Extraction of Metals

Nota:

• Reduction of Metal Oxides Using Carbon Extraction of Iron: Iron, Fe, is extracted from its metal oxide in the Blast Furnace using Carbon as a reducing agent. To the Blast Furnace, the following 'ingredients' are added: Hematite (Fe2O3), Limestone (CaCO3) and Coke (C). Hot air is also blown into the furnace which reacts with the Carbon in a highly exothermic reaction, producing much of the heat required to keep the mixture molten. In the first stage of the process, the coke reacts with oxygen in the air to form carbon dioxide. This carbon dioxide reacts further with more coke, producing carbon monoxide as another reducing agent. Finally, the Haematite compound reacts with the carbon monoxide, producing iron and carbon dioxide. The Hematite compound contains a number of impurities, including silicon dioxide, and this is where the use of calcium carbonate comes in. Calcium carbonate decomposes in the high temperatures into calcium oxide and carbon dioxide. The calcium oxide reacts with the silicon oxide impurities forming a calcium silicate, more commonly known as slag. This slag floats on top of the molten iron due to its lower density, and can be scraped off the top and used in the construction industry. Iron Impurities The iron produced directly from the Blast Furnace still contains a number of impurities which render the iron unfit for some uses. These impurities can be removed in the Basic Oxygen Converter as follows: Phosphorus impurities are removed by blowing air through the molten iron Lime (CaO) then reacts with the phosphorus oxides. Sulphur impurities are removed by adding Mg. Magnesium is used instead of oxygen to prevent the formation of SO2, which is a highly toxic gas and leads to the formation of acid rain. [edit]Reduction Using Electrolysis Some metals are above carbon in the reactivity series, and therefore can not be extracted from their ores using carbon reduction methods. Despite Aluminium being the most abundant metal on earth, it is a relatively expensive material, owing to the fact that the extraction of aluminium from its ore, Bauxite, is a costly process. First, the Bauxite ore is purified to produce aluminium oxide as a white powder. In order to electrolyse the aluminium oxide the mixture must be melted so that the ions are free to move and carry electrical charge. Aluminium oxide has a very high melting point (over ), and therefore melting this compound would prove extremely costly - instead, an aluminium-containing compound is added, cryolite, which lowers its melting point and thus reduces some of the energy costs involved with the process. Once the solution is melted the aluminium cations () and the oxygen anions () are free to move to the electrodes. At the Anode (+ve charge) the oxygen ions each lose 2 electrons to become molecules of oxygen gas. At the Cathode (-ve charge) the aluminium ions each gain 3 electrons to become atoms of aluminium with no charge - they fall to the base where molten aluminium metal is collected. Anode: (Here, the oxygen slowly reacts with the carbon electrodes to form carbon dioxide, which causes the carbon to slowly burn away. Consequently the anodes need replacing periodically.) Cathode: [edit]Extraction of Titanium Titanium is an abundant metal with some extremely useful applications and properties. Despite this, there is a limited use of this metal due to the great costs of extraction, which can be explained by looking at the process by which titanium is extracted. The first stage of this process involves the reaction between titanium oxide, TiO2, chlorine, Cl2 and coke, C at : The titanium chloride produced is then reacted with either sodium or magnesium under an inert argon atmosphere at : (The reaction is exothermic and thus the temperature increases to approx. ) Once the reaction is complete and the mixture left to cool (for a few days, an obvious inefficiency), the mixture is washed with HCl to remove the sodium chloride. Why is it so expensive? The above process is very costly. First of all it is carried out in a batch process, which renders it much more expensive compared to a continuous process. In both stages of extraction there is also a need for relatively high temperatures which result in high energy costs, and in stage 2 an argon atmosphere must be maintained to prevent oxidation of the titanium-containing product. Sodium/magnesium is also employed which must be initially produced, adding further costs to the process. Some economies can be made by recycling the products from the reactions. The  produced in the final stage can be separated by electrolysis into sodium and chlorine, used in the first and second stages respectively. [edit]Economic Issues & Recycling Both iron and aluminium are readily recycled. Recycling these metals prevents the waste of raw materials and also saves on the high costs of extraction, particularly the high costs associated with aluminium extraction. Recycling also cuts down on the waste we produce and dump, thereby preventing unsightly scrap metal piles etc. Conversely there are some other financial issues that are associated with the recycling of metals; the waste must be transported, sometimes over long distances, to reach a site where the metal can be melted down and re-used. There are also costs associated with the melting of the scrap material etc. The choice of which reductant method used is a balance between the reductant cost, the energy requirements for the particular process and the required purity of the metal.

