Limestone in industry

Hope Quarry
Coldstones Quarry

Limestone is quarried in Britain as either large blocks that are used for building, or in small pieces the size of sand or gravel, which are used for industrial purposes.

When limestone is crushed into powder, it is sometimes referred to by its chemical name, calcium carbonate, or its chemical symbol, CaCO3. Limestone and CaCO3 are therefore the same material, but used in different ways — the first is used as aggregate in the construction industry or as a 'flux' (used for removing chemical impurities) in the iron and steel industry, while the second is used in the chemical and manufacturing industries.

When limestone is heated ('calcined') to temperatures of between 900 and 1000°C, carbon dioxide (CO2) is driven off and calcium oxide (CaO) or 'lime' is the result. Lime is an important ingredient in the manufacturing and chemical industry. Not all limestones are suitable due to the impurities they contain.

The largest source of pure limestone is Carboniferous limestone, which is quarried mainly in areas of natural beauty including the North Pennines, Peak District and Mendip Hills. Chalk in southern and eastern England is the other important source of CaCO3 and is used especially for cement, but is also used in other ways such as toothpaste and in baking.

There are many uses for chalk and limestone. Most is used by the construction industry, but some is used in manufacturing and the chemical industry.

Limestone mining

Mining is much more expensive compared to quarrying and so it is rarely carried out. For a limestone to be economically mined, it would have to be either particularly pure for industrial processes or in high demand as a special building stone. There is one mine producing industrial quality limestone in Derbyshire. Bath Stone is another example — it is taken from old mines that extend for tens of kilometres beneath Bath and surrounding area (although these are largely abandoned and used for growing mushrooms).

Manufacturing industry

Lime fertiliser ready for use

When heated and in some cases made into a slurry or combined with salt, limestone is used in the manufacturing process of many products including:

  • animal feeds: as a source of calcium
  • baking powder: added to regulate acidity and in bicarbonate of soda
  • bleaching powder: this is made by passing chlorine over hydrated lime resulting in calcium hypochlorite (only very pure limestones can be used for this)
  • carpet backing: powdered limestone is used as a filler
  • cosmetics: limestone is used as a filler
  • fertiliser: limestone is used as a carrier for calcium ammonium nitrate fertiliser
  • floor tiles: limestone is used as a filler
  • glass: this is made mainly out of sand, but it is usually mixed with other things. The type of glass most often used (e.g. for windows and glassware) is called soda-lime glass. It is made by heating together sand and crushed limestone or lime and soda (sodium carbonate)
  • paint and varnish: powdered limestone is used as a filler
  • paper: lime is used for processing wood pulp and paper-mill waste; powdered limestone is also used as a filler in paper
  • pharmaceuticals: limestone is used in the manufacture of milk of magnesia, pills etc. Purity is important so limestone powder is dissolved in acid and then the pure CaCO3 is precipitated from solution
  • plastics: powdered limestone is used as a filler
  • rubber: lime is used as a filler
  • sugar: limestone is used in clarification and purification of the raw juice during sugar refining
  • textiles: for scouring and bleaching cotton and other fibres
  • toothpaste: crushed limestone is used as an abrasive

Chemical industry

BP's chemical manufacturing plant in Salt End, Hull

The chemical industry uses very pure limestone in a number ways, including the manufacture of:

  • ammonia
  • calcium carbide
  • calcium chloride
  • caustic soda
  • soda ash
  • sodium bicarbonate
  • sodium bichromate
  • lime

Neutralising acids

Lime kilns

When acid and limestone react, the limestone and the acid change. For example, when limestone (CaCO3) is added to hydrochloric acid (HCl), they form calcium chloride, water and carbon dioxide:

2HCl  +  CaCO3  →  CaCl2  +  H2O  +  CO2

This results in chemical weathering of limestone in nature, but another way of looking at this is that the acid is neutralised.

Limestone has useful applications in neutralising acids:

  • neutralising lake water that has been affected by acid rain (limestone may be heated and turned into 'slaked lime', which neutralises lake water more quickly than limestone)
  • controlling the acidity of soils (again, 'slaked lime' neutralises soils more quickly than limestone)
  • lime is used in water and sewage treatment and to neutralise acid waste

Construction industry

Giggleswick Quarry, near Settle.

Most limestone goes to the manufacture of cement for concrete and mortar. Both limestone and chalk are important in making cement.

The most common type, Portland cement, is made of crushed limestone mixed with clay and heated in large kilns.

A little calcium sulphate powder is added to the cement to stop it setting too quickly when water is added.

Cement is:

  • mixed with sand and small stones plus water to make concrete
  • mixed with sand and water to make mortar, for brick laying

Limestone and dolomite are often crushed to make aggregates and much of the limestone from the Yorkshire Dales is used for this.

Limestone aggregate is:

  • used for road building
  • turned into burnt lime and used in road base stabilisation
  • turned into mortar concrete
  • used as a filler in asphalt

Limestone blocks are used in the construction of buildings.

Iron and steel industry

Corus steel works, Scunthorpe, Lincolnshire

Limestone is poured into blast furnaces when iron is being extracted from iron ore. In the blast furnace, the limestone acts as a 'flux', which changes into calcium oxide and carbon dioxide.

The calcium oxide reacts with the silicon dioxide in sand to form calcium silicate (CaSiO3), which is called 'slag'. This floats on the surface of the molten iron and can be easily removed. If the limestone was not added, iron silicate would be formed and metallic iron would be lost. Adding limestone also has the advantage of reducing the temperature required to melt the iron, so saving on energy.

Although slag is a waste product of the iron and steel industry, it can be turned into 'slag cement' and used to make roads.

Burnt lime made from calcite and dolomite is also used in steel making. Limestone is also used as a flux in copper and lead smelting.