How lime is made
Find out how to make lime from limestone or chalk in the ground and how a lime kiln transforms these rocks into a valuable product.
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Lime is made from limestone or chalk, which are sedimentary rocks.
A key area for limestone formation in the UK was the Peak District. It 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 fossilised remains of these ancient plants and animals are easily recognisable in limestone.
Chalk deposits, found mainly in the east of the UK, were 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 and 100 million years ago). These algae can still be found in warm waters today.
To watch a video of how sedimentary rock was formed, please follow this link www.bbc.co.uk/learningzone/clips/the-formation-of-sedimentary-rock/10621.html
For further information please see the geological map of the UK by clicking here.
Shaft kilns can burn stone from a minimum of 20mm diameter up to 175mm. Some shaft kilns can be operated on natural gas, liquid and solid fuels. This type of kiln tends to produce medium reactivity quicklime, which can then be used in a number of industrial processes including the manufacture of iron and steel and aerated concrete blocks. Quicklime from shaft kilns is also processed into hydrated lime.
Rotary kilns consist of a rotating cylinder inclined at an angle of 3 to 4 degrees to the horizontal. Limestone, chalk or Dolomite is fed into the upper ‘back end’, and fuel plus combustion air is fired into the lower ‘front end’. The product is then discharged from the kiln into a cooler, where it is used to pre-heat the combustion air. Kilns of this type are usually fed with stone ranging in size from 15mm to 40mm and are fuelled by a range of fuels including coal, petroleum coke, natural gas and waste derived fuels. They are used to produce Dolomitic lime and higher purity quicklime, used for the manufacture of low-carbon steel, fibreglass and healthcare products. In particular, rotary kilns are also used to burn Dolomitic limestone at high temperatures to produce sintered Dolomite.
Twin Shaft Parallel Flow Regenerative Kilns
Twin shaft parallel flow regenerative kilns have two inter-connected, vertical shafts which are fired in sequence to achieve excellent energy efficiency. The stone size used by these kilns is usually between 90mm and 125mm. They are, on the whole, fuelled by natural gas and produce high reactivity and high purity quicklime. This is often used in industrial effluent treatment, domestic sewage treatment, manufacture of aerated concrete blocks, steelmaking and soil stabilisation.
Quicklime can be processed even further into hydrated lime by adding water. Depending on the facilities at the quarry, this can occur either on site or by transporting the lime to a separate hydrating plant. A basic hydrating plant consists of four stages:
- Quicklime handling and crushing
- Storage and despatch
Handling and Crushing
Where the removal of impurities in the hydrating plant is not important, the quicklime is often reduced in size using impact breakers. In other circumstances, rolls and jaw crushers or cone mills may be used.
Hydrators usually consist of three main sections – prehydrator, hydrator and finishing stage. The plant consists of numerous paddles that help to mix water and quicklime quickly and efficiently from start to finish. The plant is kept under slight suction to prevent any dust emissions throughout the process. The final moisture content of the raw hydrate after the finishing stage is usually about 1%.
The raw hydrate is then taken from the hydrator to the classification plant. Depending on the customers’ specification, the raw hydrate can be adjusted even further to meet their requirements.
Storage and Dispatch
All finished products, whether they be Dolomitic lime, quicklime or hydrated lime, can be stored on site prior to transportation by rail, road or ships. Alternatively, they can be packed into bags, which are then purchased by a range of customers including steelmakers, DIY stores and builders’ merchants.
Dolomitic lime is made from Dolomitic limestone.
CaCO3 MgCO3 + heat = CaO + MgO + 2CO2
It is a mixture of lime (CaO) and magnesia (magnesium oxide, MgO).
Dolomitic limestone is formed by the replacement of some of the calcium in limestone with magnesia; a process known as Dolomitisation. Dolomitic lime is also known as:
- Calcined Dolomite
- Lightly calcined Dolomite
- Burnt Dolomite
Dolomitic lime is produced using rotary kilns fired with natural gas, coal, petroleum coke and a range of waste derived fuels (such as end of life tyres and solvents) at temperatures of around 1000ºC. Horizontal rotary kilns are used to ensure complete decomposition of Dolomite, i.e. low levels of CO2 after burning and to achieve the high temperatures required for sintered Dolomite products.
Sintered or dead burnt Dolomite is produced by burning rock at temperatures of around 1000ºC and with a longer residency time in the kiln, which causes the material to densify. The small crystals of MgO grow larger and the pores in the structure disappear. The bulk density may increase from around 1600 to over 3100 kg/m3. This dense form of Dolomitic lime is the material used as a basis for a range of refractory products, which are important for making steel.
Plants producing Dolomitic lime are much less common that those producing high calcium lime. This is partially because Dolomitic limestone is less widely distributed than limestone, but also because of the more restricted uses.
The principal use of Dolomitic lime is in the steel industry. In the UK, around 250,000 tonnes per year is used for this purpose. It is used for refractory bricks and a wide range of monolithic and gunning materials. The bricks are also used in lime and cement kilns. Dolomitic lime is also used in other applications as a source of lime and magnesia, or as an alkali to raise pH. These include:
- as a slag component in the production of alloys such as ferro-chrome, ferro-manganese etc
- as a binder for pellets/sinter in iron making
- as a source of lime/magnesia in the glass fibre industry
- as a source of MgO and a pH modifier in agriculture and horticulture
- as a neutralising agent in water treatment
- as a reagent for producing magnesia from seawater and brines
- as a pH modifier in drilling muds
- as a pH modifier in sewage sludge treatment
- as a component in traditional mortars