The extraction of metals

A. Which two elements are usually combined with the metals?

Oxygen and sulfur

B. Give an example of metals found uncombined

Gold or Silver.

A. To remove the impurities what reation takes place?

Reduction reaction.

B. In metal oxides and sulfides what oxidation state does the metal have?     (positive or negative) 

 Positive. Reduction is gain of electrons, the metals become more negative because as elements all metals have an oxidation state of 0.

A. What are the other unwanted components when metals are extracted?

Materials like clay and rock. 

B. What is the scientific name for these materials?

Gangue

C. An ore must contain enough ______ to be ________ enough to mine and _______. If the metal is _______ the ore could be worth mining even if it contains ______ of the metal.                                                                                                                         

                       valuable     little    economic     metal     extract

A. Before reduction sulfide ores are converted to oxides by what process?

Roasting 

B. How is this done? 

By heating the metal sulfide in air. 

C. State the reactants and products of this process involving either Cobalt or Zinc.

ZnS(s) + 1.5O2 ® ZnO(s) + SO2(g)    Or      CoS(s) + 1.5O2 ®  CoO(s) + SO2(g)

A. What is the by-product of roasting?

Sulfur dioxide

B. What is the environmental issue with the by-product?

Its a contributer to acid rain.

C. How is this issue controlled.

The sulfur dioxide is collected and converted into sulfuric acid in controlled conditions. This can be sold, making the industrial process more economical. 

A. What is the reducing agent most commonly chosen for the extraction of metals?

Coke (an impure form of carbon)

B. How is this obtained?

Cheaply, by heating coal in the absence of air.

C. Issues with this reducing agent? 

For some metals the temperature required to react with the carbon is so high it is uneconomic, Also, at high temperatures, reactive metals will react with the carbon and form carbides

D. Which metals are reduced by this?

Iron, Manganese and Copper (are examples)

A. What may be another possible reducing agent? 

Hydrogen OR  reactive metals may be reduced by electrolysis OR a more reactive metal.                                                                                                                                 

B. Which metal(s) can be reduced by your chosen reducing agent?

Hydrogen - Tungsten,  Electrolysis - Sodium and Aluminium, More reactive metal - Titanium.                                                                                                                     

C. What is the most commonly used metal? What are its ores?

Iron. Magnetite (Fe3O4) and Haematite (Fe2O3)                                                         

D. What is its major impurity?

Silica, SiO2 (sand)

A. Coke is used in the blast furnace to extract iron, but what is the reducing agent?

Carbon Monoxide.

B. Is the reduction exothermic or endothermic?

Exothermic. (temperature is around 2000K)

C. What is the hopper charged with?

A mixture of iron ore, coke and limestone (calcium carbonate)

A. Some copper ores are ______ into solutions containing ____. This can be done by ______ copper mining waste with ______ acid in the presence of a ______ (Thiobacillus ferrooxidans) 

dilute      converted     spraying     Cu ²⁺ ions      converted     bacteria 

B. Why is this process efficient? 

The bacteria will work on low grade ores that contain little copper.

C. The copper is extracted from the solution by reduction using scrap iron. Why is this economically viable? 

The price of copper is expensive compared to that of iron. Scrap is relatively cheap and available at the moment. Also, considering there is no CO2 produced this process is also environmentally acceptable. The process uses less energy than reduction of copper by carbon.

A. What is aluminium extracted from?

Bauxite ore (mostly aluminium oxide)

B. What is the process by which aluminium is extracted by?

The Hall-Heroult process.

C. What is the Bauxite ore dissolved in?

Molten Cryolite. (Na3AlF6)  To form a solution to be electrolysed

D. What is produced at the negative electrode? What is at the positive electrode?

Positive (made of carbon): Oxygen           3O_{2  ®}  1.5O₂ + 6e^-                             Negative (the steel casing of the cell): Aluminium  2Al³⁺ + 6e^- ® 2Al                      Overall reaction: Al₂O₃  ® 2Al + 1.5O₂

E. How is the solution kept molten?

By the heat generated by the [assage of current. (the process is continuous).

F. The ______ is formed as a _______ and _______ (separated) off, while the _______ burns the carbon _______ away to carbon dioxide. Due to this, the carbon ______ have to be replaced regularly.

      electrodes       aluminium      siphoned      electrodes      oxygen     liquid

G. What is the main cost of this process? 

Electricity

F. How can we reduce this cost?

By using the process in countries where cheap hydroelectricity is available (eg. Canada)

A. Why is Titanium an excellent constructional material?

It is strong, has a low density, resistant to corrosion and is relatively abundant. 

B. Why can't Titanium be reduced by carbon?

The formation of titanium carbide makes the metal brittle.

C. What process is used instead?

Reduction using a more reactive metal (sodium or magnesium)

D. What kind of process is this, batch or continuous?

Batch

E. Why is this less efficient?

The reaction vessels have to be heated back up to optimum operating temperatures after each batch is removed. 

F. What are the two steps in this process? Describe.

Step One: Conversion of titanium oxide to titanium chloride.                                      Rutile (the ore, mainly titanium oxide) is converted to titanium chloride by reaction with coke and chlorine at 1173 K. The liquid titanium chloride is purified by distillation.

Step Two: Reduction of titanium chloride.                                                                    TiCl4 is reduced with molten sodium under an inert (unreactive) argon atomosphere at 1300K. The argon prevents the metals from reacting with nitrogen and oxygen in air. 

A. Tungsten is reduced by hydrogen, why is it only used as a reducing agent at a last resort?

Hydrogen is a flammable gas and for that reason is not very practical. 

B. Uses of Tungsten:

In light bulb filaments, due to its high melting point. 

C. Why is Tungsten like Titanium?

Tungsten also reacts with carbon to form a carbide.

A. Why are metals relatively straightforward to recycle?

They can be melted down and reformed into a new item.

B. Why is recylcling economically and environmentally better than extraction, in terms of iron? 

• reduces the scrap iron that is discarded in landfill sites
• the scrap iron has already been extracted from its ore. Its also easily separated from other materials as it is magnetic.
• melting scrap metal does not in itself produce carbon dioxide whereas extraction does. However, the energy needed to melt the metal does involve the production of carbon dioxide.

A. Name another metal which is mass recycled.

Aluminium (eg. cans)

B. Economic and environmental reasons why we recycle this metal.

• only uses about 5% of the amount of energy needed to extract aluminium from its ore.
• avoids the production of large amounts of carbon dioxide.