Tuesday 18 February 2014

GCSE chemistry - unit 1 - extracting metals

Extracting Metals

Metals are very useful as so many thing are made out of it. Metals are found in metal ores (rock with large amounts of the metal of interest; enough metal to make it worth while)

The metal ore contains impurities  and the actual metal is chemically combined with substances (usually oxygen).

First we CONCENTRATE the metal ore (basically get rid of any impurities) by SMELTING (heating) it.

The metal ore without impurities then under goes either DISPLACEMENT or ELECTROLYSIS to remove what it is chemically combined with; mostly oxygen)

All of this will happen only if the metal that is produced is worth more than the cost of getting it out if the ore (A HIGH GRADE ORE). If it is not worth while, they need to find a cheaper way of extraction or one that gets more of the metal out. If not, it's not worth it.

The reactivity series

Potassium              MOST REACTIVE
Sodium
Calcium
Magnesium
Aluminium
-----------------
*carbon*
-----------------
Zinc
Iron
Tin
Lead
Copper
Silver
Gold                      LEAST REACTIVE

If the metal is LESS reactive than carbon they undergo DISPLACEMENT. This means they are reacted with carbon and because the carbon is more reactive than them, it will take away the oxygen the the metal is bonded with to leave the metal and carbon dioxide. This is the cheaper of the two.

If the metal is MORE reactive than carbon they undergo ELECTROLYSIS because carbon is less reactive than theses metal so won't be able to steal the oxygen from them. Electrolysis uses electricity to separate the metal away from the oxygen.

IT IS IMPORTANT TO NOTE THAT GOLD IS SO UNREACTIVE THAT IT IS FOUND NATIVE IN THE GROUND (DOES NOT NEED TO BE EXTRACTED)

E.g. COPPER EXTRACTION

ELECTROLYSIS - a negative and positive electrode are placed in the melted ore solution (IN COPPER'S CASE IT IS CHEMICALLY BONDED WITH SULFUR NOT OXYGEN). The sulfur and copper have bonded ionically so the copper is made up of positive ions and the sulfur, negative ions. The positive copper ions are attracted to the negative electrode so is now separated and can be purified. Salt is left at the positive electrode.

DISPLACEMENT - scrap iron (more reactive than copper) removes the sulfate from the copper sulfate and the copper coats itself around the iron which can be easily removed and purified. This is a lot cheaper

The problem is, there's a shortage of high grade copper ores. We can use PHYTOMINING and BIOLEACHING  to extract copper from low grade ores (containing less than 1% copper) and then use electrolysis and displacement as usual after.

Phytomining

  • Grow certain type of plant on land with low grade copper ore in it
  • Plants take copper compounds into their body tissue
  • Burn plants to leave ash that contains copper which can be extracted by electrolysis or displacement
Bioleaching
  • Use certain bacteria to grow on the low grade copper ore to use it as a source of nutrition
  • The bacteria produce a byproduct solution containing copper
  • The solution goes through electrolysis or displacement to extract the copper






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