Crushing

The trucks then 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 feedstone required and the kiln in which it will fed into, the same stone can go through a second and even 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.

Kiln zone

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 in the UK, these include; shaft kilns, rotary kilns and twin shaft parallel flow regenerative kilns. Each kiln is selected depending on the nature of the feedstone used and the quality of quicklime required.  

Shaft kilns

Residence time approx 36 hours.

Produces medium carbonate / medium reactivity quicklime (Ca0).

Used mainly in steel industry processing. Major feed for hydrated lime manufacture.

Shaft kilns can use limestone from a minimum of 20mm 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 (see Hydrate zone).

Rotary kilns

Residence time approx 5 hours.	Extremely flexible processing.
Quick change-over to different specifications within 3/4 hours.	Low carbonate grades for stainless steel processing.
Can be fired on coal or gas.

The rotary kiln consists of a rotating cylinder inclined at an angle of 3 to 4 degrees to the horizontal. Limestone 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 recycled materials. They are used to produce dolomitic lime, and highier purity quicklime used for the manufacture of low-carbon steel, fibreglass and healthcare products. Rotary kilns are also used to fire dolomite at high temperature to produce sintered dolomite for the production of dolomitic refractories.

Twin Shaft Parallel Flow Regenerative kilns

Residence time approx 18 hours.	Flexible and controllable.
Produces medium and low carbonate / high reactivity quicklime (CaO).	Twin shaft operation gives good thermal efficiency.
Uses: steel industry processing, ground into fine powders for concrete block production and environmental effluent treatment markets.

Twin shaft parallel flow regenerative kilns have two inter-connected, vertical shafts which are fired in sequence to achieve excellent energy efficiency. The limestone 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 which is often used in industrial effluent treatment, domestic sewage treatment, manufacture of aerated concrete blocks, steelmaking and soil stabilisation.

Hydration plant

Quicklime can be processed even further into hydrated lime. 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:

1. Quicklime handling and crushing
2. Hydration
3. Classification
4. Storage and despatch

Sophisticated control and monitoring systems maintain end-product purity, consistency and quality.

Flexibility of using quicklime feed from different kilns to give specific characteristics to meet customers requirements.

Quicklime can be processed even further into hydrated lime. 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:

1. Quicklime handling and crushing
2. Hydration
3. Classification
4. 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.

Hydration:

Hydrators usually consist of three main sections – prehydrator, hydrator and finishing stage. The plant consists of numerous paddles that help to mix the water and quicklime quickly and efficiently from start to finish. The plant is kept under slight suction to prevent any dust emission throughout the process. The final moisture content of the raw hydrate after the finishing stage is usually about 1%.

Classification:

The raw hydrate is then taken from the hydrator to the classification plant. Depending on the customers specification, the raw hydrate can be adjusted and cut even further to meet their requirements.

Storage and despatch:

Finished products, whether they be dolomitic lime, quicklime or hydrated lime, can be either stored on site prior to dispatch by rail or road, or alternatively can be packed into bags which are then purchased by a range of customers, including steelmakers, DIY stores and builders merchants.