The lime cycle is one of nature’s best known examples of chemistry. The diagram below illustrates how limestone/chalk (calcium carbonate) turns into quicklime (calcium oxide) after heating, then into hydrated lime (slaked lime or calcium hydroxide) after adding water and finally back into limestone/chalk after it reacts with carbon dioxide from the air. Similar reactions also occur with Dolomite where a mixed Oxide of Ca.MgO (Dolomite Lime) is produced.

This classic series of chemical reactions is the basis for numerous applications of lime, many of which affect our lives every single day.

Limestone/Chalk

Calcium is the 5th most abundant element in the Earth’s crust and limestone/chalk forms 20% of the World’s sedimentary rock. In the UK, calcium carbonate is referred to as limestone or chalk.

In the Peak District, limestone was formed during the Carboniferous geological period, some 340 million years ago.  At this time, Britain was part of a large continental landmass close to the equator. In these tropical conditions rivers flowed into shallow warm seas teeming with primitive fish, molluscs, and coral reefs. Their calcium shells combined with silt to form layer upon layer of calcium carbonate rich sediments several hundred metres thick. The fossil remains of these ancient plants and animals are easily recognisable in limestone.

In the chalk deposits, found mainly on the east of the UK, the rock is formed from the skeletons of billions of microscopic marine algae called coccolithophorids, which used sunlight to synthesise food.  They died and settled on to the sea bed in the Upper Cretaceous period (between 65 – 100 million years ago).  These algae can still be found in warm waters.

The word calcium is derived from the Latin "calx", meaning lime. It was discovered early in the history of civilised man and there are references to lime in both Egyptian and Roman times, in fact as far back as the first century (see Amazing Facts for more detail).

Facts:

• Chemical formula: CaCO₃
• Molecular Mass: 100.09
• Decomposes when calcined at 900°C for form calcium oxide

Calcining (or burning)

As mentioned above, calcination or burning of limestone occurs at roughly 900°C. Kilns are often heated to 1100°C in order to ensure core regions of limestone pieces also reach this temperature. This process however can often prove harmful to the smaller pieces that become over heated and begin to sinter on the surface.

The reaction for the thermal decomposition of calcium carbonate is as follows:

CaCO₃ + heat ↔CaO + CO₂
  100g        56g       44g

Different kilns are used for different types of limestone and for different types of product (see ‘How Lime is made’ for more detail).

Quicklime

Quicklimes principal component is calcium oxide. Its quality often depends on a number of certain factors including physical properties, reactivity to water and chemical composition. As the most readily available and inexpensive alkali, quicklime plays an essential part in a wide range of industrial processes.

Facts:

• Chemical formula: CaO
• Molecular Mass: 56.08
• Melts at 2614°C, boils at 2850°C
• Hydration (or slaking)

This involves adding water to quicklime, which produces an exothermic reaction (gives out heat).

The reaction for the hydration of quicklime is as follows:

CaO + H₂0 = Ca(OH)₂ + heat

Hydrating plants are relatively complex and can be fed with surplus grades of quicklime.

Hydrated Lime

Hydrated lime despite its name is essentially dry and generally contains less than 1% of unreacted water. An estimated 10 to 15% of quicklime produced in developed countries is converted into hydrated lime (a percentage which is probably larger for countries which do not have a large steel industry). Slaking however, involves the production of a dispersion of calcium hydroxide in water, creating a product known as milk of lime or lime putty.

Facts:

• Chemical formula: Ca(OH)₂
• Molecular Mass: 74.09
• Decomposes at 580°C to form calcium oxide

Re-carbonation

The re-carbonation process is essentially the opposite of the calcining/burning process (see ‘How Lime is made’). Both quicklime and hydrated lime when exposed to the air for too long begin to draw in carbon dioxide from the atmosphere. This therefore replaces the oxide component of the chemical and turns the lime roughly back to its original state – being limestone or chalk.

The formula is as follows:

Ca(OH)₂ + CO₂ = CaCO₃ + H₂O