S6: Chemistry

UNIT 2: EXTRACTION OF METALS

Key Unit Competence

To be able to: Relate the properties of metals to their methods of extraction and uses and suggest preventive measures to dangers associated with their extraction.

Learning objectives

At the end of this unit , students will be able to:

•  Describe the extraction of copper, iron, sodium, tantalum, zinc, wolfram and aluminium;
• Outline and explain the uses of copper, iron, tantalum, zinc, wolfram, and tin ore (cassiterite);
• Explain the issues associated with the extraction of metals and preventive measures;
  
• Relate the properties of metals to their methods of extraction.

Introductory activity

1. a. Do you know any metal and mineral extracted in Rwanda?

b. If yes, name them? 

c. Give the applications of those metal and mineral .

2. Most of the metals are found in nature, not as pure metals, but as compounds, i.e. combined with other chemical elements. Such metals are extracted from their compounds using chemical reactions. The following setup shows one example of a laboratory chemical reaction. Analyze it and follow the procedure to be able to interpret the results.

Procedure

                1. Transfer one spatula measure of copper (II) oxide to a hard-glass test-tube.  

               2. Carefully add one spatula of charcoal powder on top of the copper (II) oxide without any mixing.  

               3. Strongly heat these two layers for 5 minutes in a Bunsen flame.  

              4. Allow the tube to cool and then look closely at where the two powders meet in the test tube

Questions

                1. Describe the solid copper (II) oxide before heating.

                2. Name the gas formed in the reaction. 

                 3. Describe the solid remaining after heating. 

                 4. Name the solid formed in the reaction. 

                 5. Write the word and symbol equations for the reactions. 

                  6. What does this reaction tell you about the relative reactivities of carbon and copper? 

                   7. Explain why this reaction is a redox reaction.

The chemical substances in the earth’s crust obtained by mining are called minerals. Minerals, which are source of metal economically needed, are called “Ore”. The unwanted impurities present in ore are called gangue. A native metal is any metal that is found in its metallic form, either pure or alloyed, in nature (example: Gold). The entire process of extraction of metal from its ore is called metallurgy.

Many metals are found in the Earth’s crust as ores. An ore is usually a compound of the metal mixed with impurities. An ore is any naturally-occurring source of a metal that you can economically extract the metal from. Most metals exist in in nature as compounds, usually oxides or sulphides.

Sulphide ores cannot be converted directly into the metal. Instead they must be converted to the oxide. This is achieved by roasting them in air. Roasting involves heating of ore in presence of air below melting point of metal in reverberatory furnace.

Reverberatory furnace, in zinc, copper, tin and nickel production, is a furnace used for smelting or refining in which the fuel is not in direct contact with the ore but heats it by a flame blown over it from another chamber. In steel making, this process, now largely obsolete, is called the open-hearth process.

In this process, volatile impurities escape leaving behind metal oxide and sulphur dioxide (metal sulphide is converted to metal oxide).

This process causes problems because of the large quantity of sulphur dioxide produced. Sulphur dioxide is one of the principal causes of acid rain; hence SO2 is one of the air pollutants.

However if the sulphur dioxide can be collected before being released into the atmosphere, it can be used to make sulphuric acid.

Metals in their compounds are in oxidized form, i.e. have lost electron(s) and bear a positive charge. In order to get them as metals, they need to gain electrons: this process or reaction of gaining electrons is called “reduction”. Reduction of metals can be carried out using a chemical reducing agent such as carbon (coke, charcoal) or electricity. 

When the mineral is dug up, a method must be used to separate the metal from the rest of the ore. This is called extraction of the metal.

2.1. Relating the properties of metals to their methods of extraction

Activity 2.1

Make a research (in books or internet) in order .to:

   1. Find different methods of extraction of metals referring to the reactivity.

    2. Explain why the ores have to be concentrated and different ways that can be used.

2.1.1. Methods of metal extraction according to their properties

A number of methods are used to extract metals from their ores. The best method to use depends on a number of factors. These factors are based on the properties of the metal.

The main factor here is the reactivity of the metal (the position it takes in the reactivity series). Let us use the following question method to clarify these factors.

• Will the How much do the reactants cost? Raw materials vary widely in cost.
• method successfully extract the metal? This depends on the reactivity of the metal.
  
• What purity is needed, and are the purification methods expensive? Some metals are not useful unless very pure, others are useful impure.
  
• How much energy does the process use? High temperatures and electrolysis use a lot of energy.
  
• How efficiently, and in what quantities, can the metal be made? Continuous processes are more efficient than batch processes.
  
• Are there any environmental considerations? Some processes produce a lot of pollutants.
  

Different methods of metal extraction will be considered in this unit. Some examples are given below:

• Reduction of metal oxides with carbon

Carbon (as coke or charcoal) is cheap. It not only acts as a reducing agent, but it also acts as the fuel to provide heat for the process. However, in some cases (for example with aluminium) the temperature needed for carbon reduction is too high to be economic - so a different method has to be used. Carbon may also be left in the metal as an impurity. Sometimes this can be removed afterwards (for example, in the extraction of iron); sometimes it cannot (for example in producing titanium), and a different method would have to be used in cases like this.

• Electrolysis of the metal ore

This method is used to extract metals which are difficult to reduce by chemical agents. These metals include aluminium or metals which are difficult to reduce such as Group 1 and 2 metals.

Reduction of the metal oxide with hydrogen

Hydrogen can be used as a reducing agent, it is also used in purification of cpper. This is the main method for the extraction of tungsten from its oxide.

Sustainable use of natural resources: As these extraction processes are expensive and the supply of ore is not infinite, it is essential to recycle the metals as much as possible. Recycling of metals is one way of conservation and sustainable use of natural resources.

Metallurgical operations are sources of pollution, water and air pollution. Measures must be taken to eliminate or at least to minimize that kind of pollution.

2.1.2. Concentrating the ore

Concentrating the ore is also called “Dressing” or “Benefaction of ore”. The ore from which the metal will be extracted has to be prepared before extraction. Concentrating the ore is getting rid of as much of the unwanted rocky material as possible before the ore is converted into the metal. This simply means “Removal of gangue from ore”. This may be done by chemical or physical means.

• By physical processes

Physical operations use physical techniques that do not change the chemical nature of the minerals; these techniques are based on physical properties such as: density, magnetic properties, etc.… In many cases, it is possible to separate the metal compound from unwanted rocky material by physical means.

• Mechanical sorting (Hand picking): this involves use of hands to pick the gangue and breaking away the gangue using a hammer.
• Magnetic separation: the ore is crushed and a very strong magnet is used to sort out magnetic materials from non-magnetic materials. This method is for example used to separate wolframite from cassiterite where cassiterite being non-magnetic is not attracted by the magnet.
  
• Washing: the ores are usually denser than the gangue, which may be washed away in a stream of water on an inclined table. Examples of ores separated in this way are galena and limestone; cassiterite from silicates.
  