1. Principles
   1. Most metals found in ores as metal oxides (MO) or metal sulphides (MS)
   2. Extraction involves reduction; oxygen is removed from the metal oxide.
2. Methods of Metal Extraction
   1. Heating with carbon (C)
   2. heating with a more reactive metal
   3. reduction with hydrogen gas
   4. electrolysis
   5. Extraction method is determined by:
      1. The reactivity of the metal
      2. the energy requirements of the process
      3. cost of the reducing agent
      4. The metal purity required
   6. Roasting
3. Roasting
   1. metals in oxides are generally easier to extract therefore sulphides are often converted into oxides by roasting in air
   2. ZnS + 3/2O2 -> ZnO + SO2
   3. Roasting sulphide ores leads to the production of sulphur dioxide which causes acid rain.
      1. However the sulphur dioxide can be collected and used to manufacture sulphuric acid. This makes the process more economical.
         1. SO2 + H20 + 1/2O2 -> H2SO4
4. Extraction Of...
   1. Fe using Carbon (Blast Furnace)
      1. Haematite: Fe2O3 (main ore of iron)
         1. Reduced by C and CO in a continuous process
      2. 1) Coke is burned in hot air, producing CO2 and a lot of heat.
         1. C + O2 -> CO2
         2. 2) More coke reacts with the carbon dioxide, producing 2CO
            1. C + CO2 -> 2CO
            2. 3) The iron oxide is reduced to iron by the CO
               1. Fe2O3 + 3C0 -> 2Fe + 3CO2
                  1. Carbon can also be used as a educing agent however the reaction is slower as a solid (C0 is reacting with molten iron rather than a gas (CO)
                     1. Fe2O3 + 3C -> 2Fe + 3CO
   2. Extraction of Ti using more reactive metals
      1. Ti cannot be extracted with carbon as titanium carbide (TiC) is formed, which is extremely brittle
      2. Rutile: TiO2 (main ore)
      3. 1) TiO2 (solid) is converted into TiCl4 (liquid) at 900 degrees C.
         1. TiO2 + 2Cl2 + 2C -> TiCl4 + 2CO
         2. 2) The Ti is extracted by Mg or Na in an Ar atmosphere at 1000 degrees C.
            1. TiCl4 + 4Na -> Ti + 4NaCl
            2. TiCl4 + 2Mg -> Ti + 2MgCl2
      4. Expensive process due to..
         1. energy costs to produce heat
         2. The Na/Mg (which also has to be extracted from it's ore using electrolysis)
         3. The Ar Atmosphere
         4. It is a batch process - so the process stops and starts
   3. Extraction of W using hydrogen
      1. Tungsten cannot be extracted with carbon as tungsten carbide (WC) is formed, which is extremely brittle.
      2. 1) Tungsten (VI) oxide (WO3) is heated with hydrogen at 900 degrees celcius.
         1. WO3 + 3H2 -> W + 3H2O
      3. process must be carefully managed as hydrogen gas is highly flammable.
   4. Al using Electrolysis
      1. The Aluminium Oxide (Al2O3) must be molten or dissolved to conduct electricity, therefore it is dissolved in molten cryolite.
         1. this requires lower temperatures than using molten aluminium oxide therefore lower energy costs
      2. electrodes are made of graphite, this is a continuous process.
         1. Positive (+) Electrode
            1. 202- -> O2 + 4e-
            2. positive electrode reacts with oxygen therefore burns (forming carbon dioxide) therefore requires replacing frequently
               1. C(s) + O2 (g) -> CO2
         2. Negative (-) Electrode
            1. Al3+ + 3e- -> Al
         3. Overall Reaction: 2Al2O3 -> 4Al + 3O2
         4. main cost of the process is the electricity - so a cheap supply is needed, plants are often built near hydroelectric power stations
   5. Low Grade Copper Ores
      1. Many copper ores are highly depleted so are being converted into solutions containing Cu2+ ions.
         1. This can be done by spraying the copper mining waste with dilute acid in the presence of a bacterium, Thiobacillus ferrooxidans
      2. The copper can then be extracted by reaction with scrap iron:
         1. Cu2+(aq) + Fe (s) -> Cu(s) + Fe2+ (aq)
         2. scrap iron is cheap and readily available, while the copper produced has higher economic value
5. Recycling Metal
   1. Saves resources (eg. metal ores)
   2. Creates less waste (e.g mining waste)
   3. Saves energy resources (less energy required to recycle than extract eg. Al)
   4. Reduces air pollution (e.g CO2, SO2)