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Extraction of metals
Transcript of Extraction of metals
Reducing metal oxides
Metal oxides- either from a sulfide ore or straight out of the ground is then reduced to get the pure metals.
Carbon (as coke) and Carbon monoxide are used as reducing agents for metals that are less reactive than carbon.
Carbon monoxide and carbon are the first choice for extracting metals because they're cheap.
Another possible reducing agent is hydrogen (from methane) and water. This method is used to extract tungsten. Metals higher up the reactivity series,, such as sodium and aluminium, may be reduced by electrolysis.
Reduction of iron oxide
Extraction of iron from its oxide ores is done by using coke in tall brick towers, called blast furnaces, which can reach up to 70m high. At the base of the furnace the coke burns in a blast of hot air. Heat is generated by this exothermic process so the temperature can reach around 2000K. Carbon dioxide is formed which then reacts with more carbon to form carbon monoxide. The carbon monoxide is the reducing agent which reacts with iron oxide to produce molten iron.
Electrolysis of aluminium
Extraction of titanium
Principals of metal extraction
Ores are 'a natural substance that a metal can be economically extracted from'. Metals are often found in ores for example as oxides (titanium dioxide and iron(III) oxide) or sulfides (lead sulfide and zinc sulfide).
Hematite Iron ore (Fe2O3)
Titanium is produced by reduction with a more reactive metal- sodium or magnesium. Titanium ore (rutile) is largely titanium oxide. It's converted to titanium chloride by reacting it with coke and chlorine at 1173K. The liquid titanium chloride is purified by distillation. Titanium chloride is molten sodium under an inert argon atmosphere at 1300K. The argon is needed to prevent the metals from reacting with nitrogen and oxygen in the air.
Magnesium is another reactive metal that can be used as an alternative to sodium. Unlike the blast furnace this is a batch process and therefore less efficient.
Tungsten can be extracted from its oxide with carbon but that can leave impurities which make the metal more brittle. If pure tungsten is needed, the ore is reduced using hydrogen instead.
This happens in a furnace at temperatures above 700 degrees C. Tungsten is the only metal reduced on a large scale using hydrogen. Hydrogen is more expensive than carbon but it's worth the extra cost to get pure tungsten which is much easier to work with. Hydrogen is highly explosive when mixed with air though, which is a bit of a hazard.
Titanium, Aluminium and Tungsten are not reduced by carbon because they are more reactive.
Titanium oxide cannot reduced with carbon because the formation of titanium carbide makes the metal brittle.Tungsten has the same problem with producing a brittle carbide.
Aluminium is too reactive to extract using carbon. A very high temperature is needed; so extracting aluminium by reduction is too expensive to make it worthwhile.
Converted into oxides by roasting in air
An electric current is passed though the electrolyte (an ionic substance that's molten or in solution) so that it breaks down into its elements. It relies on the flow of electrons from ions at the positive electrode (anode) to ions at the negative electrode (cathode). This is the reason the electrolyte has to be a liquid- the ions have to be free to move. As the ions gain or lose electrons they become atoms (or molecules) and are released. Al ore is bauxite- it's Al oxide with various impurities. First of all these impurities are removed. Next it's dissolved in molten cryolite (Sodium aluminium flouride, Na3AlF6) which lowers it's melting point from 2327K to 1043K. This helps reduce operating costs. At the cathode Al ions gain electrons and are reduced. The pure Al collects are molten metal at the bottom of the cell. At the anode, oxide ions are oxidised, producing oxygen gas.
Lead Sulfide (PbS)
Zinc sulfide + Oxygen -> Zinc Oxide + Sulfur Dioxide
Sulfur dioxide gas causes acid rain. This causes harm to plants, aquatic life and even damages limestone buildings
Converting sulfur dioxide into sulfuric acid both avoids a pollutant and creates a valuable in-demand product.
Reducing metal oxides (iron, manganese and copper)
Iron (III) oxide is reduced by carbon of carbon monoxide to iron and carbon dioxide in a blast furnace 700+ degrees Celsius.
Manganese(IV) oxide is reduced in the same way, but at higher temperatures of 1200 degrees Celsius.
Copper carbonate is reduced with carbon when the ore is heated directly. Alternatively it can be decomposed to copper oxide and carbon dioxide, and the oxide can be reduced with carbon.
Pros and Cons of recycling metals
Saves raw materials- ores are a finite resource.
Saves energy- recycling takes less energy than extracting metal. This saves money too.
Reduces waste sent to landfill.
Mining damages the landscape and landfills are ugly. Recycling reduces this.
Collecting and sorting metals from other waste can be difficult and expensive.
The purity of recycled metal varies - there's usually other metals and other impurities mixed in.
Recycling metals may not produce a consistent supply to meet demand.