• Froth flotation method: this process is important in treating many sulphide ores like galena, zinc blende (zinc sulphide), copper pyrites, etc. Separation is based on different abilities of mineral and gangue particles to be moistened by water. During this process, the ore is first powdered and fed into a large concentration tank containing water and a suitable oil (frothing agents).

The mixture is agitated by blowing air through at a high pressure. The sulphide ores rise to the surface in the froth while the gangue sinks to the bottom. The froth is skimmed off the surface, and an acid is added to break up the froth. The concentrated ore is filtered and dried.

By chemical processes.

For example, pure aluminium oxide is obtained from bauxite by a process involving a reaction with sodium hydroxide solution. Some copper ores can be converted into copper (II) sulphate solution by leaving the crushed ore in contact with dilute sulphuric acid for a long time and then copper can be extracted from the copper (II) sulphate solution.

Leaching with aqueous solvents: in this method the finely powdered ore is treated with a suitable reagent that dissolves the ore but not impurities, for example:

1. Bauxite is crushed and digested with sodium hydroxide solution
2. Zinc ores can be leached with dilute sulphuric acid and electrolysed.

• Roasting in air and Calcination: Here the ore is powdered and roasted in air to drive off the water (for the hydrated ores) and other volatile substances. For example, Carbonates decompose to release carbon dioxide. Roasting: Sulphides release sulphur dioxide gas:

• Calcination: It is a process heating the ore strongly either in limited supply of air ih the absence in air

Smelting is the process by which a metal is obtained, either as the element or as a simple compound, from its oxide ore by heating beyond the melting point, ordinarily in the presence of reducing agents, such as coke.

CuO + C → Cu +CO

Checking up 2.1

1. What is an ore? 

2. What is the difference between an ore and a mineral? 

3. Are ores a finite resource? 

4. Are ores renewable?

 5. When is carbon used for extraction?

 6. Name a metal that could be extracted from its ore using carbon. 

7. When is electrolysis used for extraction? 

8. What do you understand with dressing of ore, smelting, froth flotation and gangue? 

9. Name two metals that can only be extracted by electrolysis. 

10. Suggest a reason why iron is extracted using carbon rather than by electrolysis.

 11. State three factors which determine the choice of reduction method used for the extraction of metals from their ores.

 12. Complete with the following words: iron, extracted, crust, gold, native, elemental, reduced.

“Metals come from the Earth’s …….. Some metals like ……. are very unreactive and are found as ………. in their ………state. Metals such as zinc, lead and ………are found combined with oxygen in compounds. These metals can be ……… using chemical reactions. The metal oxides are ………as oxygen is removed from the compound”. 

13. Why most of metals are produced by reduction method ?

2.2. Methods of extraction of Copper

Activity 2.2

 Use the search engine and the library: 

1. To find out the name of the two main ores of Copper.

 2. To describe the full process in copper metal is extracted from its ores.

 3. To demonstrate how copper is useful in our daily life.

We are going to deal with the extraction of copper from its ores, its purification by electrolysis, and some of its uses.

2.2.1. Extracting copper from its ores

There are two main copper ore types of interest, copper  oxide ores  and copper sulphide ores. Both ore types can be economically mined, however, the most common source of copper ore is the sulphide ore mineral chalcopyrite also known as copper pyrites, which accounts for about 50 percent of copper production.

Table 2.1: Some main ores of copper

The ores typically contain low percentages of copper and have to be concentrated by, for example, froth flotation before processing.

The method used to extract copper from its ores depends on the nature of the ore. Sulphide ores such as chalcopyrite are converted to copper by a different method from silicate, carbonate or sulphate ores. Copper, Cu, is mainly extracted from the ore chalcopyrite, CuFeS₂ , in a three stage process.

• In the first stage, chalcopyrite is heated with silicon dioxide and oxygen

2 CuFeS₂ + 2 SiO₂ + 4 O₂ → Cu₂ S + 2 FeSiO₃ + 3 SO2

• In the second stage, the copper (I) sulphide is roasted with oxygen at a high temperature in a reverberatory furnace giving copper (II) oxide.

Cu₂ S + 2 O₂ →2 CuO + SO₂

_{• In the third stage, the copper (II) oxide is reduced by heating with carbon.}

_{CuO + C → Cu +CO}

_{The end product of this is called blister copper - a porous brittle form of copper, about 98 - 99.5% pure.}

_{2.2.2. Purification of copper}

_{When copper is made from sulphide ores by the first method above, it is impure. The blister copper is first treated to remove any remaining sulphur (trapped as bubbles of sulphur dioxide in the copper - hence “blister copper”) and then cast into anodes for refining using electrolysis(electrolytic refining).The purification uses an electrolyte of copper (II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.}

_{The diagram shows a very simplified view of a cell.}

• _{At the cathode, a reduction reaction takes place; copper (II) ions are deposited  as pure copper: Cu}²⁺_{(aq) + 2 − e →Cu(s)}
• _{At the anode, an oxidation reaction takes place; impure copper goes into solution as copper (II) ions: Cu(s) → Cu}²⁺_{(aq) + 2 −}
  

For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same.

All that happens is that there is a transfer of copper from the anode to the cathode. The cathode gets bigger as more and more pure copper is deposited; the anode gradually disappears.

In practice, it is not quite as simple as that because of the impurities involved. What happens to the impurities?

• Any metal in the impure anode which is below copper in the electrochemical series (reactivity series) does not go into solution as ions. It stays as a metal and falls to the bottom of the cell as “anode sludge” together with any unreactive material left over from the ore. The anode sludge may contain valuable metals such as silver and gold.
• Metals above copper in the electrochemical series (like zinc) will form ions at the anode and go into solution. However, they will not get discharged at the cathode provided their concentration does not get too high.
  

Extracting copper from other ores

Copper can be extracted from non-sulphide ores by a different process involving three separate stages:

Step 1: Reaction of the ore (over quite a long time and on a huge scale) with a dilute acid such as dilute sulphuric acid to produce a very dilute copper (II) sulphate solution.

Step 2: Concentration of the copper (II) sulphate solution by solvent extraction.

The very dilute solution is brought into contact with a relatively small amount of an organic solvent containing a substance which will bind with copper (II) ions so that they are removed from the dilute solution. The solvent must not mix with water. Copper (II) ions are removed again from the organic solvent by reaction with fresh sulphuric acid, producing a much more concentrated copper (II) sulphate solution than before.

Step 3: Electrolysis of the new solution. Copper (II) ions are deposited as copper on the cathode. The anodes for this process were traditionally lead-based alloys, but newer methods use titanium or stainless steel. The cathode is either a strip of very pure copper or stainless steel.

Checking up 2.2

1. Based on the knowledge you have on unit 1, give 3 uses of copper.
2. Describe the process by which copper is extracted from chalcopyrite, CuFeS₂
3. Give any difference between the extraction of copper from chalcopyrite and from cuprite.

