Roasting and leaching
Roasting is the process of transforming sulphide minerals into oxides. Sulphide ores are roasted or heated in plenty of air. The sulphide S changes to sulphur dioxide. The metal reacts with oxygen in the air to become a metal-oxide. Heating removes gaseous and other volatile impurities. Leaching is when copper ions are dissolved into solution using a leach solution.
Cu2+ + 4NH3 → [Cu(NH3)4]2+
ionic copper + ammonia → dissolved complex (blue)
Extraction
Leaching the ore
Put 50g of roasted ore into a conical flask; add 100cm³ of ammonia. Gently swirl mixture then place cotton wool in the neck of the flask. Place on a warming tray for 30 min.
Add 20cm³ of water and swirl solution, leave over night in a fume cupboard
Vacuum filtration
Decant the leach solution without disturbing and pouring out any of the solid. Add some water to your solid and swirl, leave until the rest of the decanted solution has been filtered.
Assemble vacuum filtration equipment; Buchner funnel with coarse filter paper, 250cm³ conical flask, attach vacuum pump, ready solution for filtering.
Filter the first leach solution changing filter paper every so often, do not over fill conical flask as some of your solution will get sucked away down the sink. Decant filtrate into an adequately sized beaker. Filter the second solution form the water you added to the solid to get all of the available copper out, add to the rest of the decanted solution.
Add some more water to the solid, swirl and leave. Repeat filtrations as for the other solutions; add to the rest of the decanted liquid.
Neutralising the leached solution
Add 4mol dm³ sulphuric acid to the solution to react with hydroxide and carbonate ions, ammonia solution. Keep adding acid until the precipitate dissolves.
Add a little sodium hydroxide and sulphuric acid to balance the solutions pH level to it being slightly acidic.
Producing and weighing the copper
Add zinc dust to the solution until it is no longer blue the copper should become displaced by the zinc, as it is higher in the reactivity series
The solution now contains zinc which has to be removed by adding sulphuric acid 1cm³ at a time until effervescence has ceased.
Filter off the displaced copper through vacuum filtration, obviously keeping what is in the filter and not the filtrate.
Wash the copper in the filter twice with water and twice with propanone. Make sure the pump is turned off so there’s time for the liquid to soak into the copper. Turn on the tap to carry away the propanone vapour from the copper solid this will also dry the solid.
Spread out the solid on a weighed watch glass then record the mass of your copper on the watch glass. Work out the weight of your copper.
Industrial copper extraction
Mining
An open pit around 4km wide and 1km deep is mined for the ore. 260,000 tonnes of waste a day and only 150,000 tonnes of ore a day.
Concentration
The first thing they do to the ore is to grind it up into a fine powder, the individual copper minerals are liberated. This means the former piece of rock, with many different types of mineral and material, has now been ground up so much that the different minerals are now individual see figure 1.
Figure 1.
The powder is now ready for froth floatation. A process which separates the rock from the copper minerals. The powder is put into water which has a collector chemical which gives the copper a hydrophobic hydrocarbon coating. Air is pumped into the bottom of the solution; because the copper is now hydrophobic it is attracted to the air bubbles which take the copper mineral to the top in the froth. The percentage of pure copper is now 30% rather than 0.6% as it was in the rock.
Smelting
The chalcopyrite is passed down a heated reaction shaft along with silica and oxygen enriched air. The exothermic reaction that occurs is called flash smelting. The iron oxide and sulphides react with the silica to make a top layer of molten slag and a bottom layer of copper sulphide (Cu2S). this copper matte is then sent into another furnace to be blown with oxygen to make copper and sulphur dioxide.
Cu2S (l) + O2 (g) 2Cu+ (l) + SO2 (g)
This copper 99% pure, it is known as Blister copper. It is purified to 99.4% before electrolytic refining.
Electrolytic Refining
The 99.4% pure copper is used as the positive electrode and sheets of pure copper are used as the negative electrodes. Both in an electrolyte of copper sulphate. Copper dissolves from the possitve electrodes and deposits on the negative electrodes.
Negative electrode:
Cu2+ (aq) + 2e- Cu (s)
Positive electrode:
Cu (s) Cu2+ (aq) + 2e-
The electrolysis takes 2 weeks, 120 kg of copper is deposited on the negative electrodes. These are washed and re-used; they are also some times made out of stainless steel. Some of the impurities that are found in the sludge at the bottom of the electrolyte are some times valuable and collected e.g. gold, silver. Some impurities are soluble so the electrolyte has to be changed regularly. The copper produced is over 99.8% pure, which is the standard for electrical components.
