper metal, it is unlikely that the carbon powder particles

caused an appreciable increase in the experimental yield. A more likely cause of the unusually

high percent yield calculation is that one or both of the roasting reactions did not go to

completion. In both Eqn. 1 and Eqn. 2 (first and second roastings, respectively), the reactants are

heavier than the product that was massed (i.e. the malachite is heavier than the copper(II) oxide,

which is in turn heaver than elemental solid copper). Therefore, if some of the reactant was not

transformed to product during the reaction, the resulting measured masses, although lower than

the initial mass of the reactant because some of the reactants in both reactions were converted to

gaseous species, would be greater than the mass if the product were pure. The argument that one

or both of the reactions did not go to completion would also corroborate the observations

(resistance, visual appearance) that suggest that the final product contained impurities.

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Adaptation of Extraction Technique for Silver Metal

A procedure similar to the one used in this experiment to extract copper metal from

malachite ore could be adapted to extract silver from acanthite ore. Acanthite is a silver sulfide

(Ag2S) mineral that is the stable form of argentite at temperatures below 180 degrees Celsius,

and it is an important ore of silver . It is dark gray to black in color. The first step in the 13

extraction of silver from acanthite would be to select pieces of the mineral that have a combined

mass of around 1 gram. After measuring and recording that mass, the silver pieces should be

placed in a ceramic crucible, which will then be placed in a metal triangle on a ring stand. A

Bunsen burner should be placed underneath the crucible such that the hottest (inner) part of the

flame is touching the crucible. For this extraction, the lid of the crucible should be left slightly

ajar. The acanthite should be heated for 20 minutes, and it should undergo the following

chemical change as a result of the roasting : 14

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It is important that this process is done in a well ventilated area due to the release of some

sulfur dioxide gas. After the 20 minutes has been completed, the Bunsen burner should be turned

off, and the crucible should be allowed to cool for 5 minutes before it is transferred (using heat

gloves) to the lab desk to cool for another 10 minutes (remove the lid). Remove the silver oxide

pieces using tweezers and record the combined mass.

A second roasting process, this time with carbon as the reducing agent, needs to be

carried out to reduce the silver oxide to elemental silver solid. A layer of pre-ground carbon

powder should be placed in the bottom of the crucible. The silver oxide particles should be

placed on top of this bottom layer and then covered with more carbon powder to fill within

2Ag2S(s)+ 3O2 (g)→ 2Ag2O(s) + 2SO2 (g)

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roughly 1-2 cm of the top of the crucible. With the lid on, heat the crucible with the same setup

as previously described, this time for 105 minutes (1 hour, 45 minutes). The chemical reaction

that will occur during this process is described by:

!

After the heating period has completed, the crucible should be cooled using the process

previously described (total of 15 minutes). The pieces should be separated from the carbon

powder by dumping the entire contents of the crucible onto a paper towel and then manually

removing the silver pieces using tweezers—the mass should be recorded after any excess carbon

powder is dusted off. From this mass of the final product, a percent yield may be calculated.

Potential challenges to this extraction may include the release of the sulfur dioxide gas

during the first roasting process and determining the length of time necessary for each reaction to

go to completion. More thermodynamic analysis on these reactions could be conducted to predict

whether this lab experiment would be energetically feasible using energy sources available in the

lab (i.e. Bunsen burner). Since the process is similar to that which was used to extract copper

from malachite, it might be reasonably expected that this set of similar reactions might also be

feasible in a laboratory. Other possible follow-ups would include the verification of the identity

of the final product. A multi-meter could be used to measure the resistance of the sample

obtained to determine whether the measured resistance is close to that which would be predicted

for silver metal (it is a good conductor of electricity and therefore should have low resistance).

Visual inspection of the final product would also aid in qualitatively estimating its purity.

2Ag2O(s)+C(s)→ 4Ag(s) +CO2 (g)

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Conclusion

In this experiment, elemental copper metal was extracted from malachite ore using a

roasting process that first facilitated the thermal decomposition of the malachite to copper(II)

oxide and second reduced the copper from copper(II) oxide to elemental copper using carbon

powder as a reducing agent. It was concluded that the final product was at least partly copper, as

the resistance of the sample were quite low and the sample was malleable. However, dull spots

on the final product seemed to indicate that it was not completely pure. Although the percent

yield was nearly 100 percent, this number could be misleading because it only took into account

the mass of the final product—not its composition. One possible explanation for the potentially

artificially large percent yield value is that one or both of the reactions did not go to completion,

leaving higher-mass reactants in the final product. A remedy for this problem could be to extend

the heating times for both the first and second roastings. That extended time could allow a larger

quantity of the reactants to react to form products, which could increase the purity of the final

copper product and potentially result in a more accurate percent yield calculation based on mass.

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References

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! Burdge, Julia. Chemistry: Atoms First. New York, NY: McGraw-Hill, 2012. Print.1

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March 2014)

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! “Malachite” http://www.mindat.org/min-2550.html (accessed March 2014)4

! “An Introduction to the Chemistry of Metal Extraction” http://chemwiki.ucdavis.edu/5

Inorganic_Chemistry/Descriptive_Chemistry/Transition_Metals/Metallurgy/An_Introduction_to_the_Chemistry_of_Metal_Extraction (accessed March 2014)

! “A Short History of Metals” http://neon.mems.cmu.edu/cramb/Processing/history.html 6

(accessed March 2014)

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Inorganic_Chemistry/Descriptive_Chemistry/Transition_Metals/Metallurgy/An_Introduction_to_the_Chemistry_of_Metal_Extraction (accessed March 2014)

! Ehret, Christopher and Wilts, Emily. “Copper Metal Extraction from Malachite”9

! Caggiano, Nick, Chem 113M Laboratory Notebook, Spring 2014, pp. 15-17.10

! Boone, Tyler, Chem 113M Laboratory Notebook, Spring 2014, pp. 12-13.11

! “Resistivity and Temperature Coefficient at 20C” http://hyperphysics.phy-astr.gsu.edu/hbase/12

tables/rstiv.html (accessed March 2014)

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2014)

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HO2.Smelting.As.pdf (accessed March 2014)