2.3. Methods of extraction of Iron

Activity 2.3 

Using your own research, use the available resources (chemistry books, notebooks, internet...) to make a succinct summary of:

1. The main ores of iron.

 2. The full process involved in the blast furnace while the iron is being extracted (Include the diagram, the raw materials introduced and the role of each and the equations of the main reactions occurring).

 3. The main different forms of iron, its properties and their respective uses.

The common ores of iron are iron oxides, and these can be reduced to iron by heating them with carbon in the form of coke. Coke is produced by heating coal in the absence of air. Coke is cheap and provides both the reducing agent for the reaction and also the heat source - as you will see below.

2.3.1. Iron ores

The most commonly used iron ores are haematite, Fe₂ O₃ , and magnetite, Fe₃ O₄ . The other examples of iron ores are: Limonite (Fe₂ O₃ •3H₂ O), Iron pyrites (FeS₂ ) and Siderite (FeCO₃ ).

2.3.2. The heat source

The air is blown into the bottom of the giant chimney called a blast furnace. 

The blast furnace is about 30 metres high and lined with fireproof bricks. This furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important not to waste any.

The coke (essentially impure carbon) burns in the blast of hot air to form carbon dioxide - a strongly exothermic reaction. This reaction is the main source of heat in the furnace.

C + O₂ →CO₂

_{2.3.3. Extraction}

_{Three substances are needed to enable extraction of iron from its ore. The combined mixture is called the “charge”:}

_{• Iron ore} 

_{• Limestone (calcium carbonate).} 

_{• Coke - mainly carbon.}

_{The charge is placed into the blast furnace. Hot air is blasted through the bottom. Several reactions take place before the iron is finally produced. This is a continuous process that needs a high temperature. The high temperature is produced by burning the carbon in a blast of hot air.  Oxygen in the air reacts with coke to give carbon dioxide.}

_{C(s) + O2 (g) → CO2(g)}

_{Limestone (calcium carbonate) is added to the blast furnace to remove sandy impurities (siO2 ). The limestone breaks down to form carbon dioxide.}

_{Carbon dioxide produced (both from carbon and from calcium carbonate) reacts with more coke to produce carbon monoxide.}

_{2.3.4. The reduction of the ore}

_{The Fe2 O3 is reduced by both carbon (C) and carbon monoxide (CO) as shown by the equations below.}

_{The limestone reacts with the sand to form slag (calcium silicate)}

_{Both the slag and iron are drained from the bottom of the furnace.}

_{The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron. It can be tapped off from time to time as slag.}

_{The slag is mainly used in the construction industry (to build roads, to make breeze blocks, as “slag cement” - a final ground slag which can be used in cement, often mixed with Portland cement…)}

_{The iron whilst molten is poured into moulds and left to solidify - this is called cast iron.}

In Rwanda tradition, iron was produced from an iron ore (heamatite, Fe₂ O₃ , and magnetite,Fe₃ O₄ ) called ubutare ,by traditional smelters called abacuzi, they used charcoal as reducing agent. Then from that, they were able to produce traditional tools such hoes, machets, arrows speras, etc…..

2.3.5. Different forms of iron

• Pig iron (Cast iron after removing some impurities), an alloy of iron that contains 2 to 4 percent carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus.

       - It is made by reducing iron ore in a blast furnace.  

       - It has a low tensile strength

       - It is used in making gates, pipes, lamp posts where high strength is not needed.

• Wrought iron is a soft, ductile, fibrous variety that is produced from a semi-

fused mass of relatively pure iron globules (haematite) partially surrounded by slag. It usually contains less than 0.1 percent carbon and 1 or 2 percent slag.

- It is purer than cast iron.

 - It is fibrous and tough

 - It can be welded (joined by hammering when red hot)

 - It is malleable and ductile

 - It is used for making sheets, wire and nails. 

- Steel is an alloy of iron, carbon, manganese, nickel and vanadium.

2.3.6. Alloys of iron and their uses

Checking up 2.3

1. Iron is extracted using the Blast Furnace

     a. What is introduced into the top of the blast furnace? 

   b. What is the source of heat used in the blast furnace? Write the involved equation

   c. What is the main reducing agent and how is it produced? (give an equation)

    d. How is the iron oxide reduced by this reducing agent? (give an equation) 

   e. What is the other reducing agent and how does it reduce the iron oxide? (give an equation)

   2. The blast furnace is a continuous process. What does this mean and why is it advantageous?

2.4. Methods of extraction of tin

Activity 2.4

Make a research using the appropriate books and internet to:

1. Find out the main tin ore and state 2 regions in Rwanda where it is mined.
2. . Describe the process taken to extract tin from its principal ore
   
3. State three properties and uses of tin.
   

The principal tin ore is a compound of tin and oxygen called cassiterite (gasegereti in Kinyarwanda).The main component of cassiterite is tin (IV) oxide or tin dioxide, SnO₂ .

From the time before independence of our country, cassiterite has been one of Rwandan exported products. It is extracted in some regions such as: Rutongo, Rwinkwavu, Gatumba, etc…

2.4.1. Extraction of tin from cassiterite

Extraction of tin from cassiterite is done into the following steps.

• Washing: Here the minerals dug from the site must be washed to remove the soil/earth accompanying the mineral.
• Concentration: The crushed cassiterite is concentrated by gravity separation and the magnetic impurities like wolframite [(Mn,Fe)WO4 ], etc, are separated from cassiterite by magnetic separators.
  
• Roasting: The tin ore is roasted in air to remove other impurities, such as arsenic and sulphur as volatile oxides. Iron compounds, which might be present as impurities are removed by electromagnetic separation and iron pyrites change to their oxides and sulphates.
  
• Leaching and washing: The roasted ore is treated with water and the soluble CuSO₄ or FeSO₄   are washed away from the main ore. Further lighter ferric oxide is washed away leaving behind heavier ore particles known as black tin containing 60 to 70 % SnO2 .
  
• Smelting: The tin metal is extracted from its ore by carbon reduction. The concentrated ore is mixed with coke/charcoal and heated in a furnace.

       SnO₂ + 2 C → Sn + 2 CO

In the furnace, there are other impurities such as silica (SiO₂ ). This one is removed using limestone. Limestone is added in the furnace and undergoes the thermal decomposition giving calcium oxide. Calcium oxide then reacts with SiO₂ to form calcium silicate which has a relatively low melting point.

Molten tin is drawn into blocks. It contains 99.5 % of tin metal and is called block tin.

(Source: http://onelearningsolution.blogspot.com/2015/02/33-reactivity-series-of-metals-and-its.html)

Refining of tin:  The tin obtained is purified. It is separated from copper, iron and any other element present as impurities by either thermal (liquation) -heating beyond its melting point of 232^o C, and running off the molten tin, leaving behind any less fusible impurities - or by electrolytic means.

