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

Extraction

  • In the quarry, explosives are used to break up limestone or chalk rock. This can dislodge up to 30,000 tonnes of rock in one explosion.
  • The broken rock is then picked up at the quarry face by huge, mechanised excavators.

Crushing and Screening

  • Trucks then tip the rock into crushers, which break down the rock into smaller pieces.
  • Screeners sort and separate the rock pieces into different sizes.

Emissions Control

  • A number of filters and scrubbers control the dust and gases generated from burning the rock.

Fuels

  • Different types of fuel are added to power the kiln.

The Kiln

  • The rock is heated to 800ºC in the preheater and then from 1200ºC to 2000ºC to make lime.
  • The burn temperature and time in the kiln depends on the type of rock that is used as the raw material.
  • The kiln can either be horizontal or vertical.

Cooling

  • The lime that leaves the kiln is cooled with air.

Hydration

  • Sometimes after cooling, water is added to lime to make hydrated lime.
  • The type of lime that is made depends on what the customer is using it for.

Storage and Dispatch

  • Finished lime products are safely wrapped, packaged and stored on site.
  • They are then sent to the customer by road, rail and even boats overseas.

Exploration

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.

Extraction

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.

Processing

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.

 

Classifying

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

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:

  1. ‘Preheating zone’ – limestone is heated to approximately 800°C by direct contact with gases leaving the calcining zone.
  2. ‘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.
  3. ‘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 kilns