Lime is made from indigenous limestone or chalk rock, one of the most abundant materials in the Earth’s crust. Limestone and chalk are both sedimentary rocks and are chemically known as calcium carbonate.
When crushed or ground, they are widely used as aggregates and building products.
As a raw material, calcium carbonate can also be processed to form lime. When the rock is heated in a specially designed kiln to over 900°C, a chemical reaction occurs and creates calcium oxide, otherwise known as lime. Learn more on each of the 4 stages in the production process:
How lime is made
Industrially exploitable deposits are characterised by the purity of the carbonate and thickness of the deposit. Therefore, any potential mining site is explored thoroughly in advance. Its viability and environmental impact are carefully assessed before opening a new quarry and its future rehabilitation is foreseen.
Geologists first create a geological mapping of the mining site, which locate the available reserves and their quality grades.
With the help of modern technologies such as GNSS, the geologist is able to create a block model.
The potential environmental impact is also carefully assessed before opening a new quarry and its future rehabilitation is already planned before operations start.
Modern technologies and expertise allow the most efficient use of these natural resources. This allows an economical, ecological and sustainable extraction of the limestone or dolomite.
The whole process of making any type of lime all begins at the limestone quarry after careful surveys.
Most limestone is extracted through blasting. Behind the rock face, holes are drilled to place the explosives. When detonated, the explosion dislodges each time up to 30,000 tonnes of stone. Today’s blasting makes use of the latest technology whereby modern electronic detonators are used to reduce vibration to a minimum.
The dislodged rocks are then picked up at the quarry ‘face’ by huge excavators, working along the rock bench.
The excavators load the big rocks into dumpers to transport them to the primary crushers.
Crushing and washing
The trucks tip the limestone into a large primary crusher which usually relies on either impact or compression to break the rock. Depending on the size of the feed stone required and the kiln in which it will fed into, the same stone can go through a second and even a tertiary crusher to reduce its mass even further.
The stone is then screened into a wide range of different sizes from 125mm kiln stone all the way down to dust. Some of the stone at this point is washed to remove any clay particles that may remain.
After crushing the limestone is classified according to its final use in different fractions. For example, bigger rocks are calcined in shaft kilns and smaller ones go to rotary kilns. Some fractions are aggregates for road construction or milled to limestone powder.
Calcination is the thermal process of heating limestone. This processed stone is then transferred by conveyors to the lime kilns. The lime burning process within the kilns requires enough heat to be transferred to the limestone in order to decompose the calcium and magnesium carbonates. Heat transfer for lime burning can be divided into three main stages:
- ‘Preheating zone’ – limestone is heated to approximately 800°C by direct contact with gases leaving the calcining zone.
- ‘Calcining zone’ – fuel is burnt in preheated air from the cooling zone. This produces heat at above 900°C and turns limestone into quicklime and CO2.
- ‘Cooling zone’ – quicklime leaving the calcining zone at 900°C is cooled by direct contact with ‘cooling’ air.
There are currently three distinct types of kiln operating, these include: shaft kilns, rotary kilns and twin shaft parallel flow regenerative kilns. Each kiln is selected depending on the nature of the feed stone used and the quality of quicklime required.
Click here to find out more on different types of kiln.