Purer tin is obtained by the electrolysis of aqueous solution of tin (II) chloride, SnCl₂  - the impure tin is made anode, while the cathode is pure tin. The electrolyte may also consist of tin sulphate containing a small amount of hydrofluorosilicic acid (H₂ SiF₆ ) and sulphuric acid

2.4.2. Uses of tin

• Tin is used to plate iron to prevent it from rusting and as alloys such as bronze (tin and copper) and solder (tin and lead). This silvery, malleable p-block metal is not easily oxidized in air and is used to coat other metals to prevent corrosion. In modern times tin is used in many alloys. The first alloy, used in large scale since 3000 BC, was bronze, an alloy of tin and copper. Most notably tin/lead soft solders, typically containing 60% or more of tin
• Another large application for tin is corrosion-resistant tin plating of steel.
  
• Because of its low toxicity, tin-plated metal is also used for food packaging, giving the name to tin cans, which are made mostly of steel.
  
• Due to its resistance to atmospheric corrosion and low melting point, it can be used to make sheet glass. 
  

Checking up 2.4

1. Name the main ore of tin.
2. Explain the extraction of tin from tin oxide.
   
3. Tin metal is obtained by removing oxygen from the metal oxide. What name do we give to this chemical reaction?
   
4. Explain why tin is said to be very useful.
   

2.5. Methods of extraction of Zinc

Activity 2.5

Use the library or other search engine to

1. Find out the main ores of zinc.
2. Describe all steps followed to obtain the purified zinc from its sulphide ore.
   
3. Demonstrate how zinc is so useful.
   

The main ore of zinc is zinc blende, ZnS which is contaminated with lead sulphide. Another example of zinc ore is calamine (ZnCO₃ ).

Zinc is extracted from zinc blende in various steps such as concentration, roasting, smelting and purification which are given below in details.

           a. Process of concentration

The first process in the extraction of zinc is the concentration. As shown in the figure 2.2, this process involves the implementation of froth floatation method for the extraction of zinc ores from zinc blende.

Zinc blende is mixed up in a large tank consisting of a mixture of pine oil and water. Later compressed air is passed through this combination. The froth containing the concentrated zinc sulphide ores settles on the surface leaving behind the impurities in water

         b. Process of roasting

The concentrated ore is then treated at 900o C in the presence of excess air, on the base of a reverberatory furnace.This process of heating is called roasting. During this process, zinc oxide is obtained from the zinc sulphide ore. The equation for this process is:

2 ZnS + 3 O₂ →2 ZnO + 2 SO₂

       c. Process of smelting

Here, the mixture of zinc oxide and coke is heated in the presence of carbon to obtain zinc. This is a reduction of zinc. The involved equation is: ZnO + C →Zn + CO

The process is employed in a vertical container. In this whole process, the zinc oxide and coke are mixed in the ratio 2:1 and in the form of a compressed mixture. This mixture is introduced into the furnace and is heated inside at a temperature of about 1400 o C. Zinc is obtained in the form of vapors which later condenses in the condenser which gives out molten zinc called spelter zinc (impure zinc).

      d. Purification process

This is the last step of the entire process of zinc extraction. The molten zinc obtained from the previous process contains some impurities. Hence the process of the removal of these impurities is called Purification. This process can be done in two ways as given below.

         • Fractional distillation

This is very useful for low boiling metals like zinc and mercury. The impure metalsis evaporated to obtain the pure metal as distillate.

       • Electrolysis process

Electrolysis is one method of removing impurities from the zinc spelter. The method uses a zinc rod which acts as a cathode, while the impure zinc serves as an anode. ZnSO₄ and dil H₂ SO₄ are mixed up and used as an electrolyte.

The current is passed through the electrolyte; Zinc deposited on the cathode leaving the impurities in the electrolyte solution.

Checking up 2.5

1.      Recall 2 uses of zinc (refer to unit 1)
2. Describe the extraction of zinc from zinc oxide.
   
3. Zinc is extracted by removing oxygen from zinc oxide.

           a. What name is given to a reaction in which oxygen is removed from a substance?

            b. Explain how oxygen can be removed from zinc oxide to make zinc.

      4.  Describe different methods by which the impurities in zinc extracted can be removed

2.6. Methods of extraction of Sodium

Activity 2.6

You are requested to use all resources about sodium (books, internet, textbooks).

1. Discuss on the importance of sodium and its compound in Chemistry and in everyday life.
2. Sodium metal does not occur in Free State in nature. Explain why these statement?
   
3. Use the available resources to describe the way this valuable metal can be obtained at large scale (include the mining process, separation methods from its compounds, reaction equations where necessary, the main drawbacks that can be encountered during this process and the way to overcome them).

On industrial scale sodium metal is extracted by “Down’s Process”. Down’s Process is based on the electrolysis of fused salt (NaCl).  This compound is found all around the world, dissolved in sea water. It is also mined from the ore called rock salt, which is made up mainly of sodium chloride (NaCl).

2.6.1. Extraction using Down’s Cell

The salt is found mixed with insoluble impurities of sand and bits of rock. Once the dissolved rock salt has been filtered, we are left with a solution of sodium chloride in water. You might think that this could be electrolysed to extract the sodium. However, because sodium is more reactive than the hydrogen in the water, hydrogen would be produced at the cathode instead of sodium. Instead, sodium chloride is crystallized out of the solution and melted. The molten sodium chloride is then electrolysed to extract the sodium metal. This is carried out in a Down’s cell as shown in figure below.

(Source:http://chem1180.blogspot.com/2010/11/199-1911-electrolysis-of-molten-salts.html)

• Down’s cell consists of a container of steel.
• Inside of the tank is lined with firebricks.
  
• Anode is a graphite rod which projects centrally up through the base of the cell.
  
• Cathode is a ring of iron, which surrounds the anode.
  
• The anode and cathode are separated from each other by a cylindrical steel gauze diaphragm (iron screen) so that Na and Cl₂  are kept apart
• A bell-like hood is immersed in electrolyte over the anode.

When an electric current is passed through the molten mixture of NaCl and CaCl₂ , NaCl decomposes into Na⁺ and Cl^-  ion. Na⁺ ions migrate towards cathode while Cl^-  ions towards the anode. The molten sodium collects in the cathode compartment where it rises to the top and is tapped off by a pipe. Chlorine is collected at the anode.

During electrolysis, calcium is also obtained at cathode but sodium and calcium are separated from each other due to the difference in density. Density of Na is 0.67 g cm^-3 and the density of Ca is 2.54 g cm^-3i.e.much higher than that of Na. That is why they do not mix with each other.

2.6.2. Problems with Down’s method
Down’s Cell NaCl is a poor conductor in its solid form, but fusing it allows for conduction of electric current by mobile ions. However, in its liquid form, its melting point is at about 800ºC, a point at which a “metallic fog”  will form between the electrolyte (NaCl) and the metallic sodium (Na) being formed and this is impossible to separate.

