Calcium Oxide and Calcium Carbonate

• Limestone consists mainly of calcium carbonate, CaCO3

• Lime which is calcium oxide, CaO, is manufactured from calcium carbonate by thermal decomposition:

  • CaCO3 →  CaO + CO2

• Slaked lime, calcium hydroxide, Ca(OH)2, is made by adding a small amount of water slowly to calcium oxide:

  • CaO+ H2O  →  Ca(OH)2

• The water is added slowly because the reaction is quite exothermic

• Limewater is a solution of calcium hydroxide in water, hence it is weakly alkaline

• Bubbling carbon dioxide through limewater produces a cloudy white precipitate of calcium carbonate, so the sequence of reactions has gone in a complete cycle, returning to the starting material:

  • CO2 + Ca(OH)2  →  CaCO3 + H20

• This reaction is the basis of the standard chemical test for CO2

• The combination of these three reactions constitute the limestone cycle

Use of limestone and limestone products

• Limestone (calcium carbonate) is used in the manufacture of iron and cement

• In the production of iron, limestone is added to the blast furnace where it decomposes to form lime (CaO) and carbon dioxide

• The CO2 produced is reduced to carbon monoxide, CO, by coke (almost pure carbon) where it acts as a reducing agent transforming iron ore into iron:

  • CO2 + C → 2CO

  • Fe2O3 + 3CO →  2Fe + 3CO2

• The lime reacts with silica impurities in the iron ore to form calcium silicate, which floats to the top of the molten iron as slag and is removed:

  • CaO + SiO2 CaSiO3

• Cement is manufactured by heating a mixture of powdered limestone and clay in a rotary kiln

• Once heated, calcium sulfate and water are added which produce cement

• Cement is a hardened, interlocked structure of calcium aluminate (Ca(AlO2)2 and calcium silicate (CaSiO3)

• CaCO3 is also used in treating excess acidity in soils and lakes where it is often preferred to lime because it does not make the water excessively alkaline

• Lime (calcium oxide) is used in lime mortar, the material used to bind bricks together, and in flue-gas desulphurisation

• Flue-gas desulphurisation involves spraying jets of slaked lime slurry into acidic sulfur dioxide emissions to reduce pollution by neutralising these gases before they leave the factory chimneys

• Slaked lime (calcium hydroxide) is used in treating acidic soils and neutralising acidic industrial wasted products