2.6.3. Steps to overcome this difficulty
In order to overcome this difficulty instead of only NaCl, a mixture of NaCl and CaCl₂  is electrolyzed in Down’s cell. The melting point of this mixture is 600 ^o C. At 600 ^o C, no metallic fog is formed. The composition of the charge in Down’s Cell is NaCl = 42 %
and CaCl2 = 58 %

2.6.4. Uses of sodium

• Molten sodium is used as a coolant in some types of nuclear reactor. Its high thermal conductivity and low melting temperature and the fact that its boiling temperature is much higher than that of water make sodium suitable for this purpose.
• Sodium wire is used in electrical circuits for special applications. It is very flexible and has a high electrical conductivity. The wire is coated with plastics to exclude moisture.
  
• Sodium vapour lamps are used for street lighting; the yellow light.
  
• Sodium amalgam and sodium tetrahydridoborate, NaBH4 , are used as reducing agents.
  
• Sodium  is also a component of  sodium  chloride (NaCl), a very important compound found everywhere in the living environment; particularly as cooking and table salt.
  
• Other uses are: to improve the structure of certain alloys; in soap, in combination with fatty acids, to purify molten metals, etc.
  
• In countries that experience very cold winter, NaCl is spread on the roads to de-ice the roads.
  

Checking up 2.6

1. Why is sodium not extracted by carbon reduction process? What is the method used?
   
2. What difficulties arise in the extraction of sodium from its ore?
   
3. Choose the suitable answer. In sodium extraction, fused (molten) NaCl is used rather than the dissolved (aqueous) NaCl because: 

•  Obtaining molten NaCl is easy than obtaining aqueous NaCl.
• In molten NaCl, the ions are free to move and not in aqueous NaCl. 
•  Molten sodium NaCl is an electrolyte and not the aqueous NaCl.
•  d. Electrolysis of molten NaCl gives Na metal at the cathode but that of aqueous NaCl gives H₂ gas
  

2.7. Methods of extraction of Aluminium

    Activity 2.7

Research in library textbook or search engine about aluminium and its extraction and make a good summary containing the following:

1. The main ores of aluminium.
2. Why aluminium extraction is suitable to be done by electrolytic method rather than reduction with carbon.
3. The way the main ore is purified to have the pure aluminium oxide
4. The process by which aluminium is extracted from the purified ore by electrolytic method (include the diagram, reaction equations at electrodes and the way to solve some difficulties which may arise during this extraction process)
5. The properties of aluminium which makes it to be very important in our daily uses and the uses of aluminium.
The main ore of aluminium is called bauxite (Al₂ O₃.2H₂ O). Other examples of ores are: Cryolite (Na₃  AlF₆ ), Feldspar (KAlSiO₃ O₈ ) and Mica [KAlSiO₁₀ (OH)₂ ].

Bauxite is purified to yield a white powder, aluminium oxide, from which aluminium can be extracted. Aluminium is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic. Instead, it is extracted by electrolysis as follows

2.7.1. Purifiying the bauxite (The Bayer Process)

The bauxite (red-brown solid) - hydrated aluminium oxide mixed with impurities - is extracted from the earth-crust. The extracted aluminium oxide (bauxite) is then treated with alkali; this separates insoluble impurities from aluminium which forms soluble complex ion Al(OH)₄ ^-.

              a. Reaction with sodium hydroxide solution

Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140 °C to 240 °C; pressures can be up to about 35 atmospheres. High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100 °C. The higher the temperature, the higher the pressure needed.

With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate (sodium aluminate).

Al₂ O₃ + 2 NaOH + 3 H₂ O →2 NaAl(OH)₄

The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. Some of the silicon dioxide reacts, but goes on to form a sodium aluminosilicate which precipitates out.

Silica (SiO₂ ) also dissolves in sodium hydroxide to form soluble sodium trioxosilicate (IV), Na₂ SiO₃ .

SiO₂  + 2 NaOH →Na2 SiO₃  + H₂ O

The impurities are filtered out and the filtrate contains sodium tetrahydroxoaluminate (III) and sodium trioxosilicate (IV) only

              b. Precipitation of aluminium hydroxide

CO₂  is bubbled through the filtrate containing sodium tetrahydroxoaluminate (III) and sodium trioxosilicate (IV). At this stage the solution may be seeded with freshly precipitated aluminium hydroxide.

NaAl(OH)₄ (aq) + CO₂ (g)  →  NaHCO₃ (aq) + Al(OH)₃ (s)

                 c. Formation of pure aluminium oxide

Aluminium oxide (sometimes known as alumina) is made by heating (decomposition) the aluminium hydroxide to a temperature of about 1100 – 1200 °C.

2 Al(OH)₃ → Al2 O₃ + 3 H₂ O

2.7.2. Extraction of the aluminium from purified bauxite by electrolysis (Hall-Heroult Process)

Alumina (purified bauxite) is then transported to huge tanks. The tanks are lined with graphite that acts as the cathode. Also blocks of graphite hang in the middle of the iron (steel) tank, and acts as anodes.

The alumina is mixed with cryolite, Na3 AlF6 and this lowers the melting point of the mixture - Cryolite is another aluminium ore, but is rare and expensive, and most of it is now synthesized.

The electrode equations are as follows:

• At the cathode: Here the aluminium ions capture electrons to become atoms again:

             Al³⁺ + 3e- → Al

•   At the anode: The oxide ions lose electrons to become oxygen molecules, O₂ :

              2 O₂-→ O₂ + 4e-

The positive electrode burns away as the carbon reacts with the oxygen produced there.

C + O₂ →CO₂

Since during electrolysis, the carbon electrodes get consumed, they have to be replaced periodically. For each kg of aluminium about 0.5 kg carbon is burnt away.

2.7.3. Uses of aluminium

• It is a good conductor of electricity. Since it is not as good conductor as copper, thicker cables of aluminium are used for transmission of electricity.
• Aluminium forms many useful alloys e.g., magnalium (Al and Mg), duralumin (Al, Cu, Mg and Mn). Aluminium alloys are used in aircraft and other transportation vehicles because of its low density.
  
• It is used as jewellery because it is a shiny metal, it has a good appearance.
  
• Resists corrosion because of the strong thin layer of aluminium oxide on its surface. This layer can be strengthened further by anodising the aluminium.
  
• It is used for making window frames.
  
• Aluminium foil is used for wrapping cigarettes, food, etc because it reflects light.
  
• Drink cans because it is not toxic.
  
• Aluminium utensils are extensively used for household purposes.
  
• Aluminium is used to produce metals such as chromium and manganese from their ores (aluminothermic process).
  

                                                                                                                    Cr₂ O₃  + 2 Al   →  Al₂ O₃  + 2 Cr

                                                                                                                     3 MnO₂  + 4 Al → 2 Al₂ O₃  + 3 Mn

2.7.4. Recycling of aluminium

Aluminum is one of the most recycled materials in the world. This is due to high cost of new aluminium that involves high cost of electricity needed to produce aluminium. Recycled aluminium cost less in production and constitutes another way of sustainable management of our natural resources.

• The consumer throws aluminium cans and foil into a recycle bin.
• The aluminium is then collected and taken to a treatment plant.
  
• In the treatment plant the aluminium is sorted and cleaned ready for reprocessing.
  
• • It then goes through a re-melt process and turns into molten aluminium, this removes the coatings and inks that may be present on the aluminium.
  
• The aluminium is then made into large blocks called ingots.
  
• The ingots are sent to mills where they are rolled out, this gives the aluminium greater flexibility and strength.
  
• This is then made into aluminium products such as cans, chocolate wrapping and ready meal packaging.
  
• In as little as 6 weeks, the recycled aluminium products are then sent back to the shops ready to be used again.
  

                Checking up 2.7

 Consider the electrolytic extraction of aluminium.

1. How is aluminium ore called?
2. Explain why cryolithe is added to aluminium oxide
   
3. 3. Describe the process by which the ore of aluminium is purified.
   
4. Write half-equations for the reactions at each electrode, and write an overall equation for the reaction.
   
5. 5. State what each electrode is made of.
   
6. Explain why                  

• The anodes need to be regularly replaced.
  
• The electrolysis of aluminium oxide is expensive.
  
• Aluminium is recycled.
  

7. Give three uses of aluminium and the properties responsible for each use.

2.8. Methods of extraction of Wolfram (Tungsten)

                Activity 2.8

1.      Visit the nearby mining sites of wolfram and make a field report.
2. Research using internet and some books and make a summary about the following:
   

•    The ores of wolfram.
• Where in Rwanda do we find wolfram?
  
• How wolfram extraction differs from that of zinc in terms of reduction.
  
• Full description of the extraction process of tungsten from its ore.
  
• The main uses of tungsten
  

Tungsten ore is a rock from which the element tungsten can be economically extracted. The ore minerals of tungsten include wolframite [(Fe, Mn)WO₄ ], scheelite (CaWO₄ ) and ferberite (FeWO₄ ).

Tungsten is a dull silver-colored metal with the highest melting point (3410 ºC) of any pure metal. Also known as Wolfram, from which the element takes its symbol, W, tungsten is more resistant to fracturing than diamond and is much harder than steel. It is unique in being a refractory metal - its strength and ability to withstand high temperatures - that make it ideal for many commercial and industrial applications.

Most tungsten ores contain less than 1.5% WO3  and frequently only a few tenths of a percent. On the other hand, ore concentrates traded internationally require 65-75% WO₃ .  Therefore, a very high amount of gangue material must be separated.  This is why ore dressing plants are always located in close proximity to the mine to save transportation costs.

2.8.1. Processing

Modern processing methods dissolve scheelite and wolframite concentrates by an alkaline pressure digestion, using either a soda ash (Na₂ CO₃ ) or a concentrated NaOH solution. The sodium tungstate solution obtained is purified by precipitation and filtration, before it is converted into an ammonium tungstate solution. This stage is carried out exclusively by solvent extraction or ion exchange resins. Finally, high purity Ammonium paratungstate (APT) is obtained by crystallization.

Wolframite concentrates can also be smelted directly with charcoal or coke in an electric arc furnace to produce ferrotungsten (FeW) which is used as alloying material in steel production. Pure scheelite concentrate may also be added directly to molten steel

Once tungsten ore has been processed and separated, the chemical form, ammonium paratungstate (APT), is produced.

2.8.2. The extraction process

APT can be heated with hydrogen to form tungsten oxide (WO₃ ) or will react with carbon at temperatures above 1050°C to produce tungsten metal. Pure tungsten cannot be obtained by reducing tungsten (VI) oxide using carbon, because it reacts with carbon to make tungsten carbide. Instead, the reducing agent is hydrogen.

Powdered tungsten (VI) oxide is heated to temperatures in the range 550 - 850°C in a stream of hydrogen.

WO₃  + 3 H₂  W + 3 H₂ O

An excess of hydrogen is used, and this carries away the steam produced during the reaction. The hydrogen is dried and recycled.

Great care obviously has to be taken to keep the whole system free of air to avoid explosion risks with the hydrogen at these high temperatures.

2.8.3. Advantages and disadvantages of the process

a. Advantages:

• It produces very pure tungsten
• Hydrogen is a cheap reagent
  

b. Disadvantages:

• The energy cost are high
• Using a flammable gas such as hydrogen at high temperatures is very dangerous
  

2.8.4. Uses

• Tungsten is mostly used in light bulb filaments which heat up to 2000ºC, when many other metals would vaporise, particularly at the pressures found inside light bulbs. This is because it has a very high melting point.
• Tungsten has the highest melting point of all metals and is alloyed with other metals to strengthen them. Tungsten and its alloys are used in many high-temperature applications, such as arc-welding electrodes and heating elements in high-temperature furnaces.
  
• Tungsten carbide (WC) is extremely hard and is very important to the metalworking, mining and petroleum industries. It is made by mixing tungsten powder and carbon powder and heating to 2200°C. It makes excellent cutting and drilling tools, including a new ‘painless’ dental drill which spins at ultrahigh speeds.
  
• Calcium and magnesium tungstates are widely used in fluorescent lighting. Tungsten mill products are either tungsten metal products, such as lighting filaments, electrodes, electrical and electronic contacts, wires, sheets, rods etc or tungsten alloys.
  
• Due to tungsten’s ability to keep its shape at high temperatures, tungsten filaments are now also used in a variety of household applications, including lamps, floodlights, heating elements in electrical furnaces, microwave ovens, x-ray tubes and cathode-ray tubes (CRTs) in computer monitors and television sets.
  
• The metal tolerance to intense heat also makes it ideal for thermocouples and electrical contacts in electric arc furnaces and welding equipment.
  
• Applications that require a concentrated mass, or weight, such as counterweights, fishing sinkers, and darts often use tungsten because of its high density (19.3 g/cm³ ).
  

Checking up 2.8

1. Nowadays, there is a special reduction method used to extract Tungsten from its ores.

• State 2 main ores of tungsten
• Write the balanced equation of the reduction reaction of tungsten (II) oxide.

2. Suggest the reason why ore concentrating plants are always located in close proximity to the mine.

3. Give 2 widely known physical properties of tungsten and the uses associated to these properties.

2.9. Methods of extraction of tantalum

Activity 2.9

• Visit the nearby mining sites of tantalum and make a field report.
• The picture below shows the miners at work in Rwanda. The metal being extracted here is very important in modern area. According to Merchant and Consulting Ltd (2018), Rwanda is the world’s largest producer of this metal which is called “tantalum”
  

                                                                                                                               

Research using any relevant source of information about tantalum:

• To find out the physical properties of tantalum metal and its main uses.
• To make a description of the extraction of tantalum from its main ore tantalite.
  

Tantalum is a hard, heavy, shiny, grayish-blue metal that is very stable, almost impervious (impermeable) to air, water and all but a few acids. It has the third highest melting point of all elements (over 3000 ^o C), and its primary use is in capacitors for electronic applications, and for vacuum furnace parts. 

It is classified as a “refractory” metal, which means it can sustain high temperatures and resist corrosion. It is a good conductor of heat and electricity, which makes it useful in various electronics. Pure tantalum can be drawn into fine wire filament, which is used to evaporate other metals.

2.9.1. Where tantalum is found

Tantalum is found in hard rock deposits such as granites, carbonites and pegmatites (igneous rock that consists of coarse granite). The chief tantalum ores are tantalite [(Fe, Mn)(Ta, Nb)₂ O₆ ], which also contains iron, manganese and niobium (former name is columbium), and samarskite, which contains seven metals. Another ore which contains tantalum and niobium is pyrochlore. In Rwanda tantalum is found with niobium in mineral known as coltan (colomb-tantalite).

Tantalum is an important component in many modern technologies, and is used in capacitors for everything from computers to mobile phones.

Despite its importance in the world today, tantalum mining takes place in very few countries and mining it is difficult. Only four countries produced tantalum in 2016, and most was mined  in the Democratic Republic of Congo (DRC). Rwanda, Brazil and China were the other top countries for tantalum mining in that year. Sites are being identified for future development, and existing sites are being evaluated for expansion.

2.9.2. How tantalum is mined

Tantalum comes from the processing and refining of tantalite. Tantalite is the common name for any mineral ore containing tantalum. Most tantalum mines are open pit; some are underground.

The process of mining tantalum involves blasting, crushing and transporting the resulting ore to begin the process of freeing the tantalum. Before transportation, the ore is concentrated at or near the mine site, to increase the percentage (by weight) of tantalum oxide and niobium. The material is concentrated through wet gravity techniques, gravity, electrostatic and electromagnetic processes.

2.9.3. How tantalum is processed

The tantalum concentrate is transported to the processor for chemical processing. The concentrate is then treated with a mixture of hydrofluoric and sulphuric acids at high temperatures. This causes the tantalum and niobium to dissolve as fluorides.

Numerous impurities are also dissolved. Other elements, such as silicon, iron, manganese, titanium, zirconium, uranium and thorium, are generally present and processed for other uses.

The concentrate is broken down into a slurry (A slurry is a watery mixture of insoluble matter such as mud, lime, or plaster of Paris). The slurry is filtered and further processed by solvent extraction. Using methyl isobutyl ketone (MIBK), or liquid ion exchange using an amine extract in kerosenes, produces highly purified solutions of tantalum and niobium. In this way, the tantalum oxide is obtained which is finally reduced with molten sodium to produce tantalum metal in powder form.

It can then be compacted (as it is for capacitors) to final shape, or may be melted (and refined) in an electron beam furnace.

2.9.4. Uses for tantalum

• Tantalum is used to make electrolytic conductors, aircraft engines, vacuum furnace parts, nuclear reactors and missile parts.
• Tantalum is unaffected by body fluids, and is non-irritating, which makes it useful for surgical appliances.
  
• It is common in the production of cell phones, personal computers, igniter chips in car air bags, cutting tools, drill bits, teeth for excavators, bullets and heat shields.
  
• Because the metal is an electrical conductor, it is useful in many consumer electronics, such as microprocessors for plasma televisions.
  

Checking up 2.9

In pairs, answer the following questions:

1. Where tantalum is found in Rwanda?
2. How tantalum is mined.
   
3. How tantalum is processed
   
4. Give 3 uses of tantalum.
   
5. What is the role of methyl isobutyl ketone (MIBK) in tantalum processing?
   

2.10. Dangers associated with extraction of metals

Activity 2.10

If you have ever visited a mining site, remember all the processes involved and suggest all possible common dangers associated with the extraction of metals.

The following are some of the dangers associated with metals extraction. Especially, these hazards are found in smelting and refining and in addition mining.

1. Injuries

Metal extraction industry has a higher rate of injuries than most other industries. Sources of these injuries include: splattering and spills of molten metaland slag resulting in burns; gas explosions and explosions from contact of molten metal with water; collisions with moving vehicles; falls of heavy objects; falls from a height,slipping and tripping injuries from obstruction of floors and passageways.

Precautions

Adequate training, appropriate personal protective equipment (PPE) [for example, hard hats, safety shoes, work gloves and protective clothing]; good storage, housekeeping and equipment maintenance; traffic rules for moving equipment (including defined routes and an effective signal and warning system); and a fall protection programme.

2.   Heat illnesses

Heat stress illnesses such as heat stroke are a common hazard, primarily due to infrared radiation from furnaces and molten metal. This is especially a problem when strenuous work must be done in hot environments.

Prevention of heat illnesses

Water screens or air curtains in front of furnaces, spot cooling, enclosed airconditioned booths, heat-protective clothing and air-cooled suits, allowing sufficient time for acclimatization, work breaks in cool areas and an adequate supply of beverages for frequent drinking.

3.   Pollutions

Mining operations are major contributors to the pollution of our environment such as: air pollution, water pollution, degradation of landscape, etc.

Exposure to a wide variety of hazardous dusts, fumes, gases and other chemicals can occur during smelting and refining operations. Crushing and grinding ore in particular can result in high exposures to silica and toxic metal dusts (containing lead, arsenic and cadmium, for example). There can also be dust exposures during furnace maintenance operations. During smelting operations, metal fumes can be a major problem especially risk of developing respiratory system illness. In metallurgical operations, the carbon dioxide released has more dangers especially the global warming because it is a greenhouse gas. Acid gases released in atmosphere such as SO₂ , SO₃ , NO_x are sources of acid rains.

Control

Dust and fume emissions can be controlled by enclosure, automation of processes, local and dilution exhaust ventilation, wetting down of materials, reduced handling of materials and other process changes. Where these are not adequate, respiratory protection would be needed.

Many smelting operations involve the production of large amounts of sulphur dioxide from sulphide ores and carbon monoxide from combustion processes. Dilution and local exhaust ventilation (LEV) are essential.

Sulphuric acid is produced as a by-product of smelting operations and is used in electrolytic refining and leaching of metals. Exposure can occur both to the liquid and to sulphuric acid mists. Skin and eye protection and LEV are needed.

The extraction of some metals can have special dangers. Examples include fluorides in aluminium smelting, arsenic in copper, etc. These processes require their own special precautions

Other dangers

•   Glare and infrared radiation from furnaces and molten metal can cause eye damage including cataracts. Proper goggles and face shields should be worn. High levels of infrared radiation may also cause skin burns unless protective clothing is worn.
• High noise levels from crushing and grinding ore, gas discharge blowers and high-power electric furnaces can cause hearing loss. If the source of the noise cannot be enclosed or isolated, then hearing protectors should be worn. A hearing conservation program including audiometric testing and training should be instituted.
  
• Electrical hazards can occur during electrolytic processes. Precautions include proper electrical maintenance with lockout/tagout procedures; insulated gloves, clothing and tools; and ground fault circuit interrupters where needed.
  
• Manual lifting and handling of materials can cause back and upper extremity injuries. Mechanical lifting aids and proper training in lifting methods can reduce this problem.
  

CASE OF RWANDA

According to Rwanda Environment Management Authority (REMA), mining activities often impact significantly on the environment. For instance, sand collecting and quarrying are already shown some significant environmental impacts, including resource depletion, energy consumption, waste generation and emissions of air pollutants. The dangers to human life and health associated with mining include the displacement of people, land use changes, dust and noise pollution.

In fact, the preparation of ores which uses a lot of water constitutes a major pollutant of stream water in Rwanda. For example, the waters draining the mining sectors of Rutongo and Gatumba pollute the rivers of Nyabarongo and Nyabugogo by sediments of clay and sand which they transport over long distances. It is this considerable mineral load which partly gives them the brown

colour that is characteristic of the rivers in Rwanda. Mining and quarrying produce massive rejects which appear in nature in the form of enormous lots of earth and rocks. Erosion from rain water transports the mineral residue towards the valleys where streams are filled and covered by the residue which may be toxic to biodiversity

Checking up 2.10

1. State 5 sources of injuries associated with the extractions of metals
2. Give 3 ways you can prevent from heat illnesses.
   
3. Describe 2 ways you can control the dangers associated with the chemicals in extraction of metals.
   
4. Suggest a way of protecting mining workers against the risk of lung disease due to dust at their working place?
   

2.11. Preventive measures associated with metal extractions

Activity 2.11

We already know that there are many dangers associated with the extraction of metals.

Recommend measures that must be taken to prevent from these dangers (risks).

Effort should be made by mining companies or planned for the future, to eliminate or minimize the environmental problems associated with metal extractions include:

1. The potential sources of air contaminants should be enclosed and isolated.
2. Brief, for any operation related to metal extraction, measures must be adopted to protect the workers in particular and the environment in general by:
   

• Elimination or reduction of air polluting gases
• Avoiding water and soil pollution.
  
• Protect the landscape.
  
• Using cleaner production techniques i.e Minimize sources of pollution and use of energy.
  

3.  Adopt technology that minimizes wastes produced through processreengineering/recycling

Checking up 2.11

1. In extraction of metals, the best and least costly preventive measures are those taken at the planning stage of a new process of extraction. Explain the main aspects that should be taken into account.
2. what are the main sources of pollution in metallurgy?
   

END UNIT ASSESSMENT

1. Which metal is extracted from Bauxite?

            a. Tin

            b. Tantalum

             c. Copper

             d. Aluminum

2. Brass is 

           a. An Element 

            b. A Compound

            c. A Mixture

            d. An Alloy

3. Bronze is an alloy of 

    a. Copper and Zinc  

    b. Lead and Copper

     c. Copper and Tin

      d. Barium, Zinc and Iron

4. Which of the following metals is often found in pure state? 

         a. Copper 

         b. Iron

          c. Gold 

          d. Aluminum

5. Which metal is extracted from Haematite?

            a. Tin

            b. Iron 

           c. Manganese 

           d. Cadmium

6. Rocks rich in metals with economic value are known as

          a. Metalloids

           b. Ores 

           c. Allotropes 

            d. Slag

7. An alloy is a

           a. Compound of three elements 

            b. Homogeneous mixture of two or more metals

             c. Heterogeneous mixture 

             d. Element in impure form

8. If a metal ore is called "pyrites" then it most probably has 

             a. Chlorine 

             b. Oxygen 

              c. Sulphur 

               d. Nitrogen

9. Often to prevent corrosion, metals are galvanized by covering them with a layer of 

             a. Copper 

             b. Sodium 

             c. Zinc 

            d. Tin

10. What is not true about Tantalum? 

         a. It is classified as a "refractory" metal

        b. Tantalum oxide is reduced with molten sodium to produce tantalum metal in powder form.

         c. Its ore minerals include scheelite

         d. It is found in hard rock deposits such as granites, carbonites and pegmatites

11. Complete with the terms applied in the Extraction of iron from haematite in industry (Blast Furnace).

            a. Raw materials: ___________________________________

              A mixture of ___________, __________ and _____________ is added at the top of the furnace. ________ air is blown into the furnace from the bottom. A chain of chemical reactions occur:

           b. Carbon reacts with oxygen in air to form ______________. Equation:

           c. The hot carbon dioxide rises in the furnace and is reduced by _________ to form __________. Equation:                                               ____________________________ _______________________________________________

            d. Carbon monoxide is a ____________ agent. It ___________ iron(III) oxide in haematite to form hot molten                                             ___________.  

          Equation: ______________________________________________ The hot molten iron is then rum out from the bottom of              the furnace.

             e. The formula of limestone: ________________ Limestone breaks up into____________ and_______________ when                       heated    

             Equation: ________________________________________________

          f. Calcium oxide helps to remove ___________ (the impurities) to form a liquid ‘_______’. Equation:                                                              ________________________________

           12. a. By giving reagents and conditions, state three different methods of extracting metals starting from their oxides. In                    each case, write equation(s) to illustrate the extraction of an appropriate metal.

                  b. i. Why are metals more usually extracted from their oxides rather than from any other compound?

                       ii. State two environmental problems associated with the extraction of metals from their oxides or sulphides and                                   give     the chemical responsible for each problem.

            13. a. Give the ores of iron.

                    b. Explain the extraction of iron from its ores.

            14. a. Give 2 or 3 uses of aluminium, copper, zinc, and iron.

                  b. Give two reasons why the extraction of aluminium is expensive.

          15. Tungsten is prepared in a pure form by high temperature reduction of tungsten (VI) oxide with hydrogen.

                   a. Construct an equation for this reaction. 

                   b. Suggest why carbon is not used as the reducing agent.

                   c. Suggest one advantage (other than purity of the product) and one disadvantage of using hydrogen as the reducing                          agent on an industrial scale.

         16. Zinc and copper are extracted in the same way as iron (in blast furnace) but exist as their sulphide ores.

                  a. How is the sulphide ore converted into an oxide and what is the problem with this process? (give an equation) 

                  b. Why can aluminium not be extracted in this way? 

                  c. Why can tungsten not be extracted in this way?

        17. Copper is a widely used metal. The main ore of copper contains copper sulphide. Copper can be extracted from copper              sulfide in a three stage process.

             a. In the first stage of extraction the copper sulfide is heated in air.

              i. Balance the symbol equation for the reaction.

                      Cu₂ S + O₂ → CuO + SO₂

              ii. Explain why there would be an environmental problem if the gas from this reaction were allowed to escape into the                      atmosphere.

               b. In the second stage copper oxide, CuO, is reduced using carbon. Describe and explain what happens during this                              reaction.

               c. During the third stage the copper can be purified as shown in the diagram.

              

              i. What is the name of the type of process used for this purification? 

              ii. Give one use of purified copper.

d. Copper-rich ores are running out. New ways of extracting copper from low grade ores are being researched. Recycling of copper may be better than extracting copper from its ores. Explain why.